U.S. patent application number 12/742292 was filed with the patent office on 2010-10-07 for polishing sheet and method of producing same.
This patent application is currently assigned to NIHON MICRO COATING CO., LTD.. Invention is credited to Kazunori Tani, Nobuyoshi Watanabe.
Application Number | 20100255764 12/742292 |
Document ID | / |
Family ID | 40667532 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100255764 |
Kind Code |
A1 |
Watanabe; Nobuyoshi ; et
al. |
October 7, 2010 |
POLISHING SHEET AND METHOD OF PRODUCING SAME
Abstract
A polishing sheet suitable for precise polishing is produced by
arranging polishing particles as a single particle layer on a base
sheet and includes a base sheet and a polishing layer made of
polishing particles and a binder resin for fixing the polishing
particles that are arranged as a single particle layer. The
polishing particles protrude partially from the surface of the
binder resin, being covered or not covered with a thin film of the
binder resin.
Inventors: |
Watanabe; Nobuyoshi; (Tokyo,
JP) ; Tani; Kazunori; (Tokyo, JP) |
Correspondence
Address: |
Weaver Austin Villeneuve & Sampson LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
NIHON MICRO COATING CO.,
LTD.
Tokyo
JP
|
Family ID: |
40667532 |
Appl. No.: |
12/742292 |
Filed: |
November 20, 2008 |
PCT Filed: |
November 20, 2008 |
PCT NO: |
PCT/JP2008/071070 |
371 Date: |
May 11, 2010 |
Current U.S.
Class: |
451/526 ;
51/298 |
Current CPC
Class: |
B24D 3/28 20130101; B24D
18/0018 20130101; B24B 37/245 20130101; B24B 37/22 20130101 |
Class at
Publication: |
451/526 ;
51/298 |
International
Class: |
B24D 11/00 20060101
B24D011/00; B24D 3/28 20060101 B24D003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 22, 2007 |
JP |
2007-303514 |
Claims
1. A method of producing a polishing sheet having polishing
particles distributed in a single-layer formation contained in a
polishing layer formed on a surface of a base sheet; said method
comprising: an application step of applying a binder resin for
fixing said polishing particles to said polishing layer; a charging
step of charging a specified quantity of said polishing particles
in a same polarity; a dispersing step of transporting and
electrostatically dispersing said charged polishing particles to
said base sheet having said binder resin applied thereto; and a
hardening step of hardening said binder resin on said base sheet
having said polishing particles dispersed thereto; wherein said
binder resin is applied to the surface of said base sheet to a
thickness no greater than the average diameter of said polishing
particles.
2. The method of claim 1 wherein the thickness of said binder resin
applied to the surface of said base sheet is two thirds or less and
one tenth or more of the average diameter of said polishing
particles.
3. The method of claim 1 wherein said polishing particles are
preliminarily classified according to their uses.
4. The method of claim 1 wherein said charging step comprises
sending said polishing particles into a metallic tube and forming
charged particles with said same polarity by frictional
charging.
5. The method of claim 1 wherein said polishing particles are
charged to 1 kV-20 kV in said dispersing step.
6. The method of claim 1 wherein portions of said polishing
particles are exposed as cutting edges from the surface of said
binder resin after said dispersing step.
7. The method of claim 1 wherein said polishing particles have an
areal density of 30%-95% on said polishing sheet.
8. The method of claim 1 wherein dry powder with water content of
5% or less is used as said polishing particles.
9. The method of claim 1 wherein an ultraviolet-setting resin is
used as said binder resin.
10. A polishing sheet comprising a base sheet and a polishing layer
formed on a surface of said base sheet, wherein said polishing
layer is formed with polishing particles and a binder resin that
fixes said polishing particles, said polishing particles are
distributed in a single layer, said polishing particles protrude in
part from a surface of said binder resin or protrude from the
surface of said binder resin with parts thereof covered with a thin
film of said binder resin.
Description
[0001] This application is a continuation of International
Application No. PCT/JP2008/71070, filed Nov. 20, 2008 which claims
priority on Japanese Patent Application 2007-303514 filed Nov. 22,
2007.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a polishing sheet that may be cut
into tapes and disks for a wide range of polishing from rough
polishing to super-precision polishing, including polishing and
abrading of materials such as metals, ceramics, glass and
monocrystals, mirror polishing of the surfaces of magnetic hard
disks and semiconductor substrates, precision final polishing of
the surfaces of magnetic heads, lenses, etc., polishing of end
surfaces of optical fibers, and surface finish polishing of
automobile bodies, as well as a method of producing such polishing
sheets.
[0003] In general, polishing sheets are produced by forming a
polishing layer including abrading particles on a flexible base
material sheet such as a plastic sheet, a woven or non-woven cloth
or paper and by cutting it into an appropriate form such as a
tape-like shape or a disk-shape for use.
[0004] The polishing layer is produced by applying slurry, which is
a mixture of polishing (or abrading) particles and an adhesive
binder, on the surface of a base material sheet, drying it and
hardening it. It may be produced also by applying an adhesive
binder on the base sheet, scattering polishing particles on the
binder surface by a method such as the electrodeposition method
after electrically charging them and drying the binder.
[0005] On such conventional polishing tapes, the polishing
particles in the polishing layer have a multi-layered structure and
these particles are intimately in contact mutually on the surface.
Microscopically viewed, however, the surface structure is by no
means uniform and aggregated lumps of polishing particles are
protruding from the surface of the polishing layer at many
locations. They tend to cause deep marks and scratches.
[0006] In high-precision polishing, fine polishing particles with
regular size distribution must be used. If the particle diameters
of a polishing material become small, the particles tend to
aggregate easily and it becomes difficult to scatter the polishing
particles uniformly inside the resin. If polishing particles are
applied in their aggregated form, the surface roughness of the
polishing tape becomes irregular, and marks are generated on the
surface of the polished object. Polishing irregularities are also
caused. Especially where the product characteristics are required
to be improved such as the mirror polishing of the surface of a
magnetic hard disk or a semiconductor substrate, the precision
final polishing of the surface of a magnetic head, a lens, etc.,
and the polishing of end surfaces of optical fibers, improvements
are being required in the smoothness and flatness of device
surfaces at each production process.
[0007] It is becoming impossible to satisfy such requirements with
conventional polishing tapes. In the case of polishing with fine
polishing particles, the binder resin covers the surfaces of the
polishing particles to increase the contact area between the
polishing layer and the object being polished such that the
friction with the polished object becomes large. Besides the marks
caused by the aggregated clumps of the polishing particles as
mentioned above, this causes deposition of the binder resin and
makes it difficult to discharge debris generated by the polishing.
Clogging is thereby caused and this adversely affects the useful
lifetime of the polishing film.
[0008] In view of this problem, it has been known, as described,
for example, in Japanese Patent Publication Tokkai 1-234169, to
scatter the polishing particles in a binder resin for adjusting the
solvent, the dispersant, their mixing ratio, the method of
application and the amount of application such that a polishing
layer is formed as a single-layer structure of the polishing
particles attached to the sheet substantially without
overlapping.
[0009] By this method, however, it is difficult to scatter the
abrading material particles and apply them uniformly, and it is not
possible to expose the cutting edges of the polishing particles on
the binder resin surface.
[0010] As a method of exposing a portion of polishing particles
from the polishing layer, on the other hand, it has been known, as
described, for example, in Japanese Patent Publication Tokkai
2-243271, to apply the abrading particles together with the binder
resin and to remove portions of the binder covering the particles
by irradiating ultraviolet light after the binder has hardened such
that the polishing particles come to be exposed on the polishing
layer surface.
[0011] By this method, however, it takes a very long time to
sufficiently remove the binder on the surface, adversely affecting
the efficiency for mass production. There was also the problem of
deterioration of the binder resin surface due to the long period of
ultraviolet irradiation, causing the polishing particles to drop
off.
[0012] According to another method, the binder resin is
preliminarily applied to the surface of the base material, and an
electric field is provided around this base material. After the
polishing particles are charged, they are transported into the
electric field for generating an electrostatic force on the
polishing particles and to thereby cause them to become attached to
the surface of the base material.
[0013] By such a method of charging polishing particles by means of
an electric field formed between an electrode and the base
material, however, there is a problem, in the case of very small
particles of less than 10 .mu.m, of becoming charged as aggregated
clumps such that the single-particle distribution becomes
difficult, as described, for example, in Japanese Patent
Publication Tokkai 2003-340730.
[0014] It is therefore an object of this invention to provide a
polishing sheet suitable for precision polishing, having polishing
particles disposed as a single-particle layer as a polishing layer
such that its flatness is improved and the generation of marks and
scratches on the polished object will be reduced, as well as a
method of producing such a polishing sheet.
SUMMARY OF THE INVENTION
[0015] In order to achieve the object described above, the present
invention provides a method of producing a polishing sheet having
polishing particles distributed in a single layer contained in a
polishing layer formed on a surface of a base sheet, the method
comprising an application step of applying a binder resin for
fixing the polishing particles to the polishing layer, a charging
step of charging a specified quantity of the polishing particles in
a same polarity, a dispersing step of transporting and
electrostatically dispersing the charged polishing particles to the
base sheet having the binder resin applied thereto, and a hardening
step of hardening the binder resin on the base sheet having the
polishing particles dispersed thereto, wherein the binder resin is
applied to the surface of the base sheet to a thickness no greater
than the average diameter of the polishing particles.
[0016] It has been found to be possible by this production method
to uniformly disperse polishing particles over the surface of a
base sheet and even to control the necessary particle density.
Since the thickness of the binder resin applied to the surface of
the base sheet is made to be no greater than the average diameter
of the polishing particles, furthermore, the polishing particles
can be distributed in a single-layer formation, the polishing
efficiency can be maintained high, and the generation of very small
scratches and unevenness can be restrained.
[0017] The thickness of the binder resin applied to the surface of
the base sheet is preferably two thirds or less and one tenth or
more of the average diameter of the polishing particles.
[0018] It should be two thirds or less such that the polishing
particles would not be buried inside the binder resin. It should be
one tenth or more such that the quantity of the binder resin would
be sufficient for preventing the polishing particles from dropping
off from the binder resin.
[0019] It is further preferable that the polishing particles be
preliminarily classified according to their uses such that a
uniform polishing layer can be easily formed in a single-layer
form.
[0020] The method is preferably carried out by sending the
polishing particles into a metallic tube by means of a compressed
gas and forming charged particles with a same polarity by friction
by their contact with the inner wall of the tube.
[0021] The dispersing step is preferably carried out such that the
polishing particles are charged to 1 kV-20 kV. It is because the
repelling force would be too weak and it would become too difficult
to form a single-layer formation if the charging potential were
below 1 kV and the binder resin would scatter off and the surface
would become damaged if it were above 20 kV.
[0022] The dispersing step may be carried out such that portions of
the polishing particles become exposed from the surface of the
binder resin such that portions of the polishing particles are
exposed as cutting edges from the surface of the binder resin after
this step. If the cutting edges are thus exposed on the surface,
the speed of fabrication can be further increased.
[0023] The areal density of the polishing particles is preferably
made to be 30%-95% on the polishing sheet. According to this
invention, this can be adjusted easily by the number of times of
emitting the particle even if the lots are small.
[0024] It is also preferable that dry powder with water content of
5% or less, and more preferably 1% or less, be used as the
polishing particles. This has the effect of stabilizing the charge
quantity and reducing the aggregation of small particles.
[0025] It is preferable to use an ultraviolet-setting resin as the
binder resin. It is further preferable to use an epoxy resin as the
binder resin for securely fixing the polishing particles.
[0026] The present invention also provides a polishing sheet
comprising a base sheet and a polishing layer formed on a surface
of this base sheet, wherein the polishing layer is formed with
polishing particles and a binder resin that fixes the polishing
particles, the polishing particles are distributed in a single
layer, the polishing particles protrude in part from a surface of
the binder resin or protrude from the surface of the binder resin
with parts thereof covered with a thin film of the binder
resin.
[0027] Since the polishing particles are distributed in a
single-layer formation without being buried inside the binder
resin, it is possible to accomplish polishing with a high
efficiency while maintaining a long useful lifetime for the
polishing sheet.
[0028] As explained above, a polishing sheet produced by the method
of this invention can polish the end surface of optical fibers and
the surface of a semiconductor substrate without forming any
scratches while reducing steps even in the case of a structure with
different materials and obtaining a flat and smooth surface.
[0029] Since polishing particles with similar diameters are used to
form the polishing layer in a single-layer formation, individual
particles work on the object to be polished effectively and hence
the speed of polishing can be significantly improved.
[0030] Since the polishing particles are in a single-layer
formation, furthermore, the amount of the particles that are used
can be significantly reduced as compared to the prior art polishing
sheets in a multi-layer formation and this contributes to the
saving of the cost.
[0031] Since the polishing particles are dispersed onto the surface
of the base sheet on which a specified amount of the binder resin
is preliminarily applied while being charged by a specified amount,
a production by a small lot becomes possible. The invention also
has the merit of making it possible to freely adjust the areal
density of the particles according to the purpose.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIGS. 1A and 1B, together referred to as FIG. 1, are
schematic sectional views of polishing sheets according to this
invention.
[0033] FIG. 2 is a schematic sectional structural diagram of a
charging-dispersing device for producing a polishing sheet of this
invention.
[0034] FIGS. 3A, 3B and 3C, together referred to as FIG. 3, are
each a microscopic photographic plan view of polishing particles
dispersed according to this invention.
[0035] FIG. 4 is a SEM image of the surface of a polishing sheet
produced by a method according to this invention.
[0036] FIG. 5 is a SEM image of the surface of a polishing sheet
produced by a method of Comparison Example.
[0037] FIG. 6 is a schematic diagonal view of a test piece
fabrication device using a polishing sheet according to this
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] The invention is described next with reference to the
accompanying drawings.
[0039] FIGS. 1A and 1B are schematic sectional views of polishing
sheets according to this invention, FIG. 2 is a schematic sectional
structural diagram of a charge dispersing device for producing a
polishing sheet of this invention, FIGS. 3A, 3B and 3C are each a
microscopic photographic plan view of polishing particles dispersed
according to this invention, FIG. 4 is a SEM image of the surface
of a polishing sheet produced by a method according to this
invention, FIG. 5 is a SEM image of the surface of a polishing
sheet produced by a method of Comparison Example, and FIG. 6 is a
schematic diagonal view of a test piece fabrication device using a
polishing sheet according to this invention.
[0040] As shown in FIG. 1, a polishing sheet 30 according to this
invention comprises a base sheet 31 made of a synthetic resin and a
polishing layer 34 formed on the front surface of this base sheet
31. The polishing layer 34 has polishing particles 32 arranged in a
single layer, these polishing particles 32 being fixed at their
positions by a binder resin 33.
[0041] FIG. 1A shows a first structure of the polishing sheet
according to this invention with polishing particles 32 arranged in
a single particle layer having their surfaces thinly covered with a
film of the binder resin 33. This is formed by the surface tension
force of the binder resin, the polishing particles 32 being
strongly fixed such that they will be prevented from dropping off.
The portions of the binder resin film covering the edges of the
polishing particles are extremely thin and peel of at the initial
contact such that not only the abrading efficiency improves
radically but also the binder resin can be prevented from becoming
attached to the object being polished.
[0042] FIG. 1B shows a second structure of the polishing sheet
according to this invention with the cutting edges of the polishing
particles 32 exposed from the binder resin 33 such that the
abrading efficiency is further improved.
[0043] Explained more in detail, portions of the cutting edges of
the polishing particles 32 (or the portions having no resin film)
can be made to appear from the surface of the binder resin 33 by
adjusting the thickness of application of the binder resin 33 and
the resin concentration. In doing so, it is desirable to make the
heights of the cutting edges uniform. For this purpose, use is made
of polishing particles that are classified and have uniform
diameters. For carrying this out, it is preferable to apply the
binder resin 33 to a thickness that is two thirds or less and one
tenth or more of the average diameter of the polishing particles
32. If the thickness of the applied binder resin is greater than
two thirds of the average diameter, more than 50% of the polishing
particles will come to be covered by a film of the binder resin 33,
and the abrading efficiency is adversely affected as the thickness
of the film increases.
[0044] The areal density of the dispersed polishing particles is
preferably 30% or more and 95% or less. If it is less than 30%, the
abrading efficiency is adversely affected. If it exceeds 95%, the
overlapping of particles takes place and it becomes difficult to
distribute them in a single-layer formation.
[0045] Although classifications according to FIGS. 1A and 1B are
separately shown, the formations shown in FIGS. 1A and 1B may be
mixed. In other words, formations with the cutting edges of the
polishing particles 32 exposed from the binder resin 33 and with
the surfaces of the polishing particles 32 thinly covered by a film
of the binder resin 33 may coexist.
[0046] Use as the base sheet 31 is made of a plastic film made of a
synthetic resin because of the necessity for the resistance
characteristics (high strength and heat resistance) against
breakage and deformation due to mechanical forces applied during
its use and deformation due to heat during its production as well
as the necessity for flexibility. Depending on the purpose of use,
however, paper, leather and rubber materials may be used. There is
no particular limitation as to its shape. It may be in the shape of
a sheet or a plate, and its surface may be flat, concave or
convex.
[0047] Examples of material for such plastic film include polyester
resins such as polyethylene terephthalate, polybuthylene
terephthalate and polyethylene naphthalate, polyolefin resins such
as polyethylene and polypropylene, polystyrene, vinyl chloride,
polyvinyl alcohol, acryl resins with methacryl alcohol as principal
component, and polycarbonates.
[0048] As a practical matter, however, it is preferable to use a
plastic film made of polyethylene terephthalate as the base film
because the handling of the film for producing a polishing film is
easy.
[0049] There is no particular limitation on the thickness of the
base sheet but it is preferably within the range of 5 .mu.m or more
and 100 .mu.m or less, and more preferably within the range of 10
.mu.m or more and 75 .mu.m or less.
[0050] There is no particular limitation on the binder resin for
fixing the polishing particles in the polishing layer.
Ultraviolet-setting resins, electron beam-setting resins, visible
light-setting resins, thermosetting resins, thermoplastic resins,
and their mixtures may be used.
[0051] For the purpose of forming the polishing layer of a
single-layer structure according to this invention, in particular,
ultraviolet-setting resins and visible light-setting resins with
low heat emission and low heating temperature are preferred for
preventing displacements of the polishing particles in the
hardening step of the production method.
[0052] If the binder resin is an ultraviolet-setting resin,
epoxy-type, polyester-type or urethane-type epoxy acrylate,
polyester acrylate and urethane acrylate as well as silicon
acrylate and their mixtures containing a photoinitiator and a
sensitizer are preferred.
[0053] There is no particular limitation on the polishing particles
to be used for the present invention. For example, inorganic
particles of alumina (Al.sub.2O.sub.3), silica (SiO.sub.2), diamond
(monocrystalline and polycrystalline), boron nitride (cBN) and
silicon carbide (SiC) and organic particles of cross-linked acryl
resins, cross-linked polystyrene resins, melamine resins, phenol
resins, epoxy resins, urea resins and polycarbonate resins may be
used.
[0054] Use of inorganic and organic particles may be appropriately
selected according to the kind of the object to be polished, the
degree of precision in the surface finishing or the protrusions to
be removed and the shape of the protrusions.
[0055] There is no particulate limitation on the diameter of these
polishing particles but particles with diameters in the range of
0.1 .mu.m or more and 200 .mu.m or less are preferred and those
with diameters in the range of 0.5 .mu.m or more and 50 .mu.m or
less are more preferred. If the diameter is less than 0.1 .mu.m, it
becomes difficult to arrange the polishing particles in a single
layer formation. If the diameter exceeds 200 .mu.m, on the other
hand, the surface unevenness of the polishing particles becomes
excessive, causing excessive scratches unsuitable for precision
polishing.
[0056] It is preferable to use polishing particles that are
classified according to the particle size distribution so as to
make the height of the cutting edges of the polishing layer uniform
because this will make it possible to form a flatter single-layer
structure of polishing particles.
[0057] Next, a production method of a polishing tape is explained,
inclusive of the principle of the dispersion of polishing particles
and a charging-dispersing device.
[0058] The single-layer particle distribution of the polishing
particles according to this invention is produced by a
charging-dispersing device shown in FIG. 2.
[0059] In order to uniformly disperse a specified quantity of
polishing particles over the surface of a base sheet preliminarily
coated with a binder resin, these polishing particles are passed
through a charging-dispersing device so as to become charged in a
single polarity and dispersed. Since these charged particles repel
one another and become mutually separated, they become attached to
the binder resin without gathering together but by keeping
appropriate distances among them as they are dispersed onto the
surface of the base sheet. Since the charged particles do not
become discharged immediately after they become attached, they do
not aggregate again on the binder resin and keep their distances.
Such charging-dispersing device is described below. The polishing
particles may be charged positively or negatively.
[0060] An example of charging-dispersing device for dry polishing
particles according to this invention will be explained with
reference to FIG. 2 which is a sectional view schematically showing
its structure including an electrostatic dispersion chamber 20 and
a polishing particle supply unit 10.
[0061] The ceiling part of the dispersion chamber is provided with
a dispersion nozzle 21 connected through a metallic pipe 16 for
pressure-transporting the polishing particles to a particle
adjustment chamber 11 inside the supply unit 10.
[0062] At a lower part inside the dispersion chamber is a stage 22
for dispersing the polishing particles, and a base sheet 23 is
placed thereon. Below the stage 22 is an air discharge opening 24.
There is also provided a collector opening (not shown) for
collecting those of the polishing particles 32 which have failed to
become attached to the base sheet 23 by dispersing.
[0063] The polishing particle supply unit 10 is provided with the
adjustment chamber 11 and a quantity control box 13. The polishing
particle adjustment chamber 11 contains therein a feeder unit (not
shown) for controlling the collection of the polishing particles
supplied from a supply hopper 12 and the quantity of the polishing
particles transported into the pressure-transporting pipe 16. The
polishing particles are led into the dispersion nozzle 21 of the
dispersion chamber 20 through the pressure-transporting pipe
16.
[0064] The polishing particles 32 become charged inside the
pressure-transporting pipe by contacting (by friction with) its
inner wall, producing charged particles. These polishing particles
repel one another and do not aggregate again since they are charged
positively or negatively in a single polarity.
[0065] Methods of accelerating friction with the inner wall of the
pressure-transporting pipe include, for preventing pressure-loss
inside the pressure-transporting pipe, (a) providing the interior
of the pressure-transporting pipe with the function of supplying a
supplementary gas (dry air, nitrogen gas, etc.), (b) providing a
negative pressure inside the pressure-transporting pipe, and (c)
adding a plurality of branches to the pressure-transporting pipe
for increasing the friction efficiency.
[0066] The charged polishing particles led to the dispersion nozzle
21 of the dispersion chamber 20 are blown onto the base sheet 23
together with a compressed gas discharged through gas nozzles
provided around the dispersion nozzle 21.
[0067] Dry air and nitrogen gas from an ordinary high-pressure
container may be used as the compressed gas.
[0068] The dispersion nozzle 21 is structured such that the charged
particles will be uniformly dispersed onto the base sheet 23 as if
blown by a spray gun. The dispersion nozzle 21 may comprise a
plurality of openings having different angles and being adapted to
open simultaneously or sequentially for dispersion. The charged
polishing particles are blown towards the base sheet placed on the
stage 22 without aggregating in the space inside the dispersion
chamber 20, repelling one another and becoming deposited without
aggregating again and by maintaining specified distances one from
another.
[0069] For controlling the quantity of the dispersed polishing
particles, a charge sensor 15 is provided near the
pressure-transporting pipe 16 for detecting the quantity of the
polishing particles being transported, and there is also provided a
controller 14 containing a power source for controlling the device
as a whole and a CPU at a lower part inside the polishing particle
supply unit 10. The quantity of the dispersed polishing particles
can thus be controlled by monitoring the charges on the dispersed
particles by means of the charge sensor 15 and feeding back this
monitored quantity of the charges.
[0070] Thus, the density of the polishing particles on the surface
of the base sheet 23 can be determined by obtaining the quantity
for each dispersing emission and by counting the number of times of
emission. The charges on the particles vary, depending on the kind
of the material of the polishing particles, the particle diameter
and shape, and are preliminarily determined and adjusted.
[0071] The polishing particles, thus electrostatically dispersed,
come to be attached to the binder resin in a single-particle layer
formation without overlapping and by maintaining a set distance
among them since they are charged with a same polarity.
[0072] The dispersion density of the polishing particles on the
base sheet can be adjusted by varying the quantity of each emission
and the number of times of emission.
[0073] There is no particular limitation on the shape of the base
sheet. Dispersion may be made on a plate-like material, a sheet or
a tape. In the case of a tape-shaped object, the production can be
continuously carried out while running the tape.
Test Example
[0074] A polishing sheet embodying this invention was prepared as
follows.
[0075] (1) A base sheet of polyethylene terephthalate (PET) of 75
.mu.m in thickness was used. Al.sub.2O.sub.3 and diamond with
average diameter of 5 .mu.m were used as polishing particles.
[0076] (2) As the binder resin, Adeka HCX200-25 (resin: PGM 17% by
weight) was used as coating solution.
[0077] (3) Polycrystalline diamond particles with average diameter
of 5 .mu.m were used as the polishing particles.
[0078] The coating solution thus prepared was applied to the
surface of the base sheet by a bar coater method to a thickness of
50% of the average particle diameter (or 2.5 .mu.m).
[0079] The polishing particles were charged and dispersed by using
a charging-dispersing device shown in FIG. 2. The quantity of the
particles per emission was 0.4 g/min and the areal density of the
polishing particles was adjusted by the number of times of
emission.
[0080] FIGS. 3A, 3B and 3C are examples of the condition of diamond
particles with average diameter of 5 .mu.m dispersed at a low
density (FIG. 3A), at a medium density (FIG. 3B) and at a high
density (FIG. 3C) by the number of times of emission. FIG. 3 shows
that the particles are uniformly dispersed.
[0081] The areal density of the polishing particles on the
polishing sheet to be used for the evaluation (to be described
below) was about 50% (for FIG. 3B).
[0082] For the dispersion, the diameter of the dispersion nozzle
was 5 mm and a carrier gas was discharged at a rate of 3
kg/cm.sup.2 for 5 seconds. This discharge was repeated for 10
times. The charging potential at this time was 5 kV.
[0083] For hardening the binder resin, used was made of an
ultraviolet irradiator J-Cure 1500 of Jatec Corporation.
[0084] For the irradiation, the output was 1 kW, wavelength was 365
nm (at the peak), the cumulative quantity of light was 600 mJ/min,
and the irradiation time (transportation by a belt) was 0.5
m/min.
[0085] A polishing sheet was produced by carrying out a post-cure
process thereafter at 130.degree. C. for 30 minutes (in an
isothermal tank).
[0086] FIG. 4 shows an image of a polishing sheet produced by a
method according to this embodiment (Test Example) of the invention
as observed by a scanning electron microscope. As shown in FIG. 4,
the cutting edges of the polishing particles are distributed in a
single-particle layer formation and, although covered by a thin
binder resin layer, are exposed from the surface of the binder
resin.
Comparison Example
[0087] A polishing sheet of Comparison Example was produced by
using polishing particles which are identical to those used for the
production of the polishing sheet of Test Example and by a
conventional method (of applying a mixture of polishing particles
with a binder resin onto a base sheet).
[0088] FIG. 5 is an image of the surface of a polishing sheet
produced under these conditions of Comparison Example as observed
by a scanning electron microscope.
Evaluation
[0089] The polishing sheets of aforementioned Test Example and
Comparison Example were individually used to carry out ball bearing
fabrication tests and comparisons were made regarding the polished
quantity for a specified length of polishing time, the central line
average surface roughness (Ra) and the maximum surface roughness
(Rmax).
[0090] The tests using the polishing sheets were carried out on
steel balls (SUJ-2) with a diameter of 4 mm of ball bearings as
test objects by using a polishing device for fabrication shown in
FIG. 6. The fabrication device 40 is adapted to have a polishing
sheet 43 of this invention pasted on a rotatable lapping plate 42,
to have a steel ball 44 as described above as an object of
polishing fixed to a jig 45 and to have a specified load 47 applied
from above onto a polishing head 41 provided to a principal shaft
46.
[0091] The fabrication test was carried out by rotating the lapping
plate 42 with a polishing sheet 43 pasted thereto, causing the
polishing head 41 with the steel ball 44 fixed thereto to contact
the surface of the polishing sheet 43 pasted onto the lapping plate
42 with a specified load and to move a specified distance from a
central position to an outer peripheral position on the lapping
plate 42 at a specified speed. The fabrication process is started
and ended automatically by moving up and down an arm 48 supported
around a supporting point 49. After the fabrication process is
completed, the steel ball 44 is removed from the jig 45, the steel
ball 44 is weighed and the change (or the reduction) in its weight
serves as the polished quantity (or the stock removal). Five
different steel balls were used for the test and the average value
from them was used for the evaluation.
[0092] The conditions of the test were as follows:
[0093] (1) Load: 500 g
[0094] (2) Diameter of the lapping plate: 8 inches
[0095] (3) Rotary speed of the lapping plate: 300 rpm
[0096] (4) Distance of motion from the central position to the
peripheral position: 100 mm
[0097] (5) Polishing time: 12 seconds
[0098] The average surface roughness Ra and the maximum surface
roughness Rmax of the polished steel balls were measured by a
surface roughness meter (SURFCON 480A produced by Tokyo Seimitsu
Kabushiki Kaisha).
Comparison Results
[0099] The comparison results are shown below in Table 1.
TABLE-US-00001 TABLE 1 Average surface Maximum surface roughness
(Ra) roughness (Rmax) Stock removal (.mu.m) (.mu.m) (mg/min) Test
Example 0.024 0.149 8.20 Comparison 0.037 0.256 7.20 Example
[0100] As shown in Table 1, the polishing by the polishing sheet of
Test Example is distinctively effective as compared to that by the
polishing sheet of Comparison Example both in the average surface
roughness (Ra) and the maximum surface roughness (Rmax) by
polishing for a specified length of time. With the polishing sheet
of Test Example, furthermore, a higher stock removal can be
achieved for finishing the surface at a lower surface
roughness.
[0101] Since the amount of polishing particles to be used for Test
Example is less than 1/50 of that in the case of Comparison
Example, the present invention serves to reduce the material cost
significantly.
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