CA2115889A1

CA2115889A1 - Coated abrasive article having diluent particles and shaped abrasive particles

Info

Publication number: CA2115889A1
Application number: CA002115889A
Authority: CA
Inventors: David E. Broberg
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1993-03-18
Filing date: 1994-02-17
Publication date: 1994-09-19
Also published as: JPH06297337A; EP0615816B1; US5496386A; EP0615816A1; BR9401140A; DE69410663T2; US5584896A; DE69410663D1; KR940021191A

Abstract

ABSTRACT OF THE DISCLOSURE
Coated abrasive articles containing particles having specified shapes. The coated abrasive article comprises:
a. a backing;
b. at least one binder;
c. an abrasive coating comprising shaped abrasive particles and diluent particles said at least one binder serving to bond the abrasive coating to the backing. The diluent particles can comprise (1) a plurality of individual abrasive particles bonded together by an adhesive to form an agglomerate, (2) a plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (3) a plurality of individual abrasive particles and a plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (4) individual non-abrasive particles, (5) individual abrasive particles, or (6) combinations of the foregoing.

Description

' - ' 49528CANlA

COAT~D A~RASIVE ARTICLE HAVING ~ILUENT PARTIÇL~S
AND ~APED A~RASI~ ~ARTI~LES
Backqround of the Invent~on 1. Field o~ thq-lnyQ~lQn Thi~ invention relates to coated abrasive articles, and, more particularly, to coatQd abrasive 10 ar~icle~ containing particles having specified shapes.

2. Discussion_o~ the Ar~
Ithree ba ic technologiQs that have been employed to produce abrasive grains having a specified shape are ~1) fusion, t2) sintering, and (3) chemical ceramic.
In the fusion process, abrasive grain~ can be shaped by a chill roll, the face of which may or may not be engraved, a mold into which molten material i8 poured, or a heat sink material immersed in an alumlnum 20 oxide melt. U.S. Patent No. 3,377,660 discloses a process comprising thQ ~teps of ~lowing molten abrasive material ~rom a furnaca onto a cool rotating casting cy~inder, rapidly solidify~ng the material to form a thin semisolid curved sheet, densifying the sem~solld 25 material with a pressure roll, and then partially fracturing the strip o~ semisolid material by reversing its curvature by pulling it away from the cylinder with a rapidly driven cooled conveyor, whereupon the partially fractured strip i8t deposited onto a collector 30 in tbe form o~ large fragments, which, upon being rapidly cooled and solidi~ied, break up into smaller fragments capable o~ being reduced in size to form conventional abrasive grains. U.S. Patent Nos.
4,073,096 and 4,194,887 disclose a process compri~lng -~
35 the steps of ~1) fusing an abrasive mix in an electric arc furnace, ~2) dipping a relatively cold sub~trate ~ into the molten material, whereby a layer of solid - abrasive material i~ qulckly frozen (or plated) on the ~-J

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substrate, (3) withdrawing the plated substrate from the molten material, and (4) bxeaking the solidified abrasive material away from the substrate and collecting it for further processing to proquce 5 abrasive grains.
In the sintering process, abrasive grain~ can be formed from refractory powders having a particle size of up to 10 micrometers in diameter. Binders can be added to the powders along with a lubricant and a 10 suitable solvent, e.g., water. The resulting mixtures, pastes, or slurries can be shaped into platelets or rods of various lengths and diameters. The resulting haped grains must be fired at high temperatures, e.g., 1,400C to 1,800C, at high pressures, or for long soak 15 times, e.g., up to lO hours. Crystal 6ize may range from under one micrometer up to 25 micrometers. To obtain shorter residence t~mes and/or smaller crystal size, either the pressure or temperature must be increased. U.S. Patent No. 3,079,242 discloses a 20 method of making abrasive grains from calcined bauxite material comprising the steps of (1) reducing the material to a fine powder, (2) compacting under affirmative pressure and forming the fine particles of said powder into grain sized agglomerations, and (3) 2S sintering the agglomerations of particles at a temperature below the fusion temperature of the bauxite to induce limited recrystallization of the particles, whereby abrasive yrains are produced directly to size.
U.S. Patent No. 4,252,544 discloses alumina abrasive 30 grains produced by sintering wherein the grain structure is constructed of alumina coarse crystal particles and alumina fine crystal particles lo~ated between the alumina coarse crystal particles. U.S.
Patent No. 3,491,492 discloses a process for making an 35 aluminous abrasive grain formed from bauxite or mixtures of bauxite and Bayer process alumina wherein the comminuted aluminous material is mixed with water 21~3889 , - 3 -and ferric ammonium citrate, or with ferric ammonium citrate and citric acid, and reduced to a state of fine subdivision by milling to give a fluid slurry of high solid content, drying said slurry to coherent cakes 5 having a thickness equal to one dimension of the final grain before sintering, breakin~ said cakes to grains, screening, optionally rounding said grains by air mulling, screening, sintering, cooling, and screening to yield the final product. U.S. Patent No. 3,637,630 10 discloses a process in which the same type of slurry disclosed in U.S. Patent No. 3,491,492 is plated or coated on a rotating anode of an electrophoretic cell.
The plated aluminous material is removed from the rotating anode, dried, broken to granules, screened, 15 sintered, and screened to final size.
Chemical ceramic technology involves converting a colloidal dispersion or hydrosol (sometimes called a sol), optionally in a mixture with solutions of other metal oxide precursors, to a gel or any other physical 20 state that restrains the mobility of the components, drying, and firing to obtain a ceramic material. A sol can be prepared by any of several methods, including precipitation of a metal hydroxide from an agueous solution followed by peptization, dialysis of anions 25 from a solutisn of metal salt, solvent extraction of an anion from a solution of a metal salt, hydrothermal decomposition of a solution of a metal salt having a volatile anion. The sol optionally contains metal oxide or precursor thereof and is transformed to a 30 semi-rigid solid state of limited mobility such as a gel by, e.g., partial extraction of the solvent, e.g., water. Chemical ceramic technology has been employed to produce ceramic materials such as fibers, filme, flakes, and microspheres. U.S. Patent No. 4,314,827 35 discloses synthetic, non-fused aluminum oxide based abrasive mineral having a microcrystalline structure of randomly oriented crystallites comprising a dominant 21 ~ 9 _ continuous phase of alpha alumina and a secondary phase. U.S. Patent No. 4,744,802 discloses an abrasive grain made by a chemical ceramic process that employs an iron oxide nucleating agent to enhance the 5 transformation to alpha alumina. This patent al~o suggests that the ~el can be shaped by any convenient ~ ~-method such as pressing, molding, or extruding. U.S.
Patent No. 4,84~,041 discloses a shaped abrasive grain made by a chemical ceramic process in which the lo abrasive grain has a mean particle volume ratio of less than 0.8.
Summary of the I~v ntion This invention provides coated abrasive articles containing both abrasive particles having specified 15 shapes and diluent particles.
The coated abrasive article comprises:
a. a backing;
b. at least one binder;
c. an abrasive coating comprising shaped 20 abrasive particles and diluent particles, said at least one binder serving to ~ond the abrasive coating to the backing. The diluent particles can comprise ~l) a plurality of individual abrasive particles bonded together by an adhesive to form an agglomerate, (2) a 25 plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (3) a plurality of individual abrasive particles and a plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (4) 30 individual non-abrasive particles, (5) individual abrasive particles, or (6) combinations of the foregoing. More than one class of diluent particles can be used in the coated abrasive article. The individual abrasive particles and the agglomerates 35 containing individual abrasive particles have irregular shapes or random shapes, i.e., they are excluded from the 8cope of the shaped abrasive particles. However, , ~ 2 1 ~

individual non-abrasive particles and agglomerates containing individual non-abrasive particles, but free from individual abrasive particles, can have shapes that are equivalent to those of the shaped abrasive 5 particles.
The abrasive coating can take on several types of configurations. However, most of the configurations fall into four major categories:
~ 1) open coat of diluent particles and open coat 10 of shaped abrasive particles;
(2) open coat of diluent particles and closed coat of shaped abrasive particles;

(3) closed coat of diluent particles and open coat of shaped abrasive particles; and (4) closed coat of diluent particles and closed coat of shaped abrasive particles.

In the first two configurations, the shaped abrasive particles reside substantially between the diluent 20 particles. In the latter two configurations, the shaped abrasive particles reside substantially between and above the diluent particles.
In one aspect of the invention, the diluent particles are coated first and then the shaped abrasive 25 particles are coated nex~. The shaped abrasive particles are preferably coated in an electrostatic field. The electrostatic field lines concentrate at the corners and along the edges of the shaped abrasive particles, and the shaped particles orient in the 30 electrostatic field in such a way that they are ~
depoæited onto the binder or diluent particles on their --thinnest edges, thereby allowing thin edges of the shaped particles to be in contact with the workpiece during abrading operations. For particles having 35 triangular-shaped faces, about 35% to about 65% of the particles are typically oriented with a ~ertex pointing away from the backing and a base in contact with the ~ .,',~"A; .. :.:-. '.', '.~. ;.' .r:;~,; ;'.V,'h ' ~ iv ~ " ~ ~;

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binder or diluent particles, with the remainder being oriented with a base pointing away from the backing and a vertex in contact with the binder or diluent particles.
In another aspect of the invention, the diluent particles and the ~haped abrasive particles are fir~t blended and then coated at the same time.
One method for preparing such shaped abrasive particles comprises the stepR of:
(a) providing a dispersion comprising particle6 that can be converted into alpha alumina, preferably particles of alpha alumina monohydrate, in a liquid, which liquid comprises a volatile component;
(b~ providing a mold having a first generally 15 planar surface and a second surface opposed to said first surface, said first surface having an opening to a mold cavity having a specified shape;
(c) introducing said dispersion into said mold cavity, preferably such that no exposed surface of said 20 disperslon extends substantially beyond the plane of said first surface df said mold;
(d) removing a sufficient portion of said volatile component of said liquid from said dispersion while said dispersion is in said mold cavity, thereby 25 forming a precursor of an abrasive particle having a shape approximately corresponding to the shape of said mold cavity;
(e) removing said precursor of the abrasive particle from said mold cavity;
(f) calcining said removed precursor of the abrasive particle; and (g) sintering said calcined precursor to form the desired abrasive particle.
In one variation of the process for making shaped 35 abrasive particles, after the dispersion is formed, it is gelled prior to being introduced into the mold cavity. As used herein, the term "to gel" means to 2'~ 7i~ 9 increase the viscosity of a substance sufficiently so that it will not flow from an inverted test tube. In a second variation, the dispersion is introduced into the mold cavity under a pressure of less than 100 psi. In 5 a third variation, at least one side of the mold, i.e., the side in which the cavity i8 formed, i6 exposed to the atmosphere surrounding the mold during the step in which the volatile component is removed. In a fourth variation, the volatile component of the dispersion is 10 removed from the dispersion while the dispersion is in the mold without ~he application of additional heat or pressure. In a fifth variation, the volatile component of the dispersion is removed from the dispersion by evaporation while the dispersion is in the mold. In a 15 sixth variation, an additional drying step i8 utilized after the precursor of the abrasive particle is removed frsm the mold.
Preferably, the mold contains a plurality of cavities, more preferably at least twenty cavities.
20 Preferably, the shapes of the cavities correspond approximately to the desired shapes of the abrasive particles. It is also preferred that the cavities are of equal size and shape.
Another method for preparing shaped abrasive 25 particles comprises the steps of:
(a) providing a dispersion comprising particles that can be converted into alpha alumina, preferably -~
particles of alpha alumina monohydrate, in a liquid, which liquid comprises a volatile component;
(b) extruding the dispersion through an orifice of a die, thereby forming an elongated precursor of an abrasive particle, said precursor having a cross-~ection substantially similar to that of the orifice of the die;
(c) removing a sufficient portion of the volatile component of the liquid from the elongated precursor of the abrasive particle such that the precursor is 3 ~ ~ rJ

sufficiently dry so as to be capable of maintaining it3 elongated shape and cross-section;
(d) converting the dried precursor of the abrasive particle to the desired lenqth;
(e) calcining said dried precursor of the abrasive particle; and (f) sintering said calcined precursor to form the desired abrasive particle.
The shaped abrasive particles can have shapes that 10 can be characterized as thin bodies having faces of triangular, rectangular, including square, circular, or other geometric shape. It is preferred that the geometric shape of the faces of the shaped abrasive partiales be triangular. The shaped abrasive particles 15 preferably have a front face and a back face, both of which faces have substantially the same geometric shape. The faces are separated by the thickness of the particle. The ratio of the length of the shortest facial dimension of an abrasive particle to its 20 thickness is at least 1 to 1, preferably at least 2 to 1, more preferably at least 5 to 1, and most preferably at least 6 to 1. The shaped abrasive particles can have shapes that can be characterized as rods. For example, the rods can be cylindrical or prismatic in 25 shape, with the ratio of length to maximum cross-sectional dimension being at least 1 to 1, preferably 2 to 1, and most preferably at least 3 to 1.
:~ .
Brief Description of ~h~_prawings FIG. 1 i~ a top view of a mold suitable for preparing shaped abrasive particles.
FIG. 2 is a perspective view of a mold suitable preparing shaped abrasive particles.
FIG. 3 is an enlarged sectional view of a fragment 35 of a coated abrasive article of this invention thatemploys shaped abrasive particles.
FIG. 4 is a photomicrograph taken at 12X

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, g illustrating shaped abrasive particles in which the planar shape is triangular.
FIG. 5 is a photomicrograph taken at 12X
illustrating shaped abrasive particles in which the 5 planar shape is rectangular.
FIG. 6 is a photomicrograph taken at 12X
illustrating shaped abrasive particles in which the planar shape is circular.
FIG. 7 is an enlarged sectional view of a fragment 10 of another embodiment of a coated abrasive article of this invention that employs shaped abrasive particles.
FIG. 8 is a side view of an apparatus for preparing abrasive particles that can be used in this invention.
FIG. 9 is a schematic perspective view of a die that can be used in the apparatus of FIG. 8.
FIG. 10 is a sectionaI view of the auger and bore of the die body of FIG. 9.

Detailed Description As used herein, the term "dispersion" means the unsolidified, undried composition comprising particles that can be converted into alpha alumina in a flowable binder precursor. The dispersion is used for preparing 25 shaped abrasive particles. Shaping can be accomplished by molding or by extruding and sizing. After the dispersion is shaped, sufficient volati~e component i5 3 removed therefrom to bring about solidification of the resultant shaped dispersion . The term "precursor of 30 ~haped abrasive particle" means the unsintered particle , produced by removing a sufficient amount of the volatile component from the dispersion so as to form a solidified body having a shape. If the shaping is effected by means of molding, the precursor of the 35 shaped particle will have a shape corresponding approximately to the shape of the mold cavity in which it was prepared. If the shaping is effected by means .
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of extrusion through the orifice of a die, the precursor of the shaped particle will have a cross-section corresponding approximately to the shape of the orifice of the extrusion die. After shaping and 5 removal of volatile component, the resulting shaped particle will be sintered, whereby density of the particle will have a value of at least 90~ of theoretical density. The term "shaped abrasive particle" means a sintered particle, which can be 10 produced by the process of assignee's copending applications U.S. Serial Nos. 07/919,179, filed July 23, 1992, and 07/918,360, filed July 23, 1992, both of which are incorporated herein by reference.
The first step of a preferred process for mak~ng 15 shaped abrasive particles involves providing a dispersion comprising a liquid containing particles that can be converted into alpha alumina, which liquid comprises a volatile component, preferably water. The dispersion should comprise a sufficient amount of 20 liquid to cause the viscosity of the dispersion to be sufficiently low to ensure ease of introduction into the mold cavity or through an extrusion die, but not so -~
much liquid as to cause subsequent removal of the liquid to be prohibitively expensive. The dispersion 25 preferably comprises from about 2 to about 90% by weight of the particles that can be converted into alpha alumina, preferably particles of alpha aluminum oxide monohydrate (boehmite), and at least 10% by weight, preferably from 50 to 70%, more preferably 50 30 to 60%, by weight, volatile component, preferably water. Conversely, the dispersion preferably contains from 30 to 50%, more preferably 40 to 50%, by weight solids. If the percentage of liquid is too high, too many cracks can develop in the resulting particles upon 35 drying thereof. If the percentage of liquid is too low, pumping of the dispersion into the mold may be difficult. Aluminum oxide hydrates other than boehmite 21.i~333 can also be used. Boehmite can be prepared by known techniques or can be obtained commercially. Examples of commercially available boehmite include products having the trademarks ~DISPERAL", available from Condea 5 Chemie, GMBH and "DISPAL", available from Vista Chemical Company. These aluminum oxide monohydrates are in the alpha form, are relatively pure, i.e., they include relatively little, if any, hydrate phases other than monohydrates, and have a high surface area. The 10 physical properties of the shaped abrasive particles will generally depend upon the type of material used in the dispersion.
It is preferred that the dispersion be in a gel state. As used herein, "a gel" is a three dimensional 15 network of solids dispersed in a liquid. A gel will not flow from an inverted test tube.
The dispersion may contain a modifying additive or ~-~
precursor of a modifying additive. The modifying additive can function to enhance some desirable 20 property of the abrasive particles or increase the effectiveness of the subsequent sintering step.
Modifying additives or precursors of modifying additives can be in the form of soluble salts, typically water soluble ~alts. They typically consist 25 of a metal-containing compound and can be a precursor of oxide of magnesium, zinc, iron, silicon, cobalt, nickel, zirconium, hafnium, chromium, yttrium, praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium, 30 titanium, and mixtures thereof. The particular concentrations of these additives that can be present in the dispersion is not critical and can be varied on the basis of convenience. Typically, the introduction of a modifying additive or precursor of a modifying 35 additive will cause the dispersion to gel. The dispersion can also be induced to gel by application of heat over a period of time.

2ll~8g3 The dispersion can also contain a nucleating agent to enhance the transformation of hydrated or calcined aluminum oxide to alpha alumina. Nucleating agents suitable for this invention include fine particles of 5 alpha alumina, alpha ferric oxide or its precursor, titanium oxides and titanates, chrome oxides, or any other material that will nucleate the transformation.
The amount of nucleating agent, if used, should be sufficient to effect the transformation of alpha 10 alumina. Nucleating such dispersions is disclosed in U.S. Patent Nos. 4,744,802 and 4,964,883, both of which are incorporated hereinafter by reference.
A peptizing agent can be added to the dispersion to produce a more stable hydrosol or colloidal 15 dispersion. Peptizing agents preferred for this invention are monoprotic acids or acid compounds such as acetic acid, hydrochloric acid, formic acid, and nitric acid, with nitric acid being preferred.
Multiprotic acids are less preferred as peptizing 20 agents because they rapidly yel the dispersion, making it difficult to handIe or to introduce additional components thereto. Some commercial sources of boehmite contain an acid titer (such as absorbed formic or nitric acid) that will assist in forming a stable 25 dispersion.
The dispersion can be formed by any suitable means, such as, for example, simply by mixing aluminum oxide monohydrate with water containing a peptizing agent or by forming an aluminum oxide monohydrate 30 slurry to which the peptizing agent is added.
The second step of the process of making shaped ! abrasive particles involves providing a mold having at least one cavity, preferably a plurality of cavities.
Referring to FIG. i, a mold 10 has a generally planar 35 surface 12 and a plurality of cavities 14. Mold 10 can be made from a rigid material, such as metal, e.g., steel. It is preferred that mold 10 be made from a 2115~8~

relatively thin aluminum or ~tainless steel sheet or belt, e.g., having a thickness of less than 5 cm, preferably less than 2 cm. Referring to FIG. 2, access to cavities 14 of mold 10 can be from an opening 15 in 5 first or top surface 16 of mold 10, from an opening tnot shown) in second or bottom surface 18 of mold 10, or from openings in both surfaces of mold 10. In some instances, cavities 14 can extend for the entire thickness of mold 10. Alternatively, cavities 14 can 10 extend only for a portion of the thickness of mold 10.
It is preferred that top surface 16 of mold 10 be substantially parallel to bottom surface 18 of mold 10.
At least one side of mold 10, i.e., the side in which the cavity is formed, can remain exposed to the 15 surrounding atmosphere during the step in which the volatile component is removed. If the cavities extend completely through the mold, both surfaces of the mold should be generally planar. As used herein, the term "planar" includes any two-dimensional surface.
20 However, it is preferred that the planar surfaces be flat or level.
The cavities 14 have a ~pecified three-dimensional shape. The preferred shape of a cavity can be described as being a triangle having a dimension of 25 depth. However, other shapes can be used, such as, circles, rectangles, squares, or combinations thereof, all having a dimension of depth. The dimension of depth is equal to the perpendicular distance from the surface 12 to the lowermost point of cavity 14. In 30 addition, a cavity can have the inverse of other geometric shapes, such as, for example, pyramidal, frusto-pyramidal, truncated spherical, truncated spheroidal, conical, and frusto-conical. There are preferably at least 20 cavities per mold, ~ore 35 preferably at least 100 cavities per mold. The depth of a given cavity can be uniform or can vary along its length and/or width. The cavities of a given mold can 2 1 ~ 9 be of the same shape or of different shapes.
It is preferred that the dimensions of cavities 14 approximately correspond to the desired dimensions of the shaped abrasive particles, taking expected 5 shrinkage into account. Accordingly, it will not be necessary to crush, break, or cut the shaped abrasive particles to reduce their size. Likewise, after the shaped abrasive particles are made by the process described herein, it is not necessary to screen them to 10 an appropriate particle size. Moreover, the size of the shaped abrasive particles will essentially remain constant between different lots, thereby assuring a ! very consistent particle size and distribution of particle sizes from lot to lot.
The third step of the process of making shaped abrasive particles involves introducing the dispersion into cavities 14 by any conventional technique. It is preferred to flood surface 12 of mold 10 with the dispersion. The dispersion can be pumped onto surface 20 12 of mold 10. Next, a scraper or leveler bar can be used to force some of the dispersion into cavities 14 of mold 10. The remaining portion of the dispersion that does not enter cavities 14 can be removed from ~-surface 12 of mold 10 and recycled. Although a small 25 portion of the dispersion can still be allowed to remain on surface 12 of mold 10, this is not preferred.
The pressure applied by the scraper or leveler bar is ~ typically less than 100 psi, preferably less than 50 3 psi, and most preferably less than 10 psi.
30 Furthermore, no exposed surface of the dispersion should extend substantially beyond the planes formed by the planar surfaces of the mold to ensure uniformity in i thickness of the shaped abrasive particles. It is also preferred that the planar surface of the mold 35 surrounding the cavities be substantially free of the dispersion.
It is preferred that a release coating be applied . ~

2~ 1J~9 to surface 12 of mold 10 and on the surfaces of cavities 14 prior to the introduction of the dispersion into cavities 14. The function of the release coating is to allow ease of removal of the precursors of the 5 shaped abrasive particles. Typical materials for preparing release coatings are silicones and polytetrafluoroethylene.
The fourth step of the process of making shaped ~
abrasive particles involves removing a portion of the -10 liquid, i.e., the volatile component thereof, from the dispersion while the dispersion is in the mold cavity, thereby resulting in an increase in the viscosity of the dispersion. It is preferred that the volatile component be removed by evaporation rather than by an lS external force such as filtration. Removal of liquid by evaporation can occur at room temperature or at elevated temperatures. The elevated temperatures can range from about 40C to about 300C. However, at higher temperatures, high drying rates are obtained 20 that may produce undesirable cracks in the resulting abrasive particle. ~hen water is the volatile component, it is preferred to heat the mold containing the dispersion at a temperature of from about 50C to about 80C for from about 10 to about 30 minutes in a 25 forced air oven. A sufficient amount of the volatile component must be re~oved from the dispersion to bring about solidification thereof, thereby forming a precursor of a shaped abrasive particle having approximately the same shape as the shape of the mold 30 cavity. It is preferred that a sufficient amount of volatile component be removed from the dispersion 80 that the precursors of the shaped abrasive particles can be easily removed from the cavities of the mold.
Typically, up to 40% of the liquid is removed from the 35 dispersion in this step.
The fifth step of the process of making shaped abrasive particles involves removing the precursors of the shaped abrasive particle from the mold cavities.
This step is made possible by shrinkagQ of the dispersion, during formation of the precursors of the shaped abrasive particles, when the liquid is removed 5 therefrom. For example, it is not uncommon for the volume of the precursor of the shaped abrasive particle to be 80% or less of that of the shaped dispersion from which it was formed. The precursors of the shaped abrasive particles can be removed from the cavities 10 either by gravity or by applying a low pressure, e.g., rotating brush, to force them out of the cavities.
The removed precursors of the shaped abrasive particles have approximately the same shape as the cavities of the mold from which they were formed.
15 Exact replication is unlikely for three reasons.
First, the dispersion will shrink, so the precursors of the shaped abrasive particles will be smaller than the cavities in which they are formed. Second, when the precursors of the shaped abrasive particles are removed 20 from the mold cavities, some of their edges may break off or become rounded. Third, when the dispersion is introduced in the cavities, the dispersion may not completely fill the cavities. It should be noted that care should be taken throughout the process to minimize 25 the forego~ng factors.
The precursors of the shaped abrasive particles can be further dried outside of the mold. If the dispersion is dried to the desired level in the mold, this additional drying step is not necessary. However, 30 in some instances it may be economical to employ this additional drying step to minimize the time that the dispersion resides in the mold. During this additional drying step, care must be taken to prevent cracks from forming in the precursors of the shaped abrasive 35 particles. Typically, when water is the volatile component, the precursors of the shaped abrasive particles will be dried for from about 10 to about 480 21i~8~3 minutes, preferably from about 120 to about 400 minutes, at a temperature from about 50C to about 160C, preferably from about 120C to about 150C.
The sixth step of the process of making shaped -~
5 abrasive particles involves calcining the precursors of the shaped abrasive particles. During calcining, essentially all the volatile material is removed, and the various components that were present in the dispersion are transformed into metal oxides. The 10 precursors of the shaped abrasive particle are generally heated to a temperature of from about 400~C
to about 800C, and maintained within this temperature range until the free water and over 90% by weight of any bound volatile material are removed. In an 15 optional step, it may be desired to introduce the modifying additive by an impregnation process. A
water-soluble salt can be introduced by impregnation into the pores of the ~alcined precursors of the shaped abrasive particles. Then the precursors of the 6haped 20 abrasive particles are prefired again. This option is further described in European Patent Application No.
293,163, incorporated herein by reference.
The seventh step of the process of making shaped abrasive particles involves sintering the precursors of 25 the shaped abrasive particles to form the shaped abrasive particles. Prior to sintering, the precursors of the shaped abrasive particles are not completely densified and thus lack the hardness to be used as shaped abrasive particles of this invention. Sintering 30 takes place by heating the precursors of the shaped abrasive particle to a temperature of from about 1,000C to about 1,650C and maintaining them within this temperature range until substantially all of the alpha alumina monohydrate (or equivalent) is converted 35 to alpha alumina and porosity is reduced to less than 15% by volume. The length of time to which the precursors o~ the shaped abrasive particles must be - ~ -- 18 --exposed to the sintering temperature to achieve this level of conversion depends upon various factors but usually from about five seconds to about 48 hours is typical. The preferred duration for ~intering ranges 5 from about one minute to about 90 minutes.
Other &teps can be used to modify the process of this invention, such as rapidly heating the material from the calcining temperature to the sinterinq temperature, centrifuging the dispersion to remove 10 sludge, waste, etc. Moreover, this process can be modified by combining two or more of the process steps, if desired. Conventional process steps that can be used to modify the process of this invention are more fully described in U.S. Patent No. 4,314,827, 15 incorporated herein by reference.
As shown in FIG. 8, a continuous process can be used to make the shaped abrasive particles that can be used in this invention. The apparatus 60 in FIG. 8 comprises a mold 62, a driving mechanism 64, a die body 20 66, leading-edge wiper blades 68, levelling doctor blades 70, an oven 72, a collecting pan 74, and a brush 76. Referring now to FIG. 9, an extrudable dispersion containing particles "P" of a material that can be converted into alpha alumina ~hereinafter "convertible 25 material") in liquid is provided to supply means 80 for delivery to die body 66. Typical supply means can comprise a combination kneader and extruder 82, which includes twin, counter-rotating mixing blades that mix and pack the convertible material into an auger channel 30 84 for delivery through exit port 86 by a supply auger 88. Mixing and packing the convertible material aids in preventing voids that may produce a nonunifor~
sheet. The exit port 86 is connected to a pump 90, which pressurizes the convertible material and supplies 35 it to a feed port 92 of die body 66.
Die body 66 includes a longitudinal bore 100 therein having first and second ends 102 and 104, 2 1 ~ 9 -: 19 .:
respectively. Feed port 92 communicates the exterior of die body 66 with bore 80 ad~acent second end 104.
An auger 106 having first and second ends 108 and 110, respectively, is disposed within bore 100. Auger 106 5 comprises a lonqitudinal root and a helical flight adjoining the root along the length thereof. The flight diameter of auger 106 is constant, and the root has a first diameter at the first end 108, and a second diameter smaller than the first diameter at the second 10 end 110. The flight depth of auger 106 is therefore greatest near feed port 92, and gradually decreases toward the first end 108 of auger 106, although the overall flight diameter is constant. The material conveying capacity of auger 106 thus gradually lS decreases along the length of the auger due to the gradually decreasing flight depth.
Die body 66 includes one or more elong~te die openings 112 that communicate the exterior of die body 66 with bore 100 along the length of auger 106. In the 20 preferred embodiment, die body 66 includes a single elongate die opening~112 that is adapted to form a uniform sheet member having a width substantially in excess of its thickness. The combination of the position of die opening 112 relative to auger 106 and 25 the configuration of auger 106 tends to produce a uniform extruded sheet 114 of convertible material.
A motor 116 rotates auger 106 within bore 100 to extrude the convertible material in sheet form. The proper rotational speed of auger 106 may be 30 experimentally or analytically determined to provide the desired uniform rate of extrusion. If auger 106 is rotated too slowly, excess convertible material may be discharged through the portion of die opening 112 nearest second end 104. Similarly, if auger 106 is 35 rotated too quickly, excess convertible material may be discharged through the portion of die openinq 112 nearest first end 102. At the proper rotational 2 1 ~

velocity, the pressure along bore lOo is uniform, thereby forcing a sheet of uniform thickness through -die opening 112.
The dispersion i8 forced into cavities (not ~hown) 5 of the mold 62 as it passes through the die opening ~-~
112. The mold 62 of FIG. 8 is a flexible belt, which is driven by the driving mechanism 64. The cavities in the mold 62 can have any desired planar shape, such as triangular, circular, or rectangular. The cavities can 10 be formed by conventional means, such as by machining, punching, or etching. The flexible belt 62 can be made of any material that will withstand the operating conditions of the process. A belt made of metal such as stainless steel or aluminum is preferable. It is 15 preferred that the mold 62 be coated with a release coating, such as polytetrafluoroethylene, to improve the release of the dried precursors of the shaped abrasive particles from the cavities of the mold 62.
It is preferred that the exposed surface or 20 surfaces of the dispersion in the cavities not extend substantially beyond the plane of the belt in order to guarantee that the shaped abrasive particles prepared from the process be substantially uniform. Any excess dispersion surrounding the openings of the cavities and 25 remaining on the non-recessed portion of the belt 62 is removed, preferably by leading-edge wiper blades 68 positioned down the belt 62 from the die body 66. The top and bottom surfaces of the ~elt 62 can be wiped by the leading-edge wiper blades 68. These blades 68 are 30 mounted between leveling doctor blades 70 and the die body 66. The leveling doctor blades 70 further ensure that abrasive precursor particles will have a uniform thickness. It i preferred that wiper blades 68 be placed very close to the die, so that the wiping action 35 does not lift the dispersion out of the cavities. If the wiper blades 68 are too far downstream from the die, excessive dispersion buildup may cause the 211~9 dispersion in the cavities to cling to the dispersion on the surface of the belt.
The filled cavities in the belt 62 are moved into the oven 72, which is preferably an air circulating 5 oven. The oven temperature is preferably set at approximately 75C. However, the oven temperature can be higher or lower depending on the speed of the belt 62 and solids content of the precursor. The volatile component of the liquid is removed from the dispersion 10 in the oven 72. Care should be taken to solidify the dispersion sufficiently slowly so that the formation of cracks in the shaped abrasive particles is minimized.
As the volatile component is removed, the precursors of the shaped abrasive particles begin to form. Because 15 their volume is less than that of the dispersion from which they are formed, they will fall out of the cavities in the belt 62, and can be collected in a collecting pan 74. The shaped, dried precursors of the shaped abrasive particles are then calcined and fired, 20 preferably in a rotary kiln (not shown). Firing is preferably carried o~t at a temperature of 1300C to 1400C for a period of 1 to 15 minutes. Any dispersion or precursor material remaining on the belt 62 or in the cavities of the belt can be removed, preferably by 25 a rotating brush 76 or other cleaning process.
In the extrusion method of preparing shaped abrasive particles, the dispersion is fed into an extruder and then extruded through an orifice of a die to form elongated precursors of abrasive particles. A
30 sufficient amount of the volatile component is then removed from the elongated precursor of the abrasive particle such that the precursor is sufficiently dry so as to be capable of maintaining its elongated shape and cross-section. The conditions for removing volatile 35 components are the same as those described previously.
The shaped abrasive particles can be converted to the desired length before, during, or after drying the 21~ ~8~

precursor. The conditions for calcining and sintering are the same as those described previously.
The shaped abrasive particles that can be used in this invention are preferably in the shape of thin 5 bodies having a front face and a back face, the front face and the back face being separated by the thickness of the particle. The front face and the back face have substantially the same geometric shape. The geometric shape can be triangular, rectangular~ circular, 10 elliptical, or that of other regular or irregular polygons~ The most preferred geometric shape is triangular. For the purposes of this invention, the sides of the geometric shapes also include polygons wherein one or more of the sides can be arcuate, for 15 example, the definition of triangular extends to spherical triangles. Of triangular shapes, that of a plane equilateral triangle is the most preferred. FIG.
4 illustrates a picture taken at 12X magnification of a triangular-shaped abrasive particle. FIG. 5 20 illustrates a picture taken at 12X magnification of a square-shaped abrasive particle. FIG. 6 illustrates a picture taken at 12X magnification of a circular-shaped abrasive particle.
In most cases, the ratio of the length of the 25 shortest facial dimension of the shaped abrasive particle to the thickness of the abrasive particle is at least 1 to 1, preferably at least 2 to 1, more preferably at least 5 to 1, most preferably at least 6 to 1. As used herein, the term "thickness", when 30 applied to a shaped particle having a thickness that varies over its planar configuration, shall mean the minimum thickness. If the shaped particle is of substantially uniform thickness, the values of minimum, maximum, mean, and median thickness shall be 35 substantially equal. For example, in the case of a particle in the shape of a triangle, if the thickness is equivalent to "a", the length of the shortest side , .~". :, ., . ,,, ' . ~

--- 21i~.3 of the triangle is preferably at least "2a". In the case of a shaped particle in which two or more of the shortest facial dimensions are of equal length, the foregoing relationship.continues to hold. In most 5 cases, the shaped abrasive particles are polygons having at least three sides, the length of each side being greater than the thickness of the shaped particle. In the special situation of a circle, ellipse, or a polygon having very short sides, the 10 diameter of the circle, minimum diameter of the ellipse, or the diameter of the circle that can be circumscribed about the very short-sided polygon is considered to be the shortest facial dimension of the shaped particle. If a ~haped abrasive parti~le is 15 prepared in a mold cavity having a pyramidal, conical, frusto-pyramidal, frusto-conical, truncated spherical, or a truncated spheroidal shape, the thickness is determined as follows: (1) in the case of a pyramid or cone, the thickness is the length of a line 20 perpendicular to the base of the particle and running to the apex of the pyramid or cone; (2) in the case of a frusto-pyramid or frusto-cone, the thickness is the length of a line perpendicular to the center of the larger base of the frusto-pyramid or of the frusto-cone 25 and running to the smaller base of the frusto-pyramid or of the frusto-cone; (3) in the case of a truncated sphere or truncated spheroid, the thickness is the length of a line perpendicular to the center of the base of the truncated sphere or truncated spheroid and ~:
30 running to the curved boundary of the truncated sphere or truncated spheroid. The length of the shortest facial dimension of the shaped particle is the length of the shortest facial dimension of the base of the particle (if the particle has only one base) or the 35 length of the shortest facial.dimension of the larger base of the particle (if the particle has two bases).
The thickness of the shaped particles preferably range 21~83 from about 25 micrometers to 500 micrometers. This ratio provides improved performance of the shaped abrasive particle as compared with conventional unshaped abrasive grits. The shaped abrasive particles 5 can also be elongated, iOe., in the shape of rods, such as, for example, rods having cylindrical or prismatic shapes. The ratio of the length of a rod to the maximum cross-sectional dimension of that rod i8 preferably at least 1 to 1, more preferably at least 2 lo to 1, and most preferably at least 3 to 1. As used herein, "maximum cross-sectional dimension of a rod"
means the diameter of a circular cross-section of a rod or the diameter of a circle circumscribing a non-circular cross-section of a rod. The cross-sectional 15 dimensions of a rod can vary along its length.
The diluent particles can comprise (1) a plurality of individual abrasive particles bonded together by an adhesive to form an agglomerate, (2) a plurality of individual non-abrasive particles bonded together by an 20 adhesive to form an agglomerate, (3) a plurality of individual abrasive particles and a plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (4) individual non-abrasive particle, (5~ individual abrasive particle~, 25 or (6) combinations of the foregoing. The individual abrasive particles and the agglomerates containing them have irregular shapes or random shapes, i.e., they are excluded from the scope of the shaped abrasive particles. However, individual non-abrasive particles 30 and agglomerates containing individual non-abrasive particles, but free from individual abrasive particles, can have shapes that are equivalent to those of the shaped abrasive particles. The diluent particles typically have a particle size ranging from about 0.1 3s to 1500 micrometers, usually from about 1 to about 1300 micrometers. It is preferred that the abrasive particles have a Mohs' hardness of at least about 8, ;"", ,,;,~,"~ ~ ,,; ,"~ ,," ~ ~" ~ , ~ .,; ~ ;
....... . ~ .

2 1 i .~

more preferably at least about 9. Examples of materials of such abrasive particles include fused aluminum oxide, ceramic aluminum oxide, heat treated aluminum oxide, silicon carbide, alumina zirconia, 5 diamond, ceria, cubic boron nitride, silicon nitride, garnet, and combinations thereof. It is preferred that the non- abrasive particles have a Mohs' hardness less than about 7. Examples of non-abrasive particle~
include metal carbonates such as calcium carbonate ~chalk, calcite, travertine, marble, and limestone), calcium magnesium carbonate, sodium carbonate, magnesium carbonate, silica (such as glass beads, glass bubbles, glass fibres), silicates, such as talc, clay (montmorillonite), feldspar, mica, calcium silicate, 15 calcium metasilicate, sodium aluminosilicate, sodium silicate, metal sulfates, such as calcium sulfate, barium sulfate, sodium sulfate, aluminum sodium sulfate, aluminum sulfate, gypsum, vermiculite, wood flour, aluminum trihydrate, carbon black, metal oxides, 20 such as calcium oxide (lime), aluminum oxide, titanium dioxide, and metal sulfites (such as calcium sulfite).
Still other examples of non-abrasive particles include ~
halide salts, e.g., sodium chloride, potassium ~ --cryolite, sodium cryolite, ammonium cryolite, potassium 25 tetrafluoroborate, sodium tetrafluoroborate, silicon fluorides, potassium chloride, and magnesium chloride.
still other examples of non-abrasive particles include sulfur, organic sulfur compounds, graphite, and metallic sulfides.
As stated previously, individual abrasive particles or individual non-abrasive particles can be bonded together by an adhesive to form an agglomerate.
The adhesive that can be used for bonding can be inorganic or organic. Examples of lnorganic adhesives 35 include metallic or vitreous binders. Examples of organic adhesives include phenolic resins, aminoplast resins, urethane resins, epoxy resins, ethylenically 211 ~ ~3 8 ~3 unsaturated resins, acrylated isocyanurate resins, urea-formaldehyde resins, isocyanurate resins, acrylated urethane resins, acrylated epoxy resins, bismaleimide resins, fluorene modified epoxy resins, 5 and mixtures thereof. Depending upon the particular adhesive, the precursor of the adhesive may further include a catalyst or curing agent. The catalyst and/or curing agent can help to initiate or accelerate the polymerization process or both. The agglomerate 10 may further include additives such as fillers (including grinding aids), fibres, lubricants, wetting agents, surfactants, pigments, dyes, coupling agents, plasticizers, and suspending agents. The amount of these materials can be selected to provide the 15 properties desired. Examples of such agglomerates are described in U.S. Patent Nos. 2,194,472; 4,311,489;
4,132,533; 4,393,021; 4,541,842; 4,652,275; 4,799,939;
5,078,753; and 5,093,311.
In general, the ratio of the size of the shaped 20 abrasive particles to the size of the diluent particles can range from about 2.5:1 to about 0.5:1. If the diluent particles are too small, relative to the shaped abrasive particles, insufficient support will be given to the shaped abrasive particles and performance sf the 25 coated abrasive article may be adversely affected. If the diluent particles are too large, relative to the shaped abrasive particles, the diluent particles could prevent the shaped abrasive particles from contacting the workpiece. The size of the diluent particles can 30 range from about 50 to about 1,500 micrometers, and preferably ranges from about 100 to about 1,200 micrometers. It is preferred that the diluent particles and the shaped abrasive particles be of approximately the same particle size ranqe.
FIGS. 3 and 7 illustrate two embodiments of the coated abrasive article of the present invention.
Referring to FIG. 3, a coated abrasive article 20 - 2 1~

comprises a backing 22, a first binder 24, an abrasive coating 26, and a second binder 28. The abrasive coating 26 comprises diluent particles 30 and shaped abrasive particles 32. The diluent particles are 5 disposed in an open coat. The shaped abrasive particles are also disposed in an open coat. The diluent particles 30 function as a support or reinforcement for the shaped abrasive particles 32.
The article in FIG. 3 has two binders. The first 10 binder 24 is commonly referred to as a make coat and is applied over the backing 22. The second binder 28 is commonly referred to as a size coat and is applied over the shaped abrasive particles 32 and diluent particles 30. The size coat reinforces the abrasive coating 26.
It is preferred that a portion of the shaped abrasive particles have a triangular-shape. These shaped abrasive particles will hereinafter be designated as triangular-shaped abrasive particles. of these triangular-shaped abrasive particles, from about 20 35% to about 65% are oriented to the backing with a vertex 34 of the tr~angle pointing away from the backing as illustrated by FIG. 3. The remainder of these triangular-shaped abrasive particles arP oriented with a base 36 of the triangle pointing away from the 25 backing. However, up to 20% of the particles may not be oriented in either of the preceding ways, e.g., they may lay such that their faces are substantially parallel to the backing. As used herein, the phrase "vertex pointing away from the backing" and the like 30 means that a base of the triangular-shaped particle is positioned toward the backing; the phrase "vertex pointing away from the backing" also includes those situations in which the line corresponding to the altitude of the triangular-shaped particle is tilted 35 from the perpendicular at a small angle, typically less than 45, preferably less than 30. As used herein, the phrase "base pointing away from the backing" and 2 1 ~

the like means that a vertex of the triangular-s~apad particle is positioned toward the backing; the phra6e "base pointing away from the backing" includes those situations in which the line corresponding to the ~ -5 altitude of the triangular-shaped particle is tilted from the perpendicular at a small angle, typically less than 45, preferably less than 30.
During the manufacture of the coated abrasive article, the triangular-shaped abrasive particles are 10 preferably applied by electrostatic coating techniques.
Electrostatic coating causes a portion of the triangular-shaped abrasive particles to be oriented with a base pointing away from the backing and a portion to be oriented with a vertex pointing away from 15 the backing.
It is to be expected that a small number of triangular-shaped abrasive particles will fail to have a base or a vertex oriented toward the backing and will have the triangular face substantially parallel to the 20 backing. These shaped particles will exhibit less cutting. The number of shaped particles lying flat will increase at lower weights of diluent particles.
During electrostatic deposition of the shaped abrasive particles, preferred orientation of the abrasive 25 particles is easier to maintain when the space between the diluent particles is so small that the shaped particles do not have sufficient room to tip over during depo~ition.
Referring to FIG. 7, a coated abrasive article 40 30 comprises a backing 42, a first binder 44, an abrasive coating 46, and a second binder 48. The abrasive coating 46 comprises diluent particles 50 and shaped abrasive particles 52. Both the diluent particles and the shaped abrasive particles are disposed in an open 35 coat. The diluent particles 50 function as a support or reinforcement for the shaped abrasive particles 52.
The first binder 44, i.e., the make coat, is applied 2~ 58g~

over the backing 42. The second binder 48, i.e., the size coat, is applied over the shaped abrasive particles 52 and diluent particles 50. The size coat reinforces the abrasive coating 46. The abrasive 5 particles 52 are in the shape of rods.
If the diluent particles are deposited i~ a closed coat, i.e., a coat in which the particles substantially completely cover the backing, a high percentage of shaped abrasive particles will reside above the diluent 10 particles.
The coated abrasive article of FIG. 3 can be prepared by applying the first binder 24 to the front surface of the backing 22. Then the diluent particles 30 are embedded in the first binder 24. The diluent 15 particles 30 can be applied by drop coating or electrostatic coating. The diluent particles 30 can be applied in an open coat or closed coat. In an open coat, a portion of the backing is free of diluent particles. In a closed coat, substantially all of the 20 backing is covered by diluent particles. Then the shaped abrasive particles 32 can be applied over the diluent particles 30 and the first binder 24 by drop coating or electrostatic coating. The shaped abrasive particles 32 can be applied in an open coat or closed 25 coat. After the shaped abrasive particles are applied, the first binder 24 is at least partially cured.
Finally, the second binder 28 is applied over the abrasive coating 26. Then the second binder 28 is cured. Curing of the second binder 28 results in 30 additional curing of the first binder 24, if the first binder 24 has been only partially cured. It is preferred to apply the diluent particles 30 by drop coating and the shaped abrasive particles 32 by electrostatic coating. Because the diluent particles 35 30 are applied first, the close or open nature of the coating of the diluent particles determine whether the ~haped abrasive particles reside substantially between :

_ 30 _ the diluent particles (diluent particles in open coat) or reside substantially between and above the diluent particles (diluent particles in closed coat~. The coated abrasive article of FIG. 7 can be prepared in 5 substantially the ~ame manner as was described to prepare the coated abrasive article of FIG. 3.
The volume ratio of shaped abrasive particles to diluent particles can vary from 95:5 to 5:95, typically from 30:70 to 70:30, and preferably from 40:60 to 10 60:40. It i8 preferred that the uppermost layer consist essentially of shaped abrasive particles. The layers underlying the uppermost layer preferably contain a majority of diluent particles.
Blends of shaped abras~ve particles having 15 different shapes can be used in the articles of this invention. The shaped abrasive particles may als~ have a ~urface coating. Surface coatings are known to improve the adhesion between abrasive grains and the binder in abrasive articles. Additionally, the surface 20 coating may prevent the shaped abrasive particle from capping. Capping is the term to describe the phenomenon where metal particles from the workpiece being abraded become welded to the tops of the abrasive particles. Such surface coatings are described in U.S.
25 Patent Nos. 5,011,508; 1,910,444; 3,041,156; 5,009,675;
5,085,671; 4,997,461; and 5,042,991, all of which are incorporated herein by reference.
The coated abrasive articles of the present invention provide a cut that compares favorably with 30 the cut provided by coated abrasive articles containing only high quality abrasive grits, such as, for example, "Cubitron" grits, available from Minnesota Mining and Manufacturing Company, with no diluent particles.
Yet, the coated abrasive articles of the present 35 invention can be prepared at a lower cost than articles containing an equivalent amount of high quality abrasive grits.

2 ~

The following examples are illustrative o~
specific embodiments of th$~ invent~on; howQver, these examples are for illustrative purposes only and are not to be construed as limitations upon the invention.
The following procedures were used for Examples 1-20.

Procedure for Makinq Shas~ed Abrasive Particles A dispersion (44~ solids) was made by the `~
10 following procedure: alpha aluminum oxide monohydrate powder (1,235 parts) having the trade designation "DISPERAL" and aqueous dispersion of FeOOH were dispersed by continuous mixing in a solution containing ~ water (3,026 parts) and 70% aqueous nitric acid ~71 15 parts).
In the case of triangular-shaped particles, the FeOOH was about 0.1 micrometer in length and 0.02 micrometer in width and consisted of about 3% by weight solids in deionized water. ~he resulting triangular-20 shaped particles contained 1.25% by weight Fe2O3, 4.5%
by weight magneæia, with the remainder being alumina.
The triangular-shaped particles contained a surface treatment of the type described in U.S. Patent No.
5,011,508.
¦ 25 In the case of square-shaped and rod-shaped particles, the FeOOH was about 0.4 micrometer in length and 0.05 micrometer in width and consisted of about 10%
, by weight solids in deionized water. The square-shaped and rod-shaped particles contained 2% by weight Fe~O3, 30 4.5% by weight magnesia, with the remainder being alumina.
The sol that resulted was mixed with magnesium nitrate (429 partæ) to form a gel whiah was then dried -at a temperature of approximately 125C in a continuous 35 dryer to produce the 44% solids dispersion. In the case of triangular-shaped particles and square-shaped .
-J

~ 2 1 1 ~ 8 ~ ~

particles, the dispersion was introduced into cavities of the desired shape in a mold by means of a rubber squeegee. The cavities were coated with a release coating, either a silicone material or 5 polytetrafluoroethylene. In the case of triangular-6haped particles, the dimensions of the mold cavities were 0.29 cm on each side and 0.05 cm in depth. In the case of square-shaped particles the dimensions of the mold cavities were 0.23 cm on each side and 0.06 cm 10 deep. The filled mold was placed in a forced air oven maintained at a temperature of 71C for 20 minutes.
The dispersion underwent substantial shrinkage as it dried, and the dried precursors of the shaped abrasive particles shrank in the cavities. The precursors of 15 the shaped abrasive particles were removed from the mold by gravity. After the precursors of the shaped abrasive particles were removed from the mold, they were dried at a temperature of 121C for three hours.
For the rods, after the precursor dispersion was 20 gelled, it was extruded into rods by means of a screw extruder. During drying, the rods broke into lengths.
They were then screened to size. The dimensions of the dried rods were about 0.6 mm diameter by about 0.6 to 2.4 ~m length, with the median length being about 1.6 25 mm.
The dried precursors of the shaped abrasive particles were introduced into the end of a calciner, which can be described as a 23 cm diameter, 4.3 m long stainless ~teel tube having a 2.9 m hot zone, the tu?be 30 being inclined at 2.4 with respect to the horizontal, and rotating at 6 rpm, providing residence time therein of about 15 minutes. The entry end temperature of the hot zone was 350C and the exit end temperature of the hot zone was 800C. The material exiting the calciner 35 was introduced into a kiln held at a temperature of about 1,390C. The kiln was a 8.9 cm diameter, 1.32 m long silicon carbide tube inclined at 4.4 with respect 211~83 ,, ~

to the horizontal, having a 76 cm hot zone, and rotating at 10.5 rpm, providing a residence time therein of about four minutes. The material exited the kiln into air at room temperature, where it was 5 collected in a metal container and allowed to cool to room temperature.

Procedure for Makina Diluent Particles A resole phenolic resin, sodium cryolite, a 10%
10 aqueous dispersion of wood pulp, a glass bubble filler (Microspheres S22, commercially available from Minnesota Mining and Manufacturing Company), and water were introduced into a Hobart mixer Model L-800, and the resulting mixture was mixed until it appeared to be 15 a homogeneous dispersion. The resulting mixture, which wa6 highly viscous, was spread out onto a shallow metal tray to a depth of about 3.5 to 6.5 cm and cured for 16 hours at a temperature of approximately 100C. The resulting material was crushed by a roll crusher to 20 reduce its æize. The crushed material was again crushed by a roll crusher, and screened. The resulting diluent particles consisted of 35.5 parts by weight cured resole phenolic resin, 61.1 parts by weight sodium cryolite, 1.0 part by weight wood pulp, and 2.4 - ~ -25 parts by weight glass bubbles. The diluent particles, hereinafter referred to as DP I, were screened to a size range of about 589 to 1350 micrometers, such that they passed through a 16 mesh stainless steel screen, but were retained on a 34 mesh stainless steel screen.
Procedure for Making Coated Abrasive ~ticles A make coat was coated onto a 0.76 mm thick vulcanized fibre disc having a diameter of about 17.8 cm and a 2.2 cm center hole. The make coat comprised 35 48% by weight resole phenolic resin and 52% by weight calcium carbonate and was diluted to 81% solids with water and glycol ether solvent. The wet weight of the make coat was 377 g/m2. The abrasive coating con~i~ted ~-of two materials. The first material was selected from the group consisting of individual abrasive particles, individual non-abrasive particles, and a plurality of 5 diluent particles agglomerated into a mass. These materials were electrostatically coated onto the make coat. The second material was selected from the group consisting of individual abrasive particles and shaped abrasive articles. These materials were also 10 electrostatically coated. The resulting construction was heated at a temperature of 77C for lS minutes and then at a temperature of 930c for 8 hours to cure the make coat. A size coat was then coated over the abrasive coating at an average weight of about 670 g/m2.
15 The size coat had been diluted to 78% solids with water and glycol ether solvent and contained 32% by weight resole phenolic resin and 68% by weight sodium cryolite. The size coat was cured at a temperature of 77C for one hour and then at a temperature of 102C
20 for 16 hours. ~he fibre discs were flexed prior to testing.

Tes~ ProcedureTI
Test Procedure I measured the cut rate of the disc 25 and the amount of metal removed from the workpiece in 12 minutes. The coated abrasive disc was mounted on a beveled aluminum back-up pad and used to grind the face of a 1.25 cm by 18 cm 1018 mild steel workpiece. The disc was driven at 5,500 rpm while the portion of the 30 disc overlaying the beveled edge of the back-up pad contacted the workpiece at a load of about 6 kg. Each disc was used to grind a different workpiece for a one minute interval for a total time of 12 minutes. Twelve different workpieces were used in this procedure for 35 each disc. The initial cut was the amount of metal removed in the first minute of grinding. The final cut was the amount of metal removed in the last minute of grinding. ~he total cut was the summation of the amount of metal removed throughout the test. In most of the examples, the performance of the abr~sive 5 article was stated as perCQnt of control, i.e., the total amount of metal removed for the control example was equated to 100% and the material removed by the abrasive articles of the examples was measured relative to 100~. Approximately three discs were tested for 10 each sample.

Test_P~ocedure ~
Test Procedure II measured the amount of metal removed in 8 minutes of grinding at high grinding 15 pressures. The test equipment included the coated abrasive disc attached to a hard phenolic backup pad (16.5 cm diameter, 1.57 mm thick) which was in turn mounted on a steel flange ~15.2 cm diameter)~ The test disc so supported was rotated at 3550 rpm. A 25 cm 20 diameter 1018 carbon steel disc-shaped workpiece was placed into contact with the abrasive face of the abrasive disc under a load of 2.9 kg. The 1.8 mm peripheral edge of the workpiece was deployed 18.5 from a position normal to the abrasive disc and rotated 25 counter clockwise at 2 rpm. At the start and end of the test, the workpiece was weighed to determine the amount of steel removed or abraded. The endpoint of the test was 8 minutes of grinding. The total cut was the amount of 6teel abraded during the entire test.
30 The values listed in the tables were measured as a percent of the Comparative Example. Approximately three discs were tested for each example.

Test Procedure III
Test Procedure III was the same as Test Procedure II except that the angle was 7 rather and 18.5 and the load at the abrading interface was 2.7 kg.

21~$$~

Test Procedure IV
Test Procedure IV measured the amount of metal removed in S minutes. The test equipment included a 12.7 cm diameter coated abrasive disc having a 2.2 cm S center hole attached to a hard black fibre backup pad tll.9 cm diameter, 1.9 mm thick). The discs were mounted on a pneumatic disc grinder from Allen Air Inc., St. Louis, ~0, with a,line air pressure of 90 psi. The workpiece was a 6 mm by 27.9 cm weld bead 10 that had been laid down onto a steel plate. The thickness or height of the weld bead was initially about 6 to 13 mm. The test consisted of moving the abrasive disc onto the horizontally mounted workpiece and applying a downward force. The total cut was the 15 amount of weld material abraded during the 5 minute test. The values listed in the tables were measured a~
a percent of the Comparative Example.

Examples 1 - 2 and Com~arative ~ample A
These examples compared various coated abrasive I constructions. The resulting fibre discs were tested ¦ according to Test Procedures II and III and the results are set forth in Table 1.
The fibre disc of Example 1 contained two abrasive 25 materials. The first abrasive material consisted of about 800 g/m2 of grade 24 heat treated fused aluminum oxide. The second abrasive material consisted of about 390 g/m2 of triangular-shaped particles.
The fibre disc of Example 2 contained only one 30 abrasive material. This material consisted essentially of about 980 g/m2 of triangular-shaped particles.
The fibre disc of Comparative Example A contained two abrasive materials. The first abrasive material consisted of about 800 g/m2 of grade 24 heat treated 35 fused aluminum oxide. The second abrasive material consisted of about 890 g/m2 o~ grade 24 l'221 Cubitron"

2 ~ 13 ~ ~ ~

particles, commercially available from Minnesota Mining and Manufacturing Company, St. Paul, MN.

Table 1 , . ~ ~ ~ , --, . ~ .~ - ~ ,_, .. , ~, ,_ Test Procedure Example No.
II (%) _ III (%) Comp. A 100 100 l 100 149 ., ,, The coated abrasive articles of Examples 1 and 2 outperformed the coated abrasive article of Comparative -15 Example A in Test Procedure III.

Examples 3 - 9 and Com~arative Example A
These examples compared various coated abraslve constructions. The fibre discs were tested according 20 to Test Procedures I through IV and the results are set forth in Table 2.
The fibre disc of Example 3 contained two abrasive materials. The first abrasive material consisted of about 800 g/m2 of grade 24 heat treated fused aluminum 25 oxide. The second abrasive material consisted of about 390 g/m2 of triangular-shaped particles.
The fibre disc of Example 4 contained one abrasive material and one material selected from the class of diluent particles. The diluent particles consisted of 30 about 210 g/m2 of DP I. The abrasive material consisted of about 390 g/m2 of triangular-shaped particles.
The fibre disc of Example 5 contained two abrasive materials. The first abrasive material consisted of about 712 g/m2 of grade 24 heat treated fused aluminum 35 oxide. The second abrasive material consisted of about 480 g/m2 of triangular-shaped particles.

21~8~
,,. .~

The fibre disc of Example 6 contained one abra~ive material and one material selected from the class of diluent particles. The diluent particles consi~ted of about 190 g/m2 of DP I. The abrasive material consisted 5 of about 480 g/m2 of triangular-shaped particles.
The fibre disc of Example 7 contained two abrasive materials. The first abrasive material consicted of about 610 g/m2 of grade 24 heat treated fused aluminum oxide. The second abrasive material consisted of about 0 590 g/m2 triangular-shaped particles.
The fibre disc of Example 8 contained one abrasive material and one material selected from the class of diluent particles. The diluent particles consisted of about 160 g/m2 of DP I. The abrasive material consisted 15 of about 590 g/m2 of triangular-shaped particles.
The fibre disc of Example 9 contained only one abrasive material. This material consisted essentially of about 1005 g/m2 of triangular-shaped particles.

T~ble 2 . ~_ .. ~ . - -- - ., Test Procedure Example ~ -I (%) II (%) III (%) IV (%) ~ ~ _, . _, . I
Comp. A 100 100 100 100 _ I . . _. . .
4 132 106 194 149 l . . . _.

. I

6 130 134 198 lS0 l I ~ ~ ....... ___ I

7 ___ 108 91 105 I _ _ 30 I 9 140 130 182 _ 165 The coated abrasive articles of Examples 3-9 outperformed the coated abrasive article of Comparative Example A in Test Procedures II and IV. The coated 35 abrasive articles of Examples 3-6 and 8-9 outperformed 2 1 ~ $.~ :- 39 -the coated abrasive article of Comparative Example A in ~ -Test Procedure6 I and II. Test Procedure I was not run for the coated abrasive article of Example 7.

~xamples 10 - 12 and Comparative Examples A and B
The~e examples compared various coated abrasive constructions. The fibre discs were tested according to Test Procedures I throuqh IV and the results are set forth in Table 3.
The fibre disc of Example 10 contained one abrasive material and one material selected from the class of diluent particles. The diluent particles consisted of about 610 glm2 of grade 24 marble particles. The abrasive material consisted of about 15 390 g/m2 of trianqular-shaped particles.
The fibre disc of Example 11 contained one abrasive material and one material selected from the class of diluent particles. The diluent particles consisted of about 460 g/m2 of grade 24 marble 20 particles. The abrasive material consisted of about 590 g/m2 of triangular-shaped particles.
The fibre disc of Example 12 contained one abrasive material and one material selected from the class of diluent particles. The diluent particles 25 consisted of about 590 g/m2 of DP I. The abrasive material consisted of about 590 g/m2 of triangular-shaped particles.
The fibre disc of Comparative Example B contained one abrasive material and one material selected from 30 the class of diluent particles. The diluent particles consisted of about 460 g/m2 of grade 24 marble particles. The abrasive material consisted of about 590 g/m2 of qrade 24 "221 Cubitron" particles, commercially available from Minnesota Mining and 35 Manufacturing Company, St. Paul, MN.

21 1 5 3 8 ~

Tabl~ 3 ~ ~ l Test Procedure Example _ I
I (%) II (%) III (%) IV (%) . . . .. I
~r~ _A~ 100 100 100 100 I
_ ... _ I
Comp. B _ 112 53 115 124 . . ___ ._ I

. . "

1 0 . . _. v-The coated abrasive article of Example 12outperformed the coated abrasive articles of Comparative Examples A and B in Test Procedures I, II, III, and IV. The coated abrasive articles of Examples 15 10 and 11 outperformed the coated abrasive article of Comparative Example B in Test Procedures I, II, III, and IV. The coated abrasive articles of Examples 10 and 11 outperformed the coated abrasive article of Comparative Example A in Test Procedures I, III, and 20 IV.

Examples 13 - 18 and Comparative Examples C and D
These examples compared various coated abraslve constructions. The resulting fibre discs were tested 25 according to Test Procedures I and II and the results are set forth in Table 4.
The fibre disc of Example 13 contained two abrasive materials. The first abrasive material consisted of about 710 g/m2 of grade 24 heat treated 30 aluminum oxide. The second abrasive material consisted -of about 480 g/m2 of triangular-shaped particles.
The fibre disc of Example 14 contained one abrasive material and one material selected fro~ the class of diluent particles. The diluent particles 35 consisted of about 190 g/m2 of DP I. The abrasive material consisted of about 480 g/m2 of triangular-,3~ ~ 3, ~

shaped particles.
The fibre disc of Example 15 contained two abrasive materials. The first abrasive mater~al consisted of about 710 g/m2 of grade 24 heat treated 5 aluminum oxide. The second abrasive material consisted of about 480 g/m2 of grade 36 rods.
The fibre disc of Example 16 contained one abrasive material and one material selected from the class of diluent particles. The diluent particles 10 consisted of about 190 g/m2 of DP I. The second abrasive material consisted of about 48~ g/m2 of grade 36 rods.
The fibre disc of Example 17 contained two abrasive materials. The first abrasive material 15 consisted of about 530 g/m2 of grade 36 heat treated aluminum oxide. The second abrasive material consisted of about 480 g/m2 of yrade 36 rods.
The f ibre disc of Example 18 contained one abrasive material and one material selected from the 20 class of diluent particles. The diluent particles consisted of about liO g/m2 of DP I. The diluent particles were screened to a size range of about 297 to 710 micrometers, such that they passed through a 25 U.S. standard screen, but were retained on a SO U.S.
25 standard screen. The abrasive material consisted of about 480 g/m2 of grade 36 rods.
The fibre disc of Comparative Example C contained two abrasive materials. The first abrasive material consisted of about 710 g/m2 of grade 24 heat treated 30 aluminum oxide. The second abrasive material consisted of about 480 g/m2 of grade 24 "221 Cubitron" particles, commercially available from Minnesota Mining and Manufacturing Company, St. Paul, MN.
The fibre disc of Comparative Example D contained 35 one abrasive material and one material selected from the class of diluent particles. The diluent particles 2 ~ 8 ~

consisted of about 190 g/m2 of DP I. The abrasive material consisted of about 480 g/m2 of grade 24 "221 Cubitron" particles, commercially available from Minnesota Mining and Manufacturing Company, St. Paul, 5 MN.

T~ble 4 ~est Procedure Example I (%) II (%) Comp. C 100 lO0 ....
Comp. D 105 112 _ .

17 85 88 ~~~~
~ 18 ~76 90 The coated abrasive articles of Examples 13 and 14 outperformed the coated abrasive articles of Comparative Examples C and D in Test Procedures I and II.
. .
Examples 13 - 20 and Comparative Examples C and D
These examples compared various coated abra~ive constructions. Examples 13-18 and Comparative Examples C and D were described previously. The resulting fibre discs were tested according to Test Procedure III and 30 the results are set forth in Table 5.
The fibre disc of Example 19 contained two abrasive materials. The first abrasive material -consisted of about 710 g/m2 of grade 24 heat treated aluminum oxide. The second abrasive material consisted 35 of about 480 g/m2 of square-shaped particles.
The fibre disc of Example 20 contained one ?.1~5~

abrasive material and one material selected from the class of diluent particles. The diluent particles consisted of about 190 g/m2 of DP I. The abrasive ~aterial consisted of about 480 g/m2 of square-shaped 5 particle Tablo 5 ~ ,, . ., _, . ., _ . . . ,_,.", . ~,.. , Example Total cut ~%) . . . - I
Comp. C 100 . .. _ I
13 174 ¦

17 __ 71 18 __ 71 ~ ~ 20 _ 168 ~
The coated abrasive articles of Examples 13, 14, 19, and 20 outperformed the coated abrasive articles of Comparative Examples C and D in total cut.
Various modifications and alterations of this invention will become apparent to those skilled in the 25 art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrated embodiments set forth herein.

Claims

1. A coated abrasive article comprising:
a. a backing having two major surfaces;
b. at least one binder;
c. an abrasive coating comprising shaped abrasive particles and diluent particles, said at least one binder serving to bond said abrasive coating to at least one major surface of said backing.

2. A coated abrasive article according to claim 1, wherein at least a portion of said shaped abrasive particles have shapes that can be characterized as thin bodies having faces of triangular, rectangular, or circular shapes.

3. A coated abrasive article according to claim 2, wherein said faces have substantially the same geometric shape.

4. A coated abrasive article according to claim 2, wherein the ratio of the length of the shortest facial dimension of said shaped abrasive particles to the thickness of said shaped abrasive particles is at least 1 to 1.

5. A coated abrasive article according to claim 2, wherein the ratio of the length of the shortest facial dimension of said shaped abrasive particles to the thickness of said shaped abrasive particles is at least a to 1.

6. A coated abrasive article according to claim 1, wherein at least a portion of said shaped abrasive particles have shapes that can be characterized as rods.

7. A coated abrasive article according to claim 6, wherein said rods have a length and a maximum cross-sectional dimension, the ratio of said length to said maximum cross-sectional dimension being at least 1 to 1.

8. A coated abrasive article according to claim 6, wherein said rods have a length and a cross-sectional dimension, the ratio of said length to said cross-sectional dimension being at least 2 to 1.

9. A coated abrasive article according to claim 1, wherein said shaped abrasive particles comprise alpha alumina.

10. A coated abrasive article according to claim 9, wherein said shaped abrasive particles comprise alpha alumina and a metal oxide selected from the group consisting of magnesia, zinc oxide, iron oxide, silicon oxide, cobalt oxide, nickel oxide, zirconia, hafnia, chromia, yttria, praseodymium oxide, samarium oxide, ytterbium oxide, neodymium, lanthanum oxide, gadolinium oxide, ceria, dysprosium oxide, erbium oxide, titanium oxide, and mixtures thereof.

11. A coated abrasive article according to claim 9, wherein said shaped abrasive particles comprise alpha alumina and a nucleating agent.

12. A coated abrasive article according to claim 11, wherein said nucleating agent is selected from the group selected consisting of alpha alumina, alpha ferric oxide or its precursors thereof, titanium oxides, titanates, and chromia.

13. A coated abrasive article according to claim 1, wherein at least a portion of said diluent particles are selected from the group consisting of (1) a plurality of individual abrasive particles bonded together by an adhesive to form an agglomerate, (2) a plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (3) a plurality of individual abrasive particles and a plurality of individual non-abrasive particles bonded together by an adhesive to form an agglomerate, (4) individual non-abrasives particles, (5) individual abrasive particles, and (6) combinations of the foregoing.

14. A coated abrasive article according to claim 13, wherein said agglomerates further comprise an adhesive.

15. A coated abrasive article according to claim 14, wherein said adhesive is selected from the group consisting of phenolic resins, urea-formaldehyate resins, acrylate resins, epoxy resins, urethane resins, and aminoplast resins.

16. A coated abrasive article according to claim 13, wherein said abrasive particles are selected from the group consisting of fused alumina, heat treated aluminum oxide, ceramic aluminum oxide, diamond, cubic boron nitride, silicon carbide, silicon nitride, ceria, alumina zirconia, and garnet.

17. A coated abrasive article according to claim 13, wherein the Mohs' hardness of said non-abrasive particles is less than about 7.

18. A coated abrasive article according to claim 13, wherein said non-abrasive particles are selected from the group consisting of metal carbonates, silica, silicates, metal sulfates, gypsum, vermiculite, wood flour, aluminum trihydrate, carbon black, metal oxides, metal sulfites, halide salts, sulfur, organic sulfur compounds, graphite, and metallic sulfides.

19. A coated abrasive article according to claim 1, wherein said abrasive coating comprises at least two layers.

20. A coated abrasive article according to claim 19, wherein at least one layer of said two layers comprises diluent particles and the other layer of said at least two layers comprises shaped abrasive particles.

21. A coated abrasive article according to claim 20, wherein said at least one layer comprising shaped abrasive particles overlies said at least one layer comprising diluent particles.

22. A coated abrasive article according to claim 1, wherein said abrasive coating comprises a blend of shaped abrasive grains and diluent particles.

23. A coated abrasive article according to claim 1, wherein said at least one binder comprises a make coat.

24. A coated abrasive article according to claim 23, further including a size coat.

25. A method for preparing a coated abrasive article according to claim 1 comprising the steps of:
a. providing said backing;
b. applying a first binder over one major surface of said backing;
c. applying said diluent particles onto said first binder;

d. applying said shaped abrasive particles onto said coated diluent particles; and e. at least partially curing said first binder.

26. A method of according to claim 25, further including the steps of:
a. applying a second binder over said shaped abrasive particles, said diluent particles, and said first binder after said first binder is at least partially cured; and b. curing said second binder.

27. A method according to claim 25 wherein said diluent particles are applied by means of drop coating.

28. A method according to claim 25 wherein said shaped abrasive particles are applied by means of electrostatic coating.

29. A coated abrasive article according to claim 1, wherein the ratio of the size of said shaped abrasive particles to the size of said diluent particles ranges from about 2.5:1 to about 0.5:1.