FR2497136A1 - ABRASIVE AGGLOMERATES AND SHEET ABRASIVE MATERIAL - Google Patents

Abstract

The invention relates to particles of agglomerates of abrasive comprising a matrix of cellular glass and grains of abrasive encapsulated in the cell walls of said glass, the said particles being present in greater concentrations in the outer walls. THESE AGGLOMERATES CAN BE PARTICULARLY USED IN PRODUCTS COATED WITH ABRASIVE IN WHICH THE AGGLOMERATES ARE BOUND TO A FLEXIBLE SUPPORT SHEET; THEY CAN ALSO BE USED IN BONDED ABRASIVES (GRINDERS).

Description

This invention relates to abrasive particle agglomerates where the

  abrasive grains are held in a friable matrix, these agglomerates being particularly suitable for use in coated abrasive products in which the agglomerates are bonded to a flexible support sheet; they can also be used in

bonded abrasives (grinding wheels).

  At least also the use of small agglomerates in particles of relatively fine abrasive grains maintained in a matrix, to be used as a substitute for conventional abrasive grains on a flexible abrasive coated with abrasive ("sandpaper") earlier than in US Patent No. 2,194,472. As far as is known, solid agglomerates of the type described in the previous patent or the products prepared from them have never been commercially satisfactory. U.S. Patent No. Re 29,808 describes hollow spheres (or other shapes, such as cylinders) consisting of abrasive grains bonded to the outer surface of a brittle matrix, such as a mineral resin or silicate. European patent application published under No. 0 008 868 on March 19, 1980 describes solid agglomerates linked by molten cryolite

or other "salts or silicates".

  British Patent No. 982,215 and U.S. Patent No. 3,156,545 describe the manufacture of solid agglomerates for use in grinding wheels, consisting of alumina or other abrasive grains bonded by glass. U.S. Patent No. 2,216,728 describes glass or metal binders for the aggregate matrix containing diamond abrasive particles. The patent indicates that the matrix can be somewhat porous to improve mechanical bonding when the aggregates are mixed with a binder to form a grinding wheel. U.S. Patent No. 2,806,772 suggests the inclusion of expanded glass in the

  abrasive agglomerates linked by a resin matrix.

  Although abrasive agglomerates of the hollow type, bound by a resin or silicate have given good results in coated abrasive applications and that agglomerates as described in the above-mentioned European application give good results, both types

  use white glass but an alumino boro- composition

  non-devitrifying silicate is superior.

  The mixture of abrasive and glass intended to form agglomerates contains from 40 to 80% (apparent dry volume) of crushed glass composition and from 20 to 60% of abrasive grains. Up to 20% of a grinding aid such as cryolite can be added to such mixtures. The final product, when it is in the form of spheres, will have a density

apparent from 320 to 880 g / dm3.

  The optimal temperature and duration of cooking depend on the particular composition used, on the density (porosity) desired for the product. In general, a temperature of 800 to 9000C or higher during

about 20 minutes is fine.

  The steps in a typical example of this invention are as follows: 1. Preparation of a batch of expandable glass by grinding scrap of white glass with 0.25% carbon black and 0.5% silicon carbide of 3 microns, for 24 hours in a discontinuous ball mill

  up to an average particle size of five microns or less.

  2. Addition of a charge of 70% by volume of the batch of expandable glass and 30% by volume of a grain

  abrasive, especially black aluminum oxide -

  fused grain N0 180, and dry mixing at high speed. After dry mixing, 1% alum is added in the form of a diluted liquid and mixed in the wet state, then 0.4%, in solids, is added of an aqueous slurry of montmorillonite. having a solids content of 4%, as a binder. Sufficient additional water is added to transform the mixture into pellets having a particle size of the order of 420-840 microns. 3. The generally spherical pellets thus formed are then dried in a fluid bed dryer and mixed dry with a partition agent which is an aluminum hydrate and baked in an oven

  rotary at a temperature of around 8500C for 20 minutes.

  249713d The resulting particles have a density of 0.48 to 0.56 g per cubic centimeter. When examined under the microscope, it is observed that the glass tends to encapsulate the alumina particles in a network of foam bubbles. It has also been observed that the alumina particles tend to be concentrated around the periphery of the bubble in a manner which resembles flotation by foaming. Particles on the surface are still covered

a layer of glass.

  The particles are sieved to a fraction of 590/840 microns and 350/590 microns then tested by using them as if they were in themselves abrasive grains and by preparing belts coated with abrasive in the conventional manner . The belts are tested in a conventional metal polishing tester and

  compared to belts Milk of black alumina of grain 180.

  It was found that the initial time to obtain a comparable polish was longer with aggregate belts than with conventional grain belts, but the total amount of metal removed and the life of the belt were two to six times that of the

normal grain belt.

  Repeated tests give erratic results, some repeating the above characteristics and others giving significantly inferior characteristics. The reasons for these varying characteristics have been determined to be the tendency of white glass to devitrify and the potential of cristobalite crystals to cause defects which sometimes cause the glass to break. Additional tests were made using the belt with an aqueous lubricant and the resulting characteristics were significantly poor. It was determined that this was caused by the

  poor durability of glass in an aqueous medium.

  It was therefore determined to use a batch which would essentially be a non-devitrifying borosilicate made of a mixture of clay or volcanic ash and chemical additives similar to those described in US Pat. No. 3,793,039. a mixture of 66% volcanic ash, 15% kaolin, 5.5% borax 5 moles, 8% dolomite, 2.7% lithium carbonate, 2% sodium bicarbonate and 0.25% black of carbon. An addition of 1% liquid alum is made before the formation of pellets. The resulting pellets, once cooked at 930 ° C, have characteristics essentially similar to those made from molten glass debris which have reproducible tests. In addition, when tested in a humid environment, the characteristics are reduced but even better than those of a conventional belt made of a grain of 180. Alumino borosilicate improved the

durability in an aqueous medium.

  It was further found that an addition of 10% of

  powder cryolite improves the grinding characteristics.

  Cryclite is a well-known grinding aid for metals and is apparently encapsulated in a similar manner

to that of alumina grains.

  The following examples show that, although it is not preferred, silicon carbide or zirconium alumine oxide may be used as the abrasive grain.

melted simultaneously.

  The first experiment uses the mixture of conventional white foam glass to which 30% of grain 180 of Crystolon 39 and 10% of fine cryolite have been added. The product is expanded at around 850 ° C. The resulting aggregate is lighter than that made from alumina, its density

  apparent being 358 g / dm3 at 840/1680 A instead of 432-

  464 g / dm3 at 840/1680 y, but otherwise seems similar.

  It has been observed that some of the grain particles are associated with large bubbles which suggest that even coarse grain SiC will influence the foaming or expansion reaction. These big bubbles will weaken

presumably the aggregate.

  The second experiment uses the same ingredients but alumina-zirconium oxide, fused together, of grain 80 containing 40% of zirconium oxide is used (grain 180 was not available). The resulting aggregate is essentially equivalent to alumina in all

  properties. A microscopic examination of alumina grains-

249,713 <

  Encapsulated zirconium oxide show that, despite the reducing foaming conditions, oxidation of the metallic components of the grain occurs. This effect

will reduce grain toughness.

  Thus, it can be seen that other abrasives can be encapsulated in aggregates as was alumina but that it

  there are side effects that may reduce their usefulness.

  The abrasive coated products are prepared from the agglomerates of this invention by bonding the aggregates on a single layer on a flexible support by conventional means well known in the art, using

  coating and thermo preparation coatings

  curable, glue, or a combination of glue and resin. After carrying out the preceding examples, it was found that aggregates containing silicon carbide were clearly superior to conventional abrasives coated with silicon carbide, for certain applications such as

  rectification of metallic titanium.

  It has further been determined that when chemical glass-forming mixtures are used, rather than pre-ground and ground glass, in the formation of aggregates, superior wetting of the abrasive can be achieved. In addition, the structure of the resulting aggregate is different from that of typical glass-containing mixtures. In the case of glass-forming mixtures, the abrasive particles are more uniformly dispersed in the mass of multicellular aggregate, compared to glass mixtures in which the abrasive particles tend to be concentrated in the peripheral cell walls.

  exterior of the multicellular matrix.

  In one implementation, an expandable assembly of a batch of glass is mixed dry, at a rate of 70%, and of raw SiC of grain 180, at a rate of 30%. To this mixture, 1% of alum, in dry solids, in aqueous solution, is added, then sufficient (0.4%) of an aqueous slurry of montmorillonite to form pellets with a dimension of 420/840 microns. These generally spherical particles are dried and baked at 850 ° C for 20 minutes in a rotary oven. If desired, an addition of 10 to 20% of cryolite, as a grinding aid, can be carried out

in the dry mixing stage.

  The baked particles are coated on a belt in the conventional manner and tested dry in the polishing of metallic titanium and in a wet way in the polishing of a glass plate. In both cases, there is a longer waiting period than with a normal SiC belt, but the useful grinding time is significantly longer. In the case of titanium, a conventional belt cuts 16 g while the experimental belt cuts a total of 245 g. Wet grinding of glass gives similar results. 18 g instead of 180 g for

  the experimental aggregate belt.

  The experiment is repeated using a mixture of chemicals and mineral products which give a vitreous oxide composition which is known to be a good binder for SiC. This binder is ground with charcoal, then mixed, transformed into pellets and cooked in a manner similar to the previous example. The dry tests on titanium are equivalent to those of a material containing a glass matrix but the wet polishing of the glass

  is improved and a cut of 248 g has been noted.

  Observations under the microscope show that the grain in the glass has migrated substantially to the periphery of the bubble and that the center of the bubble is formed by a few large cells. The nodules or agglomerates of the binder composition have SiC distributed throughout the nodule and the internal closed cells are small and have a

uniform size.

Claims (6)

  1. Abrasive agglomerate particles comprising a matrix containing particles of abrasive grains, characterized in that the matrix is a cellular glass and that the particles of abrasive grains are encapsulated in the cell walls of said glass, said particles being present in greater concentration in exterior walls.   2. Abrasive agglomerates according to claim 1, characterized in that the abrasive grain comprises of   fused alumina, alumina-zirconium oxide co-   fused or silicon carbide.   3. Abrasive agglomerates according to one or the other   claims 1 and 2, characterized in that said   agglomerates contain particles of cryolite.   4. Abrasive aggregates according to any one   claims 1, 2 and 3, characterized in that they have   a density of 0.32 to 0.88 g / cm3.   5. Abrasive agglomerates according to any one   claims 1 to 4, characterized in that they have a generally spherical in shape.   6. Abrasive agglomerates according to any one   claims 1 to 5, characterized in that the glass   is an aluminum borosilicate composition.   i. Sheet material coated with abrasive, characterized in that it is prepared by bonding   the agglomerates according to any one of claims 1 to 6 to a flexible support.