EP0705173A1

EP0705173A1 - Process for the formation of a heat mode image

Info

Publication number: EP0705173A1
Application number: EP94920462A
Authority: EP
Inventors: Herman Jozef Uytterhoeven; Luc Gustaaf Bastiaens; Rita Maria Torfs; Bart Lutgart Aerts
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1993-06-25
Filing date: 1994-06-23
Publication date: 1996-04-10
Also published as: WO1995000342A1

Abstract

A process is disclosed for the formation of a heat mode image comprising the steps of: a) exposing information-wise by intense laser light, preferably an infrared laser, a thermal imaging medium, comprising a transparent support, an image forming layer containing an image forming substance, preferably carbon black, further a release layer and an upper thermoadhesive layer with a Tg between 20 and 60 °C and a coverage of at least 10 g/m2, b) laminating by heat a plain paper foil to this imaging medium, and c) peeling-apart the support and the plain paper foil whereby the image forming layer and the release layer adhere to the support in the image parts, and whereby the image forming layer, the release layer and the thermoadhesive layer adhere to the plain paper foil in the non-image parts.

Description

DESCRIPTION

PROCESS FOR THE FORMATION OF A HEAT MODE IMAGE

1. Field of the invention.

The present invention relates to a process for the formation of a heat mode image, said process comprising only dry development steps.

2. Background of the invention.

In the past several proposals have been made for obtaining an imaging element that can be developed using only dry development steps without the need of processing liquids as for example in silver halide photographic materials.

Dry imaging elements are known that can be image-wise exposed using an image—wise distribution of heat. These types of dry imaging elements also called heat recording materials or heat mode materials offer the advantage in addition to an ecological advantage that they do not need to be handled in a dark room nor any other protection from ambient light is needed. A disadvantage of heat mode recording materials is their low sensitivity requiring powerful exposure means. This disadvantage is probably one of the major reasons why heat mode recording materials have not found wide acceptance up till recently despite their potential advantages. Since powerful exposure means especially lasers are becoming more readily available it may be expected that said disadvantage will no longer impair the wide spreading of heat mode recording materials.

Heat mode recording materials are disclosed in e.g. US-P-4.123.309, US-P- .123.578, US-P-4.157.412, US-P-4.547.456 and PCT application WO 88/04237. The latter application, which has been followed by a similar application published as WO 93/03928, discloses a web having an image forming surface and a porous layer of an image forming substance. The element further comprises a heat sensitive substance. Upon imaging with a laser the image forming surface is liquefied at the exposed parts thereby penetrating the porous layer and improving its adherence to the web while at the non-exposed parts liquefying of the image forming surface does not take place and as a consequence the adherence of the porous layer to the web remains poor. The porous layer can then be removed in the non—exposed areas using a stripping tape composed of a support and an adhesive layer. Since the thus obtained image may be easily scratched and is very poor wear resistant it is necessary to laminate a protecting layer to the image which is inconvenient and makes such a heat recording material less attractive.

Furthermore due to the subtle balance of adhesion forces between the porous layer and the image forming surface and the cohesive forces within the porous layer the removal of the porous layer with a stripping tape has to be performed under very stringent conditions and even then lateral cracks of the porous layer in the exposed parts may occur resulting in a decreased image density.

Moreover, complete materials provided with a stripping foil or stripping paper applied before laser exposure suffer from the danger of premature unwanted delamination before exposure due to the weak adherance of the porous layer to the support. This unwanted phenomenon can occur during manufacturing and cutting of the material, or during manipulation in the laser recorder.

It is an object of the present invention to provide a process for the preparation of a heat mode image which shows a clear differentiation between image and non-image parts with adequate density in the image parts.

It is a further object of the present invention to provide a process for the formation of a heat mode image using a material design wherein the danger for premature delamination is avoided.

3. Summary of the invention.

The objects of the present invention are realized by providing a thermal imaging medium, and a process for the formation of a heat mode image, comprising the following steps :

(a) exposing information-wise to intense laser radiation a thermal imaging medium comprising :

(1) a transparent support having a surface part liquefiable by intense heat,

(2) an image forming layer containing an image forming substance and a compound capable of transforming intense laser radiation into heat, said compound being the same or different from said image forming substance,

(3) a release layer,

(4) an upper thermoadhesive layer having a glass transition temperature T„ between 20 °C and 60 °C and (b) laminating by heating to the upper thermoadhesive layer of said imaging medium a plain paper foil comprising no extra coating(s) on the lamination side,

(c) peeling-apart the support and the paper foil whereby the image forming layer and the release layer adhere to the support in the image-wise exposed parts, and whereby the image forming layer, the release layer and the thermoadhesive layer adhere to the plain paper foil in the image-wise non—exposed parts.

In a preferred embodiment the image forming substance and the compound transforming laser radiation into heat are one and the same substance, most preferably carbon black.

In essence the objects of the present invention are reached by the presence on top of the heat mode element before exposure of a sufficiently thick thermoadhesive layer, preferably having a coverage of at least 10 g/m , and by performing the lamination and delamination after exposure by means of a ordinary plain paper foil having no extra coating whatsoever on the lamination side, and preferably neither on the opposite side.

4. Detailed description of the invention.

Different kinds of plain paper qualities can be applied in the practice of this process. A broad range of papers with diverging physical properties dependent on their composition, manufacturing process and post—manufacturing treatment can be used. A survey of useful commercially available paper brands are given in following table A together with a number of relevant physical properties such as surface roughness, defined by PT (profile depth) and WT (waviness depth), smoothness, rigidity, porosity and weight.

Smoothness was measured and expressed according to DIN 53107, rigidity according to SCAN-P29 (edition '69), PT according to DIN 4771 and WT according to DIN 4774. TABLE A

No Paper brand/manufacturer smoothness rigidity weiσht PT WT Bekk.s mN g/m^?

1 IKONOREX 200 (PB Paper, Brussel) 838 521 200 3.7 1.6

2 IDEAL BLANC BRILL. (") 490 362 172 3.9 1.4

3 OFFSET 80 g (Epacar) 17 93 85 30.1 9.9

4 OFFSET MAT (Epacar) 13 1041 248 28.5 7.0

5 OFFSET 80 G A3 (PB Paper) 35 119 83 24.3 6.2

6 WHITE OFFSET 90 g (Epacar) 203 56 87 10.1 3.9

7 PAMOVALOR (Albbruck) 33 33 69 29.2 7.6

8 PAMOLUX (") 33 32 59 36.7 12.1

9 PAMOREX (") 7 300 102 35.7 12.6

10 PAMOFIX (") 7 102 70 31.2 8.9

11 PAMOSOL (") 4 282 92 34.1 11.2

12 ALPAREX (") 1026 83 98 7.8 3.0

13 ALPALUX (") 64 32 65 20.8 6.7

14 ALPASET (") 15 76 79 24.8 8.2

15 ALPAWEGA (") 1029 91 99 8.3 2.9

16 ALPAFIN-LWC (") 967 45 79 12.9 5.2

17 ALPAFIN-MAT (") 18 44 70 27.0 8.0

18 REGISTER 135G (Cartaline) 20 343 129 25.8 5.8

19 MACO 100 G (Scaldia) 92 92 99 13.7 3.9

20 GREENEX (PB Paper) 43 276 164 29.4 14.0

21 WHITE 185 G (Buhrman Papers) 44 603 169 23.4 4.5

22 COUCHE 115 G (Haseldonckx) 1588 106 121 5.9 2.5

23 AϋSSEDAT REY (International Paper) 121 132 100 19.1 4.7

24 AGFA COPY PAPER (Berghuizen, NL) 30 100 81 27.1 8.1

It was found that the smoothness of the paper plays an important role in the adherance properties of the paper to the thermoadhesive layer (TAL) . So in some instances the composition of the TAL has to match the properties of the paper sort.

During the lamination / delamination process a minimal thermal load should be imposed to the material. This is favourable from the viewpoint of energy consumption. Moreover, the risk for material change becomes greater at higher temperature. Above 120 °C paper starts to degradate strongly. For these reasons the T_ of the TAL should be below 60 °C. The adherance of the TAL on paper is also determined by the flow properties of the TAL while heating above the T_. A parameter for describing this proces is the melt viscosity. This melt viscosity is also connected with the molecular weight and the cross-linking degree of the polymer(s) in the TAL. Self- evidently the T_ value of the TAL can be determined by the T_ value of the polymer(s) used. On the other hand a T_ below 60 °C can be the result of the use of polymers with a higher T_ value combined with the use of polymers with a lower T_, or the addition of polymeric or low-molecular plasticizers or thermosolvents.

In order to induce easy film formation during drying without addition of cosolvents or film forming additives a T_ value below 45 °C is most preferred. However with a T_ value between 45 °C and 20 °C there is a danger for unwanted sticking of the TAL to the backside of the imaging medium or to other materials with which the imaging medium can come into contact before or during laser exposure. This can be prevented by imposing to the TAL a high melt viscosity (greater than 7000 Poise measured at 120 °C) and an elasticity corresponding to a tg δ value greater than 1.30. The tg δ value is a measure for the elasticity as described in Polymer Chemistry : the Basic Concept, P.C. Hiemenz, 1984, edit, by M. Dekker Inc., New York.

With smooth paper sorts (> 100 Bekk.s) a good adherance to the TAL is obtained. For less smooth papers (between 5 and 100 Bekk.s) the adherance gets difficult for TAL 's with a high melt viscosity and therefore a prolonged heating or a higher temperature is required during lamination. For TAL 's with a low melt viscosity the adherance to these less smooth papers is well easy realizable. For rough paper sorts (< 5 Bekk.s) the TAL must have a low melt viscosity in order to assure adherance. However if the melt viscosity gets too low the TAL becomes so sticky so that the material produces problems during manufacturing and handling.

Polymers with a T_ lower than 20 °C cannot be used alone because they render the TAL to sticky and this cannot be compensated anymore by strongly increasing the melt viscosity. However such polymers can be used in combination with polymers with a higher T_ or with other additives like pigments, fillers, matting agents, polymeric beads, e.g. silica, titanium dioxide, polymethylmethacrylate beads, polystyrene beads, glass beads, hollow polymeric core-sheat beads and waxes, provided the resulting T_ of the layer is above 20 °C.

Papers with a smoothness lower than 5 Bekk.s are not suited because of bad quality of the final image.

The thickness of the TAL is important for the adherence during the lamination process and a minimal coverage of 10 g/ is preferred. Most preferably the thickness of the TAL is about 15 - 25 g/m , especially when using rather rough paper qualities.

For ecological and practical reasons the TAL is preferably coated from an aqueous medium. Therefore the polymers are preferably incorporated as latices. Other additives can be present into the TAL to improve the layer formation or the layer properties, e.g. thickening agents, surfactants, levelling agents, thermal solvents and pigments.

Preferred latices are styrene-butadiene latices. These latices can contain other comonomers which improve the stablitity of the latex, such as acrylic acid, methacrylic acid and acrylamide. Other possible polymer latices include polyvinylacetate, copoly(ethylene- vinylacetate) , copoly(acrylonitrile-butadiene-acrylic acid), copoly(styrene-butylacrylate) , copoly(methylmethacrylate-butadiene) , copoly(methylmethacrylate-butylmethacrylate) , copoly(methylmethacrylate-ethylacrylate) , copolyester(terephtalic acid-sulphoisophtalic acid-ethyleneglycol) , copolyester(terephtalic acid-sulphoisophtalic acid-hexanediol-ethyleneglycol) .

Particularly suitable polymers for use in the TAL layer are the BAYSTAL polymer types, marketed by Bayer AG, which are on the basis of styrene-butadiene copolymers. Different types with different physical properties are available. The styrene content varies between 40 and 80 weight %, while the amount of butadiene varies between 60 and 20 weight % ; optionally a few weight % (up to about 10 %) of acrylamide and/or acrylic acid can be present. Most suited are e.g. BAYSTAL KA 8558, BAYSTAL KA 8522, BAYSTAL S30R and BAYSTAL P1800 because they are not sticky at room temperature when used in a TAL layer. Other useful polymers are the EUDERM polymers, also from Bayer AG, which are copolymers comprising n.-butylaerylate, methylmethacrylate, acrylonitrile and small amounts of methacrylic acid. The physical properties of some polymers (all from Bayer AG) , of which some representatives will be used in the examples furtheron are summarized in following table B.

TABLE B

V ** **

No polymer melt viscosity tg δ^"1 at 120 °C

(°C) (Poise at 120 °C) 1

1 BAYSTAL KA8558 38 > 6 372 2.22

2 BAYSTAL KA8522 34 > 13 240 3.54

3 BAYSTAL S3OR 30 > 29 820 13.78

4 BAYSTAL KA8588 30 > 3 100 2.03

5 BAYSTAL KA8588V1 30 > 4 020 1.95

6 BAYSTAL P1800 24 > 11 630 4.39

7 BAYSTAL KA8525 10 > 6 330 5.48

8 BAYSTAL T425C 1 > 12 920 4.54

9 EUDERM DISP.32A 1 > 5 505 3.82

10 EUDERM DISP.92A 41 > 21 570 4.54

notes : * : measured with the 1090 THERMOANALYZER of Du Pont Co ; ** : measured with the VISCOELASTIC MELT TESTER of Rheo etrics Co, Surrey, UK

Apart from the upper thermoadhesive layer to which the plain paper is laminated and which must comply with the requirements described above the material can contain one or more supplementary thermoadhesive layer(s) positioned between the upper TAL and the release, layer. This (these) extra TAL ('s) can e.g. promote a stronger adherance to the release layer which will lead to a better image quality after the lamination - delamination process. This (these) other TAL ('s) can have a different composition not obeying the physical requirements imposed to the upper TAL. This (these) layer(s) can show e.g. a lower T„ and/or a lower melt viscosity and/or elasticity. This (these) layer(s) can contain one polymer or a mixture of polymers, optionally in combination with low-molecular additives like plasticizers or thermosolvents. Other ingredients which can be incorporated include waxes, pigments, fillers, polymer beads, glass beads, silica and titanium dioxide. In the image forming layer the image forming substance is preferably a pigment, e.g. a magnetic pigment, e.g. iron oxides, a coloured piment, e.g. copper phtalocyanine, or metal particles. However the most preferred pigment is carbon black. It can be used in the amorphous or in the graphite form. The preferred average particle size of the carbon black ranges from 0.01 to 1 μm. Different commercial types of carbon black can be used, preferably with a very fine average particle size, e.g. RAVEN 5000 ULTRA II (Columbian Carbon Co.), CORAX L6, FARBRUSS FW 2000, SPEZIALSCHWARZ 5, SPEZIALSCWARZ 4A, SPEZIALSCHWARZ 250 and PRINTEX U (all from Degussa Co.). When using carbon the image forming substance and the compound transforming intense laser radiation into heat is one and the same product. When however the image forming substance is not absorptive for the laser radiation, which is preferably infra-red laser radiation, an extra compound, preferably an infra-red absorbing compound is required for transforming the radiation into heat. It will be clear that mixtures of pigments, or mixtures of one or more pigments and one or more compounds transforming radiation into heat can be used. As binders for the image forming layer gelatin, polyvinylpyrrolidone, polyvinylalcohol, hydroxyethylcellulose, polyethyleneoxide and a broad variety of polymer latices can be considered. These latices can be film forming or non-film forming. They can comprise acid groups as a result of which they can swell in an alkaline coating medium and/or become totally or partially soluble. In this way the layer properties can be strongly influenced, e.g. less coating and drying point defects will appear.

When choosing a particular type of carbon black and a particular type of polymeric binder the ratio of the amounts of both has to be optimized for each case.

The preferred binder is gelatin. The preferred coverage of the r\ image forming layer ranges between 0.5 and 5 g/m"⁶.

The release layer contains a binder and one or more of the typical ingredients for release layers known in the art such as waxes, polyethylene, silicones, fluorated polymers such as Teflon, silica particles (e.g. SEAHOSTAR KE types, Nippon Shokukai Co), colloidal silica, polymeric beads (e.g. polystyrene, polymethylmethacrylate) , hollow polymeric core/sheat beads (e.g. ROPAQUE particles, Rohm and Haas Co) , beads of siliconised pigments like siliconised silica (e.g. TOSPEARL types, Toshiba Silicones Co), and matting agents. In a particularly preferred embodiment of the present invention the release layer contains a mixture of polyethylene and Teflon. The preferred coverage of the release layer ranges between 0.1 and 3 g/m .

As transparent support polyethylene terephtalate is preferred. However other transparent polymeric resins, e.g. polycarbonate, polyvinylchloride, polyethylene, polypropylene or polystyrene can be used. The support can consist of just one transparent resin. Alternatively the support can have a double layer stuσture comprising a transparent resin as defined above and an extra polymeric layer, a so-called "overcoat" comprising e.g. polystyrene, a copolyester, polycarbonate, a (meth) acrylic resin, a phenolic resin, a polyurethan, an epoxy resin, a cellulose derivative, or mixtures or copolymers of these monomers. Preferred polymers for use in the overcoat are polystyrene and copoly(styrene-acrylonitrile) .

To form the heat mode image the thermal imaging medium described is exposed information-wise by an intense laser beam. Especially preferred lasers are semiconductor diode lasers and YAG-lasers e.g. Nd-YAG lasers emitting at 1064 nm. The laser may have a power output between 40 and 7500 mW and preferably operates in the infra-red part of the spectrum. A series of lasers can be used arranged in a particular array.

The heat mode material can be exposed from the side of the TAL but, more preferably, the material is exposed from the backside of the support. By doing so intense heat is produced at the interface of the image forming layer and of the support. As a consequence a surface part of the support or of the overcoat liquefies and penetrates by cappilary forces into the image forming layer thus locking this layer to the support at the image parts.

After exposure the plain paper foil is laminated to the TAL. Preferably a roller laminator is used whereby the lamination parameters (roller temperature, roller impression and put-through speed) can be established dependent on the properties of the TAL and the paper type so that a good adhesion between paper and TAL is obtained.

Finally the paper and the support of the thermal imaging medium are peeled-apar . The TAL remains adhered to the paper. Only in the image-wise non-exposed areas the release layer and image forming layer wil also be removed together with the paper, while in the image-wise exposed parts the image forming layer will strongly adhere to the support and a fracture will arise between this layer and the release layer. The following examples illustrate the present invention without however limiting it thereto.

EXAMPLES

Example 1

A polyethylene foil having a thickness of 100 μ is coated by the slide hopper technique with an aqueous dispersion containing 4 % of carbon black (CORAX L6, Degussa Co), 0.8 % of gelatin, and 0.6 % of a conventional wetting agent at a total dry coverage of 2 g/m². Then the dried layer is coated with a mixture of an aqueous 1.25 % polyethylene dispersion (HORDAMMER PE02, marketed by Hoechst AG) and of an aqueous 1.25 % Teflon dispersion (HOSTAFLON TF5032, marketed by Hoechst AG) at a total dry coverage of 0.5 g/m². After drying of this release layer a TAL layer coating solution is applied containing a 50 % latex of copoly(styrene-butadiene-acrylic acid) (BAYSTAL S3OR, purchased from Bayer AG) . The dry coverage of this TAL was 25 g/m².

The thus prepared heat mode element was exposed information- wise, using a test pattern, through the polyester support by means of Nd-YAG solid state laser having an output power of 1.6 Watt and an emission wavelenght of 1064 nm.

In a test series a number of different paper sorts, chosen from table A of the description, were laminated to the TAL layer of samples of the prepared heat mode material. A roller laminator (type LPP650 of the Dutch Domed Co) was used. The roller temperature varied between 90 and 120 °C. The lamination speed varied between 0.25 and 0.6 m/minute. The pressure between the rollers corresponded to a impression of 1.5 mm.

When peeling-off the paper foil the TAL is removed at the non- exposed areas of the test pattern together with the release layer and the carbon layer provided the adherance of the TAL to the paper is sufficiently good. In the exposed parts the carbon containing layer and the release layer adhere to the polyethylene terephtalate support. In table 1 the results of the test series are summarized in terms of relative adherance quality. The paper sorts are arranged in order of decreasing smoothness. 11

TABLE 1

Paper No Smoothness Laminator Adherance*

Bekk.s °C m/min

22 1588 90 0.6 + 15 1029 " 0.6 + 2 490 " 0.6 + 6 203 " 0.6 + " " " 0.25 +

23 121 " 0.6 + 19 92 " 0.6 0 13 64 " 0.6 0

II II 0.25 +

21 44 " 0.6

0.25 0 7 33 " 0.6 -

" 0.25 0 24 30 " 0.6

" 0.25 0

3 17 0.6

0.25 0

4 13 " 0.6 -

0.25 0 11 4 " 0.6

0.25 - 105 0.6 120 0.6 0

note : * : + means good adherance ; 0 means weak adherance ; - means insufficient adherance.

The results of table 1 illustrate that a good adherance is obtained with the smooth paper sorts (> 100 Bekk.s). For paper sorts with lower Bekk value the adherance can be improved by lowering the lamination speed (to about 0.25 m/min) and/or increasing the roller temperature (at about 120 °C) . Example 2

A similar heat mode material was prepared as in example 1 with the exception that in the TAL polymer BAYSTAL S30R was replaced by BAYSTAL KA8558. A similar lamination/delamination test series as in example 1 with different papers was performed. The results are summarized in table 2.

TABLE 2

Paper No Smoothness Laminati or A Aιdherance

Bekk . s °C m/min

2 490 90 0 . 6 +

19 92 +

13 64 +

24 30 π n +

3 17 π +

4 13 π π +

11 4 n 0

As can be seen from this table a good adherance is obtained with this TAL containing a polymer with lower melt viscosity (see table B of the description) than the one from example 1 provided the lamination conditions are adequate.

Example 3

A heat mode material similar to example 2 was prepared with the exception that the 0.8 % of gelatin was replaced by 3.4 % of copoly(ethylacrylate-methylmethacrylate-methacrylic acid ; 37 % / 47 % / 16 %) (pH = 9) .

The same tests were performed with the same paper sorts as in example 2. Similar results were obtained.

Example 4

A similar heat mode material was prepared as in example 1 with the exception that in the TAL polymer BAYSTAL S3OR was replaced by BAYSTAL P1800. The melt viscosity was lower and the T_σ was significantly lower (see table B in the description) so that the adherance was easily controlable. The same adherance results were obtained with the different papers as in example 2.

Example 5

Four similar heat mode samples as in example 1 were prepared with the exception that the dry thickness from the TAL was lowered to 20 g/m², 15 g/m², 10 g/m² and 5 g/m² respectively. Paper No. 2 was used for the lamination/delamination tests. The lamination conditions were : roller temperature 90 °C, speed 0.6 m/min, pressure between the rollers 1.5 mm of impression.

The adherance results were good for 20 g/m², 15 g/m , less good for 10 g/m², and insufficient for 5 g/m .

Example 6.

A series of heat mode materials was prepared similar to example 1 with the exception that BAYSTAL S30R was replaced in the TAL by the polymers No. 2,4,5 and 10 of table B. The paper sort used was No. 2 from table A. The laminator roller temperature was 85 °C, the roller speed was 0.6 m/min and the pressure between the rollers corresponded to an impression of 1.5 mm. With all the TAL's of the test series good adhesion and a good image was obtained.

Control example 7

A series of heat mode materials was prepared similar to example 1 with the exception that BAYSTAL S30R was replaced in the TAL by the polymers No. 7,8 and 9 from table B giving rise to non-invention TAL's. The paper sort and the lamination conditions were the same as in example 6. With No. 7 a strong unwanted adherance from surface parts to support parts of the material occurred but the final image quality was still good. On the contrary, with the polymers 8 and 9 a strong adherance from surface parts to support parts of the material occurred and no good image quality could be obtained anymore.

Claims

1. Thermal imaging medium comprising :

(1) a transparent support having a surface part liquefiable by intense heat,

(3) a release layer,

(4) an upper thermoadhesive layer having a glass transition temperature T_ between 20 °C and 60 °C,

2. Thermal imaging medium according to claim 1 wherein said thermoadhesive layer has a melt viscosity of at least 3000 Poise and an elasticity corresponding to a tg δ value of at least 1.65, both physical properties measured at 120 °C.

3. Thermal imaging medium according to claim 1 or 2 wherein said thermoadhesive layer has a melt viscosity of at least 10 000 Poise and an elasticity corresponding to a tg δ value of at least 3, both physical properties measured at 120 °C.

4. Thermal imaging medium according to any of claims 1 to 3 wherein said thermoadhesive layer contains a copolymer comprising from 40 to 80 weight % of styrene, from 60 to 20 weight % of butadiene, and optionally from 0 to 10 weight % of acrylamide and/or acrylic acid.

5. Thermal imaging medium according to any of claims 1 to 4 wherein said thermoadhesive layer has a coverage of at least 10 g/m².

6. Thermal imaging medium according to any of claims 1 to 5 wherein said transparent support is composed of polyethylene terephtalate.

7. Thermal imaging medium according to any of claims 1 to 6 wherein said transparent support has a double stucture comprising a transparent resin and an overcoat, positioned between said transparent resin and the image forming layer.

8. Thermal imaging medium according to claim 7 wherein said overcoat comprises a polymer chosen from the group consisting of polystyrene and copoly(styrene-acrylonitrile) .

9. Thermal imaging medium according to any of claims 1 to 8 wherein said release layer contains a mixture of polyethylene and poly(tetrafluoroethylene) .

10. Thermal imaging medium according to any of claims 1 to 9 wherein said image forming substance is a pigment.

11. Thermal imaging medium according to claim 10 wherein said pigment is carbon black.

12. Thermal imaging medium according to claim 11 wherein said carbon black has an average particle size distribution ranging from 0.01 μm to 1 μm.

13. Process for the formation of a heat mode image, comprising the following steps :

(1) a transparent support having a surface part liquefiable by intense heat,

(3) a release layer,

(b) laminating by heating to said upper thermoadhesive layer of said thermal imaging medium a plain paper foil having no extra coating(s) on the lamination side,

(c) peeling-apart said support and said plain paper foil whereby the image forming layer and the release layer adhere to the support in the image— ise exposed parts, and whereby the image forming layer, the release layer and the thermoadhesive layer adhere to the plain paper foil in the image-wise non-exposed parts.

14. Process according to claim 13 whereby said plain paper foil has a smoothness corresponding to a Bekk vlue of at least 5.

15. Process according to claim 13 whereby said plain paper foil has a smoothness corresponding to a Bekk value of at least 100.

16. Process according to any of claims 13 to 15 wherein said intense laser radiation is produced by an infra-red laser or an array of infra-red lasers.

17. Process according to claim 16 wherein said infra-red laser is Neodymium-YAG laser or a diode laser.