This invention relates to a novel sulfone compound useful for surface-treating reinforcing fibers in producing a fiber-reinforced, thermoplastic resin composite material. The present invention is also directed to a process for surfacetreating reinforcing fibers using the above compound and to reinforced fibers obtained by the process.
Composite materials composed of reinforcing fibers and various matrixes such as synthetic resins, metals or ceramics have find wide applications such as aircraft parts, space apparatuses, precision machines, sport goods such as tennis rackets and golf shafts due to their excellent mechanical properties including specific strength and specific modulus of elasticity.
Various investigations have so far been conducted to improve the strength and modulus of fiber-reinforced composite materials. In general, in order to obtain the excellent properties of the reinforcing fibers, it has been believed to be of importance to enhance affinity and adhesion properties between the reinforcing fibers and matrix and, from this point of view, many sizing agents for activating the surface of reinforcing fibers have been proposed.
As such sizing agents, epoxy resin type sizing agents have so far been often employed. However, such sizing agents, which show excellent affinity and impregnation properties for thermosetting resin type matrixes and offer extremely favorable advantages, have the defect that they show poor wetting properties and impregnation properties when thermoplastic resins such as polyamides, polycarbonates, polyphenylene oxides, polyphenylene sulfides, polyether sulfones, polysulfones, polyether ether ketones and polyetherimides are used as the matrix resin. It follows that the adhesion between the reinforcing fibers and the matrix resin becomes poor, thus failing to offer desired physical properties.
Sulfur-containing, heat-resistant resins such as polyphenylene sulfide have recently been noted as matrix resins for fiber-reinforced composite materials because of their excellent heat resistance, flame resistance, chemical resistance and, in addition, less expensiveness and better moldability in comparison with other resins such as polyimide resins. However, the epoxy resin type sizing agents possess poor wetting properties and impregnation properties for the polyphenylene sulfide. In addition, since the molding temperature of polyphenylene sulfide is as high as 300.degree. C. or above, thermal decomposition of the sizing agents can take place upon molding to cause generation of voids.
In order to solve these problems, the use of a polysulfone or a polyphenylene sulfide as a sizing agent for carbon fibers for use in carbon fiber-reinforced composite materials containing a polyphenylene sulfide as a matrix is proposed (for example, Japanese Unexamined Patent Publication Nos. 56-120730 and 56-90837). These sizing agents are superior to the epoxy resin type sizing agents in heat resistance, impregnation properties for polyphenylene sulfide, and the like.
However, these sizing agents still involve the following problems. That is, in the case of using such a polymer as polysulfone or a polyphenylene sulfide as a sizing agent, carbon fiber bundles lose their intrinsic flexibilily and become rigid due to the large modulus of the sizing agent. As a result, it becomes impossible to sufficiently deposit the sizing agent. In addition, since there are available no suitable solvents capable of readily dissolve the polyphenylene sulfide at ordinary temperature, a special coating method must be employed in order to uniformly coat the sizing agent on carbon fiber bundles and to control the amount of the deposited sizing agent.