Performance analysis of acylphosphine oxides in photoinitiated polymerization

The initiation efficiency of acylphosphine-oxides has been evaluated in dif ferent types of UV-curable resins. Real-time infrared spectroscopy was used to monitor in situ the polymerization process in thin coatings exposed to UV-radiation. When compared to typical radical photoinitiators, acylphosphi ne oxides proved to be the most efficient ones with respect to both the pol ymerization rate and the cure extent, mainly because of their fast photolys is. Frontal polymerization proceeds readily with such photobleachable initi ators, thus allowing thick specimens to be deep-through cured by simple exp osure to sunlight. Because they absorb in the region of 350-400 nm, acylpho sphine oxides proved particularly well suited to induce the photopolymeriza tion of both pigmented systems and protective coatings used in exterior app lications, which contain UV-absorber-type light stabilizers. Superior perfo rmance was also achieved with these photoinitiators in the photocrosslinkin g of functionalized polymers, such as acrylated polyisoprene and polybutadi ene with pendent vinyl groups. The addition of small amounts of a trifuncti onal thiol was shown to drastically speed up the crosslinking polymerizatio n, insolubilization being achieved within a 0.3 s UV-exposure, even for ini tiator concentrations as low as 0.1 wt%. (C) 2001 Elsevier Science Ltd. All rights reserved.