Laminates are prepared by impregnating sheets of a fibrous substrate with a resin. The resin is partially cured to form a prepreg. The prepregs are then stacked and heated under pressure to form the laminate.
While polyimide resins have very desirable thermal properties, they have until recently been unsuitable for use in preparing laminates because they were relatively insoluble when fully imidized. Since the early 1960's there has been an industry worldwide effort to develop high performance polyimide-based laminating resins. The impetus for this research has been the ever-increasing demands by the circuit board industry for high performance laminates.
Polyimide resins are generally prepared by reacting a dianhydride with a diamine to produce a soluble polyamic acid which is then heated to condense out water and form the imide groups: ##STR1## Polyimides prepared by this route are unsuitable for use in making laminates because, if the polyamic acid stage is impregnated into the prepreg, water will be given off when the laminate is fully cured in the laminating press and the water will lead to voids in the laminate and weaken the laminate. A prepreg cannot be prepared with the polyimide itself as they are generally insoluble in organic solvents, and, due to their inherently high melting points, very high temperatures would be required in the laminating press to cause the prepregs to adhere to themselves.
U.S. Pat. No. 3,562,223 discloses a commercial polyimide laminating resin which avoids these difficulties, but requires a post cure at a high temperature. It is a polyaminobismaleimide prepared by reacting an aromatic diamine with a mixture of bismaleimide and partially addition polymerized bismaleimide. ##STR2## The product is soluble and can be used to form prepregs which can then be fully cured in the laminating press.