Conventionally well-known laminated sheets include paper base phenolic resin laminated sheets, paper base unsaturated polyester resin laminated sheets, and glass-cloth base epoxy resin laminated sheets These laminated sheets have been produced by impregnating the respective base with a resin and heat-curing a plurality of the resulting prepregs. The term "laminated sheet" as used herein means a laminated sheet having a thickness of from 0.5 to 5 mm which is used, for example, as a base of various electronic parts.
During the production of phenolic resin laminated sheets, curing of a phenolic resin is accompanied by the forming of reaction by-products, such as water, which adversely affect physical properties of the resulting laminated sheet. To avoid this, it is necessary to forcedly dissolve water, etc. in the cured product by applying a high pressure by means of a large-sized pressing machine or other machine suitable for applying high pressure to such a cured laminate. Phenolic resins generally require a long time for curing, and the additional application of high pressure also requires additional prolonged periods of time. Moreover, laminated sheets comprising phenolic resins are inferior to alternative component resins in electrical characteristics, such as dielectric constant, dielectric dissipation factor, tracking resistance,.and other relevant characteristics.
During the production of glass-cloth base epoxy resin laminated sheets, on the other hand, high pressures required for production of phenolic resin laminated sheets are not necessary, since the curing reaction is not accompanied by formation of by-products, (e.g., water), as is observed with a condensation reaction of phenolic resins. Nevertheless, curing of epoxy resins still requires long periods of time.
In order to solve these above-mentioned problems, it has been proposed to produce laminated sheets using paper as a base and an unsaturated polyester resin as an impregnating resin. According to this process, since curing is effected through radical polymerization, the time required for curing is reduced to thereby eliminate the above-described disadvantages. However, unsaturated polyester resins have poor heat resistance which results in laminated sheets exhibiting low stiffness and strength under elevated temperatures.