1. Field of the Invention
The present invention relates to a composite material for electrical applications reinforced by a para-oriented aramide fiber sheet and a process for preparing the same. More particularly, it relates to a composite material for electrical applications reinforced by a para-oriented aramide fiber sheet, which shows excellent properties well suited for use, in particular, as a reinforcing substrate such as a printed circuit board or an insulating laminate.
2. Description of the Related Art
Para-oriented aramide fibers have a variety of improved physical properties, for instance, they have excellent resistance to heat, resistance to fatigue and resistance to chemicals and they have high strength and a high modulus of elasticity. However, since they have poor adhesiveness to thermosetting resins and cannot thus be intimately bonded to thermosetting resins using conventional technology, their excellent properties are not thoroughly utilized as substrates for composite materials.
In order to solve this problem, there have been proposed a variety of improved aramide fibers such as those exposed to plasma (see, for instance, Unexamined Japanese Patent Publication Nos. 125689/1984 and 171738/1986); those treated with an aminopolyamide resin as disclosed in Unexamined Japanese Patent Publication No. 204229/1986; those treated with a polyurethane compound as disclosed in Japanese Patent Publication No. 14166/1986; and those treated with a resorcin-formalin oligomer as disclosed in Unexamined Japanese Patent Publication No. 121591/1975. Although it is stated, in these patents, that the aramide fibers treated through these prior art techniques have improved adhesiveness compared with that for untreated ones, the results obtained by these methods are not yet satisfactory.
In particular, a laminate for electrical applications should have high electric resistance even after it has absorbed water. However, a composite material for electrical applications reinforced by para-oriented aramide fibers and having satisfactory electric resistance after having immersed into a boiled water has not yet been developed or proposed.
It is known that a laminate can be prepared by first impregnating a para-oriented aramide fiber sheet with a thermosetting resin such as an epoxy resin or a polyimide to form a prepreg which is then stacked and subjected to hot-press molding, and that the laminate prepared according such a method is low in thermal expansion coefficient and improved in dimensional stability due to the excellent properties of the para-oriented aramide fibers (see Unexamined Japanese Patent Publication No. 160500/1986).
However, the para-oriented aramide fibers in the resultant laminate do not adhere sufficiently to the thermosetting resin, which results in unsatisfactory electric resistance after the laminate has absorbed a boiled water. Accordingly, the laminate prepared using conventional technology cannot be practically applied.