Effects of microwave-assisted digestion on decomposition behavior of polymethyl methacrylate (PMMA)

Fourier transform infrared spectroscopy, scanning electron microscopy, and the weight-loss method were utilized to propose a novel kinetic model for PMMA decomposition by microwave digestion with HNO3 as a digestion reagent. By Newton's method, the experimental results of the remaining weight of PMMA is closely fitted by the model combined with zero-order and first-order kinetics, in which the former dominates the reaction at lower temperatures (423–443 K) and the latter at higher temperatures (≧453 K). Accordingly, a mechanism for PMMA decomposition is proposed in this study. Kinetic parameters of PMMA decomposition under 423–453 K including rate constants and the mass fractions (α) via main-chain scission were determined by this model. Activation energies of PMMA decomposition estimated by Arrhenius equation are 2.45–2.81 and 23.5–27.0 kcal mol−1, respectively, for the zero- and first-order reactions. The pre-exponential factors of the zero- and first-order reactions are 1.96 × 10−2 to 2.92 × 10−2 g min−1 and 1.31 × 1011 to 7.08 × 1012 min−1, respectively.Effect of HNO3 volume on PMMA decomposition was investigated at 423–473 K. The digestion efficiency apparently increases as HNO3 volume is more than 3 mL at these temperatures. At 473 K, the digestion efficiency has increased to 100% as HNO3 volume is ≥3 mL. The estimated α values of the decomposition with 2–7 mL of HNO3 at 423–473 K are increasing with HNO3 volume at 423 and 443 K, yet varying insignificantly at 453 and 473 K.