company news about Methyl radicals: An underestimated secret to high efficiency in UV curing

In discussions of UV curing formulations, the focus is usually on the absorption spectrum, dark hiding power, migration, and safety of photoinitiators, with few considering "what free radicals are generated" as the primary means of performance optimization. In the efficiency game of UV curing, the deciding factor may not be the newest light source or the most expensive initiator, but rather—an overlooked free radical. In fact, small-volume, highly reactive species like methyl (·CH₃) radicals may play an underestimated but crucial role in initiation rates, early chain growth kinetics, and curing efficiency under low-energy irradiation conditions.

To understand the importance of methyl radicals, we must first address one of the core challenges of UV curing: diffusion limitation. The UV curing process essentially involves the photoinitiator absorbing UV light energy and then breaking down to produce highly reactive primary radicals. These radicals act like "igniters," rapidly attacking monomers and oligomers (acrylates) in the formulation, initiating a chain polymerization reaction and instantly transforming the liquid material into a solid state. This process is very rapid in the early stages of the reaction. However, problems soon arise: a dramatic increase in viscosity: As the polymerization reaction proceeds, the system viscosity increases exponentially, quickly entering a "gel" state. The dilemma of "heavy infantry": The primary radicals produced by the breakdown of traditional photoinitiators (such as TPO, 1173, 184, etc.) are often relatively large and bulky molecules (e.g., benzoyl radicals).

Trommsdorff Effect: In high-viscosity systems, these massive, heavily armored free radicals are rapidly trapped, their translational and diffusion capabilities severely limited. They struggle to effectively seek out and attack unreacted monomers. This is the "efficiency ceiling" of UV curing: even though unreacted monomers remain in the system, the free radicals cannot reach them, resulting in a limited conversion rate, incomplete curing, and compromised performance. This problem is particularly pronounced in thick coatings, high-pigment/filler mixtures, or high-viscosity systems (such as UV adhesives).

Methyl radicals are often seen as secondary radicals, playing a supporting role. They can arise from: deep fragmentation of initiators (some primary radicals may further break down under light); and chain transfer reactions (highly reactive radicals may abstract hydrogen atoms from other components in the formulation, such as specific auxiliaries, solvents, or even monomers). Why are they underestimated? Because they are present in small quantities, have short lifespans, and are difficult to detect precisely using conventional analytical methods, their contribution to the overall reaction kinetics is significantly underestimated. The industry tends to attribute the credit to the "main attackers"—the primary radicals.

1. Extreme Mobility: Methyl radicals are extremely small. Their size and mass are far smaller than any photoinitiator fragment. This means that while those large primary radicals are "stuck in the mud" and unable to move, methyl radicals can still move relatively freely through the "gaps" of highly cross-linked polymer networks due to their extremely small size.

2. Extremely High Reactivity: Although small, methyl radicals have extremely high reactivity. They have a very strong ability to attack acrylate double bonds and initiate polymerization. Overall Effect: Enhancing the "Last 5%" of Conversion Rate. In the later stages of UV curing, when the reaction rate drops sharply due to diffusion limitations, the final properties of the system (such as hardness, chemical resistance, and low odor) depend precisely on this "last 5%" of conversion rate.

As UV technology advances into more challenging areas (such as high-occlusion inks, water-based UV, and biomedical 3D printing), the viscosity and complexity of the systems are increasing daily. "Diffusion limitation" will become an even more difficult hurdle to overcome than "initiation efficiency."