The present invention relates to a biaxially stretch-oriented multilayer film essentially comprising a propylene homopolymer, comprised of a highly pigmented base layer and a thin modified top layer applied to one or both of its surfaces. The invention also relates to the manufacture and use of the film.
Decorative laminate panels are widely used for producing furniture, e.g., kitchen and office furniture, but also for outdoors applications, e.g., as windowsills, etc. Special types are used under extremely severe conditions, e.g., in shower cubicles or bathrooms or as benches in chemical laboratories. The last mentioned applications, in particular, require panels which are extraordinarily resistant to scratches, to organic and aqueous solvents and to detergents. Apart from imparting a special optical appearance, the surface texture is of particular importance in respect to these characteristic features.
Laminate boards of the type described above have, for example, been disclosed by German Offenlegungsschrift No. 34 18 282. As a rule, the core layer of a laminate panel is comprised of a multilayer structure comprising a plurality of paper webs impregnated with a phenolic resin. A cellulose paper which is impregnated with a melamine resin and which may be unicolored or printed with a multicolored pattern, is usually applied to this core layer as a top layer.
Prior to the actual pressing of the top layer(s) and the core layer, the resin-impregnated papers of the core and top layers are dried and thereby partially condensed. The subsequent actual pressing is in general performed by means of heatable or coolable, automatically controlled multi-level pressing units where up to 40 laminate panels can be produced simultaneously. So-called high pressure laminates (H.P.L.) are compressed at pressures of between 70 and 100 bar and temperatures of between 140.degree. and 160.degree. C. During the compression, the resins flow and are cured, whereby a compact, non-meltable, rigid, crosslinked product is formed. Suitable presses include intermittently and continuously working units and through-feed units.
If a plurality of panels are stacked on top of one another in the presses, which is economically advantageous for low-thickness core layers, the individual panels must be separated by separators. To produce smooth surfaces, polished metal sheets having plane high-gloss surfaces are used as separators. If the laminate panels are to exhibit a textured or matte surface (for example, resembling veining or fabric texture), appropriately textured templates are required.
In order to prevent a sticking-together of the templates and resinous surfaces in those cases where textured templates are used, an additional release sheet must be inserted between the actual separator and the surface of the laminate. Apart from paper and aluminum foil, biaxially stretch-oriented polypropylene foil has been increasingly used for this purpose in recent times.
When clear (transparent) stretch-oriented polypropylene films are employed as release sheets, high-gloss surface finishes are obtained. If, however, matte surface finishes are desired, the manufacturers have to fall back on brushed aluminum foils. More recently, highly pigmented, biaxially stretch-oriented polyester films have also been employed. It is a common disadvantage of these two release films, which are used to produce matte or structured surfaces, that they are relatively expensive.
Attempts have been made to employ a highly pigmented, i.e., opaque, polyolefin film as a release film, instead of the expensive, highly pigmented polyester films. Polyolefin films of this type are, for example, described in German Offenlegungsschrift No. 28 14 311 (equivalent to U.S. Pat. No. 4,303,708). There is, however, the risk that the surfaces of the laminate panels stick to these release films.