Dihydroxybiphenyl-advanced epoxy resin blends

A composite material which comprises reinforcing fibers and an epoxy resin composition comprising the following components:

• (A) a bifunctional epoxy resin;
• (B) at least one resin selected from the group consisting of a trifunctional epoxy resin and a tetrafunctional epoxy resin;
• (C) a dihydroxybiphenyl compound of the formula:
• ##STR4## wherein R.sub.1 -R.sub.4 each may be the same or different and each represents a hydrogen atom or an alkyl group of 1 to 6 carbon atoms;
• and
• (D) 4,4'-diaminodiphenylsulfone or 3,3'-diaminodiphenylsulfone;
• wherein said epoxy resin composition comprises a preliminary reaction product obtained by allowing said component (C) to react initially with all or part of component (A) and all of component (B) under the condition that at least 80% of the phenolic hydroxyl groups in component (C) react with the epoxy groups in components (A) and (B), the proportions of components (A), (B) and (C) to be used in the preliminary reaction satisfying the conditions:
• 1/0.3.gtoreq.a/b.gtoreq.1/3.0
• 5/1.gtoreq.(a+b)/c.gtoreq.0.9/1,
• and the proportions of components (A), (B), (C) and (D) in the total epoxy resin composition satisfy the conditions:
• 1/0.1.gtoreq.a/b.gtoreq.1/1.2
• 10/1.gtoreq.(a+)/c.gtoreq.1/1
• 1/0.8.gtoreq.(a+b-c)/d.gtoreq.1/1.5
• wherein a represents the molar number of epoxy groups in component (A), b represents the molar number of epoxy groups in component (B), c represents the molar number of phenolic OH groups in component (C) and d represents the molar number of NH groups in component (D).