SE451687B - AGGLOMERATED Abrasive Particles - Google Patents

Description

15 20 25 30 35 40 451 687 2 and better abrasive results with coated abrasives can be achieved by providing agglomerates of abrasive particles bound in foamed glass, the abrasive particles being included within the walls of the cellular glass matrix. Such agglomerates can be prepared by mixing suitable slip articles with conventionally known compositions to give a foamed glass structure upon firing. The glass composition, the foaming agent and, if desired, abrasive aids are mixed together, formed into small agglomerates of the desired shape, burned and cooled. The agglomerates can then be sieved to suitable sizes and used in a conventional manner to give coated grinding wheels, strips or layers. They can also be used to produce resin-bonded grinding wheels.

The present invention utilizes the basic brittleness of cellular glass and its controlled variation of brittleness as a matrix for abrasive particles. When cellular glass is at a suitable foam temperature, it expands and adheres to most of the materials around it.

In addition, it tends to encapsulate particles in its path.

This last tendency is utilized when measured abrasive particles are mixed in a cell-forming glass batch and the mass is brought to a cell-forming temperature. Surprisingly, the abrasive particles are easily distributed in the cell even if they are completely encapsulated by the glass in the walls.

Thus, mixtures of different cell-forming glass amounts with different volume contents in percent of particles are mixed, after which the mixture is formed into pellets of suitably large, unsintered spheres and these spheres are dried and fired to give the grinding unit.

Cellular glass is sold as a soft abrasive of its own accord. Its main product quality is its ease of crumbling without catastrophic degradation so that when grinding a workpiece, sharp glass surfaces are constantly formed. In addition, the material is impermeable so that no liquid is absorbed into the structure. The performance of the grinding agglomerate depends on the fragility of the matrix. Ideally, the matrix should crack or crumble as soon as the encapsulated grains begin to lose their peak value in the cutting capacity. According to the present invention there is provided a product in which fine abrasive particles are encapsulated within the walls of foam cells as a discrete contaminant. Ideally, the matrix should have a coefficient of thermal expansion that is as close as possible to that of the abrasive grains to minimize cold cracks.

The grains can be formed into extruded, cut pieces or can be formed into spheres. The brittleness or crumbling can be controlled by the ratio of pores to grains and / or the ratio of glass to grains. Higher matrix densities (D, 96 g / cm 3 and above) tend to break up such as glass while lower densities increase the crumbability: When the size of the aggregates has large variation, depending on the particular use and grain size, the aggregates are generally 250 microns or larger in diameter because the foam glass process limits the minimum size of the glass particle aggregates. The maximum size used should not exceed 5 mm, at least in coated abrasive applications. The abrasive particles are generally not finer than 10 μm, nor thicker than 2 or 3 mm.

The preferred abrasive particles consist of sintered alumina, but co-sintered abrasives of alumina-zirconia alloy can also be used, as can abrasive grains of silicon carbide.

As shown in the examples below, soda glass can be used, but a non-glass crystallizing aluminum borosilicate compound is superior.

The abrasives and glass mixtures for forming the agglomerates_contain between 40 and 80% (dry bulk volume) of ground glass composition and between 20 and 6% abrasive grains. Up to 20% addition of abrasive aids such as cryolite can be added to such mixtures. The final product, in the case where it has the shape of spheres, has a bulk density of between 0.32 and 0.88 9 / cm 2.

The optimum firing temperature and time depends on the particular composition used, the desired density (porosity) of the product. In general, a temperature of 800-90,000 or higher for 20 minutes is suitable.

The steps according to a preferred embodiment of the present invention are as follows: 1. Preparation of a foamable glass by ball milling soda glass shards with 0.25% carbon black and 0.5% silicon carbide with a size of 3 μm for 24 hours in a batch ball mill to a median particle size of 5 μm or less. H 2. Addition of a charge of 79% by volume of the foamable glass and 30% by volume of abrasive particles, especially sintered dark alumina with a particle size of 0.085 mm (180 grit), after which these are mixed dry at high speed. After the dry mixture, 1% alum is added in the form of a dilute liquid, then the mixture is mixed wet and followed by a 0.4% addition of solids of an aqueous slurry of montmorillonite at 4% solids content as binder. Sufficient additional water is added to pelletize the mixture to a pellet size of 0.853 mm / 0.422 mm. The generally spherical pellets thus formed are then dried in a liquid dryer and mixed dry with aluminum hydrate separator and fired in a rotary kiln at a temperature of about 8500 for 20 minutes.

The resulting particles have a specific gravity of 0.48-0.56 to 25 40 451 687 9 / om3.

On microscopic analysis, it is observed that the glass tends to encapsulate the alumina particles in a network of foam bubbles. It can also be observed that the alumina particles tend to concentrate at the periphery of the bubbles in a manner similar to that of foam flotation.

The particles at the surface are still covered by a layer of glass.

The particles were sieved into 0.853 mm / 0.599 mm - and 0.599 mm / 0.422 mm - fractions, after which they were tested by using them as if they were abrasive particles themselves by manufacturing coated abrasive belts in a conventional manner. The belts were tested in a standard test system for metalworking and compared with belts made of dark alumina grains with a particle size of 0.085 mm (180 grit). It was found that the initial time required to achieve comparable surface treatment was longer for the belts with aggregates than the belts with grains, but the total amount of metal removed and the life of the belt was between 2-6 times corresponding to the standard belt with grains. Repeated experiments gave irregular results, some gave the above results, others significantly worse results. It was found that the reason for the irregular result was the tendency for soda glass to glass crystallization and the possibility for the cristobalite crystals to cause defects which sometimes cause the glass to rupture. Further attempts were made using the belt with an aqueous lubricant and the resulting performance was consistently poor. It was assumed that this was caused by the poor resistance of glass to water. Thus, it was decided to use a batch which would be substantially non-glass crystallizing borosilicate made from a mixture of clay or volcanic ash and chemical additives similar to that described in U.S. Pat. No. 3,793,030; A mixture of such as: volcanic ash, 15 f: kænm fl as, msmmmma% dmmu, L7% nümmmmu; 2% sodium bicarbonate beh 1/4 carbon black maldea together. An addition of 1 9 .: liquid alum was made before the pelletization. The resulting pellets when fired at 930 ° C showed performance substantially similar to the performance of pellets made from molten glass shards, which tested the reproducibility.

In addition, when examined in wet environments, the performance decreased but was still better than the equivalent for conventional belts made of barley with a size of 0.085 mm (180 grit). The aluminum borosilicate had increased resistance to water. Z Furthermore, it was found that the addition of 10% powdered cryolite increased the cutting performance. Cryolite is a well-known abrasive additive for metals and is clearly encapsulated in a manner similar to the way in which the alumina particles are encapsulated.

The following examples show that silicon carbide or co-sintered alumina-zirconia abrasive particles can be used even if it is not preferred.

The first experiment used the mixture of standard foamed soda glass, to which 3% 39 crystallone with a particle size of 0.085 mm (180 grit) was added and 10% fine cryolite. The product was foamed at about 85,000.

The resulting aggregate was lighter than the corresponding one made of alumina, its bulk density was 0.359 g / cm; with a particle size of 1.676 mm / 0.853 mm against 0.432-0.446 g / cm3 with a particle size of 1.676 mm / 0.853 mm but otherwise seemed the same. It was noted that some of the particles were associated with larger bubbles, suggesting that even coarser size silicon carbide has an influence on the foam reaction. These large bubbles probably weaken the unit. The second experiment used the same ingredients except that a sintered alumina-zirconia with a particle size of 0.178 mm (80 grit) containing 40% zirconia was used. The resulting aggregate was essentially equivalent to the alumina in all respects.

A microscopic examination of the encapsulated alumina-zirconia particles showed that despite the reducing conditions during foaming, there were signs of oxidation of metal components in the grains. This effect reduces the breakability of the grains.

It can thus be shown that other abrasives can be encapsulated in aggregates such as alumina, provided that there are side effects that can reduce their applicability.

Coated abrasive products are made from agglomerates of the present invention by bonding the aggregates in a single layer on a flexible backing sheet, in a conventional manner, well known in the art, using thermosets and adhesive coatings, adhesives or a combination of adhesives and resins. As a result of these examples, it has been found that silicon carbide-containing aggregates, in certain applications such as in the grinding of titanium metal, are clearly superior to conventional silicon carbide coated abrasive products.

In addition, when using glass-forming chemical mixtures instead of pre-ground and ground glass in the formation of the aggregates, superior wetting properties of the abrasive were achieved. In addition, the resulting assembly is different in structure from typical glass-containing mixtures. In the glassy mixtures, the abrasive particles are more uniformly distributed within the multicellular aggregate unit compared to the glass blends where the abrasive particles tend to be concentrated in the outer periphery of the cell walls of the multicellular matrix.

Claims (7)

In a practical embodiment, a foamable mixture of a glass batch, 70%, was dry-mixed with crude silicon carbide, 30%. To this mixture was added% alum on the basis of dry solids in aqueous solution, followed by a sufficient amount (0.4%) water slurry of montmorillonite to pelletize to a particle size of 0.853 mm / 0.422 mm. These generally spherical particles were dried and fired at 85,000 for 20 minutes in a rotary kiln. If desired, 10-20% addition of cryolite as an abrasive aid can be made in the dry mixing step. The burned particles were coated on a belt in a standard manner and tested dry in the processing of titanium metal and wet in the processing of a glass sheet. In both cases, the run-in time was longer than the equivalent for a standard silicon carbide belt, but the useful slip period was longer. In the case of titanium, a standard belt ground 16 g, while the experimental belt with rounded shape ground a total of 245 g. The spring sanding of glass was similar: 18 g against 180 g for the experimental belt with rounded shape ... The experiment was repeated using a mixture of chemicals and minerals which gave a glassy oxide composition which is known to give good bonding with Siß. This binder was ball milled with carbon, then mixed, pelletized and fired in a similar manner to previous examples. The dry experiments with titanium were equivalent to the glass matrix material, but the wet processing of glass was increased and a grinding of 2.48 g was obtained. Microscopic examinations showed that the grains in the glass had migrated mainly to the periphery of the bubbles and that the center of the bubbles comprised a number of large cells. The nodules of the binder composition had Sit distributed throughout the nodules and the inner closed cells were small and uniform in size. PATENT REQUIREMENTS Agglomerated abrasive particles comprising a matrix with abrasive particles therein, characterized in that the matrix is of cellular glass and that the abrasive particles are enclosed within the cell walls of the glass, the particles being present in greatest concentration in the outer walls. . 1 »H V Abrasive agglomerate according to claim 1, characterized in that the abrasive particles comprise sintered alumina, co-sintered alumina-zirconia or silicon carbide. _¿ 7 Abrasive agglomerates according to claim 1 or 2, characterized in that the agglomerates comprise particles of cryolite. Abrasive agglomerate according to any one of the preceding claims, characterized in that it has a specific density of between 0.33 and 0.38 g / cm 3. 10 15 20 25 30 35 40 451 687 7 Abrasive agglomerate according to any one of the preceding claims, characterized in that it has a generally spherical shape. Abrasive agglomerate according to one of the preceding claims, characterized in that the glass is an aluminum-boroailicate glass. Abrasive agglomerate according to any one of the preceding claims in the form of a coated abrasive layer, characterized by a flexible substrate to which said agglomerates are adhesively bonded.