U.S. patent number RE35,570 [Application Number 08/513,219] was granted by the patent office on 1997-07-29 for abrasive article containing shaped abrasive particles.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to James G. Berg, Todd A. Berg, David E. Broberg, Donley D. Rowenhorst.
United States Patent |
RE35,570 |
Rowenhorst , et al. |
July 29, 1997 |
Abrasive article containing shaped abrasive particles
Abstract
An abrasive article comprising a binder and abrasive grits,
wherein the abrasive grits comprise abrasive particles having
specified shapes. Coated abrasive articles of this invention
comprise a backing having at least one layer of abrasive material
adhered thereto by means of a binder. A portion of this layer
contains abrasive particles having specified shapes. It is
preferred that the geometric shape of these abrasive particles be
triangular. For triangular-shaped particles, about 35% to about 65%
of the particles are oriented with a vertex pointing away from the
backing and a base in contact with the binder, with about 35% to
about 65% of the particles being oriented with a base pointing away
from the backing and a vertex in contact with the binder. It is
believed that this configuration brings about the result that the
sum of the surface areas of each of the particles in contact with
the workpiece remains essentially constant during use, even though
the surface area of each individual abrasive particle in contact
with the workpiece varies during use. The use of the abrasive
particles described herein minimizes the formation of flat surfaces
on the cutting regions of the abrasive grits.
Inventors: |
Rowenhorst; Donley D.
(Maplewood, MN), Berg; Todd A. (Lino Lakes, MN), Broberg;
David E. (Woodbury, MN), Berg; James G. (Lino Lakes,
MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
|
Family
ID: |
25441653 |
Appl.
No.: |
08/513,219 |
Filed: |
August 10, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
919180 |
Jul 23, 1992 |
05366523 |
Nov 22, 1994 |
|
|
Current U.S.
Class: |
51/293; 51/294;
51/295 |
Current CPC
Class: |
B24D
3/14 (20130101); B24D 3/28 (20130101); B24D
11/00 (20130101); C04B 35/1115 (20130101); C04B
35/626 (20130101); C09K 3/1418 (20130101) |
Current International
Class: |
B24D
3/20 (20060101); B24D 3/28 (20060101); B24D
11/00 (20060101); C04B 35/111 (20060101); B24B
011/00 () |
Field of
Search: |
;51/293,294,295,309
;264/6,7 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 084 986 |
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Jan 1983 |
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EP |
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0 293 163 |
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Nov 1988 |
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EP |
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0 325 127 |
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Jan 1989 |
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EP |
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0 395 091 |
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Oct 1990 |
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EP |
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0 395 088 |
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Oct 1990 |
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EP |
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0 395 087 |
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Oct 1990 |
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EP |
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2 354 373 |
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Jun 1977 |
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FR |
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2354373 |
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Jan 1978 |
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FR |
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2507101 |
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Dec 1982 |
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FR |
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4159387 |
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Jun 1992 |
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JP |
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4159386 |
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Jun 1992 |
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JP |
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Primary Examiner: Jones; Deborah
Attorney, Agent or Firm: Griswold; Gary L. Kirn; Walter N.
Gwin; Doreen S. L.
Claims
What is claimed is:
1. An abrasive article comprising a binder and abrasive grits,
wherein at least 10% by weight of said abrasive grits are abrasive
particles, each of said abrasive particles having a front face and
a back face, said front face having substantially the same
geometric shape as said back face, said faces being separated by
the thickness of said particle, the ratio of the length of the
shortest facial dimension of said particle to the thickness of said
particle being at least .[.1.]. .Iadd.2 .Iaddend.to 1.
2. The article of claim 1, wherein the geometric shape of said
abrasive particle is triangular.
3. The article of claim 1, wherein the geometric shape of said
abrasive particle is rectangular.
4. The article of claim 1, wherein the geometric shape of said
abrasive particle is circular. .[.5. The article of claim 1,
wherein the ratio of the length of the shortest facial dimension of
said particle to the
thickness of said particle is at least 2 to 1..].6. The article of
claim 1, wherein the ratio of the length of the shortest facial
dimension of
said particle to the thickness of said particle is at least 5 to 1.
7. The article of claim 1, wherein said abrasive grits further
comprise eroable
agglomerates. 8. The article of claim 1, further comprising a
backing having at least one layer of said abrasive grits adhered
thereto by means
of said binder. 9. A coated abrasive article comprising a backing
having at least one layer of abrasive grits adhered thereto by
means of a binder, said abrasive grits comprising abrasive
particles, each of said abrasive particles having a front face and
a back face, said faces being separated by the thickness of said
particle, said faces having the geometric shape of a triangle, the
thickness of said particle being substantially uniform and being
less than the length of the shortest side of said triangle, wherein
from about 35% to about 65% of said triangular-shaped particles are
oriented with a vertex pointing away from said backing and from
about 35% to about 65% of said triangular-shaped particles are
oriented such
that a base is pointing away from said backing. 10. The article of
claim 9, wherein up to 20% of said triangular-shaped particles have
neither a
base nor a vertex pointing away from said backing. 11. The article
of
claim 9, wherein said abrasive grits include eroable agglomerates.
12. The
article of claim 9, further including a size coat. 13. The article
of
claim 12, further including a supersize coat. 14. A coated abrasive
article comprising a backing having at least one layer of abrasive
grits adhered thereto by means of a binder, wherein at least 10% by
weight of said abrasive grits are abrasive particles, each of said
abrasive particles having a front face and a back face, said front
face having substantially the same geometric shape as said back
face, said faces being separated by the thickness of said particle,
the ratio of the length of the shortest facial dimension of said
particle to the thickness of said particle being at least .[.1.].
.Iadd.2 .Iaddend.to 1, whereby during the use of said article
during abrading, the area of each abrasive particle in contact with
the surface of the workpiece continually changes, but the sum of
the areas of each abrasive particle in contact with the surface of
the
workpiece essentially remains constant. 15. The article of claim
14, wherein said abrasive particles have faces that are in the
shape of a triangle, wherein from about 35% to about 65% of said
triangular-shaped particles are oriented with their vertices
pointing away from said backing and from about 35% to about 65% of
said triangular-shaped particles are
oriented such that their bases are pointing away from said backing.
16. The article of claim 14, wherein said abrasive grits further
comprise
erodable agglomerates. 17. The article of claim 1, wherein said
article is
a bonded abrasive article. 18. The article of claim 1, wherein
said
article is a nonwoven abrasive article. 19. The article of claim 1,
wherein the thickness of said particles is from about 25 to about
500
micrometers. .Iadd.20. The article of claim 1, further including
diluent grains. .Iaddend..Iadd.21. The article of claim 9, further
including diluent grains. .Iaddend..Iadd.22. The article of claim
14, further
including diluent grains. .Iaddend..Iadd.23. The article of claim
1, wherein said abrasive particles consist essentially of alpha
alumina and a nucleating agent selected from the group consisting
of alpha ferric oxide, titanium oxides, and chrome oxides.
.Iaddend..Iadd.24. The article of claim 9, wherein said abrasive
particles consist essentially of alpha alumina and a nucleating
agent selected from the group consisting of alpha ferric oxide,
titanium oxides, and chrome oxides. .Iaddend..Iadd.25. The article
of claim 14, wherein said abrasive particles consist essentially of
alpha alumina and a nucleating agent selected from the group
consisting of alpha ferric oxide, titanium oxides, and chrome
oxides.
.Iaddend..Iadd. 6. The article of claim 1, wherein said article is
coated abrasive article. .Iaddend..Iadd.27. The abrasive article of
claim 2, wherein the triangular geometric shape of said abrasive
particle is equilateral. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to abrasive articles, and, more
particularly, a coated abrasive article containing abrasive
particles having specified shapes.
2. Discussion of the Art
Three basic technologies that have been employed to produce
abrasive grains having a specified shape are (1) fusion, (2)
sintering, and (3) chemical ceramic.
In the fusion process, abrasive grains can be shaped by a chill
roll, the face of which may or may not be engraved, a mold into
which molten material is poured, or a heat sink material immersed
in an aluminum oxide melt. U.S. Pat. No. 3,377,660 discloses a
process comprising the steps of flowing molten abrasive material
from a furnace onto a cool rotating casting cylinder, rapidly
solidifying the material to form a thin semisolid curved sheet,
densifying the semisolid material with a pressure roll, and then
partially fracturing the strip of semisolid material by reversing
its curvature by pulling it away from the cylinder with a rapidly
driven cooled conveyor, whereupon the partially fractured strip is
deposited onto a collector in the form of large fragments, which,
upon being rapidly cooled and solidified, break up into smaller
fragments capable of being reduced in size to form conventional
abrasive grains. U.S. Pat. Nos. 4,073,096 and 4,194,887 disclose a
process comprising the steps of (1) fusing an abrasive mix in an
electric arc furnace, (2) dipping a relatively cold substrate into
the molten material, whereby a layer of solid abrasive material is
quickly frozen (or plated) on the substrate (3) withdrawing the
plated substrate from the molten material, and (4) breaking the
solidified abrasive material away from the substrate and collecting
it for further processing to produce abrasive grains.
In the sintering process, abrasive grains can be formed from
refractory powders having a particle size of up to 10 micrometers
in diameter. Binders can be added to the powders along with a
lubricant and a suitable solvent, e.g., water. The resulting
mixtures, pastes, or slurries can be shaped into platelets or rods
of various lengths and diameters. The resulting shaped grains must
be fired at high temperatures, e.g., 1,400.degree. C. to
1,800.degree. C., at high pressures, or for long soak times, e.g.,
up to 10 hours. Crystal size may range from under one micrometer up
to 25 micrometers. To obtain shorter residence times and/or smaller
crystal size, either the pressure or temperature must be increased.
U.S. Pat. No. 3,079,242 discloses a method of making abrasive
grains from calcined bauxite material comprising the steps of (1)
reducing the material to a fine powder, (2) compacting under
affirmative pressure and forming the fine particles of said powder
into grain sized agglomerations, and (3) sintering the
agglomerations of particles at a temperature below the fusion
temperature of the bauxite to induce limited recrystallization of
the particles, whereby abrasive grains are produced directly to
size. U.S. Pat. No. 4,252,544 discloses alumina abrasive grains
produced by sintering wherein the grain structure is constructed of
alumina coarse crystal particles and alumina fine crystal particles
located between the alumina coarse crystal particles. U.S. Pat. No.
3,491,492 discloses a process for making an aluminous abrasive
grain formed from bauxite or mixtures of bauxite and Bayer process
alumina wherein the comminuted aluminous material is mixed with
water and ferric ammonium citrate, or with ferric ammonium citrate
and citric acid, and reduced to a state of fine subdivision by
milling to give a fluid slurry of high solid content, drying said
slurry to coherent cakes having a thickness equal to one dimension
of the final grain before sintering, breaking said cakes to grains,
screening, optionally rounding said grains by air mulling,
screening, sintering, cooling, and screening to yield the final
product. U.S. Pat. No. 3,637,630 discloses a process in which the
same type of slurry disclosed in U.S. Pat. No. 3,491,492 is plated
or coated on a rotating anode of an electrophoretic cell. The
plated aluminous material is removed from the rotating anode,
dried, broken to granules, screened, sintered, and screened to
final size.
Chemical ceramic technology involves converting a colloidal
dispersion or hydrosol (sometimes called a sol), optionally in a
mixture with solutions of other metal oxide precursors, to a gel or
any other physical state that restrains the mobility of the
components, drying, and firing to obtain a ceramic material. A sol
can be prepared by any of several methods, including precipitation
of a metal hydroxide from an aqueous solution followed by
peptization, dialysis of anions from a solution of metal salt,
solvent extraction of an anion from a solution of a metal salt,
hydrothermal decomposition of a solution of a metal salt having a
volatile anion. The sol optionally contains metal oxide or
precursor thereof and is transformed to a semirigid solid state of
limited mobility such as a gel by, e.g., partial extraction of the
solvent, e.g., water. Chemical ceramic technology has been employed
to produce ceramic materials such as fibers, films, flakes, and
microspheres. U.S. Pat. No. 4,314,827 discloses synthetic,
non-fused aluminum oxide based abrasive mineral having a
microcrystalline structure of randomly oriented crystallites
comprising a dominant continuous phase of alpha alumina and a
secondary phase. U.S. Pat. No. 4,744,802 discloses an abrasive
grain made by a chemical ceramic process that employs an iron oxide
nucleating agent to enhance the transformation to alpha alumina.
This patent also suggests that the gel can be shaped by any
convenient method such as pressing, molding, or extruding. U.S.
Pat. No. 4,848,041 discloses a shaped abrasive grain made by a
chemical ceramic process in which the abrasive grain has a mean
particle volume ratio of less than 0.8.
SUMMARY OF THE INVENTION
This invention provides abrasive articles containing abrasive
particles having specified shapes. In particular, the abrasive
particles have shapes that can be characterized as thin bodies
having triangular, rectangular, including square, circular, or
other geometric shape. The abrasive particles have a front face and
a back face, both of which faces have substantially the same
geometric shape. The faces are separated by the thickness of the
particle. The ratio of the length of the shortest facial dimension
of an abrasive particle to its thickness is at least 1 to 1, and
most preferably at least 6 to 1. The abrasive particles of this
invention can be used in coated abrasive articles, bonded abrasive
articles, non-woven abrasive articles, and abrasive brushes. At
least 10% by weight, and preferably to 100% by weight, of the
abrasive material of the abrasive article should be of the shaped
abrasive particles described herein.
Coated abrasive articles of this invention comprise a backing
having at least one layer of abrasive grits adhered thereto by
means of a binder. A portion of this layer contains abrasive
particles having specified shapes. It is preferred that the
geometric shape of the faces of these abrasive particles be
triangular. In order to efficiently align the abrasive particles of
this invention on the backing, the abrasive particles are
preferably coated in an electrostatic field. The electrostatic
field lines concentrate at the corners and along the edges of the
abrasive particles, and by means of mutual particle repulsion, the
particles orient in the electrostatic field in such a way that they
are deposited onto the binder on their thinnest edges, thereby
allowing thin edges of the particles to be in contact with the
workpiece during abrading operations. For triangular-shaped
particles, about 35% to about 65% of the particles are oriented
with a vertex pointing away from the backing and a base in contact
with the binder, with about 35% to about 65% of the particles being
oriented with a base pointing away from the backing and a vertex in
contact with the binder. It is believed that when this
configuration is used with equilateral triangular-shaped particles,
the sum of the surface areas of each of the particles in contact
with the workpiece remains essentially constant during use, even
though the surface area of each individual abrasive particle in
contact with the workpiece varies during use. The use of the
abrasive particles described herein minimizes the formation of flat
surfaces on the cutting regions of the abrasive material. These
flat surfaces shorten the useful life of conventional abrasive
articles. During the abrading process, the shaped particles of this
invention continually fracture to expose fresh cutting surfaces.
Accordingly, they sharpen themselves during use.
One method for preparing abrasive particles useful in this
invention comprises the steps of:
(a) providing a dispersion comprising particles that can be
converted into alpha alumina, preferably particles of alpha alumina
monohydrate, in a liquid, which liquid comprises a volatile
component;
(b) providing a mold having a first generally planar surface an a
second surface opposed to said first surface, said first surface
having an opening to a mold cavity having a specified shape;
(c) introducing said dispersion into said mold cavity, such that no
exposed surface of said dispersion extends substantially beyond the
plane of said first surface of said mold;
(d) removing a sufficient portion of said volatile component of
said liquid from said dispersion while said dispersion is in said
mold cavity, thereby forming a precursor of an abrasive particle
having a shape approximately corresponding to the shape of said
mold cavity;
(e) removing said precursor of the abrasive particle from said mold
cavity;
(f) calcining said removed precursor of the abrasive particle;
and
(g) sintering said calcined precursor to form the desired abrasive
particle.
In one variation of the process, after the dispersion is formed, it
is gelled prior to being introduced into the mold cavity. As used
herein, the term "to gel" means to increase the viscosity of a
substance sufficiently so that it will not flow from an inverted
test tube. In a second variation, the dispersion is introduced into
the mold cavity under a pressure of less than 100 psi. In a third
variation, at least one side of the mold, i.e. the side in which
the cavity is formed, is exposed to the atmosphere surrounding the
mold during the step in which the volatile component is removed. In
a fourth variation, the volatile component of the dispersion is
removed from the dispersion while the dispersion is in the mold
without the application of additional heat or pressure. In a fifth
variation, the volatile component of the dispersion is removed from
the dispersion by evaporation while the dispersion is in the mold.
In a sixth variation, an additional drying step is utilized after
the precursor of the abrasive particle is removed from the
mold.
Preferably, the mold contains a plurality of cavities, more
preferably at least twenty cavities. Preferably, shape of the
cavities correspond approximately to the desired shape of the
abrasive particles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of a mold suitable for preparing abrasive
particles suitable for the coated abrasive article of this
invention.
FIG. 2 is a perspective view of a mold suitable for preparing
abrasive particles suitable for the coated abrasive article of this
invention.
FIG. 3 is a .Iadd.cross sectional .Iaddend.side view of a coated
abrasive article of this invention.
FIG. 4 is a photomicrograph taken at 12X illustrating abrasive
particles of this invention in which the planar shape is
triangular.
FIG. 5 is a photomicrograph taken at 12X illustrating abrasive
particles of this invention in which the planar shape is
rectangular.
FIG. 6 is a photomicrograph taken at 12X illustrating abrasive
particles of this invention in which the planar shape is
circular.
FIG. 7 is a .Iadd.cross sectional .Iaddend.side view of another
embodiment of a coated abrasive article of this invention.
FIG. 8 is a side view of an apparatus for preparing abrasive
particles of this invention.
FIG. 9 is a schematic perspective view of a die that can be used in
the apparatus of FIG. 8.
FIG. 10 is a sectional view of the auger and bore of the die body
of FIG. 9 .
DETAILED DESCRIPTION
As used herein, the term "dispersion" means the composition that is
introduced into the mold cavity--the composition will be referred
to as a dispersion until sufficient volatile component is removed
therefrom to bring about solidification of the dispersion; the term
"precursor of abrasive particle" means the unsintered particle
produced by removing a sufficient amount of the volatile component
from the dispersion, when it is in the mold cavity, to form a
solidified body having a shape corresponding approximately to the
shape of the mold cavity; the term "abrasive particle" means the
sintered particle produced by the process of this invention.
Referring to FIG. 3, coated abrasive article 30 comprises a backing
32 having a first layer of binder 34, hereinafter referred to as
the make coat, applied over one major surface of backing 32.
Partially embedded in make coat 34 are a plurality of abrasive
particles 36. Over the abrasive particles 36 is a second layer of
binder 38, hereinafter referred to as the size coat. The purpose of
make coat 34 is to secure abrasive particles 36 to backing 32 and
the purpose of size coat 38 is to reinforce abrasive particles 36.
It is preferred that a portion of the abrasive particles have a
triangular-shape. These abrasive particles will hereinafter be
designated as triangular-shaped abrasive particles. Of these
triangular-shaped abrasive particles, from about 35% to about 65%
are oriented on the backing with a vertex 40 of the triangle
pointing away from the backing as illustrated by FIG. 3. The
remainder of these triangular-shaped abrasive particles are
oriented with a base 42 of the triangle pointing away from the
backing. However, up to 20% of the particles may not be oriented in
either of the preceding ways, e.g., they may lay against the
backing with the triangular face of the particle being in contact
with the make coat. As used herein, the phrase "vertex pointing
away from the backing" and the like means that a base of the
triangular-shaped particle is adhered to the backing via the make
coat; the phrase "vertex pointing away from the backing" also
includes those situations in which the line corresponding to the
altitude of the triangular-shaped particle is tilted from the
perpendicular at a small angle, typically less than 45.degree.,
preferably less than 30.degree.. As used herein, the phrase "base
pointing away from the backing" and the like means that a vertex of
the triangular-shaped particle is adhered to the backing via the
make coat; the phrase "base pointing away from the backing"
includes those situations in which the line corresponding to the
altitude of the triangular-shaped particle is tilted from the
perpendicular at a small angle, typically less than 45.degree.,
preferably less than 30.degree..
During the manufacture of the coated abrasive article, the
triangular-shaped abrasive particles are applied into the make cost
by electrostatic coating techniques. Electrostatic coating causes a
portion of the triangular-shaped abrasive particles to be oriented
with a base pointing away from the backing and a portion to be
oriented with a vertex pointing away from the backing. This manner
of orientation results in improved performance of the coated
abrasive article. Additionally, this manner of orientation results
in a coated abrasive article in which the sum of the surface areas
of the triangular-shaped abrasive particles in contact with the
workpiece remains essentially constant during abrading, even though
the surface area of any individual abrasive particle in contact
with the workpiece varies during abrading.
It is to be expected that a small number of triangular-shaped
abrasive particles will fail to become adhered to the backing by
way of a base or a vertex and will lie flat on the make coat such
that the triangular face is in contact with the binder. These
particles will perform no appreciable cutting. The number of
particles lying flat will increase at lower weights of abrasive
mineral. During electrostatic deposition of the abrasive particles,
preferred orientation of the abrasive particles is easier to
maintain when the space between the particles is so small that the
particles do not have sufficient room to tip over during
deposition.
Preferably, throughout the abrading process, the total surface area
of the layer of abrasive particles in contact with the workpiece
will essentially remain constant. However, during abrading, the
surface area of the individual abrasive particles in contact with
the workpiece will vary. This effect can be achieved in the case in
which about 35% to about 65% of the abrasive particles have their
vertices pointing away from the backing and about 35% to about 65%
of the abrasive particles have their bases pointing away from the
backing. The cut and surface finish of the workpiece will remain
essentially consistent throughout the useful life of the abrasive
article.
It is also within the scope of this invention to have the
triangular-shaped abrasive particles oriented so that the vertices
of substantially all of the triangular-shaped abrasive particles
point away from the backing. This embodiment is shown in FIG. 7.
Referring to FIG. 7, coated abrasive article 50 comprises a backing
52 having a first layer of binder 54, hereinafter referred to as
the make coat, applied over one major surface of backing 52.
Partially embedded in make coat 54 are a plurality of abrasive
particles 56. Over the abrasive particles 56 is a second layer of
binder 58, hereinafter referred to as the size coat. The purpose of
make coat 54 is to secure abrasive particles 56 to backing 52 and
the purpose of size coat 58 is to reinforce abrasive particles 56.
Some of the abrasive particles 56, generally no more than 20%, may
be oriented in such a way that their vertices are not pointing away
from the backing 52. Of course, the backing, the abrasive
particles, the make coat, and the size coat can be made from the
same materials that are useful for making the coated abrasive
article of FIG. 3.
The abrasive particles useful in this invention have specified
three-dimensional shapes. The particles are preferably in the shape
of thin bodies having a front face and a back face, the front face
and the back face being separated by the thickness of the particle.
The front face and the back face have substantially the same
geometric shape. The geometric shape can be triangular,
rectangular, circular, elliptical, or that of other regular or
irregular polygons. The most preferred geometric shape is
triangular. For the purpose of this invention, triangular shapes
also include three-sided polygons wherein one or more of the sides
can be arcuate, i.e., the definition of triangular extends to
spherical triangles. Of triangular shapes, that of an equilateral
triangle is the most preferred. FIG. 4 illustrates a picture taken
at 12X magnification of a triangular-shaped abrasive particle. FIG.
5 illustrates a picture taken at 12X magnification of a
square-shaped abrasive particle. FIG. 6 illustrates a picture taken
at 12X magnification of a circular-shaped abrasive particle. In
most cases, the ratio of the length of the shortest facial
dimension of the abrasive particle to the thickness of the abrasive
particle is at least 1 to 1, preferably at least 2 to 1, more
preferably at least 5 to 1, most preferably at least 6 to 1. As
used herein, the term "thickness", when applied to a particle
having a thickness that varies over its planar configuration, shall
mean the minimum thickness. If the particle is of substantially
uniform thickness, the values of minimum, maximum, mean, and median
thickness shall be substantially equal. For example, in the case of
a triangle, if the thickness is equivalent to "a", the length of
the shortest side of the triangle is preferably at least "2a". In
the case of a particle in which two or more of the shortest facial
dimensions are of equal length, the foregoing relationship
continues to hold. In most cases, the abrasive particles are
polygons having at least three sides, the length of each side being
greater than the thickness of the particle. In the special
situation of a circle, ellipse, or a polygon having very short
sides, the diameter of the circle, minimum diameter of the ellipse,
or the diameter of the circle that can be circumscribed about the
very short-sided polygon is considered to be the shortest facial
dimension of the particle. The thickness of the particles
preferably ranges from about 25 to 500 micrometers. This aspect
ratio provides improved performance of the abrasive particle as
compared with conventional unshaped abrasive grits.
Additionally, the abrasive articles may contain a blend of the
abrasive particles of this invention along with 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 materials of conventional abrasive
grains include fused aluminum oxide, silicon carbide, garnet, fused
alumina zirconia, cubic boron nitride, diamond, and the like.
Representative examples of materials of diluent grains include
marble, gypsum, and glass. However, at least 10% by weight,
preferably 50 to 100% by weight, of the abrasive particles or
grains of the abrasive articles of this invention should be of the
type of abrasive particle of this invention. Blends of different
shapes of the abrasive particles of this invention can be used in
the articles of this invention. The alumina based, ceramic abrasive
particles useful in this invention may also have a surface coating.
Surface coatings are known to improve the adhesion between abrasive
grains and the binder in abrasive articles. Additionally, the
surface coating may prevent the 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 abrasive particles. Such surface coatings are described
in U.S. Pat. Nos. 5,011,508; 1,910,444; 3,041,156; 5,009,675;
5,085,671; 4,997,461 and 5,042,991, incorporated herein by
reference. The make coat and size coat comprise a resinous
adhesive. The resinous adhesive of the make coat can be the same as
or different from that of the size coat. 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 or size coat, 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, and combinations thereof. Examples of
fillers include calcium carbonate, silica, talc, clay, calcium
metasilicate, dolomite, aluminum sulfate and combinations thereof.
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.
In particular, it is believed that the grinding aid will (1)
decrease the friction between the abrasive grains and the workpiece
being abraded, (2) prevent the abrasive grain from "capping", i.e.
prevent metal particles from becoming welded to the tops of the
abrasive grains, (3) decrease the interface temperature between the
abrasive grains the workpiece, or (4) decreases the grinding
forces. 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 tetrachloronaphtalene,
pentachloronaphthalene; and polyvinyl chloride. Examples of halide
salts include sodium chloride, potassium cryolite, sodium cryolite,
ammonium cryolite, potassium tetrafluoroboate, 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. The preferred grinding aid of the invention is
cryolite and the most preferred is 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 abrasive
particles having specified shapes can be utilized in a bonded
abrasive or a nonwoven abrasive. A bonded abrasive comprises a
plurality of the shaped abrasive particles of this invention bonded
together by means of a binder to form a shaped mass. The binder for
a bonded abrasive can be metallic, organic, or vitreous. The shaped
abrasive particles can be designed to be suitable for cut-off
wheels. A nonwoven abrasive comprises a plurality of shaped
abrasive particles bonded into a fibrous nonwoven web by means of
an organic binder.
The first step of the process for preparing abrasive particles
useful in this invention involves preparing a dispersion containing
particles that can be converted into alpha alumina in a liquid,
which liquid comprises a volatile component, preferably water. The
dispersion should comprise a sufficient amount of liquid to cause
the viscosity of the dispersion to be sufficiently low to ensure
ease of introduction into the mold cavity but not so much liquid as
to cause subsequent removal of the liquid from the mold cavity to
be prohibitively expensive. The dispersion preferably comprises
from about 2 to about 90% by weight of particles that can be
converted into alpha alumina, preferably particles of alpha
aluminum oxide monohydrate (boehmite), an at least 10% by weight,
preferably from 50 to 70%, more preferably 50 to 60%, by weight,
volatile component, preferably water. Conversely, the dispersion
preferably contains from 30 to 50% more preferably 40 to 50% by
weight, solids. If the percentage of liquid is too high too many
cracks will develop in the resulting particles upon drying thereof.
If the percentage of liquid is too low, pumping of the dispersion
will be difficult. 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",
available from Conea Chemie, GMBH and "DISPAL", available from
Vista Chemical Company. These aluminum oxide monohydrates are in
the alpha form, 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 abrasive
particles of this invention will generally depend upon the type of
material used in the dispersion.
It is preferred that the dispersion be in a gel state. As used
herein, "a gel" is a three dimensional network of solids dispersed
in a liquid. A gel will not flow from an inverted test tube.
The 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 dispersion is not critical and can be
varied on the basis of convenience. Typically, the introduction of
a modifying additive or precursor of a modifying additive will
cause the dispersion to gel. The dispersion can also be induced to
gel by application of heat over a period of time.
The 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 invention 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 dispersions is disclosed in U.S.
Pat. No. 4,744,802, incorporated hereinafter by reference.
A peptizing agent can be added to the dispersion to produce a more
stable hydrosol or colloidal dispersion. Peptizing agents preferred
for this invention are monoprotic acids or acid compounds such as
acetic acid, hydrochloric acid, formic acid, and nitric acid, with
nitric acid being preferred. Multiprotic acids are less preferred
as peptizing agents because they rapidly gel the 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 dispersion.
The dispersion can be formed by any suitable means, such as, for
example, simply by mixing aluminum oxide monohyrate with water
containing a peptizing agent or by forming an aluminum oxide
monohyrate slurry to which the peptizing agent is added.
The second step of the process for preparing abrasive particles
useful in this invention involves providing a mold having at least
one cavity, preferably a plurality of cavities. Referring to FIG.
1, a mold 10 has a generally planar surface 12 and a plurality of
cavities 14. Mold 10 can be made from a rigid material, such as
metal, e.g., steel. It is preferred that mold 10 be made from a
relatively thin aluminum or stainless steel sheet or belt, e.g.,
having a thickness of less than 5 cm, preferably less than 2 cm.
Referring to FIG. 2, access to cavities 14 of mold 10 can be from
an opening 15 in first or top surface 16 of mold 10, from an
opening (not shown) in second or bottom surface 18 of mold 10, or
from openings in both surfaces of mold 10. In some instances,
cavities 14 can extend for the entire thickness of mold 10.
Alternatively, cavities 14 can extend only for a portion of the
thickness of mold 10. At least one side of mold 10, 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. If the cavities extend completely through the
mold, both surfaces of the mold should be generally planar. As used
herein, the term "planar" includes any two-dimensional surface. It
is preferred that the planar surfaces be flat or level.
The cavities 14 have a specified three-dimensional shape. The
preferred shape of a cavity can be described as being a triangle
having a dimension of depth. However, other shapes can be used,
such as, circles, rectangles, squares, or combinations thereof, all
having a dimension of depth. The dimension of depth is equal to the
perpendicular distance from the surface 12 to the lowermost point
of cavity 14. In addition, a cavity can have even the inverse of
other solid geometric shapes, such as, for example, pyramidal,
frusto-pyramidal, truncated spherical, truncated spheroidal,
conical, and frusto-conical. If an abrasive particle is prepared in
a mold cavity having a pyramidal, conical, frusto-pyramidal,
frusto-conical, truncated spherical, or a truncated spheroidal
shape, the thickness is determined as follows: (1) in the case of a
pyramid or cone, the thickness is the length of a line
perpendicular to the base of the particle and running to the apex
of the pyramid or cone; (2) in the case of a frusto-pyramid or
frusto-cone, the thickness is the length of a line perpendicular to
the center of the larger base of the frusto-pyramid or of the
frusto-cone and running to the smaller base of the frusto-pyramid
or of the frusto-cone; (3) in the case of a truncated sphere or
truncated spheroid, the thickness is the length of a line
perpendicular to the center of the base of the truncated sphere or
truncated spheroid and running to the curved boundary of the
truncated sphere or truncated spheroid. The length of the shortest
facial dimension of the particle is the length of the shortest
facial dimension of the base of the particle (if the particle has
only one base) or the length of the shortest facial dimension of
the larger base of the particle (if the particle has two bases).
There are preferably at least 20 cavities per mold, more preferably
at least 100 cavities per mold. 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.
It is preferred that the dimensions of cavities 14 approximately
correspond to the desired dimensions of the abrasive particles,
taking expected shrinkage into account. Accordingly, it will not be
necessary to crush, break, or cut the abrasive particles to reduce
their size. Likewise, after the abrasive particles are made by the
process of this invention, it is not necessary to screen them to an
appropriate particle size. Moreover, the size of the abrasive
particles will essentially remain constant between different lots,
thereby assuring a very consistent particle size and distribution
of particle sizes from lot to lot.
The third step of the process for preparing abrasive particles
useful in this invention involves introducing the dispersion into
cavities 14 by any conventional technique. It is preferred to flood
surface 12 of mold 10 with the dispersion. The dispersion can be
pumped into surface 12 of mold 10. Next, a scraper or leveler bar
can be used to force some of the dispersion into cavities 14 of
mold 10. The remaining portion of the dispersion that does not
enter cavities 14 can be removed from surface 12 of mold 10 and
recycled. Although a small portion of the dispersion can still be
allowed to remain on surface 12 of mold 10, this is not preferred.
The pressure applied by the scraper or leveler bar is typically
less than 100 psi, preferably less than 50 psi, and most preferably
less than 10 psi. Furthermore, it is preferred that no exposed
surfaces of the dispersion extend substantially beyond the planes
formed by the planar surfaces of the mold to ensure uniformity in
thickness of the abrasive particles. It is also preferred that the
planar surface of the mold surrounding the cavities be
substantially free of dispersion.
It is preferred that a release coating be applied to surface 12 of
mold 10 and on the surfaces of cavities 14 prior to the
introduction of the dispersion into cavities 14. The function of
the release coating is to allow ease of removal of the precursors
of the abrasive particles. Typical materials for preparing release
coatings are silicones and polyetrafluorethylene.
The fourth step of the process for preparing abrasive particles
useful in this invention involves removing a portion of the liquid
i.e. the volatile component thereof, from the dispersion while the
dispersion is in the mold cavity, thereby resulting in an increase
in the viscosity of the dispersion. It is preferred that the liquid
be removed by evaporation rather than by an external force such as
filtration. Removal of the volatile component by evaporation can
occur at room temperature or at elevated temperatures. The elevated
temperatures can range from about 40.degree. C. to about
300.degree. C. However, at higher temperatures, high drying rates
are obtained that produce undesirable cracks in the resulting
abrasive particle. It is preferred to heat the mold containing the
dispersion at a temperature of from about 50.degree. C. to about
80.degree. C. for from about 10 to about 30 minutes in a forced air
oven. A sufficient amount of the volatile component must be removed
from the dispersion to bring about solidification thereof, thereby
forming a precursor of an abrasive particle having approximately
the same shape as the shape of the mold cavity. It is preferred
that a sufficient amount of volatile component be removed from the
dispersion so that the presursors of the abrasive particles can be
easily removed from the cavities of the mold. Typically, up to 40%
of the liquid is removed from the dispersion in this step.
The fifth step of the process for preparing abrasive particles
useful in this invention involves removing the precursors of the
abrasive particle from the mold cavities. This step is made
possible by shrinkage of the dispersion, when the liquid is removed
therefrom. For example, it is not uncommon for the dispersion to
shrink 20% or more. The precursors of the abrasive particles can be
removed from the mold cavities either by gravity or by applying a
low pressure to force them out of the cavities.
The removed precursors of the abrasive particles have approximately
the same shape as the cavities of the mold from which they were
formed. Exact replication is unlikely for three reasons. First, the
dispersion will shrink, so the precursors of the abrasive particles
will be smaller. Second, when the precursors of the abrasive
particles are removed from the mold cavities, some of their edges
may break off or become rounded. Third, when the dispersion is
introduced in the cavities, the dispersion may not completely fill
the cavities. It should be noted that care should be taken
throughout the process to minimize the foregoing factors.
The precursors of the abrasive particles can be further dried
outside of the mold. If the 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 dispersion
resides in the mold. During this additional drying step, care must
be taken to prevent cracks from forming in the precursors of the
abrasive particles. Typically, the precursors of the abrasive
particles will be dried for from about 10 to about 480 minutes,
preferably from about 120 to about 400 minutes, at a temperature
from about 50.degree. C. to about 160.degree. C., preferably from
about 120.degree. C. to about 150.degree. C.
The sixth step for preparing abrasive particles useful in this
invention involves calcining the precursors of the abrasive
particles. During calcining, essentially all the volatile material
is removed, and the various components that were present in the
dispersion are transformed into metal oxides. The precursors of the
abrasive particle are generally heated to a temperature of from
about 400.degree. C. to about 800.degree. C., and maintained within
this temperature range until the free water and over 90% 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 precursors of the
abrasive particles. Then the precursors of the abrasive particles
are prefired again. This option is further described in European
Patent Application No. 293,163, incorporated herein by
reference.
The seventh step of the process for preparing abrasive particles
useful in this invention involves sintering the precursors of the
abrasive particles to form the abrasive particles. Prior to
sintering, the precursors of the abrasive particles are not
completely densified and thus lack the hardness to be used as
abrasive particles of this invention. Sintering takes place by
heating the precursors of the abrasive particle to a temperature of
from about 1,000.degree. C. to about 1,650.degree. C. and
maintaining them within this temperature range until substantially
all of the alpha alumina monohyrate (or equivalent) is converted to
alpha alumina and porosity is reduced to less than 15% by volume.
The length of time to which the precursors of the abrasive
particles must be exposed to the sintering temperature to achieve
this level of conversion depends upon various factors but usually
from about five seconds to about 48 hours is typical. The preferred
duration for sintering ranges from about one minute to about 90
minutes.
Other steps can be used to modify the process for preparing
abrasive particles useful in this invention, such as rapidly
heating the material from the calcining temperature to the
sintering temperature, centrifuging the 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 invention are more fully described in U.S. Pat. No.
4,314,827, incorporated herein by reference.
As shown in FIG. 8, a continuous process can be used to make the
abrasive particles of this invention. The apparatus 60 in FIG. 8
comprises a mold 62, a driving mechanism 64, a die body 66,
leading-edge wiper blades 68, levelling doctor blades 70, an oven
72, a collecting pan 74, and a brush 76. Referring now to FIG. 9,
an extrudable dispersion containing particles "P" of a material
that can be converted into alpha alumina (hereinafter "convertible
material") in a liquid is provided to a supply means 80 for
delivery to die body 66. Typical supply means can comprise a
combination kneader and extruder 82, which includes twin,
counter-rotating mixing blades that mix and pack the convertible
material into an auger channel 84 for delivery through exit port 86
by a supply auger 88. Mixing and packing the convertible material
aids in preventing voids that may produce a nonuniform sheet. The
exit port 86 is connected to a pump 90, which pressurizes the
convertible material and supplies it to a feed port 92 of die body
66.
Die body 66 includes a longitudinal bore 100 therein having first
and second ends 102 and 104, respectively. Feed port 92
communicates the exterior of die body 66 with bore 80 adjacent
second end 104. An auger 106 having first and second ends 108 and
110, respectively, is disposed within bore 100. Auger 106 comprises
a longitudinal root and a helical flight adjoining the root along
the length thereof. The flight diameter of auger 106 is constant,
and the root has a first diameter at the first end 108, and a
second diameter smaller than the first diameter at the second end
110. The flight depth of auger 106 is therefore greatest near feed
port 92, and gradually decreases toward the first end 108 of auger
106, although the overall flight diameter is constant. The material
conveying capacity of auger 106 thus gradually decreases along the
length of the auger due to the gradually decreasing flight
depth.
Die body 66 includes one or more elongate die openings 112 that
communicate the exterior of die body 66 with bore 100 along the
length of auger 106. In the preferred embodiment, die body 66
includes a single elongate die opening 112 that is adapted to form
a uniform sheet member having a width substantially in excess of
its thickness. The combination of the position of die opening 112
relative to auger 106 and the configuration of auger 106 tends to
produce a uniform extruded sheet 114 of convertible material.
A motor 116 rotates auger 106 within bore 100 to extrude the
convertible material in sheet form. The proper rotational speed of
auger 106 may be experimentally or analytically determined to
provide the desired uniform rate of extrusion. If auger 106 is
rotated too slowly excess convertible material may be discharged
through the portion of die opening 112 nearest second end 104.
Similarly, if auger 106 is rotated too quickly, excess convertible
material may be discharged through the portion of die opening 112
nearest first end 102. At the proper rotational velocity, the
pressure along bore 100 is uniform, thereby forcing a sheet of
uniform thickness through die opening 112.
The dispersion is forced into cavities (not shown) of the mold 62
as it passes through the die opening 112. The mold 62 of FIG. 8 is
a flexible belt, which is driven by the driving mechanism 64. The
cavities in the mold 62 can have any desired planar shape, such as
triangular, circular, or square. The cavities can be formed by
conventional means, such as by machining, punching, or etching. The
flexible belt 62 can be made of any material that will withstand
the operating conditions of the process. A belt made of metal such
as stainless steel or aluminum is preferable. It is preferred that
the mold 62 be coated with a release coating, such as
polytetraflouroethylene, to improve the release of the dried
precursor particles from the cavities of the mold 62.
It is preferred that the exposed surface or surfaces of the
dispersion in the cavities not extend substantially beyond the
plane of the belt in order to guarantee that the abrasive particles
prepared from the process be substantially uniform. Any excess
dispersion surrounding the openings of the cavities and remaining
on the non-recessed portion of the belt 62 is removed, preferably
by leading-edge wiper blades 68 positioned down the belt 62 from
the die body 66. The top and bottom surfaces of the belt 62 can be
wiped by the leading-edge wiper blades 68. These blades 68 are
mounted between leveling doctor blades 70 and the die body 66. The
leveling doctor blades 70 further ensure that abrasive precursor
particles will have a uniform thickness.
The filled cavities in the belt 62 are moved into the oven 72,
which is preferably an air circulating oven. The oven temperature
is preferably set at approximately 75.degree. C. However, the oven
temperature can be higher or lower depending on the speed of the
belt 62 and solids content of the precursor. The volatile component
of the liquid is removed from the dispersion in the oven 72. Care
should be taken to solidify the dispersion sufficiently slowly so
that the formation of cracks in the abrasive particles is
minimized. As the volatile component is removed, the precursors of
the abrasive particles begin to form. Because their volume is less
than that of the dispersion from which they are formed, they will
fall out of the cavities in the belt 62, and can be collected in a
collecting pan 74. The shaped, dried precursors of the abrasive
particles are then calcined and fired, preferably in a rotary kiln
(not shown). Firing is preferably carried out at a temperature of
1300.degree. C. to 1400.degree. C. for a period of 1 to 15 minutes.
Any dispersion or precursor material remaining on the belt 62 or in
the cavities of the belt can be removed, preferably by a rotating
brush 76 or other cleaning process.
Processes for preparing abrasive particles useful for the abrasive
articles of this invention are further described in assignee's
copending applications 46925USA1A and 48405USA1A, filed on evendate
herewith, and incorporated herein by reference.
The following examples are illustrative of specific embodiments of
this invention; however, these examples are for illustrative
purposes only and are not to be construed as limitations upon the
invention.
The following procedures were used for Examples 1-10.
Procedure for Making Shaped Abrasive Particles
A dispersion (44% solids) was made by the following procedure:
alpha aluminum oxide monohydrate powder (1,235 parts) having the
trade designation "DISPERAL" and alpha iron oxide (206 parts, 10%
FeOOH) were dispersed by continuous mixing in a solution containing
water (3,026 parts) and 70% aqueous nitric acid (71 parts). The sol
that resulted was mixed with magnesium nitrite (429 parts) to form
a gel which was then dried at a temperature of approximately
125.degree. C. in a continuous dryer to produce the 44% solids
dispersion. The dispersion was introduced into the cavities of the
desired shape in a mold by means of a rubber squeegee. The cavities
were coated with a release coating, either a silicone material or
polytetrafluorethylene. The filled mold was placed in a forced air
oven maintained at a temperature of 71.degree. C. for 20 minutes.
The dispersion underwent substantial shrinkage as it dried, and the
dried precursors of the abrasive particles shrank in the cavities.
The precursors of the abrasive particles were removed from the mold
by gravity. After the precursors of the abrasive particles were
removed from the mold, they were dried at a temperature of
121.degree. C. for three hours.
The dried precursors of the abrasive particles were introduced into
the end of a calciner, which can be described as a 23 cm diameter,
4.3 m long stainless steel tube having a 2.9 m hot zone, the tube
being inclined at 2.4.degree. with respect to the horizontal, and
rotating at 6 rpm, providing residence time therein of about 15
minutes. The entry end temperature of the hot zone was 350.degree.
C. and the exit end temperature of the hot zone was 800.degree. C.
The material exiting the calciner was introduced into a kiln held
at a temperature of about 1,390.degree. C. The kiln was a 8.9 cm
diameter, 1.32 m long silicon carbide tube inclined at 4.4.degree.
with respect to the horizontal, having a 76 cm hot zone, and
rotating at 10.5 rpm, providing a residence time therein of about
four minutes. The material exited the kiln into air at room
temperature, where it was collected in a metal container and
allowed to cool to room temperature.
Procedure for Making and Testing Coated Abrasive Articles
The abrasive particles of the examples described herein were
utilized in coated abrasive articles made according to a
conventional procedure for preparing coated abrasive articles. The
abrasive particles were first screened to a screen size of 16-20
mesh U.S. Standard. A make coat was applied to a vulcanized fiber
backing in the shape of a disc by means of a paint brush. The make
coat consisted of conventional calcium carbonate-filled resole
phenolic resin. The abrasive particles were projected into the make
coat by means of a conventional electrostatic coating technique. A
size coat consisting of conventional calcium carbonate-filled
resole phenolic resin was applied over the abrasive particles and
make coat by means of a paint brush. The concentration of calcium
carbonate was 52% by weight and the concentration of resin was 48%
by weight in the make coat and the size coat. The resin of the make
coat was precured for 90 minutes at a temperature of 88.degree. C.
and the resin of the size coat was precured for 90 minutes at a
temperature of 88.degree. C. followed by a final cure of 10 hours
at a temperature of 100.degree. C. The approximate coating weights
were 160 /m.sup.2 for the make coat, 905 g/m.sup.2 for the layer of
abrasive particles, and 987 g/m.sup.2 for the size coat.
The cured coated abrasive articles, which were in the form of discs
(having a diameter of 7 inches), were first flexed in a
conventional manner to controllably fracture the hard bonding
resins, then mounted on a beveled aluminum back-up pad, and used to
grind the face of a 1.25 cm by 18 cm 1018 mild steel workpiece. The
disc was driven at 5,000 rpm while the portion of the disc
overlaying the beveled edge of the back-up pad contacted the
workpiece at 6.81 kg load, generating a disc wear path of about 140
cm.sup.2. Each disc was used to grind a separate workpiece for one
minute each for a total time of 12 minutes for each disc or for
sufficient one minute time intervals until no more than 5 g of
metal were removed from the workpiece in any one minute time
interval. The performance of the coated abrasive article is
generally stated as a percent of Comparative Example A, that is,
the total amount of metal removed from the workpiece by the coated
abrasive article of Comparative Example A was set at 100% and the
amount of metal removed by the coated abrasive article of the
example was reported as a percent of that removed by the coated
abrasive article of Comparative Example A. For example, a coated
abrasive article made with abrasive particles according to one of
the working examples that performed 10% better than the coated
abrasive article of Comparative Example A has a performance of 110%
of the article of Comparative Example A.
EXAMPLE 1
This example demonstrates the grinding performance of coated
abrasive articles employing triangular-shape abrasive particles
prepared according to the Procedure for Making Shaped Abrasive
Particles. The mold used to make the abrasive particles had
cavities in the shape of an equilateral triangle, the length of
each side of each cavity being 0.29 cm, and the depth of each
cavity being 0.05 cm. The abrasive particles formed from this mold
were triangular-shaped and had dimensions approximately 0.157 cm on
each side and 0.028 cm thick (FIG. 1). The performance of coated
abrasive articles employing the triangular-shaped abrasive
particles was compared with coated abrasive articles employing
equivalent screen sized (16-20 mesh U.S. Standard) randomly-shaped
abrasive grains as described in Comparative Example A.
COMPARATIVE EXAMPLE A
The abrasive grains utilized in Comparative Example A were
commercially available from Minnesota Mining and Manufacturing
Company, St. Paul, Minn., under the trade designation of CUBITRON
abrasive grain. CUBITRON abrasive grain comprises 93.5% by weight
alpha alumina, 4.5% magnesium oxide, and 2% by weight iron oxide
nucleating agent. The abrasive grain was employed in a coated
abrasive article and tested as described above.
EXAMPLE 2
This example demonstrates the grinding performance of coated
abrasive articles employing disc-shaped abrasive particles prepared
according to the Procedure for Making Shaped Abrasive Particles.
The disc-shaped abrasive particles were prepared by using a mold
having cavities 0.23 cm in diameter and 0.05 cm deep (FIG. 6). The
performance of the coated abrasive articles employing the
disc-shaped abrasive particles was compared with coated abrasive
articles employing the triangular-shaped abrasive particles of
Example 1.
EXAMPLE 3
This example demonstrates the grinding performance of coated
abrasive articles employing square-shaped abrasive particles. The
abrasive particles were prepared according to the Procedure for
Making Shaped Abrasive Particles. The square-shaped abrasive
particles were prepared using a mold having cavities 0.23 cm on
each side and 0.06 cm deep (FIG. 5). The performance of coated
abrasive articles employing the square-shaped abrasive particles
was compared with the coated abrasive articles employing the
triangular-shaped abrasive particles of Example 1. Grinding
performance of the foregoing examples is set forth in Table I.
TABLE I ______________________________________ Total cut (% of
Example Shape of particle Comparative Example A)
______________________________________ Comparative A Random 100 1
Triangular 174 2 Disc 78 3 Square 114
______________________________________
The coated abrasive disc having the triangular-shaped abrasive
particles showed 74% improvement in total cut, and the disc having
the square-shaped abrasive particles showed 14% improvement in
total cut over the disc having the randomly-shaped abrasive
particles.
EXAMPLE 4
This example demonstrates the orientation of triangular-shaped
abrasive particles when coated onto a fiber backing in an
electrostatic field. Triangular-shaped abrasive particles were
prepared as in Example 1. Fiber discs were prepared according to
the Procedure for Testing and Making Coated Abrasive Articles.
The fiber discs bearing abrasive particles were observed under a
low power microscope (10X) and the number of particles with a
vertex pointing away from the backing and the number of particles
with the base pointing away from the backing in the field were
determined for four discs. Orientation of the particles is set
forth in Table II.
TABLE II ______________________________________ Percentage of
Percentage of particles having particles having vertex pointing
base pointing Disc away from backing.sup.1 away from backing.sup.1
______________________________________ I 53% 47% II 50% 50% III 65%
35% IV 55% 45 ______________________________________ .sup.1 Less
than 5% of the abrasive particles were oriented so that neither
their vertexes nor bases pointed away from the backing.
When the abrasive particles are coated in an electrostatic field,
most of the particles orient so that a vertex points either toward
or away from the backing and only a small percentage of particles
lie flat. Moreover, the triangular-shaped abrasive particles orient
such that approximately 50% have the vertex pointing away from the
backing and approximately 50% have a base pointing away from the
backing.
EXAMPLE 5
This example demonstrates the nature of the surface finish produced
by coated abrasive articles employing triangular-shaped abrasive
particles prepared as in Example 1. The coated abrasive articles of
Comparative Examples B, C, and D employed randomly-shaped abrasive
grains made by conventional methods. These grains were screened to
ANSI grades 24, 36, and 50, respectively (ANSI Standard B74.18,
1984). The chemical composition of the abrasive grains of
Comparative Examples B, C, D was the same as that of the abrasive
particles of Example 1. The coated abrasive articles, i.e., discs,
of Comparative Examples B, C, and D were made of the same material
as described in Comparative Example A. The surface finish was
determined by grinding the paint off a 15 cm.times.60 cm steel
panel with a 6,000 rpm Black & Decker electric grinder. The
surface finish of the steel was measured by using a Taylor-Hobson
Surtronic 3 profile meter. The surface finish produced by the
various discs is set forth in Table III. As used herein, "Ra" means
the arithmetical mean deviation of the profile of the scratch;
"Rtm" means the maximum peak-to-valley height of the profile of the
scratch.
TABLE III ______________________________________ Ra Rtm Example
(micrometers) (micrometers) ______________________________________
5 4.1 25.1 Comparative B 7.8 41.9 Comparative C 6.9 37.2
Comparative D 4.5 25.6 ______________________________________
The surface finish produced by the coated abrasive disc having
triangular-shaped abrasive particles was superior to the finish
produced by the discs of the Comparative Examples B and C. The
finish produced by the triangular-shaped abrasive grains was
essentially the same as that produced by the disc of Comparative
Example D.
EXAMPLE 6
This example demonstrates grinding performance of coated abrasive
articles employing triangular-shaped abrasive particles prepared as
in Example 1.
The discs were tested according to the Procedure for Testing and
Making Coated Abrasive Articles, except that the test was extended
by one-minute intervals to the point at which each disc removed the
same amount of metal in the final one-minute interval. The discs
were compared with those of Comparative Example A. The results are
set forth in Table IV.
TABLE IV ______________________________________ Duration to reach
Amount of metal Example end point (min) removed by end point (g)
______________________________________ 6 26 2515 Comparative A 12
1033 ______________________________________
This example demonstrates that a disc having triangular-shaped
abrasive grains has a longer life than does a disc employing
conventional "CUBITRON" grains. The disc of this invention removed
143% more metal before reaching the equivalent end point.
EXAMPLE 7
This example demonstrates the grinding performance of coated
abrasive articles employing blends of triangular-shaped abrasive
particles of this invention and diluent grains, such as marble. The
triangular-shaped abrasive particles were prepared according to the
Procedure for Making Shaped Abrasive Particles. The mold used to
make the abrasive particles had cavities 0.190 cm on each side and
0.03 cm deep. The particles made with this mold were
triangular-shaped and equivalent in size to 25-30 mesh U.S.
Standard screen. The triangular-shaped abrasive particles were
blended with ANSI 36 marble on an equal weight basis. The abrasive
particle/marble blend was coated at a weight of 820 g/m.sup.2. The
weight of the make coat was 160 g/m.sup.2. The weight of the size
coat was 655 g/m.sup.2. The abrasive grains in Comparative Example
E (ANSI 36) was prepared as described in Comparative Example A. The
discs were tested as in Procedure for Testing Coated Abrasive
Articles. The results are set forth in Table V.
TABLE V ______________________________________ Total cut Example (%
of Comparative Example E) ______________________________________
Comparative E 100 7 114 ______________________________________
This example demonstrates that a disc having a blend of
triangular-shaped abrasive grains and marble showed 14% improvement
in total cut over a disc having conventional sol-gel abrasive
grains of random shape.
EXAMPLE 8
This example demonstrates the grinding performance of
triangular-shaped abrasive particles at high grinding pressures.
The samples were prepared and tested in the same manner as was used
in Example 1, except that the test load applied to the rotating
disc was increased to 8.6 kg. The abrasive grains in Comparative
Example F were prepared as was described in Comparative Example A.
The disc in Comparative Example F used ANSI 24 "CUBITRON"
randomly-shaped abrasive grains.
TABLE VI ______________________________________ Total cut (% of
Example Comparative Example F)
______________________________________ Comparative F 100 8 143
______________________________________
This example demonstrates that a disc having triangular-shaped
abrasive particles showed improved grinding performance over a disc
having randomly-shaped abrasive grains at high grinding
pressures.
EXAMPLE 9
This example demonstrates the grinding performance of
triangular-shaped abrasive particles. The triangular-shaped
abrasive particles were prepared and tested in the same manner as
was used in Example 1, with the exception that magnesium nitrite
was not added to the sol. The abrasive grains in Comparative
Example G were prepared according to U.S. Pat. No. 4,964,883. The
abrasive grains contained 98% by weight alpha alumina and 2% by
weight iron oxide nucleating agent. The disc in Comparative Example
G used ANSI 36 "CUBITRON" randomly-shaped abrasive grains. The
results are set forth in Table VII.
TABLE VII ______________________________________ Total cut (% of
Example Comparative Example G)
______________________________________ Comparative G 100 9 136
______________________________________
This example demonstrates that a disc having triangular-shaped
abrasive particles that were free of magnesium oxide showed
superior grinding performance to that of a disc having
randomly-shaped abrasive grains.
EXAMPLE 10
This example demonstrates the grinding performance of
triangular-shaped abrasive particles blended with erodable
agglomerates. The triangular-shaped abrasive particles were
prepared in the same manner as was used in Example 1. The erodable
agglomerates were prepared according to U.S. Pat. No. 5,078,753,
Example 1. The erodable agglomerates used in this example were
capable of passing through a 16 mesh screen and being retained on a
30 mesh screen. The triangular-shaped abrasive particles and the
erodable agglomerates were blended. Discs were prepared and tested
in the manner described in Procedure For Making and Testing Coated
Abrasive Articles. The coating weight of the triangular-shaped
abrasive particles was 614 g/m.sup.2. The coating weight of the
erodable agglomerates was 205 g/m.sup.2. The coating weight of the
make coat was 160 g/m.sup.2. The coating weight of the size coat
was 1065 g/m.sup.2. The abrasive grain in Comparative Example H was
prepared in the same manner as was described in Comparative Example
A. The results are set forth in Table VIII.
TABLE VIII ______________________________________ Total cut (% of
Example Comparative Example H)
______________________________________ Comparative H 100 10 130
______________________________________
This example demonstrates that open coat constructions providing
good performance can be made with triangular-shaped abrasive
particles. The erodable agglomerates support the triangular-shaped
abrasive particles and provides good orientation for the
triangular-shaped abrasive particles.
EXAMPLE 11
In this example, precursors of abrasive particles were prepared by
means of the apparatus shown in FIG. 8. The dispersion for this
example was prepared under the same conditions as were described in
Procedures for Making Shaped Abrasive Particles. One lot of
triangular-shaped abrasive grains was prepared without the wiping
technique, and another lot was prepared with the wiping technique.
The abrasive grains in Comparative Example J were prepared in the
same manner as was described in Comparative Example A. Discs were
prepared and tested in the manner described in Procedure for Making
and Testing Coated Abrasive Articles. The results are set forth in
Table IX.
TABLE IX ______________________________________ Total cut (% of
Example Comparative Example J)
______________________________________ Comparative J 100 11
(without wiping) 119 11 (with wiping) 140
______________________________________
This example demonstrates that wiping of the filled web is
beneficial to the grinding performance of discs employing
triangular-shaped abrasive particles.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the
scope and spirit of this invention, and it should be understood
that this invention is not to be unduly limited to the illustrated
embodiments set forth herein.
* * * * *