U.S. patent application number 16/511962 was filed with the patent office on 2019-11-07 for sintered abrasive particles, method of making the same, and abrasive articles including the same.
The applicant listed for this patent is 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to Dwight D. Erickson, Anatoly Z. Rosenflanz.
Application Number | 20190338172 16/511962 |
Document ID | / |
Family ID | 51659148 |
Filed Date | 2019-11-07 |
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United States Patent
Application |
20190338172 |
Kind Code |
A1 |
Erickson; Dwight D. ; et
al. |
November 7, 2019 |
SINTERED ABRASIVE PARTICLES, METHOD OF MAKING THE SAME, AND
ABRASIVE ARTICLES INCLUDING THE SAME
Abstract
Sintered abrasive particles have a cellular microstructure
comprising alpha alumina crystal grains of alpha alumina having a
maximum dimension of less than about 3 microns are also disclosed.
The sintered abrasive particles have an average particle size of
less than or equal to 500 microns, and are essentially free of seed
particles and alpha alumina grain size modifiers. Abrasive articles
comprising a binder and a plurality of the sintered abrasive
particles are also disclosed.
Inventors: |
Erickson; Dwight D.;
(Woodbury, MN) ; Rosenflanz; Anatoly Z.;
(Maplewood, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
3M INNOVATIVE PROPERTIES COMPANY |
St. Paul |
MN |
US |
|
|
Family ID: |
51659148 |
Appl. No.: |
16/511962 |
Filed: |
July 15, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14782475 |
Oct 5, 2015 |
10400146 |
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PCT/US2014/032043 |
Mar 27, 2014 |
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16511962 |
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61808955 |
Apr 5, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01P 2004/61 20130101;
C04B 35/6268 20130101; C01P 2004/45 20130101; C09K 3/1409 20130101;
C04B 35/62665 20130101; C01P 2004/03 20130101; C04B 35/1115
20130101; C01F 7/027 20130101; C04B 2235/3218 20130101; C01F 7/38
20130101; C01P 2004/62 20130101 |
International
Class: |
C09K 3/14 20060101
C09K003/14; C04B 35/626 20060101 C04B035/626; C01F 7/02 20060101
C01F007/02; C01F 7/38 20060101 C01F007/38; C04B 35/111 20060101
C04B035/111 |
Claims
1. Sintered abrasive particles having a cellular microstructure
comprising alpha alumina crystal grains of alpha alumina having a
maximum dimension of less than about 3 microns, wherein the
sintered abrasive particles comprise shaped abrasive particles,
wherein the sintered abrasive particles have an average particle
size of less than or equal to 500 microns, and wherein the sintered
abrasive particles are essentially free of seed particles and alpha
alumina grain size modifiers.
2. The sintered abrasive particles of claim 1, wherein the alpha
alumina has an areal porosity of less than or equal to 5 percent,
as measured using image analysis by cross-section at 10,000 times
magnification.
3. The sintered abrasive particles of claim 1, wherein at least a
portion of the sintered abrasive particles comprises truncated
pyramids.
4. An abrasive article comprising a binder and a plurality of
sintered abrasive particles according to claim 1.
5. The abrasive article of claim 4, wherein the abrasive article is
a bonded abrasive article, a non-woven abrasive article, or a
coated abrasive article.
Description
TECHNICAL FIELD
[0001] The present disclosure broadly relates to abrasive particles
and methods of making and using them.
BACKGROUND
[0002] Sintered abrasive particles and abrasive articles including
them are useful for abrading, finishing, or grinding a wide variety
of materials and surfaces in the manufacturing of goods. Of the
wide variety of known abrasive particles, fused abrasive particles
(e.g., including fused alumina, heat treated fused alumina, and
fused alumina zirconia) and sintered ceramic abrasive particles
(including sol-gel-derived sintered ceramic abrasive particles) are
widely used in the abrasives art.
[0003] Alpha alumina abrasive particles are a major class of
abrasive particles used in the abrasives industry. Fused alpha
alumina abrasive particles are typically made by charging a furnace
with an alumina source (such as aluminum ore or bauxite), as well
as other desired additives, heating the material above its melting
point, cooling the melt to provide a solidified mass, crushing the
solidified mass into particles, and then screening and grading the
particles to provide the desired abrasive particle size
distribution. Although fused alpha alumina abrasive particles and
fused alumina-zirconia abrasive particles are still widely used in
abrading applications (including those utilizing coated and bonded
abrasive products), the premier abrasive particles for many
abrading applications since about the mid-1980s are sol-gel-derived
alpha alumina particles (also referred to as sintered ceramic alpha
alumina particles).
[0004] Sol-gel-derived alpha alumina abrasive particles may have a
microstructure made up of very fine alpha alumina crystallites
(also known as "alpha alumina crystal grains"), with or without the
presence of secondary phases added. Sol-gel-derived alumina
abrasives are conventionally produced by drying an aqueous sol or
gel of an alpha alumina precursor (typically, but not necessarily,
boehmite) to remove the water component of the gel, breaking up the
dried gel into particles of the desired size for abrasive grits;
optionally calcining the particles (typically at a temperature of
from about 400-800.degree. C.) to form a transitional alumina
(e.g., gamma alumina), and then sintering the dried and optionally
calcined particles at a temperature sufficiently high to convert
them to the alpha alumina form.
[0005] In one embodiment of a sol-gel process, the alpha alumina
precursor is "seeded" with a material having the same crystal
structure as, and lattice parameters as close as possible to, those
of alpha alumina itself. The "seed" (a nucleating agent) is added
in as finely divided form as possible and is dispersed uniformly
throughout the sol or gel. It can be added for the beginning or
formed in situ. The function of the seed is to cause the
transformation to the alpha form to occur uniformly throughout the
precursor at a lower temperature than is needed in the absence of
the seed. This seeded process produces a crystalline structure in
which individual alpha alumina crystal grains (that is, those areas
of substantially the same crystallographic orientation separated
from adjacent crystals by high angle grain boundaries), are very
uniform in size and are essentially all sub-micron in diameter.
Suitable seeds include alpha alumina itself and other compounds
such as alpha ferric oxide, chromium suboxide, nickel titanate, and
other compounds that have lattice parameters sufficiently similar
to those of alpha alumina to be effective to cause the generation
of alpha alumina from a precursor at a temperature below that at
which the conversion normally occurs in the absence of such
seed.
[0006] Similarly, one or more alpha alumina crystal grain size
modifiers (e.g., a spinel forming metal oxide) may also be added to
the sol-gel, or impregnated into the dried sol-gel particles or
calcined particles to control the size of alumina crystal grains in
the resultant alpha alumina abrasive particle.
[0007] However, the use of seeds and/or alpha alumina crystal grain
size modifiers adds complexity and cost to the sol-gel process.
SUMMARY
[0008] In one aspect, the present disclosure provides a method of
making sintered abrasive particles, the method comprising:
[0009] passing precursor particles through a flame under conditions
such that they are converted to the sintered abrasive particles,
wherein the precursor particles comprise a calcined precursor of
alpha alumina, and wherein the precursor particles have an average
particle size of less than or equal to 500 microns.
[0010] In another aspect, the present disclosure provides abrasive
particles having a cellular microstructure comprising alpha alumina
crystal grains of alpha alumina having a maximum dimension of less
than about 3 microns, wherein the sintered abrasive particles have
an average particle size of less than or equal to 500 microns, and
wherein the sintered abrasive particles are essentially free of
seed particles and alpha alumina grain size modifiers.
[0011] Alpha alumina abrasive particles made according to the
present disclosure are useful in abrasive articles. Accordingly, in
another aspect, the present disclosure provides an abrasive article
comprising a binder and a plurality of abrasive particles, wherein
at least a portion of the sintered abrasive particles are alpha
alumina abrasive particles according to the present disclosure.
[0012] Advantageously, methods according to the present disclosure
provides a route to forming alpha alumina abrasive particles via
the sol-gel process without relying on seed particles or alpha
alumina crystal grain size modifiers to provide high density alpha
alumina particles with small alpha alumina crystal grains.
Moreover, the method is typically rapid.
[0013] As used herein, the term "calcined" means heated to high
temperature (e.g., 650.degree. C.) below the melting point, for
sufficient time to remove adsorbed and chemically bound (as
hydrate) water and other volatile compounds.
[0014] As used herein, the term "essentially free of" means, on a
weight basis, containing at most a trivial amount (e.g., less than
0.1 percent, less than 0.01 percent, less than 0.001 percent, or
even less than 0.0001 percent) or completely free of.
[0015] As used herein, the term "shaped" as applied to a particle
means that the particle has a non-random shape imparted by the
method used to make it, and expressly excludes mechanically crushed
and/or milled particles.
[0016] Features and advantages of the present disclosure will be
further understood upon consideration of the detailed description
as well as the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a process flow diagram showing an exemplary method
of making abrasive particles according to the present
disclosure.
[0018] FIG. 2 is a fragmentary cross-sectional schematic view of a
coated abrasive article including abrasive particles according to
the present disclosure.
[0019] FIG. 3 is a perspective view of a bonded abrasive article
including abrasive particles according to the present
disclosure.
[0020] FIG. 4 is an enlarged schematic view of a nonwoven abrasive
article including abrasive particles according to the present
disclosure.
[0021] FIG. 5 is scanning electron micrograph of sintered abrasive
particles produced in Example 1.
[0022] FIG. 6 is a scanning electron micrograph of sintered
abrasive particles produced in Comparative Example A.
[0023] FIGS. 7-12 are scanning electron micrographs of sintered
abrasive particles produced in Examples 2-7, respectively.
[0024] FIG. 13 is a scanning electron micrograph of sintered
abrasive particles produced in Comparative Example B.
[0025] FIGS. 14 and 15 are scanning electron micrographs of
specimens used for image analysis of Examples 8 and 9,
respectively.
[0026] Repeated use of reference characters in the specification
and drawings is intended to represent the same or analogous
features or elements of the disclosure. It should be understood
that numerous other modifications and embodiments can be devised by
those skilled in the art, which fall within the scope and spirit of
the principles of the disclosure. The figures may not be drawn to
scale.
DETAILED DESCRIPTION
[0027] In one embodiment of the present disclosure, precursor
particles comprising a calcined precursor of alpha alumina are
passed through a flame under conditions such that they are
converted to the sintered abrasive particles. Referring now to the
exemplary process 100 shown in FIG. 1, calcined precursor particles
110 are dropped through nozzle 120, exiting at opening 170 into
flame 130 where conversion of the calcined precursor particles 110
into abrasive particles 140 occurs. Sintered abrasive particles 140
are collected at the bottom of containment vessel 160. Flame 130 is
fed by fuel inlet 132 and oxygen inlet 134. Argon is introduced
through argon inlet 136 and used to accelerate the velocity of the
calcined precursor particles 110 through nozzle 120.
[0028] The resultant abrasive particles have a cellular
microstructure formed of sintered alpha alumina crystal grains.
Typically, the alpha alumina crystal grains have a small size,
although this is not a requirement. In some embodiments, the
cellular microstructure comprises alpha alumina crystal grains
having a maximum dimension of less than about 3 microns, less than
about 2.5 microns, even less than about 2 microns.
[0029] Even though the method of the present disclosure typically
involves flame contact times on the order of a second or less,
which is not a requirement, the resultant sintered abrasive
particles may have low porosity. For example, the sintered abrasive
particles may have a cellular microstructure comprising alpha
alumina, wherein the alpha alumina has an areal porosity of less
than or equal to 5 percent, as measured using image analysis by
cross-section of the sintered abrasive particle at high
magnification (e.g., 10,000 times magnification).
[0030] For example, the sintered abrasive particles may have an
areal porosity of less than or equal to 4.5 percent, less than or
equal to 4 percent, less than or equal to 3.5 percent, less than or
equal to 3 percent, less than or equal to 2.5 percent, less than or
equal to one percent, less than or equal to 0.5 percent, or even
less than or equal to 0.1 percent. This is unexpected, since
conventional processes of forming alpha alumina based particles
from alpha alumina precursor particles with comparable porosities
typically involve heating times ranging from tens of minutes to
hours.
[0031] Areal porosity can be determined using conventional
techniques such as, for example, using ImageJ software available
from the U.S. National Institute of Health, Bethesda, Md. Random
cross-sections of sintered abrasive particles are independently
imaged using a field emission scanning electron microscope at high
magnification (e.g., a magnification of from 2,000.times. to
10,000.times.) using backscattered electrons. Because of the
relatively high magnification, a random area was selected on each
of the cross-sectioned surface of the abrasive particles. The
images were subsequently analyzed using the ImageJ image analysis
software. Data was obtained by manually measuring the area of
individual exposed voids and combining these individual pore area
measurements to obtain the total area of voids per image, and then
dividing this value by the area of the total field of view area to
obtain the area porosity (i.e., porosity determined on an area
basis). Conventional statistical methods (e.g., regarding the
number of measurements and samples) can be applied to ascertain
porosity.
[0032] The precursor particles comprise a calcined precursor of
alpha alumina. Examples of alpha alumina precursors that can be
calcined include: transitional aluminas (e.g., boehmite, diaspore,
gibbsite, bayerite, nordstrandite); aluminum salts and complexes
such as, for example, basic aluminum carboxylates (e.g., basic
aluminum carboxylates of the general formula
Al(OH).sub.y(carboxylate).sub.3-y, where y is between 1 and 2,
preferably between 1 and 1.5, and the carboxylate counterion is
selected from the group consisting of formate, acetate, propionate,
and oxalate, or combinations of these carboxylates, aluminum
formoacetate, and aluminum nitroformoacetate); basic aluminum
nitrates; partially hydrolyzed aluminum alkoxides; and combinations
thereof. Basic aluminum carboxylates can be prepared by digesting
aluminum metal in a solution of the carboxylic acid as described in
U.S. Pat. No. 3,957,598 (Merkl). Basic aluminum nitrates can also
be prepared by digesting aluminum metal in a nitric acid solution
as described in U.S. Pat. No. 3,340,205 (Hayes et al.) or British
Pat. No. 1,193,258 (Fletcher et al.), or by the thermal
decomposition of aluminum nitrate as described in U.S. Pat. No.
2,127,504 (Den et al.). These materials can also be prepared by
partially neutralizing an aluminum salt with a base. The basic
aluminum nitrates have the general formula
Al(OH).sub.z(NO.sub.3).sub.3-z, where z is from about 0.5 to
2.5.
[0033] Suitable boehmites include, for example, those commercially
available under the trade designation "HIQ" (e.g., "HIQ-9015") from
BASF Corp., Florham Park, N.J., and those commercially available
under the trade designations "DISPERAL", "DISPAL", and "CATAPAL D"
from Sasol North America, Houston, Tex. These boehmites or alumina
monohydrates are in the alpha form, and include relatively little,
if any, hydrated phases other than monohydrates (although very
small amounts of trihydrate impurities can be present in some
commercial grade boehmite, which can be tolerated). They have a low
solubility in water and have a high surface area (typically at
least about 180 square meters/gram). Preferred boehmites have an
average crystallite size of less than about 20 nanometers (more
preferably, less than 12 nanometers). In this context, "crystallite
size" is determined by the 120 and 031 x-ray reflections.
[0034] The precursor particles may contain water and/or organic
solvents, especially if they are formed from a sol or gel, or
contained within a slurry. Desirably, the content of volatile
components (e.g., water and/or organic solvents) is minimized so as
to avoid explosive volatilization of the volatile components
resulting in damage to or destruction of the precursor particles
upon contact with the flame. Preferably, the content of such
volatile components in the precursor particles is less than about
10 weight percent, less than about 5 weight percent, or even less
than about 1 weight percent of the precursor particles, although
this is not a requirement.
[0035] In some embodiments, the precursor particles comprise
crushed transitional alumina particles. In some embodiments, the
precursor particles are formed from a sol-gel composition
comprising an alpha alumina precursor, for example, as described
above. The sol-gel composition may be formed into particles, for
example, by processes such as extrusion (cut to length), screen
printing onto a releasable liner, or filling cavities of a mold. Of
these, the latter is typically preferred. As used herein, the term
"sol-gel composition" refers to a colloidal dispersion of solid
particles in a liquid that forms a three-dimensional network of the
solid particles on heating over a period of time, or removal of
some of the liquid. In some cases, gel formation may be induced by
addition of polyvalent metal ions.
[0036] In a typical process sequence involving a sol-gel
composition, a sol-gel composition comprising an alpha alumina
precursor is provided.
[0037] The sol-gel composition should comprise a sufficient amount
of liquid for the viscosity of the sol-gel composition 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, a sol-gel composition
comprises from 2 to 90 weight percent of an alpha alumina precursor
material (e.g., aluminum oxide monohydrate (boehmite)), and at
least 10 weight percent, from 50 to 70 weight percent, or 50 to 60
weight percent, of volatile components such as water. In some
embodiments, the sol-gel composition contains from 30 to 50 weight
percent, or 40 to 50 weight percent of the alpha alumina precursor
material.
[0038] A peptizing agent can be added to the sol-gel composition to
produce a more stable hydrosol or colloidal sol-gel composition.
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 sol-gel composition, 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 sol-gel composition.
[0039] Seed particles and/or crystal grain size modifiers may
optionally be added to the sol-gel composition, but advantageously
they are typically not needed in order to achieve small alpha
alumina crystal grain sizes.
[0040] Examples of optional alumina grain size modifiers include
Li.sub.2O, Na.sub.2O, MgO, SiO.sub.2, CaO, SrO, TiO.sub.2, MnO,
Cr.sub.2O.sub.3, Fe.sub.2O.sub.3, CoO, NiO, ZnO, ZrO.sub.2,
SnO.sub.2, HfO.sub.2, rare earth oxides (e.g., La.sub.2O.sub.3,
CeO.sub.2, Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, Sm.sub.2O.sub.3,
Eu.sub.2O.sub.3, Gd.sub.2O.sub.3, Dy.sub.2O.sub.3, Er.sub.2O.sub.3,
Yb.sub.2O.sub.3, TbO.sub.2, Y.sub.2O.sub.3), combinations thereof,
and precursors thereof. In some embodiments, the precursor
particles, and likewise the derived abrasive particles, are
essentially free of any or all of the alumina grain size modifiers:
Li.sub.2O, Na.sub.2O, MgO, SiO.sub.2, CaO, SrO, TiO.sub.2, MnO,
Cr.sub.2O.sub.3, Fe.sub.2O.sub.3, CoO, NiO, ZnO, ZrO.sub.2,
SnO.sub.2, HfO.sub.2, rare earth oxides (e.g., La.sub.2O.sub.3,
CeO.sub.2, Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, Sm.sub.2O.sub.3,
Eu.sub.2O.sub.3, Gd.sub.2O.sub.3, Dy.sub.2O.sub.3, Er.sub.2O.sub.3,
Yb.sub.2O.sub.3, TbO.sub.2, Y.sub.2O.sub.3), combinations thereof,
and precursors thereof.
[0041] The alpha alumina precursor may be "seeded" with a material
having the same crystal structure as, and lattice parameters as
close as possible to, those of alpha alumina. The "seed" particles
are added in as finely divided form as possible, and are dispersed
uniformly throughout the sol or gel. Seed particles can be added ab
initio or it can be formed in situ. The function of seed particles
is to cause the transformation to the alpha form to occur uniformly
throughout the alpha alumina precursor at a much lower temperature
than is needed in the absence of the seed. Suitable seeds include
alpha alumina itself and also other compounds such as alpha ferric
oxide, chromium suboxide, nickel titanate and a plurality of other
compounds that have lattice parameters sufficiently similar to
those of alpha alumina to be effective to cause the generation of
alpha alumina from a precursor at a temperature below that at which
the conversion normally occurs in the absence of such seed.
Examples of suitable seed particles include particles of
Ti.sub.2O.sub.3, MgO.TiO.sub.2, FeO.TiO.sub.2, NiO.TiO.sub.2,
CoO.TiO.sub.2, MnO.TiO.sub.2, ZnO.TiO.sub.2, V.sub.2O.sub.3,
Ga.sub.2O.sub.3, Rh.sub.2O.sub.3, alpha-Al.sub.2O.sub.3,
alpha-Cr.sub.2O.sub.3, and alpha-Fe.sub.2O.sub.3 particles,
preferably having an average particle size of from about 10 nm to
about 120 nanometers, although other sizes may be used. In some
embodiments, the precursor particles, and likewise the derived
abrasive particles, are essentially free of seed particles such as,
for example, alpha-Al.sub.2O.sub.3 seed particles,
alpha-Cr.sub.2O.sub.3 seed particles, or alpha-Fe.sub.2O.sub.3 seed
particles.
[0042] The sol-gel composition 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 and/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, and/or
coupling agents can be added if desired.
[0043] Next, the sol-gel composition may be dried and crushed, or
if shaped precursor particles are desired the sol-gel composition
may be used to fill one or more cavities of a mold. The mold can
have a generally planar bottom surface and a plurality of mold
cavities, which may be 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, poly(vinyl chloride), polyolefins, polystyrene,
polypropylene, polyethylene, combinations of the foregoing, and
thermosetting materials. In one embodiment, the 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, comprise
a polymeric material while other portions of the tooling can be
made from other materials. A suitable coating may be applied to a
metal tooling to change its surface tension properties by way of
example.
[0044] 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. Desirably, cavities in the production tooling have a draft
angle of from 5 to 15 degrees to facilitate removal of the shaped
precursor particles from the production tool, for example, as
described in U.S. Pat. No. 8,142,531 (Adefris et al.). More
information concerning the design and fabrication of production
tooling or master tools can be found in U.S. Pat. No. 5,152,917
(Pieper et al.); U.S. Pat. No. 5,435,816 (Spurgeon et al.); U.S.
Pat. No. 5,672,097 (Hoopman et al.); U.S. Pat. No. 5,946,991
(Hoopman et al.); U.S. Pat. No. 5,975,987 (Hoopman et al.); and
U.S. Pat. No. 6,129,540 (Hoopman et al.).
[0045] Exemplary suitable cavity shapes include triangles, 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. Exemplary suitable cavity
shapes include truncated cones and pyramids (e.g., three-sided,
four-sided, five-sided, or six-sided truncated pyramids). The depth
of a given cavity can be uniform or can vary along its length
and/or width. The cavities of a given mold (e.g., master tool or
production tool) can be of the same shape or of different
shapes.
[0046] Cavities in the mold may be at least partially (preferably
completely) filled with the sol-gel composition by any suitable
technique. In some embodiments, a knife roll coater or vacuum slot
die coater can be used. A mold release compound can be used to aid
in removing the particles from the mold if desired. Typical mold
release agents include, for example, oils such as peanut oil or
mineral oil, fish oil, silicones, polytetrafluoroethylene, zinc
stearate, and graphite.
[0047] In one embodiment, the top surface of the mold is coated
with the sol-gel composition. The sol-gel composition can be pumped
onto top surface. Next, a scraper or leveler bar is used to force
the sol-gel composition fully into cavities of the mold. The
remaining portion of the sol-gel composition that does not enter
cavity can be removed from top surface of the mold and
recycled.
[0048] Next, volatile components of the sol-gel composition are at
least partially removed to dry the sol-gel composition and form
dried shaped precursor particles. Desirably, the volatile
components are removed at a fast evaporation rates. In some
embodiments, removal of the volatile component by evaporation
occurs at temperatures above the boiling point of the volatile
component. The upper limit to the drying temperature often depends
on the material the mold is made from.
[0049] Dried shaped precursor particles 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. If desired,
the dried shaped precursor particles can be further dried outside
of the mold. Typically, the precursor shaped abrasive particles
will be dried for 10 to 480 minutes at a temperature from 50 to
160.degree. C.
[0050] Optionally, but preferably, the dried shaped precursor
particles are calcined at a temperature of from 500 to 800.degree.
C. for sufficient time (e.g., several hours) to remove bound water
and increase durability in handling. This results in calcined
shaped precursor particles.
[0051] Further details regarding sol-gel compositions comprising
alpha alumina precursor material, including methods for making them
and converting them into suitable precursor particles (e.g., shaped
or crushed), can be found, for example, in U.S. Pat. No. 4,314,827
(Leitheiser et al.); U.S. Pat. No. 4,623,364 (Cottringer et al.);
U.S. Pat. No. 4,744,802 (Schwabel); U.S. Pat. No. 4,770,671 (Monroe
et al.); U.S. Pat. No. 4,881,951 (Wood et al.); U.S. Pat. No.
5,011,508 (Wald et al.); U.S. Pat. No. 5,090,968 (Pellow); U.S.
Pat. No. 5,201,916 (Berg et al.); U.S. Pat. No. 5,227,104 (Bauer);
U.S. Pat. No. 5,366,523 (Rowenhorst et al.); U.S. Pat. No.
5,547,479 (Conwell et al.); U.S. Pat. No. 5,498,269 (Larmie); U.S.
Pat. No. 5,551,963 (Larmie); U.S. Pat. No. 5,725,162 (Garg et al.);
U.S. Pat. No. 5,776,214 (Wood); U.S. Pat. No. 8,142,531 (Adefris et
al.); and U.S. Pat. No. 8,142,891 (Culler et al.).
[0052] In some embodiments, the sol-gel composition may be dried
and crushed to form the precursor particles, while in other
embodiments the precursor particles are shaped precursor particles
formed according to one of the methods discussed herein above.
[0053] The precursor particles should generally be sufficiently
small that rapid heating occurs throughout the bodies of the
particles, and also so that any moisture within the interior of the
precursor particles can be rapidly removed without damage to the
resultant abrasive particle. Accordingly, the precursor particles
and/or the resultant abrasive particles should generally be
sufficiently small to pass through a test sieve having 220-micron
nominal sieve openings. Of course, finer particles may also be
used; for example, the precursor particles may be sufficiently
small to pass through a test sieve having a U.S. mesh size of 70
(212-micron nominal sieve openings), 80 (180-micron nominal sieve
openings), 100 (150-micron nominal sieve openings), 120 (125-micron
nominal sieve openings), 140 (106-micron nominal sieve openings),
170 (90-micron nominal sieve openings), 200 (75-micron nominal
sieve openings), 230 (63-micron nominal sieve openings), 270
(53-micron nominal sieve openings), 325 (45-micron nominal sieve
openings), 400 (38-micron nominal sieve openings), or even 500
(25-micron nominal sieve openings). As used herein, the term "test
sieve" refers to a wire mesh test sieve in compliance with ASTM
Test Method E11-09.sup..epsilon.1 entitled "Standard Specification
for Woven Wire Test Sieve Cloth and Test Sieves" (November
2010).
[0054] The precursor particles are next passed through a flame that
serves to convert at least a portion of the alpha alumina precursor
into alpha alumina. In some embodiments, at least 70 weight
percent, at least 80 weight percent, at least 90 weight percent, at
least 95 weight percent, at least 99 weight percent, at least 99.5
weight percent, or even 100 weight percent of the alpha alumina
precursor is converted to alpha alumina by passing it through the
flame. If desired, the precursor particles may be passed through
the flame multiple times to further increase the fraction converted
to alpha alumina.
[0055] Various apparatuses have been devised that are suitable for
practicing the present disclosure. In some embodiments, the
precursor particles are dropped (i.e., they are gravity fed)
through a tube into a flame along its longitudinal axis. As the
precursor particles fall through the flame they are heated and
alpha alumina precursor material is converted to alpha alumina. An
apparatus suitable for practicing this method includes a powder
feeder having a canister (8 cm diameter) at the bottom of which is
a 70 U.S. mesh screen (212-micron opening size) as illustrated in
FIGS. 1-6 and in the specification of U.S. Pat. Appln. Publ.
2005/0132655 A1 (Anderson et al.), wherein the screens are made
from stainless steel (available from W.S. Tyler Inc., Mentor,
Ohio). In use, precursor particles are filled into the canister and
forced through the openings of the screen using a rotating brush.
The flame is provided by a Bethlehem bench burner PM2D Model B
obtained from Bethlehem Apparatus Co., Hellertown, Pa. The burner
has a central feed port (0.475 cm ( 3/16 inch) inner diameter)
through which precursor particles are introduced into the flame.
Hydrogen and oxygen flow rates for the burner can be adjusted for
optimum temperature, which typically may vary with the specific
precursor particles used. The angle at which the flame hit the
water is approximately 90.degree., and the flame length, burner to
water surface, is approximately 38 centimeters (cm). An inert gas
may be mixed with the precursor particles to propel them through
the flame.
[0056] The flame temperature is preferably selected to maximize
(although this is not a requirement) conversion of the precursor
particles and densification of the resultant abrasive particles
while minimizing melting of the alpha alumina. This necessarily
will depend on various parameters such as, for example, particle
size and transit time through the flame. Representative flame
temperatures are in the range of from 1400 to 2700.degree. C.,
preferably from 1600 to 2200.degree. C., although other
temperatures can also be used.
[0057] Although the precursor particles are preferably passed
through the flame substantially along a longitudinal axis of the
flame, other configurations are also possible. For example, the
precursor particles may travel through the flame at an orthogonal
orientation (i.e., traversing the width of the flame).
[0058] Once passed through the flame, the precursor particles are
converted to sintered abrasive particles and collected at the
bottom of the containment vessel.
[0059] If desired, the sintered abrasive particles may be further
sintered by subsequent heating in an oven, for example.
[0060] Alternatively, as the precursor particles become smaller,
they may have a tendency to melt and form spheroidal particles that
are undesirable for abrading applications. In such cases, the size
and temperature of the flame and the contact time of the precursor
particles with the flame should be reduced until useful sintering
conditions are established. The exact conditions will necessarily
vary depending on size, shape, and composition of the precursor
particles and apparatus design, but are within the capability one
having ordinary skill in the art.
[0061] While the precursor particles may be accelerated through the
flame by gravity, it is also possible for them to be propelled, for
example, by a compressed gas as in the general manner of a flame
sprayer apparatus, although care should be taken to use a heating
temperature and residence time such that the precursor particles do
not become molten and/or fuse together in a mass.
[0062] The thus formed abrasive particles comprise alpha alumina
having a cellular microstructure comprising sintered alpha alumina
crystal grains. The alpha alumina crystal grains have a maximum
dimension of less than about 3 microns. In some embodiments, the
alpha alumina crystal grains have a maximum dimension of less than
about 2.5 microns, or even less than about 2 microns.
[0063] In some embodiments, the sintered abrasive particles
comprise at least 50 weight percent, at least 55 weight percent, at
least 60 weight percent, at least 65 weight percent, at least 70
weight percent, at least 75 weight percent, at least 80 weight
percent, at least 85 weight percent, at least 90 weight percent, at
least 95 weight percent, at least 99 weight percent, at least 99.5
weight percent or even at least 99.9 weight percent weight percent
of alpha alumina. Preferably, the sintered abrasive particles
consist essentially of alpha alumina, that is, they are free of
chemical impurities in quantities sufficient to degrade the
hardness of the sintered abrasive particles more than 5 percent, 4
percent, 3 percent, 2 percent, or even more than 1 percent relative
to alpha alumina particles of the same dimensions.
[0064] In some embodiments, the sintered abrasive particles have an
apparent density that is at least 95 (at least 96, at least 97, at
least 97.5, at least 98, 98.5, at least 99, or even at least 99.5)
percent of the theoretical density of alpha alumina.
[0065] Advantageously, at appropriate temperatures methods
according to the present disclosure minimizes the amount of melting
that occurs in the sintered abrasive particles. This means that
shapes of the precursor particles may be at least substantially
retained in the resultant abrasive particles. Thus sharp edges
and/or points in the precursor particles may result in
corresponding sharp edges and/or points. As used herein, the term
"sharp" means having a well-defined form, in contrast to nebulous
melted globular features. Exemplary abrasive particle shapes
include triangular prisms, cylinders, rectangular prisms, square
prisms, hexagonal prisms, star-shaped prisms, truncated cones and
pyramids (e.g., three-sided, four-sided, five-sided, or six-sided
truncated pyramids).
[0066] Abrasive particles 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.
[0067] Exemplary ANSI grade designations (i.e., specified nominal
grades) include: ANSI 80, ANSI 90, ANSI 100, ANSI 120, ANSI 150,
ANSI 180, ANSI 220, ANSI 240, ANSI 280, ANSI 320, ANSI 360, ANSI
400, and ANSI 600. Exemplary FEPA grade designations include P80,
P100, P120, P150, P180, P220, P320, P400, P500, P600, P800, P1000,
and P1200. Exemplary JIS grade designations include JIS80, JIS100,
JIS150, JIS180, JIS220, JIS240, JIS280, JIS320, JIS360, JIS400,
JIS600, JIS800, JIS1000, JIS1500, JIS2500, JIS4000, JIS6000,
JIS8000, and JIS10,000.
[0068] Alternatively, the sintered abrasive particles can be 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 test 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 -80+100 meaning that the sintered
abrasive particles pass through a number 80 test sieve and are
retained on a 100 test sieve. In various embodiments of the present
disclosure, the sintered abrasive particles can have a nominal
screened grade comprising: -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.
[0069] Abrasive particles according to the present disclosure can
be used in combination with other abrasive particles if
desired.
[0070] Abrasive particles according to the present disclosure may
be used in a loose form or slurry, and/or incorporated into
abrasive products (e.g., bonded abrasives, coated abrasives, and
nonwoven abrasives). Criteria used in selecting abrasive particles
used for a particular abrading application typically include:
abrading life, rate of cut, substrate surface finish, grinding
efficiency, and product cost.
[0071] Coated abrasive articles generally include a backing,
abrasive particles, and at least one binder to hold the sintered
abrasive particles onto the backing. The backing can be any
suitable material, including, for example, cloth, polymeric film,
fiber, nonwoven webs, paper, combinations thereof, and treated
versions thereof. Suitable binders include, for example, inorganic
or organic binders (including thermally curable resins and
radiation curable resins). The sintered abrasive particles can be
present in one layer or in two layers of the coated abrasive
article.
[0072] An example of a coated abrasive article is depicted in FIG.
2. Referring to FIG. 2, coated abrasive article 1 has a backing
(substrate) 2 and abrasive layer 3. Abrasive layer 3 includes
fused, polycrystalline ceramic abrasive particles made according to
the present disclosure 4 secured to a major surface of backing 2 by
make coat 5 and size coat 6. In some instances, a supersize coat
(not shown) is used.
[0073] Bonded abrasive articles typically include a shaped mass of
abrasive particles held together by an organic, metallic, or
vitrified binder. Such shaped mass can be, for example, in the form
of a wheel, such as a grinding wheel or cutoff wheel. The diameter
of grinding wheels typically is about 1 cm to over 1 meter; the
diameter of cut off wheels about 1 cm to over 80 cm (more typically
3 cm to about 50 cm). The cut off wheel thickness is typically
about 0.5 mm to about 5 cm, more typically about 0.5 mm to about 2
cm. The shaped mass can also be in the form, for example, of a
honing stone, segment, mounted point, disc (e.g., double disc
grinder) or other conventional bonded abrasive shape. Bonded
abrasive articles typically comprise about 3-50 percent by volume
bond material, about 30-90 percent by volume abrasive particles (or
abrasive particle blends), up to 50 percent by volume additives
(including grinding aids), and up-to 70 percent by volume pores,
based on the total volume of the bonded abrasive article.
[0074] An exemplary grinding wheel is shown in FIG. 3. Referring
now to FIG. 3, grinding wheel 10 is depicted, which includes fused,
polycrystalline ceramic abrasive particles made according to the
present disclosure 11, molded in a wheel and mounted on hub 12.
[0075] Nonwoven abrasive articles typically include an open porous
lofty polymer filament structure having fused, polycrystalline
ceramic abrasive particles made according to the present disclosure
distributed throughout the structure and adherently bonded therein
by an organic binder. Examples of filaments include polyester
fibers, polyamide fibers, and polyaramid fibers. An exemplary
nonwoven abrasive article is shown in FIG. 4. Referring to FIG. 4,
a schematic depiction, enlarged about 100 times, of a typical
nonwoven abrasive article is shown, comprises fibrous mat 150 as a
substrate, onto which fused, polycrystalline ceramic abrasive
particles made according to the present disclosure 152 are adhered
by binder 154.
[0076] Useful abrasive brushes include those having a plurality of
bristles unitary with a backing (see, e.g., U.S. Pat. No. 5,427,595
(Pihl et al.); U.S. Pat. No. 5,443,906 (Pihl et al.); U.S. Pat. No.
5,679,067 (Johnson et al.), and U.S. Pat. No. 5,903,951 (Ionta et
al.)). Desirably, such brushes are made by injection molding a
mixture of polymer and abrasive particles.
[0077] Suitable organic binders for making abrasive articles
include thermosetting organic polymers. Examples of suitable
thermosetting organic polymers include phenolic resins,
urea-formaldehyde resins, melamine-formaldehyde resins, urethane
resins, acrylate resins, polyester resins, aminoplast resins having
pendant .alpha.,.beta.-unsaturated carbonyl groups, epoxy resins,
acrylated urethane, acrylated epoxies, and combinations thereof.
The binder and/or abrasive article may also include additives such
as fibers, lubricants, wetting agents, thixotropic materials,
surfactants, pigments, dyes, antistatic agents (e.g., carbon black,
vanadium oxide, and/or graphite), coupling agents (e.g., silanes,
titanates, and/or zircoaluminates), plasticizers, suspending
agents, and the like. The amounts of these optional additives are
selected to provide the desired properties. The coupling agents can
improve adhesion to the sintered abrasive particles and/or filler.
The binder chemistry may be thermally cured, radiation cured or
combinations thereof. Additional details on binder chemistry may be
found in U.S. Pat. No. 4,588,419 (Caul et al.); U.S. Pat. No.
4,751,138 (Tumey et al.), and U.S. Pat. No. 5,436,063 (Follett et
al.).
[0078] More specifically with regard to vitrified bonded abrasives,
vitreous bonding materials, which exhibit an amorphous structure
and are typically hard, are well known in the art. In some cases,
the vitreous bonding material includes crystalline phases. Bonded,
vitrified abrasive articles made according to the present
disclosure may be in the shape of a wheel (including cut off
wheels), honing stone, mounted pointed or other conventional bonded
abrasive shape. In some embodiments, a vitrified bonded abrasive
article made according to the present disclosure is in the form of
a grinding wheel.
[0079] Examples of metal oxides that are used to form vitreous
bonding materials include: silica, silicates, alumina, soda,
calcia, potassia, titania, iron oxide, zinc oxide, lithium oxide,
magnesia, boria, aluminum silicate, borosilicate glass, lithium
aluminum silicate, combinations thereof, and the like. Typically,
vitreous bonding materials can be formed from composition
comprising from 10 to 100 percent of glass frit, although more
typically the composition comprises 20 to 80 percent of glass frit,
or 30 to 70 percent of glass frit. The remaining portion of the
vitreous bonding material can be a non-frit material.
Alternatively, the vitreous bond may be derived from a non-frit
containing composition. Vitreous bonding materials are typically
matured at a temperature(s) in a range of about 700 to about
1500.degree. C., usually in a range of about 800 to about
1300.degree. C., sometimes in a range of about 900 to about
1200.degree. C., or even in a range of about 950 to about
1100.degree. C. The actual temperature at which the bond is matured
depends, for example, on the particular bond chemistry.
[0080] In some embodiments, vitrified bonding materials include
those comprising silica, alumina (desirably, at least 10 percent by
weight alumina), and boria (desirably, at least 10 percent by
weight boria). In most cases, the vitrified bonding material
further comprises alkali metal oxide(s) (e.g., Na.sub.2O and
K.sub.2O) (in some cases at least 10 percent by weight alkali metal
oxide(s)).
[0081] Binder materials may also contain filler materials or
grinding aids, typically in the form of a particulate material.
Typically, the particulate materials are inorganic materials.
Examples of useful fillers for the present disclosure include:
metal carbonates (e.g., calcium carbonate (e.g., chalk, calcite,
marl, travertine, marble and limestone), calcium magnesium
carbonate, sodium carbonate, magnesium carbonate), silica (e.g.,
quartz, glass beads, glass bubbles and glass fibers) silicates
(e.g.; talc, clays, (montmorillonite) feldspar, mica, calcium
silicate, calcium metasilicate, sodium aluminosilicate, sodium
silicate) metal sulfates (e.g., calcium sulfate, barium sulfate,
sodium sulfate, aluminum sodium sulfate, aluminum sulfate), gypsum,
vermiculite, wood flour, aluminum trihydrate, carbon black, metal
oxides (e.g., calcium oxide (lime), aluminum oxide, titanium
dioxide), and metal sulfites (e.g., calcium sulfite).
[0082] In general, the addition of a grinding aid increases the
useful life of the abrasive article. A grinding aid is a material
that has a significant effect on the chemical and physical
processes of abrading, which results in improved performance.
Although not wanting to be bound by theory, it is believed that a
grinding aid(s) will (a) decrease the friction between the sintered
abrasive particles and the workpiece being abraded, (b) prevent the
sintered abrasive particles from "capping" (i.e., prevent metal
particles from becoming welded to the tops of the sintered abrasive
particles), or at least reduce the tendency of abrasive particles
to cap, (c) decrease the interface temperature between the sintered
abrasive particles and the workpiece, or (d) decreases the grinding
forces.
[0083] Grinding aids encompass a wide variety of different
materials and can be inorganic or organic based. 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 like
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, and magnesium chloride. Examples of metals
include, tin, lead, bismuth, cobalt, antimony, cadmium, and iron
titanium. Other miscellaneous grinding aids include sulfur, organic
sulfur compounds, graphite, and metallic sulfides. It is also
within the scope of the present disclosure to use a combination of
different grinding aids, and in some instances this may produce a
synergistic effect.
[0084] Grinding aids can be particularly useful in coated abrasive
and bonded abrasive articles. In coated abrasive articles, grinding
aid is typically used in the supersize coat, which is applied over
the surface of the sintered abrasive particles. Sometimes, however,
the grinding aid is added to the size coat. Typically, the amount
of grinding aid incorporated into coated abrasive articles are
about 50-300 g/m.sup.2 (desirably, about 80 to 160 g/m.sup.2). In
vitrified bonded abrasive articles grinding aid is typically
impregnated into the pores of the article.
[0085] The abrasive articles can contain 100 percent fused,
polycrystalline ceramic abrasive particles made according to the
present disclosure, or blends of such abrasive particles with other
abrasive particles and/or diluent particles. However, at least
about 2 percent by weight, desirably at least about 5 percent by
weight, and more desirably about 30 to 100 percent by weight, of
the sintered abrasive particles in the abrasive articles should be
fused, polycrystalline ceramic abrasive particles made according to
the present disclosure. In some instances, the sintered abrasive
particles made according to the present disclosure may be blended
with another abrasive particles and/or diluent particles at a ratio
between 5 to 75 percent by weight, about 25 to 75 percent by
weight, about 40 to 60 percent by weight, or about 50 to 50 percent
by weight (i.e., in equal amounts by weight). Examples of suitable
conventional abrasive particles include fused aluminum oxide
(including white fused alumina, heat-treated aluminum oxide and
brown aluminum oxide), silicon carbide, boron carbide, titanium
carbide, diamond, cubic boron nitride, garnet, fused
alumina-zirconia, and sol-gel-derived abrasive particles, and
combinations thereof. The sol-gel-derived abrasive particles may be
seeded or non-seeded. Likewise, the sol-gel-derived abrasive
particles may be randomly shaped or have a shape associated with
them, such as a rod or a triangle. Examples of sol gel abrasive
particles include those described in U.S. Pat. No. 4,314,827
(Leitheiser et al.); U.S. Pat. No. 4,518,397 (Leitheiser et al.);
U.S. Pat. No. 4,623,364 (Cottringer et al.); U.S. Pat. No.
4,744,802 (Schwabel); U.S. Pat. No. 4,770,671 (Monroe et al.); U.S.
Pat. No. 4,881,951 (Wood et al.); U.S. Pat. No. 5,011,508 (Wald et
al.); U.S. Pat. No. 5,090,968 (Pellow); U.S. Pat. No. 5,139,978
(Wood); U.S. Pat. No. 5,201,916 (Berg et al.); U.S. Pat. No.
5,227,104 (Bauer); U.S. Pat. No. 5,366,523 (Rowenhorst et al.);
U.S. Pat. No. 5,429,647 (Larmie); U.S. Pat. No. 5,498,269 (Larmie);
and U.S. Pat. No. 5,551,963 (Larmie). Additional details concerning
sintered alumina abrasive particles made by using alumina powders
as a raw material source can also be found, for example, in U.S.
Pat. No. 5,259,147 (Falz); U.S. Pat. No. 5,593,467 (Monroe); and
U.S. Pat. No. 5,665,127 (Moltgen). Additional details concerning
fused abrasive particles, can be found, for example, in U.S. Pat.
No. 1,161,620 (Coulter); U.S. Pat. No. 1,192,709 (Tone); U.S. Pat.
No. 1,247,337 (Saunders et al.); U.S. Pat. No. 1,268,533 (Allen);
U.S. Pat. No. 2,424,645 (Baumann et al.); U.S. Pat. No. 3,891,408
(Rowse et al.); U.S. Pat. No. 3,781,172 (Pett et al.); U.S. Pat.
No. 3,893,826 (Quinan et al.); U.S. Pat. No. 4,126,429 (Watson);
U.S. Pat. No. 4,457,767 (Poon et al.); U.S. Pat. No. 5,023,212
(Dubots et al.); U.S. Pat. No. 5,143,522 (Gibson et al.); and U.S.
Pat. No. 5,336,280 (Dubots et al.). In some instances, blends of
abrasive particles may result in an abrasive article that exhibits
improved grinding performance in comparison with abrasive articles
comprising 100 percent of either type of abrasive particle. If
there is a blend of abrasive particles, the abrasive particle types
forming the blend may be of the same size. Alternatively, the
abrasive particle types may be of different particle sizes. For
example, the larger sized abrasive particles may be fused,
polycrystalline ceramic abrasive particles made according to the
present disclosure, with the smaller sized particles being another
abrasive particle type. Conversely, for example, the smaller sized
abrasive particles may be fused, polycrystalline ceramic abrasive
particles made according to the present disclosure, with the larger
sized particles being another abrasive particle type.
[0086] Examples of suitable diluent particles include marble,
gypsum, flint, silica, iron oxide, aluminum silicate, glass
(including glass bubbles and glass beads), alumina bubbles, alumina
beads and diluent agglomerates.
[0087] Fused, polycrystalline ceramic abrasive particles according
to the present disclosure can also be combined in or with abrasive
agglomerates. Abrasive agglomerate particles typically comprise a
plurality of abrasive particles, a binder, and optional additives.
The binder may be organic and/or inorganic. Abrasive agglomerates
may be randomly shape or have a predetermined shape associated with
them. The shape may be a block, cylinder, pyramid, coin, square, or
the like. Abrasive agglomerate particles typically have particle
sizes ranging from about 100 to about 5000 microns, typically about
250 to about 2500 microns. Additional details regarding abrasive
agglomerate particles may be found, for example, in U.S. Pat. No.
4,311,489 (Kressner); U.S. Pat. No. 4,652,275 (Bloecher et al.);
U.S. Pat. No. 4,799,939 (Bloecher et al.); U.S. Pat. No. 5,549,962
(Holmes et al.), and U.S. Pat. No. 5,975,988 (Christianson).
[0088] The sintered abrasive particles may be uniformly distributed
in the abrasive article or concentrated in selected areas or
portions of the abrasive article. For example, in a coated
abrasive, there may be two layers of abrasive particles. The first
layer comprises abrasive particles other than fused,
polycrystalline ceramic abrasive particles made according to the
present disclosure, and the second (outermost) layer comprises
fused, polycrystalline ceramic abrasive particles made according to
the present disclosure. Likewise in a bonded abrasive, there may be
two distinct sections of the grinding wheel. The outermost section
may comprise abrasive particles made according to the present
disclosure, whereas the innermost section does not. Alternatively,
fused, polycrystalline ceramic abrasive particles made according to
the present disclosure may be uniformly distributed throughout the
bonded abrasive article. Further details regarding coated abrasive
articles can be found, for example, in U.S. Pat. No. 4,734,104
(Broberg); U.S. Pat. No. 4,737,163 (Larkey); U.S. Pat. No.
5,203,884 (Buchanan et al.); U.S. Pat. No. 5,152,917 (Pieper et
al.); U.S. Pat. No. 5,378,251 (Culler et al.); U.S. Pat. No.
5,417,726 (Stout et al.); U.S. Pat. No. 5,436,063 (Follett et al.);
U.S. Pat. No. 5,496,386 (Broberg et al.); U.S. Pat. No. 5,609,706
(Benedict et al.); U.S. Pat. No. 5,520,711 (Helmin); U.S. Pat. No.
5,954,844 (Law et al.); U.S. Pat. No. 5,961,674 (Gagliardi et al.);
and U.S. Pat. No. 5,975,988 (Christianson). Further details
regarding bonded abrasive articles can be found, for example, in
U.S. Pat. No. 4,543,107 (Rue); U.S. Pat. No. 4,741,743 (Narayanan
et al.); U.S. Pat. No. 4,800,685 (Haynes et al.); U.S. Pat. No.
4,898,597 (Hay et al.); U.S. Pat. No. 4,997,461 (Markhoff-Matheny
et al.); U.S. Pat. No. 5,037,453 (Narayanan et al.); U.S. Pat. No.
5,110,332 (Narayanan et al.); and U.S. Pat. No. 5,863,308 (Qi et
al.). Further details regarding vitreous bonded abrasives can be
found, for example, in U.S. Pat. No. 4,543,107 (Rue); U.S. Pat. No.
4,898,597 (Hay et al.); U.S. Pat. No. 4,997,461 (Markhoff-Matheny
et al.); U.S. Pat. No. 5,094,672 (Giles Jr. et al.); U.S. Pat. No.
5,118,326 (Sheldon et al.); U.S. Pat. No. 5,131,926 (Sheldon et
al.); U.S. Pat. No. 5,203,886 (Sheldon et al.); U.S. Pat. No.
5,282,875 (Wood et al.); U.S. Pat. No. 5,738,696 (Wu et al.), and
U.S. Pat. No. 5,863,308 (Qi). Further details regarding nonwoven
abrasive articles can be found, for example, in U.S. Pat. No.
2,958,593 (Hoover et al.)
[0089] The present disclosure further provides a method of abrading
a surface. The method comprises contacting at least one abrasive
particle, according to the present disclosure, with a surface of a
workpiece; and moving at least one of the sintered abrasive
particles or the contacted surface to abrade at least a portion of
the surface with the abrasive particle. Methods for abrading with
abrasive particles made according to the present disclosure range
from snagging (i.e., high pressure high stock removal) to polishing
(e.g., polishing medical implants with coated abrasive belts),
wherein the latter is typically done with finer grades of abrasive
particles. The sintered abrasive particles may also be used in
precision abrading applications, such as grinding cam shafts with
vitrified bonded wheels. The size of the sintered abrasive
particles used for a particular abrading application will be
apparent to those skilled in the art.
[0090] Abrading with abrasive particles according to the present
disclosure may be done dry or wet. For wet abrading, the liquid may
be introduced in the form of a light mist to complete flood.
Examples of commonly used liquids include: water, water-soluble
oil, organic lubricant, and emulsions. The liquid may serve to
reduce the heat associated with abrading and/or act as a lubricant.
The liquid may contain minor amounts of additives such as
bactericide, antifoaming agents, and the like.
[0091] Abrasive particles made according to the present disclosure
may be useful, for example, to abrade workpieces such as aluminum
metal, carbon steels, mild steels, tool steels, stainless steel,
hardened steel, titanium, glass, ceramics, wood, wood-like
materials (e.g., plywood and particle board), paint, painted
surfaces, organic coated surfaces and the like. The applied force
during abrading typically ranges from about 1 to about 100
kilograms.
SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE
[0092] In a first embodiment, the present disclosure provides a
method of making sintered abrasive particles, the method
comprising:
[0093] passing precursor particles through a flame under conditions
such that they are converted into the sintered abrasive particles,
wherein the precursor particles comprise a precursor of alpha
alumina and wherein the precursor particles have an average
particle size of less than or equal to 500 microns.
[0094] In a second embodiment, the present disclosure provides a
method according to the first embodiment, wherein the sintered
abrasive particles comprise alpha alumina having a cellular
microstructure, and the alpha alumina has an areal porosity of less
than or equal to 5 percent, as measured using image analysis by
cross-section at 10,000 times magnification.
[0095] In a third embodiment, the present disclosure provides a
method according to the first or second embodiment, wherein the
precursor particles are essentially free of seed particles.
[0096] In a fourth embodiment, the present disclosure provides a
method according to any one of the first to third embodiments,
wherein the precursor particles are essentially free of alpha
alumina grain size modifiers.
[0097] In a fifth embodiment, the present disclosure provides a
method according to any one of the first to fourth embodiments,
wherein the precursor particles are accelerated through the flame
by gravity substantially along a longitudinal axis of the
flame.
[0098] In a sixth embodiment, the present disclosure provides a
method according to any one of the first to fifth embodiments,
wherein the sintered abrasive particles comprise shaped abrasive
particles.
[0099] In a seventh embodiment, the present disclosure provides a
method according to any one of the first to sixth embodiments,
wherein the precursor particles have a shape corresponding to a
mold cavity used to shape it.
[0100] In an eighth embodiment, the present disclosure provides a
method according to any one of the second to seventh embodiments,
wherein the cellular microstructure comprises alpha alumina crystal
grains, and wherein the alpha alumina crystal grains have a maximum
dimension of less than about 3 microns.
[0101] In a ninth embodiment, the present disclosure provides a
method according to any one of the first to eighth embodiments,
wherein the sintered abrasive particles consist essentially of
alpha alumina.
[0102] In a tenth embodiment, the present disclosure provides
sintered abrasive particles having a cellular microstructure
comprising alpha alumina crystal grains of alpha alumina having a
maximum dimension of less than about 3 microns, wherein the
sintered abrasive particles have an average particle size of less
than or equal to 500 microns, and wherein the sintered abrasive
particles are essentially free of seed particles and alpha alumina
grain size modifiers.
[0103] In an eleventh embodiment, the present disclosure provides
sintered abrasive particles according to the tenth embodiment,
wherein the alpha alumina has an areal porosity of less than or
equal to 5 percent, as measured using image analysis by
cross-section at 10,000 times magnification.
[0104] In a twelfth embodiment, the present disclosure provides
sintered abrasive particles according to the tenth or eleventh
embodiment, wherein the sintered abrasive particles comprise shaped
abrasive particles.
[0105] In a thirteenth embodiment, the present disclosure provides
sintered abrasive particles according to any one of the tenth to
twelfth embodiments, wherein at least a portion of the sintered
abrasive particles comprises truncated pyramids.
[0106] In a fourteenth embodiment, the present disclosure provides
an abrasive article comprising a binder and a plurality of abrasive
particles, wherein at least a portion of the sintered abrasive
particles are sintered abrasive particles according to any one of
the tenth to thirteenth embodiments.
[0107] In a fifteenth embodiment, the present disclosure provides
an abrasive article according to the fourteenth embodiment, wherein
the abrasive article is a bonded abrasive article, a non-woven
abrasive article, or a coated abrasive article.
[0108] Objects and advantages of this disclosure are further
illustrated by the following non-limiting examples, but 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.
EXAMPLES
[0109] Unless otherwise noted, all parts, percentages, ratios, etc.
in the Examples and the rest of the specification are by
weight.
[0110] In the Examples section below, X-Ray diffraction analysis
was used to determine the presence of alpha-alumina where
indicated.
Preparation of Shaped Calcined Boehmite Particles
[0111] A sample of boehmite sol-gel was made using the following
recipe: aluminum oxide monohydrate powder (1600 parts) available as
DISPERAL from Sasol North America, Inc., Houston, Tex. was
dispersed by high shear mixing a solution containing water (2400
parts) and 70 weight percent 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 polypropylene
tooling having equilateral triangular shaped mold cavities with a
dimension of 100 microns on a side and a depth of 25 microns. The
sol-gel was forced into the cavities with a putty knife so that the
openings of the production tooling were completely filled. Prior to
filling the cavities, a mold release agent, 1 percent by weight
peanut oil in methanol, was used to coat the production tooling
with about 0.5 mg/in.sup.2 (0.08 mg/cm.sup.2) of peanut oil. Excess
methanol was removed by placing sheets of the production tooling in
an air convection oven for 5 minutes at 45.degree. C. The sol-gel
coated production tooling was placed in an air convection oven at
45.degree. C. for at least 45 minutes to dry. Dried shaped sol-gel
particles were removed from the production tooling by passing it
over an ultrasonic horn, and then calcined at approximately
650.degree. C. resulting shaped calcined boehmite particles.
Example 1
[0112] Shaped calcined boehmite particles prepared according to the
Preparation of Shaped Calcined Boehmite Particles (above) were
graded using test sieves to retain the -140+200 mesh (i.e., the
fraction collected between 106-micron opening size and 75-micron
opening size test sieves). The resulting screened particles were
fed slowly (about 0.5 gram/minute) using a vibratory feeder into a
funnel which fed a hydrogen/oxygen/argon (in a respective ratio of
18/15/0) torch flame which heated the calcined particles and
carried them directly into a 19-liter (5-gallon) rectangular metal
container (41 centimeters (cm) by 53 cm by 18 cm height) that
effectively quenched the resultant sintered abrasive particles. The
torch was a Bethlehem bench burner PM2D Model B obtained from
Bethlehem Apparatus Co., Hellertown, Pa. The torch had a central
feed port ( 3/16 inch (0.475 cm) inner diameter) through which the
feed particles were introduced vertically downward into the flame
as a mixture with argon gas along its longitudinal axis. The angle
at which the flame hit the metal container was approximately
90.degree., and the flame length, from burner to container surface,
was approximately 38 centimeters (cm). The resulting shaped
sintered abrasive particles were collected for analysis.
[0113] The resulting sintered particles were collected and a sample
was mounted and polished. Surprisingly, the resulting observed
microstructure, shown in FIG. 5, was densified and had
non-vermicular cells approximately ten microns in size. The
boundaries between cells were cracked.
Comparative Example A
[0114] Example 1 was repeated, except that the shaped calcined
boehmite particles were heated in a box kiln at 1200.degree. C. for
one hour prior to being introduced to the torch flame. Using this
firing condition, the shaped calcined boehmite particles were
completely converted to alpha alumina with minimal densification.
This material was subsequently fired using the flame firing
procedure. The resulting fired particles were collected and a
sample was mounted and polished. The microstructure of the
resulting materials was totally vermicular in character as shown in
FIG. 6. The cells of the particles were porous in character, yet
there was no cracking along the cell boundary, totally opposite to
that observed in Example 1.
Examples 2-7
[0115] Example 1 was repeated, except that the mixture of gases was
adjusted as reported in Table 1. The angle at which the flame hit
the metal container was approximately 90.degree., and the flame
length (including the post-combusting hot gas tail), from burner to
container surface, was approximately 70 centimeters (cm). The
resulting shaped sintered abrasive particles were collected for
analysis. An optical pyrometer to measure the temperature of the
particles exiting the flame. Representative microstructures for
Examples 2-7 are shown in FIGS. 7-12, respectively.
Comparative Example B
[0116] Comparative Example B was prepared identically to Example 2,
except that the shaped calcined boehmite particles were fired in a
box kiln at 1400.degree. C. for 20 minutes, whereby they were
converted to alpha alumina. Representative microstructure of
Comparative Example B is shown in FIG. 13.
Example 8
[0117] Example 1 was repeated, except that the torch used a gas
mixture composed of 16 parts hydrogen, 16 parts oxygen and 2 parts
argon. This is the gas flow mixture that produced the lowest
temperature flame that successfully produced alpha alumina grain as
determined by x-ray diffraction analysis. A sample of the shaped
alpha alumina abrasive grain was mixed with a mounting resin
(Transparent Thermoplastic Powder 165-10005 available from Allied
High Tech Products, Rancho Dominguez, Calif.) and polished using
diamond slurries to an optically smooth surface in order to
evaluate cross-sections of the fired grain by backscattered
electron imaging using scanning electron microscopy (SEM) at 10,000
times magnification.
Example 9
[0118] Example 8 was repeated, with the exception that the torch
conditions used fired through the torch using a gas mixture
composed of 16 parts hydrogen, 16 parts oxygen and 3 parts argon.
The porosity of Example 8 was compared against Example 9 that was
fired at a lower flame temperature and was significantly more
porous than Example 8. Photographs of the two examples are shown in
FIGS. 14 and 15, respectively.
[0119] In FIGS. 14 and 15, dark spots indicate pores and dark
fissures indicate cracks. The amount of porosity in the samples was
measured using ImageJ software available as free software from the
U.S. National Institute of Health, Bethesda, Md. The porosity of
each sample was evaluated using the software and was measured to be
2.8 percent of the area in the photomicrographs of the fired
abrasive particles and 12.0 percent of the area in the
photomicrographs of the incomplete fired grain. This demonstrates
that densification of alpha alumina derived from boehmite can be
achieved without the use of modifiers or seed.
TABLE-US-00001 TABLE 1 TEMPERATURE OF PARTICLES EXITING FLAME
FIGURE NO. BY OPTICAL SHOWING PARTS PARTS PARTS PYROMETER, MICRO-
EXAMPLE HYDROGEN OXYGEN ARGON .degree. C. STRUCTURE 2 18 15 1 960 7
3 18 15 2 940 8 4 18 15 3 900 9 5 18 15 4 880 10 6 18 15 5 850 11 7
18 14 5 unable to be 12 measured* Comparative na** na na [box kiln
at 1400.degree. C.] 13 Example B 8 16 16 2 unable to be 14
measured* 9 16 16 3 unable to be 15*** measured* *insufficient
optical emission from the particles exiting the flame **"na" means
not applicable ***optical examination of the particles indicated
that very few of the particles had been converted to
alpha-alumina.
[0120] Because of the inherent nature of how the particles fall
through the flame, there was significant variability in the
microstructure observed in the Examples. Again, each Example
contained numerous spherical particles indicating that the
particles see a much higher temperature than measured. However the
fraction of spherical particles obtained on firing observed became
smaller as the measured temperature became cooler.
[0121] 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.
* * * * *