Plastic films and in particular polyester films are well known for their excellent properties in terms of thermal stability, dimensional stability, chemical resistance and relatively high surface energy. They are very strong materials and are particularly in demand as substrates for various coatings. Composite materials comprising a plastic film as substrate and a coating have numerous applications: packaging, and in particular food packaging; protection of substrates; films or sheets for graphic art, for example for printing or drawing; and metallized films.
However, these composite materials have the drawback that they have insufficient adhesion between said substrates, in particular polyester films, and their coatings, thus making their use inefficient and/or unreliable and therefore unsuitable. These problems of adhesion are particularly great when the composite material is subjected to high stresses. For example, in the food sector, packaging may be subjected to conditions of humidity and high temperature in processes for hot filling, pasteurization and sterilization. It is known that in these situations, metallic coatings do not adhere properly to PET film. This lack or loss of adhesion means that the expected barrier effect against oxygen and water vapour is lost, which causes deterioration of the food contents and poses a health risk.
Several solutions have been proposed in the past in an attempt to solve this problem of bonding between the substrate and its coating. Thus, physical treatments (flame treatment, corona discharges, abrasive treatment) or physico-chemical treatments (treatments with acid, grafting of chemical functions) on the surface of the films have been tested. As these various treatments have many drawbacks, application of an intermediate coating was preferred.
This intermediate coating, also commonly called priming coat, is designed to have on the one hand good adhesion to the substrate and on the other hand good adhesion to the coating. Numerous polymers or copolymers have been proposed for producing this coating. U.S. Pat. Nos. 2,794,742, 2,627,088 and FR 1 428 831 state that the polymers that are most suitable as bonding primer are acrylic polymers. This priming coat is applied to the substrate by coating with organic solutions or aqueous emulsions. One drawback of the use of these acrylic polymers is the fact that the formulation of the acrylic-based coating must be adapted to each type of final coating and therefore to each use of the coated film to have the best possible adhesion. This means that the manufacturer needs to have a panel of various solutions. Therefore a coating has been sought that improves the adhesion of the final coating to its substrate which is as universal as possible, thus making it possible to omit the step of adapting the coating composition to each final coating. Patent application EP 0 260 203 proposes a solution to this problem. This patent application describes a modified polymer obtained by aqueous-phase radical polymerization of at least one monomer of an acrylic nature in the presence of an effective quantity of a water-dispersible polyester, derived from at least one aromatic dicarboxylic acid and at least one aliphatic diol and comprising a plurality of sulphonyloxy groups. Although the results for adhesion obtained with this formulation are good, further improvement is desirable.
Moreover, it is also desirable for the composite material to have good barrier properties to oxygen and water vapour. Although maintenance of the barrier properties of the material over time is generally linked to the adhesion between the substrate and the coating on the material, the initial quality of the barrier effect of the material is an independent property. One route used conventionally for improving the barrier effect of the material consists of increasing the degree of cross-linking in the composition of the priming coat. However, this technical solution has limited effects: it was found that when the concentration of cross-linking agent was increased, the level of the barrier effect quickly reached a plateau, the value of which remained insufficient relative to the requirements.
In this context, one of the objectives of the present invention is to propose novel coating compositions for polymer films, and in particular for polyester films, which, when used as priming coat between a polymer film and a coating, are capable of endowing the composite material obtained with improved barrier properties. In addition, it is desirable for these composite materials to have, advantageously, properties of adhesion that are as good or even better than in the prior art, and for these properties of adhesion to be maintained at high temperature and under moist conditions, in particular for this composite material to be insensitive to water. Moreover, another objective is that the coating composition should be as universal as possible.