Polymer-coated metal surfaces

The present invention relates to a metal surface coated with a thermoplastic polymer, the coating exhibiting a peel strength at 130.degree. C. (according to DIN 30 670) which is higher than 400 N/5 cm. This value can be obtained by placing between the metal and the thermoplastic a layer of epoxy resin of Tg higher than 120.degree. C. against the metal and then a layer of polypropylene-based binder. The invention is particularly useful for coating the external surface of pipes.

TECHNICAL FIELD
 The present invention relates to polymer-coated metal surfaces and more
 particularly to pipes the external surface of which is coated with a
 thermoplastic polymer. The Applicants have discovered thermoplastic
 polymer coatings which exhibit a peel strength at 130.degree. C. which is
 higher than 400 N/5 cm. A means of obtaining this peel strength is, for
 example, to place between the metal and the thermoplastic a layer of epoxy
 resin which has a glass transition temperature higher than 120.degree. C.
 and a layer of polypropylene-based binder, the layer of epoxy resin being
 against the metal.
 BACKGROUND OF THE INVENTION
 U.S. Pat. No. 4,606,953 to Suzuki et al. (and DE 3422920) describes
 coatings for steel pipes, including successively a layer of epoxy resin, a
 layer of grafted polypropylene and finally an external layer of a mixture
 of polypropylene and of a polypropylene/polyethylene block copolymer. The
 glass transition temperature (T.sub.g) of the epoxy resin is between 80
 and 94.degree. C. These coatings are suitable for hot water at 90.degree.
 C.
 U.S. Pat. Re. No. 30,006 to Sakayori et al. describes coatings for steel
 pipes including successively an epoxy resin and a polyethylene modified by
 grafting or copolymerization with maleic anhydride. Nothing is written
 concerning the T.sub.g of the epoxy resin; however, the polyethylene does
 not make it possible to work above 80.degree. C. Thus, the prior art has
 not described any coating which has a high peel strength at 130.degree. C.
 like that of the invention.
 SUMMARY OF THE INVENTION
 The present invention is therefore a metal surface coated with a
 thermoplastic polymer, the coating exhibiting a peel strength at
 130.degree. C. (measured according to DIN standard 30 670) which is higher
 than 400 N/5 cm.
 DETAILED DESCRIPTION OF THE INVENTION
 The metal surface may be of any kind; however, the invention is
 particularly useful for the external surface of pipes, it being possible
 for these pipes to have an external diameter, for example, of up to 0.8 m
 or up to 1.5 m and a thickness of 2 to 25 mm.
 The thermoplastic polymer may be of any kind, provided that its working
 temperature is higher than or equal to 130.degree. C. and preferably
 between 130 and 150.degree. C.
 To give an example, polypropylene, polyamides and polyamide blends may be
 mentioned, it being possible for these polymers to be filled with glass
 fibers. Polypropylene is intended to mean propylene homopolymers and the
 copolymers of propylene with at least one alpha-olefin and with propylene
 predominating by weight. This alpha-olefin is, for example, ethylene.
 Polyamide is intended to mean the products of condensation:
 of one or more amino acids such as aminocaproic, 7-aminoheptanoic,
 11-aminoundecanoic and 12-aminododecanoic acids or of one or more lactams
 such as caprolactam, oenantholactam and lauryllactam;
 of one or more salts or mixtures of diamines such as hexamethylenediamine,
 dodecamethylenediamine, metaxylylenediamine, bis-p-aminocyclohexylmethane
 and trimethylhexamethylenediamine with diacids such as isophthalic,
 terephthalic, adipic, azelaic, suberic, sebacic and dodecanedicarboxylic
 acids; or
 of mixtures of certain monomers, which results in copolyamides.
 Polyamide mixtures may be employed. PA-6 and PA-6,6 are advantageously
 employed.
 A polyamide blend within the meaning of the invention is intended to mean
 the mixtures of polyamide which are in the form of polyamide matrix in
 which nodules of a polymer (A) or of a rubber are dispersed, the blend
 having to exhibit a working temperature as indicated above.
 By way of example of polymer (A) there may be mentioned the above
 polypropylene, crosslinked polyethylenes or crosslinked mixtures of (i)
 ethylene copolymers including maleic anhydride and of (ii) ethylene
 copolymers including glycidyl methacrylate.
 As example of rubbers it is possible to mention: styrene-butadiene (SBR),
 nitrile-butadiene (NBR), natural rubber, polyisoprene, polybutadiene,
 butyl rubber, styrene-butadiene-styrene (SBS) block copolymers,
 styrene-isoprene-styrene (SIS) copolymers and
 styrene-ethylene/butene-styrene (SEBS) copolymers.
 The polymer (A) and the rubber may optionally carry functional groups to
 facilitate compatibilizing 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. By way of
 example of carboxylic acids it is possible to mention: 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, dimethylmaleic anhydride.
 By way of examples of derivatives it is possible to mention: salts, amides,
 imides and esters, such as sodium mono- and dimaleate, acrylamide,
 maleimide and dimethyl fumarate. The grafting processes are known to a
 person 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 of polymer (A) or of
 rubber. This product is known per se. To give an example it is possible to
 mention: polypropylene grafted with an unsaturated carboxylic acid, a
 carboxylic acid anhydride or their derivatives. These products can be
 chosen from the products of grafting which are described above.
 The compatibilizing agent may also be a mixture of grafted polypropylene
 and of an elastomer such as an EPR or EPDM rubber.
 Where polypropylene is concerned, the compatibilizing agent is
 advantageously an ethylene/propylene copolymer with propylene
 predominating and grafted with a product which has a site that is reactive
 with amines and then condensed with polyamides or polyamide oligomers
 which have a single amine end. These compatibilizing agents and the
 corresponding polyamide blends are described in U.S. Pat. No. 5,342,886 to
 Glotin et al., the content of which is incorporated into the present
 application.
 The quantity of polyamide forming the matrix of these polyamide blends may
 be between 55 and 95% by weight of the combination of the polyamide and of
 the polymer (A) or of the rubber. These polyamide blends can be
 manufactured by the usual melt-blending techniques (twin screw, Buss or
 single screw).
 The peel strength at 130.degree. C. is at least 400 N/5 cm; it can reach
 650 N/5 cm if the thermoplastic is filled with glass fibers. At
 150.degree. C. the peel strength is higher than 180 N/5 cm; it can reach
 200 N/5 cm in the case of polyamides or polyamide blends and 350 N/5 cm in
 the case of thermoplastics filled with glass fibers
 The Applicants have discovered that a means for obtaining these peel
 strengths is, for example, to place between the metal and the
 thermoplastic a layer of epoxy resin which has a glass transition
 temperature higher than 120.degree. C. and a layer of binder based on
 functionalized polypropylene, the layer of epoxy resin being against the
 metal.
 The present invention also relates to this metal surface thus coated.
 The basis of the epoxy resins is described, for example, in the Kirk-Othmer
 Encyclopedia of Chemical Technology, Vol. 9--pages 267-289, 3rd edition.
 It suffices to choose a resin which has the required T.sub.g. These resins
 are in most cases polyglycidyl ethers of a polyphenol.
 The following are advantageously employed:
 products of condensation of bisphenol A and of epichlorohydrin;
 epoxy-cresol novolak (ECN) resins;
 epoxy phenol novolaks;
 resins derived from bisphenol F;
 derivatives of polynuclear phenols and of glycidyl ethers;
 cycloaliphatic resins;
 resins derived from aromatic amines such as:
 derivatives of tetraglycidylmethylenedianiline,
 derivatives of triglycidyl-p-amino-phenol,
 derivatives of triazines such as triglycidyl isocyanurate;
 resins derived from hydantoin.
 The resins employed in the present invention are crosslinkable between 180
 and 250.degree. C. Crosslinking can be carried out, for example, with
 amines such as dimethylethanolamine and methylenedianiline or amides such
 as dicyandiamide, or else phenolic resins.
 These resins may include additives such as silicones, pigments such as
 titanium dioxide, iron oxides, carbon black, and fillers such as calcium
 carbonate, talc or mica.
 The gel time is advantageously between 20 and 30 seconds.
 Gel time is defined by Association Francaise de Normalisation (AFNOR)
 standard NFA 49-706. It is the time necessary to produce a rapid increase
 in the viscosity at a determined temperature.
 The T.sub.g is advantageously higher than 150.degree. C. These resins can
 take the form of powder or liquid which is sprayed onto the metal surface,
 which is degreased, sandblasted and heated beforehand.
 The polypropylene-based binder denotes, for example, compositions including
 essentially 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 of melt index (MI) 0.1 to 10 g/10
 min at 230.degree. C. under 2.16 kg is advantageously grafted with maleic
 anhydride in the presence of initiators such as peroxides. The quantity of
 maleic anhydride which is actually grafted may be between 0.01 and 10% by
 weight of the grafted polypropylene. The grafted polypropylene may be
 diluted with polypropylene, EPR and EPDM rubbers or copolymers of
 propylene and of an alpha-olefin. According to another alternative form it
 is also possible to perform a cografting of a mixture of polypropylene and
 of EPR or of EPDM, that is to say to add an unsaturated carboxylic acid,
 an anhydride or their derivatives to a mixture of polypropylene and of EPR
 or of EPDM in the presence of an initiator.
 The thickness of the layer of epoxy resin may be between 20 and 400 .mu.m
 and preferably between 50 and 150 .mu.m.
 The thickness of the binder layer may be between 100 and 500 .mu.m and
 preferably between 200 and 350 .mu.m.
 The thickness of the layer of thermoplastic polymer may be between 0.5 and
 5 mm and preferably 1.5 and 3 mm.
 It would not constitute a departure from the scope of the invention to add
 fillers, anti-UV agents, pigments, stabilizers, flame retardants and the
 like to the epoxy resin, to the binder, to the thermoplastic polymer or to
 any combination of the above.
 The present invention also relates to a process for the manufacture of
 these coated surfaces. The metal surface is first of all degreased,
 sandblasted, and then heated. The epoxy resin is deposited in liquid form,
 or by spraying or electrostatic spraying if it is a powder, onto the metal
 surface which is heated to about 200-240.degree. C. After approximately 20
 seconds, that is to say shortly before the end of the gel time, before the
 resin is crosslinked, so that epoxide functional groups remain for
 reacting with the graft units of the binder, the binder is deposited
 either by spraying if it is a powder, or by coating or rolling. The
 thermoplastic polymer is then deposited in the same way.
 Where the external surface of metal pipes is concerned, the procedure is
 the same in the case of the epoxy resin and then the binder is either
 deposited by spraying if it is available as a powder or, in most cases,
 extruded in an annular die arranged concentrically around the pipe. The
 binder can also be extruded in a flat die producing a continuous tape
 which is wound around the pipe, for example by virtue of the rotation of
 the pipe about itself. The thermoplastic is deposited in the same way.
 The present invention also relates to a coated metal surface including
 successively a layer of epoxy resin placed against the metal and having a
 glass transition temperature higher than 120.degree. C., a layer of binder
 based on polypropylene modified by grafting and a layer of thermoplastic
 polymer.

EXAMPLES
 In the following examples the products hereinafter are employed:
 EUROKOTE 714-31PP epoxy resin denotes an epoxy resin which has a T.sub.g
 =105.degree. C. and is supplied by the company Bitumes Speciaux and has
 the following characteristics:
 Density at 23.degree. C. (NFT 30-043): 1.5.+-.0.05 g/ml
 Moisture content (IBS 319).ltoreq.0.50%
 Particle size (IBS 316):
 median diameter 38.+-.4 .mu.m
 oversize at 96 .mu.m&lt;10%
 T.sub.g (NFA 49-706): 105.degree..+-.5.degree. C.
 Gel time 80.+-.10 seconds at 180.degree. C.
 EPOXY P405/06 epoxy resin denotes an epoxy resin which has a T.sub.g
 =160.degree. C. and is supplied by the company Bitumes Speciaux. It has a
 gel time of 25.+-.5 seconds at 210.degree. C.
 EPOXY 500 618 epoxy resin denotes an epoxy resin which has a T.sub.g
 =150.degree. C. and is supplied by Akzo and has a gel time of 25.+-.5
 seconds at 210.degree. C.
 OREVAC 1 polypropylene, OREVAC 2 polypropylene and OREVAC 3 polypropylene
 and 3 denote polypropylenes grafted with maleic anhydride (MAH),
 containing approximately 0.5% by weight of MAH and which have the
 following characteristics: