MXPA01006178A - Abrasive article bonded using a hybrid bond - Google Patents

Abstract

Bonded abrasive products comprising alumina-based abrasive grains and a geopolymer bond system are significantly improved by providing the grain with a vitreous coating before incorporation in the geopolymer bond.

Description

AGGLUTINATED ABRASIVE ARTICLE USING A HYBRID AGGLUTINANT BACKGROUND OF THE INVENTION This invention relates to abrasive articles made using a hybrid binder material. In the context of this description, it is understood that the term "abrasive article" refers to those materials most commonly described as coated abrasives and bonded abrasives. Coated abrasives are distinguished by the use of a substrate material which is usually flat, and the deposition thereon of abrasive grain bonded to the substrate by a binder material. Conventionally the binder, or a precursor thereof, is deposited on the substrate, and the abrasive grain is deposited on the binder, which is then cured to provide a suitable fixation for the grain. The first layer of binder referred to as the forming layer and a layer on the grain is referred to as the sizing layer. In an alternative configuration, the abrasive grain is mixed with a binder or binder precursor, and the mixture is deposited on the substrate before the binder or binder precursor is cured. The binder / abrasive layer can be deposited as a uniform layer or in a structured pattern which is the result of the deposition process or a subsequent treatment, prior to curing the binder. In the latter situation, the coated abrasive product is often referred to as a structured abrasive. The agglutinated abrasive articles are characterized in that they comprise a three-dimensional structure in which the abrasive grain is maintained in a matrix of a binder, which is conventionally a metal, a glassy material or an organic material. Metal binders are generally reserved for superabrasives. The bonded metal abrasives are generally obtained in the form of thin layers of superabrasive grain welded with brass to a metal surface or wheel. The present invention relates more generally to abrasive articles in which the structure is three-dimensional, and the binder is a hybrid binder. The "hybrid" binders used in the products according to the invention are binders that do not belong to the glass or organic categories. Vitreous binders, as the name suggests, are based on vitreous materials that need to melt and flow to cover the abrasive grain and form binder columns that bind adjacent grains before they are allowed to cool to solidify and hold the structure together. The vitreous bonded materials are therefore formed at high temperatures and using slow forming cycles. The product is however very rigid and particularly effective in precision grinding applications. The organic bonded materials are however formed at considerably lower temperatures, and the binder is a polymeric material that can be formed at relatively low temperatures, and which can be made to become stiff as a result of entanglement. The polymer can be a thermosetting resin such as, for example, a phenol / formaldehyde resin, a urea / formaldehyde resin or an epoxy resin, or it can be a radiation curable resin such as, for example, a urethane resin. acrylated or acrylated epoxy resin or acrylated polyester resin, or any of many variations in such chemical subjects that produce a highly interlaced rigid polymer after exposure to visible light, UV light or electron beam radiation, with or without a catalyst that activates or intensify the transformation. A useful category of hybrid polymeric materials is described in USPP 4,349,386; 4,472,199 and 4,888,311. These documents describe a family of silicoaluminates, polysialates and / or polymers of (siloxo-sialate). Said polymers have the generic formula: Mn [- (S i -O 2 -) z-AI-O 2 -] n.w.H 2 O, wherein M is sodium or potassium, or a mixture thereof, z is from 1 to 3; w has a value of up to 7, and n is the degree of condensation. Such polymers are now generally recognized with the trivial name of "geopolymers". They are conveniently obtained by the addition of a caustic-hydrated aluminosilicate to an alkali metal silicate solution. A minor variation on this subject produces the polymers known as "geosets". These are obtained by the addition of a caustic solution of an alkali metal silicate to a hydrated aluminum silicate. To facilitate the explanation, both types of product will be referred to in the sequential as "geopolymers". The use of said geopolymers in the production of bonded abrasives is recognized in the application EP 0485 966, which also describes that these binders can be modified by the addition of organic polymers. The geopolymers are characterized as "hybrid binders" because they do not resemble vitreous or organic binders, although they have certain characteristics of them. They have very significant advantages over conventional vitreous binders in the production of bonded abrasives. Of primary importance, they are formed at comparatively low temperatures (such as organic binders), are well below the temperature at which the glass melts, and have a uniform composition. In contrast, vitreous binders must be formed at the temperatures of the molten glass and maintained at such temperatures, while the glass flows to cover the abrasive grains and form binder columns. The geopolymers, however, form polymeric structures with a large part of the hardness and strength of vitreous binders and, in this respect, differ from conventional organic binders, which are much less brittle and have higher modulus values than vitreous binders.

The use of geopolymers is therefore a very attractive alternative to conventional vitreous binders from the point of view of their comparatively low formation temperature. As a result of relatively low temperature processing, many advanced technologies such as the use of active fillers which do not exist in vitreous agglutinated products can be incorporated in the binder. Added to these advantages, are the post-processing thermal stability and higher usage temperatures, compared to the organic agglutinated products. The binder materials are therefore of a truly "hybrid" nature. The low processing temperature also makes possible the moderation of part of the brittleness associated with vitreous binders by the addition of organic polymers. There is therefore the possibility of adapting the physical properties of a binder to the needs of the product to be manufactured. However, there is a serious problem with the use of geopolymers in the production of agglutinated abrasive products in which the abrasive is based on alumina. This is because the binders are formed under strongly alkaline conditions, and the surface of the abrasive grain of alumina is attacked by the alkali. The result is a significantly weakening binder between the abrasive and the binder material, so that in actual grinding tests, the performance is unimpressive.

It has now been found that geopolymers can be used with alumina-based abrasives, and this discovery forms the basis of this invention. This discovery opens up the possibility of low cost agglutinated glassy abrasives, where the properties of the bonded abrasive can be adjusted by modifying the binder, and where the binder is highly reproducible and economical to produce and use.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for the production of a bonded abrasive, which comprises providing alumina-based abrasive grains having at least a portion of the surface of said grains, covered by a vitreous layer; mixing said glass-coated alumina-based abrasive grains with a geopolymer, and curing said geopolymer to form an agglutinated abrasive product. The alumina-based abrasive can be a fused alumina or a ceramic (or concreted) alumina, optionally formed by a sol-gel process. It can also be a co-molded alumina-zirconia, or mixture of said grains, with other alumina abrasive grains. The binder attack problem is exacerbated by smaller sizes of alumina crystals, and thus the greatest benefit is ensured when the alumina-based abrasive grains are in fact obtained by a sun-gel process with seeding, such as USP 4,623,364 among others is described, since this generates alumina crystals whose size is sub-microns. Sizes of alumina crystals of up to about 10 microns are obtained by sol-gel procedures without seeding, especially in cases where the growth of the crystals during concretion is inhibited by the presence of rare earth metal oxides, yttria , magnesia, zirconia and silica, and the like. The benefits conferred by the present invention are also quite evident when used with said unseeded sol-gel aluminas. More generally, the invention is also useful with all fused aluminas. The vitreous layer can be deposited on the grain, for example, by treating the grain with fuming silica, followed by a baking process. Alternatively, the grain can be treated with a mixture of conventional glass components, and then baked at a temperature sufficient to form the glass, and allow the glass to flow and cover the grains. The mixture would then be separated to provide the glass covered beans. This procedure could be accelerated and made more uniform by using a sprayed glass frit instead of the glass components. However, the most convenient way to use the method of the present invention is, however, much more direct. During the production of conventional vitreous agglutinated abrasive products, it is found that a certain percentage of the products are outside the prescribed specifications, and should be discarded. Further, after an abrasive product such as a wheel has reached the end of its useful life, there is often a substantial volume of the remaining product. These discarded and remanent products, when compressed, produce abrasive grain at least partially covered with a vitreous layer remaining from the previously used vitreous binder. The surface area of the grains is often essentially 100% covered with glass, except when the grain has been subjected to abrasion, or when the binder column has separated, leaving a portion of the surface exposed. When said reclaimed abrasive grains are based on alumina, they can very adequately provide the coated alumina-based abrasive grains that are used in the present invention. In this way, the present invention provides the opportunity to use waste material that would otherwise have to be sent to landfill operations. The advantages of the present invention are therefore clear. It is adapted to the use of otherwise worthless materials, and is more acceptable from the environmental point of view. However, the advantages are not only economic. The invention also provides for the first time the opportunity to take advantage of processing flexibility in terms of low temperature and rapid curing, and the potential to design binders that meet the requirements of the product to be manufactured.

The preferred embodiment of the invention comprises abrasive grain with a vitreous binder coating (glass) of 0.5 to 5 microns (and more preferably 1 to 3 microns) in thickness. Said coating is quite thick to protect the grain from attack by the binder of caustic geopolymer of high alkali content and, however, still quite thin so as not to change the functions of the grain during the grinding. Obtaining coatings within the preferred scale of the grain / glass ratio may require that it be different, depending on grain size, grain density and glass density. To illustrate this, fused or concreted alumina abrasive grain with particle size of 100 grit (approximately 180 microns), covered with a typical glass vitreous binder, has a grain / glass ratio of 100: 5 by volume, if the thickness of the coating is approximately 1.5 microns, and the grain surface is assumed to be 100% covered. The coating will be slightly higher if the coverage is less than 100%. The amount of the vitreous coating deposited on the grain is preferably sufficient to cover at least 50%, and more preferably at least 70%, of the grain surface. However, it is often difficult or at least inconvenient to measure the degree of coating in this way, whereby the quantity is more conveniently expressed in terms of the percentage by weight represented by the vitreous material. In this way, the weight of the vitreous coating usually represents from 1 to 30%, preferably from 2 to 20%, and more preferably from 2 to 10% of the total weight of the coated grain. The chemical composition of the vitreous layer is preferably one that does not react significantly with the alumina during the coating operation. In this way, formulations comprising alumina, silica, alkaline earth metal oxides and boron oxide, as well as other minor amounts of other metal oxides are frequently useful. Preferred vitreous compositions comprise (by weight) > 47% silica, < 16% of alumina, 0.05-2.5% of potassium oxide, 7-11% of sodium oxide, 2-10% of lithium oxide, and 9-16% of boron oxide. Preferred vitreous compositions, especially when the alumina-based abrasive grains comprise a sol-gel alumina, are the so-called "low temperature binder" formulations, which are understood to be formulations that melt and flow at lower temperatures than approximately 1000 ° C. The geopolymer binder is generally similar to a glassy binder in the sense that it is highly entangled, and therefore rigid and brittle. The pH of the typical geopolymer formulation, before mixing with the grain, is greater than 14. However, unlike conventional glass binders, it can be entangled at temperatures that will not degrade the modifying thermoplastic polymers. In this way, by the use of geopolymers, it becomes possible to incorporate a thermoplastic modifier imparting a degree of flexibility and resistance to the binder material, and this is often a preferred feature of the present invention. Suitable thermoplastic reinforcing or modifying polymers include polyolefins, polybutadiene, polyvinyl chloride, poly (tetrafluoroethylene), polyimides and polyesters. The amount of said reinforcing and / or modification thermoplastic polymer that can be incorporated in the binder, can represent up to 30%, and preferably up to 20% of the total weight of the binder. The geopolymer binder system can also be modified by the use of filler materials. These fillers can be active fillers such as iron pyrites, sulfur or organic grinding aids, provided these are stable at binder-forming temperatures, or inorganic fillers such as mineral particles or glass or ceramic spheres, whose main purpose is to facilitate the generation of the desired degree of porosity or structure in the finished bonded abrasive product. The fillers can be used in proportions, based on the weight of the formulation, of up to 20%, and more preferably from 5 to 10% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 presents representations of bar graphs of the data of example 1. Figure 2 presents representations of bar graphs of the data of example 2.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The invention is now described with specific reference to the following examples, which is understood to imply no essential limitation on the essential scope of the invention.

EXAMPLE 1 This example describes the manufacture of hybrid binder grinding wheels, with and without a coating of vitrified binder (glass) on the abrasive grain. Also compare grinding performance between wheels that contain coated and uncoated grain. Two series of agglutinated abrasive wheels were produced. The first series comprised a conventional fused alumina bead ("Alundum 38" alumina available from Saint-Gobain Industrial Ceramics, Inc., under that trademark designation) in a geopolymer binder, and the second series comprised the same abrasive grain provided with a vitreous coating and manufactured on wheels using the same binder. The abrasive grains of the second series were obtained by compressing a vitreous bonded abrasive wheel in which the vitreous binder had a formulation within the preferred formulation scale specified above. The vitreous material was present mainly as a coating on the separated abrasive grains after compressing the wheel, and represented approximately 3% of the total weight of the coated grain. By means of optical microscopy and scanning electron microscopy, the coated grain appeared to have a smoother, brighter vitreous layer, compared to uncoated grain, and covering at least 80-90% of the total grain surface. The dispersive energy spectroscopy within the SEM revealed the characteristic X-rays emitted from the layer, which were characteristic of a silica-rich, multi-component structure. It was found that the chemical composition of the layer was consistent with that of the glass that had been used to cover the grain. In the formation of the wheels tested, the ratio of the geopolymer binder: abrasive grain, was 25:75 by weight. In each case, the geopolymer comprised the geopolymer of dry binder (GP600HT available from Geopolymere), potassium hydroxide, fuming silica and water. The dry binder material can be obtained by mixing metakaolin, sodium hexafluorosilicate and amorphous silica at the respective weight ratios of 25:18:57. The formulation used to make the wheels was the following: MATERIAL GRAMS Fused alumina (100 grain) 400 GP600HT 66 Fuming silica 21.5 KOH 44.4 Water 48.2 Both sets of wheels (that is, whether or not they contained the glass coated abrasive grain) were prepared in the following manner. Potassium hydroxide was dissolved in water, and allowed to cool. Fuming silica was stirred into the potassium hydroxide solution, producing a potassium silicate solution which was allowed to cool before the dry binder GP600HT was stirred. Finally, the abrasive was mixed in the mixture. If more water was needed, it was added at this point, and mixed into the mixture. The mixture was then poured and compacted in rubber-silicone molds. The mold of the used wheel had the dimensions of 13.65 x 1.27 x 3.18 cm. The filled mold was vibrated for approximately 1 minute. The excess of the mixture was removed, and the mold was covered with a PTFE sheet, a block of ceramic fibrous material, and then weighed with two steel plates, each weighing approximately 4.5 kilograms. The filled and heavy molds were allowed to settle for 2 to 4 hours at room temperature, and were then placed in an oven during the "A" cure cycle indicated in the following table. The wheels were then removed from the molds and placed in a Lindberg oven during the final "B" cure cycle indicated in the table.

Conditions of the curing cycle A Raise the temperature to 85 ° C for 1 hour to 1.5 hours at 85 ° C Raise the temperature to 120 ° C for 1 hour Hold at 120 ° C for 5 hours B Raise the temperature to 350 ° C during 1 hour Keep at 350 ° C for 5 hours The finished wheels each had approximately 30-40% porosity, and the final dimensions after the finishing process were 12.7 x 1.59 x 3.18 cm. Both series of wheels were then subjected to a cross-sectional surface grinding test using a Brown & Sharpe, without the use of refrigerants. The speed of the wheel was maintained at approximately 4700 r.p.m., and the table speed was 15.2m / minute. Prior to grinding, each wheel was carved using a single point diamond at a speed of 25.4 cm / minute, with a 0.025 mm carving compound. The frosted metal was steel 52100, with a hardness of 65Rc in the form of a plate with a dimension of 40.6 cm in the direction of the grinding of the wheel, and 4.6 cm in the direction of the transverse advancement of the wheel. At the transverse feed rate of 1.27 mm, each wheel had a total downward speed of 0.5 mm, with individual vertical downward speeds of 0.0125, 0.025 and 0.05 mm. The G-ratio, the grinding energy and the metal removal rate (MRR) were measured at each individual vertical downward velocity, for both series of wheels, to compare the performances. The results are shown in figure 1 in the form of two bar graphs. The first one compares the performance in terms of graphs of measurement of the G relation to the different speeds of vertical descending advance. The second compares the "crushability" (defined as the G ratio divided by the specific energy that is itself defined as the specific energy divided by MRR), at the different vertical descending velocities. From the data of Figure 1, it is clear that in the cross-sectional surface test, the wheel made with the coated grain gave a significantly higher yield than the wheel made with the non-coated grain in terms of G-ratio and grindability.

EXAMPLE 2 In this example, the effect of the addition of fillers to the binder system to modify the properties is investigated. The abrasive materials used and the molding and baking processes used are as described in Example 1, with the addition of fillers to produce two series of wheels, both containing filler, but one series being manufactured with glass coated abrasive grain. The formulation from which the wheels were manufactured was the one described in Example 1, with the difference that a filler was used which comprised a mixture of 4 parts of fine inorganic powder and 1 part of bubbled mulita beads , available from Zeelan Industries under the trademark "Z-Light". The total amount of filler added was 39.6 g. These wheels were evaluated by a cylindrical control force test (ODCF). Comparatively with the test reported in example 1, the useful contact area of the wheel was smaller, so that the force located on the abrasive grain was much more intense. The ODCF test without refrigerants was carried out in an immersion grinding mode without a sputtering procedure. The frosted metal was 52100 steel with a hardness of 59Rc. The cylindrical metal workpiece had a thickness of 6.4 mm and a diameter of 10.2 cm. Wheel speed was maintained at approximately 4950 r.p.m. and the workpiece was rotated at 150 r.p.m. For each grinding period, the wheel was advanced at a constant controlled force that started at 4.5 kg and increased at 2.3 kg intervals, until excessive wheel wear was obtained. The relation G and the grinding bias were plotted against the grinding force. The results are shown in figure 2 of the drawings in the form of bar graph, and show the same improvement pattern on the wheels manufactured without abrasive grain that lacks the glass coating, as shown in figure 1. This indicates that the economic advantages produced by the presence of filling materials are not accompanied by any deterioration of the physical advantages derived from the use of coated abrasive grain. The results obtained make it clear that, at applied force and reduced metal removal speeds, the wheels manufactured with the coated abrasive grain had a significantly higher performance than the wheels made with the uncoated grain. It is thought that at higher pressures, the predominant failure mode is failure of the binder itself, and this is reflected in the results. In this way, when binder failure is not an important factor, the coated grain used with the geopolymer binder produces a much better grinding wheel than the uncoated grain.

Claims (8)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the production of a bonded abrasive, characterized in that it comprises providing alumina-based abrasive grains having at least a portion of the surface of said grains covered by a vitreous layer; mixing said alumina-based abrasive grains coated with a geopolymer, and curing said geopolymer to form an agglutinated abrasive product.

2. The process according to claim 1, further characterized in that the vitreous layer on the grain represents from 1 to 30% by weight of the weight of the grain.

3. The process according to claim 1, further characterized in that the vitreous layer on the grain covers at least 60% of the grain surface.

4. The process according to claim 1, further characterized in that the geopolymer is mixed with the abrasive grain in proportions so that, in the final abrasive product, the geopolymer represents 10 to 50% of the weight of the product.

5. The process according to claim 1, further characterized in that the geopolymer has the formula: Mn [- (S¡-O2-) z-AI-02-] nwH2O, wherein M is sodium or potassium, or a mixture thereof, z is from 1 to 3; w has a value of up to 7, and n is the degree of condensation.

6. The method according to claim 1, further characterized in that the geopolymer is modified by the incorporation of a thermoplastic polymer.

7. The process according to claim 6, further characterized in that the thermoplastic modifier is selected from the group consisting of polyolefins, polybutadiene, polyvinyl chloride, poly (tetrafluoroethylene), polyimides, polyesters, and mixtures thereof.

8. The process according to claim 1, further characterized in that the formulation also ides up to 10% by weight finely divided filler material.