Coated abrasive material

Abstract: Coated abrasive materials having abrasive filaments of polycrystalline alumina.

Description: This invention is directed to a coated abrasive material. Sol gel aluminous abrasives, have demonstrated substantial advantages over other premium abrasives in broad areas of coated abrasive applications since their introduction some few years ago. Such abrasives are generally made by drying and sintering a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO 2 . The dried material is crushed either before or after sintering to obtain irregular blocky shaped polycrystalline abrasive grits in a desired size range. The grits may be later incorporated in abrasive products such as coated abrasive disks or belts. U.S. Patent 4,314,827 to Leitheiser et al, discloses abrasive grits made by such a method in which the sintered grits contain irregular "snowflake" shaped alpha Al 2 O 3 crystals which are on the order of 5 to 10 µm in diameter. The spaces between the arms of a "snowflake" and between adjacent "snowflakes" are occupied by other phases such as a finely crystalline alumina magnesia spinel. U.S. Patent 4,623,364, which issued on November 18, 1986 assigned to Norton Company, discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, fibers, rods or small shaped parts, having enhanced properties. In that patent the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. This can be accomplished by either wet vibratory milling of the gel or gel precursor with alpha alumina media, or by the direct addition of very fine seed particles in powder or other form. The alpha alumina so obtained (often herein called SG grain) has a very fine, uniform crystallite structure with substantially all the crystallites less than about 1 µm in size. Slightly larger sizes can be obtained by longer firing but this. is not in general desirable. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, to make shaped parts or rods, the material may be formed or molded as by extrusion before firing. In the case of extrusion of rods, the rods are later cut or broken into appropriate lengths. DE-B-11 67 725 describes alumina containing abrasive grits for the production of abrasive discs. The grits have the form of pellets with a regular geometric shape. The grits contain 50 to 95% crystalline alumina with an average crystall size of less than 5µm and 5 to 50% of a glass bond. The specific gravity of the pellets is at least 2.75 g/cm 3 , and the pellet structure does not contain cracks and tears. The pellet structure consists of a discontinuous alumina phase, which is connected by means of a continuous glassy phase. The present invention provides an improved coated abrasive material as evident from the independent claim 1. The invention provides a coated abrasive material comprising: a. a backing member, and b. a layer of coated abrasive comprising: 1. a maker coat, and 2. a layer of a plurality of preshaped abrasive particles of a filament shape adhered to said backing member bv said maker coat, said abrasive particles each being as defined in claim 1. The abrasive particles have a hardness of at least 16 and preferably 18 GPa. The average aspect ratio of the particles is at least 2:1 and a preferred diameter is about 0.33 mm. Preferably at least 80%, more preferably 95% of said filament shaped alumina abrasive particles appears as generally equiaxed crystals having a size no greater than about 0.5 µm, preferably 0.4 µm. Further aspects and advantages of the invention are evident from the dependent claims and the following description and Figures. The coated abrasive of the invention contains polycrystalline abrasive filaments. It provides a long useful life and requires less applied power for a given cutting rate by using a special form of sol gel alumina abrasive grain. The coated abrasives or the invention also are freer cutting and generate less heat during grinding. The coated abrasives or the invention are also lound to cut at a more even rate and impart a more consistent surface finish to the work piece over an unusually long useful life. The present invention provides a coated abrasive material or product in which the abrasive grain comprises sintered sol gel alpha alumina filaments having substantially consistent cross-sections along their lengths. The filaments have an aspect ratio on the average in the range of preferably at least 2:1 to 9:1 and can be up to 12:1 or more. They are formed by the seeded sol gel route. The filaments consist of alpha alumina crystallites and are essentially free of impurities forming amorphous or "glassy" material upon firing. The crystallites are below about 1 µm and most preferably below 0.5 µm in size. The abrasive filaments are attached to a flexible backing proximate one end of the filament by an adhesive maker coat. The filaments extend generally away from the backing and are normally over-coated with size coat to further anchor them into the backing. The backing may be formed of any of the known backings used for coated abrasives, such as, for example, woven or stitch bonded fabrics, film or paper. A variety of well-known cloth or paper finishing methods and materials are used in the industry to prepare the backing, depending on the application, and are equally applicable to the coated abrasive backings used in the present invention. Similarly any of the well-known standard maker coats used in coated abrasive manufacture may be used. If the aspect ratio of the abrasive filaments of the invention is fairly short, e.g. averaging 2:1 to 5:1, the size coat may be applied by standard roll coating techniques. If the abrasive filaments have a longer aspect ratio, it is preferred to coat by other means, such as spray coating, which will not squash down the filaments excessively The materials used for the size coating may also be of any of the known types used in the coated abrasive industry. It has been found that the coated abrasive products of the present invention have substantially longer lives than those incorporating the crushed grits of the prior art. They also tend to cut at a less variable rate and impart a less variable surface finish to the work piece over at least portions of their lives than do prior art coated abrasives. Unexpectedly, it has also been found that the use of the coated abrasives of the present invention is particularly effective in low pressure grinding operations. A unique advantage of the invention is that not only can coated abrasive products be provided having various length abrasive filament particles, as desired, but the size distribution for a particular application can be as exact or varied as desired, something not heretofore obtainable with roll crushed abrasive grains. A further advantage is that coated abrasive products of the invention can incorporate abrasive filaments cut to a variety of lengths or aspect ratios, to simulate a standard CAMI graded filament blend. Alternatively, the coated abrasives can have a designed blend of over grade, control and fine diameter abrasive filaments. Another advantage in the use of abrasive particles having a predetermined configuration in the manufacture of coated abrasive material is that this tends to a more economical and efficient overall manufacturing operation. Yet another advantage is that the products having a lower density of grain on the surface often perform better than products with a higher number of grains per unit surface area. Coated abrasive products according to the invention can be supplied in various forms, e.g. abrasive discs, which offer improved performance in many grinding applications. It is found that the coated abrasive products of the invention are particularly well adapted to the efficient use of grinding aids. These are usually applied to the surface of abrasive material in the form of a coating. The low profile of conventional grains limits the amount of the grinding aid that can be applied as the abrasive grains should not be covered over. However, because of the high profile of the filamentary abrasive particles used in the present invention, it is possible to incorporate far more grinding aid and this has very beneficial results. It is also possible to apply the grinding aid in a more reactive form, perhaps in the form of a foam, instead of as a mixture in a resin coat. The grinding aids used may include any of those known to be effective such as KBF 4 , K 2 TiF 6 , NaCl, sulfur and the like. The foregoing and other objects, features and advantages of the present invention will become apparent upon the reading of the following description and the examples provided, taken in conjunction with the drawings, wherein: FIG. 1 is a photograph of a plan view of a portion of a coated abrasive product of the invention having a layer of abrasive filaments; FIG. 2 is a photograph of a view in cross-section of a coated abrasive product such as shown in FIG. 1; FIG. 3 is a photograph of a view in cross-section of another coated abrasive product of the invention in which abrasive filaments having a greater average aspect ratio than the product shown in FIG. 2 are used; FIG. 4 is a photograph of a cross-sectional view of a coated abrasive product in which the abrasive filaments are of a somewhat lesser average aspect ratio than those in FIG. 2: FIG. 5 is a photograph of a cross-sectional view of a coated abrasive product in which abrasive grains are used obtained by conventional roll crushing: FIGS. 6(a), (b), (c), (d). (e), and (f) are photographs showing, respectively, 50 grit roll crushed sol-gel alumina abrasive grain: conventional 50 grit roll crushed fused alumina abrasive grain; and 50 grit sol-gel abrasive filaments of random (c) and increasing (d,e,f) average aspect ratio used in the manufacture of coated abrasive products according to the invention; FIG. 7 is a graph showing the effect of increasing aspect ratio on the total cut for coated abrasive discs according to the invention having abrasive filaments; and FIG. 8 is another graph showing the effect of the aspect ratio of the abrasive filaments on coated abrasive discs in cutting performance. FIG. 9 is an S.E.M. photograph (enlarged 100 times) of the surface of a coated product according to the invention showing the abrasive particles embedded in a size coat. FIG. 10 is an S.E.M. 100x photograph of the surface of a conventional crushed seeded sol-gel grain product with a size coat. FIG. 11 is a 50x S.E.M. photograph of the swart from use of a coated product such as that shown in FIG. 9 (product of invention). FIG. 12 is a 50x S.E.M. photograph of the swarf from use of the coated product shown in Figure 10 (Prior Art). Referring now to the drawings, there is shown in FIG. 1 thereof a photograph of a coated abrasive product 10 comprising a backing member 12 and a top surface coating of coated abrasive 14. The coated abrasive 14, as will be better seen from FIG. 2, comprises a maker coat 16, a layer of abrasive filaments 18, and a size coat 20. For purposes of this application and the invention disclosed, the terms "abrasive filament(s)" is used to refer to elongated ceramic abrasive bodies each having a generally consistent cross-section along its length and wherein the length is at least about twice (on average) the maximum dimension of the cross-section. The abrasive filaments of the invention may be bent or twisted so that the length is measured along the body rather than necessarily in a straight line. The abrasive filaments 18 are preferably obtained, in general, by extruding or spinning a seeded gel of hydrated alumina into continuous filaments, drying the filaments so obtained, cutting or breaking the filaments to the desired lengths and then firing the filaments to a temperature of not more than 1500°C. Various so gel methods of preparation of gels of hydrated alumina are described in, for example, US-A-4,314,827, and US-A-4,623,364. The sol or gel includes a dispersed submicron crystalline seed material or a precursor thereof in an amount effective to facilitate the conversion of the hydrated alumina particles to alpha alumina upon sintering. This is commonly called the "seeded sol gel process". The amount of seed material should not exceed about 10% by weight of the hydrated alumina and there is normally no benefit to amounts in excess of 5%. if the seed is adequately fine, amounts of from 0.5% to 10% may be used with 1-5% being preferred. In general, if more seed material is added, for example more alpha alumina, this can interfere with the gel stability and make extrusion very difficult. In addition large amounts of pre-existing alpha alumina in the extrudate will require higher temperatures to achieve sintering. As indicated above however, higher temperatures lead to crystal growth and such products are generally inferior. Examples of solid, microcrystalline seed materials are beta alumina, alpha ferric oxide, alpha alumina, gamma alumina, chromium oxide and other fine debris that will provide a nucleation site for the alpha alumina crystal being formed, with alpha alumina being preferred. The seeds may also be added in the form of a precursor such as ferric nitrate solution. In general the seed material should be isostructural with alpha alumina and have similar crystal lattice dimensions (within about 15%) and be present in the dried gel at the temperatures at which the conversion to alpha alumina takes place (about 1000 to 1100°C). The green abrasive filaments may be formed from the gel by a variety of methods, such as by extrusion or spinning. Extrusion is most useful for filaments between 0.25 and 1.5 mm in diameter which, after drying and firing, are roughly equivalent in diameter to the screen openings used to separate 100 grit to 24 grit abrasive grits, respectively. Spinning is most useful for fired filaments less than 100 µm in diameter. Fired filaments as fine as 0.1 µm (0.0001 mm) have been made by spinning in accordance with the invention. The green filaments shrink about 50% in diameter from their extruded diameter upon firing. Gels most suitable for extrusion should have a solids content of between 30% to 65% and preferably between 45% and 64%. The optimum solids content varies directly with the diameter of the filament being extruded, with 60% solids content being preferred for filaments having a fired diameter roughly equivalent to the screen opening for a 50 grit crushed abrasive grit (about 0.28 mm). Spinning in accordance with the invention may be performed by placing a quantity of the gel on a disk which is then spun to fling green filaments which dry almost immediately in the air. Alternatively, the gel may be placed in a centrifuge bowl having holes or slots drilled in its periphery and the bowl is spun at, for example, 5,000 rpm to form the filaments. Other known spinning methods may also be used to form the green filaments. For spinning the most useful solids content is between about 20% to 45%, with about 35% to 40% being preferred. If the filaments are being formed by spinning, it is desirable to add about 1% to 5% of a spinning aid, such as polyethylene oxide, to the sol from which the gel is formed in order to impart desirable viscoelastic properties to the gel for filament formation. The optimum amount of spinning aid varies inversely with the solids content of the gel. The spinning aid is burnt out of the filaments during calcining or firing. Since very little of it need be added (generally none at all for extrusion), it does not substantially affect the properties of the fired filaments. Various desired shapes may be imparted to extruded gel filaments by extruding the gel through dies having the shape desired for the cross-section of the filament, for example. These may be square, star-shaped, oval, triangular, or even hollow tubes. If the gel filaments are relatively large in cross-section or have been made from a gel containing a large amount of water, it may be necessary or preferable to dry them at a temperature below 100°C for 24-72 hours before subjecting them to any heating above 100°C. It the gel filaments have a relatively thin cross-section or core made from very high solid gels, drying may not be necessary. The initially formed continuous filaments are preferably broken or cut into lengths of the maximum dimension desired for the intended grinding application. In general, any shaping or partitioning operation needed to convert the continuous filaments into discrete bodies or to change their shape is best accomplished at the gel stage, or the dried stage because it can be accomplished with much less effort and expense at these points than by attempting to operate on the much harder and stronger bodies formed after final firing according to this invention. Thus, as the continuous filaments emerge from the extruder die, they may be reduced to the desired length filament by any suitable means known to the art, for example, by a rotating wire cutter mounted adjacent the face of the die. Alternatively, the dried filaments may be broken or lightly crushed and then classified to desired ranges of length. After the gel filaments have been shaped as desired and cut or crushed, and dried if needed, they are converted into final form filaments by controlled firing. The firing should be sufficient to convert substantially all the alumina content of the gel filaments into crystalline alpha alumina, but should not be excessive in either temperature or time, because excessive firing promotes undesirable grain or crystallite growth. Generally, firing can be done at a temperature between 1200°C to 1350°C for between one hour and 5 minutes, respectively. For coarser filaments the firing is preferably preceded by drying at about 400 - 600°C for from about several hours to about 10 minutes, respectively, in order to remove the remaining volatiles and bound water which might cause cracking of the filaments during firing. Particularly for filaments formed from seeded gels, excessive firing quickly causes larger grains to absorb most or all of smaller grains around them, thereby decreasing the umiformity of the product on a micro-structural scale. The abrasive filaments of this invention should have an aspect ratio, i.e. the ratio between the length along the principal or longer dimension and the greatest extent of the filament along any dimension perpendicular to the principal dimension, of at least 2:1, on average. Where the cross-section is other than round, e.g. polygonal, the longest measurement perpendicular to the lengthwise direction is used in determining the aspect ratio. It should be appreciated, however, that the aspect ratio of a particular lot of abrasive filaments may vary depending somewhat upon the particular manner of partitioning of the filaments. Thus, a particular lot may have some filaments with aspect ratios less than 2:1 and some with more than 2:1; however, on average those abrasive filaments desired for use in coated abrasive products of the invention should have an aspect ratio of at least 2:1. Preferably, the aspect ratio, on average, will range from 2 to 8, although longer filaments are also useful in many applications. In general, coated abrasive products having abrasive filaments of a lower aspect ratio will be found more suitable for high pressure grinding applications and those with filaments of a higher aspect ratio will be found more suitable in low pressure applications. The filaments most useful in the practice of the invention have a hardness of at least 16 Gpa and preferably at least 18 GPa for most applications (Vickers indenter, 500 gm load), and are preferably at least 90% and usually most preferably at least 95% of theoretical density. Pure dense alpha alumina has a hardness of about 20-21 GPa. The abrasive filaments used in the practice of the invention have a twist in their lengthwise dimension, or are somewhat curved or bent. The abrasive filaments of the present invention have been found to produce coated abrasive products that are far superior to the same type abrasive products containing normal crushed abrasive grain. even of the same material and microstructure and of equivalent diameter. The manufacture of the coated abrasive products 10 of the invention can be accomplished, for the most part, by conventional techniques known to those skilled in the art. The backing member 12 can be of any material now conventionally used in the manufacture of coated abrasives. This includes paper, film, woven and stitch bonded cloth, such as rayon. cotton drills. nylon and polyesters, vulcanized fiber, dimensionally stable polyester film and the like, sized with various materials depending somewhat upon the end use intended for the coated abrasive product. The sizing and filling materials for the backing member, if used, can be starch, glue, filled or otherwise as desired, or of a resinous material such as phenol-aldehyde. The bonding layer or maker coat 16 can be of a resinous material such as, e.g., phenol-aldehyde, epoxy resin, and the like. This layer may have a sand size coat 16 thereover, if desired. Where the sand size coat is of a heat-hardened resinous material, the maker adhesive coat can be of either glue or resinous material, as desired. An example of a conventional maker adhesive comprises a solution containing 48% phenol-formaldehyde resin solids and 52% calcium carbonate filler. Following coating of the maker coat on the backing member, the maker coat resin may conventionally be pre-cured, for instance, for 30 minutes at 107°C, depending on the formulation. The abrasive filaments are then applied according to usual electrostatic techniques, upward propulsion or projection being preferred. This may then be followed by application of the size coat or adhesive, an example of which is a conventional solution containing 48% phenol-formaldehyde resin and 52% calcium carbonate filler. The coated abrasive is then given a final cure, typically at about 107°C for 10 hours, to cure the resin in the maker and size coats to the desired hardness. Conventional radiation curable (E-beam or UV) resins used in the manufacture of coated abrasives may also be used on any or all of the cloth finishing or adhesive layers, if desired. Coating of the adhesive coats onto the backing member, in general, is accomplished according to conventional coating techniques, e.g. roll coating. However, where relatively long abrasive filaments are used, the size coat may be more desirably applied by spray coating or other techniques than roll coating to avoid undue breakage or squashing down of the abrasive filaments. Turning now to FIG. 5 of the drawings, there is shown by comparison coated abrasive material 22 according to the prior art which comprises abrasive grains 24 obtained by conventional roll crushing techniques. One can readily appreciate the differences in construction offered by the coated abrasive material of this invention as shown in FIGS. 1 - 4. The differences in configuration of the abrasive filaments used in the manufacture of coated abrasive material of the invention can be more readily appreciated by reference to FIG. 6. In that figure, the irregular three-dimensional shaped grains 26 obtained by conventional roll-crushing of sol-gel alumina abrasive are shown in FIG. 6(a). These irregularly shaped grains are similar in shape to the fused alumina grains 28 shown in FIG. 6(b), obtained by conventional roll crushing techniques. In FIGS. 6(d-f) there are shown various lots of abrasive filaments used in the manufacture of coated abrasive material according to the invention. FIG. 6C shows a lot of abrasive filaments IS of random aspect ratios, in which no effort was made to size grade the filaments. Such abrasive filaments have an average aspect ratio of 4.1:1. The abrasive filaments 18, in FIGS. 6(d) , (e), and (f) have average aspect ratios of 3.6:1; 5.1:1; and 8.5:1; respectively. As can be readily appreciated from the drawings, the sol-gel abrasive filaments 18, have substantially consistent round cross-section along their lengths. On the other hand, the roll crushed sol-gel grains 26, like the roll crushed fused alumina grains 28, are irregular in shape. Some are relatively long and others of blocky shape. None are deemed to have a consistent cross-sectional shape. Figures 9 and 10 show clearly the different physical appearance of the surface of the product of the invention and that of the prior art. Figures 11 and 12 show the swarf produced by the structures shown in figures 9 and 10. It should be noted that the product of the invention produces a clean continuous swarf, evidencing excellent cutting. Figure 12, however, shows fragmented swarf mixed with small globules of molten metal indicating relatively poor grinding performance. It has been found that the coated abrasives of the present invention have distinct advantages over the prior art products which have less regular shapes and which present a variety of configurations to the work piece. Many of these advantages stem directly from the shape of the grain. In grinding tests run under a variety of applied pressures with belt and disc product forms, the degree of performance improvement correlated closely with aspect ratio values. Total cut increased with aspect ratio up to a certain value which depended on the product type, contact pressure and other test conditions. For example, in a 5.52 bar (80 psi) belt plunge grind test, increasing the aspect ratio to greater than 3 caused no improvement in performance. Performance did not continue to increase with the high aspect ratio values without limits because the greater length produced a larger moment arm at the base of the grain and the resulting internal stress quickly fractured the grains. On fibre disc products with a lower applied pressure of about 0.34 bar (about 5 psi), performance did not drop off until a much higher aspect ratio was reached. A higher aspect ratio was found to influence grinding performance mechanisms in four ways by providing: A wider distribution of cutting edges Fewer contact points per unit area Increased swarf clearance Buttressing or aggregation of grains. With higher aspect ratios, cutting edges are distributed over a greater height or thickness above the backing allowing the coated product to grind with-several groups of cutting edges at various levels. The first group of grains with the highest aspect ratios eventually break down due to internal fiber stress and a new group of shorter, unworn grains is exposed and begins cutting. If the local edge stresses exceed the fiber stresses, then the edges chip creating new edges and a resharpening effect. Initially, high aspect ratio grains operate primarily by exposing new grains; later as the moment arm decreases, an edge resharpening mode becomes more prevalent. Several experiments and tests supported this interpretation of the mechanism. During testing, product weight loss per interval increased with an increase in aspect ratio from 2:1 to 3:1. In low pressure disc and high pressure belt tests using interrupted cycles to allow observations of grain wear and breakdown, the modes of fracturing described above occurred during grinding. Visual examinations as well as profilometry studies of the surface also indicated that the grain peaks in products according to the invention are distributed over a greater range of heights than in conventional crushed SG grain products and that the range increases with aspect ratio. In addition surface finish (Ra or arithmetic average) and working band increased with aspect ratio. Working band is determined by finding the average of the highest points and the lowest points from ten different intervals and taking the difference. Working band gives a better indication of how much material is available above the adhesive size level to perform work. For identical grain weights, a coated product according to the invention has a more "open" coat, or i.e., fewer grains per unit area on the backing since the points are distributed over a greater height and each grain is heavier The fewer potential contact points per unit area results in higher force per grain. If the grain can sustain the higher load, then a deeper and, therefore, cooler and more efficient cut will be made. While each grain edge removes more material than a crushed, pyramidal grain point, the crushed grain product cuts with more grain points and, therefore, the initial cut rates are about equal or the crushed grain may even have a slightly higher initial cut rate. However, the grains used in the products of the invention maintain a higher cut rate and last longer. Observations of the thicker, wider particles in the generated swarf supports this mechanism (See Figures 11 and 12 for example). Profile measurements also show that peak density (peaks/2.54 cm (peaks/inch)) decreases with the increasing aspect ratio. A third mechanism which occurs is the increase in swarf clearance provided by the rougher and more open products of the invention. In fibre disc tests with a large contact area, the swarf generated by the invention products increases in length with increasing aspect ratio due to the greater swarf clearance. During testing, fibre discs did not show signs of loading until the end of the test, indicating that the buildup of metal interferes with the cutting process and curtails life. As the aspect ratio of the grains increases, the grains lay against and buttress each other in an action similar to an aggregate. The group of grains support each other during cutting and while having a large mass, the finish generated in the work piece is determined by the size of the cutting edges on the individual grains. The higher aspect ratio alone cannot completely explain the superior performance of the filamentary grain. With a low mean aspect ratio of 1.9:1, the grain still outperformed conventionally crushed S.G. grain with only a slightly lower mean aspect ratio of 1.5:1. It appears therefore that the regular, cylindrical shape of the grain is also important in controlling grinding performance. The cylindrical shape affects grinding performance in four ways: Grains with zero or near zero rake angle increased More uniform stress distribution in grain Longer, continuous cutting edges Swarf controlled and directed away from cutting zone With proper orientation, the grain used in the invention provides a zero or near zero rake angle with respect to the work piece and cutting face of the grain. The rake angle will remain zero regardless of the rotational orientation about the grain axis if the grain has a radially symmetric shape. The zero rake angle allows the grain to cut a ribbon-like chip which reduces heat, friction and wear. The grain acts more like a miniature cutting tool rather than a conventional, crushed abrasive grain. The negative rake angle of the typical pyramidal shaped crushed grain causes the grain point or edge to plow through the metal; melting, tearing and smearing occurs rather than cutting. The temperatures and pressures developed during the grinding process are extremely high according to many accounts in the technical literature and by observations of metallurgical changes in the work piece material. Any mechanism which removes the swarf from the cutting zone will reduce the wear on the cutting edge and increase durability and life. The differences between the cutting plowing processes is depicted in the SEM photographs in Figures 11 and 12. These verify that the grains used in the products of the invention remove material with a cutting rather than a plowing action. With proper orientation, the cylindrical shape of the filamentary grain helps to direct swarf away from the cutting zone. Figure 11 shows that the swarf particles have a concave channel on the cut surface. The convex cylindrical grain surface keys into the swarf restricting movement of the swarf away from the cutting edge and work piece. The regular, cylindrical shape also produces a more uniform stress distribution compared to the very sharp, irregular crushed SG grain. Since a convex cylindrical surface is being plunged into the work piece, compressive stresses will develop in some regions of the grain face. Conventional, crushed SG grain often has concave surfaces and concave flake-shapes due to its tendency to fracture in a conchoidal manner. When the concave shapes are plunged into the work piece, tensile stresses tend to develop in some regions of the crushed grain. Ceramics can sustain much higher loads in compression than in tension. As a result, the grain can withstand higher loads before failing and therefore deeper cuts which extend life are possible. Crushed grains also have many flaws and notches or irregularities on their surfaces which magnify or concentrate the applied stress. With conventional crushing techniques, it is impossible to eliminate the concave surfaces and stress-concentrating notches. While the sharper edges of the crushed grain produce a higher initial cut rate in belt tests, the crushed grain has a higher tendency to crumble under grinding forces due to higher stress concentration levels. The cylindrical grains used in the products of the invention have long, continuous cutting edges which in comparison to the shorter, jagged edges of conventional crushed grain produce wider swarf ribbons and distribute the wear over a longer working edge. The longer cutting edge should take a longer time to dull and, therefore, are more durable. Observations during interrupted grinding tests indicate that the wear plateaus on the top surface of the grains develop at a slower rate compared to crushed SG grain. DESCRIPTION OF SPECIFIC EMBODIMENTS The following examples are illustrative of certain specific embodiments of this invention; however, these examples are only for purposes of better illustrating the invention disclosed herein and are not to be construed as limitations upon the invention. EXAMPLE 1 EVALUATION OF PERFORMANCE OF ABRASIVE FILAMENTS AGAINST ROLL CRUSHED SOL-GEL ALUMINA GRAIN In this example, coated abrasive discs comprising seeded gel ("S.G.") aluminum oxide abrasive filaments having an average diameter (0.33 mm (0.013")) approximately equal to 50 Grit CAMI grade (0.35mm (0.01369")) were evaluated against seeded gel aluminum oxide abrasive grains obtained by roll crushing of the dried gel as conventionally done. In each case the crystallite sizes were below 1 µm (All crystallite sizes herein are measured by the "intercept" method). The abrasive filaments were provided of varying lengths having aspect ratios varying from about 2:1 to about 12:1, in a manner somewhat analogous to the distribution of varying lengths of abrasive grain obtained by roll crushing and graded. Nevertheless, it should be appreciated that the abrasive filaments do not really lend themselves to such grading. In the case of abrasive filaments, contrary to conventional roll crushed grain, two of the dimensions of all the filaments are equal. The roll crushed gel material had a grit size distribution of + 8.2 + 3.1. Coated abrasive discs (17.78cm (7") dia., 2.22cm (7/8") central opening) were manufactured using conventional coated abrasive manufacturing procedures, conventional 0.76 mm (0.030") vulcanized fiber backings and conventional calcium carbonate-filled resol phenolic resin make (48% resin, 52% filler) and size (48% resin, 52% filler) coats. The filler used in the size coat, however, was Cryolite, instead of calcium carbonate. The maker coat resin, following application of the abrasive filaments, was pre-cured for 5 hours at 107°C (225°F) followed by a final cure, following size coat application, for 10 hours at 107°C (225°F) followed by a final cure, following size coat application, for 10 hours at 107°C (225°F). The coating was done using conventional roll coating techniques in a one pass operation with curing in a forced air oven. The resin coating weights (wet basis) were as follows: maker coat, 15#/Rm; and size coat, 23#/ Rm. A ream ("Rm") is equivalent to 30.66 m 2 (330 square feet) of coating area. The roll crushed abrasive grains and the filament-shaped abrasive particles were coated according to usual techniques, using electrostatic upward projection. The coating weights for the various discs tested are set forth in Table 1 below: Fiber Discs Characteristics For Test Evaluating Filament Shaped Abrasive Against Roll Crushed Grain Disc No. Type Abrasive Electrostatic Projection Time (Sec.) Weight Abrasive (GMS.) 1. Roll Crushed S.G. (Control) 5 52 2. Extruded S.G.#645 5 65 3. Extruded S.G. #645 20 84 - 87 The roll crushed abrasive grain (control product, Disc No. 1 ) was of the following standard composition: COMPONENT WEIGHT PERCENT Al 2 O 3 99.58 SiO 2 0.12 Fe 2 O 3 0.03 TiO 2 0.20 CaO 0.04 MgO 0.02 Na 2 O 0.01 100.00 Other properties: density 3.89 (water Pyc), 3.88 (helium Pyc); hardness 20.7 (GPa); and avg. crystallite size 0.17 µm. The abrasive filaments were of the same standard composition having an aspect ratio from about 2 to 12 (avg. Aspect Ratio 6-7). As indicated from the above data, the increase in projection time resulted in a significant increase in the weight of the coated grain. The cured discs, after being cut to size, were first conventionally 0°-90° flexed to controllably crack the hard resin bond (maker and size coats), using a rubber roll flexer, and then were curl corrected, according to conventional techniques, after which they were used in tests conventionally performed in evaluating fiber discs for low (112 DsIII) and high (112 Dsh, 112 Dss) pressure dulling. In general, such tests involve mounting the abrasive disc on a medium hard rubber back-up pad which is, in tum, mounted to a horizontally disposed motor driven spindle assembly mounted on a carriage which is free to move in a horizontal direction on frictionless bearings inwardly toward and from a hydraulically driven specimen holder. The specimen holder is adapted to accept either a 2.54 cm (1")x 2.54 cm (1 ") x 24.76 cm (9-3/4")-0.32 cm(1/8") angle or a 7.62 cm (3") x 0.48 cm(3/16") x 38.1 cm (15") long flat plate and moves horizontally in a direction 90° to the spindle, reciprocating back and forth at a predetermined rate of speed over a predetermined distance. The testing apparatus is mounted on a substantial steel table support providing stability during operation. The grinding force is applied by hanging a dead weight on a pulley system attached to the moveable carriage on which the spindle (disc) is mounted. The disc is provided at an angle of 10° from parallel to the specimen holder. Test 112 Dsh - is a high pressure (4.54 kg (10 lbs) force 50 grit grain, 5.44 kg (12 lbs) - 36 grit) dulling test wherein the fiber disc is applied to grind the 0.32 cm (1/8") thick edge of a 2.54 cm (1") x 2.54 cm (1 ") x 24.76 cm (9-3/4 inches) - 1018 carbon steel construction angle(0.32 cm (1/8")). The angleshaped work piece is first weighed and then is mounted on the specimen holder which reciprocates back and forth over a distance of 24.76 cm (9-3/4 inches), at a rate of 8-1/2 strokes/min. and a linear speed of 2.13 m/min (7ft/min). The abrasive disc is driven at 3450 rpm. The grinding cycle is two minutes after which the angle specimen is removed and the loss of weight recorded. This cycle is then repeated with new angle specimens inserted as required until a minimum grinding rate of 10 grams/min. is reached. This ends the test. The data is recorded in grams (gms. ) removed per two minute interval, member of intervals to test end, and total cut (gms.) removed by the disc being evaluated. The test results are usually shown as the percent of a control disc. Test 112 Dss - This test is identical to Test 112 Dsh except that the specimen is a 304 stainless steel angle, substituted for the carbon steel angle, the grinding interval is one minute, and the test end is 10 intervals. The grinding force, moreover, is 3.18 kg (7 lbs) for 50 grit and 4.54 kg (10 lbs) for 36 grit discs. Test 112 DsIII - This test is similar to Test 112 Dsh except that it is a low pressure test - 10 lbs. force - the specimen is a 7.62 cm (3") x 0.48 cm (3/16") x 38.1 cm (15") long cold drawn carbon steel plate and such is adpated in the mount so that the fiber disc grinds the 7.62 cm (3") face of the plate. The grinding interval is one minute, and the test is ended when the cut rate is less than 3 gms /min. Evaluation of Performance of Fiber-Shaped Abrasive Against Roll Crushed Grain TOTAL CUT (GMS.) Disc No. Type Abrasive 112 Dsh 1018 STL ANGLE 112 Dss 304 S.S ANGLE 112 DsIII 1018 STL 7.62 cm (3") FLAT (LOT B) 112 DsIII 1018 STL 7.62 cm (3") FLAT (LOT C) 1. Roll Crushed S.G. 5 Sec. UP Coating 618 50 117 365 2. Extruded S.G. #645 5 Sec. UP Coating 860 43 468 992 3. Extruded S.G. #645 20 Sec. UP Coating 1192 -- --- 937 The low pressure test was carried out on two different lots of steel (Lot B and Lot C) received at different times. As indicated, they have different grinding characteristics as evident by the total cut given in Table 2. The relative results of the above grinding tests with respect to the control disc, i.e. Disc No. 1, are shown in the following Table 3. Relative Results of Cut Performance Of Filament-Shaped Abrasive Against Roll Crushed Grain Disc No. Type Abrasives 112 DsH 1018 STL ANGLE 112 Dss 304 S.S. ANGLE 112 DsIII 1018 STL 7.62 cm (3") FLAT (LOT B) 112 DsIII 1018 STL 7.62 cm (3") FLAT (LOT C) 1. Roll Crushed S. G. 5 Sec., UP Coating 100% 100%^ 100% 100% 2. Extruded S.G. #645 5 Sec., UP Coating 139% 86% 400% 479% 3. Extruded S.G. #645 20 Sec., UP Coating 193% -- -- 1 264% It is clearly evident from the above results that on 1018 carbon steel, the fibre-backed discs having abrasive filaments therein have significantly better grinding performance than the discs having the Norton Company seeded gel aluminum oxide grains made by conventional crushing techniques. Disc No. 2 with 5 sec. UP coating gave 39% more cut in the high pressure test and from 179 to 300% more cut in the low pressure flat test. Further, increasing the amount of abrasive filaments (disc No. 3) resulted in even further greater improvement in the cut at high pressures whereas no improvement was observed in the low pressure test. The overall-results of this test indicate that abrasive filaments are more durable under high pressure grinding conditions. Such are also more versatile and freer cutting than roll crushed S.G. abrasive grains on 1018 carbon steel at low pressures. The fibrous abrasive was made by mixing 3.2 kg Pural® NG aluminum monohydrate obtained from Condea Chemie GMBH, with 1.3 kg of milled water containing 22 g of alpha alumina seeds in a conventional double shell V-blender for five minutes to form a substantially uniform sol. At this point, 200 g of 70% nitric acid diluted with 750 cm 3 distilled water was added and mixing continued for an additional five minutes to form a 595 solids gel in which the seeds were uniformly dispersed. The seeds used in the gel were prepared by milling a charge of distilled water in a model 45 Sweco mill with regular grade 88% alumina grinding media (each 12 mm diameter by 12 mm long) obtained from Diamonite Products Company, Shreve, Ohio, until the particulates (alumina seeds) in the water reached a specific surface area of at least 100 m 2 /g. The Pural® KG powder used had a purity of about 99.6% with minor quantities of carbon, silica, magnesia, and iron oxide. The seeded gel was then conventionally extruded through a multiple opening smooth walled die whose openings were 0.60 mm in diameter to produce continuous gel filaments. After drying, the extruded strands were broken to lengths averaging about 2 mm then fired to 1320°C for five minutes. After firing to convert the abrasive filaments to alpha alumina, the individual filaments had an average cross-sectional size equivalent to a standard 50 grit abrasive. At least some of the abrasive filaments were bent and twisted along their length. The abrasive fibers were substantially pure alpha alumina with an average crystallite size of 0.3 µm, as above mentioned, and a hardness of about 16 GPa. EXAMPLE 2 COMPARATIVE CUT PERFORMANCE OF ABRASIVE FILAMENTS AGAINST ROLL CRUSHED GRAIN This example compares the cut and finish of filament-shaped seeded gel aluminum oxide abrasive particles against S.G. abrasive grain obtained by conventional roll crushing. The filament-shaped abrasive particles had an average diameter 0.33 mm (0.013") approximately equal to 50 grit abrasive grains 0.35 mm (0.01369") and were of random aspect ratio varying from about 2:1 to about 8:1. Nevertheless, the batch grading used resulted in some very long fibers and a disproportionately high number of fines as below indicated. The composition of the abrasive filaments and the 50 grit S.G. roll crushed control was of the same standard composition as that set forth earlier in Example 1. Grading Results of Rolled Crushed Grain vs. Abrasive Fibers GRAIN TYPE SERIAL NUMBER GRIT SIZE GRADE Roll Crushed 08D168.3 50 +3.2 + 1.9 Abrasive Filaments 08D168.7 50 -2.5 + 30.9 As can be seen from the above table, the variation in the grading of the two different abrasive "grains" varied from one another considerably. The reading of "+3.2 + 1.9" means that the sample of the roll crushed abrasive grain was 3.2% high in over grade and 1.9% high in fines, as tested using the CAMI grading screen system. Such is within the tolerance deemed allowable for 50 grit SG abrasive grain. On the other hand, the reading of "-2.5 + 30.9" for the sample of the abrasive filaments, indicates that the sample was 2.5% low in over grade and 30.9% high in fines, a disproportionate value. The high reading on the fines end results from the abrasive filaments ("grains") all being of the same approximate diameter which is less than the size of the 50 grit control screen. Fibre discs were prepared as before described in Example 1. The coating weights were approximately: maker coat 6.8 kg (15 lbs.)/Rm; size coat 10.43 kg (23 lbs.)/Rm, abrasive filaments 65 gms, roll crushed grain 52 gms. The fibre discs made, after being conventionally flexed, were first evaluated in the low pressure dulling test (Test DsIII) earlier described. The results showing the comparative cut and finish are set forth in Table 4 below: Comparative Cut & Finish on 7.62 cm (3") Flat Plate For Filament-Shaped Abrasive Against Roll Crushed Grain CUT FINISH TIME S.G. CONTROL 50 GRIT FILAMENT-SHAPED 50 GRIT S.G. CONTROL 50 GRIT FILAMENT -SHAPED 50 GRIT INTERVAL NO. CUT/ INTERVAL GMS. ACCUM. CUT GMS. CUT/ INTERVAL GMS. ACCUM. CUT GMS. SURFACE FINISH SURFACE FINISH Ra Rtm Pc Ra Rtm Pc 1 29 29 20 20 164 1005 227 125 995 211 2 25 55 16 37 105 736 255 103 855 233 4 18 94 15 68 80 625 163 109 757 156 6 12 119 10 92 48 414 115 53 386 82 8 8 136 9 112 37 331 103 33 268 73 10 41 46 8 128 34 282 96 28 222 91 12 -- -- 7 143 28 193 92 14 -- -- 5 154 40 297 81 Ra = DEVIATION FROM MEAN LINE IN µm. Rtm = MEAN PEAK TO VALLEY HEIGHT IN µm. Pc = NUMBER OF PEAKS The finish quality of a metal work piece is commonly measured by the Ra and Rt values taken from traces at various spots (e.g. at the center and at the left, right edges) along the finished piece. The meaning of these statistical parameters is well known to those skilled in the art. Such are clearly defined in a publication entitled "An Introduction to Surface Texture and Part Geometry" by Industrial Metal Products Incorporated (IMPCO). In general, Ra is a measure of average surface roughness. Since many surfaces of differing topography might yield similar Ra values, this number is usually supplemented by other parameters generated from the same surface. In the metal finishing art, Rt is often employed to supplement the Ra measurement. The value of Rt is a measure of the depth of gauges or scratches that might remain on the work piece surface after the finishing operation. Pc is a number indicating, in general, the frequency of scratches. As can be seen from the above data, the discs having the abrasive filaments therein outperformed in cut the discs having conventional roll crushed abrasive grain. Initially, the abrasive filament disc cut at a somewhat lower rate for the first few intervals but then continued to cut at a sustained rate for a much longer time. The surface finish generated by the abrasive filaments appears about equal to that of the roll crushed grain. Nevertheless, it should be again pointed out that the batch grading of the filament shaped abrasive particles resulted in a disproportionately high number of fines, as above-disclosed and some very long abrasive filaments. With more controlled "grading", a different finish would likely result. A further test (112 DsI) was conducted to provide a comparison to the cut and the finish generated by the abrasive filaments and both 50 grit and 36 grit roll crushed abrasive grain. The results are shown in the following Table 5. This test is the same as Test DsIII except that the test piece has a 2.54 cm (1") face. As can be seen from the above results, the 50 grit abrasive fibers not only outperformed the 50 grit roll crushed abrasive grains in cut, the cut was equal to that of the 36 grit control. Again, the abrasive fibers cut at a somewhat lower rate initially than the roll crushed grains; however, it continued to cut at a sustained rate for a longer time. With this 2.54 cm (1") steel test, the fiber-shaped abrasive particles generated a finish similar to the 36 grit control. EXAMPLE 3 COMPARATIVE CUT PERFORMANCE OF ABRASIVE FILAMENTS WITH INCREASING ASPECT RATIO AGAINST ROLL CRUSHED GRAIN In this example, coated abrasive discs with abrasive filaments of different average aspect ratios are tested against roll crushed abrasive grain of similar composition, to evaluate the effect of aspect ratio on cut performance. Seeded sol-gel abrasive material was manufactured as before disclosed and was then extruded through a fine round die having a plurality of openings. The abrasive filaments were dried and lightly jaw crushed to provide different abrasive filament lengths. After firing, the lot was then screened for a short time over varying screen openings allowing for separation of different lengths of abrasive filaments. The average aspect ratio was determined and is given below. The chemical analysis of the different samples and other physical properties are set forth in Table 6. SAMPLE IDENTITY 1015 1 1016 1017 1018 CONTROL Weight (lbs) 10 24 13.5 12.5 0.01 Chemistry : Na 2 0.01 0.01 0.01 0.01 0.02 MgO 0.01 0.01 0.01 0.01 0.12 SiO 2 0.04 0.03 0.03 0.03 0.04 CaO 0.02 0.02 0.03 0.02 0.20 TiO 2 0.17 0.17 0.21 0.18 0.03 Fe 2 O 3 0.15 0.21 0.11 0.12 3.89 Density: Water Pyc 3.91 3.91 3.91 3.91 3.88 Helium Pyc 3.94 3.96 3.96 3.95 20.7 Hardness: 19.6 20.1 20.4 19.6 0.17 Avg. Crystal Size: .139 .135 .125 .135 Avg. Aspect Ratio: 4.2:1 3.6:1 5.1:11 8.5:1 Average Grit Diameter was 0.28-0.33 mm (.011 - .013") for all 4 samples. Fibre-backed abrasive discs were made according to the procedure disclosed in Example 1. The results of the grinding tests, as earlier disclosed, are given in Table 7 below: RESULTS WEIGHT OF LOSS OF DISCS Test 112 Dsh Test 112Dss Test 112 DsIII TEST 112DsIII TEST 112Ds Disc Grit Aspect Ratio LPD 1020 HR. STL. 304 STN. STL. 1018 STL C/A DISC WEIGHT LOSS SG CONTROL 50 1.4:1 180.7 705 gms 100% 48 gms 100% 180 gms 100% 0.5 gms 100% 1.8 gms 100% SG 1015 50 4.2:1 189.2 1434 gms 203% 55 gms 115% 624 gms 347% 2.6 gms 520% 6.2gms344% SG 1016 50 3.6:1 197.9 1185 gms 168% 57 gms 119% 401 gms 223% 1.5 gms 300% 3.8 gms 211 % SG 1017 50 5.1:1 180.3 1617 gms 56 gms 117% 465 gms 258% 2.3 gms 460% 7.2 gms 400% SG 1018 50 8.5:1 163.9 1346 gms 191% 53 gms 110% 639 gms 355% 2.7 gms 540% 9.5 gms 528% The abrasive filaments containing discs, as indicated above show a marked improvement in cut on the 112 Dsh and 112 DsIII tests on 1020 and 1018 steel, when compared to the roll crushed abrasive grain control disc. By comparison,however, much less of an increase in performance was shown in cutting stainless steel (Test 112 Dss). As in the high pressure dulling test, the abrasive filament containing discs appear to be highly effective in low pressure grinding application. The as is unscreened (Sample 1015) product had a total cut 347% of the control's cut on the 112 DsIII test. As shown by FIG. 7 of the drawing, in the 112 Dsh test on 1020 H.R. steel, the cut increases with increasing average aspect ratio up to a point approximately 5.1:1 (Sample 1017) and then decreases. During testing, the abrasive filaments are observed to be broken down and then fly off the fibre disc. While I do not wish to be limited to this explanation, this apparently results from the fact that, as the length of filament increases, the moment arm increases changing the stress levels in the abrasive grain (filaments). The loss of abrasive filaments relative to aspect ratio is confirmed in the above Table 7. Thus, for example, in the 112 Dsh test, the coated abrasive disc weight loss increases with increasing aspect ratio: 9.5 gms, with a 8.5:1 ratio versus 3.8 gms, with a 3.6:1 average aspect ratio. Nevertheless, as seen in FIG. 7, Sample 1018 in which the aspect ratio averaged 8.5:1 still outperformed the control disc having conventional roll crushed grain. Aspect ratio, as seen in Table 7, does not appear to effect the performance in cutting stainless steel (112 Dss). In the case of the low pressure dulling test (112 DsIII), the cut is seen from FIG. 8 to increase with the increasing aspect ratio except for an odd peak in cut with the 4.2:1 aspect ratio of sample 1015. This sample is the only one of the group of samples tested in this Example 3 provided as is, unscreened. A small number (25) of the abrasive filaments were selected at random from sample 1015 and the length of diameter of such were determined optically. The aspect ratio for those filaments varied from 2.58 to 12.66. Thus, it is believed that this difference in cut in 1015 may result from the wider distribution of lengths present in the unscreened sample than in the other samples which were all screened. Screening alters or narrows the distribution of lengths. On the other hand, with unscreened abrasive filaments, a fewer long grains (filaments) are in contact whereby to result in higher unit pressure per grain. Although not specifically shown in Table 7 above, the discs containing abrasive filaments, like in the examples earlier disclosed, have a lower initial cut rate than the SG control, but in each case maintain a higher cut rate for a much longer time. The SG control was cutting 10 gms/min by the 8th interval whereas sample 1015 was cutting higher than 10 gms/min until the 30th interval. EXAMPLE 4 CUT PERFORMANCE OF 36 GRIT ABRASIVE FILAMENT DISCS AGAINST 36 GRIT ROLL CRUSHED GRAIN In this example, a 36 grit fibre disc was evaluated, using seeded sol gel alumina abrasive having the same following composition for manufacture of the abrasive grains by conventional roll crushing and by filament extrusion: COMPONENT WEIGHT PERCENT Al 2 O 3 99.52 Na 2 O 0.01 MgO 0.01 SiO 2 0.06 CaO 0.04 TiO 2 0.19 Fe 2 O 3 0.17 The roll crushed abrasive grains and filaments had the following physical properties: Density (Water, Pyc) - 3.89; Density (Helium, Pyc) - 3.92; Hardness - 20.3; Crystal size (avg.) - 0.149 µm. This abrasive material was extruded at 0.41 mm (0.016") diameter (approximately 36 grit) and cut to random lengths varying from about 0.8 mm to about 4 mm. Thus, the aspect ratio of such abrasive filaments (grains) varied from about 2:1 to about 10:1. Fibre-backed discs were manufactured as disclosed in Example 1 except that the coating weights were: maker, 7.71 kg (17 lbs.)/Rm; size coat, 12.7 kg (28 lbs.)/Rm; abrasive grain/fibers, 27.22 kg (60 lbs.)/Rm. The grading of the abrasive materials used and the resulting cut performance, as before described, are shown in Table 8. ITEM GRIT GRADE LPO TEST RESULTS GRAMS CUT/TIME % OF CONTROL 112Ds 1018 STL 112 Dss STN STL 112 DsIII 3" Plate Control 35 -0.7 + 13.2 185.1 1214/100 80/100 264/100 Abrasive Filaments 35 -15.6 + 70.8 208.4 1842/152 91/112 465/176 In each test performed with the coated abrasive material of the invention, the initial cut rate, as in the earlier tests performed, is lower than the control but the cut rate is maintained for a much longer period. While I do not wish to be held to this theory, such low initial cut may be due to the relatively low number of extremely long grains in initial contact. On the stainless steel test, which is normally run to an end point of 10 minutes, the SG Control was cutting at a rate of six grams per minute and the total cut was 80 grams. At the end of 10 minutes, the discs having abrasive filaments on the other hand was cutting at a rate of 8 grams per minute with a total cut of 91 grams. This fibre disc product was run until a six gram/min. cut rate was reached. Such took an additional 10 minutes run time and the total cut after 20 minutes was 163 grams. Although the invention has been particularly described with respect to abrasive filaments having an approximately circular-shaped cross-section, it will be appreciated that such is not so limited. The filament-shaped abrasives used in the practice of this invention can be of any cross-sectional shape desired, e.g. round, square, triangular, diamond, polygonal, oval, x-shaped, etc. The main requirement is that such be of an elongated shape as earlier disclosed. Furthermore, filament-shaped abrasives according to the invention do not have a straight configuration. They are twisted along their lengthwise direction, or otherwise non-linear. And coated abrasive sheet material can be manufactured of blends of abrasive filaments having different shapes, or different sizes, e.g., coated abrasive material can be provided with two different diameter round-shaped abrasive filaments. Furthermore, abrasive filaments can be blended with abrasive grains conventionally crushed. Abrasive filaments of sol-gel Al 2 O 3 can be blended with abrasive grains of other compositions, e.g. fused Al 2 O 3 , garnet, etc. coated abrasive material can be provided manufactured by conventional split coat techniques, e.g., the make adhesive can be coated followed by application of a cheaper abrasive grain or filler by gravity coat and then electro-coating of the abrasive filaments as disclosed herein. Graded split coat abrasive material can be provided, e.g. by gravity coating of a fine grit first, followed by electro-coating of a more coarse abrasive filament on top. It will also be appreciated that although the invention has been particularly disclosed with respect to fibre-backed discs, such is not so limited. Various conventional coated abrasive products can be provided, e.g. belts, sheets, shapes and flap wheels. EXAMPLE 5: This Example illustrates the effect of varying crystal size on the performance of the abrasive filaments of the invention. The abrasive filaments were coated as 17.78 cm (7") diameter fiber discs and tested in the standard 112DsH, 112DsIII and 113 DsI disc test procedures. The first two are described above. The 113DsI medium pressure test procedure uses the same equipment as the 112DsH test and, as test piece, 2.54 cm (1") x 6.35 cm (2-1/2") x 24.76 cm (9-3/4") 1018 steel bar. The bar is presented to the disc so as to abrade the 2.54 cm (1") face of the bar. The grinding interval is two minutes; the bar is removed after each grinding and weighed to assess weight loss. Four bars are used alternately during the test. In all other aspects the test is like the 112DsH test. The results are recorded in Table 9 below. The "Comparative" tested is a standard commercial seeded sol gel abrasive seed with blocky grains produced by a crushing and grading procedure. SAMPLE CRYSTAL SIZE (µm) ASPECT RATIO 112DsH (1018 Steel) 112DsIII* (3 Steel Rate) 113Dsl* (1018 Steel) Comp. (SG) N/A N/A 100% 100% 100% 5-1 (Unseeded) >2 2.5:1 14% 42% 8% 5-2 a) 1.16 2.5:1 127% 127% 119% 5-3 0.65 2.5:1 148% 118% 109% 5-4 0.54 2.5:1 156% 154% 108% 5-5 0.42 2.5:1 158% 129% 118% 5-6 0.39 2.5:1 193% 136% 134% 5-7 0.26 2.5:1 211% 183% 151% From the above data, it can clearly be seen that there is significant advantage in using the smallest available crystallite size in the abrasive particles. It is also clear that the unseeded product does not perform very well at all. EXAMPLE 6: This Example compares a product made using a grain of square cross-section with one made using a standard crushed S.G. grain. In each case, the grain was formed of a seeded sol gel and the crystallites were less than 1 µm in size. The S.G. grains of 80 grit size and the product of the invention used grits with a square cross-section corresponding to an 80 grit size. The aspect ratio was 4:1. The results are expressed as a percentage of the performance of the "SG Control" product. The results obtained were as follows: 112 DsIII 7.62 cm (3") Plate 113 DsI 2.54 cm (1") Steel Bar SG (Control) (80 Grit) 100% 100% Square Grit - Invention (80 grit) 339% 249% It is clear, therefore, that grits with other cross-sections than the round shapes described above are effective in the products of the invention. EXAMPLE 7: This Example shows the effect of the amount of grain on the effectiveness of an abrasive product. A series of runs were made to evaluate the effect various parameters had on the grinding performance of products of the invention. The parameters investigated were aspect ratio, grain weight, blends of various grains, and the method of grain application. Unless otherwise noted, the backing used was finish 775, a 206.95 g (7.3 oz) polyester sateen, and the make and size adhesives were made with the following formulations: MAKE ADHESIVE SIZE ADHESIVE Varcum 2535 30.0% Varcum 2535 27.3% BM-42 13.6% BM-42 14.4% Dura 54.5% Cryolite 49.1% A-1100 0.25% A-1100 0.25% Water 1.6% Water 8.9% Table 10 summarizes the products made and the grinding test results obtained. Reviewing the results of the tests, it is apparent that the highest performing products contained the AR3.1. Furthermore, simply reducing the amount of this grain in the product had a positive effect. The product that gave the highest results, 308%, had only 66% of the surface covered with AR3.1. Results obtained with the AR2.1 were not as dramatic as the AR3.1, but they were still significantly better than the control, by as much as 172%. The same pattern of improved performance with a decrease in AR2.1 weight was noticed. Grain Weight levels between around 40 to 60% appear to provide the best results for either of the two aspect ratios evaluated. It is interesting to note that the position of the AR2.1 on the product made a significant difference in performance. Product 7-11 had AR3.1 applied as the first coat and 50 grit SG as the second whereas Product 7-12 had 50 grit SG applied as the first coat and AR3.1 as the second. 122-Ds (80) test results show that 7-11 performed little better (115%) than the control, while 7-12 was 292% of control. It should be noted that, although the AR3.1 products produced here had a particle size diameter of 0.33 cm (.013"), which is the approximate size of 50 grit, they outperformed 36 grit (see 36 R984) by as much as by 2.5 times on the 122-Ds (80) test and 2 times on the Fixed Feed test. TEST RESULTS RELATIVE TO STANDARD PRODUCT ID PROCESS GRAIN TYPE PREM. GRAIN #/ Rm TOTAL GRAIN #/ Rm % OF CLOSE COAT 122Ds (80/ (6.9 KPa psi)) 1018 Steel Fixed Feed 4140 Steel Standard One Coat SG-50 grit 39 39 100% 100% 100% 7-1 One Coat AR3.1 55.8 55.8 100% 269% 450% 7-2 One Coat AR3.1 47 47 84% 299% -- 7-3 One Coat AR3.1 37 66% 308% 406% 7-4 One Coat AR2.1 50 50 100% 136% 163% 7-5 One Coat AR2.1 42 42 84% 151% -- 7-6 One Coat AR2.1 29 29 58% 164% 184% 7-7 One Coat AR2.1 22.5 22.5 45% 172% 131% 7-8 One Coat AR3.1/FA 30.4 47.4 100% 261% 325% 7-9 One Coat AR3.1/FA 36 43.8 100% 256% 7-10 One Coat AR3.1/FA 30.4 47.4 100% 191% 7-11 1 st Coat AR3.1 28.2 47 100% 115% 97% 2nd Coat SG-50 18.8 7-12 1 st Coat SG-50 21 47 100% 292% -- 2nd Coat AR3.1 26 7-14 One Coat AR2.1/NZ 44 44 100% 138% -- 7-15 One Coat AR2.1/SiC 26.8 41.8 100% -- In each case, an electrostatic grain projection technique was used. The SG-50 grit had a loose pack density of 1.73 and had +16.0 over grade and -4.6 fines. FA indicates a 50 grit fused alumina abrasive. (This is a non -premium grain.) NZ indicates a 50 grit Norzon® from Norton Company. (This is a premium grain). SiC indicates a 50 grit silicon carbide grain. (This is a non-premium grain.). "% of Close Coat" means the percentage of the maximum possible coat weight represented by the actual coat weight. From the results shown above, it seems clear that, where a filamentary grain product is used, it should preferably be in the top coat, if multiple coat levels are used. It is also clear that a coating containing the filamentary grains should preferably comprise only about 40 to 60% of the Close Coat weight for optimum performance, with the balance being provided by a non-premium abrasive grain or, better, by air space. EXAMPLE 8 This example illustrates the effect of producing the alpha alumina in situ in the form of filaments as opposed to extruding alpha alumina particles in a suitable medium and thereafter sintering to form a coherent structure. Seeded sol-gel filaments were produced by mixing boehmite (Condea's "Disperal"), with water and 1% by weight of the boehmite of submicron sized alpha alumina in a V-blender for two minutes. An 18 weight percent solution of nitric acid was then added to give 7.2% by weight of nitric acid based on the weight of the boehmite. The mixing was continued for a further five (5) minutes to produce a boehmite gel. A series of products was then prepared for comparative purposes that corresponded to the above except that more alpha alumina (of the kind used as seed material above), was added such that total mixture had much higher proportions by weight of alumina. The boehmite was retained to give the mixture extrudability. The formulations are described in Table 11 below. Batch Variation % Solids Comparative A 30% alpha alumina 70% Comparative B 30% alpha alumina 70% Comparative C 90% alpha alumina/10% gel Comparative D 60% alpha alumina/40% gel Comparative E 60% alpha alumina/40% gel Example 1 1% alpha alumina (seed) 62% Example 2 1% alpha alumina (seed) 58% Example 3 1% alpha alumina (seed) 59% These materials were then extruded to form filaments that were dried and sintered under the conditions described below. Higher temperatures were required to sinter the high alpha alumina comparative batches than those produced by the seeded sol gel process. Samples of the filaments were then tested for their strength according to a simple three point process using an Instron test machine with a cross head speed of 0.2 cm/min. The filament was supported on a pair of edges spaced 1 cm apart (0.9 cm in the case of Comparatives C, D, and E). A downward pressure was applied midway between these points by a knife edge. The pressure was gradually increased until the filament broke and that pressure, divided by the cross-sectional area of the filaments, is reported in Table 12 below as the breaking strength. Batch Temp/Time Firing Filament Diameter (mm) kg/cm 2 Breaking Strength Average High Comp. A 1500°C 30 min. 0.32 6,831 7,465 Comp. B 1550°C 30 min. 0.3175 6,162 6,268 Comp. C 1450°C 60 min. 1.00 5,424 6,646 Comp. D 1300°C 6 min. .88 3,430 4,036 Comp. E 1350°C 6 min. .87 2,378 2,436 Ex. 1 1370°C 4 min. 0.054 11,197 13,239 Ex. 2 1350°C 30 min. 0.043 14,366 15,986 1350°C 5 min. 0.046 14,154 17,112 1325°C 30 min. 0.046 14,296 16,549 1350°C 30 min. 0.053 10,281 14,859 Ex. 3 1350°C 30 min. 0.020 16,000 18,169 The filaments of the Comparative batches were much thicker because it was very difficult to extrude finer filaments with dimensional integrity after extrusion and before firing. Higher proportions of alpha alumina were found to exacerbate this problem significantly. As can be seen from a comparison of the above data, the comparative filaments had significantly lower breaking strengths and this is believed to reflect the weaker sinter bonds developed between the alpha alumina crystals as a result of the sintering process. Therefore, the preferred filaments for use in coated products of the invention preferably have a breaking strength of at least 8,000 and preferably at least 10,000 kg per square centimeter of cross-section when measured by the test described above. This is in contrast to products made by sintering pre-formed alpha alumina where much lower strengths are obtained.

Claims: Coated abrasive material (10) comprising a backing member (12) having a layer of abrasive particles (14) adhered to the backing member by a maker coat (16) wherein the layer of abrasive particles (14) comprises a plurality of preshaped filamentary seeded sol-gel alumina abrasive particles (18) consisting of sintered sol-gel alpha alumina crystals having a size no greater than 1 µm, each such particle having a substantially uniform cross-section in the range of from 0.1 µm to 0.5 mm, an aspect ratio of at least 2:1, a hardness of at least 16 GPa, and being curved or twisted in its longer dimension. Coated abrasive material according to claim 1 wherein said alpha alumina crystals have a size below 1 µm. Coated abrasive material (10) according to one of claims 1 or 2 wherein the said sol-gel alpha alumina crystals have a density of at least 90%, preferably 95% of the theoretical density. Coated abrasive material (10) according to one of the preceding claims wherein the preshaped filamentary abrasive particles (18) have a round cross-section. Coated abrasive material (10) according to one of the preceding claims wherein the hardness of the said preshaped filamentary abrasive particles (18) is at least 18 GPa. Coated abrasive material (10) according to one of the preceding claims wherein each preshaped filamentary abrasive particle (18) has a diameter of from 0.05 to 0.5 mm. Coated material (10) according to one of the preceding claims wherein said preshaped filamentary abrasive particles (18) comprise at least 95% of generally equiaxed crystals having a crystal size no greater than 0.5 µm. Coated abrasive material (10) according to one of the preceding claims in which the preshaped filamentary abrasive particles (18) have a breaking strength of at least 8,000 kg/cm 2 . Coated abrasive material (10) according to one of the preceding claims wherein the said preshaped filamentary abrasive particles (18) are of circular cross-section, each said filamentary particle having a diameter of preferably approximately 0.33 mm. Coated abrasive material (10) according to one of the preceding claims wherein the surface of the backing member (12) receives from 40 to 60% of the theoretical weight of the preshaped filamentary abrasive particles (18) required to form a closed coat. Coated abrasive material (10) according to one of the preceding claims in which a plurality of layers of abrasive are used and at least the last-applied layer comprises the preshaped filamentary abrasives particles (18). Coated abrasive material (10) according to one of the preceding claims in which the layer of coated abrasive particles (14) comprises up to 40%, based on the weight of the abrasive present, of non-filamentary abrasive particles. Coated abrasive material (10) according to one of the preceding claims in which at least a portion of the preshaped filamentary abrasive particles (18) when in use, have a rake angle with respect to a work piece of close to zero. Coated abrasive material (10) according to one of the preceding claims in which at least a portion of the preshaped filamentary abrasive particles (18) are in groups in which the component filaments lay against and buttress one another. Coated abrasive material (10) according to one of the preceding claims in which a layer comprising a grinding aid is applied to the coated abrasive layer. Coated abrasive material (10) according to one of the preceding claims wherein a size coat (20) is applied after application of the abrasive particles and the maker and size coats (16,20) comprise a phenolic resin. Coated abrasive material (10) according to one of the preceding claims wherein an abrasive product is a disc and the backing member is vulcanized fiber. Coated abrasive material according to one of the preceding claims wherein the preshaped filamentary abrasive particles (18) have an average aspect ratio of from 2:1 to 8:1.

Inventors: KELLY ROBERT G, KELLY, ROBERT G.
Applicants: NORTON CO, NORTON COMPANY
Application year: 1990
Main classification: C09K3
IPC codes: C09K3, B24D3, B24D11