Near-Infrared Light Induced Thiol-Epoxy Step Photopolymerization

Epoxy resin is widely used in coatings and adhesives for its excellent mechanical performance, heat resistance and bonding property. Thiol-epoxy “click” reaction has become one of the hotspots in the field of photocuring due to its mild and efficient reaction conditions. As a latent photoinitiator, the photobase generator improves the storage stability of the formulation when it catalyzes the thiol-epoxy reaction. However, due to the low dose rate and poor solubility of the photobase generator, thiol-epoxy reaction rate is slow under light conditions. To solve this problem, a novel kind of near-infrared (NIR) light induced thiol-epoxy photopolymerization system was established in this paper using the up-conversion particles (UCPs) as upconversion material, 980 nm NIR light as irradiation source. The up-conversion material assisted near infrared photopolymerization (UCAP) technology was used to study the NIR light-induced thiol-epoxy step photopolymerization, and the effect of photothermal synergistic effect of NIR light on thiol-epoxy reaction catalyzed by photobase generator was explored. The temperature change of the polymerization system was measured by infrared thermal imager, the reaction rate of sulfhydryl group and epoxy group under NIR irradiation was measured by real-time infrared spectroscopy, and the molecular weight of the polymer was measured by gel permeation chromatography (GPC). Results show that under the irradiation of NIR light (980 nm), the photobase generator absorbs the light emitted by the UCPs to generate a strong base which catalyze the step photopolymerization of thiol and epoxy monomers. Under the photothermal synergistic effect of NIR, the typical step polymerization of thiol-epoxy system catalyzed by photobase generator takes place. Kinetics and molecular weight studies show that the polymerization exhibits typical step growth characteristics, and proves the time controllability of the polymerization. The results are expected to promote the application innovation of photoalkali-producing material system in the fields of photocuring 3D printing and adhesives.