This invention relates to dye diffusion thermal transfer (DDTTP or D2T2) printing, in which a dye or dye mixture is transferred from a dye sheet to a receiver sheet by the application of thermal energy ; and more particularly, to dye diffusion or sublimation thermal transfer printing in which the thermal energy is generated by converting light radiation to heat, such printing being known as light induced thermal transfer (LITT or L2T2) printing.

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, thereby causing dye to transfer to corresponding areas of that receiver sheet. The shape of the pattern transferred is determined by the number and location of the discrete areas which are subject to heating. Full colour prints can be produced by printing with different coloured dye coats sequentially in like manner and the different coloured dye coats are usually provided as discrete uniform print-size areas in a repeated sequence along the same dye sheet.

A typical receiver sheet comprises a substrate supporting a receiver coat of a dye-receptive composition containing a material having an affinity for the dye molecules, and into which they can readily diffuse when the adjacent area of dye sheet is heated during printing.

For efficient dye transfer, both the dye coat and the receiver coat need to be heated and therefore, ideally, the maximum heat should be generated at the interface between the dye coat and the receiver coat. In conventional thermal printing using a printing head, the heat is applied to the face of the dye sheet remote from the dye coat and hence dye transfer relies on the conduction of the heat through the dye sheet. There is thus a lag between the application of the heat and the transfer of the dye. This slow "turn-on" is an inherently limiting factor in conventional thermal printing. The use of a laser as the energy source can improve the "turn-on" as well as providing much higher resolution.

As is well known, the use of a laser requires that there is effective conversion of the light energy to thermal energy. Whilst in principle this conversion could be effected by the dyes themselves, in practice it is more usual, and indeed sometimes essential, to include a separate absorber material in the dye sheet. This is particularly necessary if the laser emits infra-red light. According to the present invention there is provided a thermal transfer dye sheet comprising a substrate having a dye coat comprising a transferable dye or mixture of dyes and a binder, and, either in said coating, or in a separate layer underneath the dye coat, a material capable of absorbing and converting light energy to thermal energy and having the Formula (1 )

M _k Pc(-0-R-0-), (-YR') _y (-Z) _m (-S0 ₃ A) _n wherein MkPc is a phthalocyanine nucleus of the formula :

in which M represents a metal atom, a halogeno-metal group, an oxy-metal group or hydrogen and k is the inverse of half the valency of M;

R represents an optionally substituted 1,2-arylene group;

Y represents oxygen or sulphur;

R1 represents an optionally substituted hydrocarbyl group;

Z represents a halogen or hydrogen;

A represents hydrogen, a metal or optionally substituted ammonium; x represents an integer from 1 to 8; y represents an integer from 0 to 14, m represents an integer from 0 to 14, n represents an integer from 0 to 32, the sum of 2x, y and m not exceeding 16, and the groups or atoms -0-R-0-, -YR' and Z are attached to the peripheral carbon atoms numbered 1 to 16 in Formula 2, with the proviso that when n is zero, at least oπel ,2-arylene group represented by R carries at least one alkyl group containing at least four carbon atoms. As indicated by Formula 2 above, the phthalocyanine nucleus of thecompoundsof the invention may be metal-free or may contain a complexed metal, halogeno-metal or oxy-metal, that is to say it may carry two hydrogen atoms at the centre of the nucleus or it may carry one or two metal atoms, halogeno-metal groups or oxy-metal groups complexed within the centre of the nucleus. Examples of suitable metals, halogen o-metals and oxy-metals include lithium, sodium, copper, nickel, zinc, manganese, iron, chloro-iron, chloro-aluminium, tin, lead, vanadyl and titanyl

Optionally substituted 1 ,2-arylene groups which may be represented by R include naphthylene and especially phenylene groups which may optionally be substituted by, for example, one or more alkyl groups, especially C1 -20-alkyl groups. The value of x is preferably 4or 8.

Optionally substituted hydrocarbyl groups which may be represented by R1 include optionally substituted alkyl groups and particularly optionally substituted aryl groups, for example naphthyl and especially phenyl groups The preferred value of y is in the range from 0 to 8 Halogen atoms which may be represented by Z include fluorine, bromine, iodine and, especially, chlorine The preferred value of m is in the range from 0 to 8

Metals which may be represented by A include alkaline earth metals, and, especially, the alkali metals. It is preferred that A is sodium or hydrogen. Compounds of the invention containing -S03A groups, which may be attached directly to the peripheral carbon atoms of the phthalocyanine nucleus and/or to the pendent organic groups R and R' ,are soluble in water. In compounds of the invention which contain no sulphonic acid or sulphonate groups (n=0), at least one 1 ,2-arylene group represented by R carnes at least one alkyl group containing at least four carbon atoms so as to provide solubility in organic liquids. Suitable alkyl groups may contain from 4 to 20, preferably from 4 to 12 and especially from 4 to 8 carbon atoms. Preferred alkyl groups include branched alkyl groups, especially tertiary alkyl groups such as t-butyl and t-octyl For maximum solubility, it is preferred that each 1 ,2-arylene groupcarπes at least one alkyl group having at least four carbon atoms.

It is preferred that the solubility of the sulphonate-free phthalocyanines of the invention in organic liquids is at least 3% Preferred organic liquids are selected from aliphatic

and aromatic hydrocarbons, ethers, ketones, chlorinated aliphatic and aromatic hydrocarbons, amides and substituted amides. Specific examples of suitable organic liquids are tetrahydrofuran (THF), cyclohexanone, chloroform, toluene, dichloromethane (DCM) and dimethylformamide (DMF). As specific examples of useful phthalocyanine compounds of Formula 1 , there may be mentioned the compounds

Formula 1A

Formula 1 B

Formula 1C

Formula 1 D

The use of such substituted phthalocyanine compounds which exhibit strong

absorption maxima in the near infra red region of the spectrum, particularly in the region between 750 and 900 nm gives improved optical density build up for L2T2 printing and is particularly suitable for use with a diode laser operating at 807 nm In this connection, it is to be noted that the optionally substituted catechol residues present in the compounds of Formula 1 provide a bathochromic effect of from 50 to 70 nm compared with the corresponding phenoxy-substituted phthalocyanmes The Dve Coat

The dye coat 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 and the absorber compound to the substrate which has acceptable solubility in the ink medium, i.e the medium in which the dye, absorber material and binder are applied to the transfer sheet. It is preferred however, that the dye and the absorber material are soluble in the binder so that they can exist as a solid solution in the binder on the transfer sheet In this form there is generally more resistance to migration and crystallisation during storage. Examples of binders include cellulose derivatives, such as ethylhydroxyethylcellulose (EHEC), hydroxypropylcellulose (HPC), ethylcellulose, 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, polymethyl 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 arable. 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 ethylcellulose. 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 absorber materials of Formula (1) also have strong absorbance properties and are 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 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, absorber material 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 Substrate

The substrate may be any sheet material which is transparent to the laser light, preferably having at least one smooth even surface and capable of withstanding the temperatures involved in L2T2, i.e. up to 400 ⁸ C for periods up to 20 msec. Examples of suitable materials are polymers, especially polyester, poiyacrylate, polyamide, cellulosic and polyalkylene films., including co-polymer and laminated films, especially laminates incorporating a smooth even polyester receptor layer on which the dye is deposited. The thickness of the substrate is preferably less than 20μm and more preferably less than 10μm. The L2T2 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 compπsing a transferable dye or mixture of dyes, a absorber material of Formula (1) with a receiver sheet, so that the coating is in contact with the receiver sheet and selectively exposing discrete areas of the dye sheet to radiation whereby heat is generated and the dye is transferred to the receiver sheet. The Receiver Sheet

The receiver sheet conveniently comprises a substrate of polymeπc sheet material which may be transparent or opaque. Receiver coats are generally 2-6 μm thick, and examples of suitable materials with good dye-affinity include saturated polyesters soluble in common solvents to enable them readily to be applied to the substrate as coating compositions, and then dried to form the receiver coat.

According to one further aspect of the invention, there is provided a receiver sheet for light induced thermal printing characterised by the incoφoration therein of an absorber material capable of converting light energy to thermal energy and having the Formula (1).

Whilst the absorber material is preferably incoφorated into the receiver layer, it may alternatively be in the form of a separate layer between the receiver layer and the substrate.

The location of absorber material in the receiver sheet gives flexibility to the system. By varying the relative amounts of absorber material present in the dye sheet and the receiver sheet the degree of heating in the dye sheet/receiver sheet combination can be controlled.

Faster printing or a reduced overall absorber material content whilst retaining printing speed is possible.

The phthalocyanine compounds of Formula 1 may be prepared by methods analogous to those used for the preparation of known phthalocyanine compounds. Thus, for example, an appropriately substituted phthalonitrile may be reacted with a metal or metal salt at an elevated temperature, optionally in an inert liquid and/or in the presence of catalytic materials. Sulphonation may then be effected if desired. Appropriately substituted phthalonitriles include phthalonitriles carrying two -ORO- substituents or carrying one -ORO- substituent and optionally one or two -YR1 substituents. The phthalonitriles may themselves be prepared by reacting halogen-substituted phthalonitriles with catechols and, optionally, phenols.

The invention is further illustrated by the following examples in which all parts and percentages are by weight. Example 1

Dyecoat and receiver coat solutions were made up according to the following formulations:

Dve Coat Receiver Coat

Magenta dye 12.3g Vylon 103 12.04g

Absorber material 3.0g Vyloπ 200 5.175g

PVB BX1 6.65g Tinuvin 234 0.19g

ECT 10 1.665g Ketjenflex MH 1.39g

THF 166.5g Cymel 303 1 12g

Tegomer 0.13g

R4046 0.067g

THF 128.9g (The magenta dye is

3-methyl-4(3-methyl-4-cyanoisothiazol-5-ylazo)-N-ethyl-N- acetoxyethyl aniline, the absorber material is a compound as shown in Formula 1 A (Sample 1), a compound as shown in Formula 1B (Sample 2), a compound having the formula MnPc (0-C ₆ H ₄ -0-butyl) _l6 (Sample 3) or a compound as shown in Formula 1C (Sample 4), PVB BX1 is polyvinylbutyral from Hercules, ECT 10 is ethyl cellulose from Sekisui, THF is tetrahydrofuran, Vylon 103 and 200 are high dye affinity polyesters from Toyobo, Tinuvin 234 is a U absorber from Ciba-Geigy, Ketjenflex MH is toluenesulphonamide/formaldehyde condensate from Akzo, Cymel 303 is a hexamethoxymethylmelamine oligomeric crosslinking agent from American Cyanamid, Tegomer is a bis-hydroxyyalkylpolydimethylsiloxane from Th Goldschmidt and R4046 is an amine blocked p-toluene sufphonic acid catalyst.

Dye coat solutions containing respectively Samples 1 , 2 and 3 of absorber material were applied to 23μm thick polyester film (S grade Melinex from ICI) using a K3 Meyer bar to

give a dry coat thickness of circa 1.5μm Sample 4 was applied as a subcoat in Methocel E5 beneath the dye layer

The receiver coat solution was stirred until all solids were dissolved Eight 20g batches of solution were removed 0 06g of Samples 1 to 4 of absorber material were added respectively to four of the batches with further stirring Each batch was coated on to a sheet of the same material as for the dye coat using a K3 Meyer bar to give a dry coat thickness of circa 3μm and cured at 140°C for 3 minutes

The receiver sheets were placed in turn against separate portions of the dye sheet and held together against an arc to retain laser focus by the application of 1 atmosphere pressure. An SDL 150 mw diode laser operating at 807 nm was coliimated using a 160mm achromat lens and projected on to the receiver sheet The incident laser power was about 100 mw and the full spot size (full width at half power maxima) about 30 x 20 μm The laser spot was scanned across the dye sheet by galvanometer to address the laser to locations 20 x 10 μm apart giving good overlap of adjoining dots At each location the laser was pulsed for a specific time to build up a block of colour on the receiver For each receiver, blocks of varying optical density were produced by varying the laser pulse times in increments of 50 μs between 50 and 500 μs inclusively to provide differing energy densities The optical density of each block was measured using a Sakura densitometer operating in the transmission mode. The resulting optical densities at specified energy densities for Samples 1 to 4 of absorber material in the dye sheet only are shown in the Table A more rapid build-up was achieved when the receiver sheet also contained absorber material

Energy ( j/crn ² '

Sample 2 4 6 8

1 0 0 2 068 1 8

2 0 0 2 0.68 1 32

3 0 041 1 08 1 8

4 0 0.05 0.26 065

Example 2 (Preparation of Sample 2) A mixture of 3,5-dιtertbutylcatechol (20g, 0.09 mol), tetrachlorophthalonitπle (5.31 g,

0 02 mol) and potassium carbonate (1243g, 0 09 mol) in DMF (50ml) was heated and stirred at 140 C for 1 hour under an atmosphere of nitrogen and then allowed to cool The resulting mixture was washed with methanol (2 x 400ml) and then water (2 x 400ml) and dried in air to give the product (9.05g, 80%) as an off-white solid The product was a dι-3,5-dιtertbutylcatechol substituted phthalonitrile of the formula

Lithium (0.08g, 0.012 mol) and n-butanol (4ml) were stirred at 120 C under an atmosphere of nitrogen for 2 hr. The di-3,5-ditertbutyl catechol substituted phthalonitrile (2.12g, 0.004 mol) and D.B.U. (0.57g) were added and the resulting mixture stirred at 110 C under an atmosphere of nitrogen for 4 h and then allowed to cool and drowned out into methanol. The resulting solid was collected by filtration and dried and then treated with methanol several times to remove impurities to leave the lithium phthalocyanine of the formula :

as a brown solid (0.74g) I max 832nm.

Example 3 ( Preparation of Samplel )

A proportion of the product of Example 2 (0.69g), toluene (20ml) and para-toluenesulphonic acid (0.23g) were stirred for 3 h. The solvent was then removed in vacuo and the resulting solid was dissolved in DCM and reprecipitated with methanol. The solid was then washed with methanol to leave the corresponding metal-free phthalocyanine (0.27g) as a brown solid; Imax 815nm (emax 152472).