To simplify the work and to improve the safety and hygiene, a treatment system by adhesive sheets is widely used in place of a system of coating and drying a liquid adhesive. As the adhesives for the adhesive sheets, thermosetting type adhesives capable of setting by heat treatment are proposed in the use of adhering electron parts, etc. The thermosetting type adhesive is excellent in the adhesive strength and the heat resistance, but does not have a tackiness at normal temperature to make a temporary adhesion difficult, and contains a large amount of low-molecular weight components before setting, which results in giving a problem of causing adhesive oozing, etc. Also, there is a difficulty that a thermosetting type adhesive, which is excellent in the storage stability, needs a long time for setting. On the other hand, a thermosetting type adhesive, which can set in a short time, is poor in the storage stability.
On the other hand, tacky sheets have a tackiness at normal temperature, can adhere to an adherend without need of any pre-treatment, and can immediately show the adhesive strength. However, tacky sheets have a disadvantage that they are generally inferior in the adhesive strength and the heat resistance as compared with adhesives. Also, JP-A-5-117593 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that an adhesive strength and a high heat resistance are realized by the use of a photopolymerizable composition containing an acrylic acid alkyl ester as the main component, but these effects are still at the level of the conventional tacky sheets. Thus, it has been desired to develop an adhesive sheet which exhibits not only the adhesion as the tacky sheet at the beginning of sticking to an adherend but also the same high adhesion and high heat resistance as those of an adhesive after sticking.
As a material respond to the desire, a so-called tacky adhesive having both the properties of the tacky agent and the adhesive is proposed. For example, JP-A-2-272076 discloses a thermosetting type pressure-sensitive adhesive prepared using a photopolymerizable composition, as the raw material, obtained by adding an epoxy resin to an acrylic acid ester monomer. Also, JP-A-7-2978 discloses a thermosetting type pressure-sensitive adhesive prepared using a photopolymerizable composition, as the raw material, obtained by adding the ester of an epoxy group-containing alcohol and (meth) acrylic acid and an epoxy resin to a (meth)acrylic acid alkyl ester.
In these proposals, the thermosetting type pressure-sensitive adhesives having both properties of a tacky agent and an adhesive are obtained by photopolymerizing the photo-polymerizable composition by the irradiation of ultraviolet rays. The above-described composition contains a curing agent such as imidazole, dicyandiamide, polyamine, etc. to cause a curing reaction of the curing agent and an epoxy group. In this case, there is a fear that the above-described curing reaction proceeds gradually during storage before or after the photopolymerization to thereby lower the adhesion, etc.
As described above, the thermosetting type pressure-sensitive adhesives proposed as described above have a trouble in the point of the storage stability and in particular, the thermosetting type pressure-sensitive adhesive disclosed JP-A-2-272076 is insufficient in the heat resistance because the acrylic polymer formed by the photopolymerization and the epoxy resin is not crosslinked after setting.
The present invention has been made under these circumstances and an object of the present invention is to provide a thermosetting type pressure-sensitive adhesive which has a tackiness at normal temperature, can easily temporarily adhere to an adherend, can set in a short time by heating to show a high adhesive strength and a high heat resistance, and, is excellent in the storage stability, and an adhesive sheet of a sheet form or a tape form prepared using the above-described thermosetting type pressure-sensitive adhesive.