Note: Descriptions are shown in the official language in which they were submitted.
~s~
1 SUPERIOR HIGH SODIUM ~ND C~LCIUM SOL GEL ABRA5IVE
AND PROCESS FOR ITS PRODUCTION
TECHNICAL FIELD
This invention relates o abrasive grains and mor@ par-
ticularly relates to a sintered type of abrasive grain.
BACKGROUND ART
In the prior art, abrasive grains, especially abrasive
grains comprising metal oxldes su~h as alumina, were tradi-
tionally made by fusion of the oxide ~ollowed by eru~hing the
cooled ~used oxide to ~orm the abrasive grain~
More recently, abrasive srains have been manufactured
by sintering the metal oxide, such as alumina. Such grain~O
while wearin~ reasonably well, still do not wear as long as
desirable and in addition, do not cut a~ rapidly as de~irable.
It is known that ceramic materials whieh may be in the
for~ of ab~asive products can be prepared b~ binding cera~ic
oxide particles together by using a mineral colloidal gel ~see
U.S~ Patent 2,455,358). It is also known that cvlloidal disper-
--1--
~9S~34~
1 sions can be gelled, dried and calcined to form porous ceramic
materials tsee eOg., U.S. Patent 4,181,532~.
It has also been known that free flowing spheres of
pure or mixed oxides could be made by dispersing an oxide hydrate
followed by forming a gel in the shape of spheres followed by
sintering (see "Application of Sol-Gel Processes ts Industrial
Oxides", January 13, 1968, Chemistry and Industry3.
It has also been known by others in the United States,
at least as early as lg71, that such particles in spherical or
1~ even angular shapes could be used as abrasives. Such prior
knowledge by others in the United States is evidenced by a report
from the United Ringdom Atomic Energy Authority at Harwell,
England, to Carborundum Company, Ltd. and subsequently to several
personnel at The Carborundum Company in the United States.
Recently, U.S. Patent No. 4,314,827 issued for a non-
fused aluminum oxide-based abrasive mineral, a process for its
production and abrasive products comprising the abrasive mineral.
U.S. Patent No. 4,314t827 generally discloses that an
abrasive mineral having randomly oriented crystallites with
diameters on the order of 3000 Angstroms or less can be made by
gelling a colloidal dispersion or hydrosol of alumina and a
modifying component, followed by firing the g21. The disclosure
makes it clear that the mineral must be free (less than about
0.05 total weight percent) of calcium and alkali metal. Due to
this requirement, complex purification processes must be used
when calcium and alkali metal are present, as they often arel in
the alumina or modifying component. Calcium, for example, is
--2--
i
~s~
1 usually present in commercial water supplies and in magnesium
containing modifying components unless costly and complicated
purification steps are used to remove the calcium. Similarly,
sodium i5 commonly present in many aluminas unless removed by
additional purifica~ion steps. There is no suggestion in the
U.S. patent of any process which would permit the presence of
calcium or alkali metal and no process was actually used with
high calcium or alkali metal which inherently permitted the
presence of high calcium or alkali metal~
DISCLOSURE OF THE INVENTION
In accordance with the present invention, a process is
provided for forming ceramic particles from a gelled dispersion
~sol gel~, by drying and sintering the gel. The particles of the
invention may be used for any suitable purposeO Such particles
may be used in any application where temperature resistance,
strength, hardness~ wear resistance and inertness are desirable.
Such particles may, for example, be used as fillers, in
aggregates, in distillation columns or due to the surface
porosity of some of such particles t may be used as catalyst
supports. The most common use for such particles is as
abrasives. Such par~icles will therefore be referred to herein
as abrasives, although it is to be understood that the term
abrasive particle or abrasive grain is intended to include such
particles regardless of their intended end use~
Even though the abrasive grain product obtained by the
process of this invention usually contains greater than about
0.05 weight percent sodium plus calcium, it is greatly superior
to traditional fused alumina abrasive. High sodium and calcium
3~
~ ~s~
1 in the grain is permitted due to rapid heating of the dried sol
gel through a ~ritical temperature range of from below about
800C to above about 1200C. The grain is then sintered a~ a
temperature above about 1200~C.
The preferred process comprises preparing a dispersion
comprising from about 2 to about 60 weight percent aluminum o~ide
monohydrate; a dissolved or dispersed metal containing sintering
aid in an atomic ratio of metal to aluminum of from 1:2 to 1:35,
preferably from 1:7 to 1:25; and from above about 0,05 to about
lo 1.8 percent sodium plus calcium by weight of dispersed and
dissolved metal containing solids in the dispersion.
The weight percent calcium plus the weight percent
sodium is preferably less than 0.6, and more preferably less than
0.15. The weight percent calcium can range from 0 to about 1.8
and the weight percent sodium can range from about 0 to about
0.4. The weight percent calcium is preferably from 0 to about
0.6 and more preferably from 0 to about 0.15 and the weight
percent sodium is preferably from 0 to about 0.25 and more
preferably from 0 to about O~lo
After the dispersion is prepared, it is gelled and
dried at a temperature below the frothing temperature of the gel
to remove free water. Any suitable drying method known to those
skilled in the art may be used. "Drying" as used herein means
dewatering by any method including solvent extraction. The dried
solid is then crushed to form grains. The grains are then
usually heated to between about 500 and 800C. The grains are
then rapidly heated to above about 1200C in less than 10 and
preferably less than 5 minutes. The grains then continue to be
--4--
~9x~
1 heated at a sintering temperature between about 1200C and about
1650C for a suficient sintering time to sinter the grains to a
density above about 85% of theoretical density, at leas~ a por-
tion of the heating at sintering temperature preferably occurring
above 1300C.
The resulting grain is a sintered sol gel abrasive
which has a combined sustained cutting rate and wear resistance
against carbon steel which is substantially better than prior art
fused alumina grain, and which is comparable to prior art sin-
tered sol gel abrasive grains which require low sodium and lowcalcium (less than about 0.05 weight percent) as described in U.
SO Patent No~ 4,314,827. "Substantially better" means a relative
cutting performance at least 1.5 times better and usually at
least two times better.
The invention also includes bonded, coated and nonwoven
abrasive articles comprising the novel grain. For example, not
only do coated abrasives manufactured from the preferred abrasive
grain of the invention continue to have a high cutting rate until
the grain is worn from the backing~ but the total quantity of
material cut by the abrasive article is superior to most prior
art grains.
BEST MODE FOR CARRYING OUT THE lNv~ ION
In accordance with the process of the invention, the
grain is prepared from a liquid (preferably water~ dispersion
comprising from about 2 to about 60, usually from about 10 to
about 40 weight percent aluminum oxide monohydrate (AlOOH~; a
dissolved metal containing sintering aid at an a~omic ratio of
metal in the sintering aid to aluminum of from 1:2 to 1:35; and
~L95~
1 up to about 1.8 and usually from above about 0.05 to about 1.8
percent combined alkali metal (espPcially sodium) and calcium by
weight of dispersed and dissolved solids in ~he dispersion.
The weight percent calcium plus the weigh~ percent
sodium is preferably less ~han 0.6, and more preferably less than
.15. The weight percent calcium can range from 0 to about 1.8
and the weigh~ percent sodium can range from about 0 to about .4.
The weight percent calcium is preerably from 0 to about 0.6 and
more preferably from 0 to about 0.1~ and the weight percent
lo sodium is preferably from 0 to about .25 and more preferably from
0 to about .1~
The quantity of aluminum oxide monohydrate in the
dispersion is usually from about 10 to about 40 and preferably
from about 15 to about 35 percent by weight of the dispersion.
Aluminum oxide monohydrate as used herein is intended to include
aluminum oxide hydrates having the stochiometric formula
~(A1203-XH20) where x is 0.5 to 3. Aluminum oxide monohydrate is
also known as boehmite. Aluminum oxide monohydrate, as used
herein, is also intended to include~ without limitation, pseudo
boehmite.
In general, all solids in the dispersion are preferably
dissolved or in colloidal form. The dispersion is formed by any
suitable means which may simply be mixing of ~he aluminum oxide
monohydrate with water. The liquid is almost always water but
may be another liquid such as low molecular weight alcohoI. Any
suitable mixing apparatus may be used including both high and low
shear mixers. Dispersing aids such as acid are frequently
employed. For example, from about 0~02 to 0.25 and preferably
--6--
~ 1~5~34~
1 0.05 to about 0.15 mole of HNO3 or other volatile mineral acid
per mole of aluminum oxi~e monohydrate greatly aids dispersion.
"Volatile mineral acid" means an acid which will vaporize from
the dispersion, gel or dried gel at a temperature below sintering
temperature or all of whose residues will either so vaporize or
form a part of an oxide within the finished grain. Examples of
such acids are nitric, hydrochloric, acetic and formic acids.
The solids in the finished dispersion may comprise up
to 50 weight percent of additional ingredients other than alumi-
lo num oxide monohydrate, preferably compounds such as silica,magnesia, chromia and titanium dioxide in colloidal or dissolved
form, or precursors of such compounds in colloidal or dissolved
form.
The dissolved or dispersed mQtal containing sintering
aid is added to the dispersion in an atomic ratio of metal to
aluminum of from 1:2 to 1:35 and preferably at a ratio of rom
1:7 to 1-25. The sintering aid is a dispersible or scluble metal
oxide or metal oxide precursor, i.e., a compound which will form
a metal oxide during drying, calcining or sintering. In general,
the sintering aid is usually a precursor of magnesium oxide, zinc
oxide, cobalt oxide or nickel oxide and is therefore, when the
liquid is water, a water soluble or dispersible compound of
magnesium, zinc, nickel or cobaltc Specific examples of such
precursors are the nitrates and chlorides of thos~ metals. The
nitrates of those metals and especially of magnesium are par-
ticularly preferred. The sintering aid may be prepared in situ,
for example by adding magnesium oxide or hydroxide to an aqueous
solution of inorganic acid such as hydrochloric or nitric acids
--7--
5~
to form a water soluble salt of magnesium. The sintering aid is
usually a water soluble salt but may be a water soluble base.
The sodium and calcium in the dispersion usually
results naturally from calcium impurities in other components of
the dispersion. The usual source of sodium is from alumina. The
usual source of calcium is from impuri~ies in magnesium con-
taining sintering aid or water. "Calcium" and i'sodium", as used
herein, mean chemically bound calcium and sodium, which due to
the electropositive nature of calcium and sodium, are almost
lo always calcium or sodium ion, i~e., free calcium or sodium ions
or ionically bound sodium or calcium.
In accordance with the process of the invention,
relatively high amounts of calcium and sodium can be tolerated in
the dispersion, thus complicated and expensive purification steps
are avoided. In accordance with the invention, up to about 1.8
percent combined sodium and calcium can be present in the disper-
sion and a finished abrasive grain superior to traditional fused
alumina grain will still be obtained. Preferably no more than
~.6 percent combined sodium and calcium is present~ The percent
2~ sodium and calcium as above described is by weight of dispersed
and dissolved metal containiny solids in the dispersion. The
percentages are essentially the same as the percentages in the
finished abrasives.
After the di~persion is prepared, it is gelled. The
addition of the dissolved or dispersed metal containing sintering
aid, preferably magnesium nitrate, to the dispersion usually
serves to gel the dispersion. If the solids content of the
dispersion is quite low, the liquid may either have to be
--8--
~ ~58~8
1 vaporized in order for the dispersion to gel or another method of
gelling may have to be employed, i.e , the addition of a gelling
agent.
After the dispersion is gelled, it is dried at a tem-
perature below the frothing temperature of the gel to vaporize
free water. "Dried" or "drying", as used herein, means that at
least 90~ of free (unbound) water is removed to form a solid.
The "frothing temperature" is the temperature at which the gel
will foam or froth a~ the pressure applied to the gelO Drying
lo may be accomplished by any means known to those skilled in the
art. When heat is used, the drying temperature is usually from
about 80 to about 120C. The drying time depends upon the quan-
tity of water or other liquid present, upon drying pressure, and
upon drying temperature. The drying time is usually from about 1
to about 72 hours at atmospheric pressure. The resulting dried
gel i5 usually, but not necessarily, a translucent solid, i.e., a
solid through which light will pass in diffused form.
After the solid is dry, it is crushed or broken by any
suitable means such as a hammer or ball mill to form particles or
grains. Any suitable method fvr comminuting the solid may be
used and "crushing" is intended to include all such methods.
After crushing, the grains are usually heated to a tem-
perature between about 500C and about 800C until essentially
all water is removed and until all componen~s of the grains are
either in the form of ceramics (usually metal oxides~ or else are
vaporized. When the grains reach a tempera~ure between about
250C and about 300C, the acid residues are driven off. Between
about 250C and about 300C, and usually between about 300C and
_g
?
~9S~
1 600C, essentially all water is usually removed~ nEssentially
all", as used in this context~ means that all free water and over
90 percent of bound water i5 removedO The calcining time to
remove essentially all water is usually from about 5 to about 20
minutes.
After drying and after calcining, the grains are
rapidly heated to above about 1200C. "Rapidly heated" means
sufficiently fast to permit formation of an abrasive having a
good cutting rate and good wear resistance. The rapid heating
lo usually occurs in less than 10, preferably in less than 5 and
most preferably in less than 1 minute. This rapid heating step
permits the formation of a superior abras~ve grain from a gelled
dispersion (sol gel) containing relatively high amounts of
calcium and sodium. Any suitable means or method for rapidly
heating the grains may be used such as injection of the grains in
other than bulk form, i.e., separately, into a furnace preheated
to above 1200~C and preferably to above 1300C.
It is possible to eliminate the calclning step and
instead rapidly heat the grains to above ahout 1200C after
drying. This particular procedure is particularly desirable for
producing certain type abrasives.
After the grains are rapidly heated to above 1200C,
they continue to be heated at a sintering temperature between
about 1200C and about 1650C and preferakly between 1250C and
1500C for a sufficient sintering time to sinter the grains to a
density above about 85% of~theoretical density. In the case
where the abrasive is primarily aluminum oxide with about 6%
magnesium oxide by weight of aluminum oxideJ the desired densi~y
--10--
5~
1 is above about 3.3 grams per cubic centimeter. At least a por-
tion of the heating usually occurs above 1300C~ The sufficient
time to sinter the grain depends on sintering temperature and is
usually from abou~ 5 to abou~ 30 minutes but may be less than
about 5 minutes, e.g., from about 1 to about 5 minutes.
When looking at polished ~hin sec~ions of sintered
grain manufactured in accordance with the process of the
invention in a transmission optical microscope at about 500X or
700X with crossed polarizers, one observes the microstructure of
lo material with a nominal composition of alumina-6~ magnesia to
consist of areas of from about 5,000 up to about 200,000
Angstroms in nominal diameter which extinguish as a unit upon
sample rotation. The extinction is believed to result, in this
instance, from the birefringent alpha alumina phase which is pre-
dominant~ In order for these areas to extinguish as a unit, it
is believed that they have to be either continuous alpha alumina
grains or areas of smaller non-randomly oriented alumina grains.
If the alpha alumina grains either have a diameter ranging from
about 5,000 Angstroms to about 200,000 Angstroms or have much
smaller diameters but are non-randomly orîented, the weight per-
cent combined calcium plus sodium in the grains may be less than
.05.
5~
1 The microstructure of the alumina grain of the inven-
tion differs markedly from that o~ normal sintered or fused alu-
mina grain of a similar composition in the degree of homogeneity
of distribution of metal oxide sintering aid. On firing pre-
viously calcined material, it is believed that a transformation
from an atomistically homogeneous distribution of magnesia in
gamma alumina transforms ~o a microscopically homogeneous inti-
mate mixture of alumina and spinel.
Normal sintering of abrasive grains includes con-
solidation by either a diffusional mechanism of preexistin~3 alpha
alumina crystals or a liquid phase densification mechanism.
Sin~ering of sol sel abrasives may include a displasive poly-
morphic transformation of gamma alumina to alpha alumina and
spinel involving minimum di~fusion. This unique transformation
normally results in a marked reduction in sintering temperature.
The invention fur~her includes bonded, coated and
non-woven abrasive articles comprising the abrasive grain of the
invention. In general, the abrasive grain of the invention may
be described as a sintered abrasive grain comprising alumina, a
metal oxide which aids sintering, and up to about 1.8 and usually
~`rom above about 0.05 to about 1.8, preferably to about 0.6 and
-lla-
9~
l most preferably ~o about 0.15 weight percent combined calcium
plus sodium.
As previously discussed, the preferred sintered grains
manu~actured in accordance with ~he process of the invention,
have excellent wear characteristics and in addition, maintain a
high cutting ra~e and remove larger quantities of carbon steel
stock than most prior art grains. High sodium and calcium con-
taining prior ar~ grains do not have a combination of cutting
rate and wear resistance as high as the best abrasive grains
lo manufactured in accordance with the present invention.
As previously discussed, ~he sintered sol gel abrasive
of the invention has a relative cutting performance on coated
discs against carbon steel which is better than fused alumina
abrasive. The preferred sol gel abrasives of the invention have
such a cutting rate which is at least two and usually at least
three to four times better than traditional fused alumina abra-
sive. Relative cutting performance is determined as defined and
described in the following examples.
The following examples serve to illustrate and not
limit the present invention.
EXAMPLE 1. Prior Art
A high calcium sol gel abrasive grain was made essen~
tially in accordance with a process similar to the prior art,
wherein a dried high calcium sol gel was heated slowly and essen-
tially unifor~lly from ambient temperature to 1370C. In par-
ticular, 10,199 grams of Condea Chemie Dispural~ boehmite was
dispersed in 20.5 gallons of wa~er and 573 ml of concentrated
-12-
~9S~
1 nitric acid was then added to form a sol (colloidal solu~ion).
73.9 grams of titanium IV isopropoxide dispersed in 4150 ml of
isopropyl alcohol and 36.5 grams of Nalco-Chemical Nalcoage~1034A
colloidal silica dispersion were then mixed into the sol.
3162 grams of magnesium nitrate was dissolved in 2
gallons of water and the resulting solution was added to the sol
with stirring. Gelling occurred almost immediately. Stirring
was continued for about 5 minutes.
The gel was then transferred to plastic trays at a
depth or about 10 centimeters~ The trays were placed in a steam
heated dryer to dry the gel which took about 60 hours.
The dry gel was passed through a roll crusher to reduce
it to -28 mesh granules. A size fraction -28 to +48 mesh was
separated by sieving.
The dry granules were placed in aluminum oxide coated
saggers and placed into a kiln. Th~ kiln was heated to 1370C
over a period of six hours and held at 1370C for 30 minutes.
The kiln was turned of and allowed to cool. The resulting fired
grains were porous and were analyzed as having a calcium content
of 0.14 weight percent. The density was less than 85% of theore-
tical.
The sintered granules were size classified on a conven-
tional sifter to meet the ANSI 74.18-1977 specification for 50
grit.
A single coated abrasive material was made by
electrostatically coating the 50 grit grain on a vulcanized fiber
backing.
-13-
~1~5~
1 The fiber selected was abrasive grade 0.030 inch
vulcanized fiber, having a nominal weight of 67 pounds per ream
(480-9xll sheets~.
A maker adhesive mix, consisting of a commercial one-
stage; liquid phenolic resin with a formaldehyde to phenol ratio
of about 1:1 and ground limestone with an average particle size
of between 17 and 25 microns, was made using a 1:1 net weight mix
proportion.
The maker mix was then heated to 90F and roll coated
lo on the fiber backing. About 14 pounds of adhesive per ream was
applied.
Usin~ conven~ional sandpaper making equipment, the 50
grit abrasive was electrostatically projected onto the fiber
carrying the maker mix with about 38 pounds per ream of grain
bei~g applied.
The abrasive adhesive coated backing was then heated to
175F for one hour and 200F for two hours in the maker rack.
After drying, a size coat was then applied by standard roll
coating methods with approximately 21 lbs/ream being applied.
The siæe mix consisted of the same 1:1 phenolic resin-filler
ratio. However, a non-buffered synthetic cryolite with an
average particle size of 25 microns was used as the filler.
Drying and curing was then accomplished by heating the coated
material for one hour at 150F, four hours at 175F, and 16 hours
at 22F.
After curing the material was humidified in the conven-
tional manner to a moisture content of less than 8% by weight.
-14-
~s~
1 The material was then uniformly flexed and die cut into seven
inch discs. These were then evaluated on a conventional pneuma-
tic disc grinder using 1018 cold rolled steel as a workpiece and
compared to a control disc made and handled in exactly the same
manner except the abrasive grain was fused alumina. In ~his
tes~, the abrasive disc was placed on the disc grinder in the
standard manner and a 1 x 2 x 11 inch workpiece was positioned so
that it engaged ~he disc on the 1 inch 1at side at a 10-15
angle~ ~he disc was passed back and forth along the workpiece.
lo The abrasive disc in this test was rotated at a nominal
5400 RPH's on a hard rubber type back-up pad 7" in diameter.
Eight pounds of dead weight in-feed force was exerted on the
workpiece. Testing was for 30 seconds after which stock removed
from the bar was measured (weight before grind-weight after
grind) and recordedO This se~uence was continued until the
measured stock removed was 5 grams or less per grinding interval.
Total stock removed in this manner for the test disc was compared
to the total stock removed for the control disc. (Relative
cutting performance)
The high calcium (0.14 weight percent) slowly heated
sol gel abrasive (substantially in accordance with the prior art)
had only 97% o the cutting performance of standard and inexpen-
sive fused alumina abrasive grain.
EXAMPLE 2.
Example 1. was substantially repeated except that
instead of slowly heating the dried granules as in Example 1~,
the granules were calcined at 550C for about 30 minutes and were
then introduced into a rotary tube furnace a~ 1390C to rapidly
-15-
~ i
1195B48
1 heat them to above 1200C in accordance with the present inven-
tionO The granules (grains) were heated to 1390C in about 10
minu~es and retained at 1390C for about 30 minutes to sinter the
grains, Any other differences from Example 1. were minor. The
resulting sin~ered grains were analyzed as having a calcium con-
tent of 0.11 weight percent and a specific gravity over 3.3
(greater than 85% of theoretical). Upon testing as described in
Example 1., the discs were found to have a relative cutting per-
formance which was 395 percent of the cutting performance of
fused alumina abrasive grain. In other words, the grain of the
invention had a cutting performance 3.95 times the cutting per-
formance of fused alumina abrasive grain.
EXAMPLE 3.
Example ~. was substantially repeated except that the
dried grains were calcined at 600C instead of 550C. The
finished grain was found to have a calcium content of 0.12 weight
percent and a density in excess o 85~ of theoretical. The rela-
tive cutting performance was 4.39 times the performance of fused
alumina abrasive grain.
EXAMPLE 4.
Example 20 was substantially repeat~d except that the
magnesium nitrate used was prepared by dissolving 720 grams of
magnesium hydroxide in 1.66 gallons of water containing 1660 ml
of concentrated nitric acid. The resulting solution was added to
~he sol while stirring. As in Example 2., gelling occurred imme-
diately and stirring was continued for 5 minutes.
The resul~ing grain had a calcium content of near 0~08
weight percent, a density in excess of 85% of theoretical, and
-16-
s~
1 cutting performance about 4.5 times the cutting performance of
fused alumina. In addition, the resulting grain had a cutting
performance about 2.4 times the cutting performance of a commer-
cial rapidly cooled fused alumina-zirconia grain and a cutting
performance comparable to a commercial low calcium sintered sol
gel alumina containing grain precoated on a commercial disc,
which grain is similar to the low calcium and sodium grain
described in U.S. Patent No. 4,314 r 827 .
EXAMPLE 5.
lo A high calcium and sodium sol gel abrasive grain was
made essentially in accordance with Example 2. In particular,
20,559 grams of Conden Chemie ~ispural~ boehmite was dispersed
in 33.5 gallons of water and 1250 ml of concentrated technical
grade nitric acid dilu~ed with 2 gallons of water was then added
to the dispersion to form a sol (colloidal solution).
6341 grams of magnesium nitrate was dissolved in 4
gallons of water and the resulting solution was added to the sol
with stirring. Gelling occurred almost immediately. Stirring
was continued for about 5 minutes.
The gel was then transferred to plastic trays at a
depth of between 2.5 and 3.75 centimeters~ The trays were placed
in an electrically heated dryer to dry the gel which took about
48 hours.
The dry gel was passed through a roll crusher to reduce
it to -20 mesh granules. A size fraction ~20 to +48 mesh was
~eparated by sieving.
The granules were calcined at 550C for about 30
minutes and were then fast fired in a rotary tube furnace at 1.2
-17-
~:~9513~
revolutions per minu~e at about 1395C to rapidly heat them to
above 1200C in accordance with the invention. The granules
(grains) were heated to 1390C for an additional 10 minutes. The
resulting grain had a calcium content of about 0.07 weight
percent and a sodium content of 0.015 weight percent. The
density was in excess of 85~ of theoretical.
The sintered granules were size classified on a
conventional sifter to meet the ~NSI 74.18-1977 specification for
36 grit.
lo A single coated abrasive material was ~ade by
electrostatically coating the 36 grit grain on a vulcanized fiber
backing.
The fiber selected was abrasive grade 0.030 inch
vulcanized fiber, having a nominal weight of 67 pounds per ream
(480-9xll sheets).
A maker adhesive mix, consisting of a commercial
one-stage, liquid phenolic resin with a formaldehyde to phenol
ratio of about 1:1 and ground limestone with an average particle
size of between 17 and 25 microns, was made using a 1:1 nek
weight mix proportion.
The maker mix was then heated to 90F and roll coated
on the fiber backing. About 23 pounds of adhesive per ream was
applied.
Using conventional sandpaper making equipment, the 36
grit abrasive was electrostatically projected onto the fiber
carrying the maker mix with about 62 pounds per ream of grain
being applied.
-18-
~ s~
1 The abrasive adhesive coated backing was then h~ated
to 175F for one hour and 200F for two hours in the maker rack.
After drying, a size coat was then applied by standard roll
coating methods with approximately 23 lbs/ream being applied.
The size mix consisted of the same 1:1 phenolic resin-filler
ratio. Drying and curing was then accomplished by heating the
coated material for one hour at 150F, four hours at 175F, and
16 hours at 225~F~
After curing the material was humidified in the
conventional manner to a mois~ure conten~ of less than 8% by
weight~ The material was then uniformly flexed and die cut into
seven inch discs. Five of these discs were then evaluated on a
conventional pneuma~ic disc grinder using quenched and tempered
4140 steel (hardness 285-320 BHN) as a workpiece and compared to
a control disc made and handled in exactly the s~me manner except
the abrasive grain was fused alumina. In this test, the abrasive
disc was placed on the disc grinder in the standard manner and a
1 x 2 x 11 inch workpiece was positioned so that it engaged the
disc on the 1 inch flat side at a 10-15 angle. The disc was
2~ passed back and forth along the workpiece.
The abrasive disc in this test was rotated at a nominal
5400 RPM's on a hard rubber type back-up pad 7" in diameter.
Eight pounds of dead weight in-feed force was exerted on the
workpiece. Testing was for 30 seconds after which s~ock removed
from the bar was measured (weight before grind-weight after
grind) and recorded. This sequence was continued until the
measured stock removed was 5 grams or less per grinding interval.
Total stock removed in this manner for the test disc was compared
--19--
3L~95~
1 to ~he total stock removed for the con~rol disc. (Relative
cutting performance).
The high calcium and sodium (0.07 combined weight
percent) rapidly heated sol gel abrasive had a mean cut of 612
grams (almost 7 times the cut of standard fused alumina). The
results are shown in Table 1.
EXAMPLES 6. - 19.
The procedure of Example 5 was followed except that
various known quantities of calcium and sodium were added to the
dispersion prior to the addition of magnesium nitrate. The
sodium and calcium were added as sodium and calcium nitrate
solutions. The sodium nitrate solution contain~d 0.54 grams per
ml of NaN03, equivalent to 0,2 grams/ml calculated as Na20, and
the calcium nitrate solution contained 0.58 grams per ml of
Ca(N03~2, equivalent to 0.2 grams/ml calculated as CaO. The
results are set forth in Table 1. The calcium and sodium in the
grain are analy~ed by emission spectroscopy. The slight
variation between added calcium and actual calcium in the grain
is believed due to additional calcium present in magnesium
nitrate and in water and the slight variation between added
sodium and actual sodium in the grain is ~elieved to be due to
impurities in the components of the grain, especially water, and
to some vaporization of sodium during calcining and sintering.
For comparison, the mean cut of traditional fused
alumina grain is set forth in Table 1.
-20
1~958~L~
,~
1 TABLE 1
% Ca in % Na in Mean Cut
EXAMPLE mls Ca~ mls Na+ Grain Grain Grams
~ -0~ .06 .01 612.0
6 24.9 -0- .0~ .02 644.8
7 -0- 83 ~06 .06 504.8
8 83 ~0- .13 ~01 494.6
9 -0- 166 .0~ .12 437.6
24~9 ~3 .08 .~7 424.0
lo 11 lÇ6 -0- ,1~ .~4 424.0
12 83 166 .13 - .13 383.2
`13 373.5 -0- .33 .02 369.8
14 166 166 .18 .12 326.4
166 270 .19 .21 275.4
16 -0- 270 .06 .18 270.0
17 83 27~ .13 .20 242.4
18 -0- 481.5 .06 .27 197.2
19 373.5 481.5 .36 .36 185.~
alumina 88.2
EXAMPLE 20
Examples 7, 11 and 15 were repeated except that the
grain was slowly fired in a manner similar to prior art ~xample
1. In particular, the grain was slowly fired in a stationary
kiln from ambient temperature to 1500C over a time period of 16
hours followed by 30 minutes at 1500C. The resulting abrasives
were so poor that no cutting data could be obtained.
The foregoing examples clearly demonstrate the
superiority of the cutting performance of high calcium and sodium
sol gel abrasive grains manufactured in accordance with the pro-
cess of the invention and show that when the step of rapidly
heating dried high calcium and sodium sol gel to sintering tem-
perature is omitted, as in the prior art, the resulting grain is
an inferior grain.
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