Ultraviolet cure kinetics of a low Tg polyurethane acrylate network under varying light intensity and exposure time

We investigated the photopolymerization kinetics of a low glass transition temperature (Tg) urethane acrylate network as a model soft coating system. We used real-time Fourier transform infrared (RT-FTir) spectroscopy to measure the acrylate conversion and fast sampling photorheology to measure the developing mechanical properties during cure. Experiments were conducted using a 395 nm LED ultraviolet (UV) source at different incident intensities ranging from 5 to 200 mW/cm2 at 25 degrees C. We found that the maximum rate of conversion and rate of complex shear modulus development scaled with incident UV intensity (I0) to an exponent (beta) approximate to 0.53. This finding is consistent with the classical prediction of free radical photopolymerization that cure speed should increase to the 1/2 power of intensity when a bimolecular termination mechanism takes place during the reaction. A consequence of this is that the degree of conversion and complex shear modulus were not constant at a fixed energy dose (I0t) when intensity was varied illustrating that this system does not obey the exposure reciprocity rule. Instead we observed that the degree of cure and rheological properties were a universal function of I0 beta t. We also found that beta increased only slightly throughout the reaction reaching a value of 0.65 at 95% conversion. This result indicates that bimolecular termination was predominant throughout nearly the entire reaction due to a high degree of molecular mobility in this flexible network.