GB2124791A - Production of dry transfers - Google Patents

Abstract

An imageable material useful for making labels is described. It consists of a carrier sheet bearing on one side a pair of layers the outer one of which is adhesive and protected by a removable release sheet. The pair of layers contain components which react together thermographically. Strips of such material may be exposed successively in an exposure window to heat generated by contact with a master positive which contains a selection of symbols e.g. alphabetic letters which heat up when irradiated. The result is a series of symbols which can then be adhered to a desired receptor surface e.g. in the form of a label. The carrier sheet may be peeled away if desired to leave the pair of layers (of which the then outer layer is non tacky) attached to the surface.

Description

SPECIFICATION Production of dry transfers This invention relates to the production of dry transfer materials.

In recent years dry transfer materials have become a standard item of commerce and in particular a standard tool for the graphic artist and designer. They consist generally of a carrier sheet bearing on one face thereof a plurality of detachable symbols. The surface of the symbol remote from the carrier sheet is adhesive and can be used to attach the symbol to a desired receptor surface such as an article or a piece of artwork, whereafter the carrier sheet is pulled away to leave the symbol adherent to the artwork.

In the manufacture of dry transfer materials, there are two methods which have principally been used to generate the transferable images. These may be broadly classified as photographic methods and printing methods. In the former, a photographically sensitised layer on a carrier sheet is exposed and then developed to provide visible transferable images. In the printing methods, images are formed by printing from a printing ink, most often by screen printing, on to the carrier sheet. After drying the wet ink, the symbols may be removed e.g. using the adhesive power of a coating of adhesive applied overall.

We have now found that it is possible using appropriately sensitised materials to produce dry transfer materials thermographically. This is of particular value in producing dry transfers in the form of a strip of carrier material from which a word formed on the strip may be transferred.

There is very widespread commercial demand for so-called tape labelling systems, but many of those presently on the market are unsatisfactory for some purposes. In particular, tape labelling systems generally involve the manufacture of the desired legend e.g. a word or words on a tape by means of a printing or embossing system followed by the adhesion of the tape or part of the tape to the desired receptor surface. This system clearly cannot be employed where it is desired to transfer to the receptor surface only the legend itself. Forming a tape with transferable images thereon which can then simply be transferred on to the receptor surface meets that requirement.

According to a first feature of the present invention there is provided an imageable material consisting of, in order, a transparent plastics film carrier sheet, a layer of thermographically sensitised material thereon, a layer of adhesive and a layer of release paper thereover.

Such a material may be imaged thermographically and parts of the thermographic layer and adhesive then adhered to a desired receptor surface following removal of the release paper. The material may be formulated to ensure that on then pulling away the carrier sheet the imaged areas adhere to the desired receptor surface more strongly than they adhere to the carrier sheet.

Preferably the material is so constructed that in the imaged areas, the layers of thermographically imaged material and adhesive adhere more strongly to a receptor than to the carrier, and that in areas unexposed imagewise, the thermographic and adhesive layers adhere more strongly to the carrier sheet than they do to any receptor surface. In these conditions, following imaging and removal of the release paper, the transfer material so formed may be used in the normal way and only the thermographically produced images will be transferred i.e. without any surrounding unexposed thermographically imageable material.

According to a further feature of the present invention there is provided a process for producing a dry transfer material which comprises imagewise exposing of thermographically sensitised material as defined above to produce dry transfer material.

The materials and method of the present invention are particularly valuable in the manufacture of dry transfer tapes. In particular, it is possible using appropriate equipment to produce labelling products in which instead of forming a legend on a label and then adhering that label to the desired substrate, the legend is formed on an intermediate carrier sheet and then transferred therefrom to the desired substrate. Using a labelling process in this way, the final legend often looks as though it had been printed on the article in question.

In one specific way of putting the invention into practice, sensitised material as described above is provided in the form of a reel or spool of tape e.g. 10 or 1 5 mm wide. This tape is arranged to be led through an exposure gate in which it can be selectively brought into contact with image bearing areas in a positive disc font corresponding to the various symbols desired, normally alphabetic and numeric symbols together with customary punctuation marks and the like. Remote from the font is an appropriate source of infra-red radiation which when actuated causes radiation to be absorbed in the area of the image in the gate. This area is in contact with the thermographic material and the rise in temperature in the symbol area causes the formation of a thermographically produced image of the same shape as that of the symbol on the font.It is easy to see how, using font discs, an appropriate legend may be built up along a thermographic tape. A section of the tape may then be detached and the thermographic images adhered to the desired substrate, either together with the unexposed thermographic areas of the thermographic layer or (and preferably) by themselves, leaving the remainder of the thermogra phically sensitised layer adherent to the carrier sheet.

The imageable material according to the present invention may be constructed without difficulty by conventional film coating and laminating techniques. A wide variety of materials is available for each of the various components from which the maker may easily select materials for use in any specific application.

Taking the components of the imageable material in order, the following considerations should be borne in mind: First, since the material is to be exposed to the action of heat in use, it is desirable to choose fairly heat stable plastics films as the carrier sheet. Polyethylene terephthaiate films are the materials of choice on account of their high heat resistance and excellent clarity. If it is desired to remove the carrier sheet, it must be ensured that the layer of thermographically sensitised material can release therefrom appropriately. This may occur inherently following the choice of plastics film for the carrier sheet. Otherwise, the carrier sheet may bear an appropriate release coating. Polyethylene terephthalate sheets, for example, may bear release coatings of polystyrene butadiene when the next applied layer is one based on nitrocellulose.

The thermographically sensitised layer on the carrier sheet or on a release coating on the carrier sheet preferably takes the form of a clear carrier film which can be released cleanly from the carrier sheet or its release coating. Such a clear carrier film may be sufficiently thin to be shearable or somewhat thicker if it is desired to produce e.g. labels having a high resistance to abrasion.

The layer of adhesive is preferably a layer of pressure sensitive adhesive of appropriate tack level for the desired area of use. A wide variety of pressure sensitive adhesives is known that can be used in the materials of the present invention. The coating weight of the adhesive is also variable within wide limits. One preferred class of adhesives are those based on polyisobutylene.

In order to generate images thermographically, the layer of thermographically sensitised material and the layer of adhesive each include reactants which when subjected to heat give rise to a visible colour change. Commonly, the co-reactants consist of a metal salt and a reducing agent, and either of these components may be present in either layer.

A wide variety of thermographic systems is known from the patent literature, for example United States Patent Specifications 3031329, 3080254 and 3682684. These and other disclosures describe thermographic systems in which a noble metal salt is reacted with an organic reducing agent. Preferred noble metal salts are silver salts of long chain organic acids, for example silver behenate, silver stearate, silver oleate and silver phthalate. Other metal salts may also be used either alone or in conjunction with noble metal salts, for example ferric and mercuric salts such as ferric stearate and mercuric behenate.

A wide variety of reducing agents for use with such metal salts is known, such materials generally being cyclic (preferably aromatic) organic compounds having an active hydrogen atom attached to an oxygen nitrogen or carbon atom in turn directly attached to one of the atoms in the ring. Of particular value among the very large number of such compounds are methyl gallate, 3,4-dihydroxy benzoic acid, gallic acid, pyrogallol, catechol and hydroquinone.

Generally it is found preferable to put the reducing agent or agents in the coating first applied and to disperse metal salts in the adhesive. The formulation and application of the successive coatings to the carrier sheet may be effected in any conventional fashion. For small scale work in the laboratory, Meyer bar coating is adequate but naturally for commerical production full scale coating machines and appropriate technology must be used. The drying down of the second applied layer must naturally be carried out cold in order to prevent any premature thermographic reaction.

The following Example will serve to illustrate the invention: EXAMPLE Sheets of poyethylene terephthalate film 75 micron thick (Melinex 542 ex l.C.I.) were coated with a release coating formulated as follows (percent by weight): Polystyrene Resin (type LX 4300, number average molecular weight 10,000, ex B.P. Chemicals) 20.2 Precipitated calcium carbonate (Calopake F ex Sturge Chemicals) 4.9 Anti-static agent (ASA3 ex Shell Chemicals) 0.2 Oxitol 5.5 Xylene 34.6 Ethyl acetate 34.6 This formulation after mixing and milling to Hegmann gauge 7 or more was coated using a Meyer bar. Prior to coating the composition was diluted with a mixture of ethyl acetate, xylene and oxitol in the proportions given above to give a coating composition of viscosity (Zahn Cup No. 2) 60 seconds.After coating and drying in an oven at 60"C for one minute the release coat had a coat weight of 7.0 g.s.m.

A clear carrier film containing a reducing agent was then coated on top of the release coating.

The coating composition had the following fomulation (parts by weight): Nitrocellulose solution (type E560 ex Wolff Walsrode, 30% by weight solution in oxitol) 39.30 Plasticiser (oil modified azeleic acid type Paraplex RGA2 ex Rohm a Haas) 4.90 Oxitol 47.62 Butyl Oxitol 3.43 Finely divided silica (Aerosil R972 ex Degussa) 1.90 Methyl gallate 2.85 The coating composition was applied using a Meyer bar and the wet coating dried in an oven at 60"C for one minute to give a dry coat weight of 1.0 g.s.m.

A pressure sensitive adhesive composition was then coated on top of the clear carrier film composition, the pressure sensitive adhesive coating including metal salts for reaction with the reducing agent under the action of heat.

The composition of the pressure sensitive adhesive was as follows (parts by weight): Solvent (Exsol 145/160 ex Esso) 113.80 Oxitol 9.00 Polyisobutylene resin (Oppanol B50 ex BASF, 20% by weight solution in Exsol 145/160) 15.70 Polybutene resin (Hyvis 200 ex B.P. Chemicals) 6.90 Polyethylene wax (ACP6 ex Allied Chemicals 10% by weight dispersion in Exsol 145/160) 49.60 Finely divided silica (Aerosil R972 ex Degussa) 4.00 Silver behenate (20% by weight dispersion in Exsol 145/160) 24.00 Ferric stearate (20% by weight dispersion in Exsol 145/160) 16.00 This coating composition was applied using a Meyer bar and the wet coating composition dried using a cold air drier (to prevent any thermographic reaction) to give a dry coat weight of 2.0 g.s.m.

To the pressure sensitive adhesive was then applied sheets of conventional release paper and the laminated sheets so formed then slit to tape form.

The sensitised material so formed was used by placing areas of the tape successively into contact with a photographic positive in an exposure window transparent to infrared radiation.

The exposure window was located in a sheet of metallised polymethylmethacrylate sheet adjacent an infrared lamp. During each exposure, heat from the infrared lamp passing through the window impinges on the positive and heats the black master image there. By contact the adjacent image shaped portions of the thermographically sensitised material are heated and the metal salts and reducing agent in the adhesive and clear carrier film layers react to give a visible image.

After each image is formed, the tape is advanced and brought into contact with the next desired symbol on the master positive. When all the required symbols are on the tape, this is removed from the exposure window, the release paper peeled off, the thermographically imaged layers brought into contact with a desired receptor surface and the release coated polyethylene terephthalate strip then pulled away to leave the area of adhesive adherent to the receptor surface and protected by the layer of clear carrier film thereover.

Claims (7)

1. An imageable material consisting of, in order, a transparent plastics film carrier sheet, a layer of thermographically sensitised material thereon, a layer of adhesive and a layer of release paper thereover.

2. An imageable material according to claim 1 wherein the layer of adhesive is such that following thermographic imaging of the material, and adhesion of the adhesive layer to a receptor surface, the imaged thermographic layer and the adhesive adhere more strongly to the desired receptor surface than they adhere to the carrier sheet.

3. A material according to claim 1 or 2 wherein following thermographic imaging, areas exposed imagewise adhere more strongly to a receptor than to the carrier while in areas unexposed imagewise the thermographic and adhesive layers adhere more strongly to the carrier sheet than they do to any receptor surface.

4. Imageable material according to any one of claims 1 to 3 wherein the thermographically sensitised layer is the form of a clear carrier film containing one thermographic reactant and the adhesive layer contains the other thermographic reactant.

5. An imageable material according to claim 4 wherein the thermographic co-reactants are a metal salt and a reducing agent, wherein the metal salt is dispersed in the adhesive layer and the reducing agent or reducing agents dispersed in the sensitised layer.

6. An imageable material according to claim 1 and substantially as hereinbefore described with reference to the foregoing specific example.

7. A method of producing an image which comprises thermographically imaging an imageable material according to any one of the preceding claims.