Sandwich panel

The invention relates to a sandwich panel comprisingA. a core layer of polypropylene particle foam,B. cover layers of fiber-reinforced polypropylene and also optionallyC. decorative layers,wherein the core layer includes from 1 to 30% by weight of recyclate particles of components A, B and optionally C.

This invention relates to a sandwich panel comprising

A. a core layer of polypropylene particle foam,

B. cover layers of fiber-reinforced polypropylene and also optionally

Such sandwich panels are known, for example from DE-A 195 44 451. They are used for producing motor vehicle parts. Motor vehicle part production by stamping produces appreciable waste. It is an object of the present invention to recycle this waste.

We have found that this object is achieved, surprisingly, when the comminuted waste is added in amounts from 1 to 30% by weight in the production of foam panels from which new sandwich panels can be produced with only insignificantly inferior mechanical properties.

The present invention accordingly provides a sandwich panel as claimed in claim 1 . The invention further provides a process for producing a foam panel as claimed in claim 5 .

The polypropylene of the particle foam of the core layer can be a homopolymer or a copolymer with from 0.5 to 15% by weight of ethene and/or 1-butene. Its crystallite melting point is generally in the range from 120 to 170 C. The foam particles are produced by impregnating polypropylene minipellets with a volatile blowing agent in aqueous suspension and foaming by expansion. Foam extrusion is also possible. The particle size is customarily in the range from 2 to 8 mm, the bulk density in the range from 10 to 100 g/l.

The polypropylene of the cover layers is a homopolymer or a (graft) copolymer with maleic anhydride or acrylic acid. The cover layers preferably include from 10 to 60% by weight of glass, natural or polymeric fibers in the form of mats, nonwoven scrims, wovens or short fibers. Preference is given to glass mat reinforced polypropylene having a fiber content of from 20 to 50% by weight.

The decorative panels comprise a fiber web composed of polyester or polyamide, polymeric films or a foam film optionally laminated with a film. They may also be colored.

According to the invention, the core layer includes from 1 to 20%, preferably from 2 to 10%, by weight of recyclate particles of components A, B and optionally C. The recyclate in question is obtained for example from the waste of producing motor vehicle parts by stamping sandwich panels. Corresponding parts from end of life vehicles can also be used. These waste parts are comminuted, for example in mills with screen inserts, in which case the foam structure of the core layer remains substantially intact and the polypropylene does not melt. If desired, a portion of the fibers can be separated off by a cylinder screen machine. The recycled material has an average particle size of from 5 to 10, preferably from 6 to 8 mm. This material is then premixed in amounts from 1 to 20% by weight with virgin polypropylene foam particles and fed to the fill vessel of a molding machine. There, the particles are welded with steam in a conventional manner to form a foam panel.

This foam panel is then preferably used as core layer A in the production of the sandwich panel of this invention. First, the two faces of two cover layer panels B are heated to about 200 C., and the decorative material C is placed on each of the faces. Then a foam panel A is inserted between the two cover layers and the assembly is introduced into a thermoforming mold and molded. It is also possible to pressmold the layers at temperatures above the softening point of polypropylene.

The thickness of the core layer A is in the range from 3 to 20 mm, that of the cover layers in the range from 0.5 to 2 mm; the decorative layers can be from 1 to 5 mm in thickness as fiber web or as foam film or from 1 to 3 mm in thickness as polymeric films.

For recyclate concentrations in the core layer of up to 10% by weight, no significant deterioration in the strength values is observed.

The sandwich panels of the invention can be used for producing motor vehicle parts, such as trunk floors, parcel shelves and side door trim.