Background of the Invention Definition of terms Photografting : the processes of directly or indirectly attaching dye molecules to a polymer surface. The dye molecules are typically bonded to a reactive unit, such as a monomer, which then attaches to the dedicated polymer surface.

Other possibilities include bonding of ionic (electrostatic) dye to modified polymer surface. Liquid-solid or gas-solid photografting systems are known so far.

Photopolymerization induced imasins : the imagewise formation by polymerization of monomer moieties on the surface or within a polymer matrix.

The imagewise formed structural changes may then be colored in consequent wet processing steps by attaching dye molecules in the matrix or at the matrix surface, as is used in polymer printing plates. By"imagewise"is meant herein that the said structural changes are so distributed on the surface or within a polymer matrix as to define an image.

Methods for carrying out photografting and photopolymerization on the surface of a polymer matrix are well known in the art.. Also polymerization grafting processes including photopolymerization and photo-grafting processes are limited to liquid-solid or gas-solid phase interactions, and are therefore not applicable to dry printing processes. Furthermore, all the processes known in the art produce poor color separation.

I. R. Bellobono et al in"Photochemical Grafting of Acrylated Azo Dyes onto Polymeric Surfaces, IV", Die Angewande Makromolekulare Chemie 100 (1981), p. 135-146, discuss the photochemically induced grafting and graft polymerization of 4- (N-ethyl-N-2-acryloxyethyl) amino-4'-nitroazobenzene 1, brought into contact in solid state with cellulose.

J. R. Bellobono et al., in"Photochemical Grafting of Acrylated Azo Dyes onto Polymeric Surfaces, I-Grafting of 4- (N-Ethyl, N-2-Acryloxyethyl) Amino, 4'-Nitro-Azobenzene onto Polyamide and Polypropylene Fibers", Journal of Applied Polymer Science, Vol. 26,619-628 (1981) discuss the photochemically induced grafting and graft polymerization of 4- (N-Ethyl, N-2-Acryloxyethyl) Amino, 4'-Nitro-Azobenzene brought into contact with polyamide and polypropylene fibers.

Copending application of this applicant No. 134655, the contents of which are incorporated herein by reference, discloses a-dry photoprinting process which comprises the steps of : a-providing a substrate comprising a plurality of superimposed layers of photochromic materials sensitive to different light wavelengths; b-scanning the object to be photographed; c-generating from the scanning a digital file defining the image of said object ; d-providing a plurality of sources of laser light having different wavelengths, each of which is the wavelength to which one of said photochromic materials is sensitive; and e-irradiating said layers of photochromic materials by means of the laser beams produced by said sources, according to a program determined by said digital file, whereby to develop in each pixel of the substrate the color that it has in said image; f-setting the colors thus generated in the pixels of the substrate layers; and g-setting the background areas of the substrate layers.

The said process represents an important technological advance, however suffers from two disadvantages: firstly, it requires the use of photochromic substances that are not easily available, and secondly, said substances tend to revert, after a certain length of time, to their original essentially colorless condition, so that the print is not as stable as is desirable.

The present invention has the purpose of overcoming the above limitations.

Specifically, the present invention has the purpose of overcoming the drawback of the poor color separation of the previous art.

Further, it has the purpose of providing stable color prints.

Summary of the invention The process of the invention comprises firstly providing a multilayer assembly, comprising a) at least one, but preferably three photosensitive compounds (hereinafter also called"photosensitizer"), each excitable by irradiation of a given wavelength range; b) at least one, but preferably three monomer-dye components, each capable of being activated by one of said photosensitive compounds (hereinafter, "the monomer-dye component corresponding to the photosensitizer") once this latter had been excited; and c) a binder matrix adapted to bind by photografting said photosensitive compounds, once these latter have been activated. The binder matrix preferably consists of polymeric material, but may consist of other materials such as paper of organic sol-gel materials (often termed CRMCSILS). Hereinafter the binder matrix will also be called, for descriptive purposes,"polymer binder", but this should not be construed as a limitation, since all suitable binders are intended to be included in the invention.

Three photosensitive compounds and three monomer-dye components are preferred, as they permit to obtain a complete color image; however images formed by two colors and even monochromatic images may be satisfactory in some cases, as is well known to persons skilled in the art, and therefore they are comprised in this invention. Therefore, while for descriptive purposes reference will be made hereinafter to three photosensitive compounds and three monomer-dye components, this should not be construed as a limitation.

For each color image to be printed, a digital file is provided which defines said color image in a way well known to skilled persons.

For each of said photosensitive compounds, a light source is provided having the wavelengths range suitable for exciting said compound. The multilayer assembly is irradiated by said light sources under the control of said digital file.

As a result, each of said compounds activates the corresponding monomer-dye component. Finally, said multilayer assembly is preferably irradiated uniformly, not"imagewise", whereby to cause said compounds to become grafted onto a binder.

Polymeric binders are preferably PVA, PVC, Polyester, Polycarbonate, Polystyrene, Polyethylene, etc.

The photosensitive dyes or photosensitizers are preferably selectively sensitive to several, but preferably three, different wavelength regions, typically those of the complementary colors : the blue (BS), green (GS) and red (RS) spectral regions. The photosensitizers, that may be used in carrying out the invention, include for example the xanthine derivatives, the carbocyanine and phtalocyanine dye derivatives, merocyanines, quinone derivatives, polyhalogen compounds, and many others. These photosensitizers are preferably used together with suitable electron transfer agents, such as aliphatic or aromatic amines or other electron donor or acceptor species.

The monomer-dye components preferably comprise acrylic type or other type monomers which are bonded to thermally and photochemically stable dye -molecules. One example of an acrylic type dye-monomers is the class of acrylated azo dyes. Other examples of monomers are N-Acrylomorpholine, N-Acrylotris (hydroxymethyl) methylamine, Allylbenzene, Allyl dogylcol carbonate, Allyl glycidyl ether, 2-Phenoxyethyl acrylate, 1,5-Pentandiol dimethylacrylate and Pentachlorophenyl acrylate. A non-limiting example of chemical structure of acrylated azo dyes is the following: The structure of (1) shows a diazo dye derivative attached to the acrylic monomer (lower right hand side).

The colors of the dye components will be called herein"elementary colors". The elementary colors may be the base colors yellow (Y), magenta (M) and cyan (C) for subtractive color formation (as in the embodiments illustrated hereinafter) or the complementary colors blue (B), green (G) and red (R) for additive color formation. Therefore the various possible dye-monomers will be indicated as m-Y, m-M, m-C for subtractive color processes, and as m-B, m-G, m-R for additive color formation processes.

The photografting polymer matrix or polymer layer will,, upon exposure to light, react and bond said monomer-dye molecule in its activated state (free radical or ionic species), once said dye-monomer molecule has been activated by said photoexcited photosensitizer, possibly through an electron transfer agent. The present invention comprises a photo-reactive polymer, which, upon photonic irradiation will form an ionized functional side group (process of photoionization). The monomer-dye component, which has acquired an electrostatic charge when activated, will bond itself to the ionic side group of the polymer of the opposite electrostatic charge.

A representative but not limiting photoionization reaction mechanism is shown in the following scheme, showing an anionic sulfide to cationic dye bonding.

An alternative approach would be to use the opposite charge on the polymer side group, viz. to create a cationic side group, such as amino cation, which will interact with an anionic dye.

Brief Description of the Drawings In the drawings: -Fig. 1 illustrates in schematic perspective a print substrate according to one embodiment of the invention ; -Fig. 2 illustrates in the same way a print substrate according to another embodiment of the invention; -Fig. 3 illustrates the embodiment of Fig. 2 in a later stage of the process, after the photosensitizing irradiations have been carried out; and -Fig. 4 illustrates an optional delamination of the substrate of Fig. 3, separating the photosensitizing layers from the receiving layer.

Detailed Description of Preferred Embodiments In the embodiment of Fig. 1, the print substrate is a multilayer assembly made of the following layers from bottom to top. Layer 10 is a base substrate, which has only a mechanical function and can be made of any suitable material.

Layer 11 is the photosensitizing layer and includes three monomer-dye components, yellow, magenta and cyan, and the corresponding photosensitive compounds that are sensitive to the complementary spectral regions blue, green and red, schematically indicated by BS, GS and RS. Above layer 11 is a receiving layer 12, which includes the preferably polymeric binder. Finally, above said layer 12, the substrate comprises a receiving substrate 13, which does not prevent the radiation beams, which impinge on it, from crossing to the underlying substrates.

Fig. 2 differs from Fig. 1 only because in place of a single photosensitizing layer 11 including all the photosensitizers and monomer-dye components, has three layers 14,15 and 16, which include, from bottom to top, the cyan, the magenta and the yellow monomer-dye components and the corresponding photosensitizers.

In Fig. 3, the print substrate of Fig. 2 is shown after being irradiated. The monomer-dye components have migrated into the receiving layer and have become, or are becoming, grafted to the polymer which constitutes it.

While in the embodiment of Fig. 3 each of layers 14,15 and 16 will contain what may be called a photosensitive component system, viz. a photosensitizer with a corresponding monomer-dye component having one of the three elementary colors used, and the monomer may be the same in all of them, in the embodiment of Fig. 1 the single layer 11 includes three photosensitive component systems, viz. three photosensitizers each with a corresponding monomer-dye component, and the three monomers of said component must be different from one another so that each will be activated by a different photosensitizer.

As indicated in the drawings, the aforesaid, active layers of the multilayer assembly may be separated by intermediate layers, consisting of passive polymer coatings or the like. This however is merely optional. After the process of the invention is carried out, the receiving layer, which includes the desired color image, may optionally be separated from the other layers, to obtain the image.

Several types of layer assemblies are comprised as embodiments in this invention: a) a layer assembly comprising one photosensitive layer (as shown in Fig. 1), which contains three sets of photosensitizers, each sensitive to a different spectral region (e. g. B, G, R), in conjunction with three different monomer-dye molecules (e. g. m-Y, m-M, m-C), each of which reacts specifically with only one of the photosensitizers. The combination of a photosensitizer and a corresponding monomer-dye may be called a photosensitizing system.

This type of layer assembly (a), containing only one photosensitive layer, may be fixed by either one dry self-fixing process, using an adjacent, separate receiving layer, or by a wet solvent-assisted fixing step. In the latter case, in which a quasi-dry color printing system will result, small, pressure-sensitive capsules containing a fixing agent are embodied into a polymer film. When this film is brought into contact with the imaging layer (s) with a simultaneous application of pressure, said capsules are ruptured, thereby releasing the fixing agent which will remove the dye molecules that have not been grafted b) a layer assembly comprising several but preferably three photosensitive layers (as shown in Fig. 2), each containing one set of photosensitizing systems, viz. a photosensitizer, each photosensitizer being sensitive to a different spectral region (e. g. B, G, R), in conjunction with three different monomer-dye molecules (e. g. m-Y, m-M, m-C); in this case the monomer-dye molecules, acrylic-azo, diazo dye, may differ only in the dye, but not necessarily in the monomer structure.

This type of layer assembly (b), containing several photosensitive layers may be fixed by either one dry fixing process, using a separate, adjacent photografting receiving layer, or by a wet solvent assisted fixing step such as described above.

In the latter case, a quasi-dry color printing system will result.

Preferably this invention provides an in-situ self-fixing process, incorporated into the layer assemblies; however a pressure sensitive capsule assisted fixing process may also be carried out according to the invention. Two types of imaging processes must be distinguished in connection with the self-fixing processes: 1) the photosensitive layers form the color image by an in-situ and self-developing photopolymerization process, whereby the unexposed and unreacted monomer-dye molecules are fixed by diffusion into the grafting receiving layer (as schematically indicated in Fig. 3), or 2) the grafting receiving layer form the color image by an in-situ and self-developing photografting process, by receiving the exposed and photoactivated monomer-dye molecules by diffusion from the photosensitive layers (as shown in Fig. 3), or 3) the process is as described by types 1) and 2), yet with a third dry but photo-assisted fixing process, whereby a non-imagewise post-exposure is applied to either the said grafting receiving layer or the said photosensitive layers.

In process 1, homopolymerization of the monomer of the monomer-dye component occurs in-situ in each photosensitive layer, thereby forming the image in said layer. The grafting on the polymer matrix, which occurs later, is only for fixing purposes, whereby the unpolymerized dye moieties will diffuse out and into the grafting receiving layer.

Process 3 comprises a post-exposure (non-imagewise) illumination, in order to complete the fixing-grafting process by final grafting in the receiving layer of unreacted material or for the completion of image grafted material.

Furthermore, the in-situ self-fixing process may be enhanced by a mechanically assisted layer detachment/separation process, as schematically shown in Fig. 4, which assures a better preservation of the image. This separation may be facilitated by the fact that the polymer matrices of the receiving layer, on the one hand, and those of the photosensitive layer, on the other, may be different, the first one being hydrophilic and the second ones hydrophobic. In addition, the multilayer assembly may incorporate other important additives to improve the dry color printing process, such as chemical sensitizers or desensitizers for photosensitivity enhancement or reaction kinetics control, plasticizers for inter- or intra-layer diffusion promotion, crosslinking agents for image stabilization or other additives.

While embodiments of the invention have been described by way of illustration, it will be understood that the invention can be carried out with many modifications, variations and adaptations, without departing from its spirit or exceeding the scope of the claims.