With UV curable, adhesion to plastics are particularly difficult due to the inherent polymerization process of UV resins. When exposed to UV light, the integral photoinitiator starts a chain
reaction leading to polymerization of the formulation.
But this process can include shrinking of the transferred ink, coating or adhesive. Diluting of the curable with acrylates reduces the viscosity of UV formulation. Low viscous acrylates with high functionality can positively increase cross-linking density.
But this can also increase shrinkage and therefore decreases adhesion. Choosing diluting acrylates with lower functionality can aid adhesion to the adherend.
Also a major adhesion promotion
technique when using diluting adherends is ensuring that the liquid UV formulation have a surface tension no higher (and preferably 10 mN/m lower) than the critical surface tension of the solid adherend.
Surface treating polypropylene, for example, using a functionalizing atmospheric plasma to raise its surface tension from 31 mN/m to 46 mN/m and create covalent bonding will allow most diluting acrylates to wet sufficiently to its surface. Request your free technical paper.