Process for the manufacture of filamentary abrasive particles

A device and process is disclosed for producing filamentary abrasive particles having substantially equal aspect ratios without further length reduction.

BACKGROUND OF THE INVENTION 
This invention relates to a process and device for the manufacture of 
filamentary abrasive particles having controllable and preferably 
substantially constant aspect ratios. 
Abrasive materials such as filamentary abrasive particles and grits are 
widely used in the production of abrasive devices such as grinding wheels, 
segments, belts, disks, polishing powders and the like. All of these 
materials require the use of very high temperatures in their manufacture. 
Sol-gel alpha alumina abrasive materials, in particular, have recently 
become more widely used in bonded abrasive products. Sol-gel aluminous 
abrasives have demonstrated advantages over other abrasive materials such 
as those described above. Such sol-gel abrasives are generally made by 
forming a hydrated alumina gel which may also contain varying amounts of 
additives such as MgO or ZrO.sub.2 and then drying and sintering the gel. 
See for example, U.S. Pat. No. 4,314,827. 
Seeded sol-gel abrasives such as those formed by the conversion of hydrated 
alumina to alpha alumina using a seed material have also become very 
useful in making abrasive materials, particularly coated abrasives. See 
for example, U.S. Pat. No. 4,623,364. 
Once a gel has been created, it may be shaped by any convenient method such 
as pressing, molding, or extruding and then carefully dried to produce an 
uncracked body of the desired shape. If abrasive material is desired, the 
dried extruded gel material is usually crushed or broken by suitable means 
such as a hammer or ball mill to form abrasive particles or grains. The 
same type of extrusion, grinding, and forming processes may be used with 
other abrasive forming compositions than sol-gel materials. See for 
example, U.S. Pat. No. 3,387,957 which discloses the production of 
elongated geometrical shaped and controlled grit size aluminous abrasive 
materials by means of extrusion. 
The problem with the filamentary abrasive particles produced by the 
conventional extrusion, drying, and cutting techniques, be they prepared 
from a gel or from other abrasive materials, is that they usually have a 
wide distribution of aspect ratios. As used herein, "aspect ratio" refers 
to the ratio between the length along the principal or longer dimension 
and the greatest extent of the filamentary particle along any dimension 
perpendicular to the principle dimension. Where the cross-section is other 
than round, e.g. polygonal, the longest measurement perpendicular to the 
lengthwise direction is used in determining the aspect ratio. The 
filamentary particles in the distribution with extremely high aspect ratio 
values create problems, especially during coated abrasive manufacturing 
processes. Extremely long filamentary particles can result in shorting out 
of an electrostatic field which is often used during the coating process. 
Also, during sizing operations (in which a layer of adhesive is applied by 
a rotation roller on top of the filamentary particles) extremely high 
aspect ratio materials often are either pushed over by the roller or 
become embedded in the rubber of the rollers. In addition, the high aspect 
ratio materials often fracture, resulting in a build-up of grit on the 
roller and reduction in the actual aspect ratio of the grits in the 
product. 
DISCLOSURE OF THE INVENTION 
According to the present invention there is disclosed a process and device 
for producing filamentary abrasive particles having substantially regular 
shapes and controllable aspect ratios. As used herein, "filamentary 
abrasive particles" refers to generally elongated ceramic products having 
a substantially uniform cross-sectional shape perpendicular to the 
greatest dimension, i.e. the bodies each have a generally constant 
cross-sectional shape along their lengths and have an aspect ratio of 1:1 
or greater, generally of about 1:1 to 25:1. More preferable aspect ratios 
are from about 2:1 to about 8:1. Most preferable aspect ratios are from 
about 2:1 to about 4:1. The filamentary abrasive particles of this 
invention may be bent or twisted so that the length is measured along the 
body rather than necessarily in a straight line. The process generally 
comprises forcing an aqueous dispersion of an abrasive material through a 
continuously moving belt having a multiplicity of perforations therein 
while the belt is in motion at a substantially constant rate so as to form 
filamentary particles of substantially constant length. The filamentary 
particles are formed of an aqueous dispersion of an abrasive material 
which dispersion is inherently sufficiently sticky to cause the particles 
to stick together if allowed to contact one another and which enables them 
to remain adhered to the belt after their formation. They are treated to 
render them non- sticky while still being adhered to the belt and then 
removed from the belt, preferably as the belt travels around a loop. The 
filamentary particles are fired either before removal from the belt but 
more preferably after removal. The filamentary abrasive particles which 
result may undergo conventional processing to form the finished abrasive 
particles, such as sintering, coating, irradiating, annealing, where 
appropriate. The filamentary abrasive particles may then be formed into 
final shaped bodies such as bonded abrasives, coated abrasives, and the 
like. 
The process of this invention permits the production of filamentary 
abrasive particles, particularly very fine filamentary abrasive particles, 
having regular shapes and having controllable aspect ratios without 
requiring length reduction. Filamentary abrasive particles of 
substantially the same shape and aspect ratio are produced in a single 
process operation without sticking together in clumps and generally 
without the need for further processing steps such as cutting, screening, 
or the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As best shown in the drawings, a preferred process of the present invention 
comprises forcing an extrudable gel of an aqueous dispersion of an 
abrasive material using a forcing means 12 such as a piston extruder 
(shown) through a continuous, perforated belt loop 14, while the belt is 
in motion and as it moves past the forcing means 12 while in tight 
register therewith. Filamentary particles 16 are thereby formed which are 
inherently so sticky that they would adhere to each other if permitted to 
make contact. The filamentary particles adhere to the belt loop as it 
moves away from the forcing means 12. This is an important feature of the 
invention, since the filamentary particles are formed from aqueous 
dispersions such as sol-gels which are usually soft and sticky as they are 
extruded through the belt loop and would inherently stick together if 
allowed to contact each other, i.e. if they fell off the belt loop, at 
this stage of the process. Instead, the filamentary particles stay on the 
belt loop and are then treated to make then non-sticky, preferably dried 
or de-watered by drying in a drying section 17 by drying means 18 such as 
hot air blowers positioned downstream of the forcing means 12. As used 
herein, "downstream" means a position in the direction of the forward 
motion of the belt loop 14. Also, keeping the filamentary particles apart 
until they are no longer adherent to each other permits use of lower 
percent solids dispersions. This in turn facilitates the use of smaller 
openings in the belt loop, resulting in the production of very fine grit 
sizes without the need for classification. Also the lower solids enables 
the use of lower pressures when employing an extruder as the forcing means 
12. Filamentary particles may be produced having a wide range of grit 
sizes. The process of the invention is however particularly adapted to the 
production of filamentary particles at grit sizes below 50, preferably 
below 80, and as fine as 325 grit. The average diameters of such grits in 
(i) the wet state as extruded and (ii) after firing at about 
1230.degree.-1300.degree. C. for 5-10 minutes is: 
TABLE 1 
______________________________________ 
GRIT DIAMETER (MICRONS) 
Size Extruded (Wet) 
Fired 
______________________________________ 
50 584 310 
80 280 170 
120 215 120 
180 145 75 
220 125 65 
325 85 45 
______________________________________ 
The treating means is preferably a drying means which may be any suitable 
means such as a drying chamber, hot air blowers, radiant heaters, 
microwaves, dry air or gas, or a water-extracting solvent. Alternatively, 
the treating means can be a coating means which coats the surfaces of the 
filamentary particles with a very fine dust. Suitable such dusts for 
alumina filamentary particles include alpha alumina or boehmite, since 
these materials will not be deleterious to the eventual use of the 
particles. After belt loop 14 with the filamentary particles thereon 
passes by or through the treating means, the treated filamentary particles 
are removed from the belt loop by a removing means 20 located downstream 
of the drying means. Suitable removing means 20 include such as a doctor 
blade (shown), wire, brush, air blast or other suitable means. 
The filamentary particles removed from the belt loop are collected in 
collecting means 19, fired (sintered) in accordance with conventional 
techniques known in the art and, if necessary, screened to remove dust. 
The filamentary particles so produced are finished, loose grain materials, 
which do not require further cutting to length with each filamentary 
particle having substantially the same aspect ratio, provided that the 
pressure exerted by the forcing means was substantially constant across 
its entire face. 
After the filamentary particles are removed for final processing the belt 
loop continues around its path. While not always required, the belt loop 
may pass through a cleaning means 22 such as a rotating brush (shown) so 
as to remove any remaining abrasive material and thereby avoid any 
clogging problems. Suitable cleaning means include vacuum, stiff wire 
brushes, water solvent jets, ultrasound, and air blasts. 
The forcing means 12 is preferably an extruder such as a horizontal piston 
extruder, an auger extruder, or other devices such as a pump, doctor blade 
or roller. As shown, the forcing means is positioned immediately adjacent 
to and in tight register with the belt loop 14. In the case of an extruder 
such as a horizontal piston extruder, the belt loop is stretched across 
the exit slot of the extruder so that the abrasive material that exits the 
extruder passes immediately through the perforated belt loop. 
The belt loop 14 may be made of any suitable material such as stainless 
steel or other acid and high temperature resistant material. The 
perforations in the belt loop may be obtained by using a wire mesh of the 
desired opening size or by using punched hole, laser cut, chemically 
etched, or electro-etched sheets. Alternatively, the belt may be a 
"sacrificial" belt which is used a single time and not repeatedly as in a 
continuous loop. The perforations in the belt may be of any size or shape 
depending upon the desired size and shape of the filamentary particles to 
be produced. For example, the perforations may be designed to produce 
generally cylindrical filamentary particles after firing having a diameter 
of from about 45 to about 400 microns or to produce filamentary particles 
of various other shapes including having square, rectangular, triangular, 
and star shaped cross sections. Generally the perforations are spaced such 
that the particles do not touch each other while adhered to the belt. On 
the other hand, the spacing should not be so great that the internal 
pressure of the forcing means is excessive. It is found that suitable 
belts generally contain from about 20 to about 40% of perforations in the 
surface area. Usually about 30% of the belt surface area is represented by 
the perforations. 
The length and thereby the aspect ratio of the filamentary abrasive 
particles may be controlled by controlling the velocity at which the belt 
loop moves; the greater the velocity the lower the aspect ratio of the 
filamentary particles. The belt loop may be driven by any suitable means 
24 such as a motor or the like which may be regulated to control the belt 
loop velocity and thereby the aspect ratio of the filamentary particles. 
Generally the belt loop travels at a rate of from about 0.5 to about 10 
feet per minute, though different speeds may be used depending upon the 
equipment design and the desired length of the extruded filamentary 
particles. Provided (i) the belt loop travels at a steady rate during a 
forcing run, (ii) each of the perforations is of equivalent size, and 
(iii) the pressure is constant across the entire face of the forcing means 
the aspect ratio of the filamentary particles produced during that run 
will all be substantially the same. 
In addition, the aspect ratio of the filamentary abrasive particles is 
dependent on the delivery rate of the dispersion to the orifice of the 
forcing means. This in turn is controlled by the pressure of the extruder, 
the pH and the solids content of the aqueous dispersion being processed. 
Higher delivery rates will produce greater aspect ratios, as will lower 
solids content. Generally pressures of about 2 to about 500 psi or more 
will be used with those compositions having a higher solids content 
requiring the higher pressures. In general the dispersions preferably have 
a solids content of about 40 to about 60% by weight. 
While the device of the present invention is suitable for use with numerous 
types of abrasive slurries which contain a temporary binder to hold the 
extrudates together prior to drying and sintering, it is particularly 
suitable for use with sol-gel and seeded sol-gel abrasive dispersions such 
as those disclosed in U.S. Pat. Nos. 4,314,827 and 4,623,364, the subject 
matter of which is incorporated herein by reference. Such sol-gel 
materials are generally formed from a hydrated alumina gel which may 
contain additives such as MgO or ZrO. Preferably, such gels will contain 
about 40 to 55% solids which has been found to produce very fine 
filamentary abrasive particles without requiring excessive pressures. The 
use of such relatively low solids gels with a conventional extrude, dry, 
and cut to size, process is generally avoided due to the extrudates 
sticking together before drying. Other abrasive materials such as silicon 
carbide, zirconium oxide, boron carbide and alumina based materials such 
as those obtained from bauxite, may be used provided that the composition 
extruded is sufficiently adherent to the belt, either naturally or due to 
the presence of additives. 
While the process of the present invention has been described with 
reference to certain specified operations that are performed along the 
motion of the belt, other operations may be carried out. For example, if 
it is desired to produce surface treated filamentary particles either for 
rendering them non-sticky or for some other purpose, a suitable surface 
treatment means may be positioned before the removing means.