On demand NIR activated photopolyaddition reactions†
A new approach based on a photoactivation process is proposed for the polyaddition reaction of epoxy-based systems i.e. both for epoxy/amine and epoxy/anhydride processes that are characterized by huge applications in polymers (e.g. composite and adhesive fields). Outstandingly, for the first time, near-infrared (NIR) absorbing dyes are used here for this photoactivated approach upon NIR light, providing safe irradiation conditions. Indeed, the use of NIR light remains a huge challenge i.e. the photon energy is extremely low to initiate photochemical processes. But a main advantage of the NIR light for polymer synthesis is a deeper light penetration into the material. A photoinitiating system is proposed comprising a NIR dye combined with an iodonium salt for the epoxy/amine photopolyaddition. The new proposed approach is highly robust and can be used for a broad selection of amines and epoxides. In the case of epoxy/anhydride photopolyaddition, the presence of an accelerator is required to improve the reaction. Additionally, the use of NIR light has a huge effect on the kinetics of reaction: the polymerization time is greatly improved compared to the classical reaction (e.g. in 3 hours without light vs. a few seconds or minutes upon NIR light irradiation, at room temperature). Two irradiation wavelengths are presented here: 785 nm and 1064 nm with two selected NIR dyes. The different systems presented in this paper exhibit fast polymerizations associated with full conversions. Real-time Fourier transform infrared spectroscopy is used to follow the polymerization kinetics. The applications in adhesives as well as composites are presented to highlight the interest of these NIR photoactivated processes. For the first time, on demand processes can be proposed for epoxy–amine and epoxy–anhydride polyadditions i.e. these latter systems can be polymerized within a few seconds/minutes at room temperature upon NIR stimulus vs. more than 3 h without light.