Polyurethane Casting Primer 9781439879214, 1439879214

Table of contents :

Content: Introduction Polyurethane Family of Plastics Castable Polyurethanes Comparison to Other Plastics Health and Safety Fundamentals Basics Fundamental Ingredients System Types of Polyurethane Systems Commercial Types System Selection Commercial Types of Polyurethane Systems Processing Selection Mixing and Casting Polyurethanes Basic Mixing and Casting Background to Mixing and Casting Calculations Adjustments to the Amount of Curative Used Method Curing and Postcuring Calculations Tables Supplementary (Additional) Casting Processes Thick Cross-Sections Bonding Finishing Postcure Bonding Painting Epoxy Blends Millable Polyurethanes Design Considerations Quality of Part Required Draft and Undercuts Shrinkages General Considerations in Mold Design Parts in Compression Bonded Parts Wheels Standard Polyurethane Properties Introduction Basic Comparisons Viscosity of Prepolymers Pot Life of Prepolymers Curing and Postcuring Slow Curing Effect of Backbone Type Polyether Urethanes Range of Values within a NCO Level Aliphatic Polyurethanes Testing Polyurethanes Quality Control and Assurance Testing Introduction Raw Materials Testing Finished Products Testing Standard Tests for Product Evaluation Introduction Prepolymer Properties Processing Information Cured Polyurethane Properties Details of Procedures Investigations into Structural Properties Introduction Structure from a Material Science Point of View Structure from an Engineering Point of View Infrared Spectroscopy Nuclear Magnetic Resonance (NMR) Differential Scanning Calorimetry (DSC) Dynamic Thermal Mechanical Analysis (DTMA) Thermal Gravimetric Analysis (TGA) Scanning Electron Microscopy (SEM) Atomic Force Microscopy (AFM) Small-Angle X-Ray Scattering (SAXS) Chemical Resistance Introduction Alphabetical List of Chemicals Suitability Listings Polyurethanes under Load Introduction Deformation Relationships Stress Shear Compression Set Flops Introduction Early Detection Molding Problems Problems on Demolding Potential Problems Returns from Customer Premature End of Life Health and Safety Isocyanate Vapors Curatives Catalysts General Chemicals Material Safety Data Sheets (MSDSs) Heat Electrical Equipment Compressed Gas Lines Handling Tools, Materials, and Equipment Glossary Appendix

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CLEMITSON

POLYURETHANE CASTING PRIMER

POLYURETHANE POLYURETHANE CASTING CASTINGPRIMER PRIMER

I.R. I.R. CLEMITSON CLEMITSON

POLYURETHANE CASTING PRIMER

POLYURETHANE CASTING PRIMER I . R . C L E M I TSON

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2012 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20111107 International Standard Book Number-13: 978-1-4398-7922-1 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface........................................................................ xvii The Author.................................................................. xix   1 Introduction..............................................................1 1.1 Polyurethane Family of Plastics.................................. 1 1.1.1 Fibers................................................................ 1 1.1.2 Foams................................................................ 2 1.1.3 Coatings (Films)................................................ 3 1.1.4 Thermoplastic Polyurethanes (TPU)................ 4 1.1.5 Castable Polyurethanes.................................... 4 1.1.6 Millable Polyurethanes..................................... 4 1.2 Castable Polyurethanes Principles.............................. 5 1.2.1 General............................................................. 5 1.2.2 Processing......................................................... 6 1.2.3 Skills Required.................................................. 6 1.3 Comparison to Other Plastics..................................... 7 1.4 Health and Safety........................................................ 7   2 Fundamentals.........................................................11 2.1 Basics..........................................................................11 2.2 Fundamental Ingredients...........................................12 2.3 System.........................................................................13 2.3.1 Additives..........................................................14 2.3.1.1 Catalysts..............................................14 2.3.1.2 Pigments.............................................14 2.3.1.3 Plasticizers..........................................15 2.3.1.4 Fillers..................................................16 v

vi  ◾  Contents

2.4 Types 2.4.1 2.4.2 2.4.3

2.5

2.6 2.7

2.8

of Polyurethane Systems.................................16 Single Shot.......................................................17 Prepolymers.....................................................17 Quasiprepolymer.............................................19 2.4.3.1 Isocyanate Side..................................19 2.4.3.2 Polyol Side..........................................19 Commercial Types......................................................20 2.5.1 Polyols..............................................................21 2.5.1.1 Difunctional Polyethers......................21 2.5.1.2 Polyester-Based Polyols......................21 2.5.1.3 Renewable-Based Polyols..................22 2.5.1.4 Polyfunctional Polyols........................22 2.5.1.5 Hydroxylated Polybutadiene (PBD)...22 2.5.2 Isocyanates......................................................23 2.5.2.1 Aromatic.............................................23 2.5.2.2 Naphthalenic......................................23 2.5.2.3 Aliphatic.............................................23 2.5.3 Curatives..........................................................24 2.5.3.1 Amine curative...................................24 2.5.3.2 Hydroxyl Curatives.............................25 System Selection.........................................................25 Commercial Types of Polyurethane Systems............26 2.7.1 Prepolymers.....................................................26 2.7.1.1 Type....................................................26 2.7.1.2 Quantity............................................. 28 2.7.2 Quasiprepolymers...........................................29 2.7.3 Curatives..........................................................29 Processing Selection...................................................30 2.8.1 Hand Mixing....................................................30 2.8.2 Machine Mixing...............................................31

  3 Mixing and Casting Polyurethanes........................33 3.1 Basic Mixing and Casting...........................................33 3.1.1 Machine Processing.........................................34 3.1.2 Manually..........................................................34 3.1.2.1 Method...............................................35

Contents  ◾  vii

3.2 Background to Mixing and Casting...........................38 3.2.1 Introduction.....................................................38 3.2.1.1 Dryness of System..............................38 3.2.2 Basic Requirements.........................................39 3.2.2.1 Molds..................................................39 3.2.2.2 Design Factors....................................40 3.2.2.3 Equipment..........................................40 3.2.2.4 Mixing................................................43 3.3 Calculations............................................................... 44 3.3.1 Simplest Level................................................. 44 3.3.2 Amount of Ingredients....................................45 3.3.3 Basic Level...................................................... 46 3.3.4 Allowing for the Amount of NCO in the Prepolymer..................................................... 46 3.3.4.1 Proportional Method......................... 46 3.3.4.2 Amine Equivalent Method.................47 3.4 Adjustments to the Amount of Curative Used.......... 48 3.5 Method........................................................................49 3.5.1 Casting.............................................................49 3.5.1.1 Inserts.................................................49 3.5.1.2 Time/Temperature.............................50 3.5.2 Procedures.......................................................52 3.5.2.1 Basic Pouring Procedure....................52 3.5.3 Advanced Procedures......................................52 3.5.3.1 Centrifugal Casting.............................52 3.5.3.2 Compression Molding........................53 3.5.3.3 Vacuum Casting.................................55 3.5.3.4 Pressure Assisted................................56 3.5.3.5 Multi Pour...........................................56 3.6 Curing and Postcuring................................................57 3.6.1 Initial Cure.......................................................57 3.6.2 Demolding.......................................................57 3.6.3 Postcuring........................................................58 3.7 Calculations................................................................58 3.7.1 Background to the Calculations......................58 3.7.2 Multi Curatives.................................................61

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3.8 Tables..........................................................................62 3.8.1 Temperature.....................................................62 3.8.2 Curative Details.............................................. 64   4 Supplementary (Additional) Casting Processes......67 4.1 Thick Cross-Sections..................................................67 4.2 Bonding..................................................................... 68 4.2.1 Clean Surface...................................................69 4.2.2 Prepare Surface................................................69 4.2.3 Surface Coating................................................71 4.3 Finishing.....................................................................72 4.3.1 Removal of Flash and Sprues..........................72 4.3.2 Machining Parts to Size...................................73 4.3.2.1 Methods..............................................73 4.3.2.2 Turning and Facing............................74 4.3.2.3 Parting................................................74 4.3.2.4 Band Sawing......................................74 4.3.2.5 Drilling...............................................75 4.3.2.6 Milling................................................75 4.3.2.7 Grinding.............................................75 4.4 Postcure Bonding.......................................................76 4.5 Painting...................................................................... 77 4.6 Epoxy Blends............................................................ 77 4.7 Millable Polyurethanes...............................................79   5 Design Considerations............................................83 5.1 Quality of Part Required............................................83 5.1.1 Engineering Requirements..............................83 5.1.2 Surface Finish................................................. 84 5.2 Draft and Undercuts.................................................. 84 5.2.1 Need for Drafts............................................... 84 5.2.2 Undercuts in Mold Design..............................85 5.2.2.1 Step 1..................................................85 5.2.2.2 Step 2..................................................87 5.2.2.3 Step 3..................................................87 5.2.2.4 Step 4..................................................87

Contents  ◾  ix

5.3 Shrinkages................................................................. 88 5.3.1 Mold Expansion.............................................. 88 5.3.2 Polyurethane Shrinkages................................ 88 5.3.3 Combined Effects............................................91 5.4 General Considerations in Mold Design....................91 5.5 Parts in Compression.................................................92 5.5.1 Part Design......................................................92 5.5.2 Tension Avoidance...........................................92 5.6 Bonded Parts..............................................................94 5.6.1 High Stress Areas.............................................95 5.6.2 Parts under Shear............................................95 5.6.3 Parts under Torsion.........................................95 5.6.4 Parts under Tension........................................ 96 5.7 Wheels....................................................................... 98 5.7.1 Shape Factor................................................... 98 5.7.2 Full-Width Loads............................................. 99 5.7.2.1 European Tire and Rim Technical Organization...................................... 99 5.7.2.2 Alternate Calculations in Imperial Units.................................................100 5.7.3 Part Width Loads........................................... 101   6 Standard Polyurethane Properties.......................103 6.1 Introduction..............................................................103 6.2 Basic Comparisons...................................................104 6.3 Viscosity of Prepolymers..........................................105 6.4 Pot Life of Prepolymers............................................106 6.5 Curing and Post Curing............................................107 6.6 Slow Curing..............................................................109 6.7 Effect of Backbone Type.......................................... 111 6.7.1 Analysis of the Graphs.................................. 113 6.8 Polyether Polyurethanes........................................... 114 6.8.1 Premium-Grade Polyethers........................... 114 6.8.2 Standard-Grade Polyethers............................ 117 6.9 Range of Values within a NCO Level...................... 119 6.10 Aliphatic Polyurethanes...........................................122

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  7 Testing Polyurethanes..........................................123   8 Quality Control and Assurance Testing................127 8.1 Introduction..............................................................127 8.1.1 Type Tests......................................................127 8.1.2 Production Control........................................127 8.1.3 Nonroutine Control Tests..............................129 8.1.4 Test on Raw Materials...................................129 8.1.5 Tests on Finished Products...........................130 8.2 Raw Materials Testing...............................................130 8.2.1 Moisture.........................................................130 8.2.2 Viscosity......................................................... 131 8.2.2.1 Terms................................................ 131 8.2.2.2 Types of Viscometers.......................132 8.2.3 Wet Chemical Determination of NCO Level.... 133 8.2.3.1 Determination of Percent NCO in Prepolymer.......................................134 8.2.4 Adhesion........................................................135 8.3 Finished Products Testing........................................137 8.3.1 Hardness........................................................137 8.3.2 Density...........................................................139 8.3.3 Dimensions.................................................... 141 8.3.4 Curative Levels...............................................142 8.3.4.1 Background......................................142 8.3.4.2 General.............................................142   9 Standard Tests for Product Evaluation.................143 9.1 Introduction..............................................................143 9.2 Prepolymer Properties..............................................144 9.3 Processing Information............................................144 9.4 Cured Polyurethane Properties................................ 145 9.5 Details of Procedures............................................... 147 9.5.1 Compression Modulus...................................148 9.5.2 Compression Set............................................ 149 9.5.3 Creep.............................................................. 151 9.5.4 Shear Modulus............................................... 152

Contents  ◾  xi

9.5.5 9.5.6 9.5.7 9.5.8 9.5.9 9.5.10 9.5.11 9.5.12 9.5.13 9.5.14 9.5.15 9.5.16 9.5.17 9.5.18 9.5.19 9.5.20 9.5.21

Stress Relaxation........................................ 154 Tensile and Modulus Testing.................... 156 Tension Set................................................ 158 Tear Strength............................................. 159 Fatigue Resistance..................................... 161 Frictional Properties.................................. 162 Resilience................................................... 162 Machinability.............................................165 Wear...........................................................166 Chemical Effects........................................169 Cold............................................................ 170 Dry Heat Aging......................................... 171 Flame Resistance....................................... 172 Light and Ultraviolet (UV)......................... 173 Mold and Fungus...................................... 173 Radiation.................................................... 174 Electrical.................................................... 174

  10 Investigations into Structural Properties............. 177 10.1 Introduction............................................................177 10.2 Structure from a Material Science Point of View....177 10.3 Structure from an Engineering Point of View...... 178 10.4 Infrared Spectroscopy............................................ 179 10.4.1 Near Infrared (NIR)...................................180 10.4.2 Mid-Range Infrared Spectroscopy.............181 10.4.3 Methods to Determine Infrared (IR) Spectra.......................................................183 10.5 Nuclear Magnetic Resonance (NMR).....................187 10.5.1 Principle of Method...................................187 10.6 Differential Scanning Calorimetry (DSC)..............188 10.7 Dynamic Thermal Mechanical Analysis (DTMA).191 10.7.1 Modes of Operation of the Test................ 193 10.7.1.1 Temperature............................... 193 10.7.1.2 Sample Setup............................. 193 10.7.1.3 Application Rates....................... 193

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10.7.1.4 Application Method................... 193 10.7.1.5 Output Information................... 193 10.8 Thermal Gravimetric Analysis (TGA)....................194 10.9 Scanning Electron Microscopy (SEM)....................194 10.10 Atomic Force Microscopy (AFM)...........................196 10.11 Small-Angle X-Ray Scattering (SAXS)....................196 Endnotes............................................................................196   11 Chemical Resistance.............................................197 11.1 Introduction............................................................197 11.2 Alphabetical List of Chemicals..............................198 11.3 Suitability Listings..................................................201   12 Polyurethanes under Load....................................207 12.1 Introduction............................................................207 12.2 Deformation Relationships.....................................207 12.2.1 Permanent Set............................................208 12.2.2 Creep..........................................................208 12.2.3 Stress Relaxation........................................208 12.3 Stress.......................................................................208 12.3.1 Polyether Polyurethane Stress–Strain Relationships Shape Factor 0.25 Polyether.................................................... 211 12.3.2 Polyether Polyurethane Stress–Strain Relationships Shape Factor 0.5 Polyether................................................ 212 12.3.3 Polyether Polyurethane Stress–Strain Relationships Shape Factor 1.0 Polyether.................................................. 213 12.3.4 Polyether Polyurethane Stress–Strain Relationships Shape Factor 2.0 Polyether.................................................. 214 12.3.5 Polyester Polyurethane Stress–Strain Relationships Shape Factor 0.25 Polyester.................................................... 215 12.3.6 Polyester Polyurethane Stress–Strain Relationships Shape Factor 0.5 Polyester....216

Contents  ◾  xiii

12.3.7 Polyester Polyurethane Stress–Strain Relationships Shape Factor 1 Polyester..... 217 12.3.8 Polyester Polyurethane Stress–Strain Relationships Shape Factor 2 Polyester.....218 12.3.9 Comparison of Natural Rubber to Polyurethane.............................................. 219 12.4 Shear....................................................................... 219 12.4.1 Introduction............................................... 219 12.4.2 Polyurethane under Torsion......................223 12.4.2.1 Torsion Disk...............................223 12.4.2.2 Bush Mounting..........................224 12.5 Compression Set.....................................................226 12.5.1 Introduction...............................................226 12.5.2 Chemistry...................................................226   13 Flops.....................................................................229 13.1 Introduction............................................................229 13.2 Early Detection.......................................................230 13.3 Molding Problems..................................................231 13.3.1 Striations in Polyurethane.........................231 13.3.2 Viscosity of Prepolymer Higher than Normal.......................................................231 13.3.3 Prepolymer Mix Gels Too Fast..................231 13.3.4 Excessive Shrinkage Down Sprues and Pour Holes.................................................232 13.4 Problems on Demolding........................................232 13.4.1 Cannot or Very Hard to Open Mold.........232 13.4.2 Hard to Demold Part.................................232 13.4.3 Polyurethane Lower in Sprues and Vents than Normal.....................................233 13.4.4 Cracking on Demolding............................233 13.4.5 Wet Spots on Molding...............................234 13.4.6 White Flaky Areas (Snowflakes)...............234 13.4.7 Surface Pot Marks......................................234 13.4.8 Lumps Appear in Polyurethane after Demolding.................................................234

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13.4.9 Bubbles Visible in Flash, Sprues, and Risers..........................................................235 13.4.10 Large Voids in Part....................................235 13.4.11 Physical Properties Seem Poor..................235 13.5 Potential Problems.................................................236 13.5.1 Hot Storage of Polyurethane Prepolymer.............................................. 236 13.5.2 Improper Care in Keeping Containers Closed and Flushed with Nitrogen............236 13.5.3 Faulty Scales or Estimating Weights..........237 13.5.4 Poor Mixing Techniques...........................237 13.5.5 Poor Pouring Technique............................237 13.5.6 Poor Temperature Control.........................237 13.5.7 Loss of Temperature in Ovens..................237 13.5.8 Failure to Maintain Molds in Good Condition...................................................238 13.5.9 Lack of Sufficient Mold Release................238 13.5.10 Too Much Mold Release............................238 13.5.11 Mold Release Overspray............................238 13.6 Returns from Customer..........................................239 13.6.1 Not as Ordered..........................................239 13.6.2 Not Performing Correctly..........................239 13.6.2.1 New Product..............................240 13.6.2.2 Repeat Order..............................241 13.7 Premature End of Life............................................241 13.7.1 Parts Have Broken at Sharp Intersections............................................ 242 13.7.2 Melted from Inside....................................242 13.7.3 Melted on Outside.....................................242 13.7.4 Odd Wear Patterns....................................243 13.7.5 Badly Swollen............................................243 13.7.6 Bad Cuts.....................................................243 13.7.7 Surface Cracks............................................244 13.7.8 Part Changes Color Badly.........................244 13.7.9 Polyurethane Comes Away from the Reinforcing.................................................244

Contents  ◾  xv

  14 Health and Safety.................................................247 14.1 Isocyanate Vapors..................................................247 14.2 Curatives.................................................................248 14.2.1 Amine-Based Curatives.............................248 14.2.2 Diol-Based Curatives.................................249 14.3 Catalysts..................................................................250 14.4 General Chemicals.................................................250 14.4.1 Solvents...................................................... 251 14.4.2 Plasticizers.................................................. 251 14.4.3 Pigments.....................................................252 14.5 Material Safety Data Sheets (MSDSs).....................252 14.5.1 Typical Requirements for a MSDS.............252 14.6 Heat........................................................................254 14.6.1 Ovens.........................................................255 14.6.2 Oven Temperatures...................................255 14.6.3 Molds..........................................................255 14.7 Electrical Equipment..............................................255 14.8 Compressed Gas Lines...........................................256 14.9 Handling Tools, Materials, and Equipment...........257 Glossary.......................................................................259 Suggested Further Reading.........................................267 Appendix..................................................................... 275

Preface The aim of Polyurethane Casting Primer is to give details on the casting of polyurethane products and to assist the individual carrying out the work. This book describes how to make an article successfully without the more comprehensive chemistry as explained in my earlier book Castable Polyurethane Elastomers. Polyurethanes can be used to make a multitude of items, from very simple, noncritical parts to products that are used in vital engineering applications. The versatility of polyurethane enables a wide range of uses often limited only by the imagination of the user and manufacturer. ◾◾ The casting and allied processes are fully explained. Processing may be very simple, from a “bucket and paddle mix” open pour to a machine pour with postcuring machining, bonding, and painting. ◾◾ The care needed to produce quality products continuously is explained. Precautions that must be taken to maintain the health and safety of the workers, both in the short and long term, are described. ◾◾ The aim of the book is of a practical nature and meant for day-to-day use. The amount of theory is reduced and is separated from the fundamental methods. ◾◾ Properties that are regularly specified for polyurethane systems are detailed together with how the results are obtained. Polyurethanes are most often used in compression, and particulars of these tests and typical results are described. xvii

xviii  ◾  Preface

◾◾ Successful production in an economical manner needs knowledge of the correct grade and processing to use in order to meet customer requirements. These aims are detailed throughout the primer. ◾◾ Even with the best control, “flops” are produced. The book gives details of how to evaluate what happened and the necessary corrections that should be made. ◾◾ Polyurethane Casting Primer is aimed at both professional and subprofessional people who are designing and producing polyurethane products. You can download the batch calculations most commonly used together with curative equivalent weights on the book’s Web page at http://www.crcpress.com/product/isbn/9781439879214

The Author Ian Clemitson has worked in the chemical industry for 45 years. He worked predominantly in the polymer industry in both production and development, concentrating on rubbers and polyurethanes. In 1999 he earned a master’s degree from the University of Technology in Sydney, Australia. His thesis was titled “The Influence of Polyurethane Chemistry on Erosive Wear.” During this time he was working at the research and development (R&D) laboratory of Warman International in Sydney doing research into polyurethanes and other elastomers. Before he retired he worked at Chemind in Brisbane, Australia, where he was involved in setting up the manufacture of polyurethane elastomers. He carried out some more research (with a government grant) into wear-resistant polyurethanes. On a part-time basis he lectured at the local Plastics Training Institute (PARTEC Brisbane) on polyurethane and rubber technology. He is a chartered member (MRACI CChem) of the Royal Australian Chemical Institute. In 2008 his book Castable Polyurethane Elastomers was published by CRC Press, Boca Raton, Florida.

xix

Chapter 1

Introduction Polyurethanes are a family of plastic materials that were initially developed in Germany between World War I and World War II. The driving force was to develop an alternative to nylon that had been developed in the United States by E.I. du Pont de Nemours and Company. The initial nylon that was developed was a fiber grade. Otto Bayer and his coworkers developed a similar fiber based on different chemistry. After the initial fiber development, polyurethane foams were introduced and used in aircraft during World War II.

1.1  Polyurethane Family of Plastics Castable polyurethanes are a member of the six major groups in the polyurethane family. Each of these groups has developed separately but all have the same basic chemistry, each optimized for that specific area of use.

1.1.1 Fibers Polyurethane is used extensively where stretch and strength is required in fabrics. The generic names for polyurethane fibers are “Spandex” and “Perlon.” 1

2  ◾  Polyurethane Casting Primer

Perlon is manufactured by subsidiaries of the original discoverers of polyurethane fibers. It is used in technical fabrics, ropes, fishing lines, and some agricultural products. Spandex fibers are often used in conjunction with other fabrics. Being very elastic and with special weaving, it can be made into fabrics with two-way stretch. This makes it suitable for very close-fitting garments.

1.1.2 Foams Foams are the underpinning force of the polyurethane industry. Foams represent the largest uses of polyols and diisocyanates. Foams consist of a thermoset polyurethane that has been expanded by a gas. The basic foam consists of four parts: ◾◾ Polyol −− With 2 to 10 hydroxyl (–OH) groups ◾◾ Isocyanate −− Usually a crude diisocyanate with two or more isocyanate groups (–NCO) ◾◾ Blowing agent −− Water or low boiling point organic (carbon-containing) material ◾◾ Catalysts −− Designed to promote specific reactions There are a large number of variations possible that can change the properties greatly to make a big range of products, from soft pillows to foams that stabilize cracks in a mine. The three main groups of foams are: ◾◾ Rigid foams −− These are used in insulation, both heat and acoustic. They can be produced by hand batching, machine pouring, or by spraying on.

Introduction  ◾  3

◾◾ Flexible foams −− These are found in domestic applications such as mattresses, pillows, and furniture cushions. ◾◾ Integral skins −− These foams have an outer solid skin of polyurethane with a foamed inner layer. They are used in such applications as car dashboards and steering wheel covers. The main problem with foamed polyurethanes is transporting of the bulky finished product and the use of suitable greenhouse gas friendly blowing agents.

1.1.3 Coatings (Films) These are used mainly as wear-resistant coatings that can be applied by a number of methods. ◾◾ Two-pack spray-on −− These are sprayed on using a plural spray machine and are often high build (a thick coating). −− A modified polyurethane (namely a polyurea) is often used for extremely fast curing thus avoiding problems with water. −− Spray-on systems need care in application to prevent health problems (isocyanate aerosol). ◾◾ Single component −− These systems are normally only suitable for thin coats. −− Curing is by the reaction of moisture in air. −− By-products (CO2) dissipate into the air. −− They are used in waterproof barriers and in polyurethane paints. ◾◾ Latex −− Fully cured polyurethane is made into latex. −− Film is produced on removal of carrier fluid. −− It has a low volatile organic compound (VOC) level.

4  ◾  Polyurethane Casting Primer

1.1.4 Thermoplastic Polyurethanes (TPU) Polyurethanes can be made by mixing the polyol and diisocyanate in a certain ratio that will produce a very long chained polymer. These polymers can be made with a variety of chemistries that can change the physical and chemical properties of the finished product. The processing of TPUs is similar to any other thermoplastic such as polyethylene, nylon, or polyvinyl chloride (PVC) where conventional plastics equipment such as extruders and injection molding machines can be used. There are grades that have good biocompatibility and are even used in implants such as pacemakers.

1.1.5 Castable Polyurethanes This component of the polyurethane family is a very versatile section. It can produce high-performance engineering products if required with a low initial capital outlay. The components can be blended by hand or machine and poured into a mold, then cured in an oven at 70 to 110°C to produce a part. The molds may be made from metal, epoxy, silicone, or most commonly from polyurethane itself. A pattern can be made from any impervious dry material. From this a mold can be made from which many more parts can be produced. The large number of variations that can be made to the chemistries means that materials are available for the production of simple parts such as door stops to complex cable protector pads for the mining industry. Polyurethanes are useful where a design is being evaluated and parts can be made relatively cheaply prior to the expenditure of capital to produce an injection molding die.

1.1.6 Millable Polyurethanes Millable polyurethanes are a family of polyurethanes that are processed in a similar manner to conventional rubbers.

Introduction  ◾  5

This means that companies that are highly geared to the production of rubber parts can offer the parts in polyurethane as well as the normal range of specialist elastomers. This is of importance where relatively small runs are required and the problem of exactly replicating the rubber part in castable polyurethanes is not justified.

1.2  Castable Polyurethanes Principles 1.2.1 General There are a large variety of castable polyurethanes varying from very soft to hard rigid materials. Their uses range from fishing lures to complex models of diseased human organs and to large parts used in the mining industry. Officially the abbreviation for polyurethane is PUR, but it is more commonly abbreviated to PU. The properties of the polyurethanes can be varied at different levels to give completely different materials with various optimal properties. The basic method of manufacture is as follows: ◾◾ A long-chain polyol −− A chemical with two hydroxyl groups (–OH) ◾◾ A diisocyanate (two –NCO groups) ◾◾ A curative (chain extender) These three items are reacted together to give a polyurethane. All the ratios need to be right and temperatures must be correct. Depending on the complexity and nature of the process there are three main routes: ◾◾ Prepolymer −− Bulk reaction between polyol and diisocyanate −− Curative added just before casting

6  ◾  Polyurethane Casting Primer

◾◾ Quasiprepolymer −− Portion of the polyol with curative and some catalyst −− Diisocyanate reacted with rest of polyol −− Ratios set to give easy mixing ratios −− Lower temperature cures ◾◾ One-shot process −− All ingredients mixed together and allowed to react −− Needs metering machine −− Most suited to thin section −− Heat set free needs to be removed

1.2.2 Processing The processing of polyurethanes is basically very simple. The easiest method is hand mixing or the “bucket and paddle” method. Machinery can be used to process the polyurethane. The greatest enemy in polyurethane processing is moisture, and this has to be avoided at all costs. Low-pressure molds can be used to cast the material. This is a major saving compared to the cost of molds that are used in injection molding.

1.2.3 Skills Required ◾◾ Minimal −− Recognition of health and safety requirements −− Understanding of process −− Ability to measure and mix material to specification −− Ability to perform basic machining operations (band sawing, lathing, and milling) ◾◾ Skills for proficient operation −− Knowledge of different types of polyurethane to use −− Limitations of polyurethanes −− Ability to select correct grade for application

Introduction  ◾  7

• Chemical • Engineering • Combination of chemical and engineering −− Problem solving −− Process and quality management

1.3  Comparison to Other Plastics There are three basic groups of polymers: ◾◾ Thermoplastics −− Typified by polyethylene (PE), PVC, etc. −− These materials can be remelted and reprocessed with minimal degradation ◾◾ Thermosets −− Typified by epoxy, phenolics −− Hard and rigid ◾◾ Elastomers (rubbers) −− When stretched will return to original length Castable polyurethanes have the basic properties of thermosets in that they can only be processed once, plus they have the elastic of rubber over the full range of hardness. Very hard rubbers (high Shore D and ebonite) lack the elastic properties of hard polyurethanes. The overall properties of polyurethanes make them competitive with most of the standard plastics except the expensive materials such as Kevlar.

1.4  Health and Safety All polyurethane ingredients are industrial chemicals that have health and safety risks associated with them.

8  ◾  Polyurethane Casting Primer

Note: Prior to starting work with polyurethanes, the health and safety risks of all the ingredients to be used must be fully examined and evaluated. The individual chemical properties, risks, and health implications are given in Material Safety Data Sheet (MSDS) documents that the supplier of the chemical must supply even at the sample stage. The MSDS must be the standard full format and not abbreviated. Key points to be noted are as follows: ◾◾ Isocyanates (all types) −− Vapors can cause bronchial problems including asthma attacks −− Sensitization can occur −− Worker should be medically screened prior to commencement of work −− Low and free isocyanate grades are available −− The removal of isocyanate vapors should be engineered out −− Personal protective equipment should be worn ◾◾ Curatives −− All amine curatives possess some health risks −− MOCA (MBOCA) is classed as a Class 2 suspect carcinogen • All local laws and regulations must be strictly adhered to. • All safety equipment must be used at all times. • Screening tests must be carried out. • There are different routes of absorption. • Local agencies must be consulted before use. ◾◾ Auxiliary agents −− All chemicals will exhibit some risk. • The MSDS will detail all the health, explosion, and fire risks.

Introduction  ◾  9

−− Open flames are used so the use and storage of solvents must be controlled. ◾◾ Engineering machinery −− These must be operated in accordance with local regulations and in accordance with the best practice for the operation. Operators of polyurethane processing factories need to keep abreast of all changes to health and safety regulations with regard to the chemicals they use.

Chapter 2

Fundamentals 2.1  Basics The primer deals mainly with castable polyurethanes. They are a member of the polyurethane family of plastics that all contain the urethane group. O HN

C

O

R

R

The polyurethane family consists of six main units each with a large number of subcomponents. The six groups include the following:

1. Castable 2. Foams 3. Fibers 4. Thermoplastics 5. Millable 6. Films

11

12  ◾  Polyurethane Casting Primer

The castable, thermoplastic, and millable polyurethanes have similar chemistries. The main differences are as follows: ◾◾ Castables are made with an approximate 2-to-1 polyoldiisocyanate ratio followed by further chain extension. ◾◾ Thermoplastics are made with a 1-to-1 polyol-diisocyanate ratio. ◾◾ Both rely mainly on hydrogen bonding for strength. ◾◾ Both are based on thermoplastic chemistry with some double bonds for cross-linking chains by either peroxide or sulfur cures. A close relative to the polyurethane is the polyureas where the major chemistry differs in the fact that instead of a polyol a polyamine is used. O HN

C

NH

R

R

These materials are very much more reactive than conventional polyurethanes. They find a major use in spray applications when quick cure is important and the reaction with moisture is reduced.

2.2  Fundamental Ingredients There are three vital ingredients in castable polyurethane: 1. Polyol 2. Diisocyanate 3. Chain extender (commonly called the curative) Modifiers form a fourth group that will change the appearance and properties. These include the following: a. Catalysts b. Pigments

Fundamentals  ◾  13

c. Plasticizers d. Fillers

2.3  System The formation of a polyurethane elastomer is carried out in three main steps. The polyol is reacted with the diisocyanate to form a simple chain (Figure 2.1). The second step is the chain extension that occurs by reaction of the terminal isocyanate in the reaction product with an –OH group of another polyol. The final step is the formation of an elastomer by curing (further chain extension). This can be done with either a hydroxyl (diol or higher) or a diamine. Following are some basic points: ◾◾ Final properties will depend on choice of polyol and diisocyanate. ◾◾ Properties will also depend on the exact method of preparation. Macro polyol

Diisocyanate

HC 2 HC

C

CH

C

C

N

C

CH

O

N

+1

HO

CH2 CH2 CH2 CH2 CH2 CH2 CH2 OH Heat

O HC HC

O

C

CH

C N

C CH

NH

C O

O

O CH2 CH2 CH2 CH2 CH2 CH2 CH2 Urethane bonds

First stage of reaction

C HN

O

C HC

CH

CH

C

N

C

CH

Figure 2.1  The first step in the formation of a polyurethane chain.

O

14  ◾  Polyurethane Casting Primer

◾◾ A hydroxyl compound will give a pure polyurethane. ◾◾ An amine will give a poly(urea-urethane). −− This is still called a polyurethane in the trade. ◾◾ The exact molecular ratios are adjusted to optimize properties.

2.3.1 Additives The strength of the polyurethanes is basically dependent on hydrogen bonding and not on the addition of reinforcing filler as in rubbers. Covalent bonding, which happens when triols are used, results in a softer compound. This is due to the disruption of the hydrogen bonding network. The following are the main uses of various additives.

2.3.1.1 Catalysts ◾◾ Not normally needed in standard process conditions ◾◾ Weak acids such as adipic acid for toluene diisocyanate (TDI)/amine cures ◾◾ Tin and bismuth catalysts for lower-temperature cures −− Check local regulations for levels permitted ◾◾ Range of suitable catalysts provided by major catalyst suppliers −− For example, Air Products Polycat® range

2.3.1.2 Pigments Pigments are used to color polyurethanes. They are normally added in the form of a paste. Polyurethanes based on an aromatic diisocyanate have a tendency to yellow when exposed to light. This can affect the final color considerably. Some major points in choosing a pigment are: ◾◾ They must be dispersed in a nonhydroscopic medium such as a phthalate plasticizer. ◾◾ The light stability must be suitable for the application.

Fundamentals  ◾  15

◾◾ They must be concentrated so that the minimum amount needs to be added. ◾◾ White pigment tends to go yellow with the aromatic diisocyanates. ◾◾ Yellows can be difficult to disperse in mixtures. −− Disperse first in prepolymer. ◾◾ Pigments with cadmium and lead may not always be permitted by local regulations. ◾◾ For hand mixing use squeeze bottle to dispense.

2.3.1.3 Plasticizers Plasticizers may be either reactive or nonreactive. A reactive plasticizer takes part in the cure process and must be accounted for in the calculation for the amount of curative needed. Nonreactive plasticizers act as a nonreinforcing filler that softens the material by breaking the hydrogen bonding and lubricates the molecules.

2.3.1.3.1  Reactive Plasticizers Polyols of a molecular weight (M/W) of 250 to 1000 can be used to reduce hardness. The major application is in MDI systems where additional PTMEG is added to the prepolymer. The hydroxyl groups of the polyol will react with the terminal isocyanate group of the prepolymer. The amount of the primary curative must be adjusted to compensate for the use of the reactive plasticizer. The method is detailed in the section on mixing (Chapter 3). Details of the calculations are given in the same chapter.

2.3.1.3.2  Nonreactive Plasticizers These plasticizers do not chemically react with the polyurethane but soften the material by breaking up the distribution of the polyurethane chains. They are normally a polar material that is compatible with the polyurethane. Classic examples are the phthalate range of materials. They tend to reduce the

16  ◾  Polyurethane Casting Primer

strength of the polyurethane; therefore, they find the most use in the tougher polyester materials. In this case they also help to decrease the viscosity of the prepolymer.

2.3.1.4 Fillers Polyurethanes do not require fillers in the same manner as standard rubbers. Their intrinsic strength is normally sufficient. The addition of fillers reduces many of the polyurethane’s properties. Ultrafine silica (such as Aerosil®) has been used to give thixotropic properties to spreadable polyurethanes. It is used at low levels (1% to 3%). Incorporating it into the prepolymer is difficult as the material is very fine, dusty, and bulky. Nanoparticles have been used to enhance tear properties. The technical point to overcome is that the polyurethane must come into intimate contact with the reactive groups attached to the particles. This is because the particles are often in the form of very fine stacked flakes.

2.4  Types of Polyurethane Systems There are three main routes to prepare a polyurethane elastomer: ◾◾ Single shot ◾◾ Prepolymers ◾◾ Quasiprepolymer There is a fourth route that is used in research and that is where all the reactions are carried out in solution. This is cast onto a substrate and the solvent evaporated off or the polyurethane precipitated out and dried.

Solvent

Pigment

Curative

Diisocyanate

Polyol

Fundamentals  ◾  17

Variable Pumps Heated Recirculation Lines

Mixing Head

Figure 2.2  Single shot process.

2.4.1 Single Shot In the single-shot process (Figure 2.2) all the ingredients are mixed together and the reaction is allowed to proceed to completion. The reaction often needs to be sped up by the addition of a catalyst. Due to the fact that the reaction gives off heat (exothermic), the process is mainly used in thinwalled applications where the generated heat can be readily dissipated. If the part becomes too hot (>115°C) the chemistry changes and an inferior product is created. The second effect is that stresses can be set up and the product will form stress cracks. The equipment and mold set-up is similar to that used for RIM (reaction-in-mold) processing of two-part polyurethanes.

2.4.2 Prepolymers Prepolymers are used to react the polyol and diisocyanate together so that a portion of the exotherm can be dissipated prior to final casting. There are several advantages to this route, including

18  ◾  Polyurethane Casting Primer

◾◾ Highly reduces the exposure to isocyanate vapors ◾◾ Allows more precise structure of prepolymer ◾◾ Reduces the final exotherm and hence allows thicker castings ◾◾ Reduces need for catalysts There are two main types of prepolymers: ◾◾ Unstable ◾◾ Stable Prepolymers made using NDI (naphthalene diisocyanate) have traditionally been classed as unstable prepolymers. Their shelf life was only 1 to 2 hours; hence they had to be made on-site just prior to use. Currently in some parts of the world they may be purchased from the manufacturer, and the Vulkalon has a shelf life of 6 months. Stable prepolymers are prepared by reacting the diisocyanate and polyol in a controlled manner to obtain linear chains with as low a viscosity as practicable. Excess unreacted diisocyanate is often stripped to produce a low free diisocyanate material. These prepolymers are stabilized with a weak organic acid to reduce further reactions taking place. However, there are still some reactions taking place that increase in velocity with higher storage and melting temperature. Table 2.1 gives the standard shelf life at various temperatures. Table 2.1  Shelf Life of Stable Polyurethane Prepolymers Temperature (°C)

Time

25

365 days

60

14 days

70

3 days

90

12 hrs

100

8 hrs

Fundamentals  ◾  19

Following are some key points about the life of prepolymers: ◾◾ The effect of heat is cumulative (i.e., the effect is nonreversible). ◾◾ The exact life is dependent on the manufacturing process. ◾◾ The effect of opening and closing the container must be taken into account. ◾◾ The use of a microwave oven to heat small amounts is advantageous. ◾◾ The level of isocyanate (% NCO) in aged prepolymers is reduced. ◾◾ The viscosity of aged prepolymers will not reduce as much on heating.

2.4.3 Quasiprepolymer A quasiprepolymer is a two-part polyurethane system composed of an isocyanate side and a polyol side. It is very commonly used in MDI-based systems. The main advantage is that the mixing ratios can be set close to 100:100 or similar large round numbers either on a weight or volumetric basis.

2.4.3.1 Isocyanate Side This contains a prepolymer with a very high isocyanate level normally up to a fourfold molar excess of isocyanate.

2.4.3.2 Polyol Side The polyol side contains ◾◾ Sufficient polyol to complete the reaction ◾◾ The required amount of chain extender (curative) ◾◾ A catalyst if required to speed the reaction up

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Table 2.2  Naming Variations American

European

A side

Isocyanate

Polyol

B side

Polyol

Isocyanate

Care must be taken because the A and B sides are expressed differently in various parts of the world. Table 2.2 illustrates these variations. The solubility of the curative in the polyol is close to its limit. This causes a potential for phase separation at low temperatures. The polyol side needs to be well mixed prior to use to obtain the optimum results. The manufacturer supplies the mixing ratio. There is no information given to allow for variations to the ratios for optimization as in the prepolymer route.

2.5  Commercial Types The majority of commercial systems are based on one of the following combinations: Polyol

Isocyanate

Comments

Polyether

TDI

Standard grades

Polyether

MDI

Standard plus food contact

Polyester

TDI

Extra tough oil resistant

Polyester

MDI

Extra tough oil resistant

Polyether

Aliphatic diisocyanate

Clear products

Polyether

Special diisocyanate

High-temperature goods

Within each of these grades there are many smaller groups and molecular weights. Each variation gives certain processing and property advantages.

Fundamentals  ◾  21

2.5.1 Polyols There are five main groups: ◾◾ Difunctional polyethers ◾◾ Difunctional polyesters ◾◾ Renewable polyols ◾◾ Polyfunctional polyols ◾◾ Hydroxylated polybutadiene

2.5.1.1 Difunctional Polyethers ◾◾ Polypropylene glycol (PPG) −− Most common polyether diol −− Relatively inexpensive −− Products made with it have reasonably good properties ◾◾ Monodiol PPG −− Less PPG polymers with only one hydroxyl group −− More expensive than standard PPG −− Cheaper than PTMEG −− Properties close to PTMEG based ◾◾ Polytetramethylene glycol (PTMEG) −− These give the best overall properties of the polyethers −− Prepolymers more viscous than with PPG −− More expensive than PPG ◾◾ Miscellaneous straight changed diols −− These were available depending on the feedstock available to the manufacturer

2.5.1.2 Polyester-Based Polyols Polyesters make very tough, oil-resistant polyurethanes. The traditional problem is poor resistance to water (hydrolysis).

22  ◾  Polyurethane Casting Primer

There are three main types: ◾◾ Traditional polyesters −− Made by the reaction of a dibasic acid with a diol and elimination of water −− Very tough and very good oil resistance −− Poor hydrolytic stability over 50°C ◾◾ Caprolactone based −− Caprolactone is a raw material in some nylons −− Improved hydrolytic stability −− More expensive ◾◾ Polycarbonate based −− Made from propylene and ethylene carbonates −− New on market −− Hydrolytic stability on par with PTMEG −− Currently expensive

2.5.1.3 Renewable-Based Polyols These can be made from vegetable oils or other organic matter such as cork. To make linear polyols requires careful extraction of certain fractions and appropriate chemistry. They have limited use in castable urethanes.

2.5.1.4 Polyfunctional Polyols The major use is to provide a degree of cross-linking of the polymer chains. The main uses are: ◾◾ Improves the compression set ◾◾ Used in very hard compounds ◾◾ Decreases the hardness

2.5.1.5 Hydroxylated Polybutadiene (PBD) The major use of hydroxylated polybutadiene is as a binder where a degree of flexibility is needed over a wide range of

Fundamentals  ◾  23

temperatures. A typical example is in rockets and caulking compounds.

2.5.2 Isocyanates There are three main types of isocyanates in use.

2.5.2.1 Aromatic These are used extensively in casting polyurethanes. They include: ◾◾ Toluene diisocyanate (TDI) −− Standard grade is the mixed isomers 2,4 and 2,6 (80:20) blend −− 100% 2,4 isomers give superior properties • More expensive −− Best cured with diamines ◾◾ Methylene diisocyanate (MDI) −− Purified 4,4′ isomer used • Not crude grade as in foams −− More expensive than TDI −− Cured with diols

2.5.2.2 Naphthalenic 1,5 Naphthalene diisocyanate (NDI) ◾◾ Original isocyanate use ◾◾ Lower use than the aromatic diisocyanates ◾◾ More reactive than TDI and MDI

2.5.2.3 Aliphatic Some common aliphatic diisocyanates include H12MDI, HDI, HDI, and IPDI.

24  ◾  Polyurethane Casting Primer

◾◾ Active ring is not aromatic but aliphatic ◾◾ Nonyellowing ◾◾ High usage in films ◾◾ Much slower than aromatics ◾◾ Used in polyurea systems

2.5.3 Curatives There are two main groups of curatives in common use. These are: ◾◾ Amine curatives (–NH2) ◾◾ Hydroxyl curatives (–OH)

2.5.3.1 Amine curative Diamine curatives are most suited to TDI prepolymers due to their speed of reaction. Some of the most common curatives are: ◾◾ Ethacure 300® (Albemarle Corporation, Baton Rouge, Louisiana) −− Liquid at ambient temperature −− Not as hazardous as MOCA −− Good processing speed −− Mechanical properties slightly better than MOCA ◾◾ Ethacure 100® (Albemarle Corporation, Baton Rouge, Louisiana) −− Ten times faster than E300 −− Used in polyureas −− Used with E300 for very fast cures ◾◾ MOCA (MBOCA) −− Class 2 suspect carcinogen −− Gives good all-around properties −− Used as reference curative −− Needs to be melted prior to use −− Available as a concentrate in a plasticizer

Fundamentals  ◾  25

◾◾ Variations to MOCA molecule −− Variations to molecular structure by adding extra groups −− Properties normally not as good as MOCA −− Some have special features ◾◾ M-CDEA −− Solid needs melting −− Gives better heat resistance −− Resilience of products higher (lower tan δ) −− Expensive

2.5.3.2 Hydroxyl Curatives The most common material is 1,4-butanediol (BDO). Triols such as trimethylol propane (TMP) are also used to provide cross-linking. ◾◾ 1,4-Butanediol (BDO) −− Normally used with MDI • Best reactivity for system −− Liquid at room temperature −− Very hygroscopic • Must be kept dry −− Low weight needed compared to most diamine curatives • Often best in quasiprepolymers ◾◾ Trimethylol propane (TMP) −− Triol −− Causes permanent cross-linking −− Hydroscopic −− Needs melting

2.6  System Selection The above data on the polyols, diisocyanates, and curatives indicate the large number of variables in selecting the right grade. A database of your own successful applications is very

26  ◾  Polyurethane Casting Primer

advantageous. The database can be as simple as an indexed notebook with details of the application and the materials used. Table 2.3 details the best choice for different applications. There is a sufficient range of normal hardnesses for it not to influence the choice of system. The tensile strength is adequate to cover all uses in the castable range. It must always be remembered that there is normally more than one property that needs to be optimized, but there is often one controlling factor.

2.7  Commercial Types of Polyurethane Systems 2.7.1 Prepolymers Commercial prepolymers are available in a large range of types and in a variety of sizes. The main aim is to obtain a product that is correct for the application, with the best processability and at the best price.

2.7.1.1 Type An important point is not to use a grade that is too high or too low for the application. On the other hand, it is not economical to stock too large a range of materials. There must be a good turnover of stock with the oldest being used first. Within each grade there are a number of variations. The following points need to be considered: ◾◾ Is the level of free isocyanate acceptable? −− There are often supplier-to-supplier variations −− Low level free isocyanates are available • Better properties • Higher price

Fundamentals  ◾  27

Table 2.3  System Selection Table Property

Best Choice

Second Best Choice

Poorest Option

Abrasion resistance

Ester system

Caprolactone

Ether

Compression set

PTMEG/TDI/ E300

PTMEG/TDI/MOCA

PPG/MDI/ BDO

Erosion resistance

MDI ether

PTMEG/TDI

Ester system

Fooda

PTMEG/MDI/ BDO

Hydroxyl

Amine-cured systems

Heat aging

Ester system

Caprolactone

Ester system

Heat buildup

PTMEG/MDI/ BDO

PTMEG/TDI/M-CDEA Standard ester

Hightemperature properties

PTMEG/ PTMEG/TDI/MOCA PPDI/M-CDEA plus heat treatment

Caprolactone

Hydrolysis

PTMEG/MDI/ BDO

PTMEG/MDI/BDO

Standard ester

Low temperature

PTMEG/MDI/ BDO

PTMEG/TDI/E300

Standard ester

Oil resistance

Ester system

Caprolactone

Ether

Processing

Low cost ether

Quasiprepolymers

Standard ester

Resilience

PTMEG/MDI/ BDO

PTMEG/TDI/M-CDEA Standard ester

Very low hardness

Ester/TDI

Ester/TDI

a

Low cost ether

Requires official approval such as from the U.S. Food and Drug Administration (FDA).

28  ◾  Polyurethane Casting Primer

◾◾ Is the processing profile suitable to your plant? −− Pot life −− Demold time −− Cure time ◾◾ Is the viscosity at operating temperature low enough? Chemistries can influence the state of the prepolymers at room temperatures. PTMEG and MDI softer grades are either solid or semisolid at room temperature. The drums need to be melted out completely and kept just liquid before use. Storage in a warm spot will keep them liquid for some time (often kept under ovens). The effect of heating is additive and affects the life of the prepolymer. It is not economical to purchase a high-grade polyurethane prepolymer for use in making molds. A general-purpose PPGbased material is more than sufficient. This does not preclude using the ends of a drum of suitable hardness prepolymer that has been left over from another job to make molds. In all processing it must always be remembered to flush any open containers with dry nitrogen. All bungs and bungholes must be free of polyurethane prior to closing, otherwise the moisture in the air will cure the polyurethane in the threads. Careful heating with a gas flame will soften the polyurethane in the threads and allow the bung to be removed.

2.7.1.2 Quantity Polyurethane prepolymers are available in pack sizes from 1 to 2 kg containers to intermediate bulk containers (IBCs). The normal sizes are the 20 kg pail and the 200 liter drums. The size purchased depends on the turnover of the material in the shop. Every time a drum is opened and reheated the life of the prepolymer is reduced (Table 2.1).

Fundamentals  ◾  29

2.7.2 Quasiprepolymers Quasiprepolymers come in similar pack size ranges to prepolymers. As the systems are set up for parts A and B of the same formulation, the manufacturer’s mixing ratios must be adhered to. Mixing nonmatching cans can produce unexpected results. Special care must be taken to ensure that the polyol side is fully mixed prior to use. The polyol side contains several ingredients, and a degree of phase separation has been known to occur.

2.7.3 Curatives Curatives are commercial-grade chemicals. Various manufacturers of the same chemical will have materials that may appear similar in specification but may react differently in use. This can be caused by a variety of reasons: ◾◾ The exact balance of the various isomers in the product ◾◾ Traces of different manufacturing catalysts left in material ◾◾ Different by-products remaining In critical work stick to the supplier whose material you know. Curatives such as MOCA come in a variety of forms and activities. This must be taken into account when the weight of curative to use is calculated. Liquid amine curatives are easier to use but they are best kept in the dark and under nitrogen to prevent absorption of moisture, chemical changes, and oxidation. Solid curatives must be melted before use. The following are key points to remember: ◾◾ Occupational Health and Safety regulations to be adhered to at all times ◾◾ Protective safety equipment to be worn

30  ◾  Polyurethane Casting Primer

◾◾ Only melt sufficient curative for work on hand ◾◾ Do not overheat curative as it gives off very dangerous fumes ◾◾ The molten material can give bad burns Hydroxyl curatives need to be kept dry.

2.8  Processing Selection In small to medium jobbing shops there are two main production methods: ◾◾ Hand mixing ◾◾ Machine processing

2.8.1 Hand Mixing Hand mixing is normally the initial choice for small shops where there will be short runs of many different grades. The degree of sophistication will depend on the amount and type of work that will be carried out. The shop floor needs to be divided into several zones. These include: ◾◾ Goods in and out ◾◾ Raw materials storage and preheating ◾◾ Mold preparation and heating −− Normally the same ovens as curing and postcuring ◾◾ Wet casting area ◾◾ Curing area ◾◾ Demold and trim ◾◾ Post curing ◾◾ Machining and finishing ◾◾ Mold storage

Fundamentals  ◾  31

2.8.2 Machine Mixing Machine mixing is suitable where there can be relative long runs using the same prepolymer system or the same quasi­ prepolymers. The major advantage of machine processing is that once the machine has been correctly set up and the materials stabilized, the machine should be able to produce parts with the same properties with minimal wastage. The following are some key considerations when evaluating the introduction of a processing machine: ◾◾ Is the output sufficient for the use? ◾◾ Are there sufficient inputs for your needs? −− Prepolymer or part A quasiprepolymer −− Curative or part B quasiprepolymer −− Pigment −− Plasticizer if needed −− Solvent ◾◾ Can the flows be adjusted accurately to suit your formulas? ◾◾ Can sufficient prepolymer and curative be brought up to the correct temperature for processing? ◾◾ How accurate and reproducible is the metering system? ◾◾ How easy can the head be cleaned? ◾◾ Are spare parts readily obtainable? −− Some critical parts should always be kept on site ◾◾ Can the unit be upgraded to programmable logic controller (PLC) closed-loop control? ◾◾ Does it have menu control? ◾◾ Do you have suitable staff that can learn how to run and maintain the machine? ◾◾ Is the machine economically viable?

Chapter 3

Mixing and Casting Polyurethanes This chapter is divided into two parts: ◾◾ Basic mixing and casting ◾◾ Background to mixing and casting The first part covers the steps and equipment required for making a polyurethane part; the second covers more detailed aspects of the process.

3.1  Basic Mixing and Casting There are two basic ways to mix polyurethane: ◾◾ Machine processing ◾◾ Manually Note: Before any handling of polyurethane chemicals or equipment, all staff and workers must be familiar with the Occupational Health and Safety requirements for the use of 33

34  ◾  Polyurethane Casting Primer

chemicals and machinery as well as all local rules and regulations. Copies of the relevant Material Safety Data Sheets (MSDSs) must be studied and adhered to. Chapter 14 discusses the subject of health and safety in more detail.

3.1.1 Machine Processing Machine processing should be carried out using the initial setting given by the suppliers of the raw material and the machine manufacturers. Both the machine and the polyurethane system must be compatible with each other and capable of allowing fine adjustments. The machine must be large enough for the production to be completed without a full shutdown and recharge of the system. Machine casting is ideal for one-shot processes where all the ingredients are mixed in a multi-input machine. Some of the ingredients may be preblended, such as the polyols, catalysts, and extenders.

3.1.2 Manually ◾◾ Basic equipment −− Safety gloves, mask, and goggles −− Molds −− Ovens −− Microwave oven −− Scales −− Devacuuming system −− Dry nitrogen supply −− Mixing containers −− Stirring paddles −− Melting pots for solid curatives (if required) −− Bonding agents −− Butane burner −− Trimming knives

Mixing and Casting Polyurethanes  ◾  35

−− Machining equipment (if required) −− Measuring and test equipment (as required) ◾◾ Basic raw materials −− Prepolymers −− Curatives −− Alternative quasiprepolymer system −− Mold release −− Pigments −− Fillers −− Plasticizers −− Cleaning solvents

3.1.2.1 Method The molding procedure is a multistep process with several steps running in parallel. 1. Preparation a. Clean and bring molds to temperature (1) Make any repairs (2) Clean surface dirt (3) Remove old mold buildup if necessary (4) Heat to curing temperature b. Thaw prepolymers (1) Keep in warm parts of shop ◾◾ Under ovens (2) Polytetramethylene glycol (PTMEG) based prone to solidify (3) No center milkiness (4) Roll mix to eliminate temperature variations c. Melt curative if necessary (1) Do not overheat (2) Only sufficient for day d. Weigh out prepolymer and curative (1) Into clean dry container (2) Maximum depth 25% of height

36  ◾  Polyurethane Casting Primer

(3) Can use more than one container for large pours e. Bring to correct temperature (1) Bring to supplier’s recommended temperature (2) Use microwave for smaller amounts f. Degas (1) Use vacuum pot (2) Approximately 6 kPa (28″ Hg) vacuum (3) Break vacuum if too foamy 2. Mixing a. Add pigment to prepolymer (1) Some pigments need to be predispersed into the prepolymer b. Add curative to prepolymer in gentle stream (1) Add to mixing container in a stream (2) Pour along one edge c. Mix using zigzag motion or cross-blending (1) Do not whip in air (2) Clean prepolymer from edges and corners into mix (3) Clear mixes must be streak free (4) Pigments must be completely dispersed d. Degas again (1) If required degas again provided there is sufficient pot life 3. Pouring a. Add to mold in thin stream (1) Rate of pouring must not block inlet in mold (2) Increase height to prevent blockage during pour b. Polyurethane must flow down mold and displace air upward (1) Mold fill should be from bottom up (2) All surfaces must be wetted with no adhering air bubbles c. Air must not be trapped in mold (1) Mold must be tilted and/or rotated for undercuts to be completely filled d. Polyurethane should not fold on itself

Mixing and Casting Polyurethanes  ◾  37





(1) Polyurethane that is gelling can fold over on itself and cause fault lines e. Pouring must be completed in time for air bubbles to escape before material gels (1) Gel point can be determined by drawing a thread from surface, using piece of wire with diameter of a paper clip (2) Use a “soft” butane flame to pop surface bubbles (3) Always keep flame moving 4. Curing a. Place filled mold in oven (1) Oven set to approximately 10°C higher than initial mixed material temperature (2) Polyurethane must not heat up or cool mold b. Demold when part has sufficient strength (1) Normally 1 to 2 hours (see supplier’s recommendation) (2) MDI-based systems may be hard but still brittle c. Remove any major flash and sprues (1) Use sharp knife (2) Material not as tough as when fully cured d. Prepare mold for next pour (1) Recoat mold with mold release (2) This is not always necessary as often more than one casting can be made (3) Keep hot for next run 5. Post curing a. Return part to oven for 8 to 18 hours for full cure (1) Follow supplier recommendations for time and temperature (2) Cycle is normally overnight (3) Very hard materials need extra postcure to overcome brittleness b. Final cleaning of flash lines and sprues (1) Remove excess flash with knife or by grinding (2) Clean item

38  ◾  Polyurethane Casting Primer



c. Any postmolding machining (1) Machine dimension to size (2) Remember thermal expansion

3.2  Background to Mixing and Casting 3.2.1 Introduction The mixing of polyurethane components is to complete chemical reactions that convert the various ingredients to viable products with known properties. There are four main points that must be adhered to: ◾◾ Everything dry and kept dry after use (see 3.2.1.1) ◾◾ Correct weights of all ingredients ◾◾ Correct temperatures of ingredients, mold and curing temperatures and times ◾◾ Correct mixing and casting techniques These are the basic requirements to produce satisfactory products.

3.2.1.1 Dryness of System Raw materials are normally supplied with a moisture content of less than 0.05%. The presence of moisture in the system will result in fine bubbles throughout the entire pour. These bubbles are very small and evenly dispersed. Included air bubbles are larger and more randomly dispersed. All containers must be flushed with dry nitrogen gas prior to resealing after use. Normal commercial compressed nitrogen is dry enough with a dew point of at least –40°C. Some curatives such as BDO are very hygroscopic and must be handled with care to prevent the absorption of any moisture.

Mixing and Casting Polyurethanes  ◾  39

3.2.2 Basic Requirements 3.2.2.1 Molds Molds for casting polyurethane parts can be made from a wide range of materials. The most important factor is that they must be impervious to moisture and not contain any moisture. The other main criterion is that they must not be thermoplastic at the processing temperature. Some of the main materials used to make molds include the following: ◾◾ Polyurethane ◾◾ Aluminum ◾◾ Epoxy (filled and unfilled) ◾◾ Silicones (either encased or backed) ◾◾ Steel ◾◾ Hard thermoplastics ◾◾ Polyester resins Polyurethanes, silicones, and epoxies are very useful for making molds of objects that need to be reproduced. Molds must be cost effective. A mold may be made in a single piece or in any number of parts with or without inserts. The following are some main factors to be considered in making a mold: ◾◾ Value of part being made ◾◾ Number of parts to be produced ◾◾ Finish of part ◾◾ Accuracy of size of finished casting ◾◾ Position and nature of any undercuts ◾◾ Size and shape of inserts ◾◾ Any holes in molding ◾◾ Positions allowable for pour holes and vents ◾◾ Available pot life of polyurethane Polyurethane shrinks anywhere between 0% and 2% on curing depending on the grade and the cure cycle. Room temperature

40  ◾  Polyurethane Casting Primer

cures can have zero shrinkage. The normal range of shrinkage is 1.0% to 1.5%. Shrinkage is three dimensional. The amount in thin sections may not always be noticeable.

3.2.2.2 Design Factors The following are some points that should be taken into account when designing a mold: ◾◾ Allow a slight draw on long parts to allow for removal if possible. −− ½ to 1° ◾◾ Undercuts cannot be pulled out. ◾◾ Visible areas are best from molded surfaces. ◾◾ If the shrinkage of the system is uncertain, make the mold on the smaller limits. If part is too small, material can normally be removed from the mold. −− Adding material to the mold is expensive. ◾◾ For molds in two halves pour one half, cure, mold, release, and then pour the second half. ◾◾ Molds with an undercut—make vertical casting slit into two vertical halves with scalpel or other thin, sharp knife. Make tight-fitting outer mold to keep the two halves together. More details are given in Chapter 5.

3.2.2.3 Equipment The size and complexity of the equipment needed depend on the nature of the products being produced. ◾◾ Weighing equipment −− Electronic scales are the most commonly used −− The weight range to suit weighing prepolymer for hand processing is 5 to 30 kg maximum in at least 1 gram steps

Mixing and Casting Polyurethanes  ◾  41

−− Curative range—normally up to 5 kg in 0.1 gram steps −− Processes are messy so disposable coverings are needed −− Shop scales with price and discount functions can be used to calculate curative amounts −− Standard (check) weights should be available −− Annual servicing required if no earlier problems ◾◾ Hand mixing −− Plastic containers to suit size of mix −− Sides and bottom flat −− Must be heat stable to maximum temperature of mix −− Should only fill to ⅓ maximum of volume (best ¼) • This is to allow for foaming when degassing −− Mixing paddles should be square ended • This is to provide a surface to scrape the side and bottom −− Any hard olefinic plastic is suitable −− Containers should be clean and dry after the cured remains are removed • Any wetness means poor mixing ◾◾ Ovens −− Ovens provide three functions: • Heating molds • Curing castings • Melting and heating prepolymer −− Capacity must be such that the day’s production and molds can be kept in it −− Shape dependent on molds −− Can be gas or electrically heated −− Several smaller ovens good for flexibility −− Must be temperature controlled −− Microwave ovens with turntable good for heating prepolymers Note: Ovens are not to be used for heating food. ◾◾ Finishing −− Trimming knives

42  ◾  Polyurethane Casting Primer

Clear Lid Vacuum Gauge

Vacuum Pump

Air Drier Vapour Trap

Figure 3.1  Typical vacuum degassing pot.

−− Air grinders −− Machining centers • For trimming to length • Machining to size • Squaring off molded surface ◾◾ Degassing units (See Figure 3.1) −− Rotary vacuum pumps sufficient −− Vacuum of 4 to 6 KPa (28 to 29″ Hg) −− Need to protect pump from isocyanate fumes and any boil-over with a vapor trap −− Ability to reduce vacuum quickly to break foaming HINT: A rotary vacuum pump is quite satisfactory for this operation. The main aim is always to remove isocyanate vapors from damaging the workings of the pump. A compressor has been known to work by connectiong the vacuum line to the suction side. ◾◾ Special needs −− Supply nitrogen to flush all cans after use • Standard commercial compressed nitrogen −− Flat metal or polyurethane benches for easy cleaning

Mixing and Casting Polyurethanes  ◾  43

−− Solvent dispensing cans −− Lifting equipment for large and heavy molds ◾◾ Large parts −− Multipour • Preweigh number of buckets to make up weight • Add next bucket before previous layer has gelled too far −− Handling equipment to move molds to oven

3.2.2.4 Mixing The aim is to mix the ingredients in an efficient manner to produce a homogeneous blend. The time available for mixing is a function of the reactivity and the temperature of the system. The following are the main steps: Action

Result

Degas prepolymer

The polyurethane bubbles as moisture and some free NCO is removed

Weigh out prepolymer, curative, and auxiliaries

Do not allow phases to mix

Mix without whipping in air

Cure reaction starts

This is done by carefully allowing the curative to flow down the side of the container to form a layer Initial decrease in viscosity Temperature starts to rise

Degas if pot life allows

Air mixed in is removed

Cast into mold

To be completed prior to viscosity increase Air bubbles rise to surface Temperature continues to rise slowly Viscosity slowly increases

Clear surface of air bubbles

Pop with butane flame Final quick increase in viscosity

44  ◾  Polyurethane Casting Primer

3.3  Calculations 3.3.1 Simplest Level The prepolymer supplier will provide the basic mixing data based on the average properties of the prepolymer or quasi­ prepolymer. The amount used is calculated from this data. An example is given below. A 93 Shore A prepolymer is to be reacted with Ethacure 300®. Material

Parts by Weight

Prepolymer Ethacure 300

100 12.0

To this base formula some pigment has to be added if required and the quantities adjusted to suit the mixing size of the batch. Material

Parts by Weight

Prepolymer

100

Ethacure 300

12.0

Pigment Total

0.3 112.3

If a batch size of 4.5 kg was required for the pour, each item would be proportioned as follows: Weight item =

pwb item x batch weight total pwb

e.g.



Weight Ethacure =

12.0 x 4.5 = 0.481 kg = 481 grams 112.3

Mixing and Casting Polyurethanes  ◾  45

Material

Batch Weight (kg)

Prepolymer

4.007

Ethacure 300

0.481

Pigment

0.012

Total

4.500

In the practicalities of the real world the weights would be rounded as illustrated below. Material

Batch Weight (kg)

Prepolymer

4.00

Ethacure 300

0.480

Pigment

0.012

Total

4.492

The batch weight should always allow a small overage to ensure a complete filling of the mold(s).

3.3.2 Amount of Ingredients The calculations combine a mixture of real-world factors with the scientific atomic weights and ratios. Fortunately this is simplified by the suppliers of the raw materials who supply the data at a number of levels. The calculations can be carried out at varying levels of complexity depending on the nature of the final product being made. The levels are as follows: ◾◾ The basic level −− Using average data from supplier ◾◾ Adjusting for the NCO level ◾◾ Adjusting for the actual batch NCO levels given on the can ◾◾ Adjusting for NCO level and end product being made ◾◾ Adjusting for NCO level and the amount of curative to optimize properties peculiar to application

46  ◾  Polyurethane Casting Primer

3.3.3  Basic Level In the simplest level the average NCO is taken from the supplier’s specification sheet and used in the calculation. This procedure is not recommended for any work except for the most noncritical.

3.3.4 Allowing for the Amount of NCO in the Prepolymer The actual levels of the available isocyanate vary slightly from batch to batch and are available on the can or on the Certificate of Analysis (C of A). There are two main methods of calculation: ◾◾ Proportional method ◾◾ Amine equivalent (AE)

3.3.4.1 Proportional Method A detailed description of the logic of the method is given later in this chapter. When mixing quasiprepolymers the ratios are normally set by the manufacturer of the system, and the user has to mix the material in the weight or volume ratios as given by the manufacturer. Prepolymers need to be calculated using a simple formula: parts curative per 100 parts polymer =

% NCO x EW curative x % theory EW NCO x 100

where: % NCO is the amount of terminal isocyanate in the prepolymer and is supplied by the manufacturer and is marked on the can EW curative is supplied by the manufacturer of the curative

Mixing and Casting Polyurethanes  ◾  47

EW NCO is the equivalent weight of the isocyanate group and equals 42.02 % Theory is the variation required to give the best properties for the system In normal cases the manufacturer will give a general “% theory” value for the particular curative used. In practical terms the formula can be simplified when the same curative and the best general % of theory is used. An example where MOCA is used at 95% theory the equation simplifies to parts curative per 100 parts polymer =



% NCO x 133.6 x 95 42.02 x 100

parts curative per 100 parts polymer = % NCO x 3.2

Values for a number of common curatives are given at the end of the chapter. The calculations lend themselves to use on a spreadsheet where a range of weights can be precalculated. The factors can also be programmed into some grocery scales where margins can be applied. Batch calculations can be carried out as described in 3.3.1.

3.3.4.2 Amine Equivalent Method In this method the equivalent weight of the prepolymers (amine equivalent) is supplied by the manufacturer. The amine equivalent is defined by Amine equivalent = EW prepolymer =



Amine equivalent =

42.02 x 100 % NCO

EW isocyanate x 100 % NCO

48  ◾  Polyurethane Casting Primer

For example, for a prepolymer with a % NCO of 6.32%,

Amine equivalent =

42.02 x 100 = 665 6.32

To calculate the required weight of the curative for the prepolymer the following relationship holds:

Curative weight = weight prepolymer x

EW curative x curative mole ratio EW prepolymer

Using the previous prepolymer (EW = 665), a curative of EW of 133.5 and a mole ratio of 0.9 for 10 kg of prepolymer, the equation becomes Curative weight = 10.0 x



133.5 x 0.9 665

Curative weight = 1.801 kg

3.4  Adjustments to the Amount of Curative Used It has been found that various properties can be optimized by making adjustments to the amount of curative used from the theoretical. This variation is called by several different names: ◾◾ Curative mole ratio ◾◾ % Theory ◾◾ Curative index ◾◾ Stoichiometric ratio Various curatives give the best results at different levels (see Table 3.1). Most properties optimize at different levels. Hardness is a bulk property that depends on the number and closeness of the hard segments. Elongation (modulus at maximum strain), on the other hand, depends on the ability of the molecular chains to uncoil and lengthen to the maximum.

Mixing and Casting Polyurethanes  ◾  49

Table 3.1  Effect of Curative Ratio on Properties Stoichiometric Ratio % Property

85

Hardness

90

100

105

110

115

Constant over range

Tensile

Maximum

Modulus Tear

95

Constant over range Lower

Maximum

Elongation

Increasing

Maximum

Compression

Maximum

Lower

Hysteresis

Maximum

Lower

Flex life

Increases to 100%

Maximum

As the ratio of the curative increases it changes from being more thermoset to being more thermoplastic. This is illustrated by the compression set being higher at lower (therefore more thermoset) levels. Start at a base level that is recommended for the curative from the supplier. The level can be adjusted to suit the most dominant property in the final product.

3.5  Method 3.5.1 Casting 3.5.1.1 Inserts Inserts are used with polyurethane for a number of reasons: ◾◾ Stiffness control ◾◾ Mounting points ◾◾ Flexibility control Polyurethane will adhere to metal, but an engineeringgrade bond is required. This is obtained by proper preparation and the application of an intercoat or bonding coat.

50  ◾  Polyurethane Casting Primer

The steps are as follows: ◾◾ Descale and degrease −− Remove any protection agents and/or processing scale ◾◾ Grit blast or chemical treatment −− Blast using suitable agents −− Follow manufacturer’s recommendations for chemical treatment ◾◾ Clean −− Remove any blast agents ◾◾ Use gloves to prevent recontamination ◾◾ Bond coatings −− “Chemlok” −− “Conap” Some key points in the operation are as follows: ◾◾ Use clean and dry blasting agents ◾◾ Be careful to avoid chemical cell set up (e.g., copper into steel) ◾◾ Do not allow reoxidation of surface after blasting ◾◾ Use clean gloves to prevent contamination by greasy hands ◾◾ Apply suitable grade if water or chemical attack possible ◾◾ Cover parts if dust and shop grime likely to fall on prepared inserts ◾◾ Do not get mold release on inserts ◾◾ Inserts must be above the dew point to prevent condensation ◾◾ Water-based agents must be completely dry prior to use Further discussion of bonding is presented in Chapter 4.

3.5.1.2 Time/Temperature The time available to complete a cure is of great importance in the casting process. It is of greater significance in hand casting than in machine casting. In machine casting the dwell time in the mixing head is very low compared to the time it takes to manually mix.

Mixing and Casting Polyurethanes  ◾  51

There are two opposing factors in the casting process: ◾◾ The higher the temperature the lower the viscosity −− Easier mixing −− Gas escapes easily −− Easier pouring ◾◾ The higher the temperature the shorter the gel time −− Less time for the casting period −− Bubbles in mold cannot escape Graph 3.1 shows the differences in gel time for two mixes made from the same material using Ethacure 300 as the curative. There will be a difference in the start temperature of the mix if a molten curative is used instead of the liquid curatives such as Ethacure 300. The size and position of the pour hole is of importance to allow for the material to be added as smoothly and quickly as possible. There must also be enough bleed holes for the displaced air to escape easily. Risers must be strategically placed to prevent air being trapped in corners of the mold. Viscosity/Temperature Changes Temp 95°C

Temp 83°C

8000 Viscosity cps

100 96 92

6000 4000 2000 0

88

Viscosity 83°C

84

Viscosity 95°C 0

120

240

360 480 600 Time Seconds

720

840

Graph 3.1  Viscosity/temperature build-up to gel point.

80

Temperature °C

10000

52  ◾  Polyurethane Casting Primer

3.5.2 Procedures 3.5.2.1 Basic Pouring Procedure The aim is to transfer the polyurethane from the vessel in which it was mixed to the mold—completely filling the mold with no entrapped air before the material gels. Some key points are as follows: ◾◾ Degas a second time if possible −− Impossible with hard compounds as pot life too short ◾◾ Pouring hole must be large enough to allow fast enough pouring ◾◾ There must be sufficient vent holes for the air to escape easily ◾◾ Pour so that the polyurethane flows down a surface ◾◾ Fully cover the base before filling up the mold from the bottom ◾◾ Air trap points to be eliminated either by design or air vents

3.5.3 Advanced Procedures There are several methods of assisting to obtain a completely filled mold: ◾◾ Centrifugal casting ◾◾ Compression molding ◾◾ Vacuum casting ◾◾ Pressure assisted ◾◾ Multi pour

3.5.3.1 Centrifugal Casting Centrifugal casting requires the mold to be rotated while the material is being added. The polyurethane (being denser than air) is flung to the outside of the mold with the air being

Mixing and Casting Polyurethanes  ◾  53

displaced to the center. Some venting may be needed in complex shapes. ◾◾ Recommend speeds are 3.6 to 30.5 m/s −− Use minimum speed needed ◾◾ Everything must be in balance −− Take speed up slowly ◾◾ Secure mold well to prevent flying off −− Safety guarding ◾◾ Pour into center to prevent cob webbing/fairy floss ◾◾ The polyurethane may be added into storage pot on top ◾◾ Use system with steady buildup in viscosity By spinning the mold in both the horizontal and vertical axes (or at an angle), three-dimensional hollow parts can be made. The mold may be rotated and rocked as an alternative. ◾◾ The ratio of speeds in both axes is to be adjusted so as to give even wall thickness. ◾◾ Quantity of polyurethane is added to mold that is then sealed before spinning. ◾◾ The equipment is far more complex than horizontal rotation only. ◾◾ Rotating can be carried out in a large oven to keep the mold hot.

3.5.3.2 Compression Molding Open casting leaves a surface that is normally slightly dish shaped except for any shaping in a nonpour or air vent area. Compression molding (Figure 3.2) has the advantage of making a flat or shaped surface and including spigot holes in the surface. In it simplest form a lid is placed on the mold and the whole assembly placed in a heated press until the part is cured sufficiently to be demolded. The pressure applied is

54  ◾  Polyurethane Casting Primer

Top Plate

Bottom Cavity

Polyurethane

Figure 3.2  Simple compression casting molds.

approximately 2.4 MPa or 350 psi on the projected surface area of the molding. Note: This is the surface area of the polyurethane and not the total area of the mold. The lid is placed on the mold at the point of the polyurethane gelling. The exact location of the lid in this form is not critical. More complex compression molds can be made when there are shapes on all sides (see Figure 3.3). The location of the top lid is controlled by the fitting of location dowels in the top plate. In metal molds the pins are normally made from a harder material than the location bushes. These pins and bushes should be replaceable. These molds also apply more definitive pressure to the gelled polyurethane. This will give a less porous molding if there is any moisture foaming. The mold can be adapted for reinforcings to be added to the casting. Small sprue vents may need to be added to allow for air trapped between curves and the surface of the mold. It has been known that with a new mold the lid and the land (when the lid and base have metal-to-metal contact) will close too tightly. A slight roughening of the surface will help.

Mixing and Casting Polyurethanes  ◾  55 Replacable Bushes

Hardened Location Pins

Top Plate

Bottom Cavity Polyurethane allowed to gel before compression Top Plate

Bottom Cavity

Figure 3.3  More complex compression mold.

The complexity of the mold can be increased at will with an additional transfer pot added if required.

3.5.3.3 Vacuum Casting Commercial equipment is available where the material can be mixed in one chamber and the mixed polyurethane poured into the mold that is in a second heated chamber. The whole system is kept under vacuum while the mixing and casting are taking place. The advantage of this system is that bubble-free mixes are made, and the polyurethane will completely fill the mold as there is no air to displace. Systems such as the Renishaw * units are suited for the production of silicone molds as well as polyurethane parts which is very useful in rapid prototyping. * http://www.renishaw.com/en/vacuum-casting--15266

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Quick Clamp Lid

Transfer Pot

Mold

Vacuum or Atmospheric Pressure

Figure 3.4  Transfer pot and molding.

3.5.3.4 Pressure Assisted Pressure-assisted molding is very useful in filling long molds that can be laid horizontally. The pressure can be enhanced by the application of a slight vacuum at the far end. If a vacuum is not used, there must be suitably placed air vents. The system is illustrated in Figure 3.4. This type of unit needs quick closing and locking clamps to avoid time delays after filling the transfer pot. It is suited to long molds that cannot be handled vertically.

3.5.3.5 Multi Pour Hand mixing of large pours can be carried out by using a multibucket system and a small team of workers. Machine pouring needs a machine with sufficient capacity to complete the pour without any long breaks. Each subsequent pour must be carried out before the previous one has gelled off. Care must be taken to keep the color even throughout the multi pours. The following are the basic steps: ◾◾ Weigh out all the preheated polyurethane prepolymer into separate containers. ◾◾ Melt sufficient curative for the whole job (if required). ◾◾ Add curative and additives to first container and mix.

Mixing and Casting Polyurethanes  ◾  57

◾◾ Pour into mold and degas. ◾◾ Simultaneously start mixing the next bucket load. ◾◾ Flame off the first load of polyurethane. ◾◾ Add next bucket to surface before material is completely gelled. ◾◾ Repeat until mold is full.

3.6  Curing and Postcuring 3.6.1 Initial Cure The initial cure is in the order of 1 to 3 hours at the desired cure temperature. Follow the manufacturer’s recommendations until experience is obtained. During this time the basic reaction approaches completion and the hydrogen bonding starts to form networks. Initially the material is hard to the touch but is still quite brittle. MDIs take longer than TDI systems to form strong enough networks for demolding.

3.6.2 Demolding It is possible to leave the casting in the mold for both the initial cure as well as the postcuring. This ties up the mold and reduces the potential daily production from the mold. Following are some key points in demolding: ◾◾ Material must be strong enough for demolding. ◾◾ Glass-like breaks show if demolded too soon. ◾◾ Use compressed air between mold and molding to help break the seal. ◾◾ Take demolding into account when designing a mold. −− A vertically split mold is easier to demold undercuts. ◾◾ Apply more mold release only if required. ◾◾ Note where any air is trapped in the mold. −− Change the pouring method. −− Or insert vent hole at this point.

58  ◾  Polyurethane Casting Primer

3.6.3 Postcuring The aim of postcuring is to drive the state of cure to as near to 100% as practical. Basic polymer science says 98% is not nearly enough. It must be 99.9% plus. Hydrogen bonding networks must form. The final equilibrium and properties may take over 1 week to form after the postcure. Very hard polyurethanes (65 D) may need special postcuring of several cycles at 120°C with cooling in between. Normal polyurethanes have been known to improve after an extra-hot (120 to 130°C) cycle. Extra heat during the curing cycle may cause the wrong chemistry to take place.

3.7  Calculations 3.7.1 Background to the Calculations HINT: Element or Group

Symbol

Atomic Weight

Carbon

C

12.01

Oxygen

O

16

Nitrogen

N

14.01

Hydrogen

H

Chlorine

Cl

Hydroxyl

–OH

17

Amine

–NH2

16

Isocyanate

–NCO

42.02

1 34.45

Mixing and Casting Polyurethanes  ◾  59

The fundamental calculations to determine the amount of curative to be used in mixing the polyurethane require the conversion of chemical reactions to real-world weights. An amine (–NH2) group consists of one nitrogen atom and two hydrogen atoms. The hydroxyl (–OH) group has one oxygen and one hydrogen atom. On the same basis the isocyanate (–NCO) group has one each of nitrogen, carbon, and oxygen. The groups do not exist on their own but are part of other chemical compounds. Either one amine group or hydroxyl group reacts with one isocyanate group. The diamine curative has one remaining amine (–NH2) group. This group can react with another isocyanate group from another prepolymer chain. The chains continue to lengthen until all of the curative is used up and the material is a solid. The basic chemical reactions of concern to the making of a polyurethane compound are given in Figure 3.5. There are several important definitions that must be considered when analyzing the basic mixing equations: Formula weight:  This is the sum of the weights of all the atoms in a chemical compound or reactive group. Functionality:  This is the number of reactive groups in a chemical compound that can take part in a reaction. Curative

Prepolymer

OCN

OCN

HARD Soft segment

HARD

Figure 3.5  Basic chemical reaction.

CHO HARD x

+ H2N

CUR NH2

CUR CHO NH NH2 HARD x

60  ◾  Polyurethane Casting Primer

Cl

H 2N

C C

CH

CH

CH2 C

C CH

HC

CH

CH

C C

Cl

NH2

2 -NH2 groups Functionality = 2 HO

CH2

HO

CH2

CH-OH

3 -OH groups Functionality = 3

MOCA 4,4'-methanediylbis(2-chloroaniline) Molecular formula = C13 H12 Cl2 N2 Formula weight = 267.15 Equivalent weight = 133.6 Glycerine propane-1,2,3-triol Molecular formula = C3 H8 O3 Formula weight = 92.09 Equivalent weight = 30.7

Figure 3.6  Functionality and equivalent weights of molecules.

Equivalent Weight:  The equivalent weight is the formula weight divided by the number of function groups that can take part in the reaction. The following examples (Figure 3.6) of MOCA and glycerine illustrate the terminology. In the standard polyurethane reaction one mole of the curative amine group reacts with one isocyanate group. To illustrate this, consider the following preparation of a 100 g batch of polyurethane from a 6.2% NCO prepolymer with Ethacure 300 curative and a 95% curative index. The first step is to convert the isocyanate level to chemical mass units: 6.2 % NCO x



1 mol 6.2 = mol NCO 42.02 mol NCO 42.02

The second step in the calculation is to adjust the reaction ratios. In this case because one mole of –NCO reacts with one mol of –NH2, the factor is one.



1 mol NCO 6.2 6.2 mol NCO x = mol NCO 42.02 1 mol NH 2 42.02

Mixing and Casting Polyurethanes  ◾  61

Note that this is not the case if an epoxy is added to the mix. The ratio is then 2 mols of epoxy to 1 mole of amine curative. The third step is to convert the mols of isocyanate to the equivalent amount of curative.



6.2 107 mol NCO x = 15.8 g curative ( E 300 ) 42.02 mol NH 2

The fourth and final step is to adjust the amount of curative needed for the desired curative index. 15.8 g curative ( E 300 ) x



15.8 g curativee ( E 300 ) x

desired level 100 95 = 15.8 x.95 = 15.0 g E 300 100

Combining these four steps, one has the standard formula: g Curative for 100 g prepolymer =



=

EW curative % index % NCO x x 1 100 42.02 % NCO x EW curative x % index 42.02 x 100

3.7.2 Multi Curatives When more than one curative is used the total weight must be adjusted for the molecular weights and functionality of each curative used. The calculations for the equivalent weight of the blend are given by Equivalent weight ( EW ) blend =



Wt A + Wt B Wt A Wt B + EW A EW B

62  ◾  Polyurethane Casting Primer

For example, if one wishes to make a 2-to-1 part blend by weight of Ethacure 300 and Ethacure 100®, 2+1 2 1 + 107.2 89.2 3 Equivalent weight ( EW ) Blend = 0.0187 + 0.0112 Equivalent weight ( EW ) blend =

Equivalent weight ( EW ) Blend = 100.4



3.8  Tables 3.8.1 Temperature The fundamental unit of temperature is degrees Kelvin (K) which is where the triple point of water is 273.16 K (note no ° symbol). That means that 0 K equals –273.16°C. Kelvin is used in scientific calculations. Conversion from Fahrenheit to Celsius (Centigrade): °C = 5 / 9 (° F − 32)



Conversion from Centigrade to Fahrenheit: ° F = ( 9 / 5 x °C ) + 32



The following table converts the temperatures from one set of units to the other. Find the desired unit in the middle column and the corresponding temperature in centigrade (Celsius) in the left column and the temperature in Fahrenheit in the right-hand column. °C

°F

°C

°F

– 62.2

– 80

–112

26.7

80

176

–59.4

–75

–103

29.4

85

185

–56.7

–70

–94

32.2

90

194

–53.9

–65

–85

35.0

95

203

–51.1

–60

–76

37.8

100

212

Mixing and Casting Polyurethanes  ◾  63

°C

°F

°C

°F

–48.3

–55

–67

40.6

105

221

–45.6

–50

–58

43.3

110

230

–42.8

–45

–49

46.1

115

239

–40.0

–40

–40

48.9

120

248

–37.2

–35

–31

51.7

125

257

–34.4

–30

–22

54.4

130

266

–31.7

–25

–13

60.0

140

284

–28.9

–20

–4

65.6

150

302

–26.1

–15

5

71.1

160

320

–23.3

–10

14

82.2

180

356

–20.6

–5

23

87.8

190

374

–17.8

0

32

93.3

200

392

–15.0

5

41

98.9

210

410

–12.2

10

50

100.0

212

413.6

–9.4

15

59

101.7

215

419

–6.7

20

68

104.4

220

428

–3.9

25

77

107.2

225

437

–1.1

30

86

110.0

230

446

1.7

35

95

112.8

235

455

4.4

40

104

115.6

240

464

7.2

45

113

121.1

250

482

10.0

50

122

123.9

255

491

12.8

55

131

126.7

260

500

15.6

60

140

129.4

265

509

18.3

65

149

135.0

275

527

21.1

70

158

137.8

280

536

23.9

75

167

148.9

300

572

64  ◾  Polyurethane Casting Primer

3.8.2 Curative Details Curative 1,4 Butane diol

Molecular Weight

Equivalent Weight

90.2

45.1

AH40

267.2

133.6

Baytec 1604

242.7

121.4

90.1

45

BDO

Comments BDO Commercial formula

1,4 Butane diol

Caytur 21

653

MDA complex

Conap AH-40

267

133.5

Commercial formula

Conap AH-50

180

90

Previously known as Isonol 93

Conap-AH33

560

280

Curene 3005

560

280

Cyanacure

276.4

138.2

1,2-bis(2-aminophenylthio)ethane

DEG

106

53

Diethylene glycol

DETDA

178.3

89.2

Diethyltoluene diamine

Ethacure E100

178.3

89.2

DETDA

Ethacure E300

214.4

107.2

Ethylene glycol

62.06

31.03

Glycerol

90.1

30.1

Di-(methylthio)toluene diamine DMDTA

Glycerine

HER

198

99

HQEE

198.2

99.1

Hydroquinone bis (beta hydroxyethyl)ether

Isonol 93

270

90

Range 88-93 triol

M-CDEA

379.4

189.7

MDA

198.3

99.1

MMEA

282.4

141.2

Lonzacure Methylene dianiline

Mixing and Casting Polyurethanes  ◾  65

Curative

Molecular Weight

Equivalent Weight

MOCA (MBOCA)

267.2

133.6

MOEA

254.4

127.2

Plurocol TP-440

435

145

Polacure 740M

314.3

157.2

Poly-cure 1000

310

155

PPG 1000

1000

500

PTMEG 1000

1000

500

Comments

Range 139-145 triol

Polypropylene glycol

t-BTDA

178.8

89.4

TIPA

192

64

Tri iso propane amine

TMP

134.2

44.7

Trimethyol propane

Unilink 4200

Commercial “R” groups

Versalink 740M

314

157

Versalink P-1000

1200

600

Versalink P650

415

Chapter 4

Supplementary (Additional) Casting Processes 4.1  Thick Cross-Sections Casting polyurethanes that have a thick cross-section (300 mm plus) can present some problems. Polyurethane has a low thermal conductivity and the curing reaction is exothermic (gives off heat). This can lead to problems when casting thick cross-sections. The normal aim is to have the polyurethane hot enough to reduce the viscosity to allow the easy filling of the mold and for all the entrapped air to escape. This has to occur before the polyurethane starts to increase in viscosity and gel. The heat given off will increase the temperature of the mix, and the heat in the center of a large pour will not be able to escape. Two problems arise: 1. The wrong chemical reactions can take place in the center of the molding. 2. Strains are set up because of the difference in temperature between the center of the casting and the mold wall. 67

68  ◾  Polyurethane Casting Primer

To overcome these problems the following steps will help: ◾◾ Cast at as low a temperature as practical −− Use liquid curative ◾◾ Choose lowest-viscosity grade of prepolymer ◾◾ Redesign part to have thinner cross-sections ◾◾ Use metal reinforcements to help absorb excess heat

4.2  Bonding For design reasons polyurethane often needs to be attached to a rigid base. The material may be metal, fiberglass, another polymer, or even a ceramic. Note: Polyurethane will stick to most surfaces without any pretreatment, but in most cases this is not classed as an engineering bond. If the bond is tested using the 90° peel test (see Chapter 8, “Quality Control and Assurance Testing,” for details), the material will have an adhesion of only several Newtons per meter. A proper adhesion will adhere so well to the substrate that the polyurethane will break at the bond line (stock break) when tested using the 90° peel test, or the substrate will break in the case of fiberglass. A fundamental factor in bonding polyurethane to another surface is that high stress points must be avoided. Edges must be rounded, and all angles must have a radius. The key points in the procedure are as follows: ◾◾ Clean surface −− Scale −− Oil −− Grease ◾◾ Prepare surface −− Remove oxide layer • Abrasively • Chemically

Supplementary (Additional) Casting Processes  ◾  69

◾◾ Coat surface with one or more layers of a “bonding agent” ◾◾ Allow to dry −− Must be clean −− Dust free ◾◾ Mold polyurethane on it

4.2.1 Clean Surface The initial step is to clean gross contamination from the surface of the insert. The material may be scale, oil, or grease that has been put on the insert to prevent corrosion. ◾◾ Scale removed mechanically ◾◾ Oil, grease, and cutting fluids −− Water wash −− Alkaline clean −− Water rinse −− Dry

4.2.2 Prepare Surface The surface may be prepared either mechanically or chemically. The details of formulas of the chemical treatment can be obtained from bonding agent suppliers. (See Table 4.1.) Note: Some are very dangerous (e.g., the HF Pickle). Following are some key points in preparation: ◾◾ Copper slag is not suitable −− Very prone to cause corrosion ◾◾ The grit used must be free from oil and grease ◾◾ Must clean off and dry after chemical treatment ◾◾ Clean blasted surfaces −− Mechanically cleaned surface prone to reoxidation −− Handle only with clean gloves • On nonvital surfaces only

70  ◾  Polyurethane Casting Primer

Table 4.1  Surface Preparation for Bonding Surface to which Polyurethane Can Be Bonded Ordinary steel

Mechanical Preparation

Chemical Preparation

40 grit—steel grit, Phosphate treat clean sand, aluminum oxide grit

Stainless steel

Acid etch

Aluminum

Chromate conversion

Magnesium Copper Brass Zinc

40 grit—clean sand, Chromate conversion aluminum oxide grit Ammonium persulfate etch (Steel grit will cause Ammonium persulfate etch corrosion, so do not use) Phosphate or chromic acid

Cadmium

Phosphate or chromic acid

Titanium

HF Acid pickle

Rubber

Clean surface—use coat of interlayer such as Chemlok 7701

Chemlok 7701

Nylon

Lightly blast with sand or aluminum oxide

Clean with alkaline cleaner or solvent

Phenolic, epoxy Polycarbonate Others

Consult raw materials and bonding agent suppliers

◾◾ First coat as soon as possible after blasting −− This is to prevent reoxidation of surface ◾◾ Safety equipment must be worn ◾◾ Dust and skin irritation and physical damage in blasting ◾◾ Poisoning and chemical burns in chemical treatment −− Must have antidote cream if using HF −− Attend local medical doctor to have injections −− It can continuously eat flesh and bones

Supplementary (Additional) Casting Processes  ◾  71

4.2.3 Surface Coating The type and number of coats must be considered. If the molding (especially the bond line) is going to be attacked by a fluid or moisture, the bonding system must be adjusted to suit. Some common recommendations are listed below. Chemlok

Conap

Thixon

Standard conditions

1 × Chemlok 213

Conap AD-1146 or AD 1147

423 or 403/406

Environmental conditions

2 × Chemlok 218

2 × above

423 over 403

The bonding agent may be applied in any of the following manners: ◾◾ Dipping ◾◾ Painting (brushing) −− Some grades require prebaking of coating ◾◾ Roll coating ◾◾ Spraying ◾◾ Tumbling The correct grade of the bonding agent should be used to suit the application. The following points should be observed: ◾◾ The correct thickness must be applied (as per manufacturer’s recommendations). −− Check if thickness is specified as wet or dry. ◾◾ The grade must be suitable for curing temperatures. ◾◾ The temperature of reinforcing and surroundings must be above the current dew point* temperature. ◾◾ Some grades require prebaking. * The temperature at which the air is saturated with moisture. The dew point can be calculated from the wet and dry bulb hygrometer readings.

72  ◾  Polyurethane Casting Primer

◾◾ The bonding agent, if applied by a spray gun, must still be wet when deposited on the reinforcing. ◾◾ The coated reinforcing must only be handled on either noncoated or noncritical areas with clean gloves. ◾◾ The coated reinforcing must be kept. −− Dust free (wrap in plastic if required) −− Free from mold release spray −− Free from spray from machining −− Free of any finger and palm prints

4.3  Finishing There are four different finishing operations for polyurethane including the following:

1. Removal of flash and sprues 2. Machining parts to size 3. Postcuring bonding 4. Painting

4.3.1 Removal of Flash and Sprues It is the easiest to remove any excess material straight after the curing stage. At this point the material has not built up to its maximum toughness. It is best carried out with a sharp knife. It must however be noted that the whole molding is still slightly “green” and can be damaged quite easily. After the material is fully cured, bulk flash and sprues can also be removed using a sharp knife. The excess flash can be removed by using an open grit finishing belt or a high-speed barrel grinder. Dust masks should be worn to prevent the inhalation of fine polyurethane particles generated during the operation. Excessive force applied during the grinding operations can cause melting and clogging of the grinding medium.

Supplementary (Additional) Casting Processes  ◾  73

4.3.2 Machining Parts to Size Polyurethanes can be machined in a similar manner to metals but with certain modifications. The economics of machining the part to the final shape and size must be compared to the cost of a special mold that would eliminate the machining. Soft polyurethanes need extra support to prevent distortion during machining. Note the following: 1. All machinery must be operated in accordance with local rules and regulations. 2. The inhalation of any fumes must be avoided by both engineering methods as well as personal protective masks. 3. All dust must be removed by both engineering methods as well as the use of personal protective masks. 4. Eye protection/face shield must be worn. 5. The securing of the molding must be carefully checked for complete adequacy before starting the machine.

4.3.2.1 Methods In all operations the following key points must be observed: ◾◾ The thermal conductivity of polyurethane is poor. −− Very heavy cuts should be avoided. −− The heat needs to be removed by coolants. −− Remember the next process (e.g., painting) when choosing the coolant. −− Mandrels will help where possible. ◾◾ The tools must be sharp. ◾◾ Allow ample tool clearances. ◾◾ If machining develops too much heat, the part will shrink and change shape on cooling. ◾◾ Clamping too heavily can cause distortions to the part on removal. ◾◾ 90 Shore A and harder are the easiest to machine.

74  ◾  Polyurethane Casting Primer

◾◾ 80 Shore A and softer need some modification to the technique. −− Easiest knifing, grinding, and sanding −− Can try freezing (liquid nitrogen) −− Additional supports −− Mandrels

4.3.2.2 Turning and Facing Basic points in the operation are as follows: ◾◾ Correct set should “give” −− Long continuous “chip” coming off casting −− Tool cutting very easy ◾◾ Cutting tool very sharp and well honed ◾◾ Smooth top ◾◾ High turning speed ◾◾ Feed—slow to moderate ◾◾ Greater clearance than for metals

4.3.2.3 Parting Basic points in the operation are as follows: ◾◾ Knife must be razor sharp ◾◾ Tool 0.15 to 0.25 mm wide ◾◾ Approximately 25° front rake ◾◾ Some side clearance

4.3.2.4 Band Sawing Basic points in the operation are as follows: ◾◾ Two to four teeth per inch hook carbon blade ◾◾ Racker set (teeth left and right) ◾◾ Blade speed 1200 to 2400 feet per minute ◾◾ Normally hand feed −− Operator sets to a good feed rate

Supplementary (Additional) Casting Processes  ◾  75

◾◾ Maximum diameter 200 mm ◾◾ Minimum thickness 3 mm

4.3.2.5 Drilling Basic points in the operation are as follows: ◾◾ Drill bits must allow for easy removal of chips ◾◾ 90° or more blunt point ◾◾ Remove bits frequently ◾◾ Larger diameter bits on soft materials often required ◾◾ Drill speed 600 to 800 rpm

4.3.2.6 Milling Basic points in the operation are as follows: ◾◾ Normally above 80 Shore A ◾◾ Best above 90 Shore A ◾◾ Not recommended below 80 Shore A ◾◾ Single-bladed cutters ◾◾ Two fluted end mills ◾◾ 10° Back rake −− With good clearance ◾◾ Minimum thickness 10 mm

4.3.2.7 Grinding This operation is often carried out on cast rollers to remove the surface and obtain the correct size and finish. Basic points in the operation are as follows: ◾◾ Polyurethanes 55 to 80 Shore A can be ground using a tool post grinder on a lathe ◾◾ Turning speed 150 RPM ◾◾ Some recommendations for lathe to turn in reverse ◾◾ Grinder feed rate (longitudinal) 0.125 mm per revolution −− The slower the cut the better the finish

76  ◾  Polyurethane Casting Primer

◾◾ Grit of wheel can be varied to suit finish ◾◾ Typical surface speed of wheel 35 meters per second ◾◾ Spray or water lubrication on harder grades ◾◾ Special machines are available for the grinding of rollers

4.4  Postcure Bonding Polyurethane can very successfully be bonded to a large number of other materials. The most important factors influencing a successful bond are: ◾◾ The absence of any release agent on the surface ◾◾ Clean oxidized polyurethane surface −− Surfaces that have been in contact with the mold or exposed to the atmosphere and light for some time ◾◾ The surface must be roughened sufficiently to give good “keying” of the bond coat ◾◾ Bonding or adhesives suitable for application −− These are available from leading bonding agent suppliers ◾◾ Correct mating conditions Repairs can be carried out to some defects in noncritical areas of the molding. These are best done before the molding has fully cooled down. A major cause of defects is air that has been trapped between the polyurethane and mold. These are characterized by a bright shiny surface. A method to repair this is as follows: ◾◾ Roughen surface with a high-speed air grinder ◾◾ Make a 2 to 3 mm wall around the defect −− Children’s plasticine is ideal ◾◾ Just overfill the depression with the same grade and color polyurethane ◾◾ Cure filling in oven ◾◾ Remove plasticine and blend filled area in to the rest of the surface

Supplementary (Additional) Casting Processes  ◾  77

◾◾ Color matching is often difficult ◾◾ The addition of catalysts to speed up the cure can change the color slightly

4.5  Painting Painting of polyurethane is similar to the postcuring bonding. The surface needs to be completely free of mold release. The mold releases must be removed by a solvent (such as a ketone, e.g., MIBK) and if required an alkaline bath. Care must be taken in using plasticizers in the polyurethane as these may come to the surface of the casting and cause delimitation. Lower the hardness of the polyurethane by choosing a softer grade or using a reactive polyol that takes part in the curing reaction. An epoxy or polyurethane-based paint is often the best. The paint should have similar flexibility to that of the molding. The thermal expansion and contraction rates must be very close.

4.6  Epoxy Blends Polyurethanes and epoxy resins can be used very successfully together. Polyurethane can be cured with some of the same diamine curatives as epoxies: ◾◾ Cured epoxy is a true thermoset. ◾◾ Castable polyurethane is a pseudo thermoset relying on hydrogen bonding. When epoxy resin is added to polyurethane, the hardness can be increased without losing the physical properties. The polyurethane affects the epoxy in the following manner: ◾◾ Toughness increases ◾◾ Electrical properties still very good ◾◾ Chemical properties the same

78  ◾  Polyurethane Casting Primer CH2 H2N

AR

O

NH2

CH2 CH

O CH2

Aromatic amine curative AR

HC

O

CH2

Epoxy resin

CH2 O

OH

CH2

CH2

AR

CH2

AR O

CH

N

CH2 O

HO

CH

H2C

AR

O AR

N H2C

CH2 CH

CH

AR O

OH

OH Cross-linked resin

Figure 4.1  Basic epoxy curing reaction.

Some of the commonly used polyurethane curatives can be used with epoxy resins. Epoxide terminated epoxies with a molecular weight of between 350 and 6000 have been found to be most suitable. The major difference in the usages of the amine curatives for the epoxy and polyurethane is that for the epoxy both hydrogen atoms of the –NH2 group are used in the curing, whereas for the polyurethane only one hydrogen from each –NH2 group is used. See Figure 4.1. This leads to different equivalent weights for the same curative. The main ones quoted are as shown below. MOCA Molecular weight

267

Ethacure 100 Ethacure 300 178.3

214 107

Equivalent wt for Polyurethane

Mol Wt/2

133.5

89.2

Epoxy

Mol Wt/4

66.8

44.6

53.5

The terminology used with epoxies differs from that in the polyurethane industry: ◾◾ The equivalent weight is expressed as the amine hydrogen equivalent weight (AHEW).

Supplementary (Additional) Casting Processes  ◾  79

◾◾ The strength is expressed as the epoxide value (EEW). −− This is given for each batch. To calculate the amount of hardener for 100 grams of epoxy the following relationship applies:



x =

AHEW x 100 EEW

In a mixed system the amounts of material required for each system are calculated separately. The following points should be considered: ◾◾ The reaction rate of E100 is the fastest. ◾◾ E300 needs lower temperatures than MOCA. ◾◾ The differences in the reaction rate between the polyurethane and epoxy need to be taken into account. ◾◾ Some reports infer that the polyurethane is merely a polymeric plasticizer in the epoxy matrix.

4.7  Millable Polyurethanes Millable polyurethanes are a grade of polyurethanes that were initially aimed at bridging the gap between low-pressure casting and standard rubber processing. They have all the chemical and wear properties as normal castable systems. Millable polyurethanes are processed on standard rubber processing equipment. The millable polyurethane is prepared using approximately 1:1 polyol/diisocyanate ratio. This will produce a thermoplastic polymer. The polymer is modified to add double-bond (~C=C~) sites in the backbone that can be used as cross-linking sites. The cross-linking can be carried out by either peroxide or by a sulfur cure system. The processing is carried out

80  ◾  Polyurethane Casting Primer

on standard rubber machinery. The millable polyurethane has to be compounded like standard rubber. ◾◾ Mixing −− Open mill −− Internal mixers ◾◾ Curing −− Presses • Transfer • Compression −− Injection molding machines The processing conditions are similar to standard rubber systems. The properties can be altered by the correct selection of the grade of polymer, level of plasticizer, and type and grade of fillers. Complex acrylates such as the Sartomer™ resins may also be used. These have the additional advantage of easing the processing while adding other properties such as hardness. The following are typical formulas often quoted: Peroxide Cured Ingredient

Quantity

Millable polyurethane

100

Base polyol (e.g., Millathane 76)

Stearic acid

0.25

Processing aid

Poly AC 617A

0.5

Low molecular weight polyethylene—flow

Carbon black N550

35

Semireinforcing filler

TP-95

3

Polyester adipate plasticizer (also helps incorporation of carbon black)

Dicumyl peroxide

2.5

50% Peroxide—curative

Supplementary (Additional) Casting Processes  ◾  81

Sulfur Cured Ingredient

Quantity

Millable polyurethane

100

Such as Millathane E34

Zinc stearate

0.5

Activator

N330 Black

25

Reinforcing carbon black

TP-95

5

DBEEA—plasticizer (also helps incorporation of black)

Process aid

1

Dispersion and anti-roll-sticking agent (Struktol WB222)

MBTS

4

Accelerator

MBT

2

Accelerator

Thanecure® ZM

1

Cure activator (zinc chloride/ MBTS complex)

Sulfur

1.5

Curative

Following are some general comments on millable polyurethanes: ◾◾ Shelf life limited −− Will eventually self-cure (scorch) ◾◾ Levels of accelerators much higher than normal rubbers ◾◾ Molds and presses as for rubber −− Much heavier than for polyurethane pour casting ◾◾ Hot tear strength not always the highest ◾◾ Either standard rubber or special polyurethane bonding agents can be used ◾◾ Reinforcing clays and silica such as Ultrasil VN3 can be used instead of carbon black ◾◾ Mold release very important ◾◾ Peroxide cures give the best compression set

Chapter 5

Design Considerations A number of factors must be considered in designing a part: ◾◾ Are the requirements for the part achievable in polyurethane? −− Within the temperature envelope −− At an affordable price −− Within the engineering design specified ◾◾ Does the design need to be changed to suit the use of polyurethane? −− Direct copies of metals are not always the best −− Are reinforcings needed? ◾◾ Can the part be molded and finished economically?

5.1  Quality of Part Required 5.1.1 Engineering Requirements The following engineering quality considerations need to be taken into account: ◾◾ Does the part have to be completely bubble free? −− Normal pouring with doubles degassing 83

84  ◾  Polyurethane Casting Primer

−− Centrifugal casting −− Vacuum casting ◾◾ Limits on dimensions −− Typical commercial castings* • 0 to 25 mm ±0.1 mm • 25 to 915 mm ±0.5% • 915 mm plus ±0.7% ◾◾ Is the profile the most suited to polyurethane? −− Shape factor −− Heat buildup and dissipation

5.1.2 Surface Finish The smoother the surface finish, the more expensive the mold. Polyurethane reproduces the finish of the mold. A very fine finish requires a hard scratch-resistant mold with no mold release buildup. Polyurethane makes a good mold material as it does not scratch or tear easily.

5.2  Draft and Undercuts 5.2.1 Need for Drafts The demolding process requires the bodily removal of the material from the mold. In tall cylindrical molds there is often a long length of constant diameter for the material to be moved over. A slight taper will allow the part to be removed much easier if the final product allows it. A draft of 0.5 to 1° is what is required. See Figure 5.1. The action is that once the part has been freed from the mold there is a slight clearance and the part should slip out easily.

* Gallagher Corporation. (1994). Cast and molded components of polyurethane elastomers.

Design Considerations  ◾  85

1°

0.5°

Figure 5.1  Mold draft.

5.2.2 Undercuts in Mold Design Polyurethanes are basically noncompressible. Any ridges can cause problems to the demolding if not taken into account during the mold design process. If the part has cured sufficiently the application of a large force in the correct direction may remove the article from the mold. This is not the most desirable method. The simplest method is to make a split mold at right angle to the undercuts. Figures 5.2 and 5.3 show the four steps to produce a hollow roller with a waist.

5.2.2.1 Step 1 ◾◾ Prepare a master pattern either in metal or polyurethane. −− Turn and bore the part to the desired size. −− The overall length may need to be longer if the ends need to be faced.

86  ◾  Polyurethane Casting Primer

Step One

Master Pattern

Steel Pin

Temporary Outer Mold Cure Half Filled Molding

Polyurethane Filled to Middle Line

Step Two

Mold Release Surface

Fill to Top and Fully Cure

Figure 5.2  Multipart mold Steps 1 and 2.

Step Three Mold Release Cast Polyurethane all Surfaces

Step Four

Half Molds

Keeper Sleeve

Steel Pin

Figure 5.3  Multipart mold Steps 3 and 4.

Pour Hole

Design Considerations  ◾  87

◾◾ Prepare a steel pin of the required diameter and length. −− It must be smooth. ◾◾ Make an outer holder with open top and holes for the pin. ◾◾ Use a silicone to seal pin/hole joint on outside. ◾◾ Apply mold release to inside of pattern. ◾◾ Pour polyurethane molding material to half fill the mold. ◾◾ Flame off to remove bubbles. ◾◾ Complete the initial cure of the polyurethane.

5.2.2.2 Step 2 ◾◾ Do not demold. ◾◾ Apply mold release to the surface of the polyurethane. ◾◾ Complete the casting of the mold. ◾◾ Cure the first and second halves again. ◾◾ Post cure molding(s).

5.2.2.3 Step 3 The two halves of the mold need to be kept together. One way is to have a keeper sleeve that will hold the halves in close contact when the part is being made. ◾◾ The thickness needs to be 4 to 5 mm. −− Use any available pipe or simple metal tube. −− Any flat surface may be used as a base. ◾◾ Seal all joints with silicone. ◾◾ Place mold and pin from Step 2 in outer mold. ◾◾ Tack together where necessary with a dab of silicone. ◾◾ Mold release where required. ◾◾ Heat mold and cast keeper.

5.2.2.4 Step 4 ◾◾ Demold after full curing. ◾◾ Assemble the mold. ◾◾ Carry out a check molding.

88  ◾  Polyurethane Casting Primer

The following are key points in this method: ◾◾ A keeper sleeve is needed. ◾◾ Alternatively, the halves have to be clamped together. −− Even strong elastic bands can be used. ◾◾ The mold may be cast in two halves. ◾◾ If the mold is cast in its entirety, it can be sliced vertically with a thin knife. −− Use a scalpel or “Stanley” knife. −− This is best carried out prior to full cure. ◾◾ There is always a fine flash/mold line with this method. ◾◾ A hollow part may be collapsed inward to aid removal. It is most important that the part has sufficient strength before demolding takes place, especially in parts with complex shapes.

5.3  Shrinkages 5.3.1 Mold Expansion Table 5.1 lists some mold-making materials* that may be used in making molds for casting polyurethanes. The cost, availability, and suitability of each material must be evaluated before use. The table gives expansion in Kelvin (K) which is equivalent to Celsius (°C).

5.3.2 Polyurethane Shrinkages The amount that polyurethane shrinks is a function of the initial and final cure cycle. The changes are illustrated in Table 5.2. The final shrinkage of the polyurethane will depend on the maximum temperature (exotherm) it reaches during the cure reaction. Graph 5.1 illustrates this point for a 90 Shore A material cured with MOCA. * As given in http://www.engineeringtoolbox.com/

Design Considerations  ◾  89

Table 5.1  Common Coefficients of Expansion Temperature Expansion Coefficient (10–6 m/m K)

(10–6 in/in oF)

106.5

59.2

Aluminum

22.2

12.3

Brass

18.7

10.4

Copper

16.6

9.3

Epoxy

55

31

Glass

9

5

Nylon

85

47.2

Polyester reinforced

25

14

Polypropylene unfilled

90.5

50.3

Polysulfone

55.8

31

Polyurethane

57.6

32

Steel

13

7.3

Steel stainless

14.4

8

Wood (sealed)

5.4

3

Material Acetal

Table 5.2  Shrinkages of Polyurethane Initial Cycle In mold 16 hours at room temperature

Cure Cycle Demold 4 hours at 50°C

Shrinkage