Functionality is the count of reactive acrylate groups a molecule carries — a difunctional oligomer has two, a hexafunctional one has six. It is the single most influential formulation variable in UV curing.
Low functionality (1-2) gives slower cure, lower crosslink density, a softer and more flexible film, better adhesion and less shrinkage. High functionality (4+) gives faster cure, high crosslink density, a harder and more chemically resistant film, less elongation and more cure shrinkage.
Cure shrinkage is the hidden cost of high functionality. As acrylate double bonds convert to single bonds, the film contracts — and that contraction pulls against the substrate bond. On a difficult substrate, a highly functional formulation can cure beautifully and then delaminate.
Most practical formulations blend functionalities to land on a specific point in the hardness-flexibility-adhesion triangle.