The terms "acrylic acid", "acrylate" and "acryloyloxy" are used hereinafter in this specification to mean both acrylic acid and methacrylic acid, acrylate and methacrylate, and acryloyloxy and methacryloyloxy, respectively.
A large number of acryloyloxy-terminated polyesters (also called polyester acrylates) are known wherein the polybasic acid unit and polyhydric alcohol unit constituting the polyester portion are varied. For example, crystalline acryloyloxy-terminated polyesters containing the condensate of terephthalic acid and ethylene glycol as a polyester-forming unit are known. These polyesters, however, cannot be used as a material for a liquid and non-solvent type of paint or ink because they have high melting points and are almost insoluble in monomers, oligomers and solvents generally used.
Other known polymerizable acrylates include acryloyloxy-terminated polyurethanes (also called urethane acrylates) and acryloyloxy-terminated epoxyesters (also called epoxy acrylates). It is known that these polymerizable acrylates can be used as paints or inks capable of being cured by heating or by irradiating with ultraviolet light.
Paints or inks prepared using these conventional polymerizable acrylates, however, do not have sufficient adhesion to metals, particularly tin-free steel ("TFS" for brevity) and tinplate. In applications, therefore, where post-working such as pressing, bending and squeezing is needed after the coating or printing of the paint or ink, or where the coating or printing is applied directly onto the TFS or tinplate, their performance is not sufficient. Their usefulness is thus limited in such applications.
In order to improve such poor adhesion to metals, various attempts have been made as to the resin composition per se, e.g., the structure of polymerizable acrylates and compounding technique, metal surface treating technique, and curing technique.
With regard to the resin composition, an attempt to incorporate compounds reactive with the metal surface (e.g., a silane coupling agent, a titanate coupling agent, a polyisocyanate compound and an epoxy compound), an attempt to use acrylates containing therein a phosphoric acid ester group or a carboxy group, or to introduce such functional groups into the molecular skeleton of the resin, an attempt to incorporate a surface active agent, and so forth have been made, but these approaches have failed to improve the poor adhesion to the extent that the adhesion is sufficient.
Metal surface treating techniques which have been proposed to improve the poor adhesion include chemical treatment (e.g., a phosphate treatment), an alumite treatment, a primer treatment using a silane coupling agent or a titanate coupling agent, an undercoating treatment using conventional solvent type thermosetting resins or thermoplastic resins, a primer coating treatment, and a surface coarsening treatment, e.g., partial etching of the metal surface and sand blasting. In many cases, the adhesion of the coating film to substrates subjected to these surface treatments is increased, but the improvement is not sufficient in those applications where post-working as described hereinbefore is required. These techniques, however, are not suitable for general use because special treatments are required and multiple coating is involved; that is, they are disadvantageous in both workability and economical efficiency. The primer treatments and undercoating treatments are also disadvantageous in that long periods of time are required to dry the solvent, for post-treatment and for curing and furthermore a large quantity of heat is needed. Furthermore, it is difficult to employ such treatments in the coating or printing line where an ultraviolet light-curable pigment or ink, which has a fast curing rate and needs no heating, is used.
Attempts made to improve the poor adhesion with respect to the curing technique include a method in which coating or printing is carried out while heating the substrate and a method in which heating is applied after the coating. These methods fail to increase the adhesion of the coating film to a sufficient extent and furthermore are not preferred in that a large quantity of heat is needed for heating a substrate having a high heat capacity, such as a metal.