Investigation of the thermal degradation of rigid urethane foams by means of TGA, DTA, infrared, and other techniques indicated that rigid foams prepared from a polyoxypropyl ene ether and a polymeric diphenylmethane type polyisocyanate began to decompose at ap proximately 210°C. Loss of weight occurred in two stages of approximately 50% each, oc curring at approximately 250-350°C and 400- 600°C. A similar pattern of decomposition was found for urethane foams based on a chlorinated polyester and the same polyiso cyanate. Addition of a reactive organophos phorous flame retardant to the polyoxypropyl ene ether foam lowered the temperature of initial decomposition to 150-170°C. The weight fraction of char formed at tempera tures above 400°C was increased, however.
SynopsisA series of model polyurethanw and polyureas, a polyamide, and a polyimide were prepared by reacting 4,4'-diphenylmethane diisocyanate or polyisocyanates having similar polybenzyl structures with aliphatic or aromatic coreactants. Thermogravimetric analyses indicated that the flammability of the polymers was related to the formation of volatile flammable products during early stages of decomposition. Determinations of the heat evolved during differential thermal analyses and of the thermodynamic heats of combustion suggested that the extent and rate of reaction were among the important factors governing flame propagation. Flame-resistant polymers were prepared by use of structural elements which were thermally stable and nonvolatile or which formed nonflammable decomposition products.
The acceptance of urethan foam as a commercial product has hinged to an important extent on its permanence as a foam under all conditions of actual end use. Customer experience with commercial flexible urethan foams has confirmed the satisfactory service life of these materials. However, in the development of this industry, it was necessary to be able to predict the performance of experimental systems which later were to become fully commercial. Most often, this prediction had to be made very early, long before the materials were allowed to be put into end service. The common way to do this is by accelerated testing, wherein the time axis is condensed by stepping up the aging process well beyond any normal conditions. Considerable time and expense are saved in this kind of test; however, there is always the question that such an accelerated step-up of test conditions is not representative of actual use, and/or causes reactions in the sample which do not occur in normal use. It is important that these questionable features be reconciled before the intercomparability of accelerated and shelf-life testing is accepted.
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