This invention relates to polyolefin-metal bonded structures. More particularly, it relates to a novel polyolefin-metal bonded structure and methods for making same in which a polyolefin layer is bonded to a metal substrate through a primer layer comprising a combination of a polar group-containing, ethylenically unsaturated monomer-modified resin and a coating film-forming base resin. The primer layer has a novel multilayer structure in which both resins are distributed in concentration gradients in the thickness direction.
Sealing members for crown caps or other vessel closures have heretofore been made by coating a surface-protecting paint onto a metal sheet, molding the coated metal sheet into a crown shell, cap, or the like and bonding a packing material to the inner face of the molded cap or shell. Polyolefins such as polyethylene have excellent sanitary properties with respect to food containers and the like and a very high moisture resistance. Accordingly, they are preferred for use as packing materials for vessel closures. However, polyolefins have a serious defect in that their bondability to metal substrates is very poor.
Various undercoat paints or primers for improving the bondability between a polyolefin and a metal substrate have heretofore been proposed. An example of such a primer is a composition comprising a coating film-forming epoxy base resin having dispersed therein a polar group-containing ethylenically unsaturated monomer-modified olefin resin such as polypropylene or polyethylene modified with an ethylenically unsaturated carboxylic acid. This undercoat paint performs well to some extent compared to a similar paint free from a modified olefin resin with respect to its bondability to a polyolefin. However, such undercoat paints are insufficient to bond a polyolefin packing material to the inner face of a vessel closure securely enough that the packing is not peeled off. Furthermore, the use of an undercoat paint formed by incorporating such modified resin into a coating film-forming base resin suffers a serious drawback in that the corrosion resistance of the metal substrate is drastically reduced.
It is apparent from the foregoing, therefore, that primers heretofore used for bonding a polyolefin to a metal substrate leave much to be desired with respect to the combination of peel resistance and corrosion resistance.
Accordingly, it is an object of the present invention to provide a polyolefin-metal bonded structure having a desirable and stable combination of high adhesion and high corrosion resistance even under practical mass-production conditions and wherein the bonding strength and corrosion resistance can be maintained at the necessary high levels even under severe processing conditions such as pressing.
Another object of the invention is to provide a vessel closure such as a crown cap having a polyolefin packing and which exhibits excellent sanitary properties with respect to foods and other contents, high moisture and corrosion resistance, good cushioning of the mouth portion of the vessel, and high resistance to peeling of the packing from the closure.
Yet another object is to provide a process for producing the aforementioned polyolefin-metal bonded structures and vessel closures.
These and other objects of the invention as well as fuller understanding of the advantages thereof can be had by reference to the following detailed description and claims.