Objective Photopolymerization is a widely utilized technique across various fields due to its environmentally friendly and efficient characteristics. During the process of photopolymerization, photoinitiators (PIs) are essential in determining the excitation wavelength, polymerization type, polymerization rate, and the final material properties. However, conventional PIs are hindered by two photophysical phenomena, the inner filter effect, where PIs and their photodegradation byproducts concentrated at the material’s surface absorb most of the incident light, and Rayleigh scattering, where a significant portion of incident light is scattered on the surface of a photopolymerization system. These effects prevent subsurface PIs from receiving sufficient activation energy for photolysis, leading to reduced polymerization efficiency, shallow cure depths, and impaired material performance. Additionally, the small molecules generated from conventional PIs may migrate within the cured material, potentially leading to toxicity. To address this issue, low-migration, multiband responsive polymerizable PIs have been developed.
Methods A photopolymerizable methacrylate phenacyl phenothiazinium salt (Acry-P-PTh) was synthesized by incorporating acrylic ester groups. Its structure, photophysical properties, photochemical behavior, migration ratio, and thermal stability were investigated using ultraviolet-visible absorption (UV-Vis) spectroscopy, real-time infrared (RT-IR) spectroscopy, and differential scanning calorimetry (DSC).
Results and Discussion UV-Vis analysis revealed that Acry-P-PTh was responsive to ultraviolet(UV), visible, and near-infrared (NIR) light, with the peak absorption wavelength shifting to 520 nm, a 5 nm redshift compared to phenacyl phenothiazinium salt (P-PTh). Photopolymerization kinetics demonstrated that Acry-P-PTh effectively initiated both free radical and cationic polymerization. Under 365 nm, 405 nm, and 850 nm irradiation, Acry-P-PTh generated benzoyl radicals and Brønsted superacids through C-S bond homolysis. This dual initiation mechanism enabled the polymerization of both acrylate monomers, trimethylolpropane triacrylate (TMPTA), and cationic monomers, EPOX, achieving excellent initiation performance in hybrid systems compared to single-monomer resin systems. Specifically, under 365 nm light irradiation, the final free radical conversion rate exceeded 50%, and the final cationic conversion rate exceeded 40%; under 405 nm light irradiation, the final free radical conversion rate exceeded 45%, and the final cationic conversion rate exceeded 45%. Notably, under 850 nm light, EPOX cationic polymerization achieved a 40% conversion rate within 30 min. Migration studies confirmed that Acry-P-PTh exhibited low migration, with post-curing migration ratios lower than those of P-PTh, measuring 4. 9% for the free radical system and 3. 5% for the cationic system. Thermal stability analysis further revealed high thermal resistance, with polymerization temperatures for free radical and cationic polymerization being 233 ℃ and 78 ℃, respectively. These properties make Acry-P-PTh a promising material for various applications, such as food packaging and biomedicine manufacturing.
Conclusion The synthesized Acry-P-PTh demonstrates broadband absorption across the ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrum. Compared to P-PTh, Acry-P-PTh shows reduced low migration ratio. When mixed with Acry-P-PTh with TMPTA and EPOX monomers, the photocurable system is able to resist higher thermal polymerization temperatures, demonstrating excellent thermal stability. In conclusion, Acry-P-PTh, as an advanced photoinitiator, holds significant potential for practical applications.
Keywords:photocuring; phenothiazine; polymerizable photoinitiator; low migration; near-infrared
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