CA1189327A - Abrasive agglomerates and abrasive sheet material - Google Patents

Abstract

A B S T R A C T
Abrasive agglomerate particles comprising a matrix of cellular glass and abrasive grit particles encapsulated within the cell walls of said glass, said particles being present in greatest concentration in the exterior walls, such agglomerates being particularly suitable for use in coated abrasive products in which the agglomerates are bonded to a flexible sheet backing; they can also be used in bonded abrasives (grinding wheels).

Description

33~7~

This invention relates to par~iculate abrasiv~
agglomerates in which abrasive grits are held in a friable matrix, ~uch agglomPrates being particularly suitable for use in co~tad abras~ve products in which the agglomerates are 5 bondad to a flexible sheet backing; they can also be used in bonded abracives (grindlng wheels~.
The use o~ small, particulate, agglomerates of rela~ively ~ine abxasive grits held ln a matrix, for use as a substitute ~or conventional abrasive grits on a coated abrasi~e ("~andpaper") flexible abrasive,~was suggested at least as early as U.S. Patent 2,194,472~ So far as is known, the solid agglomerates of the type disclosed in the above patent or products made from them have never been commercially successful, U.S. Pa~ent Re. 29,808, discloses hollow spheres (or o~her shapes, such as cylinders) consisting of abrasive grits bonded onto the outer surface of a friable matrix, such as resin or an inorganic ~ilicateO European published application 8868, published March 19, 1980, discloses solid agglomerates bonded by fused cryolite or other "salts or silicates", British Patent 982,215 and U.S. Pakent 3,156,545, teach maklng ~ol~d agglomerates fox use in grinding wheels consisting of glass bonded alumina or other grits. Benner U.S. Patent 2,216,728 discloses g~ass or metal bonds for the matrix of a~gregates containing diamond abrasive particles.
The patent states that ~he matrix may be made somewhat porous to enhance mechanical bonding when the aggregates are mixed with a binder to form a grinding wheel. U.S. Patent 2,806,772 ~, 3~7 suggests includ~ng foamed glass in abrasi~e agglomerates bonded by a resin matr~x.
W~ile abrasi~e agglomerates of the hollow resin or silicate bonded type have shown good results in coated abras~e applications, and agglomerates such as tauyht in the ~ e ~æc applicatlon show good results, both types of agylomerates are difficult or expensi~e to manufacture and it is dif~icult to control their friability.
More control of the physical properties of abrasive agglomerates, and excellent grinding results in coated abrasives can be achieved by providing agglomexates of abrasive particles bonded by a foamed glass in which the abraslve particles are contained within the walls of the cellular glass ma~rlx. Such agglomerates can be manufactur~d by mixlng approprlate abrasive grits with conventional known compositions which produce a foamed glass structure upon firlng. The glass composition, foaming agent, and, if deslred, grindlng aid, are mixed together, formed into small agglomerates ~ the desired shape, fired, and cooled. The 2a agglomerates may then be screened to appropriate sizes and employed in a conventional manner to produce coated abrasive discs, belts, or sheets. They may also be used to produce resin bonded grinding wheels.
The present invention utilizes the basic friability o~ the ~ellular glass and its controlled variability of friability as a matrix for abrasive grit. When a cellular glass is at the appropriate foaming temperature, it expands and will stick t~ most materials around it. In addition, it tends to encapsulate particles in its path~
This latter tendency is utilized when sized abrasive grlt is mixed with a cellular glass batch and the body brought to a cellulating temperature. Surprisingly, the grit particles are readily distribu~ed throughout the cell yet totally encapsulated by the glass in the walls.
Accordlngly, mixtures of various cellular glass batches are blended with various volume percentages of grit, the blended batch is pelletized to appropriately sized green spheroids and those spheroids dried and fired to yield the abraslve aggregate.
Cellular glass is sold as a soft abrasive in its own right. Its major product qualities are its ready ~riability without catastrophic ~ailure such that upon rubbing o~er a workpiece new sharp glass surfaces are aonstantly being formed. In addition, the material is lmpermeable so that there is no absorbtion of liquid into the structure Abrasive agglomerate performance depends upon the l.0 ~riability of the matrix~ Ideally, the matrix should ~racture or crumble as soon as the encapsulated grain begins to lose i~s peak cut~ing quality. This invention provides a product in which ~ine abrasive grit is encapsulated in the foam cell walls as a discrete impurity. Ideally, ~he matrix should be de~igned to exhibit a coefficient of thermal exp~n~ion that is as close as possible to that of the abrasive ~rit in order to minimize cooling flaws.
The subject grain can be formed into ex~ruded chopped shapes, or can be formed into spheres. Friability can ~e controlled by the ratio of pores to grain and/or the ratlo of glass to grain. Higher density matrixes (60pcf~) will tend to ~reak like a glass while lower densities will increase friabillty.
While khe ~ize of the aggregates is subject to much ~ariation, dependlng on the particular application and grit siæe, generally the aggrega~es will be 250 microns or larger in diameter, since ~he foam glass process limits the minimum s~ze glass-grit aggregate. The maximum size normally used would not be over 5mm, at least in coated abrasive applications. The abrasive grit will generally not be finer than 1~ microns, nor coarser than 2 or 3 millimeters.
The preferred abrasive grit is fused aluminum oxide, but co-fused alumina-zirconia alloy abrasives can be used, as can silicon carbide abrasive grit.
As shown in tha example below, soda lime glass can be used, but a non-devitri~ying alumino borosilicate compos~tlon is superior~

I'~e abraslve and glass mixture for forming the agglomerates contains from 40 ~o 80 percent (dry bul~ volums) of milled gla~s composition and from 20 to 60 percent of abrasive grain. Up to 2Q percen~ addi~ion o~ a grindlng aid such as cryolite can be added to such mixtures. The final productt when in the form of spheres, will have a bulk density of from 20 to 55 pounds per cu~ic foot (0.32 to 0 88g/cc).
The optlmum firing temperature and time depends upon the particular composition us~d, the desired density tPorosity) o~ the product. In general a temperature of 800 to 900 degrees C or higher for about 20 minutes is suitable.
The steps in a typical example of this invention are as follows:
lS 1. Preparing a foamable glass batch by ball milling soda lime glass cullet with 0.25% carbon black and 0O5% three micron silicon carbide for 24 hours in a batch ball mill to a median part~cle size of five microns or less.

2~ Adding a charge of 70~ by volume of the foamable glass batch and 30% by volume of an abrasive grit, in particular, a 180 grit fused dark aluminum oxide and blending them dry at high speed. Subsequent to dry blending a 1~
addition of alum is added as a dilute liquid and wet mixed followed by a 0.4% solids ad~ition of an aqueous montmorlllonite slurry at a 4% solids content as a binder.
Sufficient additional water is added to palleti~e the mix to a pellet particle size on the order of 20/40 mesh.

3. The generally ~phe~ical pellets thus formed are then dried in a fluld bed dryer and dry mixed with an aluminum 3Q hydrate parting agent and fired in a rotary kiln at a temperature of about 850 degrees C for 20 minu~es.
The resultant particles exhibit a specific gravity of 30 to 35 pcf (.48 to .56 g/cc~, When examined microscopically, it is observed that the glas~ tends to encapsula~e the alumina particles in a foam bubble network.
It is also observed that the alumina particles tend to be concentrated at the periphery of the bubble in a manner 3~

akin to froth floatation. The particles at the surEace are still covered by a layer of glass.
The particles were screened to 20/30 and 30/40 UOS. sie~e ~raction then tested by using them as if they were in themselves abrasive grits and maXing coated abrasive belts in the conventional fashion. The bel~s were tested in a standard metal finishing test system and compared with belts made from l80 grit dark alumina.
It was found that the initial time to achieve a comparable finish was longer for the aggr~gate belts than grit belts but the total amount of metal removed and the belt li~etime was between two and six times that of the grit belt standard.
~epeated testing yie~ded erratic results, some repeating the aforemen~ioned pexformance, others substantially poorer. It was determined that the reason for ~the erra~ic performance was the tendency of the soda lime ~lass to de~itrify and the potential for the cristobalite crystals to cause defects which sometimes caused the glass to fail. Additional testing was made using the belt with an aqueous lubricant and the resultant performance was consistently bad. It was determined that this was caused by the poor aqueous durability of the glass.
Accordingly, it was determined to use a batch that would be essentially a nondevitrifying borosilicate made from a mlxture of clay or volcanic ash and chemical additives similar to that described in U.S. Patent 3793039. A mix o 66% volcanic ash, 15% kaolin clay, 5.5% 5 mole borax, 8%
dolomite, 2.7% lithium carbonate, 2% sodium bicarbonate and l~4% carbon black was comilled. A l~ addition of liquid alum was made prior to pellPtizing. The resultant pellets when ~ired at 330C, exhibited performance essentially similar to those made from melted glass cullet which tested reproducibly. In addition, when tested in a we~ enviroment, the performance was reduced but still better than that of a conventional belt made with 180 grit. The alumino borosilicate has enhanc~1 aqueous durability.

3~7 It was fur~her found that a 10~ addition of powdered cryolite enhanced the cutting perEormance.
Cryolite ls a well-known grinding aid for metals and is apparently encapsulated in a fashion similar to that of the alumlna grlt.
The following examples show that while not pre~erred, slllcon carbide or co-fused alumina-zirconia abrasive grlts can be used.
The ~irst experiment used the standard soda lime glas~ foam mix to whlch we added 30~ 39 Crv~tolon 180 grit and 10% fine cryolite. The product was foamed at about 850 degrees C. The resultant aggregate was lighter than that made from alumina, its bulk density being 22.4 pcf @ 12/20 vs. 27-29 pcf @ 12/20 but seemed otherwise similar. It was obser~ed that some of the grit particles were associated with laxge bubbles which suggest that even coarse grit SiC will influence the foaming reaction. These large bubbles will probably weaken the aggregate.
The second experiment used the same ingxedients except that 80 grit ofused alumina-zirconia containing 40 zirconia was used ~180 grit was not available). The resultant aggregate was essentially equivalent to alumina in all properties. A microscopic examination of the encapsulated alumina zirconia grains showed that despite the reduclng conditions of foaming, there was evidence of oxidation of the metallic components of the grain. This ef~ect wlll reduce the facture toughness of the grain.
Thus, it can be shown that other abrasives can be encapsulated into aggregates as was alumina but there are side e~ects that may reduce their utility.
Coated abrasive products are made from the agglomerates of this invention by bonding the aggregates in a single layer on a flexible backing sheet bv conventional means well-known in the art, employing thermosetting maker and si2e coats, glue, or a combination of glue and resin.
Subsequent ~o making the above examples, it was found that silicon carbide containing aggregates, for certain puxp~se~ ~uch a3 the grinding of titanium metal, are clearly superior t~ conventlonal sillcon carbide coated abrasive products.
Addit~onally it has been determined that when glass forming chemical mixes, rather than pre-melted and g~ound glass, are used in the form~ion of the aggregates, superior wetting of the abrasive aan be achieved. In addltion the resulting aggregate is different in structure from the typlcal glass containing mixesO In the case of ~he glass forming mixes, the abrasive particles are more un~ormly dispersed within the multi-cellular aggregate body, as compared to the glass mixes in which ~he abrasive particles tend to be concentrated in the outer peripheral cell walls of the multi-cellular matrix.
lS In a reduction to practice a foamable blend of glass batch, at 70%, 180 grit green SiC at 30% was mixed dry.
To t~is mix 1% alum on a dry solids basis in aqueous solution was added, followed by enough (0.4~) montmorillonite aqueous ~lurry to pelletize to a 20/40 mesh size. These generally spher~cal particles were dried and fired ak 850Co for 20 mlnutes ~n a rotary kiln. If desired a lO to 20% addition of cryolite as a grinding aid can be added at the dry mixing stage.
The fired particles were coated on a belt in the standard fashion and tested dry in finishing titanlum metal and wet in ~inishing plate glass. In both cases there was a longer break in period than that of a regular SiC belt but the useful cut li~e was much longer. In the case of titanium a standard belt cut 16 gm while the experimental nodule belt cut a total of 245 gm. The wet cutting of glass was similar:
18 gm vs 180 gm for the experimental nodule beltO
The expeximent was repeated using a mixture of chemlcals and minerals that yield an oxide glassy composition known to achteve a good bond with SiC. This bond was ball milled with ca~bon then blended, pelletized, and ~ired in a slmilar ~ashion to the prior example~ The dry tests in titan~um were equivalent to the glass matrix material but the r wet flnishing of glass was enhanc~d and a cut of 248 gm was experisnced.
Microscopic observations showed the grit in the glass to ha~e migrated substantially to the periphery of the S bubble and the bubble center to be aomprised o a few large Cell5, The hond composltion nodules had Sic distributed throughout the nodule and the internal closed cells were sm~ll and uniform in size.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Abrasive agglomerate particles comprising a matrix of cellular foam glass and abrasive grit particles encapsulated within the cell walls of said glass.

2. Abrasive agglomerates according to claim 1, wherein said particles are present in greatest concentration in the exterior walls.

3. Abrasive agglomerates according to claim 1, in which the abrasive grit comprises fused alumina, co-fused alumina-zirconia or silicon carbide.

4. Abrasive agglomerates according to any one of claims 1, 2 or 3, in which said agglomerates include particles of cryolite.

5. Abrasive aggregates according to any one of claims 1, 2 and 3, having a specific gravity of from .32 to 0.88 grams/cc.

6. Abrasive agglomerates according to any one of claims 1, 2 and 3, having a generally spherical shape.

7. Abrasive agglomerates according to any one of claims 1, 2 and 3, in which the glass is an aluminum borosilicate composition.

8. A coated abrasive sheet material made by adhesively bonding the agglomerates according to any one of claims 1, 2 and 3, to a flexible backing.