The inner linings of motor vehicles, for example on the door panels or preassembled car roofs, are produced from heat molded rigid polyurethane foam panels. In this process, the interior furnishing layer, which is frequently a sheet of PVC, is preferably bonded to the heated foam panel during the molding process inside the appropriate shaped mold.
Solvent-free reactive adhesives, e.g. those based on polyurethanes, may be used for this purpose, but they have the disadvantage that their pot life determines the rhythm of the working cycle, apart from the fact that parts covered with wet adhesive are difficult to handle.
One method of overcoming the above-mentioned difficulties is described in U.S. Pat. No. 4,137,366, in which described are bonds produced by means of a hot melt adhesive film consisting of certain polyamide copolymers. One disadvantage of this hot melt adhesive is that it must be prepared from polymer combinations with a high softening temperature if the bonds obtained are to be heat resistant. The formation of bonds with such polymers requires prolonged heating of the part to be bonded at relatively high temperatures which are not always suitable for thermoplasts such as PVC. Moreover, heat resistance tests on these bonds show that the resistance is limited to 80.degree. C., as will be seen from the examples, whereas heat resistances of up to 110.degree. C. or higher are presently required in the motor car industry. It is therefore an object of the present invention to provide a new adhesive which would satisfy the following requirements:
The adhesives should be preparable by known methods from conventional starting materials of the kind hitherto used for the preparation of polyurethane adhesives.
The adhesive should enable highly heat-resistant bonds to be formed over a wide surface area.
The adhesive should be preparable and used solvent-free and have a pot life of at least 10 days after its preparation.
This problem has been solved by the process according to the invention as described below.