# Patent

## Title
Shaped abrasive particles with a sloping sidewall

## Abstract

Shaped abrasive particles each having a sloping sidewall, each of the shaped abrasive particles comprising alpha alumina and having a first face and a second face separated by a thickness, t. The shaped abrasive particles further comprising either: a draft angle α between the second face and the sloping sidewall, and the draft angle α is between about 95 degrees to about 130 degrees, or the sloping sidewall having a radius, R, between the first face and the second face and the radius, R, is between about 0.5 to about 2 times the thickness, t.

## Background
BACKGROUND
Abrasive particles and abrasive articles made from the abrasive particles are useful for abrading, finishing, or grinding a wide variety of materials and surfaces in the manufacturing of goods. As such, there continues to be a need for improving the cost, performance, or life of the abrasive particle and/or the abrasive article.
Triangular shaped abrasive particles and abrasive articles using the triangular shaped abrasive particles are disclosed in U.S. Pat. Nos. 5,201,916 to Berg; 5,366,523 to Rowenhorst; and 5,984,988 to Berg. In one embodiment, the abrasive particles' shape comprised an equilateral triangle. Triangular shaped abrasive particles are useful in manufacturing abrasive articles having enhanced cut rates.
SUMMARY
Shaped abrasive particles, in general, can have superior performance over randomly crushed abrasive particles. By controlling the shape of the abrasive particle it is possible to control the resulting performance of the abrasive article. The inventors have discovered that by making the shaped abrasive particle with a sloping sidewall having a draft angle between about 95 degrees to about 130 degrees several unexpected benefits occur.
First, the shaped abrasive particles with the sloping sidewall tend to rest on the make coat of a coated abrasive article at an angle corresponding to the draft angle of the sidewall. It is believed that a draft angle other than 90 degrees results in the shaped abrasive particles leaning instead of having a 90 degree orientation to the backing in a coated abrasive article since the sidewall, which the shaped abrasive particle in the coated abrasive rests on, is sloped due to the draft angle. Because the shaped abrasive particles are mostly tipped or leaning to one side due to the angled sidewall they rest on, they can have an orientation angle less than 90 degrees relative to the backing thereby enhancing cut rates.
Secondly, the shaped abrasive particles with the sloping sidewall can have different draft angles for different sides of the shaped abrasive particle. For example, an equilateral triangle can have a first sidewall at a first draft angle, a second sidewall at a second draft angle, and a third sidewall at a third draft angle, wherein the first, second, and third draft angles are different. A resulting coated abrasive article made from the shaped abrasive particles with the three different sidewall angles will tend to have an even distribution of particles landing on each of the three sidewalls. As such, the coated abrasive article will tend to have three distinct heights for the tips of the shaped abrasive particles from the backing. The sidewall contacting the make coat with the largest draft angle will have the lowest tip height, the sidewall with the intermediate draft angle will have an intermediate tip height, and the sidewall with the smallest draft angle will have the highest tip height. As a result, the coated abrasive article will possess shaped abrasive particles having three distinct orientation angles relative to the backing and three distinct tip heights. It is believed that such a coated abrasive article will possess more uniform cutting performance as the abrasive article wears due to the unused shorter tips of the shaped abrasive particles coming into contact with the workpiece as the taller tips of the shaped abrasive particles tend to wear down.
Hence, in one embodiment, the disclosure resides abrasive particles comprising shaped abrasive particles each having a sloping sidewall, each of the shaped abrasive particles comprising alpha alumina and having a first face and a second face separated by a thickness, t. The shaped abrasive particles further comprising either: a draft angle α between the second face and the sloping sidewall, and the draft angle α is between about 95 degrees to about 130 degrees, or the sloping sidewall having a radius, R, between the first face and the second face and the radius, R, is between about 0.5 to about 2 times the thickness, t.



BRIEF DESCRIPTION OF THE DRAWING
It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present disclosure, which broader aspects are embodied in the exemplary construction.
FIG. 1A illustrates a top view of one embodiment of a shaped abrasive particle.
FIG. 1B illustrates a side view the shaped abrasive particle of FIG. 1A.
FIG. 1C illustrates a side view of coated abrasive article made from the shaped abrasive particles of FIG. 1A.
FIG. 2 illustrates a photomicrograph of the shaped abrasive particles.
FIG. 3 illustrates a photomicrograph of the top surface of a coated abrasive article made from the shaped abrasive particles of FIG. 2.
FIG. 4A illustrates a top view of another embodiment of a shaped abrasive particle.
FIG. 4B illustrates a side view the shaped abrasive particle of FIG. 4A.
FIG. 4C illustrates a side view of coated abrasive article made from the shaped abrasive particles of FIG. 4A.
FIG. 5A illustrates a top view of another embodiment of a shaped abrasive particle.
FIG. 5B illustrates a side view the shaped abrasive particle of FIG. 5A.
FIG. 5C illustrates a side view of coated abrasive article made from the shaped abrasive particles of FIG. 5A.
FIG. 6 illustrates a graph of Cut Rate versus Time for shaped abrasive particles with different draft angles.
FIG. 7 illustrates a graph of Total Cut versus Time for shaped abrasive particles with different draft angles.
FIG. 8 illustrates a photomicrograph of prior art abrasive particles made according to U.S. Pat. No. 5,366,523.
FIG. 9 illustrates a photomicrograph of a cross section of the prior art abrasive particles of FIG. 8.
FIG. 10 illustrates a photomicrograph of a cross section of the prior art abrasive particles of FIG. 8.
FIG. 11 illustrates a photomicrograph of a cross section of a shaped abrasive particle with a sloping sidewall.



Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure.
DEFINITIONS
As used herein, forms of the words “comprise”, “have”, and “include” are legally equivalent and open-ended. Therefore, additional non-recited elements, functions, steps or limitations may be present in addition to the recited elements, functions, steps, or limitations.
As used herein, the term “abrasive dispersion” means an alpha alumina precursor that can be converted into alpha alumina that is introduced into a mold cavity. The composition is referred to as an abrasive dispersion until sufficient volatile components are removed to bring solidification of the abrasive dispersion.
As used herein, the term “precursor shaped abrasive particle” means the unsintered particle produced by removing a sufficient amount of the volatile component from the abrasive dispersion, when it is in the mold cavity, to form a solidified body that can be removed from the mold cavity and substantially retain its molded shape in subsequent processing operations.
As used herein, the term “shaped abrasive particle”, means a ceramic abrasive particle with at least a portion of the abrasive particle having a predetermined shape that is replicated from a mold cavity used to form the shaped precursor abrasive particle. Except in the case of abrasive shards (e.g. as described in U.S. provisional application 61/016,965), the shaped abrasive particle will generally have a predetermined geometric shape that substantially replicates the mold cavity that was used to form the shaped abrasive particle. Shaped abrasive particle as used herein excludes abrasive particles obtained by a mechanical crushing operation.
DETAILED DESCRIPTION
Shaped Abrasive Particle with a Sloping Sidewall
Referring to FIGS. 1A, 1B, and 1C an exemplary shaped abrasive particle 20 with a sloping sidewall 22 is illustrated. The material from which the shaped abrasive particle 20 with a sloping sidewall 22 is made comprises alpha alumina. Alpha alumina particles can be made from a dispersion of aluminum oxide monohydrate that is gelled, molded to shape, dried to retain the shape, calcined, and then sintered as discussed herein later. The shaped abrasive particle's shape is retained without the need for a binder to form an agglomerate comprising abrasive particles in a binder that are then formed into a shaped structure.
In general, the shaped abrasive particles 20 with a sloping sidewall 22 comprise thin bodies having a first face 24, and a second face 26 and having a thickness t. The first face 24 and the second face 26 are connected to each other by at least one sloping sidewall 22. In some embodiments, more than one sloping sidewall 22 can be present and the slope or angle for each sloping sidewall 22 may be the same as shown in FIG. 1A or different as shown in FIG. 4A.
In some embodiments, the first face 24 is substantially planar, the second face 26 is substantially planar, or both faces are substantially planar. Alternatively, the faces could be concave or convex as discussed in more detail in copending U.S. application Ser. No. 12/336,961 entitled “Dish-Shaped Abrasive Particles With A Recessed Surface”, filed on Dec. 17, 2008. Additionally, an opening or aperture through the faces could be present as discussed in more detail in copending U.S. application Ser. No. 12/337,112 entitled “Shaped Abrasive Particles With An Opening”, filed on Dec. 17, 2008.
In one embodiment, the first face 24 and the second face 26 are substantially parallel to each other. In other embodiments, the first face 24 and second face 26 can be nonparallel such that one face is sloped with respect to the other face and imaginary lines tangent to each face would intersect at a point. The sloping sidewall 22 of the shaped abrasive particle 20 with a sloping sidewall 22 can vary and it generally forms the perimeter 29 of the first face 24 and the second face 26. In one embodiment, the perimeter 29 of the first face 24 and second face 26 is selected to be a geometric shape, and the first face 24 and the second face 26 are selected to have the same geometric shape, although, they differ in size with one face being larger than the other face. In one embodiment, the perimeter 29 of first face 24 and the perimeter 29 of the second face 26 was a triangular shape that is illustrated.
Referring to FIGS. 1B and 1C, a draft angle α between the second face 26 and the sloping sidewall 22 of the shaped abrasive particle 20 can be varied to change the relative sizes of each face. In various embodiments of the invention, the draft angle α can be between approximately 95 degrees to approximately 130 degrees, or between about 95 degrees to about 125 degrees, or between about 95 degrees to about 120 degrees, or between about 95 degrees to about 115 degrees, or between about 95 degrees to about 110 degrees, or between about 95 degrees to about 105 degrees, or between about 95 degrees to about 100 degrees. As will be seen in the Examples, specific ranges for the draft angle α have been found to produce surprising increases in the grinding performance of coated abrasive articles made from the shaped abrasive particles with a sloping sidewall.
Referring now to FIG. 1C, a coated abrasive article 40 is shown having a first major surface 41 of a backing 42 covered by an abrasive layer. The abrasive layer comprises a make coat 44, and a plurality of shaped abrasive particles 20 with a sloping sidewall 22 attached to the backing 42 by the make coat 44. A size coat 46 is applied to further attach or adhere the shaped abrasive particles 20 with a sloping sidewall 22 to the backing 42.
As seen, the majority of the shaped abrasive particles 20 with a sloping sidewall 22 are tipped or leaning to one side. This results in the majority of the shaped abrasive particles 20 with a sloping sidewall 22 having an orientation angle β than 90 degrees relative to the first major surface 41 of the backing 42. This result is unexpected since the electrostatic coating method of applying the shaped abrasive particles with a sloping sidewall tends to originally orientate the particles at an orientation angle β of 90 degrees when they are first applied to the backing. The electrostatic field tends to align the particles vertically when applying them to the backing that is located above the shaped abrasive particles with a sloping sidewall. Furthermore, the electrostatic field tends to accelerate and drive the particles into the make coat at the 90 degree orientation. At some point after the web is turned over, either before or after the size coat 46 is applied, the particles under the force of gravity or the surface tension of the make and/or size coat tend to lean over and come to rest on the sloping sidewall 22. It is believed that sufficient time in the process of making the coated abrasive article is present for the shaped abrasive particles to lean over and become attached to the make coat by the sloping sidewall 22 before the make coat and size coat cure and harden preventing any further rotation. As seen, once the shaped abrasive particles with a sloping sidewall are applied and allowed to lean, the very tips 48 of the shaped abrasive particles have generally the same height, h.
To further optimize the leaning orientation, the shaped abrasive particles with a sloping sidewall are applied in the backing in an open coat abrasive layer. A closed coat abrasive layer is defined as the maximum weight of abrasive particles or a blend of abrasive particles that can be applied to a make coat of an abrasive article in a single pass through the maker. An open coat is an amount of abrasive particles or a blend of abrasive particles, weighing less than the maximum weight in grams that can be applied, that is applied to a make coat of a coated abrasive article. An open coat abrasive layer will result in less than 100% coverage of the make coat with abrasive particles thereby leaving open areas and a visible resin layer between the particles as best seen in FIG. 3. In various embodiments of the invention, the percent open area in the abrasive layer can be between about 10% to about 90% or between about 30% to about 80%.
It is believed that if too many of the shaped abrasive particles with a sloping sidewall are applied to the backing, insufficient spaces between the particles will be present to allow from them to lean or tip prior to curing the make and size coats. In various embodiments of the invention, greater than 50, 60, 70, 80, or 90 percent of the shaped abrasive particles in the coated abrasive article having an open coat abrasive layer are tipped or leaning having an orientation angle β of less than 90 degrees.
Without wishing to be bound by theory, it is believed that an orientation angle β less than 90 degrees results in enhanced cutting performance of the shaped abrasive particles with a sloping sidewall. Surprisingly, this result tends to occur regardless of the shaped abrasive particles' rotational orientation about the Z axis within the coated abrasive article. While FIG. 1C is idealized to show all the particles aligned in the same direction, an actual coated abrasive disc would have the particles randomly distributed and rotated as best seen in FIG. 3. Since the abrasive disc is rotating and the shaped abrasive particles are randomly distributed, some shaped abrasive particles will be driven into the workpiece at an orientation angle β of less than 90 degrees with the workpiece initially striking the second face 26 while a neighboring shaped abrasive particle could be rotated exactly 180 degrees with the workpiece striking backside of the shaped abrasive particle and the first face 24. With a random distribution of the particles and the rotation of the disc, less than half of the shaped abrasive particles could have the workpiece initially striking the second face 26 instead of the first face 24. However, for an abrasive belt having a defined direction of rotation and a defined point of contact with the workpiece, it may be possible to align the shaped abrasive particles with a sloping sidewall on the belt to ensure that each shaped abrasive particle runs at an orientation angle β of less than 90 degrees and that the workpiece is driven into the second face 26 first as idealized in FIG. 1C. In various embodiments of the invention, the orientation angle β for at least a majority of the shaped abrasive particles with a sloping sidewall in an abrasive layer of a coated abrasive article can be between about 50 degrees to about 85 degrees, or between about 55 degrees to about 85 degrees, or between about 60 degrees to about 85 degrees, or between about 65 degrees to about 85 degrees, or between about 70 degrees to about 85 degrees, or between about 75 degrees to about 85 degrees, or between about 80 degrees to about 85 degrees.
Referring now to FIGS. 2 and 3, photomicrographs of shaped abrasive particles 20 with a sloping sidewall 22 are shown. In FIG. 3 the draft angle α is approximately 120 degrees and the shaped abrasive particles comprised an equilateral triangle. The sides of each triangle measured approximately 1.6 mm at the perimeter of the larger first face 24. The shaped abrasive particles had a thickness of approximately 0.38 mm. The surface of the resulting coated abrasive disc made from the shaped abrasive particles of FIG. 2 is shown in FIG. 3. As seen, the majority of the shaped abrasive particles are resting in the make coat on one of the sloping sidewalls. The orientation angle β for the majority of the shaped abrasive particles with a sloping sidewall in the abrasive layer of the coated abrasive article in FIG. 3 is approximately 60 degrees.
Referring to FIGS. 4A-C, a second embodiment of the shaped abrasive particle 20 with a sloping sidewall 22 is illustrated. The material from which the shaped abrasive particle 20 with a sloping sidewall 22 is made comprises alpha alumina. Alpha alumina particles can be made from a dispersion of aluminum oxide monohydrate that is gelled, molded to shape, dried to retain the shape, calcined, and then sintered as discussed herein later. The shaped abrasive particle's shape is retained without the need for a binder to form an agglomerate comprising abrasive particles in a binder that are then formed into a shaped structure.
In general, the shaped abrasive particles 20 with a sloping sidewall 22 comprise thin bodies having a first face 24, and a second face 26 and having a thickness t. The first face 24 and the second face 26 are connected to each other by at least a first sloping sidewall 50 having a first draft angle 52 and by a second sloping sidewall 54 having a second draft angle 56, which is selected to be a different value from the first draft angle. In the illustrated embodiment, the first and second faces are also connected by a third sloping sidewall 58 having a third draft angle 60, which is a different value from either of the other two draft angles.
In the illustrated embodiment, the first, second and third draft angles are all different values from each other. For example, the first draft angle 52 could be 120 degrees, the second draft angle 56 could be 110 degrees, and the third draft angle 60 could be 100 degrees. The resulting coated abrasive article 40, as shown in FIG. 4C, made from the shaped abrasive particles with the three different draft angles will tend to have an even distribution of shaped abrasive particles landing on each of the three different sloping sidewalls. As such, the coated abrasive article will tend to have three distinct heights for the tips 48 of the shaped abrasive particles from the backing. The first sloping sidewall 50 contacting the make coat with the largest draft angle will have the lowest tip height, h1, the second sloping sidewall 54 with the intermediate draft angle will have an intermediate tip height, h2, and the third sloping sidewall, 58, with the smallest draft angle will have the highest tip height, h3. As a result, the coated abrasive article will possess shaped abrasive particles having three distinct orientation angles β relative to the backing and three distinct tip heights. It is believed that such a coated abrasive article will possess more uniform cutting performance as the abrasive article wears due to the unused shorter tips of the shaped abrasive particles coming into contact with the workpiece as the taller tips of the shaped abrasive particles tend to wear down and dull.
In some embodiments, the first face 24 is substantially planar, the second face 26 is substantially planar, or both faces are substantially planar. Alternatively, the faces could be concave or convex as discussed in more detail in copending U.S. application Ser. No. 12/336,961 entitled “Dish-Shaped Abrasive Particles With A Recessed Surface”, filed on Dec. 17, 2008. Additionally, an opening or aperture through the faces could be present as discussed in more detail in copending U.S. application Ser. No. 12/337,112 entitled “Shaped Abrasive Particles With An Opening”, filed on Dec. 17, 2008.
In one embodiment, the first face 24 and the second face 26 are substantially parallel to each other. In other embodiments, the first face 24 and second face 26 can be nonparallel such that one face is sloped with respect to the other face and imaginary lines tangent to each face would intersect at a point. The first, second, and third sloping sidewalls of the shaped abrasive particle 20 with a sloping sidewall 22 can vary and they generally form the perimeter 29 of the first face 24 and the second face 26. In one embodiment, the perimeter 29 of the first face 24 and the second face 26 is selected to be a geometric shape, and the first face 24 and the second face 26 are selected to have the same geometric shape, although, they differ in size with one face being larger than the other face. In one embodiment, the perimeter 29 of first face 24 and the perimeter 29 of the second face 26 was a triangular shape that is illustrated.
Referring to FIGS. 4B and 4C, the first, second, and third, draft angles between the second face 26 and the respective sloping sidewall of the shaped abrasive particle 20 can be varied with at least two of the draft angles being different values, and desirably all three being different values. In various embodiments of the invention, the first draft angle, the second draft angle, and the third draft angle can be between about 95 degrees to about 130 degrees, or between about 95 degrees to about 125 degrees, or between about 95 degrees to about 120 degrees, or between about 95 degrees to about 115 degrees, or between about 95 degrees to about 110 degrees, or between about 95 degrees to about 105 degrees, or between about 95 degrees to about 100 degrees.
Referring now to FIG. 4C, a coated abrasive article 40 is shown having a first major surface 41 of a backing 42 covered by an abrasive layer. The abrasive layer comprises a make coat 44, and a plurality of shaped abrasive particles 20 with either the first, the second, or the third sloping sidewall attached to the backing 42 by the make coat 44. A size coat 46 is applied to further attach or adhere the shaped abrasive particles 20 with a sloping sidewall 22 to the backing 42.
As seen, the majority of the shaped abrasive particles 20 with a sloping sidewall 22 are tipped or leaning to one side. This results in the majority of the shaped abrasive particles 20 with a sloping sidewall 22 having an orientation angle β less than 90 degrees relative to the first major surface 41 of the backing 42 as previously discussed for the first embodiment.
To further optimize the leaning orientation, the shaped abrasive particles with a sloping sidewall are applied in the backing in an open coat abrasive layer. An open coat abrasive layer will result in less than 100% coverage of the make coat with abrasive particles thereby leaving open areas and a visible resin layer between the abrasive particles as best seen in FIG. 3. In various embodiments of the invention, the percent open area in the abrasive layer can be between about 10% to about 90% or between about 30% to about 80%.
It is believed that if too many of the shaped abrasive particles with a sloping sidewall are applied to the backing, insufficient spaces between the shaped abrasive particles will be present to allow for them to lean or tip prior to curing the make and size coats. In various embodiments of the invention, greater than 50, 60, 70, 80, or 90 percent of the shaped abrasive particles in the coated abrasive article having an open coat abrasive layer are tipped or leaning having an orientation angle β of less than 90 degrees.
Without wishing to be bound by theory, it is believed that an orientation angle β of less than 90 degrees results in enhanced cutting performance of the shaped abrasive particles with a sloping sidewall as previously discussed. In various embodiments of the invention, the orientation angle β for at least a majority of the shaped abrasive particles with a sloping sidewall in an abrasive layer of a coated abrasive article can be between about 50 degrees to about 85 degrees, or between about 55 degrees to about 85 degrees, or between about 60 degrees to about 85 degrees, or between about 65 degrees to about 85 degrees, or between about 70 degrees to about 85 degrees, or between about 75 degrees to about 85 degrees, or between about 80 degrees to about 85 degrees.
Referring now to FIGS. 5A-B a third embodiment of the invention is shown. In this embodiment, the sloping sidewall 22 is defined by a radius, R, instead of the draft angle α for the embodiment shown in FIGS. 1A-1C. A sloping sidewall 22 defined by a radius, R, has also been found to result in the shaped abrasive particles 20 tipping or leaning when forming a coated abrasive article as shown in FIG. 5C. Grinding tests have shown that shaped abrasive particles comprising an equilateral triangle with the sides of each triangle measuring approximately 1.6 mm at the perimeter of the larger first face 24, and having a thickness of approximately 0.38 mm, have the same cut performance with a draft angle of 120 degrees or a radius, R, of 0.51 mm. In various embodiment of the invention, the radius, R, can be between about 0.5 to about 2 times the thickness, t, of the shaped abrasive particle.
As with the second embodiment, the radius, R, can be varied for each of the sidewalls to result in shaped abrasive particles leaning or tipping to varying degrees in the coated abrasive article.
It is believed that if too many of the shaped abrasive particles with a sloping sidewall are applied to the backing, insufficient spaces between the shaped abrasive particles will be present to allow for them to lean or tip prior to curing the make and size coats. In various embodiments of the invention, greater than 50, 60, 70, 80, or 90 percent of the shaped abrasive particles in the coated abrasive article having an open coat abrasive layer are tipped or leaning having an orientation angle β of less than 90 degrees.
For either the first embodiment, the second embodiment, or the third embodiment, the shaped abrasive particles 20 with a sloping sidewall 22 can have various three-dimensional shapes. The geometric shape of the perimeter 29 can be triangular, rectangular, circular, elliptical, star-shaped or that of other regular or irregular polygons. In one embodiment, an equilateral triangle is used and in another embodiment, an isosceles triangle is used. For the purpose of this disclosure, a substantially triangular shape also includes three-sided polygons wherein one or more of the sides can be arcuate and/or the tips of the triangle can be arcuate.
Additionally, the various sloping sidewalls of the shaped abrasive particles can have the same draft angle or different draft angles. Furthermore, a draft angle of 90 degrees can be used on one or more sidewalls as long as one of the sidewalls is a sloping sidewall having a draft angle of about 95 degrees or greater.
The shaped abrasive particles 20 with a sloping sidewall can have various volumetric aspect ratios. The volumetric aspect ratio is defined as the ratio of the maximum cross sectional area passing through the centroid of a volume divided by the minimum cross sectional area passing through the centroid. For some shapes, the maximum or minimum cross sectional area may be a plane tipped, angled, or tilted with respect to the external geometry of the shape. For example, a sphere would have a volumetric aspect ratio of 1.000 while a cube will have a volumetric aspect ratio of 1.414. A shaped abrasive particle in the form of an equilateral triangle having each side equal to length A and a uniform thickness equal to A will have a volumetric aspect ratio of 1.54, and if the uniform thickness is reduced to 0.25 A, the volumetric aspect ratio is increased to 2.64. It is believed that shaped abrasive particles having a larger volumetric aspect ratio have enhanced cutting performance. In various embodiments of the invention, the volumetric aspect ratio for the shaped abrasive particles with a sloping sidewall can be greater than about 1.15, or greater than about 1.50, or greater than about 2.0, or between about 1.15 to about 10.0, or between about 1.20 to about 5.0, or between about 1.30 to about 3.0.
The shaped abrasive particles with a sloping sidewall can have a much smaller radius of curvature at the points or corners of the shaped abrasive particles. The equilateral triangular shaped abrasive particles disclosed in U.S. Pat. No. 5,366,523 to Rowenhorst et al. and pictured in FIG. 8, had a radius of curvature for the points of the triangle (measured from one side around the point to the next side) of 103.6 microns for the average tip radius. The radius of curvature can be measured from a polished cross-section of the first or second face using image analysis such as a Clemex Image Analysis program interfaced with an inverted light microscope or other suitable image analysis software. The radius of curvature for each triangular apex can be estimated by defining three points at each apex when viewed in cross section at 100× magnification. A point is placed at the start of the tip's curve where there is a transition from the straight edge to the start of a curve, at the apex of the tip, and at the transition from the curved tip back to a straight edge. The image analysis software then draws an arc defined by the three points (start, middle, and end of the curve) and calculates a radius of curvature. The radius of curvature for at least 30 apexes are measured and averaged to determine the average tip radius. The shaped abrasive particles made by the current method are much more precisely made as best seen by comparing FIG. 2 to FIG. 8. As such, the average tip radius for the shaped abrasive particles is much less. The average tip radius for shaped abrasive particles made according to the present disclosure has been measured to be less than 19.2 microns. In various embodiments of the invention, the average tip radius can be less than 75 microns, or less than 50 microns, or less than 25 microns. It is believed that a sharper tip promotes more aggressive cutting an improved fracturing of the shaped abrasive particles during use.
In addition to having a sharper tip, the shaped abrasive particles can have a much more precisely defined sidewall. Referring now to FIGS. 9 and 10, photomicrographs of polished cross sections taken perpendicular through the faces of the prior art shaped abrasive particles of FIG. 8 are shown. As seen, the sidewall (top surface) tends to be either concave or convex and is not uniformly planar. Depending on where you take the cross section, the same sidewall may transition from one shape to another. Referring to FIG. 10, in the foreground the sidewall is convex while in the background it is concave.
Referring to FIG. 11, a polished cross section taken perpendicular through the faces of a shaped abrasive particle with a sloping sidewall having a 98 degree draft angle is shown. The first face 24 (right hand vertical surface) is concave as disclosed in the pending patent application attorney docket number 64716US002 referred to above. A concave surface is thought to enhance grinding performance by removing more material during use similar to a scoop, spoon, or hollow ground chisel blade. The second face 26 is substantially planar (left hand vertical surface). Finally, the sidewall (top surface) is uniformly planar. By uniformly planar it is meant that the sidewall does not have areas that are convex from one face to the other face, or areas that are concave from one face to the other face and at least 50%, or at least 75%, or at least 85% or more of the sidewall surface is planar. As seen in the cross section, when the sidewall is cut at a 90 degree angle and polished, a substantially linear edge appears (where the top sidewall surface meets the cut cross section's front surface). The uniformly planar sidewall would typically have that substantially linear edge at substantially all cross sectional planes along the length of the sidewall. The uniformly planar sidewall provides better defined (sharper) edges where the sidewall intersects with the first face and the second face, and this is also thought to enhance grinding performance.
Shaped abrasive particles 20 with a sloping sidewall 22 made according to the present disclosure can be incorporated into an abrasive article, or used in loose form. Abrasive particles are generally graded to a given particle size distribution before use. Such distributions typically have a range of particle sizes, from coarse particles to fine particles. In the abrasive art this range is sometimes referred to as a “coarse”, “control”, and “fine” fractions. Abrasive particles graded according to abrasive industry accepted grading standards specify the particle size distribution for each nominal grade within numerical limits. Such industry accepted grading standards (i.e., abrasive industry specified nominal grade) include those known as the American National Standards Institute, Inc. (ANSI) standards, Federation of European Producers of Abrasive Products (FEPA) standards, and Japanese Industrial Standard (JIS) standards.
ANSI grade designations (i.e., specified nominal grades) include: ANSI 4, ANSI 6, ANSI 8, ANSI 16, ANSI 24, ANSI 36, ANSI 40, ANSI 50, ANSI 60, ANSI 80, ANSI 100, ANSI 120, ANSI 150, ANSI 180, ANSI 220, ANSI 240, ANSI 280, ANSI 320, ANSI 360, ANSI 400, and ANSI 600. FEPA grade designations include P8, P12, P16, P24, P36, P40, P50, P60, P80, P100, P120, P150, P180, P220, P320, P400, P500, P600, P800, P1000, and P1200. JIS grade designations include JIS8, JIS12, JIS16, JIS24, JIS36, JIS46, JIS54, JIS60, JIS80, JIS100, JIS150, JIS180, JIS220, JIS240, JIS280, JIS320, JIS360, JIS400, JIS600, JIS800, JIS1000, JIS1500, JIS2500, JIS4000, JIS6000, JIS8000, and JIS10,000.
Alternatively, the shaped abrasive particles 20 with a sloping sidewall 22 can graded to a nominal screened grade using U.S.A. Standard Test Sieves conforming to ASTM E-11 “Standard Specification for Wire Cloth and Sieves for Testing Purposes.” ASTM E-11 proscribes the requirements for the design and construction of testing sieves using a medium of woven wire cloth mounted in a frame for the classification of materials according to a designated particle size. A typical designation may be represented as −18+20 meaning that the shaped abrasive particles 20 pass through a test sieve meeting ASTM E-11 specifications for the number 18 sieve and are retained on a test sieve meeting ASTM E-11 specifications for the number 20 sieve. In one embodiment, the shaped abrasive particles 20 with a sloping sidewall 22 have a particle size such that most of the particles pass through an 18 mesh test sieve and can be retained on a 20, 25, 30, 35, 40, 45, or 50 mesh test sieve. In various embodiments of the invention, the shaped abrasive particles 20 with a sloping sidewall 22 can have a nominal screened grade comprising: −18+20, −20+25, −25+30, −30+35, −35+40, −40+45, −45+50, −50+60, −60+70, −70+80, −80+100, −100+120, −120+140, −140+170, −170+200, −200+230, −230+270, −270+325, −325+400, −400+450, −450+500, or −500+635.
In one aspect, the present disclosure provides a plurality of shaped abrasive particles having an abrasives industry specified nominal grade or nominal screened grade, wherein at least a portion of the plurality of abrasive particles are shaped abrasive particles 20 with a sloping sidewall 22. In another aspect, the disclosure provides a method comprises grading the shaped abrasive particles 20 with a sloping sidewall 22 made according to the present disclosure to provide a plurality of shaped abrasive particles 20 with a sloping sidewall 22 having an abrasives industry specified nominal grade or a nominal screened grade.
If desired, the shaped abrasive particles 20 with a sloping sidewall 22 having an abrasives industry specified nominal grade or a nominal screened grade can be mixed with other known abrasive or non-abrasive particles. In some embodiments, at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or even 100 percent by weight of the plurality of abrasive particles having an abrasives industry specified nominal grade or a nominal screened grade are shaped abrasive particles 20 with a sloping sidewall 22 made according to the present disclosure, based on the total weight of the plurality of abrasive particles.
Particles suitable for mixing with the shaped abrasive particles 20 with a sloping sidewall 22 include conventional abrasive grains, diluent grains, or erodable agglomerates, such as those described in U.S. Pat. Nos. 4,799,939 and 5,078,753. Representative examples of conventional abrasive grains include fused aluminum oxide, silicon carbide, garnet, fused alumina zirconia, cubic boron nitride, diamond, and the like. Representative examples of diluent grains include marble, gypsum, and glass. Blends of differently shaped abrasive particles 20 with a sloping sidewall 22 (triangles and squares for example) or blends of shaped abrasive particles 20 with different draft angles (for example particles having an 98 degree draft angle mixed with particles having a 120 degree draft angle) can be used in the articles of this invention.
The shaped abrasive particles 20 with a sloping sidewall 22 may also have a surface coating. Surface coatings are known to improve the adhesion between abrasive grains and the binder in abrasive articles or can be used to aid in electrostatic deposition of the shaped abrasive particles 20. Such surface coatings are described in U.S. Pat. Nos. 5,213,591; 5,011,508; 1,910,444; 3,041,156; 5,009,675; 5,085,671; 4,997,461; and 5,042,991. Additionally, the surface coating may prevent the shaped abrasive particle from capping. Capping is the term to describe the phenomenon where metal particles from the workpiece being abraded become welded to the tops of the shaped abrasive particles. Surface coatings to perform the above functions are known to those of skill in the art.
Abrasive Article Having Shaped Abrasive Particles With A Sloping Sidewall
Referring to FIGS. 1C, 4C, and 5C, a coated abrasive article 40 comprises a backing 42 having a first layer of binder, hereinafter referred to as the make coat 44, applied over a first major surface 41 of backing 42. Attached or partially embedded in the make coat 44 are a plurality of shaped abrasive particles 20 with a sloping sidewall 22 forming an abrasive layer. Over the shaped abrasive particles 20 with a sloping sidewall 22 is a second layer of binder, hereinafter referred to as the size coat 46. The purpose of make coat 44 is to secure shaped abrasive particles 20 with n sloping sidewall 22 to backing 42 and the purpose of size coat 46 is to reinforce shaped abrasive particles 20 with a sloping sidewall 22. The majority of the shaped abrasive particles 20 with a sloping sidewall 22 are oriented such that the tip 48 or vertex points away from the backing 42 and the shaped abrasive particles are resting on the sloping sidewall 22 and tipped or leaning as shown.
The make coat 44 and size coat 46 comprise a resinous adhesive. The resinous adhesive of the make coat 44 can be the same as or different from that of the size coat 46. Examples of resinous adhesives that are suitable for these coats include phenolic resins, epoxy resins, urea-formaldehyde resins, acrylate resins, aminoplast resins, melamine resins, acrylated epoxy resins, urethane resins and combinations thereof. In addition to the resinous adhesive, the make coat 44 or size coat 46, or both coats, may further comprise additives that are known in the art, such as, for example, fillers, grinding aids, wetting agents, surfactants, dyes, pigments, coupling agents, adhesion promoters, and combinations thereof. Examples of fillers include calcium carbonate, silica, talc, clay, calcium metasilicate, dolomite, aluminum sulfate and combinations thereof.
A grinding aid can be applied to the coated abrasive article. A grinding aid is defined as particulate material, the addition of which has a significant effect on the chemical and physical processes of abrading, thereby resulting in improved performance. Grinding aids encompass a wide variety of different materials and can be inorganic or organic. Examples of chemical groups of grinding aids include waxes, organic halide compounds, halide salts, and metals and their alloys. The organic halide compounds will typically break down during abrading and release a halogen acid or a gaseous halide compound. Examples of such materials include chlorinated waxes, such as tetrachloronaphthalene, pentachloronaphthalene; and polyvinyl chloride. Examples of halide salts include sodium chloride, potassium cryolite, sodium cryolite, ammonium cryolite, potassium tetrafluoroborate, sodium tetrafluoroborate, silicon fluorides, potassium chloride, magnesium chloride. Examples of metals include tin, lead, bismuth, cobalt, antimony, cadmium, iron, and titanium. Other grinding aids include sulfur, organic sulfur compounds, graphite, and metallic sulfides. It is also within the scope of this invention to use a combination of different grinding aids; in some instances, this may produce a synergistic effect. In one embodiment, the grinding aid was cryolite or potassium tetrafluoroborate. The amount of such additives can be adjusted to give desired properties. It is also within the scope of this invention to utilize a supersize coating. The supersize coating typically contains a binder and a grinding aid. The binders can be formed from such materials as phenolic resins, acrylate resins, epoxy resins, urea-formaldehyde resins, melamine resins, urethane resins, and combinations thereof.
It is also within the scope of this invention that the shaped abrasive particles 20 with a sloping sidewall 22 can be utilized in a bonded abrasive article, a nonwoven abrasive article, or abrasive brushes. A bonded abrasive can comprises a plurality of the shaped abrasive particles 20 with a sloping sidewall 22 bonded together by means of a binder to form a shaped mass. The binder for a bonded abrasive can be metallic, organic, or vitreous. A nonwoven abrasive comprises a plurality of the shaped abrasive particles 20 with a sloping sidewall 22 bonded into a fibrous nonwoven web by means of an organic binder.
Method of Making Shaped Abrasive Particles with a Sloping Sidewall
The first process step involves providing either a seeded on un-seeded abrasive dispersion that can be converted into alpha alumina. The alpha alumina precursor composition often comprises a liquid that is a volatile component. In one embodiment, the volatile component is water. The abrasive dispersion should comprise a sufficient amount of liquid for the viscosity of the abrasive dispersion to be sufficiently low to enable filling the mold cavities and replicating the mold surfaces, but not so much liquid as to cause subsequent removal of the liquid from the mold cavity to be prohibitively expensive. In one embodiment, the abrasive dispersion comprises from 2 percent to 90 percent by weight of the particles that can be converted into alpha alumina, such as particles of aluminum oxide monohydrate (boehmite), and at least 10 percent by weight, or from 50 percent to 70 percent, or 50 percent to 60 percent, by weight of the volatile component such as water. Conversely, the abrasive dispersion in some embodiments contains from 30 percent to 50 percent, or 40 percent to 50 percent, by weight solids.
Aluminum oxide hydrates other than boehmite can also be used. Boehmite can be prepared by known techniques or can be obtained commercially. Examples of commercially available boehmite include products having the trademarks “DISPERAL”, and “DISPAL”, both available from Sasol North America, Inc. or “HiQ-40” available from BASF Corporation. These aluminum oxide monohydrates are relatively pure, i.e., they include relatively little, if any, hydrate phases other than monohydrates, and have a high surface area. The physical properties of the resulting shaped abrasive particles 20 with a sloping sidewall 22 will generally depend upon the type of material used in the abrasive dispersion.
In one embodiment, the abrasive dispersion is in a gel state. As used herein, a “gel” is a three dimensional network of solids dispersed in a liquid. The abrasive dispersion may contain a modifying additive or precursor of a modifying additive. The modifying additive can function to enhance some desirable property of the abrasive particles or increase the effectiveness of the subsequent sintering step. Modifying additives or precursors of modifying additives can be in the form of soluble salts, typically water soluble salts. They typically consist of a metal-containing compound and can be a precursor of oxide of magnesium, zinc, iron, silicon, cobalt, nickel, zirconium, hafnium, chromium, yttrium, praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium, titanium, and mixtures thereof. The particular concentrations of these additives that can be present in the abrasive dispersion can be varied based on skill in the art. Typically, the introduction of a modifying additive or precursor of a modifying additive will cause the abrasive dispersion to gel. The abrasive dispersion can also be induced to gel by application of heat over a period of time.
The abrasive dispersion can also contain a nucleating agent to enhance the transformation of hydrated or calcined aluminum oxide to alpha alumina. Nucleating agents suitable for this disclosure include fine particles of alpha alumina, alpha ferric oxide or its precursor, titanium oxides and titanates, chrome oxides, or any other material that will nucleate the transformation. The amount of nucleating agent, if used, should be sufficient to effect the transformation of alpha alumina. Nucleating such abrasive dispersions is disclosed in U.S. Pat. No. 4,744,802 to Schwabel.
A peptizing agent can be added to the abrasive dispersion to produce a more stable hydrosol or colloidal abrasive dispersion. Suitable peptizing agents are monoprotic acids or acid compounds such as acetic acid, hydrochloric acid, formic acid, and nitric acid. Multiprotic acids can also be used but they can rapidly gel the abrasive dispersion, making it difficult to handle or to introduce additional components thereto. Some commercial sources of boehmite contain an acid titer (such as absorbed formic or nitric acid) that will assist in forming a stable abrasive dispersion.
The abrasive dispersion can be formed by any suitable means, such as, for example, simply by mixing aluminum oxide monohydrate with water containing a peptizing agent or by forming an aluminum oxide monohydrate slurry to which the peptizing agent is added. Defoamers or other suitable chemicals can be added to reduce the tendency to form bubbles or entrain air while mixing. Additional chemicals such as wetting agents, alcohols, or coupling agents can be added if desired. The alpha alumina abrasive grain may contain silica and iron oxide as disclosed in U.S. Pat. No. 5,645,619 to Erickson et al. on Jul. 8, 1997. The alpha alumina abrasive grain may contain zirconia as disclosed in U.S. Pat. No. 5,551,963 to Larmie on Sep. 3, 1996. Alternatively, the alpha alumina abrasive grain can have a microstructure or additives as disclosed in U.S. Pat. No. 6,277,161 to Castro on Aug. 21, 2001.
The second process step involves providing a mold having at least one mold cavity, and preferably a plurality of cavities. The mold can have a generally planar bottom surface and a plurality of mold cavities. The plurality of cavities can be formed in a production tool. The production tool can be a belt, a sheet, a continuous web, a coating roll such as a rotogravure roll, a sleeve mounted on a coating roll, or die. The production tool comprises polymeric material. Examples of suitable polymeric materials include thermoplastics such as polyesters, polycarbonates, poly(ether sulfone), poly(methyl methacrylate), polyurethanes, polyvinylchloride, polyolefins, polystyrene, polypropylene, polyethylene or combinations thereof, or thermosetting materials. In one embodiment, the entire tooling is made from a polymeric or thermoplastic material. In another embodiment, the surfaces of the tooling in contact with the sol-gel while drying, such as the surfaces of the plurality of cavities, comprises polymeric or thermoplastic materials and other portions of the tooling can be made from other materials. A suitable polymeric coating may be applied to a metal tooling to change its surface tension properties by way of example.
A polymeric or thermoplastic tool can be replicated off a metal master tool. The master tool will have the inverse pattern desired for the production tool. The master tool can be made in the same manner as the production tool. In one embodiment, the master tool is made out of metal, e.g., nickel and is diamond turned. The polymeric sheet material can be heated along with the master tool such that the polymeric material is embossed with the master tool pattern by pressing the two together. A polymeric or thermoplastic material can also be extruded or cast onto the master tool and then pressed. The thermoplastic material is cooled to solidify and produce the production tool. If a thermoplastic production tool is utilized, then care should be taken not to generate excessive heat that may distort the thermoplastic production tool limiting its life. More information concerning the design and fabrication of production tooling or master tools can be found in U.S. Pat. Nos. 5,152,917 (Pieper et al.); 5,435,816 (Spurgeon et al.); 5,672,097 (Hoopman et al.); 5,946,991 (Hoopman et al.); 5,975,987 (Hoopman et al.); and 6,129,540 (Hoopman et al.).
Access to cavities can be from an opening in the top surface or bottom surface of the mold. In some instances, the cavity can extend for the entire thickness of mold. Alternatively, the cavity can extend only for a portion of the thickness of the mold. In one embodiment, the top surface is substantially parallel to bottom surface of the mold with the cavities having a substantially uniform depth. At least one side of the mold, i.e. the side in which the cavity is formed, can remain exposed to the surrounding atmosphere during the step in which the volatile component is removed.
The cavity has a specified three-dimensional shape. In one embodiment, the shape of a cavity can be described as being a triangle, as viewed from the top, having a sloping sidewall such that the bottom surface of the cavity is slightly smaller than the opening in the top surface. A sloping sidewall is believed to enhance grinding performance and enable easier removal of the precursor abrasive particles from the mold. In another embodiment, the mold comprised a plurality of triangular cavities. Each of the plurality of triangular cavities comprises an equilateral triangle.
Alternatively, other cavity shapes can be used, such as, circles, rectangles, squares, hexagons, stars, or combinations thereof, all having a substantially uniform depth dimension. The depth dimension is equal to the perpendicular distance from the top surface to the lowermost point on the bottom surface. The depth of a given cavity can be uniform or can vary along its length and/or width. The cavities of a given mold can be of the same shape or of different shapes.
The third process step involves filling the cavities in the mold with the abrasive dispersion by any conventional technique. In some embodiments, a knife roll coater or vacuum slot die coater can be used. A mold release can be used to aid in removing the particles from the mold if desired. Typical mold release agents include oils such as peanut oil or mineral oil, fish oil, silicones, polytetrafluoroethylene, zinc sterate, and graphite. In general, between about 0.1% to about 5% by weight mold release agent, such as peanut oil, in a liquid, such as water or alcohol, is applied to the surfaces of the production tooling in contact with the sol-gel such that between about 0.1 mg/in2 to about 3.0 mg/in2, or between about 0.1 mg/in2 to about 5.0 mg/in2 of the mold release agent is present per unit area of the mold when a mold release is desired. In one embodiment, the top surface of the mold is coated with the abrasive dispersion. The abrasive dispersion can be pumped onto top surface. Next, a scraper or leveler bar can be used to force the abrasive dispersion fully into the cavity of the mold. The remaining portion of the abrasive dispersion that does not enter cavity can be removed from top surface of the mold and recycled. In some embodiments, a small portion of the abrasive dispersion can remain on top surface and in other embodiments the top surface is substantially free of the dispersion. The pressure applied by the scraper or leveler bar is typically less than 100 psi, or less than 50 psi, or less than 10 psi. In some embodiments, no exposed surface of the abrasive dispersion extends substantially beyond the top surface to ensure uniformity in thickness of the resulting shaped abrasive particles 20.
The fourth process step involves removing the volatile component to dry the dispersion. Desirably, the volatile component is removed by fast evaporation rates. In some embodiments, removal of the volatile component by evaporation occurs at temperatures above the boiling point of the volatile component. An upper limit to the drying temperature often depends on the material the mold is made from. For polypropylene tooling the temperature should be less than the melting point of the plastic.
In one embodiment, for a water dispersion of between about 40 to 50 percent solids and a polypropylene mold, the drying temperatures can be between about 90 degrees C. to about 165 degrees C., or between about 105 degrees C. to about 150 degrees C., or between about 105 degrees C. to about 120 degrees C. Higher temperatures can lead to the formation of larger openings but can also lead to degradation of the polypropylene tooling limiting its useful life as a mold.
In one embodiment, a sample of boehmite sol-gel was made using the following recipe: aluminum oxide monohydrate powder (1600 parts) having the trade designation “DISPERAL” was dispersed by high shear mixing a solution containing water (2400 parts) and 70% aqueous nitric acid (72 parts) for 11 minutes. The resulting sol-gel was aged for at least 1 hour before coating. The sol-gel was forced into production tooling having triangular shaped mold cavities of 28 mils depth and 110 mils on each side and having a sloping sidewall having a predetermined draft angel a between the sidewall and bottom of the mold.
The sol-gel was forced into the cavities with a putty knife so that the openings of the production tooling were completely filled. A mold release agent, 1% peanut oil in methanol was used to coat the production tooling such that about 0.5 mg/in2 of peanut oil was applied to the mold surfaces. The excess methanol was removed by placing sheets of the production tooling in an air convection oven for 5 minutes at 45 C. The sol-gel coated production tooling was placed in an air convection oven at 45 C for at least 45 minutes to dry. The precursor shaped abrasive particles were removed from the production tooling by passing it over an ultrasonic horn. These precursor shaped abrasive particles can be fired to produce shaped abrasive particles 20 with a sloping sidewall 22.
The fifth process step involves removing the precursor shaped abrasive particles with a sloping sidewall from the mold cavities. The precursor shaped abrasive particles with a sloping sidewall can be removed from the cavities by using the following processes alone or in combination on the mold: gravity, vibration, ultrasonic vibration, vacuum, or pressurized air to remove the particles from the mold cavities.
The precursor abrasive particles with a sloping sidewall can be further dried outside of the mold. If the abrasive dispersion is dried to the desired level in the mold, this additional drying step is not necessary. However, in some instances it may be economical to employ this additional drying step to minimize the time that the abrasive dispersion resides in the mold. Typically, the precursor shaped abrasive particles will be dried from 10 to 480 minutes, or from 120 to 400 minutes, at a temperature from 50 degrees C. to 160 degrees C., or at 120 degrees C. to 150 degrees C.
The sixth process step involves calcining the precursor shaped abrasive particles with a sloping sidewall 22. During calcining, essentially all the volatile material is removed, and the various components that were present in the abrasive dispersion are transformed into metal oxides. The precursor shaped abrasive particles are generally heated to a temperature from 400 degrees C. to 800 degrees C., and maintained within this temperature range until the free water and over 90 percent by weight of any bound volatile material are removed. In an optional step, it may be desired to introduce the modifying additive by an impregnation process. A water-soluble salt can be introduced by impregnation into the pores of the calcined, precursor shaped abrasive particles. Then the precursor shaped abrasive particles are prefired again. This option is further described in European Patent Application No. 293,163.
The seventh process step involves sintering the calcined, precursor shaped abrasive particles to form alpha alumina particles. Prior to sintering, the calcined, precursor shaped abrasive particles are not completely densified and thus lack the desired hardness to be used as shaped abrasive particles. Sintering takes place by heating the calcined, precursor shaped abrasive particles to a temperature of from 1,000 degrees C. to 1,650 degrees C. and maintaining them within this temperature range until substantially all of the alpha alumina monohydrate (or equivalent) is converted to alpha alumina and the porosity is reduced to less than 15 percent by volume. The length of time to which the calcined, precursor shaped abrasive particles must be exposed to the sintering temperature to achieve this level of conversion depends upon various factors but usually from five seconds to 48 hours is typical.
In another embodiment, the duration for the sintering step ranges from one minute to 90 minutes. After sintering, the shaped abrasive particles with a sloping sidewall can have a Vickers hardness of 10 GPa, 16 GPa, 18 GPa, 20 GPa, or greater.
Other steps can be used to modify the described process, such as rapidly heating the material from the calcining temperature to the sintering temperature, centrifuging the abrasive dispersion to remove sludge, waste, etc. Moreover, the process can be modified by combining two or more of the process steps if desired. Conventional process steps that can be used to modify the process of this disclosure are more fully described in U.S. Pat. No. 4,314,827 to Leitheiser. Additionally, the shaped abrasive particles with a sloping sidewall can have grooves on one of the faces as described in copending application U.S. Ser. No. 12/627,567 entitled “Shaped Abrasive Particles With Grooves”, and filed on Dec. 17, 2008. The grooves are formed by a plurality of ridges in the bottom surface of the mold cavity that have been found to make it easier to remove the precursor shaped abrasive particles from the mold. More information concerning methods to make shaped abrasive particles is disclosed in U.S. patent application Ser. No. 12/337,001 entitled “Method Of Making Abrasive Shards, Shaped Abrasive Particles With An Opening, Or Dish-Shaped Abrasive Particles”, and filed on Dec. 17, 2008.
EXAMPLES
Objects and advantages of this disclosure are further illustrated by the following non-limiting examples. The particular materials and amounts thereof recited in these examples as well as other conditions and details, should not be construed to unduly limit this disclosure. Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.
Preparation of REO-Doped Shaped Abrasive Particles with Sloping Sidewall
A sample of boehmite sol-gel was made using the following recipe: aluminum oxide monohydrate powder (1600 parts) having the trade designation “DISPERAL” was dispersed by high shear mixing a solution containing water (2400 parts) and 70% aqueous nitric acid (72 parts) for 11 minutes. The resulting sol-gel was aged for at least 1 hour before coating. The sol-gel was forced into production tooling having triangular shaped mold cavities of 28 mils depth and 110 mils on each side. The draft angle α between the sidewall and bottom of the mold was different for each production tooling. The draft angle α was 90 degrees for the first tooling, 98 degrees for the second tooling, 120 degrees for the third tooling and 135 degrees for the last tooling. The 98 degree draft angle production tooling was manufactured to have 50% of the mold cavities with 8 parallel ridges rising from the bottom surfaces of the cavities that intersected with one side of the triangle at a 90 degree angle and the remaining cavities had a smooth bottom mold surface. The parallel ridges were spaced every 0.277 mm and the cross section of the ridges was a triangle shape having a height of 0.0127 mm and a 45 degree angle between the sides of each ridge at the tip as described in patent application attorney docket number 664792US002 referred to above. The sol-gel was forced into the cavities with a putty knife so that the openings of the production tooling were completely filled. A mold release agent, 1% peanut oil in methanol was used to coat the production tooling with about 0.5 mg/in2 of peanut oil applied to the production tooling. The excess methanol was removed by placing sheets of the production tooling in an air convection oven for 5 minutes at 45 degrees C. The sol-gel coated production tooling was placed in an air convection oven at 45 degrees C. for at least 45 minutes to dry. The precursor shaped abrasive particles were removed from the production tooling by passing it over an ultrasonic horn. The precursor shaped abrasive particles were calcined at approximately 650 degrees Celsius and then saturated with a mixed nitrate solution of the following concentration (reported as oxides): 1.8% each of MgO, Y2O3, Nd2O3 and La2O3. The excess nitrate solution was removed and the saturated precursor shaped abrasive particles with openings were allowed to dry after which the particles were again calcined at 650 degrees Celsius and sintered at approximately 1400 degrees Celsius. Both the calcining and sintering was performed using rotary tube kilns.
After making the shaped abrasive particles with sloping sidewalls having the four different draft angles, coated abrasive discs were made. The shaped abrasive particles with sloping sidewalls were electrostatic coated onto a 7 inch diameter fiber disc with a 7⁄8 inch center hole using phenolic make coat and size coat resins as shown in Table 1. The phenolic resin can be made from resole phenol-formaldehyde resin, a 1.5:1 to 2.1:1 (phenol:formaldehyde) condensate catalyzed by 1 to 5% potassium hydroxide.







TABLE 1







Make and Size Coat Formulation











Ingredient
Make Coat
Size Coat






Phenolic Resin
49.15%
29.42%



Water
10.19%
18.12%



Calcium Carbonate
40.56%
 0.0%



Cryolite
 0.0%
50.65%



Emulon A (BASF)
 0.10%
 1.81%




100.0%
100.0%









The grinding performance of the shaped abrasive particles with a sloping sidewall was evaluated by grinding 1045 medium carbon steel using the following procedure. 7-inch (17.8 cm) diameter abrasive discs for evaluation were attached to a rotary grinder fitted with a 7-inch (17.8 cm) ribbed disc pad face plate (“80514 Extra Hard Red” obtained from 3M Company, St. Paul, Minn.). The grinder was then activated and urged against an end face of a 0.75×0.75 in (1.9×1.9 cm) pre-weighed 1045 steel bar under a load of 12 lb (5.4 kg). The resulting rotational speed of the grinder under this load and against this workpiece was 5000 rpm. The workpiece was abraded under these conditions for a total of fifty (50) 10-second grinding intervals (passes). Following each 10-second interval, the workpiece was allowed to cool to room temperature and weighed to determine the cut of the abrasive operation. Test results were reported as the incremental cut for each interval and the total cut removed. If desired, the testing can be automated using suitable equipment.
Referring to FIGS. 6 and 7, the Cut Rate versus Time and the Total Cut versus Time are plotted. As seen, shaped abrasive particles having a sloping sidewall with a draft angle greater than 90 degrees significantly out performed similarly shaped abrasive particles having a 90 degree draft angle. As the draft angle approached 135 degrees, the performance of the shaped abrasive particles with a sloping sidewall begin to rapidly deteriorate. When shaped abrasive particles having a 135 degree draft angle are compared to the shaped abrasive particles having a draft angle of 98 degrees, the initial cut rate was similar but the total cut was significantly reduced. The shaped abrasive particles having a draft angle of 120 degrees had approximately a 20% improvement in initial cut and approximately the same total cut as blend of shaped abrasive particles having a 98 degree draft angle (grooves and no grooves), which was unexpected. Even more surprising, shaped abrasive particles having only an 8 degree change in draft angle from 90 degrees to 98 degrees had a huge jump in performance. The initial cut rate was almost doubled and the cut rate remained relatively constant during the entire test duration. It is believed that the more consistent cut rate is a result of using a blend of 50% shaped abrasive particles with grooves and 50% shaped abrasive particles without grooves as discussed in pending patent application attorney docket number 664792US002 referred to above.
Other modifications and variations to the present disclosure may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present disclosure, which is more particularly set forth in the appended claims. It is understood that aspects of the various embodiments may be interchanged in whole or part or combined with other aspects of the various embodiments. All cited references, patents, or patent applications in the above application for Letters Patent are herein incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.

## Claims

1. Abrasive particles comprising:
shaped abrasive particles without a binder each having a sloping sidewall, each of the shaped abrasive particles comprising alpha alumina and having a first face and a second face separated by a thickness, t,
the shaped abrasive particles further comprising either:
a draft angle α between the second face and the sloping sidewall, and the draft angle α is between about 95 degrees to about 130 degrees, or
the sloping sidewall having a radius, R, between the first face and the second face and the radius, R, is between about 0.5 to about 2 times the thickness t.

2. The abrasive particles of claim 1 wherein a perimeter of the first face and the second face comprises a substantially triangular shape.

3. The abrasive particles of claim 2 wherein the draft angle α is between about 95 degrees to about 120 degrees.

4. The abrasive particles of claim 2 wherein the draft angle α is between about 95 degrees to about 110 degrees.

5. The abrasive particles of claim 3 or 4 wherein the substantially triangular shape comprises an equilateral triangle.

6. The abrasive particles of claim 5 comprising a first sloping sidewall having a first draft angle, a second sloping sidewall having a second draft angle, and a third sloping sidewall having a third draft angle, and wherein the first draft angle, the second draft angle, and the third draft angle are equal.

7. The abrasive particles of claim 1 comprising a first sloping sidewall having a first draft angle and a second sloping sidewall having a second draft angle, and wherein the first draft angle is a different value than the second draft angle.

8. The abrasive particles of claim 1 comprising a first sloping sidewall having a first draft angle, a second sloping sidewall having a second draft angle, and a third sloping sidewall having a third draft angle, and wherein the first draft angle, and the second draft angle, and the third draft angle are different values from each other.

9. The abrasive particles of claim 7 or 8 wherein a perimeter of the first face and the second face comprises a substantially triangular shape.

10. The abrasive particles of claim 9 wherein the substantially triangular shape comprises an equilateral triangle.

11. The abrasive particles of claim 1 comprising a volumetric aspect ratio and the volumetric aspect ratio is greater than about 1.15.

12. The abrasive particles of claim 2 comprising an average tip radius and the average tip radius is less than 75 microns.

13. The abrasive particles of claim 2 or 12 comprising a uniformly planar sloping sidewall.

14. The abrasive particles of claim 13 wherein the draft angle a is between about 95 degrees to about 100 degrees.

15. The abrasive particles of claim 1 comprising a binder forming an abrasive article selected from the group consisting of bonded abrasive articles, coated abrasive articles, nonwoven abrasive articles, and abrasive brushes.

16. A coated abrasive article comprising the abrasive particles of claim 1 and a make coat on a first major surface of a backing and a majority of the shaped abrasive particles adhered to the make coat by the sloping sidewall and having an orientation angle β between about 50 degrees to about 85 degrees, the shaped abrasive particles forming an abrasive layer, the abrasive layer coated with a size coat, and wherein the abrasive layer comprises at least 5 percent by weight of the shaped abrasive particles.

17. The coated abrasive article of claim 16 wherein the abrasive layer is an open coat abrasive layer and a percent open area in the abrasive layer is between about 10% to about 90%.

18. The coated abrasive article of claim 17 wherein the orientation angle β is between about 60 degrees to about 85 degrees.

19. The coated abrasive article of claim 17 wherein the orientation angle β is between about 70 degrees to about 85 degrees.

20. The coated abrasive article of claim 19 wherein the abrasive layer comprises at least 50% by weight of the shaped abrasive particles.

21. A method of making the abrasive particles of claim 1 comprising:
providing a mold having a plurality of cavities, the plurality of cavities comprising polymeric surfaces;
filling the plurality of cavities with a sol-gel, the sol-gel comprising particles that can be converted into alpha alumina in a liquid, the liquid comprising a volatile component;
removing at least a portion of the volatile component from the sol-gel while the sol-gel resides in the plurality of cavities thereby forming a plurality of precursor shaped abrasive particles each having a sloping sidewall; and
sintering the plurality of precursor shaped abrasive particles each having the sloping sidewall.