Binder for abrasive greenware

Abrasive greenware having improved strenght is prepared using a binder comprising poly(ethyloxazoline).

BACKGROUND OF THE INVENTION 
The present invention relates to ceramic greenware, particularly greenware 
which can be fired into abrasive articles. 
Abrasive articles such as sharpening stones, honing stones, mold stones, 
dressing sticks, grinding wheels, and microfinishing stones are useful in 
polishing, sharpening, dressing, shaping and the like. For example, an 
Arkansas stone can be used to sharpen knives Arkansas stones can be 
prepared synthetically, or, as indicated by the name, can be cut from 
natural stone. Synthetic Arkansas stones and other synthetic abrasive 
articles are prepared by firing the appropriate greenware. The greenware 
is prepared by cold pressing a mixture comprising abrasive particles, a 
temporary binder and vitreous bond components. Without the binder the 
greenware would lose its shape or fall apart upon removal from the cold 
pressing mold. The strength of the greenware, i.e. green strength, needs 
to be high enough to prevent damage to the greenware during firing and 
handling. For example, during transfer from the press to the firing 
furnace greenware of insufficient strength can be deformed resulting in, 
e.g. chipped edges or finger marks. 
Following cold pressing, the greenware is fired. The purpose of firing is 
to decompose the binder and melt the vitreous bond component of the 
greenware. A binder commonly employed in the abrasive industry is dextrin. 
This dextrin is added to the pregreenware batch as a fine powder and is 
mixed with the abrasive grit and bond components. When making small 
articles using what is referred to as "press to size" technology it is 
common to use large amounts of dextrin. This is especially true for fine 
grain (e.g. 400 grit) materials with high (e.g. 10) grit to bond ratios. 
This large amount of dextrin is required to provide sufficient green 
strength. Large amounts of dextrin require long burn off times to ensure 
complete removal of the dextrin. Incomplete removal of dextrin leaves 
carbonaceous residue in the greenware. This residue leads to bubbles and 
imperfections, such as bloating, upon firing. 
In view of the disadvantages associated with dextrin, it would be 
advantageous to have a readily removable temporary binder for abrasive 
greenware. It also would be advantageous if this binder could be employed 
in lower amounts than dextrin to give equal or greater green strength, and 
if it could be used with shorter binder burn out cycles, thereby improving 
productivity. 
SUMMARY OF THE INVENTION 
The present invention employs poly(ethyloxazoline) as such a binder in the 
preparation of abrasive greenware. The invention includes a process for 
preparing abrasive greenware by pressing a mixture comprising an abrasive 
material, poly(ethyloxazoline), and a vitreous bond material to form an 
abrasive greenware article. Surprisingly, in comparison with dextrin, much 
less poly(ethyloxazoline) is required to form greenware of sufficient 
strength. The use of poly(ethyloxazoline) is further advantageous in that 
it readily mixes with the abrasive formulation, it reduces the tendency 
for greenware to stick to the pressing dies and molds, and it requires 
less time to "burn out" than does dextrin. These advantages result in 
significant economic benefits. For example, the productivity of the cold 
pressing operation is increased due to the high strength of the greenware 
and its reduced tendency to stick to the dies. Additionally, shorter burn 
out time results in increased furnace turn around time and, therefore, 
greater furnace productivity.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the present invention requires an abrasive material, a 
vitreous bond material, poly(ethyloxazoline), and optionally, a carrier 
medium. 
The poly(ethyloxazoline) is employed in an amount sufficient to provide a 
green ceramic article with enough strength to retain its shape during 
normal handling and processing. Preferably the ceramic greenware comprises 
between about 0.1 and about 20 weight percent poly(ethyloxazoline) and more 
preferably between about 0.1 and about 10 weight percent based on the 
weight of the ceramic material. Even more preferably, said greenware 
comprises between about 0.2 and about 5 weight percent of 
poly(ethyloxazoline), and most preferably from about 0.5 to about 3.0 
weight percent. The poly(ethyloxazoline) preferably has a weight average 
molecular weight ranging from about 1,000 to about 1,000,000 and more 
preferably from about 50,000 to about 500,000. 
An abrasive material is employed in the process of the present invention. 
This abrasive material typically is granular and commonly is referred to 
as grit. While virtually any size grit can be employed, common grit sizes 
range from submicron size to in excess of 1 mm. The grit forms the bulk of 
the abrasive article to be produced. Any abrasive material can be employed 
as the grit. Preferably, the abrasive material comprises ceramic material 
such as metal oxides, carbides and nitrides. Examples of preferred 
abrasive materials include alumina, silicon carbide, diamond, silica, 
boron carbide, tungsten carbide, titanium carbide, cubic boron nitride, 
aluminum nitride and the like. Alumina and silicon carbide are examples of 
more preferred abrasive materials. 
The abrasive grit is held together in the final article by a vitreous bond 
material, also called the "permanent" bond. The vitreous bond material is 
employed in an amount which is sufficient to maintain the integrity of the 
finished abrasive article. The use of vitreous bonds is well known in the 
art. For example, see U.S. Pat. Nos. 1,364,849: 1,548,145; 2,281,526; and 
2,423,293, the teachings of which, with respect to bonds, are incorporated 
herein by reference. Preferably, the bond comprises a powdered glass frit 
and, optionally, a clay, which preferably is a ball clay. Preferably, for 
the sake of convenience, the glass has a low softening point. For example, 
a preferred glass frit has a softening point ranging from about 500.degree. 
C. to about 600.degree. C. Aluminum borosilicate glasses are more 
preferred. Preferably, from about 0.05 to about 1 part of vitreous bond 
material is employed per part of abrasive material. More preferably, from 
about 0.1 to about 0.5 part of vitreous bond material is employed per part 
of abrasive material. The clay typically is employed in an amount which 
ranges between about zero and about 40 weight percent of the total 
vitreous bond material. It is preferred to employ from about 0.1 to about 
0.4 part of ball clay per part of glass. 
The grit, the bond, and the poly(ethyloxazoline) are mixed together using 
well known methods. For example, molten poly(ethyloxazoline) can be added 
to a previously formed mixture of grit and bond with stirring or blending 
until the grit and bond particles are thoroughly wetted. A more preferred 
method of mixing the grit, bond, and binder involves the use of a carrier 
medium. The carrier medium serves to suspend the solid grit and bond 
particles, and further serves to disperse the poly(ethyloxazoline) binder 
in a manner such that the solid particles of grit and bond are thoroughly 
wetted. Preferably, the carrier medium is substantially capable of 
dissolving poly(ethyloxazoline). Examples of preferred carrier media 
include water, acetone, methanol, ethanol, other polar organic solvents, 
and the like, and mixtures thereof. Water is the most preferred carrier 
medium in view of its ease of use, and in view of the fact that 
poly(ethyloxazoline) is water soluble. However, polar organic solvents, 
such as methanol, ethanol and acetone, are particularly useful in this 
invention if it is desired to avoid the chemical reactions that may occur 
if the ceramic grit is in the presence of water. For example, nitrides may 
form oxides in the presence of water, and this may or may not be desired. 
As is well known in the art, other optional materials, such as lubricants, 
coloring agents, surfactants, dispersants, fillers, such as sawdust, and 
the like can be added to the mixture of grit, bond, and binder. For 
example, a lubricant can be employed in order to reduce the friction 
between particles during cold pressing. Examples of lubricants include, 
for example, calcium stearate, zinc stearate, synthetic waxes, stearic 
acid, and the like. Coloring agents can be employed for the purpose of 
altering the color of the final article. Examples of coloring agents 
include ceramic pigments and glass enamels, e.g. colored glazes. 
Advantageously, the greenware of the present invention does not require 
biodegradable polymers, such as alpha amino acid polymers, and can be 
prepared in the absence of such polymers. 
The mixed material can be formed into greenware by known methods such as, 
for example, casting, cold pressing or extrusion. As is well known, cold 
pressing can be dry, semi-dry, isostatic, and the like. The resulting 
greenware is a porous article. The greenware must have sufficient strength 
to be handled without breakage or significant deformation. For example, 
greenware has insufficient strength when picking it up with the bare hand 
in a normal fashion would leave finger indentations or rounded or chipped 
edges. 
The greenware is fired using methods well known in the art. The purpose of 
firing is to remove the temporary binder and to convert the bond material 
into a glassy phase that will form the permanent bond between the abrasive 
particles. Complete removal of the temporary binder, i.e. 
poly(ethyloxazoline), is desirable. Incomplete removal of the temporary 
binder can have consequences such as leaving carbon residue that can later 
be trapped in the vitreous bond material, causing bubbles which lead to 
bloating, warping, cracking and the like. 
Poly(ethyloxazoline) can decompose rapidly if heated above certain 
temperatures, such as about 600.degree. F. (316.degree. C.) unless it is 
slowly heated to reach said temperatures. Rapid decomposition produces 
large volumes of gas which can crack the greenware. Thus, it is preferred 
to perform the binder burn out step at a temperature below about 
460.degree. F. (238.degree. C.) for a time sufficient to remove slowly 
most of these gases before the temperature is increased to the next step 
of the firing sequence. 
The fired abrasive articles typically are very porous. Abrasive articles 
can be made with varying degrees of porosity and cohesion, as is well 
known to those skilled in the art. The articles can be prepared with 
sharp, well defined edges in view of the improved strength of the 
greenware. 
SPECIFIC EMBODIMENTS OF THE INVENTION 
The following examples and comparative experiments are included for 
illustrative purposes only, and are not intended to limit the scope of the 
invention. All parts and percentages are by weight unless otherwise 
indicated. 
EXAMPLE 1 
An aqueous solution of poly(ethyloxazoline) having a weight average 
molecular weight of about 200,000, is prepared by adding 30 weight parts 
poly(ethyloxazoline) into 70 parts water. A high speed/high shear mixer is 
employed. Mixing is continued until the poly(ethyloxazoline) is dissolved. 
The solution (5.4 parts) is added to a vessel containing 30.3 parts of 100 
mesh mined novaculite. The resulting mixture is blended vigorously for 
about 8 minutes to ensure that the grit is thoroughly wetted with the 
solution. Then 10.1 parts of 200 mesh novaculite are added to the wetted 
mass and the resulting mixture is blended for about 5 minutes at a slower 
speed to ensure that the grit is thoroughly wetted. 
Calcium stearate (1.2 parts) is added as a lubricant to 13.9 parts of a 
finely ground (&lt;320 mesh) aluminum borosilicate fritted glass powder. The 
lubricant and the glass powder are blended and then added to the vessel 
and the resulting mixture is blended for about 5 minutes. 
Eight parts of Tennessee ball clay are added to the vessel and the contents 
are mied for another 5-10 minutes at a lower mixing speed. At this point, 
the appearance of the mixture is similar to that of damp sand. 
The mixture is then sieved through a 20-mesh screen to remove large 
particles. The screened powder is then dried in air until the moisture 
content is approximately 1 weight percent. The strength of the greenware 
can be detrimentally affected if the powder is too dry. On the other hand, 
a powder with an excessive moisture content will not have good flow 
characteristics and may result in an abrasive article having non-uniform 
density. 
The powder is then passed through a 40-mesh screen, and is ready to be 
formed into greenware. The powder is cold pressed at 2500 psi into blocks 
having the following dimensions: 1/2".times.15/8".times.6". The modulus of 
rupture (flexural strength) of the greenware is measured using a 3-point 
bend test, and is determined to be 102 psi. 
A number of greenware blocks are placed on edge on a refractory batt. The 
batt is then placed in a kiln and is subjected to the following firing 
schedule: 
______________________________________ 
200.degree. F. 
for 1 hour for drying 
200.degree. F.-460.degree. F. 
in 1 hour get to burnoff plateau 
460.degree. F. 
for 6 hours 
low temp. burnoff 
460.degree. F.-900.degree. F. 
in 3 hours get to burnoff plateau 
900.degree. F. 
for 2 hours 
high temp. burnoff 
900.degree. F.-1900.degree. F. 
in 4.5 hours 
to reach vitrification temp. 
______________________________________ 
Hold at vitrification temperature for 2 hours. 
The kiln and the abrasive articles are allowed to cool. The cooled articles 
are uniform in appearance and have well defined edges. The articles have a 
porosity of 34.5 volume percent and a density of 1.80 g/cm.sup.3. 
COMATIVE EXPERIMENT 1 
(not an embodiment of the present invention) 
The procedure of Example 1 is repeated except that dextrin is employed 
rather than poly(ethyloxazoline). The modulus of rupture of the greenware 
is determined to be 59 psi. 
EXAMPLE 2 
An aqueous solution of poly(ethyloxazoline), having a weight average 
molecular weight of about 200,000, is prepared by adding 45 weight parts 
poly(ethyoxazoline) granules into 55 parts water. A high speed/high shear 
mixer is employed. Mixing is continued until the poly(ethyoxazoline) is 
dissolved. 
The solution (3.6 parts) is added to a vessel containing 25.1 parts of 180 
mesh fused alumina. The resulting mixture is blended vigorously for about 
6 minutes to ensure that the grit is thoroughly wetted with the solution. 
Then 25.1 parts of 200 mesh fused alumina are added to the wetted mass and 
the resulting mixture is blended for about 5 minutes at a slower speed to 
ensure that the grit is thoroughly wetted. A finely ground (&lt;320 mesh) 
aluminum borosilicate fritted glass powder (9.4 parts) is then added to 
the vessel and the resulting mixture is blended for about 5 minutes. 
Tennessee ball clay (3.8 parts) is added to the vessel and the contents are 
mixed for another 5-10 minutes at a lower mixing speed. At this point, the 
appearance of the mixture is similar to that of damp sand. The mixture is 
then sieved through a 20-mesh screen to remove large particles. The 
screened powder is then dried in air until the moisture content is 
approximately 1 weight percent. The powder is then passed through a 
40-mesh screen, and is ready to be formed into greenware. The powder is 
cold pressed into blocks at a pressure (ca. 2800 psi) that gives a green 
density of 2.17 g/cm.sup.3. The blocks have the following dimensions: 
1/4".times.2".times.7". The modulus of rupture of the greenware is 
measured using a 3-point bend test, and is determined to be 446 psi. 
A number of greenware blocks are stacked flat on a refractory batt. The 
batt is then placed in a kiln and is subjected to the firing schedule of 
Example 1. The kiln and the abrasive articles are allowed to cool. The 
cooled articles are uniform in appearance and have well defined edges. The 
articles have a porosity of 42.0 volume percent and a density of 2.08 
g/cm.sup.3. 
COMATIVE EXPERIMENT 2 
(not an embodiment of the present invention) 
The procedure of Example 2 is repeated except that dextrin is employed 
rather than poly(ethyloxazoline). The modulus rupture of the greenware is 
determined to be 114 psi. 
The preceding Examples and Comparative Experiments demonstrate the 
unexpectedly improved green strength of greenware prepared using 
poly(ethyloxazoline) versus dextrin as a binder. The following additional 
observations are applicable to the preparation of greenware in the 
Examples and Comparative Experiments: compared to greenware prepared using 
poly(ethyloxazoline), greenware prepared using dextrin has weak edges, is 
dusty during cold pressing, and is fragile and easily broken during normal 
handling.