Method of improving adhesion to plastic substrates

DETAILED DESCRIPTION What I claim is: 1.
A method for improving the adhesion of an acrylate ester or methacrylate ester based radiation photocurable coating or ink vehicle to a plastic substrate, wherein said vehicle comprises (i) a photoinitiator or photosensitizer and (ii) a mixture of photopolymerizable monofunctional and polyfunctional acrylate or methacrylate esters; said method comprising adding to said vehicle from 10 to 50 weight percent based on the weight of the total composition of poly(vinyl acetate) or a poly(vinyl chloride/vinyl acetate) polymer having a polymerized vinyl chloride content of from about 55 to about 90 mole percent, said poly(vinyl acetate) or poly(vinyl chloride/vinyl acetate) polymer being essentially soluble in said esters mixture, applying the modified ink or coating to a plastic substrate, and exposing the applied ink or coating to ultraviolet light radiation for a period of time sufficient to cure said applied ink or coating on said plastic substrate to a dry tack-free state.
2.
A method as claimed in claim 1, wherein said vehicle contains from 10 to 35 weight percent of said poly(vinyl acetate) or said poly(vinyl chloride/vinyl acetate) copolymer.
3.
A method as claimed in claim 2, wherein said vehicle contains a poly(vinyl acetate).
4.
A method as claimed in claim 2, wherein said vehicle contains a poly(vinyl chloride/vinyl acetate).




Description:
BACKGROUND OF THE INVENTION Radiation curable compositions for use in coating and printing ink applications have been in commercial use for several years, but, have all generally been lacking in their ability to adhere well to plastic substrates.
These compositions, unlike conventional solvent-based or water-based, thermally dried systems, experience high shrinkage during the conversion from their applied monomeric form to their high density, cured, polymeric form.
The shrinkage, due to polymerization and crosslinking, can in some instances approach about 30 percent by volume.
Furthermore, because of the rapidity of the polymerization, usually less than one second, there is insufficient exposure to heat during the cure, hence the stresses induced in the newly formed polymer cannot relax and a strong bond between the coating and substrate cannot form or persist