WO1994006636A1

WO1994006636A1 - Thermal transfer printing receiver

Info

Publication number: WO1994006636A1
Application number: PCT/GB1993/001911
Authority: WO
Inventors: Ian Richard Stephenson; Kenneth West Hutt
Original assignee: Imperial Chemical Industries Plc
Priority date: 1992-09-11
Filing date: 1993-09-10
Publication date: 1994-03-31
Also published as: GB9219237D0

Abstract

A receiver sheet for light-induced thermal printing which incorporates absorber material capable of converting laser light to heat gives increased speed of dye transfer and greater flexibility to the overall printing system.

Description

THERMAL TRANSFER PRINTING RECEIVER

This invention relates to light-induced thermal transfer printing and particularly to receiver sheets therefor.

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. Such 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.

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.

Whilst in light-induced thermal printing, (ie where the required thermal energy is generated by converting light radiation to heat) the use of a laser as the energy source can improve the "turn-on" ( as well as providing much higher resolution) the inherent limitation still exists.

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.

The absorber material may be located in the dye coat or in a separate layer underneath the dye coat. Both locations mean that the heat is generated in a more appropriate position than when a print head is used and hence "turn-on" is improved. However, there is still room for improvement as there is a limit to the amount of separate absorber material that can be accomodated in the dye coat without affecting the amount of dye available for transfer and having the absorber material in a separate layer means that the conduction factor, although reduced, is still present.

A further improvement can be achieved by locating absorber material in the receiver sheet so that heat is generated on both sides of the interface.

Hence, according to one aspect of the present invention, there is provided a receiver sheet for light induced thermal printing characterised by the incorporation therein of an absorber material capable of converting light energy to thermal energy.

Whilst the absorber material is preferably incorporated into the receiver layer, it may alternatively be in the form of a separate layer between the receiver layer and the substrate. Any absorber material may be used depending on the wavelength of the laser light. Thus it may be a broad spectrum absorber for use with any laser such as carbon black (as dislosed in UK Patent No 2083726) or it may be an absorber capable of absorbing strongly at a particular wavelength. For example, poly(substituted)phthalocyanine compounds, such as hexadeca-β-thionaphthalene copper(II) phthalocyanine or pentadeca (4-methoxyphenylthio) copper(II) phthalocyanine, absorb strongly at around 800nm and are, therefore, very effective absorbers for use with diode lasers operating at 807nm. Alternatively, materials absorbing at around lOOOnm, such as

are suitable for use with a YAG laser.

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 amount of absorber material present in the receiver sheet depends on the particular construction of the receiver sheet and the conditions of use. Generally speaking, 10 to 70Z of the total amount of absorber material present in the dye sheet/receiver sheet combination may be located in the receiver sheet.

However, if the receiver sheet is to be used for reproducing photographic slides (ie it must be transparent to visible light) then the main factor influencing the amount of absorber material is to what extent it discolours the receiver sheet. Different absorber materials have different absorption characteristics and therefore the amount that can be tolerated varies but an indication can be ascertained by measuring the transmision optical density to visible light.

According to a preferred aspect of the invention, the substrate and the receiver coat are transparent and the absorber material is present in such an amount that the transmission optical density to visible light is no more than 0.3.

The value of 0.3 for the optical density to visible light is relevant because this is the minimum value obtainable for transparancies prepared by the conventional silver halide photographic process.

Hence, the incorporation of absorber material in the receiver sheet has the advantage that tranparancies produced by thermal printing can be matched with transparancies produced photographically in terms of optical density and, consequentially, in terms of contrast.

Additionally, the discolouration may be reduced by the addition of further material to neutralise the colour of the absorber material. For example, the brown/yellow discoloration produced by the phthalocyanine compounds mentioned above may be altered to a light grey by the addition of a suitable combination of magenta and cyan dyes.

A transparent receiver sheet according to the invention is particularly suitable for a printing method in which the laser is projected through the receiver sheet into the dye sheet. This arrangement gives various advantages such as increased printing speed and a more, efficient arrangement of the mechanical components of the printer.

A receiver sheet according to the invention, whether having a transparent or opaque substrate/receiver layer combination is also suitable for the more conventional printing method, in which the laser is projected into the dye sheet, either by adjusting the optical density of the dye sheet so as to allow sufficient light to pass through to the receiver layer whilst still generating enough thermal energy in the dye sheet to effect dye transfer, or by irradiating a combination of a dye sheet and a receiver sheet with laser light of two different wavelengths, the dye sheet and the receiver sheet containing different absorber materials, each absorbing respectively at the wavelength of one of the lasers.

For example, two diode lasers emitting light of different wavelengths ( a range of 780nm to 900nm is commercially available) may be used. The two beams may be combined into one imaging spot by a galvanometer or a polygon in known manner or alternatively, one beam may be passed through a half wave plate and combined with the other beam using a polarisation beam splitter, allowing easy adjustment of the path length of the beams to ensure overlap at the focus point.

Alternatively, a diode pumped Nd/YAG doped fibre laser may be used. In such a laser, the laser light from the diode is converted to the Nd/YAG wavelength of 1060nm. However, not all the light is converted leaving a residual emission at the diode wavelength (780nm to 900nm) in addition to the emission at 1060nm.

In either case, the wavelengths may be matched to appropriate absorber materials, for example those mentioned above, in the dye and receiver coats respectively or vice-versa. Alternatively, a broad spectrum absorber may be used as one of the absorber materials, but it must be located in the receiver sheet.

Similar systems may be used for projection into the receiver sheet but if a broad spectrum absorber is used then it must be located in the dye sheet.

The receiver sheet may also contain known additives such as silicone release agents for preventing total transfer of dye coat.

The invention will be more readily understood from the foilowing examples. Example 1

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

Dye coat Receiver coat

Magenta dye 0.833g Absorber material 0.197g PVB BX1 0.444g

ECT 10 O.lllg

THF 11.lg

(The magenta dye is

3-methyl- (3-methyl-4-cyanoisothiazol-5-ylazo)-N-ethyl-N-acetoxy- ethyl aniline, the absorber material is hexadeca-β-thionaphthalene copper(II) phthalocyanine, 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 UV 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 sulphonic acid catalyst.

The dye coat solution was applied to 23μ thick polyester ^■ film (S grade Melinex from ICI) using a K2 Meyer bar to give a dry coat thickness of circa 1.5μm.

The receiver coat solution was stirred until all solids were dissolved.Two 20g batches of solution were removed to one of which 0.06g of the same absorber material as used in the dye coat were added 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 resulting two receiver sheets were measured as having optical densities at 807 nm of circa 0.3 and zero respectively.

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 collimated 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. The optical density of each block was measured using a Sakura densitometer operating in the transmission mode and as clearly shown by Curves 1 & 2 in the plot of optical density against laser-on time, the receiver sheet containing the absorber material (Curve 1) has a more rapid build up rate.

100 200 300 400 500

Laser on time (uS)

Example 2

Example 1 was repeated except that the absorber material added to the receiver coat consisted of 0.3g of carbon black (Efweko NC18/2 from Degussa)

Similar results were obtained in terms of the build-up of the optical density. Example 3

Example 1 was repeated except that 0.0005g of the magenta dye and 0.001 of cyan dye (CI Solvent Blue 63) dye was added to the receiver coat batches to alter the yellow-brown discolouration to a neutral grey.

There was negligible effect on the visible light optical density of the receiver sheet and the build-up rate was the same.

Example 4 Further dye and receiver sheets were made in the same way as in Example 1 except that the receiver coat contained carbon black (as per Example 2) as the absorber material.

The receiver sheets were printed in the same way as in Example 1 except that a second diode laser emitting radiation at 900nm was included, and the two beams were combined into one imaging spot suitable for scanning by passing one beam through a half-wave plate and combining it with the other beam using a polarisation beam splitter, the paths of the two beams being adjusted to give overlap at the focus position. Again, in comparison with a dye/receiver sheet combination in which absorber material is only present in the dye sheet, a more rapid build-up rate of image optical density was obtained.

Example 5 Further dye sheet and receiver sheet samples were prepared and printed as in Example 1 except that the dye sheet samples contained progressively less absorber material. The optical densities of the resulting prints were measured and it was found that the same build up rate as a dye sheet/receiver sheet combination where only the dye sheet contained absorber material could be achieved with 20Z less absorber material.

Claims

1. A receiver sheet for light induced thermal printing comprising a substrate supporting a receiver coat of a dye receptive composition characterised by the incorporation of an absorber material capable of converting light energy to thermal energy.

2. A receiver sheet according to claim 1, in which the absorber material is incorporated into the receiver layer.

3. A receiver sheet according to claim 1, in which the absorber material is in the form of a separate layer between the receiver layer and the substrate.

4. A receiver sheet according to any preceding claim, in which the substrate and the receiver coat are transparent and the absorber material is present in such an amount that the transmission optical density to visible light is no more than 0.3.

5. A receiver sheet according to claim 4, containing additional material to neutralise the colour of the absorber material.

6. A method of light-induced thermal printing in which a dye sheet comprising a substrate having thereon a dye coat containing one or more thermally transferable dyes and an absorber material capable of converting light energy to thermal energy, and a receiver sheet comprising a transparent substrate having thereon a transparent receiver coat of a dye receptive composition and an absorber material capable of converting light energy to thermal energy, are pressed together and laser light is projected through the receiver sheet into the dye sheet.

7. A method of light-induced thermal printing in which a dye sheet comprising a substrate having thereon a dye coat containing one or more thermally transferable dyes and a first absorber material capable of converting light energy to thermal energy, and a receiver sheet comprising a substrate having thereon a receiver coat of a dye receptive composition and a second absorber material capable of converting light energy to thermal energy, are pressed together and laser light of two different wavelengths is projected into the dye sheet/receiver sheet combination so formed, one of said absorber materials being such that only laser light at one of said wavelengths is absorbed thereby.

8. A method according to claim 7, in which each absorber material absorbs respectively the laser light at one or other of said wavelengths.

9. A method according to claim 7, in which one of said absorber materials absorbs at both wavelengths.

10. A method according to claim 7,8 or 9, in which the laser light is generated by two diode lasers.

11. A method according to claim 7,8 or 9, in which the laser light is generated by a diode pumped, Nd/YAG doped fibre laser.