THERMAL TRANSFER PRINTING DYE SHEET

Abstract: A thermal transfer printing dye sheet comprises a substrate having on one surface a dye coat comprising a thermally transferable dye, wherein a copolymer of polycaprolactone and polydimethylsiloxane is provided on at least part of said surface, e.g. by being incorporated in a dye coat. The copolymer can reduce significantly the friction between the surface of the dye sheet and a receiver sheet and so can alleviate printer jamming problems.

Description: Title: Thermal Transfer Printing Dye Sheet Field of Invention This invention relates to a thermal transfer printing (TTP) dye sheet. Background of the Invention Dye diffusion thermal transfer printing is a generic term for processes in which one or more thermally transferable dyes are caused to transfer from a dye sheet to a receiver sheet in response to thermal stimuli. Using a dye sheet comprising a thin substrate supporting a dye coat containing one or more such dyes uniformly spread over an entire printing area of the dye sheet, printing can be effected by heating selected discrete areas of the dye sheet whilst the dye coat is pressed against a receiver sheet. The shape of the pattern transferred is determined by the number and location of the discrete areas which are subject to heating. Complex images can be built up from large numbers of very small pixels placed close together, the resolution of the final image being determined by the number, size and spacing of such pixels. Full colour prints can be produced by printing with different coloured dye coats sequentially in like manner. The thermal stimuli are conventionally produced by a thermal printing head having a matrix of tiny heating elements (typically six or more to the millimetre) which are selectively energisable to transfer individual pixels. By programming the printing head to respond to electronic signals representing monochrome or full colour images (eg from a video camera, electronic still camera or computer), hard copies of those images can be produced. Alternatively, the thermal stimuli can be produced by means of a laser beam which is scanned across the dye coat in a raster pattern, the intensity of the beam being modulated in accordance with the aforesaid electronic signals to transfer the individual pixels. Usually, the dye sheet is in the form of a ribbon, typically of polyethylene terephthalate, with the different coloured dye coats being in the form of discrete stripes transverse to the axis of the ribbon in a repeated sequence along the ribbon, printing of the three colours (yellow, magenta and cyan) being effected by moving the dye ribbon axially relative to the receiver sheet and whatever means are used to generate the thermal stimuli. Each dye coat conventionally consists of the thermally transferable dye and a binder, typically a vinyl polymer such as polyvinyl butyral or polyvinyl chloride or a cellulosic polymer such as ethyl cellulose. Receiver sheets conventionally comprise a substrate with a dye receiving surface on one side into which a dye is thermally transferable and retainable. The dye-receiving surface may be provided by one side of the substrate. However, receiver sheets typically comprise a substrate supporting a receiving layer which layer presents a dye-receiving surface. The receiving layer typically comprises a dye-receptive polymer, a cross-linking agent and a release system. Suitable materials for the dye receptive polymer are polyesters or poly vinyl chloride. During the course of printing, the dye sheet and the receiver sheet are held together under pressure whilst experiencing a heating/cooling cycle. This tends to fuse the two surfaces of the sheets together and it is the function of the release system to ensure that the surfaces peel apart cleanly after printing. However, even materials that give clean separation can have a high residual friction, which can lead to the problem of the receiver sheet failing to feed out of a printer after imaging because the frictional force between the dye sheet and the receiver sheet is sufficiently large to overcome the drive force feeding the receiver sheet through the printer. The release system can be applied to the receiver sheet as a separate coat, but is more conventionally a low surface energy material such as a silicone or polyfluorinated compound which is applied as part of the receiver coat and is normally chemically bound as part of a crosslinked structure. By increasing the amount of low surface energy material it is possible to decrease the friction between the dye sheet and the receiver sheet to a very low value, but this has the undesirable consequence of reducing the ability of d e receiver sheet to accept ink from, for example, ball point pens that could otherwise be used to annotate the image. A lack of writability of this kind is a particularly severe defect if, say, the image forms part of a medical record and the physician wishes to highlight a particular feature. EP 0227092 discloses various release agents for incorporation in dye layers of dye-donor elements, to facilitate clean separation of a dye-donor element and dye-receiving element after processing. Summary of the Invention According to one aspect of the invention there is provided a thermal transfer printing dye sheet, comprising a substrate having on one surface a dye coat comprising a thermally transferable dye, wherein a copolymer of polycaprolactone and polydimethylsiloxane is provided on at least part of said surface. It is found that the presence of the copolymer can reduce significantly the friction between the surface of the dye sheet and a receiver sheet. The copolymer is conveniently incorporated in the dye coat, which typically also comprises a polymeric binder. Thus, in a further aspect the invention provides a thermal transfer printing dye sheet comprising a substrate having on one surface a dye coat comprising a thermally transferable dye, a polymeric binder and a copolymer of polycaprolactone and polydimethylsiloxane . The frictional effect between the dye sheet and a receiver sheet is only relevant when printing is complete and the receiver sheet is to be removed from the printer, so it is only necessary to include the friction reducing material in or downstream of the last portion of the dye sheet printed in each sequence (usually cyan), although the friction reducing material may be included in all dye coats, or alternatively in, say, an overlay panel or a dedicated panel of the dye sheet. According to a further aspect of the invention, there is provided a thermal transfer printing dye sheet comprising a substrate in the form of a ribbon having on one surface thereof a repeated sequence of dye coats in the form of discrete stripes transverse to the axis of the ribbon, each dye coat comprising a thermally transferable dye and a polymeric binder, at least the last dye coat to be used in each sequence also contenting a copolymer of polycaprolactone or polydimethylsiloxane. The last dye coat to be used in each sequence is generally cyan. Copolymers of polycaprolactone and polydimethylsiloxane are commercially available, eg as Tegomer 6420, 6440 or 6460 (Tegomer is a Trade Mark) from TH Goldschmidt. The different Tegomer materials are chemically and structurally similar, differing in molecular weight. Mixtures of suitable copolymers may be used. The copolymer is conveniently present in an amount in the range 0.001 to 1 % , preferably in the range 0.01 to 0.1%, w/v of a dye formulation. Good results have been obtained using an amount of 0.05% w/v of Tegomer 6440 in a cyan dye formulation. The dye sheet may be otherwise of generally conventional formulation, and may be produced in known manner. The coefficient of friction between the surface of the dye sheet and a receiver sheet is preferably less than about 1.0, more preferably less than about 0.9 and may be less than about 0.5. The invention will be further described, by way of illustration, in the following Examples and with reference to the accompanying drawing, which is a schematic cross sectional view through part of a printer. Detailed description of the drawing Referring to the drawing, an ink cassette comprises a cassette body 10 containing a dye ribbon 12 extending between a feed spool 14 and a take-up spool 16. Ribbon 12 comprises a repeated sequence of dye coats in the form of stripes extending across the width of the ribbon, as described above, each sequence comprising a yellow panel, a magenta panel and then a cyan panel, with each panel comprising a dye coat of the relevant colour. The cassette is positioned so the ribbon between the spools extends across a cylindrical platen roller 18. A receiver sheet 20 is fed through the gap between a static guide 22 and a spring-loaded arm 24 carrying a pair of rollers 26, onto the surface of the platen. The leading edge of the receiver sheet is held in position by a retaining clip 28. Platen 18 is rotatably driven so as to transport the receiver sheet into an imaging position beneath the first dye panel or coat of the dye ribbon 12, wititi the receiver sheet 20 being smoothed out by passage under the rollers 26 of arm 24. Imaging is effected by means of a thermal printing head (not shown) operating through an aperture (not shown) in the cassette, the head being moved to pinch the dye ribbon 12 against the receiver sheet 20. During printing, the rotation of the platen 18 and the friction between the dye ribbon and receiver sheet (held together by the thermal head) cause the dye ribbon to move as one with the receiver sheet, unwinding dye ribbon from the feed spool 14 as printing progresses. Used dye ribbon is wound onto the take-up spool 16, which is kept under constant low torque sufficient in value to separate the dye ribbon and receiver sheet after printing. The tfiree colours are printed sequentially, the platen roller 18 continuing to rotate in the same sense (clockwise) throughout. The length of the print is slightly shorter than the circumference of the platen 18, so that the receiver sheet 20 is comfortably wrapped around it. After the final colour is printed, the thermal head is retracted, so that me dye ribbon is no longer pinched against the receiver sheet, and d e retaining clip 28 is released. The receiver sheet is now free to move through exit rollers 30, as indicated in the drawing. If there is excessive friction between dye ribbon and receiver sheet under these low load conditions, then diis prevents forward motion of the receiver sheet. Because the drive is tangential to the desired direction of travel, the receiver sheet bows up and increases the area of contact still further. This creates such a high braking force that the receiver sheet 20 remains stationary while the platen 18 moves underneath it, as the pressure from the rollers 26 on the spring-loaded arm 24 does not provide sufficient grip to drive die receiver sheet. The receiver sheet 20 therefore does not reach the exit drive rollers 30, thereby jamming die printer. A solution to mis problem is illustrated in the following examples. Example 1 A series of experiment dye sheet samples were prepared by coating polyemylene tereph alate sheets 4.5μm thick made by Diafoil-Hoechst under product name K203E with cyan dye coat solutions having me following formulations: Cyan Cl (3-acetylamino-4-(3-cyano-5-phenylazothiophenyl- 2-ylazo)-N,N-diemyaniline) 1.25% w/v Cyan C2 (CI solvent blue 63) 5.0% w/v Poly vinylbutyral (binder) 3.75% w/v Tetrahydrofuran (THF) solvent 100ml Additive as stated, w/v The additives used were as follows: 1. Tegomer 6440, an α,ω-hydroxyorganofunctional polycaprolactone- polydimethylsiloxane-copolymer, from TH Goldschmidt. 2. Tegomer 2311, a polydimethylsiloxane, from TH Goldschmidt (for comparison purposes). 3. Paraffin wax (for comparison purposes). The formulations were all hand coated with a number 2 wire-wound Mayer bar producing 12 gsm wet coat weight. The coating was then dried with a hair drier and then placed in the oven for 30 seconds at 110°C. Each dye sheet sample was securely fixed to a flat glass plate. A receiver sheet comprising a polyethylene terephthalate sheet coated with polyester receiver coat was fixed to a sled having a curvature equivalent to that of a printer platen and having a weight suitable to simulate the condition within the printer corresponding to a downward force of 0.3N. The sled was placed with the receiver coat surface in contact with the dye coat surface and attached an Instron Force Gauge. The force in Newtons required to draw the receiver sheet over the dye sheet was measured. The results are shown in Table 1. Table 1 The paraffin wax appeared to increase the frictional properties of the system and so to be unsuitable as a friction reducing additive. Both Tegomer 6440 and 2311 significantly reduce friction. Example 2 A similar series of experiments was carried out using Tegomer 2311 and Tegomer 6440 to test me frictional force at various different times after production of the dye sheets, to test stability of the additive in the cyan dye formulation with time. The results are show in Table 2. Table 2 Tegomer 2311 thus appears to produce a somewhat unstable system, with friction decreasing and then increasing with time over a time scale of 8 days. In contrast, Tegomer 6440 produced a stable system in which friction did not change significantly over me measured time frame. Long term tests have shown Tegomer 6440 is also stable over the time scale of at least 1 year. Tegomer 6440 thus enables controllable friction reduction, is effective at very low concentrations and does not impair dye sheet stability. Similar performance is to be expected from the closely related materials, Tegomer 6420 and 6460. Example 3 In order further to test the feed characteristics of dye sheets including Tegomer 6440, a series of factory coated dye ribbons were produced having successive sequences of yellow, magenta and cyan dye panels. Various amounts of Tegomer 6440 were included in the cyan panels only, using dye formulation as described in Example 1. Similar dye formulations, with different dyes, but not including Tegomer 6440, were used for the yellow and magenta panels. The ribbons were coated by direct gravure coating with a wet coat weight of approximately 12gsm. Hot air is impinged onto the coated surface, at approximately 100°C to dry the coating. The ribbons were men placed in an over for approximately 30 seconds at 110 °C. The main difference between this factory coating and the hand coating described in Example 1 is the residence time of the wet coating before being dried: with the factory coating me time is approximately 1 second, and for the hand coating the time could easily be 10 times as long and the coating could be virtually dry by the time the hair drier is eventually used. Friction measurements were carried out as in Example 1, and the dye ribbons were also used to print receiver sheets having polyester receiver coats in a Sony UP-5500 printer. The results are shown in Table 3. Table 3 There is a known difference in the effect of hand coated samples and factory coated samples. This is reflected in the actual fπctional measurements obtained. It will be seen that good results were obtained with 0.05% w/v Tegomer 6440.

Claims: Claims 1. A thermal transfer printing dye sheet, comprising a substrate having on one surface a dye coat comprising a thermally transferable dye, wherein a copolymer of polycaprolactone and polydimethylsiloxane is provided on at least part of said surface. 2. A thermal transfer printing dye sheet, comprising a substrate having on one surface a dye coat comprising a thermally transferable dye, a polymeric binder and a copolymer of polycaprolactone and polydimethylsiloxane. 3. A thermal transfer printing dye sheet, comprising a substrate in the form of a ribbon having on one surface mereof a repeated sequence of dye coats in the form of discrete stripes transverse to the length of the ribbon, each dye coat comprising a thermally transferable dye and a polymeric binder, at least the last dye coat to be used in each sequence also containing a copolymer of polycaprolactone and polydimethylsiloxane. 4. A dye sheet according to claim 2 or 3, wherein the copolymer is incorporated in a dye coat comprising cyan dye. 5. A dye sheet according to any one of the preceding claims, wherein the copolymer is selected from Tegomer 6420, Tegomer 6440 and Tegomer 6460. 6. A dye sheet according to any one of the preceding claims, wherein the copolymer is present in an amount in the range 0.001 to 1 % w/v of a dye formulation. 7. A dye sheet according to claim 6, wherein the copolymer is present in an amount in the range 0.01 to 0.1 % w/v of a dye formulation. 8. A dye sheet according to claim 7, wherein the copolymer comprises Tegomer 6440 and is present in an amount of about 0.05% w/v of a cyan dye formulation.

Inventors: PAGE GRAHAM ALISTAIR, PAGE, GRAHAM, ALISTAIR
Applicants: ICI PLC, PAGE GRAHAM ALISTAIR, IMPERIAL CHEMICAL INDUSTRIES PLC, PAGE, GRAHAM, ALISTAIR
Application year: 1998
Main classification: B41M5
IPC codes: B41M5