GB2071521A

GB2071521A - Shot blasting medium

Info

Publication number: GB2071521A
Application number: GB8106185A
Authority: GB
Original assignee: Colebrand Ltd
Current assignee: Colebrand Ltd
Priority date: 1980-03-05
Filing date: 1981-02-27
Publication date: 1981-09-23
Also published as: GB2071521B

Abstract

A shot blasting medium, comprising a plurality of discrete particles which in use is able to clean a surface at which it is directed, substantially all of the particles having a coating comprising means for protecting the surface and a carrier for that means. The coating may include a biocide which inhibits the inception of marine growth for as much as eight minutes.

Description

SPECIFICATION Shot blasting medium TECHNICAL FIELD OF THE INVENTION The invention reiates to shot blasting medium particularly such a medium which is used to shot blast to clean and to protect a surface which is in wet or damp conditions, such as a steel or concrete underwater structure, for example a pile.

BACKGROUND ART Protective coating systems have been formulated for application to surfaces under wet or damp conditions. However, in order to take advantage of these systems particularly those formulated for underwater and splash zone applications, the substrate or surface to be treated requires special preparation prior to the application of the particular formulation being used.

Blasting the surface with grit is a known procedure. This cleans the surface, but the surface immediately begins to corrode and the permissible time between the shot blasting with grit and the application of the system can be as low as a few seconds depending on the temperature and the biological activity of the water.

DISCLOSURE OF THE INVENTION It is an object of the invention to provide a shot blasting medium which seeks to mitigate the disadvanLages of the prior art and which seeks to provide a coherent and adherent coating.

According to the invention there is provided a shot blasting medium, comprising a plurality of discrete particles which in use is able to clean a surface at which it is directed, characterised in that substantially all of the particles have a coating comprising means for protecting the surface.

Thus using a medium in accordance with the invention it is possible to provide a surface which is cleaned by the shot blasting medium and which has an adherent corrnsion-resisting coating transferred from the particles tci the surface, which subsists for a time suitable to allow the application of a subsequent protective coating system e.g.

paint.

The coating may also comprise a biocide. This helps to inhibit the inception of marine growth on the cleaned surface.

The carrier may comprise an organic resin and the coating may comprise corrosion inhibitor or prevention agent. A suitable corrosion prevention agent may be zinc dust. The carrier may alternatively comprise an organo-titanate compound.

The particles may each comprise a grit of a mineral slag material, for example a copper slag material. This is hard yet light enough to be projected in a stream against a surface to be cleaned.

An embodiment of the invention is described with respect to the following specific examples.

A standard grit of a copper slag base or core was coated with resin such as an epoxy resin, preferably solid, or rosin, and its derivatives, prior to the addition of the coating of zinc dust or other corrosion inhibiting agents and a biocide.

A target surface was subjected to shot blasting using this medium, both above and below the water-line. It was found that the surface was cleaned by the grit and the coating inhibited corrosion, and particularly the settlement of marine growth for u9 to eight minutes (during which time the protective system could be applied). It will be understood that when the particles impinge on the target surface, the copper slag base cleans that surface and the zinc dust and resin are transferred to the cleaned surface. The resin both acts as a carrier to adhere the zinc dust to the particles and, subsequently, to adhere the zinc dust to the cleaned surface, which is protected prior to subsequent treatment as by painting. The biocide was the agent inhibiting the marine growth formation.

The grit can be coated with a resin as follows: The resin, preferably a solid epoxy resin, is dissolved in solvent to say a 50% solids content by weight. The grit (particles) to be coated is put in a rotatable drum. The solution is added to the drum which is then rotated until each particle of grit is evenly coated with resin solution. A calculated quantity of zinc dust is then added to the drum which is again rotated until the particles are evenly coated with zinc dust. The coated grit is then allowed to dry, for example by blowing heated air through the drum. Any additional materials such as biocides for the coating are added to the resin solution prior to coating the particles.

The zinc dust is very fine, the particle size being such that 99 cm% passes a B. S. 240 mesh sieve.

The average particle size of such a dust varies between 2.25 and 3 microns.

In a typical coated grit forming the shot blasting medium, there is 5% by weight of zinc dust to the weight of uncoated grit.

Grit used in shot-blasting may be directed at the surface to be treated from a nozzle to which there is secured a shroud or skirt. The shroud or skirt is then between the surface to be treated and the nozzle which is interiorly of the shroud or skirt and set back from the opening thereof. This arrangement provides a relatively more efficient blast as its profile is better than previous ones because there is more efficient use of the grit as it passes through the air void between the nozzle and the surface.

Preferably, the nozzle may be inclined at an angle to the surface to be cleaned. Also, there may be guide means such as wheels or rollers mounted on the shroud to control the angle of the shroud and its distance from the surface. The guide means may be adjustable to alter the angle and distance as required.

In a second example embodying the invention, the shot blasting medium comprises a plurality of sand particles which are coated with a coating comprising, preferabli an epoxy resin. A clear epoxy resin is preferred. Such coated particles of sand, which is suitably silica-free, is used to clean the facades of buildings of dirt. The dirt is removed and the epoxy resin coating is transferred from the particles to the target surface. The surface is thus coated with a clear coat which protects it from atmospheric corrosion the coat being waterproof.

Rain impinging on the building is thus deflected by the coat away from the fabric of the building.

This is particularly advantageous in the protection of sand-stone structures which are inherently porous.

The particles are coated with resin solution as in the previous example, in other words the clear epoxy resin is dissolved in a solvent and the solution is added to the sand in a rotatable drum.

The drum is rotated until each particle is evenly coated with the solution, and the coated particles are allowed to dry. The shot blasting medium can then be used. It will be understood that a grit other than sand can be used.

Claims

1. A shot blasting medium, comprising a plurality of discrete particles which in use is able to clean a surface at which it is directed, characterised in that substantially all of the particles have a coating comprising means for protecting the surface.

2. A shot blasting medium according to Claim 1, characterised in that the coating comprises a carrier and in that the carrier carries a further protective means.

3. A shot blasting medium according to Claim 2, characterised by the further protecting means comprising zinc dust.

4. A shot blasting medium according to any preceding claim characterised by the coating including a biocide.

5. A shot blasting medium according to any of Claims 2 to 5, characterised in that the carrier comprises an organo-titanate compound.

6. A shot blasting medium according to any preceding claim, characterised in that the coating comprises an organic resin.

7. A shot blasting medium according to Claim 6, characterised by the resin comprising an epoxy resin.

8. A shot blasting medium according to any preceding claim, characterised by the particles each comprising a core of a mineral slag material.

9. A shot blasting medium according to Claim? 8, characterised by the core of each particle comprising a copper slag material.

10. A method of making a shot blasting medium according to Claim 1, characterised by dissolving a resin in solvent to provide a carrier by providing a plurality of particles, by mixing the particles and the resin solution, and then allowing the coated particles to dry.

11. A method of making a shot blasting medium according to Claim 10, comprising dissolving a resin in solvent to provide a solution, providing a plurality of particles, mixing the particles and the resin solution, agitating the mixture of solution and particles, adding protective means of the mixture, re-agitaring, and then allowing the coated particles to dry.

12. A shot blasting medium according to Claim 1, substantially as hereinbefore described.

13. A method of making a shot blasting medium according to Claim 1, substantially as hereinbefore described.