CA1179558A

CA1179558A - Process for coating metal substrates with plastics and the composite metal-plastics products thus obtained

Info

Publication number: CA1179558A
Application number: CA000396539A
Authority: CA
Inventors: Simon R. Kabalo; Franco Marchese
Original assignee: Atochem SA
Current assignee: Arkema France SA
Priority date: 1981-02-23
Filing date: 1982-02-18
Publication date: 1984-12-18
Also published as: EP0059136B1; EP0059136A3; FR2500371A1; JPS57156070A; FR2500371B1; EP0059136A2; JPH0230308B2; DE3266260D1

Abstract

IN THE PATENT OFFICE
APPLICATION

ENTITLED : A Process for coating metal substrates with plastics and the composite metal-plastics products thus obtained.
IN THE NAME OF : ATO-CHIMIE

ABSTRACT OF THE DISCLOSURE
The process comprises the following steps :
- coating the metal with a primer - drying and cross-linking said primer - coating with a suspension of a fine polyamidic powder in an organic phase - drying the organic phase components and fusing the fine powder into a continuous layer.
The primer contains an epoxyphenolic resin or an epoxy resin associated with a phenolic resin, both resins being added with one or more compound belonging to the following group :
- thermoplastic linear polyesters _ blocked aromatic or aliphatic isocyanates - one or more corrosion-inhibiting mineral salts, The polyamide suspension consists of a fine polya-mide powder (grain size lower than 40 µ ) dispersed in an organic liquide phase.

Description

- ~1'7gS~
The invention relates to the field of metal substrates coating More particularly, one obJect of this invention is a process for coating metal substrates with plastics so as to provide multilayer metal-plastics pro-ducts. This invention also encompasses the products obtai-ned by such a process.
For decoration or anti-corrosion protection pur-poses, the manufacturers were conduced to provide multi-layer metalplastics composite products or complexes. The method used therefor consisted in gluing, a plastics sheet onto a metal strip continuously fed from a coil, or in hot-rolling, in a calender or a rolling mill, a metal sheet previously treated by application thereto of an adhesion primer.
The metalsforming the metal sheet were usually steel or galvanized steel, or else aluminium or an alu-minium-based alloy.
The plastics used were principally polyvinyl chloride and~ to a lesser extend, polyethylene The present invention relates to the application onto a metal substrate of one or more plastics layer, the external layer being always o~ the polyamidic type.
Theob~ect of the invention is, on the one hand, to provide a process more economical than the known proces-ses starting from previously made films and, on the other hand, to prov~e a process applicable to polyamides.
As a matter of fact, said polyamides are par-ticularly valuable because of their resistances to tempe-rature, abrasion and chemical agents, which are better ~-D

~ i'7~5~1~
than those of the plastics used heretofore, such as PVC, and afford full protection against corrosion to the basic metal substrate.
One obJect of this invention is a process for coating a metal substrate by application thereto of one primer layer and at least one plastics layer, characte-rized in that the primerlayer comprises a resin selected from epoxy-phenolic resins, mixtures of epoxy resins and phenolic resins and combinations of said resins and resin mixtures, in that the last plastics layer is a layer of polyamide applied under the form of a suspension, in an organic liquid phase, of a polyamide powder having a particle size lower than 40 ~, and in that the whole is heated until fusion o~ the external polyamide layer and obtention of a continuous coating.
The advantageous ~eatures of the~invention are derived form the combined selection of the primer of the external plastlcs layer, which is applied under the form of a polyamide suspension, The invention includes the application to the substrate of an organic tack primer, having a structure liable to be modified by heat.
In fact, it is indispensable that the metal substrate to be covered with a polyamide suspension be previously coated with a primer because, among its other functions, said primer affords binding between the metal and the polyamide coating, due to its very good adhesive-ness to both metals and polyamides.

55~3 The primer acts to reinforce the metal protec-tion afforded by the polyamidic coating, first by preven-ting the see page between the metal and the polyamide of gases such as oxygen, carbon dioxide, steam, and further by adding its own protective action to¦that of the polya-mide.
The causes for corrosion are multiple and will differ according to the intended use for the coated metal articles; for example, the following may be mentioned, among others :
- boiling water and detergents in the case of washers for clothes or for plates, - various chemical agents if the metal-sheets are used to construct storage tanks for such products, - weathering agents if the coated articles are used outside~
The provision o~ the primer is still more in-dispensable when the metal sheet to be coated was not sub-~ected to a preliminary treatment.
According to the present invention, the primer contains an epoxy resln in an amount ranging from 1 to 20%
of the total primer weight, in association with a phenolic resin in an amount ranging form 1 to 20% of the total pri-mer weight, or an epoxyphenolic resin in an amount ranging from 1 % to 20% of the total primer weight, or a mixture of the above resins in the same amounts by weight, as re-lated tOthe primer, as those mentioned above.

According to the invention, the anti-corrosion ac-tion afforded by the primer may be promoted by adding to the primer corrosion-inhibiting chemical compounds such as zinc phosphate, zinc tetrahydroxychromate, or strontium chromate, those salts being usable alone or in mixture.
In a preferred embodiment, the compositionof the primer according to the invention includes one or more compounds selected among the : -- thermoplastic linear polyesters having a FP ran-ging from lO0 to 150C, soluble in the solvent mixture used, the content thereof preferably ranging from 10 to 25% of the total primer weight, - blocked aliphatic or aromatic isocyanates which are releasable from a temperature of 120C, the content thereof ranging from 1 to 6% of the total primer weight, - a corrosion-inhibiting mineral salt selected from the group consisting of zinc phosphate, zinc tetrahy-droxychromate, strontium chromate, used alone or in mixtu-re~ the total content thereof ranging from O.S to 15% of the primer weight.
The preparation of those primers may be readily effected in usual mixing devices, such as those for paints, it is recommended to use dispersing stirrers having impel-lers with peripheral teeth particularly suited for prepa-ring suspensions and pastes; it is advisable to previous-ly mix the solid additives ~silica, mineral salts) or in-soluble additives in a small porti~,of the solvents and to add the resultant paste to remaining composition.

~ 1'7~ 5 ~ ~
These formulation techniques are within the range of those skilled in the art and therefore require no fur-ther description, According to the invention, the second step of the coating process consists in coating the primer with a suspension of a fine polyamide-based thermoplastic powder, in an organic solvent or a mixture of organic solvents.
Once the metal substrate has been coated with the primer, it is generally more advantageous to dry it be-~ore applying the polyamide suspension. However, this step is not compulsory as far as the final heating step of the coating is liable to afford suitable bonding of the primer to the substrate, The final heating step cau-ses fusion o~ the polyamide particles and formation on the substrate of a smooth continuous layer. This invol-ves ln the polyamide layer a purely physical change occur-ring under the effect o~ the temperature rise, e.g. in a thermal treatment furnace. This ~eature is advantageous and constitutes an important difference with the processes of the prior art wherein the thermal treatment was inten-ded to cause a chemical change comparable to cross-linking within the final plastics layer, On the contrary, according to the invention, the final layer or ~inishinglayer is obtained by evaporating the liquid phase having the polyamide particles suspended therein and by fusing said particles to form the final layer, ~ 7~

According to an essential feature of the inven-tion, the grain size of the polyamide powder corresponds to a specific granulometry lying in a range lower than about 40~ and preferably ranging from about 10 to 40 ~L, more particularly from about 10 to 30 ~ .
The powders used in suspension are powders of polyamides~ viz. resins of a hi~h molecular weight derived from lactams or amino-acids wherein the hydrocarbon chain has a number of carbon atoms ranging from 4 to 20 as~ for example, caprolactam, oenantholactam, dodecalactam, unde-canolac~amS dodecanolactam~ amino-undecanoic acid, 12-amino-dodecanoic acid.
Thus the present invention provides in one aspect the process for coating a metal substrate by application thereto of one primer layer and at least one plastics layer, characterized in that the primer layer comprises a resin ~elected from epoxy-phenolic resins, mixtures of epoxy resins and phenolic resins and combinations of said resins and resin mixtures, in that the last plastics layer is a layer of polyamide applied in the form of a suspension, in an organic liquid phase, of a polyamide powder having a particle size lower than 40 ~, and in that the whole is heated until fusion of the external polyamide layer and obtention of a continuous coating, wherein, for the primer application, the content of epoxy-phenolic resin or epoxy resin associated with the phenolic resin ranges from 1 to 20% of the total primer weight.

~7~5~3 ITI a preferred embodiment the process is charac-terized in that the primer includes one or more compounds selected from: ~
- thermoplastic linear polyesters with a FP ranging from 100 to 150C, soluble in the solvent mixture used, the content thereof ranging from 10 to 25% of the total primer weight, - blocked aliphatic or aromatic isocyanates releasable from a temperature of 120C, the content thereof ranging from 1 to 6% of the total primer weight.
In another embodiment the present invention pro-vides a process as above characterized in that the organic liquid phase containing the polyamide powder in suspension comprises a solvent or a mixture of solvents including one or more of the materials from the group consisting of:
- the product sold by American Cyanamid Company under the Trade Mark CYMBL 303, and which is a hexamethoxy methyl melamine, - the product sold by Bayer Company under the Trade Mark SILICONE OL which is a surfactant, - the product sold by Esso Company under the Trade Mark SOLVBSSO 150 which is an aromatic solvent with 9 carbon atoms, - the product sold by the Rohm and Haas Company under the Trade Marks PARALOID B 48 and DM 54, said products facilitat-ing the mixing and the adhesion of the dispersion on a substrate.
In still another embodiment the invention provides such a process wherein the organic liquid phase contains the polyamide powder in suspension and includes a solvent or r 9 A 7a -11'7955~

or solvent mixture containing at least one c~mponent from the group consisting of:
- hexamethoxy methyl melamine, polymethyl siloxane, an aromatic hydrocarbon solvent, a thermoplastic acrylic ester, and a thermoplastic acrylic resin.

The polyamlde may also be a condensation product of a dicarboxylic acid with a diamine such as, for example, the 6.6~ 6.9, 6 10, 6.12, 9.6 polyamides, which are conden-sation products of hexamethylene diamine with adipic acid, azelaic acid, sebacic acid, 1.12 dodecanedioic acid and of nonamethyl~ne diamine with adipic acid Of course, the powders may consist of copolyamidesresulting from a polymerization of the various above-recited monomers.
Particularly suitable in practizing the process are the powders of :
Polyamide 6, obtained by polymerization of -caprolactam~
Polyamide 11, obtained by polycondensation of ll-aminoundecanoic acid.

~ 7b -7~SS~3 Polyamide 12, obtained by polycondensation o~ 12 aminododecanoic acid or of dodecano-lactam , or of copolyamides obtained by interpolymerization of two or three of the above-mentioned monomers Valuable copolyamides are derived from a polyconden-sation of ~-caprolactam , of amino-undecanoic acid and/or of lauryl lactam.
The higher the molecular weight of the polyamidic component of the powders, the better their resistance to ageing and their mechanical prsperties. However, their fusion viscosity is all the more high, this being detri-mental to their spreading onto the substrate upon fusion;
according to the present invention, the suspension may con-tain ad~uvants adapted to promote spreading of the fused polyamide in spite of a high fusion viscosity. Conversely, the lower the molecular weight, the higher the ~luidity of the coating in the ~used state to the pre~udice of its mechanlcal properties and its resistance to ageing and to weathering agents, In general, the Inherent viscosity (as measured at 20C ~or a solution of 0.5g in 100 ml of metacresol) of the polyamide powders, which reflects their molecular mass, ranges between values of 0 75-1.40; the nature of the poly-amide and final use intended for the coated substrate are the determinant parameters ~or selecting the most favora-ble value.

5~i~

The organic liquid phase, which contains suspended polyamide powder, includes a solvent or mixture of sol-vents selected from the group consisting of alcohols, es-ters, cetones and aromatic hydrocarbons, as it is known by man skilled in the art in view of choosing an appro-priate solvent for the polyamides.
Specific examples of appropriate solvents are ben-zyl alcohol, ethylglycol acetate, dibutylglycol acetate, N-methylpyrrolldone and the mixtures available under the trade name Solvesso 100 and 150.
The polyamide powder based suspension may contain various additives providing the formation of the suspen-sion, such as expanded silica (for example the compound available under the trade name Aerosil) or additivesacting as tensio_actives, such additives being known from the man skilled in the art for preparing paints and lacquers.
The polyamide powders used according to the inven~
tion can be obtained by all sorts of techniques known to those skilled in the art, e.g. by grinding of polyamide pellets previously cooled with liquid nitrogen, or by hot dissolution of such pellet~ in solvents and precipitation by cooling, among others.
Methods involving anionic polymerizationof the mono-mer or monomers in solvents, wherein the polymers are separated under powder from the reaction medium wherein they became insoluble, are recommended and, more particu-larly, those described in FR patents 1,601,194, 1,601,195 and 1,602,751; in fact, the particles forming the powder * Trade mark.
t g _ ~:~795~i8 are small spheres especially suitable to provide satis-factory suspensions and smooth coatings after fusion onto the metal sheet, Such powders may be in their naturally occurring state and therefore provide colorless coatings, or be pigmented, with possible introductionof the pigment or pigments in the suspension through a blending resin spe-cifically selected for this purpose, this representing a further advantage of the invention.
According to the invention, it is possible to coat the most diversified metal substrates, in particular sheets or plates of steel, galvanized steel, aluminium or aluminium-based alloys, It is to be noted that coating may be effected either on one or on both sides of such a metal-sheet, The process may be applied most advantageous-ly to metal strips of a thickness ranging from about 0,2 mm to about 2 mm, In this case, the thickness of the ex-ternal finishing polyamidic layer ranges ~rom about 20~to about 50 ~, The process according to the invention will now be illustrated by no way of limitation by the following exam-ples and description, with reference to the appended dra-wings wherein.
Fig, l diagrammatically shows a process for conti-nUouscoating of a metal strip, Fig, 2 shows an alternate embodiment of the pri-mer application step, Referring to figure l, there may be seen at le~t side thereoff the strip uncoiling station, This station includes a coil l and an accumulator la adapted to take ~17955~3 up the variations in length and tension caused by coil-replacements. Numeral 2 generally designates a surface-treatment step which in fact includes several stations, viz.
- rough pickling - brushing - fine pickling - hot rinsing - phosphatizing - chromizing - cold rinsing - chromic rinsing.
Said stations consist of separate vats through which the strip is fed.
The strip then enters a primer-application station 3, As schematically illustrated in figure 1, station 3 comprises a two-high mill operating according to the known so-called "reverse roll" technique. An alternative embodiment of this primary application step is shown in figure 2. This figure diagrammatically shows a coating step by the so-called rotogravure technique. In this case, the solution to be applied is contained in a vat 10 and de-posited onto the strip by a two-high mill 11, 12.
Of course, for coating the two sides of the strip, another coating station of the same type is to be provi-ded.
Usually, a final thickness of 6-7 microns is deemed sufficient for the primer.

~L~ 7~5~3 Referring again to figure 1, numeral 4 generally designates a furnace for drying and cross-linking the primer deposited at station 3. The temperature to be reached in furnace 4 will of course depend upon the natu-re of the primer. In practice, the temperature reached by the strip metal (called peak metal temperature or PMT) is used as a measure of the real primer curing temperature.
The plant shown in figure 1 for practizing the process of the invention further includes a station 5 for pulsed air cooling of the primer-coated strip. Station 6 servesto apply on the strip already coated with the pri-mer a layer of a dispersion of a fine polyamidic powder according to the invention. As illustrated, station 6 com-prises a two-high mill operating according to the roto-gravure principle (similar to figure 2). A drying and ge-lifying furnace 7 is used to remove the solvents compri-sing the organic phase and to fuse the fine powder parti-cles into a continuous layer, When issuing from furnace 7, the strip goes through a cooling station 8, then to a coiler 9 preceded by its accumulator 9a.
The temperature conditions in furnace 7 will depend upon the nature of the polyamides used.
It is admitted that the area lying between the cur-ve of the temperatures recorded on the metal strip bet-ween the furnace inlet and outlet, the abscissae axis (indifferentlygraduated in terms of spa ce or time) and the two verticals corresponding to the inlet and the outlet of the furnace represents the energy used to effect the ~7~5~3 steps of solvent removal~ and polyamidic powder fusion.
Experience showed that according to the intended use, to the sought properties and to their degree of in-tenseness, the above PMT's could be varied in the range from 230 to 260.C. These directions to those skilled in the art are merely illustrative and it will be unter-stoodthat departure therefrom may be comtemplated within the scope of the invention It will thus be appreciated that the process of the invention makes it possible to treat metal substrates under coil form, by means of a continuous production li-ne of the kind shown in figure 1.
In the ~ollowing, merely illustrative examples, the-re were used various formulations for the primer and the finishing polyamidic layer(s). In these examples, all parts are given by weight, unless otherwise, stated, and the various components are recited in the order wherein they operate and are introduced within the composition.
FORMULA I (primer) % by weight Dynapol L 441 17 tthermoplastic linear polyester o~ Dynamit Nobel) Ethylglycol acetate 10.5 *

Solvesso 100 10.5 ( a solvent of Esso) Aerosil 300 0.3 tmicronized dispersant silica of the firm Degussa) * Trade Mark.

.~

~1'7~55~3 FORMULA I (primer) % bY weight Zinc phosphate 7.5 Titanium dioxide 11 Phenolic Resin PR 897 6 (unplastified phenolic condensate of the firm Hoechst) Epikote 1001 1.5 (polycondensate of epichlorhy-drine and diphenylolpropane of the firm Shell) Solution of lOX phosphoric acid in butanol Ethyl glycol acetate 34.7 This primer is characterized by being wholly free o~ yellowing and having good flexibility due to the pre-sence of polyester; however, its boiling water strength is poor.
FORMULA II (Primer) X by weicht Phenoxy PK HH 12 ~polycondensate of epochlorhy-drine and diphenylolpropane manufactured by Union Carbide) Zinc tetrahydroxychromate 10 Aerosil 300 0~5 Ethyl glycol 25 Toluene 29,S
Phenolic resin PR 897 2 Desmodur L 2248 2 (blocked aliphatic isocyanate of Bayer) * Trade Mark.

13l7~55~
FORMULA II (primer) X by weight Ethylglycol acetate 19 This primer is also characterized by being free of yellowing :
FORMULA III lprimer) % by weight Ethyl glycol acetate 8 Phenolic resin PR 897 12 Dimethyl formamide 6.5 Titanium dioxide 3B
Strontium chromate 3.8 Ethylglycol acetate 3.1 Solvesso 100 3.1 (solvent of the firm ESSO~
Epikote 1007 4.3 (polycondensate of epichlorhydrine and diphenylolpropane o~ the firm Shell) Ethylglycol acetate 4.3 Solution of 10% phosphoric acid in butanol 2.5 Phenoxy resine PK HH 2.4 (polycondensate of epichlorhydrine and diphenylolpropane of a high molecular weight manufactured by Union Carbide) Ethylglycol acetate 12 This primer is characterized by a good boiling water strength. It is thus clear that the primer formulation may be adapted to the final use of the coated metal-sheet.

* Trade Mark.

~17955~3 Thus, the formulas disclosed in examples I and II
will be recommended for their non-yellowing properties.
Yellowing is undesirable in some applications, since the sub-layer may sometimes be visible by transparent effect through the polyamide coating On the other hand, their boiling water strength is not very good.
The contrary is true in the case of the primer of Example III which is yellow but has very good resistance to hot water, at 750C!
FORMULA IV (polyamide) Polyamide dispersion Polyamide powder with 25% o~ TiO2 ~iller 40%
Benzyl alcohol 60X
lOOX
In this same type o~ ~ormula, the benzyl alcohol may be replaced by the ~ollowing solvents :
- isophorone ~ éthylglycol acetate - dibutylglycol acetate The polyamide powder may, according to the contemplated applications, be present in the dispersion at a concentra-tion ranging ~rom 20X to 50%.
FOflMULA V (polyamide) The components being added, as previously stated ~or the primer, in the order indicated below, the ~ormula is as ~ollows :

_ 16 -~7~5~

Component Amount % Possible Nature range %
benzyl alcohol 20 Paraloid DM54(1) 2 0-6 thermoplastic acrylic resin polyamide powder with 25% TiO2 filler 38 25-45 Paraloid B 48(2)1.5 0-6 thermoplastic acrylic ester benzyl alcohol 38.5 100 parts ~1) promotes wetting and adherence ~2) promotes adherence to the primer . supplier : Rohm and Haas FORMULA Vl (Polyamide) Component Amount % Possible ~ture range %
N-methylpyrrolldo-ne 40 Epikote 1001(1) 3 0-8 polycondensate of eplchlorhydrine and diphenylpro-pane Silicone OL (2) 0.1 polymethylsilo-Disperbyk (3) 0,3 barium salt of a polycarboxylic acid polyamide powder with 25% of TiO2 filler 40 25-45 * Trade Mark.

~7~?SS~3 Component Amount X Possible Nature range %
Cymel 303(4) 0.3 0-1 hexamethoxyme-thylmelamine N-methyl pyrro- 16.3 lidone (l) product of SHELL

(2) product of UAYER - a surfactant promoting spreading

(3) product of MALLINCKRODT - a dispersion-stabilizing agent

(4) product of AMERICAN CYANAMID - an agent for cross-linking the Epikote resin.
The latter also serves as a blending resin to carry the various additives or further fillers for co-lor-ad~ustment of the finishing layer.

A continuous strip of mild steel having a~thickness o~ 0,7 mm, previously sub~ected to a preliminary surface treatment, for the twofold purpose o~ anti-corrosion protection and adhesion promotion, with the commercial product designated BONDERITE 914 o~ the firm PARKER, i~
fed to the primer application station 3 at a velocity o~ about 30 m/mn, It is provided therein with a layer of primer accor-ding to abOve formula III having a 7-microns thickness in the dry state, resides in furnace 4 for a time period of about 30-60 seconds and reaches a Peak Metal Temperature (PMT) of 241 C. It goes through cooler 5 before reaching * Trade Mark.

li7~35S~3 the inlet of coating station 6 where it receives a layer of polyamide 11 according to formula IV ha~ing a 30-micro~ thickness in the dry state. In furnace 7, the strip reaches a PMT of 230C before cooled and colled.
The resulting metalloplastic multilayer complex is capable of successfully undergoing, without suffering stripping of the plastic coating, the test oflOOO_hours immersion in boiling water on samples provided with crossed slits in the coating.

A continuous aluminium strip having a 0.5 mm thick-ness, previously sub~ected to a preliminary surface treat-ment wlth the commercial product designated COLARDIN
6204 of the firm C.F.P.I., is provided at priming station 3 with a layer of primer according to formula II, having a thickness of 4-5 microns ln the dry state, and reaches in furnace 4 a PMT of 249C. Similarly, in the second fur-nace 7, the strip also reaches a PMT of 249C after ap-plication thereto of a coat of formula VI, wherein the polyamide is a polyamide 12. The residual thickness cf the polyamide coating after drying is of 25 microns.

* Trade Mark.

Claims

1. A process for coating a metal substrate by application thereto of one primer layer and at least one plastics layer, characterized in that the primer layer comprises a resin selected from epoxy-phenolic resins, mixtures of epoxy resins and phenolic resins and combinations of said resins and resin mixtures, in that the last plastics layer is a layer of polyamide applied in the form of a suspension, in an organic liquid phase, of a polyamide powder having a particle size lower than 40 µ, and in that the whole is heated until fusion of the external polyamide layer and obtention of a continuous coating, wherein for the primer application, the content of epoxy-phenolic resin or epoxy resin associated with the phenolic resin ranges from 1 to 20% of the total primer weight.

2. A process according to claim 1, characterized in that the metal substrate is a ferrous alloy, steel, galvanized steel, aluminium or aluminium alloy.

3. A process according to claim 1, characterized in that metal sheets or strips, of a thickness ranging from about 0.2 mm to about 5 mm, are treated on a continuous basis.

4. A process according to claim 2, characterized in that metal sheets or strips, of a thickness ranging from about 0.2 mm to about 5 mm, are treated on a continuous basis.

5. A process as in claim 3 or 4, wherein the metal sheets or strips have a thickness of 0.2 mm to 2 mm.

6. A process according to claim 1 characterized in that the primer contains a corrosion-inhibition mineral salt belong-ing to the group which consists of zinc phosphate, zic tetra-hydroxychromate, strontium chromate, as used alone or in mixture, the total content thereof ranging from 0.5 to 15% of the primer weight.

7. A process according to claim 2, 3 or 4, characterized in that the primer contains a corrosion-inhibition mineral salt belonging to the group which consists of zinc phosphate, zinc tetrahydroxychromate, strontium chromate, as used alone or in mixture, the total content thereof ranging from 0.5 to 15% of the primer weight.

8. A process according to claim 1, characterized in that the primer includes one or more compounds selected from:
- thermoplastic linear polyesters with a FP ranging from 100 to 150°C, soluble in the solvent mixture used, the content thereof ranging from 10 to 25% of the total primer weight, - blocked aliphatic or aromatic isocyanates releasable from a temperature of 120°C, the content thereof ranging from 1 to 6% of the total primer weight.

9. A process according to claim 8, characterized in that the primer is dried and cross-linked by heating, between about 230 and about 260°C.

10. A process according to claim 1, 2 or 8, characterized in that the grain size of the polyamide powder, which is applied in suspension form onto the primercoated substrate ranges approximately from about 10 to 40 µ.

11. A process according to claim 1, 2 or 8, characterized in that the polyamide powders forming the suspensions mainly consist of polyamides containing 4-20 carbon atoms in their hydrocarbon chain.

12. A process according to claim 1, 2 or 8, characterized in that the polyamide powders mainly consist of polyamides pertaining to the group comprising polyamide 6, polyamide 11, polyamide 12 or the copolyamides 6/11, 6/12, 6/11/12, 11/12.

13. A process according to claim l, 2 or 8, characterized in that the inherent viscosity (as measured at 20°C on a solution of 0.5 g in 100 ml of metacresol) of the polyamide powders, which reflect the molecular mass thereof, ranges between values of about 0.75-1.40.

14. A process according to claim 1, 2 or 8, characterized in that the polyamide powders mainly consist of polyamides pertaining to the group comprising polyamide 6, polyamide 11, polyamide 12 or the copolyamides 6/11, 6/12, 6/11/12, 11/12, and characterized in that the inherent viscosity (as measured at 20°C on a solution of 0.5 g in 100 ml of metacresol) of the polyamide powders, which reflect the molecular mass thereof, ranges between values of about 0.75-1.40.

15. A process according to claim 1, 2 or 8, characterized in that the organic liquid phase, which contains suspended polyamide powder, includes a solvent or mixture of solvents selected from the group consisting of alcohols, esters, cetons and aromatic hydrocarbons.

16. A process according to claim-l, 2 or 8, characterized in that the polyamide powders are used either in their natural state or as added with one or more pigments, which are intro-duced into the suspension through a blending resin.

17. A process according to claim 1, 2 or 8, characterized in that the final heating step which provides the continuous polyamidic external layer through a purely physical change (fusion of the polyamide particles) is effected between about 230 and about 260°C.

18. A process according to claim 1, 2 or 8, characterized in that the primer thickness is of the order of 6-7 microns.

19. A process according to claim 1, characterized in that the treatment is effected on metal strips of a thickness ranging from about 0.2 to about 2 mm and in that the thickness of the polyamidic external layer ranges from about 20 to about 50 µ.

20. A process according to claim 19, characterized in that the substrate in the form of a metal strip is coated on both sides.

21. Metal-plastics complexes comprising a metal strip or sheet of a thickness ranging from about 0.2 to about 5 mm, a primer and a continuous polyamidic external layer of a thick-ness ranging from about 20 to about 50 µ, whenever obtained by the process according to claim 3 or 4.

22. Metal-plastics complexes comprising a metal strip of a thickness ranging from about 0.2 to about 2 mm and in that the thickness of the polyamidic external layer ranges from about 20 to about 50 µ, whenever obtained by the process according to claim 19 or 20.

23. A process according to claim 1, 3 or 8, characterized in that the organic liquid phase containing the polyamide powder in suspension comprises a solvent or a mixture of solvents including one or more of the materials from the group consisting of:
- the product sold by American Cyanamid Company under the Trade Mark CYMEL 303, and which is a hexamethoxy methyl melamine, - the product sold by Bayer Company under the Trade Mark SILICONE OIL which is a surfactant, - the product sold by Esso Company under the Trade Mark SOLVESSO 150, which is an aromatic solvent with 9 carbon atoms, - the product sold by the Rohm and Haas Company under the Trade Marks PARALOID B 48 and DM 54, said products facilitating the mixing and the adhesion of the dispersion on a substrate.

24. A process according to claim 1, wherein the organic liquid phase containing the polyamide powder in suspension includes a solvent or solvent mixture containing at least one component from the group consisting of:
- hexamethoxy methyl melamine, polymethyl siloxane, an aromatic hydrocarbon solvent, a thermoplastic acrylic ester, and a thermoplastic acrylic resin.