PUR Facts: Conservation of Polyurethane Foam in Art and Design 9789048512072

Table of contents :
Table of Contents
Preface
Introduction
1. Polyurethanes, manufacture and applications
2. Chemistry, properties and degradation
3. History of polyurethane foam conservation
4. Ageing behaviour of polyurethane foam
5. Assessing the condition of polyurethane foam
6. Case studies
Acknowledgements
Glossary
General information
Bibliography

Citation preview

PUR Facts

Publications in the Cultural Heritage Agency of the Netherlands series (Rijksdienst voor het Cultureel Erfgoed) provide knowledge about conservation, restauration, and collections for the purpose of practical use.

Other titles in this series Klimaatwerk, Richtlijnen voor het museale binnenklimaat Bart Ankersmit 2009, ISBN 9789085550259 Gekregen! Aanwinsten van de staat 1990-2010 Fransje Kuyvenhoven (red.) 2011, ISBN 9789089642998 Inside Installations, Theory and Practice in the Care of Complex Artworks Tatja Scholte and Glenn Wharton (eds.) 2011, ISBN 9789089642882

PUR Facts Conservation of Polyurethane Foam in Art and Design

PUR Research Project by Thea van Oosten Aleth Lorne Olivier Béringuer

Cover illustration: The polyurethane foam work of art Untitled by Kirsten Hutsch (1998) no longer exists. Due to oxidation of the polyurethane foam the surface of the artwork was crumbled to such an extent that exhibition was unacceptable. In 2007 the artist remade the work of art (see Figure 31 on page 28) Cover design: Neon, design and communications, Sabine Mannell, Amsterdam Lay-out: V3Services, Baarn isbn 978 90 8964 210 3 e-isbn 978 90 4851 207 2 nur 647 © Thea van Oosten / Amsterdam University Press, 2011 All rights reserved. Without limiting the rights under copyright reserved above, no part of this book may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the written permission of both the copyright owner and the author of the book. Every effort has been made to obtain permission to use all copyrighted illustrations reproduced in this book. Nonetheless, whosoever believes to have rights to this material is advised to contact the publisher.

Table of Contents

Preface

7

Introduction

9 13

1

Polyurethanes, manufacture and applications

2

Chemistry, properties and degradation

3

History of polyurethane foam conservation

4 4.1 4.2 4.3

Ageing behaviour of polyurethane foam 57 Materials and methods 57 Consolidation of polyurethane foam 61 Sun Block for polyurethane ether foam 69

5 5.1 5.2

Assessing the condition of polyurethane foam 81 Condition of polyurethane ester and ether foam 81 Condition of polyuerethane foam of works of art and design objects 86 Instruction for the consolidation of new and aged polyurethane ether foam 87

5.3

6 6.1

6.2

29 47

Case studies 91 Zuccaia (1991) by Piero Gilardi 91 The adventurous consolidation of a polyurethane ether foam Nature Carpet by Aleth Lorne Sassi (1972), by Piero Gilardi 107 Restoration project carried out by the Barbara Ferriani Conservation Studio, Milan Acknowledgements 115 Glossary 117 General information 121 Bibliography 123

Preface

At about the time that Thea van Oosten was born, plastics were the other promise for the future. Now that Thea reaches retirement, many plastic objects have already crumbled to pieces, but she is stronger than ever. The years in between, known as the plastic era (for us the Thea era) have seen the rise in production and artists’ use of plastics. Plastics entered the museum collections, the first conservation problems came to light and conservation science developed a new direction with synthetic polymers and modern materials. This book represents the lifework of a conservation scientist PUR-sang. It shows how the scientist, together with conservator and collection manager, investigates the collection and identifies the preservation problems. It describes how research is then initiated for the development of analytical techniques to study material composition and degradation processes. And how then, based on an understanding of these processes, materials and methods can be developed for the preventive conservation and consolidation treatment of plastic objects. All of this based on multi-disciplinary co-operation with manufacturers, artists, historians and collection managers, but above all with conservators as the case studies and pilot treatments described in the latter chapters of the book show. Working from a real conservation problem in the collections, with the equipment and knowledge of the scientific laboratory, towards a solution that can be applied in museum practice; putting science to work to support conservators. Facts is a fine example of conservation science bridging the gap between scientific theory and conservation practice. That has always been the motivation of the former Netherlands Institute for Cultural Heritage (ICN) and now, after the merger, that will be continued to be supported by the Cultural Heritage Agency of the Netherlands (RCE).

PUR

Cees van ’t Veen, Director Janneke Ottens, Head Research Movable Heritage Cultural Heritage Agency of the Netherlands (RCE)

Introduction

In 1969 scientist Paul F. Bruins wrote the following in the preface to the book Polyurethane Technology: “Polyurethane technology is both a science and an art. Remarkable advances have been made in the past few years in both areas, thus creating the need for a review of the present state of knowledge and a projection into future developments.”

During my research into the conservation of plastics, particularly polyurethanes, I observed that 40 years later, they are even more widely used in science and art than when the above was written. Polyurethanes with tailor-made properties have been developed, and numerous applications for this material and works of art have been created. This has brought with it new challenges for the conservation of these ‘new materials’, such as the consolidation of polyurethane foams in works of art. The main motivation behind this book PUR Facts was to describe the conservation and consolidation treatment of endangered polyurethane ether foams, especially those used in cultural heritage objects. To place polyurethane ether foam in its proper context, however, the book also deals with its history, manufacture and applications and gives examples of works of art and design objects made of other polyurethanes. PUR Facts is a semi-technical publication presenting a brief overview of the field of polyurethane production without going into too many technical details. It seeks to demonstrate general principles so that readers not so well versed in chemistry can obtain an understanding of the versatility and potential of polyurethanes. Although I wanted to keep this book accessible to non-scientists, there are some parts that delve a little deeper into the chemistry of polyurethanes, as this is needed for a fuller understanding. Hence this book is aimed not only at conservators and conservation scientists but also at curators and keepers of collections, art collectors and artists and designers who wish to know something about fields of expertise other than their own.

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Chapter 1 describes the production and use of flexible polyurethane foams in daily life and in objects of cultural heritage. Ever since polyurethane foam first became available, artists and designers have had a great appreciation for this ‘new’ material. Forty years later, we see ourselves confronted with the limited durability of the early polyurethane foams: loss of flexibility, cracking, crumbling and powdering of the foam. The polyurethane ether foam carpets by the artist Piero Gilardi and the history of the ICN Polyurethane (PUR) Research Project at the Netherlands Institute for Cultural Heritage (ICN) are closely intertwined. Research into the conservation of polyurethane foam started in 1997 with Angurie, better known as Still Life of Watermelons, which is part of the collection of Museum Boijmans van Beuningen in Rotterdam. Dating from 1967, Still Life of Watermelons is a large carpet representing a field of watermelons. By 1997 the polyurethane ether foam had become very brittle. At the time the conservation possibilities for degraded polyurethane foam objects were still very limited. The materials of Still Life of Watermelons were investigated, and several solutions were proposed for conservation. In 1999 the results were published in Modern Art: Who Cares [de Jonge 1999]. This first experience of conservation was somewhat empiric but nevertheless important. It made us aware of the lack of precise knowledge about the degradation process of polyurethane foam and the limitations of the conservation methods at our disposal. The problem with polyurethane foam was acknowledged, but there was an enormous lack of knowledge in art conservation circles worldwide, although some attempts at preservation had been made in the early years (see Chapter 3 on the history of polyurethane foam conservation). This formed the principle motive for starting the ICN PUR Research Project. To start with, a risk analysis of polyurethane foam objects was carried out. The main question: What is the significance of the object and to what extent will the object gain or lose its significance when treated? That same year, another polyurethane foam carpet by Piero Gilardi named Zuccaia crossed the path of the ICN PUR Research Project. Zuccaia represents a brightly coloured field of squashes and pumpkins. Part of the field is uncultivated and branches lie on the furrows drawn in the ground. Grass has invaded part of the field, and logs lie at the edges. An important characteristic of the carpet is its large size: 279 x 188 x 35 cm. Zuccaia shares several similarities with Still life of Watermelons, but the works differ in age. In 2000, Zuccaia was only nine years old. It had been commissioned in 1991 by the city of Zoetermeer. After a short temporary exhibition in 1992 at the Floriade World Agricultural Fair in Zoetermeer, the carpet was moved to the city hall where it was exhibited within reach of the public. The carpet’s unusual combination of bright colours and soft materials appealed

introduction

11

greatly to city hall visitors, who could not resist pressing the materials and even pulling tiny parts off. All the same, Zuccaia’s visual qualities were so impressive that despite this minor damage and the slowly growing dust layer, the carpet remained presentable. The turning point in its condition came about in 1997 when a melon was pulled off. After that, Zuccaia was moved to a carpenter’s workshop where it was exposed to the wood dust of the sawing machines. ICN received the carpet in 2000. The Collection Department of the ICN had been asked to take it into temporary storage and to propose solutions for conservation treatment. This provided a wonderful opportunity for the ICN PUR Research Project to assess the developing theoretical knowledge about polyurethane foam on the basis of a real case. The art historical context was studied, the materials of the carpet were investigated, and the artist was interviewed about recent developments in his working technique. Research into the materials of Zuccaia and the assessment of the condition grew simultaneously with the PUR Research Project. The two projects were in permanent communication with one another with the advancement of one project modifying the order of steps taken for the other. Applying the theoretical research to a real case raised methodological and ethical problems. An important step in defining a proposal for treatment was the precise condition assessment of the polyurethane ether foam. The polyurethane ether foam was curatively and preventively consolidated with the nebulisation method developed within the ICN PUR Research Project. Consolidation also included the repair of the mechanical damage. Some parts were restored by inlaying new pieces of polyurethane ether and retouching them. The conservation and restoration of Zuccaia started in 2007 and was completed in 2008. These examples of works of art created from ‘short life’ consumer material demonstrate important conservation problems caused by degradation. Factors causing and accelerating the photo-oxidation of flexible polyurethane ether foams are light, oxygen and heat, leading to discolouration, loss of strength and flexibility and, ultimately, total crumbling. Due to the many open pores, polyurethane foam has a large internal surface and is therefore very prone to degradation. Polyurethanes are both (photo)-oxidised and hydrolysed by exposure to light, heat, relative humidity and chemicals. In general, polyurethane ester foams are more sensitive to hydrolysis, whereas polyurethane ether foams are more sensitive to oxidation. The degradation of polyurethanes is described in Chapter 2. From the 1990s onwards, conservators and curators had concerns about light and oxygen-vulnerable polyurethane foam. An increasing number of degraded polyurethane foam works of art were presented as case studies at conferences.

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At first, only preventive measures were advised, and occasionally, attempts at curative treatment were mentioned. As described in Chapter 3, most forms of treatment between 1990 and 2000 made use of already proven conservation materials such as sturgeon glue and acrylic resins. Several conservators performed impregnation of flexible PUR foam with various consolidating agents as a curative method on degraded objects, thereby hoping to slow down degradation. As part of the PUR research project at ICN, various consolidating agents for the conservation of flexible polyurethane foams were tested and evaluated for their appearance and behaviour, as described in Chapter 4. Due to the decreasing cell struts, polyurethane ether foam will collapse; it cannot withstand the stress imparted during the manufacture of the foam, and the struts break, causing the foam to crumble. In order to protect new, still flexible polyurethane ether foams from degradation or crumbling and to restore the flexibility of old degraded foams, the foams were coated with a light stabilising system (Sun Block). Impranil® DLV combined with Tinuvin®B75 proved to be the best sun block agent for protecting and simultaneously consolidating. As a method and a material, ‘Sun Block’ was tested with special emphasis on applicability, effectiveness in preventing oxidation and influence on the visual, textural and chemical properties of the treated polyurethane ether foam. The decrease in cell strut size, discolouration and increasing hydroxyl index of a polyurethane foam could be related to the condition assessment of polyurethane ether foam as described in Chapter 5. A better insight into the condition of a foam makes it easier to decide whether to recommend treatment in due course or whether immediate consolidation is required. Instructions for consolidation can also be found in Chapter 5. Overall, Chapter 4 is more suitable for conservation scientists, whether Chapter 5 is more for conservators. Chapter 6 clearly describes the consolidation of two works of art by Piero Gilardi with ‘Sun Block’, the solution of Impranil® DLV + Tinuvin®B75. Zuccaia was treated at ICN in 2007 and another work of art by Piero Gilardi called Sassi was treated in Milan in 2009. Methods and materials are discussed in relation to the original meaning and function of the objects and their physical integrity. At the moment, works of art around the world – both polyurethane ether foams and closed cell polyurethane foams – are about to be consolidated using the ‘Sun Block’ method. An example is the work of art ‘Untitled’ by the artist Kirsten Hutsch, shown on the cover of this book. Consolidation using Sun Block is a first step in safeguarding our polyurethane cultural heritage.

1 Polyurethanes, manufacture and applications

What are polyurethanes? Polyurethanes (PUR) are a family of plastics with different compositions and numerous applications, including cellular materials (flexible and rigid foams), fibres, soft and hard rubber (elastomers), surface coatings and adhesives. Polyurethanes are formed by the reaction of a diol (alcohol with two reactive hydroxyl groups) or a polyol (an alcohol with more than two reactive hydroxyl groups per molecule) with a diisocyanate (cyanate with two reactive sides) or a polyfunctional isocyanate (with more than two reactive sides) in the presence of suitable catalysts and additives (Bruins 1969). Polyurethanes can be found as thermoplasts, thermosets and rubber (elastomeric) materials. Thermoplastic polyurethanes consist of linear, long-chained polymers (Figure 1), and they are formed when difunctional reactants (a diol and a diisocyanate) are used.

Figure 1: thermoplast

Thermosetting polyurethanes are branched or cross-linked polyurethanes which are formed if three or more functional reactants (a triol and polyfunctional isocyanate) (Figure 2) are used.

Figure 2: thermoset

Rubber (elastomeric) polyurethanes are co-polymers of hard polyurethane and very flexible polyurethane in which microphase segregation of the hard phase occurs (Figure 3). Figure 3: elastomer

14

pur facts

The clusters of hard polyurethanes shown as thicker lines act as ‘pseudo cross-links’ and allow the material to behave as a rubber (Figure 4).

Figure 4: pseudo cross-links

These thermoplastic-thermosetting polyurethanes come about when an excess of the diisocyanate component is charged during polymerisation. This excess allows the formation of cross-links subsequent to the formation of linear polymer chains and during the post-cure of the formed article. When the temperature is raised, the clusters disassociate, and the material can be made to flow; when subsequently cooled, the clusters reform, and the material again exhibits elastomeric properties. These materials exhibit elastomeric behaviour at room temperature, but can be processed as thermoplastics. Hence the name of this class of materials: thermoplastic urethane elastomer. When hard clusters predominate, thermosetting polyurethanes are formed, whereas soft clusters will give thermoplastic polyurethanes (Dombrow 1957) (see Figure 1-4).

Polyester is a family of organic polymers characterised by the presence of ester groups RO-C=O within the molecule. Polyesters can be prepared so that they have reactive hydroxyl groups and can thus be used as a polyol in the preparation of polyurethane ester foam. Polyether is a family of organic polymers characterised by the presence of ether groups R-O-R within the molecule. Polyethers can be prepared so that they have reactive hydroxyl groups and can thus be used as a polyol in the preparation of polyurethane ether foam. For more information on the chemistry of polyurethanes, see Chapter 2.

When When the chemist Würtz formed urethanes from isocyanates and alcohols in 1864, it signified the birth of the polyurethane industry. The development of polyurethanes in the USA and Europe started after World War II as a result of the studies of Dr. Otto Bayer in 1937 at the laboratories of I.G. Farben (now Bayer) in Leverkusen in Germany [Bayer 1963]. Bayer succeeded in producing

15

polyurethanes, Manufacture and Applications

fibre-forming polymers by reacting aliphatic diisocyanates and aliphatic diols (glycols). In 1938 Du Pont in the USA started research into polyurethanes, but it was not until the 1950s that polyurethanes became commercially available. Before the 1950s, the production of polyurethanes initially focused on fibres, coatings and flexible foams by using the polyaddition reaction and producing polyurethanes from liquid diisocyanates and liquid polyols. During World War II these polyurethanes were applied as aircraft coatings and used only for military purposes. Stiffness, hardness and toughness can be modified using a combination of reactants containing both large and small chain extenders or cross-linking agents. In addition, the diisocyanate component can exert a significant influence on the thermoplastic polyurethane properties. Fillers are added to lower the cost and increase the hardness, tear, strength and modulus. The effect of most filling materials on the mechanical properties of polyurethanes begins to show at a level of around 5 parts per 100 parts of polymer (5%).

Thermoplastic Polyurethane Thermoplastic urethanes (TPU) have been developed since the 1950s. With their high elasticity, flexibility and resistance to abrasion, impact and weather resistance, thermoplastic polyurethanes are used for coatings, adhesives, sealants, paints and elastomers. Furthermore, thermoplastic polyurethanes are used in elastic fibres for textile applications such as artificial leather, in medical devices and footwear, in ski boots, transparent films for laminating, cable jacketing, hot-melt coatings, hoses and medical tubing. Polyurethane adhesives and sealants are used in construction, transportation, marine use and other applications requiring high strength, moisture resistance and durability (www.pfa.org). Thermoplastic coatings are formed when dibasic acid and diethylene glycol form a polyether polyol that reacts with a diisocyanate forming polyurethane ether. Can be applied on textile in the form of synthetic leather

O HO

OH O

Figure 5: adipic acid

HO

O

OH

Figure 6: diethylene glycol Figure 7: Polyurethane (pur) coating on textile. Photo Ulrike Müllners.

16

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Thermosetting Polyurethanes Thermosetting polyurethanes are mostly used for foamed materials and are found as the flexible foam providing comfort in upholstered furniture, mattresses, carpet underlay, vehicle interiors and packaging. Flexible foam can be formed into almost any kind of shape and firmness. It is light, supportive and comfortable. Polyurethanes can be found as rigid foams in the insulation of walls and roofs and as tough and hard elastomers when used for roller blade wheels and for floors and automotive interiors. Production technology, additives and formulations such as reinforced and structural mouldings for exterior automotive parts and one-component systems were developed, and nowadays polyurethanes can be found almost everywhere: in our desks, chairs, cars, clothes, footwear, appliances, beds, cosmetics and insulation in walls and roofs.

Thermosetting flexible polyurethane foam formed by dibasic acid and diethylene glycol forming polyether polyol, using a trifunctional polyol as chain extender reacting with a diisocyanate to form flexible foam.

O

CH3

O HO

OH HO O

Figure 8: adipic acid

O

OH HO

Figure 9: diethylene glycol

OH

Figure 10: trimethylolpropane

Figure 11: Polyurethane foam puppet from the early 1970s television series ‘Larry the Lamb’. Courtesy Victoria & Albert Museum, London. Photo Brenda Keneghan.

polyurethanes, Manufacture and Applications

17

Thermosetting rigid polyurethane formed by three different acids forming a polyester polyol that reacts with a diisocyanate resulting in a polyurethane ester. Found as mouldable objects as well as in foam. HO

O HO

O

OH

OH O

O

Figure 12: adipic acid

Figure 13: phthalic acid O

CH3

O OH HO

Figure 14: oleic acid

OH

Figure 15: Trimethylolpropane

Figure 16: Noah by Benjanin Houlihan. Kunstmuseum, Bonn, Germany. Photo Reni Hansen.

Rubber (Elastomers) Polyurethane rubber originated at the same time as the polyurethane foam industry. However, the development of elastomers was somewhat delayed due to the newness of the engineering characteristics and the difficulty in evolving the totally new engineering concepts required for the successful utilisation of urethane elastomers.a Polyurethane elastomers have evolved along with three different production techniques: first, as millable gums, polyurethane backbones containing unsaturation can be compounded and cured in the manner of conventional rubber; second, as injection and extrusion moulding using polyurethane backbones with controlled molecular weights; and third, as casting when mixed in the liquid state with appropriate co-reactants and poured into the final mould. Elastomers are amorphous polymers with glass transition temperatures below their service temperature. The degree of elasticity is obtained with highly flexible segments, generally low intermolecular forces and little or no crystallinity. In order to reduce creep and high compression set, it is usually a lightly cross-linked polymer. For high tensile strength, tear resistance and abrasion resistance, a higher crystallinity in the polymer is desired. By careful formulation it is possible to produce polyurethane elastomers with various desired properties. The first pur rubbers were developed in Germany around the 1940s. They were known as I Gummi and were produced by reacting polyester with a diisocyanate. At first, it exhibited poor properties until successful investigation eventually led to the Vulkollan types (Bayer) of polyurethane rubber (Dombrow 1957).

18

pur facts

Elastomeric polyurethanes formed by dibasic acid, ethylene glycol, a diisocyanate and using 1,4 butane diol as chain extender results in a polyurethane ether rubber.

O HO

OH

HO

OH

HO

OH

O Figure 17: adipic acid

Figure 18: ethylene glycol

Figure 19: 1,4-butane diol

Figure 20: Polyurethane rubber bracelet (Gaetano Pesce design). Photo author.

Figure 20a: Roller Skatewheel. Photo author.

Thermosetting, thermoplastic and elastomeric polyurethanes differ in their manufacturing techniques and resistance to weathering (ageing) due to different properties. This will be explained further in Chapter 4. In 1954 production started of commercial flexible polyurethane foam based on toluene diisocyanate (TDI) and polyester polyols for flexible and rigid foams (Figure 21). The early polyurethane foams produced from the 1950s to the 1960s were flexible polyurethane ester foams (Bayer 1956).

Figure 21: Production of polyurethane foams at the beginning of pur foam industry. 250

150

polyurethane ester foam polyurethane ether foam

100 50 0 19 54 19 55 19 56 19 57 19 58 19 59 19 60 19 61 19 62 19 63 19 64 19 65

Pounds (millions)

200

Year

polyurethanes, Manufacture and Applications

19

Due to their lack of resilience in comparison with the latex foams mostly used for cushioning at the time, these first polyurethane ester foams were not successful for use in upholstery applications (Bruins 1969). The development of low-cost polyether polyol flexible foams by General Tire in 1956 opened the door for upholstery and automotive applications as we know them today and provided the industry with market-acceptable cushioning foam. The improved properties of polyurethane ether foam sounded the death knell for polyurethane ester foam. All the same, the latter remains widely used, due to its excellent resistance to solvents, fine and even cell structure, and the ease with which it can be bonded to fabrics. Due to the many structural variations possible in their formation, polyurethanes can be described as versatile polymers (Table 1). Polyurethane can be produced in a variety of densities and hardnesses by varying the type of monomer(s) used and adding other substances to modify their characteristics – notably density – or enhance their performance. Other additives can be used to improve the fire performance, stability in difficult chemical environments and other properties.

Table 1 1937

Dr. Otto Bayer discovers PUR chemistry, IG Farben (Bayer) Bayer patents the process

1940

Rigid foam first introduced for aircraft

1941

Adhesive

1948

First insulation application (beer barrel)

1949

Vulcanised polyurethane rubber

1953

Shoe soles, thermoplastic polyurethane films (Epurex) for synthetic leather

1954

Flexible foam upholstery, foam cushioning

1958

Introduction of Spandex fibre Lycra, lamp by Cometa

1960

Steel sandwich building panels

1966

Integral skin for mattresses and shoe soles

1969

Automobile bumpers

1970

Imitation wood, orthopaedics and medical applications

1979

Spray building insulation

1980s

Acoustic insulation

1980s

Profiles/ packaging materials

1980s

Household sponges

1981

Surfboards, skate wheels

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Table 1 1985

Energy-absorbing foams for safety of people

1993

Thin-walled medical hoses, i.e. catheters

1995

Bicycle tyres

2005

Special effects, body and costume display

Although the properties of polyurethane are determined mainly by the choice of polyol, the diisocyanate also exerts some influence and must be suitable for the application. The cure rate is influenced by the functional group reactivity and the number of functional isocyanate groups. The choice of diisocyanate also affects the stability of the polyurethane upon exposure to light. Polyurethanes made with aromatic diisocyanates yellow with exposure to light, whereas those made with aliphatic diisocyanates are stable. Polyurethanes are compounded in the reaction of equivalent amounts of polyether polyols or polyester polyols and diisocyanate. Depending on the chemical structure of the basic polyol components, specific properties for specific applications can be tailor-made, e.g. flexibility for coatings or stiffness for rigid materials. Thermoplastic polyurethanes can be processed using many different techniques. These include extrusion, injection moulding, compression moulding, blow moulding, transfer moulding, calendaring, extrusion coating, film blowing, solution, heat sealing and solvent sealing. Scrap can be reprocessed repeatedly. Thermoplastic polyurethanes are successfully applied as adhesives, film, moulded parts, laminates and coatings (Uhlmann 1977).

Adhesives Polyurethane is used as an adhesive, especially as woodworking glue. Its main advantage over more traditional wood glues is its water resistance. As a step towards improved glue for bookbinders, a new adhesive system was introduced for the first time in 1985. The basis of this system is polyether or polyester while polyurethane is used as a pre-polymer. Its special feature is coagulation at room temperature and reaction to moisture. Polyurethane adhesives and sealants provide strong bonding and tight seals in a variety of applications. Polyurethane adhesives are used in the assembly of shoes, automotive interiors, windshield bonding and as textile laminates. Conveyor belts are usually sealed using polyurethane adhesives, and polyurethane binders are mixed with wood chips or saw dust to form fibre board. Polyurethane sealants are used in road repair, plumbing, construction and any other application where a high-strength, water-resistant seal is required (Randall 2002).

polyurethanes, Manufacture and Applications

21

Fibres Softer, elastic and more flexible polyurethanes result when linear difunctional polyethylene glycol segments, commonly called polyether polyols, are used to create the urethane links. This strategy is used to make spandex elastomeric fibres. The most famous brand name associated with spandex is Lycra, a trademark of Invista (formerly part of DuPont). Lycra ® fibre was originally created in 1958 by DuPont to replace rubber in corset making. Today, the stretch fibre can be found in almost any garment made from natural and man-made fibres: from denim and leather to the finest silks and cottons. Under the name Spandex or Elastane, it is known for its exceptional elasticity (stretchability). It is stronger and more durable than rubber, its major non-synthetic competitor. When first introduced, it revolutionised many areas of the clothing industry. It is one of the most commonly used and best materials in lingerie and its use in garments is still growing, for example in the lining of bra cups. In clothing, flexible polyurethane foam provides thermal insulation, tear resistance, fire resistance and lightweightness to a variety of textiles and fibres including leather products, shoe uppers, tents, liferafts, labels, hand bags, insulation liners and more. It is utilised in circular knit, weaving, covering and other textile applications where fabric stretch and recovery are required. Elaspan® can be found in hosiery, denim (such as stretch jeans), intimate apparel (underwear, bras, etc.), sportswear, and ready-to-wear garments such as shirts and dresses. Products are never manufactured using 100% polyurethane fibre but by blending with other fibres. The fabric known as “two-way tricot” in which a polyurethane fibre is knitted with a nylon or polyester filament yarn is highly elastic and used for swimsuits, leotards, underwear and other stretchable clothing. When scientists discovered that polyurethanes could be made into fine threads, they were combined with nylon to manufacture more lightweight, stretchable garments. Over the years, polyurethanes were improved and developed into Spandex fibres. Spandex has a generic name which, unlike most manufactured fibres, is not derived from the chemical name of the fibre but from jumbling up the syllables in the word expand. Spandex is the preferred name in North America; elsewhere it is referred to as Elastane or Lycra. It is stronger and more durable than rubber, its major non-synthetic competitor (www.polyurethanes.org).

Foams Flexible polyurethane foam is manufactured by the reaction of a polyol and a diisocyanate with water. When the raw materials are combined, the reaction forms bubbles, and the mixture expands, much like the rising of a cake. In a

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matter of minutes the reaction is complete, and the raw materials are converted into a usable product. The most widely used preparation method of foamed polyurethanes is the addition reaction between di- or polyfunctional hydroxyl compounds such as hydroxyl-terminated polyethers or polyesters and di- or polyfunctional isocyanates. Due to the variety of diisocyanates and the wide range of polyols – either polyethers or polyesters – which can be used to produce polyurethane foam, a broad range of materials can be created to meet the needs of specific applications. Polyurethanes have acquired widespread commercial applications within the last 50 years. Foams make up nearly 75% of the worldwide consumption of polyurethane products, with flexible and rigid types being roughly equal in market share. Flexible polyurethane foam is currently one of the most popular materials for adding support and comfort to furniture, bedding and carpet underlay. Flexible and semi-flexible polyurethane foams are used extensively for interior components of automobiles, in seats, headrests, armrests, interior panels, roof liners and instrument panels. Flexible polyurethane foams are lightweight, water resistant, shock absorbent and resilient, making them attractive for use in packaging and for providing protection and cushioning to packaged products. Polyurethane foams are often used to package highly sensitive equipment such as electronics, printed circuit boards, jewellery and delicate foods. Microcellular foam variants are widely employed in the tyres of wheelchairs, bicycles and other such uses. These latter foam types are also widely encountered in car steering wheels and other interior and exterior automotive parts (Harrington 1991). There are two main foam variants: one in which most of the foam bubbles (cells) remain closed and the gas remains trapped and the other involving systems which have mostly open cells (Figure 22, 23, 24).

Figure 22: flexible foam low/high density

Figure 23: microcellular foam

Figure 24: rigid foam

polyurethanes, Manufacture and Applications

23

A third foam variant, called microcellular foam, yields the tough elastomeric materials typically found in the coverings of car steering wheels and other interior automotive components. Carefully controlling the visco-elastic properties by modifying the catalysts and polyols used can lead to memory foam, which is much softer at skin temperature than at room temperature. In many cases, foam is positioned behind other materials; flexible foams are behind upholstery fabrics in commercial and domestic furniture, rigid foams are inside the metal and plastic walls of most refrigerators and freezers, or behind paper, metals and other surface materials in thermal insulation panels in the construction industry. Polyurethane foams are available in many forms for use in insulation, as sound-deadening material, floatation, industrial coatings, packing material, and even cast-in-place upholstery padding. Since they adhere to most surfaces and automatically fill voids, they have become quite popular for these applications. Flexible polyurethane foam is produced according to two production processes: the slabstock and the moulding process (www.pfa.org). The slabstock production process is used to produce most foam for furniture cushioning, carpet cushion and bedding. The polymer mix is poured onto a moving conveyor with sides of 3 to 4 meters high, where it reacts and expands into a slab. The process that produces slab urethane foam mixes these reactants in a suitable mixing head and deposits them while still in liquid form onto a moving conveyor where foaming and part of the final cure takes place (Figure 25). The continuous slab is then cut, stored and allowed to cure for up to 24 hours, before it undergoes fabrication into useful shapes for a wide range of applications. The moulding production process is used for automotive and office and design furniture. This process produces individual items by pouring the foam mixture into shaped moulds where the foam reaction taken place within the enclosure.

Figure 25: Slabstock pur foam production (Caligen Europe b.v.).

24

pur facts

Rigid Foams Rigid polyurethane foam (1960s) and urethane modified polyisocyanurate foams (1967) are used for the insulation of buildings, water heaters and household refrigerators, and they significantly cut fuel and construction costs. Rigid polyurethane foam is a durable, strong, low-cost and effective insulating material. It has many good properties such as low thermal conductivity, low density, and dimensional stability, high strength-to-weight ratio, low moisture permeability and low water absorption. During the 1960s, automotive interior safety components such as instrument and door panels were produced by back-filling thermoplastic skins with semirigid foam.

Moulded Objects Polyurethane elastomers can be injection moulded or cast into moulds of almost any shape. Examples of castable polyurethanes include caster wheels, industrial tyres, skate wheels, rolls and roll covers. Polyurethane is also used in making solid tyres. Car bumpers, electrical housing panels and computer and telecommunication equipment enclosures are among the parts produced with polyurethanes via Reaction Injection Moulding (RIM). The fibre-reinforced model is placed in a mould, polyol and diisocyanate are injected into the mould, polymerisation takes place, and polyurethane is formed. Polyurethanes are commonly used in a number of medical applications including catheter and general purpose tubing, hospital bedding, surgical drapes and wound dressings, as well as a variety of injection-moulded devices. They are also used for mouldings including door frames, columns, balusters, window headers, pediments, medallions and rosettes.

Figure 26: pur foam moulding production process (Bayer). component A

recirculation lines component B

component A

component B precise heaters large volume capacity pumps

this way to mold

mixing head (pull back to mix and mold)

polyurethanes, Manufacture and Applications

25

Surface Finishes/Coatings Polyurethane is used as coatings on fabric for use as artificial leather. Polyurethane coatings show a good combination of properties such as toughness, chemical and abrasion resistance, durability and formulation versatility. Polyurethane coatings make a product look better and last longer, thanks to their relative insensitivity to moisture. Polyurethanes are selected for their excellent abrasion resistance, good electrical properties, superb adhesion, impact strength, and low temperature flexibility. A disadvantage of polyurethanes is their limited upper service temperature (120°C). Polyurethane coatings are applied to products to improve their appearance and lifespan. Electronic components are protected from environmental influences and mechanical shock by enclosing them in polyurethane. For production an electronics manufacturer would purchase a two-part urethane (resin and catalyst) to be mixed and poured onto the circuit assembly. In most cases, the final circuit board assembly would be non-repairable after the urethane has cured. Because of its physical properties and low cost, polyurethane encapsulation (potting) is a popular option for circuits and sensors in the automotive manufacturing sector. Polyurethane coatings on cars give the exterior a high gloss and better colour retention, while offering improved scratch and corrosion resistance. Other types of polyurethane coatings are used in construction and for floors. Coatings used in the aerospace industry protect the external parts of aircraft from extreme temperature differences – from summer heat at ground level to below freezing conditions at cruising altitudes – and help protect the skin of the aircraft from rust and pitting. Polyurethane materials are commonly formulated as paints and varnishes for finishing coats to protect or seal wood. The coating results in a hard, abrasionresistant and durable coating which is popular for hardwood floors. However, it can sometimes be problematic or unsuitable as a finish for furniture or other detailed pieces. When compared to oil or shellac varnishes, polyurethane varnish forms a harder film that tends to de-laminate when subjected to heat or shock, fracturing the film and leaving white patches. This tendency increases when it is applied on softer woods such as pine. This is also partly due to polyurethane penetrating into the wood to a lesser extent. Various priming techniques are employed to overcome this problem, including the use of “oil-modified” polyurethane designed especially for this purpose. Unlike drying oils and alkyds, which cure after evaporation of the solvent upon reaction with oxygen from the air, polyurethane coatings cure after evaporation of the solvent by various reactions of chemicals within the original mix, or by a reaction with moisture from the air. Certain products are “hybrids” and combine different aspects of their parent components. “Oil-modified” polyurethanes, whether water-borne or solvent-borne, are currently the most widely used wood floor finishes.

26

pur facts

The exterior use of polyurethane varnish may be problematic due to its susceptibility to deterioration through ultraviolet light exposure. It must be noted, however, that all clear or translucent varnishes and indeed all film-polymer coatings (i.e. paint, stain, epoxy, synthetic plastic, etc.) are susceptible to this light-induced damage to varying degrees. Pigments in paints and stains protect against UV damage, while UV-absorbers are added to polyurethane to shield against UV damage. Polyurethanes are typically the most resistant to water exposure, high humidity, temperature extremes and fungi. The use of diamine chain extenders and trimerisation technology yielded poly (urethane urea), poly(urethane isocyanurate), and polyurea RIM. The addition of fillers such as milled glass, mica, and processed mineral fibres gave rise to RRIM, reinforced RIM, which provided improvements in flexural modulus (stiffness) and thermal stability. This technology enabled the production of the first plastic-body automobile in the United States in 1983, the Pontiac Fiero. In 1969, Bayer AG exhibited an all plastic car in Dusseldorf, Germany. Parts of this car were manufactured using a new process called RIM, Reaction Injection Moulding. RIM technology uses high-pressure impingement of liquid components followed by the rapid flow of the reaction mixture into a mould cavity. Polyurethane RIM evolved into a vast number of different products and processes. Further improvements in physical properties were obtained by incorporating preplaced glass mats into the RIM mould cavity, also known as SRIM, or structural RIM (www.polyurethane.org).

Works of Art and design In museum collections, polyurethanes can be found in all types of collections as flexible and rigid foams, adhesives, coatings, paints, fibres, soft and rigid elastomers, tubing, packaging, etc. In modern and contemporary art museums, polyurethanes can be found in works of art mostly as flexible foams and rigid foams, in design furniture as upholstery foams, in clothing as textile fibres and as imitation leather. Polyurethanes used for everyday objects are durable enough for their intended short service life. However, works of art and objects composed of these ‘short-life’ consumer polyurethane show severe problems due to ageing, and in museum collections degrading foams are considered an important conservation problem (Figure 27-32).

polyurethanes, Manufacture and Applications

27

Number seven from Gaetano Pesce’s up collection: Il Piede foot made of polyurethane ether flexible foam. The skin of Il Piede is not uniform due to the production process. Foam that was not protected by the skin showed more degradation than areas that were protected (see Chapter 5).

Figure 27: ‘UP7, Il Piede’ (Gaetano Pesce). ©Vitra Design Museum, Weil am Rhein. Photo author.

Figure 28: (detail) ‘UP7, Il Piede’ (Gaetano Pesce). ©Vitra Design Museum, Weil am Rhein. Photo author.

The deterioration of polyurethane foam in foam laminated 1960s dresses is shown on the decorated seams of the dress of the couturier Courrèges. (Photo courtesy: Doon Lovett©2003)

Figure 29: Dress Courrèges (1960s). Photo Doon Lovett© 2003.

Figure 30: Detail Dress Courrèges (1960s). Photo Doon Lovett© 2003.

28

pur facts

The Dutch artist Kirsten Hutsch made the sculpture Untitled (pur foam, 160 x 160 x 110 cm) in 1998 and the work of art is part of the Océ Art Collection. In 2007 the Océ Art Collection asked her to make a new work of art because the sculpture had become severely degraded and discoloured as a result of being exhibited under non-museum conditions in the hall of the Océ Company. In 2007 the artist remade the work of art, and today (2010, photo taken) it is kept under the same non-museum conditions in the building’s corridor, where it is exposed to environmental factors such as light, temperature and relative humidity.

Figure 32: Detail Untitled (2007) by Kirsten Hutsch (pur ether foam). Photo author.

Figure 31: Untitled (2007) by Kirsten Hutsch (pur ether foam). Photo author.

2

Chemistry, properties and degradation

Introduction The history of polyurethanes dates back to the chemistry of isocyanates in the year 1849, when Würtz reported the synthesis of isocyanate. It was not until after 1945, however, that the synthesis of isocyanates became commercially important for the polyurethane industry. The simple reaction of an alcohol and an isocyanate results in the formation of an urethane linkage (1).

1 2 R – N=C=O + R – O–H  Isocyanate

alcohol

H O | || 1 2 R –N–C–O–R

[1]

urethane

Polyurethanes are formed by the reaction of a polyol (an alcohol with at least two reactive hydroxyl groups per molecule) with a diisocyanate or a polymeric isocyanate. The most widely used method for the preparation of polyurethane is the additional reaction between di- or polyfunctional hydroxyl compounds, such as hydroxyl-terminated polyethers or polyesters and di- or polyfunctional isocyanates in the presence of suitable catalysts and additives. In polyurethanes, the polyol component is the long (1000-2000 nm), flexible segment and the diisocyanate is the short (150 nm), rigid segment, chemically and hydrogen bonded together (Figure 33). The final polymer is formed in two separate steps. The diisocyanate and the polyol react together to form an intermediate polymer called the prepolymer, which is usually a viscous thick liquid or low melting solid (Figure 33). The prepolymer is converted into the high-molecular-weight polymer by further reaction with a diol or diamine chain extender. This step is the chain extender stage, though sometimes the term cross-linking is used if this better represents the character of the final polymer. Alternatively, the entire polymer

30

pur facts

Figure 33: (Kerr 1993) Formation of polyurethanes. Polyol Polyether or Polyester

HO OCN

NCO H

OCN

OH

OCN

O

NCO O

Polyol Polyether or Polyester

N C O Urethane group

H

NCO

O C N Urethane group

PREPOLYMER

O O

R

O

C

Hard segment

H N

H N

O C

O O

O

Soft segment

C

H N

H N

O C

O

1 R O

O

H

H

O

C

N

N

C

Hard segment

formation may be carried out by simultaneously mixing together polyol, diisocyanate, chain extender and catalysts in the so-called one-shot process. Depending on the polyols, isocyanates, chain extenders, cross-linkers and other additives used, a variety of polyurethanes can be produced, varying from thermoplastic soft to rigid elastomers, thermoplastic fibres, thermoplastic adhesives and coatings, and thermoplastic-thermosetting coatings and thermosetting (cross-linked) foams, from flexible to semi-rigid to rigid and to micro-cellular foams (memory foams). In all polyurethanes the hardness ranges from soft jelly structures to hard rigid plastics. Properties are related to segmented flexibility, chain entanglement, interchain forces and cross-linking (Figure 34). Depending on the polyols, isocyanates, chain extenders, cross-linkers and other additives used, various reactions take place, e.g. the end-standing urethane can react again with a polyol. The more reactive the polyol, the more this reaction will take place. If enough isocyanate is available, the urethane can react with the isocyanate, and urethanes with end-standing allophanate groups will be formed. Branches or cross-linked polyurethanes are formed when the functionality of the hydroxyl or isocyanate components is three or more. The more than 25 pathways of reactions between isocyanates and hydroxyl containing compounds can be divided into different steps: Reactions responsible for the formation of the urethane links (for overall polymerisation),

31

Chemistry, properties and degradation

Figure 34: Polyurethanes, densities, stiffness and applications.

Density Solid Polyurethane Elastomers

1200 kg/m3

RIM solid plastics Cast elastomers Print rollers Coatings, adhesives, sealants, elastomers Fabric coating and synthetic fibres

Microcellular foams and elastomers

800 kg/m3

Vehicle parts and exterior parts

Structural foams Footwear outsoles High-density foams

400 kg/m3

Simulated wood Footwear midsoles Integral skin foam for vehicle interiors

Low-density foams

6 kg/m3

Highresilience foam for bedding and upholstery Insulation foam Packaging foam

Stiffness

flexible

semi-rigid

rigid

32

pur facts

Reactions in which carbon dioxide gas formation prevails (for foam production), Chain extension reactions (for formulating and choosing properties), Cross-linking reactions (for thermosetting polyurethanes). The addition reaction between di- or polyfunctional hydroxyl compounds such as hydroxyl-terminated polyethers or polyesters and di- or polyfunctional isocyanates in the presence of suitable catalysts and additives forms the polymerisation reaction. The end-standing urethane can react again with a polyol. The more reactive the polyol, the more this reaction will take place (polymerisation). Cross-linked polyurethanes are formed when the functionality of the hydroxyl or isocyanate components is three or more.

Foaming Reaction The foaming process involves three steps: the chain extension reaction, the gas foaming reaction and the cross-linking reaction. If carbon dioxide is formed by the reaction of the diisocyanate and water, flexible open-pore polyurethane foam is made (2). The production of consistent end products is dependent on mixing the ingredient chemicals in the precise ratio and maintaining the appropriate processing temperatures. As the liquid poly-isocyanate and polyol react to form the polyurethane, the liquid mix becomes increasingly viscous, eventually forming a solid mass. The reaction is exothermic, and therefore heat is produced. Other ingredients will be included in the polyol blend, for example the catalyst that controls the rate at which the liquid mixture reacts to become solid. The reaction responsible for the foaming of polyurethane in the manufacture of flexible foams is the liberation of carbon dioxide and at the same time the formation of urea groups. The first step in this reaction is the formation of unstable carbamic acid, which decomposes to form amine and carbon dioxide. The amine immediately reacts with additional isocyanate to form a substituted urea (3).

O || step 2 gas R–N=C=O + H O  R–N–C–O–H  R–NH + CO 2 2 2 | decomposes H step 1

[2]

33

Chemistry, properties and degradation

O step 3 || R–N=C=O + R–NH  –R–N–C–N–R 2 | | H H

[3]

Chain Extension Reaction The urethane and urea reactions are chain extension reactions with the carbon dioxide formed in the urea reaction serving as the primary source of blowing for the foam. If enough isocyanate is available, the urethane can react with it, and urethanes with end-standing allophanate groups will be formed (4).

R–N=C=O Isocyanate

+

R’NHCO

RNHC–N–CO

(urethane)

(allophanate)

[4]

Isocyanates may react with themselves to give dimers, trimers (isocyanurates) and carbodi-imides. Nowadays, modified isocyanates (for instance modified TDI types containing allophanates, urethane and urea groups) are in use in flexible foam manufacture (Brydson 1995, 1999).

Cross-Linking Reactions The amine cross-linking agent can react with the diisocyanate and forms urea groups (5). Urea compounds can react with the diisocyanate and form biuret compounds (6). The biuret and allophanate linkages are a principal form of cross-linking in all diol foams, but in almost all one-shot foams the polyol is a triol, giving the polymer its three-dimensional structure in which the biuret and allophanates also formed are a secondary source of cross-linking.

34

pur facts

OCN–R –NH 1 2

+

R NH-CO-NH-R 1 1

+

Amine

OCN–R –NCO 1

OCN–R –NH–CO–NH–R –NCO 1 1

R1NCO

R NH-C-N-C-NHR 1 1

di-isocyanate

Urea

isocyanate

[5]

urea

[6]

biurete

Polyurethane Foam Components Polyurethane foams can be flexible, semi-rigid and rigid. Depending on their chemical composition and the rigidity of the resin used, they are thermosetting materials, i.e. they cannot be melted and reshaped as thermoplastic materials. Flexible foams have a glass transition below room temperature whereas rigid foams have one above room temperature. Most polyurethane flexible foams are three-dimensional block polymers in which flexible linear chains of polyethers or polyesters are linked together with rigid di-isocyanate segments.

Polyurethane ETHER The hydroxyl-terminated polyethers are prepared by the base-catalysed reaction of propylene, ethylene or butylene oxide with di- or polyfunctional alcohols such as diols (propylene glycol, glycerol, diethylene glycol, trimethylol propane), hexitols (sorbitol) and octols (sucrose) (Figure 35). Cross-linked polyethers are formed when not two but three or more active H atoms are used. The early polyurethane ether foams were fabricated in a two-step procedure that was a little too complex for commercial manufacture, but after the development of organo-metallic catalysts, the one-shot process became possible. The earliest polyether urethane foam is based on glycerol and propylene oxide with MW 3000.

Figure 35: Polyurethane ether foam reactants.

O Propylene oxide

35

Chemistry, properties and degradation

Figure 35: Polyurethane ether foam reactants.

HO

Diethylene glycol

OH

O O

CH3

Trimethylol propane HO

OH

Polyurethane ESTER Hydroxyl-terminated polyesters were the first hydroxyl-containing materials used in the fabrication of flexible polyurethane ester foams. The principal components are di-basic acids such as adipic acid, sebacic acid, phthalic acid or phthalic anhydride, dimerised linoleic acid; and diols such as ethylene glycol, 1,2-propylene glycol, 1,4 butylene glycol and 1,6 hexylene glycol. If chain-branching or crosslinking is needed, higher functional glycols such as trimethylolpropane, glycerol, pentaerythriol, sorbitol and 1,2,6-hexanetriol are used (Figure 36). Most of the polyurethane ester flexible foams are based on ethyleneglycoladipic acid polyesters. All polyurethane esters have fine, even cell structures. They form products with superior tensile, abrasion, flexing and oil resistance properties. A disadvantage of polyester-based polyurethanes is their lower hydrolysis resistance. In practice, these failings are multiplied due to the fact that after polyester cleavage, non-reacted carboxyl groups will react further, reducing chemical resistance. In spite of their shortcomings, the polyurethane esters also have advantages: they can be formulated to contain fire inhibitors or self-extinguishing characteristics. Polyurethane ether foams have largely replaced polyurethane ester foams in the flexible urethane foam industry, but some applications still exist.

Figure 36: Polyurethane ester foam reactants.

Adipic acid

O HO

OH O

Ethylene glycol HO

OH

36

pur facts

Catalysts Catalysts play an important role in the manufacture of polyurethanes because they affect the ultimate properties of the polyurethane, i.e. the rate of chain propagation, chain extension and cross-linking. The combination of the highly complex polyurethane chemistry and various processing and moulding conditions makes great demands on the catalyst. Two main classes of catalyst are used in polyurethane production. Catalysts have to balance either the chainpropagating reaction (hydroxyl–isocyanate) or the foam formation (water-isocyanate reaction). A balance has to be established between polymer growth and gas formation in order to entrap the carbon dioxide efficiently and to develop sufficient strength in the cell walls at the end of the foaming reaction in order to maintain their structure without shrinkage or collapse. Therefore, catalysts can be chosen on the basis of whether they favour the urethane reaction, such as diazobicyclo-octane (DABCO), or the urea (blow) reaction, such as dimethyl amino ethylether. Another important function of the catalyst is to bring the reaction to an end, resulting in an adequate cure of the polymer. Most of the catalysts used are tertiary amines such as triethylene diamine and dimethyl cyclohexylamine or organometallic salts such as stannous octoate, stannous oleate and dibutyl tin dilaurate (Figure 37).

Figure 37: Catalysts.

N Di-azobicyclo octane DABCO N O

H3C Di-methyl amino ethylether

Triethylene amine

N

CH3

N

N

CH3

CH3

CH2

CH2

CH2

CH2

CH2

CH2

N

37

Chemistry, properties and degradation

Figure 37: Catalysts.

O OSn++O

Stannous octoate

O

Polyols The first essential component of a polyurethane polymer is the polyol. Molecules containing two hydroxyl groups are called diols, those with three hydroxyl groups are called triols, etc. In practice, polyols are long-chain, highmolecular-weight components and can be distinguished from short-chain or low-molecular-weight glycols such as ethylene glycol (EG), 1,4-butanediol (BDO), diethylene glycol (DEG), glycerine and trimethylol propane (TMP), which are mostly used as chain extenders and cross-linkers. Two main types of polyols are used in the polyurethane industry, polyethers and polyesters. The choice of initiator, extender and the molecular weight of the polyol greatly affects the physical state and the physical properties of the polyurethane polymer. Important characteristics of polyols are their molecular backbone, initiator, molecular weight, percentage of primary hydroxyl groups, functionality and viscosity. The functionality of the polyol (number of active hydroxyl groups per molecule) can vary and is usually 2 for elastomers, approximately 3 for flexible foams and up to 6 or more for rigid foams. Polyester polyols can be formed by esterification of a di-acid (such as adipic acid) with a glycol, such as ethylene glycol (EG) or dipropylene glycol (DPG) (7).

ester groups

O

O

H

H

H

O

H

C N

C

N C O

C O C

C

H

H

O

H

H m O

urethane diphenyl urethane group methane group group

polyethylene glycol segment H H

C O

C O O p H H [7]

n

38

pur facts

Polyether polyols are formed by the free radical addition of propylene oxide (PO) or ethylene oxide (EO) and a hydroxyl- or amine-containing initiator (8).

polyethyleneglycol segments

O O

C

H

H

N

C

N

H

H

O

diphenyl methane group

urethane group

urethane group

C

O

H

H

C

C

H

H

O

ether group

m

n [8]

Polyether polyols Polytetramethylene ether glycol (PTMEG), a polyether polyol also known under the trade name Terathane®, comes from a family of linear diols in which the hydroxyl groups are separated by repeating tetramethylene ethane. Terathane® is available in a variety of molecular weights (MG). Below MG 650 they are liquids at room temperature, while above 1000 MG they are low melting waxy solids. PTMEG is used as a soft-segment building block in high-performance polyurethanes and provides outstanding dynamic properties, low temperature flexibility, resistance to microbes and hydrolytic stability. Typical end uses include Spandex fibres, solid elastomers (skate wheels, belts and rollers), as well as flexible adhesives and coatings.

Polyester polyols Polyester glycols are also used as soft segments in polyurethanes and are made by the reaction of 1.4-butanediol with various dibasic acids such as adipic acid or sebacic acid. 1.4-Butanediol is used in both the hard and soft segments of polyurethanes, in the hard segment as a cross-linker and in the soft segment as a chain extender. Polyesters such as poly (butylene adipate) glycol yield urethanes with an excellent resistance to oils and solvents and ultraviolet light. These urethanes also show very good mechanical properties at room and elevated temperatures as well as high resistance to flexing and abrasion.

Diisocyanates The second largest component of polyurethane formulation is the diisocyanate, which can be classed as aromatic, such as diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI), or aliphatic, such as hexamethylene diisocyanate (HDI)

39

Chemistry, properties and degradation

or isophorone diisocyanate (IPDI). Each diisocyanate will give different properties to the end product, requiring different curing systems and, in most cases, different processing systems. Important characteristics of isocyanates are their molecular backbone, percentage of NCO content, functionality i.e. the number of isocyanate groups (-NCO) per molecule and viscosity. In cross-linked polyurethanes the average functionality of the used diisocyanate is usually more than two. The most commonly used diisocyanate for flexible foams is a 80:20 mixture of 2.4 and 2.6 toluene diisocyanate (TDI). However, a 65:35 ratio mixture is also commercially available. This blend is used extensively in the manufacture of polyurethane flexible slabstock foam. The other important isocyanate used is methylene diphenyl diisocyanate (MDI), which has a functionality of about 2.8, making it suitable for the manufacture of high-performance polyurethane elastomers. Diphenylmethane diisocyanate (MDI) has three isomers, 4,4’-MDI, 2,4’-MDI, and 2,2’-MDI, and is also polymerised to provide oligomers of functionality three and higher. The higher functionality diisocyanates are used for special applications. Aliphatic diisocyanates are used only when special properties are required for the final product. For example, light stable coatings and elastomers can only be obtained with aliphatic diisocyanates. Some diisocyanates are shown in figure 38.

Figure 38: Diisocyanates. 3

MDI

OCN

1

2

2

4

1 5

1

3

1

CH2

1

4

1

1

6

6

NCO

1

5

Methylene diphenyl diisocyanate 1

TDI

6

2

5

3

N=

C=

O

4

=N

C O= Toluene diisocyanate

HDI

N C=

C N=

O= Hexa methylene diisocyanate

=O

40

pur facts

N=

C=

O

Figure 38: Diisocyanates.

IPDI C N=

=O

Isophorone diisocyanate

Cell Structures The cell structure of foam determines some of its properties. In flexible opencell foams the passage of liquids and gases is not restricted, while in closed-cell foams the membranes act as barriers to gases and liquids. Closed-cell foams have lower water absorption and lower vapour permeability than open-cell foams. In open-cell foams the gas phase is air. Flexible open-cell foams have sound-absorbing properties and cushioning characteristics, while closed-cell foams have thermal insulation, floatation and packaging properties.

Density The density of polyurethane foams is determined by the ratio of gas and solid plastic components during manufacture and affects the physical properties of the foam. Thermal resistance, mechanical strength and heat capacity are related to the density of the foam.

Common Additives Additives are added to improve the properties and workability of the material. The blend of polyols and other additives is commonly called a ‘resin’ or ‘resin blend’ and may include chain extenders, cross-linkers, surfactants, flame retardants, blowing agents, pigments, light stabilisers and fillers. Sometimes small amounts of antioxidants such as tert-butyl catechol, resorcinol and tartaric acid are used (Foerst 1963). The physical and chemical character, structure and molecular size of these compounds all have an influence on the polymerisation reaction and on the ease of processing and final physical properties of the finished polyurethane.

Chain Extenders Chain extenders are reactive, low-molecular-weight, difunctional compounds such as hydroxyl amines, glycols or diamines, and they are used to influence the end properties of the polyurethanes. The chain extender reacts with the isocyanate to affect the hard/soft segment relationship and therefore the stiffness/

Chemistry, properties and degradation

41

softness and glass transition temperature (Tg) of the polymer. The Tg provides a measure of the polymer’s softening point and some indication of the safe upper limit of its working temperature range. A long polymer is formed with a soft block (polyol) alternating with ridged blocks (the urethane). The long strands of polymer are bonded to each other by hydrogen bonds (Harrington 1991).

Blowing Agents Cellular or foamed PUR is manufactured by using blowing agents to form gas bubbles in the reaction mixture as it polymerises. They are usually low-boilingpoint liquids, which are volatilised by the heat generated by the exothermic reaction between the isocyanate and polyol. Rigid foams yield sufficient exothermic heat from the reaction to allow foam expansion in association with the blowing agent. Flexible PUR foams are usually blown by the CO2 generated by the reaction of water and isocyanate. The foam can also be blown by the direct injection of air or gas into the foam. Chloro-fluorocarbons (CFCs) have been used as blowing agents, but their effects on the ozone layer have led to restrictions on their use, and they are being replaced by more environmentally acceptable alternatives such as pentane (Oertel 1985).

Flame Retardants In their material selection, certain end-use sectors now increasingly take into account possible ‘worst case scenarios’. These considerations will include the effects of smoke and toxic decomposition products on people, property and equipment. Fire inhibition can be achieved by the addition of fluorine, chlorine, bromine or iodine compounds to the polyol. Solid compounds such as melamine and aluminium tri-hydrate are also important flame retardants. Materials and products are continuously evolving and developing, and the trends are now to lower smoke and fume generation and, in the much longer term, ‘lower toxicity’. There is an increasing commitment to tougher guidelines, and in certain sectors of the PUR industry, this has led to the development of low or halogen-free systems (Uhlig 1998).

Pigments Many polyurethane types of foam tend to turn yellow in light, albeit without any adverse affect on their physical properties. Coloured polyurethanes are produced by adding pigmented pastes to the polyol formulation. The pigments, both inorganic and organic, improve the light stability of PUR products. Organic dyes or other colour pigments are sometimes added to polyurethanes, but highly coloured foams tend to show through many upholstery fabrics, which is considered undesirable (Uhlmann 1977).

42

pur facts

Fillers As with other polymers, the use of fillers in polyurethanes will yield products with a modified performance. Calcium carbonate and glass fibres are most commonly used. Calcium carbonate is used primarily to make cheaper formulations, and glass fibres are of growing interest in reaction injection moulding (RIM) technology. Plasticisers such as didecylphthalate (ddp) are sometimes used to give higher resilience or a softer tactile feel to slab polyether urethane. Although antioxidants are used to protect the polyether foam before foaming, additional antioxidants can be added to the foam formulation. The use of zinc dibutyl thiocarbamate is used to keep the polyether urethane foams from yellowing when exposed to light. While the yellowing is not due to deterioration, this additive is also believed to provide protection against ageing (Randall 2002).

Degradation Materials such as glass and wood have been used for such a long period and under such widely different conditions that it is safe to predict their performance in almost any application. Plastics, however, have only been in active service for about 60 years (Shashoua 2008; Waentig 2009). Only limited amounts of data on the natural ageing of plastics have been obtained during this period. The service life of a product is determined by a decrease in product properties and a diminished ability to fulfil product requirements. Due to ageing mechanisms, the properties of a product change with time. The degradation of materials is identified by the user/observer as an unacceptable change in physical and chemical characteristics. During their processing and normal service life, most plastics are exposed to conditions that favour photo-oxidation or thermal oxidation and atmospheric attack by oxygen or ozone. The oxidation of polymers by molecular oxygen is in most instances a free radical chain reaction, consisting of initiation, propagation and termination processes (Dolezel 1978, Domininghaus 1988). Antioxidants can be usefully described as interfering with the initiation or propagation of oxidation; the suppression of either will drastically reduce the rate of oxidation. Polyurethanes in general are degraded during processing and end-use as a result of the combined effects of UV radiation and daylight exposure, thermal oxidation, hydrolysis by moisture and mechanical shear (Dombrow 1957; Rek 1988, 1989; Rabek 1995). Thermal and photo-oxidation can induce change in the chemical composition. Hydrolysis leads to the breakage of polymer chains. Flexible polyurethane foams are especially prone to degradation because the

Chemistry, properties and degradation

43

many open pores give them a large internal surface, being made up of around 3% solid material and 97% air by volume. Polyurethane foams undergo two systems of degradation: (photo-)oxidation and hydrolysis. Polyurethane ether foam is more prone to photo-oxidation and polyurethane ester foam is more prone to hydrolysis; however, the two pathways of degradation can be found in polyether-polyester urethanes. The useful lifetime of a polyurethane object depends on internal factors, its product characteristics and external factors. It is noticed that within 20 to 25 years, depending on the thickness of non-coated foam (either by its own skin or a paint layer), the material is completely crumbled (Kau 1992).

Internal Factors The product characteristics depend on the type of polyurethane, e.g. its components and hence its structural composition (cross-linking), the presence of protective agents such as antioxidants and UV absorbers, built-in stress, the size and shape of the object and its physical properties (Schwarz 1988; Brydson 1995). Weak points in a polyurethane molecule are the urethane, ester, ether and amide linkages. These bonds vary in their degree of resistance to oxidation and hydrolysis. During degradation, chain scission and cross-linking occur, but over a period of time cross-linking prevails, resulting in the foam’s decrease of flexibility. During fabrication processes like moulding and extruding, materials are subjected to high temperatures in air, and thermal and oxidative degradation processes occur. The loss in mechanical properties is accelerated by impurities in the starting materials and certain organo-metallic compounds used as curing agents. Therefore, polyether polyols already contain 0.05 – 0.3% phenolic antioxidants for improved storage. Physical properties such as the molecular weight, degree of cross-linking, stiffness of chain segments, crystallinity and effective intermolecular forces will change during ageing of the polyurethane, resulting in the brittleness of polyurethane ether foams and the total loss of resilience of polyurethane ester foams.

External Factors Light/UV Radiation Photo-oxidation is the oxidation of a polyurethane surface in the presence of oxygen or ozone facilitated by UR radiation and daylight. Photo-oxidation results in chemical changes and, more often than not, a reduction in the polyurethane’s molecular weight. Consequently, the polyurethane becomes more brit-

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tle, coinciding with a reduction in tensile strength, impact strength, elongation at break. Discolouration, yellowing and loss of gloss often accompany photooxidation within a few weeks to several years, with yellowing even occurring in the dark. The stability of polyurethane when exposed to light depends on the diisocyanate and polyol used. Aromatic diisocyanates are more sensitive to oxidation than aliphatic ones, resulting in yellowing of the polyurethane. The accumulation of coloured chromophores in aromatic polyurethanes arises from a secondary reaction of amino and hydroperoxide groups formed initially upon photo-oxidation. The best light-resistant polyurethanes are those formed from polyester and an aliphatic diisocyanate. An increased resistance to light can be achieved by incorporating antioxidants and UV absorbers. High temperature and stress concentration are significant factors that increase the reaction rate of photo-oxidation. Moisture, Solvents and Temperature Important factors causing the degradation of flexible polyurethane ester foams are moisture and heat, leading to discolouration, loss of strength and flexibility, and finally totally loss of resilience. Hydrolytic degradation of flexible polyurethane ester foam by hydrolysis or dissociation by water occurs under conditions of constant exposure. Esters are more susceptible to hydrolysis than ethers. The hydrolyse stability depends on the polyester used. Polyester based on glycols and di-carbonic acids with at least five C-atoms has improved stability. If the acid number of the polyester is comparatively high (around 10), there will be less ageing. Temperature affects ageing in a manner that indicates the normal dependency on temperature found in most chemical reactions. In polyester urethanes the more methylene groups in the adipate ester, the more stable the polyester urethane; and the lower the acid number of the polyester, the more stable the resulting polyurethane. Since the hydrolysis reaction of the polyester group in polyester urethanes produces acid fragments that catalyse the breakdown, degradation becomes autocatalytic and rapid. Likewise, the addition of acid, for example stearic acid as a lubricant, can be particularly harmful to the hydrolytic stability of polyester urethanes.The best polyurethane esters are based on 1.6 hexanediol-adipic acid polyester. Less hydrolysis-resistant polyurethanes are based on adipic acid and ethyleneglycol/1.4 butanediol, which have lower production costs. Information about the glycols and dicarbonic acids used are given on the manufacturers’ datasheets. Impurities imparted during manufacture can catalyse the hydrolysis of polyurethanes, but the catalyst dibutyl stannous laureate has also been shown to have negative side-effects: as little as 0.005% results in an eight-fold increase in the rate of hydrolysis.

Chemistry, properties and degradation

45

When polyurethanes are new and in good condition, they show good resistance to organic solvents like aliphatic, aromatic and chlorinated hydrocarbons and mildly polar solvents such as acetone, methanol and diethylether. However, highly polar solvents like dimethylformamide will dissolve polyurethanes. The resistance to chemicals depends on the structure and composition of the polyurethane: the more cross-linked, the less soluble. However, the resistance to acids and alkalis depends on the urethane group and can be described as poor. Moreover, polyurethanes are resistant to water and to inorganic salt solutions. During ageing the resistance to the above-mentioned solvents, decreases. Polyester-based polyurethanes are easily hydrolysed by alkalis that cause saponification of the ester group. On the other hand, polyether-based polyurethanes exhibit good stability in alkaline solutions, but will be readily hydrolysed by acids. Elevated temperatures affect ageing in the usual way, i.e. by increasing the rate of chemical reactions. Polyurethane ester foams degrade much faster than polyurethane ether foams, implying that the ester group is more susceptible to hydrolysis than any of the urethane-based groups in the polymer. Studies have shown that the polyurethanes themselves are relatively stable when immersed in water.

3

History of polyurethane foam conservation

Introduction Ever since polyurethane foams first became available, artists have had a great appreciation for this ‘new’ material. Nowadays, we find ourselves confronted with the limited durability of the early polyurethane ether and ester foams: loss of resilience, cracking, crumbling and powdering. From 1990 onwards, conservators carried out consolidation of flexible PUR foam using various consolidating agents as a curative method on degraded objects, thus expecting to slow down or inhibit degradation. As long as polyurethane foam has not lost its structural integrity, objects can in some cases be restored using traditional techniques that have proven their suitability and are reversible. Objects that have degraded to such an extent that they will soon lose their significance require invasive treatment. For these objects there is simply no time to wait for a better form of treatment because the condition of the foam is so bad that the object will soon cease to exist altogether. Reversibility is not an issue when the chosen method is essentially a last resort. In this chapter conservation treatments are categorised alphabetically and chronologically according to the consolidants and adhesives used (Table 2). (1990, 1992, 1997). One of the first research studies into the consolidation of objects of cultural heritage using plastic polymers was a work by David Grattan dating from 1990. He researched the possibility of consolidating degrading objects with Parylene by graft polymerisation. In 1992, this line of research was continued by Malcolm Bilz, who investigated the possibilities of restoring a foamed work of art by Marcel Breuer. In 1997 Lisa Nilson performed tests with Parylene. However, this irreversible method, which takes place under vacuum, is limited by the fact that the vacuum chamber only allows rather small objects to be consolidated. Moreover, vacuuming is often inappropriate for cultural heritage objects. (1994/1995). Brenda Keneghan researched ‘Larry the Lamb’, a foam figure of the children’s storybook character from the collection of the Bethnal Green Museum of Childhood in London (Figure 11). The animation polyure-

48

Malcolm Bilz

Brenda Keneghan

Don Sale

Patricia Langen

Lisa Nilson

Aleth Lorne/ Iris Winkelmeyer

Conservation treatments and agents

David Grattan

Table 2

pur facts

1990

1992

1995

1995

1996

1997

1996

Consolidation agent Acrilem IC 15 BEVA 371 Butylmethacrylate 639 Epoxy resin

X

Gelatine Impranil DLV Impranil DLV + Tinuvin B75 Inlays

X

Isovin V Lascaux 498 HV Lascaux hydrogrund Methylcellulose Parylene

X

X

X

Plextol B 500

X

Primal AC35 PVAc (Mowiol 4-88)

X

PEG 3350 PEG 8000 Rubber lacquer (Dunlop R) Sturgeon glue

X X

Judith Bützer

Iris Winkelmeyer

Kathrin Gill

Andorfer

Van Oosten/Lorne

Tim Bechthold

Verteramo, Chiantore /Rava Anna Laganá

VanOosten/Lorne/ Kievits/ Barbara Ferriani/ Anna Volpe

History of polyurethane foam conservation

X

X

X

X

X X

X

X X X

X X

X

X

X

X

49

2000 2000 2001 2001 2002 2002 2004 2005 2006 2009

X X

X

X

X

X X

X X

X X X X

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thane foam puppets were made between 1975 and 1978. The puppets that had been painted were best conserved because their coating had restricted the entry of light and oxygen and hence slowed down photo-oxidation. Consolidation with Dunlop S300 clear was the proposed treatment. The unpainted polyurethane foam of ‘Larry the Lamb’ was seriously degraded and some pieces of foam had already fallen of. Handling the figure resulted in a great deal of crumbling and further loss of material. The museum wished to keep the severely degraded Toyland figures as long as possible. Therefore, a decision was made to restore the most degraded figures by impregnating the polyurethane foam with a flexible rubber lacquer (Dunlop®). The foam became stiffer and the colour changed, but the shape was retained, and the crumbling stopped. (1995). Don Sale restored the sculpture ‘Thumb’ (1965) by the artist Cesar. The polyurethane ester foam was degraded in the parts where the skin of the sculpture protecting the inside foam had become damaged. In these specific parts, the polyurethane foam had discoloured from yellow to dark brown through photo-oxidation. In order to prevent further damage, the sculpture was not cleaned, since polyurethane esters are vulnerable to hydrolysis. Broken parts of the surface were restored by covering the exposed parts with a consolidant. In this way the inside foam was protected from further damage. (1996). Patricia Langen reported on the conservation/restoration of new pieces of foam (inlays) used in the restoration of an artwork by F. Spindel (1969). The work consists of a relief of polyurethane foam, painted red, framed in a black painted showcase covered with Perspex®. The red polyurethane foam sheet (1.5 cm) is folded irregularly and nailed to a chipboard bottom (Figure 39, 40). The

Figure 39: Polyurethane foam object by F.Spindel (1969). Photo Patricia Langen.

Figure 40: Detail of polyurethane foam object by F.Spindel. Photo Patricia Langen.

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51

red paint penetrated the foam to a depth of approximately 1 mm. This work is part of the collection of the ‘Bundesministerium des Innern’ in Bonn, Germany. Due to severe degradation of the foam, several deep cracks of around 15 cm long and 2 cm wide were visible, showing the uncoloured inside of the foam. The foam had lost its flexibility; when pressed it did not immediately return to its original shape. The cracks had given the work a damaged appearance. Besides dealing with the aesthetic damage, the inside of the foam had to be protected against further degradation. To prevent the severe cracks in the red painted foam from tearing further, some form of treatment was needed. The cracks had opened so far that joining them together would cause an enormous degree of tension, which was to be avoided. Therefore, inserts of red painted new polyester urethane foam of the same cell size as the original foam were placed into the cracks and adhered using a 2% PVAc (Mowiol 4-88) glue with a 1% Tylose MHB 3000 thickening agent. (1996). Iris Winkelmeyer did some consolidation tests on broken parts of the work of art ‘Still life of Watermelon’ (1967) by the Italian artist Piero Gilardi using sturgeon glue. Also in 1996, Aleth Lorne carried out the restoration of half of this artwork for the Conservation Research project ‘Modern Art Who Cares’. Lorne used both Plextol B 500 and sturgeon glue consolidation on degraded parts of ‘Still life with Watermelons’ (Figure 41, 41 a).

Figure 41 and 41a: Still Life with Watermelons, 1967, Piero Gilardi (154 x 306 x 25 cm), polyether urethane flexible foam, coated with pvac paint, Museum Boijmans van Beuningen. Photos author.

(2000). Judith Bützer restored the design series ‘Fun-Foam’ by the Italian company Gufram using sturgeon glue, BEVA 371 and Plextol B500. ‘Pratone’, the anti-design chair designed by the Italian group ‘Gruppo Strum’ is one of the

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most important design experiments from this period (Figure 42, 42a). ‘Pratone’ was acquired by the Kunstmuseum Düsseldorf (Museum Kunst Palast) in 1997 and showed severe damage to the coating and the foam.

Figure 42 and 42a: Block of foam and 42 blades of grass made of flexible polyurethane foam, coated with layers of paint (140 x 140 x 95 cm) Pratone, 1966, by Gruppo Strum. Photo author.

(2000). Iris Winkelmeyer restored the sculpture ‘Funburn’ (1967) by the American artist John Chamberlain. Methods of cleaning, inlaying and consolidation are discussed in her diploma thesis. In addition, storage and exhibition conditions are proposed. Methylcellulose, Impranil DLV (polyester, polyether urethane dispersion of aliphatic isocyanate by Bayer) was introduced, as was the use of an aerosol nebuliser as a method of application (Figure 43, 43a).

Figure 43 and 43a: “Funburn” by the American artist John Chamberlain, created between 1966 and 1972 using slabs of polyurethane ester foam which were sliced, squashed and tied together with ropes. Photo after restoration (Iris Winkelmeyer ©).

History of polyurethane foam conservation

53

(2001). Andorfer described the conservation of polyurethane foam in the natural tapestry ‘Pietre di Fiume’ by the Italian artist Piero Gilardi. Andorfer used PEG 3350 and PEG 8000, methylcellulose, combined with acrylic emulsions: Plextol B500, Primal AC35, Lascaux 498 HV and Lascaux Hydrogrund. Spraying was chosen as the application method of the consolidants. (2001). Kathrin Gill carried out the conservation of the degraded polyurethane upholstery of a ‘Heron’ chair (1955) designed by Ernest Race. Polyester, polyether and latex foam were among the materials used for the chair. The most degraded foams were conserved using a polyester textile covering and less degraded parts were adhered and consolidated using butylmethylacrylaat 639, which was sprayed on the degraded polyurethane surfaces. Parts that could not be consolidated or restored were removed, described and stored. (2002). Tim Bechthold used Impranil, Isovin V and Lascaux 498 HV for the restoration of design furniture. (2002). Research into the influence of consolidating agents on the ageing of PUR foams was carried out in 2002 and 2003 by Van Oosten and Lorne (Bern report, University of Bern, Switzerland). The use of consolidating agents for the preservation of flexible PUR foams is a recognised conservation method. Various agents have already been applied and tested. The best consolidation agents to date, Plextol B-500 and Impranil DLV, were used in this research. Plextol B-500 showed more unwanted yellowing after ageing than Impranil, so overall Impranil gave a better result. (2003, 2004). Van Oosten and Lorne presented “Further research into the influence of impregnating agents on the ageing of PUR foams using FTIR spectroscopy” at the IRUG 6th conference in Florence, Italy. Various impregnating agents were tested for the conservation of flexible polyurethane (PUR) foams. In this study, the two most promising impregnating agents, Plextol B-500 and Impranil, and an antioxidant (Vitamin E) were tested with special emphasis on their influence on the visual, textural and chemical properties of the treated PUR foams. (2004). Verteramo, Rava and Chiantore reported on testing Plextol B500, Lascaux 360 HV, sturgeon glue and Impranil DLV for the consolidation of polyurethane and the conservation of two works of art by Piero Gilardi. (2005). Laganá researched Plextol B-5oo, Acrilem IC15 and Impranil DLV. Research into consolidating agents using Tinuvin B75 as a light stabiliser was performed by Van Oosten, Lorne and Beringuer in an internal ICN report in 2005 and 2006. In order to protect new, hence flexible polyurethane ether foams from degradation or crumbling and to restore the flexibility of old degraded foams, the option of coating the foams with light-stabilising systems was investigated. Impranil®DLV combined with Tinuvin®B75 was tested with special emphasis on applicability, effectiveness in preventing oxidation and influence on the visual, textural and chemical properties of treated polyurethane ether

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foams. Polyurethane ether foam test samples impregnated with a Tinuvin®B75 + Impranil® DLV solution using a nebuliser provided a deep and homogenous penetration of the impregnating agent to a depth of around 20mm without filling the cells of the foam. It formed a 15μm-thick layer on the foam cell struts, whilst keeping its polyurethane ether foam-specific intrinsic flexibility.

An example of a work of art made from polyurethane ether foam is ‘Still life of Watermelons’ (1967) by Piero Gilardi. The work is part of the collection of Museum Boijmans Van Beuningen in Rotterdam and represents a huge carpet of grass, watermelons and leaves. The work consists of a base layer, the lawn, made from one large piece of yellow coloured polyurethane foam measuring approximately 150 x 300 x 10 cm. The melons and the stalks have also been cut from polyurethane and are painted. Thin sheets of polyurethane (0.5 cm) were used for the leaves. Gilardi adhered the melons, stalks and leaves of the work using rubber adhesive. The leaves were shaped using metal staples. In the parts where the material is relatively thick, such as the melons and the lawn, the resilience and suppleness of the foam are reasonably intact. However, the work shows quite a few cracks in the stalks and the leaves, and the foam of the leaves has degraded: any form of transport or contact may cause a piece to break off. In addition, the object contains a great deal of dust and crumbled, original material. Nevertheless, the brightness of the colours is still impressive. In the 1960s, Gilardi’s artworks were always painted using pvac paint emulsion (Vinuvil, Montedison, communication with the artist). Non-consolidated parts not protected by paint are in a worse state than the painted pur foam parts. In 1990 Gilardi modified his techniques; he impregnated his foam objects with rubber paint or with an acrylic emulsion or a mixture of both ingredients. Gilardi also mentioned that he was using Plextol b500 (communication with the artist 2001, Amsterdam, icn) (see also Figure 41 and 41a, page 51).

History of polyurethane foam conservation

55

No treatment for polyurethane ester foams The modern art object ‘59-18’, created in 1959 by the Dutch artist Henk Peeters, is made from polyurethane ester foam (Figure 44 and 44a). It belongs to the Netherlands Institute for Cultural Heritage (icn). The work consists of a sheet of polyurethane foam (64.5 x 61.5 cm) adhered to a soft board of the same size. Holes were burned in the foam: this type of work is called a ‘pyrography’. Henk Peeters, an artist belonging to the ‘zero movement’, did not take very much notice of the materials he used. The title of the work refers to the year 1959 in which Peeters created the work and 18 refers to it being the 18th work of this type that he created that year. In 1984, when the foam was 25 years old, the work was framed behind glass. At the time crumbling and loss of flexibility of the foam were observed, but no cracks were visible. When the work was examined in 1996, it was in a deplorable state. The foam was in an advanced state of decomposition. It was severely cracked and crumbling to dust. ‘59-18’ was intended as a sheet of polyurethane foam with burned holes. Considering the work’s age and its rather vulnerable material, some degree of degradation like discolouration or a little crumbling would be acceptable. However, the present condition is far beyond what is considered acceptable, it has degraded too far. The object has lost its original meaning, and no conservation treatment can restore the object. Therefore, the work was declared ‘unexhibitable’ and with the consent of the artist the ‘corpus’ was donated to science. For this artwork made of polyurethane ester foam, no possible form of treatment has yet been found.

Figure 44 and 44a: Henk Peeters, 59-18 (1959). Condition of the work in 1996. Photo Tim Koster (rce).

At present, the preventive conservation of polyurethane ester foam is rather problematic: keeping the foam in a cold and totally dry space is probably the best solution (Lovett 2004). A research project into the consolidation of polyurethane ester foam is ongoing at the 7th framework EU project POPART (Pellizini 2011).

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Research performed into the consolidation and restoration of polyurethane ether and polyurethane ester foam in works of art and design since the first study in 1990 has shown the use of water-based resins in nearly all cases. Each of the water-based resins used has advantages and disadvantages and served their intended purpose. However, the problem of photo-oxidation in polyurethane foam remains unsolved. For this reason the ICN PUR research project was started. Its aim was to protect new, hence flexible polyurethane ether foams from degradation or crumbling and to restore the flexibility of old degraded polyurethane ether foams by coating the cells of the foam with a light-stabilising system (Chapter 4 and 5).

4 Ageing behaviour of polyurethane foam

It is a known fact (see also Chapter 2, degradation) that polyether urethanes are more sensitive to oxidation and photo-degradation than polyester urethanes. A higher resistance to light can be achieved by incorporating antioxidants and UV absorbers. The best light-resistant polyurethanes are those made from polyester and an aliphatic diisocyanate. As it progressed, the ICN research project began to confirm that polyurethane ester foam is generally more sensitive to hydrolysis whereas polyurethane ether foam is more sensitive to oxidation. Test samples of polyurethane foam were subject to artificial light and thermal ageing. Samples (5 x 5 cm) were cut from large sheets of both polyester and polyether urethane foam. Both sheets had a thickness of 2.5 cm and were obtained from ‘Caligen’ (now Vitafoam), a polyurethane manufacturer.

4.1

Materials and methods

Polyurethane ETHER Foam The polyurethane ether foam sheet was light yellow in colour, and the foam was supple and elastic. When compressed, the foam immediately regained its original position. Under the microscope, the cells were irregular, and their sizes varied from 0.2 – 1 mm. Most of the walls between the cells were open, but about 25% were closed.

Polyurethane ESTER Foam At the time of our experiments, polyurethane ester foam sheet for our tests was only available in a dark anthracite-like shade of grey. In the factory, each type of polyurethane foam is produced separately (see chapter on manufacturing polyurethane foams). The foam was supple but denser than the ether type. When compressed, the foam immediately regained its original position. Under the microscope, the size of the cells was more regular than those of the ether foam. Their diameter varied approximately from 0.2 to 0.5 mm. As in the ether foam, some of the walls between the cells were closed, but most of them were open.

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Artificial Light Ageing (Xenotest) All polyurethane test samples – both polyurethane ether and ester urethane foam test samples – were artificially light aged using a Xenotest Alpha High Energy (Atlas®) and exposed to the radiation of a filtered Xenon Arc lamp (105 Klx, 50°C, 40% RH) for up to 216 hours to induce photo-oxidation. Test samples were examined for discolouration and surface changes at 0, 4, 8, 16, 24, 48, 72, 96, 120 and 216 hours. A 4-hour Xenotest is equivalent to one year’s exposure to museum conditions at 200 lux. Ergo 216-hour Xenotest is equivalent to 54 years of indoor ageing at 200 lux.

Thermal Ageing (Heat Chamber) Artificial thermal ageing experiments were carried out on both polyurethane ether and ester urethane foam test samples. Ageing took place in the dark in a Vötsch Vc 0200 climate chamber at 90°C to speed up hydrolysis, hence ageing. Samples were exposed to large amplitude relative humidity of 35-80% in 3-hour cycles to enhance hydrolysis. Test samples were taken from the climate chamber for visual examination, and lead bullet testing was carried out at 1, 2, 3, 4, 5, 7, 10, 14, 20 and 27 days.

Resilience (Lead Bullet Test) A non-standardised test using a lead bullet was set up to help conservators assess the resilience of polyurethane foam. A lead bullet weighing 14 grams was pressed into the polyurethane test foam until it reached the base of the block. When the pressure on the bullet was released, observations could be made regarding the extent to which the bullet returned to its initial position on the surface. In new polyurethane foam the bullet comes to the surface rapidly.

Results after artificial light ageing

Figure 45: Polyurethane ether foam artificially aged, discolouration and hours of light ageing (0,4,8,16,24,48,72,96,120 Hours).

59

Ageing behaviour of polyurethane foam

Figure 46: Polyurethane ether foam artificially aged, degraded surface layer (in mm) against hours of light ageing.

polyurethane ether foam artificially light aged 4

mm

3 2 no consolidant 1

no consolidant duplo

0 0

50

100

150

200

250

300

hours

Polyurethane ether foam exposed to light ageing degrades by forming a discoloured, thin, powdery layer on the surface (Figure 45). This layer can be easily removed by scraping gently. The thickness of an un-degraded test sample and that of the aged test sample after removal of the powdery layer were measured. The aim of these measurements was to observe whether the thickness of the powdery layer grew proportionately with exposure time to light. This experiment could not be carried out on polyurethane ester foam, which does not form a powdery layer at ageing. The powdery layer did not grow regularly but in stages; it started at 48 hours, and by 72 hours the foam was really powdery. After 96 hours of exposure to light, the surface of the foam was totally powdered and wrinkled, and cracks began to appear. Up to 120 hours of light ageing, the layer suddenly grew thicker and stabilised once again. After 216 hours, wrinkles had developed all over the surface. Under this damaged layer, the original properties of the foam remained unchanged: colour, structure and resilience were not modified. The results of the measurements were plotted on a graph showing clearly that with time the brittle surface layer increased (Figure 46). After 100 hours of artificial light ageing (equal to 10404 Kluxhrs), a 2cm-thick polyurethane ether foam object will have a degraded surface layer of 1mm but will survive, while a 1mm-thick polyurethane ether foam object will be completely destroyed after 100 hours of artificial light ageing.

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Results after artificial thermal ageing The polyurethane ether foam was minimally affected by thermal ageing. There was little discolouration and the properties of the polyurethane ether foam test remained unchanged: they retained their relatively good resilience. The polyurethane ester foam was extremely sensitive to thermal ageing. Hydrolysis particularly affected the polyester polyol of the soft segment of the polyurethane foam. Discolouration was difficult to observe due to the grey colour of the foam. After 7 days, however, the grey foam had become brownish, the surface was less even, and the response to compression was slightly slower. After 14-15 days, the test samples of aged polyurethane ester foam had become even browner, and the resilience was really poor. A lead bullet pressed into the foam came only halfway back, and when the bullet was removed, an impression remained in the test sample. Moreover, test samples exposed to thermal ageing for 20 days were torn when rubbed on the surface with a soft brush. Also, the lead bullet pressed into the foam remained inside the foam and did not come back at all (Figure 49). Hydrolysis of polyurethane ester foam not only took place on the surface but throughout the test sample, since moisture responsible for hydrolysis could enter all sides of the test samples and penetrates inside. Result after artificial light and artificial heat ageing polyurethane foam is shown in Table 3.

Table 3

Ageing

Type of foam

Degradation pathway

Effect after ageing

Artificial light ageing

Polyurethane ether foam

Sensitive to photooxidation

Discoloured, powdery surface layer, underneath foam was in good condition

Polyurethane ester foam

Sensitive to hydrolysis

_

Polyurethane ether foam

Sensitive to photooxidation

Little discolouration throughout the foam sample

Polyurethane ester foam

Sensitive to hydrolysis

Total loss of resilience throughout the foam sample

Artificial thermal ageing

Ageing behaviour of polyurethane foam

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Polyurethane ETHER Foam Although polyurethane ether foam is highly resistant to thermal ageing, it is highly sensitive to light ageing. It discoloured quickly and became light brown after 120 hours of artificial light ageing. Furthermore, the surfaces exposed to light became powdery after 72 hours, and the powder fell off very easily after 96 hours, while the mechanical properties of the foam under the superficial brittle layer remained unchanged. Since the definition of an object lies in the character of its original surface, it has to be recognised that an object ceases to exist from the moment it loses that surface. Therefore, after 72 hours of artificial light ageing (equivalent to 18 years of museum lighting at 200 lux), the surface of a museum object begins to lose its original characteristic surface. It is thus advisable to exhibit polyurethane ether foam objects for short periods only. For the remainder of the time the object should be kept in dark storage. In this way the life of the object can be prolonged for a considerable period. A normal exhibition period would be three months every 5 years. However, a popular object might travel around the world and be exhibited for many months. In such cases special care should be taken.

Polyurethane ESTER Foam In contrast, polyurethane ester foam is highly resistant to light ageing but sensitive to thermal ageing due to hydrolysis. The polyurethane ester foam discoloured during thermal ageing, and its resilience decreased considerably. The degradation process did not take place only on the surface of the foam, but throughout the entire test sample.

4.2

Consolidation of polyurethane foam

The outcome of the first part of the PUR research project performed at ICN in 2001 (Table 2) confirmed that polyurethane ester-type foams predominantly undergo hydrolysis during ageing, while polyurethane ether-type foams are especially sensitive to oxidation. Therefore, further tests using consolidants for both artificial light ageing and artificial thermal ageing were carried out on polyurethane ether foams, while only artificial thermal ageing was performed on polyurethane ester foams. Various conservation and restoration methods for conserving flexible polyurethane foams have already been performed and described (see Chapter 3). Plextol B -500 (Lascaux) and Impranil ® DLV (Bayer) – two consolidating agents with good ageing behaviour and easy applicability – were selected and tested further with special emphasis on their effect on the visual, textural and chemi-

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cal properties (Van Oosten 2003) and their ability to prevent degradation of polyurethane foams. Polyurethane ether and polyurethane ester foam test samples were consolidated with a certain amount (8 grams) of either Plextol B-500 or Impranil ® DLV by soaking the samples in the emulsion. After consolidation, the test pieces were dried on a tray. Plextol B500 is an acryl copolymer, and Impranil ® DLV is based on isocyanate components (polyurethanes). The emulsions were too thick to penetrate the foam easily. Therefore, each consolidant was diluted with three parts of demineralised water. Polyurethane foam in good condition is fairly water repellent. In order to facilitate the wetting of the foam and the penetration of the consolidant into the foam, the blocks of foam were placed in a bath of Plextol B-500 or Impranil ® DLV and pressed with both hands. In order to observe just the behaviour of the polyurethane foam and consolidant, no wetting agent or other product was added to the consolidant.

Resilience Test The changes in appearance and resilience of the consolidated and non-consolidated polyurethane test samples were described, measured with the lead bullet test and compared with the results of the ‘Compression set’ test, a standard test for determining the flexibility of polyurethane foams according to DIN 553572. DIN 553572 covers the testing of flexible cellular materials: the determination of the compression set (CS) after constant deformation. The compression set (CS) is the ratio of the difference between the sample height before and after compression and the original height of the sample. CS (%, C, h) = (ho – hr)/ ho X 100. The height of the samples is measured with Vernier callipers.

Fourier Transform Infrared Spectroscopy (FTIR) To verify the composition and study the molecular changes upon artificial ageing, the FTIR spectra of the test samples were recorded before and after ageing using a Perkin Elmer Spectrum 1000 FTIR spectrometer combined with a Golden Gate, Single Reflection Diamond ATR unit (sample size 0.6 mm2). Spectra were recorded from 4000-600 cm-1 (Figure 47). Polyurethanes can be differentiated as urethanes by the presence of the absorption bands at approximately 1538 cm-1 of the amide II (NH deformation), 1724 cm-1 of the amide I (C=O stretch) and around 3300 cm-1 (NH stretch). The presence of polyether or polyester linkages in the polymer chain can be established by examining the relative intensity of the rather large bands around 1100 cm -1 due to the C-O-C (ether linkage) and O=C-O-C (ester linkage). In the case of polyurethane ester foam, the ester linkage at 1124 cm -1 is more intense, while the ether linkage at 1088 cm -1 is more dominant in

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Ageing behaviour of polyurethane foam

Figure 47: FTIR spectra of polyurethane ether (blue spectrum) and polyurethane ester foam (black spectrum).

4000 3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800

600

polyurethane ether foam. Specific ester absorptions of polyurethane ester are the carbonyl at 1725 cm -1 and the O=C-C-O-C absorption at 1218, 1170, 1124 and 1078 cm -1.

Hydroxyl Index Chemical changes in polyurethane foams due to degradation can be seen in the FTIR spectrum by the increase of hydroxyl and carbonyl absorption bands. The increase in carbonyl absorption could not be used due to the obscuring effect of the infrared absorptions of the consolidating agents Plextol B-500 and Impranil® DLV. Therefore, only the hydroxyl absorption was measured. The increase of the hydroxyl index was determined by calculating the relative absorbance (A3450/A1850), which is the height of the hydroxyl absorption peak at 3450 cm-1 (A3450) divided by the height of the absorbance at 1850 cm-1 (A1850) where no organic infrared absorption occurs.

Results of consolidation after artificial light ageing After artificial light ageing, the exposed surface of the polyurethane ether foam discoloured, and a superficial layer of foam became degraded while the foam underneath kept its original properties. In the consolidated polyurethane ether foam test samples, however, the superficial degradation of the foam occurred later. Impranil® DLV and Plextol B-500 consolidated foams had a degradation layer that could be scraped off after 96 hours of ageing, while non-consolidated polyurethane ether foam already had a brittle surface layer after 48 hours of ageing.

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Results of consolidation after artificial thermal ageing Polyurethane ether foam test samples consolidated with Plextol B-500 became very dark after 27 days of thermal ageing. The brown colour was not due to the discolouration of the foam but the discolouration of the consolidant itself, which formed small dark brown points distributed irregularly inside the foam. This indicated that the emulsion was not distributed equally within the foam during consolidation. A more equal distribution may have been achieved if a wetting agent had been used. The discolouration of the test samples consolidated with Impranil® DLV was less intense after thermal ageing than those consolidated with Plextol. As the distribution of Impranil® DLV within the foam was more equal, the discolouration of the sample was more homogenous than the samples consolidated with Plextol (Figure 48).

Figure 48: Thermal aged polyurethane ether foam samples. From left to right (10, 15, 21 and 27 days) Upper row: Impranil DLV consolidated Bottom row: Plextol B 500 consolidated

The resilience of the polyurethane ether foam consolidated with Impranil® DLV remained very good after thermal ageing and was comparable to the resilience of the non-consolidated samples. The polyurethane ether foam consolidated with Plextol B-500 had become more rigid; moreover, its resilience was not as good as that of the non-consolidated foam. This indicated that the presence of Plextol B-500 modifies the elasticity of the foam. After thermal ageing all consolidated polyurethane ester foams showed better handling properties than the non-consolidated samples. A total loss of flexibility for the non- consolidated test samples was observed after 20 days of ageing, whereas consolidated polyurethane ester test samples required 27 days of ageing for total loss. Degradation observed in the resilience of the samples consolidated with Plextol B 500 occurred faster: after 7 days of artificial thermal ageing, the compression set measured was equivalent to that of 15 days of artificial ageing of

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Ageing behaviour of polyurethane foam

the same foam consolidated with Impranil® DLV. After 20 days of thermal ageing, the compression set of all consolidated test samples was the same as nonconsolidated foams: all polyurethane ester foams were totally degraded (Figure 49). The results of the compression set test of the polyurethane ester foam, both consolidated and non-consolidated, were plotted on a graph against artificial thermal ageing in days (Figure 50).

Ageing of PUR(ester) 60

compression set (%)

50

Figure 49: Thermal aged polyurethane ester foam samples after compression set test. From left to right (10, 15, 21 and 27 days aged) Upper row: non-consolidated Middle row: Plextol B 500 consolidated, Bottom row: Impranil DLV consolidated

40 30

plextol impranil PUR ester

20 10 0 0

5

10

15

20

25

30

days

Figure 50: Compression set after artificial light ageing of polyurethane ester foam.

These compression set test results were compared with the lead bullet measurement results to establish the condition of polyurethane ester foams. The degradation observed in the resilience of the foam consolidated with Impranil® DLV occurred at a slower rate than in the non-consolidated samples.

Hydroxyl Index The FTIR spectra of artificially light-aged, consolidated and non-consolidated polyurethane ether and ester foam test samples were recorded. All infrared spectra showed a decrease in the intensity of absorption bands characteristic of the urethane functional group at the beginning of ageing. The appearance of broad absorption bands for carbonyl groups occurred with longer ageing. These broad carbonyl bands included new absorption bands resulting from the formation of ketonic and aldehyde groups as well as amino-containing components.

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The increase of photo-oxidation can be seen in the infrared spectra as the increase of hydroxyl groups of non-consolidated polyurethane ether foam after 216 hours of artificial light ageing (Figure 51). The hydroxyl index was determined by calculating the relative absorbance (A3450/A1850), which is the height of the absorption peak at 3450 cm-1 (A3450) divided by the height of the absorbance at 1850 cm-1 (A1850). Degradation due to photo-oxidation in both consolidated and non-consolidated polyurethane ether foams expressed as the relative absorbance (A3450/ A1850) is plotted against ageing in hours (Figure 52). One hour of artificial light ageing in a Xenotest is equivalent to 0.3 Mlxhour. Figure 51: FTIR spectra of polyurethane ether foam, 216 hours of artificial light ageing (black spectrum) and nonaged foam (blue spectrum).

4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000

800 600

PUR ether foam, light aged 5

impranil A3450/A1850)

4

plextol

3 2

no impregnation

1 0 0

100

hours

200

300

Figure 52: Hydroxyl index plotted against light ageing in hours.

The infrared spectra of non-consolidated polyurethane ester foam after 27 days of thermal ageing and non-aged polyurethane ester foam are shown (Figure 53).The position of the hydroxyl absorption band of the polyurethane ester differs slightly from that of polyurethane ether foam, so therefore the increase of hydroxyl absorption was determined by calculating the relative absorbance (A3470/A3900), which is the height of the absorption peak at 3470 cm-1 (A3470)

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Ageing behaviour of polyurethane foam

Figure 53: FTIR spectra of thermal aged (27 days) polyurethane ester foam (black spectrum) and non-aged foam (blue spectrum).

98,9 90 80 70 60 50 40 30 22,2 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600

PUR ester foam artificially heat aged A3470/A3900 cm-1

3 2,5 2

purester blanco

1,5

impranil

1

plextol

0,5 0 0

7

11

14

21

27

days

Figure 54: Hydroxyl index plotted against thermal ageing in days.

divided by the height of the absorbance at 3900 cm-1 (A3900). Degradation due to hydrolysis of all consolidated and non-consolidated polyurethane ester foams expressed as the relative absorbance (A3470/A3900) is plotted against thermal ageing in days (Figure 54). After 15 days of thermal ageing, polyurethane ester foam cannot be touched or lifted without leaving fingerprints on the surface. It has also totally lost its resilience. The complete loss of flexibility of non-consolidated polyurethane ester foams occurred after 20 days and the total loss of flexibility for consolidated polyurethane ester foams after 27 days. This showed that the impregnating agents Impranil® DLV and Plextol B-500 do not prevent degradation, but prolong the “life” of polyurethane foam for 7 days by “an adhering effect”. Unlike artificial light ageing, seven days of artificial thermal ageing cannot be related to ‘real museum life’. However, when compared to examples of natu-

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rally aged polyurethane ester foams that showed no resilience whatsoever after 25-30 years of natural ageing, it might be estimated that due to this adhering effect, consolidated polyurethane ester foams will last 5-10 years longer than non-consolidated polyurethane foams. Overall, it can be said that the consolidation of polyurethane foam with Plextol B-500 or Impranil® DLV to prevent the degradation of polyurethane foams did not yield any good and lasting results (Table 4 and 5). From a physical and chemical point of view, the same kind of degradation could be observed on the non-consolidated foam and the consolidated foam. However, the presence of a consolidant, in this case an emulsion, helped to keep the loose particles together and thereby preserved the surface of the polyurethane ether foam object for longer. Impranil® DLV gave better results than Plextol B-500 with regard to discolouration and resilience.

Table 4

Effects of Artificial Light Ageing Type of foam

ConsoliColour dating agent change

Degradation Resilience layer after compression set test

Hydroxyl index FTIR analysis

Plextol B500 Polyurethane ether foam

Yellowing

Same degradation as nonconsolidated foam

–

No positive effect

Impranil® Polyurethane DLV ether foam

Yellowing

Same degradation as nonconsolidated foam

–

No positive effect

Plextol B500 Polyurethane ester foam

–

–

–

–

Impranil® Polyurethane DLV ester foam

–

–

–

–

– = not measured

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Ageing behaviour of polyurethane foam

Table 5

Effects of Artificial Thermal Ageing Type of foam

Consolidating agent

Colour change

Resilience after compression set test and lead bullet test

Hydroxyl index FTIR analysis

Polyurethane ether foam

Plextol B500

Dark brown

–

1

Polyurethane ether foam

Impranil® DLV

Yellowing –

1

Polyurethane ester foam

Plextol B500

–

7 days gained, no resilience after 27 days

No positive effect

Polyurethane ester foam

Impranil® DLV

–

7 days gained, no resilience after 27 days

No positive effect

– = not measured

In order to protect new, hence flexible polyurethane ether foams from degradation or crumbling and restore the flexibility of old degraded polyurethane ether foams, the option of coating the foams with a light-stabilising system was investigated (see below). At present, conserving polyurethane ester foam preventively is rather problematic: keeping the foam in a cold and completely dry space is probably the best solution (Doon Lovett 2003). A research project into the consolidation of polyurethane ester foam is ongoing in the 7th framework EU project POPART (Pellizini 2011).

4.3

Sun Block for polyurethane ether foam

On the basis of previous research (van Oosten and Lorne 2003) into consolidating agents for conserving polyurethane foams, it was decided to select Impranil® DLV, a polyether/polyester urethane dispersion based on aliphatic isocyanate. Although the impregnating agent prolonged the service life of polyurethane ether foams, it did not protect them against photo-oxidation. Incorporating light stabilisers containing antioxidants, UV and heat stabilisers can slow down the degradation of polyurethanes. Studies into the effective-

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ness of UV stabilisers on polyurethane ether foams showed good results (Ciba). Polyurethane ether foams only need to be protected from the penetration of UV radiation and daylight when exposed under non-museum conditions, so only the outer surface has to be protected. Therefore, a light stabiliser system (Tinuvin®B75) was added to the Impranil® DLV to test its usefulness for polyurethane ether foams used in cultural heritage. This light-stabilising system was tested with special emphasis on its effects on the visual, textural and chemical properties of treated polyurethane foams. Tinuvim ® B75 is composed of 20% Irganox (a phendic antioxidant), 40% Tinuvin 571 (a UV absorber) and 40% Tinuvin 765 (a hindered amine). Impranil®43032 dispersion and Impranil® DLF, two new dispersions developed by Bayer were also investigated (Kessler 2005; Bayer). When testing the usefulness of consolidants in earlier research, test samples were soaked in the agents. For objects of cultural heritage, however, this method could not be applied. A medical air-pressure nebuliser that allowed the user to regulate the size of the droplets and the velocity of flux was selected for the application of the consolidant (Kessler 1997; Pataki 1997; Winkelmeyer 2000). The stabiliser system combined with the consolidant was tested for application on both new polyurethane ether foam objects that cannot be protected against degradation due to daylight and on aged, crumbled polyurethane ether foams.

Stabilisers Stabilisers preventing the processing-, light- and weather-induced degradation of polyurethane ether foams can be divided into UV radiation stabilisers, light stabilisers and heat stabilisers. UV light absorbers function by absorbing harmful UV radiation and dissipating it as thermal energy. The stabilisers function according to the Lambert Beer’s law, which specifies that the amount of UV radiation absorbed is a function of both sample thickness and stabiliser concentration. In the polymer industry, Hindered Amine Light Stabiliser (HALS) is used, which is extremely efficient against the light-induced degradation of most polymers. HALS’s high efficiency and longevity are the result of a cyclic process wherein the HALS is regenerated rather than consumed during the stabilisation process. Light stabilisers do not absorb UV radiation, but act to inhibit degradation of the polymer. Significant levels of stabilisation are achieved at relatively low concentrations. Hindered amines and light absorbers are used in combination to provide a level of stability that is higher than either type of stabiliser would provide by itself. Such combinations are used effectively in many plastics. The stabilisation of polyurethane ether foams is best achieved using a threecomponent system containing an antioxidant, a UV absorber and a light protec-

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71

tion agent. Research has shown that a three-component system has better workability than the sum of the separate components; this is due to the synergistic working of the system (Ciba 1963). The exact mechanisms of hindered amine stabilisers against thermo-oxidation appear to be complex and are not fully understood.

Polyurethane Ether Foam Test Samples In order to test which degree of light stabiliser system would give the best result in preventing polyurethane ether foams from photo-oxidation, another series of 60 smaller sized test samples (20 x 40 mm) were cut from the same batch of polyurethane ether foam sheet (thickness 25 mm, density 17 kg/m3) used for the earlier ageing and consolidation tests (see previous chapters).

Consolidation (Nebuliser System) A good consolidation method should provide a deep and homogenous penetration of the consolidant without filling the cells or losing the intrinsic flexibility of the foam. An air pressure nebuliser (inhalation set VE 125, Medisize®) was used. The recommended air pressure of around 2 bars was provided by an air compressor (Whispair®, CW 50/24 AL) (Pataki 2003). The mist of droplets was generated by an air compressor with adjustable pressure. The size of the droplets of this system is usually 10 microns. The concentration of the stabiliser solution affects the time needed to apply a certain volume of the product and the length of the treatment: the higher the concentration of the solution, the shorter the nebulisation time. The higher the amount of stabiliser distributed in the foam, the longer the effectiveness of the treatment will be.

Light Stabiliser System Impranil® DLV + Tinuvin® B75 Solution (sun block) on Polyurethane Ether Foams. Consolidation was applied on polyurethane ether test samples, and they were subjected to artificial light ageing. Chemical changes due to photo-oxidation of the polyurethane ether foams were investigated using FTIR spectroscopy whilst colour changes and dimensional changes of cell struts were visually examined under a microscope. In order to examine the smallest amount of light stabiliser system that had to be added for the optimum prevention of photo-oxidation, various amounts were tested, nebulised onto the polyurethane ether foam test samples, artificially light aged and examined by microscopy according to the measurements of the decrease in cell strut and the changes in the chemical structure of the polyurethane ether foams using FTIR spectroscopy.

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Artificial Ageing Consolidated and non-consolidated test samples were artificially light aged in a Xenotest, Alpha High Energy (Atlas ®), and exposed to the radiation of a filtered xenon arc lamp (E = 3665 KJ/m 2 for 24 hours, T 50°C, 40 %RH) for up to 1100 hours to induce photo-oxidation. Aged test samples were examined after 48, 96, 144, 192, 300,400, 570, 594, 618, and 666, 764 and 1100 hours.

Microscopic Examination To measure the foam cell strut thickness before and after consolidation and before and after artificial light ageing, foam test samples were examined using a Zeiss Axioplan 2 imaging research microscope equipped with a digital camera (Zeiss Axiocam MRc with AxioVision software). Measurements of the cell struts were made using the measurement tool with calibrated scaling of the AxioVision software (Figure 57).

Layer of Consolidant with Light Stabiliser Polyurethane ether foams test samples consolidated with a combined solution of Tinuvin®B75 + Impranil® DLV using a nebuliser provided a deep and homogenous penetration of the impregnating agent. To visualise the consolidation, polyurethane ether foam was nebulised for 5 minutes with blue coloured (methylene blue) Impranil® DLV + Tinuvin® B75. A consolidation layer of about 20 mm inside the foam was observed (Figures 55, 56). Consolidation of 1 – 2 minutes using the nebuliser provided a consolidation layer of 10-15 microns on the cell struts (Figure 57).

Figure 55: Blue coloured layer of consolidant.

Figure 56: Detail of blue coloured layer.

Figure 57: Cell strut consolidated with blue coloured consolidant.

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73

Visual Observations of Yellowing, Discolouration and Crumbling The yellowing (discolouration) of all test samples was examined with the naked eye. After artificial ageing, non-consolidated polyurethane ether foam test samples showed signs of degradation such as yellowing and crumbling only on the surface layer of the test pieces, indicating that photo-oxidative degradation reactions are limited to the surface and penetrate only slowly into the interi- Figure 58: Artificial light ageing and discoor layers of the polyurethane foams. loured layer of polyurethane ether foam. After 800 hours of artificial light ageing, the discoloured layer of non-consolidated polyurethane ether foam was visible and measured about 800 microns with a ruler (Figure 58).

Tinuvin® B75-Consolidated Polyurethane Ether Foam Polyurethane ether foam test samples consolidated with only Tinuvin® B75 started yellowing after 48 hours of light ageing. The discolouration was different to that of non-consolidated polyurethane ether foam. It was brown-pink in colour rather than yellow ochre, due to the stabilising components used in the Tinuvin® B75 solution. After 400 hours of artificial light ageing, discolouration increased until it turned a certain colour (dark beige) and no further changes occurred. Non-consolidated polyurethane foam started crumbling after 100 hours, whereas the consolidated polyurethane foams with only 5% Tinuvin® B75 and only 3% Tinuvin® B75 series started crumbling after 350 hours of ageing. In the case of the consolidated polyurethane foam with only 10% Tinuvin® B75 series, crumbling started after 500 hours.

Impranil® DLV + Tinuvin® B75 For the polyurethane ether foam test samples consolidated with Impranil® DLV + Tinuvin® B75 series, discolouration started after 96 hours of artificial light ageing. The discolouration was the same as that observed in the Tinuvin® B75consolidated test samples (Figure 58). After 500 hours of artificial light ageing, the Tinuvin® B75-consolidated polyurethane ether foams showed significant protection against discolouration and retained their original flexibility. Even after 1100 hours of artificial light ageing, no crumbling of the upper layer of the consolidated polyurethane ether foams was observed.

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Measuring Cell Struts For non-consolidated polyurethane ether foam, an average cell strut size of 95 microns was measured, whereas after 96 hours of artificial light ageing, the strut size had decreased to 50 microns (Figures 59, 60). This signified a decrease of around 40% in the cell strut thickness of polyurethane ether foam, and from this point onwards cracks in the cell struts were visible, and the foam broke down when touched. The decrease in the width of the polyurethane foam cell struts due to photo-oxidation was a result of chain scission and cross-linking of the polymer, which resulted in a decrease in molecular weight. At a certain molecular weight, the stress already imparted during the manufacture of the foam was higher than the strength of the bonds in the polymer chains, resulting in breakage of the cell struts at their weakest point in general, namely in the middle of the cell strut.

Figure 59: Polyurethane ether foam cell strut width is about 100 microns.

Figure 60: After artificial light ageing cell strut size diminishes to 55 microns.

Cell Strut Size after Consolidation before and after Artificial Light Ageing The foam cell strut thickness of consolidated and non-consolidated polyurethane ether foam test samples before and after artificial ageing was measured, and the results were plotted in a graph. After consolidating polyurethane ether foam with the Impranil® DLV and Tinuvin® B75 solution using the nebuliser, the cell strut size increased from 95 micron to 120 microns. The application of a Tinuvin® B75 solution alone using the nebuliser did not affect the polyurethane ether foam strut thickness; the cell strut size remained 95 microns. After artificial light ageing, all polyurethane ether test samples consolidated (or non-consolidated) with the light stabiliser system showed a decrease in cell strut size due to photo-oxidation (Figures 61, 62, 63, 64).

Ageing behaviour of polyurethane foam

75

Figure 61: Naturally aged (degraded) polyurethane foam, untreated.

Figure 62: Naturally aged (degraded) polyurethane foam untreated, after artificial light ageing.

Figure 63: Naturally aged (degraded) polyurethane foam consolidated before artificial light ageing.

Figure 64: Naturally aged (degraded) polyurethane foam consolidated after artificial light ageing.

Polyurethane ether foams consolidated with the light stabiliser Tinuvin® B75 alone showed a decrease in cell strut size from 95 to 75 microns (21%) after 1100 hours of light ageing. No breaks were visible in the cell struts, and there was no brittleness. Polyurethane ether foam consolidated with the Impranil® DLV + Tinuvin® B75 solution showed the smallest decrease in cell strut size, from 120 microns to 105 microns (only 12.5%) after 1100 hours of artificial light ageing (Figure 65). As in former chapters, to establish the condition of the polyurethane ether foam, the increase of the hydroxyl index was determined by calculating the relative absorbance (A3450/A1850), which is the height of the absorption peak at 3450 cm -1 (A3450) divided by the height of the absorbance at 1850 cm -1 (A1850) in the FTIR spectrum. The degradation due to photo-oxidation of both consolidated and non-consolidated polyurethane ether foams was

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140 120

120

115 105

100 diameter in micrometer

120

95

95 Tinuvin B75

85 80

75 Tinuvin B75 + Impranil DLV

60

55

55

55

not consolidated

40 20 0 0

500

1000

1500

hours

Figure 65: Cell strut size at artificial light ageing with and without consolidation.

consolidated PUR ether foam artficially light aged

A(3450/A1850) Index

5 4

Impranil

3

no consolidation

2

Tinuvin B75 Impranil + Tinuvin B75

1 0 0

200

400

600

800

1000

hours

Figure 66: Hydroxyl index expressed as the relative absorption (A3450/A1850) of artificial light aged polyurethane ether foam.

expressed as the hydroxyl index and plotted against ageing in hours (Figure 66). Upon ageing, the FTIR spectra of artificial light aged polyurethane ether foams consolidated with the Impranil® DLV + Tinuvin® B75 solution and with Impranil® DLV alone and Tinuvin® B75 alone showed an increase in hydroxyl

Ageing behaviour of polyurethane foam

77

absorption from 1, for new non-consolidated foam, up to a value of 4 for totally degraded crumbled foam. When they leave the factory, new polyurethane ether foams already have a hydroxyl index of about 1. The foam used for the experiments had already aged naturally for five years and therefore had a hydroxyl index of around 2, reaching a maximum of 4 after 200 hours of artificial light ageing. At this point the polyurethane ether foam had completely crumbled, resulting in a total loss of flexibility. However, the relatively large variations in the hydroxyl index values measured (A3450/A1850) were due to the heterogeneity of the foam samples and the large variation coefficient (±1) of the absorption measurements. From FTIR measurements of naturally aged polyurethane ether foam objects, we learnt that degraded, crumbled polyurethane ether foam has a hydroxyl index of 3 (see Chapter 5). Brittle and broken degraded polyurethane ether foams regained flexibility after consolidation with Impranil ® DLV + Tinuvin ® B75 solution. Polyurethane ether foam consolidated with Tinuvin® B75 and polyurethane ether foam consolidated with the Impranil® DLV + Tinuvin® B75 solution showed no significant increase in the hydroxyl index, even after more than 1100 hours of artificial light ageing (Figure 66, page 76). The exposure to 1100 hours artificial lighting (114,444 Kluxhrs) is approximately equal to 275 years of exposure to museum light conditions at 200 lux.

Consolidation of Degraded Polyurethane Ether Foams The applicability of the light stabilising system using a nebuliser was also tested on degraded polyurethane ether foam test samples in three different states of degradation: discoloured; discoloured and stiff; and discoloured, stiff and brittle. For testing the application method, the degraded polyurethane ether foam samples were first nebulised with the 10% Tinuvin® B 75 solution in cyclohexane, followed by nebulising with a 25% Impranil® DLV solution and a 10% Tinuvin®B75 solution (Figures 63, 64).

Consolidation of Painted Polyurethane Ether Foams The final test on painted degraded polyurethane ether foams used in works of art was performed on some test pieces from the artist Piero Gilardi (Figures 67, 68, page 78). These test pieces were donated by Gilardi when he was interviewed in his studio in Turin by Thea van Oosten in 1998. Test pieces of polyurethane ether foam with yellow and green paint were nebulised with Tinuvin®B75 alone and with the Impranil® DLV + Tinuvin® B75 solution. After artificial light ageing for 320 hours, Impranil® DLV + Tinuvin® B75 showed the best results (Table 6, page 78):

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the polyurethane ether foam showed no serious discolouration and no crumbling. Furthermore, the paint had not faded, and the flexibility of the foam had not changed. The nebulising system and the consolidant also worked well for the painted polyurethane ether foam, not filling the cells but only applying a layer of about 10-15 microns of the consolidant on the surface of the painted foam cell struts.

Figure 67: Artificial light ageing of polyurethane foam test pieces donated by Piero Gilardi.

Figure 68: Artificial light ageing of polyurethane foam grass blades.

Table 6

Artificial Light Ageing for 320 hours

Discolouration

Condition

Grass blade 1 Impranil® DLV + Tinuvin® B75

Slight discolouration: greyish and brownish.

Good condition: PUR-ether is not brittle but still flexible.

Grass blade 2 Tinuvin® B75 3% Isopropanol

Discolouration; greyish.

Good condition: PUR-ether is not brittle but less elastic. Less ‘body’.

Grass blade 3 No consolidation

Strong discolouration: brownish

Very fragile: brittle, crumbs

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79

From the point of view of consolidation alone, the results of these tests were in accordance with previous ageing tests carried out during the PUR Research Project. On the subject of discolouration, the ageing tests of the grass blades indicated that discolouration of the carpet would occur anyway and that discolouration of the coloured polyurethane ether foam of Zuccaia would vary depending on the presence or absence of consolidation mediums (Figure 78). Polyurethane ether foam that had not been impregnated discoloured more than impregnated polyurethane ether foam. The painted polyurethane ether foam impregnated with Tinuvin® B75 alone showed little discolouration, but the colour intensity decreased. The polyurethane foam became more greyish. The painted polyurethane foam impregnated with Impranil® DLV + Tinuvin® B75 discoloured more than in the previous case. The discolouration was rather brownish, and there was only little loss of colour intensity. Although a greater degree of discolouration had occurred on grass blade 1, the colour change was considered less disturbing than the loss of colour intensity of grass blade 2. Similar ageing tests were carried out on samples from a yellow polyurethane ether foam banana provided by Piero Gilardi as study material. The same results were observed, showing that the discolouration of the samples did not depend on the colour of the paint.

Conclusion Essentially, consolidating polyurethane ether foams with a Tinuvin ® B75 solution considerably protects the foam from photo-oxidation. However, nebulising an Impranil ® DLV + Tinuvin ® B75 solution by forming a 15μm-thick consolidation layer on the cell struts gives the foam even better protection against degradation. The application method of the consolidation agent, using a nebuliser, is technically feasible and provides a homogeneous layer of Impranil® DLV on the foam structure. In this way, it improves and conserves the flexibility of already aged and hence brittle polyurethane foam. Although consolidated, discolouration of polyurethane foam nevertheless still occurs after artificial light ageing, but at a slower rate. Eventually, both consolidated and non-consolidated polyurethane foam show the same discolouration after 1100 hours of light ageing, which is equal to 114,444 Kluxhrs. Application of the Impranil® + Tinuvin® B75 consolidant on degraded, painted polyurethane foam did not have any visual side-effects on the painted surfaces after artificial light ageing. The yellowing of the consolidant upon ageing was slighter than the yellowing of the polyurethane foam without consolidation. Preventive consolidation treatment by applying a Tinuvin® B75+ Impranil® DLV

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solution on polyurethane foams can be considered before losses of mechanical properties occur. Therefore, it is also possible to achieve a curative treatment by applying the stabilising system combined with the impregnating agent onto extremely degraded foam, thus improving the foam structure, strength and flexibility and offering protection against light.

5 Assessing the condition of polyurethane foam

5.1

Condition of polyurethane ester and ether foam

Polyurethane foam used in modern and contemporary art works and design objects eventually degrades, thus confronting curators and conservators with the consequences of the limited durability of these materials: brittleness, crumbling, disintegration, deformation, cracking, delaminating and discolouration. Open (flexible) foams as well as closed (rigid) foams have a limited lifespan. In order to predict the longevity of both polyurethane ester and ether foam, the current condition and rate of decay of the object need to be ascertained. Two decades of conservation research at ICN have shown that these two parameters depend on the composition, production method and added anti-ageing components, the past life of the objects, their use and the conditions surrounding them. Polyurethane ether foam is very sensitive to light ageing and hardly affected by thermal ageing, whereas polyurethane ester foam is vulnerable to thermal ageing and hardly affected by light ageing. A better insight into the condition of a polyurethane foam will make it easier to decide whether to consolidate or postpone consolidation treatment. At present, there is no available consolidation treatment for polyurethane ester foams due to the fact that the consolidant needs to penetrate the foam object from top to bottom. Existing consolidation application techniques do not currently allow this. Even so, it is essential to establish the condition of a polyurethane ester foam work of art or design object in order to carry out preventive measures for this endangered material such as low RH storage. When it is unclear whether the polyurethane foam is of the ester or the ether type, two identification methods are available: FTIR analysis which requires only a very small sample of 0.6 mm2 or by dissolving the polyurethane foam in a 10% KOH solution, which requires a sample of at least 1 mm2. Polyester urethane foam dissolves in a 10% KOH solution while polyether urethane does not. Both methods are invasive. A non-invasive identification involves NIR analysis, which does not require taking samples at all.

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POLYURETHANE ESTER FOAMS In order to get a first general impression of the condition of polyurethane foam, the mechanical strength (the cohesion of the surface) is evaluated by touching the foam with the finger or gently rubbing it with a soft brush. This is done to assess whether the foam still has its flexibility and to see if any crumbling has occurred. A simple test method to establish the condition of polyurethane ester foams involves the use of a lead bullet. A lead bullet weighing 14 grams is pressed into the polyurethane ester foam until the base of the artwork or foam object is reached. The aim is to observe the extent to which the bullet can return to its initial position on the surface. When the foam is new, the bullet comes to the surface rapidly. When the foam has started to degrade or is degraded to a certain extent, the bullet comes halfway back; and when foam is totally degraded, the bullet remains at the base of the block of foam or artwork. Needless to say, this test can only be performed after obtaining a first overall idea of the foam’s condition by touching it with the fingers or a brush. If the material is too degraded, the condition is obvious, and the lead bullet test should not be performed. Data obtained from compression set measurements according to DIN 53572 – a standard Din test (see Chapter 4) – were used and enabled classification of polyurethane ester foams into three types of conditions. Since most conservators do not have access to analytical test methods, a threecategory condition table of polyurethane ester foam was set up using the data from the lead bullet test, the compression set test and colour change (Table 7). Condition 3 prescribes immediate consolidation, condition 2 recommends consolidation in due course, and condition 1 does not require consolidation but needs regular inspection.

Table 7

Polyurethane ester foam condition Condition of polyurethane ester foam

Colour change

Lead bullet test

Compression set DIN 53572

Recommendation

1 good

Slight discolouration

bullet comes back to the surface immediately

< 10%

Only general preservation required

2 poor

Discolouration

bullet comes halfway back

10-20%

Consolidation treatment recommended

Assessing the condition of polyurethane foam

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Table 7

Polyurethane ester foam condition 3 bad

Strong discolouration

bullet stays at base of artwork/test sample

20-50%

Highly urgent consolidation treatment required

POLYURETHANE ETHER FOAMS Polyurethane foam is a ‘short-life’ consumer material and shows huge conservation problems due to degradation. Polyurethane ether flexible foams degrade relatively fast. The main cause of degradation is photo-oxidation due to light and UV-radiation and oxygen, often accelerated by pigments, leading to discolouration, loss of strength and flexibility, and ultimately, total crumbling (see also Chapter 2). Polyurethane ether foam is very sensitive to light ageing, and due to the presence of aromatic structures in polyurethane foam formulation, the artificially aged samples yellowed very fast. Moreover, after 72 hours of artificial light ageing, the surface of the polyurethane became powdery. After 96 hours of exposure to light, the superficial layer of the foam was totally powdery, and wrinkles or cracks had begun to appear. After 216 hours, wrinkles had developed all over the surface. Underneath this damaged layer, the original colour and structure properties of the foam remained unchanged. Upon ageing, the functional groups within the polyurethane ether foam change (see Chapter 2). Fourier Transform Infrared Spectroscopy (FTIR) is a very useful and convenient technique for detecting changes caused by degradation in functional groups such as carbonyl or hydroxyl. In the FTIR spectrum, chemical changes in polyurethane ether foam caused by photo-oxidation can be seen in the increase of hydroxyl or carbonyl absorptions. The increase of the hydroxyl index is determined by calculating the relative absorbance (A3450/A1850), which is the height of the hydroxyl absorption peak at 3450 cm-1 (A3450) divided by the height of the absorbance at 1850 cm-1 (A1850) where no organic infrared absorption occurs (Figure 51). The hydroxyl (C-OH) index shows a variation in values from 1 for new foam increasing with artificial light ageing (up to 216 hours) to 4 for totally degraded, crumbled foam. The increase in carbonyl absorption could not be used due to the obscuring effect of other infrared radiation-absorbing components present in the polyurethane ether foam test samples. Hydroxyl index expressed as the relative absorption (A3450/A1850) of artificial light aged PUR ether foam against exposure in hours.

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New polyurethane ether foams have a hydroxyl index of around 1. The foam used for the experiments had already aged naturally for five years and therefore showed an index value of around two, reaching a maximum of four after 216 hours of artificial light ageing (Figure 62). At this point the foam was completely crumbled, resulting in a total loss of flexibility. In keeping with previous research, we observed that polyurethane ether foam starts crumbling when a hydroxyl index of 3 is measured. Using FTIR, the changes in the hydroxyl index as a function of exposure to light were used as an indicator of the stability of polyurethane foam. The higher the index, the more oxidised the polymer and the more brittle or breakable an object. The increase of the hydroxyl index could be related to the condition assessment of polyurethane ether foams.

MICROSCOPIC EXAMINATION In order to measure the foam cell strut thickness before and after artificial light ageing, foam test samples were examined using a Zeiss Axioplan 2 imaging research microscope equipped with a digital camera (Zeiss Axiocam MRc with AxioVision software). Measurements of the cell struts were made using the AxioVision software measurement tool with calibrated scaling. An average cell strut size of 95 microns was measured for non-consolidated polyurethane ether foam, whereas after artificial light ageing, the strut size decreased to 50 microns, which signified a decrease of around 40% in thickness. From this point onwards, breaks in cell struts were visible under the microscope, and the foam was brittle and crumbly when touched. The decrease in cell strut size due to photo-oxidation is a result of chain scission and cross-linking of the polymer, resulting in a decrease in molecular mass. Its ‘critical’ molecular mass means that at a certain point, the stress already imparted during the manufacture of the foam is higher than the strength of the polymer itself, resulting in breaking of the struts at their weakest point, which is generally located in the middle. The cell strut thickness of the polyurethane ether foam test samples before and after artificial ageing was measured and the results plotted on a graph (see Figure 61). Upon artificial light ageing, non-consolidated polyurethane ether foam test samples showed signs of degradation such as yellowing and crumbling only in the surface layer of the test pieces, indicating that photo-oxidative degradation reactions are limited to the surface and penetrate only slowly into the interior layers. After 1100 hours of artificial light ageing, the discoloured layer of nonconsolidated polyurethane ether foam was about 900 microns. Signs of degradation such as yellowing and crumbling only occurred in the surface layer of the test pieces (Figure 69).

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Assessing the condition of polyurethane foam

Figure 69: Yellowing of polyurethane ether foam (900 microns after 1100 hours of artificial light ageing (275 years of museum lightning at 200 lux).

Examinations using strut size measurement with a Zeiss Axioplan 2 imaging research microscope or measuring the hydroxyl index by FTIR could provide sufficient information to determine the condition of an artwork made of polyurethane ether foam. A three-category condition table of polyether urethane foam was set up using the data from the hydroxyl index, the cell strut measurements and the colour change (Table 7). Condition 3 prescribes immediate consolidation, Condition 2 recommends consolidation in due course and Condition 1 does not require consolidation but needs regular inspection.

Table 8

Polyurethane ether foam condition.

Condition of Colour polyurethane ether change foam

C-OH index

Cell strut size

Recommendation

1 good

Slight discolouration

3

Highly urgent 55 microns consolidation treatment required

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5.2

pur facts

Condition of polyurethane foam of works of art and design objects

The condition of naturally aged polyurethane ether artworks was assessed by comparing data acquired from naturally aged works of art during 20 years of plastics research at ICN with the degradation indices of the artificially lightaged test samples. Figure 70 shows examples of hydroxyl index values of naturally aged polyurethane ether foams in works of art and artificially light-aged test foam samples. Non-degraded foam has an index of 1-2. Artificially light-aged foam has an index of 4 and is totally degraded by that stage. A foam that had been kept for 8 years in the dark showed an index of 1.7. After 32 years of natural ageing, the foam of the design chair “ UP7, Il Piede” from the collection of the Vitra Design museum in Weil am Rhein showed an index of 3.4 (AXA-Art 2007). When the hydroxyl index of the foam of the same object was measured three years later, the foam had degraded, and the index had increased to 4.2. The foam of the red coloured design chair ‘UP7, Il Piede’ from the collection of the ‘Kunstgewerbemuseum’ in Berlin showed an index of 2.6. (Figures 71, 72). There is a difference between the condition of the outer surface and that of the inner foam. The outer layer has a higher index (more degraded) than the foam directly under the surface because light only penetrates 300 microns into an open-structured surface, leaving the layer underneath unaffected.

PUR ether foam not aged anti-oxidant applied on foam

4

artificially aged foam

3

8 years in the dark foam red Il Piede

2

foam Il Piede 2001

1 35 years

32 years

35 years

8 years

216 hours

192 hours

foam Il Piede 2004

0 hours

0

skin Il Piede 2001

skin Il Piede 2004

Figure 70: Polyurethane foam, naturally and artificially light aged.

Assessing the condition of polyurethane foam

Figure 71: UP, Il Piede, Vitra Design Museum, Berlin. Photo Kathrin Kessler.

5.3

87

Figure 72: UP, Il Piede Vitra Design museum, Weil am Rhein. Photo author.

Instruction for the consolidation of new and aged polyurethane ether foam

Condition of foam Investigations using an empirical (strut width measurement) or scientific (measuring the hydroxyl index, A3450/A1850) method provide sufficient information to determine the condition of an artwork made of polyurethane ether foam. New polyurethane ether foams have a hydroxyl index of about 1-2. Polyurethane ether foam starts crumbling when a hydroxyl index of 3 or higher is measured. At this point, the width of the cell struts has diminished from 95 to 45/55 micrometers. (Figures 59, 60).

Consolidation Impranil® DLV in combination with the light stabilising system Tinuvin® B75 prolongs the service life of new or degraded polyurethane ether foam, both by repairing broken cells and by inhibiting oxidation. When the impregnating agent is applied using a nebuliser, it results in the formation of a 10-15 μm-thick homogeneous layer on the cell struts of the foam that does not fill the cells of the foam. The higher the amount of stabiliser distributed inside the foam, the longer the effectiveness of the treatment. The concentration of the

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stabiliser solution affects the time needed to apply a certain volume of product and therefore influences the lifespan of treated polyurethane foam: the higher the concentration of the solution, the shorter the nebulisation time. New and painted polyurethane ether foams are nebulised directly with a 5% Tinuvin® B75 + Impranil® DLV solution to prevent oxidation. Crumbly (degraded) polyurethane ether should be nebulised first with 3% Tinuvin® B75 in isopropanol and afterwards with 5% Tinuvin® B75 + Impranil® DLV. Materials Impranil® DLV (40% emulsion) from Bayer, a polyester/polyether-urethane dispersion of aliphatic isocyanate as consolidating agent and the light stabiliser Tinuvin® B75 from Ciba-Geigy. Tinuvin® B75 is a synergistic blend of three components: 20% Irganox® 1135 as heat stabiliser, 40% Tinuvin® 571 as UV absorber and 40% Tinuvin® 765 as light stabiliser. It was developed specifically to stabilise thermoplastic polyurethane foam and/or synthetic leather.

Figure 73: Tinuvin® B75 and Impranil® DLV.

Solution The preparation of 200ml of a 5% Tinuvin® B75 + Impranil® DLV solution requires 10ml of Tinuvin® B75, 25ml of isopropanol, 50ml of Impranil® DLV and 115ml of demineralised water. The mixture will not be homogeneous if the following mixing order is not followed. – mix 10ml of Tinuvin® B75 with 12.5ml of isopropanol until the Tinuvin® B75 is dissolved. – mix 50ml of Impranil® DLV with 12.5ml of isopropanol – mix the two solutions and dilute carefully with 115ml of demineralised water while stirring with a magnetic stirrer for several minutes.

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For the preparation of a 3% Tinuvin® B75 solution, dilute 3ml of the abovedescribed Tinuvin® B75 solution in 97ml of isopropanol.

Figure 74 : Nebuliser system.

Figure 75 : Mouthpiece of nebuliser. Photo Olivier Béringuer.

Application Apply the solution using an air pressure nebuliser (Inhalations-set VE 125, Medisize®) developed for medical treatment. An air compressor (Eurotech) provides the recommended air pressure of around 2 bars. Consolidation of 1 – 2 minutes using the nebuliser provides a consolidation layer of 10-15 microns on the cell struts (Figure 76, 77).

Figure 76: Cells of polyurethane ether foam without consolidation. Photo Olivier Béringuer.

Figure 77: Cells of polyurethane ether foam with consolidation layer of about 15 micron. Photo Olivier Béringuer.

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Risk analysis Preventive impregnation treatment using the Tinuvin® B75 + Impranil® DLV solution on polyurethane ether foams could be considered before the loss of mechanical properties occurs. It is also possible to achieve curative treatment by applying the stabilising system combined with the impregnating agent on extremely degraded foam, thereby improving the foam structure, strength and flexibility. The discolouration of the consolidant in consolidated polyurethane ether represents a smaller loss of significance than the complete loss of crumbled foam when consolidation is not applied. Even when consolidated, however, discolouration of polyurethane ether foam still occurs upon artificial light ageing, but at a slower rate. Eventually, both consolidated and untreated polyurethane foam test samples show the same extent of discolouration after 1100 hours of light ageing (which is similar to 250 years under museum conditions at 200 lux). The extended lifespan of more than 250 years after consolidation is striking.

Safety The use of the Impranil® DLV solution, light stabiliser and solvent isopropanol requires safety precautions. The consolidant cannot be nebulised in a spraying chamber since the nebulising mist must not be air sucked before reaching the cells of the polyurethane foam. Working in a fume hood without controlling air suction would not provide a good consolidant mist either. Controlled air suction is absolutely essential, as is the use of gloves and an air mask. At ICN the best result was achieved by using the transportable fume hood suction installation (Figure 83).

6

Case studies

6.1

The adventurous consolidation of a polyurethane ether foam Nature Carpet by Aleth Lorne

Original work Zuccaia (1991), by Piero Gilardi

Figure 78: Zuccaia, 1991, Piero Gilardi. Photo Aleth Lorne.

The Nature Carpets1 of Piero Gilardi (Figure 78) In the 1960s, Piero Gilardi was a young Italian artist2 nourished by American Pop Art, Italian Arte Povera and European New Realism. These art movements were inspired by industrial production methods and the growing consumption of industrial goods in daily life. Artists belonging to these movements used quick and immediate techniques, and their materials were picked up on the

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shelves of the supermarket or manufactured objects that they recycled. The work of art placed these objects in a new perspective and in some cases gave them a playful dimension. In 1965, Piero Gilardi began to make his Nature Carpets from polyurethane foam, playing with the concept of ‘artificial nature’.3 When asked about the title ‘Still Life of Watermelons’ under which his carpet Angurie was registered at the Boijmans van Beuningen, he answered promptly that a big misunderstanding had taken place. His carpets were not meant to be arranged still lifes but rather ‘cuts’4 showing the disorder of nature. Yet they are not realistic because they represent the artist’s subjective vision and show how contradictory situations happen by coincidence in nature as in real life. Fresh and dead wood, green and ripe pumpkins, a ripe apple that has dropped in the snow, a dead tree branch amid the waves of the sea: these are some of the recurrent representations in his carpets. Piero Gilardi’s production of polyurethane foam carpets is huge. The polyurethane foam material enabled him to give shape to his visions very rapidly. The fruit and leaves arranged on Gilardi’s carpets were made in series, and cut from uncoloured blocks or plates. The leaves were cut using templates, and the clumps of grass were made from a rolled strip of foam. Along one of the strip’s edges, narrow triangles were cut off to represent grass blades. The colouration was also carried out en masse. The parts got their first coloured base by being pressed into a paint bath by hand. A final tonality was sprayed using an airbrush, and final details were added with a brush. The paint did not fill the open structure of the polyurethane foam; it only formed a thin layer on the skeletons. The preparation of all the separate parts involved quite a lot of work and skill, but the execution of the carpet was rapid. According to a sketch from his drawing book, Piero Gilardi arranged the loose parts on a large and thick rectangular layer of polyurethane foam that had also been impregnated with paint. The parts were fixed onto the underlying carpet or to one another using rubber adhesive, and the large leaves were put in place with iron staples. The simplicity of the assemblage allowed the artist to concentrate on the composition. In their finished state, the carpets are pieces of idealised nature and extremely colourful. The texture of the polyurethane foam that remains visible suggests an unreal lightness and softness. The sides of the carpet contrast with the upper part; they are straight and even surfaces coated with a thick black skin, as a reminder that the carpet is a ‘cut’ of nature.

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Piero Gilardi’s search for durable materials The Nature Carpets of the 1960s had taken shape very spontaneously without any concerns regarding the durability of the materials. The carpets were made of polyether urethane and painted with water-based polyvinyl acetate (PVAc) paint. At the time, PVAc was the most common synthetic paint used by artists. Gilardi’s carpets proved a success and were purchased by many private collectors and museums. In the 1970s, however, Piero Gilardi stopped his production of the carpets for political reasons. By the time the artist resumed his activities in the 1980s, the degradation of the first polyether urethane carpets has become a matter of concern. The oxidation of the polyether urethane, which had been delayed by the use of the PVAc paint, had finally occurred. Piero Gilardi comprehended the limits of the lifespan of polyether urethane foam. Nevertheless, he decided to continue using the material that offered him so many possibilities, and along with some more experimental forms of art, the production of the Nature Carpet series continued. Thea van Oosten interviewed Piero Gilardi in his studio in Turin in March 2000.5 He explained how he adapted his technique for improved conservation of the carpets in the 1980s. Aware that the consolidation with PVAc paint had slowed down the oxidation process of the polyether urethane, he searched for performing consolidation mediums that could postpone oxidation even further. With this in mind and with the advice of a conservator, Piero Gilardi developed a consolidation system in two steps. The polyether urethane parts of his carpets were first impregnated with Plextol B500 and then impregnated a second time with a water-based acrylic paint. Plextol B500 is an uncoloured acrylic emulsion widely used as consolidant in the field of conservation and well known for its good ageing properties. It was hoped that the additional layer of Plextol B500 on the skeleton of the polyurethane ether would reinforce the protection of the polyurethane ether. As an additional measure against the oxidation process, Piero Gilardi advised collectors to protect their Nature Carpet under a Plexiglas cap equipped with UV filters. Another major technical innovation by Piero Gilardi at the time was the development of a paint based on natural rubber.6 Next to elasticity this paint had the essential quality of being able to form a rather thick, smooth and resistant skin on the surface of the polyurethane foam. Since rubber can oxidise rather quickly into a hard and brittle material, an antioxidant was added to the paint. This paint solution was intended as a finishing layer for seats or other designed objects produced by the artist. The Nature Carpets mostly had this paint applied to their sides to form the black skin described above.

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Identification of the materials of Zuccaia Zuccaia belongs to the second generation of carpets made by Piero Gilardi. Did the artist effectively apply the new developments of his technique to this carpet? How did the new technique influence the ageing of the polyether urethane foam? Questions were also raised about the discolouration of the stems of the pumpkins and squashes, which contrasted with the good state of conservation of other parts. The polyether urethane foam of the stems had become dark yellow and was covered in places by a thin brownish layer that had begun to crack. The presence of a paint based on natural rubber was suspected. For a correct interpretation of the condition, it therefore appeared necessary to identify the materials. A few representative samples were taken and analysed with Fourier Transform Infrared spectroscopy (FTIR). The results of the analyses can be summarised as follows: – All the samples of foam appeared to be PUR-ether as expected. – The samples, with the exception of those taken from the edges, were impregnated or painted with acrylic materials. The FTIR spectrum did not show any evidence of the presence of Plextol B500 since – like the paint – it is based on acrylic copolymers. Traces of natural rubber were not found in these samples. – The samples taken from the sides of the carpet consisted of two layers: a whitish underlayer and a black top layer. The whitish layer appeared to be latex based on natural rubber. The binding medium of the black layer was PVAc. – The strongly discoloured adhesive used to assemble the parts was a rubberbased adhesive. The analyses indicated that the difference in the degree of ageing of the stems and the other parts of the carpets could not be explained by the presence of natural rubber. An answer to this specific discolouration was soon found in the results of the artificial ageing tests of PUR-ether carried out by the PUR Research Project. Small blocks of polyether urethane impregnated with Plextol B500 had been artificially aged. After a few hours of exposure to UV light the samples had already discoloured and had even become browner than the samples that were not impregnated. This observation about the ageing of Plextol B500 could explain the strong discolouration of the stems. They may have been impregnated by Plextol B500 without having being impregnated with the acrylic paint. The light green hue of the stems had been suggested only by painting a few green lines on the surface. Piero Gilardi had made use of the original light yellow colour of the unpainted polyether urethane foam, hoping that a consolidation with Plextol B500 would

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protect the foam against discolouration. Unfortunately, after nine years, the stems were more discoloured than the other parts of the carpet. This last interpretation about the consolidation technique of the stems was later confirmed by the artist himself.7 The consolidation of Zuccaia using Plextol B500 had been assumed but could not be detected since all the parts, with the exception of the stems, were impregnated with acrylic paint. Evidence of the use of Plextol B500 was discovered at a later stage in the hollowed part of a detached melon. The unpainted polyether urethane was impregnated with a consolidant that was identified as Plextol B500. Because the polyether urethane foam and Plextol in this hollowed part had never been exposed to light, they retained their original light colour.

The condition of the carpet In 2000, when Zuccaia arrived at the Collection Department of the ICN, the presence of dust and the mechanical damage seemed to be the principal forms of decay. A thin layer of dust had formed in the open structure of the materials and a large amount of rough sawdust was present between the grass blades and under the large leaves. City hall visitors had caused mechanical damage: one melon was detached and fissures were seen on the surrounding leaves. Small pieces of foam had been pulled off along the edges of some leaves. A number of grass blades had also been pulled off, and several of the thin points were broken. Visitors had also pressed their fingers onto the black skin of the sides, causing several tears and holes in the black layer and in the carpet itself. The black skin was severely cracked at the corners where it was exposed to more physical stress. Small flakes of paint had started to fall off along these cracks. Despite this damage, the visual qualities of the carpet remained strong. The colours were still bright and the composition unaltered. Most importantly, the condition of the foam seemed to be rather good. The surface of the melons and squashes was dense but elastic and the large leaves of the squash plants had remained supple. Cracks had not yet formed on the parts stressed by the staples. The beginning of an oxidation process was observed only on the discoloured stems, the thin points of the grass and at the edges of a few thin leaves of a grape plant. The rubber adhesive used to assemble the parts had strongly discoloured. Dark brown stains had formed all over the carpet wherever the adhesive had been applied.

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From theory to practice: problems posed when applying a consolidation method to a real object The treatment of Zuccaia finally became possible in 2007. For several years, the carpet had been carefully stored in the dark. The condition of the polyurethane foam had remained reasonably stable. In the meantime, PUR Research Project had studied the degradation mechanisms of polyether urethane foam, and excellent consolidation results had been achieved with Impranil DLV, an anionic aliphatic polyester polyurethane dispersion. Applied in an early stage of the oxidation process, the presence of the resin would considerably prolong the foam’s lifespan. The combination of Impranil DLV and the antioxidant Tinuvin B75 gave even better results. The method’s success did not lie in the properties of the consolidation mixture alone but also in the application method by nebulisation. With the help of an air compressor, the liquid mixture for the consolidation was transformed into a thin mist. This mist made it possible to penetrate the polyether urethane foam to a depth of a centimetre and form a very thin film of 10 microns on the cell struts. The Impranil DLV/Tinuvin B75 mixture could be applied curatively as well as preventatively. Accumulated knowledge modified the approach to the conservation of Zuccaia. The consolidation of the carpet needed to be regarded as an integral process including consolidation of the foam and the repair of mechanical damage. The complexity of the choices that had to be made became apparent. They implied both ethical and practical considerations. The main points were: – Question 1 – Local or general consolidation? Should the integrity of the original materials be respected by limiting the treatment to local consolidation, with the knowledge that further consolidation would become necessary in a few years’ time? Or should the entire carpet be impregnated as a preventive measure, modifying irreversibly the composition of the non-degraded original materials in order to prolong their lifespan? What are the risks posed by general consolidation? – Question 2 – Risks of discolouration? Research had shown that the consolidation mixture would prolong the lifespan of PUR-ether considerably and limit the discolouration process to a light greyish hue. The main risks of consolidation seemed to be the discolouration of the materials. This risk was especially great for Zuccaia since its bright colours were one of its key characteristics.

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What influence would the consolidation treatment have on the discolouration of the painted foam of Zuccaia? If only part of the carpet was impregnated, would differences in discolouration appear? – Question 3 – Choice of products involved in the treatment Would it make sense to impregnate the healthy parts with the antioxidant Tinuvin B75 alone? Could the risks of discolouration be limited that way? – Question 4 – Toxicity and duration of treatment Vapours evaporating during the drying process of the Impranil DLV/ Tinuvin B75 mixture are irritating. The consolidation of polyurethane foam by nebulisation is a slow process. Was it realistic to plan the consolidation of such a large object? How would one solve the safety problems? Was there a means of shortening the treatment?

Deciding upon a general consolidation of the object One way of deciding for a local or a general treatment was to evaluate the risk of discolouration. Artificial ageing tests on original parts of the carpet were carried out. Three broken dark green grass blades were used for this purpose and impregnated in different ways. They were exposed to artificial light ageing for 320 hours (Figure 68). Half the surface of the blades was protected by a mask to enable comparisons with (dis)colouration before and after ageing.

Table 9

Artificial light ageing on original foam samples

Artificial light ageing for 320 hours

Discolouration

Condition

Grass blade 1 Impranil DLV + Tinuvin B75

Slight discolouration: Greyish and brownish.

Good condition: PUR-ether is flexible and not brittle.

Grass blade 2 Tinuvin B75 3% isopropanol

Discolouration: Greyish.

Good condition: PUR-ether is not brittle but less elastic. Less ‘body’.

Grass blade 3 No consolidation

Strong discolouration: brownish

Very fragile: brittle, crumbling

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From the point of view of consolidation alone, the results of these tests were in accordance with previous ageing tests carried out during the PUR Research Project. From the standpoint of discolouration, the ageing tests of the grass blades indicated the following: – The discolouration of the carpet would occur anyway. – The discolouration of Zuccaia’s coloured polyether urethane foam would vary depending on the presence or absence of consolidation media. – Polyurethane foam that had not been impregnated underwent greater discolouration than impregnated polyurethane foam. – The coloured polyurethane foam impregnated with Tinuvin B75 alone exhibited little discolouration but the colour intensity decreased. The polyurethane foam became greyish. – The dyed polyurethane foam impregnated with Impranil DLV and Tinuvin B75 underwent a greater degree of discolouration than in the previous case. The discolouration was rather brownish and only a small loss of colour intensity occurred. Although a greater degree of discolouration had occurred in grass blade 1, the colour change was experienced as less disturbing than the loss of colour intensity of grass blade 2. Similar ageing tests were carried out on samples from a yellow polyether urethane foam banana supplied by Piero Gilardi as study material. The same results were observed, showing that the discolouration of the samples did not depend on the colour of the paint. It was decided that Zuccaia should discolour in a homogenous way, and therefore the same treatment should be applied to the whole carpet. Consolidation with the Impranil DLV/ Tinuvin B75 mixture was the chosen option. The first reason was that Tinuvin B75 alone would not be able to consolidate the most oxidised parts. A second argument concerned the manner of discolouration. With Tinuvin B75 alone, the carpet would lose colour intensity. The brownish hue that could appear when impregnating with the Impranil DLV/ Tinuvin B75 mixture would more closely resemble the ageing of the nonimpregnated polyurethane foam. The intensity of the colours would also be better preserved.

Safe working conditions The consolidation method by nebulisation is a slow process (time-consuming), and the ingredients of the consolidant and the antioxidant are irritating when inhaled.

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Was it realistic from a safety point of view to plan the general consolidation of such a large object? Solutions were found by playing with two parameters: – The working conditions – The time limitations of the treatment Solutions for safe working conditions were sought within the Collection Department of the ICN. Treating the carpet in a spray room was one option. In addition to considerations relating to the size of the room and the power of the air extraction, it was found that the strength of the motor was too high for the mist of the nebulising consolidant. The consolidation of polyurethane foam with the Impranil DLV/ Tinuvin B75 mixture requires rather incompatible conditions. On the one hand, the mist has to be capable of penetrating the materials, and the gentle flow should therefore not be disturbed by an exhaustion system. On the other hand, the irritating solvent vapours resulting from the quick drying process need to be exhausted for the safety of the operator. Practically speaking, the flow of the mixture must be allowed to penetrate the foam before the vapours are exhausted. A very slow exhaustion system with a large Plexiglas cap preventing the vapours from being dispersed into the room was finally selected. The cap was placed parallel and close to the area to be consolidated. The central part of the cap, where the exhaustion is strongest, would be placed slightly further away than the treated area. The cap was broad enough to allow space for hands, and its transparency allowed a clear field of view. The operator would need good protective clothing: solvent-proof overall, gloves, glasses and solvent mask. Consolidation would be executed in short daily periods to prevent prolonged exposure to these working conditions.

Limiting nebulisation time and precise condition assessment of the PUR-ether One way of limiting the consolidation time was to shorten the time of application in accordance with the potential degradation of parts of the carpet. The potential degradation could only be evaluated on the basis of the present condition. Therefore, an accurate assessment of the conditions of the PUR-ether in different parts of the carpet was necessary. The present condition of the foam was influenced by three factors: – The density of the polyether urethane foam: Different densities of polyurethane foam had been used by the artist. The degradation of high-density foam is slower.

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– The thickness of the parts: Thin parts were likely to be more fragile because the thickness of the layer that can be penetrated by light is similar to the thickness of the parts. The process is accelerated when light can penetrate from both sides of the part. – The painting technique: The consolidation of Zuccaia’s parts was not homogenous. Some parts contained more paint than others. The presence of a thick layer of paint on the surface of some of the parts formed another potential for decay. The thick acrylic paint layers such as the blue/green layers on the unripe pumpkins displayed a tendency to shrink and pull at the skeleton of the PUR-ether. Finally, the latex layer on the sides of the carpet would also degrade in a specific manner. Guided by these criteria, the present condition of the polyurethane ether foam was evaluated into five different grades. These grades were mapped on a drawing with different colours, the order from the most oxidised to the least oxidised being red, orange, yellow, ochre yellow and dark brown (Figure 79).

Figure 79: Colour scheme showing condition of polyurethane ether foam using cell strut thickness.

Legend figure 79

69 μm

The most oxidised parts. The very thin parts: tip of the grass blades, grape plant leaves.

72 μm

Parts that are strongly impregnated with Plextol B500 and carry very little paint: stems and thin branches.

93 μm

Thick parts showing some fragility on the surface caused by a thick paint layer stressing the underlying foam.

88 μm 108 μm

Thick parts that are superficially impregnated by paint. The less oxidised parts. Thick parts that are heavily impregnated by paint.

The red grade represents the points of the grass blades and the thin leaves of the grape plant. The oxidised stems have an orange grade. The large leaves of

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the pumpkin plants would be expected to have a high potential for degradation due to their thinness. These parts are in fact so heavily impregnated with paint that they were classed as a dark brown grade. It seemed relevant to check the value of this empirical evaluation of the condition. Tiny samples were taken on representative areas to measure the thickness of the skeleton under the optical microscope (Zeiss microplan 100×). With the progress of the oxidation of polyurethane ether foam, the skeleton gradually got thinner until cracks and breaks appeared. Measuring the thickness of the skeleton turned out to be more fastidious, time-consuming and problematic than expected. Even the relevance of the measurements was questioned, since large differences in thickness could be found on samples from the same condition grade. This was due to several factors: the thickness of the sample itself, the exposure to light of the area from which the sample was taken, the thickness of the paint layer and also the density of the foam. This last factor was very important: how does one evaluate the oxidation of a skeleton when the original thickness related to the density is unknown? The lack of information was offset by making a large number of measurements. The average numbers for each grade were calculated and compared. Surprisingly, the classification of the average numbers of each grade in ascending order corresponded to the evaluation done empirically, with the exception of the yellow grade. The yellow grade characterised the parts thickly painted on the surface, such as the blue pumpkins. The fragility of these surfaces not only related to the degree of oxidation of the polyurethane ether but also to the behaviour of the thick paint layer. It seems indeed quite coherent that the skeletons of the yellow grade would be thicker than those of the ochre yellow grade. The results of these tests indicate that an empirical evaluation of the polyurethane ether foam is a reliable method.

Legend figure 80

Grade 1. The consolidants are applied for longer time: Approx. 1 min/ 5cm2. Grade 2. The consolidants are applied for a shorter time: Approx. 0,5 min/ 5cm2.

Figure 80: Colour scheme showing nebulisation time.

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In of to –

practice, it did not seem possible to design a mode of consolidation for each the five condition grades (Figure 80). The classification was finally reduced two grades: Grade 1: polyurethane foam parts showing significant signs of oxidation and requiring consolidation. These parts belonged to the red and orange grades. – Grade 2: polyurethane foam parts that had remained in sound condition and required preventive impregnation. These parts had been given the yellow, ochre yellow and dark brown grades. The application time of the consolidation mixture would be linked to these two grades. The parts classified as Grade 1 would be impregnated twice as long as those classified as Grade 2. Parts classified as Grade 2 essentially needed surface protection, rather than in-depth protection. Studying the samples under the microscope also provided an opportunity to evaluate the technical developments of Piero Gilardi. The darkened layer of Plextol B500 was observed on samples taken from the oxidised parts of the stems: the Plextol B500 layer had cracked in several places, allowing oxidation of the polyurethane foam to take place. The thin films of acrylic paint on the skeleton were in a much better state of conservation than the films of Plextol B500 and therefore provided a better protection against oxidation of the polyurethane foam. Once the paint layers are oxidised, the Plextol layer underneath will crack very quickly and cannot slow down the oxidation process of the polyurethane foam. This underlines the fact that acrylic paint applied by consolidation in the depth of the material is the only effective protection of the polyether urethane foam Nature Carpets.

Performing consolidation To begin with, Zuccaia was cleaned. Dust was removed mechanically using brushes of different sizes and hardnesses depending on the areas. Dust was simultaneously vacuumed using a small museum vacuum cleaner equipped with a HEPA filter. The rough sawdust stuck in the open texture of the polyurethane foam could barely be removed by brushing or vacuuming. Most of it was removed using tweezers. After mechanical cleaning, the carpet was consolidated. The preparation of the products and the setting of the devices are broadly described in other chapters of this book and will therefore not be described here. The Grade 1 areas were consolidated for 1 minute and the Grade 2 areas for 30 seconds.

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Depending on the evaporation rate of the solvents and the distance from the nozzle to the object, small white crystals were seen to form after a certain time of application, often after 50 seconds. It was possible to avoid the formation of these small crystals by consolidating in two periods of 30 seconds. The progress of the consolidation was mapped daily on a drawing. Parts that had been impregnated for a longer time were indicated in dark green. The original plan of consolidation had been modified slightly. Extra consolidation was carried out on the parts subject to stress, such as the junction of the stems and the leaves and at the fixation points of the staples. The quality of the consolidation was checked under a microscope. In total, the consolidation of Zuccaia (288 x 178 x 35 cm.) took 50 hours. The latex skin and the black PVAc paint on the sides were brushed with the mixture Impranil DLV/ Tinuvin B75, although the effect of the mixture on latex had not yet been tested. This mixture was also injected into the cracks of the skin.

Repair of mechanical damages With the repair of the mechanical damages, the consolidation of Zuccaia was complete. The principles on which the polyurethane foam repairs were based had been developed several years earlier with the conservation of ‘Still life with watermelons’ (Lorne 1999). The edges of the broken surfaces were reinforced by an additional application of Impranil DLV/ Tinuvin B75 with a brush. This was done for two reasons, first, because the oxidation along the tears was expected to be greater, and second, more stress could be expected on these parts in the future. Lascaux 360 HV, an acrylic emulsion with a very low glass transition temperature, was chosen for this purpose. Using a wooden stick, tiny drops of adhesive were applied on the surfaces that needed to be glued. To ensure that the tiny drops would remain at the interface, the thick adhesive was used in its pure form. The flexibility of the adhesive combined with the discontinuous application resulted in a flexible system that suited the open structure and the elasticity of the polyurethane foam. The broken surfaces of parts such as the grass blades were too narrow to be assembled with an adhesive. These were assembled using very thin pins made of 1mm-thick nylon thread. The extremities of the pins were sharply pointed. Tiny 0.5mm indentations were made near the tips of the pins, their orientation corresponding to that of the tips. Introducing the pins into the polyurethane foam was a straightforward matter. The indentations hooked into the material’s open structure, thereby preventing any movement of the pin (Figure 81).

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Pulling at broken parts to assemble them in their original position can generate local stress in the material. This tension can be easily released by spraying the material with a mixture of demineralised water and ethanol. This makes the foam extremely soft so that the parts can be brought together very gently. They can then be positioned into the desired position and left to dry. Once dry, the polyurethane ether regains its rigidity and retains the given shape. Tears can then be glued. In the case of large or heavy parts, however, releasing the tension and reshaping the polyurethane ether will not suffice to keep the broken fragments together. It would not be possible to bring the edges of tears together in the large and heavy leaves of the pumpkin plants without the help of supports at the base (Figure 82). Supports were made using casting tape lined with nonwoven polyester fibres stained with green paint. When placed under the leaves, these supports proved very efficient and invisible.

Figure 81: Nylon thread pin in tip of broken grass blade. Photo Aleth Lorne.

Figure 82: Non-woven polyester support underneath leave. Photo Aleth Lorne.

Inlays and retouches A number of losses were disturbing, such as the holes on the black sides. New uncoloured PUR ether was purchased in several thicknesses and impregnated with the Impranil DLV/ Tinuvin B75 mixture. A piece of polyurethane ether foam from which inlays for the green leaves were to be cut was prepared according to Piero Gilardi’s technique: the foam was first impregnated with light green acrylic paint and a thin dark green layer was sprayed on the surface. The inlays were precisely cut and fixed with points of Lascaux 360 HV. A similar method was used to fill the holes on the sides of the carpet. Tiny losses that had occurred at the corners between the cracks of the black paint were filled with tiny chips of new polyurethane.

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At this stage, retouches were carried out. Being aware that the discolouration of the retouches would be unpredictable and would very probably differ from the discolouration of the rest of the carpet, the retouches were strictly limited to the new parts of the carpet. The inlays and small fillings were retouched with acrylic paint in emulsion. A few spots on the original parts were retouched using dry pigments and the PVAc resin Mowilith 20 that remains reversible. The texture of the latex skin was imitated by applying several successive layers of thickened Impranil DLV. For this purpose the glass powder Aerosil was used. Black pigments were added to the mixture.

Conclusion Zuccaia has always been an impressive work of art. Its strength lies in the unusual combination of materials, the powerful colours and the idealised theme. After having being exhibited for a period of nine years, the dust, dirt and mechanical damage gave a slightly desolate feel to the work. Following treatment, the colours had become even brighter and the composition more distinct. The desolate feel had vanished and been replaced by sparkle and intensity. The function of the black edges had been retrieved: they once again provided the composition with sharp boundaries. The consolidation of this large polyurethane foam object required a methodical approach. A good knowledge of the technique proved essential for the interpretation of the condition. The condition assessment of the polyurethane foam was a useful aid in designing a specific consolidation plan for the carpet. Impranil DLV and Tinuvin B75 are tools that can be combined in different ways, depending on the problems posed by the polyurethane foam object.

Figure 83: Movable fume hood at consolidation. Photo Aleth Lorne.

Figure 84: Mouthpiece of nebuliser and aerosol of consolidating agent. Photo Aleth Lorne.

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The success of conserving polyurethane foam is related to the degree of oxidation. One takes into account that the oxidation process is already advanced when the struts at the surface of the foam object begin to break. This turning point in the condition may occur at variable moments during the lifetime of a polyurethane foam object, depending on the properties of the polyurethane, the artist’s technique and the conditions of conservation. Consolidation of polyurethane foam can be carried out at this precise moment or before this. The treatment possibilities for crumbled polyether urethane foam are very limited: the removal of dust is no longer possible, and the consolidation process may alter the appearance of the object: the surface may be flattened, and discolouration may occur. Therefore, the care of polyether urethane foam objects in collections involves three essential preventive measures of conservation: – Minimal exposure to light in time and intensity, filtration of UV light, total darkness during storage. – Good protection against dust. The objects are placed under caps or showcases when exhibited. – Consolidation of the polyurethane ether foam with Impranil DLV and/ or Tinuvin B75 before surface damage occurs. Hopefully, the lifespan of Zuccaia can now be prolonged to a hundred years or more, provided it is kept under the proper conditions (e.g. low light levels of 200 lux and no dust). The city of Zoetermeer has plans to exhibit the work in its new museum. Zuccaia is to be placed below floor level and protected by safety glass or Perspex so that visitors can walk over it. Lighting will be controlled by a sensor indicating the presence of visitors.

Notes 1 2 3

4 5 6 7

Name given by the artist to his polyurethane ether foam carpets. Piero Gilardi was born in 1942 in Turin, Italy. He still lives and works there. In the artist’s own words, from an interview with Antonio RAVA: ‘Piero Gilardi’, in: Corso per la Manutenzione di Opere d’Arte Contemporeanea, Castello di Rivoli, 1991, p.p. 219-227. Ditto. Unpublished interview with Thea van Oosten at the artist’s studio on March 23, 2000, in the presence of A. Rava and O. Chiantore. From the unpublished interview with Thea van Oosten at the artist’s studio on March 23, 2000. Piero Gilardi visited the Research Department of ICN in August 2001.

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Restoration project carried out by the Barbara Ferriani Conservation Studio, Milan*

Original work Sassi (1972), by Piero Gilardi

Figure 85: Sassi, 1972, Piero Gilardi. Photo Barbara Ferriani.

The artwork Sassi by Piero Gilardi forms part of a decorative interior realised in 1972. It belongs to a series known as ‘Tappeti-natura’ (Natural Carpets), in which the artist proposed almost perfect copies of natural subjects utilising an entirely synthetic material – polyurethane flexible foam (Figure 85). Gilardi created the first ‘Tappeti-natura’ between 1963 and 1964: “The idea for these carpets came to me one afternoon while discussing with a friend the landscape that would surround humankind in the future. I supposed this landscape would be different and from images projected by present-day science fic-

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tion literature I imagined, with some emotion, a natural environment that for reasons of hygiene and comfort was artistically interpreted using synthetic materials. From that moment onwards I became curious and experimented in my home with circumstances of this kind. Having found a material that was both soft and crushproof, namely polyurethane flexible foam, I reconstructed 4 m2 of a pebble shoreline, basing my work on actual forms taken from the banks of a river. I obtained an impressively realistic result, which continued to amaze me even after walking over the carpet many times. From this point on, the work developed.”1 In accordance with the artist’s wishes, these ‘tappeti natura’ became functional objects that one could walk over or lie down on: objects that could be used in everyday life in order to “free art from conventional stereotypes and allow it to live within the context of quotidian life, with a functional mechanism of a recreational sort”.2 The work titled Sassi, which is the subject of this case study, forms part of a series of natural carpets created by the artist in 1972 as a commission for the architect Alessandro Monteforte and intended for a living room. As the artist specified in an interview, “it involves works of applied art which portray the subject of a river bank, realised with pebbles created industrially and in series from flexible polyurethane”.3 According to the testimony of architect Monteforte4, the entire installation, which had been acquired by photographer and interior decorator Eda Urbani, was dismembered and sold on to collectors over the next few years. The current owner, although unable to recall the exact year in which the work was bought, remembers that when her children were small in the mid 1970s, they loved to play on the carpet. The visual effect created is that of a bed of large pebbles on which two broken branches and some leaves are placed. The colouration of this work is sombre, composed of varying tonalities ranging from grey to green. Visual aspects such as the flawless imitation of the striations on the rock surfaces, the silvery areas of light creating the illusion of a wet surface, the veining of the leaves and the dry textured bark of the branches combine to create a composition that is remarkably true to life.

Condition before treatment In the autumn of 2008, the work arrived in the studio in an advanced state of deterioration (Figures 86, 87). Its adoption by the owners’ children as a place to rest and play, as indeed its original purpose was intended to be, appeared to be the primary cause of the work’s deterioration.

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Figure 86: Detail of damaged, degraded polyurethane foam. Photo Barbara Ferriani.

Inconsistent accumulations of dust, hairs and general debris were found over the entire surface and particularly in the interstices – between the pebbles and in those parts not covered by the paint layer and also in the areas where the polyurethane foam was visible. Pressure strains due to use had damaged and caused losses in both the polyurethane and the paint layer. The entire surface of the pebbles appeared to be grooved with microcracks and deeper cracks leading into detachments and breaks in the surface. The strong adherence of the paint layer to its support, aggravated by the age-related stiffening of the material, implied that the detachments in the paint would naturally be accompanied by losses in the polyurethane base itself. Both superficial and deeper cracks were apparent, varying according to the various densities of the polyurethane, which was notably disintegrating in areas not covered by the paint layer.

Figure 87: Detail of damaged, degraded polyurethane foam. Photo Barbara Ferriani.

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Making process Visual observation and FTIR5 analysis showed the use of five different types of PUR ether flexible foams by the artist – each characterised by different densities and colourations (see diagram).

— Very High Density Polyurethane — High Density Polyurethane — Medium density Polyurethane — Low density Polyurethane — Deep lacuna — Superficial lacuna

The primary material, a product by the company OLMO GIUSEPPE S.p.a, was probably sold by BOMBE S.r.l., where Piero Gilardi usually purchased it in a semi-processed form ready for serial moulding. In this panel, Gilardi realised the base and leaves by working the raw sheets of polyurethane ‘a freddo’, or without heat. The rocks and branches were created in series using moulds. The results of the FTIR analyses confirmed that the paint layer was composed of a PVAc-based synthetic latex consisting of a chemical resin similar to acrylic, with added quartz. At Gilardi’s directions, the company E.O.C. produced it especially for him under the name ‘Mescola Gilardi’ (Gilardi’s Mix). The moulded pieces were coloured by immersion, while the base was brush painted. Decorative motifs such as the striations reproducing the schistose of the minerals, the details on the branches and the veining of the leaves were also painted by brush. Finally, the artist applied shimmering aluminium dust to the entire surface to create an effect of superficial wetness.

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Aim of restoration The principal aims of the restoration project were to improve the visual reading of the work and to recover the cohesiveness of the polyurethane material as much as possible. Due to the condition of the work and the lack of choices in conservation methods for polyurethane foams, the decision was made to apply the conservation treatment that had been developed during the PUR Research Project of ICN (2002-2006).

Tests and treatment After the removal and conservation of detached fragments, loose superficial deposits were removed by micro-aspiration using a surgical micro-pump. Subsequently, deposits adhering to the surface were removed using a chelating agent of pH 6 applied as a liquid solution and removed using a surgical micro-pump. During the consolidation treatment (carried out with reference to the studies described in Chapter 4), a solution for works in PUR ether flexible foams formulated by the researchers of ICN was tested. Its properties allowed for the recreation of a correct cohesion of the material and a filter for ultraviolet radiation. The solution consisted of 2 composite elements: 1) Impranil DLV/16, which acts as a consolidant for the areas of polyurethane that are not protected by the paint layer and are therefore susceptible to a faster rate of auto-catalysing deterioration, and 2) Tinuvin B757, which is composed of 20% Irganox 1135 (a phenolic antioxidant stabiliser for colour), 40% Tinuvin 571 (a UV filter) and 40% Tinuvin 765 (a hindered amine which acts as a stabiliser for light). During the preliminary phase, tests were carried out to ascertain the compatibility of the solution (particularly the mixing solvent used) with the polyurethane and the paint layer and to evaluate the degree of penetration and efficacy. Following the manual provided by ICN, and bearing in mind the impossibility of creating a solution containing these two pure elements without a base solvent, 10 ml of Tinuvin B75 were dissolved in 12 ml of isopropyl alcohol and 50 ml of Impranil DLV/1 were in turn mixed with 12 ml of the same alcohol. The two solutions were combined and diluted with 115 ml of demineralised water and mixed with a magnetic stirrer. Tests were carried out on aged polyurethane samples provided by the same supplier used by Gilardi, and on small, detached fragments of the original material. The mixture was applied to the surfaces according to the instructions using the nebulisation method. This technique, which involves breaking up the consolidant of the microscopic particles, al-

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lows for an increase in the diffusion time of the compound and consequently a deeper penetration where deterioration is already taking place, without needing to soak the piece excessively. The nebulisation treatment was carried out using a compressor, as it guaranteed a more regular jet than an ultrasound nebuliser. Tests were carried out on the application of the solution, both from a distance and by direct contact, over time periods varying from one to four minutes (Figure 88). Different degrees of pressure were tested, and the cells of the different samples were photographed before and after the application. It was discovered that the solution tended to create a film during the nebulisation treatment, so 10 ml of H2O (82%) and isopropyl alcohol (18%) were added. For the consolidation to be effective without changing the elasticity of the material, an application time of two minutes was set8 for most of the elements. The product was applied to the leaves from a slight distance (approx 1 cm) for only 30 seconds, due to their extreme delicacy. On the other hand, the application to the grey-green rocks, which were in a far more advanced state of deterioration due to the porous nature of their surface and the consequently increased depth of superficial damage, was increased to three minutes. The partial detachments of the paint layer from the base and the fragments found on the base or in the cavities of the work – which had been collected and numbered before cleaning – were returned to their original positions. This was done using Lascaux 360 HV adhesive, applied in tiny drops along the internal surface of the fragment with a fine brush or probes and needles where greater levels of precision were required. Those rocks which had become detached from the base were also fixed and stabilised with this adhesive in correspondence to the base of the piece and its support, taking care not to force the object back to its exact original position so as not to further compromise the elasticity of the material. Since the consolidating solution contained a UV filter and had been applied only to those areas of polyurethane visible to the eye, it was decided to apply Tinuvin to the entire surface. In this way an attempt was made to protect even the micro-cracking present in the paint by covering the entire surface so as to further prevent photo-oxidisation and any resultant chromatic alteration. A solvent in which to dilute the Tinuvin was sought, and seeing as the microcracking in the paint surface would have channelled the solution into the underlying polyurethane layer, this solvent had to be safe for use with polyurethane. Considering the solubility of Tinuvin in various solvents, including isopropyl alcohol, the behaviour of the substance was also tested with ethyl acetate.

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Isopropanol had already been used during the consolidation treatment as a mixing agent in the Impranil DLV/1 – Tinuvin B75 solution, but in relation to polyurethane, it had the disadvantage of a significant percentage of water present in the formula. 9 In order to avoid this inconvenience, a solvent that was less aggressive on the polyurethane was needed – in this case ethyl acetate, which thanks to its volatility, allowed for a reduced retention period. Once the solubility of Tinuvin in both of the solvents had been verified, the product was applied to four samples and several original fragments and nebulised at 2% and at 3%, dissolved in both solvents, respectively, for different time intervals. The cells of the samples did not display significant changes in any of the cases. Given the higher volatility of ethyl acetate in comparison to that of alcohol, it was decided that the Tinuvin should be dissolved in this solvent at a 3% concentration. Upon conclusion of the project, no chromatic or tonal variations were noted in the paint layers, nor was there evidence of the polyurethane surfacing through the micro-fissures. On completion of the restoration process, the original aesthetic value and correct visual rendering of the work had almost been completely reclaimed, while the small signs testifying to its history were toned down but left in place. Obviously, it would no longer be possible for the work to live fully as a carpet by walking or playing on it, but the few remaining stains testifying to its former life can at least remind us of its past use. Although it may have been prudent to protect the work from the adverse effects of UV radiation, the accumulation of atmospheric particles and variations in humidity by placing it in a transparent case with UV protection, both the artist and the owner preferred not to have this done. Without a protective case the work will obviously degrade more quickly. As the subject of two theses, this treatment was preceded by an extensive phase of research which allowed the students involved to trace and follow the process that had led the ICN PUR Research Project to identifying the appropriate materials, while also creating an opportunity to formulate a comprehensive research methodology which provides procedure guidelines and control checks that are easy to apply throughout the process. With regard to the treatment process itself, the time required to complete the intervention was longer than the average time required to reach a proper state of conservation in three-dimensional objects. In fact, the consolidation phase took longer as it was extremely important that the consolidant be applied only to the areas where the colour was missing and the polyurethane was exposed. In order to do this, a very small diffuser had to be used, and held on the same spot for a few minutes at a time.

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Figure 88: Consolidation treatment. Photo Barbara Ferriani.

This lengthened the treatment time compared with other methods such as spray or brush application and led to slightly higher labour costs. However, this treatment was the only option that avoided altering the nature of the work while conserving it for prolonged existence.

Notes *

1 2 3 4 5 6 7 8 9

With the assistance of Anna Cecilia Volpe, Accademia di Brera, Milan: Tutors Barbara Ferriani and Dario Trento; Maddalena Camera, Università Statale di Milano: Tutors Barbara Ferriani and Silvia Bignami. P. Gilardi. Dall’arte alla vita dalla vita all’arte, Milan, La Salamandra, 1982, p. 23. Interview with Piero Gilardi, in ‘Juliet’, n.40, Trieste, 1986. P. Gilardi, email to Studio Barbara Ferriani s.r.l, of 29 July 2009. Interview with Alessandro Monteforte, Vercelli, 23 November 2009. Mirella Baldan – R&C Scientifica – Altavilla Vicentina, Italy. We thank Mr. Colombo of IRSEA in Casorezzo, distributor of Bayer products, for the sample of Impranil DLV/1. We thank Mr. Giorgetti of Bodo di Origgio, distributor of Ciba products, for the sample of Tinuvin B75. 2 bars of pressure. To avoid contact between the alcohol and the polyurethane, we hope that further experiments will be carried out in order to evaluate if the consolidation treatment can be realised in two phases: in an initial phase the water-soluble materials could be applied (Impranil), bringing about a partial recovery in the cells, and following the drying process, the solvent-soluble antioxidants (like Tinuvin) could be applied. The first treatment would further protect the porous structure of the polyurethane from contact with the alcoholic solvent necessary for the antioxidant solution.

Acknowledgements

Special thanks go to Olivier Béringuer and Aleth Lorne for their commitment and valuable work for the ICN PUR Research Project. And thanks go to Beatriz Lorente Álvarez, intern at ICN for her help with the Zuccaia research. I would also like to thank Claudine Hellweg, curator of the art collection of the Océ Company, for providing the work of art for the cover of this book. Special thanks also go to Piero Gilardi who was always helpful to answer all our questions and providing the spare polyurethane ether foam test pieces. Thanks go to Caligen Europe BV, Breda, the Netherlands, (www.Caligen. com) for providing polyurethane ether and ester foam, to Mr. Yves Martelle from Bayer, for providing the various antioxidants and heat stabilisers and Impranil® DLV. Luc Megens and Suzan de Groot of the Cultural Heritage Agency of the Netherlands (rcf) are thanked for their help with microscopy and ageing experiments. Lydia Beerkens, Bart Ankersmit and Anna Laganá are thanked for their careful reading of the manuscript and their useful advice. And above all, without the support of rce (former icn), this book would not have been written.

Glossary

ALIPHATIC

organic compound with an open chain structure

ANTIOXIDANT

compound that slows oxidation processes

AMORPHOUS

a solid that does not have a repeating, regular geometric arrangement of atoms, molecules, or ions

AROMATIC

organic compound with a ring structure with conjugated double bonds

BLOWING

moulding of a heated thermoplastic sheet by using compressed air

CALENDER

a machine consisting of rollers for the production of foil

CELL

the cavity remaining in the structure of flexible polyurethane foam surrounded by polymer membranes or the polymer strut after blowing is complete

CELL SIZE

the average diameter of the cells in the final flexible polyurethane foam product, often measured in micron units

CELL STRUT

length and diameter of each side of the dodecahedron polyurethane foam

CHAIN EXTENDER COMPRESSION SET

COPOLYMERISATION

chain extenders couple polycondensate end groups

permanent deformation of a specimen after it has been exposed to compressive stress for a set time period

linking of two or more chains of polymers

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CROSS LINK

bonds (covalent or ionic) that link one polymer chain to another

CRYSTALLINE

molecules arranged in a regular order and pattern

DENSITY

measurement of the mass per unit volume. Measured in pounds per cubic foot (pcf) or kilograms per cubic meter (kg/m3).

ELASTOMER

polymer that, when deformed, springs back into its original shape as rubber does

EXTRUDER

a machine for the continuous forming of sheets, tubes and profiles by forcing the base material through a die

EXOTHERMIC REACTION

chemical reaction that releases energy in the form of heat

FIRE RETARDENT

material that reduces flammability in plastics

FLOCKING

applying short textile fibres to create a textile-like surface

GLASS TRANSITION TEMPERATURE (Tg) temperature under which a material is in a glossy state and above which it is in a rubbery state GRANULATE

compound transformed into grains

HYDROLYSIS

chemical process in which a certain molecule is split into two parts by the addition of a molecule of water

INDUCTION TIME

period before the first signs of degradation become evident

INJECTION MOULDING

manufacturing process for producing parts from thermoplastic plastics

ISOCYANATE

functional group of atoms –N=C=O

LAMINATING

technique of covering paper or wood with a layer of plastic

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MACROMOLECULAR

macromolecular materials (polymers) are made up of macromolecules (giant molecules), consisting of smaller molecules (monomers). Giant means that the molecules are 100 to 100,000 times larger than molecules of substances like water or salt.

MIGRATION

transmission of substances (usually plasticisers) from one material to another

MODIFYING

alter, change through chemical reaction

MONOMER

a small molecule that is linked with large numbers of other small molecules to form a chain or a network

PHOTO0XIDATION

photo-oxidation is the oxidation of a polyurethane surface in the presence of oxygen or ozone facilitated by Uv radiation and daylight.

PLASTIC

generic name for certain synthetic or semi-synthetic materials that can be moulded or extruded into objects or films or filaments or used for manufacturing, e.g. coatings and adhesives

PLASTICISE

ability to transform or mould a material in the presence of pressure and heat

PLASTICISER

compound, usually a liquid, in which other substances can be dissolved. Plasticisers are added in small quantities to plastic materials to make them more flexible

POLYMERISATION

the process in which many small molecules are joined together to form larger molecules

POLYOL

alcohol containing multiple hydroxyl groups

PREPOLYMER

polymer of relatively low molecular weight; intermediate between monomer and final polymer capable of further polymerisation

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RESILIENCE

indicator for surface elasticity. Can be measured by dropping a steel ball onto the foam and measuring how the steel ball rebounds

RIM

Reaction Injection Moulding

SEMI-SYNTHETIC

semi-synthetic materials are produced from natural materials, e.g. celluloid made from wood and casein made from milk. In some chemical processes cellulose can be modified into cellulose-nitrate or cellulose-acetate.

STABILISER

additive to increase heat resistance during production and use of the product

SYNTHETIC

manufactured or man-made

THERMOSET

a plastic material that has undergone a chemical reaction so that it remains hard, even when reheated

THERMOPLASTIC

a polymer that becomes soft and deformable when heated

UV ABSORBER

additive used in plastics and rubbers to decrease light sensitivity

VISCOELASTICITY

the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.

General information

Health and safety Fully reacted polyurethane polymer is chemically inert. Like all organic materials such as paper, cotton and wood, it is flammable. It should be kept away from heat sources and open flames. When polyurethane foam is ignited, it can burn rapidly. Toluene diisocyanate (TDI) is considered an irritant rather than a toxic chemical. Isocyanate vapours are extremely irritating to the respiratory tract. Efficient ventilation systems should be provided in any work areas. Liquid isocyanates are irritating to the skin; care should be taken when working with these chemicals (BRMA 1979).

Suppliers Bayer (www.bayer.com) Impranil® DLV (40% emulsion), a polyester/polyether-urethane dispersion of aliphatic isocyanate. Tinuvin®B75 Caligen Europe b.v. (www.caligen.com) Polyurethane ether foam Polyurethane ester foam

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