An analysis of photopolymerization kinetics and stress development in multifunctional acrylate coatings

Polymerization kinetics and stress buildup of multifunctional acrylic coati ngs have been studied simultaneously using a cantilever deflection method a nd real time one-bounce infrared reflectance spectroscopy. Cantilever defle ction due to stress buildup in the film is correlated with crosslink densit y in the polymerizing mass, Stress buildup was observed not to occur instan taneously after radiation was introduced but rather after a certain time in terval. The time delay between onset of polymerization and initiation of st ress indication is attributed to the introduction of crosslinks, leading to a gel state. The measured stress-conversion relationship can be predicted using an approach incorporating monomer reaction probability. Stress buildu p can vary considerably as a function of processing conditions such as film thickness, temperature at solidification point, and the type, concentratio n and absorptivity of photoinitiator, which govern the degree of crosslinki ng. Under certain processing conditions, an uneven distribution of the mono mer conversion in the film may severely alter the stress-film thickness rel ationship. in some cases the cantilever material anisotropy can be detected based on the cantilever deflection measured.