DYE DIFFUSION THERMAL TRANSFER PRINTING

Abstract: A thermal transfer dye sheet giving improved stability images on printing comprises a substrate having thereon a dye coat comprising a dye or mixture of dyes having a melting point greater than 150 DEG C and a molecular weight of less than 440.

Description: Dve Diffusion Thermal Transfer Printing Introduction This invention relates to dye diffusion thermal transfer printing (DDTTP or D2T2 printing, D2T2 is a trade mark of Imperial Chemical Industries PLC), especially to a dye sheet carrying a dye or a dye mixture which has an improved print stability and to a transfer printing process in which the dye or the dye mixture is transferred from the transfer sheet to a receiver sheet by the application of heat. It is known to print woven or knitted textile material by a thermal transfer printing (TTP) process. In such a process a sublimable dye is applied to a paper substrate (usually as an ink also containing a resinous or polymeric binder to bind the dye to the substrate until it is required for printing) in the form of a pattern, to produce a transfer sheet comprising a paper substrate printed with a pattern which it is desired to transfer to the textile. Substantially ail the dye is then transferred from the transfer sheet to the textile material, to form an identical pattern on the textile material, by placing the patterned side of the transfer sheet in contact with the textile material and heating the sandwich, under light pressure from a heated plate, to a temperature from 180-220°C for a period of 30-120 seconds. As the surface of the textile substrate is fibrous and uneven it will not be in contact with the printed pattern on the transfer sheet over the whole of the pattern area. It is therefore necessary for the dye to be sublimable and vaporise during passage from the transfer sheet to the textile substrate in order for dye to be transferred from the transfer sheet to the textile substrate over the whole of the pattern area. As heat is applied evenly over the whole area of the sandwich over a sufficiently long period for equilibrium to be established, conditions are substantially isothermal, the process is non-selective and the dye penetrates deeply into the fibres of the textile material. In DDTTP, a dye sheet is formed by applying a heat- transferable dye (usually in the form of a solution or dispersion in a liquid also containing a polymeric or resinous binder to bind the dye to the substrate) to a thin (usually <20 micron) substrate having a smooth plain surface in the form of a continuous even film over the entire printing area of the dye sheet. Dye is then selectively transferred from the transfer sheet by placing it in contact with a material having a smooth surface with an affinity for the dye, hereinafter called the receiver sheet, and selectively heating discrete areas of the reverse side of the dye sheet for periods from about 1 to 20 milliseconds (msec) and temperatures up to 300°C, in accordance with a pattern information signal, whereby dye from the selectively heated regions of the dye sheet diffuses from the dye sheet to the receiver sheet and forms a pattern thereon in accordance with the pattern in which heat is applied to the dye sheet. The shape of the pattern is determined by the number and location of the discrete areas which are subjected to heating and the depth of shade in any discrete area is determined by the period of time for which it is heated and the temperature reached. Heating is generally, though not necessarily, effected by a line of heating elements, over which the receiver and transfer sheets are passed together. Each element is approximately square in overall shape, although the element may optionally be split down the centre, and may be resistively heated by an electrical current passed through it from adjacent circuitry. Each element normally corresponds to an element of image information and can be separately heated to 300°C to 400°C, in less than 20 msec and preferably less than 10 msec, usually by an electric pulse in response to a pattern information signal. During the heating period the temperature of an element will rise to about 300-400°C over about 5-8 msec. With increase in temperature and time more dye will diffuse from the dye sheet to the receiver sheet and thus the amount of dye transferred onto, and the depth of shade at, any discrete area on the receiver sheet will depend on the period for which an element is heated while it is in contact with the reverse side of the dye sheet. As heat is applied through individually energised elements for very short periods of time the process is selective in terms of location and quantity of dye transferred and the transferred dye remains close to the surface of the receiver sheet. As an alternative heating may be effected using a light source in a light-induced thermal transfer (LITT or L2T2 printing, L2T2 is a trade mark of imperial Chemical Industries PLC) printer where the light source can be focused, in response to an electronic pattern information signal, on each area of the dye sheet to be heated. The heat for effecting transfer of the dye from the dye sheet is generated in the dye sheet which has an absorber for the inducing light. The absorber is selected according to the light source used and converts the light to thermal energy, at a point at which the light is incident, sufficient to transfer the dye at that point to the corresponding position on the receiver sheet. The inducing fight usually has a narrow waveband and may be in the visible, infra-red or ultra violet regions although infra- red emitting lasers are particularly suitable. It is clear that there are significant distinctions TTP on to synthetic textile materials and DDTTP on to smooth polymeric surfaces and thus dyes which are suitable for the former process are not necessarily suitable for the latter. In DDTTP it is important that the surfaces of the dye sheet and receiver sheet are even so that good contact can be achieved between the printed surface of the dye sheet and the receiving surface of the receiver sheet over the entire printing area because it is believed that the dye is transferred substantially by diffusion in the molten state in condensed phases. Thus, any defect or speck of dust which prevents good contact over any part of the printing area will inhibit transfer and lead to an unprinted portion on the receiver sheet on the area where good contact is prevented, which can be considerably larger than the area of the speck or defect. The surfaces of the substrate of the dye and receiver sheets are usually a smooth polymeric film, especially of a polyester, which has some affinity for the dye. Important criteria in the selection of a dye for DDTTP are its thermal properties, fastness properties, such as light fastness, and facility for transfer by diffusion into the substrate in the DDTTP process. For suitable performance the dye or dye mixture should transfer evenly and rapidly, in proportion to the heat applied to the dye sheet so that the amount transferred to the receiver sheet is proportional to the heat applied. After transfer the dye should preferably not migrate or crystallise and should have excellent fastness to light, heat, rubbing, especially rubbing with a oily or greasy object, e.g. a human finger, such as would be encountered in normal handling of the printed receiver sheet , and when in contact with plastics materials containing plasticisers, particularly poly(vinyi chloride) e.g. by being placed in a wallet of such material. The dye should be sufficiently mobile to migrate from the transfer sheet to the receiver sheet at the temperatures employed, 100-400°C, in the short time-scale, generally <20 msec. Many potentially suitable dyes are also not readily soluble in the solvents which are commonly used in, and thus acceptable to, the printing industry; for example, alcohols such as i-propanol, ketones such as methyl ethyl ketone (MEK), methyl i-butyi ketone (MIBK) and cyclohexanone, ethers such as tetrahydrofuran and aromatic hydrocarbons such as toluene. The dye can be applied as a dispersion in a suitable medium or as a solution in a suitable solvent to the substrate from a solution. In order to achieve the potential for a high optical density (OD) on the receiver sheet it is desirable that the dye should be readily soluble or readily dispersible in the ink medium. It is also important that a dye which has been applied to a dye sheet from a solution should be resistant to crystallisation so that it remains as an amorphous layer on the dye sheet for a considerable time. Crystallisation not only produces defects which prevent good contact between the dye receiver sheet but gives rise to uneven prints. The following combination of properties is highly desirable for a dye which is to be used in DDTTP:- Ideal spectral characteristics (narrow absorption curve) Correct thermochemical properties (high thermal stability and efficient transferability with heat). High optical densities on printing. Good solubility in solvents acceptable to printing industry: this is desirable to produce solution coated dyesheets alternatively good dispersibility in acceptable media is desirable to produce dispersion coated dyesheets. Stable dyesheets (resistant to dye migration or crystallisation). Stable printed images on the receiver sheet (resistant to heat, migration, crystallisation, grease, rubbing and light). DDTTP is used for printing images on suitable substrates. The achievement of good stability in DDTTP is extremely difficult because of the unfavourable environment of the dye, close to the surface of the polyester receiver sheet. To preserve an image on a receiver sheet it is important to minimise migration and/or crystallisation of the dye. An objective of the present invention is to overcome the above problems by providing a convenient means of enhancing print stability and minimising migration and crystallisation of dyes in DDTTP without the disadvantage of substantially changing the absoφtion maximum of the dye. One way in which improved stability can be achieved is to use dyes with enhanced thermal properties, more particularly dyes with higher melting points.. However, it is possible that an increase in melting point can be accompanied by an increase in the molecular weight, and an increase in the size of the molecule. Whilst an increase in molecular size means that the dye is less likely to migrate and/or crystallise after transfer to the receiver coat, it also means that there are problems in the actual transfer. Therefore it is desirable that the molecular weight of the dye is not too high. According to one aspect of the invention, there is provided a dye sheet comprising a substrate having thereon a dye coat comprising a dye or mixture of dyes having a melting point greater than 150 °C and a molecular weight of less than 440. Preferably, the dye has a melting point greater than 190 °C, more preferably greater than 210 °C. According to a preferred aspect of the invention, the dye has a solubility in tetrahydrofuran of greater than 1% to allow the dye coat to be easily applied to the substrate. According to a further preferred aspect of the invention, the dye is a propyl analogue of azopyridone having a melting point of 288 °C, a molecular weight of 340 and a solubility in THF of 5.5% According to a further preferred aspect of the invention the dye has the Formula ( R a Y„ > , Ch \ ( R D Y n ) ι in which: Ch is a chromogen Ra and Rb each independently is a spacer group Y is an interactive functional group w and x each independently is 0 or an integer equal to or greater than 1 , and m and n each independently is an integer equal to or greater than 1 , provided that w and x are not both equal to 0 and when one of w or x is 0 at least one of m and n is equal to or greater than 2. In this specification, the term "chromogen is defined as meaning the arrangement of atoms which substantially governs the absorbance of electromagnetic radiation by the dye molecule and, particularly in the case of visible radiation, the arrangement of atoms which causes the dye molecule to be coloured. The spacer groups represented by R" and R" may be any groups capable of carrying one or more interactive functional groups (Y) and minimising steric and electronic effects of the Y group and thereby minimising any changes in the absorption characteristics of the chromogen group Ch and thus shade which the Y group would otherwise cause. Preferably the spacer groups each comprise an atom or group of atoms connected to the chromogen by at least one sigma bond and to the interactive group by at least one sigma bond. The spacer groups may contain at least one of a carbon, silicon or sulphur atom, preferably two carbon atoms and more preferably from three to ten carbon atoms. The interactive functional group represented by Y are such that the Y groups on different dye molecules may be interact with each other to form dye complexes of larger size and thus of lower mobility and/or the Y groups may interact with a dye receptive polymer on the receiver sheet. In the dyes of Formulae (1) to (5) the Y groups may be the same or different and the R" and R" may carry one more Y groups. The interactions between different Y groups or between the Y groups and the dye receptive polymer produces an image on the receiver sheet which is resistant to crystallisation and migration of the dyes is minimised. The Y groups are preferably selected from OH, NH 2 , NHR, NR 2 , COOH, CONH-, NHCOR, CONHR, S0 2 NH 2 SO-NHR, S0,H, NHCONH,, NHCONHR , =NOH, and P0 3 H, According to a further preferred aspect of the invention, a release layer comprising a styrene /butadiene copolymer is provided on the surface of the dye coat. The Coating The coating suitably comprises a binder together with a dye or mixture of dyes of Formula (1). The ratio of dye to binder is preferably 1 :2 to 4:1 in order to provide good adhesion between the dye and the substrate and inhibit migration of the dye during storage. The coating may also contain other additives, such as curing agents, preservatives, etc., these and other ingredients being described more fully in EP 133011 A, EP 133012A and EP 111004A. The Binder The binder may be any resinous or polymeric material suitable for binding the dye to the substrate which has acceptable solubility in the ink medium, i.e. the medium in which the dye and binder are applied to the transfer sheet. It is preferred however, that the dye is soluble in the binder so that it can exist as a solid solution in the binder on the transfer sheet. In this form it is generally more resistant to migration and crystallisation during storage. Examples of binders include cellulose derivatives, such as ethylhydroxyethylcellulose (EHEC), hydroxypropylcellulose (HPC), ethyicellulose, methylcellulose, cellulose acetate and cellulose acetate butyrate; carbohydrate derivatives, such as starch; alginic acid derivatives; alkyd resins; vinyl resins and derivatives, such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetoacetal and polyvinyl pyrrolidone; polycarbonates such as AL-71 from Mitsubishi Gas Chemicals and MAKROLON 2040 from Bayer (MAKROLON is a trade mark); polymers and co-polymers derived from acrylates and acrylate derivatives, such as polyacrylic acid, polymethyi methacrylate and styrene-acrylate copolymers, styrene derivatives such as polystyrene, polyester resins, polyamide resins, such as melamines; polyurea and polyurethane resins; organosilicones, such as polysiloxanes, epoxy resins and natural resins, such as gum tragacanth and gum arabic. Mixtures of two or more of the above resins may also be used, mixtures preferably comprise a vinyl resin or derivative and a cellulose derivative, more preferably the mixture comprises polyvinyl butyral and ethyicellulose. It is also preferred to use a binder or mixture of binders which is soluble in one of the above-mentioned commercially acceptable organic solvents. The dye or mixture of dyes has good thermal properties giving rise to even prints on the receiver sheet, whose depth of shade is accurately proportional to the quantity of applied heat so that a true grey scale of coloration can be attained. The dye or mixture of dyes also has strong absorbance properties and is soluble in a wide range of solvents, especially those solvents which are widely used and accepted in the printing industry, for example, alkanols, such as i-propanol and butanol; aromatic hydrocarbons, such as toluene, ethers, such as tetrahydrofuran and ketones such as MEK, MIBK and cyclohexanone. Alternatively the mixture of dyes may be dispersed by high shear mixing in suitable media such as water, in the presence of dispersing agents. This produces inks (solvent plus mixture of dyes and binder) which are stable and allow production of solution or dispersion coated dyesheets. The latter are stable, being resistant to dye crystallisation or migration during prolonged storage. The combination of strong absorbance properties and good solubility in the preferred solvents allows the achievement of good OD of the dye or mixture of dyes on the receiver sheet. The transfer sheets of the present invention have good stability and produce receiver sheets with good OD and which are fast to both light and heat. The Substrate The substrate may be any sheet material preferably having at least one smooth even surface and capable of withstanding the temperatures involved in DDTTP, i.e. up to 400C for periods up to 20 msec, yet thin enough to transmit heat applied on one side through to the dyes on the other side to effect transfer of the dye onto a receiver sheet within such short periods. .Examples of suitable materials are polymers, especially polyester, polyacrylate, polyamide, cellulosic and polyalkyiene films, metallised forms thereof, including co-polymer and laminated films, especially laminates incorporating a smooth even polyester receptor layer on which the dye is deposited. Thin (<20 micron) high quality paper of even thickness and having a smooth coated surface, such as capacitor paper, is also suitable. A laminated substrate preferably comprises a backcoat, on the opposite side of the laminate from the receptor layer, which, in the printing process, holds the molten mass together, such as a thermosetting resin, e.g a silicone, acrylate or polyurethane resin, to separate the heat source from the polyester and prevent melting of the latter during the DDTTP operation. The thickness of the substrate depends to some extent upon its thermal conductivity but it is preferably less than 20μm and more preferably less than 10μm. The DDTTP Process According to a further feature of the present invention there is provided a dye diffusion thermal transfer printing process which comprises contacting a dye sheet comprising a coating comprising a dye or mixture of dyes having a melting point greater than 150°C and a molecular weight less than 440 with a receiver sheet, so that the coating is in contact with the receiver sheet and selectively applying heat to discrete areas on the reverse side of the dye sheet whereby the dye on the opposite side of the sheet to the heated areas is transferred to the receiver sheet. Heating in the selected areas may be effected by contact with heating elements, which can be heated to 200-450C, preferably 200-400°C, over periods of 2 to 10 msec, whereby the dye mixture may be heated to 150-300°C, depending on the time of exposure, and thereby caused to transfer, substantially by diffusion, from the transfer to the receiver sheet. Good contact between coating and receiver sheet at the point of application is essential to effect transfer. The density of the printed image is related to the time period for which the transfer sheet is heated. Alternatively, the heat may be generated by means of a laser, in which case the dye coat also contains an absorber material for absorbing and converting the laser light to heat. The Receiver Sheet The receiver sheet conveniently comprises a polymeric sheet material substrate, preferably a white polymeric film. The design of receiver and transfer sheets is discussed further in EP 133,011 and EP 133012. According to a further aspect of the invention, there is provided a thermal transfer dye sheet/receiver sheet combination, in which the dye sheet comprises a dye having a melting point greater than 150 °C and a molecular weight of less than 440 and in which the receiver sheet comprises a substrate having thereon a receiver coat consisting of a polymer more hydrophilic than polyester. Where the dye has the formula given above, the receiver coat preferably contains at least one polymer capable of interacting with Y. Preferably, the receiver coat polymer is poly (vinyl pyridene), poly (vinyl pyrrolidone) or a vinyl pyrrolidone. vinyl acetate copolymer. The following non-limiting examples illustrate the invention. Example 1 A dye sheet was produced by coating a 6 micron thick substrate supplied by Diafoil with a solution containing 1.4% w/w dye and 2.8%w/w poly (vinyl butyral) in tetrahydrofuran using a K3 wire bar and drying the coating at 110 ° C for 20 seconds to 1.0 μm thickness and then applying a 1.2% w/w solution in hexane of a copolymer containing equal amounts of styrene and butadiene (Europrene ® Sol S141 from Enichem) with a K2 wire bar to produce a release layer 0.15μm in thickness. A receiver sheet was prepared by coating a Melinex 990 ® substrate with a solution containing 11.1 %w/w of an amorphous polyester ( Vylon 103 ®) using a K4 wire bar and dried at 140 °C for 3 minutes to give a final coat of 4 μm. Printing was carried out at different print times using a laboratory thermal printer having a head voltage of 12 v. After measuring the optical density using a Macbeth densitomiter , the receiver sheets were placed in wallets of PVC containing dioctyl phthalate as plasticiser and maintained under pressure in an oven at 45°C/85% RH for 16 hours. The sheets were removed from the wallets and the OD of the dye which had transferred to the PVC sheet was measured. The ratio of transfer OD/original OD gives a measure of the stability of the image. The results for dyes Y1 and Y2 are shown in Table 1. Y1 which has a melting point of 141°C and a molecular weight of 404 has the formula Y2 which has a melting point of 288°C and a molecular weight of 340 has the formula % LOSS 17.9 22.5 12.1 11.65 14.3 12.1 4.62 4.3 SUBSTITUTE SHEE Example 2 Example 1 was repeated except that the polyester binder was replaced with a vinyl pyrrolidone/ vinyl acetate resin. The results are shown in Table 2. TABLE 2 DYE PRINT TIME mS ORIGINAL OD TRANSFER % LOSS OD EExample 4 Example 1 was repeated except that isothiazole dyes were used at a print time of 12.7ms. The results for dyes M1 to M7 are shown in Table 4. TABLE 4 Dyes M1 to M7 have the formulae Example 5 Examples 1 to 3 were repeated at print times of 8.4 and 14.8 ms using dye sheets containing 2.1%w/w of dye Y2 and 2.1%w/w poly vinyl butyral. The results are shown in Table 5. TABLE 5 Examples 1 to 4 were repeated except that the receiver sheet contained cross-linked hydroxy functional silicone as release agent and the release layer on the dye sheet was omitted. Similar results were obtained.

Claims: claims 1. A thermal transfer dye sheet comprising a substrate having thereon a dye coat comprising a dye having a melting point greater than 150 °C and a molecular weight of less than 440. 2. A dye sheet according to claim 1, in which the dye has a melting point greater than 190 °C. 3. A dye sheet according to claim 1 or 2, in which the dye has a solubility in tetrahydrofuran of greater than 1%. 4. A dye sheet according to claim 3 in which the dye is a propyl analogue of azopyridone having a melting point of 288 °C, a molecular weight of 340 and a solubility in THF of 5.5%. 5. A dye sheet according to claim 1 , 2 or 3, in which the dye has the Formula ( R Y m m ') w Ch \ ( R Y in which: Ch is a chromogen Ra and Rb each independently is a spacer group Y is an interactive functional group w and x each independently is 0 or an integer equal to or greater than 1 , and m and n each independently is an integer equal to or greater than 1 , provided that w and x are not both equal to 0 and when one of w or x is 0 at least one of m and n is equal to or greater than 2. 6. A dye diffusion thermal transfer printing process which comprises contacting a dye sheet comprising a coating comprising a dye or mixture of dyes having a melting point greater than 150°C and a molecular weight less than 440 with a receiver sheet, so that the coating is in contact with the receiver sheet and selectively applying heat to discrete areas on the reverse side of the dye sheet whereby the dye on the opposite side of the sheet to the heated areas is transferred to the receiver sheet. 7.A thermal transfer dye sheet/receiver sheet combination, in which the dye sheet comprises a dye having a melting point greater than 150 °C and a molecular weight of less than 440 and in which the receiver sheet comprises a substrate having thereon a receiver coat consisting of a polymer more hydrophilic than polyester. 8. .A thermal transfer dye sheet/receiver sheet combination, in which the receiver coat polymer is poly (vinyl pyridene), poly (vinyl pyrrolidone) or a vinyl pyrrolidone/viπyl acetate copolymer.

Inventors: BUTTERS ALAN, SLARK ANDREW, BUTTERS, ALAN, SLARK, ANDREW
Applicants: BUTTERS ALAN, ICI PLC, SLARK ANDREW, BUTTERS, ALAN, IMPERIAL CHEMICAL INDUSTRIES PLC, SLARK, ANDREW
Application year: 1996
Main classification: B41M5
IPC codes: B41M5, D06P5