WO2001023324A1 - Method of making abrasive grain - Google Patents

Abstract

Method of making alpha alumina-based abrasive grain. The abrasive grain can be incorporated into abrasive products such as coated abrasives, bonded abrasives, non-woven abrasives, and abrasive brushes.

Description

METHOD OF MAKING ABRASIVE GRAIN

Field of the Invention This invention pertains to a method of making abrasive grain. The abrasive grain can be incorporated into a variety of abrasive articles, including bonded abrasives, coated abrasives, nonwoven abrasives, and abrasive brushes.

Description of Related Art In the early 1980's a new and substantially improved type of alumina abrasive grain, commonly referred to as "sol gel" or "sol gel-derived" abrasive grain, was commercialized. This new type of alpha alumina abrasive grain had a microstructure made up of very fine alpha alumina crystallites. The grinding performance of the new abrasive grain on metal, as measured, for example, by life of abrasive products made with the grain was dramatically longer than such products made from conventional, fused alumina abrasive grain.

In general, sol gel abrasive grain are typically made by preparing a dispersion or sol comprising water, alumina monohydrate (boehmite), and optionally peptizing agent (e.g., an acid such as nitric acid), gelling the dispersion, drying the gelled dispersion, crushing the dried dispersion into particles, calcining the particles to remove volatiles, and sintering the calcined particles at a temperature below the melting point of alumina. Frequently, the dispersion also includes one or more oxide modifiers (e.g., CeO₂, Cr₂O₃, CoO, Dy₂O₃, Er₂O₃, Eu₂O₃, Fe₂O₃, Gd₂O₃, HfO₂, La₂O₃, Li₂O, MgO, MnO, Na₂O, Nd₂O₃, NiO, Pr₂O₃, Sm₂O₃, SiO₂, SnO₂, TiO₂, Y₂O₃, Yb₂O₃, ZnO, and ZrO₂,), nucleating agents (e.g., α-Al₂O₃, α-Cr₂O₃, and α-Fe₂O₃) and/or precursors thereof. Such additions are typically made to alter or otherwise modify the physical properties and/or microstructure of the sintered abrasive grain. In addition, or alternatively, such oxide modifiers, nucleating agents, and/or precursors thereof may be impregnated into the dried or calcined material (typically calcined particles). Further details regarding sol gel abrasive grain, including methods for making them, can be found, for example, in U.S. Pat. Nos. 4,314,827 (Leitheiser et al.), 4,518,397 (Leitheiser et al), 4,623,364 (Cottringer et al.), 4,744,802 (Schwabel), 4,770,671 (Monroe et al.), 4,881,951 (Wood et al), 4,960,441 (Pellow et al.) 5,011,508 (Wald et al.), 5,090,968 (Pellow), 5,139,978 (Wood), 5,201,916 (Berg et al.), 5,227,104 (Bauer), 5,366,523 (Rowenhorst et al.), 5,429,647 (Larmie), 5,547,479 (Conwell et al.), 5,498,269 (Larmie), 5,551,963 (Larmie), 5,725,162 (Garg et al.), and 5,776,214 (Wood).

Examples of alumina monohydrate (boehmite) that have been reported for use in making sol-gel-derived abrasive grain include those available under the trade designations "DISPERAL" from Condea Chemie, GmbH of Hamburg, Germany, and "DISPAL" and "CATAPAL" (including "CATAPAL A," "CATAPAL B," and "CATAPAL D") from Condea Vista Company of Houston, TX. Such aluminum oxide monohydrates are in the alpha form, and include relatively little, if any, hydrated phases other than monohydrates (although very small amounts of trihydrate impurities can be present in some commercial grade boehmite, which can be tolerated). They typically have a low solubility in water, and have a high surface area (typically at least about 180 m²/g). Boehmite typically includes at least about 2-6 percent by weight free water (depending on the humidity) on its surface.

In addition, PCT Application having International Publication No. WO 99/22912, published May 14, 1999, reports that boehmite suitable for making sol gel derived alumina abrasive grain is available under the trade designation "HI-Q" from the Alcoa Industrial Chemicals.

The boehmite gel, which is produced from the boehmite sol is typically a viscoelastic body having interconnected pores of submicrometric dimensions. The gelled dispersion is known to contain a solid network that entraps a liquid phase (i.e. water with the optional acid in most cases). The network consists of boehmite particles linked through hydrogen bonds of -OH groups on the surface of the particles. When the gel is dried, most of the water entrapped in the solid network is removed, leaving behind a network of empty pores.

The dried gel is usually calcined at a temperature range of 400°C to 800°C. At around 300°C, the formation of gamma-alumina from boehmite is initiated. Further, in the calcining step, the dried particles are densified by dehydroxylation (i.e. removal of the remaining free water and -OH groups), and by vaporizing volatiles. During the sintering step, the number of pores and their connectivity are substantially reduced. The density of the material increases and the volume fraction of porosity decreases. The temperature at which densification occurs is known as the densification temperature. The density of a material is theoretically the highest if all pores are eliminated.

It is typically preferred to produce an abrasive grain at a low sintering temperatures to reduce processing costs (e.g., energy costs), and to minimize detrimental crystal or grain growth of the alpha alumina which tends to increase as the sintering temperature increases. It is also typically preferred to have highly dense (e.g., greater than 90 percent, or even greater than 95 percent of theoretical density) abrasive grain.

However, such densities are generally obtained by sintering at higher temperatures and/or for longer times, which leads undesirable grain growth. In addition, larger alpha alumina crystals may form if the boehmite particles are not properly dispersed (i.e., agglomerated particles) or a non-uniform sol is formed. Over the past fifteen years sintered alumina abrasive grain, in particular sol gel-derived abrasive grain, have been used in a wide variety of abrasive products (e.g., bonded abrasives, coated abrasives, and abrasive brushes) and abrading applications, including both low and high pressure grinding applications.

There is a continuing need for abrasive grain having dense, fine grained (e.g., a microstructure comprised of submicrometer alpha alumina) and high grinding performance characteristics (e.g., high substrate removal rates and long useful life) that are made using low cost raw materials and processes.

Summary Of The Invention The present invention surprisingly provides method for making alpha alumina-based ceramic abrasive grain, the method comprising: preparing a dispersion by combining components comprising liquid medium, peptizing agent (e.g., nitric acid), and boehmite, wherein the dispersion exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis (DTA), of less than 1185°C (preferably, less than 1184°C, 1183°C,

1182°C, 1181 °C. or even 1180°C and wherein at least 25 % by weight (at least 35 % by weight, at least 50 % by weight, at least 75 % by weight, at least 90 % by weight, or even 100% by weight) of the boehmite, based on the total boehmite content of the dispersion, has a dispersability value in the range from 97.5 to 99% (preferably, 97.6% to 99%, 97.7% to 99%, 97.8% to 99%, 97.9% to 99%, or even 98.5% to 99%) and exhibits an alpha alumina transition transformation temperature, as determined by DTA, of less than 1185°C

(preferably, less than 1184°C, 1183°C, 1182°C, 1181 °C, or even 1180°C); converting the dispersion to alpha alumina-based ceramic abrasive grain precursor material; and sintering the precursor material to provide alpha alumina-based ceramic abrasive grain having a density of at 95% (or even at least 97%, or preferably at least 98%) of theoretical density, wherein the precursor material is essentially free of nucleating material (i.e., contains, on a metal oxide basis, less than 0.1 percent by weight nucleating material).

Due to the high degree of dispersability needed to create non-seeded (i.e., made in the absence of (alpha alumina) nucleating material), defect free abrasive grain derived from boehmite, boehmites such as those marketed by Condea Chemie, GmbH under the trade designation "DISPERAL", and by Condea Vista Company under the trade designations "DISPAL 23480" and "CATAPAL D", have been the most preferred raw materials. Non-dispersible material typically becomes entrapped in the dried gel and ultimately leads to undesirable defects in the sintered abrasive grain. Surprisingly,

Applicant has discovered that certain boehmites (e.g., certain boehmites marketed by Alcoa Industrial Chemicals under the trade designation "HI-Q" boehmites) which are lower in dispersability than the boehmites listed above, have been found to produce non- seeded abrasive grain with excellent grinding performance and high density (i.e., greater than 95 percent of theoretical).

Further, titania is known in the art as a densification aid and grain growth enhancer for alpha alumina. Titania is present as an impurity in some commercially available boehmites (including those marketed by Condea Chemie, GmbH under the trade designation "DISPERAL", and by Condea Vista Company under the trade designations "DISPAL" and "CATAPAL"). For a given sintered density (e.g., 90 percent of theoretical), the presence of the titania in such boehmites allows the use of a lower sintering temperature and/or time than would be needed if such titania were not present. That is, the presence of the titania allows for a lower relative sintering temperature and/or time. Surprisingly, Applicant, has discovered that certain boehmites that contain little if any titania can also be processed at a relatively low sintering temperatures, even in the absence of nucleating materials, to make high density alpha alumina abrasive grain. In addition, the absence of titania in the dispersion leads to reducing or minimizing unwanted grain growth in the sintered alpha alumina abrasive grain.

In certain embodiments of the method according to the present invention, the boehmite contains, on a theoretical metal oxide basis, less than 600 ppm TiO₂ (less than 300 ppm TiO₂, less than 150 ppm TiO₂), based on the theoretical Al₂O₃ content of the boehmite. In another aspect, certain embodiments of abrasive grain made according to the method of the present invention contain, on a theoretical metal oxide basis, less than 600 ppm TiO₂ (less than 300 ppm TiO₂, less than 150 ppm TiO₂), based on the theoretical Al₂O₃ content of the abrasive grain. Another surprisingly result obtained using certain boehmite materials (e.g., boehmites market by Alcoa Industrial Chemicals under the trade designation "HIQ" is that the resulting alpha alumina-based abrasive grain may be "sharper". In certain coated abrasive applications, these "sharp" abrasive grains, remove more metal than conventional abrasive grains (including sol-gel-derived alpha alumina-based abrasive grain made using boehmites market by Condea Chemie, GmbH under the trade designation "DISPERAL", and by Condea Vista Company under the trade designations "DISPAL" and "CATAPAL"). One means to measure "sharpness" of an abrasive grain is through a bulk density measurement as described in with ANSI Standard B74.4-1992, published November, 1992. For example, if the bulk density of a conventional sol-gel-derived abrasive grain is about 1.75 g/cm³, the bulk density of abrasive grain made according to the present invention would typically be about 1.66 to about 1.69 g/cm³. In this application:

"Abrasive grain precursor" or "unsintered abrasive grain" refers to a dried alumina-based dispersion (i.e., "dried abrasive grain precursor") or a calcined alumina- based dispersion (i.e., "calcined abrasive grain precursor"), typically in the form of particles, that has a density of less than 80% (typically less than 60%) of theoretical, and is capable of being sintered or impregnated with an impregnation composition and then sintered to provide alpha alumina-based ceramic abrasive grain.

"Alpha alumina-based ceramic abrasive grain," "alumina-based abrasive grain," or "abrasive grain" as used herein refers to a sintered abrasive grain that contains, on a theoretical oxide basis, at least 60%) by weight Al₂O₃, wherein at least 50% by weight of the total amount of alumina is present as alpha alumina.

"Dispersion" or "sol" refers to a solid-in-liquid two-phase system wherein one phase comprises finely divided particles (in the colloidal size range) distributed throughout a liquid. A "stable dispersion" or "stable sol" refer to a dispersion or sol from which the solids do not appear by visual inspection to begin to gel, separate, or settle upon standing undisturbed for about 2 hours.

"Impregnation composition" refers to a solution or dispersion of a liquid medium and a source of metal oxides that can be impregnated into abrasive grain precursor. "Impregnated abrasive grain precursor" refers to a dried alumina-based dispersion (i.e., "impregnated dried abrasive grain precursor") or a calcined alumina-based dispersion (i.e., "impregnated calcined abrasive grain precursor") that has a density of less than 80% (typically less than 60%) of theoretical, and has been impregnated with an impregnation composition, and includes impregnated dried particles and impregnated calcined particles.

"Sintering" refers to a process of heating at a temperature below the melting temperature of the material being heated to provide densification and crystallite growth to provide a tough, hard, and chemically resistant ceramic material. The alpha alumina-based ceramic abrasive grain according to the present invention is not made by a fusion process wherein heating is carried out at a temperature above the melting temperature of the material being heated.

Abrasive grain made according to the method of the present invention can be incorporated into abrasive products such as coated abrasives, bonded abrasives, non- woven abrasives, and abrasive brushes. Brief Description of the Drawing

FIG. 1 is a fragmentary cross-sectional schematic view of a coated abrasive article including abrasive grain made according to the method of the present invention;

FIG. 2 is a perspective view of a bonded abrasive article including abrasive grain made according to the method of the present invention;

FIG. 3 is an enlarged schematic view of a nonwoven abrasive article including abrasive grain made according to the method of the present invention; and

FIGS. 4 and 6 are elevational plan views of an extruder useful in the methods according to the present invention, while FIG. 5 is an enlarged top plan of the extruder feed port.

Detailed Description

The dispersion is prepared by combining components comprising liquid medium (e.g., water, preferably, deionized water), peptizing agent (e.g., an acid such as nitric acid), and boehmite (i.e., alpha alumina monohydrate and/or what is commonly referred to in the art as "pseudo" boehmite (i.e., Al₂O₃-xH₂O, wherein x=l to 2)). The boehmite is capable of converting to alpha alumina upon the appropriate sintering conditions.

More particularly, the dispersion may be prepared, for example, by gradually adding a liquid component(s) to a component(s) that is non soluble in the liquid component(s), while the latter is mixing or tumbling. For example, a liquid containing water, nitric acid, and optionally metal salt may be gradually added to the boehmite, while the latter is being tumbled such that the liquid is more easily distributed throughout the boehmite. Alternatively, for example, the dispersion may be formed by combining boehmite, liquid medium and acid and then mixed to form essentially a homogeneous dispersion. Next, metal oxide precursors may be added to this dispersion.

The alpha alumina transition transformation temperatures referred to herein with regard to the boehmite of dispersion are those as determined by the procedure set forth in the working examples below. The boehmite(s) used to prepare the dispersion is selected and combined, optionally with other boehmite, to provide, with the other components of the dispersion (e.g., liquid medium and peptizing agent), dispersion that exhibits an alpha alumina transition transformation temperature, of less than 1185°C (preferably, less than 1184°C, 1183°C, 1182°C, 1181°C, or even 1180°C.

In another aspect, at least 25 % by weight (at least 35 % by weight, at least 50 % by weight, at least 75 % by weight, at least 90 % by weight, or even 100% by weight) of the boehmite, based on the total boehmite content of the dispersion, has a dispersability value in the range from 97.5 to 99% (preferably, 97.6% to 99%, 97.7% to 99%, 97.8% to 99%o, 97.9% to 99%, or even 98.5% to 99%), wherein "dispersability value" as referred to herein is determined as set forth in the working examples below, and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.

Suitable boehmite for practicing the present invention includes that commercially available from Alcoa Industrial Chemicals under the trade designations "HIQ-30" and "HIQ-40". The average particle size for these two boehmites is reported by the manufacturer to be 50 micrometers; the weight percent Al₂O₃ is reported to be, by weight, 72%. Further the reported crystallite size for these two boehmites is reported to be, at the 020 peak, 33 angstroms and 42 angstroms, respectively; and at the 021 peak, 50 angstroms and 70 angstroms, respectively. It may be preferable to dry mill (e.g., ball milling, attritor milling, or jet milling the boehmite for a time sufficient to increase the average crystallite size of the boehmite by at least ten percent prior to making the dispersion. For additional details regarding dry milling the boehmite see application having U.S. Serial No. 09/407,780. The liquid medium in which the boehmite is dispersed is typically water (preferably deionized water), although organic solvents, such as lower alcohols (typically C,^ alcohols), hexane, or heptane, may also be useful as the liquid medium. In some instances, the liquid medium may be heated (e.g., 60-70°C). The peptizing agent(s) is generally a soluble ionic compound(s) which is believed to cause the surface of a particle or colloid to be uniformly charged in a liquid medium (e.g., water). Preferred peptizing agents are acids or acid compounds. Examples of typical acids include monoprotic acids and acid compounds, such as acetic, hydrochloric, formic, and nitric acid, with nitric acid being preferred. The amount of acid used depends, for example, on the dispersability of the boehmite, the percent solids of the dispersion, the components of the dispersion, the amounts, or relative amounts of the components of the dispersion, the particle sizes of the components of the dispersion, and/or the particle size distribution of the components of the dispersion. The dispersion typically contains at least, 0.1 to 20%, preferably 1% to 10% by weight acid and most preferably 3 to 9% by weight acid, based on the weight of boehmite in the dispersion. In some instances, the acid may be applied to the surface of the boehmite particles prior to being combined with the water. The acid surface treatment may provide improved dispersability of the boehmite in the water.

The boehmite containing dispersions typically comprise greater than 15% by weight (generally from greater than 20% to about 85%) by weight; typically greater than 20% to about 80% by weight; more typically greater than 30%) to about 80%) by weight) solids (or alternatively boehmite), based on the total weight of the dispersion. Certain preferred dispersions, however, comprise 35% by weight or more, 45%) by weight or more, 50%) by weight or more, 55%) by weight or more, 60% by weight or more and 65% by weight or more by weight or more solids (or alternatively boehmite), based on the total weight of the dispersion. Weight percents of solids and boehmite above about 80 wt-% may also be useful, but tend to be more difficult to process to make the abrasive grain provided by the method according to the present invention.

Optionally, the boehmite dispersion includes metal oxide (e.g., particles of metal oxide which may have been added as a paniculate (preferably having a particle size (i.e., the longest dimension) of less than about 5 micrometers; more preferably, less than about 1 micrometer) and/or added as a metal oxide sol (including colloidal metal oxide sol)) and/or metal oxide precursor (e.g., a salt such as a metal nitrate, a metal acetate, a metal citrate, a metal formate, or a metal chloride that converts to a metal oxide upon decomposition by heating). The amount of such metal oxide and or metal oxide precursor (that is in addition to the alumina provided by the boehmite) present in a dispersion or precursor (or metal oxide in the case of the abrasive grain) may vary depending, for example, on which metal oxide(s) is present and the properties desired for the sintered abrasive grain. For dispersions containing such metal oxides (and/or precursors thereof), the metal oxides (that are in addition to the alumina provided by the boehmite) are typically present, on a theoretical metal oxide basis, up to about 25 percent by weight, preferably from about 1 to about 15 percent (more preferably, about 2 to about 15 percent) by weight; although the amount may vary depending, for example, on which metal oxide(s) is present.

Examples of such other metal oxides include: praseodymium oxide, dysprosium oxide, samarium oxide, cobalt oxide, zinc oxide, neodymium oxide, yttrium oxide, ytterbium oxide, magnesium oxide, nickel oxide, manganese oxide, lanthanum oxide, gadolinium oxide, dysprosium oxide, europium oxide, hafnium oxide, and erbium oxide.

Metal oxide precursors include metal nitrate salts, metal acetate salts, metal citrate salts, metal formate salts, and metal chloride salts. Examples of nitrate salts include magnesium nitrate (Mg(NO ) -6H₇O), cobalt nitrate (Co(NO )_^-6H,O), nickel nitrate

(Ni(NO )_?-6H2θ), lithium nitrate (LiNO₃), yttrium nitrate (Y(NO ) -όH_^O), praseodymium nitrate (Pr(NO ) -6H O), samarium nitrate (Sm(NO ) -6H O), neodymium nitrate

(Nd(NO )₃-6H₂O), lanthanum nitrate (La(NO₃)₃-6H O), gadolinium nitrate

(Gd(NO ) -5H O), dysprosium nitrate (Dy(NO ) -5H O), europium nitrate (Eu(NO₃)₃-6H₂O), zinc nitrate (Zn(NO₃)₃-6H₂O), erbium nitrate (Er(NO₃)₃-5H O), zirconium nitrate (Zr(NO ) -5H O), and zirconium hydroxynitrate. Examples of metal acetate salts include zirconyl acetate (ZrO(CH COO) ), magnesium acetate, cobalt acetate, nickel acetate, lithium acetate, yttrium acetate, praseodymium acetate, samarium acetate, ytterbium acetate, neodymium acetate, lanthanum acetate, gadolinium acetate, and dysprosium acetate. Examples of citrate salts include magnesium citrate, cobalt citrate, lithium citrate, and manganese citrate. Examples of formate salts include magnesium formate, cobalt formate, lithium formate, manganese formate, and nickel formate.

The colloidal metal oxides are discrete finely divided particles of amorphous or crystalline metal oxide typically having one or more of their dimensions within a range of about 3 nanometers to about 1 micrometer. The average metal oxide

(including in this context silica) particle size in the colloidal is preferably less than about 150 nanometers, more preferably less than about 100 nanometers, and most preferably less than about 50 nanometers. In some instances, the particles can be on the order of about 3- 10 nanometers. In most instances, the colloidal comprises a distribution or range of metal oxide particle sizes. Preferably, the colloidal metal oxide sols are a stable (i.e.. the metal oxide solids in the sol or dispersion do not appear by visual inspection to begin to gel, separate, or settle upon standing undisturbed for about 2 hours) suspension of colloidal particles (preferably in a liquid medium having a pH of less than 6.5).

Metal oxide sols for use in methods according to the present invention include sols of ceria, silica, yrtria, titania, lanthana, neodymia, zirconia, and mixtures thereof. Metal oxide (including silica) sols are available, for example, from Nalco of Naperville, IL; Nyacol Products, Inc. of Ashland, MA; Eka Nobel of Augusta, GA; Rhone-Ploulenc of Shelton, CT; Transelco of Perm Yan, NY; and Fujimi Corp. of Japan. Silica sols include those available under the trade designations "NALCO 1115", "NALCO 1130", "NALCO 2326", "NALCO 1034A", and "NALCOAG 1056" from Nalco Products,

Inc. of Naperville, IL, wherein the latter two are examples of acidic silica sols; and "NYACOL 215" from Eka Nobel, Inc. For additional information on silica sols see, for example, U.S. Pat. Nos. 5,611,829 (Monroe et al.) and 5,645,619 (Erickson et al.).

If a basic colloidal silica is used, it is preferable to combine the basic colloidal silica with an acid source and convert it to an acidic colloidal silica dispersion

(preferably having a pH of about 1-3) prior to combining it with either boehmite or a source of iron oxide.

The zirconia sol typically comprises finely divided particles of amorphous or crystalline zirconia having one or more dimensions preferably within a range of about 3 nanometers to about 250 nanometers. Zirconia sols are available, for example, from

Nyacol Products, Inc. under the trade designations "ZR100/20" and "ZR10/20". Ceria sols are available, for example, from Rhone-Ploulenc of Shelton, CT; Transelco of Penn Yan, NY; and Fujimi Corp. of Japan. For more information on ceria, silica, or zirconia sols, see, for example, U.S. Pat. Nos. 5,429,647 (Larmie), 5,498,269 (Larmie), 5,551,963 (Larmie), 5,611,829 (Monroe et al.), and 5,645,619 (Erickson et al.).

Whether from colloidal metal oxide directly, or from other forms or sources of colloidal metal oxide, the average metal oxide particle size in the colloidal metal oxide is preferably less than about 150 nanometers, more preferably less than about 100 nanometers, and most preferably less than about 50 nanometers. In some instances, the metal oxide particles can be on the order of about 3-10 nanometers. In most instances, the colloidal metal oxide comprises a distribution or range of metal oxide particle sizes. Typically, the use of a metal oxide modifier may decrease the porosity of the sintered abrasive grain and thereby increase the density. Certain metal oxides may react with the alumina to form a reaction product and/or form crystalline phases with the alpha alumina which may be beneficial during use of the abrasive grain in abrading applications. For example, the oxides of cobalt, nickel, zinc, and magnesium typically react with alumina to form a spinel, whereas zirconia and hafnia do not react with the alumina. Alternatively, the reaction products of dysprosium oxide and gadolinium oxide with aluminum oxide are generally garnet. The reaction products of praseodymium oxide, ytterbium oxide, erbium oxide, and samarium oxide with aluminum oxide generally have a perovskite and/or garnet structure. Yttria can also react with the alumina to form Y₃Al₅O₁₂ having a garnet crystal structure. Certain rare earth oxides and divalent metal cations react with alumina to form a rare earth aluminate represented by the formula LnMAl_πO₁₉, wherein Ln is a trivalent metal ion such as La³⁺, Nd³\ Ce³⁺, Pr³⁺, Sm³\ Gd³⁺, Er^3*, or Eu³⁺, and M is a divalent metal cation such as Mg²⁺, Mn²⁺, Ni²⁺, Zn²⁺, or Co²⁺. Such aluminates have a hexagonal crystal structure. For additional details regarding the inclusion of metal oxide (and/or precursors thereof) in a boehmite dispersion see, for example, in U.S. Pat. Nos. 4,314,827 (Leitheiser et al.), 4,770,671 (Monroe et al.), 4,881,951 (Wood et al.) 5,429,647 (Larmie), and 5,551,963 (Larmie).

General procedures for making sintered alpha alumina-based abrasive grain are also described, for example, in U.S. Pat. Nos. 4,518,397 (Leitheiser et al.), 4,770,671

(Monroe), 4,744,802 (Schwabel), 5,139,978 (Wood), 5,219,006 (Wood), and 5,593,647 (Monroe).

A high solids dispersion is typically, and preferably, prepared by gradually adding a liquid component(s) to a component(s) that is non-soluble in the liquid component(s), while the latter is mixing or tumbling. For example, a liquid containing water, peptizing agent, and optional metal salt(s) may be gradually added to boehmite, while the latter is being mixed such that the liquid is more easily distributed throughout the boehmite.

Suitable mixers include pail mixers, sigma blade mixers, ball mill and high shear mixers. Other suitable mixers may be available from Eirich Machines, Inc. of

Gurnee, IL; Hosokawa-Bepex Corp. of Minneapolis, MN (including a mixer available under the trade designation "SCHUGI FLEX-O-MIX", Model FX-160); and Littleford- Day, Inc. of Florence, KY.

The dispersion may be heated to increase the dispersability of the alpha alumina monohydrate and/or to create a homogeneous dispersion. The temperature may vary to convenience, for example the temperature may range from about 20°C to 80°C, usually between 25°C to 75°C. Alternatively, the dispersion may be heated under a pressure ranging from 1.5 to 130 atmospheres of pressure.

The dispersion typically gels prior to, or during, drying. The addition of most modifiers may result in the dispersion gelling faster. Alternatively, ammonium acetate or other ionic species may be added to induce gelation of the dispersion. The pH of the dispersion and concentration of ions in the gel generally determines how fast the dispersion gels. Typically, the pH of the dispersion is within a range of about 1.5 to about 5.

The dispersion may be extruded. It may be preferable to extrude a dispersion where at least 50 percent by weight of the alumina content is provided by particulate (e.g., boehmite), including in this context a gelled dispersion, or even partially deliquified dispersion. The extruded dispersion, referred to as extrudate, can be extruded into elongated precursor material (e.g., rods (including cylindrical rods and elliptical rods)). After firing, the rods may have an aspect ratio between 1.5 to 10, preferably between 2 to 6. Alternatively the extrudate may be in the form of a very thin sheet, see for example U.S. Pat. No. 4,848,041 (Kruschke). Examples of suitable extruders include ram extruders, single screw, twin screw, and segmented screw extruders. Suitable extruders are available, for example, from Loomis Products of Levitown, PA, Bonnot Co. of Uniontown, OH, and Hosokawa-Bepex of Minneapolis, MN, which offers, for example, an extruder under the trade designation "EXTRUD-O-MIX" (Model EM-6).

Preferably, the dispersion is compacted, for example, prior to or during extrusion (wherein the extrusion step may inherently involve compaction of the dispersion). In compacting the dispersion, it is understood that the dispersion is subjected to a pressure or force such as experienced, for example, in a pellitizer or die press (including mechanical, hydraulic and pneumatic or presses) or an extruder (i.e., all or substantially all of the dispersion experiences the specified pressure). In general, compacting the dispersion reduces the amount of air or gases entrapped in the dispersion, which in turn generally produces a less porous microstructure, that is more desirable. Additionally the compaction step results an easier means to continuously feed the extruder and thus may save on labor producing the abrasive grain. If the elongated precursor material is a rod, it preferably has a diameter such that the sintered abrasive grain will have a diameter of, for example, about 150-5000 micrometers, and preferably, an aspect ratio (i.e., length to width ratio) of at least 2.5:1, at least 4:1, or even at least 5:1. The rod may have any cross sectional shape including a circle, an oval, a star shape, a tube and the like. The rod abrasive grain may also be curved.

A preferred apparatus for compacting the dispersion (gelled or not) is illustrated in FIGS. 4-6. Modified segmented screw extruder 40, has feed port 41 and auger 42 centrally placed within barrel 44. FIG. 5 is a view of the interior of extruder 40 looking through feed port 41. Barrel 44 has grooves (not shown; generally known as "lands") running parallel down its length. Pins 48 extend centrally into barrel 44. Further, helical flight 46 extends the length of auger 42. Flight 46 is not continuous down the length of auger 42 but is segmented so that flight 46 on auger 42 does not come into contact with pins 48.

The dispersion (including in this context gelled dispersion) (not shown) is fed in feed port 41. Packer screw 43 urges the dispersion against auger 42 so that the dispersion is compacted by auger 42 and extruded through die 49. Die 49 can have a variety of apertures or holes therein (including a single hole or multiple holes). The die apertures can be any of a variety of cross sectional shapes, including a circle or polygon shapes (e.g., a square, star, diamond, trapezoid, or triangle). The die apertures can be any of a variety of sizes, but typically range from about 0.5 mm (0.02 inch) to 1.27 cm (0.5 inch), and more typically, from about 0.1 cm (0.04 inch) to about 0.8 cm (0.3 inch).

The extruded dispersion can be can be cut or sliced, for example, to provide discrete particles, and /or to provide particles having a more uniform length. Examples of methods for cutting (or slicing) the dispersion include rotary knife, blade cutters and wire cutters. The compacted dispersion can also be shredded and/or grated. In general, techniques for drying the dispersion are known in the art, including heating to promote evaporation of the liquid medium, or simply drying in air. The drying step generally removes a significant portion of the liquid medium from the dispersion; however, there still may be a minor portion (e.g., about 10% or less by weight) of the liquid medium present in the dried dispersion. Typical drying conditions include temperatures ranging from about room temperature to over about 200°C, typically between 50 to 150°C. The times may range from about 30 minutes to over days. To prevent salt migration, it may be desirable to dry the dispersion at low temperature.

After drying, the dried dispersion may be converted into precursor particles. One typical means to generate these precursor particles is by a crushing technique.

Various crushing or comminuting techniques may be employed such as a roll crusher, jaw crusher, hammer mill, ball mill and the like. Coarser particles may be recrushed to generate finer particles. It is also preferred that the dried dispersion be crushed, as, for example, it is generally easier to crush dried gel versus the sintered alpha alumina based abrasive grain. Thus it is preferred to generate the desired particle size distributions prior to sintering.

Alternatively, for example, the dispersion may be converted into precursor particles prior to drying. This may occur for instance if the dispersion is processed into a desired grit shape and particle size distribution. For example, the dispersion may be extruded into rods that are subsequently cut to the desired lengths and then dried.

Alternatively, the dispersion may be molded into a triangular shape particle and then dried. Additional details concerning triangular shaped particles may be found in U.S. Pat. No. 5,201,916 (Berg et al.).

Alternatively, for example, the dried dispersion is shaped into lumps with a high volatilizable content which then are explosively communited by feeding the lumps directly into a furnace held at a temperature above 350°C, usually a temperature between 600°C to 900°C.

Typically, the dried dispersion is calcined, prior to sintering, although a calcining step is not always required. In general, techniques for calcining the dried dispersion or ceramic precursor material, wherein essentially all the volatiles are removed, and the various components that were present in the dispersion are transformed into oxides, are known in the art. Such techniques include using a rotary or static furnace to heat dried dispersion at temperatures ranging from about 400-1000°C (typically from about 450-800°C) until the free water, and typically until at least about 90 wt-% of any bound volatiles are removed. It is also within the scope of the present invention to impregnate a metal oxide modifier source (typically a metal oxide precursor) into a calcined precursor particle. Typically, the metal oxide precursors are in the form metal salts. These metal oxide precursors and metal salts are described above with respect to the boehmite dispersion.

Methods of impregnating sol gel-derived particles are described in general, for example, in U.S. Pat. No. 5,164,348 (Wood). In general, ceramic precursor material

(i.e., dried alumina-based dispersion (or dried ceramic precursor material), or calcined alumina-based dispersion (or calcined ceramic precursor material)) is porous. For example, a calcined ceramic precursor material typically has pores about 2-10 nanometers in diameter extending therein from an outer surface. The presence of such pores allows an impregnation composition comprising a mixture comprising liquid medium (typically water) and appropriate metal precursor to enter into ceramic precursor material. The metal salt material is dissolved in a liquid, and the resulting solution mixed with the porous ceramic precursor particle material. The impregnation process is thought to occur through capillary action. The liquid used for the impregnating composition is preferably water

(including deionized water), an organic solvent, and mixtures thereof. If impregnation of a metal salt is desired, the concentration of the metal salt in the liquid medium is typically in the range from about 5% to about 40% dissolved solids, on a theoretical metal oxide basis. Preferably, there is at least 50 ml of solution added to achieve impregnation of 100 grams of porous precursor particulate material, more preferably, at least about 60 ml of solution to 100 grams of precursor particulate material.

After the impregnation, the resulting impregnated precursor particle is typically calcined to remove any volatiles prior to sintering. The conditions for this calcining step are described above. After the precursor particle is formed or optionally calcined, the precursor particle is sintered to provide a dense, ceramic alpha alumina based abrasive grain. In general, techniques for sintering the precursor material, which include heating at a temperature effective to transform transitional alumina(s) into alpha alumina, to causing all of the metal oxide precursors to either react with the alumina or form metal oxide, and increasing the density of the ceramic material, are known in the art. The precursor material may be sintered by heating (e.g., using electrical resistance, microwave, plasma, laser, or gas combustion, on batch basis or a continuous basis). Sintering temperatures are usually range from about 1200°C to about 1650°C, typically, from about 1200°C to about 1500°C. The length of time which the precursor material is exposed to the sintering temperature depends, for example, on particle size, composition of the particles, and sintering temperature. Further, the sintering temperature and/or time needed to achieve a specified density decreases as the pore size of the calcined abrasive grain decreases. Typically, sintering times range from a few seconds to about 60 minutes (preferably, within about 3-30 minutes). Sintering is typically accomplished in an oxidizing atmosphere, although inert or reducing atmospheres may also be useful. The longest dimension of the alpha alumina-based abrasive grain is typically at least about 1 micrometer. The abrasive grain described herein can be readily made with a length of greater than about 50 micrometers, and larger abrasive grain (e.g., greater than about 1000 micrometers or even greater than about 5000 micrometers) can also be readily made. Generally, the preferred abrasive grain has a length in the range from about 100 to about 5000 micrometers (typically in the range from about 100 to about

3000 micrometers), although other sizes are also useful, and may even be preferred for certain applications. In another aspect, abrasive grain according to the present invention, typically have an aspect ratio of at least 1.2:1 or even 1.5:1, sometimes, at least 2:1, and alternatively, at least 2.5:1. Dried, calcined, and/or sintered materials provided during or by the method according to the present invention, are typically screened and graded using techniques known in the art. For example, the dried particles are typically screened to a desired size prior to calcining. Sintered abrasive grain are typically screened and graded prior to use in an abrasive application or incorporation into an abrasive article. Screening and grading of abrasive grain made according to the method of the present invention can be done, for example, using the well known techniques and standards for ANSI (American National Standard Institute), FEPA (Federation Europeenne des Fabricants de Products Abrasifs), or JIS (Japanese Industrial Standard) grade abrasive grain.

It is also within the scope of the present invention to recycle unused (typically particles too small in size to provide the desired size of sintered abrasive grain) deliquified dispersion material as generally described, for example, in U.S. Pat. No.

4,314,827 (Leitheiser et al.). For example, a first dispersion can be made as described above, dried, crushed, and screened, and then a second dispersion made by combining, for example, liquid medium (preferably, aqueous), boehmite, and deliquified material from the first dispersion, and optionally metal oxide and/or metal oxide precursor. Optionally, the first dispersion includes nucleating material. The recycled material may provide, on a theoretical metal oxide basis, for example, at least 10 percent, at least 30 percent, at least 50 percent, or even up to (and including) 100 percent of the theoretical Al₂O₃ content of the dispersion which is deliquified and converted (including calcining and sintering) to provide the sintered abrasive grain. It is also within the scope of the present invention to coat the abrasive grain with a surface coating such as described in U.S. Pat. Nos. 1,910,440 (Nicholson), 3,041,156 (Rowse), 5,009,675 (Kunz et al.), 4,997,461 (Markhoff-Matheny et al.), and 5,042,991 (Kunz et al.), 5,011,508 (Wald et al.), and 5,213,591 (Celikkaya et al.).

Abrasive grain made by a method according to the present invention can be used in conventional abrasive products, such as coated abrasive products, bonded abrasive products (including vitrified and resinoid grinding wheels, cutoff wheels, and honing stones), nonwoven abrasive products, and abrasive brushes. Typically, abrasive products (i.e., abrasive articles) include binder and abrasive grain, at least a portion of which is abrasive grain made by a method according to the present invention, secured within the abrasive product by the binder. Methods of making such abrasive products are well known to those skilled in the art. Furthermore, abrasive grain made by a method according to the present invention can be used in abrasive applications that utilize slurries of abrading compounds (e.g., polishing compounds).

Coated abrasive product generally include a backing, abrasive grain, and at least one binder to hold the abrasive grain onto the backing. The backing can be any suitable material, including cloth, polymeric film, fibre, nonwoven webs, paper, combinations thereof, and treated versions thereof. The binder can be any suitable binder, including an inorganic or organic binder. The abrasive grain can be present in one layer or in two layers of the coated abrasive product. Methods of making coated abrasive products are described, for example, in U.S. Pat. Nos. 4,734,104 (Broberg), 4,737,163 (Larkey), 5,203,884 (Buchanan et al.), 5,378,251 (Culler et al.), 5,417,726 (Stout et al.), 5,436,063

(Follett et al.), 5,496,386 (Broberg et al.), and 5,520,711 (Helmin).

An example of a coated abrasive product is depicted in FIG. 1. Referring to this figure, coated abrasive product 1 has a backing (substrate) 2 and abrasive layer 3. Abrasive layer 3 includes abrasive grain 4 secured to a major surface of backing 2 by make coat 5 and size coat 6. In some instances, a supersize coat (not shown) is used.

Bonded abrasive products typically include a shaped mass of abrasive grain held together by an organic, metallic, or vitrified binder. Such shaped mass can be, for example, in the form of a wheel, such as a grinding wheel or cutoff wheel. It can also be in the form, for example, of a honing stone or other conventional bonded abrasive shape. It is typically in the form of a grinding wheel. Referring to FIG. 2, grinding wheel 10 is depicted, which includes abrasive grain 11, at least a portion of which is abrasive grain made by a method according to the present invention, molded in a wheel and mounted on hub 12. For further details regarding bonded abrasive products, see, for example, U.S. Pat. Nos. 4,997,461 (Markhoff-Matheny et al.) and 4,898,597 (Hay et al.). Nonwoven abrasive products typically include an open porous lofty polymer filament structure having abrasive grain distributed throughout the structure and adherently bonded therein by an organic binder. Examples of filaments include polyester fibers, polyamide fibers, and polyaramid fibers. In FIG. 3, a schematic depiction, enlarged about lOOx, of a typical nonwoven abrasive product is provided. Such a nonwoven abrasive product comprises fibrous mat 50 as a substrate, onto which abrasive grain 52, at least a portion of which is abrasive grain made by a method according to the present invention, are adhered by binder 54. For further details regarding nonwoven abrasive products, see, for example, U.S. Pat. No. 2,958,593 (Hoover et al.).

Useful abrasive brushes include those having a plurality of bristles unitary with a backing (see, e.g., U.S. Pat. No. 5,679,067 (Johnson et al.)). Preferably, such brushes are made by injection molding a mixture of polymer and abrasive grain. Suitable organic binders for the abrasive products include thermosetting organic polymers. Examples of suitable thermosetting organic polymers include phenolic resins, urea-formaldehyde resins, melamine-formaldehyde resins, urethane resins, acrylate resins, polyester resins, aminoplast resins having pendant α,β-unsaturated carbonyl groups, epoxy resins, and combinations thereof. The binder and/or abrasive product can also include additives such as fibers, lubricants, wetting agents, thixotropic materials, surfactants, pigments, dyes, antistatic agents (e.g., carbon black, vanadium oxide, graphite, etc.), coupling agents (e.g., silanes, titanates, zircoaluminates, etc.), plasticizers, suspending agents, and the like. The amounts of these optional additives are selected to provide the desired properties. The coupling agents can improve adhesion to the abrasive grain and/or filler.

The binder can also contain filler materials or grinding aids, typically in the form of a particulate material. Typically, the particulate materials are inorganic materials. Examples of particulate materials that act as fillers include metal carbonates, silica, silicates, metal sulfates, metal oxides, and the like. Examples of particulate materials that act as grinding aids include: halide salts such as sodium chloride, potassium chloride, sodium cryolite, and potassium tetrafluoroborate; metals such as tin, lead, bismuth, cobalt, antimony, iron, and titanium; organic halides such as polyvinyl chloride and tetrachloronaphthalene; sulfur and sulfur compounds; graphite; and the like. A grinding aid is a material that has a significant effect on the chemical and physical processes of abrading, which results in improved performance. In a coated abrasive product, a grinding aid is typically used in the supersize coat applied over the surface of the abrasive grain, although it can also be added to the size coat. Typically, if desired, a grinding aid is used in an amount of about 50-300 g/m² (preferably, about 80-160 g/m²) of coated abrasive product.

The abrasive products can contain 100%) abrasive grain made by a method according to the present invention, or they can contain a blend of such abrasive grain with conventional abrasive grain and/or diluent particles. However, at least about 5%> by weight, and preferably about 30-100% by weight, of the abrasive grain in the abrasive products should be abrasive grain made by a method according to the present invention.

Examples of suitable conventional abrasive grain include fused aluminum oxide, silicon carbide, diamond, cubic boron nitride, garnet, fused alumina zirconia, and other sol-gel abrasive grain, and the like. Examples of suitable diluent particles include marble, gypsum, flint, silica, iron oxide, aluminum silicate, glass, and diluent agglomerates. Abrasive grain ade by a method according to the present invention can also be combined in or with abrasive agglomerates. An example of an abrasive agglomerate is described in

U.S. Pat. Nos. 4,311,489 (Kressner), 4,652,275 (Bloecher et al.), and 4,799,939 (Bloecher et al.).

Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. All parts and percentages are by weight unless otherwise indicated.

Examples Any reference to the percent solids levels of the dispersion used in the following examples are the approximate solids levels, as they do not take into account the 2-6%) water commonly found on the surface of boehmite, nor the solids provided by any non-boehmite additives.

The following designations are used in the examples: D DWWTT deionized water that was at a temperature of 60-65°C, unless otherwise specified

HNO₃ nitric acid, 70% concentrated AAMH an alpha-alumina monohydrate (boehmite) (obtained from Condea Chemie, Hamburg, Germany, under the trade designation "DISPERAL")

H-40 alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial Chemicals, Houston, TX, under the trade designation "HIQ-40")

H-30 alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial Chemicals under the trade designation "HIQ-30")

H-20 alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial Chemicals under the trade designation "HIQ-20") H-10 alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial

Chemicals under the trade designation "HIQ- 10")

H-X alpha-alumina monohydrate (boehmite) (obtained from Alcoa Industrial

Chemicals under the trade designation "HIQ", wherein the manufacturer indicated that this boehmite had properties of, or in properties between, their boehmites marketed under the trade designations "HIQ-30" and "HIQ- 40")

C-A alpha-alumina monohydrate (boehmite) obtained from Condea Vista

Company, Houston, TX, under the trade designation "CATAPAL A"; C-B alpha-alumina monohydrate (boehmite) obtained from Condea Vista

Company under the trade designation "CATAPAL B"

C- D alpha-alumina monohydrate (boehmite); obtained from Condea Vista

Company under the trade designation "CATAPAL D";

AAMH-DIS alpha-alumina monohydrate (boehmite); obtained from Condea Vista Chemical Company under the trade designation "DISPAL 23N480"

MEM a rare earth nitrate solution prepared by mixing a lanthanum, neodymium, and yttrium nitrate (having, on a theoretical metal oxide basis, 23% rare earth oxide (i.e., La₂O₃, Nd₂O₃, and Y₂O₃); available from Molycorp of

Lourviers, CO) with a sufficient amount of magnesium nitrate (Mg(NO₃)₂-6H₂O) solution (having, on a theoretical metal oxide basis, 11%

MgO; available from Mallinckrodt Chemical of Paris, KY) and cobalt nitrate (Co(NO₃)₂-6H₂O) solution (having, on a theoretical metal oxide basis 19% CoO; available from Hall Chemical of Wickliffe, OH) to provide a solution containing, on a theoretical metal oxide basis 5.8% La(NO₃)₃-6H₂O, 5.8% Nd(NO₃)₃-6H₂O, about 7.1% Y(NO₃)₃-6H₂O, about

14.4% Mg(NO₃)₂-6H₂O, about 0.4% Co(NO₃)₂-6H₂O, and the balance deionized water. Differential Thermal Analysis (DTA) And Thermogravimetric Analysis (TGA)

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) was conducted for each of the following boehmites: AAMH, H-40, H-30, H-20, H-10, C- A, C-B, C-D, and AAMH-DIS. For each boehmite, except AAMH-DIS, a dispersion was made by mixing together 200 grams of the boehmite, 10 grams of HNO₃, and 1333 grams of DWT in the conventional 4 liter, food grade blender (Waring blender available from Waring Products Division, Dynamics Corp. of America, New Hartford, CT; Model 34BL22(CB6)) at low speed for one minute. A dispersion was also prepared in the same manner using AAMH- DIS except no nitric acid was added as this boehmite, as-received, includes nitric acid.

About 100 grams of each the resulting sols were poured into 22.9 cm x 30.5 cm (9 inch x 12 inch) glass trays (obtained under the trade designation "PYREX"). The sols were allowed to dry at room temperature over a weekend. The resulting dried, friable materials were each screened to retain particles that were in the 300-400 micrometer size range.

Two independent DTA and TGA runs were made (using a equipment obtained from Seiko Instruments Inc., Chiba, Japan under the trade designation "SEIKO SSC-5200 TG/DTA 320") for each of the samples made from AAMH, C-D, and H-40. One DTA run was made of the remaining samples. Approximately 40 mg each of sample and an alpha alumina powder reference were heated in separate 100 ul (i.e., microliter) Pt sample vessels using an air flow of 100 ml/min over the samples. The samples were heated at a rate of 10°C/minute from room temperature to 1350°C. The DTA and TGA results are shown in Tables 1 and 2 respectively, below.

Table 1

Table 2

There was no significant weight loss observed for any sample after the conversion to transitional alumina. An unknown peak was observed for AAMH for both the DTA and the TGA. For the two DTA runs, the peak was at 192.0°C and 194.9°C, respectively. For the two TGA runs, the peak was at 189.1°C and 192.0°C, respectively. Dispersability Test

Sols were prepared mixing 444.3 grams of DWI, 5 grams of HNO₃, and 66.7 grams of the respective boehmite powders using a conventional household blender (obtained under the trade designation "OSTERIZER" from Oster Products, Sunbeam Coφ., Boca Raton, FL) at its lowest speed for one minute, except no acid was used to make the AAMH-DIS sol. Each sol was centrifuged at 2400 φm for 15 minutes (using a centrifuge obtained under the trade designation 'TEC CENTRA-7 CENTRIFUGE" from Damon/IEC Division, Needham Heights, MA). The supernate for each sample was carefully removed and discarded. The remaining "sludge" was dried at 90°C for one hour. The dried "sludge" was then weighed. The dispersability value, reported in Table 1, above, is one minus the weight ratio of the dried sludge over the weight of the original powder used (i.e., 66.7 grams), times 100%.

Titania Content Of Various Boehmites The titania content of AAMH, H-10, H-20, H-30, H-40, C-A, C-B, C-D, and AAMH-DIS were determined by x-ray fluorescence using a wavelength dispersive x- ray fluorescence spectrometer obtained under the trade designation "SIEMENS SRS 200 DISPERSIVE X-RAY FLUORESCENCE SPECTROMETER" from Siemens AG, Munich, Germany. The titania contents of the boehmite materials are summarized below in Table 3.

Table 3

Comparative Example A and Example 1

Comparative Example A was prepared and tested as follows. A dispersion was prepared by mixing 680 grams of AAMH, 36 grams of HNO₃, and 1.284 grams of DWT together in the 4 liter, food grade blender (Model 34BL22(CB6)). The contents were then mixed at low speed setting for 60 seconds.

The resulting dispersion was poured into a glass tray ("PYREX") and dried overnight at about 93 °C to provide dried, friable solid, material. The dried material was crushed with a pulverizer (obtained under the trade designation "BRUAN" Type UA,

Braun Coφ.) and screened to retain the particles in the -20+60 mesh (U.S.A. Standard Testing Sieves) size range.

The retained particles were fed into a calcining kiln to provide calcined abrasive grain precursor material. The calcining kiln consisted of a 15 cm inner diameter, 1.2 meter in length, stainless steel tube having a 0.3 meter hot zone. The tube was inclined at a 2.4 degree angle with respect to the horizontal. The tube rotated at about 7 φm, to provide a residence time in the tube of about 15 minutes. The temperature of the hot zone was about 650°C.

The calcined material was impregnated by mixing 70 grams of MEM per 100 grams of calcined material. More specifically, impregnation solution was sprayed onto the calcined material as it was being tumbled in a container. The impregnated material was dried at 80°C. and calcined again at 650°C.

The re-calcined particles were fed into a rotary sintering kiln. The sintering kiln consisted of an 8.9 cm inner diameter, 1.32 meter long silicon carbide tube inclined at 4.4 degrees with respect to the horizontal and had a 31 cm hot zone. The heat was applied externally via SiC electric heating elements. The sintering kiln rotated at 10.5 φm, to provide a residence time in the tube of about 4 minutes. The sintering temperature was about 1410°C. The product exited the kiln into room temperature air where it was collected in a metal container and allowed to cool to room temperature.

The density of the abrasive grain was determined with a helium gas pycnometer (obtained under the trade designation "MICROMERITICS ACCUPYC 1330" from Micromeritics Instruments Coφ., Norcross, GA). The density of the Comparative

Example A abrasive grain was 3.94 g/cm³. The sintered alpha alumina-based ceramic abrasive grain was graded to retain the -30+35 and -35+40 mesh fractions (U.S.A. Standard Testing Sieves). These fractions were blended in a 1 :1 ratio and incoφorated into coated abrasive discs, which were tested for grinding performance. The coated abrasive discs were made according to conventional procedures. The abrasive grain were bonded to 17.8 cm diameter vulcanized fiber backings (having a 2.2 cm diameter center hole) using a conventional calcium carbonate-filled phenolic make resin (48% resole phenolic resin, 52% calcium carbonate, diluted to 81% solids with water) and a conventional cryolite-filled phenolic size resin (32%» resole phenolic resin and 68% cryolite, diluted to 78% solids with water). The wet make resin weight was 150 g/m². Immediately after the make coat was applied, the abrasive grains were electrostatically coated. The resulting construction was heated at 77°C for 15 minutes, and then at 93 °C for 90 minutes to partially cure the make resin. The wet size weight was about 670 g/m². The size resin was precured for 1 hour at 77°C for one hour, followed by a final cure for 16 hours at 102°C. The fibre discs were flexed prior to testing.

The coated abrasive discs were tested by attaching a disc to a 16.5 cm diameter, 1.57 mm thick hard, phenolic backup pad which was in turn mounted onto a 15.2 cm diameter steel flange. The mounted disc was rotated counterclockwise at 3550 φm. The 1.8 mm peripheral edge of a 25 cm diameter disc of a 304 stainless steel (workpiece) was deployed 7 degrees from a position normal to the abrasive disc under a load of about 4 kg. The workpiece was weighed at the start of the test, and at 2 minute intervals, to determine the amount of steel removed (i.e., abraded). Two 2 minute intervals (for a total of 10 minutes of grinding) were conducted for each abrasive disc. The average (based on 4 discs) amount of metal removed during the 10 minute test was 124.3 grams. Example 1 was prepared and tested as described for Comparative Example

A, except the AAMH was replaced with H-X. The density of Example 1 abrasive grain was determined as described for Comparative Example A to be 3.93 g/cm³. The average (based on 4 discs) amount of metal removed during the 10 minute Example 1 grinding performance test was 157.8 grams. Example 2

Approximately 1200 grams of H-40 powder was ball-milled at 80 φm in a polyurethane lined vessel (30 cm) 12 inches) x 23 cm (9 inches); Paul O. Abbe, Inc., Little Falls, NJ) containing about 8000 grams of 1.27 cm (0.5 inch) zirconia media (U.S. Stoneware, East Palestine, OH) for 24 hours. After recovering the ball-milled powder,

1000 grams of the powder was mixed with an acid- water solution (prepared by mixing 60 grams of HNO₃ with 607.6 grams of DWT) by hand in a plastic wash-tub. The resulting powder material was allowed to stand over a weekend in sealed-plastic bags. The powdered material was then extruded through a single screw extruder containing a die with 36 2.54 mm (0.1 inch) openings. The extrudate was dried through a tunnel oven at a rate of 1 m/min. The tunnel oven had first zone, 2.43 meters in length, set at 71°C (160°F), and the second zone, 2.43 meters in length, set at 82°C (180°F). The dried extrudate as collected in aluminum pans and allowed to stand for 2 hours in a forced air oven set at 80°C. The resulting dried material was crushed into particles using a pulverizer

(having a 1.1 mm gap between the steel plates; obtained under the trade designation "BRAUN" Type UA from Braun Coφ., Los Angeles, CA) and screened to sizes +30-16 mesh (0.68 mm) to (1.44 mm) using a conventional screener (obtained under the trade designation "EXOLON SCREENER" from Exolon-ESK, Tonawanda, NY). The retained particles were calcined as described in Comparative Example

A, except the tube was inclined at a 3.0 degree angle with respect to the horizontal, the tube rotated at about 3.5 φm, and the residence time in the tube was about 4-5 minutes.

The calcined particles were impregnated with MEM, wherein the ratio of solution to particles was 60 ml of solution to 100 grams of particles. The impregnated particles were dried using a blow gun. The dried, impregnated particles were then calcined again at 650°C.

The re-calcined abrasive grain precursor particles were sintered as described in Comparative Example A, except the sintering kiln rotated at 5.0 φm, the residence time in the tube of about 7 minutes, and the sintering temperature was about 1400°C. The sintered alpha alumina-based ceramic abrasive grain was used to make coated abrasive discs as described in Comparative Example A, except abrasive grain was screened to retain the -25+30 and -30+35 mesh fractions (U.S.A. Standard Testing Sieves). The coated abrasive discs were tested according to the following test procedure. Each coated abrasive disc was mounted on a beveled aluminum back-up pad, and used to grind the face of a pre- weighed 1.25 cm x 18 cm 304 stainless steel workpiece. The disc was driven at 5,000 φm while the portion of the disc overlaying the beveled edge of the backup pad contacted the workpiece at a load of 8 Kg (18 lbs). Each disc was used to grind individual workpiece in sequence for ten one-minute intervals. The total cut was the sum of the amount of material removed from the workpieces throughout the 10 minutes test period. The average total cut (based on 4 discs) was 295 grams.

Example 3

Example 3 was prepared and tested as described for Example 2 except two identical alumina ball mills (17.78 cm in height and 15.24 cm in diameter), each with 1500 grams 0.635 cm (0.25 inch) alumina media, were used, and the samples were milled for 18 hours. Each mill contained 600 grams of H-40. The milled powder was combined into one sample. The average total cut (based on 4 discs) was 293 grams.

Comparative Example B

Comparative Example B was prepared and tested as described in Example 2 except the H-40 powder was not ball-milled. The average total cut (based on 4 discs) was 245 grams.

Example 4

An alumina ball mill, measuring 17.78 cm height (without cover) and 15.24 cm in diameter, was filled with 1200 grams of H-30 powder and 3300 grams of 0.635 cm (0.25 inch) alumina media (U.S. Stoneware). The sample was milled at 80 φm for 48 hours. The milled powder was recovered from the mill. The median particle size of the milled powder was measured using a laser scattering particle size analyzer (obtained under the trade designation "HORIBA LA-910" from Horiba Laboratory Products, Irvine, CA) and found to be 7.3 micrometers.

A sol was prepared by combining 338 grams of the milled powder, 31 grams of HNO₃ and 1550 grams of DWT together in a conventional 4 liter, food grade blender (Model 34BL22(CB6), Waring Products Division, Dynamics Coφ. of America,

New Hartford, CT). The contents of the blender were mixed at low speed for one minute. The resulting sol was poured into a 23 cm (9 inch) x 30 cm (12 inch) glass pan (obtained under the trade designation "PYREX"). The sol was dried overnight in a forced air oven at 93°C (200°F). The resulting friable material was crushed into particles using the pulverizer, screened, calcined, impregnated, re-calcined, and sintered as described in

Example 2. Coated abrasive discs were prepared as described in Example 2.

The coated abrasive disc was attached to a 16.5 cm diameter, 1.57 mm thick hard, phenolic backup pad which was in turn mounted onto a 15.2 cm diameter steel flange. The mounted disc was rotated counterclockwise at 3550 φm. The 1.8 mm peripheral edge of a 25 cm diameter disc of 304 stainless steel workpiece was deployed 7 degrees from a position normal to the abrasive disc under a load of about 4 kg. The workpiece was weighed at the start of the test and at 2-minute intervals to determine the amount of steel removed (i.e., abraded). Six 2-minute intervals (for a total of 12 minutes of grinding) were done for each abrasive disc. The average total cut (based on 4 discs) was 117.8 grams.

Comparative Example C

Comparative Example C was prepared and tested as described in Example 4 except the H-30 powder was not ball-milled. The average total cut (based on 4 discs) was 109.1 grams.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims

What is claimed is:

1. A method for making alpha alumina-based ceramic abrasive grain, said method comprising: preparing a dispersion by combining components comprising liquid medium, peptizing agent, and boehmite, wherein said dispersion exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1 185°C, and wherein at least 25 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C; converting said dispersion to alpha alumina-based ceramic abrasive grain precursor material; and sintering said precursor material to provide alpha alumina-based ceramic abrasive grain having a density of at 95 percent of theoretical density, wherein said precursor material is essentially free of nucleating material.

2. The method according to claim 1 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99%> and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.

3. The method according to claim 2 wherein the boehmite present in said dispersion contains, on a theoretical metal oxide basis, less than 600 ppm TiO₂, based on the theoretical Al₂O₃ content of the boehmite.

4. The method according to claim 2 wherein the boehmite present in said dispersion contains, on a theoretical metal oxide basis, less than 300 ppm TiO₂, based on the theoretical Al₂O₃ content of the boehmite.

5. The method according to claim 2 wherein the boehmite present in said dispersion contains, on a theoretical metal oxide basis, less than 150 ppm TiO₂, based on the theoretical Al₂O₃ content of the boehmite.

6. The method according to claim 2 wherein said converting includes calcining, and wherein between said calcining and said sintering, said method further comprises (i) impregnating calcined precursor material with a mixture prepared by combining components comprising a second liquid medium and at least one of metal oxide or metal oxide precursor to provide impregnated precursor material; (ii) drying the impregnated precursor material; and (iii) calcining the dried, impregnated precursor material.

7. The method according to claim 2 wherein said abrasive grain comprises at least 0.1 percent by weight of oxide selected from the group consisting of MgO, CoO, NiO, Ce₂O₃, ZrO₂, HfO₂, Y₂O₃, Pr₂O₃, Sm₂O₃, Yb₂O₃, Nd₂O₃, La₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃, Eu₂O₃, SiO₂, TiO₂, ZnO, and ZrO₂, based on the total metal oxide content of said abrasive grain.

8. The method according to claim 2 wherein said abrasive grain comprises at least 0.1 percent by weight of each of: (a) MgO;

(b) Y₂O₃; and

(c) oxide selected from the group consisting of Pr₂O₃, Sm₂O₃, Yb₂O₃, Nd₂O₃, La₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃, Eu₂O₃, and combinations thereof, based on the total metal oxide content of said abrasive grain.

9. The method according to claim 1 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1184°C.

10. The method according to claim 1 wherein at least 50 %> by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than

1183°C.

11. The method according to claim 1 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1182°C.

12. The method according to claim 1 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1181°C.

13. The method according to claim 1 wherein at least 50 %> by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1180°C.

14. The method according to claim 1 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99%> and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1179°C.

15. The method according to claim 11 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.8 to 99%.

16. The method according to claim 1 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99%).

17. The method according to claim 1 wherein at least 75 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.

18. The method according to claim 1 wherein at least 90 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.

19. The method according to claim 1 wherein at least 100%) by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than

1185°C.

20. A method for making an abrasive article, said method comprising: preparing a dispersion by combining components comprising liquid medium, peptizing agent, and boehmite, wherein said dispersion exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C, and wherein at least 25 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than

1185°C; converting said dispersion to alpha alumina-based ceramic abrasive grain precursor material; sintering said precursor material to provide alpha alumina-based ceramic abrasive grain having a density of at 95 percent of theoretical density; and combining a plurality of said alpha alumina-based ceramic abrasive grain with binder to provide an abrasive article, wherein said precursor material is essentially free of nucleating material.

21. The method according to claim 20 wherein at least 50 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.

22. The method according to claim 21 wherein said abrasive article is a coated abrasive article that includes a backing.

23. The method according to claim 21 wherein combining at least a plurality of said alpha alumina-based ceramic abrasive grain with binder includes combining fused alumina abrasive grain with said binder.

24. The method according to claim 20 wherein at least 75 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99%) and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.

25. The method according to claim 20 wherein at least 90 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than

1185°C.

26. The method according to claim 20 wherein at least 100 % by weight of said boehmite, based on the total boehmite content of said dispersion, has a dispersability value in the range from 97.5 to 99% and exhibits an alpha alumina transition transformation temperature, as determined by differential thermal analysis, of less than 1185°C.