The present invention relates to composite plastic moldings having a solid, thin skin and a foamed backing layer and a process for their production. Such moldings are widely used in the manufacture of interior parts for motor vehicles, such as instrument panels, center consoles, arm rests or door inner panels.
In the production of such composite plastic moldings, the solid skin is generally produced in a known manner from a thermoplast in a first step. This step is carried out in a suitable mold, for example by deep drawing of an ABS film or by slush molding from a PVC plasticizer plastisol or a PVC powder containing polymeric plasticizers. A PVC skin is often preferred to an ABS skin by virtue of its more pleasant feel.
In a second step, the solid skin produced in the first step is back-foamed with a suitable plastic either in the same mold (deep drawing) or after transfer to a second mold (slush molding). By virtue of their excellent foamability and the range of variation of the physical properties of the foam, polyurethane(urea) systems are generally used for this back-foaming step.
Under certain loads, however, composite plastic moldings of this type are affected by serious problems, particularly where the skin is based on PVC. The PVC contains stabilizers and plasticizers which have a certain vapor pressure and, hence, can lead to the emission of unpleasant odors and to a permanent coating on the windows of the automobile from inside.
In addition, the effect of heat on the composite material (dashboards can become heated to surface temperatures of &gt;100.degree. C., depending upon color and insulation) produces considerable interaction between the outer skin and the foam backing, which can lead to delamination, embrittlement of the outer skin and degradation of the backing foam.
Several attempts have been made to overcome this drawback. For example, special stabilizers for the PVC skin have been used. An intermediate layer between the skin and the backing foam to prevent the migration of plasticizers and additives (See EP No. 0,161,477) has also been employed.
However, these measures which seriously restrict the choice of additives and/or lead to additional cost generating process steps do not result in a significant improvement.
Consideration has been given to producing the solid skin and backing foam in a single step from a polyurethane system in the form of a so-called integral foam having a compact skin using the reaction injection molding process. In this case, however, reaction injection molding produces certain disadvantages, despite the saving of time. In such moldings, the skin and foam backing would have to be made of the same material. It would not be possible to make the skin, for example, from a light-stable, pigmented system and the foam backing from a simple, inexpensive system or to use systems having different mechanical properties for the skin and backing. In addition, it would be difficult to adjust the thickness of the skin as required within wide limits. The occasionally large and complicated undercuts of the moldings to be produced would also represent a certain problem in terms of mold design.
Another alternative for the production of a polyurethane skin is the in-mold coating process, in which the inside of the mold is first sprayed using a one-component or two-component spray or mixing head with enough material to form a layer a few tenths of a millimeter thick. However, considerable problems are created by the necessarily thin coating applied by a spray or mixing head. For example,the coating materials tend to run down vertical walls and mold undercuts are virtually impossible to achieve by this process.
The deep drawing of a polyurethane film is another method of forming a polyurethane skin. However, this process is subject to the same problems as the deep drawing of films of other materials. The deep-drawn skin has an irregular thickness (uneven stretching) and complicated mold geometries, such as undercuts are virtually impossible to achieve.
A polyurethane skin could also be formed by the slush molding of a polyurethane thermoplast powder. This process is known in the case of PVC powders. However, mold temperatures above 180.degree. C. would be necessary and the energy costs would be very high.