An amine synergist solves the most common complaint in UV curing: a coating that is hard underneath but tacky on top.
That tack is oxygen inhibition. Atmospheric oxygen quenches free radicals at the air-film interface faster than polymerisation can propagate, leaving an undercured surface layer. The tertiary amine functionality in an amine synergist acts as a hydrogen donor — it scavenges oxygen and regenerates radicals precisely where the problem is.
The practical payoff is significant: the same formulation cures at higher line speed, at lower photoinitiator loading, or under lower lamp power. Amine synergists are the standard chemical alternative to nitrogen inerting, and they matter more in LED UV systems, where lower peak intensity makes oxygen inhibition more pronounced.
The caveat is yellowing. Amine functionality contributes to it, particularly at higher loadings and over time. In pigmented systems this is usually invisible; in clear coats and whites it is worth evaluating at your intended loading.