Fire and Polymers V - American Chemical Society

Chapter 23

Effect of Ambient Oxygen Concentration on Thermal Decomposition of Polyurethanes Based on MDI and PMDI 1

Kenneth L. Erickson and John Oelfke

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Downloaded by COLUMBIA UNIV on June 26, 2012 | http://pubs.acs.org Publication Date: April 27, 2009 | doi: 10.1021/bk-2009-1013.ch023

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Sandia National Laboratories*, Albuquerque, NM 87185 O R I O N International Technologies, Inc., Albuquerque, NM 87106

The presence of O accelerated and increased char formation during the decomposition of polyurethanes based on M D I and PMDI. This appeared to occur by exothermic reaction between O and polymer. The extent of char formation and exothermic heat release depended strongly on the ambient O concentration. The char appeared to be chemically similar to the char that formed during decomposition in N . The increased char formation appeared to result from increased H abstraction due to reaction of O with polymer. Ultimate reaction of the char to gaseous products depended strongly on the O concentration and temperature. 2

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NOTE: Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC0494AL85000. © 2009 American Chemical Society

In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction Thermal decomposition of organic polymers is important in many scientific and engineering applications. Thermal decomposition of polymers has been studied from a scientific perspective to gain insight into molecular structure, and from an engineering perspective to determine how specific materials behave at elevated temperatures. A particular area of interest is the behavior of polymer materials in fire environments. Organic polymer materials are used frequently in structures and transportation systems. In an oxidizing environment, polymer materials can provide the fuel that propagates a fire. In a non-oxidizing environment, polymer materials can be damaged catastrophically as a result of an incident heat flux. Modeling the response of such structures and systems in fire environments has important applications in safety and vulnerability analyses. In either oxidizing or non-oxidizing environments, the thermal decomposition chemistry of the organic polymer materials is often an important factor. Specific applications include predicting fluxes of volatile species to a flame region, predicting the extent of thermally induced mechanical damage in structural composite materials, predicting pressure growth in closed containers, modeling liquefaction and flow of decomposing materials (particularly foams), determining the toxicity of evolved gases and vapors, and characterizing char formation. To provide input to numerical models for hazard and vulnerability analyses involving polymer materials in inert environments subjected to external heat fluxes, thermal decomposition of several organic polymers in N atmospheres was investigated previously using TGA-FTIR, pyrolysis-GC-FTIR, D S C , and infrared microprobe analysis of solid residues (1-3). Results were used to determine decomposition mechanisms and to develop rate expressions for use in numerical simulations. Materials studied included poly(methyl methacrylate) ( P M M A ) , poly(diallyl phthalate) (DAP), polyvinyl chloride) (PVC), polycarbonate (PC), poly(phenylene sulphide) (PPS), and two rigid polyurethanes. One polyurethane (referred to below as M D I RPU) was based on methylene-4,4'diphenyl diisocyanate (MDI) and polyhydroxy polyethers. The other polyurethane (referred to below as P M D I RPU) was based on polymeric diisocyanate (PMDI) and polyhydroxy polyethers. T G A results and FTIR analysis of gaseous decomposition products from M D I R P U and PMDI R P U in N atmospheres indicated two decomposition steps: (1) reaction at the urethane moieties to form isocyanates and alcohols or reaction to form anilines and C 0 , either directly or by secondary reaction, and (2) fragmentation and secondary reaction of the polyether moieties. These results were consistent with results from previous studies (4, 5). Thermal decomposition of the same polymers in air atmospheres was subsequently investigated (6). To varying degrees, the presence of 0 appeared to alter the decomposition mechanisms in all of the materials studied. In constant-heating-rate T G A experiments, the initial stage of decomposition of 2

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389 each polymer in air generally proceeded similarly to the initial stage of decomposition in N . Small deviations between decomposition in N and decomposition in air were noted with P M M A and D A P . However, with the exceptions of P M M A and P V C , decomposition of each polymer in air involved an intermediate stage in which 0 appeared to react with the decomposing condensed phase to form a more thermally stable product or char. That product then decomposed slowly until the temperature increased sufficiently to substantially increase the rate of reaction between the condensed phase and 0 . Samples were ultimately consumed by reaction with 0 to form H 0 , C O , or C 0 . This behavior was most pronounced with polymers that formed a substantial amount of carbonaceous char during decomposition in N atmospheres. In the case of both polyurethanes, complete consumption of the condensed-phase in air did not occur until temperatures of 700° C or higher. Both M D I and PMDI based polyurethanes are frequently of interest in hazard and vulnerability analyses. Those analyses can involve a variety of scenarios in which 0 may or may not be readily available to interact with the thermally decomposing polymers. Formation of an intermediate char could provide resistance to heat transfer to un-reacted polymer, could reduce the rate of pressurization in sealed containers, and could reduce liquefaction and flow. To gain insight into the mechanism for formation of the intermediate char, the interaction of 0 with M D I and PMDI based polyurethanes during thermal decomposition was investigated using several methods. Experiments were done with samples in atmospheres of N , air, or intermediate mixtures of 0 in N . Chemical analyses of gas-phase and condensed-phase decomposition products were done using multiple techniques. 2

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Experimental The experimental techniques used to examine thermal decomposition of the polymers have been discussed in detail previously (1-3, 6). The experimental data that will be discussed were obtained as described below. Thermal gravimetric analysis (TGA) and simultaneous evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) were used to obtain rate data, to examine gas-phase decomposition products as a function of time, and to obtain condensed-phase samples for postmortem analyses. The furnace purge gas exhaust from the T G A (TA Instruments Model 2950) was connected by a heated stainless steel transfer line to the T G A interface module of the FTIR spectrometer (Nicolet Magna 750). The purge gas was U H P N , air, or intermediate mixtures of UHP 0 in U H P N flowing at 50 to 60 ml/min. The transfer line temperature was set at 300° C. The TGA-FTIR interface module in the auxiliary experiment compartment of the FTIR spectrometer also was set at 2

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390 300° C. The spectrometer provided concurrent chemical analysis of evolved gases. Multiple spectra were collected and averaged over consecutive 30-second intervals to provide time-averaged spectra as a function of time. Both differential scanning calorimetry (DSC) and simultaneous D S C - T G A (SDT) were used to examine enthalpy changes during decomposition. A T A Instruments model 2920 DSC was used for temperature ranges from ambient to 600° C. The purge gas was U H P N or air flowing at 60 ml/min. A T A Instruments Q600 SDT was used for temperature ranges from 200° C to 1200° C. The purge gas was U H P N , air, or intermediate mixtures of U H P 0 in U H P N flowing at 100 ml/min. Partially decomposed samples of M D I R P U , as well as P M D I R P U , were prepared in the T G A by lowering the furnace and quenching the samples at preselected values of m/m from 90% to 10%. Samples were prepared using N and air as purge gases. During postmortem analyses, condensed-phase FTIR spectra were obtained using a Thermo Scientific 6700 FTIR spectrometer equipped with a Thermo Scientific "Smart Orbit" A T R accessory with a diamond objective. Elemental analyses for total C, H , and Ν were obtained using a Perkin Elmer 2400 Elemental Analyzer. Certified acetanilide was used as a standard. Results from experiments with M D I R P U and P M D I R P U were similar. Results for M D I R P U are discussed first in detail. Results for P M D I R P U are then summarized, and small differences relative to M D I results are noted. 2


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Results: MDI Based Polyurethane Figure 1 shows residual mass ratio m/mo (ratio of instantaneous sample mass to initial sample mass) versus temperature from replicate T G A experiments that were done with M D I R P U samples heated at 20° C/min. Results are shown from experiments using N , 1% 0 (99% N ) , 2% 0 (98% N ) , 5% 0 (95% N ) , or air as purge gas. Figure 2 shows rate of mass loss corresponding to one of the T G A experiments in Figure 1 done with N purge gas, one of the experiments with 2% 0 (98% N ) purge gas, and one of the experiments with air purge gas. Relative to experiments using N purge gas, the presence of 0 had little effect on initial mass loss. In the temperature range between 250° C and 320° C, the curves representing residual mass versus temperature (Figure 1) essentially overlaid each other for each of the purge gases used. This is further illustrated in Figure 2, where the first peak in each of the rate-of-mass-loss curves occurred between about 320° C and 330° C. With N as the purge gas, the rate of mass loss steadily increased until m/m =54% (T=330° C). The rate of mass loss then gradually decreased until m/m =45% (T= 337° C). Between m/m =45% and m/m =40% (T=346° C), the 2

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Figure 1. TGA results from MDI RPU and several purge gases.

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Figure 2. Rate of mass loss for selected curves shown in Figure 1.

rate of mass loss decreased abruptly. Below m/m =40%, the rate of mass loss continued to steadily decrease and became small at m/m =20% (T=547° C). As the temperature increased above 320 °C (m/m =70%), decomposition in the presence of 0 deviated substantially from decomposition in N . The decomposing condensed phase appeared to form a more thermally stable product, or char, having a much larger residual mass fraction than the char that formed in experiments using N . The mass fraction of the char that formed increased with the 0 concentration in the purge gas. A concentration of only 1% 0 approximately doubled the amount of char that formed relative to the char that formed with N purge gas. In air, the rate of mass loss began to gradually deviate from that in N between m/m =70% (T=318 °C) and m/m =60% (T=332 °C). Between m/m =60% and m/m =54% (T=351 °C), the rate of mass loss decreased abruptly. Between m/m =54% and m/m =50% (T=435 °C), the rate of mass loss was slow. Between m/m =50% and m/m =40% (T=542 °C), the rate of mass loss gradually increased with temperature. For m/m less than 40%, the rate of 0

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393 mass loss increased significantly. These results are consistent with previous work reported by Pielichowski, et al. (7) who examined thermal degradation of polyurethanes based on M D I and several polyhydroxy polyethers. During their T G A experiments using air purge gas and samples that were heated at 10° C/min to 500 °C, about 50% to 60% of the original sample mass remained at 500 °C. Ultimate reaction of the char to gaseous products was also highly dependent on 0 concentration in the purge gas, as well as temperature. As the 0 concentration in the purge gas increased, the temperature range over which the char decomposed decreased and shifted to lower temperatures. This is illustrated in Figure 2. The second peak and third peaks in the curves for 2% 0 and for air correspond to reaction of char to gaseous products. The second and third peaks in the curve for 2% 0 are broad and occur at T=695 °C (m/mo=23%) and at T=938 °C (m/m =5%), respectively. In the curve for air, however, the second and third peaks are sharper, larger, and occur at T=567° C (m/mo=33%) and T=769 °C (m/mo=6%), respectively. Figure 3 shows heat flow versus temperature from DSC and SDT experiments using N , 2% 0 (98% N ) , or air for purge gas. Experiments were done with open pans. DSC results obtained with N showed primarily endothermic behavior during decomposition. A slight exothermic peak occurred at about 340 °C. The SDT results obtained using 2% 0 or air for purge gas showed three exothermic peaks during decomposition. With 2% 0 purge gas, the two most prominent peaks were at 337 °C and 674 °C, which were similar to the temperatures corresponding to the peaks in the rate of mass loss curves in Figure 2. The exothermic energy associated with decomposition of the char was greater than the energy associated with the preceding decomposition of the polymer. With air for purge gas, the two most prominent peaks were at about 337 °C and 572 °C, which also were similar to the temperatures corresponding to the peaks in the rate of mass loss curves in Figure 2. In this case, the exothermic energy associated with decomposition of the char was much greater than the energy associated with the preceding decomposition of the polymer. The D S C and SDT results indicated that although the presence of 0 had little effect on the initial mass loss in the T G A experiments, the presence of 0 resulted in exothermic behavior throughout decomposition. The onset of exothermic behavior coincided with initial mass loss, and the exothermic heat flow increased during decomposition of the char. Furthermore, the exothermic heat flow that occurred throughout decomposition increased with the 0 concentration in the purge gas. Gas-phase FTIR spectra obtained during a T G A experiment using N purge are shown in Figure 4 and correspond to m/m =80% (T=305 °C), m/m =60% (T=326 °C), and m/mo=40% (T=346 °C). Spectra obtained using air purge are shown in Figure 5 and correspond to m/m =80% (T=304 °C), m/m =60% (T=332 °C), and m/mo=40% (T=542 °C). 2

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Fire and Polymers V - American Chemical Society

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