Coated Abrasive Material Containing Abrasive Filaments

This invention is directed to coated abrasive products in which the coated abrasive contains polycrystalline abrasive filaments.

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.sub.2. The dried material is crushed either before or after sintering to obtain irregular 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. Pat. No. 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.sub.2 O.sub.3 crystals which are on the order of 5 to 10 microns 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. Pat. No. 4,623,364, which issued on Nov. 18, 1986 assigned to Norton Company, the assignee of this application, 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 micron 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.

The present invention provides a coated abrasive with a long useful life and which require less applied power for a given cutting rate using a special form of abrasive grain.

The coated abrasives of the invention are freer cutting and generate less heat during grinding.

The coated abrasives of the invention are also found 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 product in which the abrasive grain comprises sintered sol gel alpha alumina filaments having substantially consistent cross-sectional shapes along their lengths. The filaments have an aspect ratio on the average of at least about 1:1, preferably at least about 2:1 to about 9:1 and can be up to 12:1 or more. The preferred filamentary particles comprise alpha alumina crystals up to about 5 microns in size. The alpha alumina filaments may contain additives such as MgO or other spinel alumina formers, ZrO.sub.2, or other compatible materials. They are preferably formed by the sol gel route and preferably formed from a seeded sol gel.

The filaments preferably comprise at least 95% and more preferably at least 98% of alpha alumina crystallites and are essentially free of impurities forming amorphous or "glassy" material upon firing. The crystallites are up to about 2 microns in size, but the preferred filaments have crystallites that are below about 1 micron and most preferably below 0.5 micron in size.

The abrasive filaments are attached to a flexible backing proximate one end of the filament by an adhesive maker coat. The filaments generally extend 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 o 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 at least about 1:1, preferably about 2:1 to about 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.sub.4, K.sub.2 TiF.sub.6, NaCl, sulfur and the like.

The foregoing and other objects 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. 100.times. photograph of the surface of a conventional crushed seeded sol-gel grain product with a size coat.

FIG. 11 is a 50.times. S.E.M. photograph of the swarf from use of a coated product such as that shown in FIG. 9 (product of invention).

FIG. 12 is a 50.times. S.E.M. photograph of the swarf from use of the coated product shown in FIG. 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-sectional shape along its length and wherein the length is greater than, and preferably 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 preferably 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 1,500.degree. C.

Various sol gel methods of preparation of gels of hydrated alumina are described in, for example, U.S. Pat. Nos. 4,314,827, and 4,623,364. In addition to the hydrated alumina, as disclosed in said patents, the sol may include up to 10-15% by weight of spinel, mullite, manganese dioxide, titania, magnesia, ceria or zirconia in the form of a powder. A precursor of such components may be present can be also added in a larger amount, e.g., 40% or more, or other compatible additives or precursors thereof. It is, however, preferred that the gel be substantially free of material that forms an amorphous glassy phase on firing. The fired sol gel grain used in the product of the invention should therefore preferably be at least 95% and more preferably at least 98% by wt. alpha alumina. In a most preferred embodiment, 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 about 5%. If the seed is adequately fine, amounts of from about 0.5% to 10% may be use 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 1,000 to 1,100.degree. 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 about 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 about 100 microns in diameter. Fired filaments as fine as 0.1 micron (0.001 mm) have been made by spinning in accordance with the invention. The green filaments shrink about 50% in diameter from their extruded diameter upon drying and firing.

Gels most suitable for extrusion should have a solids content of between about 30% to about 65% and preferably between about 45% and 64%. The optimum solids content varies directly with the diameter of the filament being extruded, with about 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 of 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 they 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.degree. C. for 24-72 hours before subjecting them to any heating above 100.degree. C. If 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 mush 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 1,200.degree. C. to 1,350.degree. C. for between one hour and 5 minutes, respectively. For coarser filaments the firing is preferably preceded by drying at about 400-600.degree. 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 uniformity 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 about 1:1, preferably at least about 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 preferably have an aspect ratio of at least about 2:1.

Preferably, the aspect ratio, on average, will range from about 2 to about 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. In some instances, at least, the abrasive filaments used in the practice of the invention may have a twist in their lengthwise dimension, or be somewhat curved or bent.

The coated abrasive products of the present invention are for 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 as well 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.degree. 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.degree. 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 shaped 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. 6(c) shows a lot of abrasive filaments 18 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 a 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.

FIGS. 9 and 10 show clearly the different physical appearance of the surface of the product of the invention and that of the prior art.

FIGS. 11 and 12 show the swarf produced by the structures shown in FIGS. 9 and 10. It should be noted that the product of the invention produces a clean continuous swarf, evidencing excellent cutting. FIG. 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 an 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 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 FIGS. 11 and 12 for example). Profile measurements also show that peak density (peaks/inch) decreases with 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 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 FIGS. 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. FIG. 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 the 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.

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.

In this example, coated abrasive discs comprising seeded gel ("S.G.") aluminum oxide abrasive filaments having an average diameter (0.031") approximately equal to 50 Grit CAMI grade (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 micron (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 (7" dia., 7/8" central opening) were manufactured using conventional coated abrasive manufacturing procedures, conventional 0.030" vulcanized fibre 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 225.degree. F. followed by a final cure, following size coat application, for 10 hours at 225.degree. F. followed by a final cure, following size coat application, for 10 hours at 225.degree. 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 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:

The roll crushed abrasive grain (control product, Disc No.1) was of the following standard composition:

Other properties: density 3.89 (water Pyc), 3.88 (helium Pyc); hardness 20.7 (GPa); and avg. crystallite size 0.17 microns.

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 applied.

The cured discs, after being cut to size, were first conventionally 0.degree.-90.degree. 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 turn, 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 1".times.1".times.93/4"-1/8" angle or a 3".times.3/16".times.15" long flat plate and moves horizontally in a direction 90.degree. 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.degree. from parallel to the specimen holder.

Test 112 Dsh--is a high pressure (10 lbs force 50 grit grain, 12 lbs.--36 grit) dulling test wherein the fibre disc is applied to grind the 1/8" thick edge of a 1".times.1".times.93/4"-1,018 carbon steel construction angle (1/8"). The angle-shaped work piece is first weighed and then is mounted on the specimen holder which reciprocates back and forth over a distance of 93/4 inches, at a rate of 81/2 strokes/min. and a linear speed of 7 ft./min. The abrasive disc is driven at 3,450 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 7 lbs. for 50 grit and 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 3".times.3/16".times.15" long cold drawn carbon steel plate and such is adapted in the mount so that the fiber disc grinds the 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.

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.

It is clearly evident from the above results that on 1,018 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 i 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 1,018 carbon steel at low pressures.

The fibrous abrasive was made by mixing 3.2 kg Pural.RTM. 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 cc distilled water was added and mixing continued for an additional five minutes for 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.sup.2 /g.

The Pural.RTM. NG 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 1,320.degree. 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 microns, as above mentioned, and a hardness of about 16 Gpa.

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 (0.013") approximately equal to 50 grit abrasive grains (0.01369) and was 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.

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 15 lbs./Rm; size coat 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:

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 out performed 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 i the following Table 5. This test is the same as Test DsIII except that the test piece has a 1" face.

As can be seen from the above results, the 50 grit abrasive fibers not only out-performed 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 1" steel test, the fiber-shaped abrasive particles generated a finish similar to the 36 grit control.

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.

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:

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 1,018 in which the aspect ratio averaged 8.5:1 still out performed 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.

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:

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 microns.

This abrasive material was extruded at 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, 17 lbs./Rm; size coat, 28 lbs./Rm; abrasive grain/fibers, 60 lbs./Rm.

The grading of the abrasive materials used and the resulting cut performance, as before described, are shown in Table 8.

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, such filament-shaped abrasives need not necessarily have a straight configuration. They can be twisted along their lengthwise direction, or otherwise non-linear, if desired. 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 conventional crushed abrasive grains of other compositions, e.g. fused Al.sub.2 O.sub.3, garnet, etc. Coated abrasive material can be 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.

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 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, a 1".times.21/8".times.93/4" 1018 steel bar. The bar is presented to the disc so as to abrade the 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 10 below. The "Comparative" tested is a standard commercial seeded sol gel abrasive seed with blocky grains produced by a crushing and grading procedure.

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 unseeded product does not perform very well at all.

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 micron in size. The S.G. grains of 80 grit size and the product of the invention used qrits 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 affirmed were as follows:

It is clear, therefore, that grits with other cross-sections than the round shapes described above are effective in the products of the invention.

This Example shows the effect of the amount of grain in 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 7.3 oz. polyester sateen, and the make and size adhesives were made with the following formulations:

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 AR3.1 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 0.013", which is the approximate size of 50 grit, they outperformed 36 grit by as much as by 2.5 times on the 122-Ds (80) test and 2 times on the Fixed Feed test. The 122 Ds (80) test is a high pressure plunge grind test in which a 1 inch steel bar is plunged into the belt surface under a constant 80 lb. force for three seconds, followed by a thirty second cool cycle. The test is ended when the cut per cycle is down to 0.030 linear inches.

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.