US 2811430 A
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nite States ABRASIVES No Drawing. Application April 25, 1955, Serial No. 503,795
11 Claims. (Cl. 51-298) The present invention relates generally to the production of abrasives and more particularly to the production of grinding wheels for heavy duty use such as in snagging in the steel industry, and the like.
In the production of abrasives a mixture of abrasive grains and a bond material including fillers, binder resin, plasticizers and other ingredients are combined and molded in various ways. The filler in a molded abrasive is believed to reduce cutting temperatures, prevent loading of the surface of the abrasive, and to otherwise improve cutting efiiciency. The filler material usually employed is a finely ground or powdered natural mineral known as cryolite, having the general formula NasAlFs. Even though cryolite significantly improves the cutting efficiency of grinding wheels, in the grinding of stainless and alloy steels the performance of cryolite-filled grinding wheels has left something to be desired. Moreover, since cryolite is largely imported, its supply and price are subject to wide fluctuations in times of international crisis.
It is an object of this invention, therefore, to provide an abrasive article which contains an inexpensive, readilyavailable filler or fillers and which has an efliciency at least equal to that of cryolite.
Another object is to provide a filled abrasive article which will more satisfactorily grind stainless steel and other tough alloy steels. Still other objects and advantages of the present invention will be apparent, or will become apparent, in the more specific description of the invention to follow.
In accordance with the present invention an abrasive article is provided containing at least one filler selected from the class consisting of the alkali metal salts of sulfuric, hydrochloric, and hydrobromic acids and mixtures of at least one of the latter salts with zinc sulfide. Most of these fillers or combinations of fillers are inexpensive and readily-available in vast quantities at all times. With combinations of the alkali metal salts and zinc sulfide in certain proportions, synergistic effects are noted, that is, the combination is much more effective than either of its components used alone and an abrasive article is produced which cuts faster, lasts longer, and otherwise performs more satisfactorily. The abrasive articles of this invention are most effective in the cutting or grinding of stainless and alloy steels.
While we do not wish to be bound by theory due to the fact that the function of fillers is imperfectly understood, the following explanation is offered only because it appears to fit the facts as presently known. The stainless alloy steels and various of the other alloy steels high in total alloying ingredients, are characterized by their toughness and abrasion resistance. Consequently, grinding pressures and/ or grinding times, must be increased to remove a given weight of metal, as compared to iron or mild steels. Consequently, the abrasive wheels wear away at an increased rate and temperatures generated at the metal-to-abrasive interface will usually be higher and may, in fact, be so high that a thin surface layer of the Patented Oct. 29, 1957 metal itself may be burned, oxidized, or otherwise heatmodified to make it unsightly. The high temperatures may cause the heated metal to load the wheel and reduce its cutting efliciency. Likewise, the bond material, and particularly the filler content thereof, may be modified, volatized or otherwise rendered ineffective at the temperatures obtained. We have observed that many materials which have melting points above about 1200 C. are not effective as fillers in abrasives for grinding the alloy steels. This might indicate that a good filler has a fluxing, protective or other function and that such function requires a liquefaction of the filler. Further sup port for the latter surmise is furnished by the observation that many of the most effective mixtures or combinations of fillers of the invention either are at or near their eutectic composition, or are compositions exhibiting lower melting points than one or both of its components used alone. It is believed that the sulfur of the sulfides is dissolved by the molten alkali metal salts to form a low melting fiuxing composition that greatly facilitates grinding. e
The abrasive articles of this invention may be any molded abrasive made from a mixture including abrasive grains and a bond material containing one or more of the above fillers and a resinous thermosetting binder material. The abrasive articles of this invention which are particularly effective are molded abrasives having a high density, that is, an abrasive article having a low percentage of void spaces (i. e. lessthan 15%). it is the latter type that is utilized in heavy duty grinding such as snagging. Any abrasive grain or mixture of grains may be employed including silicon carbide, boron carbide, tantalum carbide, tungsten carbide or other hard metal carbides; alumina such as emery, and including electric furnace fused alumina such as corundum, diamond grains, glass, quartz, garnet, etc.
The resinous binder material in the bond material must be curable, heat-hardenable or heat-convertible to form a hard, strong bond which will not readily deteriorate. at elevated temperatures. An illustrative binder resin found particularly satisfactory in the abrasives of this invention is the fusible B-stage phenolic resins (phenol-aldehyde or cresol-aldehyde resins) such as the standard Bakelite types of phenol-formaldehyde resins which are available in either powdered or liquid form. If desired, the heatconvertible binder resin can be modified with small proportions of other resinous or rubbery materials such as the epoxy resins, vinyl resins including those of vinyl chloride, vinyl butyral and others, styrene-acrylonitrile resins, butadiene-acrylonitrile rubbers, andothers and may contain varying proportions of cross-linking or curing agents such as hexamethylene tetramine, paraformaldehyde, and others. In some cases solvents or plasticizers such as furfuraldehyde, propylene sulfite, furfuryl alcohol, cresol, and others may be present in the bond mix, if desired. The bond mix may also contain, in addition to the fillers of this invention, any of the conventional fillers and other additive materials such as cryolite, feldspar, iron oxide, and others. If desired, the bond may also contain lime, and Where furfuraldehyde is employed, the
presence of lime is recommended.
poorer in cutting efficiency than cryolite.
furfuraldehyde for every pound of powdered phenolaldehyde resin, if the latter type is employed.
The above proportions used in the abrasives of this invention relate to the total volume of the finished abrasive. The proportion of filler, as related to the bonding material, can also vary considerably depending on the use to be made of the finished abrasive. Generally, the total filler content of the bonding material should range from about 5 to about 40% by volume, more preferably from about to about 35%. These proportions include both the fillers of this invention and any other conventional filler or fillers which may be present. In general, any filler in addition to the filler of this invention should constitute no more than half the total filler and in no case should the proportion of the fillers of the present invention constitute less than about 2% by volume of the total bonding material.
In combination with zinc sulfide any of the alkali metal salts of sulfuric, hydrochloric and hydrobromic acids may be employed as fillers in the abrasives of this invention including those of lithium, sodium, potassium, rubidium and cesium. Illustrative salts of this type include sodium sulfate, potassium sulfate, lithium sulfate, cesium sulfate (CszSOr), rubidium sulfate (RbzSOr), sodium bromide, sodium chloride, potassium bromide, potassiumchloride, and many others. All of the known alkali metal salts of these acids melt at temperatures below about 1100 C. and many of the more common salts of this type melt at temperatures between 400 to about 900 C. Of these, the sodium and potassium sulfates, sodium and potassium chlorides, and sodium and potassium bromides are most readily available, areless expensive and have been found to be very e'flicient fillers when used in admixture with zinc sulfide.
When zinc sulfide is added to the abrasive along with one or more of the alkali metal salts, a great improvement in the cutting efiiciency of the resulting abrasive is usually noted. This is surprising since zinc sulfide, when used as a sole filler, produces abrasives which are' much As little as S to 10% zinc sulfide by volume, based on the total volume of alkali metal and sulfide fillers, significantly improves cutting efficiency. In most cases, as the proportion of sulfide is increased, large increases in cutting efficiency are realized until a point is reached where further increases in cutting efliciency are not obtained. For example, with mixtures of potassium sulfate and zinc sulfide, the maximum cutting efiiciency is obtained with mixtures of from 70 to 80% potassium sulfate and to Zinc sulfide. *Further increases in Zinc sulfide seem to decrease cutting 'eificiency slightly, yet with proportions as high as 1:1 the cutting efficiency is still considerably higher than is obtainable with cryolite as a sole filler in an. otherwise similar formulation. Efficient abrasives can be made with a ratio of potassium sulfate to zinc sulfide as high as 3:7. Efiicient abrasives can be produced in most cases with the mixed fillers of this invention containing as high as 70% by volume of sulfide.
The abrasive products of: this invention can be made in any conventional manner. For example, the conventional coldor hot-pressing methods may be employed wherein the abrasive grains are first wet with furfuraldehyde or another plasticizer or solvent and the binder resin, fillers, and other dry, powdery or liquid ingredients then added to the wetted grains and mixed until homogeneous. The resulting damp mixture then is placed in either a hot or cold mold and pressed under high pressure to form a coherent green abrasive which is removed from the mold and heated in an oven to cure the bond. The displacement method disclosed in the copending application of John R. Gregor and Samuel L. Kistler, Serial No. 499,443, filed of abrasive and furfuraldehyde.
4 April 5, 1955, may also be employed to advantage. In the latter method, dry, loose abrasive grains are placed in a suitable mold and a continuous layer of a fluid or fluidized bond mix applied to one exposed surface of the grain layer in the mold. Air is removed or is permitted to escape from the surface of the grain layer opposite the continuous bond layer to produce a flooding, directional movement of the bond layer toward the opposite surface to ensure complete filling of the voids. Gravity, vacuum, gas pressure, or centrifugal force may be applied to one side or the other of the abrasive layer to increase the speed of the directional movement of the bond. The abrasive is then baked to cure the bond. Any other method may be employed which is capable of producing a sound, compact, well-bonded abrasive article.
The invention will now be described in greater detail in connection with a number of illustrative specific examples. Unless otherwise specified in the examples, all proportions will be expressed as percent by volume.
Example 1 A series of abrasive wheels were made by the cold pressing technique using a mixture containing by volume of abrasive and approximately 38 to 40% by volume of bonding material including plasticizer. The bonding material had the following general composition, by volume:
Resin 1 Lime 8 Filler 27 1 Powdered B-stage phenol-formaldehyde. resin.
To the dry abrasive grains there was first added cc. of furfuraldehyde for every pound of binder resin to be employed. The liquid and abrasive were then mixed until a uniform consistency was obtained. The powdered bonding materials were first combined with 30 cc. of a neutral tar oil before being added to the somewhat moist mixture Mixing was then resumed until a homogeneous mixture was obtained. Wheels were made from this mixture by cold pressing.
Four control wheels were made employing powdered cryolite, in the same proportion as the sole filler. Another set of four wheels was made using zinc sulfide as the sole filler. The latter two sets of wheels were compared to a third set of four wheels wherein potassium sulfate was employed as the sole filler and to a fourth set of four wheels wherein the filler was a 1:1 mixture of potassium sulfate and zinc sulfide. The comparison was made on a standard abrasive wheel grinding test machine for a five minute grind using Type 302 stainless steel (18-8) as the metal ground. Each datum given below represents the average of the four wheels of each set tested.
Wheel Steel Filler Wear Removal, S/W SI/W lbS./hr.
(1:1) with potassium sulfate, a wheel of high cutting efficiency was obtained.
Example 2 As a result of the above tests, further four wheel sets w re m de wherein the proportion of zinc sulfide was increased from zero to 50% in increments. Again, a set of four cryolite-filled wheels were made and tested under identical conditions as in Example 1 (controls). The data are as follows:
It will be seen that the greatest cutting efiiciency (ratio S/W or S /W) appears to occur somewhere between the 80/20 and 70/30 mixtures. The increased cutting efficiency is derived both from reduced wheel wear and from greater ability-to-cut the steel. The above data, it should be noted, were obtained on an exceptionally tough stainless steel.
What is claimed is:
1. An abrasive article comprising abrasive grains, a thermostat binder resin and a filler consisting of at least one alkali metal salt of sulfuric, hydrochloric and hydrobromic acids in admixture with from 5% to 70% zinc sulfide by volume of the total mixed filler, said admixed filler constituting at least about 2% by volume of the total bonding material.
2. A cured, molded abrasive article comprising abrasive grains, a thermoset phenolic binder resin, and a filler consisting essentially of a mixture of potassium sulfate and from 5% to 70% of zinc sulfide by volume of the total mixed filler, said admixed filler constituting at least about 2% by volume of the total bonding material.
3. A cured, molded abrasive article comprising abrasive grains, a thermoset phenolic binder resin, and, as
a filler, a mixture consisting essentially of potassium chloride and from 5% to of zinc sulfide by volume of the total mixed filler, said admixed filler constituting at least about 2% by volome of the total bonding material.
4. A cured, molded abrasive comprising abrasive grains, a thermoset phenolic binder resin, and, as a filler, a mixture of potassium bromide and from 5% to 70% of zinc sulfide by volume based on the total mixed filler, said admixed filler constituting at least about 2% by volume of the total bonding material.
5. An abrasive article as defined in claim 1 wherein said alkali metal salt is potassium sulfate.
6. An abrasive as defined in claim 1 wherein the alkali metal salt is potassium chloride.
7. An abrasive as defined in claim 1 wherein the alkali metal salt is potassium bromide.
8. A cured molded abrasive article comprising abrasive grains, a thermoset phenolic binder resin and a filler consisting essentially of a mixture of potassium sulfate and zinc sulfide wherein the cured article has less than 15% by volume of voids, the proportion of potassium sulfate is between 50% and and the proportion of zinc sulfide is between 10% and 50% by volume of the mixture of fillers, said fillers constituting at least about 2% by volume of the total bonding material.
9. An abrasive article as defined in claim 1 wherein said alkali metal salt is sodium bromide.
10. An abrasive article as defined in claim 1 wherein said alkali metal salt is sodium chloride.
11. An abrasive article as claimed in claim 1 wherein the ratio of said alkali metal salt to said zinc sulfide is in the range of about 4:1 to about 3:7.
References Cited in the file of this patent UNITED STATES PATENTS 1,918,996 Weger July 18, 1933 2,216,135 Rainier Oct. 1, 1940 2,253,235 Hempel Aug. 19, 1941 2,308,981 Kistler Jan. 19, 1943 2,308,982 Kistler Jan. 19, 1943 2,325,180 Egeberg July 27, 1943 2,377,995 Coes June 12, 1945