EP0059136A2 - Process for coating metallic substrates with plastic materials and metal-plastic complexes thus obtained - Google Patents

Abstract

1. Process for coating a metal substrate, particularly to produce an anticorrosion covering, according to which a primary layer and at least one layer of plastic are applied to the said substrate, the primary layer comprising a resin chosen from epoxyphenolic resins, mixtures of epoxy resins and phenolic resins and combinations of the said resins and mixtures of resins, while the last layer of plastic material is a layer of polyamide, after which the whole is heated until the outer layer of polyamide melts, the said process being characterised in that the said polyamide is applied continuously in the form of a suspension, in an organic liquid phase, of polyamide powder the particle size of which is below 40 microns, the thickness of the outer layer of polyamide being between 20 and 50 microns, the process being carried out continuously and in a single step.

Description

The invention relates to the field of coating of metallic substrates. It relates more particularly to a process for coating metal substrates using plastics making it possible to produce multi-layer metal-plastic complexes. It also relates to the products obtained by such a process.

For decorative or corrosion protection purposes, the industry has had to produce metal-plastic multilayer complexes. The processes used consisted of gluing a plastic sheet on a strip supplied continuously from a reel, or in hot rolling, on a calender or rolling mill, of a sheet previously treated by the application of a adhesion primer.

The metals constituting the sheet were generally steel or galvanized steel or aluminum or an aluminum-based alloy.

The plastics used were mainly polyvinyl chloride, and to a lesser extent polyethylene.

The present invention relates to the application to a metal substrate of one or more layers of plastics, the outer layer always being polyamide.

The object of the present invention is on the one hand to provide a more economical method than the known methods which start from films produced beforehand, and on the other hand, to provide a method applicable to polyamides.

These polyamides are indeed particularly advantageous by their resistance to temperature, to abrasion, to chemical agents which are better than those of plastics used until now, such as PVC and provide total protection against corrosion to the metallic substrate of based.

The subject of the invention is a method for coating a metal substrate by applying a primary layer and at least one layer of plastic material, characterized in that the primary layer comprises a resin chosen from epoxy resins - phenolics, mixtures of epoxy resins and phenolic resins and combinations of said resins and mixtures of resins, in that the last layer of plastic material is a layer of polyamide applied in the form of a suspension, in an organic liquid phase, polyamide powder, the particle size of which is less than 40 μm and in that the assembly is heated until the outer polyamide layer melts and a continuous coating is obtained.

The advantageous characteristics of the invention are obtained by the combination of the choice of primer and the external layer of plastic material, which is applied in the form of a polyamide suspension.

The invention involves the application to the substrate of an organic bonding primer, susceptible to a structural modification by heat.

Indeed it is essential that the substrate me-. metal intended to be covered with polyamide suspension is previously coated with a primer, because, among other functions, this ensures the bond between the metal and the polyamide coating, due to the very good adhesiveness which it has both towards metals and polyamides.

The primer strengthens the metal protection provided by the polyamide coating firstly by preventing the path between metal and polyamide of gases such as oxygen, carbon dioxide, water vapor, and then, by superimposing its own protective action to that of polyamide.

• -l'eau bouillante et les détergents dans le cas des machines à laver le linge ou la vaisselle,
• -des agents chimiques divers si les tôles servent à construire des cuves de stockage de ces produits,
• -les intempéries si les objets revêtus sont utilisés au dehors.

The causes of corrosion are multiple and different depending on the application of the coated metallic article; we can for example cite, among others:

• - boiling water and detergents in the case of washing machines or dishes,
• - various chemical agents if the sheets are used to build storage tanks for these products,
• -the bad weather if the coated objects are used outside.

The presence of the primer is even more essential when the sheet to be coated has not undergone a preparation treatment.

According to the present invention, the primer contains an epoxy resin in an amount between 1 and 20% of the total weight of the primer, associated with a phenolic resin in an amount between 1 and 20% of the total weight of the primer, or an epoxy resin -phenolic in an amount between 1% and 20% of the total weight of the primer or a mixture of the above resins in the same weight proportions relative to the primer. as those mentioned above.

According to the invention, the anti-corrosion action provided by the primer can be reinforced by adding to the primer corrosion-inhibiting chemical compounds such as zinc phosphate, tetrahydro zinc xychromate, strontium chromate, these salts can be used alone or as a mixture.

• -polyesters linéaires thermoplastiques de PF compris entre 100 et 150°C ,solubles dans le mélange de solvants utilisés,dont la teneur est comprise de préférence entre 10 et 25% du poids total du primaire,
• -isocyanates aliphatiques ou aromatiques bloqués et libérables à partir d'une température de 120°C et dont la teneur est comprise entre 1 à 6% du poids total du primaire,
• -un sel minéral inhibiteur de corrosion appartenant au groupe constitué par le phosphate de zinc, le tétrahydroxychromate de zinc,le chromate de strontium, utilisés seuls ou en mélange,la teneur totale étant comprise entre 0,5 et 15% du poids du primaire.

In a preferred embodiment, the composition of the primer according to the invention comprises one or more of the compounds belonging to the

• linear thermoplastic polyesters of PF of between 100 and 150 ° C., soluble in the mixture of solvents used, the content of which is preferably between 10 and 25% of the total weight of the primer,
• -aliphatic or aromatic isocyanates blocked and releasable from a temperature of 120 ° C and whose content is between 1 to 6% of the total weight of the primer,
• -a corrosion inhibiting mineral salt belonging to the group consisting of zinc phosphate, zinc tetrahydroxychromate, strontium chromate, used alone or as a mixture, the total content being between 0.5 and 15% of the weight of the primer.

The preparation of these primers is carried out without difficulty in the usual mixing apparatuses, for example those for paints; it is recommended to use dispersing agitators provided with turbines with peripheral teeth particularly suitable for dispersions and mashings; it is good to '' paste the solid additives (silica, mineral salts) or insoluble in a small part of the solvents beforehand and add the paste obtained to the rest of the composition.

These formulation techniques are within the reach of ordinary skill in the art and therefore need not be described in more detail.

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, presented in a solvent or mixture of organic solvents.

After coating the metal substrate with the primer, it is generally more advantageous to dry it before applying the polyamide suspension. This step is however not essential insofar as the final step of heating the entire coating can allow proper attachment of the primer on the substrate. The final heating step causes the polyamide particles to melt and forms a continuous and smooth layer on the substrate. In the polyamide layer, this is a purely physical transformation carried out under the effect of the rise in temperature, for example in a heat treatment oven. This characteristic is advantageous and constitutes an important difference with the processes of the prior art in which the heat treatment was intended to effect a chemical transformation comparable to a crosslinking in the plastic layer. final.

On the contrary, in the invention, the final layer or finishing layer is obtained by evaporation of the liquid phase in which the polyamide particles are suspended and by fusion of said particles to form the final layer.

According to an essential characteristic of the invention, the grain size of the polyamide powder corresponds to a specific particle size, being in a range of less than 40 μm and preferably between 10 and 40 μm, particularly between 10 and 30

The powders suspended are powders of polyamides, that is to say resins of high molecular weight obtained from lactams or amino acids whose hydrocarbon chain has a number of carbon atoms between 4 and 20 such as, for example, caprolactam, oenantholactam, dodecalactam, undecanolactam, dodecanolactam, 11 amino-undecanoic acid, 12 amino-dodecanoic acid.

The polyamide can also be a condensation product of a dicarboxylic acid with a diamine such as, for example, the polyamides 6.6, 6.9, 6.10, 6.12, 9.6, condensation products of hexamethylene diamine with adipic acid, azelaic acid, sebacic acid, 1.12 dodecane-dioic acid and nonamethylene diamine with adipic acid.

Of course, the powders can consist of copolyamides resulting from the polymerization of the various monomers mentioned above.

• Polyamide 6, obtenues par polymérisation de l'ε -caprolactame
• Polyamide 11 ,obtenues par polycondensation de l'acide 11 amino-undécanoique
• Polyamide 12, obtenues par polycondensation de l'acide 12 amino-dodécanoique ou du dodécano- lactame

ou des copolyamides obtenus par la polymérisation de deux ou de trois des monomères cités ci-dessus.Particularly suitable for implementing the process are powders of:

• Polyamide 6, obtained by polymerization of ε -caprolactam
• Polyamide 11, obtained by polycondensation of amino-undecanoic acid 11
• Polyamide 12, obtained by polycondensation of amino-dodecanoic acid 12 or dodecano-lactam

or copolyamides obtained by the polymerization of two or three of the monomers mentioned above.

Interesting copolyamides result from the polycondensation of ε-caprolactam, amino-undecanoic acid and / or lauryl lactam.

The higher the molecular weight of the polyamides constituting the powders, the better their resistance to aging and their mechanical properties. On the other hand, their melting viscosity is all the higher, which harms their spreading on the substrate during melting; according to the present invention, the suspension may contain adjuvants promoting the spreading of: polyamide melt despite a viscosity of high fusion. Conversely, the lower the molecular weight, the more the coating will be fluid in the molten state to the detriment of its mechanical properties, its aging and its resistance to weathering.

In general, the inherent viscosity (measured at 20 ° C. for a solution of 0.5 g in 100 ml of metacresol) of the polyamide powders, reflecting their molecular mass, is between the values 0.75 and 1.40; the nature of the polyamide and the final application of the coated substrate are the determining factors for choosing the most favorable value.

The polyamide powders used according to the invention can be obtained by all kinds of techniques known to a person skilled in the art, for example by grinding polyamide granules previously cooled with liquid nitrogen, or dissolving these granules in solvents hot and precipitation by cooling, among others.

The processes by anionic polymerization of the monomer (s) in solvents, in which the polymers separate in the form of powders from the reaction medium in which they have become insoluble, are recommended and, more particularly, those described in patents FR 1,601,194, 1,601,195 and 1,602,751; indeed, the particles constituting the powder are small spheres particularly suitable for giving good suspensions and regular coatings after melting on the sheet.

The organic liquid phase, containing the polyamide powder in suspension, comprises a solvent or mixture of solvents chosen from alcohols, esters, aromatic hydrocarbons, and ketones, as is known to those skilled in the art to choose a suitable solvent for polyamides. _:

Specific examples of such solvents are benzyl alcohol, N-methyl pyrrolidone, ethyl glycol acetate, di-butyl glycol acetate or mixtures of solvents marketed under the name Solvesso 100 and 150.

The suspension based on polyamide powder may contain various additives which promote suspension, such as expanded silica (for example the product known as Aerosil) or which modify the surface tension, as is known to man of the art. art in the field of paints and varnishes.

These powders can be in their natural state and, consequently, generate colorless or pigmented coatings, the pigment or pigments that can be introduced into the suspension using a cutting resin as is known to those skilled in the art. art for the formulation of pigmented compositions.

According to the invention, it is possible to coat the most varied metallic substrates, in particular sheets of galvanized steel, aluminum or aluminum-based alloys. It goes without saying that the coating can be make on one or both sides of such a sheet. The process is applicable with the most advantages to metal strips of thickness between 0.2mm and 2mm approximately. In this case, the thickness of the external polyamide finishing layer is between approximately 20 and 50 μ.

• Fig. 1 illustre schématiquement un procédé d'enduction en continu d'un feuillard métallique.
• Fig. 2 représente une variante d'exécution de l'opérarion d'enduction primaire.

The process of the invention will now be illustrated without being in any way limited by the examples and the description below, made with reference to the appended drawings in which:

• Fig. 1 schematically illustrates a process for continuously coating a metal strip.
• Fig. 2 shows an alternative embodiment of the primary coating operation.

Referring to Figure 1, we can see on the left of the figure the strip unwinding station. It includes a coil 1-and an accumulator allowing it to absorb variations in length and tension due to coil changes.

• -dégraissage grossier
• -brossage
• -dégraissage fin
• -rinçage à chaud
• -phosphatation
• -chromatation
• -rinçage à froid
• -rinçage chromique

Reference 2 schematically designates a surface treatment installation which actually comprises several stations, namely:

• - coarse degreasing
• -brushing
• - fine degreasing
• - hot rinse
• -phosphatation
• -chromation
• - cold rinse
• - chromic flushing

These stations consist of different trays where the strip circulates.

The strip then passes through a primary coating station 3. In the example schematically shown in Figure 1, the station 3 includes a duo of cylinders working according to the known technique called "reverse roll". An alternative embodiment of this coating / priming step is shown in FIG. 2. This schematically represents a coating by the technique called / rotogravure. In this case the solution / apply is contained in a tank 10 and deposited on the strip by a duo of cylinders 11,12.

It goes without saying that to coat the two sides of the strip it is necessary to provide another coating station of the same type.

In general, a final thickness of 6 to 7 microns is considered sufficient for the primer.

Referring again to FIG. 1, the general reference 4 designates a drying and crosslinking oven for the primer deposited at station 3. The temperature to be reached in the oven 4 will obviously depend on the nature of the primer. In practice, we use the temperature reached by the metal of the strip (called peak metal temperature or PMT) as a measure of the true baking temperature of the primary.

The installation shown in FIG. 1 for implementing the method of the invention then comprises a station 5 for cooling by pulsed air the strip coated with primer. Station 6 is used to apply a thin polyamide powder dispersion layer according to the invention to the strip already coated with primer. In the example shown, station 6 comprises a duo of cylinders working according to the principle of rotogravure (similar to FIG. 2). A drying and gelling oven 7 is used to remove the solvents from the organic phase and to melt the grains of fine powder into a continuous layer. On leaving the oven 7 the strip passes through a cooling station 8 and then into a reel 9 preceded by its accumulator 9a.

The temperature conditions in the oven 7 will depend on the nature of the polyamides used.

We admit that the area between the temperature curve recorded on the sheet between the entry and exit of the oven, the x-axis (indifferently graduated in space or time) and the two verticals corresponding to the entry and to leaving the oven, corresponds to the energy used to carry out the operations for removing the solvents, crosslinking the cutting binder and melting the polyamide powder.

Experience has shown that, depending on the application envisaged, the properties sought and their degree of intensity, the above PMTs can be varied in the range 230 ° to 260 ° C. These indications are given to those skilled in the art for purely illustrative purposes and it goes without saying that we can do some. set aside without departing from the scope of the invention.

It can therefore be seen that the method of the invention makes it possible to treat metallic substrates presented in coils, by means of a continuous manufacturing line of the kind shown in FIG. 1.

In the purely illustrative examples which follow, various formulations of primer and of polyamide finishing layers have been used. In these examples, all the parts are indicated by weight, unless otherwise indicated and the various components are listed in the order of their integration and introduction into the composition.

This primer is characterized by a total absence of yellowing and good flexibility due to the presence of polyester; on the other hand, its resistance to boiling water is poor.

This primer is also characterized by an absence of yellowing.

This primer is characterized by good resistance to boiling water. It can therefore be seen that the formulation of the primer can be adapted to the end use of the coated sheet.

This is how the formulas described in Examples I and II will be recommended for their absence of yellowing. The latter, in certain applications, is not desirable because it sometimes happens that the undercoat is visible through transparency through the polyamide coating. However, their resistance to boiling water is not very good.

It is the opposite in the case of the primer of Example III which is yellow but very resistant to very hot water at 75 ° C.

Dans ce même type de formule,on peut substituer à l'alcool benzylique les solvants suivants:

• -isophorone
• -acétate d'éthylglycol
• -dibutylglycolacétate

In the same type of formula, the following solvents can be substituted for benzyl alcohol:

• -isophorone
• - ethyl glycol acetate
• -dibutylglycolacetate

The polyamide powder may, depending on the desired applications, have a concentration in the dispersion varying from 20 to 50%

FORM V (polyamide)

The components being, as it was said previously for the primary, added in the order indicated below, the formula is established as follows:

FORM VI (polyamide)

After

• SHELL product
• (2) BAYER product-surfactant promoting spreading
• (3) MALLINCKRODT product - dispersion stabilizer
• (4) AMERICAN CYANAID product - Epikote resin crosslinking agent

It also serves as a cutting resin, a vector for various additives or a complement of fillers allowing the finish layer to be tinted.

EXAMPLE 1

A mild steel strip in a continuous strip, 0.7 mm thick, having undergone a surface preparation for the dual purpose of protecting against corrosion and promoting adhesion with the commercial product referenced BONDERITE 914 of the PAR1KER, is introduced to station 3 for primary coating with a speed of around 30 m / min.

It receives a layer of primer there according to formula III of 7 microns thick when dry, and stays in the oven 4 for a period of approximately 30 to 60 seconds and reaches a "Metal Temperature Peak" (PMT) of 241 °. vs. It passes through the cooling 5 before being presented at the entrance to the coating station 6 where it receives a layer of polyamide 11 according to formula IV with a dry thickness of 30 microns. In the oven 7, the strip reaches a MTP of 230 ° C before being cooled and rolled up.

The multilayer metalloplastic complex obtained can undergo successfully without exhibiting the phenomenon of detachment from the plastic coating the immersion test for 1000 hours in boiling water on test pieces having undergone cross-shaped notches in the coating.

EXAMPLE 2

A strip of aluminum in a continuous strip 0.5 mm thick, having undergone a surface preparation with the commercial product referenced COLARDIN 6204 from the company CFPI, receives at the coating station 3 a primary layer according to II the formula, / with a dry thickness of 4 to 5 microns and reaches in the oven 4 a PMT of 249 ° C Similarly, in the second oven 7 the strip also reaches a PMT of 249 ° C after application of a layer of formula VI where the polyamide is a polyamide 12 of which it will retain a dry thickness of 25 microns.

Claims (19)

1. Process for coating a metal substrate by applying a primary layer and at least one layer of plastic material, characterized in that the primary layer comprises a resin chosen from epoxyphenolic resins, mixtures of resins epoxy and phenolic resins and combinations of said resins and mixtures of resins, in that the last layer of plastic material is a layer of polyamide applied in the form of a suspension, in an organic liquid phase, of polyamide powder the size of which of particles is less than 40 y and in that the whole is heated until the outer polyamide layer melts and a continuous coating is obtained. 2. Method according to claim l, characterized. in that the metal substrate is made of ferrous alloy, steel, galvanized steel, aluminum or aluminum alloy. 3. Method according to one of claims 1 or 2 characterized in that it treats metal sheets or strips, for example of thickness between 0.2 mm and 2 mm approximately, preferably continuously. 4. Method according to any one of claims 1 to 3, characterized in that, for the application of the primer, the content of epoxyphenolic resin or epoxy resin associated with the phenolic resin is between 1 and 20 percent of the total primary weight. 5. Method according to any one of claims 1 to 4, characterized in that the primer contains a corrosion inhibiting mineral salt belonging to the group consisting of zinc phosphate, zinc tetrahydroxychromate, strontium chromate, used alone or mixed, the total content being between 0.5 and 15% of the weight of the primer. 6. Method according to any one of claims 1 to 5, characterized in that the primer comprises one or more of the compounds belonging to linear thermoplastic polyesters of PF of between 100 and 150 ° C., soluble in the mixture of solvents used, the content of which is preferably between 10 and 25% of the total weight of the primer, -aliphatic or aromatic isocyanates blocked and releasable from a temperature of 120 ° C and whose content is between 1 to 6% of the total weight of the primer. 7. Method according to any one of claims 1 to 6, characterized in that the primary is dried and crosslinked by heating, in particular between 230 and 250 ° C approximately. 8. Method according to any one of claims 1 to 7, characterized in that the grain size of the polyamide powder, applied in the form of a suspension on the substrate coated with the primer, is between 10 and 40 µ, in particular between 10 and 30 µ approximately. 9. Method according to any one of claims 1 to 8, characterized in that the polyamide powders constituting the suspensions are based on polyamides containing 4 to 20 carbon atoms in their hydrocarbon chain. 10 Method according to which one of the resells a a tions 1 to 8, characterized in that / are based on polyamides belonging to the group consisting of polyamide 6.1 polyamide 11, polyamide 12 or copolyamides 6/11, 6/12, 6/11 / 12,11 / 12. 11. Method according to any one of claims 1 to 10, characterized in that the inherent viscosity (measured at 20 ° C for a solution of 0.5 g in 100 ml of metacresol) polyamide powders reflecting their molecular mass, is between the values 0.75 and 1.40 12. Process according to any one of claims 1 to 11, characterized in that the organic liquid phase, containing the polyamide powder in suspension, comprises a solvent or mixture of solvents chosen from alcohols, esters, aromatic hydrocarbons and ketones. 13. Method according to any one of claims 1 to 12, characterized in that the polyamide powders are used in the natural state or added - born of one or more pigments, which can be introduced into the suspension thanks to a resin cutting. 14. Process according to any one of Claims 1 to 13, characterized in that the final heating carried out has the continuous polyamide outer layer by purely physical transformation (melting of the polyamide particles) is carried out between 230 and 260 ° C approximately. 15. Method according to any one of claims 1 to 14, characterized in that the thickness of the primer is of the order of 6 to 7 microns. 16. A method according to any one of claims 1 to 15, characterized in that metal strips of thickness between 0.2 mm and 2 mm are treated and in that the thickness of the polyamide outer layer is between about 20 and 50 µ. 17. The method as claimed in any one of claims 1 to 16, characterized in that the substrate presented in sheet or strip is coated on one or on two faces. 18. Metal-plastic complexes obtained by the process according to any one of claims 1 to 17. 19.Metal-plastic complexes comprising a metal strip of thickness between 0.2 mm and 5 mm approximately, a bonding primer and a continuous polyamide external layer of thickness between 20 and 50 μ approximately.

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