CA1252674A - Coating process - Google Patents

Abstract

ABSTRACT

Coating Process A substrate is coated with a coating material by rubbing substantially dry, discrete particles of the coating material across the surface of the substrate with a sufficient rate of energy input to cause them to adhere. Preferably, the particles are carried on the surface of a soft, resilient buffing wheel rotating sufficiently rapidly to give peripheral speeds of from 2 to 200 m/s. Exemplified coating materials include metals, metal oxides and plastics.

Description

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COATING PROCESS
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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 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~

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.

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lZ~2674 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 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 . ~ .

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 ~

~ 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, ~
!

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~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 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 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.

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.

~_ .
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.

¦ 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 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.

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.

Uncoated, unsized paper of 105 g/m2 (manufactured by Tullis Russell) was coated with PMMA using a soft fabric 2~'7 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.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of coating a substrate with a material other than PTFE, comprising rubbing discrete substantially dry particles of the coating material across the surface of the substrate by means of a felt or fabric buffing wheel and with sufficient force and at sufficient speed relative to said surface to cause the material to become deposited on the surface of the substrate in an adherent film.

2. A method according to claim 1, wherein the particles are less than 100 microns in diameter.

3. A method according to claim 1, wherein the substrate is a wire thread, filament, tube or flexible web.

4. A substrate having deposited thereon an adherent substantially non-microporous film having a non-granular appearance at magnifications of at least 2000x and 12,000x, which film is less than 3 microns thick.

5. A method of coating a substrate with a coating material, comprising rubbing substantially dry, discrete particles of the coating material across the surface of the substrate by means of a felt or fabric buffing wheel and with a rate of energy input which is equal to or greater than the critical rate of energy input.