GB2313560A - Treatment of painted surface to improve adhesion of an overcoat paint - Google Patents

Abstract

A process for treating a cured paint, particularly an epoxy paint, which is to be overcoated comprises applying to the surface of the cured paint a water-dispersible preferably thixotropic composition comprising a surfactant and a low-molecular-weight hydrophobic organic material such as a terpenic hydrocarbon which interacts with the paint surface, and subsequently washing the treated paint surface with water, preferably under pressure, to remove the waterdispersible composition, thereby improving the adhesion of subsequently overcoated paint to the treated cured paint surface.

Description

Treatment of Painted Surfaces This invention relates to a process for the rejuvenation of the surface of a cured paint to improve the adhesion of a subsequently applied overcoating to the paint.

During the construction of a ship or other large steel structure various coatings are applied. An anticorrosive "shop primer" is applied to the steel at an early stage before fabrication. A main permanent anticorrosive primer paint is applied to the structure at a convenient stage during fabrication; in shipbuilding for example it can be applied after steel plates and reinforcing members have been assembled to form a "block" but before the blocks are joined up to form the ship. The primer paint is generally a cured or thermoset paint (although the term "thermoset" does not imply that the paint is heated to cure it, which would be impractical when coating large structures such as ships).

Such cured paint generally comprises a resin, for example an epoxy resin or polyurethane, and a curing agent for the resin, and it is commonly supplied as a 2-pack paint in which the resin and the curing agent are packaged separately and are mixed either just before the paint is to be applied or during paint application in a twin-feed spray.

The cured primer paint is generally overcoated with a top coat paint. The top coat paint is not usually applied until the whole structure has been assembled. Different top coat paints are often used on different parts of a structure; for example on a ship the underwater hull area is coated with antifouling paint, the superstructure and topsides may be coated with a weather-resistant decorative paint and different specialist paints may be used on the deck, the hold and/or the cargo tanks. The top coat paint is applied to the cured primer paint a variable time after the primer paint has itself been applied; for example during shipbuilding blocks which have been fabricated and primer painted early may be left without overcoating for a much longer period than blocks which have been fabricated and primer painted later. We have found that this gives rise to problems of adhesion of the top coat paint to the cured primer paint.

Cured paints, particularly 2-pack paints such as 2-pack epoxy primers, have a limited period or "window" within which they can be overcoated with good adhesion. The length of this period varies for different cured paints (including different epoxy paints) and for different top coat paints, and also varies with the conditions of weathering of the cured paint such as temperature, humidity, rain and exposure to UV rays present in sunlight, but we have found that there is generally a time limit, for example 3 to 9 months, or even 1 or 2 months, after which the cured paint cannot be overcoated without loss of adhesion of the top coat.

Shipbuilders do not like this time constraint, and would prefer to be free to overcoat the cured paint an unpredictable time after it has been applied.

A process according to the invention for treating a cured paint which is to be overcoated is characterised in that a water-dispersible composition comprising a low molecular weight hydrophobic organic material which interacts with the paint surface and a surfactant is applied to the surface of the cured paint, and the paint surface thus treated is subsequently washed with water to remove the said water-dispersible composition, thereby improving the adhesion of the overcoated paint to the cured paint.

From another angle, the invention consists of the use of a treatment composition to improve the adhesion of a cured paint to a subsequently overcoated paint, in which use the treatment composition is a water-dispersible composition comprising a low molecular weight hydrophobic organic material which interacts with the paint surface and a surfactant and is capable of being removed from the surface of the cured paint by water washing.

The cured paint which is to be overcoated is most commonly based on an epoxy resin, for example a condensed glycidyl ether of bisphendl A, and a curing agent, for example an amino-functional curing agent such as an amino functional polyamide or other amino-tipped polymer and/or an aralkylene diamine. Alternative types of cured paint which may be overcoated are cured polyurethane paints derived from a polyisocyanate and a polyol, moisture-curing paints such as moisture-cured polyurethanes or organic silicate paints or paints cured by autoxidation such as alkyds. The paints are usually pigmented with anticorrosive pigments such as zinc phosphate, calcium phosphate, zinc molybdate and/or powdered zinc and optionally with extender pigments such as barytes, talc, calcium carbonate and/or titanium dioxide.

The paints may contain organic plasticisers or extenders such as coal tar or pitch, for example epoxy coal tar paints.

The water-dispersible composition is particularly effective in treating cured paints which have been allowed to cure and weather for such a long time that they have reduced adhesion on overcoating, for example cured paints which have been allowed to cure and weather for more than 4 weeks, particularly 2 months or more. The water-dispersible composition does not impair the overcoatability or other properties of paints which have cured for less time, and it can be applied routinely by shipyards when overcoating cured paint.

The water-dispersible composition applied to the cured paint must at least disperse in water and may be watermiscible or dissolve in water. It may itself be an aqueous composition containing for example at least 20 or 40% up to 80 or 90% water, by weight.

The low molecular weight hydrophobic organic material generally has a molecular weight of less than 300 and preferably less than 200. This hydrophobic organic material, together with the surfactant contained in the waterdispersible composition, interacts with and may penetrate the surface of the cured paint, but it does not substantially dissolve the cured paint. Curable paints sold as primers for marine and protective coatings, particularly epoxy primers, are highly solvent-resistant when cured. The low molecular weight hydrophobic organic material preferably comprises a hydrocarbon. A hydrocarbon which we have found to be particularly useful in rejuvenating cured epoxy resin paints is a terpenic hydrocarbon such as limonene, for example D-limonene, or pinene. The hydrocarbon can alternatively be a "white spirit", which is a predominantly aliphatic, especially paraffinic, hydrocarbon mixture. The white spirit is preferably one having some aromatic content, for example 10 to 30% by weight aromatics, although entirely aromatic hydrocarbon solvents are less effective than white spirit. White spirit is generally less effective than Dlimonene, that is to say it may need to be used at a higher concentration in the water-dispersible composition or with a longer contact time between application of the composition and water washing, but white spirit may be preferred because of lower cost. Alternatively, a mixture of white spirit and a terpenic hydrocarbon may be used. The proportion of hydrocarbon in the water-dispersible composition is preferably at least 5 and may be up to 50 or even 60% by weight, with the most preferred proportion being in the range 8 or 10 to 30% by weight.

The water-dispersible composition preferably contains sufficient surfactant to ensure that the hydrophobic organic material, for example the hydrocarbon, is stably dispersed in the water-dispersible composition and that the composition is water-dispersible on washing with water. The surfactant can be an anionic, nonionic or cationic surfactant; an example of a suitable anionic surfactant is an ammonium, amine or alkali metal salt of a long-chain sulphonic acid such as dodecylbenzenesulphonic acid. The surfactant can be present at up to 30% by weight of the solvent composition, most preferably at 5 to 25%, particularly 15 to 25%, by weight.

The water-dispersible composition may additionally contain a water-miscible organic solvent such as an ether alcohol, for example methoxypropanol, ethoxyethanol, butoxyethanol or ethoxyethoxyethanol. Esters of such ether alcohols, for example methoxypropyl acetate, can also be used. Other alcohols or carboxylate esters may also be useful, including those of limited water-solubility such as butanol or benzyl alcohol. The proportion of water-miscible solvent, for example ether alcohol, in the water-dispersible composition can be up to 50% by weight, preferably up to 25%, for example 1 to 20%, by weight. The presence of such a water-miscible organic solvent in the composition may aid in the solution or dispersion of a hydrocarbon or other hydrophobic organic material.

The water-dispersible composition is preferably thickened before application to the cured paint surface, for example by incorporation of a thickening agent which makes the composition thixotropic or pseudoplastic. The waterdispersible composition may even be gelled, provided it is capable of being applied as a coating. The thickening agent used is preferably a fine-particle silica gel such as that sold under the Trade Mark "Aerosil 200", which can for example be used at 2 to 6% by weight of the composition, and/or a bentonite clay. The thickened water-dispersible composition most preferably has a viscosity at low or zero shear higher than is usual for paint; this allows application of a thick layer of the composition, preferably at least 100 microns, for example 150 to 200 microns up to 250 or even 500 microns thick (wet film thickness), and it inhibits the penetration of the water-dispersible composition into the cured paint layer. The thickened waterdispersible composition is preferably applied by spray, for example airless spray, but it can alternatively be applied by brush or roller. The thickened composition is preferably able to be applied to a ship's hull when the ship is in a substantially vertical position, for example in a shipyard, substantially without the composition running down the hull.

The water-dispersible composition can contain one or more additional ingredients, for example it may contain a colouring pigment or dye to aid visibility so that there is an easy check that the water-dispersible composition has been removed before overcoating.

After the water-dispersible composition has been applied to the cured paint it is washed off by water. A light wash (hosing down with water at a normal pressure of up to 10.5 MPa (1500 psi) may be adequate but a highpressure water wash at at least 10.5 MPa (1500 psi), particularly 14 to 35 MPa (2000-5000 psi), may be preferred.

In general, water washing can take place at any time from 5 minutes to 5 days or more after the water-dispersible composition, most preferably at least 2 or 3 hours, or up to 24 hours, for example 4 to 16 hours, after application.

Water washing should be continued until all the waterdispersible composition has been washed from the hull before overcoating.

Paints which can be overcoated on the cured paint surface which has been treated with the water-dispersible composition include decorative and weather-resistant top coats which can for example be polyurethane paints, thermosetting or thermoplastic acrylic paints or alkyd paints; antifoulings which can for example be acrylic paints such as self-polishing organic copolymer paints or rosinbased paints; abrasion-resistant paints for decks and holds which can be based on polyurethanes, elastomers or filled epoxy resins; and solvent-resistant coatings for cargo and ballast tanks such as highly crosslinked epoxy or polyurethane coatings. All of these adhere to a cured epoxy paint which has been treated with a water-dispersible composition according to the invention better than to the same cured epoxy paint which has been allowed to cure and weather and has not been treated before overcoating. The paint surface is preferably overcoated less than 5 days, for example 4 hours to 2 days, after the water-dispersible composition has been washed off the paint surface, provided that the surface has dried.

The process of the invention is particularly applicable during shipbuilding, or the construction of other large steel structures such as oil rigs, bridges or mining installations, when overcoating intermediate structures which have been fabricated and primer-painted but then left to cure and weather before being overcoated. In some shipyards it is the practice to apply the whole coating scheme at intermediate fabrication (block stage) in areas where this is possible. Even when using this procedure large areas of cured primer paint are left uncoated for a long time in areas which are liable to damage during further construction, and treatment by the process of the invention is beneficial in those areas; it avoids the need for the manual mechanical treatment now used.

The process of the invention can also be used to treat a cured paint, particularly an epoxy resin paint, which has weathered in use and has a chalked appearance due to UV degradation of the resin of the paint. The chalked appearance is due to a surface layer depleted of resin and comprised of lightly-bound pigment. Such a chalked paint layer is unsightly and moreover prevents good adhesion of the top coat paint when overcoated. Such chalked layers are sometimes found at the surface of cured epoxy paints on decks, superstructures and topsides of ships which have been weathered and exposed to a lot of bright sunlight. When the water-dispersible composition is applied to the chalked surface of the cured and weathered paint, it softens and may partially dissolve the degraded resin, so that on water washing the chalked paint layer is washed away with the water-dispersible composition. When the process of the invention is thus used to remove a chalked layer at the surface of a cured paint, it gives an improved surface for overcoating; alternatively, if there is still a substantial thickness of cured paint below the chalked layer, it can provide an improved appearance without overcoating.

Terpene hydrocarbons, particularly limonene, are particularly effective in interacting with cured epoxy paints which have been allowed to cure and age for so long that they are difficult to overcoat. In some cases improved adhesion can be achieved by incorporating a terpene in the paint which is used to overcoat the cured epoxy paint. This can be a top coat paint based for example on a polyurethane or acrylic or alkyd resin as described above or can be an adhesion-promoting tie coat comprising a film-forming resin, for example an epoxy, polyurethane, alkyd or acrylic resin, dispersed in a solvent blend including the terpene. Such a tie coat might particularly be used when overcoating a degraded or chalked epoxy resin paint.

Thus according to another aspect of the invention a process for overcoating a cured epoxy paint with a coating composition comprising a film-forming resin in a solvent is characterised in that the coating composition contains a terpene hydrocarbon.

The proportion of terpene hydrocarbon in the overcoating composition is preferably at least 3% by weight and can be up to 30 or 40% by weight, with the most preferred proportion being in the range 5 to 20% by weight.

The terpene hydrocarbons are also particularly effective in interacting with the binders used in antifouling paints, especially rosin, and can be used to give improved adhesion to an aged antifouling, for example an aged antifouling paint whose surface comprises an outer layer of paint binder from which seawater-soluble material has been leached. International Patent Application PCT/GB95/02816 describes a process for the removal from a hull of a ship of an outer leached layer of an antifouling paint which originally comprised a paint binder and a biocide but from which seawater-soluble material has been leached, which process comprises applying to the surface of the antifouling paint a water-dispersible composition comprising a solvent for the paint binder remaining in the leached layer. The surface thus treated is subsequently washed with water to remove the said water-dispersible composition containing dissolved or emulsified material of the leached layer, thereby exposing a surface of underlying intact antifouling paint. According to the present invention good adhesion to the leached layer can be achieved without a washing step by incorporating a terpene in the coating composition used to overcoat the leached layer.

This coating composition can be an adhesion-promoting tie coat as described above based on a film-forming acrylic, epoxy, alkyd or polyurethane resin or can be an antifouling coating composition based on a film-forming resin which can for example be a self-polishing acrylic copolymer or can comprise rosin.

Thus according to another aspect of the invention a process for overcoating an aged antifouling paint whose surface comprises an outer leached layer of paint binder from which seawater-soluble material has been leached is characterised in that the leached layer is overcoated with a coating composition comprising a film-forming resin in a solvent which contains a terpene hydrocarbon.

The proportion of terpene hydrocarbon in the overcoating composition is preferably as described above for overcoating cured epoxy paints.

The invention also includes a tie coat for improving adhesion of a subsequently applied paint to cured epoxy paint or to a leached layer of antifouling, characterised in that the tie coat comprises a film-forming resin in a solvent containing a terpene hydrocarbon.

The invention is illustrated by the following Examples: Example 1 96% by weight degreasing composition sold under the name "Citrisolve Concentrate", believed to consist by weight of 64% water, 8-10% limonene, up to 5% methoxypropanol, 2025% surfactants (including an alkyl benzene sulphonate as the main surfactant) and 1% 2,6-di-t-butyl cresol, was mixed with 3% by weight "Aerosil 200" fumed silica thickener and 1% by weight of a tinter comprising 20 wt% green dye in a 40% solution of rosin in trimethyl benzenes, to give a thixotropic water-dispersible composition.

This thickened water-dispersible composition was sprayed at an average wet-film thickness of 200 microns on a steel panel coated with a cured epoxy paint. The epoxy paint had a pigment volume concentration of 25% and was a 2pack paint based on a bisphenol A epoxy resin and an aminofunctional curing agent. It had been applied to the gritblasted steel 2 months before and had been allowed to cure and weather since then. After 4 hours the epoxy paint surface coated with the water-dispersible composition was washed by water at a pressure of 24.5 MPa (3500 psi). The water washing removed the water-dispersible composition, exposing a rejuvenated surface of cured epoxy resin paint.

Four hours after the washed paint surface had dried, a 2-pack polyurethane top coat paint based on a hydroxyacrylic polymer and an aliphatic polyisocyanate and having a pigment volume concentration of 15% was applied by airless spray to the rejuvenated cured epoxy resin paint surface and allowed to set. The polyurethane top coat paint adhered well to the rejuvenated cured epoxy resin paint.

By comparison, when the 2-pack polyurethane top coat paint was applied to a similar steel panel which had been primer coated at the same time with the same epoxy resin paint but had not been treated with the water-dispersible composition, the polyurethane top coat paint did not adhere so well to the cured epoxy resin paint and became delaminated in places on subsequent immersion in water.

Examples 2 to 4 The process of Example 1 was repeated except for the use of the following water-dispersible compositions (t by weight based on liquid ingredients) in place of the composition used in Example 1:

Example 2 Example 3 Example 4 Water 50 50 60 D-Limonene 25 15 20 White Spirit 15 (18-20% aromatic content) Methoxypropanol 10 5 Dodecylbenzene sulphonate 15 15 20 Aerosil 200 4 4 4 Tinter 1 1 1 In each case the top coat paint adhered well to the rejuvenated cured epoxy resin paint.

Claims (21)

1. A process for treating a cured paint which is to be overcoated, characterised in that a water dispersible composition comprising a low molecular weight hydrophobic organic material which interacts with the paint surface and a surfactant is applied to the surface of the cured paint, and the paint surface thus treated is subsequently washed with water to remove the said water-dispersible composition, thereby improving the adhesion of the overcoated paint to the cured paint.

2. A process according to claim 1, characterised in that the water-dispersible composition is thixotropic.

3. A process according to claim 1 or claim 2, characterised in that the water-dispersible composition is an aqueous composition containing at least 40% by weight water.

4. A process according to any of claims 1 to 3, characterised in that the low molecular weight hydrophobic organic material comprises a hydrocarbon.

5. A process according to claim 4, characterised in that the hydrocarbon comprises a terpenic hydrocarbon.

6. A process according to claim 4 or claim 5, characterised in that the water-dispersible composition contains 5 to 50% by weight of the hydrocarbon.

7. A process according to any of claims 1 to 6, characterised in that the composition contains 5 to 25% by weight of the surfactant.

8. A process according to any of claims 1 to 7 characterised in that the composition contains an ether alcohol.

9. A process according to any of claims 1 to 8, characterised in that the composition contains a silica gel thickener.

10. A process according to any of claims 1 to 9, characterised in that the water-dispersible composition is applied to the cured paint surface at a wet film thickness of at least 100 microns.

11. A process according to any of claims 1 to 10, characterised in that the washing with water is carried out 2 to 24 hours after the water dispersible composition has been applied to the surface of the cured paint.

12. A process according to any of claims 1 to 11, characterised in that the washing with water is carried out at a pressure of 10.5 MPa to 35 MPa.

13. A process according to any of claims 1 to 12, characterised in that the cured paint is a cured epoxy resin paint.

14. A process according to any of claims 1 to 13, characterised in that the cured paint has a chalked surface layer comprising pigment lightly bound by degraded resin, and in that the said chalked surface layer is washed away with the water-dispersible composition during the washing step.

15. A process for overcoating a cured paint with a top coat paint, characterised in that a water-dispersible composition comprising a low molecular weight hydrophobic organic material which interacts with the paint surface and a surfactant is applied to the surface of the cured paint, and the paint surface thus treated is subsequently washed with water to remove the said water-dispersible composition before being overcoated with the top coat paint.

16. A process according to claim 14 characterised in that the top coat paint is overcoated on the cured paint within 5 days after the water-dispersible paint has been washed off the cured paint surface.

17. The use of a treatment composition to improve the adhesion of a cured paint to a subsequently overcoated paint, in which use the treatment composition is a waterdispersible composition comprising a low molecular weight hydrophobic organic material which interacts with the paint surface and a surfactant and is capable of being removed from the surface of the cured paint by water washing.

18. A process for the rejuvenation of a cured and weathered epoxy resin paint surface which has an outer chalked layer comprising pigment lightly bound by degraded epoxy resin, characterised in that a thickened water-dispersible composition comprising a low molecular weight hydrophobic organic material which interacts with the degraded epoxy resin and a surfactant is applied to the surface of the cured paint and the paint surface thus treated is subsequently washed with water to remove the said waterdispersible composition together with the chalked layer thereby revealing a surface of underlying intact cured epoxy resin paint.

19. A process for overcoating a cured epoxy paint with a coating compositibn comprising a film-forming resin in a solvent, characterised in that the solvent contains a terpene hydrocarbon.

20. A process for overcoating an aged antifouling paint whose surface comprises an outer leached layer of paint binder from which seawater-soluble material has been leached, characterised in that the leached layer is overcoated with a coating composition comprising a filmforming resin in a solvent which contains a terpene hydrocarbon.

21. A tie coat for improving adhesion of a subsequently applied paint to a cured epoxy paint or to a leached layer of antifouling, characterised in that the tie coat comprises a film-forming resin in a solvent containing a terpene hydrocarbon.