The present invention relates to polyester resins cured with isocyanate cross-linking agents and more particularly to such a system which is curable in the presence of a vaporous tertiary-amine catalyst and is especially adapted for application to flexible substrates.
Vapor Permeation Cure (VPC) has been proposed to a limited basis by the prior art. For example, U.S. Pat. No. 2,967,117 shows a coating composed of a polyhydroxy polyester and a polyisocyanate which are cured in the presence of a gaseous tertiary amine or phosphine. The polyester contains aliphatic hydroxyl groups and heating of the coated substrate at about 60.degree.-120.degree. C. prior to or concomitant with the vaporous tertiary amine exposure is taught in the specification and examples. U.S. Pat. No. 3,874,898 shows a coating composed of a polymer containing only isocyanate functionality which can be cured in the presence of a vaporous amine. VPC technology additionally has been used for curing foundry binder compositions of a phenol-aldehyde resin (U.S. Pat. No. 3,409,579). For coatings applications, a hydroxy functional polyester may be end-capped with hydroxybenzoic acid as taught in U.S. Pat. No. 3,836,491, a phenol reacted with an unsaturated resin as taught in U.S. Pat. No. 3,822,226, or an epoxy polymer capped with hydroxybenzoic acid as taught in U.S. Pat. No. 3,789,044. British Pat. No. 1,351,881 modifies a polyhydroxy, polyepoxy, or polycarboxyl resin with the reaction product of a phenol and an aldehyde. The reaction product is curable with an isocyanate cross-linking agent in the presence of a vaporous tertiary amine. British Pat. No. 1,369,351 proposes a hydroxy or epoxy compound which has been reacted with diphenolic acid. The resulting compound is curable with a polyisocyanate according to VPC techniques.
For application on flexible substrates, such as flexible vinyl substrates, it has been determined that resins capped with hydroxybenzoic acid and phenol-aldehyde resins do not provide acceptable properties. The same would be expected to be true of resins containing only isocyanate functionality. The diphenolic acid-capped polyester coating composition of British Pat. No. 1,369,351 provides only minimal performance requirements based upon the particular polyester resins disclosed therein.
Suitable coating compositions for use on flexible substrates must meet certain minimum performance specifications prior to being acceptable. These performance specifications include mar resistance, abrasion resistance, flexibility, good color retention, and even embossability on occasion. Acceptable coating compositions additionally must retain attributes of acceptable conventional coating compositions including flow, leveling, pot life, high non-volatile solids content, acceptable application viscosity, acceptable gloss, and like coatings properties. Moreover, since many thermoplastic substrates can be damaged by exposure to too much heat or too prolonged heating, acceptable coatings compositions for such substrates must be curable at low temperature.
The present invention provides a coating system which meets or exceeds all performance requirements outlined above for flexible substrates as well as retains application and other conventional coating characteristics. Moreover, the coating system of the present invention is fully curable at room temperature for minimizing heat damage to the flexible substrate to which it is applied. Additional advantages of the coating system of the present invention is that such system, with at most modest variation, is entirely suitable for application to exterior substrates. Such coating system provides requisite toughness, exterior durability, adhesion including adhesion to metal, resistance to UV degradation, and the like, which properties are necessary for providing a suitable exterior coating. The truly unique coating system which possesses these and other advantages will be appreciated by the skilled artisan according to the disclosure contained herein.