METALLIC SURFACES COATED WITH POLYMERS. Field of the Invention The present invention relates to metal surfaces coated with polymers and, more particularly, to pipes whose outer surface has been coated with a thermoplastic polymer. The applicant entity has discovered thermoplastic polymer coatings that exhibit a peel strength at 130 ° C which is greater than 400 N / 5 cm State of the Art One means of obtaining this peel strength consists, for example, of placing, between metal and thermoplastic, a layer of epoxy resin which has a vitreous transition temperature higher than 120 ° C and a layer of binder based on polypropylene, the epoxy resin layer remaining against the metal, DE 3 4222 920 describes coatings for pipes steel, including, in turn, a layer of epoxy resin, a layer of grafted polypropylene and, finally, an outer layer of a polypropylene mixture and a polypropylene / polyethylene block copolymer.The vitreous transition temperature (Tg) of the epoxy resin is between 80 and 94 ° C. These coatings are suitable for hot water at 90 ° C. Re 30006 describes coatings for pipe Steel bars that include, in turn, an epoxy resin and a polyethylene modified by grafting or copolyzing with maleic anhydride. Nothing is mentioned in relation to the Tg of the epoxy resin; however, polyethylene makes it impossible to work above 80 ° C. Thus, the prior art has not disclosed a coating that has a high peel strength at 130 ° C such as occurs in the present invention. Summary of the Invention. Therefore, the present invention consists of a metal surface coated with a thermoplastic polymer, the coating exhibiting a peel strength at 130 ° C (measured according to DIN 30 670) which is greater than 400 N / 5 cm. Objects and Preferred Modalities of the Invention The metallic surface can be of any type; however, the invention is particularly useful for the outer surface of pipes, it being possible for these pipes to have an outer diameter, for example, of up to 0.8 or 1.5 m and a thickness of 2 to 25 mm. The thermoplastic polymer can be of any type, provided that its working temperature is greater than or equal to 130 ° C and preferably between 130 and 150 ° C. As examples, polypropylene, polyamides and mixtures of polyamides can be mentioned, it being possible that these polymers can be loaded with glass fiber. By
"polypropylene" is meant to mean propylene homopolymers and propylene copolymers with at least one alpha-olefin and with propylene predominating in weight percent. This alpha-olefin is, for example, ethylene. By "polyamide" it is meant the condensation products: - of one or more amino acids such as aminocaproic, 7-aminoheptanoic, 11-aminoundecanoic and 12-aminododecanoic acids or one or more lactams such as caprolactam, enantolacta and lauryl-lactama; - of one or more salts or mixtures of diamines such as hexamethylenediamine, dodecamethylenediamine, meta-xylylene diamine, bis-p-aminocyclohexyl methane and tri ethylhexamethylene diamine with diacids such as isophthalic, terephthalic, adipic, azelaic, suberic, sebacic acids and dodecanodicarbo-xyl; or - of mixtures of certain monomers, which result in copolyamides. Mixtures of polyamides can be used. Conveniently, PA-6 and PA-6,6 are used. The term "mixture of polyamides" within the meaning of the invention means the polyamide mixtures which are in the form of a polyamide matrix in which nodules of a polymer (A) or a rubber are dispersed, the mixture having that exhibit a working temperature as indicated above. As examples of polymers (A), mention may be made of said polypropylene, crosslinked polyethylenes or crosslinked mixtures of (i) copolymers of ethylene including maleic anhydride and (ii) copolymers of ethylene including glycidyl methacrylate. As examples of rubbers can be mentioned styrene-butadiene (SBR), nitrile-butadiene (NBR), natural rubber, polyisoprene, polybutadiene, butyl rubber, styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene (SIS) copolymers and styrene-ethylene / butene-copolymers styrene (SEBS). The polymer (A) and the rubber can optionally carry functional groups to facilitate compatibilization with the polyamide. These functional groups can be obtained by grafting at least one unsaturated carboxylic acid, an anhydride and the derivatives of these acids and anhydrides. As examples of carboxylic acids, mention may be made of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, itaconic anhydride, nadic anhydride, maleic anhydride and substituted maleic anhydrides such as, for example, ethyl dihydride. maleic As examples of derivatives may be mentioned salts, amides, imide and esters, such as sodium mono- and dimaleate, acrylamide, aleimide and dimethyl fumarate. The grafting processes are already known to those skilled in the art. The formation of the polyamide matrix can also be facilitated by adding a compatibilizing agent to the mixture of polyamide and polymer (A) or rubber. This product is known per se. As an example, polypropylene grafted with an unsaturated carboxylic acid, a carboxylic acid anhydride or its derivatives can be mentioned. These products can be chosen from the graft products that have been described above. The compatibilizing agent can also be a mixture of grafted polypropylene and an elastomer such as an EPR rubber or EPDM. As far as polypropylene is concerned, the compatibilizing agent is conveniently an ethylene / propylene copolymer wherein propylene predominates and grafted with a product having a point which is reactive with amines, and subsequently condensed with polyamides or polyamide oligomers having a single end amine. These compatibilizing agents and the corresponding mixtures of polyamides are described in patent US 5 342 886, the content of which is incorporated herein for reference purposes only. The amount of polyamide forming the matrix of these polyamide mixtures can be between 55 and 95% by weight of the combination of the polyamide and the polymer (A) or of the rubber. These mixtures of polyamides can be prepared by the usual techniques of mixing in the molten state (with double screw, Buss or with a single screw). The peel strength at 130 ° C is at least 400 N / 5 cm; this value can reach 650 N if the thermoplastic material is loaded with fiberglass. At 150 ° C, the peel strength is greater than 180 N / 5 cm; it can reach a value of 200 N / 5 cm in the case of polyamides or mixtures of polyamides and a value of 350 N / 5 cm in the case of thermoplastics loaded with glass fiber. The applicant entity has discovered that a means for obtaining these peel strengths consists, for example, of placing, between the metal and the thermoplastic, a layer of epoxy resin having a glass transition temperature greater than 120 ° C and a layer of binder based on functionalized polypropylene, leaving the epoxy resin layer against the metal. The present invention also relates to these metallic surfaces thus coated. The base of the epoxy resins is described, for example, in the Kirk-Othmer Encyclopedia of Chemical Techno-logy, Vol. 9 - pages 267-289, 3rd edition. It is sufficient to choose a resin that has the required Tg. These resins are in most cases polyglycidyl ethers of a polyphenol. Conveniently, the following are used: - condensation products of bisphenol A and epichlorohydrin; - Novolac epoxy-cresol resins (ECN); - epoxy-phenolic novolacs; - Resins derived from bisphenol F; - Derivatives of polynuclear phenols and glycidylleters; - Cycloaliphatic resins; Resins derived from aromatic amines such as: tetraclicidylmethylenedianiline derivatives, triglycidyl-p-aminophenol derivatives and triazine derivatives such as triglycidyl isocyanurate; - Resins derived from indantoin. The resins used in the present invention are crosslinkable between 180 and 250 ° C. The crosslinking can be effected, for example, with amines such as dimethylene-tanolamin and methylenedianiline or amides such as dicyandia-mida, or with phenolic resins. These resins may include additives such as silicones, pigments such as titanium dioxide, iron oxide, carbon black and fillers such as calcium carbonate, talc or mica. The gelling time is conveniently comprised between 20 and 30 seconds. The gelling time is defined by the AFNOR NFA 49-706 standard. It is the time necessary to produce a rapid increase in viscosity at a certain temperature. The Tg is suitably higher than 150 ° C. These resins may be in the form of powder or liquid which is sprayed onto the metal surface, which has previously been degreased, sandblasted and heated. The binder based on polypropylene represents, for example, compositions which essentially include polypropylene functionalized by grafting with at least one unsaturated carboxylic acid, an unsaturated carboxylic acid anhydride or derivatives of these acids and anhydrides. These products have already been referred to above. A polypropylene with a melt index (MI) of 0.1 to 10 g / 10 minutes at 230 ° C, under 2.16 kg, is suitably grafted with maleic anhydride in the presence of initiators such as peroxides. The amount of grafted maleic anhydride can actually be from 0.01 to 10% by weight of the grafted polypropylene. The grafted polypropylene can be diluted with polypropylene, EPR rubbers and EPDM or copolymers of propylene and an alpha-olefin. According to another alternative, it is also possible to carry out a cografting of a mixture of polypropylene and EPR or EPDM, that is, adding an unsaturated carboxylic acid, an anhydride or its derivatives to a mixture of polypropylene and EPR or EPDM in the presence of an initiator. The thickness of the epoxy resin layer can be from 20 to 400 μm, preferably from 50 to 150 μm. The thickness of the binder layer can be from 100 to 500 μm, preferably from 200 to 350 μm. The thickness of the thermoplastic polymer layer can be 0.5 to 5 mm, preferably 1.5 to 3 mm. It should not constitute a deviation from the scope of the invention to add fillers, anti-UV agents, pigments, stabilizers, flame retardants and similar to epoxy resin, binder and thermoplastic polymer. The present invention also relates to a process for the preparation of these coated surfaces. First, the metal surface is degreased, sandblasted and then heated. The epoxy resin is deposited in liquid form or by electrostatic spray or spraying and is in the form of powder, on the metal surface, which is heated to approximately 200-240 ° C. After about 20 seconds, ie shortly before the end of the gelling time, before the resin is crosslinked, so that functional epoxy groups remain to react with the graft units of the binder, the binder is deposited either by spraying it into the binder. whether it is a powder or by coating or roller application. The thermoplastic polymer is then deposited in the same manner. In relation to the outer surface of metallic pipes, the procedure is the same in the case of the epoxy resin and then the binder is deposited either by spraying it in the case that it is in powder form or, in most cases, by extrusion by an annular nozzle arranged concentrically around the pipe. The binder can also be extruded by a flat nozzle to produce a continuous ribbon which is wound around the pipe, for example by means of the rotation of the pipe around itself. The thermoplastic is deposited in the same way. The present invention also relates to a coated metal surface that includes, in turn, a layer of epoxy resin placed against the metal and having a vitreous transition temperature greater than 120 ° C, a layer of binder based on graft-modified polypropylene. and a layer of thermoplastic polymer. Examples The following products are used in the following examples: Eurokote 714-31PP represents an epoxy resin having a Tg-105 ° C and is supplied by the company Bitumes Spéciaux and has the following characteristics: Density at 23 ° (NFT 30 -043): 1.5 ± 0.05 g / ml Moisture content (IBS 319) = 0.50% Particle size (IBS 316): Mean diameter 38 ± 4 μm Size greater than 96 μm <; 10% Tg (NFA 49-706): 105 ° ± 5 ° C Gel time 80 ± 10 s at 180 ° C. Epoxy P405 / 06 represents an epoxy resin that has a Tg - 160ßC and is supplied by the company Bitumes Spéciaux. It has a gel time of 25 ± 5 s at 210 ° C. Epoxy 500618 represents an epoxy resin having a Tg = 150 ° C and is supplied by Akzo and has a gel time of 25 ± 5 s at 210"C. Orevacs 1, 2 and 3 represent polypropylenes grafted with maleic anhydride (MAH) containing approximately 0.5% by weight of MAH and having the following characteristics:
Orgalloy 1 represents a blend of: 60% PA-6 with melt index 2-3 (235 ° C) 30% polypropylene with melt index 1,5-2 (235 ° C) - 10% copolymer ethylene / propylene backbone containing 12% ethylene, grafted with maleic anhydride (1% anhydride with respect to the spine) and then condensed with a caprolactam monoamine oligomer with a degree of polymerization of 22, the amount of these oligomers being of 25% with respect to the spine. PP represents a polypropylene of MI 1 (230ßC, 2.16 kg) and MI 4 (230 ° C, 5 kg) and Shore D hardness 63 (ISO 868). Example 1 The following three-layer coating is produced: Steel / Epoxy / Orevac / PP / 20% short fiberglass (t - 2.5 mm) (70 μm) (250-300 μm) Application conditions? Pipe - 200-220 ° C Time (epoxy / adhesive) = 22-25 s; This is the time between the application of the epoxy resin and the application of Orevac. Time (epoxy / cooling) - 3 'Table 1
Division between steel and epoxy resin at 130 ° C. Example 2 The following three-layer coating is produced: Steel / Epoxy / Orevac / Orgalloy 1 (t = 2.5 mm) (70 μm) (250-300 μm)? Pipe - 200-220 ° C Time (epoxy / adhesive) = 22-25 s; This is the time between the application of the epoxy resin and the application of Orevac. Time (epoxy / cooling) »3 'Table 2
Division between steel and epoxy resin between 130 ° and 150 ° C. Example 3 The following three-layer coating is produced: Steel / Epoxy / Orevac / PP (70 um) (250-300 μm) t - 2.5 mm? Pipe = 200-220"C Time (epoxy / adhesive) = 22-25 s Time (epoxy / cooling) = 3 '