GB2237812A - Pretreatment primer for metal substrates - Google Patents

Abstract

A primer for application to metal substrates particularly as part of a coil coating process uses, as a vehicle, an aqueous acrylic polymer dispersion containing a reactive steric stabiliser, thereby allowing the incorporation of oxidising acids, especially chromic acid, without destabilising the dispersion. Epoxy resins and crosslinking resins may also be intimately incorporated in the polymer particles. The primer also includes opacifying and optionally anticorrosive pigments. Use of the primer avoids the need for a pretreatment stage in the coating process.

Description

PRETREATMENT PRIMER This invention relates topretreatment primers for metal substrates, in particular for application to prepared metal substrates as part of a coil-coating process.

Coil coating is an established process which is widely used for application of industrial paints to metal substrates in sheet or strip form, in which the metal, usually aluminium or steel, is supplied in rolls or coils, is unwound, coated and, generally, re-wound for supply to the customer. The process is continuous and, in the coating stage of the process, the substrate is physically cleaned by brushing and/or abrasion, optionally passed through a solvent degreaser, passed through detergent or alkali cleaner and rinse solutions, pretreated to improve corrosion resistance and adhesion, rinsed and dried and finally coated firstly with primer and secondly with a top coat.The pretreatment generally comprises iron or zinc phosphate for cold rolled steel, zinc phosphate, a chromate or a complex amorphous oxide for hot dipped galvanised steel and a chromate conversion coating for aluminium.

The rinsing stage generally includes a rinse with chromic acid or other strong oxidising acid to passivate the metal surface, followed by a water wash. The top coat may be different for the upper and lower sides of the sheet or strip; both the primer and top coat(s) are oven-cured.

In modern coil coating installations, the lines run at speeds of up to about 120 metres per minute, although speeds of up to about 275-300 metres per minute are attainable for some aluminium lines. The economics of the process are dependent largely qn throughput and in general terms anything which contributes to increased line speed or decreased total treatment time will enhance the economics. In particular, it has in the past been recognised that the process economics would be considerably improved by effectively combining the pretreatment and primer stages, that is by using a primer which has the effect of pretreating the metal substrate as it is applied and, therefore, rendering unnecessary a separate pretreatment stage.

EP-A 0264471 describes a coating composition which can be applied, without a prior pretreatment step, to hot metal surfaces. The composition includes an aqueous dispersion of an organic polymer as vehicle, a mineral acid, preferably phosphoric acid, hexavalent chromium and a reducing agent to reduce the Cr(VI) to Cr(III) on the hot metal surface, pigments and optionally additives such as zinc. The polymer dispersion has, however, limited stability in the presence of chromic acid, which necessitates its being manufactured and sold as a two-part composition, and the performance is inferior compared with traditional systems using separate pretreatment and primer process steps.

EP-A 0264472 proposes an aqueous composition containing organic polymers and chromate ions for passivation of zinc- or cadmium-plated surfaces, in which the organic polymer is an acrylic emulsion polymer. The application process does not require a subsequent rinse stage, whereby pollution caused by the chromate is avoided, but a separate primer coat is required, and again the polymer dispersion has limited stability in the presence of chromic acid.

EP-A 0274543 relates to compositions which improve the miscibility of acrylic emulsions and chromic acid to enable chromate treatment without the necessity for a subsequent water rinse. This is achieved by using a nonionic emulsifying agent including a polyoxyethylene-polyoxypropylene block copolymer. However, the described composition, while enhancing the known pretreatment processes by eliminating the need to dispose of waste water containing chromium species, does not provide the additional function of a primer.

EP-A 0219976 describes a non-aqueous primer composition intended to avoid the need for a separate pretreatment operation. The primer contains strontium chromate pigment but does not include any other chrome species, in particular Cr(VI) or chromic acid. The primer as described is therefore highly corrosion resistant but this does not compensate for the absence of a moiety having a pretreatment function.

EP-A 0291260 is concerned with an aqueous pretreatment coating composition including a water-dispersible epoxy resin, chromium trioxide (chromic acid), water and optionally zinc powder and/or a ferro alloy powder. Metal substrates coated with such compositions can then be primed and topcoated. X In summary, the art has failed to provide a primer composition which can be applied to a metallic substrate and which as such also provides a pretreatment function, which composition is stable to chromic acid and which would in use render unnecessary a separate pretreatment stage in the coating process.

We have found that the use, as vehicle in a primer formulation, of an aqueous polymer dispersion stabilised with a reactive steric stabiliser enhances the resistance to instability on incorporation of acid material, especially chromic acid.

According to the present invention, therefore, we provide a primer for metal substrates, the primer comprising an aqueous dispersion based on an acrylic polymer as vehicle, an oxidising acid for passivating the substrate surface, and one or more pigments, in which the acrylic polymer dispersion includes a reactive steric stabiliser which is chemically bound as part of the polymer particles during the polymerisation process.

By reactive steric stabiliser we mean a material or precursor component thereof that is capable of stabilising an oil in water emulsion and which additionally contains one or more reactive functional groups that may take part in the polymerisation reactions that can take place in the oil droplet when it contains acrylic monomer materials, or other polymerisation reactions.

Where a precursor of a reactive steric stabiliser is used, the stabilising species is produced in-situ from the percursor by its reaction with other reactive components present in the oil droplet or the aqueous phase.

In the above formulation, the pigment generally comprises an opacifying moiety such as titanium dioxide, optionally together with extenders such as china clay, for the purpose of masking the colour of the substrate, and may also comprise an anti-corrosive pigment such as strontium chromate, especially for steel substrates. For aluminium substrates, however, the anti-corrosive pigment may be omitted. The use of ion-exchange pigments or pigments which are unstable at low pH values is precluded due to the presence of the oxidising acid.

The use of a water-borne system is environmentally preferable to a solvent-borne system and is in any event necessary in order to solvate the oxidising acid, which preferably comprises chromic acid. The chromic acid may be added to the formulation in the form in which the chromium is present as Cr(VI) or, alternatively, in the partially-reduced form in which the chromium is present as a mixture of Cr(VI) and Cr(III) species. The partially-reduced form is prepared by mixing chromic acid in which the chromium is present as Cr(VI) with a mild reducing agent such as formaldehyde.

The presence of chromic acitd or other oxidising acid in the primer formulation passivates the substrate surface in a similar way as it does as part of a traditional pretreatment stage and in addition, due to its presence in and throughout the cured film, may provide a repair capability should the substrate surface become subsequently damaged. Coil-coated substrates are especially vulnerable at their edge regions, and to impacts and abrasion during handling, and the sites of such impacts and abrasion can subsequently become the source of incipient corrosion; the present invention mitigates this effect.

Additionally, the fact that the oxidising acid is retained within the cured film, rather than being rinsed off the substrate, provides an economic and environmental advantage in that there is no need to carry out a process for removal of oxidising acid from waste water.

Preferably, primers according to the present invention also include an epoxy resin to enhance the binding properties of the vehicle, increase corrosion resistance in the cured film, and also the flexibility thereof. For this purpose, an epoxy resin having a reasonably high molecular weight is preferred, especially an epoxy resin having a molecular weight in the range 2000-5000 (number average molecular weight). One suitable epoxy resin is "Epikote" (Rtm) 1009, manufactured by Shell Resins. In place of an epoxy resin, primers may include a urethane resin to give the same desirable features provided by the epoxy resin. Suitable urethane resins include those derived as the reaction product of an isocyanatei especially toluene diisocyanate, with a low molecular weight polyester resin having a high hydroxyl functionality.A suitable polyester resin is that derived from the reaction of hydrogenated bisphenol A with adipic acid.

It is preferred that a cross linking agent is also included in the formulation. The purpose of the crosslinking agent is to assist in crosslinking during curing, to increase the strength and chemical resistance of the cured film. The crosslinking agent may be any of a wide range of oil soluble amino resins, including melamine formaldehyde based resins with or without alkylation, imino-functional melamine formaldehyde resins, urea formaldehyde resins, benzoguanamine formaldehyde resins, glycoluril formaldehyde resins or phenol formaldehyde resin.Alternatively, an oil soluble isocyanate resin may be used L as the crosslinking resins, especially a blocked isocyanate resin, which is not reactive at room temperature but which becomes unblocked and reactive at the temperatures used in the paint curing process, an example being a resin prepared by reacting toluene diisocyanate with 2-ethyl hexanol. Alternatively, a water soluble amino resin, based on melamine formaldehyde, glycoluril formaldehyde or benzoguanamine formaldehyde may be used.

The use of an aqueous dispersion of acrylic polymer including a reactive steric stabiliser renders the primer stable to acids and also permits the intimate ilicorporation in the polymer particles of the epoxy resin and crosslinking resins. Preferably the acrylic polymer is formed from one or more of a range of acrylic monomers, including but not restricted to methyl methacrylate, butyl acrylate, methyl acrylate, ethyl acrylate, 2-ethyl hexyl acrylate and styrene. Additionally, the monomers which form the acrylic polymer may include as part of their composition one or more monomers which have functional groups including for example hydroxyl and carboxylic acid groups which will be available for crosslinking during curing, especially with the crosslinking agent, if present.

One suitable type of acrylic polymer dispersion is that described in EP-B 0094386, in which an aqueous dispersion of pre-formed polymer and unsaturated monomer which is substantially insoluble in water is caused to react so that the monomer polymerises in situ as a blend with the pre-formed polymer, in the presence of a stabiliser which is an amphipathic compound with an H.L.B. value of at least 8, the lipophilic portion containing at least one ethylenic double bond.Another type is a sterically stabilised aqueous dispersion polymer as described in GB-B 2127835, prepared by a polymerisation process in which an ethylenically unsaturated monomer of limited water solubility is emulsified in water and polymerised in the presence of a redox initiator comprising hydrogen peroxide and ascorbic acid, a non-ionic surfactant, and a water-soluble polymer having a molecular weight of at least 400 and containing an unsaturated grouping copolymerisable with the monomer.Yet a further type is a sterically stabilised dispersion of polymer particles in an aqueous medium prepared by a process as described in GB-A 2039497, in which an ethylenically unsaturated monomer is subjected to free radical-initiated polymerisation in a medium containing water and a water-miscible second constituent, at a temperature at least 100C higher than the glass transition temperature of the polymer formed, in the presence of a block or graft copolymer stabiliser containing solvatable and non-solvatable components, the second constituent serving to maintain at least a limited solubility of the monomer in the water while being a non-solvent for the polymer being formed.

It will be appreciated that the above types of polymer dispersions all include a reactive stabiliser, which may typically be the reaction product of pentaerythritol triallyl ether and one or more alkylene oxides, for example butylene oxide and ethylene oxide, or methoxy poly (ethylene glycol) methacrylate. Such stabilisers become chemically bound as part of the polymer particle during the polymerisation process, making them very difficult to remove and increasing the stability of the aqueous polymer dispersion.

Primers according to the present invention may also include additives such as levelling agents, anti-foam agents, thickeners, coalescing solvents and the like.

In primers according to the invention, the pigments are generally present at a total amount of from 5 to 40% pigment volume concentration (pvc) in the dry film, preferably 10 to 25% pvc. Strontium or other chromate pigment, present as an anti-corrosive agent, generally occupies from 0 to 20% pvc, amounts towards the upper end of this range being preferred for steel substrates coated with zinc and/or aluminium.

The amount of epoxy resin, if present, should be, in general, not less than 20% by weight based on the total quantity of non-volatile resin, although we prefer to use concentrations in the range 25 to 35%.

The upper concentration limit of oxidising acid is determined according to the amount which renders the cured film water-sensitive. For most purposes and with reference to chromic acid, a maximum of 10% (as Cr03) by weight on the weight of the dried and cured film is considered desirable, with a minimum of 1% being considered necessary.

Primers according to the invention may be manufactured by dispersing the pigments in the aqueous dispersion polymer according to known methods, for example ball-milling, to provide a millbase, and adding further vehicle and other ingredients. However, the oxidising acid may be added as a separate solution in water before use of the primer, although this is not absolutely necessary in view of the stability resulting from the vehicle.

The substrate primed with these compositions may be coated with a topcoat from a wide range of systems used in coil coating applications, including those that are polyester-based, silicone modified polyester-based, polyurethane-based, acrylic-based, and those based on polyvinylidene fluoride polymers.

Embodiments of the invention will now be described with reference to the following examples.

EXAMPLE 1: Preparation of an aqueous dispersion polymer vehicle.

A vehicle was prepared according to the following formulation, in which the amounts are in weight percent: Methyl methacrylate 14.115 4-hydroxy butyl acrylate 1.775 "Epikote" 1009 14.201 Urea formaldehyde resin solution 21.516 Pentaerythritol triallyl ether/ 2 butylene oxide/35 ethylene oxide 2.366 Butyl acrylate 5.411 Butyl perbenzoate 0.577 Demineralised water (1) 22.628 (2) 15.592 Ascorbic acid 0.242 dissolved in demineralised water 1.041 Dimethyl aminoethanol 0.268 dissolved in demineralised water 0.268 The preparative method was as follows: the methyl methacrylate and 4-hydroxy butyl acrylate were heated in a reactor to 50-60 0C. The epoxy resin was then added slowly while maintaining the temperature at 50-60 C until all the epoxy resin was dissolved.The solution was then removed from the heat and the urea formaldehyde resin solution added, followed by the modified pentaerythritol, butyl acrylate and t-butyl perbenzoate. The mixture was stirred well, demineralised water (1) added, stirring continued for a further hour, and the mixture passed through a homogeniser until the particle size was less than 300nm.

Demineralised water (2) was then added and the mixture stirred for a further 30 minutes, the ascorbic acid solution was added, the mixture placed under a blanket of inert gas (nitrogen) and the resulting exotherm allowed to proceed. When the temperature had fallen to less than 400C, the dimethylaminoethanol solution was added to pH 7.5. The product was filtered; the specification is: solids content 48-50%; viscosity 15-25 seconds at 25OC (A10 cup); particle size 250-350 nm.

EXAMPLE 2: Preparation of an aqueous dispersion polymer vehicle.

A vehicle was prepared according to the following formulation, in which the amounts are in weight percent: Part A: Blocked isocyanate crosslinker (76% solids solution in n-butanol) 10.69 Methyl methacrylate 15.59 4-hydroxybutyl acrylate 1.17 Epikote 1009 14.98 Pentaerythritol triallyl ether/ 2 butylene oxide/35 ethylene oxide 2.50 Butyl acrylate 5.71 Dibutyl tin dilaurate 0.24 t-Butyl perbenzoate 0.61 Part B: Demineralised water 45.36 Part C: Ascorbic:acid 0.26 Demineralised water 1.10 Part D: Dimethylaminoethanol 0.53 Demineralised water 1.26 The preparative method was as follows: The blocked isocyanate, methyl methacrylate and 4-hydroxybutyl acrylate were heated in a reactor to 50-600C. The epoxy resin was then added slowly while maintaining the temperature at 50-600C until all the epoxy resin was dissolved.

The solution was then removed from the heat and the pentaerythritol triallyl ether/2 butylene oxide/35 ethylene oxide was added, followed by the remainder of the components of Part A. the mixture was stirred well and Part B added slowly with continual stirring. The mixture was passed through a homogeniser until the particle size was less than 2000nm. Part C was then added, the mixture placed under a blanket of inert gas, and the resulting exotherm allowed to proceed.

When the temperature had fallen to less than 400C, sufficient of Part D was added to give pH 7.5.

EXAMPLE 3: Preparation of an aqueous dispersion polymer vehicle.

A vehicle was prepared according to the following formulation, in which the amounts are in weight percent: Part A: Urea formaldehyde resin solution 18.00 Methyl methacrylate 14.20 4-hydroxy butyl acrylate 1.78 Polyester urethane resin solution 14.30 Pentaerythritol triallyl ether/ 2 butylene oxide/35 ethylene oxide 2.40 Butyl acrylate 5.44 Butyl cellosolve 1.36 t-Butyl perbenzoate 0.58 Part B: Demineralised water 40.65 Part C: Ascorbic acid 0.24 Demineralised water 1.05 The preparative method was'as follows: Part A was prepared by mixing the components together.

This mixture was stirred well and Part B added slowly. The mixture of Part A and Part B was passed through a homogeniser until the particle size was less than 300 microns. Part C was added and then stirring stopped. The emulsion was placed under a blanket of inert gas, and the resulting exotherm allowed to proceed. The pH of the resulting polymer dispersion was adjusted to pH 6-7 by addition of a 30% solution of dimethylaminoethanol in demineralised water.

EXAMPLE 4: Preparation of an aqueous dispersion polymer vehicle.

A vehicle was prepared by blending together, with stirring, the following ingredients, in which all the amounts are in weight percent: Part 1 (as detailed below) 51.28 Part 2 (as detailed below) 43.72 Cymel 303 (Melamine formaldehyde resin) 5.00 Part 1 Part A: Demineralised water 24.493 Levelan P208 0.426 Part B: Methyl methacrylate 2.695 Buytlacrylate 1.033 Part C: Demineralised water 0.533 Ascorbic acid 0.010 Part D: Demineralised water 0.533 Hydrogen peroxide 0.100 Part E: Demineralised water 19.173 Levelan P208 1.065 Methyl methacrylate 25.531 Butyl acrylate 9.778 4-Hydroxybutyl acrylate 3.552 Part F: Demineralised water 2.610 Ascorbic acid 0.053 Part G:- Demineralised water 2.175 Hydrogen peroxide 0.488 Part H:Demineralised water 4.793 Pentoerythritol triallyl ether/ 2 butylene oxide/35 ethylene oxide 0.959 The preparation method was as follows: Part A was added to the reactor and heated to 400C under a blanket of inert gas, with steady stirring. Part B was added, followed by Part C and Part D. Stirring was maintained for 60 minutes. The temperature was maintained at 400C throughout the rest of the preparation, together with steady stirring and an inert gas blanket.

Part E, Part F and Part G were then added to the reactor, separately and at a constant rate, over a period of 3 hours 20 minutes, after 2 hours 50 minutes of which ths additions were stopped and Part H was added. The additions of Parts E, F and G were continued fo,r the remaining 30 minutes. After completion of these additions, stirring was maintained for a further 60 minutes.

Part 2 Part A: Methyl propoxol acetate 21.07 Epikote 1009 31.60 Pentaerythritol triallyl ether/ 2 butylene oxide/35 ethylene oxide 2.47 Part B: Methyl propoxol acetate 6.46 Demineralised water 38.40 The preparation method was as follows: Part A was prepared by heating the methyl propoxol acetate in a reactor to 1000C. The epoxy resin was then added slowly, maintaining the temperature at 1000C, until it had all dissolved. The heat was removed, and the remaining ingredient added the temperature had fallen below 60 C. The mixture was stirred well and Part B added slowly with continual stirring. The mixture was passed through a homogeniser until the particle size was less than 2000nm.

These products were used in primers according to the following Examples.

EXAMPLE 5: Formulation of a primer for coating a hot dip galvanised steel substrate.

A two-pack pretreatment-primer was prepared.

All amounts are in weight %.

Pack 1 Part A: Vehicle (from Example 1) 4.049 Demineralised water 13.362 Titanium dioxide pigment 9.995 Strontium chromate pigment 6.309 Part B: Demineralised water 6.527 Part C: Vehicle (from Example 1) 51.621 Ethylene glycol 0.850 Butyl carbitol 3.687 Polyox WSR 3000 0.040 dissolved in demineralised water 1.560 Surfonic LF17 dissolved in demineralised water 1.000 The ingredients of Part A were ground in a ball mill, and then let down with Part B to provide a millbase. This millbase was formulated as a paint by blending with Part C, to give a paint with a pigment volume concentration within the dried and cured film (pvc) of 15.1%.

Pack 2 Chromic acid (as Cr03) 20 Demineralised water 80 In use, 1000 parts by weight of Pack 1 are charged to a mixing vessel and 27.1 parts by weight of Pack 2 are added with stirring. The complete pretreatment-primer is then storage stable for at least one month without loss of 1coating performance.

EXAMPLE 6: Formulation of a primer for coating an aluminium substrate.

A two-pack pretreatment-primer was prepared. All amounts are in weight %.

Pack 1 Part A: Vehicle (from Example 1) 4.740 Demineralised water 10.230 Titanium dioxide pigment 18.220 Part B: Demineralised water 2.800 Part C: Vehicle (from Example 1) 55.870 Ethylene glycol 0.850 Butyl carbitol 3.690 Surfonic LF17 1.000 dissolved in demineralised water ; 1.000 Polyox WSR 3000 0.040 dissolved in demineralised water 1.560 The ingredients of Part A were ground in a ball mill, and then let down with Part B to provide a millbase. This millbase was formulated as a paint by blending with Part C, to give a pvc of 15.9%.

Pack 2 Chromic acid (as Crow) 20 Demineralised water 80 In use, 100 parts by weight of Pack 1 are charged to a mixing vessel and 8 parts by weight of Pack 2 are added with stirring. The complete pretreatment-primer is then storage stable for at least one month without loss of coating performance.

EXAMPLE 7: Formulation of a pretreatment-primer for coating hot dip galvanised steel substrate.

A two pack pretreatment-primer was prepared.

All amounts are in weight percent: Pack 1 Part A: Vehicle (from Example 3) 4.56 Demineralised water 11.08 Titanium dioxide pigment 12.00 Strontium chromate pigment 2.76 Part B: Vehicle (from Example 3) 62.13 Ethylene glycol 0.78 Butyl carbitol 3.39 Surfonic LF17 0.92 in demineralised water 0.92 Polyox WSRN 3000 0.03 in demineralised water 1.43 The ingredients of Part A were ground in a ball mill to provide a millbase. This millbase was formulated as a paint by blending with Part B, to give a paint pvc of 13.8%.

Pack 2 Chromic acid (as Cr03) 20 Demineralised water 80 In use, 1000 parts by weight of Pack 1 are charged to a mixing vessel and 115 parts by weight of Pack 2 are added with stirring. The complete pretreatment-primer is then storage stable for at least one month without loss of coating performance.

EXAMPLE 8: Formulation of a pretreatment-primer for coating hot dip galvanised steel substrate.

A two-pack pretreatment-primer was prepared.

All amounts are in weight percent.

Pack 1 Part A: Vehicle (from Example 4) 5.134 Titanium dioxide pigment 9.863 Strontium chromate pigment 5.694 Demineralised water 10.760 Part B: Vehicle (from Example 4) 59.772 Ethylene glycol 0.918 Butyl cellosolve 3.984 Surfonic LF17 1.074 in demineralised water 1.074 Polyox WSRN 3000 0.035 in demineralised water 1.692 The ingredients of Part A were ground in a ball mill to provide a millbase. This millbase was formulated as a paint by blending with Part B,to give a paint pvc of 15.1%.

Pack 2 Chromic (as CrO3) 20 Demineralised water 80 In use, 1000 parts by weight of Pack 1 are changed to a mixing vessel and 26.2 parts by weight of Pack 2 are added with stirring. The complete pretreatment-primer is then storage stable for at least one month without loss of coating performance.

EXAMPLE 9: Formulation of a pretreatment-primer for coating an aluminium substrate.

A 2-pack pretreatment-primer was made. All amounts are in weight percent: Pack 1 Part A: Vehicle (from Example 2) 4.622 Titanium dioxide pigment 18.241 Demineralised water 13.326 Part B: Vehicle (from Example 2) 55.676 Ethylene glycol 0.847 Butyl carbitol 3.688 Surfonic LF17 1.000 in demineralised water 1.000 Polyox WSRN 3000 0.036 in demineralised water 1.564 The ingredients of Part A were ground in a ball mill to provide a millbase. This millbase was formulated as a paint by blending with Part B, to give a paint pvc of 13.5%.

Pack 2 Chromic acid (as Cr03) 20 Demineralised water 80 In use, 100 parts by weight of Pack 1 are charged to a mixing vessel and 11.3 parts by weight of Pack 2 are added with stirring. The complete pretreatment-priwer is then storage stable for at least one month without loss of coating performance.

EXAMPLE 10: The paint prepared according to Example 5 was applied and tested according to the following procedure: A 12 inch x 4 inch (30cm x 10cm) panel of hot dip galvanised steel was cleaned and activated using a proprietary alkali cleaner ("Ridoline" (RTM) 1089) according to the manufacturer's recommendations. The panel was then dried in a hot air oven at 1000C for 10 minutes. The pretreatment-primer was applied using a wire-wound coater bar to give a dry film thickness of 4-6 microns after stoving. The paint was stoved in a hot air oven using a stoving schedule typical of that used on a production coil coating line.

This panel was then coated with a proprietary polyester-based coil topcoat to give a dry film thickness (topcoat) of 21-23 microns, and stoved using the procedure specified by the manufacturer.

Testing was carried out on this coated system, and the results are detailed in Table 1.

EXAMPLE 11: The paint prepared according to Example 6 was applied and tested according to the following procedure: A 12 inch x 4 inch (30cm x 10cm) panel of aluminium was cleaned and activated using a proprietary alkali cleaner ("Ridoline" (RTM) 1089), according to the manufacturer's recommendations. The panel was then dried in a hot air oven at 1000C for 10 minutes. The pretreatment-primer paint was applied using a wire-wound coater bar to give a dry film thickness of 3-5 microns after stoving. The paint was stoved in a hot air oven using a stoving schedule typical of that used on a production coil coating line.

The panel was then coated with a proprietary polyester-based coil topcoat to give a dry film thickness (topcoat) of 22-25 microns, and stoved using the procedure recommended by the manufacturer.

Testing was carried out on this coated system, and the results are detailed in Table 2.

EXAMPLE 12: The paint prepared according to Example 7 was applied and tested as a pretreatment-primer for a hot dip galvanised steel substrate as detailed in Example 10.

Results on the testing of this coated system are detailed in Table 1.

EXAMPLE 13: The paint prepared according to Example 8 was applied and tested as a pretreatment-primer for a hot dip galvanised steel substrate as detailed in Example 10.

Results on the testing of this coated system are given in Table 1.

EXAMPLE 14: The paint prepared according to Example 9 was applied and tested as a pretreatment-primer for an aluminium substrate as detailed in Example 11.

Results on the testing of this coated system are given in Table 2.

COMPARATIVE EXAMPLE 15: A coated sample of aluminium substrate was prepared according to current coil coating practice, that is the metal was cleaned, pretreated, primed and topcoated in successive operations. A 12 inch x 4 inch (30cm x 10cm) aluminium panel was cleaned and activated using a proprietary alkali cleaner ("RidolineB' [RTM] 1089), according to the manufacturer's recommendations. The panel was then pretreated using a proprietary chromate pretreatment process ("Alocrom" (RTM) 1225) followed by rinsing with the specified chromate rinse ("Deoxylite" ERTh) 41), according to the procedures of the manufacturer. The panel was then primed with an epoxy-based primer, following the manufacturer's instructions, and topcoated with the same polyester-based topcoat used in Examples 11 and 14.Test results for this system are detailed in Table 2.

COMPARATIVE EXAMPLE 16: A coated sample of hot dip galvanised steel substrate was prepared according to current coil coating practice of cleaning, pretreating, priming and topcoating as successive operations. A 12 inch x 4 inch (30cm x 10cm) hot dip galvanised steel panel was cleaned and activated using a proprietary alkali cleaner ("Ridoline" (RTMl 1089), according to the manufacturer's recommendations. The panel was then pretreated using aL proprietary chromate process ("Granodine" [RTM] 768 followed by a "Deoxylite "RTM) 41 rinse) following the manufacturer's procedures. The panel was then primed with an epoxy-based, strontium chromate-pigmented, primer according to the manufacturer's instructions. Finally the panel was topcoated with the same polyester-based topcoat used in Examples 10, 12 and 13.Test results for this system are detailed in Table 1.

COMPARATIVE EXAMPLE 17: A paint was prepared according to the teaching of EPA 0264471. This paint was applied according to the procedure detailed in Example 10, to a cleaned hot dip galvanised steel substrate, and topcoated with the same polyester-based paint used in Examples 10, 12, 13 and 16. Test results for this system are detailed in Table 1.

COMPARATIVE EXAMPLE 18: A composition was prepared according to the teaching of EPA 0274543. This composition was applied according to the procedure detailed in Example 11, to a cleaned aluminium substrate, and topcoated with the same polyester-based paint used in Examples 11, 14 and 15. Test results for this system are detailed in Table 2.

Testing of these applied panels was carried out according to the following standard procedures: "T" bend: National Coal Coaters Association standard "T" bend test method (Bulletin 19) Reverse Impact: European Coil Coaters Association standard method T5, where 1 joule is equivalent to 8.85 in. lib Salt Spray: Hot dip galvanised steel according to ASTM B117, Aluminium according to ASTM B287 Cleveland Condensing humidity: ASTM D4585, with a water temperature of 60 C, and assessment according to ASTM D714.

400C water soak:European Coil Coaters Association method T9.

TABLE 1 Hot Dip Galvanised Steel Substrate

MECHANICAL PROPERTIES RESISTANCE PROPERTIES Example ASTM B117 SALT CLEVELAND CONDENSING 40 C WATER SOAK T-BEND REVERSE IMPACT SPRAY(1000hrs) HUMIDITY (600hrs) 600 hrs 2T no pick-off OK 18.08 J. SCRIBE: NO CREEP NO ADHESION LOSS NO BLISTERS Example 10 no cracks: 5T no cracks 13.56 J. EDGE: 2-8 mm NO BLISTERING NO ADHESION LOSS 2T no pick-off OK 11.30 J. SCRIBE: OK CREEP 8F BLISTERS Example 12 no cracks: 3T no cracks 9.04 J. EDGE: 5-10 mm NO BLISTERING NO ADHESION LOSS 2T no pick-off OK 18.08 J. SCRIBE: OK 9F BLISTERS Example 13 no cracks: 4T no cracks 9.04 J. EDGE: 3-10 mm NO ADHESION LOSS Comparative 2T no pick-off OK 18.08 J. SCRIBE: OK NO BLISTERS no cracks: Example 16 3T no cracks 13.56 J. EDGE: 2-7 mm NO ADHESION LOSS Comparative 3T no pick-off OK 11.30 J. SCRIBE: 1-3mm 8M BLISTERS no cracks: EDGE: 5-15mm 20% ADHESION LOSS Example 17 5T no cracks 9.04 J. ADHESION LOSS ON PANEL FACE TABLE 2 Aluminium Substrate

MECHANICAL PROPERTIES RESISTANCE PROPERTIES Example ASTM CLEVELAND CONDENSING T-Bend REVERSE IMPACT SPRAY (1000hrs) HUMIDITY (600hrs) 1T no pick-off PASS 6.10 J. SCRIBE: 1mm NO BLISTERS Example 11 no cracks: 2T no cracks-off 4.52 J. EDGE: 2 mm NO ADHESION LOSS 1T no pick PASS 6.10 J. SCRIBE: 1-2mm 9F BLISTERS Example 14 no cracks: 2T no cracks 6.10 J. EDGE: 1-2 mm NO ADHESION LOSS Comparative 2T no pick-off PASS 6.10 J. SCRIBE: 1mm 9F BLISTERS no cracks: Example 15 3T no cracks 4.52 J. EDGE: 1mm 1% ADHESION LOSS Comparative 1T no pick-off PASS 6.10 J. SCRIBE: 2-3mm 8M BLISTERS no cracks: Example 18 2T no cracks 6.10 J. EDGE: 2-4mm 20% ADHESION LOSS

Claims (13)

1. ClAIMS 1. A primer for metal substrates, the primer comprising an aqueous dispersion based on an acrylic polymer as vehicle, an oxidising acid for passivating the substrate surface, and one or more pigments, in which the acrylic polymer dispersion includes a reactive steric stabiliser which is chemically bound as part of the polymer particles during the polymerisation process.
2. 2. A primer according to Claim 1, in which the oxidising acid comprises chromic acid.
3. 3. A primer according to Claim 1 or Claim 2, in which the primer additionally includes an epoxy resin or a urethane resin.
4. 4. A primer according to any preceding claim, in which the primer also includes a cross linking agent.
5. 5. A primer according to Claim 4, in which the crosslinking agent comprises an amino resin or an isocyanate resin.
6. 6. A primer according to any preceding claim, in which the acrylic polymer is formed from monomers which include one or more monomers having a functional group which will be available for crosslinking during curing.
7. 7. A primer according to any preceding claim, in which the reactive steric stabiliser includes the reaction product of pentaerythritol triallyl ether with one or more alkylene oxides, or methoxy poly (ethylene glycol) methacrylate, or methoxy poly (ethylene glycol) acrylate.
8. 8. A primer according to any preceding claim, in which the amount of pigment is such as to occupy a pigment volume concentration of from 5 to 40% in the dried and cured film.
9. 9. A primer according to Claim 8, in which the amount of pigment is such as to occupy a pigment volume concentration of from 10 to 25% in the film.
10. 10. A primer according to any preceding claim, in which one or more of the pigments is an anticorrosive pigment, including strontium or other chromate pigment.
11. 11. A primer according to any of Claims 3 to 9, in which the amount of epoxy resin occupies not less than 20% by weight oninon-volatile resin.
12. 12. A primer according to any of Claims 2 to 11, in which the concentration of chromic acid is in the range 1-10% as Cr03 in the dried and cured film.
13. 13. A coated metal substrate including as at least part of the coating system a primer according to any of Claims 1 to 11.