Structure-property relationship and influences of phenolic compounds on the mechanical and thermomechanical properties of UV-cured acrylic resin networks

Trimethylolpropane triacrylate (TMPTA) resin was cured under UV radiation, with 2,2-dimethoxy-2phenylacetophenon (DMPA) and a mixture of benzophenone and methyl-diethanolamine (BP/MDEA) as initiators and three phenolic compounds as inhibitors, respectively. For each formulation, the curing enthalpy was measured by photocalorimetry and the modulus of elasticity (MOE) of cured resin films by thermomechanical analysis. The DMPA resulted in networks with higher rigidity that was slightly reduced by the addition of a phenolic compound; while the networks were more flexible and the effects in reducing the MOE by the phenols were much more pronounced when the BP/MDEA was used as an initiator. For the three phenolic compounds, their importance in reducing the MOE can be ranked in the order of eugenol > dimethoxyphenol > phenol. Three equations are proposed to model the structure property relationship of TMPTA networks. These models suggest that 1) the MOE increases with the crosslinking density, which is proportional to the conversion degree to a power of about 1/3, indicating that cyclization becomes progressively more important as the photopolymerization advances; 2) the decrease of the MOE with temperature is mainly due to relaxation of pending chains and chain segments, and the activation energy needed to overcome such an energy barrier is greater for a less crosslinked network than for a more crosslinked one; and 3) the overall contribution of the crosslinking and the interactions between pending chains (secondary forces) can be expressed by a simple equation in which the network rigidity (MOE) is a function of the degree of conversion, the activation energy and the temperature. (C) 2004 Wiley Periodicals, Inc.