Ultraviolet light curing technology of coatings

Many photocrosslinkable ink and coating compositions in commercial use today consist of prepolymers or oligomers that are combined with polyfunctional monomers and photoinitia- 

There are two types those that crosslink by free-radical mechanisms and those that crosslink by ionic mechanisms. In the first the monomers are often mixtures of di- and tiiacrylate esters. They are combined with photosensitive prepolymers, but lack adequate photosensitivity to crosslink by themselves. Instead, they rely upon photoinitiators as the sources of free radicals for the cure. The diacrylate esters usually serve as diluents to lower the viscosity of the mixtures. These can be various glycol diacrylates, such as butanediol diacrylate o o 

Sometimes, other diluents that are not acrylic esters, like vinyl pyrrolidone, are also used. The triacrylate esters may be compounds, like pentaerythritol triacrylate or trimethylolpropane triacrylate  

The prepolymers can be different types of materials. They must, however, contain residual unsaturation in order to react and crosslink with the monomers. Examples of such materials may be polyurethane acrylates that are prepared from urethane prepolymers. The excess isocyanate groups are treated with hydroxyethyl or hydroxypropyl acrylates. Other prepolymers with terminal and/or pendant hydroxy groups are also often esterified with acrylic acid. The oligomers might also be bisphenol A diglycidyl ethers prereacted with acrylic acid to form terminal acrylate groups. An example of these would be  

Unsaturated polyesters are also used as prepolymers. Such polyesters might, in addition, have the unreacted hydroxy groups esterified with acrylic acid. Photocrosslinkable unsaturated polyesters can also be prepared by many other means. One example is intr uction of unsaturation into the backbones or into the pendant groups via a Knoevenagel reaction 

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