The invention relates to a composite material comprising a support with at least two major surfaces and at least one first and at least one second plastic layer arranged on at least one major surface, a process for the preparation thereof, and the use thereof as a mold or support in the baking, meat or sweets industries.

Objects with an anti-adhesion coating enjoy an extensive use in the household and industry. Thus, teflon-coated pans have been used for decades. To prepare such objects, fluoropolymers are applied to the corresponding supports as powders, especially by means of electrostatic methods. Such powder coatings result in an extremely thin coating thickness. Due to this thin coating thickness, the corresponding objects suffer from mechanical loads, which often lead to damage to the coating and thus adversely affect the anti-adhesion properties of such equipment up to the extent where they become useless. Thus, in the baking industry, for example, this means that only very short service lives of the coated baking plates are to be noted in part. The damage to the anti-adhesion coatings arising from mechanical loads also lead to problems with hygiene. The damaged sites are often contaminated with residues utilizable by microorganisms, which results in an increased cleaning expenditure, which means increased stress, either mechanical or chemical from washing liquors, and accelerates the destruction of the anti- adhesion coating.

WO 00/12622 discloses an improved heat transfer release finish on cookware which is formed by coating the cookware interior (sidewall and flat bottom) with a mixture of fluoropolymer and magnetic flakes and magnetically orienting said flakes to run in the thickness direction of the coating, followed by baking the coating. Advantageously, the coating mixture also contains compatibilizing polymer which enables the flakes to be so-oriented without creating minute fissures in the coating. US-4,252,859 discloses blends of PTFE and perfluorinated ether copolymers of TFE and perfluoro(propyl vinyl ether) provide coating compositions which have reduced crystallinity and useful lives superior to what would be expected from averaging the properties of the unblended resins themselves.

US-5,705,017 discloses a method of producing a steel sheet coated with a fluororesin film by heat-fusing the fluororesin film onto a surface of the steel sheet, comprising heating the surface of the steel sheet to a temperature higher by 30- 120 ⁰ C than a melting point of the fluororesin of the fluororesin film, and contact bonding the fluororesin film onto the steel sheet surface between a pair of rolls, while a surface of a roll facing the fluororesin film, in the pair of rolls, is heated to a temperature lower by 100-210 ⁰ C. than the melting point of the fluororesin of the fluororesin film.

WO 93/11935 discloses a fluororesin film coated steel plate obtained by forming a substrate treatment film consisting of a mixed resin of a fluororesin and a heat resisting resin on the surface of an aluminium plated steel plate, an aluminium-zinc alloy plated steel plate, an alloyed molten zinc plated steel plate or a stainless steel plate, and further forming a thermally fused film consisting of a thermoplastic fluororesin film on the substrate treatment film. This resin coated steel plate is superior not only in heat resistance, durability and adhesion but also in the adhesion, wear resistance, processability and corrosion resistance of the film.

JP 1986/0274867 discloses a surface of Al(alloy) plate which is chemically converted to form a hydroxide film. The chemical conversion film is coated with thermoplasticfluor containing resin and then melt coated with a PTFE resin film. The degreased Al plate is immersed in an aqueous 0.2-1.5 vol.% solution of an organic basic compound e.g. triethanol-, diethanol- or mono-ethanol amine, in deionized water having specific resistivity of at least 500 x 10 ⁴ ohm. cm at 90-100 ⁰ C for 1-15 min. and then optionally after treating it by immersing in an aqueous 0.35-1.05 vol.% solution of water glass having molar ratio of SiO ₂ ZNa ₂ O of 2.0-4.0 in the same deionized water at 90-100 ⁰ C for 1-15 min. The pretreated Al plate is washed and optionally, coated with a solution of an aminosilane compound by immersion, brushing, spraying or by a roll coater and then coated with a thermo- plastic fluor containing resin e.g. tetrafluoroethylene copolymer with hexafluoro- propylene, perfluoroalkoxyethylene or ethylene, by coating and baking the powder or dispersion or melt laminating the thin resin film. The precoated plate is then hot pressed with a PTFE film.

Due to the preparation process, objects prepared in this way have a microporosity on their surfaces bearing the fluoropolymer coating. If other objects are molded in molds provided with fluoropolymer coatings, for example, in industrial baking processes, problems with the release effect often occur.

Thus, one object of the invention is to avoid the above mentioned drawbacks of the prior art. In particular, the invention is to provide an object having a plastic layer that has a low microporosity and is mechanically more robust than the previously known plastic-coated composite materials, especially objects coated with fluoropolymers. Another object of the invention is to provide a process for the preparation of said object.

Figure 1 schematically shows the laminate structure of the composite material according to the invention.

According to the invention, one object of the invention is achieved by the composite material according to the invention comprising a support with at least two major surfaces 2/3 and at least one first 10 and at least one second plastic layer 11 arranged on at least one major surface 2/3, characterized in that

said support is a metallic support 1;

- said at least one first plastic layer 10, is a plastic sheet, and said at least one second plastic layer 11 essentially consist of perfluorinated plastic material;

- said at least one first plastic layer 10 has chemical groups forming a permanent bond, especially a chemical bond, on the major surface 3 of the metallic support 1, the plastic layer is a sheet essentially consisting of a copoly- mer of PTFE and PPVE or a copolymer of PPVE with MVE (methyl vinyl ester - preparation of methylethylvinyl modified PTFE (MFA));

said at least one first plastic layer 10 has chemical groups forming a cova- lent bond with said at least one second plastic layer 11 upon thermal treatment, optionally under a molding pressure; and

- said at least one second plastic layer 11 has first and second major surfaces 111/112, wherein said first major surface 111 of said second plastic layer 11 forms said bond with the surface of the first plastic layer 10 that does not face towards the support 1; and

- said second plastic layer cannot form permanent chemical bonds with the support 1 and has a lower number of chemical groups capable of forming a covalent bond with said first plastic layer 10.

In general the chemical structure of the first layer 10 being a polypropyl vinyl ether (PPVE) modified polytetrafluoro ethylene (PTFE) enables both, the adhe- sion to the second plastic layer, preferably being perfluoro alkoxy (PFA), as well as the release from the metal plate of the hot press upon completion of the lamination process. Tetrafluoromethylene vinylether (TFM), a PTFE like polymeric material which contains traces of PPVE, low enough to have inherent PTFE- like release properties, but high enough to allow a sufficient covalent bond to the second layer, preferably being PFA, combines both features to enable the manufacturing of durable laminations as described in this invention.

The above mentioned PTFE or PFA sheets are available as Teflon ^® films from Dupont USA; and the TFM is available from Dyneon, USA whereas MFA is commercially available as Hyflon ^® from the company Solexis, Belgium.

In particular, the composite material according to the invention has a second plastic layer 11 with a surface that has no microporosity. This can be achieved by laminating the second plastic layer 11 as a sheet onto the support. In one embodiment of the composite material according to the invention, it has a first plastic layer 10 with an adhesion promoter, whereby the adhesion of the first plastic layer 10 to the major surface 2 of the metallic support 1 is improved.

In particular, the first plastic layer 10 of the composite material according to the invention is a sheet of perfluoropolyethylene having perfluoroalkoxy groups for improving the adhesion of the first plastic layer 10 to the major surface 2 of the metallic support 1.

The adhesive effect of the first plastic layer 10 to the metallic support can be significantly increased by gamma irradiation of the perfluorinated material. The irradiation of the plastic layer 10 significantly increases the number of reactive groups, probably through the formation of free radicals. The first plastic layer 10 may be an irradiated sheet or powder of a perfluorinated material. The chemical bond can be subjected to supplementary treatment by chemical or physical treatment of the metallic support 1, for example, by sand blasting, undercutting the metallic support or controlled etching. The supplementary treatment improves the bonding of the plastic layers and is advantageous in terms of permeative processes occurring in aggressive preparation processes, for example, alkaline operation.

In another embodiment of the composite material according to the invention, said first and/or second plastic layers 10, 11 include adhesion promoters that improve the adhesion of the first plastic layer 10 to the second plastic layer 11.

The second plastic layer 11 may contain fillers, preferably plate-like additives (mica), for reducing the permeation of alkalis, for example.

In case of a thermo/vacuum forming manufacturing process the composite material according to the invention may comprise in an other embodiment of the invention that each of said first and second plastic layers 10, 11 is a sheet essentially consisting of a copolymer of tetrafluoroethylene (TFE) and PPVE. A copolymer with MVE (methyl vinyl ester - preparation of MFA) may also be used. The technology of thermo/vacuum forming is well-known to the skilled person. In the second plastic layer 11, the PPVE content may be clearly reduced in order to optimize TFE-like properties, especially the anti-adhesion properties to adjust the release effect.

The structure of the composite material according to the invention as a laminate of sheets 11 and 12 and metallic support 1 has the advantage that the sheet creates a closed structure to avoid microporosities. Further, the service life of the coating/metal plate can be adjusted to virtually any value by selecting the sheet thickness.

The PPVE or MVE content in the second plastic layer 11 of the composite material according to the invention may be lower than that in the first plastic layer 10.

The use of PTFE sheets as plastic layers of the laminate enables the best possible release effect. Of all plastic materials, PTFE has the lowest surface tension, which results in the very good release effects.

The second plastic layer 11 of the composite material according to the invention may be provided with a macroscopic structure at the surface thereof that is not bonded to the first plastic layer 10. The macroscopic structure may be created, for example, by correspondingly structures sheets or by subsequent embossing. The use of structures or, in particular, corrugated molding plates enables the cover layer to be embossed in order to avoid slipping of the baking product on auto- mated industrial baking lines. The corrugation advantageously causes an alternative conduction of hot air to the baking product as compared to a uniformly flat surface (point contact of the baking product) and thus a reduction of baking times.

The composite material according to the invention is a laminate of a metallic support 1, first and second plastic layers 10, 11.

If the composite material according to the invention is to find use as a support for lye-glazed bakery products (salted pastry), it is recommendable to apply two plastic layers 10 to the two major surfaces of the metallic support. When lye- glazed bakery products are prepared, the use of lyes at a high temperature (220 ⁰ C) usually has an extremely corrosive effect on the metallic support. For this reason, the metallic support is provided with a plastic sheet on both sides thereof, virtually "housed in", wherein the first and second plastic layers 10, 11 are applied to both sides of the metallic support, binding the edges. The second plastic layer 11 preferably contains inorganic fillers, especially mica, in order to reduce or avoid lye permeation.

The composite material according to the invention can be prepared by the process according to the invention comprising the following steps:

applying to the metallic support 1 a first plastic layer 10 in the form of a sheet of essentially PTFE with chemical groups forming a permanent bond to the metallic support 1; a plastic layer is a sheet essentially consisting of a copolymer of PTFE and PPVE or a copolymer of PPVE with MVE (methyl vinyl ester-preparation of methylethylvinyl modified PTFE (MFA));

applying a second plastic layer 11 in the form of a sheet of essentially PTFE with chemical groups forming a covalent chemical bond with said first plastic layer 10 upon the action of heat, the second plastic layer 11 optionally being provided with fillers;

realizing the application of said first and second plastic layers 10, 11 simultaneously or successively; and

- the stack of metallic support 1 and first and second plastic layers 10, 11 is laminated by the action of heat and pressure.

In the hot molding of the plastic sheets, it is advantageous that the use of a release film is avoided in the preparation process if a TFM sheet with PTFE-like release effect properties is employed as the second plastic layer 11. Otherwise, a separate release film, for example, based on polyimide or made of a flexible ceramic, can be employed in the hot lamination process. In particular, the composite material according to the invention can be used as a mold or support in the baking, meat or sweets industries.