EP0739274B1

EP0739274B1 - Printing optical patterns on polymer articles

Info

Publication number: EP0739274B1
Application number: EP95905182A
Authority: EP
Inventors: Michael Glenn Arndt; Steven Allen Barth; John Bennett Fenn, Jr.; William Douglas Snider
Original assignee: Akzo Nobel UK PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1994-01-13
Filing date: 1995-01-09
Publication date: 1999-10-27
Anticipated expiration: 2015-01-09
Also published as: ES2139880T3; DE69513009D1; ATE186020T1; WO1995019266A1; DE69513009T2; AU1388995A; EP0739274A1

Abstract

An optical pattern can be placed within a hydrophobic polymer article, for example a polyester film, by printing disperse dyestuff directly onto the surface of the article followed by heat treatment to cause the dyestuff to diffuse into the surface of the article without use of any swelling agent. The pattern may be a visible pattern, and is highly resistant to scratching, scuffing and migration, bleeding or blooming. The dyestuff is preferably applied to the article together with a polymeric thickening agent. The dyestuff may be coated onto the article from a dispersion in a volatile liquid such as water or a mixture of water with a low-boiling organic solvent.

Description

Technical Field

This invention relates to methods of forming an optical, for example visible, pattern on articles including sheets and films made of hydrophobic polymers such as polyesters.

Background Art

Dyeing and dyestuffs are described generally in an article entitled "Dyeing" in Encyclopaedia of Polymer Science and Engineering, Volume 5 (1986), John Wiley & Sons (New York) at pages 214-277. Hydrophobic polymers such as polyester, cellulose acetate and triacetate, polyamide and acrylic polymers may be dyed with disperse dyestuffs. Disperse dyestuffs are coloured substances of low molecular weight which are sparingly soluble in water. They have an affinity for hydrophobic polymers and are believed to form solid solutions therein. Dyeing may be carried out by causing the dyestuff to diffuse into the polymer either from a liquid phase (a solution or dispersion of the dyestuff) or from the vapour phase (by contact diffusion or sublimation).

Disperse dyestuffs may migrate within or out from a dyed polymer article on storage. Such migration is generally undesirable. Migration within an article can lead to blurring or loss of a visible dyed pattern in the article. Migration out from an article may be referred to as blooming or bleeding. Blooming is migration of dyestuff to the surface of the article, forming a layer which can be wiped off. Bleeding is migration of dyestuff from the dyed article into another article with which it is held in contact. Migration can be caused or accelerated by the presence of plasticisers or other additives within the polymer article. Plasticisers are typically organic liquids of low volatility which act as swelling agents for the polymer and are solvents for disperse dyestuffs.

One known method of dyeing a hydrophobic polymer, for example a polyester film, with a disperse dye relies upon swelling the film with an organic solvent. The dye is generally applied to the film in solution in the solvent, either at the same time as or subsequently to the swelling treatment. The former of these techniques is the one more commonly used. The dye diffuses into the swollen polyester film, which is then washed to remove the solvent and dried. The process of diffusion may be assisted by heating. This technique can be referred to as solvent dyeing. Another known method of dyeing a polyester film utilises a suspension of disperse dye in a mixture of water and an organic solvent which swells the film. This method may be called solvent-assisted dyeing, and is otherwise similar to solvent dyeing. Neither solvent dyeing nor solvent-assisted dyeing is well suited for the production of optical patterns such as visible patterns, for example printed or traced designs, on hydrophobic polymers, because of problems of dye migration. Both techniques rely on the use of organic solvents (swelling agents) which swell and plasticise the polymer and dissolve the dyestuffs. Furthermore, such organic solvents may be difficult or expensive to remove from the film and to reclaim or to dispose of, and in general have a significant environmental impact.

Another known method of dyeing a hydrophobic polymer such as a polyester film with disperse dyestuffs relies upon thermal diffusion of the dye into the film. The dye is brought into contact with the film which is then heated to cause migration of the dye into the film. The dye may be coated onto the film and the coated film dried and heated, or the dye may be provided on a separate sheet which is brought into contact with the film and the sheet and film are then heated. The former process may be referred to as thermofixation or thermosol fixation dyeing and the latter process as transfer printing. The heating step is generally carried out at or around the sublimation temperature of the dye. The process of transfer of dye into the film may be referred to as fixation. Transfer printing may impart a grainy appearance to a film as a consequence of the surface roughness of a paper transfer sheet. Transfer printing has been used to prepare both uniformly coloured hydrophobic polymer sheets and sheets carrying a permanent printed pattern, but thermofixation dyeing has previously only been thought to be generally suitable for the manufacture of uniformly coloured sheets.

Description of the Prior Art

British patent Application 2,224,974 describes a means for decorating a substrate comprising a flexible, transparent or translucent film, having a design or colouration printed thereon by sublimation printing, such that the design or colouration at least partially penetrates the film. The film may be of polyester. Transfer printing techniques are used in order to print the design

Non-prepublished European Patent Application 587,282 (published 16th March 1994)(prior art according to Art. 54(3) EPC) describes a process for producing a coloured polyester film including the steps of (1) providing a dye mixture which is a suspension of at least one disperse dyestuff in a solution of a thickener in water; (2) coating the dye mixture onto a polyester film to form a layer; and (3) heating the film to cause the at least one dyestuff to migrate from the layer into the film, the viscosity of the dye mixture being no more than 500 mPa.s (centipoise) at ambient temperature. This process provides a coloured film with high uniformity of colouration and freedom from streaks.

British Patent Application 2,008,034 describes a process for the production of a synthetic plastics or rubber sheet structure having a surface applied pattern in one or more colours extending thereinto, in which process said pattern is applied to the surface of the sheet structure using one or more colouring agents capable of undergoing migration through the synthetic plastics material, migration of the colouring agent(s) is effected in the presence therein of at least one cross-linkable component until said colouring agents have penetrated to a predetermined depth and cross-linking of the cross-linkable components is initiated thereby to stop further migration of the colouring agent(s) on cross-linking of the cross-linkable components. Suitable plastics materials include polyvinyl chloride, polyolefines, styrene polymers, acrylic resins, polyacetals and polycarbonates. The sheet structure preferably contains auxiliary substances, for example plasticisers, lubricants and stabilisers which partially dissolve the colouring agents so that the migration of said agents is enhanced. The cross-linkable component may be included either in the street structure or in a printing ink used for the application of colour.

French Patent Application 2,318,193 describes a variety of methods for colouring polyester and other polymer films with disperse dyes. A dyestuff-bearing paper sheet for use in transfer printing can be prepared by printing the sheet with a water-based polymer-thickened ink of high viscosity (at least 1000 centipoise) containing disperse dyestuff. An ink consisting of a fine aqueous dispersion of a disperse dye, a cellulose material and, optionally, a wetting agent can be applied directly to the film to be coloured, uniformly or in patterned manner, followed by heating to cause the dyestuff to penetrate into the film. An indelible mark may be obtained on a polyester or other film by writing on the film with the aid of a liquid dispersion, preferably a water/glycol mixture which may optionally contain dispersing and/or wetting agents, of the disperse dyestuff followed by heating.

Disclosure of Invention

The invention provides a method of producing an optical pattern which absorbs light at one or more wavelengths or ranges of wavelengths within the range from 300 to 2500 nm, within a hydrophobic polymer article, the method including the steps in sequential order of:

(1) coating at least one dye composition directly onto a surface of the article in the form of the optical pattern, each at least one dye composition comprising at least one disperse dyestuff in dispersion in a volatile liquid which comprises water, the total amount of dyestuff in the composition being in the range from 0.5 to 15 percent by weight;

(2) drying the coated article under conditions which cause substantially complete evaporation of the volatile liquid; and

(3) heating the article so as to cause each at least one disperse dyestuff to diffuse into the surface of the article, the article being substantially free of any plasticiser for the hydrophobic polymer during this heating step, thereby producing the optical pattern within the article,

characterized in that the dye composition comprises from 0.2 to 10 percent by weight of a water-soluble polymeric thickener in solution, the viscosity of the dye composition being in the range 2 to 500 mPa.s (centipoise) measured under low shear conditions at ambient temperature.

The polymer is a hydrophobic polymer having affinity for disperse dyestuffs. Examples of such polymers include cellulose acetate, polyamide and especially polyester, in particular poly(ethylene terephthalate), although other polyesters including polycarbonates may be used.

The polymer article may take the form of a sheet, in particular a sheet in the form of an unsupported film having a thickness in the range 5 to 250 µm, more often 10 to 50 µm, for example 12.5 or 25 µm. Such a film is preferably biaxially oriented film. Uncoloured polymer films including polyester and acetate films of this type are readily available commercially. A polymer sheet may alternatively take the form of a thicker sheet or of a coating on an article, for example a tile. The surface of such a polymer sheet is preferably smooth, so that the dye composition may be applied thereto by conventional printing techniques.

The polymer article, for example in the form of a film or other type of sheet, is preferably substantially free of plasticisers and suchlike substances before, during and after application of the optical pattern. If such plasticisers are present, dyestuff in the optical pattern in the finished article may migrate, with the undesirable consequence of blooming and/or bleeding. Furthermore, addition of plasticisers to a hydrophobic polymer generally alters the physical properties of the polymer. It is an advantage of the invention that no plasticiser need be used. It is a further advantage of the invention that no cross-linking agent need be used, and preferably both the polymer article and the dye composition are substantially free of any cross-linking agent.

The at least one disperse dyestuff may in general be any disperse dyestuff having substantivity for hydrophobic polymers such as polyester. Many such dyestuffs are available commercially. The dye composition may contain one or several dyestuffs. The dye composition may contain an ultraviolet absorber, for example a compound of the benzophenone or benzotriazole class which is capable of diffusion into the article in the heating step.

Disperse dyestuffs are commonly classified as type A (having a molecular weight around 250), B, C or D (having a molecular weight around 450). The vapour pressure of type A dyes increases the most rapidly as the temperature is raised and consequently fixation occurs more rapidly and at lower temperatures with type A dyes than with the other types. The vapour pressure of type D dyes increases the least rapidly with temperature.

The dye composition contains a water-soluble polymeric thickener to improve its adhesion to the surface of the coated article. The polymeric thickener may, for example, be a water-soluble polymer such as sodium carboxymethyl cellulose, sodium alginate or polyvinyl alcohol. Use of such water-soluble polymers has the advantage that they can be subsequently removed by washing with water as described hereinafter.

The dry coating weight of the article, that is, the dry weight of dye composition per unit area of surface on the coated article prior to the heating step, is preferably in the range 0.1 to 2.0 gm^-2. All or part of the surface may be coated. It will be understood that two or more, for example four, dye compositions of different colours may be applied to defined areas of the surface to create the optical pattern.

The dyestuff penetrates into the surface of the polymer article during the heating step, so that the optical pattern becomes permanently fixed into the article itself. The optical pattern retains definition through the heating step, so that its degree of definition is substantially the same both before and after the heating step. The optical pattern is highly resistant to erasure, smearing or loss of definition if the surface of the article is scratched or scuffed. The optical pattern retains definition over time. No special precautions or further treatments are necessary to prevent or hinder subsequent dyestuff migration, and this is a particular advantage of the invention.

The heating step may be carried out for example at 160°C for 60 seconds or 180°C for 30 seconds, although higher or lower temperatures and other times can also be used. The heating step is carried out at a temperature below that at which degradation of polymer article or dyestuff would occur. For example, temperatures up to 190°C or 200°C may be used for poly(ethylene terephthalate). The time of the heating step is often in the range 5 to 60 seconds. The heating step can frequently be carried out at temperatures lower than conventionally used for transfer printing of disperse dyes, such printing often being performed at 200-210°C. It is generally thought that disperse dyes move into the substrate during transfer processes by sublimation. It was therefore surprising to find that a wide variety of disperse dyes could be caused to diffuse into a polymer article at temperatures at or below the sublimation temperature of the dyestuff. The ability to use relatively low fixation temperatures is an advantage of the invention. Thermoplastic films, for example polyester films, may be damaged, for example they may shrink, and dyestuffs may be chemically degraded by excessive heating. The most appropriate heating time at any particular temperature can be selected on the basis of experiments in which the increase in colouration of samples of the article with time at that temperature is monitored. Different dyestuffs may diffuse at different rates. It is an advantage of the invention as compared with transfer printing that diffusion and fixation takes place as contact diffusion through a zero air-gap. The heating step is preferably carried out in a hot air oven, although other heating methods may also be used.

The optical pattern produced in the article by the method of the invention may be visible to the naked eye, that is to say absorb light at one or more wavelengths or ranges of wavelengths within the range 400 to 700 nm. Alternatively, the optical pattern may comprise or consist of a pattern which absorbs ultraviolet light and/or infrared light and/or which fluoresces on exposure to ultraviolet light. The optical pattern may be photochromic. The optical pattern generally absorbs light at one or more wavelengths or ranges of wavelengths within the range 300 to 2500 nm.

The coating layer becomes exhausted of dyestuff during the heating step as dyestuff diffuses into the article. After the heating step, the exhausted layer comprises any residual dyestuff which has not diffused into the sheet and thickener. This exhausted layer is generally removed from the article by washing, preferably with water. The article may be washed by passage through hot or cold water, preferably hot water. The article may be passed through an agitated bath of water. The article may alternatively or additionally be sprayed with water. Alternatively, the article may be washed by passage through cold or warm water while being subjected to ultrasonic vibrations. If desired, such a water wash may be preceded by a washing treatment with a water-miscible organic solvent, for example a ketone solvent such as acetone or an amide solvent such as N-methyl-2-pyrrolidinone. The development of colouration after different heating times as mentioned hereinabove may also be monitored by spectroscopic or other measurement of the amount of dye washed off the article in this step. A further advantage of the present invention is that, because no swelling agent is required, and because such swelling agents are generally solvents for disperse dyestuffs, as a consequence less dye is washed out of the article in the washing step. The invention therefore provides efficient dyestuff use and washing liquors containing only low levels of contaminant.

The heating step may be performed under conditions in which essentially all the dyestuff, for example 95% or 98% or more of the dyestuff, diffuses from the layer into the article, and such conditions are preferred. Under such conditions, the washing liquor may be very lightly coloured or nearly colourless.

The at least one disperse dyestuff generally penetrates only a relatively small distance into the surface of the article during the heating step. It penetrates into the surface region adjacent to and underlying the coated surface and is substantially all present in this surface region. Dye distribution through a sheet or film may be studied for example by spectroscopy, for example infrared spectroscopy, of thin sections cut with a microtome. For example, in a dyed film 25 micron thick it may be observed that 90% of the dyestuff is to be found in a surface region amounting to about 10 or 20% of the thickness and that the remaining 10% of the dyestuff is to be found in the remaining 80 or 90% or so of the thickness. If the degree of levelling through the film is greater, it may be observed that 90% of the dyestuff is to be found in a surface region amounting to about 50% of the film thickness. This effect may be advantageous if one or more of the dyes used is sensitive to light, in particular ultraviolet light. At least the sensitive dye may be printed on the side of the film intended in use to be remote from the source of light, for example sunlight. The polyester provides some protection to the sensitive dye, which is an advantage of the invention. The film may contain a substance which absorbs the harmful wavelengths of light, for example an ultraviolet absorber. The ultraviolet absorber may be incorporated into the film in a number of ways. For example, it may be incorporated into the film during its manufacture. Alternatively, the film may be dyed with the ultraviolet absorber, preferably on the side of the film intended in use to be nearer the source of light. The amount of harmful light reaching the sensitive dye is thereby reduced to a minimum.

The dye composition may contain a minimum of 0.1%, preferably 0.5%, more preferably 2%, by weight total dyestuff solids. The dye composition may contain a maximum of 20%, preferably 15%, more preferably 10%, by weight total dyestuff solids.

The dye composition contains the at least one disperse dyestuff in dispersion in a volatile liquid or mixture of volatile liquids. Suitable volatile liquids include water and low-boiling organic solvents such as alcohols, esters and ketones. Suitable organic solvents preferably boil the range 50 to 150°C, more preferably 70 to 120°C, at atmospheric pressure. Such organic solvents are generally neither plasticisers for the polymer nor solvents for disperse dyestuff to any substantial extent, and solvents of this type are preferred. A mixture containing a major proportion of water and a minor proportion of a water-miscible volatile organic solvent such as ethanol or isopropanol may be preferred as the volatile liquid. It is believed that the organic solvent in such a mixture acts as a wetting agent. It was nevertheless surprising to observe that water-based dye mixtures could be satisfactorily applied to a hydrophobic polymer sheet to provide well-defined reproducible patterns on the sheet. Use of mixtures containing a major proportion of water is inherently safer and cheaper than the use of liquids which contain a high proportion of inflammable and relatively expensive organic solvents. The dye mixture may contain 5 to 50% by weight of the organic solvent, more preferably 5 to 40%, further preferably 10 to 25%, with the balance of the volatile liquid being water. In an alternative preferred embodiment, the volatile liquid is water and the dye composition contains a surface active wetting agent. Commercial liquid disperse dyestuffs commonly contain a surface active dispersing agent, and such an agent may also serve as the wetting agent in the dye composition. In some cases, it may be desirable to add additional wetting agent to the dye composition to achieve the best results.

The dye mixture contains a water soluble polymeric thickener, which is preferably completely dissolved in the mixture. If the dye mixture contains a major proportion of water, the thickener may be, for example sodium carboxymethyl cellulose, sodium alginate or polyvinyl alcohol. Alternative water-soluble polymers include polymers based on acrylic acid or acrylamide. Use of water-soluble polymers as thickeners ensures they can be removed from the article after the heating step by washing with water. The thickener is preferably biodegradable.

The polymeric thickener and its concentration are chosen to provide a desired viscosity for the dye mixture. The viscosity of the dye mixture can be controlled by suitable choice of the concentration and molecular weight of the polymeric thickener. The dye mixture contains a minimum of 0.2% by weight of the thickener, preferably 0.5%. The dye mixture will contain a maximum of 10% by weight of the thickener, preferably 5%, more preferably 2%. It is an advantage of the invention that low concentrations and amounts of thickener can be used, because the thickener can be washed off the film and discarded after use.

The viscosity of the dye mixture should be chosen having regard to the method by which it is to be coated onto the polymer article. If the dye mixture is to be coated onto the polymer article using a pen or suchlike implement, the viscosity or the dye mixture may be that of a conventional ink for such pens, for example in the range 2 to 200, preferably 5 to 50 mPa.s (centipoise), measured under low shear conditions at ambient temperature.

The solids in the dye mixture comprise the disperse dyestuff and the thickener. The minimum total solids content of the dye mixture by weight is preferably 0.1%, more preferably 0.2%, further preferably 2% or 5%. The maximum total solids content of the dye mixture by weight may be up to about 25% and is preferably 20%, more preferably 15%, further preferably 10%.

The dye mixture may be made up using a disperse dyestuff in powder or liquid form. Liquid disperse dyestuffs are commercially available and comprise a suspension of a disperse dyestuff in an aqueous solution, often also containing a water-miscible high-boiling organic liquid such as ethylene glycol or propylene glycol, for example 10 to 25% or around 20% by weight of a glycol. Such liquid disperse dyestuffs commonly contain around 20 to 50% by weight, for example about 40% by weight, of the dyestuff, and are viscous pastes which commonly have a viscosity around 2000 to 5000 mPa.s (centipoise). Some disperse dyestuffs are only available commercially in liquid form. Use of liquid disperse dyestuffs may be preferred for convenience. It will be understood that liquid disperse dyestuffs generally need to be diluted before coating onto a hydrophobic polymer article.

The dye mixture may be filtered, for example through a fibre or steel woven gauze, to remove oversize particles. The dye mixture preferably contains essentially no particles larger than 50 µm in size, more preferably 20 µm, further preferably 10 µm. Average particle size is preferably less than about 1 µm.

The dye mixture may be coated onto the surface of a polymer article, for example a film or other sheet, by conventional printing techniques, for example silkscreen printing or gravure, flexographic or offset printing. The dye mixture may alternatively be coated onto the polymer sheet mechanically, for example using a plotter, or manually using a pen, for example a tubular, marker or ballpoint pen, a brush, for example an artist's brush, a stamp head, inkjet or spray, or suchlike implements.

The wet coated article is dried either separately from the heating/diffusion step or as a preliminary part of that step. The wet coating may be dried at ambient or elevated temperature, for example in an electrically-heated oven at around 80-120°C or by exposure to infrared radiation. A film may be dried on steam-heated rollers. The time between coating and drying should not be so long that deterioration or contamination of the wet coating takes place.

It is known that polyester film containing swelling agent tends to shrink, perhaps by up to 5 or 6%, during heating. The substantial absence of swelling agents from the dye mixture is an advantage in the heating step. It has been found that films treated according to the process of the invention generally shrink by no more than 0.5 to 2% during the heating step, for example around 1% at 180°C or around 2% at 200°C.

It is known to be difficult in the prior art processes of solvent dyeing and solvent-assisted dyeing to remove all the swelling agent after dyeing. The presence of residual swelling agent alters the properties, of the dyed film. Many of the known swelling agents have undesirable toxicological or environmental properties. It is an advantage of the present invention that no swelling agent need be present in the dye mixture. If a dye mixture containing a low-boiling organic solvent is used, such solvent is readily evaporated from the coating during the drying step. Dye mixtures which are substantially free of swelling agent are preferred.

Polyester film may be metallised, for example for use in solar control applications. It is sometimes found that the metal does not adhere well or uniformly to the surface of the film during the metallisation process. This defect has been attributed to the presence of residual swelling agent in the film. This disadvantage can be overcome by the present invention, since no swelling agent is required. Even if the dye mixture contains a low level of swelling agent, for example when a liquid disperse dye containing a swelling agent as mentioned hereinabove is used, much less swelling agent is present than in prior art solvent dyeing or solvent-assisted dyeing processes.

An article containing within it an optical pattern produced by the method of the invention may subsequently have applied to it a scratch-resistant coating or hardcoat or the like using known techniques.

Industrial Applicability

The optical pattern on an article produced by the method of the invention is highly resistant to erasure, scratching, scuffing and the like, because the pattern is contained within the sheet itself. Polymer films, such as polyester films, bearing therein an optical pattern can be laminated to other films including metallised films. Films bearing an optical pattern can be used in the manufacture of laminated labels, useful for example as security labels for personal identification and the like. The dyestuff may be caused to diffuse from the dried coating layer into the film during a hot lamination step during the manufacture of such labels. Polymer films bearing an optical pattern can be adhered to glass, for example window glass, and suchlike materials. Patterned films made using the invention are useful for decorative purposes. The method of the invention is suitable for the production of patterned films for archival purposes or for use as durable transparencies for overhead projectors and the like. The method of the invention is useful for printing charts, maps, plans, engineering drawings and suchlike information onto hydrophobic polymer films.

Description of the Drawing

The accompanying Figure is a schematic diagram of a process for producing a polyester film bearing an optical pattern according to the invention.

Modes for Carrying out the Invention

Referring to the Figure, a roll of polyester film 1 is unwound and the running film indicated generally at 2 is fed over a patterned reverse gravure printing roll 3 which serves to coat it with a dye mixture, so yielding a film coated with a wet patterned layer of the dye mixture as indicated generally at 4. The dye mixture is a suspension having a viscosity of no more than 500 mPa.S. (centipoise) at ambient temperature of at least one disperse dyestuff in a solution of a thickener in water as hereinbefore described. The dye mixture is contained in pan 5 through which gravure roll 3 rotates. Metering roll 6 is in near contact with gravure roll 3 and serves to ensure that the printing areas of gravure roll 3 are coated with a thin uniform layer of dye mixture when gravure roll 3 contacts the film. Gravure roll 3 rotates in the opposite sense to the direction of travel of the film as indicated by the curved arrow. The film is held in grazing contact with gravure roll 3 by means of the pair of

yoke rollers

7, 8 to ensure transference of the dye mixture from gravure roll 3 to the film. It will readily be understood that other forms and configurations of gravure printing equipment known in the art may be used, for example involving more complex film paths or the use of a doctor blade instead of a metering roll to provide the necessary thin layer of dye mixture on the gravure roll. It will also be understood that other printing techniques can be used, and that several dye mixtures of different colours, for example four dye mixtures, may be applied to the film by conventional printing techniques. The film is then fed through oven 9 maintained at a temperature of about 80-120°C to dry the layer of dye mixture on the film. The path of the film through oven 9 may for example be about 4.5 (15 feet) long, this length being chosen to suit the conditions of operation of the process. The film bearing a dry layer of dye mixture in the form of an optical pattern indicated generally at 10 is then passed through oven 11 maintained at a temperature of for example about 160-180°C in order to cause the dyestuff to migrate from the dry layer into the film. The path of the film through oven 11 may for example be about 4.5 (15 feet) long, this length being chosen to suit the conditions of operation of the process. The dyed film coated with an exhausted layer of thickener indicated generally at 12 is passed through wash bath 13 containing an organic solvent such as acetone or N-methyl-2-pyrrolidinone and then through wash bath 14 containing water. The washed film dyed with a coloured pattern indicated generally at 15 is then passed through oven 16 to dry the film, and the coloured film taken up on roll 17 at the conclusion of the process.

The invention is illustrated by the following Examples, in which all parts, percentages and proportions are by weight unless otherwise specified:-

Example 1

A dye mixture can be produced by diluting a commercially-available liquid disperse dyestuff with a solution of a water-soluble polymer such as sodium carboxymethyl cellulose (CMC) or sodium alginate in water. Such a dye mixture may contain about 10-20% dyestuff and about 2-3% water soluble polymer and have a viscosity in the range about 300 to about 500 mPa.s measured under low-shear conditions at ambient temperature. Lower viscosity dye compositions, for example of viscosity about 170 mPa.s, may alternatively be used, but may be found to give less satisfactory results in silkscreen printing. The dye mixture may contain about 10% by weighs isopropanol. Multicoloured patterns can be printed onto a polyester film, for example 25 µm thick, by silkscreen printing using several such dye mixtures. The wet coated film so produced can be dried at about 100°C in an air oven. The dry coated film can be passed through two 1.5m (five foot) long ovens maintained at 185°C at a speed of 0.12ms^-1 (24 ft/min) to cause diffusion of the dyestuff from the dry coating layer into the film. The film can then be washed with hot water to remove the water-soluble polymer and any residual dyestuff from the surface of the film. This process has been used to reproduce coloured pages from a house journal onto polyester film with a high degree of fidelity. The printed patterns so produced were highly resistant to erasure and scuffing.

Example 2

A dye mixture can be produced in the manner described in Example 1, but containing about 5% dyestuff and about 1% water-soluble polymer and having a viscosity in the range about 25 to about 50 mPa.s measured under low-shear conditions at ambient temperature, by choosing a suitable low-viscosity grade of water-soluble polymer. Such dye mixtures can be coated onto the surface of a polyester film using one or more plotter pens so as to reproduce engineering and suchlike drawings prepared using a computer-assisted drafting system. The optical pattern can alternatively be coated onto the film by freehand drawing using felt-tip marker pens containing such inks. It will be understood that the film is coated in this way over only part of its surface. The wet coated film may be dried and heated as described in Example 1 to provide a permanent dyed record of the drawing within the film.

Claims

A method of producing an optical pattern which absorbs light at one or more wavelengths or ranges of wavelengths within the range from 300 to 2500nm, within a hydrophobic polymer article, the method including the steps in sequential order of:

(1) coating at least one dye composition directly onto a surface of the article in the form of the optical pattern, each at least one dye composition comprising at least one disperse dyestuff in dispersion in a volatile liquid which comprises water, the total amount of dyestuff in the composition being in the range from 0.5 to 15 per cent by weight;

(2) drying the coated article under conditions which cause substantially complete evaporation of the volatile liquid; and

(3) heating the article so as to cause each at least one disperse dyestuff to diffuse into the surface of the article, the article being substantially free of any plasticiser for the hydrophobic polymer during this heating step, thereby producing the optical pattern within the article,

characterized in that the dye composition comprises from 0.2 to 10 per cent by weight of a water-soluble polymeric thickener in solution, the viscosity of the dye composition being in the range 2 to 500 mPa.s (centipoise) measured under low shear conditions at ambient temperature.
The method according to claim 1, characterized in that the volatile liquid is a mixture containing a major proportion of water and a minor proportion of a water-miscible organic solvent of boiling point in the range 50 to 150°C.
The method according to one of claim 1 or claim 2, characterised in that the polymeric thickener comprises sodium carboxymethyl cellulose, sodium alginate or poly(vinyl alcohol).
The method according to any one of the preceding claims, characterised in that the dye composition is coated onto the hydrophobic polymer article by means of a pen.
The method according to any one of the preceding claims, characterised in that the viscosity of the dye composition is in the range about 5 to about 50 mPa.s (centipoise) measured under low shear conditions at ambient temperature.
The method according to any one of the preceding claims, characterised in that the hydrophobic polymer article is a polyester film.
The method according to claim 6, characterised in that the polyester is poly(ethylene terephthalate).
The method according to any one of the preceding claims, characterised in that the optical pattern is visible to the naked eye.
The method according to any one of the preceding claims, characterised in that it additionally includes the step (4) of washing the article with water to remove residual coated dye composition therefrom.
The method according to claim 9, characterised in that it includes the step (5) of coating the hydrophobic polymer article with a scratch-resistant coating or hardcoat subsequent to the washing step (4).