Note: Descriptions are shown in the official language in which they were submitted.
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FINING ABRASIVE MATERIALS
Backqround of the Invention
The present invention relates to coated abrasives and
specifically to coated abrasives that are particularly adapted
to producing a fine finish on a hard surface.
When polishing hard surfaces such as glass for optical
purposes or silicon wafers for the production of computer chips,
it is common to use lapping or fining materials. These are
often applied in the form of a slurry using a flexible and
resilient pad. However in many circumstances, notably when the
surface to be lapped or finished is curved as in the case of
optical lenses, it is preferred to use a coated abrasive.
- For abrading non-planar surfaces, a conformable flexible
abrasive material is preferred. Flexible abrasive materials are
disclosed in European Application 0 111 765 A3 which describes a
flexible and extensible textile backing material, a continuous
abrasive layer comprising abrasive particles and a base binder,
and an elastic polymeric interlayer bonded between the backing
and the abrasive layers. These products are designed foe
abrading profiles with complex geometries.
One type of conformable fining product comprises a disc cut
to resemble a multibladed propeller. Such discs are variously
referred to as "daisy wheel" or "snowflake" pads. This
configuration enables the pad to conform more closely to the
surface of the curved surface. The backing on such pads, though
flexible, is essentially non-elastic and can not readily conform
completely to the surface to be finished and thus is not fully
satisfactory. In a different way of using such discs, the disc
is supported on a lap tool having the desired curvature and a
lens blank is pressed against the supported disc and rotated so
as to abrade the surface till it possesses the same curvature as
the lap tool. Such a process is described in USP 4,733,502.
In all fining or lapping processes applied to curved
surfaces and employing a coated abrasive material, the key
problem is that of conforming the coated abrasive exactly to the
curved surface that is to be finished. Because the coated
abrasive is essentially planar and because the abrasive grits
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are usually held in place by relatively inflexible and inelastic
maker and size coats, no matter how flexible the backing, the
planar coated abrasive cannot easily conform itself to a curved
surface so as to substantially equalize the pressure at all
locations on the surface.
To diminish the problem as it relates to the relatively
stiff maker and size coats, it has been proposed that a more
flexible bond material be used and that the maker and abrasive
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be applied to the surface of a "snowflake" pad so as to produce
a series of spaced islands of abrasive, or a pattern of abrasive
areas. This approach is described for example in USP 5,014,468
and the result is certainly increased flexibility, especially
when used in conjunction with more W-curable, flexible maker
coats.
A novel approach has now been developed to this problem.
It provides a coated abrasive that is very flexible and
resilient such that it can readily be adapted to the difficult
abrading situations referred to above. This approach allows the
use of conventional maker and size coat materials which are well
understood and highly versatile. It can also be used with
conventional fine grit abrasive materials. The invention
therefore provides a versatile and effective coated abrasive of
particular utility in fine finishing curved surfaces such as
plastic lenses.
The present invention also provides a process for making a
fining coated abrasive material having the above desirable
characteristics.
Descri~tion of the Invention
The present invention provides a coated abrasive material
having length and width dimensions wherein the material has at
least 80%, and preferably substantially 100%, recovery when
subjected to a strain producing a 50%, and preferably 90%,
elongation in the length direction, or 25%, and more preferably
35%, in the width direction, said coated abrasive comprising
abrasive materials deposited on a surface of the material in
spaced discontinuous patterns.
In order to achieve the necessary elasticity in the coated
abrasive, it is clearly necessary to have at least the same
degree of elasticity in the substrate on which the abrasive
materials are deposited and preferably more. This can clearly
be achieved by the use of elastic fibres or filaments in a woven
structure. An alternative approach which is often preferred is
the use of a knit fabric that includes laid-in elastic fibers.
Such knit fabrics are well known in the art and are used to
produce elastic garments such as womens' hose and support
garments of various sorts. It is also possible in principle to
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use woven or non-woven fabrics made from basically inelastic
fibers that have been subjected to extensive crimping or bulking
so as to give them the capacity to expand, (by straightening
out), upon being strained and yet return to the crimped
configuration when the strain has been removed. However unless
the fabric construction is specially adapted, such materials are
not well suited for use in the present coated abrasive products
since the force tending to restore the unstretched dimensions
tends to diminish at elevated temeperatures and with time.
A suitable fabric for use in the production of the coated
abrasives of the invention is an elastic knit fabric. Such
fabrics are generally knit from nylon or polyester continuous
filament yarns and have an elastic yarn filament laid in between
the knit yarns. The knit can be of the tricot or raschel or any
similar knitted or stitch-bonded system that gives the same
elastic possibilities. Other suitable fabrics include for
example a lofty tangled continuous filament material with the
filaments bonded together at at least some of their points of
contact. One suitable elastic filament/yarn is available from
DuPont company under the registered Trademark "Spandex" and
comprises an elastic yarn sold under the registered Trade mark
"Lycra". Suitable knit fabrics are available from Guilford
Mills.
It is important for the intended applications that the
overall weight and thickness of the backing be not excessive.
It is therefore preferred that the weight of the elastic fabric
used as the backing in the present invention be less than about
220, and more preferably less than about 180, g/m2. The lower
limit is determined at least in part by the conditions of use
but generally the weight should not be less than about lOO g/m2.
The weight is in part dependent on the denier of the yarns used
and this is preferably less than about 140 denier and more
preferably still, less from about 20 to about 40 denier.
It is important that the abrasive material be deposited on
the surface of the substrate in a discontinuous pattern. This
places m;n;mum constraint on the stretching of the coated
abrasive as a result of the bond material by which the abrasive
is retained on the surface of the coated abrasive. As will be
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WO 96t21539 PCT/US9~/16740
appreciated a continuous coat of a relatively inextensible resin
will tend to lead to inextensibility and the same problems in
conforming the coated abrasive material to the surface to be
finished that have made the prior art materials less than
satisfactory. Additionally the spaces between the abrasive
areas allow abraded material to collect without interfering with
continued abrading operations.
In one embodiment of the invention the coated abrasives are
made by applying the abrasive grit in admixture with the bond
material in the form of a slurry by means of a rotogravure
roller so as to lay a discontinous pattern of applications of
the grit/bond slurry on the surface. It is also possible to lay
down the bond material in a first operation and then, in a
separate operation, deposit the abrasive grits in conventional
fashion so as to adhere only in those places having the bond
resin. This second alternative is not generally preferred as it
introduces a second stage and requires a means to remove the
excess grit that does not adhere.
The pattern in which the grit is applied should not be
selected so as to result in significant constraint on the
extensibility of the substrate when the bond component is cured.
Thus a pattern of spaced round patches is often preferred.
In a preferred embodiment the backing material has a knit
structure with a surface comprising a plurality of raised areas,
referred to as knuckles, where the knit yarn stands out of the
general plane of the knit fabric. This is a common feature of
tricot or raschel-knit fabrics. When such surfaces are treated
with a slurry of bond and abrasive grits using a roller
applicator the slurry adheres only to the knuckles and thus
forms a pattern of isolàted areas corresponding to the knuckles
in the fabric. To a lesser extent the same result may be
obtained using certain woven structures where the warp and fill
yarns are elastic. Such fabrics also have surfaces with raised
portions corresponding to the passage of a fill yarn over a warp
yarn or vice versa. ~ ~
The abrasive used is typically a fine particle size alumina
having a narrow particle size range. Typical materials have a
particle size from about 1 to 10 microns and preferab]y from 3
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WO96/21539 PCT~S95/16740
to 5 microns with at least 50% of the particles having sizes
within a micron of the no~ l average particle size. Aluminas
of this type include white calcined alumina, (WA 3000) available
s from Fujimi Corporation; and Electronics Grade Precision
5 Alumina, available from Saint-Gobain/Norton Industrial Ceramics,
Materials Division.
Because the slurry can tend to flow to the spaces between
the desired patterns it is often preferred to apply the slurry
to the substrate while the substrate is in the relaxed state and
10 then to place the substrate under strain so as to G~p~ the
material and separate the treated areas so as to ensure that the
treated areas are well separated in discontinuous patterns on
the substrate.
The discontinuous pattern of abrasive areas in the finished
l5 product is very advantageous as it results in a more open
structure in which abraded material can be collected in the
spaces between the abrasive areas without interfering with the
abrading operation.
A very significant advantage of the materials of the
20 invention is their great flexibility and extensibility. This
makes it possible to conform the abrading surface to curved
surfaces and to subject them to uniform abrading forces.
Description of the Preferred Embodiments
The backing material used in the resilient coated abrasive
25 materials according to the invention is preferably a fabric knit
from an elastic bicomponent yarn in which one component has been
induced to shrink so as to compel the yarn to adopt a coiled or
otherwise crimped configuration. The knit technique used can
be any of those commonly used to produce knit fabrics,
30 particularly those that result in a pattern of raised knuckles
on at least one surface of the knit fabric. Suitable knitting
patterns 1nclude for example raschel and tricot.
It is also possible to use a non-woven fabric in which two
or more arrays of elastic yarns are laid across one another and
35 stitch bonded together using a third yarn in such a fashion as
to result in a spaced pattern of raised knuckles on one surface
of the fabric.
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Certain weaves of elastic fabrics can also be used if they
can satisfy the elasticity requirements for the woven fabric.
Once again however it is preferred that the weave of the fabric
should result in a pattern of isolated raised areas on one
surface. Weaves that give rise to such patterns include for
example terry or towel loops and jacquard weaves with at least
one warp being a "Spandex"-type yarn.
The binder used can conveniently be any of those used in
the manufacture of coated abrasives including phenolic resins,
melamine/formaldehyde resins, polyurethanes, radiation or W -
curable acrylate resins and the like. For most purposes a W -
curable acrylate-modified polyurethane resin is the preferred
binder.
The abrasive grit is conveniently alumina, which may be
sintered or fused, but in some applications diamond or cubic
boron nitride may be preferred. For most applications an
alumina is preferred either alone or in admixture with another
abrasive such as diamond.
The pattern of application of the abrasive is a matter of
choice provided that the abrasive areas are separated from one
another and the deposited structure does not impair the
elasticity of the coated abrasive material to the extent it does
not meet the standards described above. Often two or more
coatings may be desirable to ensure that a sufficient amount of
abrasive is applied while still observing the limitations
regarding spaced patterns and discontinuity recited above.
The coated abrasive product of the invention may be in the
form of a disc, such as a "daisy wheel" disc, or a belt
depending on the equipment with which it is to be used. In one
approach a "bean bag" support is used that conforms itself to
the shape to be ground. To be effective the coated abrasive
must also conform to the desired shape and thus a disc may be
the preferred configuration. The support may also be formed
from a resilient material having elastic memory such that it can
return to a predetermined shape after deformation. One such
material is sold under the registered Trademark "Sorbothane" by
Sorboth~ne Inc.. In such case the preferred configuration may
be a strip.
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DescriPtion of Specific Embodiments
The invention is now described in the context of certian
specific materials and their performance as fining abrasives.
These are for the purpose of illustration only and are not
intended to imply any necessary limitation on the essential
scope of this invention.
Example 1
This Example describes the use of abrasive products made using a
number of different substrates together with the same abarsive
binder mix. In each case the products were evaluated in the
fining of the surface of a plastic lens.
The abrasive/binder formulation used in the evaluations
comprised 70~ by weight of 3 micron alpha alumina abrasive
particles and 30~ by weight of a binder material comprising an
acrylate-modified polyesterurethane sold under the trade name
Celord 3600 by Unirez Corporation. The formulation was applied
to the fabric in an untensioned form, (beyond that imposed by
passage over a roller), using a rotogravure roller depositing
the mixture in a series of repeating patterns according to the
technique described in USP 5,014,468. The abrasive formulation
was cured to provide the finished product.
The expandable coated abrasive material thus formed had the
shape of a strip which was wound onto a reel. This strip was
then used to carry out a second fining on a 6.25 cm,plastic lens
under a 20 pounds applied pressure for two minutes. The
smoothness, (measured by Ra and Rt) were measured at three
locations, (left of center, center and right of center), on the
lens face both before and after the fining operation.
The fabrics tested were:
1. a control product which is a standard commercial product
comprising the same abrasive/binder formulation applied by
a rotogravure technique on a non-expandable film backing.
This product is sold by Norton Company under the trade
designation Q-135 - 3 micron;
2.' twelve samples in which the same abrasive/binder mixture
was applied to a Spandex material in the manner described
above to produce a product according to the invention; and
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3. one sample in which the backing fabric was a knitted fabric
available from Guilford Mills under the designation "Navy-l
Style 55051" and the resin/binder formulation was applied
as described above to produce a product according to the
invention.
The results of these evaluation are set forth in Table 1
below.
In the Table the Rt value is the mean peak to valley
distance for 9 traces each 2.5 cm long. The Ra value is the
roughness. Foe each parameter, lower is better.
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TABLE 1
SAMPLE AMOUNT AS RECEIVED AFTER FINING
REMOVED RA(miCrOn) Rt~m1CrOn~ Ra~m~CrOn) Rt(miCrOn)
Control . 052 g 14L 132L 5L 38L
17C 133C 5C 37C
16R 141R 6R 41
Spandex 1 . 020 g 18L 152L 5L 38L
22C 161C 4C 36C
21R 150R 4R 30R
Spandex 2 .020 g 18L 189L 3L 26L
20C 174C 4C 30C
17R 167R 4R 32R
Spandex 3 .019 g 18L 161L 4L 30L
20C 185C 3C 26C
17R 155R 5R 33R
Spandex 4 .019 g 15L 132L 4L 26L
21C 156C 4C 29C
18R 166R 6R 36R
Spandex 5 .020 g 15L 115L 4L 31L
20C 173C 3C 24C
18R 152R 5R 32R
Spandex 6 .020 g 17L 164L 5L 33L
22C 152C 4C 31C
18R 176R 4R 23R
Spandex 7 .019 g 19L 140L 4L 32L
18C 147C 4C 31C
18R 163R 5R 33R
Spandex 8 .020 g 17L 150L 5L 29L
20C 140C 4C 33C
18R 176R 5R 32R
Spandex 9 . 018 g 18L 162L 4L 30L
21C 153C 4C 33C
18R 156R 4R 23R
Spandex 10 . 021 g 17L 137L 4L 30L
21C 143C 3C 33C
19R 127R 3R 26R
Navy-l . 032 g 19L 166L 4L 29L
21C 188C 4C 30C
16R 138R 5R 36R
AS can be seen' all according to the invention performed better
than the standard.
Example 2
In this Example, two samples of backing with the same
abrasive/binder composition as is described in Example 1 were
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evaluated in second fining a plastic lens. The figures given
are ~ ~
averages over several evaluations. Other comm~nts are
subjective evaluations of the experimenter.
SPandex Backinq Material
No wrinkling of the abrasive sheet when applied or at the end of
the cycle. Material moves with the tool.
Stock Removal: 0.007mm after 90 secs; 0.012 mm after 120 secs.
Final Surface Finish: 1.80 micron Rt; Ra 0.318 micron.
Navy-1 Backinq Material
No wrinkling of the abrasive sheet when applied or at the end of
the cycle. Material moves with the tool.
Stock Removal: 0.008 mm after 30 secs.
0.010 mm after 60 secs.
0.020 mm after 360 secs.
Final Surface Finish: Rt 0.943 micron; Ra 0.205 micron.
