The UV radiation-curing process is based on two chemistries
The radiation-curing process is based on two chemistries: the radical polymerization of unsaturated oligomers (mainly acrylate functionalized) and the cationic polymerization reaction of cycloaliphatic epoxides and vinyl ether.
Typical liquid radiation-curing formulations are composed of oligomers, reactive diluents, photoinitiators and additives, the general performance of the final coating mainly influenced by the oligomers. The oligomers are characterized by a relatively low molecular weight (between 500 and 5,000 daltons) and are based on different types of backbones: epoxyacrylates, urethane acrylates, polyesters acrylates, acrylic acrylates and cycloaliphatic epoxides.
This variety of chemical structures offers a wide range of properties and applications and gives a high degree of freedom to the formulator to meet target performances. Epoxy acrylates are mainly based on bisphenol-A derivatives. They generally give a fast cure response and provide hard coatings characterized by high solvent resistance and high gloss. Recent developments mean that more-flexible products and also aliphatic versions, or nonyellowing formulations, are now available.
Urethane acrylates represent a wide range of products in terms of structures, functionality and molecular weights, mainly due to the variety of available raw materials that can be used to build them up. This results in a wide range of mechanical properties after curing, from very flexible to hard and brittle materials. They are generally considered the highest-performing oligomers.
Polyesters acrylates have, like urethane acrylates, a versatile chemistry. A variety of products is therefore available, covering a wide range of functionality, molecular weights and viscosities. They are mostly used where high reactivity is required, such as for printing inks. Their performances are mostly between those of epoxy acrylates and urethane acrylates.
Acrylic acrylates are used only in very particular applications at this time, mainly for adhesion as a primer onto difficult supports. Today, only a few cycloaliphatic epoxides for cationic cure are commercially available. The main benefits of this technology are a good balance of elasticity and film hardness, plus excellent adhesion to difficult substrates, such as metal and polyolefins, and improved barrier properties.
A common point among the different families of oligomers is the relatively high viscosity. This can be adjusted by adding reactive diluents to meet the requirements of the final application. The reactive diluents also influence, to a certain extent, the performance of the cured films, depending on their concentration, structure and functionality. They generally have a detrimental effect on the general performance of the oligomers, but they can help to control surface properties and properties such as flexibility and adhesion.
Acrylate-based UV liquid formulations are characterized by a relatively low unsaturation equivalent weight, resulting, after cure, in a material with a high crosslinking density. This accounts for some outstanding properties, such as solvent, water and stain resistance. The main drawbacks are shrinkage and loss of flexibility and adhesion on difficult substrates.