Note: Descriptions are shown in the official language in which they were submitted.
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, PTFE COA~ING PROGESS
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This invention relates to a process and apparatus
' for coating a substrate with polytetrafluoroethylene
I (PTFE), and to PTFE-coated products.
;~` PTFE is extraordinarily resistant to chemical
! 5 attack, and the surface free energy of solid PTFE is very
j' low. This means t~at liquids do not readily wet the solid,
and other solids do not adhere strongly. These properties
` render PTFE very valuable for forming protective surface
coatings, in a wide range of applications from non-stic~
cookware to surgical sutures. However, the very properties
which make PTFE so use~ul in such applications also make
it very difficult to form PTFE coatings which are
sufficiently adherent to their substrates. In practice,
PTFE coatings formed hitherto have relied on mechanical
keying for their adherence to their substrates. For
. example, one common method of forming PTFE coatings is to
coat the intended substrate first with a primer and then
to implant solid PTFE particles in the primer layer,
while the latter is still tacky, so that it can trap the
PTFE particles in a mechanical matrix. These PTFE particles
then form anchor points for subsequent layers of PTFE
il particles which are attached thereto by sintering at
¦ temperatures of between 350 and 400C, depending on the
type of PTFE used.
In an alternative method for forming PTFE coatings,
an extruded or sXived PTFE sheet is fibrillated by
subjecting it to substantial mechanical stresses. The
voids created in the PTFE allow adhesives or other
~ materials to penetrate the PTFE sheet and form a mechanical
¦ 30 bond therewith.
j A still further method employed for coating with
I PTFE telomers is to spray, brush or otherwise spread onto
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the substrate a disperslon of low molecular weight solids
of PTFE suspended in a suitable liquid such as*Freon or
water, and then to evaporate the solvent. Such coatings
are sometimes sintered or even buffed in order to increase
the uniformity of the coating. However, coatings formed
by this method are only poorly adherent and usually
intended for single applications such as in mould release
applications or as release layers for decalcomanias.
The present invention provides an entirely new method
of depositing PTFE films. The method is applicable to
forming PTFE coatings on a wide range of substrates, and
produces a type of PTFE coating which is believed to be
unique.
The present invention is based on the unexpected
discovery that PTFE can be made to form films of un-
precedented adherence merely by rubbing minute particles
of PTFE with sufficient force across the surface of a
substrate. It is believed that the bond obtained between
the PTFE coating and the material of the substrate when
the method of the invention is used ls not merely the
result of mechanical keying between the PTEE and micro-
scopic rugosities on the surface of the substrate,
because friction coatings of the latter type are known
to be only very weakly adherent. For example, a method
which has been used in the study of friction wearing of
PTFE has been to cause a solid rod of PTFE having a pointed
end bearing perpendicularly onto a plate of glass to
traverse back and forth over the same track until a
layer of the desired magnitude is deposited. Coatings
formed in this way can be lifted from the surface of the
glass merely by immersion in water containing a surfactant.
The difference between the PTFE coating of the
present invention and the PTFE friction coating just
described is believed to be due to the high energies which
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are involved in forming the coatings of the ~nvention.
While the exact mechanism of coating is not knownt it is
thought that at sufficiently high energies, the PTFE
particles serve to decontaminate the surface of the
substrate, the surface thus being activated in some way
so as to be highly receptive to any molecule with which
it might come into contact. When a sufficient number
of PTFE particles are rubbed across the surface at a
sufficient rate, fresh PTFE particles are presented to
the decontaminated surface, and thus bond therewith,
before recontamination by other molecules can occur.
- A possible alternative mechanism is that under
' the very high energy conditions which obtain at the
¦ interface between the PTFE particle and the substrate,
an intimate molecular mixture or complex is fon~d between the PTFE
and the material of the substrate, analogous to a metallic alloy,
I notwithstanding that the two materials would not non~ly fonm an
j alloy with each other.
j Quite apart from its adherence, the PTFE coating
of the present invention has a number of characteristics
which are not found, at least in combination, in con-
1~ ventional PTFE coatings. 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 nmthick. Typical film thicknesses are from 10 to 100 nm
thick, for example from 20 to 50 nm thick. A most
unusual characteristic of the process of the invention
is that the PTFE 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 PTFE powder is rubbed over the surface.
Another characteristic of the PTFE films formed by
the process of the invention is that they are substantially
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non-porous. This is hlghly unexpected in such a thin
l PTFE coating.
Yet a further characteristic of the coatings formed
by the method of the invention is that they are sub-
stantially free of voids. Even sintered PTFE coatings
include a sufficient number of voids to brin8 the bulk
; density of the coating down to 1.5 g/cm3 or even less.
In contrast, the coatings formed by the method of the
present invention have a bulk density which is at least
1.70 g/cm3, most usually greater than 1.80 g/cm3 and
often greater than 1.90 g/cm . Typical coatings
according to the invention have a bulk density of
2.0 g/cm3 or greater, for example from 2.1 g/cm3 to
j 2.25 g/cm3.
r 15 Accordingly, the present invention provides a methocl
of coating a substrate with PTFE, comprising causing
discrete, substantially dry particles of PTFE to move
, across the surface of the substrate with sufficient
¦ force and at sufficient speed relative to the sur~ace
to cause PTFE to become deposited on the surface of the
substrate in an adherent film.
Viewed from a different aspect, the present
invention provides a method of forming a PTFE coating
on a substrate, comprising rubbing discrete, sub-
stantially dry particles of PTFE across the surface of
the substrate with sufficient force and at sufficient
speed relative to the surface of the substrate to cause
a PTFE coating of self-limiting thickness to be
deposited on the substrate
Also provided by the present invention is an
apparatus for coating a substrate using the method, said
apparatus comprising a support for the substrate,a rotary
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applicator arranged to bear against a substrate supported
on said support, means for delivering a supply of
substantially dry particles of PTFE to the surface of
Il the applicator, or of the substrate, or both, and means
`1~ 5 for rotating the rotary app:Licator to cause the surface
thereof to rub said particles against the substrate,
whereby to coat the substrate with the coating material.
Further provided by the invention is a substrate having
deposited thèreon an adherent substantially non-microporous
PTFE film which is less than 3 microns thick.
The application of PTFE to the substrate with the
requisite rate of energy input may be achieved by
bombarding the intended substrate with particles of
PTFE carried on the surface of larger particles of the
same or different resilient material such as cork. The
carrier particles may be projected at the surface to be
;~ treated by entrainment in a cold or heated high velocity
jet of gas. Alternatively, the carrier particles may
be caused to vibrate acoustically (ultra-sonically),
magnetically or mechanically against a substrate.
Preferably, however, the PTFE particles are rubbed
i across the surface of the substrate by means of an
applicator having a resilient surface which is in
¦ sliding contact with the substrate. The applicator
i, 25 may bel for example, a rotary applicator such as a
¦' roller or wheel.
¦~ 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
¦ 30 W. Canning Materials Limited, Great Hampton Street,
I Birmingham, England. These buffing wheels generally
¦ comprise a plurality of fabric discs clamped together
in a way which allowsthe density of fabric at the
periphery of the wheel to be adjusted,or are made of felt.
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,~ The term "PTFE" as used herein is intended to
'' embrace both PTFE homopolymers and polymers formed by
copolymerising tetrafluoroethylene with other monomers.
`, Polymers of fluoroethylene containing other halogens
,j 5 are also included, as are mixtures of polymers of
different composition. Polymers may be of different
chain lengths (molecular weights), molecular weight
distribution and crystallinity. Oligomers and telomers
of tetrafluoroethylene are also included.
If desired, other particles, such as mineral
particles of a pigmentary character, may be included
with PTFE particles.
~ The PTFE particles are preferably less than 100
! microns in diameter, and more preferably less than 50
micror.s in diameter. Particularly preferred are PTFE
particles having a maximum diameter less than 30 microns.
J The PTFE particles may be delivered to the surface
!,~ of the applicator in the dry state, but it has been found
to be more convenient to deliver the PTFE particles to
the surface of the applicator in the form of a liquid
dispersion. Preferably, the dispersing solvent is
sufficiently volatile to evaporate almost instantly,
leaving the particles in a substantially dry state.
A suitable dispersing solvent is trichlorotrifluoroethane,
though other low-boiling halogenated hydrocarbons can
also be used.
The method of the invention can be used for coating
; virtually any substrate, but it is of particular utility
in the formation of thin PTFE films on plastics sheets,
foils, tapes and films. Remarkably, the process may
also be used to great advantage for coating paper and
woven and nonwoven fabrics (whether of natural fibres
such as cellulosic fibres, or synthetic fibres such as
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polyesters, polyolefins, polyamides and substituted
celluloses) and other materials of a soft nature.
When the substrate has an uneven surface 9 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,
substantially nonmicroporous film. However, when such
substrates are coated by the method of the invention,
it is found that both the micro and macro interstices
between and within the fibres are ~illed with loosely
compacted sub-particulate materials which are thought to
be micro platelets and it is thought that these platelets
are formed when the PTFE particle is violently scraped
across the protruding fibres on the surface of the
substrate. It will be understood that the PTFE
deposited on the highlights does not require sintering
to ensure an effective and adherent film. Depending
on the end use for which the coated substrate is intended,
however, it may be desirable to increase the coherence
and adherence of the micro platelets which accumulate in
the depressions between the highlights. This can be
achieved by sub~ecting the coated substrate to a fla~h
sintering operation. This flash sintering involves the
passing of a coated substrate through a nip where at
least one roller is heated, for example to a surface
temperature of above ~50~, e.g. 410C. The coated
substrate has to pass through rapidly so as not to cause
scorching or other structural damage. The thicker the
platelet deposits, the longer is the dwell time necessary
in the heated nip. Therefore there is a natural
restriction on the thickness of sintered coatings which
may be formed on substrates which are liable to thermal
damage.
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In certain cases, the above-described method of
flash sintering will not be appropriate. For example,
if a PTFE-coated bank note is flash sintered using
heated rollers, the elevated temperature and pressure at
the nip of the heated sintering roller cause ink at the
raised images produced by the Intaglio process to soften
and flatten. Consequently it is appropriate in thls
instance to use a non-contact heat source such as high
intensity radiation.
In cases where the PTFE coating of the invention
is deposited on a relatively une~en surface, the thin
film which is formed on the ~igh points of the substrate
constitutes an anchor to which further PTFE layers ~ay
be bonded by conventional sintering processes.
It will be appreciated that the nature of the
present invention is such as to preclude precise
enumeration of the appropriate process conditions for
forming a PTFE film 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 the PTFE particles 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 between the
surface of the wheel and the substrate may all be varied.
However, alteration of any one of these parameters may
require that one or more of the other parameters be
adjusted in order to compensate. Moreover, the conditions
necessary for forming a PTFE coating according to the
invention will depend greatly on the physical and chemical
nature of both the PTFE and the substrate. In all cases,
however, the appropriate process conditions will be
readily determinable by the person skilled in the art,
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particularly having regard to the guidelines and specific
examples set out below.
Generally, we have found that the more delicate
the substrate, the lower the pressure with which the
PTFE particles should be pressed against the substrate,
in order to avoid damage thereto Thus, for example,
a very light~eight nonwoven farb~c may be coated using a
30 cm diameter soft ~abric ~uf~ing wheel, by training
the fabric round the buffing wheel, and applying only
10 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/cm2 to a few grams/cm2. However,
such low pressures are compensated for by the fact that
individual PTFE particles are drawn over a very sub-
stantial length of the nonwoven fabric, such as from
one quarter to three quarters of the circumference of
the wheel. In the example first 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 paper of weight 100 g/m2, 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 PTFE particles 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 leas-t lOO g/cm2, preferably at
least 200 g/cm2, a,nd more preferably from 300 g/cm2 to
10 kg/cm , e.g. from
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SOO g/cm2 to 2kg/cm2.
Generally the harder the intended substrate, the
harder the applicator which may be used. For example, we
have coated a 50 micron thick polyester sheet using a soft
fabric buffing wheel (see Example 2 below), but it was
preferred to use a ha~d felt polishing wheel ( 20 cm x
2.5 cm, 1700 rpm, 3kg/cm2 estimal;ed dynamic nip pressure)
for coating aluminium.
Although the factors which determine the appropriate
operating conditions for different substrates are
imperfectly understood, it will be apparent that
identifying the 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 a desired coating
is formed.
The method of the invention can be used for coating
an enormous range of products. Particular exa~ples
include the coating of magnetic recording media, such as
video recorder tapes, audio recording tapes, computer
tapes, computer floppy discs and computer hard discs.
The PTFE coating services to protect both the recording
medium itself and the information stored in it against
dirt, liquids and other materials which might interfere
with proper recording and reading of stored information.
Because the process of the invention results in the
formation of a very thin film of PTFE. the recording and
reading heads which are used for storing and retrieving
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information from magnetic recording media can be placed
in their normal very close relationship to the surface
of the recording medium.
Another area of application for the method of the --
invention is in the coating of medical products9 such
as bandages, wound dressings, burns dressings, personal
hygiene products, surgical needles, staples, sutures,
catheters, surgical drapes and surgical gowns. ~ound
dressings which are PTFE coated in accordance with the
invention are dirt-repellant, and therefore more
hygienic, and also non-adherent to wounds. PTFE-coated
fabrics in accordance with the invention may b~ used
to form water-repellant, but air-permeable gowns and
drapes. Needles and staples may be given a low-friction
coating to decrease the discomfort caused to the patient
by their use, and PTFE-coated sutures according to the
invention are more easily removed.
Yet further applications of the invention are in
forming anti-soiling coatings for security paper, bank
notes, stamps, maps, charts, paper bags, envelopes,
food wrappings, cookware, fabrics for curtains, wallpaper,
yarns such as carpet yarns, threads and ropes. Still
further applications include the formation of low-friction
coatings for nip rollers, calenders, process machinery,
missile and aircraft s~in coatings, helicopter and air-
craft blades, impellers and propellors, boat and shiphulls, low speed bearings, razor blades and conveyor
tube coatings. Further applications include the
formation of water-repellant coatings for tent fabrics,
clothing fabrics, and incontinence products including
diapers, and the formation of release coatings for
pressure sensitive adhesive tape backings, dry print
foils, mould release papers and foils, heat transfers
and decalcomanias.
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Still further examples of the applications of the ,
present invention are set out in our earlier British
Patent Application ~o. 8401838, filed 24th January 1984 .
and published in EPO Publication No. 0152203A on
August 21st, 1985.
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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 thisinvention; and
Figure 2 shows diagrammatically the applicator
in the context of apparatus for use in carrying ou~
the method of this invention.
The apparatus shown in Figure 2 will be carried
within a metal frame of such mass and proportions so as
to withstand 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 deli~ering
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 heartof the apparatus of the present example
are two rollers 10, 11 forming a nip 12 through which the
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 travel-
ling. 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.
~ he 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.
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Apart from the small segment of its clrcumference
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
to convey the PTFE particles at a nozzle pressure of
480 P.S.I. Although in the above-mentioned airless
spray the PTFE particles are dispersed in a sol~ent~
which being FREON ~Registered Trade Mark~ TF is highly
volatile and is thought to "flash off" almost completely
before the PTFE ~artlcles 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 for commercial
and environmental reasons.
The airless spray is equipped with a switch
mechanism (not shown) which is operated by a cam which
is rotating at 38 RPM and has lifting knobs having 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 gearing in such a way that with each squirt
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of the nozzle approximately one quarter of the applicator's
surface areaalong its circumference receives a deposit of
the coating material and 40 revolutions later the appli-
cator 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 mrn 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 the perimeter face of the compacted
cotton mass appropriate to the material to be coated. We
have found that delicate substrates require softer rollers
than resilient substrates. When using polyester films
and other delicate materials the applicator is considered
to be of sufficient density ~or use on a polyester film
when it cannot be compressed by more than 6 mm when
reasonable thumb pressure is applied.
When a softer applicator is desired intermediate
' nuts 18 and ~ashers 20 may be used on the sha~t at say
every 1 to 2 cm along the length of the applicator. Alter-
natively, the nuts may be tightened further in order tocompact the cotton sheets into a more solid mass.
Once the correct applicator density is achieved it
is then ground in by running it at high speed against
the retaining roller, the surface of which is closely
covered with a sheet or coarse abrasive material such as
emery cloth and running in a counter direction to the
rotation of the applicator for 1 or 2 hours or until such
time as a smooth enough surface corresponding to the con-
tours of the retainer roller is produced. Following this
operation the coarse abrasive mater~al is rsmoved and the
52~5
the deposition process is ready to commence.
In general, we have found that the rate of energy
input required for the best operation of the invention
increases with the molecular weight and/or the
crystallinity of the PTFE.
The invention is now further illustrated by the
following examples:-
EXAMPLE 1
To coat a common grade of paper we have used the
following formulation:-
Fluon L168 (PTFE) 150 gm
Freon TF (trichlorotri-
fluoroethane),-as
dispersant 2,750 gm
2,900 gm
Fluon is a Trade Mark of ICI, and Freon is a
Trade Mark of Du Pont.
In this particular example the paper used was a
stencil based paper manufactured by Tullis Russell. It
was an uncoated, unsized paper of 105 gm per metre.
The pressure applied by the applicator roller was
0.77 kgs/Sq.cm. The substrate (paper) web moved at 27
metres per minute. The speed of rotation of the appli-
cator roller was 1550 rpm but the retainer roller
rotated at only 92 rpm.
EXAMPLE 2
The same apparatus as in Example 1 was used to coat
a polyester film of 50 microns thickness (Melinex S grade
polyester sold by ICI). The applicator roller applied
pressure was 0.5 kgs/Sq.cm. which was reduced to this
level because the substrate is in this case a relatively
smooth surface material. It was also found best to use a
softer roller for this type of substrate than was used for
coating paper. Also the best results on this type of
substrate occurred when a low molecular weight PT~E
was used with a formulation of the following proportions:-
Vydax AR (20% solid ln Freon) 600 gmsFreon TF 2,900 gms
3,500 gms
Vydax fluorotelomers are dispersions of a white,
waxy comparatively short-chain telomer of tetrafluoro-
ethylene in Freon TF.
EXAMPLE 3
Stainless steel surgical wire was coated with PTFE
by rubbing with a felt buffing wheel (20 cm diameter x
2.5 cm) rotating at 3000 rpm, to which the PTFE dispersion
described in Example 1 was continuously applied. By way
of comparison, a sample of the same wire was coated by
the method currently recommended for coating surgical
staples. This method is to dip the wire into a dispersion
of Vydax AR and Freon, and to allow to dry. The coating
is then sintered at 350C until it develops a glossy
appearance.
When the wire coated with the conventional method
was contacted with acidic ferric chloride solution, it was
attacked and etched within seconds. By contrast, when the
wire coated with the method of the present invention was
contacted with the same ferric chloride solution, it
showed no sign of attack even after days.
In order to test the friction properties of wire
coated by the two methods, the conventionally coated wire
was inserted into a f~abric which is commercially referred
to as mole-skin, left in place for 30 minutes at 25C,
and then pulled through by an Instron instrument. It
showed an initial resistance of 50 gms and a dynamic
resistance of 20 gms. When a wire coated with the method
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of the invention was sub~ected to the same test, the
corresponding values were 26 gms and 9 gms.
The applicator roller pressures mentioned in the
above examples were estimated on the basis of the degree
of deformation of the surface of the roller, when station-
ary. It is believed, however, that very substantially
larger pressures (e.g. from 10 to 50 times the values given
above) are developed when the roller rotates against the
substrate at the high rotational speeds used for coating,
which will typically involve peripheral speeds of from 2
to 200 m/sec, more usually from 5 to 100 m/sec, e.g.
from 10 to 50 m/sec.
It will be understood that the present invention has
been described above purely by way of example, and modi-
fications of detail may be made without departing fromthe scope of the invention.
