DE68911960T2 - Thermal transfer material. - Google Patents

Description

The invention relates to a thermal transfer material which is useful in a thermal transfer process.

The thermal wax transfer process uses a transfer sheet or film made of paper or a plastic material. The sheet or film has a coating on one side containing a pigment and a wax such as carnauba wax, paraffin wax, montan wax or beeswax. When a thermal head, activated by supplied color signals corresponding to the color of a colored original, generates heat on the opposite side of the sheet or film, the wax coating is melted and transferred to an image receiving material or a copy sheet, reproducing an image of the colored original thereon. A process of this type is described, for example, in GB-PS 2 069 160.

Because of the deficiencies exhibited by the waxes normally used, and in particular by some paraffin waxes, which cause a progressive deterioration of the transfer properties, it was proposed in Japanese patent application JP 58-16267B to use a pigment together with a chlorinated paraffin wax which is easily miscible with the other waxes normally used in wax transfer printing. A printing ink composition containing carbon black and equal weights of a chlorinated paraffin (mp 50-53ºC) and carnauba wax is particularly described.

Unfortunately, pigments, when used in conjunction with the waxes previously described for this purpose, generally produce coatings that are not suitable for some applications have insufficient transparency. It has now been found that when pigments are used together with certain halogenated hydrocarbons, transparent films having excellent transparency are obtained which, due to the non-crystalline nature of the halogenated hydrocarbon, are suitable for such applications as overhead light projection.

Accordingly, the invention relates to a thermal transfer sheet made of a substrate with a coating consisting of a dispersion of a finely divided pigment in a medium containing at least 90% by weight of a halogenated hydrocarbon having a chlorine content of at least 65% by weight and a softening point of at least 50°C.

The substrates used in the preparation of the transfer sheets of the present invention are suitably plastic films or papers having a thickness of 3 to 25 µm. Specific examples of such substrates are polyethylene terephthalate films, polyethylene films, polypropylene films, polystyrene films, glassine papers, synthetic papers and laminated papers.

The coating preferably has a thickness of about 1 to 10 µm and in particular contains 2 to 30 wt.% of the pigment. The remainder of the coating preferably consists of the transferable medium.

The coating can be applied to the substrate as a melt, but it is preferably fluidized with an organic liquid in which the transfer medium is soluble and the pigment is dispersed, such as tetrahydrofuran

The transferable medium preferably contains more than 90 and preferably at least 95 and most preferably 100% by weight of halogenated hydrocarbon. The formability of the halogenated hydrocarbon can be adjusted by incorporating a small proportion of up to 10% by weight, preferably not more than 5% by weight, of a plasticizer such as stearic acid, chlorostearic acid, oleic acid or a liquid chlorinated hydrocarbon based on a hydrocarbon having a shorter carbon chain or a lower degree of chlorination.

The halogenated hydrocarbon preferably has a softening point of 50 to 100°C. It is also preferably a chlorinated hydrocarbon having a chlorine content of 65 to 72% by weight. Such a product can be prepared by chlorinating a paraffin or a mixture of paraffins having 18 to 35 carbon atoms and preferably 18 to 26 carbon atoms.

As indicated above, the chlorinated paraffin may be based on a single paraffin or on a mixture of paraffins. While single paraffins are known and can be prepared, the usual commercial products consist of mixtures obtained by chlorinating mixed paraffins having a range of carbon chain lengths. It should be noted that, for example, a nominal C18-26 paraffin fraction such as might be used to prepare a chlorinated paraffin for use in accordance with the invention may contain a minor proportion of paraffins having a chain length outside the range specified.

The chlorinated paraffins can be prepared by chlorinating a suitable paraffin starting material in the usual way to the desired chlorine content. All the known processes for preparing chlorinated paraffins can be used. These processes generally consist of introducing chlorine gas into the liquid paraffins at a temperature above about 80°C.

The coating present on the transfer sheet according to the invention may contain any additives, such as stabilizers and modifiers, that are commonly incorporated into chlorinated paraffin compositions. Commercial chlorinated paraffins usually contain a stabilizer or a mixture of stabilizers to impart light stability, high temperature stability and storage stability to the compositions. A common stabilizer is an epoxy, eg epoxidized soybean oil. The coating may also contain other common ingredients.

The pigment or pigment mixture present in the transfer sheet according to the invention is preferably of organic nature.

Examples of such organic pigments are those of the azo, disazo, thioindigo, anthraquinone, anthanthrone, isobenzanthrone and triphendioxazine series, vat dye pigments, phthalocyanine pigments such as copper phthalocyanine, the core-halogenated derivatives thereof and copper tetraphenyl and copper octaphenyl phthalocyanines, quinacridone pigments, lakes of acidic, basic and mordanting dyes and in particular copper phthalocyanines of the alpha and beta form as well as carbon black, which for dispersion purposes is classified as an organic pigment for the sake of simplicity.

Such pigments are described, for example, in Volume 2 of the Colour Index (second edition 1956 or third edition 1971) under the heading "Pigments" and in the subsequent authorised amendments thereto.

Examples of suitable inorganic pigments include transparent iron oxides.

The pigments are preferably formulated as a dispersion by dissolving them in a suitable organic solvent, in particular the solvent in which the coating to be applied to the substrate, such as tetrahydrofuran, together with dispersants and/or fluidizing agents. It has been shown that the use of suitable dispersants is important in order to achieve a good transfer sheet, since chlorinated paraffins have poor dispersing properties.

Any suitable dispersant known in pigment technology can be used, but preferred dispersants are polymeric dispersants in which the solvatable chain is derived from an aliphatic hydroxy acid such as hydroxystearic acid, ricinoleic acid or caprolactone, or a mixture thereof. Particularly preferred dispersants are those which also contain an alkylamine, in particular a polyalkyleneimine, as described for example in GB 1 373 660, EP 158 406A and EP 208 041A, such as poly-C 2-4 alkyleneimines carrying at least two mono- or poly-(carbonyl-C 1-7 alkyleneoxy) groups.

Suitable fluidising agents are described in GB 1 508 576 and GB 2 108 143. The fluidising agent of GB 1 508 576 is a substituted ammonium salt of a coloured acid in which there are 19 to 60 carbon atoms in at least three chains attached to the N atom of the substituted ammonium ion. In a preferred fluidising agent of this type for use with a phthalocyanine pigment, the coloured acid is a copper phthalocyanine sulphonic acid containing on average 1 to 2 sulphonic acid groups. The coloured acid, unlike the ammonium salt, can itself be used as a fluidising agent. The fluidising agent of GB 2 108 143 is a water-insoluble disazo compound containing a central divalent group free from acidic and other ionic substituents which is bonded via azo groups to two monovalent end groups, one end group being free from acidic and other ionic substituents and the other containing a single substituted ammonium salt group Such fluidizing agents are useful for improving the fluidity of the dispersion of the pigment and wax in the organic liquid so that it can be efficiently ground and applied to the substrate.

When the thermal transfer sheet of the invention is used in the thermal transfer process, a good gray scale is obtained, i.e. the amount of dye transferred from the substrate to the copy sheet increases uniformly with an increase in the applied thermal energy.

The invention is further illustrated by the following examples, in which all parts and percentages are by weight.

example 1

A mixture of tetrahydrofuran (54.5 g), Pigment Blue 15:3 (35 g), Dispersant 8 (7 g) described in EP 208 041A and Fluidising Agent 1 (3.5 g) described in EP 208 041A was milled with 3 mm glass beads for 24 h on a flat shaker. The dispersion was diluted with a 10% solution of a chlorinated aliphatic C18-26 hydrocarbon having a chlorine content of 70% in tetrahydrofuran (700 g) to give a finely divided flowable dispersion with a pigment content of 4.4%.

The dispersion was coated onto a 6 µm thick polyester sheet using a K-bar wrapped with a 24 µm wire and then dried to give a clear blue translucent film on the polyester substrate suitable for thermal transfer printing.

Further formulation examples are given below, which are suitable for coating a 6 µm thick polyester film,

Example 2

A mixture of toluene (48 g), Pigment Blue 15:3 (40 g), Dispersant 8 (8 g) described in EP 208 041A and Fluidising Agent 1 (4 g) described in EP 208 041A was milled with 3 mm glass beads for 24 h on a flat bed shaker. The dispersion was diluted with a 10% solution of a chlorinated aliphatic C 18-26 hydrocarbon having a chlorine content of 70% in toluene (809 g) to give a finely divided flowable dispersion with a pigment content of 4.4%.

Examples 3 to 5

The toluene of Example 2 was replaced by the following solvents:

Example 3 Methyl ethyl ketone

Example 4 Methyl isobutyl ketone

Example 5 Xylene

Example 6

A mixture of toluene (51 g), Pigment Red 48:2 (35 g) and Dispersant 8 (14 g) described in EP 208 041A was milled with 3 mm glass beads for 24 h on a flat bed shaker. The dispersion was diluted with a 10% solution of a chlorinated aliphatic C₁₈₋₂₆ hydrocarbon having a chlorine content of 70% in toluene (700 g) to give a finely divided flowable dispersion with a pigment content of 4.4%.

Example 7

The toluene in Example 6 was replaced by xylene.

Example 8

Pigment Red 48:2 of Example 6 was replaced by Pigment Red 57:1.

Examples 9 to 12

The toluene of Example 8 was replaced by the following solvents:

Example 9 Tetrahydrofuran

Example 10 Methyl ethyl ketone

Example 11 Methyl isobutyl ketone

Example 12 Xylene

Example 13

A mixture of methyl isobutyl ketone (52 g), Pigment Yellow 12 (30 g), Dispersant 8 (12 g) described in EP 208 041A and SOLSPERSE 22000 (6 g) was milled with 3 mm glass beads for 24 hours on a flat bed shaker. The dispersion was diluted with a 10% solution of a chlorinated aliphatic C18-26 hydrocarbon having a chlorine content of 70% in methyl isobutyl ketone (581.8 g) to give a finely divided liquid dispersion with a pigment content of 4.4%.

Example 14

Pigment Yellow 12 of Example 13 was replaced by Pigment Yellow 13.

Claims (13)

1. Thermal transfer sheet comprising a substrate with a coating consisting of a dispersion of a finely divided pigment in a medium containing at least 90% by weight of a halogenated hydrocarbon having a chlorine content of at least 65% by weight and a softening point of at least 50ºC. 2. Transfer sheet according to claim 1, wherein the coating contains 2 to 30 wt.% pigment. 3. A transfer sheet according to claim 1 or 2, wherein the coating comprises a medium containing at least 95% by weight of the halogenated hydrocarbon. 4. The transfer sheet of claim 3, wherein the coating comprises a medium consisting of 100% by weight of the halogenated hydrocarbon. 5. Transfer sheet according to one of the preceding claims, in which the halogenated hydrocarbon has a softening point in the range of 50 to 100°C. 6. Transfer sheet according to one of the preceding claims, in which the halogenated hydrocarbon is a chlorinated hydrocarbon having a chlorine content in the range of 65 to 72 wt.%. 7. Transfer sheet according to claim 6, wherein the chlorinated hydrocarbon is a product obtained by chlorinating a paraffin or a mixture of paraffins having 18 to 35 carbon atoms. 8. Transfer sheet according to claim 7, wherein the halogenated hydrocarbon is a product obtained by chlorination a paraffin or mixture of paraffins having 18 to 26 carbon atoms. 9. Transfer sheet according to one of the preceding claims, in which the pigment is an organic pigment. 10. Transfer sheet according to one of the preceding claims, in which the dispersion of the pigment in the halogenated hydrocarbon contains a pigment dispersant and/or a fluidizing agent. 11. The transfer sheet of claim 10, wherein the dispersant is a polymeric dispersant having a solvable chain derived from an aliphatic hydroxy acid. 12. Transfer sheet according to claim 10 or 11, wherein the dispersant contains a polyalkyleneimine residue. 13. Transfer sheet according to claim 12, wherein the dispersant is a poly(C₂₋₄-alkyleneimine) bearing at least two mono- or poly(carbonyl-C₁₋₇-alkylenoxy) groups.