Photopolymerizable compositions have been intensively studied at the request for energy-saving, space-saving and prevention of environmental pollution in the fields of printing inks, paints, coatings, liquid resist inks, etc. and it has been attempted to put them to practical use. However, most of these studies are based on radical polymerization reaction of double bond. Cation polymerizable substances such as epoxy resins are excellent in properties, but are difficult to photopolymerize. Thus, hitherto, such substances having a double bond introduced by acrylic modification have been mainly used.
As a method for curing epoxy resins by exposure to light, for example, U.S. Pat. No. 3794576 proposes to use photosensitive aromatic diazonium salts as a photopolymerization initiator. However, aromatic diazonium salts generate nitrogen gas upon decomposition with light, and if the thickness of epoxy resin coat is more than 15 .mu., the coat foams and therefore, the initiator is not suitable for the use of thick coating. Further, the mixture with an epoxy resin gradually cures even in the absence of light and thus has the problem in storage stability and cannot be used as one-pack type compositions.
Various studies have been made in an attempt to solve the defects of the diazonium salt photopolymerization initiators and as a result, aromatic sulfonium salt or aromatic iodonium slat initiators and curable resin compositions containing the same are reported to improve the thick coatability and the storage stability in JP-B-52 14278 and JP-B-52 14277, U.S. Pat. Nos. 4161478 and 4273668. However, the compositions containing these aromatic onium salts are inferior in curability as compared with those containing diazonium salts. Further, aromatic sulfonium salts have the problem in the smell of cured products. For solving these problems, use of aromatic sulfonium salts having a specific group is proposed in U.S. Pat. No. 4374066. However, the problems have not yet been sufficiently solved. Moreover, with expansion of the field where photopolymerizable compositions are used, development of novel photopolymerization initiators and compositions containing the same is important for meeting the market's demand.