company news about A Breakthrough in UV Curing: How to Achieve Both Deep Curing and Low Yellowing via Photobleaching Mechanisms

In the field of UV curing, we often face an "impossible triangle": deep curing, high color density (or high transparency), and low yellowing.Traditional UV formulations are like a student who excels in one area but struggles in another. Seeking deep curing? In thick coatings or high-pigment systems (such as titanium dioxide and carbon black), UV light is almost completely absorbed by the initiator and pigments as soon as it enters the surface, resulting in a "dry surface, not a dry base." Seeking low yellowing? Many highly efficient initiators (especially amine synergists or certain ketones) leave behind chromophore "residues" after the reaction, instantly turning the originally crystal-clear coating yellowed and faded. We always seem to have to compromise on performance. It wasn't until the advent of photobleaching initiators that a brilliant solution to this dilemma was offered—a solution that kills two birds with one stone.

Traditional photoinitiators (PIs) are like trees in a forest. After absorbing UV light (nutrients), they break down to produce free radicals (soldiers), but their "remains" (decomposition products) are still trees, even more dense, blocking subsequent light. This is the "internal friction effect" or "shielding effect." The surface PI absorbs a large amount of light energy, causing an exponential decrease in UV light intensity, preventing it from penetrating deep into the coating.

In paints, pigment particles further scatter and absorb light, exacerbating the situation. Photobleaching initiators (PBIs), especially the acylphosphine oxide family (such as TPO, TPO-L, BAPO, etc.), have a completely different mechanism. When a PBI molecule absorbs photons and breaks down, the UV absorption rate of its resulting free radical fragments is significantly lower than that of the original PI molecule at the original excitation wavelength. In other words, during the reaction, PBIs "sacrifice themselves," transforming themselves from a "light barrier" into a "light channel."

As the surface cures, PBI continuously degrades and bleachs, increasing the coating's "transparency" to UV light. Subsequent UV light can then penetrate deeply, achieving "penetrating" curing. This is the fundamental reason why they perform exceptionally well in thick-film and color paint systems.

Yellowing of coatings is largely due to unwanted absorption of initiator degradation products in the visible light region (especially the blue-violet region), resulting in a complementary color—yellow. The brilliance of photobleaching initiators lies in the fact that their degradation products not only exhibit reduced absorption in the UV region but also extremely low absorption in the visible light region.

They are "clean" initiators. Take the classic TPO (2,4,6-trimethylbenzoyl-diphenylphosphine oxide) as an example; its degradation fragments themselves are low-chromophores, producing almost no color.This makes them ideal for manufacturing high-transparency varnishes, white coatings, and light-colored inks. Therefore, photobleaching achieves two goals at once: for UV light bleaching: it opens physical pathways, enabling deep curing; for visible light bleaching: it eliminates chromophore residues, resolving the yellowing problem.