CH652414A5 - ABRASIVE AGGLOMERATES AND SHEET ABRASIVE MATERIAL. - Google Patents

Description

The invention relates to abrasive agglomerates in particles where the abrasive grains are held in a friable matrix, said agglomerates being 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 the outer walls. These agglomerates are 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. 2194472. 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. US Patent No. Re 29808 describes hollow spheres (or other shapes, such as cylinders) consisting of abrasive grains bonded to the outer surface of a friable matrix, such as a mineral resin or silicate. European patent application published under No. 0008868 on March 19, 1980 describes solid agglomerates linked by molten cryolite or other "salts or silicates".

British Patent No. 982215 and U.S. Patent No. 3 156545 describe the manufacture of solid agglomerates for use in grinding wheels, consisting of alumina or other abrasive grains bonded by glass. US Patent No. 2216728 describes glass or metal binders for the aggregate matrix containing abrasive diamond particles. The patent indicates that the matrix may be somewhat porous to improve mechanical bonding when the aggregates are mixed with a binder to form a grinding wheel. US Patent No. 2806772 suggests the inclusion of expanded glass in the abrasive agglomerates bonded by a resin matrix.

Although abrasive agglomerates of the hollow type, bound by a resin or a 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 of Abrasives are difficult or expensive to manufacture and it is difficult to control their friability.

Better control of the physical properties of abrasive agglomerates and excellent grinding results of coated abrasives can be obtained by using agglomerates of abrasive particles bound by an expanded glass in which the abrasive particles are contained inside the walls of the cellular glass matrix. Such agglomerates can be produced by mixing the appropriate abrasive grains with known conventional compositions which give a structure of expanded glass by baking. The glass composition, the blowing agent or porophore agent and, if desired, a grinding aid, are mixed together, transformed into small agglomerates of the desired shape, baked and cooled. The agglomerates can then be screened to the appropriate sizes and conventionally used to produce discs, belts or sheets coated with abrasive. They can also be used to produce grinding wheels bound by resins.

The present invention uses the basic friability of cellular glass and its determined variations in friability as a matrix for abrasive grains. When a cellular glass is at the proper temperature for expansion, it will expand and stick to most materials around it. In addition, it tends to encapsulate particles in its path.

The latter trend is used when calibrated abrasive grains are mixed with a batch of cellular glass and the mass is brought to the cell formation temperature. Surprisingly, the abrasive particles are easily distributed in the cell while being completely encapsulated by the glass inside the cell walls.

Consequently, mixtures of various batches of cellular glass are assembled with various volume percentages of grains, the assembled batch is transformed into pellets in order to obtain raw spheres of appropriate size and Fou dries these spheroids which Fon bakes to obtain the agglomerate. abrasive.

Cellular glass is sold as a soft abrasive by itself. Its main product qualities are its easy friability without catastrophic breakage, so that when Fon rubs on a workpiece, new sharp glass surfaces are constantly being formed. In addition, the material is waterproof so that there is no absorption of liquid in the structure. The characteristics of the abrasive agglomerates depend on the friability of the matrix. Ideally, the die should break or break as soon as the encapsulated grain begins to lose its maximum cutting quality. This invention provides a product in which fine abrasive grains are encapsulated in the walls of the expanded cells, as a separate impurity. Ideally, the matrix should be designed to have a coefficient of thermal expansion that is as close as possible to that of the abrasive grains to minimize defects or "flaws" upon cooling.

The grains in question can be prepared in cut out extruded shapes, or in a sphere shape. Friability can be determined by the ratio of pores to grain and / or the ratio of glass to grain. Higher density matrices (960 g / dm3 and above) will tend to break like a glass while lower densities will increase friability.

Although the size of the agglomerates is subject to significant variations depending on the particular application and the grain size, in general the agglomerates will have a diameter of 250 n or more because the expanded glass process limits the minimum size of the glass-grain agglomerates . The maximum size normally used will not be more than 5 mm, at least in abrasive coating applications. The abrasive grains will generally be no finer than 10 µm or coarser than 2 or 3 mm.

The preferred abrasive grains consist of molten aluminum oxide, but abrasives of alloy alumina-zirconium oxide melted simultaneously, as well as abrasive grains based on silicon carbide.

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As indicated in the previous example, white glass can be used, but a non-devitrifying alumino-borosilicate composition is superior.

The mixture of abrasive and glass intended to form agglomerates preferably contains from 40 to 80% (dry apparent 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 an apparent density of 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 900 ° C or higher, for about 20 min, is suitable.

The steps in a typical example of this invention are as follows:

1. Preparation of a batch of expandable glass by crushing white glass debris with 0.25% carbon black and 0.5% 3% silicon carbide for 24 h in a batch ball mill until '' with an average particle size of 5 (i or less.

2. Addition of a charge of 70% by volume of the batch of expandable glass and 30% by volume of an abrasive grain, in particular of black aluminum oxide fused with grain N "180, and their mixing with dry at high speed. After dry mixing, 1% alum is added in the form of a dilute liquid and mixed in the wet state, then 0.4%, in solids, is added. 'An aqueous montmorillonite slurry having a solids content of 4% as a binder, enough additional water is added to transform the mixture into pellets having a particle size of the order of 420-840 l.

3. The generally spherical pellets thus formed are then dried in a fluid bed dryer and dry mixed with a partition agent which is an aluminum hydrate, and baked in a rotary oven at a temperature of about 850 °. C for 20 min.

The resulting agglomerates have a density of 0.48 to 0.56 g / cm3. 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. The particles on the surface are still covered with a layer of glass.

The agglomerates are sieved to a fraction of 590/840 p. and 350/590 | i 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 made of 180 grit black alumina.

It was found that the initial time to obtain a comparable polish was longer with agglomerate belts than with conventional grain belts, but the total amount of metal removed and the life of the belt were two at 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 done 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. mixture of 66% volcanic ash, 15% kaolin, 5.5% 5 mol borax, 8% dolomite, 2.7% lithium carbonate, 2% sodium bicarbonate and 0.25% black carbon, 1% liquid alum is added before the formation of pellets. The pellets are baked at 930 ° C and the resulting agglomerates have characteristics essentially similar to those of agglomerates obtained 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. The alumino-borosilicate has improved durability in an aqueous medium.

It has further been found that an addition of 10% powdered cryolite improves the grinding characteristics. Cryolite is a well-known grinding aid for metals and is apparently encapsulated in a manner similar to that of alumina grains.

The following examples show that, although it is not preferred, silicon carbide or alumina-zirconium oxide can be used simultaneously as the abrasive grain.

The first experiment uses the mixture of classic white foam glass to which 30% of grain 180 of Crystolon 39 and 10% of fine cryolite are added. The product is expanded at around 850 ° C. The resulting agglomerate is lighter than that made from alumina, its apparent density being 358 g / dm3 at 840/1680 [X instead of 432-464 g / dm3 at 840/1680 p., But seems otherwise 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 large bubbles will likely weaken the agglomerate.

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 agglomerate is essentially equivalent to alumina in all properties. A microscopic examination of the encapsulated alumina-zirconium oxide grains shows that, despite the reducing foaming conditions, an oxidation of the metallic components of the grain appears. This effect will reduce the toughness of the beans.

Thus, it can be seen that other abrasives can be encapsulated in agglomerates as has been alumina, but that there are side effects which can reduce their usefulness.

The abrasive coated products are prepared from the agglomerates of this invention by bonding said agglomerates in a single layer on a flexible support by conventional means well known in the art, using coating and thermosetting preparation coatings, glue. , or a combination of glue and resin.

After carrying out the previous examples, it was found that agglomerates containing silicon carbide were clearly superior to conventional abrasives coated with silicon carbide, for certain applications such as the grinding 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 the agglomerates, superior wetting of the abrasive can be achieved. In addition, the structure of the resulting agglomerate 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 agglomerate, compared to glass mixtures in which the abrasive particles tend to be concentrated in the outer peripheral cell walls 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 is added 1% of alum, in dry solids, in aqueous solution, then enough (0.4%) of an aqueous slurry of montmorillonite to form pellets with a dimension of 420/840 p. These generally spherical particles

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are dried and baked at 850 ° C for 20 min in a rotary oven. If desired, an addition of 10 to 20% cryolite, as a grinding aid, can be carried out in the dry mixing step.

The cooked agglomerates are coated on a belt in the conventional manner and tested dry in the polishing of metallic titanium and in the 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 the glass gives similar results: 18 g instead of 180 g for the experimental agglomerate 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 5 sec 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.

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Claims (7)

652,414 1. Abrasive agglomerates 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 in greater concentration in the outer walls. 2. Abrasive agglomerates according to claim 1, characterized in that the abrasive grain comprises fused alumina, co-fused alumina-zirconium oxide or silicon carbide. 2 3. Abrasive agglomerates according to one of claims 1 or 2, characterized in that said agglomerates contain particles of cryolite. 4. Abrasive agglomerates according to one of claims 1, 2 or 3, characterized in that they have a density of 0.32 to 0.88 g / cm3. 5. Aggregates of abrasive according to one of claims 1 to 4, characterized in that they have a generally spherical shape. 6. Abrasive agglomerates according to one of claims 1 to 5, characterized in that the glass is an aluminum borosilicate composition. 7. Sheet material coated with abrasive, characterized in that it comprises abrasive agglomerates according to one of claims 1 to 6, bonded by adhesion to a flexible support.