Open Access

Fereshteh Habibollahi

• The high rate of photoinitiation is the reason for ultrafast curing in photopolymerization. Investigation about the mechanism and kinetics of this decisive step are important for optimizing the parameters of the photopolymerization and their effects. In this work, the initiation kinetics of BAPO and TPO (type I) photoinitiators were quantified as a function of thickness d, light intensity [I]_0, irradiation time t, temperature T, initiator concentration C0 type of monomer and air presence. We quantified the radical formation rate for defined thicknesses with two newly developed methods: A) time-dependent light absorption of photoinitiator decay via radiometer measurements with UV-pad radiometer. B) By observing polymerization behavior and trapping of radicals with defined concentrations of TEMPO as inhibitor for different conditions of intensities and pulse times and thicknesses via ATR-FTIR spectroscopy measurements. It was shown by independent NMR experiments that TEMPO veryThe high rate of photoinitiation is the reason for ultrafast curing in photopolymerization. Investigation about the mechanism and kinetics of this decisive step are important for optimizing the parameters of the photopolymerization and their effects. In this work, the initiation kinetics of BAPO and TPO (type I) photoinitiators were quantified as a function of thickness d, light intensity [I]_0, irradiation time t, temperature T, initiator concentration C0 type of monomer and air presence. We quantified the radical formation rate for defined thicknesses with two newly developed methods: A) time-dependent light absorption of photoinitiator decay via radiometer measurements with UV-pad radiometer. B) By observing polymerization behavior and trapping of radicals with defined concentrations of TEMPO as inhibitor for different conditions of intensities and pulse times and thicknesses via ATR-FTIR spectroscopy measurements. It was shown by independent NMR experiments that TEMPO very effectively quenches the radical species formed in the course of the photo-induced initiation of both of BAPO and TPO. Finding out the necessary amount for complete quenching of just first pulse precisely, the initiation step will be just evaluated and the propagation and termination steps will not be involved.The results of both independent methods agreed very well with each other, and the data were adjusted to the initiator-specific models using POLYMATH as a fitting program for the non-linear optimization. The experimental results from initiation step quenching with TEMPO are in quite good accordance with the kinetic parameters, found from radiometer results via photoinitiator decay. On doing so for the first time we quantified the dissociation rate constant Kα of α-scission for the extinction coefficient ε_365 of the initiators at 365 nm wavelength. With these parameters in hands, the complete kinetic behavior of these two initiators for different experimental conditions can be precalculated. The extinction coefficients, the effective rate constant and the number of radicals per molecule of BAPO and TPO are different which relates to the chemical structure and chromophore groups of photoinitiator. Theoretically and also experimentally BAPO releases 4 radicals in two photodissociation steps and TPO releases 2 radicals per molecule in just one step. The measured extinction coefficient at wavelength of 365 nm of the LED lamp for BAPO photoinitiator is 0.008 ± (3,19 E-4) and for TPO is 0,005109 ± (5,14 E-4) [mol%-1. µm-1] respectively, these values are logical because BAPO has two active chromophore groups and TPO has just one, which increases the absorption of BAPO compared to TPO. The evaluated rate constant for decomposition of BAPO is 11*10-6 ± (4,43 E-7) and for TPO is 9.13*10-6 ± (6,86 E-7) [(Mw /cm2)-1.ms-1]; their rate constants have quite close values since the type of broken bonds in BAPO and TPO are the same. For the evaluation of photoinitiation kinetics in different acrylate media, the radiometric measurements showed that the initiator decay rate is quite similar however the polymerization behavior and conversion degree is different. The monomers can have two parallel competing effects due to their reactivity level and also due to their inhibiting effect. An unmentioned inhibiting effect of the metacrylates on the initiation was found in this study. Expectedly, it has been observed that temperature does not play a role in photoinitiation but rather it influences the propagation step. The evaluation of several parameters and application of the final derived equations very precisely defines how much photoinitiator is necessary for any desired percentage of curing degree, and also it can be predicted for different thicknesses which initiator has a superior performance, opening up the possibility to optimize the irradiation time and photoinitiator concentration. The presented procedure in this research to quantify the initiation kinetics of BAPO and TPO photoinitiators can in principle be transferred for initiation rate evaluation of any other UV-triggered initiators in the future.…