Heretofore heat-shrinkable foamed films have had wide applications, for example, as sleeves for covering containers. Printing on such heat-shrinkable foamed films is usually conducted on the surface of the skin layer, i.e. the laminated layer of unfoamed film superposed on foamed film. The method adapted to print the skin frequently produces a print of poor gloss, thus rarely giving an aesthetically attractive print. This problem may be alleviated by laminating a foamed film on the printed surface of the skin. However, such film laminate poses another problem that the printed surface side can be scarcely smooth-surfaced because of the uneven surface of the foamed film and the air introduced into the film laminate in the laminating process due to the surface irregularities of the foamed film. Conventional heat-shrinkable foamed films formed from polystyrene have the drawbacks of being susceptible to impact and brittle, hence prone to damage when used for covering bottles or like containers.
To improve the printability of conventional heat-shrinkable foamed films, we proposed "a heat-shrinkable foamed composite film composed of a laminate of foamed film and unfoamed film which is superposed on a printed heat-shrinkable film" (Unexamined Utility Model Publication No. Sho 63-43722). We investigated foamed materials substituting for polystyrene and filed patent applications on the research results in Japan (Examined Patent Publications Nos. Sho 62-33251 and 62-22787 and No. Hei 1-38662) to propose foamed film laminates of olefin polymers as disclosed therein for example as "a laterally heat-shrinkable foamed composite material composed of a laminate of a laterally heat-shrinkable thermoplastic film and a laterally heat-shrinkable foamed material prepared by laterally orienting a film formed from a composition comprising an ethylene-propylene copolymer having an ethylene content of 1 to 10 mole %, a polymer for improving the flexibility (such as an ethylene-vinyl acetate copolymer, low-density polyethylene, etc.) and a foaming agent".
The proposed film laminates, however, are low in heat shrinkage factor and thus unsuitable for applications which require a higher heat shrinkage factor. Such film laminates are also defective in that they are generally limp and insufficient in rigidity and cause faults during a continuous operation for covering bottles or like containers, leading to unavoidable shutdown of the operation.
To overcome the drawbacks of the above foamed film laminates of olefin polymers, we have researched the properties of composite films for covering containers or the like which are composed of a laminate of such foamed film laminate and a heat-shrinkable film of a polymer. Yet the investigated olefin polymers had the problems of failing to improve fully the heat shrinkability of the foamed film laminate and to meet completely the rigidity requirement and giving a film of poor printability.