Investigations of Thermoplastic Poly(imide-urethanes) Flame

May 16, 2014 - Author: A series of novel intrinsically flame-retardant thermoplastic poly(imide-urethanes) (TPIUs) has been synthesized by 4,4′-diph...
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Investigations of Thermoplastic Poly(imide-urethanes) FlameRetarded by Hydroxyl-Terminated Poly(dimethylsiloxane) Qiheng Tang, Rongjie Yang, and Jiyu He* National Engineering Research Center of Flame Retardant Materials, School of Materials Science and Engineering, Beijing Institute of Technology, Zhongguancun South Street 5, Haidian District, Beijing 100081, People’s Republic of China ABSTRACT: Author: A series of novel intrinsically flame-retardant thermoplastic poly(imide-urethanes) (TPIUs) has been synthesized by 4,4′-diphenylmethane diisocyanate (MDI), poly(tetrahydrofuran) (PTMG), pyromellitic anhydride (PMDA), and hydroxyl-terminated poly(dimethylsiloxane) (PDMS) used as flame retardants. The obtained TPIU/PDMS exhibited good tensile strength and elongation at break. TPIU/PDMS has a higher thermal decomposition temperature than that of commercial PTMG−MDI−1,4-butanediol-based thermoplastic polyurethane (TPU) according to thermal gravimetric analysis. The total heat released and peak heat release rate of TPIU/12.4%PDMS were found to be lower than those of the pure TPIU by 8.0% and 26.3%, respectively, based on cone calorimeter testing. The condensed phase of TPIU/PDMS has been investigated by scanning electron microscopy, Fourier transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy. The results indicate that the good flame retardancy of TPIU/PDMS can be attributed to thermally stable compounds with Si−O structures on the surface of its char layers.

1. INTRODUCTION Thermoplastic polyurethane elastomers (TPUs) can be used as coating materials in a wide variety of commercial and technical applications and are becoming more and more important due to their excellent abrasion resistance, chemical resistance, good processing, and mechanical properties.1,2 TPUs consisting of soft and hard segments are synthesized by either the one-step or the prepolymer method.3−5 The soft segments are usually amorphous diols with relatively high molecular weight. The hard segments are made up of diisocyanates and chain extenders, both of which have low molecular weight. As is known to all, traditional TPUs exhibit poor heat resistance, with thermal degradation occurring just above 200 °C.6−8 Therefore, there has recently been increased emphasis on the development of technologies to enhance the thermal stability of TPUs. In our previous work,9 we successfully synthesized thermally stable thermoplastic poly(imide-urethanes) (TPIUs) by incorporating thermally stable heterocyclic imide groups into the main chain. These materials showed excellent mechanical properties, antidripping effect, a low heat release rate, and so on. However, the applications of TPIUs are still limited by their flammability. Generally, halogen-containing compounds have been widely used as additives or co-monomers in TPUs to obtain fireretardant materials. However, flame-retardant TPUs with halogen can produce corrosive and poisonous smoke upon combustion, as well as highly toxic halogenated dibenzofurans and dibenzodioxins.10−12 Thus, it is essential to develop more environmentally friendly flame retardants to replace those containing halogens. Organic−inorganic hybrid composites as environmentally friendly flame retardants emitting no pollution to the environment, such as octa(tetramethylammonium) polyhedral oligomeric silsesquioxanes (OctaTMA-POSS),13 polyhedral oligomeric silsesquioxanes (POSS),14 [OSiO1.5]x (x = 6, 8, 10) octasilsesquioxanes,15 and numerous POSS derivatives,16 © 2014 American Chemical Society

are broadly considered as a new generation of multifunction materials, for they combine the advantages of organic polymers with those of inorganic materials.17−20 The effects of various silicon-containing compounds on the flame retardancy of polyurethanes have been explored. Modesti et al.21 investigated the synergy between a phosphorus-based flame retardant (aluminum phosphinate) and layered silicates in the flame retardancy of polyurethane foams. In 2009, Bourbigot et al.22 studied the flame retardancy of TPUs containing POSS; the results confirmed that a large reduction in peak heat release rate compared with pristine TPU could be achieved due to the generation of intumescent structures. Poly(dimethylsiloxane) (PDMS), a typical reactive organic− inorganic hybrid material, appears to be an attractive flame retardant because a silica ash layer remains on the surface of the carbon residue after combustion,23,24 acting as a shield against pyrolysis gas and heat transfer. There have been many literature reports on the use of PDMS as a flame retardant in polypropylene (PP),25 polycarbonate (PC),26 and epoxy resins.27 In 2011, Chen et al.28 used microencapsulated ammonium polyphosphate (MAPP) and hydroxyl silicone oil in TPUs, and they showed that this agent lowered the heat release rate (HRR) of TPUs more effectively than pristine ammonium polyphosphate (APP). All of these methods for achieving flame retardancy in polymers are by physical means. However, all kinds of problems, such as migration and poor compatibility, are associated with systems prepared in this manner. Hence, there is a need to develop intrinsic flame-retardant polymers or Received: Revised: Accepted: Published: 9714

February 1, 2014 May 13, 2014 May 16, 2014 May 16, 2014 dx.doi.org/10.1021/ie500473t | Ind. Eng. Chem. Res. 2014, 53, 9714−9720

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Scheme 1. Preparation of TPIU/PDMS

to modify existing polymers by copolymerizing with a flameretardant structure either in the side chains or in the backbones. As a flame retardant containing reactive terminal dihydroxyl groups, PDMS can be incorporated into thermoplastic polyurethanes to reduce their flammability. However, as far as we know, there have been few literature reports concerning the use of silicon-containing macromolecules as reactive flame retardants in TPIUs. Thus, the object of this work is to study the use of PDMS to replace part of the poly(tetrahydrofuran) (PTMG) in TPIUs. The flammability of the resultant material has been characterized by cone calorimeter tests. Its thermal degradation has been studied by thermogravimetry (TG). Moreover, the flame-retardant mechanism of PDMS in TPIUs has also been investigated by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDXS) in conjunction with SEM.

TG analysis was performed by a NETZSCH 209 F1 thermal analyzer under nitrogen atmosphere from 40 to 600 °C at a heating rate of 10 °C/min.” Tensile tests were carried out according to ASTM D 638-91 method. The dumbbell-shaped specimens of polymers had a width of thickness of about 1−2 mm and 10 mm in the neck. At least five specimens were tested for each kind of polymer. Combustion experiments were characterized by a cone calorimeter device (50 kW/m2, Fire Testing Technology Co., Ltd.). The dimension of the samples are 100 × 100 × 3 mm3. SEM experiments were characterized by a scanning electron microscope (Hitachi S-4800). Samples for SEM were the char residue from cone calorimeter test. The C, O, and Si elements in the char residue were characterized through energydispersive X-ray spectroscopy (EX-350) in SEM. 2.3. Preparation of the TPIU and TPIU/PDMS. The polymers were prepared by two-step polymerization through MDI, PTMG, and PDMS, as well as PMDA as chain extender, according to Scheme 1. The synthesis was conducted by mechanical agitation in a three-necked flask. The appropriate amounts of PTMG and PDMS were poured into the flask containing melted MDI, and the reaction was stirred at 80 °C for an hour; the right amount of PMDA (as chain extender) was dissolved in DMF and then poured into the previously mentioned flask. The system was rigorously stirred for another 12 h under nitrogen atmosphere. Ultimately, the product was poured into a poly(tetrafluoroethylene) (PTFE) mold and then placed in a blast oven at 40 °C (to prevent the formation of bubbles in the sample) for 48 h to evaporate some of the DMF; then the oven was heated to 70 °C until the sample achieved constant weight. The mole ratios of reagents for various TPIU/

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-Diphenylmethane diisocyanate (MDI) was provided by Nippoh Polyurethane Industry Co., Ltd. and used as received. Poly(tetrahydrofuran) (Mn = 2000 g/mol) was purchased from Aladdin Reagent (China) Co., Ltd. PDMS (Mn = 1000 g/mol) was purchased from Dow Corning Corp., as shown in Scheme 1. Pyromellitic dianhydride (PMDA) was purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethylformamide (DMF) was purchased from Beijing Chemical Reagents Co. 2.2. Measurements. FTIR analysis was tested by NICOLET 6700 IR spectrometer. The spectra with a spectral resolution of 4 cm−1 were collected by 32 scans. 9715

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Table 1. Effect of PDMS Content on the Mechanical Properties of TPIU/PDMS samples

molar ratio (MDI:PDMS: PTMG:PMAD)

PDMS content (%)

pure TPIU TPIU/3.8%PDMS TPIU/12.4%PDMS TPIU/22.5%PDMS

2:0:1:1 2:0.1:0.9:1 2:0.3:0.7:1 2:0.5:0.5:1

0 3.8 12.4 22.5

tensile strength (MPa) 34.7 29.8 20.1 17.5

± ± ± ±

elongation at break (%)

Mw × 104

Mw/Mn

± ± ± ±

8.2 6.3 5.9 5.2

1.6 2.1 2.8 2.9

4.3 5.2 3.5 4.2

798 828 747 694

33 51 37 25

Compared to the maximum tensile strengths of inherent flame-retardant polyurethanes reported by Chen-Yang et al.29 (approximately 9 MPa) and Tang et al.30 (approximately 3.56 MPa), the minimum value for TPIU/PDMS (17.5 MPa) is enhanced by more than 94.4% and 391.6%, respectively. The tensile strength of pure TPIU is also 5.5 MPa higher than that of conventional TPU211,30 so its mechanical properties meet the requirements for practical use. 3.3. Thermal Properties. Figure 2 displays the TG curves of pure TPIU and TPIU/PDMS measured in an atmosphere of

PDMS are shown in Table 1; meanwhile, the percent in Table 1 is the weight percent of PDMS in total mass.

3. RESULTS AND DISCUSSION 3.1. FTIR Analysis. Figure 1 displays the FTIR spectra of the reactants, pure TPIU and TPIU/PDMS. The spectra show

Figure 1. FTIR spectra of MDI, PTMG, PDMS, TPIU, and TPIU/ PDMS. Figure 2. TG and DTG curves of TPIU and TPIU/PDMS.

the characteristic absorptions of aromatic imide rings at 1780 and 1720 cm−1 (stretching vibration for CO), 1357 cm−1 (stretching vibration for C−N−C), and 726−730 cm−1 (deformation vibration for the imide ring), confirming that the imide rings had been introduced to the main chains through the reaction. In these spectra, the characteristic band of -NCO in MDI at about 2270 cm−1 had almost disappeared; meanwhile, bands at 2700−2900 cm−1 belong to the stretching vibration of -CH2- groups in PTMG, indicating that MDI and PTMG had been incorporated into the TPIU. Furthermore, it should be noted that not only were the characteristic bands at ν = 1256, 1010, and 793 cm−1 of PDMS seen in the spectra of TPIU/PDMS but also their intensity increased with increasing PDMS content. All of the aforementioned bands are fully supportive of successful preparation of TPIU and TPIU/ PDMS. 3.2. Mechanical Properties. The mechanical properties of TPIU/PDMS were characterized by tensile testing. Relative data for the tensile strength and elongation at break are summarized in Table 1, which give a clear indication of the mechanical properties of each TPIU/PDMS copolymer. It can be observed that the tensile strength and elongation at break show significant variations compared to those of pure TPIU and that they decrease monotonically with increasing PDMS content. This phenomenon can be rationalized in terms of the poor mechanical properties of PDMS at room temperature. Moreover, this phenomenon could be attributed to the decrease in the weight-average molecular weight (Mw) of the polymers. The values of Mw determined by gel permeation chromatography (GPC) decreased with increasing the content of PDMS, which is due to the lower molecular weight of PDMS compared to PTMG.

nitrogen. The corresponding thermal decomposition data, containing the temperatures of T1% (1% weight loss), T5% (5% weight loss), Tmax (maximum weight loss rate), DTGmax (the maximum weight loss rate), and the char at 600 °C, providing information on their thermal degradation behavior, are given in Table 2. Table 2. Thermal Properties of TPIU and TPIU/PDMS samples

T1% (°C)

T5% (°C)

DTGmax (%/min)

Tmax (°C)

char at 600 °C (%)

PTMG PDMS pure TPIU TPIU/3.8%PDMS TPIU/12.4%PDMS TPIU/22.5%PDMS

243.6 152.1 306.2 306.0 285.8 274.9

288.4 294.8 347.7 341.0 341.3 341.5

−20.4 −14.8 −19.4 −18.7 −17.8 −16.6

363.5 387.2 425.8 419.5 420.4 421.5

0.0 0.5 12.1 13.5 15.7 17.6

TPIU displays one-stage thermal degradation in nitrogen. It begins to degrade between around 300 and 450 °C, undergoing a rapid weight loss of 80%, and the residue at 600 °C is 12.1%, indicating that TPIU has a high charring yield. The flame-retarded TPIU/PDMS showed similar thermal degradation behavior to that of pure TPIU. The DTG curves of TPIU/PDMS samples shown in Figure 2 provide insight into the decomposition process. PDMS is well-known for its good thermal stability. However, from Figure 2 and Table 2, it can be seen that the T1% of TPIU/3.8%PDMS are slightly lower than those of pure TPIU. This abnormal phenomenon can be explained by the thermal stabilities of PDMS and PTMG. As 9716

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Table 3. Cone Calorimeter Data for TPIU and TPIU/12.4%PDMS samples

TTI (s)

p-HRR (kW/m2)

TSR (m2/m2)

mean SEA (m2/kg)

mean COY (kg/kg)

mean CO2Y (kg/kg)

pure TPIU TPIU/12.4%PDMS

45 36

654.4 428.2

1091.4.5 1681.8

326.0 473.1

0.03 0.0247

2.54 1.8664

showed in Table 2, the T1% of PDMS is much lower than that of PTMG, which may be due to the elimination of some less thermally stable impurities from PDMS. Moreover, the initial degradation temperature (T1%) of TPIU/PDMS decreases markedly with increasing PDMS content, which further supports this inference. As the temperature is increased in TG, the impurities gradually evaporate. Therefore, the samples of TPIU incorporating PDMS did not show such obvious changes in their T5% and Tmax values compared to those of pure TPIU, as also shown in Figure 2 and Table 2. As shown in Table 2, it can be seen that the T5% and Tmax of PDMS are much higher than those of PTMG; however, the T5%s and Tmaxs of TPIU/PDMS are a little lower than those of pure TPIU, which may be explained by the microphase separation. As is known to all, the semipolar soft segments (PTMG) and polar hard segments result in thermodynamic incompatibility, leading, to some extent, to microphase separation. The solubility parameters of urethane bonds and PDMS are 25.4 and 15.6 J/cm3.31 When PDMS incorporated into the main chains, the significant difference between the solubility parameters of urethane bonds and PDMS results in more obvious microphase separation, which is consistent with results given in a multitude of literature reports.32−36 The obvious microphase separation forms a phase-separated structure; that is, each phase is composed of a rich phase, resulting in the Tmaxs of TPIU/PDMS shifting to the lower Tmaxs of PDMS and PTMG (as shown in Table 2). Furthermore, it may be noted that the T5% and Tmax of TPIU/PDMS increased with increasing PDMS content. This result indicates that the thermal stabilities of TPIU/PDMS increase with PDMS contents. This conclusion is supported by the DTGmax values in Table 2; it can be observed that the DTGmax values of TPIU/ PDMS decrease with increasing PDMS contents. T5%s and Tmaxs of TPIU/PDMS are approximately 340 and 420 °C, respectively, which are higher than those of conventional PTMG−MDI−1,4-BDO-based TPU at 308.9 and 410.9 °C.37 Moreover, compared to intrinsic flameretarded polyurethanes reported in the literature,30,37 T5% and Tmax of the series of TPIU polymers are higher than those of PPPZ−PU-20 (289 and 327 °C for T5% and Tmax, respectively) and PUIG321 (311.8 and 408.8 °C for T5% and Tmax, respectively). In Figure 2A, the TG curves of TPIU/PDMS in the range of 450−600 °C are always higher than that of pure TPIU, indicating that the TPIU/PDMS samples yielded more char residue than pure TPIU. Moreover, in Table 2, it is obvious that the final char at 600 °C is affected by the PDMS content in each TPIU/PDMS copolymer. That is to say, with increasing PDMS content, the char increases, implying that the TPIU/ PDMS copolymers with higher PDMS content are more thermally stable. This may be ascribed to the movement of free radicals into the inclusions of siloxane in TPIU/PDMS, and is consistent with the results of Monakhova et al.,38 who reported that PDMS could delay the thermal oxidative aging of polypropylene. 3.4. Cone Calorimeter Tests. To evaluate the effect of PDMS on the combustion behavior of TPIU, cone calorimeter

tests were performed for TPIU and TPIU/12.4%PDMS as a representative of TPIU copolymers. Cone calorimetry is able to quantitatively analyze the combustibility of materials by offering data such as time to ignition (TTI), heat release rate (HRR), total smoke release (TSR), and total heat released (THR). Experimental results obtained by cone calorimetry for pure TPIU and TPIU/12.4%PDMS are summarized in Table 3. The HRR curves and p-HRR values of TPIU without and with PDMS are presented in Figure 3(A) and Table 3. It is

Figure 3. Curves of HRR (A) and THR (B) for TPIU and TPIU/ 12.4%PDMS.

observed that the pure TPIU burned rapidly with a p-HRR of 654.4 kW/m2. After PDMS was incorporated into the TPIU, the HRR curve of the flame-retarded TPIU/PDMS increased rapidly at first but then attained a plateau stage with a much lower p-HRR value of 482.2 kW/m2. This p-HRR value was thus 26.3% lower. Figure 3B shows the THRs for pure TPIU and TPIU/12.4%PDMS. From Figure 3B, it is observed that the THR of TPIU/12.4%PDMS was lower than that of TPIU by 8.0%. From the aforementioned results, it may be inferred that PDMS served to inhibit the combustion of the TPIU matrix because of PDMS producing a stable char layer containing silicon oxide on the surface. Moreover, the fire spread can be assumed to the slope of the THR curve. It is clear that the flame spread of TPIU/12.4%PDMS was less than that of TPIU. Therefore, it can be inferred that the Si−O structures on the surface can inhibit the rapid propagation of a flame. The influence of a flame retardant on ignitability is usually determined by the TTI. To our surprise, an obvious decrease in TTI by 9 s was measured after incorporating 12.4% PDMS into TPIU. This may be ascribed to the lower thermal stability of TPIU/12.4%PDMS, leading to accelerated thermal decomposition of the material after irradiation of the surface of the composite by the cone heater. Thereafter, some small volatile molecules are produced and burned, and this is analyzed in the TG section. The total smoke release curves of TPIU and TPIU/12.4% PDMS are displayed in Figure 4; the TSR and specific extinction area (SEA) values are shown in Table 3. It can be noted that TPIU/12.4%PDMS produced more smoke than did pure TPIU. This was because the char layer with Si−O structures protected the underlying material from further burning, thereby resulting in insufficient combustion and the products are mainly in charge of smoke emission.39,40 The curves of the CO2 production rate (CO2PR) and CO production rate (COPR) of TPIU and TPIU/12.4%PDMS 9717

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Figure 4. Curves of TSR for TPIU and TPIU/12.4%PDMS with time.

Figure 6. Photographs of char residues of TPIU(A) and TPIU/12.4% PDMS(B) obtained from cone calorimeter tests.

during combustion are presented in Figure 5. The COPR and CO2PR of pure TPIU are observed to be higher than those of

that the exterior and interior chars had Si contents of 1.30% and 1.28%, respectively. The results further indicated that the presence of Si−O structures in the residues. The residue with Si−O structures on the surface of a flaming polymer creates a physical protective layer. This layer can inhibit heat transfer from the flame zone to the burning substrate, but also limit oxygen diffusion to the flaming substrate, thereby protecting the polymers from further combustion and delaying their pyrolysis. In order to study the structures of the char layers more closely, the exterior char layers at 50× magnifications are shown in Figure 7A,B. From the structure of char residues, the

Figure 5. CO (A) and CO2 (B) production rates of TPIU and TPIU/ 12.4%PDMS with time.

TPIU/12.4%PDMS. These results indicate that PDMS can efficiently restrict the release of CO and CO2. Furthermore, the mean COY and CO2Y (CO and CO2 yields) of TPIU were both higher than those of TPIU/12.4%PDMS. This was because the char layer protected the underlying material from further burning. 3.5. Analysis of the Char Residue. 3.5.1. EDXS Analysis. To better understand the flame-retardant mechanism of PDMS, chars were investigated by digital photographs, SEM, and EDXS. The concentrations of Si, C, and O in the interior and exterior chars are listed in Table 4.

Figure 7. SEM micrographs of exterior residues from TPIU(A) and TPIU/12.4%PDMS(B).

Table 4. EDXS Results of the Chars from TPIU and TPIU/ 12.4%PDMS

combustion phenomenon of the samples can be gleaned. Based on SEM images, the morphologies of the residues from TPIU and TPIU/12.4%PDMS were distinctly different. As shown in Figure 7A, the char layer from TPIU was loose and alveolate, the structure was contributive to heat transfer and gas diffusion, and hence the sample burned rapidly. Although the char from TPIU/12.4%PDMS also contained lots of holes, the char layer exhibited a continuous structure (Figure 7B). We speculate that this char layer is more efficient in inhibiting flame propagation than a loose char layer, leading to a sample with lower p-HRR and THR. 3.5.2. FTIR Analysis. To confirm the constituents of the char layers, the exterior char layers from TPIU and TPIU/12.4% PDMS were characterized by FTIR. The FTIR spectra of the char obtained from cone calorimeter tests are shown in Figure 8. Only a broad band at approximately ν = 1620 cm−1 was observed for the pure TPIU, indicating the formation of polyaromatic carbon structures. The FTIR spectrum of the white substance is also shown in Figure 8. It can be seen that there are two absorbances, at ν = 1170 cm−1 due to a stretching vibration of Si−O−Si units and a small peak at ν = 800 cm−1 due to a deformation vibration of Si−O−C units (and/or the

element concentration (%) pure TPIU

TPIU/12.4%PDMS

elements

exterior

interior

exterior

interior

C O Si

75.34 24.66

74.65 25.35

77.50 21.20 1.30

78.84 19.88 1.28

After the tests of the cone calorimeter, some arresting phenomena were observed by visual observation of the char residues. As shown in Figure 6A,B, the char residues from TPIU and TPIU/12.4%PDMS gradually shrink during combustion, eventually leaving a piece of porous carbon layer. However, in Figure 6B, the char layer from TPIU/12.4% PDMS is seen to be covered with a large amount of a soft and crumbly white substance. This is assumed to consist of Si−O compounds produced by the combustion of PDMS. According to the EDXS data, the chars from both TPIU and TPIU/12.4%PDMS contain abundant C and O contents (Table 4). Moreover, for TPIU/12.4%PDMS, it can be seen 9718

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(5) Mark, H. F. Encyclopedia of Polymer Science and Technology, 3rd ed.; John Wiley & Sons: New York, 2003; p 26. (6) Takeichi, T.; Suefuji, K.; Inoue, K. Preparation of hightemperature polyurethane by alloying with reactive polyamide. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3497−3503. (7) Cervantes-Uc, J. M.; Espinosa, J. I. M.; Cauich-Rodriguez, J. V.; Avila-Ortega, A.; Vazquez-Torres, H.; Marcos-Fernandez, A. TGA/ FTIR studies of segmented aliphatic polyurethanes and their nanocomposites prepared with commercial montmorillonites. Polym. Degrad. Stab. 2009, 94, 1666−1677. (8) Petrovi, Z. S.; Zavargo, Z.; Flyn, J. H.; Macknight, W. J. Thermal degradation of segmented polyurethanes. J. Appl. Polym. Sci. 1994, 51, 1087−1095. (9) Tang, Q. H.; Song, Y.; He, J. Y.; Yang, R. J. Synthesis and characterization of inherently flame-retardant and antidripping thermoplastic poly(imides-urethane)s. J. Appl. Polym. Sci. 2014, DOI: 10.1002/app.40801.. (10) Wang, J. S.; Liu, Y.; Zhao, H. B.; Liu, J.; Wang, D. Y.; Song, Y. P.; Wang, Y. Z. Metal compound-enhanced flame retardancy of intumescent epoxy resins containing ammonium polyphosphate. Polym. Degrad. Stab. 2009, 94, 625−631. (11) Chow, W. S.; Neoh, S. S. Dynamic Mechanical, Thermal, and Morphological Properties of Silane-Treated Montmorillonite Reinforced Polycarbonate Nanocomposites. J. Appl. Polym. Sci. 2009, 114, 3967−3975. (12) Becker, L.; Lenoir, D.; Matuschek, G.; Kettrup, A. Thermal degradation of halogen-free flame retardant epoxides and polycarbonate in air. J. Anal. Appl. Pyrolysis 2001, 60, 55−67. (13) Liu, L.; Hu, Y.; Song, L.; Nazare, S.; He, S. Q.; Hull, R. Combustion and thermal properties of OctaTMA-POSS/PS composites. J. Mater. Sci. 2007, 42, 4325−4333. (14) Su, C. H.; Chiu, Y. P.; Teng, C. C.; Chiang, C. L. Preparation, characterization and thermal properties of organic−inorganic composites involving epoxy and polyhedral oligomeric silsesquioxane (POSS). J. Polym. Res. 2010, 17, 673−681. (15) Laine, R. M. Nanobuilding blocks based on the [OSiO1.5]x (x = 6, 8, 10) octasilsesquioxanes. J. Mater. Chem. 2005, 15, 3725−3744. (16) Li, G. Z.; Wang, L. C.; Ni, H. L.; Pittman, C. U. Polyhedral oligomeric silsesquioxane (POSS) polymers and copolymers: A review. J. Inorg. Organomet. Polym. 2001, 11, 123−154. (17) Orme, C. J.; Klaehn, J. R.; Harrup, M. K.; Lash, R. P.; Stewart, F. F. Characterization of 2-(2-methoxyethoxy)ethanol-substituted phosphazene polymers using pervaporation, solubility parameters, and sorption studies. J. Appl. Polym. Sci. 2005, 97, 939−945. (18) Zhu, L.; Zhu, Y.; Pan, Y.; Huang, Y. W.; Huang, X. B.; Tang, X. Z. Fully crosslinked poly[cyclotriphosphazene-co-(4,4′-sulfonyldiphenol)] microspheres via precipitation polymerization and their superior thermal properties. Macromol. React. Eng. 2007, 1, 45−52. (19) Camino, G.; Martinasso, G.; Costa, L. Thermal degradation of pentaerythritol diphosphate, model compound for fire retardant intumescent systems. Part I. Overall thermal degradation. Polym. Degrad. Stab. 1990, 27, 285−296. (20) Su, C. H.; Chiu, Y. P.; Teng, C. C.; Chiang, C. L. Preparation, characterization and thermal properties of organic-inorganic composites involving epoxy and polyhedral oligomeric silsesquioxane (POSS). J. Polym. Res. 2010, 17, 673−681. (21) Modesti, M.; Lorenzetti, A.; Besco, S.; Hrelja, D.; Semenzato, S.; Bertani, R.; Michelin, R. A. Synergism between flame retardant and modified layered silicate on thermal stability and fire behaviour of polyurethane nanocomposite foams. Polym. Degrad. Stab. 2008, 93, 2166−2171. (22) Bourbigot, S.; Turf, T.; Bellayer, S.; Duquesne, S. Polyhedral oligomeric silsesquioxane as flame retardant for thermoplastic polyurethane. Polym. Degrad. Stab. 2009, 94, 1230−1237. (23) Rochery, M.; Lewandowski, M. High temperature resistant plastics. In Plastic flammability handbook, 3rd ed.; Troitzsch, J, Ed.; Hanser: Munich, Germany, 2004; pp 99−107.

Figure 8. FTIR spectra of char residues from pure TPIU, TPIU/12.4% PDMS, and the white compound after cone calorimeter tests.

stretching vibrations of Si−C bonds).41 No other absorbances, such as those of C−C stretching in aliphatic or aromatic structures, can be detected. This implies that the white product is mainly made up of −Si−O−Si- structures. In the case of TPIU/12.4%PDMS, as expected, the band belonging to polyaromatic carbon structures at 1620 cm−1 can be seen, as well as corresponding to the stretching vibration of Si−O−Si, which is consistent with the results of the EDXS analysis.

4. CONCLUSION PDMS has been incorporated into TPIU as a flame retardant by copolymerization to investigate its effect on the thermal and mechanical properties as well as flame retardancy The tensile strength and elongation at break for TPIU/PDMS have been measured as 17.5−29.77 MPa and 694−828%, respectively, and thus meet the requirements for engineering applications. TG tests have indicated that TPIU/PDMS samples did not show obvious changes in their T5% and Tmax values compared with pure TPIU. The only difference was that TPIU/PDMS gave slightly more residue than neat TPIU. The analyses of HRR, pHRR, and THR for TPIU/PDMS have shown that PDMS imparts good flame retardancy. The THR and p-HRR of TPIU/12.4%PDMS were found to be lower than those of pure TPIU by 8.0% and 26.3%, respectively. The condensed phases of TPIU and TPIU/PDMS have been investigated by SEM, FTIR, and EDXS. The results confirmed that there are many Si−O structures in the char residue, producing a physical protective layer on the surface of TPIU/PDMS that prevent them from burning further.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-6891-3456. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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