GB2189408A

GB2189408A - Coating surfaces

Info

Publication number: GB2189408A
Application number: GB08708760A
Authority: GB
Inventors: Geoffrey Landon-Browne
Original assignee: Smith Meters Ltd
Current assignee: Smith Meters Ltd
Priority date: 1986-04-25
Filing date: 1987-04-13
Publication date: 1987-10-28
Anticipated expiration: 2007-04-13
Also published as: GB2189408B; GB8708760D0

Description

SPECIFICATION Coating surfaces This invention relates to coating surfaces with materials which have poor adhesion thereto, and particularly, but not exclusively, to coating metal surfaces.

Previous methods of coating surfaces with materials having poor adhesion thereto, for instance painting lead alloys, have comprised shot blasting the surface with an abrasive, cleaning the abrasive material from the surface and then using a wet paintto cover the surface. This process has the disadvantage that an appreciable amount of lead is removed from the surface, and that the surface must be made free of contaminating abrasive before it can be painted.

This invention is concerned with providing a method of coating surfaces which does not suffer these problems and which provides a particularly good bond to the surface.

The invention provides a method of coating a surface with a material which has poor adhesion thereto, comprising first bombarding the surface with particles of a conditioning material and then applying a coating material, the bombardment being such as to effect a mechanical bond between the conditioning material particles and the surface, the conditioning material and the coating material being selected so thattheywill bond together.

The said bombardment of the surface is such that the particles of conditioning material penetrate small irregularities in the surface forming a mechanical key.

The particles may also form indentations in the surface and seat in the indentations. The relative characteristics of hardness of the surface and particles and the kinetic energy ofthe particles impinging on the surface determines the extent of penetration ofthe surface by the particles.

The conditioning material may be a dry epoxy resin powder and the coating material may be an epoxy resin paint applied either in a dry powder coating process or wet in a solvent.

The conditioning material and the coating material may be the same dry epoxy resin powder paint.

Some specific examples ofthe invention are now described with reference to the accompanying diagrammatic drawing.

A casing (11) is formed of pressedterne steel, i.e.

steel coated with a lead alloy which improves the corrosion resistance of the steel. Great difficulty has previously been found in applying paint or other decorative or protective surface coating to this material. The casing is first subjected to a conventional degreasing operation, for instance in aTRICHLOR solvent bath. It is then bombarded with dry particles of a conditioning material discussed hereinafter. As shown, an airgun (12) hasa reservoir(13) containing the dry powdered conditioning material. A high pressure airstream (14) propelled byairfrom a 5.5 bar source is passed across a nozzle (15) of 9.5 mm diameterto pick up the particles and propel them through a mixing nozzle (16) in a jet (17)to impinge on the surface of the casing.The whole surface to be coated is subjected to this bombardment.

The conditioning powder material comprises particleswhich when projected with sufficient energy penetrate small crevices in the surface ofthe casing and stay embedded therein. They also make indentations in the surface and embed themselves therein. A strong physical bond between the conditioning pow der material andthe surface ofthecasing isthus made. The conditioning process depends on selection of the pressure of the air stream, the size and hardness of the particles and the hardness of the casing.

Selection between these criteria is made so that the powder is mechanically locked to the surface of the casing without destroying the surface. It is emphasised that the casing is not subjected to an electrostatic charge, the bond being the result of impact pressure ofthe particles.

The choice of material for the conditioning powder material is made with relation to the coating material it is desired to use, and is such that the coating material adheres well to the powder material. In one example, the conditioning material is a fusible powder paint mixture of dry polyester and epoxy resin particles of between 1.26 and 80.64 microns diameter, a material having no appreciable abrasive qualities. This is propelled by a 3.5 bar pressure airsource ata terne steel casing so that the surface is evenly coated.A conditioning machine may, for instance, have ten guns of the kind shown at (12), all operating together about 10 cms from the casing to coat the exterior surfaces of the rotating casing.The conditioning is maintained for 4.5 seconds, at the end ofwhich the casing is visibly coated with the paint. The coating adheres to the casing sufficiently so that it is not possible to remove all the embedded powder by rubbing with a finger.

Immediately, or at least within 2 hours of the conditioning treatment, and with no intermediate processing, the casing is passed into a conventional electrostatic powder coating plant, using the same powder paint mixture as was used for the conditioning treatment. In this process the casing is electrostatically charged and the dry paint mixture is propelled towards it at low pressures (e.g. 0.3 - 0.7 bar air source pressure), so that a powder layer clings to the charged conditioned surface. Thecasing is then heated in an oven to fuse the paint. For instance, the casing may be held at between 160P- 180"fr8to 15 minutes.

The paint coating so formed is of about 50 microns thickness, and is found to have a good bond to the terne steel, performing well in corrosion tests.

Itwill be appreciated that in this process, the conditioning material has negligible abrading effect on the terne steel, and forms an essential base layer of the final paint coating. Thus, there is no problem with spent conditioning material contaminated with lead, and there is no need to clean the surface after conditioning.

In another example, the conditioning material is a dry epoxy resin powder of between 2 - 50 microns diameter, propelled by air at 5.5 bar onto a lead/tin alloy surface. The coating material is a wet epoxy resin paint in a solvent which is also a solventforthe conditioning material. When the wet paint is applied, the solvent attacks the surface of the conditioning material, dissolving it and forming a liquid interface layer between the conditioning and coating materials comprising the solvent and a dissolved mixture of the conditioning materials and the coating material. When the solvent evaporates, it leaves the paint layer adhering closelyto the conditioning material, thus strongly bonded to the casing.

In anotherexamplethe dry powder conditioning material comprises a catalyst, e.g. a hardener and accelerator, while the coating material comprises a liquid epoxy resin which hardens on contact with the catalyst. The coating material may include a pigment sothata coloured coating isformed.The hardening maytake place at room temperature or heat may be used to speed the hardening process. In a variation of this example the coating material is also a dry powder.

The base surfaces onto which the coating is made may include terne steel, galvanised steel, aluminium, copper, zinc or plastics materials. Since a mechanical bond is being madeto the base surface, the main requirements concern the pressure and particle size of the conditioning material being adjusted to give satisfactory penetration of the base surface. For instance, in the examples given above using terne steel, ifthe air pressure is reduced to 2 bar, very good conditioning is still achieved, but if it is reduced to 1.3 bar the bond is rated only as fairly good. A poor adhesion is revealed by wear and corrosion tests in which the coating peels, blisters or cracks.

Different pressures are required for other base surfaces. It is found that aluminium can be conditioned at lower pressures than terne steel, buttrial and errorfor each material quickly reveais suitable operating pressures. Generallythe harderthesurface the higherthe pressure required. Higher air pressures, e.g. of 13 bar or more, may be used.

In orderto increase the penetration of the particles, they may be chilled before use, and/orthe base surface may be softened by heating.

The invention extends to items coated by the method described, and also to apparatus for carrying outthe new method.

Claims

1. A method ofcoating a surface with a material which has pooradhesion theretocomprising first bombarding the surface with particles of a conditioning material and then applying a coating material, the bombardment being such as to effect a mechanical bond between the conditioning material particles and the surface, the conditioning material and the coating material being selected so thattheywill bond together.

2. The method as claimed in claim 1, wherein said conditioning material comprises an epoxy resin powder.

3. The method as claimed in claim 2, wherein said epoxy resin powder is a paintsuitable for use in powder coating.

4. A method of coating a surface of lead alloy or a metal of similar hardness, comprising first bombarding the surface with dry epoxy resin particles projected in a jet of pressurised gas from a supply pressurised at between 1.3 barand 13 bar, andthen applying a coating material compatible with the epoxy resin.

5. The method as claimed in claim 4, wherein said supply is pressurised at substantially 3.5 bar.

6. The method as claimed in any of claims 1 to 5, wherein said conditioning material comprises dry epoxy resin powder paint containing mainly particles of diameter between 1.26 and 80.64 microns.

7. The method as claimed in any of claims 1 to 5, wherein said conditioning material comprises dry epoxy resin powder paint containing particles of between 2 and 50 microns diameter and said bombardment is of particles projected in a jet of pressurised airfrom a source at substantially 5.5 bar pressure.

8. The method as claimed in any of claims 1 to 7, wherein said conditioning material is substantiallythe same as the coating material and the step of applying the coating material isanelectrostaticpowdercoating process.

9. The method as claimed in claim 1, wherein the conditioning material is a dry powder catalystforthe coating material.

10. A member coated with a material which has poor adhesion thereto by the method as claimed in any of claims 1 to 9.

11. A member as claimed in claim 10, having a coated surface ofterne steel, galvanised steel, aluminium, copper, zinc or a plastics material.

12. A member coated with a material which has poor adhesion thereto by a method substantially as described hereinbefore with reference to the accompanying drawing.

13. Apparatus for coating a surface with a material which has poor adhesion thereto substantially as described hereinbefore with reference to the accompanying drawing.