EP0657293A1

EP0657293A1 - Method for making an image according to a thermal dye transfer process

Info

Publication number: EP0657293A1
Application number: EP93203449A
Authority: EP
Inventors: Geert C/O Agfa Gevaert N.V. Defieuw
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1993-12-09
Filing date: 1993-12-09
Publication date: 1995-06-14
Also published as: JPH07195854A

Abstract

The present invention provides a method for making an image comprising the steps of:

placing a dye donor element, having on a support a dye layer containing a thermally transferable dye, with said dye layer in face-to-face relationship with an image receiving element;
image-wise heating a thus obtained assemblage by activating a number of heating elements of a thermal head being in contact with said dye donor element, each of said heating elements corresponding to an image pixel and
separating said dye donor element from said image receiving element;

characterised in that during said image-wise heating, heating elements corresponding to an image pixel for which no density is required are activated such that they generate an amount of heat (H_e) in accordance with the following formula:

{0.5 H}_{D} {< H}_{e} {< H}_{D} (I)

wherein H_D represents the amount of heat required to cause visible dye transfer from said dye donor element to said image receiving element. The life-time of the thermal head can be prolonged when the above method is used.

Description

1. Field of the invention.

The present invention relates to a method for making an image according to the thermal transfer process wherein the image is formed by heating an assemblage of a dye donor element and an image receiving element by means of a thermal head. More in particular, the present invention relates to a thermal transfer process wherein wearing of the thermal head is reduced.

2. Background of the invention.

Thermal dye transfer methods include thermal dye sublimation transfer also called thermal dye diffusion transfer. This latter is a recording method in which the amount of dye that is transferred to an image receiving element can be controlled by controlling the amount of heat supplied to the assemblage of a dye-donor element and receiving element.
Image-wise heating can be carried out by means of a thermal head having a plurality of heating elements i.e. heat-generating resistors. Generally an assemblage of a dye-donor element and a receiving element will be heated by a thermal head from the back of the dye donor element i.e. the side of the support of the dye-donor element opposite to the side containing the dye layer. The back of such a dye donor element is commonly provided with a backing layer containing a heat-resistant binder and a number of additives such as particles and lubricants to assure a smooth transport of the dye donor element over the thermal head.
Image-wise heating of the assemblage is generally carried out line by line wherein the image information is divided in a number of lines which are consecutively supplied to the thermal head. Such a line of image data consists of a number of image pixels representing an image value to be reproduced. By activating each of the heating elements, which are arranged in a line one along the other, in accordance with the image value of the image pixel to which they correspond the image data of one line can be reproduced. For example if the image value of an image pixel is zero the corresponding heating element for the image pixel will not be activated whereas when the image value of an image pixel represents a certain density the image pixel will be activated so as to obtain the desired density on the image receiving element. This process is repeated for all lines of the image so as to obtain a complete reproduction thereof.
A problem with the above method is that the heating elements of the thermal head wear with the number of prints being made. As a consequence their efficiency decreases causing a reduction in quality of the image. This problem is especially observable when images need to be printed on an image receiving element based on a resin support e.g. polyester film and in particular on such transparent image receiving elements, due to a larger hardness of the surface of a resin support as compared to a paper support.

3. Summary of the invention.

It is an object of the present invention to provide a method for making an image according to a thermal transfer method using a thermal head wherein less wear of the thermal head is caused.
It is a further object of the present invention to provide an improved method for making an image on a resin based image receiving element.
Still further objects of the present invention will become clear from the description hereinafter.
According to the present invention there is provided a method for making an image comprising the steps of:

{0.5 H}_{D} {< H}_{e} {< H}_{D} (I)

_D

4. Detailed description of the invention.

In accordance with the present invention it has been found that the wear of a thermal head can be reduced by also activating those heating elements of a thermal head corresponding to an image pixel for which no density is required. Activation of these heating elements is carried out such that the heating elements generate an amount of heat (H_e) in accordance with the above formula (I). Preferably the activation of heating elements corresponding to image pixels representing a density of 0 is such that an amount of heat is generated between 0.7H_D and H_D.
The amount of heat supplied to a heating element can be easily controlled by controlling the magnitude of the current flowing through the heat-generating resistors of the heating elements and/or the time during which current flows through the heating element. The actual amount of heat H_D will depend on a number of factors such as e.g. the type of dye donor element and receiving element, the temperature of the heat sink of the thermal head and the heater line of the thermal head, the ambient temperature etc... The value of H_D for a particular combination can be easily determined by a simple experiment wherein the amount of heat supplied to a heating element is step-wise increased untill a detectable density builds up. This value may then be stored in the printer so that for a particular combination of dye donor element and receiving element the correct value for H_D may be manually or automatically retrieved and used during printing.
It will be understood that the minimal amount of heat to cause a visible dye transfer (H_D) will decrease during printing as a consequence of an increasing temperature of the heat sink of the thermal head. Accordingly it is required to adjust H_e during printing so as to keep it in accordance with formula (I). This can be accomplished by a continuous measurement of the temperature of the heat sink during printing and using the results of this measurement to adjust H_e during printing to keep in accordance with the present invention.
The force exerted by unit of length of the heater line by the thermal head on the back of the dye donor element is preferably between 150g/cm and 400g/cm and more preferably between 250g/cm and 400g/cm. When too low forces are used, the thermal head becomes strongly contaminated with the consequence of a low image quality whereas too large forces will rapidely wear the thermal head and cause breakage of the glass bulb of the thermal head. Wearing of the thermal head at high pressures is further increased by inorganic particles that are added to an optional backing layer of the dye donor element to avoid contamination of the thermal head. It is a particular advantage of the present invention that forces of upto 400g/cm can be used so that contamination of the thermal head can be decreased without substantially increasing the wear of the thermal head.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer. Preferably, the opposite side is covered with a heat-resistant layer. An adhesive layer may be provided between the support and the heat-resistant layer.
The heat resistant layer can comprise a lubricating material such as a surface active agent, a liquid lubricant, a solid lubricant or mixtures thereof preferably in a heat-resistant polymeric binder. The surface active agents may be any agents known in the art such as carboxylates, sulfonates, phosphates, aliphatic amine salts, aliphatic quaternary ammonium salts, polyoxyethylene alkyl ethers, polyethylene glycol fatty acid esters, fluoroalkyl C₂-C₂₀ aliphatic acids. Examples of liquid lubricants include silicone oils, synthetic oils, saturated hydrocarbons and glycols. Examples of solid lubricants include various higher alcohols such as stearyl alcohol, fatty acids and fatty acid esters. Preferred lubricants are polyethylene oxid, modified polydimethylsiloxane and zinc stearate. The heat resistant layer preferably also contains solid particles e.g. inorganic particles such as salts derived from silica such as e.g. talc, clay, china clay, mica, chlorite, silica, or carbonates such as calcium carbonate, magnesium carbonate or calcium magnesium carbonate (dolomite) can be further added to the heat resistant layer.
It is highly preferred to add mixtures of particles to the heat resistant layer having a Mohs hardness below 2.7 and particles having a Mohs hardness above, 2.7 such as mentioned in EP-A-93201642.1. A mixture of talc and dolomite particles is highly preferred.
The binder for the heat-resistant layer can be a cured binder or a polymeric thermoplast. A cured binder can be produced by a chemical reaction as described in e.g. EP 153,880 and EP 194,106, or by the influence of moisture as described in e.g. European Patent Application N° 91202098.9, or by irradiation of a radiation-curable composition as described in e.g. EP 314,348 and EP 458,538.
Thanks to the fact that the coating procedure of polymeric thermoplasts is very convenient, they are preferably used as binder for the heat-resistant layer. Preferred polymeric thermoplasts are those having a glass transition temperature above 100°C; these thermoplasts are suited for use as binder in the heat-resistant layer, because they are dimensionally stable at higher temperatures. Polymers having a glass transition temperature above 170°C are especially preferred. Even more preferred polymeric thermoplasts are those that are soluble in ecologically acceptable solvents such as ketones (e.g. ethyl methyl ketone and acetone) and alcohols (e.g. isopropanol).
Representatives of polymeric thermoplasts that are suited for use as binder in the heat-resistant layer are e.g. poly(styrene-co-acrylonitrile), polycarbonated derived from bisphenol A, polyvinyl butyral, polyvinyl acetal, ethyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, and polyparabanic acid.
Especially preferred polymeric thermoplasts are the polycarbonates derived from a bis-(hydroxyphenyl)-cycloalkane corresponding to general formula (II) :

wherein :
R¹, R², R³, and R⁴ each independently represents hydrogen, halogen, a C₁-C₈ alkyl group, a substituted C₁-C₈ alkyl group, a C₅-C₆ cycloalkyl group, a substituted C₅-C₆ cycloalkyl group, a C₆-C₁₀ aryl group, a substituted C₆-C₁₀ aryl group, a C₇-C₁₂ aralkyl group, or a substituted C₇-C₁₂ aralkyl group; and
X represents the atoms necessary to complete a 5- to 8-membered alicyclic ring, optionally substituted with a C₁-C₆ alkyl group, a 5- or 6-membered cycloalkyl group or a fused-on 5- or 6-membered cycloalkyl group. The heat-resistant layer thus formed has a thickness of about 0.1 to 3 µm, preferably 0.3 to 1.5 µm.
The dye layer of the dye donor element can be a monochromic dye layer or it may comprise sequential repeating areas of differently coloured dyes e.g. dyes having a cyan, magenta, yellow, and optionally black hue. When a dye-donor element containing three or more primary colour dyes is used, a multicolour image can be obtained by sequentially performing the dye transfer process steps for each colour.
A primary coloured dye layer e.g. a magenta or cyan or yellow dye layer may comprise only one primary coloured dye (a magenta, cyan or yellow dye respectively) or may comprise a mixture of two or more primary colour dyes of the same hue (two magenta, two cyan or two yellow dyes respectively).
Any dye can be used in such a dye layer provided it is easily transferable to the receiver sheet or element by the action of heat.
Typical and specific examples of dyes for use in thermal dye sublimation transfer have been described in e.g. EP 209,990, EP 209,991, EP 216,483, EP 218,397, EP 227,095, EP 227,096, EP 229,374, EP 235,939, EP 247,737, EP 257,577, EP 257,580, EP 258,856, EP 279,330, EP 279,467, EP 285,665, US 4,743,582, US 4,753,922, US 4,753,923, US 4,757,046, US 4,769,360, US 4,771,035, US 5,026,677.
The dye layer generally also comprises a binder in which the dye or dyes are dissolved or dispersed. The amount ratio of dye or dye mixture to binder generally ranges from 9:1 and 1:3 by weight, preferably from 3:1 and 1:2 by weight.
The following polymers can be used as polymeric binder : cellulose derivatives, such as ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose nitrate, cellulose acetate formate, cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate pentanoate, cellulose acetate benzoate, cellulose triacetate; vinyl-type resins and derivatives, such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, copolyvinyl butyral-vinyl acetal-vinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetoacetal, polyacrylamide; polymers and copolymers derived from acrylates and acrylate derivatives, such as polyacrylic acid, polymethyl methacrylate and styrene-acrylate copolymers; polyester resins; polycarbonates; copoly(styrene-co-acrylonitrile); polysulfones; polyphenylene oxide; organosilicones, such as polysiloxans; epoxy resins and natural resins, such as gum arabic. Preferably, the binder for the dye layer of the present invention comprises poly (styrene-co-acrylonitrile) or a mixture of poly(styrene-co-acrylonitrile) and a toluenesulphonamide condensation product.
The dye layer may also contain other additives such as i.a. thermal solvents, stabilizers, curing agents, preservatives, organic or inorganic fine particles, dispersing agents, antistatic agents, defoaming agents, and viscosity-controlling agents, these and other ingredients being described more fully in EP 133,011, EP 133,012, EP 111,004, and EP 279,467.
Addition of beads of polyolefin waxes or amid waxes, and/or of polymethylsilylsesquioxan particles, as described in European Patent Application No. 92203496.2, to the dye layer, said beads and/or particles protruding from the surface of said layer, is especially preferred.
A magnetic coating may be provided on one ore both sides of the dye donor element. The magnetic layer may be coated on the whole surface of the dye donor element, or on specific areas such as e.g. in the intermediate region between the dye frames in a 3 color dye donor element, along the dye frames where usually the detection marks are provided or at the beginning or the end of the dye donor element. This magnetiC coating may be used to write information concerning the dyes used in the dye donor element, the production date, the type of receiving element to be used in combination with the dye donor element and the like.
Any material can be used as the support for the dye-donor element provided it is dimensionally stable and capable of withstanding the temperatures involved, up to 400°C over a period of up to 20 msec, and is yet thin enough to transmit heat applied on one side through to the dye on the other side to effect transfer to the receiver sheet within such short periods, typically from 1 to 10 msec. Such materials include polyesters such as polyethylene terephthalate, polyamides, polyacrylates, polycarbonates, cellulose esters, fluorinated polymers, polyethers, polyacetals, polyolefins, polyimides, glassine paper and condenser paper. Preference is given to a support comprising polyethylene terephthalate. In general, the support has a thickness of 2 to 30 µm. The support may also be coated with an adhesive of subbing layer, if desired. Examples of suitable subbing layers have been described in e.g. EP 433,496, EP 311,841, EP 268,179, US 4,727,057, and US 4,695,288.
The support for the receiver receiving element that is used in combination with the dye-donor element may be a transparent resin support e.g. polyethylene terephthalate, a polyether sulfone, a polyimide, a cellulose ester, or a polyvinyl alcohol-co-acetal. The support may also be a reflective one such as a baryta-coated paper, polyethylene-coated paper or opaque resin supports such as e.g. white polyester i.e. white-pigmented polyester. Blue-coloured polyethylene terephthalate film can also be used as support.
To avoid poor adsorption of the transferred dye to the support of the receiving element this support is preferably coated with a special layer called image receiving layer, into which the dye can diffuse more readily. The image receiving layer may comprise e.g. a polycarbonate, a polyurethane, a polyester, a polyamide, polyvinyl chloride, polystyrene-co-arcylonitrile, polycaprolactone, or mixtures thereof. The image receiving layer may also comprise a heat-cured product of poly (vinyl chloride-co-vinyl acetate-co-vinyl alcohol) and polyisocyanate. Suitable image-receiving layers have been described in e.g. EP 133,011, EP 133,012, EP 144,247, EP 227,094, and EP 228,066.
In order to improve the light resistance and other stabilities of recorded images, UV absorbers, singlet oxygen quenchers such as HALS-compounds (Hindered Amine Light Stabilizers) and/or antioxidants may be incorporated into the image-receiving layer.
The dye layer of the dye-donor element or the image-receiving layer of the receiver sheet may also contain a releasing agent that aids in separating the dye-donor element from the receiver sheet after transfer. The releasing agents can also be applied in a separate layer on at least part of the dye layer or of the image-receiving layer. Suitable releasing agents are solid waxes, fluorine- or phosphate-containing surfactants and silicone oils. Suitable releasing agents have been described in e.g. EP 133,012, JP 85/19,138, and EP 227,092.
A subbing layer can be provided between the receiving layer and the support. Suitable subbing layers are based on vinylidenechloride copolymers, aromatic copolyesters and polystyrene sulphonic acid. Hydrophilic layers inbetween the subbing layer and the dye receiving layer can be applied in order to enhance the recyclability of the support. This hydrophilic layer comprises usually a water-soluble binder such as gelatin, polyvinylalcohol, hydroxypropylcellulose, hydroxyethyl cellulose or polystyrenesulphonic acid (or sodium salt), or a mixture thereof, with or without an anionic, a kationic, a nonionic or a zwitter ionic surfactant. A mixture of hydroxyethylcellulose and polystyrene sulphonic acid is especially preferred.
With the following example the invention will be illustrated without the intention to limit the present invention thereto. All parts are by weight unless otherwise stated.

EXAMPLE

Polyethylene terephthalate film having a thickness of 5.7 µm was provided on both sides with a subbing layer from a solution of copolyester comprising isophthalic acid units/terephthalic acid units/ethylene glycol units/neopentyl glycol units/adipic acid units/glycerol units in ethyl methyl ketone.
A solution comprising 9% by weight of dye A, 2 % by weight of dye B, and 10 % by weight of poly(styrene-co-acrylonitrile) as binder in ethyl methyl ketone as solvent was prepared.
From the resulting solution a layer having a wet thickness of 10 µm was coated on the subbed polyethylene terephthalate film. The resulting dye layer was dried by evaporation of the solvent.

A heat-resistant layer having a wet thickness of 10 µm was coated on the subbed back of the polyethylene terephthalate film from a solution in ethyl methyl ketone containing a polycarbonate binder having a structure as shown below (13% by weight) and 0.5% by weight Microdol Super (Norwegian Talc).

Polycarbonate for the heat resistant layer:

wherein n has a value giving a polycarbonate with a relative viscosity of 1.295 (measured in a 0.5% by weight solution in dichloromethane).
The side of the donor elements that showed the heat-resistant layer was coated with a solution forming a topcoat, said solution being a 0.5% by weight solution of Tegoglide 410 (commercially available from Goldschmidt) in isopropanol.

Preparation of the receiving element:

A receiving element was prepared by coating a polyethylene terephthalate film support having a thickness of 175 µm with a dye-image-receiving layer from a solution in ethyl methyl ketone of 3.6 g/m² of poly(vinyl chloride/co-vinyl acetate/co-vinyl alcohol) (Vinylite VAGD supplied by Union Carbide), 0.336 g/m² of diisocyanate (Desmodur N75 supplied by Bayer AG), and 0.2 g/m² of hydroxy-modified polydimethylsiloxan (Tegomer H SI 2111 supplied by Goldschmidt).

Printing procedure:

200 meters of the above described dye donor element were printed with a blanc image, i.e. an image containing only zero densities, while its dye layer was in face-to-face relationship with the image receiving layer of an image receiving element as described above. The back side of the dye donor element was in contact with a thermal head (Kyocera type KGT-219-12MP4-12-SPM having a length of the resistor line of 20cm (8 inch)) exerting a force of 160g/cm on the assemblage of dye donor element an receiving element. According to printing procedure A the heating elements of the thermal head were not activated while according to procedure B they were activated with an amount of heat corresponding to 85% of H_D.

Results:

The wearing of the thermal head was measured using an optical profilo meter (Rodenstock). In the case of printing procedure A, 1.7µm at the centre of the heating elements was weared away, while in case of printing procedure B this was less than 0.1µm.

EXAMPLE 2

A dye donor element was prepared similar to the dye donor element described in example 1 with the modification that to the heat resistant layer there was further added 0.5% of zinc stearate.
2200 meters of a thus prepared dye donor element were used to print on a receiving element as described in example 1 in accordance with printing procedure B described in example 1 with the modification that the thermal head exerted a force of 350g/cm on the dye donor element.
It was found that less than 0.1µm at the centre of the heating elements was weared away.

Claims

A method for making an image comprising the steps of:
- placing a dye donor element, having on a support a dye layer containing a thermally transferable dye, with said dye layer in face-to-face relationship with an image receiving element;

- image-wise heating a thus obtained assemblage by activating a number of heating elements of a thermal head being in contact with said dye donor element, each of said heating elements corresponding to an image pixel and

- separating said dye donor element from said image receiving element;
characterised in that during said image-wise heating, heating elements corresponding to an image pixel for which no density is required are activated such that they generate an amount of heat (H_e) in accordance with the following formula:

${0.5 H}_{D} {< H}_{e} {< H}_{D} (I)$

wherein H_D represents the minimal amount of heat required to cause visible dye transfer from said dye donor element to said image receiving element.
A method according to claim 1 wherein said thermal head contacts the side of the support of said dye donor element opposite to the side carrying the dye layer and wherein said thermal head exerts a force of not more than 400g/cm.
A method according to claim 1 or 2 wherein said dye donor element comprises on the side of the support opposite to the side carrying the dye layer, a heat-resistant layer comprising inorganic particles dispersed in a heat-resistant binder.
A method according to any of the above claims wherein said image receiving element comprises on a support an image receiving layer, said image receiving layer being in face-to-face relationship with said dye layer during image-wise heating.
A method according to claim 4 wherein said support of said image receiving material is a resin support.
A method according to claim 4 or 5 wherein said image receiving material is transparent.