US 20040137834 A1
Molded articles in the form of grinding/cut-off wheels, drill bits, reamers, knife, blade, etc., having a core section or sections comprising a moldable resin of some intensive property, e.g., hardness, concentration, etc., and a surface layer or rim or section(s) comprising another moldable resin of different magnitude of the same intensive property e.g., hardness, concentration, etc., and abrasive articles selected from one or more of diamond, cubic boron nitride (CBN), alumina, SiC, garnet, and mixtures thereof. The articles are characterized in that a) the resins comprising the sections are miscible and diffusively or convectively mixed; b) a composition ratio of the abrasive articles in the resins is decreased inwardly from a surface of said molded article on a scale of the mesh size of the abrasive and article size; and c) the resins comprising the sections are miscible and diffusively mixed.
1. An abrasive article, which comprises:
a first part comprising a first moldable resin having an intensive property;
a second part being adjacent to the first part forming an interface, said second part comprising a second moldable resin having an intensive property of different magnitude from the first moldable resin,
said second moldable resin being at least partially miscible with the first moldable resin; and wherein
the interface between said first part and said second part being diffuse and graded and varying in the magnitude of the intensive property, on at least a molecular scale, limited only by the scale of the abrasive article.
2. The abrasive article of
4. The abrasive article of claim 3, wherein the interface between said first part and said second part being diffuse and graded and varying in the magnitude of the intensive property, on a scale of mesh sizes of said abrasive particles.
5. The abrasive article of
6. The abrasive article of
7. The abrasive article of
8. The abrasive article of
9. The abrasive article of
10. The abrasive article of
11. The abrasive article of
12. The abrasive article of
13. The abrasive article of claim 3, wherein said first moldable resin further comprises secondary abrasive particles selected from the group of ceramic boride, nitric oxide, Cu, Fe, Sn, bronze, carbon, silica, alumina, Si, and mixtures thereof.
14. The abrasive article of
15. A molded article, which comprises a gradient composite resinous material containing a polymer resin and abrasive particles having various mesh sizes, and wherein
a composition ratio of said abrasive articles changes gradually on a scale of the molded article,
said abrasive articles comprise one or more of diamond, cubic boron nitride (CBN), alumina, SiC, garnet, and mixtures thereof.
16. The molded article of
17. The molded article of
18. The molded article of
19. The molded article of
20. A method for forming a cutting element, which method comprises the steps of:
introducing into a mold or a press at least two resinous materials having different concentrations of abrasive particles selected from one or more of diamond, cubic boron nitride (CBN), alumina, SiC, garnet, and mixtures thereof;
allowing for controlled convective mixing of the contact surface between the resinous layers;
applying sufficient temperature and pressure on said mold or press such that said resinous materials are at least partially miscible, thus forming an intermediate body having a diffuse interface between said at least two resinous materials which is graded in concentrations of said abrasive particles on at least a molecular scale level; and
machining the intermediate body into a desired shape and size to form said cutting element.
 This application claims priority of U.S. Provisional Patent Application No. 60/440,455, filed on 15 Jan. 2003 and U.S. Provisional Patent Application No. 60/464,517, filed on 22 Apr. 2003.
 The invention relates to abrasive cutting and grinding articles comprising superabrasive materials.
 Conventional abrasive cutoff wheels of the prior art largely comprise abrasive grit, e.g., aluminum oxide, and resin throughout the entire wheel, making the wheel brittle. These discs are frequently covered in fiber mesh to help contain the ballistic fragments that result from bending or abuse.
 In another prior art embodiment of grinding wheels, wires, sanding discs and cutting blades of various shapes, superabrasive materials are used. The superabrasive materials or “grit,” comprising diamond or cubic boron nitride (cBN) are used in the outer surface or periphery of a circular grinding wheel or grinding cup for use in sawing, drilling, cutting, dressing, grinding, lapping, polishing, and other abrading applications. The grit typically is surrounded in a matrix of a metal, such as Ni, Cu, Fe, Co, Sn, W, Ti, or an alloy thereof, or of a resin, such as phenol formaldehyde or other thermosetting polymeric material. By attaching the matrices to a body, blade core or backing membrane or other support, tools may be fabricated having the capability to cut through such hard, abrasive materials, such as, for example, concrete, asphalt, masonry, ceramic, brick, or granite, marble, or other rock. In this method, when the blade or disk is bent in use, only segments break away from the blade, not whole blade or disk fragments.
FIG. 1A illustrates a typical abrasive wheel or blade or disk in the prior art, formed as a composite of a hard abrasive-containing surface (item 1) bonded to a separate blade core (item 2). The hard, brittle surface is typically a mixture of glass, metal, or resin and abrasive grit and other additives. The core is typically a metal or resinous disc. In another embodiment of a cross-section view of a typical abrasive wheel or blade or disk in the prior art as shown in FIG. 1B, the wheel comprises a hard grit-resin surface layer (item 3) to prevent wall erosion in deep cutting with hard rim (item 1) or alone, as in a sanding disc. The surface abrasive layer is attached to the core by compression and/or adhesion improved by geometric profiles, e.g., notches, ridges, channels, dovetails or slots with or without use of a 3rd-body adhesive.
 The bonded article of the prior art frequently suffers delamination at the bond between the core and surface if the blade, disc or wheel is bent, such as in the mold in manufacture, or in thermal annealing, or by bending abuse in use. Cracks that form in the surface propagate to the discontinuous interface and travel freely along it, delaminating large portions of the surface layer. This creates a dangerous situation during use or scrap parts in wheel fabrication.
 U.S. Pat. No. 5,167,674 discloses one embodiment of the wheel or blade arrangement in the prior art, with grinding wheels manufactured from a metal or resin-metal tough core. The core is adhesively bonded to a mixture of superabrasive grit, a bis-maleimide triazine addition copolymer resin, free-radical initiator, and catalyst, which mixture is compression molded to form a grinding segment annulus. This article has a brittle, hard rim bonded to a core through a discontinuous and discrete interface that will attract and confine cracks, and delaminate if the wheel is bent, dropped or otherwise abused.
 In another reference, U.S. Pat. No. 5,314,512, injection mold saw segments from superabrasive particles and a non-porous thermoplastic polymer are disclosed. The molded saw segments then are affixed to the periphery of a saw blade. The interface between abrasive segment and core is discontinuous and a delamination will occur if the wheel is bent or subjected to dynamic tension.
 U.S. Pat. No. 3,960,516 discloses a grinding cup, wherein the interface between hard rim and cup core is discontinuous and susceptible to delamination in bending. U.S. Pat. No. 4,369,046 teaches a wheel that comprises resin and metal powders cured simultaneously in a mold to obtain a good bond. The bond of this wheel is a discrete interface whose strength relies on adhesion between immiscible resin and metal.
 U.S. Pat. No. 5,679,067 teaches injection of abrasive-filled resinous bristles bonded to a resinous handle. One resin pushes the other into the mold. Bonding is by adhesion at a discrete interface between the abrasive bristles and the non-abrasive handle. There is no interdiffusion or convection described at the interface between the non-abrasive-filled and abrasive-filled resins. The abrasive-filled part is homogeneously filled and forms a discrete, and thus, weaker interface.
 U.S. Pat. Nos. 6,074,278 and 6,102,789 teach the use of epoxy cement to bond rim to core in high-speed grinding wheel, wherein this interface is not graded as the adhesive is not required to be miscible with the rim or core resin nor is any intentional interdiffusion described to achieve grading.
 U.S. Pat. No. 3,722,831 teaches another way to resolve the delamination problem with slotted resin-grit segments attached to a wheel that allows operation at >125 ml/s speed without delamination. Slots, notches, filaments, fibers and the like function to reduce the stiffness gradient at the interface on a scale of the dimensions of the slot or notch. However, cracks from bending will still follow the slot interfaces freely.
 EP 0794850 B 1 proposes cutting segments manufactured from superabrasive particles molded with a thermoplastic material wherein the superabrasive particles are oriented in a chosen direction and there is porosity in the molded segments. The porosity in the hard, brittle rim limits delamination failures at the interface. However, it does so by weakening the rim with porosity.
 As an improvement, U.S. Pat. No. 5,885,149 teaches casting of metals to infiltrate porous rims of grit and resin to achieve an interface free bond between core and rim of the grinding wheel, with the scale of the grading depending on the pore size and porosity of the surface. In this reference, metals are not miscible with resins and the interface is weakened accordingly.
 In another embodiment to resolve the delamination problem, U.S. Pat. No. 6,187,069 teaches a porous metal core which is infiltrated by resin-grit to form a core-rim grinding wheel. There is no miscibility metal-to-resin and the scale of the grading is that of the pores in the resin, leaving the interface weak.
 The invention relates to a surprisingly safe abrasive cutting/grinding tool without the delamination failures typically experienced in the prior art tools, the tool comprises a cutting blade having a diffuse or graded interface, wherein there is a discontinuous, or diffuse, or graded interface between hard surface or rim layer and core of the blade, and wherein the binder phase of the surface layer and core comprises partially or completely miscible resin(s) with the scale of the grading being molecular up to visible scale.
FIG. 1A is a top view of a typical wheel or blade in the prior art, formed as a composite of a hard abrasive-containing surface layer (1) bonded to a separate blade core (2).
FIG. 1B is a cross-section view of a wheel in the prior art, wherein the wheel further comprises a hard grit-resin surface layer (1, 3) in either or both, radial and axial directions.
FIG. 2A is a cross-section view of one embodiment of the invention with a graded interface with the gradient/diffuse interface denoted by shades of gray.
FIG. 2B is a top view of another embodiment of the invention for an abrasive wheel having a gradient/diffuse interface denoted by shades of gray.
FIG. 3 is a graph illustrating the change in an intensive property, e.g., the concentration, of the abrasive surface layer.
FIG. 4 is a graph illustrating the variation in the intensive property hardness from the edge of a molded disk to the center.
FIG. 5 is an SEM scan of an embodiment of the invention, of a blade demonstrating the gradedness of the copper concentration in the blade.
 The present invention relates to resinous abrasive cutting and grinding articles having an abrasion-resistant outer surface comprising hard grits and resin, integrally bonded through a graded, diffuse, continuous interface to a resinous core, for use as tools for material removal or cutting, such as cut-off saws, grinding or sanding wheels, cutting blades and the like.
 In one embodiment of the invention, the resins comprising the outer surface and core of the articles are partially or completely miscible, optionally containing any variety of fillers to provide useful functions to the parts of the bonded article such as abrasion-resistance, toughness, inertness, conductivity, and/or color.
 The invention further relates to a molded article comprising a gradient composite resinous material containing a polymer resin and abrasive particles, and wherein a composition ratio of the abrasive articles is decreased inwardly from the outer surface of the molded article, and the abrasive articles are selected from one or more of diamond, cubic boron nitride (CBN), alumina, SiC, garnet, and mixtures thereof.
FIG. 2A illustrates one embodiment of a cutting element of the invention with abrasive filled surface. As illustrated in the Figure, the composition of the polymer resin matrix gradually changes on a scale of the article, i.e., at the molecular and/or up to the visual scale from the outer surface toward the core, such that interface between the outer surface and core is diffuse and graded in at least one intensive property.
 As known in the art, “intensive property” refers to any property that can exist at a point in space and not dependent on how much of the substance in the core and/or the outer surface. Examples of intensive properties include but are not limited to density, hardness, toughness, melting point, composition (concentration), and the like.
 In one embodiment of the invention, the concentration of the abrasive grits in the polymer resin varies gradually at the scale of the mesh size of the abrasive grits or larger. It should be noted that the scale of the mesh size of the abrasive/superhard grits varies according to the applications and specific embodiments, i.e., the mesh size can be in microns, millimeters, or centimeters.
 In yet another embodiment of the invention, the scale of the article limits the gradient. Surface cracks formed in the article of the invention are absorbed in the diffuse interface and do not propagate along the interface and thus do not lead to delamination.
FIG. 2B illustrates another embodiment of a graded abrasive wheel of the present invention. The varying shades indicate the varying concentration of the abrasive particles or other component from core to outer surface. As shown in the figure, there is no bond line, no knit line, no weld line, no visible sharp interface between the abrasive surface and the non-abrasive core.
 Molding Resins: As indicated above, the article of the present invention comprises a polymer resin (or polymer resins) as the matrix and varying concentrations of abrasive particles in the polymer matrix, forming molding fluid(s) that are at least partially miscible and moldable.
 Molding of the abrasive wheel is conducted under conditions such that a diffuse, discontinuous, smoothly changing interface forms between the resin(s) of the core and the surface or outer rim. The resins forming the wheel are sufficiently miscible and moldable such that when contacted, they interdiffuse and/or intermix due to controlled convection in the mold, before freezing or curing begins.
 Abrasive Component Abrasive particles are incorporated in both the surface and core of the article of the invention. The abrasive component comprises superabrasive or superhard particles (grits) with a hardness of greater than about Mohs 8. These include, but not limited to, diamond, cBN, hexagonal boron nitride, alumina, SiC, garnet, and the like and mixtures thereof. The abrasive particles can be used neat or coated to improve wetting and adhesion to the matrix resin. Suitable coatings include, for example, metals, including, e.g., Ni, Cu, Cr, Fe, Co, Sn, W, Ti, or an alloy or compound thereof.
 In one embodiment, the abrasive particles are diamond particles, which can be natural or synthetic. Synthetic diamond particles can be made by conventional high pressure/high temperature (HP/HT) techniques or by conventional chemical vapor deposition (CVD) techniques. In another embodiment, the abrasive component comprises cBN or cubic boron nitride. The manufacture of CBN by the high pressure/high temperature (HP/HT) process is known in the art and is typified by the process described in U.S. Pat. No. 2,947,617, which discloses a process to make a basic monocrystalline CBN. An improvement on the direct conversion process is disclosed in U.S. Pat. No. 4,289,503, wherein boric oxide is removed from the surface of the HBN powder before the conversion process.
 In one embodiment of the invention, the particle sizes of the abrasive component range from microscopic particles (e.g., 0.1 microns) to 1 mm. Within this range, the particle size of the abrasive grit in the surface may be about 1 μm for thin, hard coatings and up to about 0.4 mm for thick, very hard coatings on cutting tools and blades. There is virtually no limitation on the shape of the abrasive particles suitable for use in the present invention. Such particles can be in the shape of regular geometric solids, irregular geometric solids, fibers, agglomerates, and the like.
 In one embodiment of the invention and depending on the abrasive particles used, the abrasive grits comprising the outer layer of the article of the invention may be selected for specific functions or properties such as pigments for color, electrical conductivity, thermal conductivity, blowing agents for porosity, and the like.
 Polymer Resin Component. In one embodiment of the invention, the polymer resin for use in the resin matrix which comprises the surface and core of the article of the invention is a moldable resin. The resin may be liquid or solid or plastic. In a second embodiment of the invention and depending on the use and required characteristics of the molded article, the polymer matrix is selected from one of a thermoplastic resin; a thermoplastic elastomer, a thermosetting resin, and a vulcanized rubber.
 Examples of thermoplastic resin include polyethylene, polypropylene, ethylene α-olefin copolymer such as ethylene-propylene copolymer, polymethylpentene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, ethylene vinyl acetate copolymer, polyvinyl alcohol, polyacetal, fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polystyrene, polyacrylonitrile, styrene acrylonitrile copolymer, ABS resin, polyphenylene ether (PPE) and modified PPE resin, aliphatic and aromatic polyamides, polyimide, polyamide imide, polymethacrylic acid and polymethacrylates such as polymethyl methacrylate, polyacrylic acids, polycarbonate, polyphenylene sulfide, polysulfone, polyether sulfone, polyether nitrile, polyether ketone, polyketone, liquid crystal polymer, silicone resin, and ionomer.
 Examples of thermoplastic elastomers include repeatedly moldable and recyclable thermoplastic elastomers such as styrene-butadiene or styrene-isoprene block copolymers and hydrogenated polymer thereof, styrene thermoplastic elastomer, olefin thermoplastic elastomer, vinyl chloride thermoplastic elastomer, polyester thermoplastic elastomer, polyurethane thermoplastic elastomer, and polyamide thermoplastic elastomer.
 Examples of thermosetting resins include epoxy resin, polyimide, bis-maleimide resin, benzocyclobutene, phenol resin, unsaturated polyester, diallyl phthalate, silicone resin, polyurethane, polyimide silicone, thermosetting polyphenylene ether resin and modified PPE resin.
 Examples of vulcanized rubbers and analogues thereof include natural rubber, butadiene rubber, isoprene rubber, styrene-butadiene copolymer rubber, nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, ethylene-propylene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, butyl rubber and butyl rubber halide, fluorine rubber, urethane rubber, and silicone rubber.
 In one embodiment of the invention, thermosetting resins are used. The thermosetting resins are molded and cured outside the mold, i.e., polymerized and/or crosslinked upon exposure to heat and/or other sources of energy, such as E-beam, ultraviolet, visible, etc., or with time upon the addition of a chemical catalyst, moisture, and the like.
 In another embodiment of the invention, polyetherimide resins are used. The basic polyetherimides are known in the art and generally are prepared by reacting an organic diamine with an aromatic bis(ether dicarbonyl), i.e. an aromatic bis(ether anhydride) or an aromatic bis(ether dicarboxylic acid). Such polyetherimides are disclosed, for example, in U.S. Pat. Nos. 3,803,805, 3,787,364, 3,917,643, and 3,847,867, the disclosures of which are expressly incorporated herein by reference.
 In yet another embodiment of the invention, the polymer resins are selected from bis-maleimide-triazine addition copolymer resins, polyetheretherketone (PEEK) resins, polyetherketone (PEK) resins, polyaryletherketone resins, poly(amide-imide) resins, polyphenylene sulfide (PPS) resins, liquid crystal polymers (LCP), aromatic polycarbonate resins, and the like, and mixtures thereof.
 Depending on the final application, the polymer resin for use in the articles of the present invention is used in the form of either or both, blended pellets or powder. In one embodiment, the polymer resin is used in the form of a powder. In another embodiment, the polymer resin may be formulated as a pellet or other solid form derived from compounding. In another embodiment, the polymer resin may be a fluid or slurry or mixtures thereof.
 Optional/Additional Components. In one embodiment of the invention, wherein the abrasive element is used in the cutting of hard concrete and brick with considerable deflection and heat, hot strength and toughness are required. In this embodiment, porous fillers or pore formers may be added to the polymer resin. The fillers/formers burn out in operation, leaving holes in the outer layer to allow temporary accumulation of fluid and debris to minimize wear.
 The porous fillers or pore formers may be virtually any material so long as the material is substantially porous (30% to 99.5% porosity). In one embodiment, the materials comprise a plurality of non-randomly spaced openings. Suitable materials are organic or metallic non-woven, or woven mesh materials, such as copper, bronze, zinc, steel, or nickel wire mesh, or fiber meshes (e.g. carbon or graphite). In another embodiment, the pore formers are stainless steel wire meshes, expanded metallic materials, and low melting temperature mesh-type organic materials.
 In another embodiment of the invention, surfactants are added to the polymer matrix composition for establishing folding flow at the interface, i.e., creating viscous fingering interfacial convection, and thus accelerate and control diffusive mixing of the interface to create the grading between the outer layer and the core layers of the article.
 In yet another embodiment of the invention, filler that has high thermal conductivity may be optionally included in the polymer matrix composition. Fillers include powder- or fiber-shaped metal and ceramic such as silver, copper, gold, aluminum oxide, magnesium oxide, aluminum nitride, silicon nitride, aluminum hydroxide; metal coated resin; conventional graphitized carbon fiber, non-graphitized carbon fiber, graphite, graphite, mesocarbon micro beads, whisker-, micro coil- or nanotube-shaped carbon, and the like.
 Process Forming the Article of the Invention. Compounding procedures known in the art may be useful to form the “molding compositions” comprising the outer surface and core of the article. Certain compounding procedures may be specifically used to improve dispersion and wetting of the filler, compared to dispersion and wetting achieved with dry powder mixing methods. Dispersion and wetting of abrasive fillers may also be improved by the use of coatings applied to the abrasive filler prior to being mixed into the polymer resin.
 Depending whether the molding composition is for the outer surface layer or the core layer, the amount of hard grit (abrasive and/or superabrasive particles) to be compounded into the polymer resins may range up to 95% v/v (volume by volume) of the total solids, with sufficient matrix resin to fully wet, i.e., conformally contact and coat, each grit without porosity, fissures, cracks, or delamination. In one embodiment of the surface layer, the amount of hard grit ranges from about 1 to 95% v/v solids.
 With respect to the core part of the article of the invention, in one embodiment of the invention, the amount of hart grit (abrasive and/or superabrasive particles) to be compounded into the polymer resins may be little to none, while the amount of fillers such as fibers may be an amount of up to 70% v/v (volume by volume) of the total solids.
 Processes that can be used to manufacture the inventive articles include, for example, injection, over-molding, reaction injection molding, resin-transfer-molding (RTM), reaction injection molding, vacuum assisted resin transfer molding, Seeman's composite resin infusion manufacturing process (SCRIMP), chemically assisted resin transfer molding, atmospheric pressure molding, open mold casting, bulk molding, sheet molding, lamination, co-injection and compression molding, and combinations comprising at least one of the foregoing processes, for forming an abrasive molded article in which the binding phases between the outer surface layer and the core are moldable, miscible, partially or completely.
 In one embodiment of the invention, a co-injection molding process is used for fast, inexpensive resin molding with controlled interdiffusion and convection technology used to establish gradedness at the interface as opposed to the prior art process of expensive assembly of separate parts.
 In one embodiment of the co-injection molding process, a first matrix composition comprising a polymer resin filled with abrasives and optional fillers is fed/placed/or pushed into a mold in the shape of the finished article. A second matrix composition comprising a second polymer resin filled with optional toughening fillers such as fibers or plasticizers is fed/pushed/placed into the same mold in the same molding cycle, thus forming a bond (physical and chemical) between the first and second compositions comprising the outer surface and core of the article of the present invention.
 In another embodiment, different matrix compositions may be fed/placed/pushed into the mold in the same molding cycle, for multi-layers or multi-annular layers (for abrasive wheels) of resins having varying abrasive concentrations. The amount of diffusion between the layers is controlled to eliminate any sharp gradient in density, hardness, toughness, or composition at the interface(s) between the resin matrix materials.
 In one embodiment of the invention, the molded articles have a property profile as illustrated in FIG. 3, showing the gradient change in an intensive property of the article, e.g., density, hardness, toughness, or composition of the abrasive surface layer, as opposed to the sharp gradient or change in the property profile of an article in the prior art having a non-graded profile.
 In yet another embodiment of the invention, a small and controlled amount of convective flow in the mold during fill and/or cure helps accelerate interdiffusion, interlocking the miscible resinous phases in the mold to improve the bond between abrasive-rich, brittle outer layer and abrasive-depleted, tough core, on a molecular scale. This interlocking at the scale of the convective or diffusive flow at the interface improves the toughness of the interface, eliminating delamination, especially useful for thin and hard surface layers.
 In another embodiment of the invention, the “finished” abrasive article may undergo further post-mold processing steps. Namely, a water-repellant material such as a silicone or fluorine resin, is applied to or splayed onto the outer surface layer of the article as disclosed in U.S. Pat. No. 5,079,875, the disclosures of which are expressly incorporated herein by reference. As a result, the water-repellant material enters the pores of the outer surface layer, thereby preventing various oxidants or fluids from entering the inside of the abrasive wheel.
 In yet another embodiment of the invention, the post-mold processing step may include pigmenting the outer surface layer, or rendering the surface layer electrically insulative relative to the core.
 Articles of the Present Invention. The abrasive element or part of the present invention can be post-molded processed, such as, for example, ground to a particular size or shape, sharpened, annealed, fixtured, or the like, forming cutting/grinding articles in the form of blades, wheels, discs, etc.
 In one embodiment of a blade configuration, one side of the resinous blade is filled with hard particles (i.e., superabrasive or abrasive particles); the other side is filled with fillers such as fibers to render it tough. The interface between the two sides of the blade is diffuse and graded in superabrasive concentration. In one embodiment, a grinder may be used to form a sharp and hard abrasive-resinous edge. Because the interface of the blade is graded, the blade can be bent without delamination.
 In another embodiment of the invention, the abrasive element of the invention is in the form of a cutting disc, having a thick outer layer for increased wear life. The disc surface may further comprise a thin surface layer of grit-resin to eliminate binding of disk in the deep slot (relative to tool size) being cut or ground, as well as to increase the life of the core when deflected against the work wall in the cut (see, for example, U.S. Pat. No. 5,495,844). In one embodiment of a cutting disc, the disc comprises hard grits only on the surface material, with no hard material on the outer rim, e.g., to act as a sanding disc.
 In yet another embodiment of the invention, the abrasive elements are in the form of cut-off wheels and grinding wheels, and the like, having extended life and improved properties compared to the wheels of the prior art.
 The examples below are merely representative of the work that contributes to the teaching of the present invention, and the present invention is not to be restricted by the examples that follow.
 A polymer matrix composition is prepared by blending a poly (arylene ether) and an allylic monomer to form a first intimate blend. This first blend is then blended with an acryloyl monomer to form a second blend. This second blend is then ground at a temperature of about −75° C. or less, wherein the resin is embrittled and ground into small particles having particle sizes less than about 50 grit. The cryogenic grinding is done using a Retsch/Brinkmann ZM-1 Grinder.
 Ten parts by weight of the powdered resin (“second blend”) are combined with 40 parts by weight of nickel-coated diamond to form a third blend. The nickel-coated diamond has 44 weight percent diamond and 56 weight percent nickel, and 31.5 parts by weight of silicon carbide. The nickel-coated diamond had a mesh size of 120-140.
 Forty parts by weight of the “third blend” are combined with 10 parts by weight of the powdered resin (second blend) to form a fourth blend of a gradually reduced or depleted concentration of abrasive particles.
 Forty parts by weight of the “fourth blend” are combined with 10 parts by weight of the powdered resin (second blend) to form a fifth blend with a more depleted concentration of abrasive particles.
 Forty parts by weight of the “fifth blend” are combined with 10 parts by weight of conventional graphitized carbon fiber to form a sixth blend with an enriching concentration of fibers.
 Forty parts by weight of the “sixth blend” are combined with 10 parts by weight of conventional graphitized carbon fiber for a more enriched fiber concentration, forming a seventh blend.
 Forty parts by weight of the “seventh blend” are combined with 10 parts by weight of a conventional graphitized carbon fiber, forming the core composition, an eighth blend.
 The first, second, third, etc. blends are placed onto a mold in annular layers starting with the core blend (mostly fiber filled) forming the inner hub to the surface blend (mostly abrasive filled) placed at the outermost rim.
 The mold is then closed and heated to 140° C. as a force of 3 metric tons is applied to compress the resin/abrasive mixtures. After 25 minutes, the specimen is removed from the mold and examined. A hardened, cross linked, disk having good physical integrity is obtained with a gradient composition, hardness, brittleness, and abrasiveness varying from the core blend layer (most tough) to the third blend layer (most brittle).
 Ten parts by weight of a powdered poly (etherimide) resin are combined with 40 parts by weight of nickel-coated diamond to form a blend. The nickel-coated diamond has 44 weight percent diamond and 56 weight percent nickel, and 31.5 parts by weight of silicon carbide. The nickel-coated diamond had a mesh size of 120/140. Sequential blends are prepared as per example 1, finishing with the wheel fabrication by hot pressing at the blends in layers at 850° F. and 14 tons/inch2 for 1 hour, for a hardened, disk having good physical integrity with a gradient composition, hardness, brittleness, and abrasiveness varying from the core blend layer (most tough) to the outer rim blend layer (most brittle).
 The same resin blends are made as described in Example 2. The resin blends are placed in the mold, not in annular sections, but axial layers (as in a cake) with the high-abrasive content forming one side and fiber-filled blend forming the other side, and layers of different and sequentially decreasing abrasive contents in between. The article is hot pressed and cured at 850° F. and 14 tons/inch3 for 1 hour as in Example 2 to form a one-sided sanding disk that is durable and will not delaminate when bent.
 The same resin blends are made as described in Example 2 but comprising 3 annular layers. The first or outermost layer is a well-blended mixture of 5% v/v diamond MBS910 Ti-coated 40/50 mesh size, 38% v/v copper powder (AEE, <25 microns) and 57% v/v polyetherimide resin powder (grade Ultem 1000P, commercially available from GE Plastics of Pittfield, Mass.). The 2nd innermost layer is 2.5% v/v diamond, 58.5% v/v polyetherimide resin powder and 39% v/v copper powder. The innermost of core layer is 60% resin and 40% v/v copper powder.
 The layers are manually intermixed at the interface, prior to being compacted, compressed, and heated, then cooled to cause densification and hardening of the wheel.
 A comparative wheel is fabricated with only 2 annular layers, comprising 5% v/v diamond and no diamond, same materials, blending and molding procedures. Hardness is measured every 1 mm from the 74 mm OD blade.
 The results are shown in FIG. 4, wherein within the gauge error, hardness of the graded blade decreases less swiftly within the outer layers of the molded disk than the bi-layer blade.
FIG. 5 shows an image of the interface of one embodiment of the invention demonstrating gradedness via copper concentration for an abrasive rim containing copper and tin powder additives. The visually dark region delineates the abrasive rim by its dark secondary abrasive filler package and embedded diamond crystals.
 While the invention has been described with reference to a preferred embodiment, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Also, all citations referred herein are expressly incorporated herein by reference.