An oligomer sits between a monomer and a polymer in molecular weight, carrying reactive acrylate groups at its ends or along its chain. In a UV formulation it is the structural component — typically 30-60% of the formulation — and it determines most of what the cured film will do.
The three main classes each have a characteristic profile. Epoxy acrylates give hardness, gloss, chemical resistance and the best cost-performance. Urethane acrylates (PU-acrylates) give flexibility, elongation and adhesion. Polyester acrylates sit between them and are often used to adjust viscosity.
Functionality — the number of reactive acrylate groups per molecule — is the main dial. Low functionality gives a softer, more flexible, more adhesive film that cures more slowly. High functionality gives faster cure and a harder, more crosslinked film with less elongation.