This invention relates to a method of blast cleaning and a composition suitable for use therein.

In the blast cleaning of surfaces, particularly those (eg of soft alloys or plastics composites) which might be damaged by the use of highly abrasive shot such as steel, it is known to employ high velocity particles comprising a thermoset resin. These particles may for example be suspended in a high-velocity stream of air or other gas or liquid such as water directed at the surface to be cleaned.

By "cleaning" we include not only the removal of dirt but also the stripping of paint or like protective coating. One procedure for making suitable abrasive particles is to take granules of an ordinary moulding composition, eg of cellulose-filled urea- formaldehyde resin, mould it conventionally under heat and mechanical pressure, ie with use of a moulding tool, comminute the heat-set moulded product, and sift the comminuted material into fractions of various particle sizes (eg BS mesh 10-20, 20-40 and 40-60) suited to particular surface-cleaning operations.

This, however, is a rather laborious route to a suitable composition; and a much simpler one is to use, as feedstock for comminution, the scrap that is available from commercial moulding establishments. The disadvantage of this latter approach is that the scrap may well be contaminated, as with inclusions of metal, and the presence of such contaminant material in a composition used for blast cleaning a surface could seriously harm the substrate.

According to the present invention, we provide a method of blast- cleaning a surface by exposing it to high-velocity abrasive particles comprising a thermoset resin, wherein the resin is a urea formaldehyde resin having a molar ratio urea : formaldehyde in the range 1:1.2 to 1:1.6 and is heat-set without the application of mechanical pressure.

Besides dispensing with the use of a moulding tool, the heat- setting operation referred to leads direct to a product that is utilisable in blast cleaning. Heat-setting, which can be carried out in conventional steam-heated or electrically heated batch ovens, or in hot air driers such as fluid bed driers, with the resin in conventional granular form, need not be taken right to completion; materials of intermediate but still suitable hardness can be made by reducing the time for which the granules are held at curing temperature, eg to obtain a cure of only 90%.

The thermosetting resin employed is a urea-formaldehyde resin in which the molar ratio of urea : formaldehyde is in the range

1:1.2 to 1:1.6, preferably 1:1.3 to 1:1.5.

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According to a preferred feature of the invention, the resin which is subjected to heat-setting without the application of mechanical pressure is in the form of granules, in which it is present either admixed with or absorbed in a filler such as cellulose, wood flour, alumina, alumina trihydrate, silica, talc, barium sulphate, clay or mixtures of these substances. Admixture or absorption affords a means of varying the abrasive properties of the end product. The granules can be made by first mixing the filler with a solution of the resin, driving off solvent from the mixture, and heating to a temperature at which the resin undergoes a degree of curing (up to about 50%) , sufficient to leave the mixture in a form known as "popcorn", capable of being fine-ground. The ground mixture is extruded hot eg 80°C and the extrudate is cut into granular form. Alternatively, solvent in limited amount (eg water, in the case of a resin which is a formaldehyde condensate) is added with stirring to the ground material, and the granules thus formed are then dried.

According to another feature of the invention, the popcorn referred to earlier is used direct to make the abrasive particles, by further heating to increase the degree of cure of the resin in it without the application of mechanical pressure.

Whilst the method just described gives satisfactory results, the particles may be too friable for some applications. According to a further preferred feature of the invention, the particles are

mixed with aqueous urea formaldehyde resin followed by heat setting the mixture in the absence of pressure to form a friable mass of resin reinforced particles. Such resin reinforced particles have proved to be more durable in the blast cleaning process. This is thought to be due to the aqueous resin at least partially filling the pores in the original heat set particles, thereby conferring improved strength/hardness.

According to another aspect of the invention, a free-flowing composition suitable for use in the method defined earlier consists essentially of abrasive particles which are predominantly in the size range BS 10 - 60, said particles comprising (a) resin reinforced granules of a urea formaldehyde resin which is heat-set without the application of mechanical pressure, and (b) a filler.

The invention is further illustrated by the following Example.

EXAMPLE 1

The starting material employed was a commercially available granular urea-formaldehydemoulding composition passing BS 8 mesh (2mm) and retained on BS 100 mesh (0.15mm), or which about 30% by weight was BS 10 - 20 mesh (1.7 - 0.8mm) and about 70% by weight was BS 20 - 60 mesh (0.8 - 0.25mm). Particles passing BS 60 mesh formed less than 5% by weight of the composition.

The main ingredients were (a) thermosetting resin of molar ration

urea : formaldehyde = 1:1.4, and (b) cellulose, in the weight proportion resin : cellulose = 70:30. The composition contained the usual small amounts of curing catalyst, stabiliser and mould lubricant.

The composition (25 kg) was heated in an oven at 150°C for 1.5 hours, to bring the resin content from an initial 40 - 50% cure (too low to confer abrasiveness on the particles) to full cure.

The cured product, whose particle size distribution was substantially unchanged from that of the starting material, was separated into free-flowing BS 10 - 20 and BS 20 - 60 mesh fractions.

These fractions were tested separately in a conventional air- blast cleaning rig using a nozzle pressure of 205 kN/m ² , to compare their ability to (i) strip a steel plate painted with standard undercoat and topcoat, and (ii) finish a standard aluminium panel. Results were as follows:

Test (i) Test (ii)

Area of paint Effect on removed/minute Al panel

Granular starting None Material impacted material on surface; no cleaning achieved

Cured material 270cm ² Uniform, fine BS 10 - 20 mesh finish

Cured material 230cm ² Uniform, fine BS 20 - 60 mesh finish

EXAMPLE 2

A 25kg batch of commercially available, granular urea formaldehyde moulding material (UF ratio in the range 1:1.2 to 1:1.6) was mixed at room temperature with 10 litres of an aqueous urea formaldehyde resin of UF ratio 1:1.4 and 65% resin solids content. At small amount of an acid catalyst was mixed in and the wet granular material was dried/heat-set (cured) on trays in an oven at 110°C for 16 hours. The product was a readily friable mass of resin reinforced particles. These were comminuted and sieved to give a particle size range of BS 10 - 60.

This material was compared with that of Example 1 by testing in a conventional air blast rig, using a test sample a steel panel

coated with a standard automobile primer and top coat. The particles were sprayed through a nozzle of diameter 9.5mm using a spraying pressure of 240 kN/m ² . The following results were achieved.

Area of paint removed per

Blast Material Source minute

Example 1 116cm ²

(Particle size BS 10 - 60)

Example 2 193cm ²

(Particle size BS 10 - 60)

The resin reinforced particles were thus significantly more effective than the unreinforced particles.