Mechanisms controlling the degradation of poly(methyl methacrylate) prior to piloted ignition

Degradation of type G poly(methyl methacrylate) (PMMA) (Rohm and Haas) plates was studied prior to piloted ignition. The onset of pyrolysis was estimated by means of an IR camera with an MMA filter, a small load cell, thermocouples, and flow visualization. Onset of gasification due to material pyrolysis occurs significantly earlier than piloted ignition. Therefore, models of standard test methods such as ASTM-E-1321 must incorporate preignition gasification to properly model the ignition process. To better establish the mechanisms controlling the degradation process, controlled mass loss tests were conducted with shavings from the PMMA plates. Tests in nitrogen and oxygenated atmospheres were conducted with thermogravimetric analysis and evolved gas measurements using a quadrupole mass spectrometer. Results indicated a one-step degradation process in pure N-2, while a two-step reaction could be observed in the presence of O-2. As the O-2 fraction increased, the onset of degradation shifted to lower temperatures, but the second step correlated well with that of N-2 degradation. The evolved gas measurements suggest that both reactions were limited by mass transport, with the first reaction limited by O-2 diffusion into the sample. Mass transport limitations were identified by abrupt changes in the slope of the production rate of evolved gases, and this indicates the importance of coupling thermogravimetric analyzer measurements with evolved gas analysis for detecting mass transport limitations. Implications of these results on interpreting material flammability tests are discussed in this paper.