EP0367653B1 - Method of coating metal substrates with a powder primer and a surface coat applied by dipping, powder-primer composition and product obtained - Google Patents

Abstract

The present invention relates to the field of coatings of metal substrates applied by dipping. It relates to a method of coating a metal substrate using a surface coat applied by dipping after coating of said substrate with a powder primer. It also relates to powder compositions which can be used as adhesion primer, as well as to composite materials: metal substrates/adhesion primer/surface coat.

Description

The present invention relates to a method for coating a metal substrate using a surface coating applied by dipping after coating said substrate with a powdered primer.

The soaking process in a fluidized bed requires prior preheating of the metal part to be coated before its immersion in the tank with a porous bottom in which the powder of the suspension coating is kept in suspension by air circulation: the powder then melts at contact with hot metal surfaces and forms a deposit, the thickness of which depends on the temperature of the substrate and its duration of immersion in the powder.

The part is preheated in an oven at a temperature determined according to the nature and thickness of the part to be coated.

However, for a given part, there is a minimum temperature below which it is not possible to obtain a coating of good quality from the point of view of its appearance and of its adhesion to the substrate.

But, on the other hand, an excessively high preheating temperature can be harmful, more particularly in the case where the metal part has previously been coated with an adhesion primer which can deteriorate during the passage of the part in the oven and / or no longer ensuring adhesion with the surface coating and the metal substrate.

At present, the adhesion primers for surface coatings applied by dipping are exclusively in liquid form, that is to say the resins of the primer are either in suspension or in solution in one or more solvents. The coating of metal substrates with these liquid primers is done for example using a pneumatic gun.

These solvents, often toxic to the environment, require the provision of recovery and / or recycling systems, thus resulting in additional cost to the installation.

The adhesion primers developed by the applicant are in the form of powder compositions, which can be applied as they are to metallic substrates.

• - un primaire d'adhérence en poudre à base de résine(s) époxyde(s) et de durcisseurs de résine(s) époxyde(s),
• - et un revêtement superficiel appliqué par trempage en lit fluidisé.

The process for coating metallic substrates developed by the applicant implements:

• - a powder adhesion primer based on epoxy resin (s) and epoxy resin hardener (s),
• - And a surface coating applied by soaking in a fluidized bed.

• a) enduction du substrat avec une ou plusieurs couches de primaire d'adhérence selon une technique d'application en poudre,
• b) chauffage du substrat et immédiatement après :
• c) application par trempage en lit fluidisé du revêtement superficiel en poudre.

The process developed by the applicant is characterized in that it comprises the following stages:

• a) coating the substrate with one or more layers of adhesion primer according to a powder application technique,
• b) heating of the substrate and immediately after:
• c) application by soaking in a fluidized bed of the surface powder coating.

The metal substrate which has previously been able to undergo one or more surface treatments such as alkaline degreasing, brushing, shot blasting, hot rinsing, etc., is coated with one or more layers of powder adhesion primer.

The metal substrate can be chosen from a wide range of products. These can be ordinary or galvanized steel parts, aluminum or aluminum alloy parts, the steel parts being more particularly targeted for the invention.

Although the thickness of the metal substrate is not critical in itself, it will most often be between 1 and 50 mm.

The application of the powder primer composition according to the invention can be carried out according to the powder application techniques usually used. Among the powder application techniques, there may be mentioned electrostatic spraying, a technique particularly preferred for the application of primer according to the invention, soaking in a fluidized bed.

In electrostatic projection the powder is introduced into a gun where it is conveyed by compressed air and passes through a nozzle brought to a high potential, generally between ten and one hundred kilovolts.

The applied voltage can be of positive or negative polarity, negative polarity being generally preferred.

The powder flow in the gun is generally between 10 and 200 g / min.

In general, it is possible to use powders with an average particle size of between 5 and 100 μm, and preferably between 5 and 80 μm.

The average thickness of the primer can be between 5 and 60 µm, and preferably between 10 and 20 µm.

The powder compositions which constitute the adhesion primer according to the invention are based on solid thermosetting resins and preferably based on epoxy resins and epoxy resin hardener (s).

By thermosetting resin is meant in the present description epoxy resins, phenolic resins, epoxyphenolic precondensates, alone or as a mixture.

By way of example of advantageously preferred epoxy resins, mention may be made of the products obtained by reaction of bisphenol-A and of epichlorohydrin and the degree of polymerization of which is greater than two, as well as the high molecular weight polycondensates of phenoxy type without free reactive group.

As examples of advantageously preferred phenolic resins, mention may be made of the resins obtained by polycondensation of formaldehyde and of a phenol.

Among the various solid hardeners of epoxy resin (s), compounds of the amine, acid anhydride or isocyanate type are preferably used.

The term “amine-type compound” means aliphatic or aromatic amines and their derivatives, such as dicyandiamide, benzyldimethylamine, boron trifluoride monoethylamine.

By way of example of acid anhydride, mention may be made of aromatic anhydrides such as phthalic anhydride or cycloaliphtalic anhydrides such as hexahydrophthalic anhydride.

The term “isocyanate type compound” means a monomer, prepolymer or polymer compound containing at least two free or blocked isocyanate groups. Polyisocyanate compounds with an aromatic or aliphatic structure whose isocyanate groups are blocked by condensation with phenol or caprolactam are particularly preferred.

The proportion of epoxy resin (s) and hardener (s) of the amine, acid anhydride or isocyanate type must be such that the number of epoxy functions is equal to the number of reactive functions of the amine of the acid anhydride or isocyanate compound.

However, for reasons of coating quality, it may be preferable to vary the stoichiometric proportion indicated above between 0.1 and 1.5 reactive anhydride amine or isocyanate functions for an epoxy function.

The primers in accordance with the invention may also contain various additives and agents, alone or as a mixture, such as pigments, fillers, corrosion inhibiting agents, etc. Among the compounds usually encountered, mention may be made of strontium chromate. , zinc phosphate, titanium dioxide, silica.

Once the primer has been applied to the substrate, the substrate is placed in an oven where it is heated to a determined temperature depending in particular on the nature of said substrate, its shape and the desired coating thickness. The energy stored during the passage through the oven of the substrate coated with primer allows, in addition to the crosslinking of the primer, the melting and the adhesion of the surface coating applied by dipping on the substrate as soon as it leaves the oven.

The powder adhesion primers developed by the applicant resist particularly well at the high temperatures which may be necessary to ensure effective preheating of the substrate to be coated.

This is not the case for liquid primers for which too high a temperature adversely affects the adhesion results of the final coating; this is why the normal conditions of use of liquid primers are limited to coatings of metal parts of geometric configuration such that the temperature of preheating before soaking which is necessary is less than 270-320 ° C.

The powder primers according to the invention can not only be heated to 270-320 ° C, but they can, without damage to the quality of the final coating, undergo heating at higher temperatures, such as for example between 320 and 380 ° C, when necessary.

The average residence time of the substrate in the oven is generally between 1 and 30 min and preferably between 3 and 10 min.

As soon as it leaves the oven, the substrate is immersed in a soaking tank containing the surface coating as defined above.

The surface coatings in powder form, applied by dipping and in accordance with the invention are preferably based on polyamide and / or polyetheresteramide.

By polyamide entering the powder surface coating according to the invention is meant the aliphatic polyamides obtained from lactams or amino acids whose hydrocarbon chain has a number of carbon atoms between 4 and 20 such as, for example, caprolactam, enantholactam, dodecalactam, undecanolactam, amino 11-undecanoic acid, amino 12-dodecanoic acid, the condensation products of a dicarboxylic acid with a diamine such as, for example, polyamides 6.6 , 6.10, 6.12, 9.6, (products of the condensation of hexamethylene diamine with adipic acid, azelaic acid, sebacic acid, dodecanedioic acid-1,12 and nonamethylene diamine with acid adipic), the copolyamides resulting from the polymerization of the various monomers mentioned above or the mixtures of several polyamides mentioned above.

• - le polyamide 11, obtenu par polycondensation de l'acide amino-11 undécanoïque,
• - le polyamide 12, obtenu par polycondensation de l'acide amino-12 dodécanoïque ou du dodécanolac- tame et,
• - les copolyamides obtenus par la polymérisation des monomères cités ci-dessus.

Among these polyamides, special mention will be made of:

• polyamide 11, obtained by polycondensation of amino-11 undecanoic acid,
• polyamide 12, obtained by polycondensation of 12-amino-dodecanoic acid or of dodecanolactam and,
• - the copolyamides obtained by the polymerization of the monomers mentioned above.

In general, the inherent viscosity (measured at 20 ° C for a solution of 0.5 g per 100 g of metacresol) of the polyamides can be between 0.20 and 2.0, and preferably between 0.60 and 1.30 dlg- ¹ .

The term “polyamides” also means semi-aromatic amorphous polyamides, and in particular as defined in French patents FR 1,588,130, 2,324,672 and 2,575,756, in European patent EP 53,876, in Japanese patents 59,015,447 and 60,217,237.

The term “polyetheresteramides” is intended to mean both the random polyetheresteramides (that is to say formed by the random sequence of the various monomer components) and the block polyetheresteramides, that is to say formed from blocks having a certain length of chain. their various constituents.

• - séquences polyamides à fins de chaîne dicarboxyliques avec des séquences polyétherdiols.

Polyetheresteramides are products of the copolycondensation of reactive end polyamide blocks with reactive end polyether blocks such as:

• - polyamide sequences for dicarboxylic chain ends with polyetherdiol sequences.

The number-average molecular mass of these polyamide blocks is generally between 500 and 10,000 and more particularly between 600 and 5,000. The polyamide blocks of the polyetheresteramides are preferably formed from polyamide 6, 6.6, 6.12, 11 or 12 or from copolyamides resulting from the polycondensation of their monomers.

The number average molecular weight of the polyethers is generally between 200 and 6000 and more particularly between 600 and 3000.

The polyether blocks preferably consist of polytetramethylene glycol (PTMG), polypropylene glycol (PPG), or polyethylene glycol (PEG).

The inherent viscosity of polyetheresteramides is advantageously between 0.8 and 2.05, and preferably between 0.80 and 1.20.

The inherent viscosity is measured in metacresol at 25 ° C with an initial concentration of 0.5 g per 100 g of metacresol. It is expressed in dlg- ¹ .

The polyetheresteramides according to the invention can be formed from 5 to 85% by weight of polyether and from 95 to 15% by weight of polyamide, and preferably from 30 to 80% by weight of polyether and from 70 to 20% by weight of polyamide.

The particle size of the surface coating powders can be between 20 and 300 μm, and preferably between 40 and 200 μm.

The soaking technique according to the invention is carried out in a non-electrostatic fluidized bed, the electrostatic fluidized beds being unsuitable and difficult to use for soaking in powders based on polyamide and / or polyetheresteramide which are particularly preferred by the plaintiff.

In general, the thickness of the surface coating can be between 150 and 600 μm, and preferably between 200 and 400 μm.

Once the soaking operation has ended, the substrate is cooled, for example with ambient air, by immersion in water or in any other suitable solvent, after having possibly undergone a post-melting.

The following examples illustrate the invention without, however, limiting it.

EXAMPLE 1 A) CONSTITUENTS

• 1 °) The metal substrate consists of a steel plate 1 mm thick. This plate has previously been degreased and then shot blasted.
• 2 °) The composition of powdered primer includes (in grams):
  • - epoxy resin obtained by reaction of epichlorohydrin and bisphenol A. (molecular weight: 1400 g; weight of epoxide equivalent: 850-950; softening point: 90 ° C) 90
  • - Isocyanate compound (blocked aromatic polisocyanate with -N = C = O content equal to 14%; density: 1.27) 10
    The particle size of the primer composition is less than 80 μm.
• 3) The surface coating consists of PA-11, in the form of a powder with a particle size between 40 and 200 μm.

The inherent viscosity of PA-11, measured at 20 ° C for a solution of 0.5 g of polymer in 100 g of m-cresol, is equal to 1.

B) IMPLEMENTATION

The powdered primer composition as described in A. 2 °) is deposited at room temperature on the steel plate by electrostatic spraying under a negative electrostatic charge of 40 kV, the metal surface being at potential O.

The substrate thus coated passes through an oven maintained at 380 ° C where it remains for 3 min.

Immediately afterwards, it is immersed in a soaking tank in a fluidized bed containing PA-11 powder as defined in A.3 °).

After approximately 4 seconds of immersion, the substrate thus coated is removed from the soaking tank and then cooled in air after post-melting.

C) CHARACTERISTICS OF THE MATERIAL

• 1 °) The material is a composite comprising successively:
  • - a sandblasted steel plate (thickness 1 mm)
  • - a primer layer of average thickness equal to 20 µm.
  • - a surface coating layer of thickness between 200 and 250 µm.
• 2 °) The material described in C.1 °) undergoes an adhesion test carried out according to standard NF T 58-112.

• Classe 4 (très bonne adhérence).

The following adhesion result is obtained:

• Class 4 (very good adhesion).

The material described above undergoes a salt spray aging test carried out according to standard NF X 41-002.

• - adhérence mesurée selon la norme NF T 58-112 : Classe 3,5-4 (très bonne adhérence).
• - cheminement à partir d'une entaille en croix : 11 mm.
• - cloquage mesuré selon la norme ASTM D 56(81) : Classe 10 (pas de cloquage).

After 2000 hours of testing, the following results are obtained:

• - adhesion measured according to standard NF T 58-112: Class 3.5-4 (very good adhesion).
• - routing from a cross notch: 11 mm.
• - blistering measured according to ASTM D 56 (81): Class 10 (no blistering).

EXAMPLE 2

The test of EXAMPLE 1 is repeated using different powdered primers, the composition of which comprises (in grams):

EXAMPLE 2.A

• - epoxy resin obtained by reaction of epichlorohydrin and bisphenol A.
  (molecular weight: 1400 g; weight of epoxy equivalent: 850-950; softening point: 90 ° C) 90
• - catalyzed or micronized dicyandiamide resin 10

EXAMPLE 2.B

• - epoxy resin with the same characteristics as in 2.A 92.5
• - phthalic anhydride 7.5

EXAMPLE 2.C

• - epoxy resin with the same characteristics as in 2.A 92
• - diamino diphenylsulfone 8

EXAMPLE 2.D

• - epoxy resin with the same characteristics as in 2.A 50
• - saturated polyester resin (acid number: 70-85; Tg = 55 ° C) 50

EXAMPLE 2.E

• - epoxy resin with the same characteristics as in 2.A 50
• - phenol / aldehyde resin 50 (melting point: 100 ° C; aldehyde / phenol: 1.2 (in M); Mw = 2000 - 3000)

The metal substrate is a sandblasted steel plate and the surface coating has the same characteristics as those of EXAMPLE 1.

The substrate is coated with the primer by electrostatic spraying under the same conditions as in 1.B.

It then passes into an oven maintained at 330 ° C where it remains for 10 minutes.

Immediately afterwards, it is immersed in a soaking tank in a fluidized bed under the same conditions as those described in 1.B.

• - une plaque d'acier sablé (épaisseur 3 mm)
• - une couche de primaire d'épaisseur moyenne égale à 20 µm
• - une couche de revêtement superficiel d'épaisseur comprise entre 200 et 250 µm.

The material obtained is a composite comprising successively:

• - a sandblasted steel plate (thickness 3 mm)
• - a primer layer of average thickness equal to 20 µm
• - a surface coating layer of thickness between 200 and 250 µm.

The materials undergo an adhesion test carried out in accordance with standard NF T 58-112 and a salt spray aging test carried out in accordance with standard NF X 41-002.

The results are collated in Table I.

EXAMPLE 3

• A) - résine époxyde modifiée novolaque (poids d'équivalent époxyde : 500-575 ; point de ramollissement compris entre 90 et 98°C ; d = 1,19) 92
  • - dicyandiamide micronisée 8
• B) - poly p. vinylphénol 100 de Mw compris entre 2000 et 30000, de poids d'équivalent époxyde = 120 et possédant un poids de ramollissement compris entre 140 et 210°C.

The test of EXAMPLE 1 is repeated using a powdered primer which comprises (in g).

• A) - modified novolak epoxy resin (epoxy equivalent weight: 500-575; softening point between 90 and 98 ° C; d = 1.19) 92
  • - micronized dicyandiamide 8
• B) - poly p. vinylphenol 100 from Mw between 2000 and 30000, of epoxy equivalent weight = 120 and having a softening weight between 140 and 210 ° C.

The metal substrate and the surface coating have the same characteristics as in EXAMPLE 2 and the conditions for processing and evaluating the materials obtained are identical to those described in EXAMPLE 2.

The results obtained are collated in Table I.

EXAMPLE 4 (COMPARATIVE)

• - résine époxyde obtenue par réaction de l'épichlorhydrine et du bisphénol A dont la masse moléculaire est comprise entre 3000 et 3800 g et le poids d'équivalent époxyde entre 1600 et 4000.
• - résine formo-phénolique de type résol
• - résine aminoplaste mélamine-formol

The test of EXAMPLE 1 is repeated using a liquid primer which comprises the following resins:

• - epoxy resin obtained by reaction of epichlorohydrin and bisphenol A whose molecular mass is between 3000 and 3800 g and the weight of epoxide equivalent between 1600 and 4000.
• - resol phenophenol type resin
• - aminoplast melamine-formaldehyde resin

dissolved in a mixture of ethylene glycol, solvent naphtha, butanol, isobutanol and methyl isobutyl ketone.

The metal substrate and the surface coating have the same characteristics as those of EXAMPLE 1.

The conditions of implementation are identical to those described in 1.B.

The material obtained is such that the initial adhesion of the coating is zero (Class O) and the corrosion path is complete in a few hours.

EXAMPLE 5 (COMPARATIVE)

The test of EXAMPLE 1 is repeated using the primer described in 2Adans the operating conditions described in French patent application No. 2,340,140.

• - Application au pistolet électrostatique (V = - 40 kV) du primaire (épaisseur = 100 µm) sur une plaque d'acier sablé d'épaisseur 1 mm.
• - Chauffage de la plaque recouverte de poudre à 200°C pendant 3 mm.
• - Application du polyamide-11 après refroissement de la plaque par projection électrostatique (V =-40 kV) Epaisseur≃140 µm.
• - Post-fusion du système à 300°C pendant 3 mn.
• On obtient ainsi un revêtement bi-couche comprenant
• - une sous-couche époxyde (42 % de l'épaisseur totale du revêtement)
• - une couche de polyamide, l'épaisseur totale du revêtement étant égale à 260 µm.

Or the succession of the following stages:

• - Application with an electrostatic gun (V = - 40 kV) of the primer (thickness = 100 µm) on a sanded steel plate 1 mm thick.
• - Heating of the plate covered with powder at 200 ° C for 3 mm.
• - Application of polyamide-11 after cooling the plate by electrostatic spraying (V = -40 kV) Thickness≃140 µm.
• - Post-fusion of the system at 300 ° C for 3 min.
• A two-layer coating is thus obtained comprising
• - an epoxy undercoat (42% of the total thickness of the coating)
• - a polyamide layer, the total thickness of the coating being equal to 260 μm.

The results obtained in the salt spray adhesion and aging tests are given in Table I.

The surface appearance of the coating is quite poor with in particular bubbling on the edges corresponding to degradation of the primer.

Claims (6)

1. Process for coating metal substrates with the aid of an adhesion primer and of a surface coating, characterised in that it comprises the following stages:

a) coating of the substrate with one or more layers of adhesion primer powder based on thermosetting resins according to a powder application technology,

b) heating of the substrate thus coated and, immediately afterwards:

c) application of the surface coating powder by dipping in a fluidised bed.

2. Process according to Claim 1, characterised in that the temperature of heating of the substrate in stage b) can reach 380°C.

3. Process according to Claim 1 or 2, in which the surface coating is based on polyamide, preferably PA-11 and/or PA-12, and/or based on polyetheresteramide.

4. Adhesion primer powder composition based on epoxy resin(s) and hardener(s) for epoxy resin(s) employed in the process such as defined in Claim 1, 2 or 3.

5. Composition according to Claim 4, characterised in that the hardeners for epoxy resin(s) are chosen from resins of the amine, acid anhydride or isocyanate type.

6. Composite materials consisting:

- of a metal substrate,

- of one or more layers of adhesion primer such as defined in Claims 1 to 5, with a mean thickness preferably between 10 and 20 µm,

- of a surface coating such as defined in Claims 1 to 3, with a mean thickness preferably between 200 and 400 µm.