Photoacoustic spectroscopy applied in the monitoring of photocuring process in epoxy diacrylate resins: Positive effect

Abstract

The main goal of this study is to show that photoacoustic spectroscopy (PAS) can be applied in the monitoring of photocuring processes in epoxy diacrylate resins. The resin used was EBECRYL 3720‐TP25 with the photoinitiator Omnirad 808. Comparison between the conventional transmission and absorption photoacoustic spectra of the resin is presented. Amplitude and phase of the photoacoustic signal were used to monitor the photocuring. The wavelength of the modulated photocuring light and the concentration of the photoinitiator were the parameters varied in the monitoring. The novelty was found when the resin was irradiated by a continuous light, centered at the absorption peak of the photoinitiator and simultaneously with the modulated light. In the time window when this continuous light was on (off) the amplitude of the photoacoustic signal increased (decreased). This is called “positive effect,” as it has been observed when PAS was applied to study photosynthesis in leaves. A mechanism is proposed to correlate the positive effect with the oxygen inhibition effect during the photocuring of the epoxy diacrylate resin. The results suggest that the amplitude of the photoacoustic signal and the positive effect can be used as monitoring parameters in the studies of photocuring process in epoxy diacrylate resins.

Review of quantitative and qualitative methods for monitoring photopolymerization reactions

“…The limitation of PAS in quantitative analysis is the need to define a reference peak because the intensity of the photoacoustic signal depends on the depth of penetration and heat diffusion in the composition layer. 173,180…”

“…The intensity of the photoacoustic signal also depends on the amount of heat emitted during the photoinitiated polymerization process, which is related to the rate of polymerization, which allows the analysis of the process kinetics. 173,177,181 This technique has proved itself in the study of UV or electron beam (EB) cured acrylate coatings, where monomer or photoinitiator degradation was monitored, in the study of the kinetics and degree of polymerization of dental resins and epoxygraphite composites. PAS also provides information on the energy state and quantum efficiency of the excited state of the molecule and the lifetime of intermediate products in photochemical reactions.…”

“…Photoacoustic spectroscopy Photoacoustic spectroscopy (PAS), also called optoacoustic spectroscopy, classified as photothermal spectroscopy, is a non-invasive and non-destructive technique for monitoring changes in the thermal and optical properties of a material as a result of energy absorption. [173][174][175] The possibility of using any source of monochromatic radiation (e.g., IR, UV-VIS) makes it an attractive technique for investigating changes in a sample at the molecular level. During measurement, the modulated radiation intensity interacts with the absorbing…”

“…The limitation of PAS in quantitative analysis is the need to define a reference peak because the intensity of the photoacoustic signal depends on the depth of penetration and heat diffusion in the composition layer. 173,180 The PAS technique is particularly suitable for evaluating the cure along with the thickness profile of the coating, which is difficult to analyze by other techniques, in particular with regard to the thickness of paint and varnish samples. The intensity of the photoacoustic signal also depends on the amount of heat emitted during the photoinitiated polymerization process, which is related to the rate of polymerization, which allows the analysis of the process kinetics.…”

“…PAS also provides information on the energy state and quantum efficiency of the excited state of the molecule and the lifetime of intermediate products in photochemical reactions. 173,175,182…”