WO2003046661A1

WO2003046661A1 - Printed uv cured optical films

Info

Publication number: WO2003046661A1
Application number: PCT/GB2002/005322
Authority: WO
Inventors: Rifat Iqbal; Dosten Baluch
Original assignee: Durand Technology Limited
Priority date: 2001-11-27
Filing date: 2002-11-27
Publication date: 2003-06-05
Also published as: CA2412712A1; GB0412568D0; GB2398074A; AU2002343096A1; GB0128381D0

Abstract

A photopolymer blend or system is disclosed capable of deposition onto a substrate (such as fabric, paper, plastics, metal or wood) to form text, drawings or shapes by a printing method such as by stencil printing or inkjet printing and capable of polymerisation to form a solid, light transmitting material having volume refractiveindex variations or surface relief features by the exposure of the deposits of the blend to electromagneticor other radiation. The blend includes a monomer, prepolymer, macromonomer or co-monomer comprising silicone groups, and at least one component of the system incorporates ethylenic unsaturation such that the system is capable of undergoing free radical initiated polymerisation. The blend or mixture may also contain various combination of thixotropic agents, and/or thickeners and/or dyes or pigments.

Description

PRINTED UV CURED OPTICAL FILMS

THIS INVENTION relates to optical recording materials and to methods of forming optical devices such as difiusers and holograms and security encoding from such materials. The present specification discloses, inter alia, various systems or mixtures comprising organic and pre-polymer components which are of such a character so as to enable stencil or inkjet printing such that when the printed deposit is exposed to some form of electromagnetic radiation, at least some components of the system undergo polymerisation in the areas so exposed, whereby an end product is obtainable, (after any necessary processing steps), which comprises a solid transparent, or at least light-transmitting, sheet or layer having refractive index variations and/or having variations in the layer thickness, i.e. having surface relief features. Such systems are useful, for example as holographic recording materials, or in the production of microlens arrays, or light- diffusing or de-pixelating screens or in security applications such as printing on currency notes, certificates, credit cards, bank cards or other secure documents.

A typical, known system for the purpose referred to comprises a mixture including

1 a monomer or monomers capable of undergoing free-radical-initiated polymerisation, a photo-initiator capable of generating free radicals on exposure to radiation of pedetermined wavelengths and a polymeric matrix or binder. Examples of such known systems or mixtures are disclosed in for example U.S. Patents Nos. 5470662, 4963471, 4942112, 3658526. It is among the objects of the present invention to provide an improved photopolymerisable system or mixture suitable for stencil or inkjet printing, for example, useful for the formation of security encoding films.

According to one aspect of the invention there is provided a photopolymerisable system capable of polymerisation to form a solid, light-transmitting material having volume ref active index variations and/or surface relief features dependent on the exposure of the system to polymerising radiation, wherein the system includes a silicone acrylic compound or compounds.

The mixture may also contain various combinations of a thixotropic agent(s), and/or thickener(s) and a dye(s) or pigments(s).

According to another aspect of the invention there is provided a system capable of being modified by electromagnetic radiation to provide a transparent or substantially transparent material having volume refractive index variations, or surface contour features, determined by exposure of the system to such radiation, the system comprising:-

(a) a silicone pre-polymer, or a monomer, co-monomer, macromonomer or prepolymer incorporating a silicone monomer or pre-polymer,

(b) an ethylenically unsaturated monomer capable of undergoing free-radical initiated polymerisation, and

(c) a photo-initiator capable of generating free radicals upon excitation by such radiation. The system may additionally include a compatible polymer, e.g. polymers or copolymers with similar groups to the silicone prepolymer. Compatible polymers which may be used include PVA, polysiloxane polymers or acrylic type polymers such as PMMA. The system may also include thixotropic agents or thickeners to impart to the system rheological properties suitable for stencil printing or inkjet printing, i.e.such that the system or mixture may be used as a printing fluid or "ink" in stencil printing or in an inkjet printer.

It is another object of the invention to provide an improved light-modifying structure using a system or mixture in accordance with the above noted aspect as a starting material.

According to this aspect of the invention, there is provided a diffuser, hologram or other light-modifying structure formed by polymerisation of a system in accordance with the previously noted aspects of the invention.

According to a further aspect of the invention there is provided a photopolymer blend capable of deposition onto a substrate (such as fabric, paper, plastics, metal or wood) to form text, drawings or shapes by the methods of stencil printing or inkjet printing and capable of polymerisation to form a solid, light transmitting material having volume refractive index variations or surface relief features by the exposure of the deposits of the blend to 'electromagnetic or other radiation wherein the blend includes a monomer, prepolymer, macromonomer or co-monomer comprising silicone groups, that is to say groups with a

backbone (Structure I), and wherein at least one component of the system incorporates ethylenic unsaturation such that the system is capable of undergoing free radical initiated polymerisation.

It is also among the objects of the present invention to provide a light-diffusing material by exposure of a photopolymerisable system or mixture to polymerising radiation without requiring an optical mask or screen.

According to another aspect of the invention there is provided a method of making a light-diffusing material comprising providing a photopolymerisable system capable of polymerisation to form a solid, light-transmitting material having volume refractive index variations and/or surface relief features dependent on the exposure of the system to polymerising radiation, and which method comprises exposing a layer provided by said system to polymerising radiation through an optical aperture screen to polymerise the- material and subsequently polymerising any of the material still unpolymerised

According to yet another aspect of the invention there is provided a method of making a light-diffusing material comprising providing a photopolymerisable system capable of polymerisation to form a solid, light-transmitting material having volume refractive index variations and/or surface relief features dependent on the exposure of the system to polymerising radiation, and which method comprises exposing a layer provided by said system to polymerising radiation without any intervening mask and without other means of producing small scale variation of light intensity with position in the layer.

Preferably said system includes a silicone acrylate or similar compound or compounds. The polymerising radiation is preferably parallel (collimated) or substantially parallel radiation.

According to still another aspect of the invention, there is provided a diffuser formed by the above method.

According to still another aspect of the invention, there is provided a method of making a light-diffusing material comprising providing a photopolymerisable system, for example in a layer on a substrate, the system including a solvated or dispersed dye having a selective affinity for a part of the photopolymerisable system, the method further including exposing the material to electromagnetic or other radiation so as to cause polymerisation of the system and thereby to form a solid, light-transmitting material having volume refractive index variations and/or surface relief features coupled with colour graduations within the solid material, dependent on the exposure of the system to polymerising radiation.

Where the solid, light-transmitting material is in the form of a layer, sheet or film, the density of the dye may also vary with distance from one surface of such layer, i.e. may vary along a perpendicular to the plane of the layer.

Preferably the method comprises, in a first stage, exposing a layer provided by said system to polymerising radiation through an optical aperture screen to polymerise the material and subsequently, in a second stage, polymerising any of the material still unpolymerised, for example by blanket exposure to polymerising radiation.

According to a still further aspect of the invention, there is provided a method of providing printed text, legends, shapes, drawings or pictures on a substrate in a light diffusing, hologram or other light modifying material, such method comprising printing, for example stencil printing or inkjet printing, a liquid photopolymer blend onto said substrate and exposing the printed photopolymer blend to electromagnetic or other radiation to bring about polymerisation.

In preferred embodiments of the invention, the system comprises:-

(b) an ethylenically unsaturated monomer or prepolymer capable of undergoing free-radical-initiated polymerisation, and

(c) a photo-initiator capable of generating free radicals upon excitation by electromagnetic or other radiation,

(d) at least one thixotropic agent and/or at least one thickener, (such as Rheocin or Thixin) capable of imparting thixotropic properties, tack and slump characteristics for stencil printing or inkjet printing, and

(e) a dye (for example blue, red or green) with preferential affinity for one part of the polymer system such that upon curing the dye becomes heterogeneously distributed in the polymer matrix with selectively localised intensity.

Examples of component (b) which may be used include :-

ACRYLATE/METHACRYLATE GROUPS

PPTTA (polyethe olyoltetraacrylate) RI = 1.4772

ACMO (Acryloyl Morρholine)RI = 1.53

Sartomer 355, SR-355, DTMPTTA, (Di-Trimethylolpropane tetraacrylate

RI = 1.4758

Sartomer 399, SR-399, DiPEPA, (Dipentaerythritol pentaacrylate) RI = 1.4885 Ebecryl 2047 (Trifunctional acrylate) RI - 1.4757

Genomer 4302 (Aliphatic Polyester Triurethane Triacrylate, hazardous component being urethane acrylate 100%) RI - 1.509

Photomer 4810 F (Ester of acrylic acid and isoCIO alcohol) or Isodecyl (IDA)

CIO acrylate RI = 1.4395

Photomer 4072 (Trimethylol Propoxylate (3) Triacrylate) RI = 1.464

Sartomer 415 or SR-415 (Ethoxylated (20) Trimethylolpropane Triacrylate)

RI = 1.4699

Sartomer 492 or SR-492 (Propoxylated Trimethylolpropane Triacrylate)

RI = 1.459

Genomer 4269/M22 (aliphatic diranctional urethane acrylate diluted in Genomer

1122 (monofunctional, aliphatic urethane acrylate)) RI = 1.4785

Trimethylopropane triacrylate (TMPTA)

2-Phenoxyethyl acrylate (POEA)

2-Phenoxyethyl methacrylate

Phenyl ethoxylate monoacrylate

2-(p-Chlorophenoxy) ethyl acrylate

Phenyl acrylate

2-(l-Naphthyloxy) ethyl acrylate

Urethane methacrylates such as supplied under the Trade name "Genomer 4302", or any of the ethylenically unsaturated monomers capable of free radical initiated addition polymerisation identified as such in any of U.S. Patent Nos. 5470662;

4963471; 4942112; or 3658526. ±n particular, it may be beneficial to use polyester or polyether acrylates, which may impart flexibility to the polymerised material.

VINYL GROUPS

Poly(dimethylsiloxane) , vinyl terminated - viscosity 850-1150 cst N-vinyl pyrrolidinone (NVP)

Ethyl l-benzoyl-2-vinyl-l -cyclopropane carboxylate

The system may additionally include a compatible polymer, e.g. polymers or copolymers with similar groups to the silicone prepolymer. Compatible polymers or binders which may be used include PVA, polysiloxane polymers or acrylic-type polymers such as PMMA. Thus the formulation may include:- BINDERS PVA MW 12800 PVA MW 10-15000 PMMA MW 3300 - 996000

The system may additionally include a thixotropic and/or thickening agents such as rheocin and or thixcin.

The system may additionally include solvent or dispersed dyes in red, blue and/or green such as:

Solvent Red 19 (Sudan Red 7B), Solvent Red 23 (Sudan III), Solvent Red 24 Sudan IV), Solvent Red 26 (Oil Red EGN), Solvent Red 27 (Oil RedO), Solvent Red 41 (Basic Fuchin), Solvent Red 43 (Eosin Y, free acid), Solvent Red 45 (Ethyl Eosin), Solvent Red 49 (Rhodamine B base), Solvent Red 72 (4',5'- Dibromofluorescein), Solvent Blue 14 (Oil Blue N), Solvent Blue 35 (Sudan Blue II), Solvent Blue 38 (Luxol® Fast Blue MBSN), Solvent Blue 59 (Atlasol Blue 2N, l,4-bis(emylammo)-9,10-anthraquinone), Solvent Green 1 (Malachite Green base), Solvent Green 3, Solvent Green 7 (8-hydroxy-l,3,6-pyrenetrisulfonic acid, trisodium salt), Solvent Green 11 (Luxol® Brilliant Green BL).

In practice, there may be added to the formulations of the above system (i.e. silicone acrylate and monomer blends; silicone acrylate or silicone methacrylate alone; silicone/acrylate monomer blends), various other components such as flexibilisers, binders, other monomers, stabilisers, defoamers, thixotropic agents, thickening agents, dyes, antioxidants, photoinitiators and amine synergists.

Examples of materials in these classes which may be used are set out below:-

FLEXLBΓLIZER

Tri(ethylene glycol) RI = 1.4550

Poly(ethylene glycol)methyl ether (average Mn ca. 5,000,T_m52°)

Photomer 481 OF (Ester of acrylic acid and isoCIO alcohol) or Isodecyl (IDA)

CIO acrylate RI = 1.4395

The addition of such flexibilisers was found to be beneficial, allowing the production of good diffuser films with good to adequate flexibility.

BINDERS PVA MW 12800 PVA MW 10-1.5000 PMMA MW 3300 - 996000

Other monomers (vinyl, acrylate groups)

DEFOAMERS Rhodoline 646 Dehydran 1620

STABILIZERS 4-Methoxyphenol 2,4-Dihydroxybenzophenone

PHOTOLNITIATORS

PHOTOLNITIATOR Type I

Daracure 1173 or 2-hydroxy-2-methyl-propriphenone -1 -phenyl- 1-propanone

Genocure DMHA or 2-hydroxy-2-methyl-l -phenyl- 1-propanone

Genocure MBF (methyl benzoyl formate aromatic ketone) RI=1.5270

Irgacure 184 (1-hydroxycyclohexylphenylketone)

Irgacure 369 (2-beri2yl-2-(dimethylamino)-4'-moφholino-butyrophenone)

Lucrin TPO (diphenyl(2,4,6-frimethylbenzoyl)phosplιine oxide)

PHOTOLNITIATOR Type II

4-(p-tolylthio)benzophenone (Type II initiator)

Speedcure ITX (a mixture of ^* 2-isopropylthioxanthanone and 4- isopropylthioxanthanone - a Type II initiator which requires an amine synergist)

Speedcure DETX [(2,4-diethylthioxanthanone) - a Type II initiator which requires an amine synergist]

Speedcure EDB (ethyl-4-(dimethylamino)benzoate (amine synergist))

Speedcure PDA (poly[oxy(methyl-l,2-ethanediyl)] alpha-

[(dimemylanιino)benzyl-w-butoxy ] (amine synergist))

OXYGEN INHIBITORS WHICH MA¹ Y BE USED INCLUDE ethyl-4-dimethylaminobenzoate aminobenzoate derivative such as that supplied under the Trade name "Speedcure

PDA" (amine synergist) triethylamine oligoamines , such as supplied under the Trade name "Genomer 5248 and 5275" Ethyl -4- (dimethylamino) benzoate (amine synergist). Trade name Speedcure EDB.

Actilane 800 is a silicone difunctional acrylate product from Akros Chemicals Ltd., Manchester; Daracure 1173 is 2-hydroxy-2- methyl- 1 -phenyl- 1-propanone; NVP is N-vinyl pyrrolidinone; PVA is poly(vinyl acetate) MW 12,800; TMPTA is trimethylolpropane triacrylate; POEA is 2-phenoxyethyl acrylate; Rahn 99-662 is a silicone acrylate a product from RAHN AG, Switzerland.

The Actilane 800 and Rahn 99-622 materials referred to are examples of materials referred to generally as silicone acrylates, and are also known as acrylate terminated poly (dimethylsiloxanes) ( PDMS). Silicone acrylates useful in the context of the invention may also include aliphatic, phenyl, other aromatic or cyclic groups, or vinyl groups in addition to or as an alternative to methyl groups.

In variants of the invention, other silicone copolymers such as silicone methylacrylates or silicone acrylamide and silicone epoxy pre-polymers may be used instead of the silicone acrylate prepolymers referred to herein.

In some embodiments of the invention described in the following, a light-diffusing screen, for use as a rear projection screen or a depixelating screen, is made by a technique similar to that disclosed in European Patent No. 0294122 or European Patent No. 0530269, and in which 'technique, a layer of photopolymerisable material is applied to an optical mask featuring a plurality of light-transmitting apertures or windows in an opaque background or a plurality of opaque spots or patches in a light-transmitting background, these apertures or patches being on a microscopic scale (typically about 2 microns, and up to about 10 microns maximum dimension). In what follows, some examples are given of the manufacture of optical diffusers using masks substantially of the kind disclosed in EP-0801767. In these examples, the mask took the form of a photographic negative or transparency in the conventional form of a glass plate bearing on one side a gelatine layer incorporating opaque silver grains. In the examples of this character given below, the photopolymerisable mixture or system was coated directly onto the gelatine side of the prepared mask. In other examples below, optical diffusers are manufactured by a process which is similar but in which no mask is used, the photopolymerisable mixture or system being coated directly onto a plain, transparent sheet or film.

In the examples in which masks were used, they were prepared by being cleaned in acetone or chloroform or a 5% solution of "Decon Neutracon" in acetone or a 5% solution of "Decon Neutracon" in distilled water. After such cleaning, the mask was dried, then a 1% solution of Glassclad 18 (a proprietary release agent) in distilled water was applied for 10 to 20 seconds to the gelatine side of the mask, whereafter the mask was air-dried for 24 hours. In the examples in which no mask was used, a plain glass plate or tile was used instead of a developed photographic plate, the plain glass plate being prepared by cleariing in acetone.

In the stencil printing method, a laser cut stainless steel stencil was placed on a plastic tile or mylar sheet or paper and secured with appropriate pressure. A stirred polymerisable formulation of appropriate rheology as characterised by rheological studies was placed on the stencil and rolled across the stencil by using a rubber or stainless steel squeegee. The stencil was lifted off the substrate and the tile, sheet or paper was placed in a ultra-violet light exposure apparatus under a source of ultra-violet light for a standard period, (e.g. two minutes). Thus, where an optical mask of the kind referred to was used, the photopolymerisable material was first exposed through the mask (a "contact printing" technique). The tile or plate was then turned turned over and similarly exposed for a further period to provide a "blanket" exposure. As the polymerisation of the systems in accordance with the invention is in general inhibited by contact with oxygen, it is necessary to exclude oxygen, either by providing a covering layer of impermeable material such as plastics film or by a blanket of inert gas, during exposure.

In the inkjet printing method, a photopolymerisable system of appropriate rheology is loaded onto a drop-on-demand or continuous jet printer and dots of the photopolymer system are printed onto paper or plastic and cured in ultra-violet light for a standard period (e.g. two minutes) in an inert atmosphere..

In the examples in which a mask was used, the optical density of the mask was typically 1.6. Optical density is defined in accordance with the following equation:-

Transmittance = 1

10 OD

Where transmittance is the proportion of the light striking the mask which is allowed to pass through it and OD is the optical density. An optical density of 1.6 allows about 3% of the light to pass through.

Table 1 below sets out, for each of a plurality of trials, the respective formulation in accordance with the invention, (the components of each formulation or system being set out under the heading "Forrnulation"), the portions of the respective components, (these being set out in the column headed "Parts" in the same sequence as set out in the column "Formulation"), the optical density of each mask being set out in the column headed "Mask" and the characteristics of the resulting diffuser being summarised in the column headed "Comments". In the first column of the Table 1 the characters in the brackets in the first column are codes identifying the particular system of formulation concerned. These codes are used to identify the respective formulations in the discussion below.

In the "Comments" section in Table 1, the indications "SURFACE RELIEF" and "VOLUME EFFECT" indicate respectively that the light-diffusive effect appeared to be entirely or almost entirely due to surface relief produced in accordance with the mask pattern, and that at least a significant part of the light-diffusing effect appeared to be due to refractive index variations within the bulk of the photopolymer layer. Whether the light-diffusing effect was due to surface relief or to refractive index variations in the volume of the photopolymer layer was assessed by application of an index matching fluid such as propan-2-ol to the free surface of the photopolymer. If such application of propan-2-ol removed the light-diffusive effect, the diffusion was considered to be due to surface relief, whereas if it did not, the diffusion was considered to be due to refractive index variations in the volume of the material.

TABLE 1: UV - curing of various formulations

The photopolymerisation of some of the systems in accordance with the present invention is believed to be somewhat complex, in that it is believed that significant copolymerisation with the monomeric components ( such as the NVP, POEA or TMPTA, etc. (also illustrated in Figure 11)), takes place and quite possibly a degree of crosslinking between the monomers (TMPTA, etc., ) and the silicone acrylate and even between these and the PVA (where present).

It is hypothesised that, during exposure of a system in accordance with the invention to UV, through an optical mask, as described, the mechanism involved is as follows, :-

Initially, during the exposure, photopolymerisation of the silicone acrylate and monomer commences in the immediate area exposed to ultra-violet light, followed by migration, by diffusion, of monomer and silicone prepolymer from the imexposed regions into the exposed regions, accompanied by further polymerisation, including co-polymerisation, and even a degree of cross-linking, in the exposed regions, leaving a deficit of prepolymer and monomer in the unexposed areas.

The PVA incorporated in some of the formulations indicated is added in order to increase the viscosity of the system, to assist in handling and processing of the coated masks, and to improve the mechanical properties of the end product, by improving flexibility etc., the PVA being effectively dissolved in the monomer and polymer. It is believed however, that during the UV curing described, some (beneficial) chemistry involving the PVA may also take place, including bonding or cross-linking with the monomer and/or with the functional groups of the silicone acrylate. The alternatives to PVA noted herein by way of further compatible polymers likewise serve to increase viscosity of the uncured system and to improve flexibility in the end product.

As noted in Table 1, Formulation MCL 63 (Actilane 800 with NVP) forms a diffuser that exhibits surface relief. The addition of PVA, as in Formulation MCL 65, does not change this result. The use of a cross-linker such as TMPTA with the Actilane 800 as in Formulation MCL 70 forms a diffuser that exhibits volume graded refractive index effects, i.e. a diffuser in which within the photopolymer material, the refractive index varies from position to position. The addition of PVA as in Formulation MCL 73 does not change this result. This particular formulation (MCL 73) formed a very hard film. The mixture was very viscous before being UV-cured. The addition of POEA to the Actilane 800 Formulation MCL 71 led to a diffuser which exhibited graded refractive index volume effects. POEA is an aromatic acrylate also known as ethylene glycol phenyl ether acrylate and has a reasonably high refractive index (RI) of 1.5180. Acrilane 800 has a RI 1.4590. Formulations MCL 70 and MCL 71 in the Table had favourable light- diffusing characteristics. A favourable formulation is MCL 77 in Table 1.

The Actilane 800 material and the Rahn 99-622 material referred to above, (and the Rahn 00-225 material referred to below), are prepolymers or macromonomers comprising molecules of significant molecular weight comprising a plurality of acrylic and siloxane groups, the molecules typically comprising an inorganic silicone backbone with pendant methyl groups. These materials, in the presence of free radicals, are able to undergo still further polymerisation. More particularly, these materials may comprise solutions of said prepolymers or macromonomers in further compatible monomers. The NVP, TMPTA and POEA referred to are ethylenically unsaturated monomers capable of polymerisation in the presence of free radicals. The Daracure 1173 (2-hydroxy-2-methyl-l -phenyl- 1-propanone) is a photoinitiator capable of generating free radicals when exposed to UV light, to bring about polymerisation of the silicone acrylate materials and the ethylenically unsaturated monomers.

It may also be appropriate to incorporate an oxygen inhibitor or anti-oxidant in the formulation to improve shelf life and /or to minimise interference by atmospheric oxygen in the polymerisation of the material whilst it is exposed to atmosphere during exposure to UV light.

Claims

1. A photopolymer blend capable of deposition onto a substrate (such as fabric, paper, plastics, metal or wood) to form text, drawings or shapes by a printing method such as by stencil printing or inkjet printing and capable of polymerisation to form a solid, light transmitting material having volume refractive index variations or surface relief features by the exposure of the deposits of the blend to electromagnetic or other radiation wherein the blend includes a monomer, prepolymer, macromonomer or co-monomer comprising silicone groups, that is to say groups with a

backbone, and wherein at least one component of the system incorporates ethylenic unsaturation such that the system is capable of undergoing free radical initiated polymerisation.

2. A system according to Claim 1 wherein the silicone monomer, macromonomer, co-monomer or prepolymer comprises silicone acrylate or multifunctional silicone acrylate.

3. A system according to Claim 2 wherein the silicone monomer, macromonomer, co-monomer or prepolymer comprises a difunctional silicone acrylate.

4. A system according to any previous claim in which the silicone monomer, macromonomer, co-monomer or prepolymer is a substituted multifunctional silicone acrylate.

5. A system according to any previous Claims wherein the silicone monomer, macromonomer, co-monomer or prepolymer is a substituted multifunctional silicone acrylate such as urethane acrylate or acrylamide.

6. A system according to any of the preceding claims which incorporates further compatible polymer(s) and/or monomer(s).

7. A system according to claim 6 in which one or more of the additional compatible components has a refractive index significantly different to the polymerisable silicone comprising component.

8. A system according to claim 6 in which the compatible polymer or polymers includes polyvinyl acetate, polymethyl methacrylate and silicone gum materials.

9. A system according to claim 6 in which the compatible monomer or monomers are unsaturated organic compounds.

10. A system according to claim 9 in which the unsaturated organic compounds are multifunctional.

11. A system according to claim 9 or 10 in which the unsaturated organic compounds are monomers which include acrylate, methacrylate, epoxy or ethylenic groups.

12. A system according to claim 6 which incorporates a compatible polar organic solvent

13. A system according to claim 6 in which the polar organic solvent is methyl ethyl ketone.

14. A system according to any previous claim in which the polymerizing radiation is high energy electrons or UV light.

15. A system according to claim 14 which incorporates at least one UV photoinitiator to facilitate polymerisation.

16. A system according to claim 15 in which the photoinitiator is selected from the group comprising:- a) Methyl benzoyl formate b) 1-Hydroxycyclohexylphenylketone c) 2-Hydroxy-2-methyl- 1 -phenyl- 1 -propanone d) Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide e) 2,4-Diethylthioxanthone plus suitable amine synergist or mixtures of two or more of the above.

17. A system according to any previous Claims wherein the blend incorporates a pigment in a percentage range of 10-15% by weight.

18. A system according to Claim 1 wherein the blend incorporates a soluble or dispersed dye in weight range of 0.5 - 5% by weight.

19. A diffuser, hologram or other light-modifying structure formed by polymerisation of the system of any of the preceding claims.

20. A system according to any of claims 1 to 18 or a diffuser, hologram or other light-modifying structure according to claim 17 in which the silicone content comprises 15-90% by weight of the reactive components.

21. A system according to any of claims 1 to 18 or a diffuser, hologram or other light-modifying structure according to claim 17 in which the silicone content comprises 25-80% by weight of the reactive components.

22. A system according to any of claims 1 to 18 or a diffuser, hologram or other light-modifying structure according to claim 17 in which the silicone content comprises 30-60% by weight of the reactive components.

23. A method of providing printed text, legends, shapes, drawings or pictures on a substrate in a light diffusing, hologram or other light modifying material, such method comprising printing, for example stencil printing or inkjet printing, a liquid photopolymer blend onto said substrate and exposing the printed photopolymer blend to electromagnetic or other radiation to bring about polymerisation.

24. A method according to Claim 23 in which the said substrate is paper.

25. A method according to Claim 23 in which the said substrate is plastic.

26. A method according to Claim 23 in which the said substrate is wood.

27. A method according to Claim 23 in which the said substrate is metal.

28. A method according to Claim 23 in which the said paper substrate is a currency note or a secure certificate or secure document.

29. A method according to Claim 25 in which the said plastic substrate is a currency note, credit card, bank card, identification card, certificate or other document.

30. A method according to any of Claims 23 to 29 in which the electromagnetic radiation is UV light.

31. A method according to Claim 30 in which the UV light is substantially collimated.

32. A method according to Claim 31 in which the major part of the radiation directed onto the polymerisable material to polymerise the same is directed along directions which are within 5 degrees of the nominal exposure direction.

33. A method according to any of Claims 23 to 32 in which prior to irradiation, the liquid layer is overlaid with an optical mask and after exposure from one side, through said mask, is subjected to a second flood or blanket irradiation from the reverse side.

34. A method according to any of Claims 23 to 33 in which prior to irradiation the liquid layer is overlaid with a clear film.

35. A method according to any of Claims 23 to 33 in which irradiation is carried out in an inert atmosphere such as nitrogen.

36. A method according to any of Claims 23 to 35 in which the exposure to electromagnetic or other radiation is followed by a thermal process to complete polymerisation.

37. A method according to any of Claims 23 to 36 in which stencil printing is used and the printing method employed uses a stainless steel laser cut or electroformed stencil or a silk screen.

38. A method according to Claim 37 in which the stencil printing is carried out using a rubber or metal squeegee.

39. A method according to any of Claims 23 to 36 in which inkjet printing is used and in which the inkjet printer used for printing onto the substrate is a drop-on-demand or continuous inkjet printer.

40. A method of making a light-diffusing material comprising providing a photopolymerisable system, for example in a layer on a substrate, the system including a solvated or dispersed dye having a selective affinity for a part of the photopolymerisable system, the method further including exposing the material to electromagnetic or other radiation so as to cause polymerisation of the system and thereby to form a solid, light-transmitting material having volume refractive index variations and/or surface relief features coupled with colour graduations within the solid material, dependent on the exposure of the system to polymerising radiation.