Since a hot melt adhesive (hereinafter referred to as "HMA") is a non-solvent type adhesive and fast setting, it is widely employed in various fields. For example, in the field of the production of an optical disc, HMA is expected to be used in place of the conventional epoxy adhesive and solvent type adhesive due to its advantage such as easiness in handling.
There has hitherto been known, for example, HMA comprising a block thermoplastic elastomer, a tackifier and a softening agent wherein a softening point or a melt viscosity is regulated (see Japanese Patent First Publication "Kokai" No. 108044/1983). In application to a disc, it is desired to apply at as low temperature and low viscosity as possible so that the disc is not impaired. Under these conditions, however, a heat resistant property is limited. The HMA having a disadvantage of insufficient heat resistance as mentioned above, it is particularly difficult to keep a balance between an application property and a heat resistance property and the HMA is thus limited in an applicable range as an adhesive.
Under the above-mentioned circumstances, it has been attempted to improve a heat resistance of the HMA by blending it with a compound curable by irradiation of radiation (e.g. acrylate monomer, etc.), followed by irradiation of radiation to cure (see Japanese Patent First Publication "Kokai" No. 1086/1972, West Germany Offenlegungsschrift No.2350030). However, since the acrylate monomer easily provokes thermal polymerization with heating during blending or application, increased viscosity or gellation of the HMA frequently occurs. Therefore, it is necessary to carry out a series of blending and application procedures in a short period of time (see "Setchaku (adhesion)" Vol. 29, No. 6 (1985), pages 36-37).
An optical disc comprising a pair of substrates, which is provided with an optically readable information recording layer in at least one of the substrates, has been used as a video disc, a digital audio disc, or a disc for documents and data.
The optical disc is usually prepared by adhering a pair of substrates. For the adhesion of the pair of substrates, there have been used an epoxy adhesive, an urethane adhesive, an ultraviolet curable adhesive, a hot melt adhesive and the like. However, these adhesives have disadvantages, i.e. the epoxy adhesive has a low curing rate and the disadvantage of lower productivity. In case of the ultraviolet curable adhesive, the monomer component contained in the adhesive may corrode the substrate and the hot melt adhesive has low heat stability.