Electron-beam initiated polymerization of acrylate compositions, 2. Simulation of thermal effects in thin films

Abstract

The thermal effects taking place during the electron beam-induced polymerization of acrylate type formulations were numerically simulated on the basis of the general heat equation applied to a one-dimensional system, The nature, the dimensions and the environment of the polymerizing medium were defined for representing the actual conditions of kinetic experiments performed with a 175 kV laboratory accelerator and FTIR monitoring. The modeled system was constituted of a polymerizable composition coated onto a NaCl plate, initially at 20°C in gaseous nitrogen at the same constant temperature, with or without a PET film covering the reactive layer, Polymerization profiles describing the progress of the reaction as a function of dose were modeled on a phenomenological basis from actual data obtained by discontinuous FTIR monitoring of typical epoxy acrylate or polyurethane acrylate compositions. The influence of the reactive layer thickness (10/100 μm), dose rate (10-110 kGy.s -1 ), maximum polymerization heat (200-400 J.g -1 ) on the temperature-time variations was examined for continuous irradiation. In spite of the relatively small thickness of the reactive layer, significant temperature rise is simulated when heat production is large and fast compared to energy dissipation at the reactive layer boundaries. The obtained data substantiate the fact that upon fractionated EB-treatment with small dose increments (down to 0.6 kGy per pass) at low dose rate (down to 10 kGy.s -1 ) the heat release can be considered weak and without noticeable influence on the conversion data processed for a detailed kinetic analysis. For example, a maximal temperature rise of 6°C was calculated for a fractionated irradiation of 2 kGy increments at 19 kGy.s -1 applied to a polymerizable formulation releasing a maximum enthalpy of 300 J.g -1 .