GB2090275A - Abrasive agglomerates and abrasive sheet material - Google Patents

Abstract

Abrasive agglomerate particles comprising a matrix of cellular glass and abrasive grit particles encapsulated within the cell walls of said glass, said particles being present in greatest concentration in the exterior walls. The agglomerates can be used particularly in abrasive sheets or in grinding wheels.

Description

SPECIFICATION Abrasive agglomerates and abrasive sheet material This invention relates to particulate abrasive agglomerates in which abrasive grits are held in a friable matrix, such agglomerates being particularly suitable for use in coated abrasive products in which the agglomerates are bonded to a flexible sheet backing; they can also be used in bonded abrasives (grinding wheels).

The use of small, particulate, agglomerates of relatively fine abrasive grits held in a matrix, for use as a substitute for conventional abrasive grits on a coated abrasive ("sandpaper") flexible abrasive, was suggested at least as early as U.S.

Patent 2,194,472. So far as is known, the solid agglomerates of the type disclosed in the ab.ove patent or products made from them have never been commercially successful. U.S. Patent Re.

29,808, discloses hollow spheres (or other shapes, such as cylinders) consisting of abrasive grits bonded onto the outer surface of a friable matrix, such as resin or an inorganic silicate.

European published application 8868, published March 19, 1980, discloses solid agglomerates bonded by fused cryolite or other "salts or silicates".

British Patent 982,215 and U.S. Patent 3,156,545, teach making solid agg!omerates for use in grinding wheels consisting of glass bonded alumina or other grits. Benner U.S. Patent 2,216,728 discloses glass or metal bonds for the matrix of aggregates containing diamond abrasive particles. The patent states that the matrix may be made somewhat porous to enhance mechanical bonding when the aggregates are mixed with a binder to form a grinding wheel. U.S. Patent 2,806,772 suggests including foamed glass in abrasive agglomerates bonded by a resin matrix.

While abrasive agglomerates of the hollow resin or silicate bonded type have shown good results in coated abrasive applications, and agglomerates such as taught in the above EPC application show good results, both types of agglomerates are difficult or expensive to manufacture and it is difficult to control their friability.

More control of the physical properties of abrasive agglomerates, and excellent grinding results in coated abrasives can be achieved by providing agglomerates of abrasive particles bonded by a foamed glass in which the abrasive particles are contained within the walls of the cellular glass matrix. Such agglomerates can be manufactured by mixing appropriate abrasive grits with conventional known compositions which produce a foamed glass structure upon firing. The glass composition, foaming agent, and, if desired, grinding aid, are mixed together, formed into small agglomerates of the desired shape, fired, and cooled. The agglomerates may then be screened to appropriate sizes and employed in a conventional manner to produce coated abrasive discs, belts, or sheets. They may also be used to produce resin bonded grinding wheels.

The present invention utilizes the basic friability of the cellular glass and its controlled variability of friability as a matrix for abrasive grit. When a cellular glass is at the appropriate foaming temperature, it expands and will stick to most materials around it. In addition, it tends to encapsulate particles in its path.

This latter tendency is utilized when sized abrasive grit is mixed with a cellular glass batch and the body brought to a cellulating temperature.

Surprisingly, the grit particles are readily distributed throughout the cell yet totally encapsulated by the glass in the walls.

Accordingly, mixtures of various cellular glass batches are blended with various volume percentages of grit, the blended batch is pelletized to appropriately sized green spheroids and those spheroids dried and fired to yield the abrasive aggregate.

Cellular glass is sold as a soft abrasive in its own right. Its major product qualities are its ready friability without catastrophic failure such that upon rubbing over a workpiece new sharp glass surfaces are constantly being formed. In addition, the material is impermeable so that there is no absorption of liquid into the structure. Abrasive agglomerate performance depends upon the friability of the matrix. Ideally, the matrix should fracture or crumble as soon as the encapsulated grain begins to lose its peak cutting quality. This invention provides a product in which fine abrasive grit is encapsulated in the foam cell walls as a discrete impurity. Ideally, the matrix should be designed to exhibit a coefficient of thermal expansion that is as close as possible to that of the abrasive grit in order to minimize cooling flaws.

The subject grain can be formed into extruded chopped shapes, or can be formed into spheres.

Friability can be controlled by the ratio of pores to grain and/or the ratio of glass to grain. Higher density matrixes (60 pcf+) will tend to break like a glass while lower densities will increase friability.

While the size of the aggregates is subject to much variation, depending on the particular application and grit size, generally the aggregates will be 250 microns or larger in diameter, since the foam glass process limits the minimum size glass-grit aggregate. The maximum size normally used would not be over 5 mm, at least in coated abrasive applications. The abrasive grit will generally not be finer than 10 microns, nor coarser than 2 or 3 millimeters.

The preferred abrasive grit is fused aluminum oxide, but co-fused alumina-zirconia alloy abrasives can be used, as can silicon carbide abrasive grit.

As shown in the example below, soda lime glass can be used, but a non-devitrifying alumino borosilicate composition is superior.

The abrasive and glass mixture for forming the agglomerates contains from 40 to 80 percent (dry bulk volume) of milled glass composition and from 20 to 60 percent of abrasive grain. Up to 20 percent addition of a grinding aid such as cryolite can be added to such mixtures. The final product, when in the form of spheres, will have a bulk density of from 20 to 55 pounds per cubic foot (0.32 to 0.88 g/cc).

The optimum firing temperature and time depends upon the particular composition used, the desired density (porosity) of the product. In general a temperature of 800 to 900 degrees C or higher for about 20 minutes is suitable.

The steps in a typical example of this invention are as follows: 1. Preparing a foamable glass batch by ball milling soda lime glass cullet with 0.25% carbon black and 0.5% three micron silicon carbide for 24 hours in a batch ball mill to a median particle size of five microns or less.

2. Adding a charge of 70% by volume of the foamable glass batch and 30% by volume of an abrasive grit, in particular, a 180 grit fused dark aluminum oxide and blending them dry at high speed. Subsequent to dry blending a 1% addition of alum is added as a dilute liquid and wet mixed followed by a 0.4% solids addition of an aqueous montmorillonite slurry at a 4% solids content as a binder. Sufficient additional water is added to pelletize the mix to a pellet particle size on the order of 20/40 mesh.

3. The generally spherical pellets thus formed are then dried in a fluid bed dryer and dry mixed with an aluminum hydrate parting agent and fired in a rotary kiln at a temperature of about 850 degrees C for 20 minutes.

The resultant particles exhibit a specific gravity of 30 to 35 pcf (.48 to .56 g/cc). When examined microscopically, it is observed that the glass tends to encapsulate the alumina particles in a foam bubble network. It is also observed that the alumina particles tend to be concentrated at the periphery of the bubble in a manner akin to froth floatation. The particles at the surface are still covered by a layer of glass.

The particles were screened to 20/30 and 30/40 U.S. sieve fraction then tested by using them as if they were in themselves abrasive grits and making coated abrasive belts in the conventional fashion. The belts were tested in a standard metal finishing test system and compared with belts made from 1 80 grit dark alumina.

It was found that the initial time to achieve a comparable finish was longer for the aggregate belts than grit belts but the total amount of metal removed and the belt lifetime was between two and six times that of the grit belt standard.

Repeated testing yielded erratic results, some repeating the aforementioned performance, others substantially poorer. It was determined that the reason for the erratic performance was the tendency of the soda lime glass to devitrify and the potential for the cristobalite crystals to cause defects which sometimes caused the glass to fail.

Additional testing was made using the belt with an aqueous lubricant and the resultant performance was consistently bad. It was determined that this was caused by the poor aqueous durability of the glass.

Accordingly, it was determined to use a batch that would be essentially a nondevitrifying borosilicate made from a mixture of clay or volcanic ash and chemical additives similar to that described in U.S. Patent 3793039. A mix of 66% volcanic ash, 1 5% kaolin clay, 5.5% 5 mole borax, 8% dolomite, 2.7% lithium carbonate, 2% sodium bicarbonate and 1/4% carbon black was comilled.

A 1% addition of liquid alum was made prior to pelletizing. The resultant pellets when fired at 9300C exhibited performance essentially similar to those made from melted glass cullet which tested reproducibility. In addition, when tested in a wet environment, the performance was reduced but still better than that of a conventional belt made with 1 80 grit. The alumino borosilicate has enhanced aqueous durability.

It was further found that a 1 0% addition of powdered cryolite enhanced the cutting performance. Cryolite is a well-known grinding aid for metals and is apparently encapsulated in a fashion similar to that of the alumina grit.

The following examples show that while not preferred, silicon carbide or co-fused aluminazirconia abrasive grits can be used.

The first experiment used the standard soda lime glass foam mix to which we added 30% 39 Crystolon 1 80 grit and 10% fine cryolite. The product was foamed at about 850 degrees C. The resultant aggregate was lighter than that made from alumina, its bulk density being 22.4 pcf C 12/20 vs. 27-29 pcf (Z 12/20 but seemed otherwise similar. It was observed that some of the grit particles were associated with large bubbles which suggest that even coarse grit SiC will influence the foaming reaction. The large bubbles will probably weaken the aggregate.

The second experiment used the same ingredients except that 80 grit cofused aluminazirconia containing 40% zirconia was used (180 grit was not available). The resultant aggregate was essentially equivalent to alumina in all properties. A microscopic examination of the encapsulated alumina zirconia grains showed that despite the reducing conditions of foaming, there was evidence of oxidation of the metallic components of the grain. This effect will reduce the fracture toughness of the grain.

Thus, it can be shown that other abrasives can be encapsulated into aggregates as was alumina but there are side effects that may reduce their utility.

Coated abrasive products are made from the agglomerates of this invention by bonding the aggregates in a single layer on a flexible backing sheet by conventional means well-known in the art, employing thermosetting maker and size coats, glue, or a combination of glue and resin.

Subsequent to making the above examples, it was found that silicon carbide containing aggregates, for certain purposes such as the grinding of titanium metal, are clearly superior to conventional silicon carbide coated abrasive products.

Additionally it has been determined that when glass forming chemical mixes, rather than premelted and ground glass, are used in the formation of the aggregates, superior wetting of the abrasive can be achieved. In addition the resulting aggregate is different in structure from the typical glass containing mixes. In the case of the glass forming mixes, the abrasive particles are more uniformly dispersed within the multi-cellular aggregate body, as compared to the glass mixes in which the abrasive particles tend to be concentrated in the outer peripheral cell walls of the multi-cellular matrix.

In a reduction to practice a foamable blend of glass batch, at 70%, 1 80 grit green SiC at 30% was mixed dry. To this mix 1% alum on a dry solids basis in aqueous solution was added, followed by enough (0.4%) montmorillonite- aqueous slurry to pelletize to a 20/40 mesh size.

These generally spherical particles were dried and fired at 8500C for 20 minutes in a rotary kiln. If desired a 10 to 20% addition of cryolite as a grinding aid can be added at the dry mixing stage.

The fired particles were coated on a belt in the standard fashion and tested dry in finishing titanium metal and wet in finishing plate glass. In both cases there was a longer break in period than that of a regular SiC belt but the useful cut life was much longer. In the case of titanium a standard belt cut 1 6 gm while the experimental nodule belt cut a total of 245 gm. The wet cutting of glass was similar: 18 gm vs. 1 80 gm for the experimental nodule belt.

The experiment was repeated using a mixture of chemicals and minerals that yield an oxide glassy composition known to achieve a good bond with SiC. This bond was ball milled with carbon then blended, pelletized, and fired in a similar fashion to the prior example. The dry tests in titanium were equivalent to the glass matrix material but the wet finishing of glass was enhanced and a cut of 248 gm was experienced.

Microscopic observations showed the grit in the glass to have migrated substantially to the periphery of the bubble and the bubble center to be comprised of a few large cells. The bond composition nodules had SiC distributed throughout the nodule and the internal closed cells were small and uniform in size.

Claims (9)

1. Abrasive agglomerate particles comprising a matrix of cellular glass and abrasive grit particles encapsulated within the cell walls of said glass, said particles being present in greatest concentration in the exterior walls.

2. Abrasive agglomerates according to claim 1, in which the abrasive grit comprises fused alumina, co-fused alumina-zirconia or silicon carbide.

3. Abrasive agglomerates according to claim 1 or 2, in which said agglomerates include particles of cryolite.

4. Abrasive aggregates according to any one of the preceding claims, having a specific gravity of from .32 to 0.88 grams/cc.

5. Abrasive agglomerates according to any one of the preceding claims, having a generally spherical shape.

6. Abrasive agglomerates according to any one of the preceding claims, in which the glass is an aluminum borosilicate composition.

7. A coated abrasive sheet material made by adhesively bonding the agglomerates according to any one of the preceding claims, to a flexible backing.

8. Abrasive agglomerate particles substantially as herein described with reference to any one of the examples.

9. A coated abrasive sheet material substantially as herein described with reference to any one of the examples.