US2327218A - Abrasive article and method of manufacturing the same - Google Patents

Description

Patented Aug. 17, 1943 ABRASIVE ARTICLE AND METHOD OF MANUFACTURING THE SAME Norman Robie, Niagara Falls, N. Y., assignor to The Carborundum Company, Niagara Falls, N. Y., a corporation of Delaware N Drawing. Application October 30, 1939. Serial No. 301,980

7 Claims.

This invention relates to abrasive articles including bonded articles such as abrasive wheels and stones and coated articles such as abrasive paper and cloth and flexible abrasive discs. More specifically, the invention is concerned with such articles where there is employed as a. binder either in whole or in part a resinous polymer including a substantial proportion of an ester of an acrylic acid.

This application is a continuation-impart of my copending application Serial No. 92,937, filed July 27, 1936, and also my copending application Serial No. 100,920, filed September 15, 1936.

An object of the invention is the provision of abrasive articles comprising improved binders. Another object of the invention is to provide an abrasive article containing a binder which ineludes as an essential ingredient a resinous polymer including a substantial proportion of an ester of an acrylic acid. Another object of the invention is the production of abrasive articles in which the binder comprises a substantial proportion of a. resinous polymer which includes in the polymer an ester of methacrylic acid. Another object or the invention comprises an improved method for manufacturing abrasive articles in which the binder includes a resinous polymer. Other objects of the invention will appear from the description of the invention which follows.

Broadly speaking, my improved binders include resins formed by polymerizing monomeric esters of acrylic acid, either alone or coniointly with other monomeric compounds which include the ethenoid linkage. Such compounds may be represented by the formula CHz=CRR' and, as an example, suitable binders may be made by polymerizing methyl methacrylate in which compound R is CH3 and R. is COOCHa. Other suitable polymers of methacrylic acid include the esters of propyl and butyl alcohol and particularly the iso and secondary alcohols. For example, the polymer oi the secondary butyl al cohol ester oi methacrylic acid has properties which make it particularly well adapted as a binder for abrasive products.

As has been stated, my binders may also be prepared by co-polymerizing two or more of the acrylic acid esters, or one or more of these esters with either acrylic acid or with vinyl compounds. A specific composition which I have found to be quite satisfactory for many purposes is the conjoint polymer of the methyl and secondary butyl alcohol esters of methacrylic acid. Generally speaking, the esters of methacrylic acid are preferable to those of acrylic acid CH2=CHCOOH although for some purposes, where the articles are to be used wet, the lower softening point of esters of acrylic acid are not objectionable and such esters may be usefully employed. A second example of the conjoint polymers comprises the co-polymer of methyl methacrylate and vinyl chloride. Both of these compounds contain the double bond which opens up under suitable conditions to join a plurality of the monomers into a long chain polymer which is resinous in character. The monomers themselves are liquid materials but when polymerized, form solids having differing properties, depending upon the composition of the monomer or monomers em ployed and the number of monomers joined into each polymeric molecule.

In making abrasive articles with my improved binders, the polymers may be formed before admixture with or application of the abrasive grains, or a. part or all of the acrylic compounds may be added in the monomeric condition and polymerized with the abrasive grains in place. For example, an abrasive wheel may be prepared by first wetting the abrasive grains with a monomeric compound or a. mixture of such compounds, a polymer suitably pulverized may then be added to the moistened grains, the article suitably shaped, and then a reaction be brought about to polymerize the liquid monomeric materials to provide a completely solidified binder.

As an alternative, some of the polymeric materials may be dissolved in suitable monomers and substituted for the monomer in the process just described. By using a solution of polymers in the monomer, a thicker liquid may be obtained which is desirable in some instances and particularly where the abrasive grains are of coarser grits. Likewise, in the preparation of coated abrasive articles, suitable backings such as webs of paper of the class commonly used in the production of sandpaper, may be first coated with a solution of a polymer in a monomer, or a mixture of monomers, abrasive grains distributed upon the liquid coated web in the conventional manner and this abrasive coated web may then be suitably treated to solidify the liquid and form the so-called making binder for the abrasive grains. In accordance with the usual practice, it is generally desirable to apply a second coat of adhesive over the abrasive grains to attach them more firmly to the backing. The sizing speaking, the esters of acrylic acid are coat may be a liquid similar to that used in the making coat, although it is generally preferable to use a somewhat less viscous adhesive for the sizing coat than is employed in the making coat. The making coat of the abrasive grain may be conveniently applied in the usual making machine and the sizing coat may also be applied by the use of rolls or may be sprayed on or otherwise applied to the abrasive grains.

Alternatively, in making either the coated or bonded type of article, solutions of polymeric materials in suitable solvents may be used to pro vide liquids. In another modification, which is of use particularly in the preparation of bonded articles, the solidified resinous composition may be admixed with the abrasive grains and heated to soften it sufficiently to cause adhesion to the grains and thus provide a mixture of the grains and the binder which can be conveniently molded. An abrasive article may be then formed by placing such mix in a mold and applying suitable pressure, usually accompanied by heat to cause a fiuxing of the resin according to the usual hot molding methods which causes the adhesion of the bond to the grains and the unification of the grains into a finished article which can be removed from the mold upon cooling.

In polymerizin the monomeric compounds, the usual methods of causing the polymerization reaction to occur may be employed, including the utilization of catalysts in accordance with the processes well known in the abrasive art. The monomeric esters polymerize readily unde the influence of heat, light and oxygen which may be conveniently supplied by employing an oxidizing agent such as sodium, hydrogen or benzoyl peroxide as catalysts. The peroxides are particularly suitable where it is desired to hasten the polymerization action since they may be incorporated in the liquid monomers and the speed of reaction can be still further hastened by heating, although heat alone is sufiicient to bring about polymerization.

The particular polymers which are to be employed are determined by the uses to which the articles are to be put. All of these compositions are of the so-called "thermally reversible type as distinguished from the heat-hardening resins of the class represented by the well-known phenol formaldehyde condensation products. Consequently, they have the advantage that the mix 01' any defective article can be reused because the binders can be repeatedly softened by heat and solidified by cooling without a change in their properties.

As has been stated, the properties of the monomers employed are dependent upon both the composition of the monomers employed and upon the chain length of the polymers. Generally more elastic than those of methacrylate acid and have lower softening points, and likewise, the elasticity increases while the softening point decreases with increase in the number of carbon atoms in the alcohol employed to form the ester with the acid. The softening point and th elasticity of the esters varies from the secondary alcohols through the iso alcohols to the normal alcohols in the order stated, the secondary alcohols having the highest softening point and the lowest elasticity. The secondary butyl alcohol ester has roughly the same properties as the methyl ester, while the propyl and other butyl esters are lower in softening point and more elastic. Consequently, it is possible to obtain various degrees ofelasticity and rigidity by the employment of difl'erent esters and such flexibility in the character of the binder is of very great advantage in the production of abrasive products, some of which should be readily flexible, while others should be more rigid, depending upon the use to which they are put. 4

Since the resinous polymers are softenable by heat, it is usually desirable to employ them in wet grinding operations where the products are not allowed to become heated from the cutting operations. In such applications, these resinous polymers have been found to be highly satisfactory as bonds for various types and sizes of abrasive grain.

I will now illustrate my invention with reference to specific examples, it being understood that such examples are for illustrative purposes only and are not limitative.

Example I A methyl methacrylate polymer was dissolved in toluene to form an adhesive containing 40% solid resin. A coating of this adhesive in the amount of 2.5 pounds of resin solids per ream of 480 sheets 9 inches by 11 inches was spread on 130# rope cylinder paper. mesh fused alumina particles were electrostatically projected onto the adhesive coating. The coating of grain particles was dried sufficiently to set the grains firmly in position. The grain coating was then sized with a second application of adhesive. The sizing adhesive, consisting of a 25% solution of methyl methacrylate polymer in toluene, was applied to the grain coating at the rate of 3 pounds of resin solids per ream.

The abrasive coated paper was air dried to remove the toluene solvent and finally baked 6 hours to complete the hardening of the adhesive.

Example [I A mixture of 70% methyl methacrylate polymer and 30% of very finely ground flint was pulverized to a fine powder. 80 mesh fused alumina particles were moistened with a 20% solution of methyl methacrylate polymer in toluene. The resin-flint powder was then stirred into the moistened grains and the solvent was removed to provide a mass of resin coated abrasive particles. The amount of resin on the abrasive approximated 10%. The resin coated granules were uniformly distributed in a suitable steel mold and were compressed at a pressure of 2000 pounds per square inch while heating the mold and its contents to 350 F. The molded article was cooled and removed from the mold and baked for 16 hours at 300 F. to remove all traces of volatile materials.

As has been stated, my improved binders have the great advantage that they are obtainable in a wide variety of physical properties, particularly the properties of rigidity and elasticity, and it is thus possible to prepare articles having desired combinations of properties which have not been obtainable with the resinous binders most commonly used heretofore; namely, the phenol formaldehyde condensation products. Furthermore, the binders may still be further modified by admixture with other binders such as phenolic resins, to raise the softening point and to increase the rigidity, or the usual modifications of admixing filling materials or plasticizing agents may be used to still further vary the properties of the binder material. The abrading characteristics of abrasive articles are determined largely by the properties of the binder, such as its elasticity, tensile strength and adhesiveness to the abrasive grains. The first two of these properties may be varied within a very considerable range by the selection of a suitable polymer or by admixture with plasticizers or fillers. The resinous polymers of my invention are all possessed of a high degree of adhesiveness which is employment of binders of the class included within my invention provides abrasive articles having greatly improved properties for many purposes,

I and it will be seen that the methods which I have developed in the production of abrasive products with these binders have numerous advantages over methods heretofore commonly used. For example, in the production of a resinous bonded abrasive with the phenol resins, there is the problem resulting from the fact that gases are evolved from the curing of the resins which has the tendency of making the binder spongy, and which require very careful heating of the abrasive article, particularly through the early stages of the reaction to prevent the entrapment of the gases in the binder with the consequent loss of strength and sponginess of the binder. In polymerizing the esters employed in my invention, there is no such problem and consequently, the

process may be carried out without exercising the extreme degree of care necessary where phenolic resins are employed.

This feature is of a considerable value in commercial operations where processes which require careful control are likely to result in considerable spoilage with consequent loss. Furthermore, as

atrix of the has been stated, articles in which the binder is thermoplastic, as in the case of my improved binders, can be reclaimed and it is not necessary to discard the mixtures, if the abrasive article is for any reason unsatisfactory, as is the case when heat-hardening resins are used.

While I have described my invention in considerable detail in respect to certain specific materials and steps of processes, it will be understood that this description is for the purpose of illustrating my invention and is not limitative, the scope of the invention being defined by the appended claims.

I claim:

1. An abrasive article comprising abrasive grains and a binder comprising a polymer in- A eluding a substantial proportion of an ester of an acrylic acid.

2. An abrasive article comprising abrasive grains and a binder comprising a substantial proportion of polymethyl methacrylate.

3. An abrasive article comprising abrasive grains and a binder comprising a substantial proportion of a co-polymer of an acrylic ester with a vinyl ester.

4. An abrasive article comprising abrasive grains and a binder comprising a substantial proportion of the co-polymer of an acrylic acid and an ester of acrylic acid.

5; An abrasive coatedarticle comprising a backing and a layer of abrasive grains adhesively securedthereto by a binder comprising a polymer including a substantial proportion of an ester of an acrylic acid.

6. An abrasive coated article comprising a backing and a layer of abrasive grains adhesively secured thereto by a binder comprising a substantial proportion of the co-polymer of an acrylic acid and an ester of an acrylic acid.

7. In the process of making abrasive articles, the step of bonding abrasive grains with a bond comprising a substantial proportion of a resinous polymer of an ester of an acrylic acid.

NORMAN P. ROBIE'.