CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application under 35 U.S.C. § 371 of international application number PCT/CN2017/098399, filed Aug. 22, 2017.

FIELD OF THE INVENTION

The present invention relates to urethane oligomers with allyl and more particularly allyl and (meth)acrylate groups, with good surface drying properties by reducing oxygen inhibition, a method of producing the urethane oligomers and their specific uses in curable compositions in particular in ultraviolet (UV) curable surface dry coating applications.

BACKGROUND OF THE INVENTION

Compared to traditional solvent based coatings, the principal advantage of UV curable coating is the environmental friendly solvent-free formulations with no volatile organic compound (VOC). Urethane acrylate oligomers are often used in coating composition with acrylate monomers as reactive diluents. By formulating with photo-initiators, UV coating are cured by photo-chemically initiated radical polymerization which would result in an incomplete cure with tacky surface due to the oxygen inhibition from atmosphere. Oxygen inhibits the curing by quenching the excited triplet state of photo-initiator and scavenging the radicals to form per-oxy radicals to terminate polymerization. This oxygen inhibition effect results in reduced properties or tacky surface on the coating. In extreme cases such as low intensity cure processes would result in uncured liquid surface.

Strategies to reduce the oxygen inhibition in industry process have been investigated in the past decades. Physical ways includes removing oxygen with inert gas such as nitrogen, or forming a barrier on surface using wax. Chemical ways includes increasing free radical concentration by formulating with higher content of photo-initiator, or using chemicals to react with per-oxy radicals, or using UV cationic epoxy to eliminate oxygen inhibition.

U.S. Pat. No. 8,268,104 reported using cationic epoxy to repair cracks on windshields which eliminates oxygen inhibition and shrinkage. However, application of this strategy is limited when epoxy could not provide the required properties and acrylates must be used. In fact, most of the UV coating systems could not be replaced by cationic epoxies.

In industry, about 3-5% (w/w) photo-initiator is added to generate enough free radicals under UV irradiation for polymerization. Oxygen inhibition could be reduced by formulating with even more than 10% (w/w) photo-initiator to increase the concentration of radicals. Due to the high cost of photo-initiators, the final product would be more expensive with higher content of photo-initiator. Another aspect is that the final formulation will be compromised and properties after curing could be affected. Moreover, since photo-initiators are small molecules, it's derives migrates even after curing. In many industries such as food packaging, migration of organic compounds is restricted.

One alternative way to eliminate oxygen inhibition effect is excluding oxygen by inert gas such as nitrogen while curing the compositions. In many cases, this process means that special designed and manufactured system is requested, which is not economically feasible and could be extraordinary costly.

Despite current effects to reduce oxygen inhibition effect in UV curing compositions, it still needs new oligomers with fast curing, air drying properties.

SUMMARY OF THE INVENTION

A subject of the present invention relates to a urethane oligomer, wherein according to a) or b) as follows, it has:

a) at least two backbone residues R issued from a polyisocyanate without the NCO groups, linked between them with a diol residue RB(diol without the two OH groups) by two urethane bonds (—NHCOO—) and each of said backbone residues R carrying (or linked to) at least two urethane segments each one of which, contains at least one terminal allyl group, preferably from 1 to 5 terminal allyl groups,
b) at least one backbone residue R issued from a polyisocyanate without the NCO groups, said backbone residue R carrying (or linked to) at least two urethane segments each linked to R by one urethane bond withat least one urethane segment (arm) containing at least one terminal allyl group, preferably from 1 to 5 terminal allyl groups, andat least one urethane segment (arm) containing at least one terminal (meth)acrylate group, preferably from 1 to 5 terminal (meth)acrylate groups.

Another subject relates to a method for preparing said urethane oligomer.

Another subject relates to a curable composition comprising at least one of said urethane oligomers according to the present invention.

Another subject covered by the present invention relates to the use of said urethane oligomer in curable compositions in the presence of air.

Finally the present invention covers a finished product selected from the group consisting of a coating, an adhesive, a sealant or a resin matrix, which results from the curing of the urethane oligomer or of the curable composition as defined according to the invention.

