Thermal characteristics of thermosets formed by free radical photocuring

Abstract

Thermal analysis (TA) is useful for studying the reaction and thermal properties of free radical cured photopolymers. Starting with reactive liquid monomers, the photocuring reactions are initiated by the interaction of UV radiation with an added photoinitiator to form free radicals. The monomers generally are acrylates or methacrylates. Some of the important characteristics of these systems that can be illustrated by TA methods are: 1) the polymers are heterogeneous with more than one phase present even when only one monomer is involved; 2) because of this heterogeneity they have unusually broad glass transitions; 3) the degree of conversion achieved in many UV cured systems is in the 60–80% range, so that considerable residual monomer is often present; 4) partially cured, vitrified samples contain trapped free radicals that will continue to react slowly; 5) when a partially cured photopolymer is heated above its current T _g a reaction exotherm is evident.

Some other aspects of photocuring are not easily disclosed by thermal analysis. Studying fast photoreactions by DSC may not give valid kinetic data because the reaction occurs faster than the DSC instrument time constant. Optical methods (IR, Raman) can be used to advantage in such cases. While photocuring resins are usually exposed to light at ambient temperatures, the local temperature in the resin will be quite elevated, resulting in T _g values that are much higher than ambient. This has been demonstrated by thermal modeling of the reaction and verified by infrared thermography.