SHEET FOR USE IN THERMAL TRANSFER PRINTING

Abstract: A sheet for use in a thermal transfer printing process which comprises a substrate having on one side a surface coat and a sub-layer comprising a metal oxide-containing electro-conducting material below said surface coat is disclosed. Sheets in which the surface coat is a dye receptive layer are particularly useful.

Description: Sheet for use in Thermal Transfer Printing This invention relates to a sheet for use in thermal transfer printing (TTP) and especially to a TTP sheet having a sublayer between a substrate and a surface coat. Thermal transfer printing is a printing process in which a dye is caused, by thermal stimuli, to transfer from a dye sheet to a receiver sheet. In such processes, the dye sheet and receiver sheet are placed in intimate contact, the thermal stimuli are applied to the dye sheet and the dye sheet and receiver sheet are then separated. By applying the thermal stimuli to pre-determined areas in the dye sheet, the dye is selectively transferred to the receiver to form the desired image. 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 typically comprises a receiver layer specifically tailored to receive the dye which is coated onto the substrate. Dye-sheets conventionally comprise a substrate having on one surface thereof a dye coat which typically comprises a thermally transferable dye dispersed in a binder. Both receiver sheets and dye sheets may also comprise a backcoat on the opposite surface to the receiver layer or the dye coat respectively which is typically employed to impart desirable characteristics to the sheet for example good handling properties. It is desirable that receiver sheets and dye sheets possess good anti-static characteristics otherwise problems for example, jamming of the sheets in the printing apparatus, and other handling problems may occur due to charge build up on the receiver sheet and/or the dye sheet. TTP sheets having suitable anti-static properties have been produced by employing an ionic material in a surface coat for example, a receiver layer, a dye coat and a backcoat. However ionic materials are dependent on the presence of moisture to function as anti-static agents as a consequence of which the anti static performance may vary with a change in the humidity of the environment. Carbon black has been employed in dye sheets to act as an absorber of light in light induced thermal transfer processes however this material is unacceptable for use in receiver layers as an anti-static agent due to its colour. We have now found that by employing an electro-conducting material below a surface coat of a dye and/or receiver sheet, a TTP sheet having excellent anti-static properties which are substantially independent of environmental humidity may be secured. Accordingly, a first aspect of the invention provides a sheet for use in a thermal transfer printing process which comprises a substrate having on one side a surface coat and a sub-layer comprising a metal oxide-containing electro-conducting material below said surface coat. It is not necessary that the surface coat comprises a conductive material as the presence of the sub-layer provides for a surface resistivity which secures advantageous anti-static characteristics for the sheet. A further aspect of the invention provides a thermal transfer printing sheet which comprises a substrate having on one side a surface coat and a sub-layer comprising a metal oxide-containing electro-conducting material below said surface coat, the surface coat of said sheet having a resistivity in the range lxlO 8 to lxlO 13 Ohms per square. Preferably, the surface coat has a surface resistivity of lxlO 9 to lxlO 12 Ohms per square. If the resistivity is too high, there may be an undesirable build up of static charge on the printer in which the sheets are processed. A TTP sheet, for example a receiver sheet, which has a small charge of the same polarity on both sides may provide optimum feed and stacking performance. The surface resistivity of the surface coat is dependent upon its coat weight and composition and it will be appreciated that, for a given surface coat composition, the coat weight will be selected to provide the desired surface resistivity. The improved antistatic properties of TTP sheets according to the invention reduce handling problems and for example, the tendency for adjacent receiver sheets to stick to each other whilst in a stack. The electro-conductive material suitably conducts electronically rather than ionically. This provides the advantage that the conductance of the surface of the sheet is independent of the moisture content of the environment and the antistatic performance hence does not vary with humidity. The metal oxide-containing electro-conductive material may comprise particles of metal oxide-containing electro-conductive material but preferably comprises a particulate material for example alumina, silica, china clay and titanium dioxide, which is coated with a metal oxide-containing conductive coating for example a metal oxide doped with a metal. Particles of barium sulphate which are coated with tin oxide doped with antimony which is available from Sachtleben Chemie under the trade name SACON P401, are particularly preferable, especially for use in receiver layers, due to their very light colour. It is preferred that the the electro-conductive material is present as very finely ground particles and is distributed evenly over the whole area of the TTP sheet to provide an electro-conductive effect over the whole sheet. Preferably a discrete electro-conducting layer comprising the electro-conductive material is provided interposed between the substrate and the surface layer as the sub-layer but, if desired, the sub-layer may comprise the electro-conductive material embedded in the substrate itself and the surface layer may be coated directly onto the embedded substrate. Desirably, the sub-layer comprises the metal oxide-containing electro-conductive material dispersed, preferably evenly, in a binder material which is coated onto the substrate. Materials which are conventionally employed as resin binders may be used as a binder for the electro-conducting material. Preferably, the binder is cross-linkable, polyvinyl butyral being particularly preferred as its high viscosity for a given molecular weight facilitates the dispersion of the electro-conducting material when in a coating solution. Suitable cross-linking agents include formaldehyde resins, particularly urea formaldehyde and melamine formaldehyde and for example BEETLE RESINS BE685 available from British Industrial Plastics; cross-linking of the binder improves its durability and facilitates adhesion of the binder to adjacent layers in the TTP sheet. Desirably, a catalyst for the cross-linking reaction is present in the binder system. Suitably, a dispersing agent is present in the binder system. The conductive material is suitably present in the sub-layer in a sufficient quantity to provide improved antistatic performance, preferably at least 152 by weight of the said layer. Suitably the amount of the electro-conductive material does not exceed 85Z by weight as this may render the said sub-layer unacceptably fragile. Suitably the electro-conducting layer has a thickness of at least 0.2μm and preferably no more than 2μm for example lμm. To achieve a desired anti-static performance, the thickness of the sub-layer is selected depending on the nature and thickness of the surface coat. TTP receiver and dye sheet substrates known in the art may be employed as the substrate in the present invention including cellulose fibre paper desirably with a polymer coating, thermoplastic films for example polyethylene terephthalate (desirably biaxially orientated), filled and/or voided thermoplastic films for example pearl film, and laminates of two or more substrate materials. We have found that by employing an electro-conductive sub-layer as herein described into a receiver sheet, significant advantages may be secured. A further aspect of the invention provides a thermal transfer printing receiver sheet which comprises a substrate having a dye-receiving surface on one side, optionally a backcoat on the other side and a sub-layer comprising a metal oxide-containing electro-conductive material below the dye-receiving surface and/or, if present, the backcoat. As consumers typically require a white receiver sheet for many applications, the conductive material, if not white in colour, may impart an undesirable colour to the receiver sheet if present in the sub-layer in large quantities. The quantity of the electro-conducting material employed is selected according to the desired colour of the receiver sheet. Substantially transparent TTP receiver sheets are also required by consumers to allow projection of an image on the sheet onto a screen. We have surprisingly found that a substantially transparent receiver sheet having an electro-conducting sub-layer as herein described may be secured. This is particularly surprising in view of the fact that in producing such a transparent sheet, prior to the application of the receiver layer the sheet may appear translucent or opaque and only becomes transparent on application of the receiver layer. The invention further provides a substantially transparent thermal transfer printing receiver sheet which comprises a substrate having a dye-receiving surface on one side and a sub-layer comprising a metal oxide-containing electro-conductive material below the dye-receiving surface wherein the said sheet has an optical haze value of not more than 102 and preferably not more than 82. Optical haze values referred to herein are measured using a Gardner Haze Meter unless otherwise specified. The dye-receptive surface suitably comprises a discrete receiver layer coated onto the sub-layer. The receiver layer preferably comprises at least one dye-receptive polymer which is an amorphous polyester. The polymer may comprise other polymers for example polyvinyl chloride, polyvinyl alcohol/polyvinyl chloride copolymer, polyacrylonitrile, polystyrenes and acrylonitrilebutadiene styrene (ABS). Commercially available examples of suitable amorphous polyesters include VITEL PE200 (Goodyear) and VYLON polyesters (Toyobo) especially grades 103, 200 and 290. Different grades of polyester may be mixed to provide a suitable composition as desired. Suitably a receiver sheet according to the present invention is laminatable with a cover sheet on both sides to provide protection for the image on the sheet. The cover sheet may be the same or different on the different sides of the sheet and is preferably transparent on at least one side of the sheet. The cover sheet suitably comprises a thermoplastic film, for example polyvinyl chloride, polyethylene terephthalate and polycarbonate compositions. A dye sheet having improved anti-static performance may be secured by employing an electro-conductive sub-layer as herein described. A further aspect of the invention provides a thermal transfer printing dye sheet which comprises a substrate having a dye coat comprising a thermally transferable dye and a dye binder on one side, optionally a backcoat on the other side and a layer comprising a metal oxide-containing electro-conductive material below the dye coat and/or, if present, the backcoat. The dyecoat binder can be selected from such known polymers as polycarbonate, polyvinylbutyral, and cellulose polymers, such as methyl cellulose, ethyl cellulose and ethyl hydroyethyl cellulose, for example, and mixtures of these. A preferred dyecoat is one comprising one or more thermally transferable dyes dispersed throughout a polymeric binder comprising a mixture of polyvinylbutyral and cellulosic polymer, wherein the percentage by weight of polyvinylbutyral in the mixture lies within the range 65-852, the range 70-852 being particularly preferred. Any dye capable of being thermally transferred may be selected as required. Dyes known to thermally transfer, come from a variety of dye classes, eg from such nonionic dyes as azo dyes, anthraquinone dyes, azomethine dyes, methine dyes, indoaniline dyes, naphthoquinone dyes, quinophthalone dyes and nitro dyes. The dye may also include dispersing agents, antistatic agents, antifoaming agents, and oxidation inhibitors, and can be coated onto the absorber layer as described for the formation of the latter. The thickness of the dyecoat is suitably 0.1-5 μm, preferably 0.5-3 μm. Where light induced themal transfer (LITT) is to be employed, it is necessary to employ a light-absorbing material which may be present in the dye coat or may be present in a separate layer preferably located between the dye coat and the substrate in order to allow absorption of the inducing light, For lasers operating in the near infra-red, there are a number of organic materials known to absorb at the laser wavelengths. Examples of such materials include the substituted phthalocyanines described in EP-B-157,568, which can readily be selected to match laser diode radiation at 750-900 nm, for example and carbon black pigment which has a broad absorption spectrum and is thus useful for a wide range of visible light and infra red emitting lasers. The inducing light is desirably a laser, for example Nd:YAG, Argon ion and Ti:sapphire and preferably a laser diode. Also of importance is the provision of sufficient absorber for the system used. It is desirable to use sufficient to absorb at least 502 of the incident inducing light. We prefer to use sufficient to absorb at least 902 of the inducing light, to obtain an optical density of 1 in transmission, although higher proportions may be used if desired. The dye sheet may be elongated in the form of a ribbon and housed in a cassette for convenience, enabling it to be wound on to expose fresh areas of the dyecoat after each print has been made. Dyesheets designed for producing multicolour prints have a plurality of panels of different uniform colours, usually three: yellow, magenta and cyan, although the provision of a fourth panel containing a black dye, has also previously been suggested. When supported on a substrate elongated in the form of a ribbon, these different panels are suitably in the form of transverse panels, each the size of the desired print, and arranged in a repeated sequence of the colours employed. During printing, panels of each colour in turn are held against a dye-receptive surface of the receiver sheet, as the two sheets are imagewise selectively irradiated, the first colour being overprinted by each subsequent colour in turn to make up the full colour image. A receiver or dye sheet according to the present invention may possess a back coat as desired, to impart desirable properties for example, to improve handling characteristics during and after printing and for example to aid adhesion of a protective cover sheet to a receiver sheet. Suitably, the back coat, if present, comprises a cross-linked polymer binder as herein described as being suitable for use as the binder for the electro-conductive sub-coat. If desired, the back coat may have a textured surface which may be imparted by a filler material or by the polymer per se. Any filler present in the backcoat may be ionic or electo-conductive as desired. In applying a coating to a substrate, either for the dye sheet or the receiver sheet, various coating methods may be employed including, for example, roll coating, gravure coating, screen coating and fountain coating. The coating may be deposited as a solution or a dispersion as desired from any suitable solvent for example water, acetone, methyl ethyl ketone and methanol which is then suitably removed by drying. Suitable drying conditions include, heating in air at a temperature of so to 110°C for a period of 30 seconds to 2 minutes according to the coating solvent employed. The coating can be cured, as desired, for example by heating or by irradiation with for example ultra violet light, electron beams and gamma rays. The invention is illustrated by the following non-limiting examples. Example 1 A TTP sheet according to the present invention was produced by coating onto a sample of MELINEX 990 (available from ICI) using a Meier bar, a dispersion containing 82 by weight of solids of the compositions in Table 1 to knes of lun. Table 1 SACON P401 - electro-conductive particles from Sachtlebenchemie; BEETLE Be685 - urea formaldehyde resin from British Industrial Plastics; PVB Bx-1 - Polyvinyl butyral from Sekisui; PVB B72 - Polyvinyl butyral from Monsanto; HYPERMER SCS1117, SOLSPERSE 20k and SOLSPERSE 24k-dispersing agents from I The electro-conductive layer was then coated with a dye-receptive surface coating composition as below to a thickness of 4μm and dried for 3 minutes at 140°C. Coating Composition VYLON 200 (Toyobo) 100 (parts by weight) CYMEL 303 (American Cyanamid) 1.4 TEGOMER Si2210 0.7 TINUVIN 900 (Goldschmidt) 1.0 NACURE 2530 0.2 Methylethyl ketone/ to provide 122 solids toluene (50/50) Example 2 TTP sheets produced according to Examples 1A to ID were tested (according to the tests below) to assess various characteristics thereof. The results are listed in Table 2. Resistivity; The TTP sheet was stored for a period of 1 hour at a temperature of 25°C in a relative humidity of 202, after which time, the resistivity of the surface coat was measured using a Model TI500 Surface Resistivity meter from Static Control Services. Static Charge; The static charge on the surface coat was measured at 202 relative humidity using a field strength meter after the receiver sheet had been passed through a Hitachi VY200 printer. Decay Time; The static charge of the surface coat was measured at 202 relative humidity using a field strength meter to determine the reduction in charge after a period of time. Adhesion to Dye-Receotive Coat: The sheet was delaminated and it was observed whether the substrate tore (acceptable adhesion) or whether the sheet fractured between different layers. Table 2 1 1C ID Resistivity (Oh s/sq) 10 10 - 10 10 Static Charge (kV) 0.6 Decay Time 15 sees to 0V Adhesion good good good good The results illustrate that a receiver sheet having a desirable combination of properties may be secured by employing an electro-conductive material below the surface coat.

Claims: Claims 1. A sheet for use in a thermal transfer printing process which comprises a substrate having on at least one side a surface coat and a sub-layer comprising a metal oxide-containing electro-conducting material below said surface coat. 2. A sheet according to claim 1, in which the surface coat of said sheet has a a resistivity in the range lxlO 8 to lxlO 13 Ohms per square. 3. A sheet according to claim 2, in which the surface coat has a resistivity of lxlO 9 to lxlO 12 Ohms per square. 4. A sheet according to any preceding claim in which the electroconductive material comprises a particulate material which is coated with a metal oxide-containing conductive coating. 5. A sheet according to claim 4 in which the particulate material is alumina, silica, china clay, titanium dioxide or barium sulphate. 6. A sheet according to claim 4 or 5, in which the particulate material is coated with tin oxide doped with antimony oxide. 7. A sheet according to any preceding claim, in which the electro-conductive material is present as very finely ground particles and is distributed evenly over the whole area of the sheet. 8. A sheet according to any preceding claim, in which the metal oxide-containing electro-conductive material is dispersed in a binder material which is coated onto the substrate. 9. A sheet according to claim 8, in which the the binder is cross-linkable, polyvinyl butyral. 10. A sheet according to any preceding claim, in which the conductive material is present in an amount of between 152 and 852 by weight of the sub layer. 11. A sheet according to any preceding claim, in which the sub layer has a thickness of between 0.2 and 2μm. 12. A sheet according to any preceding claim, having an optical haze value of not more than 10Z.

Inventors: HANN RICHARD ANTHONY, MORRISON GARY WAYNE, HANN, RICHARD, ANTHONY, MORRISON, GARY, WAYNE
Applicants: HANN RICHARD ANTHONY, ICI PLC, MORRISON GARY WAYNE, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1993
Main classification: B41M5
IPC codes: B41M5