WO1994016835A1

WO1994016835A1 - Blast media containing sanitizer

Info

Publication number: WO1994016835A1
Application number: PCT/US1993/012336
Authority: WO
Inventors: Benny S. Yam; Amy L. Joseph; Antohony E. Winston; Keith A. Jones
Original assignee: Church & Dwight Company, Inc.
Priority date: 1993-01-21
Filing date: 1993-12-22
Publication date: 1994-08-04
Also published as: AU5984994A

Abstract

A blast media is provided for blast cleaning contaminants from a solid surface and effecting a secondary sanitizing treatment on said surface includes abrasive particles and an antimicrobial agent.

Description

BLAST MEDIA CONTAINING SANITIZER

FIELD OF THE INVENTION The present invention relates to improvements in blast media utilized to remove adherent material such as paint, scale, dirt, grease and the like from solid surfaces. In particular, the present invention is directed to abrasive blast media which has incorporated therein an antimicrobial agent to sanitize the surface being blast cleaned.

DESCRIPTION OF THE PRIOR ART In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, remove graffiti from stone or simply to degrease or remove dirt from a solid surface, it has become common practice to use an abrasive blasting technigue wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminates. Various abrasive blasting technigues have been utilized to remove coatings, grease and the like from solid surfaces. Thus, blasting technigues comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technigue such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technigue, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation. The blast media or abrasive particles most widely used for blasting surfaces to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technigue, there are disadvantages in using sand as a blast media. For one, sand, i.e., crystalline silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.

An alternative to sand as a blast media, particularly, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, composite surfaces, plastics, ceramic tile, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subseguent to blasting can be simply washed away without yielding environmental harm. Since sodium bicarbonate is water soluble and is benign to the environment, this particular blast media has found increasing use in cleaning dirt, grease and oil and the like from harder surfaces such as steel as well as interior surfaces such as those which contact food such as in environments of food processing or handling.

Sodium bicarbonate is also a friable abrasive and, like sand, will form a considerable amount of dust during the blast cleaning process.

To control the dust formed by the sodium bicarbonate blast media as it contacts the targeted surface, water is included in the pressurized fluid carrier medium. Thus, water can be used as the carrier fluid or, more preferably, injected into a pressurized air stream which carries the blast media from the blast nozzle to the targeted surface. Water as a means to control dust has been mixed with the air stream internally in the blast nozzle or into the air stream externally of the nozzle. The addition of water to the pressurized air stream has been very effective in controlling dust formed by the sodium bicarbonate blast media.

In U.S. 4,125,969, a wet abrasion blasting process is disclosed in which a water soluble abrasive material is directed against a surface for cleaning same and which abrasive media may be such to effect a further, secondary treatment of the work surface. Thus, the patent suggests including a rust inhibiting composition for the protection of various working surfaces or an etching agent such as phosphoric acid or unnamed surface protection compositions. Although this patent recognizes the advantages of utilizing water soluble abrasive media at least with respect to disposal thereof, the patent is not necessarily concerned with cleaning surfaces contained within interior building structures. The patent is specifically concerned with cleaning the interior of tanks such as tanks of oil tankers.

However, it has been found that with water soluble media, especially water soluble media which are environmentally benign such as sodium bicarbonate, indoor surfaces can be blast cleaned. As stated above, sodium bicarbonate blast media has found increasing use in cleaning surfaces of ceramic, aluminum, stainless steel, etc., which come into contact with or are in the environment of food stuffs, such surfaces including ones involved in food storage, food processing and food handling and including structural surfaces as well as machinery, conveying eguipment and the like. Such surfaces must be maintained in a clean condition and must be free of microorganisms which can contaminate and spoil any food stuffs present in the nearby environment. It would be most worthwhile to provide a secondary treatment of sanitizing such surfaces during a blast cleaning operation.

Accordingly, it is a primary objective of the present invention to blast clean a surface with an abrasive media and to effect a secondary sanitizing treatment to such surfaces.

Another object is to blast clean indoor surfaces with a water soluble abrasive media and to effect a secondary sanitizing treatment to such surfaces.

Still another object of the invention is to provide a powdery, free-flowing abrasive blast media which can provide a secondary sanitizing treatment to solid surfaces. SUMMARY OF THE INVENTION

The objectives of the present invention are achieved by incorporating with a water soluble blast media a small amount of an antimicrobial agent. The antimicrobial agent can be solid or liguid and incorporated with the blast media either by mixing the antimicrobial agent with the solid abrasive particles of the blast media or by adding the agent in the water stream which is utilized either as the carrier fluid for the blast media or added to a pressurized air stream for the purpose of dust control. The addition of the antimicrobial agent to the blast media effects a secondary sanitizing treatment on the substrate which is being blast cleaned and stripped of contaminants.

DETAILED DESCRIPTION OF THE INVENTION

The blast media to be utilized include a powdery, water soluble abrasive having an average particle size of from about 50 to 1,000 microns in diameter. The abrasive particles should not exceed 1,000 microns and, thus, a useful blast media will not contain greater than 1% particles over 1,000 microns. Water soluble blast media are advantageous since such blast media can be readily disposed of by a water stream, are readily separated from the insoluble paints and resins which have been stripped to facilitate waste disposal, and since most water soluble blast media are relatively soft, i.e., Mohs hardness less than 3.0, such media can be utilized to remove coatings, grease, dirt and the like from a variety of substrates including relatively soft metals such as aluminum as well as plastic, ceramic tile, concrete, wood and composites of such materials. Water soluble media having a Mohs hardness of less than 5.0 are generally useful to remove contaminates from relatively soft substrates. Non-limiting examples of water soluble blast media which can be utilized include the water soluble alkali metal and alkaline earth metal salts such the chlorides, chlorates, carbonates, bicarbonates, sulfates, silicates, the hydrates of the above, etc. The preferred blast media are the alkali metal salts and, in particular, the sodium and potassium carbonates, bicarbonates, silicates and sulfates. The most preferred blast media are the alkali metal bicarbonates as exemplified by sodium bicarbonate. Also preferably useful are sodium sesguicarbonate, natural sodium sesguicarbonate known as trona, sodium bicarbonate, sodium carbonate, potassium carbonate, potassium bicarbonate, sodium chloride and sodium sulfate which is described in commonly assigned U.S. Patent No. 5,112,406. It is important to note that by water soluble is not meant completely water soluble as some salts and natural minerals such as trona may contain minor amounts of insoluble materials. For example, trona which is a natural sodium sesguicarbonate may contain up to 10 wt.% of insolubles. Thus, by water soluble is meant to include those materials which are substantially soluble in water.

The antimicrobial agent to be incorporated with the abrasive in the blast media of this invention can be any compound or substance that kills microorganisms or prevents or inhibits their growth and reproduction. Antimicrobial agent in accordance with this invention is meant to include disinfectants, sanitizers, bacteriostats, sterilizers, fungicides and fungistats. Such materials are widely known and a list of same can be found in the Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., 1991, Vol. 13, pp. 223-252 under the heading "Industrial Antimicrobial Agents", the content of which is herein incorporated by reference. The antimicrobial agents of this invention can be inorganic or organic, liguid or solid, water soluble or water insoluble. The method of incorporating the antimicrobial agent in the blast media of this invention will vary depending upon the physical state of the antimicrobial agent and the solubility thereof in water.

Non-limiting examples of antimicrobial agents include various halogen compounds such as chlorine or iodine compounds, phenols, alcohols, quaternary ammonium compounds, amines, alkanol amines and various heavy metal compounds including organic mercurial compounds. Other metallic compounds are particularly useful inasmuch as these compounds are solids and can be mixed with the powdery abrasive media without difficulty and without adversely affecting the free flow of blast media through the blast nozzle to the targeted surface. A preferred metallic compound is zinc oxide which is known to have antimicrobial properties, in particular, fungicidal properties. A particularly useful zinc oxide is a submicron size zinc oxide marketed as "Sachtotec" from Sachtleben Chemie. This particular submicron zinc oxide has a primary particle size of 5-15 nm which results in a high specific surface area. The very small sized zinc oxide is also easily dispersed in water and, thus, can find particular use in wet blasting systems in which water is used as the primary carrier fluid or in those instances wherein the water stream is utilized to control dust formation of the blast media as it contacts the targeted substrate surface. Also useful are antimicrobial metal salts which are not only solid but many of which are water soluble. Thus, non-limiting examples of useful metal salts include ZnCl₂, Znl₂, Zn(C₂H₃0₂)₂.2H₂0, ZnS0₄.7H₂0, NiCl₂.6H₂0, Ni(N0₃) _≥.6H₂0, CθCl₂.6H₂0, Co(N0₃)₂, CθCl₂.2H₂0, Co(N0₃)₂.6H₂0, Pb(N0₃)₂, Pb(C₂H₃0₂)₂.3H₂0, SnCl₂.2H₂0, FeCl₃.6H₂0, and Fe₂(S0₄)₃. Other specific non-limiting examples of useful sanitizing agents include potassium, sodium or calcium hypochlorite and dischloroisocyanuric acid or the sodium or potassium salts thereof with or without the bromides of potassium, sodium or calcium. Other useful compounds and solutions are described in Current Good Manufacturing Processes, by Wilber A. Gould PhD, CTI Publications, Inc., Baltimore, MD, 1990, pp. 158-163, herein incorporated by reference. If the antimicrobial agent of the present invention is a liguid, such agent can be incorporated into the water soluble blast media in a variety of ways. For example, the antimicrobial agent can be sprayed directly onto the blast media particles. While this method is the most direct way of incorporating the antimicrobial agent, the flow of blast media through the metering means which meters the amount of abrasive particles into the fluid carrier stream may be adversely affected by incorporating the antimicrobial agent in this manner. Thus, the very fine particles of blast media may agglomerate and otherwise cake or bride together and render particle flow through the metering microns difficult. Alternatively, the antimicrobial agent can be sprayed directly onto the abrasive blast media particles and the antimicrobial agent- coated particles then dusted with a very finely divided material to reduce the caking and bridging between the abrasive particles. Thus, finely divided silicates such as clays, talc, mica, diatomaceous earth and metal silicates such as aluminosilicates including zeolites may be used for dusting the liquid antimicrobial agent-coated abrasive. Obviously, the addition of a significant amount of water insoluble additives reduces the advantages of the water solubility of the abrasive blast media with respect to disposal and, as such, the amount of dusting agent should be minimized. Still another method of incorporating a liquid antimicrobial agent into the blast media is to apply the antimicrobial agent to solid carrier particles similar to those described above which are then added to the abrasive particles. Thus, various silicates can be utilized as the carrier particles including clays such as kaolin clay, talc, mica, aluminosilicates such as zeolites, as well as water insoluble carbonates, sulfates, etc. Again, the amount of water insoluble materials should be minimized so as to not adversely affect the advantages of the water soluble blast media. An alternative to adding a liquid antimicrobial agent to any of the solid materials which form the blast media is to add the antimicrobial agent to the water which is utilized as the primary fluid carrier medium or as a dust control agent. Thus, the antimicrobial agent can be added at the supply of water or can be added to the water stream at the blast nozzle. By incorporating the antimicrobial agent into the water stream, the disadvantages of adding additional water insoluble materials to the blast media is avoided and so is the agglomerating and caking, bridging and restriction to flow of the blast media.

If the liguid antimicrobial agent is not water soluble and not readily dispersable in water, it will be necessary to include a dispersing agent with the liquid antimicrobial agent. Such dispersing agents are liguid surface active compounds which contain a hydrophobic portion and a hydrophilic portion in the molecule. Obviously, the addition of a further liquid to the blast media may result in additional agglomeration or caking of the abrasive particles and, thus, the amount of any dispersant should be the minimum needed to disperse the antimicrobial agent in the-water stream which acts as the primary carrier medium or, more preferably, as provided for dust control.

Most preferably, the antimicrobial agent is added to the abrasive blast media as a particulate solid which can be readily mixed with the abrasive particles and provide a powdery, free- flowing blast media. One useful technigue of incorporating a liguid antimicrobial agent as a solid particle in the blast media may be to form a granulated clathrate of the liguid antimicrobial agent and a water soluble compound having clathration capability. A particularly useful compound which has clathration properties is urea. It may not be possible to clathrate every liquid antimicrobial agent. The granulated antimicrobial agent- clathrate compound may be formulated in a variety of ways, if possible, and subsequently incorporated into the abrasive blast media as a solid by several techniques. In one method, the antimicrobial agent- clathrate compound is prepared such as by melting the clathrate and the liguid antimicrobial agent in a ratio of about 2:1 to about 10:1 clathrate to antimicrobial agent, cooling the mixture to form a dry crystalline solid and then grinding the solid into particles which are then directly added to the abrasive blast medium as a dry powder. To ensure intimate and uniform mixture of the antimicrobial agent-clathrate compound with the blast medium, the clathrate can be prepared in the presence of the abrasive blast media particles to produce a dry particle of similar density to that of the medium. Alternatively, the hot molten liguid antimicrobial agent and urea mixture can be sprayed directly onto the abrasive blast media particles. In this method, when the clathrate solidifies, it will be tacked onto the abrasive media particles. Still another method, the clathrate and antimicrobial agent can be dissolved in an appropriate solvent and sprayed onto the abrasive blast media particles. The solvent is subsequently allowed to evaporate and the clathrate forms directly on the abrasive blast media particles. If provided in the form of a clathrate compound, the antimicrobial agent will be released from the clathrate once the clathrate particle is contacted with the water stream which is provided as the fluid carrier medium or used for dust control since the clathrate is readily water soluble. Again, if the antimicrobial agent is not water soluble, the clathrate compound will have to include a dispersant which can be included in the clathrate compound by simply adding it with the liquid antimicrobial agent during the clathrate compound formation and/or method of incorporating such clathrate compound in the abrasive blast media as described above.

Still another method is to mix or spray the abrasive solid with the liquid antimicrobial agent, compact the mixture under heavy pressure to form pellets, sheets or sticks of the blast media, granulate, and screen the granulated solids to the desired size.

As previously stated, it is most preferable to incorporate the antimicrobial agent as a solid in the abrasive blast media. Thus, those antimicrobial agents which are natural solid materials and which can be provided in particulate form are most preferable. Thus, antimicrobial agents such as zinc oxide and the other water soluble metal salts described above can be readily incorporated into the blast media of this invention by simply mixing the particles of the antimicrobial agent as is with the abrasive particles.

The amount of the antimicrobial agent to be incorporated into the blast media will vary widely depending upon the purpose of the antimicrobial agent as well as the activity of the specific antimicrobial agent to achieve the desired sanitizing effect. Thus, minimum amounts of antimicrobial agent are those which provide the desired sanitizing treatment of the solid surface which is being blast cleaned. Excessive amounts will be those which adversely affect the free-flow of the blast media through the metering device or blast nozzle or which substantially reduce the ability of the blast media to strip contaminants such as paints, resins, dirt, grease, oil and the like from the targeted substrate surface. In the most general terms, the antimicrobial agent should be incorporated at a minimum of at least about 0.1 wt.% relative to the blast media and at a maximum should not exceed about 10 wt.% of the blast media. It is also useful in accordance with the present invention to include a flow aid or a decaking agent with the blast media. Most preferably, the flow aid is a hydrophilic silica, hydrophobic silica, hydrophobic polysiloxane or mixture thereof. Hydrophobic silica, unlike known hydrophilic silicas, is substantially free of non- hydrogen bonded silanol group and absorbed water. One preferred hydrophobic silica which may be utilized in the blasting media hereof is Aerosil R 972, a product which is available from DeGussa AG. This material is a pure coagulated silicon dioxide aerosol, in which about 75% of the silanol groups on the surface thereof are chemically reacted with dimethyldichlorosilane, the resulting product having about 0.7 mmol of chemically combined methyl groups per 100 m² of surface area and containing about 1% carbon. Its particles vary in diameter from about 10 to 40 nanometers and have a specific surface area of about 110 m²/gram. It may be prepared by flame hydrolysis of a hydrophilic silica as more fully described in Angew. Chem. , 72, 744 (1960); F-pS 1,368,765; and DT-AS 1,163,784. Further details respecting such material are contained in the technical bulletin entitled "Basic Characteristics and Applications of AEROSIL", DeGussa AG, August 1986. The hydrophobic silica particles are admixed with the abrasive blasting agent in the proportion of at least about 0.1 and up to about 1.0% by weight thereof. Another hydrophobic silica is Quso, marketed by DeGussa A.G.

Hydrophobic polysiloxanes, preferably non- halogenated polysiloxanes, suitable for use in the blasting media hereof are commercially marketed by Dow Corning and General Electric.

The blast media of the present invention as constituted from the water soluble abrasive particles and antimicrobial agent as described above are useful for efficient cleaning or decoating of relatively soft substrates such as sensitive metals including aluminum or aluminum alloys or magnesium or composite substrates formed of plastic, masonry, stucco, plaster, ceramic or wood. In particular, the blast media of the present invention are most useful in treating ceramic, aluminum, steel, including stainless steel substrates and the like and which are in the environment of food stuffs including internal structural components of food storage, handling and processing operations as well as the equipment or machinery which come into contact with food stuffs such as during processing, packaging, transport and handling such as during marketing of particular food items. Additionally, it may even be useful to provide a secondary sanitizing effect to surfaces which are not involved in the food industry but which are required or advantageous to be free of mold, mildew, fungi, etc. If such surfaces include exterior building structures and the like, it is not necessary that the abrasive particles be water soluble. Thus, the invention also encompasses the addition of an antimicrobial agent to any type of blasting media including the water soluble blasting media described above and water insoluble blasting media including sand, plastic media, rice hulls, walnut shells, water insoluble alkali metal and alkaline earth metal salts such as calcium carbonate and the like. The blast media containing abrasive particles and antimicrobial agent are preferably applied in commercial pressurized water streams and, more preferably, compressed air streams which contain water either added at the blast nozzle or externally therefrom for the purpose of controlling dust formation. Blasting equipment _for the blast media of the present invention are commercially available. The blast media flow rates through the nozzle typically range from about 0.5 to 15, desirably from about 1.0 to 10.0 lbs. per minute and under air pressures from 10 to 100 psi and water pressures for dust control typically ranging from about 10 psi and above. EXAMPLE

A square foot aluminum panel is blasted with various blast media using the Accustrip™ system at the following operating conditions: 60 psi blast air pressure, 4 lbs per minute media flow rate, 0.5 gpm water flow rate. The blast media compositions are set forth in Table 1.

A potato-glucose agar medium (pH: 6.0) is steam-sterilized under elevated pressure and pipeted into petri dishes (diameter: 9 cm) in 25 ml portions, followed by coagulation. Next, a sample piece of the blasted aluminum sample (4 x 4 cm) is placed on the agar plate medium and 1 ml of a spore suspension of the fungi penicillium citrinum is uniformly sprayed on the test piece. After incubating at 28°C for 14 days, the growth of hyphae on the surface of the sample is examined and the mildew-resistance evaluated by the following three ranks.

Expression of mildew-resistance:

1: Greater than 1/3 of the total area of the sample piece contains growing hyphae.

2: Less than 1/3 of the total area of the sample piece contains growing hyphae.

3: No hyphae is observed on the sample piece. The results of testing are expressed in Table 1.

TABLE 1

Blast Media Compositions

A B C D sodium bicarbonate 99.75 98.75 97.75 100.0

Sylox® 15¹ 0.25 0.25 0.25 —

ZnO 1.00 —

Zn(S0₄) 2.00

Mildew resistance Test 2

¹Hydrophilic silica flow aid

Claims

WHAT IS CLAIMED IS:

1. A method of blast cleaning a solid surface comprising; propelling a blast media against said surface by means of a pressurized fluid stream to clean contaminants from said surface, said blast media comprising abrasive particles and an antimicrobial agent to effect a secondary sanitizing treatment of said surface.

2. The method of claim 1 wherein said abrasive particles are water soluble.

3. The method of claim 1 wherein said abrasive particles comprise sodium bicarbonate.

4. The method of claim 1 wherein said antimicrobial agent is present in said blast media as separate particles.

5. The method of claim 4 wherein said antimicrobial agent comprises particles of a metallic compound.

6. The method of claim 5 wherein said metallic compound is zinc oxide.

7. The method of claim 1 wherein said antimicrobial agent is water soluble or dispersible and said pressurized fluid stream includes water.

8. The method of claim 1 wherein said pressurized fluid stream consists essentially of water.

9. The method of claim 1 wherein said pressurized fluid stream is primarily air and said water is included as a separate water stream in said pressurized air stream so as to control dust formation.

10. The method of claim 9 wherein said separate water stream is included in said pressurized air stream externally from a nozzle which propels said blast media to said surface.

11. The method of claim 5 wherein said abrasive particles are water soluble.

12. The method of claim 11 wherein said water soluble abrasive particles comprise sodium bicarbonate.

13. The method of claim 6 wherein said zinc oxide comprises submicron size particles.

14. The method of claim 1 wherein said blast media further includes a flow aid.

15. The method of claim 14 wherein said flow aid comprises hydrophilic silica, hydrophobic silica, hydrophobic polysiloxane or mixtures thereof.

16. The method of claim 2 wherein said solid surface is contained in the interior of a building structure.

17. The method of claim 16 wherein said solid surface comes into contact or in the immediate environment of food stuffs.

18. The method of claim 1 wherein said solid surface comes into contact or is in the immediate area of food stuffs.

19. The method of claim 16 wherein said solid surface is metallic.

20. The method of claim 19 wherein said solid surface is aluminum or stainless steel.

21. The method of claim 1 wherein said solid surface comprises conveying machinery.

22. A blast media for blast cleaning a solid surface and for effecting a secondary sanitizing treatment thereto comprising abrasive particles and an antimicrobial agent.

23. The blast media of claim 22 wherein said antimicrobial agent is in the form of solid particles separate from said abrasive particles.

24. The blast media of claim 23 wherein said antimicrobial agent comprises metallic compounds.

25. The blast media of claim 24 wherein said antimicrobial agent comprises zinc oxide.

26. The blast media of claim 25 wherein said zinc oxide comprises submicron size particles.

27. The blast media of claim 24 wherein said antimicrobial agent is water soluble.

28. The blast media of claim 22 wherein said abrasive particles are water soluble.

29. The blast media of claim 28 wherein said abrasive particles comprise sodium bicarbonate.

30. The blast media of claim 22 wherein said abrasive particles are water insoluble.

31. The blast media of claim 30 wherein said abrasive particles comprise calcium carbonate.

32. The blast media of claim 22 further including a flow aid.

33. The blast media of claim 31 wherein said flow aid comprises hydrophobic silica, hydrophobic polysiloxane or mixtures thereof.

34. The blast media of claim 22 wherein said abrasive particles have an average diameter in the range of 50-1,000 microns.

35. The blast media of claim 34 wherein no greater than 1% of said abrasive particles are larger than 1,000 microns.