Thermal degradation of light-cured dimethacrylate resins: Part I. Isoconversional kinetic analysis

The thermal degradation of resins based on bis-phenol A glycidyl dimethacrylate (Bis-GMA), bis-phenol A ethoxylated dimethacrylate (Bis-EMA), urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) was investigated using thermogravimetric analysis. Four different heating rates were employed for its polymer in order to have adequate data to perform an isoconversional analysis. Activation energies were estimated as a function of the extent of degradation using a differential and an integral method. The observed differences were interpreted in terms of degradation mechanisms, which are highly depended on the structural characteristics of the corresponding resin. The existence of only a small number of defects in the network structure of Bis-GMA and Bis-EMA resins together with their stiff aromatic nuclei lead to the formation of a rigid network with high thermal stability and to decomposition in one-step. In contrast, inhomogeneities in the network structure of TEGDMA and UDMA resins, mainly due to the formation of primary cycles during polymerization, result in a two-step degradation mechanism at much lower temperatures. The hydrogen bonding ability of Bis-GMA and UDMA monomer units was also found to play an important role during thermal degradation of these resins.