Photoinitiator 819 (Bis-Acylphosphine Oxide, White Pigment Systems)

TiO2 white pigment scatters UV light intensely, reducing intensity below the coating surface exponentially (Beer-Lambert). BAPO photoinitiators have extended absorption tailing into the 380–420 nm visible range, where TiO2 scattering is lower than at 254–320 nm. This long-wavelength absorption enables photoinitiator molecules deeper in the film to generate radicals. Additionally, BAPO produces two sets of reactive radicals per photon (biradical cleavage), making it twice as efficient per molecule as mono-functional photoinitiators. Use at 2–4% with a surface-active co-initiator (thioxanthone or amine) for best results.