CA1297304C

CA1297304C - Abrasive grains in the shape of platelets

Info

Publication number: CA1297304C
Application number: CA000581184A
Authority: CA
Inventors: Howard Lavern Kruschke
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1987-11-23
Filing date: 1988-10-25
Publication date: 1992-03-17
Anticipated expiration: 2009-03-17
Also published as: NO884663L; BR8806120A; ATE117012T1; US4848041A; JPH01167375A; KR960015735B1; NO173130B; DE3852752T2; EP0318168A2; KR890007848A; DE3852752D1; NO884663D0; EP0318168A3; NO173130C; EP0318168B1

Abstract

ABSTRACT

This invention relates to abrasive grains, and, more particularly, abrasive grains having a particular shape and side.
Three basic technologies that can be employed in the production of shaped abrasive materials suitable for use as abrasive grains are fusion, sintering, and chemical ceramic. However, it is not known whether any particular shape or size of abrasive grains would provide enhanced properties with respect to abrading.
This invention provides abrasive grains having a shape such that the mean particle volume ratio thereof is less than about 0.8, and preferably ranges from about 0.3 to about 0.7. Typically, the grains have the shape of a thin platelet, the average thickness of which must be no greater than about 460 micrometers.
The abrasive grains of this invention can be used in coated abrasive products, bonded abrasive products, nonwoven abrasive products, and abrasive brushes.
The products made with the grains of this invention exhibit higher initial cut and higher total cut, along with lower grinding forces than do products having equivalent weight loadings of conventional abrasive grains.

Description

F.N. 41819 CAN lA

ABRASIVE GRAINS IN THE SHAPE OF PLATELETS

BACRGROUND OF TE~E INVENTION

1. Field of the Invention This invention relates to abrasive grains, and, more particularly, abrasive grains having a particular shape and size.
~ 2. Discussion o the Prior Art : Three basic technologies that can be employed in the production of shaped abrasive materials suitable for use as abrasive grains are fusion, sintering, and chemical ceramic.
In general, abrasi~e grains prepared by means of a fusion process can be shaped by a chill roll with or without an engraved roll face, a mold into which molten material is poured, or a heat sink material immersed into an aluminum ox~de melt. U.S. Patent No. 3,377,660 discloses an apparatus utilizing a trough for flowing molten abrasive material from a furnace onto a cool rotating casting cylinder, rapidly solidifyins it into a :: 25 thin semi solid curved sheet, densifying the semi-solid material by reversing its curvature by pulling it away from the cylinder with a driven rapidly cooled conveyor means whereupon the partially fractured strip is deposited onto collecting means in the form of large fragments which upon : 30 being rapidly cooled and solidified break up into smAller fragm~nts well adapted to be reduced in size to form conventional abrasive particles. U.S. Patent Nos.
4,073,Q96 and 4,194,887 disclose a process for the manu~acture of abrasive material in which (1) an abrasive mix is brought to fusion as in an electric arc furnace, ~2) a relatively cold substrate is dipped into the molten material whereby a layer of solid abrasive material is , ~2 g3 0';30 ~

quickly frozen (or plated) on the substrate, (3) the plated substr~te is withdrawn from the molten material, and (4) the solidified abrasive material is broken away from the substrate and collected for further processing to produce abrasive grains.
Abrasive minerals prepared by a sintering process can be formed from refractory powders ha~ing a particle size of 1 to 10 micrometers in diameter. ~inders can be added to the powders along with a lubricant and a suitable solvent, e.g. water. The resulting mixtures, pastes, or slurries can be shaped into platelets or rods of various lengths and diameters. In firing the resulting shaped grains to density, high temperatures, e.g. 1400 to 1800C, high pressures, and long soak times, up to 10 hours, must be used. Crystal size may range from under one micrometer up to 25 micrometers. To obtain shorter residence times and/or smaller crystal size, either the pressure or temperature must be increased. U. S. Patent No. 3,079,242 discloses a method of making abrasive grain from calcined bauxite material comprising reducing the material to a fin~
powder, compacting under affirmative pressure and ~ormin~
the fine particles of said powder into grain sized agglomerations, and sintering said agglomerations of particles at a temperature below the fusion temperature of the bauxite to induce limited recrystallization of said particles, whereby to produce abrasi~e grains directly to size. U.S. Patent No. 4,252,544 discloses alumina abrasive grains wherein the grain structure is constructed of electrofused or high temperatuse calcined alumina coarse crystal particles and alumina fine crystal particles which are located between said alumina coarse crystal particles, the latter having particle sizes smaller than that of the former and being processed at a calcination temperature lower than that of the former, wherein said fine crystal particles are sintered inte~rally with said coarse crystal particles. U.S. Patent ND. 3,491,492 dlscloses a process for making an aluminous abrasive grain formed from bauxite, " , :3LX~'735~ 3-or mixtures of bauxite and say~r process alu~ina wherein the comminuted aluminous material is mixed with water 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 ca~es having a thickness equal to o~e dimension of the final ~rain before sintering, breaking said cakes to grains, screening, optionally roundinq said grains by air mulling, screening, sintering, cooling, and screening to yield the final product.
U.S. Patent No. 3,637,630 discloses a process in which the ~ame aluminous slurry disclosed in U.S. Patent 3,491,492 is plated or coated on a rotatinq anode of an electrolytic cell. The wet, high density platelets of aluminous material are removed ~rom the rotating anode via air impingement, dried, crushed, sized and sintered to density.
Chemical ceramic technology involves converting a colloidal dispersion or hydrosol (sometimes called a sol) in a mixture with solutions or other sol precursors to a ~ 20 gel or any other physical state which restrains the mobility of the components, drying, and firing to obtain a ceramic material. A sol can be prepared by precipitation ~ of a metal hydroxide from an aqueous solution followed by ; peptization, dialysis of anions from a solution o~ metal 2~ salt, sol~ent extraction of an anion from a solution of a metal salt, hydrothermal decomposition of a solution of a metal salt having a voiatile anion, etc. The sol contains metal oxide or precursor thereof and is transformed to a semi-rigid solid state of limited mobility such as a gel by eOg., partial extraction of the solvent. Chemica~ ceramic technology has been employed to produce ceramic materials such as fibers, films, flakes, and microspheres. U.S.
Patent No. 4,314,827 discloses synthetic, non-fused aluminum oxide-based abrasive mineral having a microcrystalline structure of randomly oriented crystallites comprising a dominant continuous phase of alpha-alumina and a secondary phase.

3~ 4~

Although the foregoing references, and others, allude ,o shaped abrasive grains, none have indicated that any particular shape or size would provide enhanced properties with respect to abrading.

SUMMARY OF THE INVENTION
This invention provides abrasive products comprising abrasive grains having shapes wherein the mean pa~ticle volume ratio is less than about 0.8, and preferably ranges from about 0.3 to about 0.7. Generally, the grains have a shape that can be characterized as a thin platelet. It has been found that the grains must have an average thickness no greater than about 460 micrometers.
The abrasive products of this invention exhibit higher initial cut and higher total cut, along with lower grinding forces than do abrasive products having equivalent weiqht loadings of conventional abrasive grains.
Although abrasive products utilizing the abrasive grains of this invention have the same abrasive grain weight a6 products made with conventional abrasive grain, the increased number of cutting points due to the inc~eased number o abrasive particles present results in a more effective and a higher performing abrasive article.
The abrasive grains of this invention can be used in coated abrasive products, bonded abrasive products, nonwoven abrasive products, and abrasive brushes. At least 15~ by weight, and preferably 50 to 100% by weight, o the grains of the abrasive product should be of the type described herein.
BRIEF DESCRIPTION OF T~E DR~WINGS
.
FIG. 1 is a picture of abrasive grains of this invention, photographed at 32X using a scanning electron microscope, said grains being made of ceramic aluminum oxide.
FIG. 2 is a picture of conventional brown .

, ~73~4 -5~

aluminum oxide mineral, photographed at 32x using a scanning electron microscope.
FIG. 3 is a picture of conventional ceramic aluminum oxide mineral (Cubitron~, available from Minnesota Mining and Manufacturing Company), photographed at 32X
using a scanning electron microscope.
FIG. 4 is a sectional view of a coated abrasive article that can utilize the abrasive grains of FIG. 1.

DETAILED D~:SCRIPTION
_ _ FIG. 1 shows abrasive grains of the present invention. The abrasive grains of this invention can be - formed from materials known to be useful for forming abrasive grains. These materials include natural or synthetic materials such as, for example, flint, garnet, aluminum oxide, alumina: zirconia, diamond, and silicon carbide, and ceramic minerals such as modified aluminum oxide (e.g. Cubitron~). The two critical ~actors of the abrasive grains of this invention are mean particle volume ratio and particle thickness.
Mean particle volume ratio of the grains must be le55 than 0.80, and preferably ranges from 0.30 to 0.80, more preferably from 0.30 to 0.70. Mean particle volume ratio can be determined by the following procedure:
1. Mean particle weight is calculated by weighing a random sample of abrasive grains, counting the number of grains in the sample, preferably by means of an electronic particle analyzer, and dividing the weight by the number of particles to obtain an average weight per particle.
2. The density of the sample is measured by means of a gas pycnometer.
3. The mean particle weight is then divided by 3S the density to obtain mean particle volume.

4. The mean particle volume can be compared with the volume determined for standard sands of :

~ 30~ -6-the same grade. Relative volume can be determined by dividing the volume of the sample grain by the volume of the standard sand.

TXe following table indicates the weight/particle and volume/particle for standard sands. (ANSI Standard #B74 .18, 1984 ) .

Weight/particle Volume/particle Grade (~ x 10 6 ) (CC X 10 6 610 lS9 36 3~2 89 20~ 54 6~ 42 10.9 11.2 2.9 100 4.g 1.3 120 2.4 0.63 150 1.6 0.42 2~ Average thickness of the abrasive grains of this invention cannot exceed 460 micro~eters. AS used herein, the ter~ "average thickn~ss" mea~s approxima~ing an arithmetic mean thickness, i.e. midway between extremes o~
thickness. If the average thickness were to exceed 460 3~ micrometers, the result would be fewer grains for a qiven loading weight, and fewer cutting edges for a given loading weight Aspect ratio, i.e. the ratio of length to width, is not critical, and it typically varies from about 1.6 to about 2Ø According to conventional thinking, the mineral with the highest aspect ratio, i.e. the sharpest mineral, should cut the most material during grinding.

~ 3~4 -7-Surprisingly, ho~e~ec, it was found that aspect ratio did not ~ignificantly affect ~utting performance.
Other properties, e.g. hardness, strength, density, do not differ significantly from those properties of conventional abrasive grains having a random shape.
The grains can be used in a wide variety of abrasive products, such as, for example, coated a~rasive belts, abrasive discs, bonded wheels, brushes, and nonwoven abra~ive products. Abrasive products made with the abrasive grains of the present invention exhibit higher initial cut and higher total cut than do abrasive products having equivalent weight loadings of conventional abrasive grains. In addition, abrasive products made with the grains of the present invention require lower grinding forces than do abrasive products made with an equivalent amount by weight of conventional abrasive grains. It should be noted, however, that the benefits provided by the abrasive grains described herein are only available when these grains are positioned so that they are substantially normal with respect to the surface that is to be abraded, i~e. the surface of the workpiece.
In order to obtain the advantages provided by the abra~ive yrains of the present invention, at least about 15% by weight, and preferably 50 to 100% by weight, of the abrasiv~ grains of the abrasive product should be the shaped abrasive grains of this invention.
The preparation of alumina-based ceramic materials by means of a chemical ceramic proces~ usually begins with the preparation of a dispersion comprising from about 2 to almost 60 weight percent alpha aluminum oxide monohydrate ~boehmite). The boehmite can either be prepared ~rom various techniques well ~nown in the art or can be acqulred commercially from a number of suppliers.
~xamples of commercially available materials include 3~ Disperaln, produced by Condea Chemie, GM~ and Catapa ~ SB, produced ~y Vi~ta Chemical Company. These aluminum oxide monohydrates are in the alpha-form, are relatively pure ~2~3~ -8-(including relatively little, i~ any, hydrate phases other than monohydr~tes), and have a high surface area.
The dispersion may contain a precursor of a modifying additive which can be added to enhance some desirable property of the finished product or increase the effectiveness of th0 sintering step. These additives are in the form of soluble salts, typically water solu~le, and typically consist of a metal~containing compound and can be a precursor of the oxides of magnesium, zinc, cobalt, nickel, zironium, hafnium, chromium, titanium, yttrium, rare earth oxides, and mixtures thereof. The exact proportions of these components that are present in the dispersion are not critical to this invention and thus can vary to convenience.
A peptizing agent is usually added to the boehmite dispersion to produce a more stable hydrosol or colloidal dispersion. Monoprotic acids or acid compounds which may be used as the peptizing agent include acetic, hydrochloric, formic and nitric acid. Nitric acid is a preferred peptizing agent. Multiprotic acids are normally avoided since they rapidly gel the dispersion making it difficult to handle or mix in additional component~. Some commercial sources of boehmite contain an acid titer (such as absorbed formic or nitric acid) to assist in forming a stable dispersion.
; The dispersion can be formed by any suitable means which may simply be the mixing of aluminum oxide monohydrate with water containing a peptizing agent or by forming an aluminum oxide monohydrate slurry to which the peptizing acid is added. Once the dispersion is formed, it preferably is then gelled. The gel can be formed by any conventional technique such as the addition of a dissolved or dispersed metal containing modifying additive, e.g.
magnesium nitrate, the removal of water from the dispersion or some combination of such techniq~les.
once the gel has formed it may be shaped by any convenient method such as pressing, molding, coating, or --9~
~2~,30~
extrusion and then carefully dried ~o produce an uncracked body of the desired shape.
The gel can be extruded or simply spread out to any convenient shape and dried, typically at a temperature below the frothing temperature of the gel. Any of several dewatering methods, including solvent extraction, can be used to remove the ~ree water of the gel to form a solid.
After the solid is dried, the dried solid can be crushed or broken by any suitable means, such as a hammer or ball ~ill, to form particles or grains. Any method for comminuting the ~olid can be used and the term "crushing"
is used to include all such methods.
After crushing or breaking the dried gel can then be calcined to remove essentially all volatiles and transform the various components of the grains into ceramics (metal oxides). The dried gel is generally heated to a temperature between about 400C and about 800C and held within this temperature range until the free water and over 90 weight percent of any bound water is removed.
The calcined material is then fired by heating to a temperature of between about 1200C and about 1650C and holdiny within this temperature range until substantially all of the alpha alumina monohydrate is converted to alpha alumina. Of course, the length of time to which the ceramic must be exposed to the firing temperature to achieve thi~ level of conversion will depend upon various factors but usually from about 5 to about 30 minutes is sufficient.
Other steps can be included in this pro~ess, such as rapidly heating the material from the calcining temperature to the firing temperature, sizing granular material, centrifuging the dispersion to remove sludge waste, etc. Moreover, this process can be modified by combining two or more of the individually described steps, if ~8ired.
~ hese conventional process steps and materials are more fully describ~d in U.S. Patent No. 4,574,003, .

~73~ o-incorporated herein by reference.
The abrasive g~ains of this invention can also be prepared by means of fusion technology and sintering technology.
The following non~limiting example~ further iilustrate the advantages oP the present invention. All parts and percentages are by weight unless otherwise indicated.

Example 1 An aqueous dispersion of boehmite alumina sol (27.5% solids~ and an aqueous solution of magnesium nitrate (65% solids) were combined in the ratio of 6 parts sol to l part magnesium nitrate. The mixture was extruded throuqh a s,heet die of a conventional coat hanger design to a film thickness of 1590 micrometers. The resultinq film was carried into a drier, dried at 230C for 5 to 10 minutes, and collected as a dried platelet-like material. ~he dried platelets were crushed by means of a roll crusher and sized to a -1350 micrometer diameter to a +335 micrometer diameter particle size distribution. The crushed platelets were calcined at 600DC for approximately 5 to 10 minutes and fired to substantially full density at 1400~C for less than 5 minutes. The abrasive grain produced was graded to a grit size of 24. Mean particle volume was determined in the manner previously described. The platelets had a mean particle volume of 158 x 10-6 cc per particle, and a mean particle volume ratio of 0.66.
Coated abrasive discs 10 of the type shown in FIG. 4 were prepared as follows:
A conventional 0.76 mm vulcanized fiber backing 12 was provided. Both make and size coats 14, 16 respectively, were conventional calcium carbonate filled phenolic resin. Make coat 14 was applied by means of a roll coater and the shaped abrasive grains 18 were applied by means of an electrostatic coater. The electros.a~ic coater tend~ to deploy shaped abrasive grains 18 in such a 3~
way that they are in a substantially normal position with respect to the backing, thereby allowing the grains to be oriented substantially normal to the workpiece during abrading operations. Make coat 14 was precured for 75 minutes at ~8DC. Size coat 16 was applied by means of a roll coater and precured for 90 minutes at 88C, and final cured for 24 hours at lOO~C. The cured product was converted to 17.8 cm x 2.2 cm discs. The coating weights of make coat 14, size coat 16, and mineral coat 18 were as ~llows:
Coatinq Coating we ~_t (g/cm2) Make 0-039 Mineral 0.125 Size 0.057 The cured discs were first conventionall~ flexed to controllably crack the hard bonding resins, then mountèd on a beveled aluminum back-up pad, and finally used to grind the face of a 2.54 cm x 18 cm 1018 cold rolled steel workpiece. The disc was driven at 5000 rpm while the portion of the disc overlying the beveled edge of the back-up pad contacted the workpiece at a pressure of 0.2 kg/cm2, generating disc wear path of about 140 cm2. Each disc wa~ used to grind four separate workpieces for a duration of 1 minute each in rotation to an accumulated endpoint of 20 minutes. The control was a ~egal~ Resin Bond Fibre disc available from Minnesota Mining and Manufacturing Company. The control employed aluminum oxide grains having a grit size of 24. The results of-the test were as follows:
Disc Initial cut (g) Total cut (g) Control 48 683 Example l 61 837 The coated abrasive of this invention showed 27 improvement in initial cut and 30% improvement in total cut over the control.

Example 2 Abrasive platelets were formed in the same manner as were the platelets in Example 1. The abrasive grain produce~ was graded to a gri~ size of 36. The mean particle volume was 62 x 10-6 cc/particle and the mean particle volume ratio was 0.70. The mean particle volume for grade 36 Cubitro ~ mineral, available from Minnesota Mining and Manufarturing Company, was 93 x 10 6 cc/particle, and the mean particle volume ratio was 1.04.
The coated abrasive belts used a filled polyester Y weight sateens cloth as the backing. The make coat was a conventional solution of phenol-formaldehyde resin and calcium carbonate filler. The size coat was a conventional solution containing phenol-formaldehyde resin and calcium carbonate ~iller. The make coat was applied to the backing by means of a roll coater. Abrasive grains were applied by means of elec~rostatic coating. The make coat precured for 75 minutes at 88Co The size coat was applied by means of a roll coater. The size coat was precured for 90 minutes at 88C~ The size coat was final cured ~or 10 hours at 100C. The coated abrasive sheets were converted to 7.6 cm x 335 cm endless belts. ~he coating weights of the make coat, size coat, and mineral coat were as follows:

~ Coating weight (g/cm2) Make 0.029 Mineral 0.091 Siz~ 0.047 The coated abrasive belts were tested on a reciprocatang bed ~rinder. The belts were run at 40 m/~ec over a 40 cm diameter steel contact wheel. The coated abrasive belts were tested on lOg5 steel with a ~%9~3~

depth of cut of 0.5 mm and a through feed rate of 10 cm/sec. The grinding forces, cut rate, and surface temperature were compared to those values obtained with grade 36 Regal~ Resin sond cloth belts available from Minnesota Mining and ~anufacturing Company. The results of the tests were as follows:

Belt Cut ( cm3 ~/ Tangential force Normal force Surface material path (cm ) (kg/cm width) 1~9~ temp (C) Control 0.349 11.6 81.4 188 Example 2 O.S18 8.6 50.1 127 The belt of Example 2 showed 48% improvement in c~t rate over the control while reducing the tangential grinding force by more than 25~ and reducing the normal grinding force by 38%. The reduction in grinding force indicates a more efficient coated abrasive product. As a result of the reduced grinding forces, the surface temperature of the workpiece decreased by 50C.

Example 3 A sample of grade 24 platelets similar to those prepared in Example 1 were analyzed to det~rmine the effect oE shape on cutting performance.
A Jef~ries Table was used to separate mineral into various fractions, from blocky shaped mineral particles to sharp, platelet-like particles. Conventional abrasive grain (grade 24) contained 47% blocky mineral and 22~ sharp, platelet-like particles. The grade 24 platelets contained 0.8% blocky mineral and 97.5~ sharp, platelet-like particles.
Mean particle volumes and aspect ratios were measured for each sample and were as follows:
Mean Relative particle particle Aspect ~ineral volume volumeratio __ _ ~locky Cubitro ~ 243 x 10-~ 1.02 1.31 Conventional C~bitro ~ 214 x 10- 6 0 . 90 1 . 50 Sharp Cubitro~ 192 x 10- 6 0.80 1.69 Platelets of invention 158 x 10- 6 0 . 66 1.62 .

~ 3~ 14-Fiber discs were prepared from the four abrasive grain samples by means of the same procedures employed in Example 1 for coated abrasive fiber discs.
The di~cs were tested with a slide action disc tester on 1018 steel, as in Example 1. The results are shown below:
Abrasive material Total cut (g) slocky Cubitron~ 506 10 Conventional C~bitron~ 601 Sharp Cubitro~ 705 Platelets o~ invention ~28 Discs using platelets of this invention showed a 31%
improvement over discs using conventional Cubitron, a 57%
improvement over discs using blocky Cubitron, and a 13 improvement over discs using sharp Cubitron.

ExamRle 4 This example demonstrates the superiority of the abrasive grains of this invention in abra~ive articles other than coated abrasive articles.
Mean particle volume for grade 36 aluminum oxide was 66 x 10-6 cc/particle, and mean particle volume ratio was 0.97. Mean particle volume for grade 36 platelet~ was 25 62 x 10- 6 cc/particle, and mean particle volume ratio was 0.70.
~n open-mesh substrate of polyvinyl chloride was coated with a 50/50 blend of epoxy resin ("Epon" 82~) and polyamide ("Versamid" 125). Mineral was then drop coated on the resin substrate so as to allow the mineral to lie flat on the substrate. The resin was cured, and the substrate converted into wheels by wrapping the substrate around a 7.6 cm diameter core helically to expose the sharp edges of the mineral.
The wheels were tested on a robot tester consisting of a grinder which held the 7.6 cm diameter x 2.5 cm wide wheels. The wheels were run at ll90 rpm -~9~3~ -15-against a 5.0 cm wide by 0.63 cm thick 1018 steel workpiece. A dead load of 0.9 kg/cm width was applied to the workpiece against the wheel. The test period consisted of two five minute passes per wheel. I~he control was a wheel containing conventional brown aluminum oxide.
The test results were as follows:
Material Fîrst cut (~) Second cut (~) Total cut (~) .
Conventional brown aluminum oxide 5.6 3.2 8.8 Platelets of invention 7.0 6.2 13.2 Non-woven abrasive product using platelets of this invention showed a 50% improvement over non-woven abrasive product u ing conventional brown aluminum oxide.
Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the ~cope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims

1. An abrasive article of a type which has at least one abrading surface and includes abrasive grains, at least a portion of said abrasive grains comprising shaped abrasive grains having a shape such that the average thickness of said grains is no greater than about 460 micrometers, said shaped abrasive grains having a mean particle volume ratio of less than about 0.8.

2. An article according to claim 1 wherein said shaped abrasive grains have a mean particle volume ratio from about 0.3 to about 0.7.

3. An article according to claim 1 wherein said shaped abrasive grains comprise at least about 15% by weight of the abrasive grains of said article.

4. An article according to claim 1 wherein the thickness of said shaped abrasive grains is substantially uniform throughout each grain.

5. An article according to claim 1 wherein said abrasive grains are deployed in said article in a substantially normal position with respect to said abrading surface.

6. An article according to claim 1 wherein said shaped abrasive grains comprise non-fused aluminum oxide-based abrasive mineral.

7. An article according to claim 1 wherein said article is a coated abrasive product.

8. An article according to claim 1 wherein said article is a bonded abrasive product.

9. An article according to claim 1 wherein said article is a nonwoven abrasive article.

10. An article according to claim 1 wherein said article is an abrasive brush.