Flame retardant polymer nanocomposites 9780470109021, 0470109025, 9780470109038, 0470109033, 9781280839153, 1280839155, 9786610839155, 6610839158

Table of contents :
Content: Cover --
COVER --
FLAME RETARDANT POLYMER NANOCOMPOSITES --
CONTENTS --
Contributors --
Preface --
Acronyms --
1 Introduction to Flame Retardancy and Polymer Flammability --
1.1 Introduction --
1.2 Polymer Combustion and Testing --
1.2.1 Laboratory Flammability Tests --
1.2.2 Polymer Combustion --
1.3 Flame Retardancy --
1.3.1 General Flame Retardant Mechanisms --
1.3.2 Specific Flame Retardant Mechanisms --
1.3.3 Criteria for Selection of Flame Retardants --
1.3.4 Highly Dispersed Flame Retardants --
1.4 Conclusions and Future Outlook --
References --
2 Fundamentals of Polymer Nanocomposite Technology --
2.1 Introduction --
2.2 Fundamentals of Polymer Nanocomposites --
2.2.1 Thermodynamics of Nanoscale Filler Dispersion --
2.2.2 Synthetic Routes for Nanocomposite Formation --
2.2.3 Dispersion Characterization: Common Techniques and Limitations --
2.3 Effects of Nanofillers on Material Properties --
2.3.1 Effects on Polymer Crystallization --
2.3.2 Effects on Mechanical Properties --
2.3.3 Effects on Barrier Properties --
2.4 Future Outlook --
References --
3 Flame Retardant Mechanism of Polymer-Clay Nanocomposites --
3.1 Introduction --
3.1.1 Initial Discoveries --
3.2 Flame Retardant Mechanism --
3.2.1 Polystyrene Nanocomposites --
3.2.2 Polypropylene-Clay Nanocomposites --
3.2.3 Thermal Analysis of Polymer-Clay Nanocomposites --
3.3 Conclusions and Future Outlook --
References --
4 Molecular Mechanics Calculations of the Thermodynamic Stabilities of Polymer-Carbon Nanotube Composites --
4.1 Introduction --
4.2 Background and Context --
4.3 Description of the Method --
4.4 Application to PS-CNT Composites --
4.5 Uncertainties and Limitations --
4.6 Summary and Conclusions --
References --
5 Considerations Regarding Specific Impacts of the Principal Fire Retardancy Mechanisms in Nanocomposites --
5.1 Introduction --
5.2 Influence of Nanostructured Morphology --
5.2.1 Intercalation, Delamination, Distribution, and Exfoliation --
5.2.2 Orientation --
5.2.3 Morphology During Combustion or Barrier Formation --
5.3 Fire Retardancy Effects and Their Impact on the Fire Behavior of Nanocomposites --
5.3.1 Inert Filler and Char Formation --
5.3.2 Decomposition and Permeability --
5.3.3 Viscosity and Mechanical Reinforcement --
5.3.4 Barrier for Heat and Mass Transport --
5.4 Assessment of Fire Retardancy --
5.4.1 Differentiated Analysis with Regard to Different Fire Properties --
5.4.2 Different Fire Scenarios Highlight Different Effects of Nanocomposites --
5.5 Summary and Conclusions --
References --
6 Intumescence and Nanocomposites: a Novel Route for Flame-Retarding Polymeric Materials --
6.1 Introduction --
6.2 Basics of Intumescence --
6.3 Zeolites as Synergistic Agents in Intumescent Systems --
6.4 Intumescents in Polymer Nanocomposites --
6.5 Nanofillers as Synergists in Intumescent Systems --
6.6 Critical Overview of Recent Advances --
6.7 Summary and Conclusion --
References --
7 Flame Retardant Properties of Organoclays and Carbon Nanotubes and Their Combinations with Alumina Trihydrate --T$83.

Citation preview

Flame Retardant Polypropylene Nanocomposites Qingliang He, Tingting Yuan, and Daowei Ding Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas, U.S.A.

Suying Wei Department of Chemistry and Biochemistry, Lamar University, Beaumont, Texas, U.S.A.

Zhanhu Guo Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas, U.S.A.

Abstract In this entry, the unique function of polypropylene-graft-maleic anhydride (PP-g-MA) has been demonstrated, which serves as a synergist to stimulate the catalytic effect of magnetic cobalt–cobalt oxide core–shell nanoparticles (NPs) on reducing the flammability of polypropylene (PP). Specifically, through a one-pot wet chemistry, PP polymer matrix nanocomposites (PNCs) with in situ synthesized cobalt–cobalt oxide core–shell NPs were stabilized by 5.0 wt% PP-g-MA, and the heat combustion parameters including heat release rate was reduced more than 50% compared with the PP PNCs with only these NPs. Even though these cobalt–cobalt oxide core–shell NPs can be solely used as heat shielding components to decrease the fire hazards of PP, this highly effective synergistic effect of nonflame-retardant PP-g-MA on stimulating the catalytic flame retardant PP was rarely reported before.

INTRODUCTION Transitional metals (oxides) have been extensively used as synergist for flame retardant polymer nanocomposites (PNCs);[1–3] however, the majority of the research focused on collaborating with traditional flame retardants like intumescent systems[4–6] or clays[7] as major fillers for the polymer matrix. The mechanism is to promote the flame retardant efficiency of major flame retardant additives by these transition metal (oxide) particles via catalytic carbonization. For example, cobalt, as a widely used catalyst, has been extensively studied in both academic and industrial field because of its merits including a wealth of structure-dependent catalytic properties. [8,9] Cobalt nanostructures were widely used as catalysts for reduction of nitrophenols[10] and synthesis of long-chain hydrocarbons and clean fuels.[11] Among the reported versatile synthesis methods, in situ bottom-up approach by thermal decomposing neutral organometallic precursors such as dicobalt octacarbonyl in organic solvents is one of the most commonly used approaches for synthesizing cobalt nanoparticles (NPs).[12,13] Besides the advantage of size and shape control from the bottom-up approach, its solution-based condition also facilitates using inert polymer such as polypropylene (PP) to serve as host to these cobalt NPs for preparing PNCs. [13,14] The reason is because the steric repulsion from the PP hydrocarbon

main chains can effectively stabilize the magnetic NPs through counteracting with the strong attraction forces among magnetic NPs. [15,16] Hence, this bottom-up method is able to compensate the weak filler–polymer interfacial adhesion for inert polymers like PP, which favors the particle dispersion. More importantly, enhancing the bonding at the polymer–nanofiller interface further favors the improvements in thermal stability and flame retardancy of the target PNCs.[17,18] Nonetheless, how the thermal stability and flame retardancy will be improved is obscure. The catalytic effect of the transition metal or metal oxide on the flame retardant treatment is widely used during the 1990s;[3] however, the catalytic mechanism is not well elucidated. For PP-based PNCs with traditional flame retardants such as organoclay,[17,19] carbon nanotubes (CNTs),[20–24] intumescent flame retardant/ CNT,[25,26] and layered double hydroxide PNCs,[27,28] compatibilizer such as polypropylene-graft-maleic anhydride (PP-g-MA) is commonly used to improve the filler dispersion. Previously, we have reported the use of PP-g-MA as surfactant and stabilizer for synthesizing magnetic cobalt and Fe2O3 NPs with tunable size, morphology, crystalline structures, assembly patterns, and magnetic property.[13,29,30] In this entry, we demonstrate a unique magnetic PNC, i.e., in situ formed cobalt–cobalt oxide core–shell NPs reinforced PP nanocomposites stabilized by PP-g-MA (two molecular

Dekker Encyclopedia of Nanoscience and Nanotechnology, Third Edition DOI: 10.1081/E-ENN3-120053555 Copyright © 2015 by Taylor & Francis. All rights reserved.

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weights are selected, Mn = 800 and Mn = 2500 g/mole). The detailed synthesis procedure is published by He et al.[14,31] We here emphasize on the illustration of how the PP-g-MA stimulates the catalytic effect of cobalt–cobalt oxide core– shell NPs on reducing the flammability of PP. DISCUSSION Fig. 1a and 1b shows the thermogravimetric analysis (TGA) curves of PP and the PNCs conducted under both nitrogen and air, respectively. The detailed data are listed in Tables 1 and 2. Here, the initial degradation temperature (Tini) is defined as a 5% weight loss of the tested sample, while the Tmax (obtained from the differential thermogravimetric analysis, DTG) is defined as the temperature when the tested sample experiences maximum weight loss rate. The thermal degradation of pure PP was mainly initiated by the thermal scissions of aliphatic C–C backbone associated

Flame Retardant Polypropylene Nanocomposites

Table 1 TGA characteristics of the samples measured in nitrogen. Composition

Residue at Tmax (°C) 700°C (%)

Pure PP

421.0

475.7

0.0

PP/PP-g-MA (Mn = 800)

391.0

481.4

0.0

PP/PP-g-MA (Mn = 2,500)

415.3

477.3

0.1

PP/20.0% NPs

409.0

481.4

20.2

PP/20.0% NPs/PP-g-MA (Mn = 800)

383.4

484.6

22.1

PP/20.0% NPs/PP-g-MA (Mn = 2,500)

404.6

482.2

20.9

Source: From He, Yuan, et al.[31] ©2009 The Royal Society of Chemistry (RSC).

with hydrogen transfer at the scission sites under nitrogen.[32] Specifically, pure PP has an one-stage thermal degradation with Tini of 421.0°C and Tmax of 475.7°C and no char residue left at 700°C.[33] The addition of 5.0 wt% PP-g-MA in the PP matrix, decreased Tini, but increased the Tmax. When forming 20.0 wt% cobalt–cobalt oxide core– shell NPs in the PP matrix (Table 1), Tini of the PNCs decreased by 12°C (from 421.0°C to 409.0°C) and Tmax increased by 5.7°C (from 475.7°C to 481.4°C). When 20.0 wt% cobalt–cobalt oxide core–shell NPs in situ are synthesized in PP matrix in the presence 5.0% PP-g-MA, the PNCs were found to have significantly decreased Tini and slightly increased Tmax. For the PP PNCs with 5.0% PP-g-MA (Mn = 800) and 20.0 wt% particles, Tini decreased to 383.4°C and Tmax increased to 484.6°C. Meanwhile, Tini decreased to 404.6°C and Tmax increased to 482.2°C for the PP–5.0% PP-g-MA (Mn = 2500) –20.0% particles system. The final residues at 700°C are 20.2, 22.1, and 20.9% in the PP PNCs without PP-g-MA, with PP-g-MA (Mn = 800), and with PP-g-MA (Mn = 2500), respectively. The decreased Tini indicated that earlier weight loss took place when PP-g-MA was introduced in the PP/cobalt system and the increased Tmax suggested a better thermal stability at

Table 2

TGA characteristics of the samples measured in air.

Composition

Fig. 1 TGA curves of pure PP and the PNCs under (a) nitrogen and (b) air atmosphere. Source: From He, Yuan, et al.[31] ©2009 The Royal Society of Chemistry (RSC).

Tini (°C) N2

Tini (°C) air

Residue at Tmax (°C) 550°C (%)

Pure PP

266.5

328.0

0.0

PP/PP-g-MA (Mn = 800)

280.4

359.9

0.0

PP/PP-g-MA (Mn = 2,500)

266.5

375.0

0.0

PP/20.0% NPs

330.6

432.0

25.0

PP/20.0% NPs/PP-g-MA (Mn = 800)

345.0

443.3

24.5

PP/20.0% NPs/PP-g-MA (Mn = 2,500)

330.4

435.2

26.4

Source: From He, Yuan, et al.[31] ©2009 The Royal Society of Chemistry (RSC).

Flame Retardant Polypropylene Nanocomposites

higher temperatures. The decreased Tini indicates a different degradation pathway, in which more gas volatiles were generated from initial thermal degradation. The probable reason is because the chain scissions took place at low temperature in the PP PNCs in the presence of PP-g-MA. Compared with thermal degradation under nitrogen, the thermal oxidative stability of the polymer material was prominently reduced by oxidative dehydrogenation accompanied by hydrogen abstraction.[34] The Tini of pure PP was drastically reduced to 266.5°C in air compared with that of 421.0°C in nitrogen, and Tmax also reduced to 328.0°C (Table 2). Tini and Tmax increased to 330.6°C and 432.0°C when 20.0% cobalt–cobalt oxide core–shell NPs were formed in PP matrix, which were 64.1 and 104.0°C higher than those of pure PP. The oxidative residue of the PP/ cobalt PNCs at 550°C was 25.0%, indicating that the further oxidation of these cobalt–cobalt oxide core–shell NPs takes place under high temperature in air. It is probably because that the cobalt–cobalt oxide core–shell NPs act as effective heat barrier and thus significantly enhanced the thermal oxidative stability of the PP matrix through deferring oxidative degradation. More importantly, Tini and Tmax were further increased to 345.0°C and 443.3°C (14.4°C and 11.3°C higher than PP/cobalt or 78.5°C and 115.3°C higher than pure PP) when 5.0% PP-g-MA (Mn = 800) was added in the PP-20.0 wt% cobalt–cobalt oxide core–shell NPs system. However, the PP-g-MA (Mn = 2500) was found to have limited effect in further increasing the Tini and Tmax of the PP matrix when added into PP PNCs with 20.0 wt% cobalt–cobalt oxide core–shell NPs. The enhanced thermal oxidative stability by PP-g-MA (Mn = 800) is obviously attributed to the enhanced interfacial adhesion effect between PP and cobalt–cobalt oxide core–shell NPs, which requires more energy to decompose the PP/PP-g-MA/ cobalt complex in air; meanwhile, this effect is limited by the fewer bonding through PP-g-MA (Mn = 2500) on these cobalt–cobalt oxide core–shell NPs.[14] Microscale combustion calorimetry (MCC) was utilized to evaluate the fire hazards of the PP and its PNCs by investigating the heat combustion parameters including heat release capacity (HRC), heat release rate (HRR), peak heat release rate (PHRR), temperature at PHRR (TPHRR), and total heat release (THR). Fig. 2 depicts the HRR curves as function of temperature. The higher PHRR one material behaves under a specific heat flux, the more dangerous it will act under a fire accident. Pure PP is a highly flammable material with a measured PHRR value of 1513.0 W/g. When forming cobalt–cobalt oxide core–shell NPs in PP PNCs, PHRR decreased from 1513.0 to 1024.0 W/g (more than 32% reduction), THR decreased from 40.6 to 27.7 W/g (more than 31.8% reduction) and the initial decomposition temperature was enhanced upon adding these cobalt– cobalt oxide core–shell NPs (Fig. 2). Obviously, excluding the dilution effect of 20.0 wt% noncombustible cobalt– cobalt oxide core–shell NPs in PP matrix, an additional 12.3% decrease in HRR and 11.8% decrease in THR

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Fig. 2 Heat release rate curves of pure PP, PP/PP-g-MA, PP/ 20.0% cobalt PNCs, and PP/20.0% cobalt PNCs stabilized with two different molecular weight PP-g-MAs. Inset is enlarged initial low temperature decomposition between 100°C and 400°C (S: Mn = 800, L: Mn = 2500). Source: From He, Yuan, et al.[31] ©2009 The Royal Society of Chemistry (RSC).

indicated a flame retardancy effect of these in situ obtained cobalt–cobalt oxide core–shell NPs. A barrier effect from these cobalt–cobalt oxide core–shell NPs is believed to be responsible for this flame retardancy similar to the enhanced thermal oxidative stability. When the PNCs were exposed under high temperature, heat and mass transfers between gas and condense phases were slowed down by an insulating layer formed from these NPs, which suppressed the fast decomposition of the PP matrix.[35] Therefore, the lower flammability in terms of HRR reduction suggests a slower speed of combustible volatiles generated from the random chain scission of PP backbones in the presence of cobalt– cobalt oxide core–shell NPs. Synergistic effect in reducing HRR was observed when adding 5.0% PP-g-MA in the PP-20.0 wt% cobalt–cobalt oxide core–shell NPs system (Fig. 2). HRC and PHRR were observed to decrease significantly, i.e., PHRR further decreased to 532.4 W/g in the case of PP–20.0 wt% cobalt– cobalt oxide core–shell NPs PNCs with PP-g-MA (Mn = 2500) or to 500.8 W/g in the case of PP-g-MA (Mn = 800). Meanwhile, THR further slightly decreased from 27.7 to 25.6 kJ/g with 5.0 wt% PP-g-MA (Mn = 2500) or to 25.1 kJ/g with 5.0 wt% PP-g-MA (Mn = 800) in PP-20.0% cobalt– cobalt oxide core–shell NPs PNCs. Although conventional synergistic effect including nitrogen–phosphorus,[33,36,37] phosphorus–silicon, [38,39] or nitrogen–phosphorus– silicon[40]on flame retardant PNCs has been extensively studied, the synergistic effect between PP-g-MA and cobalt–cobalt oxide core–shell NPs have rarely been reported. In addition, when PP-g-MA was added in the PP–20.0% cobalt–cobalt oxide core–shell NPs system, a

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broad HRR peak was observed during the initial decomposition stage (around 100–300°C) and TPHRR decreased to 471.0°C (shown in the inset of Fig. 2). X-ray photoelectron spectroscopic (XPS) analysis was further investigated to identify the atomic composition of solid char residues of these PP PNCs after the MCC test and thus to determine flame retardancy mechanism. It was observed that the carbon species mass percentage increased sharply from 61.80% for the PP–20.0 wt% cobalt–cobalt oxide PNCs to 71.08% for PP PNCs in the presence of PP-g-MA (Mn = 2500) and to 80.92% in the presence of PP-PP-g-MA (M n = 800), corresponding to 83.16%, 89.43%, and 90.89% carbon atomic percentage, respectively. This proves that the combination of cobalt–cobalt oxide core–shell NPs and PP-g-MA in the PP matrix can facilitate the char formation at high temperature combustion. Possible reason is that the anhydride groups from PP-g-MA promoted carbonization catalytically during the chain scission and chain transfer process, thus increasing the char yield. In order to further elucidate the synergism between PP-g-MA and cobalt–cobalt oxide core–shell NPs in reducing the flammability of PP, a fast thermal degradation test was performed by TGA using the same conditions as MCC measurements—a heating rate of 60°C/min (1°C/sec) under nitrogen (TGA and DTG curves shown in Fig. 3). MCC measurements here were performed using an inert sample thermal degradation procedure to pyrolyze the sample into combustible gas volatiles followed by a nonflaming oxidation of these volatiles. The fast thermal degradation by TGA can illustrate the dynamic sample weight loss under temperature ramping at a constant high heating rate (1°C/sec); meanwhile, the DTG (%/°C) from the inset of Fig. 3 can reproduce the thermal degradation stage of

Fig. 3 TGA and DTG curves of PP/20.0% cobalt PNCs and PP/ 20.0% cobalt PNCs stabilized with two PP-g-MAs under a heating rate of 1°C/sec. Source: From He, Yuan, et al.[31] ©2009 The Royal Society of Chemistry (RSC).

Flame Retardant Polypropylene Nanocomposites

MCC clearly. Therefore, the degradation and real-time weight loss processes can be demonstrated simultaneously. It is noticed from Fig. 2 that the addition of 5.0% PP-g-MA barely decreased the initial thermal degradation temperature of PP, while 20.0% cobalt–cobalt oxide core–shell NPs delayed the initial degradation of PP as evidenced by the higher thermal degradation temperature than that of pure PP (no detectable HRR increase before 400°C, shown in the inset of Fig. 2). However, the degradation of PP–5.0% PP-g-MA–20.0% cobalt–cobalt oxide core– shell NPs PNCs was definitely altered by the evidence of broad HRR peak appeared in the range of 130–310°C (inset of Fig. 2) and ~18.0% weight loss within the thermal degradation temperature of 100–310°C (dashed rectangle zone in Fig. 3). When exposed to heat at elevated temperature from 80°C to 650°C, the inert thermal degradation of PP was initiated mainly by chain scission and chain transfer, and then reductions in molecular weight were first observed at 227–247°C and gas volatiles became significant above 302°C. Finally, ignition of PP was observed at a surface temperature of 337°C,[41] which is consistent with the initial HRR jump at ~330°C observed from MCC. Meanwhile, the addition of 5.0% PP-g-MA has limited influence on initiating the degradation of the PP matrix. With adding 20.0% cobalt–cobalt oxide core–shell NPs, only heat barrier effect was found to reduce the HRR through slowing the release of gas volatiles.[35] When adding PP-g-MA with cobalt–cobalt oxide core–shell NPs in the PP matrix, the catalytic effect was found to lower the initial thermal degradation temperature of the resulted PNCs (from 330°C for pure PP to ~100–130°C for the PNCs) and lead to a smaller HRR in the range of 100– 310°C due to a small amount of gas volatiles released from bulk material. Probable mechanism includes random chain scission of C–C bond of PP backbone to generate hydrocarbon radicals during initial decomposition, the formation of lower hydrocarbons such as propylene from further degradation of these hydrocarbon radicals, the β-scission and abstraction of H radicals from other hydrocarbons to produce a new hydrocarbon radicals during propagation stage, and finally the disproportionate or recombination of two radicals as termination reaction.[42] Meanwhile, slightly similar to a “smoldering,” a substantial fraction (~18.0%) of the total mass of PP/cobalt PNCs in the presence of PP-gMA was consumed at 100–310°C. Therefore, only small amount of heat release in a slow speed was generated, effectively decreasing the total available gas fuels, which would generate large quantity of heat if placed under high temperatures. One can also observe that the peak width of the PP/PP-g-MA/cobalt PNCs was much wider than those of pure PP, PP/PP-g-MA, and PP/Co PNCs (Fig. 2), further indicating a longer combustion period upon introducing the PP-g-MA and cobalt–cobalt oxide core–shell NPs in the PP matrix. This is another sign of low fire hazard for the PP/PP-g-MA/cobalt–cobalt oxide core–shell NPs

Flame Retardant Polypropylene Nanocomposites

PNCs. Although the catalytic effect has been long proposed to be responsible for enhancing the flame retardancy for the PNCs filled with flame retardants with transitional metals, this synergistic catalytic effect from combining cobalt–cobalt oxide core–shell NPs with nonflame retardant PP-g-MA has rarely been reported. CONCLUSION The catalytic and synergistic effect on significantly suppressing the combustion behaviors of PP has been demonstrated by adding small amount (5.0 wt%) of nonflame retardant additive—PP-g-MA in the PP PNCs reinforced with the in situ prepared cobalt–cobalt oxide core–shell NPs. Combining cobalt–cobalt oxide core–shell NPs with PP-g-MA has changed the thermal decomposition pathway of PP by accelerating the low temperature weight loss rate (observed from MCC) and promoting the high temperature carbonization catalytically (confirmed by XPS). Those changes further result in significant reduction in PHRR compared with PP/cobalt PNCs without PP-g-MA. Without using typical flame retardants, this new pathway in reducing the flammability of polyolefin will give rise to further insight to the design of novel catalytic flame retardant composition for practical applications. ACKNOWLEDGMENTS This project was supported by the Lamar University. Partial financial support from National Science Foundation— Chemical and Biological Separations (CBET: 11-37441) is appreciated. We also appreciate the support from National Science Foundation Nanoscale Interdisciplinary Research Team and Materials Processing and Manufacturing (CMMI 10-30755). Partial financial support from Baker Hughes Inc., is also appreciated. REFERENCES 1. Lewin, M.; Endo, M. Catalysis of intumescent flame retardancy of polypropylene by metallic compounds. Polym. Advan. Technol. 2003, 14, 3–11. 2. Lewin, M. Synergism and catalysis in flame retardancy of polymers. Polym. Advan. Technol. 2001, 12, 215–222. 3. Lewin, M. Synergistic and catalytic effects in flame retardancy of polymeric materials—An overview. J. Fire Sci. 1999, 17, 3–19. 4. Song, R.; Zhang, B.; Huang, B.; Tang, T. Synergistic effect of supported nickel catalyst with intumescent flameretardants on flame retardancy and thermal stability of polypropylene. J. Appl. Polym. Sci. 2006, 102, 5988–5993. 5. Liu, Y.; Wang, Q. Catalytic action of phospho-tungstic acid in the synthesis of melamine salts of pentaerythritol phosphate and their synergistic effects in flame retarded polypropylene. Polym. Degrad. Stabil. 2006, 91, 2513–2519.

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