FIELD This invention relates to abrasive articles and to a method and compositions for making abrasive articles. In particular, the invention relates to bonded abrasive tools comprising superabrasive grits.

BACKGROUND The term "superabrasive" commonly refers to an abrasive material with exceptional hardness. Typical 'conventional' abrasives such as, aluminium oxide and silicon carbide, have a hardness in a range of from 2000 to 2500 kg/mm^, whereas "superabrasive" cubic boron nitride (cBN) and diamond have hardness of the order of 4500 and 8000 kg/mm^, respectively. There are a variety of bonded abrasive articles that comprise superabrasive grits such as diamond and cubic boron nitride. Examples of such abrasive articles include honing stones, polishing sticks, saw blades, cutting sticks, mounted points, snagging wheels, dressing tools, cup wheels, depressed centre wheels, grinding wheels, and flap wheels. Grinding wheels generally comprise a hub and a working rim secured to the periphery of the hub. The hub may be made of metal, resin plastics or a combination thereof and generally comprises means to facilitate attachment to a grinding machine. The working rim may be a single annular piece or formed from a plurality of segments. The working rim typically comprises superabrasive particles dispersed in a metal matrix, vitreous matrix, or in a thermosetting resin such as a phenol-formaldehyde, urea- formaldehyde or melamine-formaldehyde resin. Thermosetting resins have been used because of their ability to withstand the high temperatures associated with the operation of grinding wheels, and because the brittle nature of many thermosetting resins enables them to break down during grinding. The manufacture of bonded abrasive products such as grinding wheels, using thermosetting resins is generally tedious and slow. The articles are typically made by compression molding or constant volume molding in which a mold is filled with a precise volume of particulate abrasive filler and binder. The binder may be in powdered form, liquid or a combination thereof. It is necessary that the mold volume be filled very accurately such that the top and bottom are flush with the mold walls since heating occurs by conduction, and the heating relies on there being adequate surface contact between the mold and the heating pressure platens. The molding materials typically have poor heat conductivity, and it may be necessary to heat the powder mix for several hours to ensure adequate curing throughout the abrasive article. The molding process also gives rise to emissions that are not only unpleasant but may also have undesirable environmental consequences. It has been proposed to manufacture resin-bonded abrasive products using an injection molding process. EP 0551714, for example, discloses an abrasive article comprising a molded abrading body produced from an injection molded polymeric material with an abrasive material and a secondary filler material interspersed homogeneously therethrough, the abrading body comprising from 1 percent to 20 percent by volume of a diamond hardness abrasive grit; from 5 percent to 80 percent by volume of a secondary filler; and from 5 percent to 90 percent by volume of a thermoformable polymer selected from thermoplastic polymeric materials having a softening point temperature greater than 100 0C and less than 250 0C, and thermoset polymers. The thermal characteristics of resin-bonded abrasive products that comprise such thermoplastic polymeric materials are not satisfactory for all applications. It has also been proposed to manufacture certain resin-bonded abrasive products using, as the binder resin, a thermoplastic polymer having a higher softening point temperature. U.S. Pat. No. 5,314,512, for example, discloses a method for preparing an injection molded saw segment comprising the steps of:

(a) heating a mass of ultra-hard abrasive particles dispersed in a non-porous thermoplastic polymer matrix in the barrel of an injection molding machine at a temperature of from about 280 0C to 400 0C, wherein the abrasive content of the segment is at least 4 volume percent;

(b) injecting said heated product of step (a) into a mold at a pressure of from about 70 MPa to 150 MPa; and (c) maintaining said injected product of step (b) in said mold at a pressure of about 35 MPa to about 75 MPa for a period of from at least about 2 to about 10 seconds.

SUMMARY According to one aspect of the present invention, there is provided a composition comprising: superabrasive particles, thermoplastic polymer having a processing temperature at least 280 0C and filler wherein the thermoplastic polymer is present in an amount sufficient to bind the composition, and the filler comprises spherical particles present in an amount of at least 40 percent by volume of the composition. According to a second aspect of the invention, there is provided a bonded abrasive product comprising a plurality of abrasive particles bonded together by a bonding medium into a shaped mass formed of a composition as described above. According to a further aspect of the invention, there is provided a method of making a bonded abrasive article comprising: heating a composition as described above at a temperature in a range of 280 °C up to and including 400 °C to provide a heated composition; injecting the heated composition into a mold; and cooling the heated composition to provide the bonded abrasive article. The mold may conveniently be heated at a temperature in a range of from at least 150 °C up to and including 250 0C prior to injection. The injection pressure is generally in a range of from at least 70 MPa up to and including 210 MPa. As used in the context of the present invention, the term "superabrasive grit" refers to an abrasive grit having a hardness of greater than 4000 kg/mm^ According to the present invention, compositions comprising superabrasive particles and a thermoplastic polymer having a comparatively high crystallization melting point, intended for the manufacture of bonded abrasive products, can be improved by the addition of a comparatively large amount of spherical particles as filler in that the composition can be injection-molded without undue difficulty and the resulting abrasive products typically show good performance in terms of stock removal. It is found that spherical fillers having an average particle size in a range of from at least 10 up to and including 2000 micrometers may be used at high filler levels in a range of from at least 40 up to and including 70 percent by volume of the composition, or even higher, and provide compositions having a suitable viscosity for injection molding. Many conventional fillers have irregularly shaped particles or acicular particles. During injection molding it is necessary for the filler particles to flow past one another and inter- particle friction forces between irregularly shaped particles are high and resist flow thereby increasing the viscosity of the composition. In contrast spherical particles flow easily past one another and the viscosity increase imparted by inter-particle friction is significantly reduced allowing higher loading levels of filler to be successfully used for injection molding.

BRIEF DESCRIPTION QF THE DRAWINGS Fig. 1 is a bar chart of mass removed from tiles (cut) by the wheels of various Examples; Fig. 2 is a bar chart of loss of mass from wheels of various Examples; and Fig. 3 is a bar chart of diamond efficiency index for the wheels of various Examples.

DETAILED DESCRIPTION Suitable spherical filler particles for use in the invention include glass spheres, ceramic spheres and mixtures thereof. Preferred filler particles comprise soda-line borosilicate glass spheres, calcium carbonate spheres and silica-alumina ceramic spheres. The filler particles generally have an average particle size in a range of from at least 10 up to and including 2000 micrometers, or more, preferably from at least 10 up to and including 400 micrometers. Particles having an average particle size in a range of from at least 25 up to and including 50 micrometers are particularly useful. The spherical particles are present in an amount in a range of from at least 40, 45, or 50 percent up to and including 60, 65, or 70 percent or more, by volume of the composition; for example, from at least 45 up to and including 65 percent, preferably from at least 50 up to and including 60 percent by volume of the composition. The compositions may include minor amounts of other fillers, such as silicon carbide, aluminium oxide, copper powder, aluminum powder, silica, fiberglass, etc. provided the additional filler does not deleteriously effect the melt viscosity of the composition to prevent injection molding. The choice of thermoplastic polymer is generally dependent upon the temperatures likely to be generated in the end use of the abrasive article. The thermoplastic polymers have a processing temperature of at least 280 °C, generally in the range 280 to 420 0C, preferably 280 to 400 °C. The processing temperature is the temperature at which the polymer will flow under pressure to render it suitable for injection molding. Processing temperatures for thermoplastic polymers are widely quoted in the literature. The thermoplastic polymer is typically selected from engineering thermoplastics, such as polyetheretherketone (PEEK), polyetherketone (PEK), polyaryletherketone, polyaryletheretherketone, poly(amide-imide) (PAI), polyphenylene sulfide (PPS), polyarylene sulfide (PAS), polyethersulphone (PES), polyetherimide (PEI) and liquid crystal polymers (LCP). Such materials are readily commercially available. For example PEEK is commercially available under the trade designation "VICTREX" from Victrex pic, U.K. and PPS is commercially available under the trade name "FORTRON" from Fortran Industries, USA. Two or more polymers may be used simultaneously in the polymer matrix in order to use the beneficial characteristics of each polymer. For instance, liquid crystal polymer (LCP) may be used in conjunction with polyetheretherketone (PEEK) in order that the low melt viscosity of the LCP may assist in the free flowing characteristics of the relatively highly viscous PEEK. The thermoplastic polymer is present in an amount sufficient to bind the composition which is generally in an amount of from at least 20, 30, or 35 percent up to and including 45, 50, or 59 percent, or more, by volume of the composition; for example, from at least 30 up to and including 50 percent, preferably from at least 35 up to and including 45 percent by volume of the composition. The superabrasive grits used in the invention are generally selected from diamond, cubic boron nitride and mixtures thereof. The superabrasive grits have an average particle size in a range of from at least 30 up to and including 300 micrometers or more, generally from at least 50 up to and including 150 micrometers, and preferably from at least 90 up to and including 105 micrometers. The superabrasive grits generally occupy from at least 2.5 up to and including 20 percent by volume of the composition, preferably about 5 percent by volume of the composition. Examples of diamond that can be used are: Resin bond grade 200/230 commercially available under the trade designation "ED A2021 " , Resin bond grade 140/170 commercially available under the trade designations "SYN GREEN" and "SYN BLACK", and Metal bond grade 140/170 commercially available under the trade designation "EDA2050", all from Edel Industrial Diamond (EID), London, UK. Particle sizes of the various diamond grades: 200/230 = 63 - 75 micrometers, 140/170 = 90 - 105 micrometers. LGDlO 120/140 (105 - 125 micrometers) and LGDlO 100/120 (125 - 150 micrometers) commercially available from Longer Inc., San Jose, California, USA. Diamond agglomerate particles may be used where the particle size of the agglomerate is greater than 360 micrometers. "Metal bond" and "Resin bond" diamonds are two designations recognized in the superabrasive industry, and refer to the type of formulations in which they are normally used, viz. "Metal bond" diamonds are generally used in superabrasive products largely composed of metal and "Resin bond" diamonds are generally used in superabrasive products largely composed of organic resins. "Metal bond" diamonds differ from "Resin bond" diamonds in that metal bond diamonds are typically blockier in appearance and are tougher. This toughness is required since metal bond applications are much more demanding than resin bond applications and so the metal bond diamonds must be capable of withstanding the harshness of the environment. Resin bond diamonds on the other hand are typically much more friable and less blocky - designed to break down and 'sharpen themselves' such that fresh diamond is continuously exposed at the cutting interface. 'Blockiness' refers to the shape of the diamond particles, which in turn serves as an indicator to the performance of the diamond. The shape of blocky diamonds tends towards a cubo-octahedral geometry and provides a very tough, hard wearing diamond, whereas less blocky diamonds tend to be more acicular in shape, breaking down via a 'self sharpening' mechanism with the formation of shards. It is found that the presence of a coupling agent, particularly in particulate form, may improve the molding properties of the compositions by reducing the melt viscosity. It is postulated that the coupling agent may become associated with the filler and/or abrasive particles exerting a charge, which tends to reduce contact between particles thereby reducing the frictional resistance between particles during injection molding. Suitable coupling agents include organo-silanes, zircoaluminates, zirconates, and titanates. The coupling agent is generally present in an amount of from at least 0.1 up to and including 2 percent by weight of the filler. The coupling agent may be applied directly into the mixture of bonding medium, abrasive grit and filler. Alternatively, the abrasive grit and/or filler may be pre-treated with the coupling agent. The injection molding process may utilize conventional equipment. A suitable injection mold structure or cavity is prepared according to the desired shape of the abrasive article. The components of the abrasive composition are mixed and heated at a temperature to provide a viscosity suitable for injection molding. The temperature will generally depend upon the melt viscosity of the thermoplastic material, the degree of filler and the presence of coupling agent etc. Generally, the temperature will be in a range of from at least 280 0C up to and including 400 0C. The heated composition is injected into the mold, typically under an injection pressure in a range of from at least 70 MPa up to and including 210 MPa. The mold may be preheated (for example, at a temperature in a range of from at least 150 0C up to and including 250 0C) to facilitate injection of the composition. Thereafter the mold is cooled to solidify the abrasive composition and the molded article removed. The molded abrasive article may be in any desired form. A preferred form is a grinding wheel. The grinding wheel typically can range in diameter from at least about 0.1 cm up to about 2 meters, more typically from at least 1 cm up to about 2 meters. Typically, the grinding wheel thickness can range from at least about 0.001 cm up to about 1 meter, more typically from at least about 0.01 cm up to about 0.5 meter. However, the abrasive article may take other forms, including honing stones, polishing sticks, saw blades, cutting sticks, mounted points, snagging wheels, dressing tools, cup wheels, honing stones, cut off wheels, depressed centre wheels, flap wheels and the like. The invention will be illustrated by the following Examples which disclose the production of a diamond edging wheel suitable for edging or arising ceramic or vitreous- type materials; such materials include ceramic tiles, glass and other materials of a hard, brittle nature. The edging wheel comprised a working rim made by injection molding the abrasive composition. The working rim had an outside diameter of 150 mm, an inside diameter of 90 mm and a thickness of 8 mm.

TEST METHODS The performance of the edging wheels to edge tiles was assessed using a Thibaut machine, which is a rotary grinding/polishing machine, ref. TIlOS available from Thibaut S. A., rue de Caen, 14500 Vire, France. The tiles were "KEOPE GRANITI" 30 cm x 30 cm porcelain stoneware, manufactured by Keope S.p.a, Via Statale 467,21 - 42013, Casalgrande (RE) Italy.

The test procedure is as follows: 1. Record mass of a fresh dry tile 2. Record mass of unused edging wheel 3. Load tile and secure into position on the Thibaut machine 4. Load edging wheel into position on the Thibaut machine 5. Activate Thibaut machine to allow water to flow through the head of the Thibaut 6. Once all apparatus are soaked with cooling water, engage the rotating edging wheel with the tile edge surface 7. Traverse the Thibaut head manually back and forth at a controlled speed along the tile edge for a total duration of 60 seconds at a pressure of 80 psi (0.6 MPa). 8. Disengage edging wheel from tile edge and switch off Thibaut machine. 9. Remove tile from Thibaut machine bed 10. Dry the tile using absorbent paper, blow dry with compressed air and place tile into a heated oven at 105 °C for 5 minutes to ensure that all water traces have been removed. 11. Remove tile from oven and weigh, recording mass loss 12. Reload tile onto Thibaut bed and repeat test (steps 4 - 11) for the remaining 3 sides of the tile 13. After all 4 sides of the tile have been edged, remove the edging wheel from the Thibaut machine, dry the wheel using absorbent paper, blow dry with compressed air and place tile into a heated oven at 105 °C for 30 minutes to ensure that all water traces have been removed. 14. Record mass loss of the dry wheel 15. Repeat steps 1-14 for a total of 5 tiles 16. Results are expressed as "Total Cut" in grams of total mass loss for 5 tiles.

Definition of "Total Cut": Total Cut is the cumulative total mass of tile removed by the edging wheel during the Thibaut processing of 5 tiles (20 edges).

Definition of "Total Wheel Mass Loss": Total Wheel Mass Loss is the cumulative total of the mass of wheel lost after edging 20 tile edges (5 tiles in total).

Definition of "Diamond Efficiency Index": Diamond efficiency index is calculated in the following manner:

Diamond Efficiency Index equals α multiplied by Tile Mass Loss divided by Wheel Mass Loss; where a = proportion by mass of diamond present in the initial formulation.

Since the volume proportion of diamond is known for each formulation, then with a knowledge of the density and the proportions of the other constituents the mass of diamond loss can be determined, since one has measured the total mass of wheel lost. The diamond efficiency index is then calculated by dividing the mass of tile lost in the test by the mass of diamond lost in the test. It is assumed that during injection molding there is a homogeneous distribution of diamond throughout the construction.

MATERIALS

In the Examples the following materials were used:

Glass Bubbles - available under the trade designation "3M SCOTCHLITE GLASS BUBBLES S60/10000" from 3M Company, St. Paul, Minnesota, USA. The glass bubbles have an average diameter of 30 micrometers with 80% by volume being between 15 and 55 micrometers and a top size of 65 micrometers. Microspheres - available under the trade designation "3M ZEEOSPHERES TM G-850" from 3M Company. The microspheres have an average diameter of 40 micrometers with 80% by volume being between 12 and 100 micrometers and a top size of 200 micrometers.

Microbeads - solid glass spheres available under the trade designation "MB300/400" from Microbeads AG, Bragg, Switzerland. The glass spheres have a particle size in the range 300 to 400 micrometers.

CaCU3 spheres - calcium carbonate spheres with an upper particle size of 2000 micrometers available under the trade designation "SPHERICARB" from Lawrence Industries Ltd., Staffordshire, UK.

PEEK - polyaryletheretherketone commercially available under the trade designation "150 PF" from Victrex pic, Lancashire, UK.

PPS - polyphenylene sulfide commercially available under the trade designation "FORTRON 0020A9" from Fortran Industries, Ticona GmBH, European Customer Service, Frankfurt am Main, Germany.

KR135SP/H - monoalkoxy titanate coupling agent commercially available under the trade designation "CAPOW KR135SP/H" from Kenrich Petrochemicals, Bayonne, New Jersey, USA

KR12/H - titanate coupling agent commercially available under the trade designation "CAPOW KR12/H" from Kenrich Petrochemicals.

PTS - phenyltrimethoxysilane commercially available under the trade designation "DYNASILAN 9165" from Degussa AG, Aerosil & Silanes, Frankfurt am Main, Germany.

DL70 - powdered vinyl silane commercially available under the trade designation "SILAN 9116 DL70 DRY LIQUID" from Degussa AG, Aerosils & Silanes. Diamond 1 - Resin bond grade 140/170 commercially available under the trade designation "SYN BLACK" from Edel Industrial Diamond (EID), London, UK.

Diamond 2 - 360/120 vitrified diamond agglomerates commercially available from National Research Company, Chesterfield, Michigan, USA. 360/120 refers to 360 micrometers cubic glass dimension encapsulating a 120 micrometers diamond.

EXAMPLES The compositions reported in Table 1, in which the figures are in volume percent of the composition, were used to produce working rims for an edging wheel by injection molding. The figures are in volume percent since due to density variations mass percent values are misleading because the difficulties of injection molding are affected by the surface area of filler particles which in turn depends on the number of particles present (as well as particle size) - volume percent illustrates the formulations on a normalized level playing field'. For example, in volume percent all formulations contain the same quantity of diamond, however if the results are converted into mass percent then the diamond is not represented proportionately and so it appears as if the diamond quantity varies between each formulation, when it is actually the same. TABLE 1

T = Trace amount The resulting wheels were tested as described above. The results are reported in Table 2. TABLE 2

U) I

"Standard" refers to a standard phenolic formulation edging wheel, 3M 6700J, available from 3M Corporation, 3M Center, St. 5 Paul, Minnesota 55144-1000, USA. The results of these tests are recorded in Figures 1 to 3 in which: Figure 1 represents a bar chart of mass removed from tiles (cut) by the wheels of various Examples; Figure 2 represents a bar chart of loss of mass from wheels of various Examples; and Figure 3 represents a bar chart of diamond efficiency index for the wheels of various Examples. Referring to Figure 1, which shows the total cut for each wheel it will be noted Example 1 gives the lowest cut of the PEEK containing formulations tested and this is attributed to the absence of destabilizing spherical filler. Therefore, the tough PEEK matrix was not destabilized to encourage breakdown and the exposure of fresh diamonds. Of the two PPS containing formulations tested (Examples 2 and 9), it is seen from Figure 1 that the Total Cut for each are very similar, although Figure 2 shows that there is slightly more wheel loss associated with Example 9. Example 9 contains a coupling agent whereas Example 2 does not. The observed similarity of Total Cut for each suggests that the coupling agent does not promote diamond retention in the PPS formulations. However, the increased wheel loss for the PPS containing formulation containing the coupling agent suggests that the agent did not bond well to the spherical S60 filler, allowing it to be lost during use. It is seen that Example 6 gives increased cut compared with Example 5 Example 6 contains less PEEK than does Example 5, and so it tends to break down more readily than Example 5, this being due to the presence of the microspheres which serve to weaken the PEEK matrix. The opposite effect is observed with the formulations containing PTS as the additive. Here, it is seen that the sample with increased PEEK content (Example 3) gives increased cut compared to Example 4. During testing, the coolant had a dark cloudy gray appearance, indicative that the wheel was breaking down and losing the microsphere filler. This is concluded since PEEK is pale in color whereas the microsphere filler is dark gray. It is theorized that destabilization of the PEEK in Example 5 may be due to the non- bonding of the silanized filler with the PEEK resin - effectively this will give a similar effect to the presence of porosity. Comparison of Example 6 with Example 8 shows that the formulation containing the coupling agent (Example 6) exhibits increased Total Cut. This suggests that in the PEEK formulations the coupling agent aids in diamond retention in addition to serving as a processing aid. The results shown in Figure 2 are directly proportional to those in Figure 1 by v virtue of the fact that in order for a formulation to exhibit high stock removal, it must itself breakdown so as to continuously expose fresh diamonds. Example 1 shows a low level of wheel loss, due to the absence of spherical filler to weaken the PEEK matrix. Accordingly the total cut (Figure 1) is also low. In similarity to Example 1, Example 10 also shows a relatively low value of Wheel Loss, likely due to the reduced level of destabilization imparted by the glass bubbles, which are smaller, hollow, glass spheres. This suggestion is further supported by the increased wheel loss observed for all of the Examples that contain the larger microspheres. In addition, it is seen that Example 7 (containing microbeads, slightly larger in average particle size than the microspheres) exhibits slightly greater Wheel Loss (and correspondingly slightly increased Total Cut) than Example 5, which contains the same amount of PEEK. This further suggests that the size of the spherical filler plays an important role in destabilizing the PEEK matrix. In the case of Examples 3 and 4 (employing PTS as a coupling agent) there is an interesting phenomenon whereby Example 3 has a higher proportion of PEEK than does Example 4, yet Example 3 has a higher amount of wheel loss. As mentioned previously it is theorized that the PTS has bonded to the microspheres but has not bonded to the PEEK matrix. This then results in the loss of microspheres during use, which leads to the increased rate of attrition of the wheel. Effectively the theorized non-bonding of the microspheres to the PEEK matrix is equivalent to the presence of the porosity in the wheel. While stock removal and wheel mass loss are very important parameters from a performance perspective, in order to gain a better, quantifiable understanding of the behavior of the wheels the "diamond efficiency index" was assessed. Simply stated, diamond efficiency index is the mass of tile removed per equivalent gram of diamond and is calculated based on the knowledge of the diamond concentration within each formulation. Knowledge of the stock removal or the amount of wheel loss in themselves is insufficient to describe the performance of a formulation since it is frequently the case that a formulation which gives a high stock removal rate will also itself suffer a high degree of attrition or mass loss. Similarly, a formulation with a low index of wheel loss typically gives a low stock removal, and while this formulation may indeed last a very long time in use, it is unlikely to satisfy the stock removal requirements. It is seen from Figure 3 that Example 1 gives a high diamond efficiency index, however when coupled with data from Figures 1 and 2 it is revealed that the reason for this is due to the low stock removal and the low amount of wheel loss. The case is similar for Example 4. Examples 6, 5 and 7 both have relatively high diamond efficiency indices and inspection of data from Figure 1 shows that they each give relatively high stock removal. Examples 3 and 4 have widely differing properties, since while Example 4 has a high diamond efficiency index, it also has low stock removal. Conversely while Example 3 has high stock removal, this is at the expense of wheel mass loss, as evidenced with the lower diamond efficiency index. It is clear that Examples 1 to 13 have improved diamond efficiency index compared with the Standard and hence have higher efficiency of diamond usage.