The present invention relates to a low-viscosity epoxy resin, a resin composition containing said resin, and a fiber-reinforced composite material containing a cured product of said composition as matrix.  2.  Description of the Prior Art
Having excellent curability, heat resistance, mechanical properties, adhesive property and bonding property, epoxy resins are extensively in use.  One of the uses of epoxy resin is its use in the electric and electronic fields, and another is lamination and composite materials mainly in airplanes.  In these fields, more and more improved performances are required currently. 
That is, on the one hand, improvements in heat resistance and in mechanical properties at high temperature and high humidity are required.  On the other hand, a low viscosity of resin is required from the viewpoint of workability in the impregnation of epoxy resin into fiber material for making a prepreg.  Although single nucleus type aromatic epoxy resins are hopeful as an epoxy resin fulfilling the above-mentioned requirements, none of the existing epoxy resins can fulfil all the requirements mentioned above. 
In view of above, the present inventors have conducted elaborated studies on an epoxy resin which is excellent in heat resistance and mechanical properties at high temperature and high humidity and has a low viscosity.  As the results, there has been found a method for easily producing epoxy resins (polyglycidyl derivatives) which satisfy all the above-mentioned requirements.  Based on this finding, the present invention has been accomplished. 
Apart from above, fiber-reinforced composite materials containing a cured epoxy resin composition as matrix and a fiber such as carbon fiber, alumina fiber, polyamide fiber or the like as reinforcing material are in use as structural materials for various industries and as sports goods, leisure goods and the like owing to their excellent mechanical properties. 
However, with the expansion of the application field of composite material in the recent time, there has become desired a material which retains its excellent chemical stability and mechanical properties even under more severe conditions, i.e. higher temperature and higher humidity, than ever. 
The resin used as matrix is important from the viewpoint of making a fiber-reinforced composite material exhibiting the characteristic properties of reinforcing fiber to the greatest extent.  Accordingly, development of fiber-reinforced composite material using various epoxy resin compositions as matrix has actively been studied, For example, in Japanese Patent Publication No.  25,217/80, there is proposed a carbon fiber-reinforced composite material comprising an epoxy resin composed of N,N,N',N'-tetraglycidylaminodiphenylmethane and the like and diphenyldiaminosulfone as the matrix.  Although this composite material has a high interlaminar shear strength and a high heat resistance, its cured product is small in elongation and therefore is hard and brittle.  In addition, it is not resistant to thermal shock.  Accordingly, it is not satisfactory when put to special uses such as airplanes.  Further, a mixture consisting of N,N,N',N'-tetraglycidyldiaminodiphenylmethane and diphenyldiaminosulfone has a viscosity as high as 500 poises or above at 50.degree.  C., so that it is difficult to handle at room temperature.  Further, the mixture is poor in reactivity and its curing reaction can sufficiently be completed only by after-curing it at a high temperature of 180.degree.  C. or above for a period of at least four hours.  If boron trifluoride-monoethylamine complex or boron trifluoride-piperazine complex is added as a curing accelerator to this composition in order to overcome the above-mentioned fault, its reactivity may be improved.  However, heat resistance of the composition is deteriorated by its addition.  On the other hand, regarding the improvement of flexibility, addition of a reactive elastomer may be possible.  However, it has been found that its addition markedly deteriorates heat resistance and bending strength of the composition. 
As the epoxy resin which can be put to the above-mentioned uses, there are also known, for example, Bisphenol A diglycidyl ether type epoxy resin excellent in adhesion to carbon fiber and the like, its pre-polymer with diaminodiphenylsulfone (DDS) and the like, phenolnovolac and cresol-novolac type epoxy resins, triglycidylaminophenol, and mixtures thereof. 
However, these resin compositions are also disadvantageous in the following points.  Thus, the prepolymer prepared from Bisphenol A diglycidyl ether type epoxy resin and DDS is disadvantageous in that it has too high a curing temperature and its molded products involve a great residual stress and often involve a defect.  Triglycidylaminophenol is disadvantageous in that it is poor in water-resistance and its flexibility, interlaminar shear strength and bending strength are unsatisfactory and must be further improved.  Apart from above, there is also proposed a resin composition prepared by combining Bisphenol A diglycidyl ether type epoxy resin with dicyandiamide (curing agent) and a curing accelerator.  However, such a combination brings about a marked decrease in the heat resistance of cured product.  Thus, an epoxy resin of which heat resistance is not greatly deteriorated even if combined with those curing agent and curing accelerator is desired. 
The present inventors have conducted elaborated studies with the aim of achieving the above-mentioned objects.  As the result, the aimed resin composition and fiber-reinforced composite material have been found.  Based on these findings, the present invention has been accomplished.