Ultraviolet (UV) curing of phosphated polyurethane-acrylic dispersions
Phosphate-containing polyurethane-acrylic dispersions were synthesised for UV curing studies. The effects of light intensity, substrate-dependent temperature increase, soft-segment content and water on conversion were investigated. The effect of the light intensity on conversion was twofold. At first, conversion increased with light intensity. This was attributed to the inability of shrinkage to keep pace with the polymerisation and crosslinking, resulting in the creation of free volume, thereby facilitating reaction and enhanced conversion. At higher intensities, conversion was found to be reversed or, at least, it remained constant, owing to increasing radical-radical termination reactions. Phosphated-polyurethane coatings with high soft-segment content show improved conversion with exposure time. This was found to be related to the chain mobility, caused by the low glass transition temperature of the soft segment. The effect of water on conversion was also twofold. On the one hand, water had a plasticising effect on the UV curing and the polymerisation rate was fast. On the other hand, the gel content was found to be lower when films were cured before the evaporation of water.