Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~5~;79~
-- 1 --
COATING PROCESS
____~
This invention relates to a process ~or depositing
thin films of coating material onto a substrate, and to
substrates having thin film coatings thereon.
Thin films have an enormously varying range of
industrial applciations. For example, thin films of gold,
silver and chromium are used for decorative purposes, thin
films of aluminium and nickel-boron have been used for
corrosion protection, and thin films of magnesium fluoride,
aluminium oxide and silicon oxide have all been used as
non-reflective coatings for optical lenses.
Kirk-Othmer's l'Encyclopaedia of Chemical Technology",
3rd EdItion (198Q) Vol. 10, pages 247 to 283 describes the
following types of process for depositing thin films:-
A Deposition of Films from Solution
.
1. Electrolytic deposition - cathodic and anodic
films.
2. Chromate conversion coatings.
I 3. Electroless plating.
¦ 20 4. Polymeric coatings.
B. Vacuum Deposition of Films
' 1. Evaporation of inorganic materials.
2. Evaporative coating with polymers.
3. Vapour-phase polymerisation.
1 25 4. Sputtering.
`; 5. R-f sputtering of polymers.
6. Ultra-violet irradiation, photopolymerisation.
C. Deposition of Films in Gaseous Discharge
D. Deposition of Films at Atmospheric Pressure
. _ _ .
1 30 1. Metallo-organic deposition.
2. Electron-beam polymerisation.
3. Gamma irradiation.
4. W solid polymerisation.
The present invention provides a method of depositing
~ .
~'~S2674
-- 2
films which falls into none of the above-mentioned
categories. The method has application to a vast range
of substrates and coating materials~ and produces a type
of thin film which is believed to be unique.
The present invention is based on the unexpected
discovery thatthin films of unprecedented characteristics
can be made merely by rubbing small particles of a coating
material (such as copper) with sufficient force across the
surface of a substrate (such as a sheet of glass). Our
investigations have shown that the bond obtained between
the copper coating and the glass substrate in the above-
mentioned example was not merely the result of mechanical
keying between the copper and microscopic rugosities on
the surface of the substrate, but is a quite different
kind of bond which is only achieved at or beyond certain
critical rates of energy input. This was demonstrated
by an experiment in which copper particles were rubbed
across the surface of glass by means of a rotating
buffing wheel, while gradually increasing the force
with which the wheel was pressed against the glass.
Measurement of the frictional force acting on the glass,
(i.e. the force acting on the glass in a direction tan-
gential to the circumference of the wheel) gave a most
unexpected result. It was found that the frictional
force increased gradually, and generally in proportion
to the load on the glass, until a critical load was
reached. At this point the frictional force increased
very markedly upon only slight increase in the applied
load. It was only at and beyond this point that copper
was deposited on the glass. Had the bond between the
copper coating and the substrate been merely the result
of mechanical keying, it might have been expected that
the extent of coating would have increased gradually
with the applied load.
3~ It is therefore believed tha-t the copper coating
.,
'7~1
described above is totally unrelated in character to the
type of coating which may be formed by drawing a relatively
soft material across a microscopically or macroscopically
rough surface, so that fragments of the soft material
are mecha~ically held in fissures or on microscopic pro-
tuberances in or on the coated surface. Fxamples of such
mechanically keyed coatings are those obtained when waxes
are applied to wood, graphite or paper, and when copper
is applied to iron or steel as described in U.S. Patent
Specification No. 826628, Thurston of July 24, 1906.
The exact nature of the copper/glass bond obtained
in the experiments described above is imperfectly under-
stoodr However, it is thought that the critical conditions
of roller pressure and peripheral speed represent the
conditions necessary to remove contaminants ~rom the
surface of the substrate, and to present fresh copper
particles to the decontaminated surface before
recontamination can occur. In the extremely short period
of time for which the s~lrface remains uncontaminated,
the surface molecules are thought to be in some way
activated, and highly receptive to any molecule with which
they might come into contact.
~ possible altemative mechanism is that under the very high
energy conditions which obtain at interface between the particle of
coating material and the substrate, an intimate lecular mixture or
complex is fon~ed between the coating material and the materi~ of the
substrate, analogous to a metallic alloy, notwithstand ~ that the two
materials would not non~ly fonm an alloy with each other.
A similar mechanism of film formation to the first
mechanism propounded above is apparently disclosed in
U.S. Patent No. 2,640,002 Clayton, May 26, 1953. In the introductory
passages of this specification, it is suggested that an "atomic bond can
be created between a metallic coating and a metallic sub-.
strate by dry tumbling the metallic substrate, crushed
iron shot or the like, and metal dust (such as zinc dust)
7~
- 4
in a barrel. However, it is believed that the bond
which is in ~act obtained is merely mechanical in charac-
ter, because it is said ~n US-A-2640002 to be necessary
to the plating mechanism that the surface of the substrate
be sufficiently rough.
Other instances of coatings being formed by rubbing
a coating material across the surface of a substrate are
also to be found in the prior art. For e~ample, U.S.
Patent Specifica~on No. 2284590 RDgers of May 26, 1942 discloses a method of
applying a plastic material to a curved surface, and more
particularly to a method of applying a coatlng of poly-
vinyl alcohol or polyvinylacetal to a headlight lens.
The method lnvolves rubbing a belt of the pla tic material
across the surfacè of the substrate until a coating is
formed. It is believed, however, that the mechanism of
film formation in this case is also quite different from
the mechanism of film formation by the process of the
present invention. Firstly, U.S. specification No.
2284590 indicates that the method may be practised by
merely stroking the substrate with a mass of polyvinyl
alcohol held in the hand of the operator. In contrast,
we have found that power necessary to deposit a coating
by the method of the present invention is many times ~e.g.
from 10 to 100 times) that which can be achieved manually.
Secondly, U.S. Specification No. 2284590 suggests that
the coating mechanism involves gross melting of the PVA
belt, whereas the method of the present invention has
been found to be applicable to the formation of coatings
to materials whi¢h have melting points substantially above
the melting point of PVA, for example, materials having
melting points of 300C or more, and more particularly to
materials having melting points above S00C. In some
cases, we have found that coatings can be formed using
materials having melting points over 800C, and even over
1000C. Most remarkably, the process o~ the present
l~Ztj'7~
invention has been u~ed to obtain coatin~s of materials
which decompose before meltir~ or which are not normally
thought of as having any melting point, such as diamond.
Thirdly, the implication of Specification No. 2284590 is
that melting alone is sufficient to effect a bond between
the plastic film and the substrate, whereas the process
of the present invention has been found to be applicable
to the formation of adherent coatings on substrates to
which the coating material will not normally adhere, even
when molten,
A ~urther type of coating dlsclo~ed in the prior art
as being ~btained by means of rubbing is that disclosed in
U.S. Patent Specification No. 3041140, Alexander, June 26, 1962. This
specification discloses ~he formation of non-reflecting coatings on glass
lenses by rubbing very fine powders o~ silica using light
pressure. Again, it is believed that the mechanism of film
~ormation in this prior art specification is quite unrelated
to the mechanism of film formation in the process of the
present invention. Firstly, the energiesn~K~for forming
the prior art coating are very much smaller than those
typically used in the process of the present invention.
Secondly, the present invention has been ~ound to be
applicable to the formation of coatings even on substrates
for which the coating material would not normally be
regarded as having any chemical affinity.
As noted above, we have found that coatings of an
enormous range of materials can be deposited merely by
rubbing with sufficient force and at sufficient speed
across the surface of the desired substrate. In each
case, we have observed the same phenomena of the coating
being deposited and the friction increasing greatly, at
or above a critical rate of energy input. Accordingly,
as used herein, the expression "critical rate of energy
input" means the rate of energy input at which these
phenomena are observed.
~.
lZ~2674
-- 6
Moreover, in each case the coatlng formed is very
thin, but nonetheless highly adherent, non-granular in
appearance and substantially free of micropores. Even
in cases when the coating material had a very high melting
point, the coating had a characteristic smeared appearance
under high magnification scanning electron microscopy,
strongly suggesting plastic deformation of the particles
¦ of coating material at the time of film formation.
! The coatings formed by the method of the present
invention have a number of important characteristics.
Firstly, they are very thin, being less than 3 microns in
thickness. More usually, they are substantially thinner
than this, very often being less than 500 nm thick and often
less than 200 nm thick. Typical film thicknesses are from
1- to 100 nm thick, for example from 5 to 50 nm thick. A
most unusual characteristic of the process of the invention
is that in many instances, the coatings produced thereby
are effectively self-limiting in thickness, in the sense
that the coating, once formed, will generally not increase
in thickness even when more of th~s~ecoating powder is
rubbed over the surface.
Another characteristic of the films formed by the
process of the invention is that they may~be substanti~ly
non-porous.This i,s highly unusual in such thin coatings.
Yet a further characteristic of the coatings formed
by the method of the invention is that they are substanti-
ally free of voids. This is in marked contrast to the
coatings formed by many prior art techniques, such as
sputtering.
The present invention thus provides a method of
coating a substrate with a coating material, comprising
rubbing discrete, substantially dry particles of the
, coating material across the surface of the substrate with
sufficient force and at sufficisnt speed relative to said
surface to cause the coating material to become deposited
.
,
'. ~,
~SZ~;74
-- 7
on the surface of the substrate in an adherent, sub-
stantially non-microporous, non-granular thin film.
Differently expressed, the invention provides a method of
coating a substrate with a coating material, comprising
rubbing discrete, substantially dry particles of the
coating material across the surface of the substrate with
a rate of energy input which is greater than the critical
rate of energy input as hereinbefore defined.
According to a further aspect of the present
invention, there is provided a substrate having deposited
thereon a thin, highly adherent, non-granular, substant-
ially non-microporous smeared coating.
The application of the coating material to the
substrate with the requisite rate of energy input may
be achieved by bombarding the intended substrate with
particles of the coating material carried on the surface
of larger particles of the same or different resilient
material such as çork e.g. by means of a wheelabrator. The
carrier particles may be projected at the surface to be treated by
20 entrainment in a cold;F heated high velocity jet of gas.
Alternatively, the carrier particles may be caused to vibrate
acoustically (ultra-sonically), magnetically or mechanic~ly
against a substrate.
Preferably, however, the particles of coating mater-
ial are rubbed across the surface of the substrate by
means of an applicator having a resilient surface which
is in sliding contact with the substrate. The appli-
cator may be, fo'r example, a rotary applicator such as a
j roller or wheel.
t 30 Accordingly, the present invention also provides
apparatus for coating a substrate using the method, said
apparatus comprising a support for the substrate, a
¦ rotary applicator arranged to bear against a substrate
supported on said support, means for delivering a supply
of substantially dry particles of coating material to the
,
l~S~6
. ~ .
- 8
surface of the applicator, or of the substrate, or both,
¦ and means for rotating the rotary applicator to cause
¦; the surface thereof to rub said particles against the
substrate, whereby to coat the substrate with the coating
material.
A particularly preferred applicator for use in the
' method of the invention is a jeweller's buffing wheel.
Suitable buffing wheels include those available from
W. Canning Materials Limited, Great Hampton Street,
Birmingham, England. These buffing wheels generally
comprise a plurality oX fabric discs clamped together
in a way-which allow the density of fabric at the
periphery of th~ wheel to be ad~usted.
As mentioned above, the coating material can be
1 15 selected from an enormous variety of materials. For
example, it may be an organic polymer. Illustrative
examples include; polyolefins such as polyethylene,
polypropylene, polybutylene and copolymers of the fore-
going; halogenated polyolefins such as fluorocarbon
polymers; polyesters such as polyethyleneterephthalate;
vinyl polymers such as polyvinylchloride and polyvinyl
alcohol; acrylic polymers such as polymethylmethacrylate
and polyethylmethacrylate; and polyurethanes. Alterna-
tively, the coating material may be a metal such as gold,
silver, platinum, iron, aluminium, chromium or tantalum.
Further examples of suitable coating materials include
magnetic oxides such as magnetic iron oxide and magnetic
chromium dioxide, ~iner~s such as quartz, organic and inorganic
pigment, and even such materials as diamond and ch~a clay. Yet
further examples include metalloid elements such as phosphorus,
silicon, germanium, gallium, selenium and arsenic, optionally doped
? with other materials to confer desired semiconductor prDperties.
If desired, mixtures of different kinds of particle
may also be used.
Products which may be made by the process of the
'7~
g
invention include magnetic recording medla and electrical
components having conducting resistive, dielectric or
semiconducting layers thereon. Other applications include
the formation of protective coatings, decorative coatings,
si~ing coatings, key coats, light or heat absorbing
coatings, light or heat reflective coatings, heat conduct-
ing coatings, slip coatings, non-slip coatings, anti-
corrosion coatings, anti-static coatings and even abrasive
coatings on substrates such as metal, paper, glass,
ceramics, fabrics and plastics. Yet further applications
of the process of the invention are set out in our British
Patent Application No. 8401838, filed 24th January 1984,
(EPO Publication No. 0152203A August 21, 1985)~
The par~icles of coating material will generally
be less than lOO microns in size. However, the most
appropriate particle size will depend to some extent on
the chemical nature of the coating material and on the
physical and chemical nature of the substrate. Usually,
the particles will have a maximum diameter of less than
50 microns, and more usually a maximum diameter less
than 30 microns. For example, the particles may have a
maximum diameter of from 0.5 to 30 microns, such as from
1 to lO microns.
The particles of coating material may be delivered to the sur-
face of the applicator in ~he dry state, for ex~mple in a gas stream,
but is often ~ound to be more convenient to deliver the particles to
the surface of the applicator in the form of a liquid di~ersion, such
dispersions,beir~ readily controllable. Preferably, the dispersir~
li ~ d is sufficientl~ volatile to evaporate almost instantly,
leaving the particles in a substantially dry state. A suitable dis-
persing liquid is trichlorotrifluoroethane, though other low-boiling
o~nated hy~carbons can ~so be used, as can other liquids
such as water.
The method of the invention can be used for coating
virtually any substrate, whe-ther flexible or rigid~smooth or
rough. Remarkably, the process may also be used to
~ff ~,tj~ ~ 7 ~
10 -
~ great advantage for coating paper and woven and nonwoven
¦ fabrics (whether of natural fibres such as cellulosicfibres 9 or synthetic fibres su~h as polyesters, polyolefins,
i polyamides and substituted celluloses) and other materials
5 of a soft nature.
When the substrate has an uneven surface, such as
the surface of a nonwoven fabric, the coating may be
macroscopically discontinuous, in that only the high
points of the substrate are coated with a thin, adherent,
J 10 substantially non-microporous film. However, when such¦ substrates are coated by the metnod of the invention, it
1~ is found that both the micro and macro interstices between
I and within the fibres are filled with loosely compacted
sub-particulate material.
1~ In the case of certain, relatively low-melting
, coating materials, the sub-particulate material which
! collects in the interstices in this way may be rendered
more coherent and adherent by subsequent sintering or ~using, e.g.
flash heating. This flash heating involves the
passing of a coated substrate through a nip where at least
one roller is heated to the required sintering or fusi~gtemperature
If the substrate is one which may be damaged by prolonged
exposure to this temperature, the coated substrate has to
pass through rapidly so as not to cause scorching or
other structural damage. The thicker the deposits which
it is desired to sinter or fuse; the longer is the dwell time
necessary in the heated nip. Therefore there is a natural
restriction on the thickness~ sintered or fused coatings which
may be formed oh substrates which are liable to thermal
I 30 damage.
f In certain cases, the above-described method of
¦ flash sinteringor fusing will not be appropriate. For example, if
¦ a plastics-coated bank note is flash heated using
heated rollers, the elevated temperature and pressure at
! 35 the nip of the heated roller will cause ink at
,._
.
I
1;2S267
the raised images produced by the Intagllo process to
soften and flatten. Consequently it is appropriate in
this instance to use a non-contact heat source such as
high intensity radiation.
In cases where a sinterable or fusible coating o~ the invention
; is deposited on a relatively uneven surface, the thin
film which is formed on the high points of the substrate
constitutes an anchor to which further layer~ of coating
material may be bonded by conventional sintering or ~using processes.
It will be appreciated that th~ nature of the
present invention is such as to preclude precise
enumeration o~ the appropriate process condltions for
~orming film of a given material on a given substrate~
' This is because coatings can be formed using a wide range
of process conditions, which are all dependent on each
other. Thus, for example, when a buffing wheel is
' used to rub particles of coating material across the
; substrate, the pressure applied by the wheel, the area
of contact between the wheel and the substrate, the
peripheral speed of the wheel, and the relative speed
3 between the surface of the wheel and the subs~rate may
all be varied. However, alteration of any one of these
parameters may require that one or more of the other
1~ parameters be adjusted in order to compensate.
I 25 In addition, of course, the conditions which are
! appropriate for forming a coating of a given material on
a given substrate may not be appropriate for coating a
different substrate or for coating with a different
coating material. In all cases, however, the appropriate
i 30 process conditions will be readily determinable by the
person skilled in the art, particularly having regard to
¦ the guidelines and examples hereindescribed.
Generally, we have found that the more delicate the
substrate, the lower the pressure with which the particles
of coating material should be pressed against the substrate,
~
!
~ .
~ZS26'7
in order to avoid damage thereto. Thus, for example,
a very lightweight nonwoven fabric may be coated with
plastics materials using a 30 cm diameter soft fabric
buffing wheel, by training the fabric round the buffing
wheel, and applying only a slight tension (e.g. from 10
to 100 grams/cm width of fabric, depending on the strength
of the fabric). With this arrangement, the pressure with
which the wheel bears against the fabric is very low indeed,
for example from less than 1 g/cm~ to a few grams/cm2.
However, such low pressures are compensated for by the
fact that the individual particles of coating material
are drawn over a very substantial length of the nonwoven
fabric, such as from one quarter to three quarters of the
circumference of the wheel. In the example just described,
the roller can conveniently be rotated at 2000 rpm, while
the nonwoven fabric web is drawn through at about 10 metres/
minute.
When the substrate is rather more robust, such as a
a paper of weight 100 g/m2 9 a convenient coating technique
is to feed the substrate into the nip between a buffing
wheel and a retaining roller. In this case, the distance
for which individual particles of coating material are in
contact with the substrate is very much smaller (generally
from 1 to 20 mm, e.g, from 2 to 10 mm), and substantially
larger pressures are therefore appropriate. Conveniently,
the static pressure of the roller on the substrate will be at
least 100 g/cm2, preferably at least 200 g/cm2, and more preferably
from 300 glcm2 to 10 ~ cm2~ e.g. ~00 g/cm2 to 2 kælcm2.
When even harder to less easily damaged substrates are
used, it may be appropriate to use still larger contact
pressures between the applicator and the substrate. For
example, we have found that for coating metals with other
relatively hard materials (such as metals, metal oxides,
etc) pressures greater than 1 kg/cm2 may be appropriate.
35 ~namic pressures of from 2 to 100 kg/cm2 are most frequently -
'" ."~R~'
~ . ..
~ 2~2~i'7
used for this kind of coating, for example from S to
50 k/cm .
Although the factors which determine the appropriate
operating conditions for different substrates are im-
perfectly understood, it will be apparent that identifyingthe appropriate conditions for a given substrate is merely
a matter of trial and error. The operator need only
choose a coating technique which is appropriate to the
strength and flexibility of the substrate in question,
and then increase the applicator pressure and/or applicator
speed until ~ desired coating is formed.
A number of embodiments of the invention will now be
particularly described with reference to the accompanying
drawings in which:-
Figure 1 illustrates diagrammatically a rotary
applicator for carrying out the method of this invention;
Figure 2 shows diagrammatically the applicator in the
conte~t of apparatus for use in carrying out the method of
this invention; and
Figure 3 shows diagrammatically a form of apparatus
suitable for determining the frictional force acting on a
substrate when being coated by the method of the invention.
The apparatus shown in Figure 2 will be carried within
a metal frame of such mass and proportions so as to with-
stand the loadings and stresses imposed upon it by the
operation. A rotary motive power unit, in this case an
electric motor (not shown), capable of delivering
rotational speeds at the torque necessary for the
operation, is mounted to drive the apparatus. Within
this description we shall consider only the coating of a
moving web of approximately 20 cm width. The apparatus
therefore also requires the means of conveying the web
through the apparatus.
At the heart of the apparatus of the present example
are two rollers 10, 11 forming a nip 12 through which the
! 12S26 79~
- ~4 -
substrate 13 must pass. One of these rollers 10 is the
applicator and the other is the retainer 11. The
retainer roller rotates in the same direction as the web
is travelling. The applicator roller is driven and
rotates so that its surface in the region of the nip
moves in the same direction as the web, but at a
, different speed, or in the opposite direction, all as
indicated by arrows in Figure 2.
The two rollers 10, 11 are mounted within the
frame in such a way that the centre lines of their axis
may be moved relative to each other and possess the
necessary facility to be firmly fixed in the desired
position after the correct nip pressure has been set.
Apart from the small segment of its circumference
at the nip and the aperture required through which the
coating Material is conveyed or any surplus which may
be extracted via a flexible duct 14A, the applicator is
contained in an enclosure 14.
The coating material may be applied to the
applicator by any means so long as the particulate
material is in a dry form when it reaches the nip and
it is uniformly deposited over the face of the
applicator.
~ In the present example an airless spray 15 is used
¦ 25 to convey the particles of coating material at a nozzle
¦ pressure of 480 P.S.I. Although in the above-mentioned
airless spray the particles are dispersed in a sotvent,
which being FREON (Registered Trade Mark) TF is highly
. I volatile and is thought to "flash off" almost completely
before the particles hit the surface of the applicator,
the preferred method is to apply the coating material
uniformly in a dry particulate state. One benefit of
using the dry particulate state is to avoid using solvents
which are unattractive ~or commercial and environmental
reasons.
~252~;74
- 15 -
The airless spray is equipped with a switch
mechanism (not shown) which is operated by a can which
is rotating at 38 RPM and has lifting knobs ha~ing an
effective operating dwell of 3 arc on the cam. The
number of lifting knobs used is determined by the
surface roughness of the substrate and or the quantity
of particulate material that is desirable to be deposited
on the substrate.
The spray nozzle is adjusted to produce a fan-shaped
spray pattern 16 in which the particles are evenly dis-
tributed when they contact the applicator roller 10. The
applicator roller 10 and the spray cam (not shown) are
linked through gèaring in such a way that with each
squirt of the nozzle approximately one quarter of the
applicator's surface area along its circumference
receives a deposit of the coating material and 40
revolutions later the applicator receives a second
squirt of material which should land on the second
quadrant and so forth.
The applicator is made from sheets of cotton
fabric 17 cut in 10 cm diameter discs with a hole in the
centre of each disc of 2.5 cm diameter. These cotton
discs are then pulled onto a threaded steel shaft 18 of
2.5 cm diameter and are retained by 6 mm thick steel
washers 19 of 8.9 cm diameter to form an applicator
30 cm wide. The washers in turn are retained by
suitable nuts. The cotton discs are compacted by
tightening the retaining nuts to produce a density at
I the perimeter face of the compacted cotton mass appropriate
1 30 to the material to be coated. We have found that delicate
substrates require softer rollers than resilient sub-
strates. When using polyester films to be of sufficient
density for use on a polyester film when it cannot be
compressed by more than 6 mm when reasonable thumb pressure
is applied.
. J ~2S~;7~
- lfi -
When a so~ter appllcator is desired lntermedlate
nuts 28 and washers 20 may be used on the shaft at æay
every 1 to 2 cm along the length o~ the applicator.
Alternatively, the nuts may be tightened further in order
; 5 to compact the cotton sheets into a more solid mass.
Once the correct appli~ator density is achieved it
is then ground in by running it at high speed against
j the retaining roller, the sur~ace o~ which is closely
covered with a sheet or coarse abrasive material such
; as emery cloth and running in a counter direction to the
rotation o~ the applicator ror 1 or 2 hours or until such
time as a smooth enough surface corresponding to the
contours oi~ the retainer roller is produced. Following
this operation the coarse abrasive material is removed
and the deposition process is ready to commence.
Depending
on the substrate to be coated, the retaining roller may
have a resilient or a hard surface.
In Figure 3, there is shown a test rig 60 mounted
on a firm level surface ~2. The test rig comprises a
base portion 64 to which is attached an arm 66, mounted
for pivotal movement about pivot 68. One end 70 of arm
25 66 carries a weight 72 for biassing the other end 74 of
arm 66 against a felt applicator disc 76 (30 cm dia. x
5 cm). The applicator disc is rotatably mounted on
spindle 78, and is connected to electric motor 80 by
means of belt drive 82.
The operation of the test rig is as follows:
A sample 84 of the desired substrate is interposed between
the arm 66 and applicator disc 76. Particles of the
desired coating material are applied to the cylindrical
sur~ace of the disc, and the disc is driven at an
~;~S26'^~4
- 17 -
arbitrarily chosen speed, for example 3000 r.p.m. The
force with which the applicator disc 76 bears against
the sample 84 is gradually increased by increasing the
weight 72. The frictional force acting on the substrate
in a direction tangential to the disc (i.e. out of the
plane of the paper in Figure 3) is continuously monitored
by means of strain gauges 86 (only one shown) on either
side of arm 66, using a carrier wave frequency bridge
connected to a chart recorder. When the load on the
substrate is sufficiently great for coating to take place,
the strain measured by the strain gauges suddenly increase.
For commercial purposes, it will usually be desired
to coat the substrate on a continuous basis by driving it
past the applicator. For this purpose, it may be
desirable to modify the apparatus of Figure 3 so as to
simulate more closely the dynamics of such a continuous
process. This can be done by causing the test rig 60,
or at least arm 66 to traverse in a direction tangential
to the disc.
~25~6;7~
' - 18- .
The invention is now further illustrated by the
following examples:
, EXAMPLE 1
A hard felt applicator disc (W. Canning Materials
Ltd.,12" (30.5 cm) x 2" (5.1 cm)) was used to rub
il particles of polymethylmethacrylate (PMMA) over a glass
' plate~ using-.the rig of Fig~ 3.- me ~M~ particles were of 5
i microns average diameter. With-the applicator disc ~ ning at 17Q0
r.p.m., a load of 7.5 kg hung on the arm was found to be adequate to
' 10 cause an adherent coating of:P~ to be deposited on the glass.
The film was estimated to have a thickness of ~ 20 nm,
and had a smooth appearance ~ith no micropores visible
under scanningelectron microscopy at 2000 x and 12,000 xma~lfication.
The area of contact between the disc and the plate
was estimated to be about 0.4 to 0.5 cm , and the apparent
¦ dynamic roller pressure is therefore estimated to be approximately
8.5 kg/cm2.
EXAMPLE 2
The procedure of Example 1 was repeated, except
that the glass plate was traversed across the applicator
disc at speed from 0.1 to 10 cm/sec. It was found that
satisfactory coatings were still formed, but higher roller
pressures were found to be desirable at the higher traverse
speeds.
EXAMPLE 3
~_ .
Exa~ple 1 was repeated, using 1 to 10 micron diameter iron
powder instead of PMMA, and increasing the roller speed
to 3000 r.p.m. A load of 4 kg was found to be sufficient
to cause the iron to be deposited in a film which was
i 30 estimated to be 10 nm thick. Scanning electron microscopy
at 2000 x and 12,000 x magnificationshowedit to have the smeared,
! n on-microporous, nongranular appearance which is character-
istic of coatings according to the invention.
~ 67~
- 19 - ' '
¦ EXAMPLE 4
, Example 3 was repeatedusing 0.5 to 20 micron diameter
¦ copper particles instead or iron powder. A load of 5 kg was
found to be sufficient to cause coating with the applicator
disc turning at 3000 r.p.m., but a load of 7 kg was re-
quired at 2640 r.p.mO
In each case, the coating had an estimated thickness
of ~ 25 nm.
EXAMPLE 5
¦ 10 ~xample 3 was repeated using alumina powder (particle
size, 1-10 microns)0 Coating occurred at an applicator
disc loading of 3 kg.
EXAMPLE 6
Example 3 was repeated using diamond dust (particle
size,C 1 microns). Coating occurred with the usual
characteristic increase in friction between the applicator
and the glass, at a load of 4 kg.
EX~NPLE 7
The general procedure of Example 1 was followed,
using a felt applicator disc of diameter 20.3 cm and
thickness 3.2 cm, to apply iron powder to a polished
I aluminium plate. A coating of thickness C 25 nm was
¦ obtained at a load of 10 kg.
EXAMPLE 8
When the product of this Example was heated in a flame,
the aluminium coated with iron was found to be markedly
more resistant to melting than uncoated aluminium.
Example 7 ~as repeated using copper powder instead
of iron powder. A coating of estimated thickness ~ 25 nm
was obtained at a load of 8 kg.
EXAMPLE 9
Uncoated, unsized paper of 105 g/m2 (manufactured
by Tullis Russell) was coated with PMMA using a soft fabric
2~'7
-- 20
r~ller (10 cm dlam x 30 cm) in the apparatus of Figure 2.
The ~tatic pressure applied by the applicator roller was
estimated to be 0.8 kg/cm2, and the roller was rotated
at 1600 r.p.m. The paper web was delivered to the nip
between the applicator roller and the retalner roller at
a speed of 10 metres/min. Satisfactory coatings were
Plso obtained both at higher and lower web speeds, e.g.
from 0.1 to 100 ~/mln.
-
Our copending Canadian application serial No. 495,628 entitled
"PTFE Coating Process", filed contemporaneously herewith, disclosesyet further e~a~ples of suitable operating conditions
for forming coatings on substrates. While the said
copending application is concerned exclusi~ely with PTFE
coatings, the operating parameters exemplified therein
will also be applicable to the formation of other plastics
coatings within the scope of the present invention~
It will be understood that the present invention
has been described above purely by way of example, and
modifications of detail may be made without departing
from the scope of the invention.
