WO2008040992A1

WO2008040992A1 - Improved transfer paper for selective image printing

Info

Publication number: WO2008040992A1
Application number: PCT/GB2007/003783
Authority: WO
Inventors: Simon Daplyn; Ged Hastie; Andrew Hancock
Original assignee: Kiss Cut Technology Limited
Priority date: 2006-10-06
Filing date: 2007-10-05
Publication date: 2008-04-10

Abstract

The present invention relates to a transfer medium for use when printing images onto textiles. In particular, the invention relates to a transfer paper (1) suitable for use in the printing of images onto textiles where the transfer paper (1) comprises a base substrate (6), a layer of thermoplastic film-forming polymer (3) and a barrier layer (5).

Description

Improved Transfer Paper for selective image printing

The present invention relates to a transfer medium for use when printing images onto textiles. In particular, the invention relates to a transfer paper for the selective transfer of inkjet printed images on to fabric material.

Textile printing involves the application of dyes or inks etc. in a localised design or pattern to a textile material such as the fabric of a T-shirt or other item of clothing.

Various methods for performing textile printing are known in the art which allow images to be transferred onto clothing and other textile articles.

Traditionally, screen-printing methods such as flat-bed, rotary and cylinder techniques have been used to print on to textiles. These screen-printing processes involve the sequential application of coloured dyes Lo the fabric, followed by a drying stage. However, there are inherent disadvantages to screen-printing techniques. As these are "wet-printing" techniques, the procedure necessitates a drying or curing step, often resulting in harmful emissions due to solvent elimination (organic solvents). It is also recognised that these screen-printing processes are not capable of matching the quality of digital print images. Furthermore, it is both time-consuming and laborious to change or alter the printed design once the screen-print is set up, making these techniques only suitable and cost- effective for large print-runs.

It is known in the art to print images directly on to fabrics using digital printers, an example of which can be seen described in US2006/0210719. However, these techniques are often time-consuming, comprising pre-treatment and drying steps. Furthermore, as direct digital textile printing is still an emerging technology, the high costs of suitable laser printers can be prohibitive, making these methods particularly unsuitable for smaller-scale printing.

Transfer printing is also a well known process for producing images on fabrics. This technique involves the printing of the desired print, such as a photograph, graphic design or text etc., onto an appropriate specialised transfer paper by techniques such as screen or digital printing, followed by the transfer of the image to the fabric. Transfer sheets for this purpose commonly consist of a release paper (typically a paper where at least one side is coated with a material, such as silicone, wax, paper/polythene laminates, chrome complexes etc., that can be "released" from adhesive,) coated first with a layer of hot-melt adhesive, followed by an inkjet receptive layer if using such technology. In use, the ink is typically printed onto the inkjet receptive layer, forming a mirror image of the desired print on the surface of the transfer sheet. Transfer onto the fabric is then performed by placing the transfer sheet image-face down onto the fabric (which has been pre-pressed to remove any creases) and using a heat press to apply heat to the reverse of the transfer sheet (the time and temperature are dictated by the characteristics of the transfer paper that is used) . The heat application causes the hot-melt to release from the transfer sheet and adhere to the fabric taking the ink image with it. The transfer sheet is then removed either immediately (hot-peel) or after a cooling period (cold- peel) .

Transfer printing is advantageous as the "dry printing" process eliminates the solvent waste associated with the "wet printing" techniques. Furthermore, transfer sheets can be readily prepared in large quantities with the desired images for application to a variety of different fabrics.

However, there are also inherent problems associated with the transfer printing techniques. The first of these is the so-called "A4 effect" or "polymer window" effect. A noticeable impression of the transfer sheet is discernable on the fabric surrounding the print. This is the trace of the transfer sheet itself which is often visible on the resultant printed product. This is caused by the transfer of the entire hot-melt layer from the transfer sheet onto the printed textile, including the sections which do not have image printed onto them. Furthermore, this hot-melt trace often absorbs colours during washing, ultimately causing greying of the polymer window. A further disadvantage of the transfer printing technique is the colour-fastness of the transferred image. Repeated washing of the printed fabric can cause the ink to degrade over time, fading the print. This can render many transfer printed articles unsuitable for industrial purposes where frequent washing is often required.

Digital printing directly onto textiles is known. However, these processes require the use of expensive laser technology and a multi-step process to form the print^ again making it unsuitable for smaller print runs.

Another area of rapid growth within the digital market is the application of conductive polymers. Such technology has potentially wide-ranging applications including, for example, tracking of products such as laundered clothing, smart packaging and traceability.

Whilst digital printing of electronics is known, again these techniques require the use of sophisticated ink jet printers. Furthermore, the inks themselves are expensive, limiting the potentially wide-ranging use of this technology.

The high cost of the inks required for printing electronics results from the stringent criteria required of such inks in relation to particle size and shape (in particular of metallic pigments), agglomeration of particles, and the difficulty in laying down a thick enough layer of the ink to produce the required conductivity. In particular, the necessary compromise between rheology and conductivity places limitations on the potential applications of the technology.

The present invention identifies the drawbacks of conventional fabric printing techniques and proposes a transfer sheet and method of using said sheet which mitigates one or more of the problems associated with the prior art.

According to a first aspect of the present invention there is provided a transfer paper suitable for use in the printing of images onto textiles, comprising a base substrate; a layer of thermoplastic film-forming polymer; and a barrier layer wherein the barrier layer is adapted to prevent the transfer of the thermoplastic film forming polymer to textiles upon heating and is soluble in one or more solvents .

Throughout this document the term "solvent" is intended to be taken in the broadest sense, to include any substance, usually a liquid, capable of dissolving another substance, and includes water, aqueous media and organic solvents.

Advantageously, when an image is printed onto the transfer paper using solvent or solvent-based ink, the barrier layer dissolves only where it is in contact with the solvent. Thus, upon heating, the layer of thermoplastic film forming polymer is only able to pass the barrier layer and adhere to a separate piece of fabric at those points where the image has been printed. At those points where no image is printed the barrier layer remains intact and the thermoplastic film forming polymer layer cannot pass through and adhere onto fabric upon heating.

Preferably, the transfer paper further comprises a conductive element.

The conductive element may be, for example, a conductive polymer, or it may be a conductive particle such as a metal particle .

Conductive polymers suitable for use with the transfer paper of the invention may comprise material selected from the group comprising; polyacetylene, polypyrrole, polyaniline, poly ( tiophene) , polyfluorene, poly (3-hexylthiophene) , polynapthalene, poly (p-phenylene sulfide) and poly (para- phenylene vinylene) . Examples include Poly(ethylene- dioxythiophene) or PEDT, polyaniline and polypyrrole-coated carbon powders, soluble emeraldine base, polyaniline powders and aqueous and non-aqueous dispersions.

Preferably, the barrier layer overlays the layer of thermoplastic film-forming polymer.

Optionally, the transfer paper comprises at least one additional layer.

Advantageously, where there is an additional layer or layers present on the transfer paper, upon heating the transfer- paper will transfer not only the bottom-most hot-melt layer but also any subsequent layer or layers present on the transfer paper. Preferably, the at least one additional layer comprises a conductive element .

Providing the transfer paper with an additional layer provides a convenient means to introduce materials with desirous properties into the transferred image. For example, the additional layer may comprise metallic pigments or flakes, or may be a conductive polymer layer.

Alternatively, the conductive element is incorporated into the thermoplastic film-forming polymer layer.

Preferably the layer of thermoplastic film-forming polymer comprises a hot-melt adhesive.

Preferably the layer of thermoplastic film-forming polymer has a melting point in the range of 60⁰C to 220⁰C.

Optionally the barrier-layer is water-soluble.

As water-based ink sets that have been developed by the inventors provide significant improvements over other inks when considering wash-fastness, producing better overall products, it is preferable to use a barrier layer that is soluble with respect to water as it can be utilised with these inks .

Preferably the thermoplastic film-forming polymer comprises a film-forming binder and a thermoplastic polymer.

Advantageously, such a polymer can exist as a single (one part) liquid prior to application. Preferably the layer of thermoplastic film-forming polymer comprises material selected from the group comprising; polyamides, polyolefins, polyesters, polyurethanes, poly(vinyl chloride), poly(vinyl acetate), polyethylene oxide, polyacrylates , polystyrene, polyacrylic acid, and polymethacrylic acid; or copolymers or mixtures thereof.

Preferably, the thermoplastic polymer is an ethylene vinyl acetate based polymer.

Optionally, the thermoplastic film-forming polymer is a conductive polymer.

When the thermoplastic film-forming polymer is a conductive polymer, this advantageously means that the conductive properties of the polymer can be exploited in the transferred image.

Alternatively, the thermoplastic film-forming polymer comprises a conductive element.

Conductive elements suitable for this purpose include metal particles such as copper, gold, nickel, silver alloys, silver plated metals and silver.

Alternatively, the layer of thermoplastic film-forming polymer comprises material selected from the group comprising; ethylene/acrylic acid, ethylene/methacrylic acid, and ethylene/vinyl acetate copolymers. Preferably, the thermoplastic film-forming polymers have a particle size of less than 50 microns.

Selecting polymers of this size improves the film-forming ability of the thermoplastic and allows for a thin coating of hot-melt, and as such confers less handle to the fabric upon transfer.

Most preferably the thermoplastic film-forming polymer has a particle size of between 2 and 20 microns.

Optionally the layer of thermoplastic film-forming polymer comprises additional pigments, dyes or products which provide visual effects.

Incorporating additional pigments, dyes or visual effect products such as glitter, pearlescent or luminescent particles into the layer of thermoplastic film forming polymer or hot melt adhesive would produce a transfer paper where a colourless ink pattern could be printed onto it allowing the printing of a pre-coloured design onto textiles .

Optionally the thermoplastic film-forming polymer may be overlayed with a one or more image-receptive films.

If the thermoplastic film-forming polymer is not itself suitably ink- or image-receptive an additional layer of image-receptive material can be provided with similar melt properties, and the barrier layer is able to perform this function if desired. Preferably the barrier layer comprises solvent, a solvent- soluble film-former and an image enhancing material.

Preferably the image-enhancing material comprises silica.

Alternatively, the image-enhancing material may be selected from the group comprising; polyvinyl alcohol, gelatine, alginate, complexing salts, clays, precipitated carbonate and aluminium.

Most preferably, the barrier layer comprises water, a water- soluble film-former and silica.

Advantageously, the water component acts as a carrier and silica is used to increase the affinity of ink to the barrier layer to ensure a high quality image is formed as well as to ensure that the barrier layer dissolves sufficiently in the presence of external water or water based ink to allow good image transfer.

Typically the water-soluble film-former is selected from the list comprising; agar, alginate, gelatins, starches or synthetic polymers.

Preferably the soluble film former is a synthetic polymer selected from the list comprising; poly vinyl acetates, polyurethanes , polyvinyl alcohols, polyvinyl pyrrolidones , acrylics, carboxymethyl cellulose and other cellulose derivatives, starch derivatives, and guar derivatives.

Most preferably the water soluble film former is polyvinyl alcohol . Preferably, the barrier layer is formed from a 5-50% solution of polyvinyl alcohol in water, mixed with colloidal silica.

Most preferably the barrier layer is formed from a 17.5% solution of polyvinyl alcohol in water, mixed with colloidal silica.

Preferably the barrier layer is less than 50 microns thick.

Most preferably the barrier layer is between 1 and 15 microns thick.

Selecting a barrier layer of this thickness allows for the rapid and complete dissolving of the barrier layer when required.

According to a second aspect of the present invention there is provided a barrier layer-forming composition for application to a transfer paper, comprising solvent, a solvent-soluble film-former and silica, wherein the solvent- soluble film-former is adapted to form a heat-stable film which is soluble in one or more solvents.

Preferably the barrier layer-forming composition is adapted to form a heat stable film which is stable at temperatures - up to approximately 22O⁰C.

In order to provide a better understanding of the present invention embodiments will now be described, by way of example only, and with reference to the following drawings in which;

Figure 1 is a cross-section view of a release paper of the present invention prior to ink addition (Ia) and post-ink addition (Ib); and

Figure 2 is a schematic showing the effect of prior art transfer papers when printing (2a) and the effect of the transfer paper of the present invention (2b) .

In a first embodiment of the present invention there is provided a transfer paper 1, generally depicted in figure 1, which can be used for transferring a design 2 onto a T-shirt 4.

The transfer paper 1 is laminate in nature. In the preferred embodiment the first layer or base substrate is a paper 6, which is readily prepared by one of ordinary skill in the art. However, it is clear that any suitable base substrate can be used including papers, non-woven webs, films etc. The thickness of the base substrate may also vary, although it is typically in the range of approximately 50μm - 250μm which allows for both strength and flexibility.

In certain embodiments the paper is coated on its upper surface with silicone to provide a "stick-resistant" coating which allows the paper to be peeled away from other layers after heat transfer has occurred.

As shown in figure 1, the paper base 6 is overlaid with a film layer of image/ink receptive hot-melt adhesive 3. The hot-melt adhesive 3 comprises between 15 and 80 percent film forming binder and from 20 to 85 percent of a thermoplastic polymer. The hot melt adhesive 3 melts in the temperature range of approximately 60⁰C to 180⁰C such that, when heated, it can then flow from the base paper and adhere to a separate piece of fabric such as the front of a T-shirt 4.

Although the preferred embodiment utilises a barrier layer that is itself image-receptive on top of a hot-melt layer, it will be clear to one skilled in the art that, if required, the melt-transfer film layer may itself act as an image-receptive layer.

The topmost layer is a barrier layer 5, which in the preferred embodiment comprises polyvinyl alcohol containing silica This layer is cast to act as a barrier for the hot melt adhesive 3 (or potentially as a barrier to the melt- transfer film layer and an image receptive layer if this is what is being used) such that if the transfer paper 1 is heated to a temperature where the hot-melt adhesive 3 melts and flows, the polyvinyl alcohol barrier layer 5 does not melt and prevents the hot-melt adhesive 3 from transferring to fabric. However, as the barrier film 5 is highly water- soluble it will dissolve when it comes into contact with a water-based ink.

In alternative embodiments, the hot melt adhesive 3 can further comprise colour pigments or dyes giving possible effects such as pearlescence or luminescence or can be mixed with other products such as glitter which provide a visual effect . Using transfer paper to produce textile images

When using the transfer paper 1, an ink jet printer is used to print a mirror image of the image of interest 2 onto to the upper surface of the transfer paper 1 (the upper surface being the barrier layer 5) using a water-based ink. The application of the water-based ink causes the barrier layer 5 to dissolve where the image 2 is printed and the image is then taken into the hot-melt adhesive. At points where no image is printed the barrier layer 5 remains intact. A T-shirt 4 is pressed using a heat press at 180 ²C for 3 to 15 seconds to remove any ambient moisture and then the transfer paper 1 is placed on top of the T-shirt 4, design side/upper surface down, and pressed using a heat press for 4 seconds at 180^aC. The temperature increase causes the hot-melt adhesive 3 to melt and flow onto the material of the T-shirt only where the barrier layer 5 has been dissolved by the water-based ink. Thus, upon application of heat, only the imaged areas are transferred to a T-shirt 4. The transfer paper 1 is then peeled quickly and smoothly from the T-shirt, leaving only the sections of hot-melt adhesive which contain the image on the T-shirt. The T-shirt 4 is then pressed again for 30-90 seconds at 160 to 180^aC.

Figure Ib shows a transfer paper 1 which has had aqueous or water-based ink printed onto its upper surface removing some of the barrier layer 5. If an image 2 is printed in this manner, using water based-ink prior to the heating of the transfer paper, the ink will dissolve the barrier 5 only at the points where the image 2 has been printed. This allows the hot melt adhesive to melt and flow through the barrier at the points where the image has been printed but leaves the barrier intact where no ink design has been printed and thus there is no transfer from these areas.

As can be see in figure 2, the method described above results in only the image-bearing hot melt adhesive being transferred to the T-shirt 4 (figure 2b) , with the remainder of the hot-melt adhesive layer which did not have image printed remaining on the transfer paper when it is peeled from the T-shirt after heat pressing. With prior art transfer paper, only the base substrate paper is peeled away after heating leaving all of the hot melt adhesive layer on the T-shirt (see figure 2a), even where no image has been printing leaving an unsightly and problematic "A4 effect" or "polymαr window" .

If an alternative transfer paper is used where the hot melt adhesive also comprises colour pigments or dyes giving possible effects such as pearlescence or luminescence or where the hot-melt adhesive has been mixed with other products such as glitter which provide a visual effect, or conductive particles an image can be printed using water alone, or a colourless ink formulation. The water simply dissolves the barrier layer on contact, and allows the pre- coloured or pre-mixed hot melt adhesive to transfer onto fabric in a desired shape or design. This is particularly useful if printing, for example, lettering in a particular colour or with a particular effect as a pre-coloured transfer paper can be selected and there is no need for additional expensive inks.

In a further alternative using the above transfer paper where only the printed design has been transferred to the substrate (as opposed to the entire hot-melt window as in conventional transfer techniques) , a secondary layer may subsequently be applied to the transferred hot-melt image. Once the printed design has been transferred to the substrate it has the hot-melt adhesive as the outermost layer on the substrate. A secondary layer can therefore be applied to the printed design, and heat pressed onto the existing hot-melt design. As an example, a fully conductive metallic foil could be placed on top of the printed hot melt image and heat pressed onto it. Once cooled, the foil is peeled off and the metallised particles will have adhered to- the hot-melt areas only, thus imparting a metallised layer to the existing image design. Any area of the foil not in contact with the hot-melt adhesive will remain on the polyester film

Such foils are prepared by sputter-coating the required conducting particles onto a polyester film.

Manufacture of transfer paper

When making the transfer paper it is well known in the art how to manufacture papers and apply a hot melt adhesive layer, see for example US 7 087 274 or US 5 242 739. In order to provide the transfer paper of the present invention, the manufacture will involve the additional step of preparing a solution of polyvinyl alcohol (provided by Monosol, Hatrlebury, Worcestershire or from AMC (UK) Ltd. Wirral Merseyside) in water and mixing with, the colloidal silica (Ludox CL provided by GRACE Davison, Columbia, Maryland) to form a water soluble film forming polymer. The solution of polymer and colloidal silica is then hand applied using a K-Bar to drawdown a thin film with complete coverage of the sheet. Typically, a 6 micron layer is applied, although thicker layers up to 50 microns may work. Other application methods would work equally successfully such as roller, blade, wire bar, dip, solution-extrusion, air-knife, gravure, spray coating, lithographic printing, and screen printing. The coated paper is then dried by air drying at room temperature (alternatively the coated paper could be dried by oven tunnel drying at elevated temperatures (40-180°C), hot roller, infra red or using any direct or indirect heat source) .

If desired, rather than manufacturing the entire transfer paper, it is possible to simply modify existing papers which already comprise a hot-melt adhesive layer. For example, the abovementioned solution of polymer and colloidal silica can be applied to Forever Classic + Universal A4R transfer paper supplied by Forever Promotions, Mannheim, Germany. In certain cases it will be necessary to remove any contaminants that may be present on pre-existing transfer paper by wiping down with a cleansing agent, e.g. with isopropyl alcohol, and then allowing the paper to dry before applying the barrier layer.

It will be clear to a man skilled in the art that modifications may be made without departing from the scope of the invention herein intended. In particular, although the description above relates to a water soluble barrier layer for use with water or water based inks, it is clear that the barrier layer could be soluble in a different solvent and the printing inks would simply be based on the solvent in which the barrier layer will dissolve.

Claims

1. A transfer paper suitable for use in the printing of images onto textiles, comprising a base substrate; a layer of thermoplastic film-forming polymer; and a barrier layer wherein the barrier layer is adapted to prevent the transfer of the thermoplastic film forming polymer to textiles upon heating and is soluble in one or more solvents.

2. A transfer paper as in Claim 1 wherein the transfer paper further comprises a conductive element.

3. A transfer paper as in Claim 2 wherein the conductive element is a conductive polymer.

4. A transfer paper as in Claim 2 wherein the conductive element is a conductive particle such as a metal particle .

5. A transfer paper as in Claim 3 wherein the conductive polymers comprise material selected from the group comprising; polyacetylene, polypyrrole, polyaniline, poly ( tiophene) , polyfluorene, poly (3-hexylthiophene) , polynapthalene, poly (p-phenylene sulfide) and poly (para-phenylene vinylene) .

6. A transfer paper as in Claim 3 wherein the conductive polymers are selected from the group comprising; Poly (ethylene-dioxythiophene) or PEDT, polyaniline and polypyrrole-coated carbon powders, soluble emeraldine base, polyaniline powders and aqueous and non-aqueous dispersions .

7. A transfer paper as in any of the previous Claims wherein the barrier layer overlays the layer of thermoplastic film-forming polymer.

8. A transfer paper as in any of the previous Claims wherein the transfer paper comprises at least one additional layer.

9. A transfer paper as in Claim 8 wherein the at least one additional layer comprises the conductive element.

10. A transfer paper as in any of the Claims 1 to 8 wherein the conductive element is incorporated into the thermopl"astic film-forming polymer layer.

11. A transfer paper as in any of the previous Claims wherein the layer of thermoplastic film-forming polymer comprises a hot-melt adhesive.

12. A transfer paper as in Claim 11 wherein the layer of thermoplastic film-forming polymer has a melting point in the range of 60⁰C to 220⁰C.

13. A transfer paper as in any of the previous Claims wherein the barrier-layer is water-soluble.

14. A transfer paper as in any of the previous Claims wherein the thermoplastic film-forming polymer comprises a film-forming binder and a thermoplastic polymer.

15. A transfer paper as in any of the previous Claims wherein the layer of thermoplastic film-forming polymer comprises material selected from the group comprising; polyamides, polyolefins, polyesters, polyurethanes, poly (vinyl chloride) , poly(vinyl acetate) , polyethylene oxide, polyacrylates, polystyrene, polyacrylic acid, and polymethacrylic acid; or copolymers or mixtures thereof .

16. A transfer paper as in any of the Claims 1 to 14 wherein the thermoplastic polymer is an ethylene vinyl acetate based polymer.

17. A transfer paper as in any of the previous Claims wherein the thermoplastic film-forming polymer is a conductive polymer.

18. A transfer paper as in any of Claims 1 to 16 wherein the thermoplastic film-forming polymer comprises a conductive element.

19. A transfer paper as in Claim 18 wherein the conductive element is selected from the group comprising; metal particles such as copper, gold, nickel, silver alloys, silver plated metals and silver.

20. A transfer paper as in any of Claims 1 to 17 wherein the layer of thermoplastic film-forming polymer comprises material selected from the group comprising; ethylene/acrylic acid, ethylene/methacrylic acid, and ethylene/vinyl acetate copolymers.

21. A transfer paper as in any of the previous Claims wherein the thermoplastic film-forming polymers have a particle size of less than 50 microns.

22. A transfer paper as in Claim 21 wherein the thermoplastic film-forming polymer has a particle size of between 2 and 20 microns.

23. A transfer paper as in any of the previous Claims wherein the layer of thermoplastic film-forming polymer comprises additional pigments, dyes or products which provide visual effects.

24. A transfer paper as in any of the previous Claims wherein the thermoplastic film-forming polymer may be overlayed with one or more image-receptive films.

25. A transfer paper as in any of the previous Claims wherein the barrier layer comprises solvent, a solvent- soluble film-former and an image enhancing material.

26. A transfer paper as in Claim 25 wherein the image- enhancing material comprises silica.

27. A transfer paper as in any of Claims 1 to 25 wherein the image-enhancing material may be selected from the group comprising; polyvinyl alcohol, gelatine, alginate, complexing salts, clays, precipitated carbonate and aluminium.

28. A transfer paper as in any of the previous Claims wherein the barrier layer comprises water, a water- soluble film-former and silica.

29. A trans fer paper as in Claim 28 wherein the water- soluble fi lm- former is selected from the list comprising ; agar , alginate , gelatins , starches or synthetic polymers .

30. A transfer paper as in Claim 28 wherein the soluble film former is a synthetic polymer selected from the list comprising; poly vinyl acetates, polyurethanes, polyvinyl alcohols, polyvinyl pyrrolidones, acrylics, carboxymethyl cellulose and other cellulose derivatives, starch derivatives, and guar derivatives.

31. A transfer paper as in Claim 28 wherein the water soluble film former is polyvinyl alcohol.

32. A transfer paper as in any of the previous Claims wherein the barrier layer is formed from a 5-50% solution of polyvinyl alcohol in water, mixed with colloidal silica.

33. A transfer paper as in Claim 32 wherein the barrier layer is formed from a 17.5% solution of polyvinyl alcohol in water, mixed with colloidal silica.

34. A transfer paper as in any of the previous Claims wherein the barrier layer is less than 50 microns thick.

35. A transfer paper as in Claim 34 wherein the barrier layer is between 1 and 15 microns thick.

36. A barrier layer-forming composition for application to a transfer paper, comprising solvent, a solvent-soluble film-former and silica, wherein the solvent-soluble film-former is adapted to form a heat- stable film which is soluble in one or more solvents.

37. A barrier layer-forming composition as in Claim 36 which is adapted to form a heat stable film which is stable at temperatures up to approximately 220⁰C.