Improved abrasive particles

Abstract

The present invention relates to a shaped abrasive particle, comprising opposing and spaced apart first and second major surfaces (1), the spacing comprising a sidewall, each major surface comprising an inner surface (2) and at least five and at most seven sharp outward protrusions (3), and where the sidewall comprises multiple intersecting sides which continuously interconnect the opposing surfaces and define the protrusions. The present invention further provides abrasive articles containing such particles and a process to produce such articles.

Description

[0001] RACAB.1-PCT01 IM P R O V E D A B R A S I V E P A R T I C L E SFIELD OF THE INVENTION 5 The present invention provides a shaped abrasive particle. The present invention further provides abrasive articles containing such particles and a process to produce such articles. 10 BACKGROUND OF THE INVENTION Abrasive particles typically rely on a difference between the hardness of the abrasive and the material being worked upon, with the abrasive being the harder substance to affect their abrasion. Abrasive particles may be mined (natural abrasive minerals) but they may also be 15 manufactured (synthetic abrasive minerals or synthetic abrasives). Abrasive articles may be coated or bonded. Coated abrasive articles, e.g. grinding belts, typically comprise the particles mounted on a backing substrate. Bonded abrasive articles, e.g. grinding wheels, typically comprise the particles embedded in a matrix forming the body of the article. 20 Minerals used as abrasives typically have a hardness rated 7 or above on the Mohs scale of hardness and they may be classified or crushed to a specified average particle size ranging from around 1 μm to 2 mm for the particles which are often referred to as “grit”. These particles typically have rough edges which tend to increase the localized contact pressure with the workpiece. 25 Common synthetic abrasive materials include garnet, diamond (synthetic), silicon carbide, aluminium oxide, boron nitride, ceramic materials, zirconia alumina, and many others. The grit size is a number used to describe the number of openings per linear inch in a sieve to classify the particle size according to FEPA standards. Low grit numbers are coarser and 30 higher grit numbers are finer. Factors that influence the rate of abrasion include for instance: — the difference in hardness between the abrasive particles and the workpiece; — the size of the abrasive particles (also known as “grit size”), wherein larger particles 35 usually cut faster and deeper; — the contact force applied to the process; — the shape of the abrasive particles, and — the orientation of the abrasive particles, e.g. the orientation of the abrasive particles on the backing substrate in the case of coated abrasive articles. In the past, different shapes for abrasive particles were proposed to improve the rate of abrasion. Despite significant efforts, no optimum particle shape has yet been identified which provides an improved rate of abrasion and durability of the abrasive particle during use, combined with an easy way to orientate properly the abrasive particles. During the production of a coated abrasive article, electrostatic strewing of the abrasive particles is typically used. It was observed that the shaped abrasive particles of the past (for example triangular particles) generally do not orientate properly, which is detrimental to the abrasive performance of the produced article. Consequently, additional constraining industrial process steps are required to improve their orientation. The present invention aims to provide an improved abrasive particle, an improved abrasive article and / or an improved process for the production of an abrasive article. GENERAL DESCRIPTION OF THE INVENTION According to the present invention, there is provided an abrasive particle as defined in the accompanying claims as well as the manufacture of the abrasive article comprising such particles and an abrasive article. Some common abrasive grains are based on polycrystalline alpha alumina, like sol-gel derived abrasive grains. Such abrasive grains are well known and are established commercially. It was found that the grinding performance of this alpha alumina abrasive grain on metal, for example as measured by stock removal rate or g-ratio (quotient of stock removal and abrasive loss), may be higher than that obtained by using the previous fused alumina abrasive grain. Sol-gel abrasive grains are typically prepared by dispersing an alumina monohydrate in acidified water, gelling the dispersion, drying the obtained gel, crushing the dried gel into particles, calcining the dried particles to remove the water and other volatiles, and sintering the calcined particles at a temperature far below the melting point of alumina. Frequently, one or more oxide modifiers, nucleating agents, grain growth inhibitors or other additives are additionally used in order to further improve the properties and the grinding performance of the alumina abrasive grains. The present invention therefore provides an abrasive particle comprising opposing and spaced apart first and second major surfaces, the spacing comprising a sidewall, wherein each major surface comprises an inner surface and at least five and at most seven sharp outward protrusions, and wherein the sidewall comprises multiple intersecting sides which continuously interconnect the opposing surfaces and define the protrusions. This means in particular that the inner surface of the first major surface is formed by a portion of the first major surface and the inner surface of the second major surface is formed by a portion of the second major surface. The five to seven protrusions constitute another portion of the first and second major surface, respectively. The five to seven protrusions extend from the respective inner surface. In other words, each major surface may be divided into an inner surface and five to seven protrusions such that the protrusions protrude from the inner surface. Abrasive particles are widely used in industrial activities such as cutting, grinding and polishing. In the case of coated abrasive articles, the particles are provided on a support base such as a belt, fibre disc, flap disc, paper backing such as in sandpaper, etc. In the manufacture of such articles the abrasive particles are deposited onto the support base where they are retained by an adhesive layer. These deposited abrasive particles may be coated, the coating being selected according to the use to which the abrasive product is envisaged. One important aspect of the manufacturing process is that the abrasive particles are deposited on the support base in the optimum orientation to optimise their abrasive performance in use. There is a need of abrasive particles which are easy to deposit with an optimum orientation on a support base and which have good abrasive properties. We have now found that the abrasive particle according to the present invention solves this problem. In particular, we have found that the abrasive particle according to the present invention may for example allow for an advantageous particle orientation, in particular when strewn onto a substrate, and yet a relatively simple and robust particle such that excellent grinding performance even after longer periods of use and / or an excellence balance between manufacturability and grinding efficiency may be obtained. Preferably, each major surface further comprises a regular polygon with 5 to 7 vertices constituting the inner surface, wherein each protrusion connects two vertices of the regular polygon. An aspect of the present invention is that the particles according to the present invention can be readily deposited by electrostatic strewing only (without the need of a further method pre- orientating the particles) on a substrate in a preferred orientation. By regular polygon is meant a convex regular polygon, being substantially equiangular (all angles are substantially equal in measure) and substantially equilateral (all sides have substantially the same length). The regular polygon has 5 to 7 vertices, i.e. is a pentagon, a hexagon or a heptagon. The first and second surfaces are preferably substantially parallel. The intersecting sides which define the protrusions may be at an angle from about 10 to 90° in relation to a major surface. Herein, it is preferred that the intersecting sides are substantially perpendicular to the first and second major surfaces. Preferably, the number of intersecting sides is twice the number of protrusions. Preferably, the intersecting sides may be rectangular, trapezoidal and / or triangular. The abrasive particle preferably comprises a maximum of 6 protrusions. In preferred embodiments, the number of the protrusions is five. With only five protrusions, the particle structure is relatively simple and stable, which may be advantageous in view of manufacturability. Further, an odd number of protrusions (such as 5) may be advantageous in view of the particle orientation, e.g. if the strewn or otherwise applied particles have the tendency to be oriented with two protrusions on the substrate to have a stable position (i.e. “to stand with both feet on the ground” so to speak). This may be in particular the case if the inner surface is polygonal. A protrusion is typically formed by two intersecting sides. One or both of the surfaces of the intersecting sides may be concave, convex or planar. Preferably they are planar or concave. The protrusions are preferably substantially identical (for example, identical size and / or shape). The protrusions are preferably equidistantly positioned around the perimeter of the inner surface, for example a regular polygon. Preferably, each protrusion connects two vertices of the regular polygon, i.e. each protrusion connects two subsequent vertices. It means that two vertices of the protrusions coincide with two vertices of the polygon. The term sharp when used in relation to the protrusions is used to require that the cross section of the protrusion decreases as it extends away from the inner surface (for example the regular polygon). In other words, the protrusion tapers in the direction outwards from the inner surface, i.e. it has lateral sides that converge towards each other such that the tapered shape results. The direction outwards from the inner surface may also be referred to as “distal direction” because it points away from the inner surface being the central portion of the major surfaces. The expressions “outwardly tapered protrusion” and “sharp outward protrusion” may therefore be considered as synonymous expressions. It is not required that the protrusion reduces to a single point (called “opposite point”). If the protrusion reduces to a single point, i.e. if it tapers in such a way that a single tip is formed constituting the point of the protrusion that is most distant from the inner surface, the protrusion could also be called “pointed”. The extent to which the cross section should reduce may depend on the particular use envisaged for the abrasive particle. Preferably, the sharp protrusions may be pointed. Alternatively, they may not be pointed, for example, they may reduce to two (opposite) points. The protrusions may for instance be substantially triangular. For example, they may be an equilateral triangle or an isosceles triangle. A triangular protrusion is an example of a pointed protrusion. The protrusions may for instance be substantially trapezoidal. For example, they may be an isosceles trapezoid. A trapezoidal protrusion is an example for a protrusion reducing to two points. A trapezoid (also referred to as “trapezium”) is a quadrilateral that has one pair of parallel sides. Preferably, the major surfaces are star-shaped, wherein the protrusions constitute the (triangular, trapezoidal, or otherwise shaped) arms of the star. The protrusions may have a protrusion angle from about 36° to less than 108°. The protrusion angle of a protrusion is the taper angle of the sharp outward protrusion. In other words, the protrusion angle is the angle between the lateral sides of the protrusion which converge towards each other such that the tapered shape results. This is illustrated in Figs.1a-1e for exemplary protrusion shapes and an exemplary pentagonal inner surface. In preferred embodiments, the protrusions are triangular, i.e. they substantially have the shape of a triangle, wherein the first side of the triangle coincides with a side of the inner surface. In this case, the protrusion angle is the angle opposite to the first side, i.e. the angle between the second side and the third side of the triangle (as illustrated in Fig.1a for a pentagonal inner surface shown as an example, wherein the reference signs are as follows: 1 – first or second major surface, 2 – inner surface, 3 – protrusions, p – protrusion angle). In this case, the protrusion angle may also be called “tip angle”. In the case the protrusion is pointed, and the opposite point of the protrusion is connected to the vertices of a regular polygon through curved lines, then the protrusion angle is the angle drawn by the two tangents to the curves from the opposite point (as illustrated in Figs.1b and 1c for a pentagonal inner surface shown as an example, wherein the reference signs are as follows: 1 – first or second major surface, 2 – inner surface, 3 – protrusions, p – protrusion angle). In the case the protrusion reduces to two points and the inner surface is a regular polygon, then the protrusion angle is the angle formed by the two prolonged lines between the vertices of a regular polygon and the opposite points (as illustrated in Fig.1d). In further embodiments, the protrusions are rounded triangular, i.e. they substantially have the shape of a triangle whose tip that is most distant from the inner surface is cut off along a curved line such that a rounded tip results. In further preferred embodiments, the protrusions are truncated triangular, i.e. they substantially have the shape of a triangle whose tip that is most distant from the inner surface is cut off. Preferably, the cutting line along which the tip of the triangle is cut off is a straight line. Said straight line may be parallel to the side of the triangle coinciding with a side of the inner surface such that a trapezoid results or it may be non-parallel such that an irregular quadrilateral results, see below. Hence, in further preferred embodiments, the protrusions are trapezoidal, i.e. they substantially have the shape of a trapezoid, wherein the first side (a) of the trapezoid coincides with a side of the inner surface and wherein the second side (b) of the trapezoid is parallel to the first side. In this case, the protrusion angle is the angle between the third side (c) and the fourth side (d) of the trapezoid, i.e. the intersection angle of the extensions of the third and fourth sides. This is illustrated in Fig.1e for a pentagonal inner surface shown as an example, wherein the reference signs are as follows: 1 – first or second major surface, 2 – inner surface, 3 – protrusions, p – protrusion angle, a – first side of the trapezoidal protrusions, b – second side of the trapezoidal protrusions, c – third side of the trapezoidal protrusions, d – fourth side of the trapezoidal protrusions (a, b, c, d are shown for one protrusion only because they are analogous for the other protrusions). In the case of trapezoidal protrusions, it is preferred that the ratio of the length of the second side (b) and the length of the first side (a), i.e. b / a (ratio of the lengths of the parallel sides), is at least 0.1, more preferably at least 0.15, even more preferably at least 0.20 and / or 0.75 or less, more preferably 0.60 or less, even more preferably 0.45 or less. In further preferred embodiments, the protrusions are irregular quadrilateral, i.e. they substantially have the shape of an irregular quadrilateral, wherein the first side (a) of the quadrilateral coincides with a side of the inner surface. The second side (b) of the irregular quadrilateral is not connected to the first side (a) and not parallel to the first side (a). In this case, the protrusion angle is the angle between the third side (c) and the fourth side (d) of the irregular quadrilateral, i.e. the intersection angle of the extensions of the third and fourth sides. This is illustrated in Fig.1d for a pentagonal inner surface shown as an example, wherein the reference signs are as follows: 1 – first or second major surface, 2 – inner surface, 3 – protrusions, p – protrusion angle, a – first side of the quadrilateral protrusions, b – second side of the quadrilateral protrusions, c – third side of the quadrilateral protrusions, d – fourth side of the quadrilateral protrusions (a, b, c, d are shown for one protrusion only because they are analogous for the other protrusions). In the case of protrusions having the shape of irregular quadrilaterals, it is preferred that the ratio of the length of the second side and the length of the first side, b / a, is at least 0.1, more preferably at least 0.15, even more preferably at least 0.20 and / or 0.75 or less, more preferably 0.60 or less, even more preferably 0.45 or less. In addition, it is preferred that the ratio of the length of the third side and the length of the fourth side, c / d, is at least 0.1, more preferably at least 0.15, still more preferably 0.2 and / or 1 or less, more preferably 0.75 or less, even more preferably 0.6 or less. It is noted that the individual protrusions may have different shapes, in particular shapes selected from the above-mentioned shapes (triangular, truncated triangular, trapezoidal, irregular quadrilateral), optionally with different protrusion angles and / or optionally of different size. Preferably, the protrusions are substantially the same in terms of shape, protrusion angle, and size to provide a high symmetry, which may be advantageous for instance in terms of manufacturability of the abrasive particles and / or in connection with the orientation of the particles on a substrate and / or in connection with the homogeneity of the grinding behaviour. Further, preferably, the protrusion angle is at least 40°, more preferably at least 45°. In addition or alternatively, it is preferred that the protrusion angle is 90° or less, more preferably 72° or less, even more preferably 60° or less. It is noted that in the case of a flat major surface with a pentagonal inner surface and triangular protrusions, a protrusion angle of 108° would result in a major surface having the shape of a pentagon. That is, the inner surface and the protrusions would then be rather mental subdivisions of the major surface because it would not be visible where portions of the major surface project to form sharp outward protrusions. In contrast, triangular protrusions with a protrusion angle smaller than 108° results in a major surface having the shape of a star (with tips cut off, rounded tips, etc.) if the inner surface is a pentagon (and also if the inner surface is a hexagon or heptagon). Star-shaped major surfaces and therefore star-shaped abrasive particles may be preferred because they have uniformly arranged and exposed sharp protrusions (in the case of stars also referred to as “arms”). Such a uniform arrangement and therefore uniform exposure may be advantageous both in terms of the relative simplicity of the shape and in connection with the manufacturing process of the abrasive particles as well as the orientation of the particles during a possible strewing process. Herein, the arms of the star are not necessarily triangular as is usually the case for a star. For example, it is possible that the tips of the arms are truncated or rounded as discussed above. The protrusions may be co-planar with the inner surface of a major surface, i.e. part of a same plane. Preferably, all the protrusions may be co-planar with the inner surface of a major surface. Alternatively, they may be configured at an angle to a major surface and different protrusions may be configured at different angles. For example, they may be configured at an angle from about 5° to about 90° to the inner surface of a major surface. Here again the selected configuration can depend on the particular use envisaged for the abrasive particle. The diameter of the circumcircle of a major surface is the diameter of the smallest circle that encloses the entire major surface (or in the case of a non-planar major surface its projection onto the plane). This circumcircle diameter is also referred to as “circumscribed diameter”. Hereinafter, the circumcircle diameter is also denoted as Dc. Herein, the dimensions of the major surfaces may for instance be measured by analysis of a microscopic image of particles, wherein preferably the particles are substantially horizontally arranged. Preferably, the average values obtained from multiple individual measurements are used. Further, the diameter of the incircle of a major surface is the diameter of the largest circle that is within the major surface (or in the case of a non-planar major surface its projection onto the plane). This incircle diameter is also referred to as “inscribed diameter”. Hereinafter, the incircle diameter is also denoted as Di. In preferred embodiments, the major surface is such that each pointed end of the protrusions can be circumscribed by a circle that is centred at the centre of the inner surface, which is in particular a regular polygon. Herein, more preferably, the pointed ends all essentially lie on this circle, i.e. the protrusions all have essentially the same length. In this case, the circumcircle is hereinafter also referred to as “circlepro”. Analogously, in the case the protrusions each reduce to two opposite points (one of which, O1, being more distant from the inner surface than the other one, O2), then all opposite points O1 preferably lie on the same circle (namely the circumscribed circle). The diameter of such a circleproD(circlepro) corresponds to Dcas defined above. Preferably the inner surface has vertices. The vertices of the inner surface, for example, the regular polygon, can preferably be circumscribed by a circle that is centred at the centre of inner surface, for example the centre of a regular polygon (circlepoly). The diameter of such a circlepolyD(circlepoly) corresponds to Dias defined above. Figure 2 represents examples of abrasive particles according to the present invention (top image) and particles with a shape not part according to the present invention (bottom image) with circleproand circlepolydrawn. In preferred embodiments, the diameter ratio Di / Dc(in particular D(circlepoly) / D(circlepoly) if circleproand circlepolyaccording to the definition above exist) is from about 0.10 to about 0.80, preferably from about 0.20 to about 0.70, more preferably from about 0.40 to about 0.70. In preferred embodiments, the abrasive particles according to the present invention have a chemical composition comprising from about 0.1 % to about 40% by weight zirconia, preferably from about 1 % to about 20% by weight zirconia, more preferably from about 1.5% to 7.5% per weight. In this case the crystal structure may comprise a dominant continuous phase of α-alumina crystals and a secondary phase of substantially intergranular oriented zirconia crystals, wherein the average crystal size of the alumina and zirconia crystals is between 100 and 300 nm and wherein the crystal size of the zirconia crystals is less than 100 nm. We have found that an amount of zirconia between 1 % and 10% by weight is sufficient for obtaining high performing abrasive particles for special grinding operations. Thus, in a preferred embodiment of the present invention, the abrasive particles comprise from about 1 to about 10 wt.% zirconia. More preferably, the abrasive particles comprise from about 1 to about 7 wt.%, or from about 1 to about 6 wt.% zirconia. Preferably, the abrasive particles according to the present invention comprise from about 50 to about 99.9 wt.% alumina, from about 85 to about 99.7 wt.%, from about 85 to about 98.8 wt.%, or from about 85 to 94 wt.% alumina. The alumina may comprise ^-alumina, preferably the alumina comprises from about 75 to about 100 wt.% ^-alumina. Preferably, the chemical composition of the abrasive particles according to this invention additionally comprises between 0.1% and 10%, preferably from 0.2 to 1 % by weight MgO, due to the fact that a magnesium nitrate solution is usually added as crystal growth inhibitor and peptizing agent during the sol-gel process. Preferably, the abrasive particle is a platelet-like particle. The abrasive particle according to this invention comprises a first major surface and a second major surface opposite the first surface whereby both surfaces are separated by a sidewall which preferably has a thickness (T) from about 20 µm to about 500 µm. Preferably, the thickness (T) is from about 50 to about 400 µm. Another suitable feature for characterizing the abrasive particles is the length-to-thickness ratio which ranges typically from 2 to 10, preferably from 3 to 7, more preferably from 4 to 6. It is to be noted that to calculate this ratio, the length corresponds to the maximum dimension of the two major surfaces, i.e. the highest distance between two points of a major surface (i.e. the maximum Feret points, wherein the distance between these points usually correspond the diameter of the circumcircle of the major surface). The thickness corresponds to the thickness of the sidewall, which may for instance be measured by analysis of a microscopic image of particles. Preferably, the maximum thickness value is considered as thickness T if the thickness of the sidewall is not constant. Preferably, the particles are substantially vertically arranged for the thickness measurement. Preferably, the average of multiple individual thickness measurements is used as thickness. The thickness is hereinafter also denoted as T. The length-to-thickness ratio may be calculated as Dc / T (in particular D(circlepro) / T if circleproaccording to the definition above exists), i.e. by dividing the diameter of the circumcircle by the thickness T. Accordingly, the length-to-thickness ratio is hereinafter also referred to as “diameter-to-thickness ratio”. If the major surfaces have different lengths / diameters, the higher value is used to calculate the length-to-thickness ratio / diameter-to-thickness ratio. It was found that, in order to be successfully applied to abrasive articles, the abrasive particles preferably have a certain degree of compactness with a corresponding length-to- thickness ratio. In preferred embodiments, the length-to-thickness ratio is from 2 to 8. We have found this preferable range and believe that thinner particles (with a length-to-thickness ratio above 8) might have a relatively low mechanical resistance, especially under severe grinding conditions involving high pressure and heat, whereas, on the contrary, more compact particles (with a length-to-thickness ratio below 2) might be too bulky and chunky such that grinding efficiency and self-sharpening efficiency might be too low. We have found particularly favourable results for length-to-thickness ratios from 3 to 7, especially from 4 to 5. The first and second major surfaces may be substantially parallel, i.e. the thickness of the sidewall interconnecting the opposing surfaces is substantially the same, i.e. does not differ from more than 15% of the maximum thickness of the sidewall. Alternatively, the first and second major surfaces may not be parallel. For example, the two major surfaces might be at angle from about 5° to about 85°, for example from about 10° to about 50°. Preferably, the first and second major surfaces may be identical. Preferably, the first and second major surfaces may be superposable, i.e. may be identical in shape and size, and in 2 dimensions they share the same projected centre. Alternatively, they may be identical in shape and size, but the first major surface is rotated on its centre compared to the second major surface, at a certain twisted angle. For example, the twisted angle may be from about 1° to about 54°. Alternatively, the first major surface may be translated at a certain distance (translation distance) compared to the second major surface. For example, the translation distance may be from about 10 µm to about 500 µm. The present invention also provides an abrasive article comprising the abrasive particles according to the present invention. Preferably, the abrasive article is a coated abrasive article. The coated abrasive article may comprise a backing substrate carrying the abrasive particles according to the present invention. Preferably, the grit density (mass of abrasive particles per area of the coated abrasive article) is between 600 g / m2and 700 g / m2. The coated abrasive article may further comprise a top coat. The present invention also provides a process for the production of a coated abrasive article comprising applying abrasive particles according to the present invention to a backing substrate provided with an adhesive by electrostatic strewing. SPECIFIC DESCRIPTION OF THE INVENTION The present invention concerns shaped abrasive particles, abrasive articles containing these particles and a process for making the article. Particle sizes and shapes as well as thickness values were measured by means of an optical microscope STEMI SV6 (Carl Zeiss GmbH). Particle size and shape are analysed by means of ImageJ software for calculating the respective dimensions and / or angles of each single abrasive particle conveniently deposited and separated when making the picture, based on at least 10 particles. An analogous approach was used for the particle thickness. The preferred method of making the abrasive particles according to this invention comprises the steps of: - preparing a dispersion of alumina hydrate in acidified water comprising α-alumina seeds having a particle size of less than 300 nm; - gelling the dispersion by addition of aqueous solution of zirconium and magnesium salts, such as zirconium nitrate and magnesium nitrate; - forming the gel to a layer of homogeneous thickness preferably by moulding; - drying the shaped gel to obtain an abrasive grain precursor material; - optionally calcining the abrasive grain precursor particles; - sintering the abrasive grain precursor particles at temperatures below 1450 °C to obtain the abrasive particles. The preparation of α-alumina seeds maybe carried out by wet ball milling, starting with a fine grained α-alumina powder having a mean particle size of less than 1 μm and optionally centrifuging the ball milled dispersion, to obtain α-alumina seeds having a particle diameter of less than 100 nm. The amount of seeds used for the production of abrasive particles is typically within the range of 1 % and 5% by weight, advantageously about 3% by weight, based on the weight of the abrasive particle product. In one process embodiment according to this invention in order to prepare a seeded dispersion of alumina hydrate in acidified water, the centrifuged seed dispersion is added to about 30% by weight aqueous solution of alumina monohydrate whereby the pH of the solution is adjusted to about 2.5 by adding a sufficient amount of nitric acid. The obtained solution is homogenized by using a high shearing homogenizer and subsequently gelled at room temperature by the addition of a salt solution comprising a sufficient amount of zirconium and magnesium salts, such as zirconium nitrate and magnesium nitrate, to obtain abrasive particles having a chemical composition comprising from 1 % to 20% by weight zirconia and from 1 % to 5% by weight magnesium oxide. The gel is then cast into shaped moulds and may then optionally be calcined at a temperature between 600 and 700 °C; and is sintered at about 1400 °C in for example a rotary kiln for 5 to 30 minutes. In general, the resulting abrasive particles have a hardness Hv of more than 20 GPa, preferably more than 22 GPa, and a density of more than 97% of the theoretical density. The abrasive particles according to this invention may be used inter alia in grinding, cutting and polishing applications. The present invention also provides an abrasive article comprising the abrasive particles according to the present invention. Preferably the abrasive article may have a grit density from about 100 to about 1500 g / m2, more preferably from 600 to 700 g / m2. Preferably, the abrasive article is a coated abrasive article. The coated abrasive article may comprise a backing substrate carrying the abrasive particles according to the present invention. Abrasive articles typically comprise abrasive particles secured to the surface of a backing substrate such as a belt, a disc or paper depending upon the use to which the article is to be part. Three coatings can be used in the manufacture of abrasive articles, an adhesive (called make coat) coating on the backing substrate to fix the abrasive to the substrate. A first size coating (called size coat) on the particles once applied to the substrate to secure the fixing of the abrasive to the substrate, by filling voids between the particles. A top coating (called top coat) is to provide a degree of temperature control during use of the abrasive article. One method generally employed for the manufacture of coated abrasive articles is electrostatic strewing. In this method, the backing is provided on one of a pair of electrodes, while the particles are provided on the other electrode of the pair. When the electric field is applied a charge differential on the particles will be created to direct the particles onto the backing substrate where they are fixed by an adhesive. The charge density on the particle will be greatest at the sharpest points of the particle and which will therefore be attracted to the backing substrate, having an opposite charge. For example, a triangular particle would preferably orientate by fixing the backing substrate at one sharp point instead of one side of the triangle, as intended. It is desirable that some sharp elements of the particles protrude away from the backing substrate to enable them to enhance the abrasive performance of the obtained abrasive article. We have found that the provision of five to seven protrusions on the particle enables such an orientation of the particles on the substrate and results in good abrasive performance. A process for the production of a coated abrasive article comprising applying abrasive particles according to the present invention to a backing substrate provided with an adhesive by electrostatic strewing. For use the particles are deposited on a backing substrate such as, for example, a belt, a grinding disc or paper for producing sand paper and the like. The backing substrate will typically be provided with an adhesive surface to secure the abrasive particles which are typically applied to the surface by electrostatic strewing. A make coat comprising a liquid phenolic resin is a suitable adhesive material for the abrasive particles according to this invention. After application of the abrasive particles to the backing substrate a size coat is typically applied following which the material is cured. If desired a further top coat (also called supersize coating), comprising a phenolic resin and cooling agents may be applied and a final curing may be performed to provide the finished article. The coated abrasive article may further comprise a top coat. The conditions for the electrostatic strewing of the abrasive particles should be controlled according to the size and dimensions of the abrasive particles to obtain the desired orientation of the particles on the backing substrate and we have found that the use of the particles according to this invention (particularly the particles with five protrusions) enables an improved orientation of the particles to be achieved. DRAWINGS AND EXAMPLES The present invention will now be further clarified by way of example only with reference to the following Examples and Figures in which Figures 1a to 1d show the protrusion angle of different exemplified particles according to the present invention; Figure 2 shows the circleproand the circlepolyon different shaped particles; Figure 3 shows a comparative shaped abrasive particle with three protrusions (not according to the present invention); Figures 4 and 5 show comparative shaped abrasive particles with four protrusions (not according to the present invention); Figures 6 and 7 show shaped abrasive particles with five protrusions according to embodiments of the present invention. Figure 8 shows shaped an abrasive particle with five protrusions according to another embodiment of the present invention. Example 1: Multiple abrasive particles were prepared. They differed with respect to their shape, i.e. with respect to the moulds that were used for their preparation, and / or their chemistry, i.e. the chemical composition of the material they were made of. Test results and various properties are in particular discussed for abrasive grains hereinafter referred to as P1 to P10, which are described below. Several properties of P1 to P10 are summarized in Tables 1 and 2 below. For the preparation of P1 to P10 and further abrasive grains, a dispersion with about 30% by weight solids content was made by mixing aluminium oxide monohydrate powder (boehmite) together with 3% by weight alpha aluminium oxide seeds, based on the total weight of the desired product, in a solution containing water and concentrated nitric acid (25%), whereby the pH was adjusted to 2.5. The resulting sol was mixed with 0.5 % by weight equivalent oxide of magnesium nitrate and 2 or 5 % by weight equivalent oxide of zirconium nitrate, each based on the weight of the desired product. Gelling occurred at room temperature after the addition of zirconium nitrate and magnesium nitrate. Then, the various gels were casted into moulds of different shapes before drying in a static oven at 100°C for about 10-13 min. Afterwards, the dried particles were recuperated by tapping on the back of the mould. Finally, these ZTA dried grits are sintered in a vertical piston kiln within a crucible at around 1400°C for 2 to 10 min. The different plastic moulds were manufactured by a 3D-printing process, called stereolithography or SLA method, of an UV-curable resin with a relatively good resolution of Z~50 µm and XY~28 µm. The printer is a “Anycubic Photon M3 Premium”. Table 1 1)comparative example2)equivalent to D(circlepro)3)calculated as D(circlepoly) / D(circlepro)4)calculated as D(circlepro) / T Table 2 1)equivalent to D(circlepro) 2)calculated as D(circlepoly) / D(circlepro) 3)calculated as D(circlepro) / T 4)length of side a coinciding with a side of the inner surface = 606 µm; length of side b parallel to side a = 145 µm; length of other two sides c = d = 506 µm The sintered density was measured by helium pycnometry on a Micromeritics Accupyc II 1330 device. The Vickers hardness was measured on a LM100 microindentation hardness tester, based on 15 abrasive particles, with a load of 200 g force, for example according to ISO 6507-1- 2018. The average grain size was measured on SEM micrographs with a SEM microscope JSM 6610 from JEOL, based on the mean linear intercept method. The obtained particles P1, P2, P3, P4, P5, and P10 are respectively shown in Figures 3, 4, 5, 6, 7, 8, by way of example. The particles according to the present invention were found to have a good friability performance when tested in the MKZ ball milling test which is performed at dried conditions (ambient), where 10 g of abrasive particles are poured into an iron cylinder. Twelve heavy tungsten carbide beads of about 20 mm diameter are also placed into the cylinder and will erode and / or break the surface of the abrasive particles during a testing time of 2.5 min. Then, the milled abrasive particles are re-sieved and compared to their original size distribution. Thus, for example, a value of 10 wt% measured below sieve #50 (300 µm) in FEPA standards corresponds to 1g of weighted worn residue versus 10g of original abrasive grits. Similarly, the value measured below sieve #35 (500 µm) corresponds to the weighted worn residue (including the sieved part below 300 µm) versus 10g of original abrasive grits. MKZ ball milling tests were carried out for the abrasive articles. Exemplary results are shown in Table 3.

[0002] Table 3 1)comparative example 2)chemistry different from P1-P4 and P6-P10, see Tables 1-2 Comprising for instance the particles P1, P2, and P6 with substantially the same protrusion angle (60° and 62°), the particles P6 according to the present invention show a much lower friability, which gives a longer life-time expectation to the particles, which is particularly beneficial. At the same time, the abrasive grains according to the present invention show a high grinding efficiency, see below. In other words, well-balanced properties including high grinding efficiency and high life-time and therefore durability of the manufactured abrasive articles are to be expected for the abrasive particles according to the present invention. The same effect was not only observed for P6, which is shown as an exemplary sample above, but also when comparing other abrasive grains according to the present invention with comparative examples. Particularly preferable results in the sense of both a long life- time and a high grinding efficiency were observed for particles according to the present invention with a protrusion angle between 40° and 90°. For instance, the tests made with the particles P4, P6, and P9 demonstrate that in particular with protrusion angles between 45° and 72°, favourable MKZ values may be achieved, which are low enough for a long life-time, whereas the protrusion are sharp enough for efficient grinding. It is noted that P4 and P5 do not have the same chemistry, cf. Table 1, so that the occurrence of different MKZ values for these particles, which are similarly shaped, is assumed to be because of to this difference. Example 2 Multiple grinding belts comprising the abrasive particles of Example 1 were manufactured as described below. A backing cloth (i.e. J625-H39) was cut into pieces in order to prepare a belt of 2000 mm length and 50 mm width. The backing cloth was covered with an adhesive (make coat) composed of bakelite (56.6 wt%), calcium carbonate Imercarb (37.7 wt%) and water (5.7 wt%) for dilution. The Bakelite used is a Bakelite resole from Hexion named PF0361SW. The abrasive particles were normally strewed on the make-coated backing cloth at fixed conditions in the climate room: 20°C, 50% humidity, 50 kV electric-field for 6s holding time with an electro-distance between plates of 2.5 cm. A curing was carried out with a thermal cycle of 6h. A size coat was then applied, comprising the same bakelite (54.5 wt%), calcium carbonate Imercarb (18.2 wt%), cryolite (18.2 wt%) and water (9.1 wt%). Then the curing was carried out with a long thermal cycle of 12h as a standard procedure. Then a top coat, containing 50 wt% of the cooling agent KBF4mixed with 16.7 wt% of liquid Phenol-Formaldehyde resin PF805055A (ex. Prefere resin) and 33.3 wt% of deionized water, is applied on the already-cured size coat with a paint brush. The mass of top coat mix applied is on average about 50 g for 2000 x 50 mm2belt, which approximately corresponds to 500 g / m2. The thermal cycle is 5h30, faster than for the two first resin layers. Testing grinding belts with abrasive particles having different protrusion angles, it has been found that protrusion angles of at least 36° and less than 108° may provide a favourable compromise in the sense that the abrasive particles and their protrusions are not too friable and do not break too easily but sharp enough to provide sufficient grinding performance measured e.g. as stock removal (see below). Preferably, the protrusion angle is 40° or more and 90° or less, more preferably 45° or more and 72° or less as confirmed inter alia by the experimental data presented herein. The following Table 4 summarises the parameters and grinding performance values measured for exemplary belts on 20 mm diameter bars of stainless steel 1.4571 after 12 min cycle at 88N. The total cycle of 12 minutes grinding actually consists of 12 cycles of one minute of grinding, each cycle consisting of 6 times 10 seconds of grinding interrupted by pauses of 20 seconds (for cooling down the metal bar). Thus, after each cycle of one-minute effective grinding, the stock removal of abraded metal is weighted. Thus the total stock removal, which is the sum of the stock removal of the twelve cycles, is finally calculated. The belt wear is determined at the end of the test, after 10 minutes cooling. The belt is then weighted and compared to its original mass, i.e. before the test. The grit density corresponds to the mass of abrasive particles strewed on one square meter backing. Table 4 1)comparative example 2)chemistry different from P1-P4 and P6-P10, see Tables 1-2 It was clearly shown that the abrasive particles according to the present invention (e.g. P4, P6-P10) resulted in grinding belts with an improved grinding performance and an improved belt wear as compared to comparative examples (e.g. P2). For protrusion angles between 45° and 60°, particularly advantageous results were observed. The same effect was not only observed for the particles characterised in Table 4 above, which are shown as an exemplary sample above, but also when comparing other abrasive grains according to the present invention with comparative examples. It is noted that P4 and P5 do not have the same chemistry, cf. Table 1, so that the occurrence of different MKZ values for these particles, which are similarly shaped, is assumed to be because of this difference. Further, grinding belts with different diameter-to-thickness ratios were tested. Favourable results have been found in particular if the diameter-to-thickness ratio lies between 2 and 8, more preferably between 3 and 7, even more preferably between 4 and 6. We assume that thinner particles (with a length-to-thickness ratio above 8) might have a relatively low mechanical resistance, especially under severe grinding conditions involving high pressure and heat. On the contrary, we assume that more compact particles (with a length-to- thickness ratio below 2) might be too bulky and chunky such that grinding efficiency and self- sharpening efficiency might be too low. Further, grinding belts with pointed and non-pointed protrusions were tested, in particular triangular protrusions, which are pointed, and irregular quadrilateral or trapezoidal protrusions, which are not pointed. Reference is made to exemplary particles P10, which may in particular be compared to P7 having triangular protrusions with the same protrusion angle (55°). The tests showed that, surprisingly, abrasive particles with such non-pointed (especially trapezoidal but also other quadrilateral) protrusions have a grinding performance that is comparable to the grinding performance of pointed abrasive particles, cf. e.g. stock removal and belt wear of P10 vs. P7 in Table 4. Especially the belt wear is not increased for P10. The stock removal is somewhat lower for P10 than for P7 but still much better than for conventional abrasive particles such as P2. Herein, it is important to note that pointed abrasive particles are usually more difficult to produce, e.g. because the pointed portions lead to difficulties when demoulding the precursor particle. Moreover, pointed portions break more easily when processing or manipulating the particles (e.g. during manufacture and storage or during application onto a substrate). Probably related to the fact that the pointed portions break easily, the grinding behaviour might be inhomogeneous for pointed particles, namely very aggressive in the beginning but weakening quickly as a considerable part of the pointed portions is expected to break off already after a short period of use. Also these problems may be solved when using particles with protrusions that are not pointed from the outset.

Claims

C L A I M S1. An abrasive particle comprising opposing and spaced apart first and second major surfaces (1), the spacing comprising a sidewall, wherein each major surface comprises an inner surface (2) and at least five and at most seven sharp outward protrusions (3), and wherein the sidewall comprises multiple intersecting sides which continuously interconnect the opposing surfaces and define the protrusions (3).

2. An abrasive particle according to claim 1, wherein the number of protrusions (3) of each major surface (1) is five.

3. An abrasive particle according to claim 1, wherein the inner surface (2) is a regular polygon with five to seven vertices, and each protrusion (3) connects two vertices of the regular polygon.

4. An abrasive particle according to claim 3, wherein the inner surface (2) is a pentagon.

5. An abrasive particle according to any of the preceding claims, wherein the first and second major surfaces (1) are substantially parallel.

6. An abrasive particle according to any of the preceding claims, wherein the first and the second major surfaces (1) are essentially star-shaped.

7. An abrasive particle according to any of the preceding claims, wherein the protrusions (3) are substantially triangular, for example wherein the protrusions substantially have the shape of an equilateral triangle or an isosceles triangle.

8. An abrasive particle according to any of the preceding claims, wherein the protrusions (3) are substantially quadrilateral, preferably substantially trapezoidal, more preferably having substantially the shape of an isosceles trapezoid.

9. An abrasive particle according to claim 8, wherein the protrusions (3) have substantially the shape of a trapezoid, wherein a first side of the trapezoid coincideswith a side of the inner surface (2) and has a length a, wherein a second side of the trapezoid is parallel to the first side and has a length b, and therein b and a have a ratio b / a that is at least 0.1, preferably at least 0.15, more preferably at least 0.20 and / or 0.75 or less, preferably 0.60 or less, more preferably 0.45 or less.

10. An abrasive particle according to any of the preceding claims, where the protrusion angle (p) is 36° or more and less than 108°.

11. An abrasive particle according to claim 10, wherein the protrusion angle (p) is at least 40°, preferably at least 45, and / or wherein the protrusion angle (p) is 90° or less, preferably 72° or less, more preferably 60° or less.

12. An abrasive particle according to any of the preceding claims, wherein the intersecting sides are substantially perpendicular to the first and second major surfaces (1).

13. An abrasive particle according to any of the preceding claims, wherein the number of intersecting sides is twice the number of protrusions (3).

14. An abrasive particle according to any of the preceding claims, wherein the intersecting sides are rectangular, trapezoidal and / or triangular.

15. An abrasive particle according to any of the preceding claims, wherein the first major surface (1) has a first circumcircle having a first circumcircle radius, wherein the second major surface (1) has a second circumcircle having a second circumcircle radius, wherein Dcdenotes the larger one of the first circumcircle radius and the second circumcircle radius, or wherein Dcdenotes the first circumcircle radius and the second circumcircle radius if the first circumcircle radius and the second circumcircle radius are equal, wherein the sidewall has a maximum thickness denoted as T, and wherein Dc / T is at least 2, preferably at least 3, more preferably at least 4, and / or wherein Dc / T is 8 or less, preferably 7 or less, more preferably 6 or less.

16. An abrasive particle according to any of the preceding claims, wherein one or both of the surfaces of two intersecting sides defining a protrusion (3) is / are concave, convex or planar.

17. An abrasive particle according to any of the preceding claims comprising alumina.

18. An abrasive particle according to any of the preceding claims comprising from 0.1% to 40% by weight of zirconia and / or from 0.1% to 10 wt.% magnesia.

19. An abrasive particle according to any of the preceding claims, wherein the abrasive particle comprises a material having a hardness rated 7 or above on the Mohs scale of hardness, and / or wherein the abrasive particle comprises a material having a Vickers hardness Hv of at least 18 GPa, preferably at least 20 GPa, more preferably at least 21 GPa, most preferably at least 22 GPa, wherein the abrasive particle in particular consists of said material.

20. An abrasive article comprising the abrasive particles of any of claims 1 to 19.

21. An abrasive article according to claim 19, wherein the abrasive article is a coated abrasive article, preferably wherein the coated abrasive article comprises a backing substrate carrying abrasive particles according to any of claims 1 to 19.

22. A coated abrasive article according to claim 21, further comprising a top coat.

23. A process for the production of a coated abrasive article comprising applying abrasive particles according to any of claims 1 to 19 to a backing substrate provided with an adhesive by electrostatic strewing.

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