Oligomers are too viscous to apply on their own. A reactive diluent — a low-molecular-weight acrylate monomer such as TPGDA, TMPTA or HDDA — reduces viscosity to something a press or coater can handle.
The critical distinction from solvent: a reactive diluent does not evaporate. It carries acrylate groups, participates in the cure and becomes part of the crosslinked film. That is why UV systems are described as 100% solids and essentially VOC-free.
Because it stays in the film, the diluent affects final properties as much as the oligomer does. Monofunctional diluents thin effectively and improve flexibility and adhesion, but cure slowly and can leave extractable residue. Multifunctional diluents cure fast and add crosslink density, but increase shrinkage and brittleness.
Monomer selection also drives skin irritation potential, which is a genuine handling consideration in UV formulation.