Meaning
Compounds belonging to the class of Norrish Type I photoinitiators initiate polymerization when exposed to ultraviolet radiation. Within radiation-curable polymer formulations, acylphosphine oxides are employed to achieve deep through-cure in thick sections and heavily pigmented systems. The molecular structure of these compounds undergoes cleavage upon excitation to generate free radicals that trigger the polymerization of acrylic monomers.
The reaction terminates once the radiation source is removed or the active radical species are quenched by oxygen, defining the physical boundary of the cured region.
Absorption Profile
Long-wavelength radiation in the near-ultraviolet region triggers the activation of these photoinitiators. While conventional ketone-based photoinitiators absorb primarily in the short-wave ultraviolet region, acylphosphine oxides extend their absorption bands above three hundred and eighty nanometers. This spectral shift permits curing through thick layers of resin.
Highly loaded titanium dioxide coatings rely on this characteristic to allow light to penetrate past the surface.
Curing Response
Rapid radical generation occurs when the formulation receives the correct dose of energy. Because acylphosphine oxides undergo photo-bleaching during the reaction, the cured film becomes clearer as the initiator decomposes. This optical change prevents yellowing in clear topcoats.
Run speeds in continuous extrusion lines can be adjusted to match this transition time.
Thermal Stability
Storage conditions require careful temperature regulation to prevent premature cross-linking. These initiators can degrade when exposed to prolonged heat before ultraviolet exposure. High compounding temperatures must be avoided.
