US3502453A - Abrasive article containing hollow spherules filled with lubricant - Google Patents

Description

E. L. BARATTO ABRASIVE ARTICLE CONTAINING HOLLOW SPHERULES March 24, 1970 3,502,453

FILLED WITH LUBRICANT Original Filed Aug. 9, 1965 IN VENT OR. 50 5/? L. 63424 rro United States Patent Int. Cl. B24d 3/00 US. Cl. 51-295 8 Claims ABSTRACT OF THE DISCLOSURE An abrasive article containing a multiplicity of separately prepared hollow spherules filled with a lubricant.

The spherules rupture under grinding conditions and release the lubricant.

This application is a continuation of application Ser. No. 478,043, filed Aug. 9, 1965, and now abandoned.

This invention relates to abrasive articles such as coated abrasive sheet material, grinding wheels, and the like.

A perennial problem in any abrading, sanding, or grinding operation is the generation of heat. This problem is particularly acute when the workpiece being finished is metal which may distort or which is required to have a uniform attractive appearance. The generation of excess heat causes buckling, discoloration, and, in some instances, destruction of the temper of the workpiece. For many years, it has been common to supply a grinding aid, such as a lubricating oil, to the surface of the abrasive article, thereby reducing the frictional heat and increasing the ease of cut. Although this technique is sometimes satisfactory, it is inconvenient, messy, requires additional equip ment and may create a fire hazard.

The prior art discloses many attempts to include grinding aids within an abrasive structure. For example, US. Patent No. 2,327,846 suggests that chlorinated hydrobon filler can be included in a grinding wheel, the filler decomposing to form acidic metal-reactive substances during the abrading operation. Likewise, nearly half a century ago it was known (cf. US. Patent 1,325,503) to impregnate a porous grinding wheel with molten paraflin, grease, oil, wax, or fat. Chlorinated naphthalenes have been similarly used; cf. US. Patent 1,900,430. Although each of these techniques is sometimes effective, highly acidic or corrosive grinding aids may also corrode the equipment on which they are used. In abrading operations, a grinding wheel often gets so hot that much of the parafiin, wax, or similar filler material melts and is spun off before the wheel is consumed. US. Patent 2,734,812 suggests soaking particles of expanded perlite in lubricating oil and including them in a phenolic resin bond. Not only does the lubricating oil tend to reduce the adhesion of the resin for the abrasive grains, but the oil also tends to seep prematurely from the wheel during storage.

The present invention provides a simple and convenient way of making abrasive articles which contain grinding aids or lubricants and which are extremely useful in sanding operations where generation of heat is a problem. Grinding aids which are not receptive to the adhesive which bonds the abrasive grains may be used, and far more grinding aid may be included than was ever before possible. Products can be made to possess widely varied performance characteristics, and results are closely reproducible. Resin-bonded wheels made according to my invention may be designed to run cool throughout their useful abrading life while still retaining the ability to be run at high speed with superior rate of cut which distinguishes 3,502,453 Patented Mar. 24, 1970 them from vitreous-bonded wheels. The products do not weep during storage, even at elevated temperatures.

In accordance with my invention I prepare abrasive products in which grinding aids are encapsulated in minute shells, typically hollow spherules, which maintain their integrity under normal handling conditions but which rupture under grinding conditions. I have found that in some conditions as little as 1% by volume of the abrasive article (including abrasive grains) produces a noticeable improvement in abrading efficiency, while up to 25% is desirable under certain other operating conditions; generally speaking, the abrasion of regular workpieces requires more lubricant than the abrasion of sharp or jagged workpieces.

The lubricant-containing capsules which I employ may be distributed throughout the bond adhesive in a grinding wheel or in at least the sandsize of a conventional coated abrasive structure. One product in which my invention is effectively embodied is the abrasive wheel shown in Nestor U.S. Patent No. 2,862,806. The Nestor device is a circumferentially uniform accurately balanced molded abrasive article comprising a permanent hardened resinous annulus. The radially outer portion of the annulus is filled to a relatively substantial depth with abrasive grains, while the radially inner portion of the annulus may be devoid of grains. I prefer to distribute encapsulated grinding aids only in the portion of the annulus actually containing abrasive grains.

The lubricant-filled spherules themselves may be formed from either organic or inorganic materials, as will be shown. Ethyl cellulose, carboxymethylcellulose, polymers of methyl methacrylate, styrene, divinyl benzene, ethyl acrylate, methyl acrylate, or vinyl acetate gelatin, gum arabic, starch, sugar, casein, alginates, pectin, Irish moss, styrene-divinyl benzene copolymers, glass, ceramics, and metals all maybe employed under appropriate conditions. The term lubricant is intended to embrace any nonabrasive substance which reduces either the force required to cut metal, the tendency of freshly exposed metal surfaces to weld to the abrasive grains, or both. Among substances found to function effectively as lubricants in steel grinding are petroleum oil, chlorinated hydrocarbons, iron pyrites, parafiin, and divalent sulfur compounds. For grinding or sanding such other metals as zinc or aluminum, strongly acidic or basic materials (e.g., NaHSO, or NaOH) may be employed as lubricants, a possibility which never before existed. Even wheels made with corrosive grinding aids of this type are perfectly safe to handle but release controlled amounts of grinding aid. at the site of stock removal.

Lubricants may be enclosed in spherules by any of several known procedures, simple coacervation, as described in US. Patent 2,800,458 being suitable. In this process a hydrophilic colloid such as gelatin is dispersed in water, lubricant oil added, and the mixture agitated to form an oil-in-water emulsion. A coacervating salt is then added to decrease solubility of the colloid and cause it to form a fluid sheath around the oil droplets. The temperature is then lowered to solidify the colloid, and the resultant capsules washed and filtered. By varying the relative amounts of colloid and oil and the size of the oil droplets, one can produce capsules with a predetermined wall thickness. Speaking broadly, the thinner the wall and/or the larger the capsule, the more readily the capsule will be ruptured during grinding conditions. Capsules having a diameter much below 5 microns tend to resist fracture unduly and to contain insufficient lubricant to be commercially attractive. Likewise, capsules having a diameter in excess of 500 microns tend to be hard to handle, often rupturing prematurely.

Lubricants may also be enclosed in capsules by other techniques. For example, a low molecular weight monomer may be dissolved in oil, which in turn is dispersed in water as described in the preceding paragraph. A catalyst is then added to the water and the shells formed around the oil by interfacial polymerization of the monomer.

Solid lubricants may also be encapsulated mechanically. For example, refrigerated particles of lubricant may be exposed to heated vapors of encapsulating material in an evacuated system, the vapors condensing around the particles. Particles may also 'be encapsulated in a film-forming material by simultaneously passing the material and particles through orifices and into a film-hardening bath.

My invention is further illustrated in the attached drawings in which:

FIGURE 1 is a view in perspective of a grinding wheel illustrating my invention;

FIGURE 2 is a greatly enlarged cross-section of the wheel shown in FIGURE 1 taken along the section line 2-2;

FIGURE 3 is a view in cross-section of a coated abrasive sheet material embodying my invention; and

FIGURE 4 is a cross-sectional view of a spherule of the type I employ in my invention.

As shown in FIGURES 1 and 2, solidified resinous material 11 bonds abrasive grains 12 firmly in position at the radially outer portion of the wheel. Contained within resinous material 11 are a large number of spherules 14 and, optionally, filler particles 13, which may be, for example, graphite, calcium carbonate or similar material.

Turning now to FIGURE 3, flexible backing material 31 is provided with a make adhesive 32 in which are embedded abrasive granules 33. Overlying make adhesive 32 and abrasive granules 33 is size adhesive 34, distributed within which are lubricant-containing spherules 14. Although it is possible to distribute these spherules throughout the make adhesive as well as the size adhesive, it has been found unnecessary to do so, and, further, such action may tend to weaken the make coat unnecessarily.

In FIGURE 4, spherule 14 comprises rupturable shell 15 and nucleus 16, the latter constituting the grinding aid.

My invention will be further illustrated by the following non-limiting examples in which all parts are by weight unless otherwise indicated.

EXAMPLE 1 Preparation of lubricant-filled spherules A precondensate is formed by blending 6 moles of 37% formaldehyde in Water and 4 moles of urea, adding triethanolamine to render the system alkaline, and heating at 70-80 C. for 1 hour. The system is then diluted with water and acid added to lower the pH to about 2-4. The system is continuously agitated and reprocessed SAE motor oil added slowly; agitation is continued for about Manufacture of grinding wheel The following materials were directly loaded into a cylindrical mold having a diameter of 8 inches and a width of /2 inch:

Parts Grit aluminum oxide 278.4 Graphite, medium particle size 20.8

50-100 micron urea-formaldehyde spherules containing reprocessed SAE No. 10 oil, prepared as described above 10.8

The mold was rotated about its axis at 500 r.p.m. to distribute the solid materials, after which the following were added to the rotating mold:

Parts DEN 438 resin 86 Methylene dianiline 24 The speed of rotation was then increased to 3000 r.p.m. and a 300 F. oven placed over the rotating mold for 45 minutes. The wheel was then removed from the mold and postcured 4 hours at 400 F. Rim thickness of the finished wheel was 1% inches.

The spherules employed in this example consist of 70% oil and 30% shell by weight, the total shell thickness being on the order of 1 micron. DEN 438 is a polyfunctional epoxy novolac resin available from the Dow Chemical Company and having a molecular weight of 6 00, an epoxide equivalent weight of 176, and the following idealized formula:

The wheel of this example was used to grind a fiat tool steel plate having a Rockwell C hardness of 55-60, traversing the plate in cross-feed increments of 0.250 inch with 0.001 inch of down-feed. After 30 passes, the wheel had cut 198 grams of steel, while a commercial vitrified wheel had cut only 186 grams of steel in the same length of time. No burning or discoloration of the steel plate was evident with either wheel. Conventional resin bonded grinding wheels are generally unsatisfactory for this operation because of localized overheating of the workpiece. Based on total volume, the wheel of this example contained approximately 11.7% encapsulated grinding aid, i.e., approximately 9% oil.

EXAMPLE 2 To a 14 inch diameter x 1 inch Wide rotating cylindrical mold was added 850 grams of Epon 828 epoxy resin catalyzed with 18% N-aminoethyl piperazine (NAEP). The speed of rotation was then increased to 1000 r.p.m., and the following materials added:

Parts Grade 600 aluminum oxide particles 731 Graphite, medium particle size 222 50-100 micron urea-formaldehyde spherules of the type described in Example 1 222 The speed of rotation was then increased to 1500 r.p.m., an oven placed over the mold and the resin allowed to precure for 45 minutes at 190 F. The wheel was then removed from the mold and cured for 2 hours at 250 F.; rim thickness was 2 inches.

Epon 828, available from the Shell Chemical Company, consists essentially of the diglycidyl ether of Bisphenol A, having a viscosity of -160 cps. at room temperature and a molecular Weight of -192 per epoxy equivalent.

Based on total volume, the wheel of this example contained 14.0% aluminum oxide granules, 7.7% graphite, 18.8% encapsulated oil, and 59.9% resin binder. This wheel was useful in polishing operations, where it cut uniformly and smoothly with less tendency to burn than an identical wheel containing no encapsulated oil.

, EXAMPLE 3 Into an 8 inch diameter x 1 inch wide compression mold, haivng a 1% inch diameter bore were placed the following ingredients:

The mold was subjected to a pressure of 12,000 p.s.i. at a temperature of 300 F. and 30 minutes, after which the molded wheel was removed and cured for two additional hours at 350 F.

EXAMPLE 4 The following components were mixed together and then centrifugally formed into a wheel as in Example 1:

- 7 Parts 78:22 DEN 438:methylene dianiline 21.4 Oil-containing capsules as in Example 1 2.9 Grade 24 aluminum oxide particles 75.7

The finished wheel, having a rim thickness of 1% inches was tested in the snagging of mild steel. Both the rate of cut and the tendency to resist burning were significantly better than for a control wheel containing no encapsulated oil. The wheel of this example contained 45.5% resin, 7.7% encapsulated oil, and 46.8% abrasive granules by volume.

EXAMPLE 5 Two wheels, A and B, were made up according to the composition and procedure of Example 1, the only diiference being that the capsules were replaced in wheel A with 11.7 volume percent of 20-60 micron ureaformaldehyde spherules containing Excelene NF and in Wheel B with 12.8 volume percent of 30-80 micron urea-formaldehyde capsules containing *Excelene NF. (Excelene NF is a chlorinated mineral oil, particularly recommended as an extreme pressure lubricant.) When subjected to the test described in Example 1, Wheel A had a cut rating of 105% compared to a conventional vitreous wheel, while Wheel B had a cut rating of 117%. It is believed that the superior performance of Wheel B stems from the fact that the relatively greater quantity and availability of the oil contained in each capsule.

EXAMPLE 6 A wheel identical to that of Example 1 was prepared, with the single difference that the urea-formaldehyde capsules had an average diameter of 75-200'microns. When subjected to the test described in Example 1, the cut rating of this wheel was approximately 103%; it is believed that the rating was depressed by the comparative fragility of larger capsules and their tendency to rupture prematurely during formation of the wheel. Such occurrences may be minimized by increasing either the thickness or the toughness of the capsule shell.

EXAMPLE 7 A conventional Grade 46 vitreous bonded grinding wheel was weighed, placed in a Buchner funnel, and a vacuum of approximately 20 mm. of mercury applied. A slurry of 20% encapsulated oil (as described in Example 1) and 80% water was poured over the wheel in three separate applications. The wheel was then removed from the funnel, dried 12 hours at F., and weighed again, an increase of 1% being noted. When the wheel Was broken and examined under a microscope it was found that oil-containing spherules were uniformly distributed throughout. With smaller capsules and/or greater vacuum, the concentration of oil in the wheel can be increased.

EXAMPLE 8 A coated abrasive product having a starchand gluefilled jeans cloth backing was coated with a glue make adhesive in the conventional manner and a standard quantity of Grade 150 aluminum oxide particles applied. The product was then sandsized with a basecatalyzed phenol-formaldehyde resin, the sole deviation from standard practice being the inclusion of 16% by weight (21% by volume) of 50-100 micron ureaformaldehyde spherules' containing a proprietary lubricant (Myers Miracle Oil), and the resultant coated abrasive material cured. A 4 inch x 84 inch belt was formed from this material and evaluated on an aluminum sanding test. After 30 minutes the belt had cut 261 grams, while a conventional belt (i.e., identical except for the absence of encapsulated oil) out only 205 grams under the same test conditions. Tendency for the aluminum to load the surface of the belt of this example was significantly lower than for the control.

EXAMPLE. 9

To a 24-inch x 2-inch centrifugal mold were added the following materials:

Parts Grade 16 aluminum oxide 78 78:22 DEN 438:methylene dianiline 19.8 50-100 micron urea-formaldehyde capsules containing SAE 20 oil 2.2

The wheel was precured for 45 minutes at 300 F. while rotating the mold at 600 r.p.m. and then removed from the mold and cured an additional 5 hours at 300 F. The finished wheel, having a rim thickness of 6 inches, contained 50.2% abrasive grains, 43.3% resin, and 6.5% encapsulated oil by volume.

When tested on a snag grinder driven at 9500 surface feet per minute, the wheel of this example showed a ratio of lbs. malleable iron removed to cubic inches: wheel loss of 1.26, while a conventional resinoid wheel (of the type recommended for and commonly used in this operation) had a ratio of less than 1.0.

EXAMPLE 10 To a 12 inch x 1 inch mold were added the following materials:

The wheel was precured by heating for 30 minutes at 300 F. while rotating at 2200 r.p.m., after which it was removed from the mold and postcured for one hour at 300 F. Rim thickness was 2% inches.

Aroclor 1248 is a polychlorinated polyphenyl, yellowtinted oily water-insoluble liquid having a specific gravity of about 1.45 and a distillation range of 330-370 C. frequently used in cutting oil; it is available from the Monsanto Chemical Company.

In titanium snagging operations, the wheel of this example cut 10.0 grams of titanium in one minute, with a wheel loss of 32 grams. N0 undue heat was generated. In contrast, the commercial resin bonded wheel ordinarily recommended for such operations out only 1 gram of titanium with 1 gram of wheel loss under identical conditions; when using this commercial wheel the workpiece became red hot and supported combustion.

EXAMPLE 11 Preparation of paraffin beads One gallon of tap water was heated to 65 C. in a 2 /2 gallon stainless steel beaker and 2 grams of liquid detergent (Lever Brothers DW-300) added. The solution was stirred rapidly with a propeller mixer and 120 grams of melted parffin slowly added. The resultant dispersion was then quenched by adding 1 gallon of ice water, stirred for several additional minutes, and the water removed by suction filtering.

Encapsuation of paraffin beads To a stainless steel beaker were added 440 grams of a 37% aqueous solution of formaldehyde, 164 grams of urea, and 3.2 ml. of a 25% (volume) solution of triethanolamine in water. The reaction mixture was stirred rapidly for 1 hour at 70 F., after which 800 ml. of cold tap water was added to form a urea-formaldehyde polymer stock solution.

To a stainless steel beaker containing 350 grams of the urea-formaldehyde polymer stock solution was added 2 ml. of a 25 :75 volume blend of concentrated HCl:H O while stirring rapidly. Stirring was continued, and 120 grams of paraflin wax globules added, after which an additional 2 m1. of HCl:H O solution was added, temperature being maintained below 30 C. at all times. After 20 minutes of stirring, 100 ml. of water was added and the temperature raised to 30 C. Stirring was com tinned, the temperature being maintained at 30 C. for 1 hour, 35 C. for 2 hours, and 40 C. for 2 hours. The mixture was then neutralized to pl-I7 by adding a dilute solution of sodium bicarbonate, suction-filtered, washed with water and dried. Average capsule diameter was approximately 50-100 microns.

Manufacture of grinding wheel A blend of 73.8 parts by weight Epon 828, 16.2 parts by weight NAEP, and 10.0 parts by weight encapsulated paraflin was prepared, and 230 grams thereof introduced into an 8 inch diameter x /2 inch wide mold spinning at 475 r.p.m. To the mold was then added 640 grams of Grade 46 aluminum oxide and the speed of rotation increased to 2,000 r.p.m. After 1 hour the cured wheel (containing, by volume, 50,8% cured resin, 6.7% encapsulated paraffin, and 42.5% abrasive grain) was removed from the mold and postcured 16 hours at 130 F. When evaluated in the grinding of tool steel having a Rockwell C hardness of 60, this wheel caused noticeably less burning of the workpiece than did a wheel which was identical except for omission of the encapsulated paraffin.

Although the foregoing examples are by no means exhaustive, it is believed that they suffice to show the nature of this invention. No attempt has been made to provide specific examples for all the variations which have been found effective. For example, it has been found that the particular bonding resin is not critical, assuming it has sufiicient strength to perform in that capacity. Thus, such materials as polyurethanes, styrene-crosslinked polyesters, mixtures of monoand di-furfuryl acetone, and similar bonding materials have been found to function effectively. Likewise, there has been no exhaustive attempt made in this application to provide specific examples directed to all those grinding aids which function effectively. Many of these are well-known in the art, for example, chlorinated naphthalene, mineral oil, kerosene, and sulfurcontaining oils. It will also be apparent that certain adjustments in the composition employed should be made dependent upon the nature of the workpiece, the severity of the grinding operation, the rate of cut desired, and

so on.

What is claimed is:

1. In a coated abrasive article comprising a flexible backing, a make adhesive firmly bonded to one surface of said backing, abrasive grains embedded in said make adhesive, and a sandsize overlying said grains and anchoring them firmly in position, the improvement which comprises including in said sandsize a multiplicity of small hollow urea-formaldehyde or gelatin spherules having a thin shell which completely surrounds a lubricant and which is distinguishable from said sandsize adhesive, whereby the shells normally prevent contact between said grinding aid and said sandsize but rupture under abrading conditions to release said lubricant.

2. The abrasive article of claim 1 in which said sandsize adhesive consists essentially of a phenol-formaldehyde resin and said lubricant is a lubricating oil.

3. In a circumferentially uniform accurately balanced molded rotative abrasive article comprising a permanently hardened epoxy resin annulus whose radially outer portion is filled to a substantial depth with abrasive grain and whose radially inner portion is devoid of abrasive grain, the improvement which comprises including in said radially outer portion finely divided graphite and a mul tiplicity of small hollow spherules 5-500 microns in diameter having a thin urea-formaldehyde shell which completely surrounds a lubricant said spherules constituting up to 40% by volume of the portion of said article exclusive of abrasive grains, whereby the shells normally prevent contact between said grinding aid and the resin in said radially outer portion of the resinous annulus but rupture under grinding conditions to release said grinding aid.

4. An abrasive article comprising abrasive granules and a hardened organic binder therefor, said binder containing a multiplicity of small closed hollow spherules having a thin urea-formaldehyde or gelatin shell which completely surrounds a lubricant andwhich is distinguishable from said binder, whereby said shells normally prevent contact between said lubricant and said binder but rupture under grinding conditions to release said lubricant.

5. The abrasive article of claim 4 wherein the thinshelled spherules have an average diameter in the range of 5-500 microns.

6. The product of claim 4 wherein the article is an abrasive wheel structure.

7. The product of claim 4 wherein the lubricant is hydrocarbon oil. I

8. The product of claim 4 wherein the lubricant is paraffin.

References Cited UNITED STATES PATENTS 1,325,503 12/1919 Katzenstein 51295 1,573,061 2/1926 Hartmann 51295 1,900,430 3/1933 Daniels 51-295 1,986,849 1/1935 Pohl 51296 2,806,772 9/ 1957 Robie 51296 2,862,806 12/1958 Nestor 51-298 2,986,455 5/ 1961 Sandmeyer 51-296 DONALD J. ARNOLD, Primary Examiner U.S. C1. X.R. 51 29s, 305, 306 y

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