Abstract
A kinetic analysis on pyrolysis of Polymethylmethacrylate (PMMA) under fire conditions was performed by the thermal gravimetric analysis (TG) method. Heating rates of PMMA samples adopted in this study to simulate fire conditions were 10, 20, 30, 40 and 50 °C/min which were relatively high heating rates in comparison with those adopted in usual studies for examining thermal properties of polymers. The amount of a sample (ca. 5 mg) adopted in each run was restricted to be as small as possible to minimize temperature difference in the sample.
Two peaks were found in each weight loss rate (DTG) curve. These peaks correspond to two different reactions. Initiation temperatures of these reactions show that the first stage in pyrolysis reaction of PMMA is end-initiated and the second is initiated by random chain scissions. End-initiated pyrolysis reaction takes place for molecules with unsaturated chain-ends. Experimentally confirmed fraction of molecules with unsaturated chain-ends were about 0.22 and 0.25-0.28 for Acrylite AR (Mitsubishi Rayon Co.) and Plexiglas G (Rohm and Haas Co.), respectively.
Activation energies for each reaction under different conditions and different samples were estimated from experimentally obtained TG and DTG curves. Estimated activation energies for pyrolysis reaction of Acrylite AR were 51 kcal/mol in N2 and 44 kcal/mol in air at the first peak and 22 kcal/mol in N2 and 18 kcal/mol in air at the second peak, and those of Plexiglas G were 39 kcal/mol in N2 and 40 kcal/mol in air at the first peak and 20 kcal/mol in N2 and 21 kcal/mol in air at the second peak. Activation energies for pyrolysis reaction of Acrylite AR depend on its atmosphere. On the other hand, Plexiglas G were nearly independent of its atmosphere. For either Acrylite AR or Plexiglas G, the activation energy at the first peak was almost twice of that at the second peak.
