DE69703062T2 - Process for producing a color image with reduced dot gain and color image obtained by this method - Google Patents

Description

The present invention relates to a method for producing a color image for color proofing of originals for multi-color printing, a layer material for carrying out the method and a color image obtained by this method.

As a rule, printers use such a color image as an aid to checking and, if necessary, correcting the color separations used for exposing the printing plates in multi-color printing. This color proof image must be a consistent duplicate of the desired halftone or line image. The proof image must embody a faithful reproduction of the tonal values of the colors that fully corresponds to the image to be printed later.

When producing such a color proof image, primary color images that correspond to the primary colors of the multi-color copy are usually completed one after the other on an image receiving material, or primary color images are transferred to it.

Each primary color image is obtained by exposing and developing a light-sensitive layer, with development either being a wash-off process or being accomplished by peeling off a carrier film.

There are numerous materials and methods for producing such color proof images. For descriptions, see, for example, the following documents: US 3,649,268, US 3,642,474, US 4,260,673, US 4,650,738, US 4,656,114, US 4,659,642, US 4,895,787, US 5,049,476 and EP-A 447829.

In order for the color tone values in a color proofing process to visually correspond to those of the image to be printed, the color proofing process must be able to simulate the dot gain that occurs during the actual printing. Dot gain is the enlargement of the halftone dot during printing compared to the foil template. Depending on the case, a distinction is made between physical (or mechanical) dot gain and optical dot gain. Physical dot gain is an actually measurable value that depends on various parameters such as the type of printing paper, the type of printing ink used, the type of The enlargement of the screen dot during the printing process depends on the type of printing press and the type of printing plate, etc. Optical dot gain, on the other hand, is based on light scattering and light absorption properties (shadow effect) without changing the actual dot size. Optical dot gain depends on the nature and opacity of the paper and, in the case of the color proof, also on any adhesive and separating layers that may be present. In offset printing, dot gain depends on the parameters mentioned above to a magnitude of around 8-30% in the midtones, i.e. with a 40-50% area coverage of the screen tones. Low dot gain values are preferred for high-quality prints and the associated higher image sharpness.

The materials used to produce the color proof described in the references mentioned at the beginning consist of a large number of layers on temporary carrier films. The use of colorless separating layers and/or adhesive layers is the reason why an optical dot gain of generally about 20-25% can be observed in the finished color proof. Within certain limits, the dot gain in the color proof can be controlled by the thickness of the adhesive layers and separating layers, as can be seen, for example, from US 4,262,071 and EP-A 339,860. By using thin adhesive layers, a reduction in the dot gain can be achieved, but this possibility is limited by difficulties in processing, since the lamination temperature must be increased or the lamination speed reduced. Another possibility for reducing the dot gain is to incorporate white pigments such as titanium dioxide into the adhesive layer of the color films or the image receiving material. If titanium dioxide is added to the adhesive layer of the color foil, only the color foil of the first primary color image can be provided with such an adhesive layer containing TiO₂, because the white pigment would cause a loss of color quality in the subsequent layers. This method therefore has the disadvantage that the order of the colors in the image construction has to be determined in advance and only one A slight reduction in dot gain can be achieved. Such a material is described in EP-A 420 675.

Preference is given to processes in which the dot gain can be controlled without affecting the color film material, for example by adding TiO2 to the image receiving material, as described, for example, in EP-A 305599 and EP-A 625224. Instead of titanium dioxide, pearlescent pigments containing barium sulfate, calcium carbonate or titanium species can also be used, as described in EP-A 186902. The white pigment can also be incorporated into the image receiving material as an additional primer layer, as described in EP-A 639736. In order to ensure good adhesion to the receiving sheet, the primer layer in this case must be composed of two layers, i.e. an adhesive layer and a layer containing the white pigment.

As another technique for reducing dot gain by using a specific receiving material, the literature mentions the use of air-filled films, optionally in combination with titanium dioxide (EP-A 186902). The laborious production of such films is described by Mathews et al. in US 3,944,699. A mixture of polyester and polyethylene or polypropylene is extruded and then biaxially stretched and tempered. The polymers must be mixed in their granular state before extrusion. Coextrusion of the melts does not produce air-filled films. Mathews et al. describe the use of the films produced in this way as substrates for photographic purposes, but do not mention anything about an influence on the optical dot gain when the films are used as a substrate for halftone images. Apart from their laborious production, these films have another disadvantage in that they have nothing in common with printing paper.

The present invention describes a process which uses a film or paper support as the image-receiving material for a color proof image, which support consists of a light-sensitive layer in which gas bubbles are formed upon irradiation. The present invention describes in particular a process for producing a color image, in which a light-sensitive material comprising a temporary carrier film (i), a colored photosensitive layer and a heat-activatable adhesive layer, laminated to an image-receiving material at elevated temperature and under pressure, the photosensitive layer is image-wise exposed and developed to form the image and, if appropriate, these steps are repeated with at least one further photosensitive film in a different colour, characterized in that an image-receiving material is used which has at least one vesicular layer on a carrier material (ii), which is exposed to actinic radiation before the first lamination stage, whereby a gas is photolytically formed which remains permanently contained in the photosensitive vesicular layer and gives the layer an opaque appearance when heated.

The heating, which causes the formation of gas bubbles, can preferably also be carried out simultaneously with the lamination of the first primary color image. The order of the primary color images can be freely selected. For a description of materials according to the invention, known as vesicular film, see, for example, EP 038016. If the vesicular film is cast onto a paper carrier, the resulting color proof image will be particularly similar to the print output to be simulated.

The preferred embodiment of the process according to the invention comprises, as a first step, the production of a layer material consisting of a temporary support (1) and a vesicular layer (2). The temporary support (1) preferably consists of polymer or polymer-coated special paper. The composition of the vesicular layer (2) is described, for example, in EP-B 038016, FR-A 2 304 944, US-A 3 622 336 and DE-A 24 38 157. The vesicular layer (2) preferably contains a binder with a maximum glass transition temperature of 130°C, so that development takes place at a maximum temperature of 150°C. Preferred polymers of this type include, for example, copolymers of methacrylonitrile and vinylidene chloride. The light-sensitive compounds used for the vesicular layer (2) are preferably diazonium compounds which release nitrogen when irradiated. In In this context, diazonium salts are particularly preferred, as they form a vesicular layer when exposed to light and developed. However, any other compounds that form a gas when photolytically produced can also be used.

The vesicular layer (2) on the temporary support (1) is evenly exposed to ultraviolet light until the photosensitive compound has completely disintegrated. When using diazonium compounds, the disintegration is visually evident as the disappearance of the yellow discoloration. When using gallium-doped ultraviolet light sources with an output of 5 kW in combination with diazonium salts, for example, a few seconds (20 s) are sufficient to cause complete disintegration. Longer exposure times are no longer of critical importance.

A color proofing film (F) is laminated onto the exposed vesicular layer (2) at elevated temperature and under pressure. Preferred color proofing films for the process according to the invention are those consisting of a temporary carrier (3), a colored light-sensitive layer (4) and an adhesive layer (5). Such materials are described, for example, in EP-A 352 055, US-A 4 895 787, US-A 5 049 476 and EP-A 525 624. The color proofing film is laminated onto the vesicular layer (2) with the adhesive layer (5). The lamination temperature is preferably set sufficiently high, whereby the vesicular layer is thermally developed during the lamination process and at the same time the adhesive layer (5) is softened, thus obtaining a layered assembly of color proofing film and vesicular layer. The lamination temperature is usually 120ºC. This technique is preferred because it does not require an additional step to develop the vesicular layer. However, the vesicular layer can also be thermally developed using another technique, for example in an oven, before lamination of the first color proof film.

The light-sensitive layer (4) of the color proofing film is exposed imagewise under a color separation film before or preferably after lamination through the carrier film (3) and developed after lamination onto the vesicular layer (2). This development must not damage the vesicular layer, preference is given to using dry-developable peelable colour proof materials, the development of which is carried out by peeling off the temporary carrier film (3) after exposure. The colour proof materials described in EP-A 352 055, US-A 5 049 476 and EP-A 525 624 are developed in this way.

After developing the first primary color image, the lamination, exposure and development of the second primary color image and the further primary color images can be carried out until a complete color proof image is obtained on the vesicular layer (2). This color proof image has the advantage of a lower dot gain compared to a color proof image that is produced directly on a polymer-coated paper carrier (1).

Examples

The following examples illustrate the present invention. The calculation of the dot gain (S₂-S₁) is carried out using test foils with calibrated area coverage (S₁) as an exposure mask according to the Murray-Davies equation:

The density values Ds and Dv are measured with a densitometer.

Example 1: Preparation of a vesicular film

A solution with the following composition is cast onto a biaxially stretched polyethylene terephthalate film with a thickness of 150 µm and dried:

Components Parts by weight

Copolymer of methacrylonitrile and vinylidene chloride (comonomer ratio 70:30) 12.8

Copolymer of methacrylonitrile and vinylidene chloride (comonomer ratio 20:80 - Saran® F310, Dow Chemical) 3.2

4-Morpholino-2,5-diisopropoxybenzenediazonium tetrafluoroborate 0.8

Silicone oil (as flow regulator) 0.05

2-Butanone 80.0

The layer is first dried for 60 s at 70ºC and then for 30 s at 130ºC. The weight of the dried layer is 8 g/m². The resulting film is yellow in color.

Example 2: Production of color test films for producing a color test image according to the invention

Four light-sensitive color proof films are produced with the standard printing inks for multi-color printing (cyan, magenta, yellow and black). For each film, casting solutions with the following compositions are first cast onto a 50 µm thick, biaxially stretched, heat-set polyethylene terephthalate film with improved adhesion and dried. Drying takes 2 minutes at 70ºC. The weight of the layer is between 0.4 and 0.8 g/m² and is selected so that the density values of a standard multi-color copy are achieved in the color proof image that is subsequently produced.

Pigments 1 to 4 are ball-milled with a portion of the binder (5) and the solvent (8) before being added to the respective casting solution.

An adhesive layer solution with the following composition is poured onto the dried light-sensitive layers and dried for 2 minutes at 100ºC. The dry adhesive layers all have a weight of 7 g/m².

Components Parts by weight

Copolymer of vinyl acetate and crotonic acid (comonomer ratio 95:5 - Mowilith® CT5, Hoechst AG) 50.0

Polyvinyl methyl ether (Lutanol® M40, BASF) 1.0

Water 253.3

Ethanol 24.0

Ammonia solution (25% solution) 5.0

Example 3: Production of a color proof image on a vesicular film

The vesicular film from example 1 is exposed for 20 seconds in a contact copying frame (5 kW, gallium-doped burner). After exposure, the film no longer shows the typical pronounced yellowing, but is still transparent. The cyan film from example 2 is laminated with its adhesive layer side to the layer side of the exposed vesicular film at 120ºC and under pressure. This gives the vesicular film an opaque white appearance. The cyan film of the laminate is exposed image-wise through its 50 μm thick carrier film under a color separation film (60 lines/cm grid). The exposure time is set so that the tonal value range is between 3% and 98%. After exposure, this carrier film is peeled off the color proof film. The non-exposed areas remain with the complete adhesive layer on the vesicular layer, whereas the exposed areas of the color layer are peeled off together with the carrier film. The stages of

- Lamination of the colour proof film,

- Exposure under the respective colour separation foil and

- Peel off the carrier film from the color test film is repeated for the colors magenta, yellow and black. Finally, a matt or light protective layer can be applied as a finishing layer.

The color densities are measured using a commercially available incident light densitometer (D186 from Gretag). A number of layers of white paper are used as the measurement background. The dot gain can be calculated from the color density values using the Murray-Davies equation (see page 6). The tonal value range is checked optically using a monocle.

The results are presented in Table 1. Table 1

Example 4 (comparative example): Color proof image on a support material without vesicular layer

The color test films from example 2 are laminated, exposed and developed one after the other, in accordance with the order given in example 3, onto a polymer-coated special paper (Ozasol® TF01) instead of onto a vesicular film. The exposure is such that the tonal value range of each color is the same as the range of example 3 and example 4. This is the only way to compare the dot gain values. In this case too, the dot gain values of the primary colors are determined in the finished color test image. The results are entered in table 2. Table 2

A comparison with the values in Table 1 shows a reduction in dot gain in Example 3 of 3-5% with an area coverage of 40% and of 1-2% with an area coverage of 80% in the slide template.

Example 5: Preparation of a paper carrier with vesicular layer

A solution with the following composition is applied to the polymer-coated special paper of example 4 (Ozasol® TF01) and dried. Drying takes 1 minute at 70ºC. The layer weight is approximately 10 g/m².

Components Parts by weight

Copolymer of methacrylonitrile and vinylidene chloride (comonomer ratio 70:30) 18.4

Copolymer of methacrylonitrile and vinylidene chloride (comonomer ratio 20:80 - Saran® F310, Dow Chemical) 0.8

3-Chloro-4-N,N'-dimethylaminobenzenediazonium hexafluorophosphate 0.76

Silicone oil (as flow regulator) 0.03

Citric acid 0.13

2-Butanone 20.0

The resulting support is yellow on the vesicular layer side. After exposure, which is carried out in the manner described in Example 3, the yellow color disappears. This support can either be used directly to produce a color proof or first coated with an additional adhesive layer to ensure better adhesion of the image. To do this, a layer with the following composition is applied to another temporary support film made of biaxially stretched and heat-set polyethylene terephthalate, the adhesion of which is not improved, and dried. The weight of the dried layer is 20 g/m².

Components Parts by weight

Polyvinyl acetate (Mowilith® 30, Hoechst) 25

2-Butanone 75

This adhesive layer film is laminated with the layer side facing downwards onto the special paper provided with the exposed vesicular layer. The carrier film of the adhesive layer film is peeled off. The additional adhesive layer remains on the vesicular layer, the thermal development of which takes place simultaneously with the lamination of the additional adhesive layer film.

Example 6: Production of a color proof on a paper support with a vesicular layer

A color proof is produced on the special paper from Example 5, which is coated with an exposed vesicular layer and an additional adhesive layer, in the same way as in Examples 3 and 4 by laminating, exposing and peeling off the four color proof films, after which the dot gain in the finished proof is determined. The values obtained are compared with the values of the color proof on the TF01 special paper, which does not contain a vesicular layer but is coated with the additional adhesive layer from Example 5 (for the sake of comparability). The results are entered in Table 3. Table 3

VS = vesicular layer

HS: Adhesive layer

From the comparison of the values in Table 3, it is clear that the presence of the vesicular layer causes a reduction in dot gain of 1-7% with an area coverage of 40% and of 0-1% with an area coverage of 80% in the film template. The effect is clearly less pronounced in the fourth primary color image (black) than in the first two primary color images (cyan and magenta). With the mere presence of the additional adhesive layer without a vesicular layer, an increase in dot gain can clearly be observed compared to Example 4. The increase in dot gain through additional adhesive layers is a known phenomenon (see page 1 and following). The use of a vesicular layer therefore makes it possible to take advantage of the benefits of an additional adhesive layer (for example, a reduction in the risk of dust being trapped in the material during lamination) without the disadvantage of an increase in dot gain. By using a polymer-coated paper support, the color proof obtained in this example is much more similar to the final proof than a color proof obtained on a vesicular film in Example 3.

Claims (13)

1. Process for producing a color image, in which a light-sensitive material which has a temporary carrier film (i), a colored light-sensitive layer and a heat-activatable adhesive layer is laminated to an image-receiving material at elevated temperature and under pressure, the light-sensitive layer is exposed imagewise and developed to form the image and, if necessary, these steps are repeated with at least one further light-sensitive film in a different color, characterized in that an image-receiving material is used which has at least one vesicular layer on a carrier material (ii), which is exposed to actinic radiation before the first lamination stage, whereby a gas is photolytically formed which remains permanently contained in the light-sensitive vesicular layer and gives the layer an opaque appearance when heated. 2. Process according to claim 1, characterized in that the light-sensitive layer of the image-receiving material contains a diazonium salt. 3. Process according to claim 2, characterized in that the diazonium salt is a halogen-containing diazonium hexafluorophosphate. 4. Process according to claim 3, characterized in that the halogen-containing diazonium hexafluorophosphate is 3-chloro-4-N,N'- dimethylaminobenzenediazonium hexafluorophosphate. 5. Process according to any one of claims 1 to 4, characterized in that the image receiving material is exposed before lamination of the light-sensitive color film. 6. Process according to any one of claims 1 to 5, characterized in that the exposed image-receiving material is thermally developed. 7. Process according to claim 6, characterized in that the thermal development of the image receiving material takes place simultaneously with the lamination of the first light-sensitive color film. 8. Process according to any one of claims 1 to 7, characterized in that the carrier material (ü) of the image-receiving material consists at least partly of paper. 9. Process according to claim 8, characterized in that the carrier material (ii) of the image receiving material consists of a polymer-coated paper carrier. 10. Process according to any one of claims 1 to 9, characterized in that a further heat-activatable adhesive layer is arranged between the vesicular layer of the image-receiving material and the thermally activatable adhesive layer of the light-sensitive color film. 11. Process according to any one of claims 1 to 10, characterized in that the image development of the color image is carried out by pulling apart the carrier film (i) and the image-receiving sheet. 12. Image receiving material as defined in any of claims 1 to 11. 13. A color image obtained by any of the processes defined in claims 1 to 11. Attachment to description Fig. 1 Structure of the image carrier according to the invention before lamination and the 1st colour proof film F 3 Carrier film Color test film 4 light-sensitive color layer 5 Adhesive layer 2 Vesicular layer 1 Carrier vesicular layer Fig. 2 Structure of the color image according to the invention after lamination of the 1st partial color 3 Carrier film Color test film 4 light-sensitive color layer 5 Adhesive layer 2 Vesicular layer, developed 1 Carrier vesicular layer Fig. 3 Structure of the color image according to the invention after development of the 1st partial color image 3 Carrier film color test film 4* exposed color layer 4 unexposed color layer 5 Adhesive layer 2 Vesicular layer, developed 1 Carrier vesicular layer