GB2437714A

GB2437714A - Printing on optical discs

Info

Publication number: GB2437714A
Application number: GB0608948A
Authority: GB
Inventors: Robin Mcmillan; James Welch; Stewart Kessel
Original assignee: Sun Chemical Ltd
Current assignee: Sun Chemical Ltd
Priority date: 2006-05-05
Filing date: 2006-05-05
Publication date: 2007-11-07
Also published as: JP2009536425A; EP2024973A2; WO2007131098A3; GB0608948D0; WO2007131098A2; CN101484946A

Abstract

A composition comprising a prepolymer, at least one monomer, a photoinitiator and a levelling additive, the composition being free from any surface-modifying additives that would interfere with printability and the cured composition being water-insoluble and non-absorbing to water, is applied to one side of an optical disc and is then radiation-cured e.g. with UV or electron beam. The levelling additive may be a polymeric acrylate, a mineral oil blend, a low temperature wax dispersion or a paraffinic wax dispersion which is free of silicone. The prepolymer may be an epoxy acrylate, an acrylated oil, a urethane acrylate, a polyester acrylate, a polyether acrylate, an acrylated amine, an acrylic saturated resin, an acrylic acrylate, a cycloaliphatic epoxide, a phenol epoxy novolac or an acrylated epoxidised soya bean oil. The monomer may be an acrylate, methacrylate, a vinyl compound, or a compound capable of polymerising by a ring-opening mechanism. The resulting coated surface is durable and receptive to printing using a radiation-curable ink jet ink.

Description

1 2437714

A METHOD OF PRINTING ON OPTICAL DISCS

The present invention relates to a method of printing on optical discs.

Optical discs include compact discs (CD5) and digital versatile discs (DVD5) of various kinds including future formats such as Blu-ray Disc and HD-DVD. They are designed to carry data (including audio or video information) which may be read by a laser or other optical device. Pre-recorded or replicated discs commonly have, on the side opposite that read by the laser or other optical device, a top layer of printing either to indicate the nature of the information on the disc or simply for decorative purposes.

In the case of compact discs, for example, the area to be decorated is both the disc material, typically polycarbonate, and a spin coated layer used to protect the recording medium. It is desirable that this printing should be durable, water-and grease-resistant and inexpensive.

For longer print runs, say over 1000 discs, the replicator industry would commonly use screen andlor offset printing. These techniques can produce good strong images and durable print quality relatively inexpensively per disc.

However, for shorter print runs, say 500 copies or less, the cost per disc of such techniques becomes too high. For a run of, say, 50 discs, the cost per disc of good quality screen printing could easily approach GBP 1 per disc.

Accordingly, such short print runs are nowadays commonly dealt with by ink jet printing. These runs are also usually duplicated (i.e. not replicated) unless they are for a repeat, top up order.

Since conventional ink jet inks will not adhere well to the surface of optical discs (typically polycarbonates or, in the case of CD's, also UV curing spin coatings), many proposals have been made for base layers that will bond well to the optical disc and to the ink jet ink. Such a base layer may be applied, for example, by a screen printing or spin coating technique. Examples include: EP 0 574 860, EP 0 628 956, EP 1 294 570, US 5 573 831 and US 6 437 017, all of which describes the use of an absorbent, preferably hydrophilic, primer layer as a substrate for subsequent printing using a standard ink jet printer. The resultant products are attractive and relatively inexpensive per disc.

Although these prior art proposals succeed at what they attempt to do, they use water-based ink jet inks and do not address a fundamental problem of such inks when used in this way -they are simply not sufficiently durable. They will come away from the optical disc in water or even with normal handling. Moreover, they generally have poor abrasion resistance. These problems can be overcome by covering the printing with a transparent film or by spraying or printing on a varnish or lacquer coating.

However, either of these expedients adds an extra processing step, which always adds expense and can be inconvenient, and which requires extra materials, thus adding to the expense.

All of the above proposals use standard ink jet inks. These are commonly water-or solvent-based. Such inks "dry" by a combination of evaporation and absorption into the primer substrate onto which they are printed, which results in the various disadvantages mentioned above. None of the prior proposals for dealing with this problem has, so far as we know, used any other type of ink jet ink.

Radiation-curable, e.g. UV energy curable, ink jet inks are known. These are normally free from water or organic solvents and "dry" by curing as a result of the polyrnerisation induced by exposure to UV (ultraviolet) or other radiation, e.g. electron beam. Unlike their water-based equivalents, once cured they can provide a hard, durable surface which is resistant to water, oils, bodily fluids and most domestically available fluids, and which is resistant to abrasion. However, in experiments, we have found that UV ink jet inks suitable for use with high definition print heads such as XAAR Omnijet 318 do not print well onto conventional optical discs, either with or without a base coating of the types described above. For example, the coating can often be removed by scratching, even with a fingernail, and the image printed is generally of poor quality, giving a "washed-out" or faded appearance due to poor printed dot formation. It should also be noted that best visual results are obtained when the jet printed image is backed-up with a solid colour and, in particular, white.

If the base coating is employed wet, i.e. before it has dried or cured, we have found that good print quality may be obtained when jet printed. However, once the coating is dried or cured, print quality becomes unacceptable due to poor receptivity.

This is unfortunate, since the duplicating industry, which is responsible for short runs of optical discs, is unaccustomed to the need for two separate printing processes (coating the base layer, and then ink jet printing the top layer). Apart from the expense of acquiring two separate coating machines, one for each process, they would either have the increased labour cost of manually transferring discs from one machine to the other or would have to invest in yet more equipment to achieve this mechanically. In either case, this not only adds to the cost but also risks damage to the discs being transferred.

This industry would prefer to use blank recordable optical discs already coated with the base coating, so that they only have to be concerned with the information or decoration to be printed thereon.

We have now found that these problems may be overcome by the use of a base coating material having certain specific physical and chemical properties.

Thus, the present invention consists in an optical disc having an optically readable side and a printed side, the printed side bearing a radiation-cured receptive coating formed by radiation curing a composition comprising a prepolymer, at least one monomer, and a levelling additive, said composition being free from any surface-modifying additives that would interfere with printability, and the cured composition being water-insoluble and non-absorbing to water.

Notable among the surface-modifying additives that would interfere with printability are silicone surface-modifying additives, and thus, in one embodiment, the invention provides an optical disc having, on an optically readable side and a printed side, the printed side bearing a radiation-cured receptive coating formed by radiation curing a composition comprising a prepolymer, at least one monomer, and a levelling additive, said composition being free from any silicone surface-modifying additives, and the cured composition being water-insoluble and non-absorbing to water.

In the present invention, unlike the prior art, the receptive coating does not absorb the ink applied to it; instead, the ink sits on top of the coating and is cured by radiation, e.g. UV or electron beam. Where the composition is to be cured by UV energy, it should include, in addition to the components mentioned above, a photoinitiator. Where it is to be cured by electron beam (EB), a photoinitiator is not necessary.

Provided that the composition forming the receptive layer is free from silicone or similar surface modifying agents, a radiation-curable ink jet ink will adhere well to the surface of the cured composition. In order to achieve good compatibility between the cured receptive coating and the cured ink, the cured receptive coating should also be water-insoluble and non-absorbing to water.

Thus, the invention further provides a process in which a radiation-curable jet ink is jet printed onto a cured receptive coating, as defined above, on one side of an optical disc, and the ink is then cured by exposure to curing radiation.

Provided the constraints defined above are adhered to, the composition of the receptive coating may vary over a wide range. It should contain at least the following components: 1. a radiation-curable prepolymer; 2. a radiation-curable monomer; 3. a photoinitiator, if the composition is to be cured by UV energy; and 4. a levelling additive, which is not a silicone-based surface-modifying agent.

In addition, the composition optionally and preferably contains either one or both of: 5. a pigment and/or filler andlor extender; and 6. a structuring additive.

The composition used to prepare the receptive coating advantageously comprises at least one prepolymer. Suitable prepolymers (also referred to as oligomers) include epoxy acrylates, acrylated oils, urethane acrylates, polyester acrylates, polyether acrylates, acrylated amines, acrylic saturated resins, acrylic acrylates, cycloaliphatic epoxides (cationic), phenol epoxy novolacs and acrylated epoxidised soya bean oils.

Preferably, the prepolymer has a number average molecular weight greater than 500 and more preferably greater than 800. The receptive coating preferably comprises no less than 10% and preferably no less than 20% by weight of one or more such prepolymers, based on the total weight of the composition. Preferably, the receptive coating comprises not more than 50% and more preferably not more than 40% by weight of prepolymer based on the total weight of the composition. For example, the prepolymer may be present in an amount if from 1% to 60% and preferably from 20% to 40% by weight based on the total weight of the composition. In a preferred embodiment, the prepolymer has a number average molecular weight of more than 500 and is present at a level of from 20% to 40% by weight based on the total weight of the composition.

Preferred prepolymers exhibit a low level of shrinkage on curing. Further details and examples are given in "Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints", Volume II: Prepolymers & Reactive Diluents, edited by G Webster, the disclosure of which is incorporated herein by reference. Of these, the epoxy acrylates are preferred, bisphenol A epoxy acrylates and modified bisphenol A epoxy acrylates (modified, for example by the incorporation of amino, hydroxy and ethoxy groups during manufacture) being more preferred. Particularly preferred are those epoxy acrylates available under the trade names Ebecryl 3701, Ebecryl 3700, Ebecryl 3500 and Ebecryl 605 from UCB, Craynor CN1O4A8O, Craynor CNUVE1 10/95, and Actilane 320TP20 ex Cray Valley.

The radiation-curable monomer is preferably an ethylenically unsaturated compound, for example an acrylate, a methacrylate or a vinyl compound, or a compound capable of polymerising by a ring-opening mechanism.

Examples of suitable acrylate monomers include: butanediol diacrylate; trimethyloipropane triacrylate; di-trimethyloipropane tetraacrylate; pentaerythritol triacrylate; di-pentaerythritol pentaacrylate; polyether acrylates, such as ethoxylated trimethylol propane triacrylate, glycerol propoxylate triacrylate, ethoxylated pentaerythritol tetraacrylate, epoxy acrylates such as dianol diacrylate ( the diacrylate of 2,2-bis [4-(2-hydroxyethoxy)phenyl]propane, Ebecryl 150 from UCB); glycol diacrylates such as tripropylene glycol diacrylate, polyethylene glycol diacrylates (for example, tetraethylene glycol diacrylate), neopentyiglycol diacrylate and dipropylene glycol diacrylate; acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate and alkyl acrylates, such as hexanediol diacrylate, isobornyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate, lauryl acrylate, stearyl acrylate, isodecyl acrylate, octyl acrylate, decyl acrylate, isobornyl acrylate, phenoxyethyl acrylate, oxyethylated phenol acrylate, tetrahydrofuryl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate and 4-t-butylcyclohexyl acrylate.

Methacrylate monomers include the methacrylate equivalents of the acrylates described above, and notably hexanediol dimethacrylate, trimethyloipropane triacrylate, triethylene glycol dimethacrylate, diethylene glycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate.

Non-acrylated reactive diluents that may be incorporated include vinyl components such as N-vinylpyrrolidones, N-vinylcaprolactams, N-vinylformamide, vinyl ethers and styrenes. N-Vinylcaprolactam (VCAP) has been found to be a particularly suitable vinyl monomer for inclusion in the compositions used to prepare the receptive coating of the invention due to its good adhesion to optical disc substrates.

Vinyl ether compounds including uj3-unsaturated ether monomers, such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether, as well as ethyl 1 propenyl ether, triethylene glycol methyl propenyl ether, triethylene glycol methyl vinyl ether and 2-cyclopenten-I -yl ether may also be used. Other monomers include acryloyl morpholine.

Suitable oxygen-containing ring opening monomers include those comprising an oxetane ring or an oxirane ring. Oxirane species include cycloaliphatic oxiranes (also known as epoxides), such as 3,4-epoxy cyclohexyl methyl-3,4-epoxy cyclohexyl carboxylate and the glycidyl ethers of polyols. Oxiranes derived by the epoxidation of unsaturated materials may also be suitable, for example, epoxidised soybean oil, epoxidised polybutadiene or epoxidised alkenes. Oxetane species include mono-functional and multi-functional oxetanes, for example, 3-ethyl-3-hydroxymethyl- oxetane, bis [(1 -ethyl-3 -oxetanyl)methyl] ether, 3 -ethyl-3 -[2-ethylhexyloxy)m ethyl] oxetane, [1,4-bis(3 -ethyl-3 -oxetanylmethoxy)methyl]benzene and trimethyloipropane oxetane. Many further suitable materials are known to the skilled person.

It is preferred that the prepolymer and the monomer(s) should be selected for high surface tension, so that the composition has a surface tension higher than that of the ink jet ink which is to be printed onto it. For example, the prepolymer preferably has a surface tension of at least 35 mN per m (dynes per cm2), more preferably at least 38 mN per m (dynes per cm2), whilst the monomer(s) should preferably have a surface tension of at least 32 mN per m (dynes per cm2), more preferably at least 35 mN per m (dynes per cm2).

The receptive coating may be free-radically curable, cationically curable or it may be curable by a combination of the two mechanisms. It may also be curable by electron beam. If the coating is to be cured by UV energy, then a photoinitiator may be necessary. The nature of the photoinitiator used will depend on the curing mechanism desired, but is otherwise not critical to the present invention. The photoinitiators may be of the cleavage or hydrogen abstraction type and are preferably selected from the following photoinitiator classes: benzophenones, thioxanthones and related compounds, hydroxyalkyiphenones, aminoalkylphenones, anthraquinones, standard and modified acyl phosphine oxides (such as Lucirin TPO and TPO-L), bis-acyl phosphine oxides (such as Irgacure 819), benzil ketals, benzoin ethers, acetophenones, beta ketosuiphones, oxime esters and phenyl glyoxic acid esters. Specific examples of such cationic photoinitiators include sulphoniurn salts (such as the mixture of compounds available under the trade name UV16992 from Dow Chemical), thianthrenium salts (such as Esacure 1187 available from Lamberti), iodonium salts (such as 1GM 440 from 1GM), phenacyl sulphonium salts, and the thioxanthonium salts, such as those described in WO 03/072567 Al, WO 03/072568 Al, and WO 2004/055000 Al, the disclosures of which are incorporated herein by reference, such as those sold under the trade marks 1GM 550 and 1GM 650 by 1GM.

Further examples of photoinitiators, synergists and sensitisers can be found in standard textbooks such as "Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints", Volume III, "Photoinitiators for Free Radical Cationic and Anionic Polymerisation", 2' edition, by J.V. Crivello & K. Dietliker, edited by G. Bradley and published in 1998 by Joim Wiley & Sons in association with SITA Technology Limited; and "Exploring the Science, Tecimology and Applications of UV and EB Curing", R.S. Davidson, SITA Technology Ltd., London, 1999, the disclosures of which are incorporated herein by reference.

The photoinitiator used is preferably one which does not require the additional use of an amine synergist, and, in particular, it is preferred that the composition should not include a non-acrylated free amine synergist.

Where, as is preferred, the receptive coating is to be white, it is generally preferred that the photoinitiator should be non-yellowing. Where, however, it is to be another colour, this is unnecessary.

In general, the receptive coating should be such that the receptive coating wets the optical disc substrate sufficiently well to form a film on the substrate when applied by screen printing. The receptive coating adheres well to the disc material or spin coat, enabling the receptive coating to adhere well to the disc, without swelling or otherwise distorting the disc material, thereby promoting good adhesion.

The composition of the present invention also contains surface active agents to improve for example levelling, wetting and defoaming properties. These surface active materials should have a low tendency to migrate to the coating surface and thereby adversely affect its receptivity. Suitable materials are silicone free and may be polymeric in nature. Examples of such compounds include polymeric acrylates, mineral oil blends, low temperature wax dispersions and paraffinic wax dispersions, for example Modaflow TM (Cytec), BYK A501 (BYK), Bevaloid 6681 (Rhone Poulenc), BYK 1790, BYK A500, BYK A560, Tego Airex 910 and Tego Airex 920. (Goldschmidt), which are able to assist dearation of the uncured composition. If desired, a fluorosurfactant may be added to the composition to improve surface wetting during application.

Commercial examples of such fluorosurfactants include Zonyl FSN100, Zonyl FSO100 ex Dupont and Fluorad 4430 ex 3M.

The composition used to prepare the receptive coating the present invention may optionally contain a pigment or other colorant, preferably a pigment. In the context of the present invention, the term colorant' covers materials which endow an actual visual colour, which may be, and preferably is, white.. White pigments are typically included in amounts of about 40-50% by weight of the total composition. However, the actual amount of colorant used may vary widely depending on the nature of the colorant, for example within the range of from 1 to 60% by weight, more preferably from 7 to 50% by weight. If it is desired that the coating should be clear, then no colorant need be included.

Broadly speaking, colorants may be considered as falling into two classes, namely dyes, which are substantially soluble in the ink composition, and pigments, which are dispersed in the ink composition in the form of fine particles, if necessary with the aid of a suitable dispersant. Pigments may be selected from a wide range of classes, for example, Pigment Red 146, Pigment Red 170, Pigment Red 48:2, Pigment Red 122, Pigment Blue 15:3, Pigment Violet 23, Pigment Green 7, Pigment Yellow 83, Pigment Yellow 13, Pigment Yellow 17, Pigment Black 7. Other examples of suitable pigments are given in "Printing Ink Manual", fourth edition, Leach R. H. et a!. (eds.), Van Nostrand Reinhold, Wokingham, (1988), the disclosure of which is incorporated herein by reference. However, the pigment, if used, is preferably a white pigment, for example Pigment White 6 titanium dioxide e.g. Kemera RDI-S, Kronos 1070, Kronos 1071, Tipure R706 or zinc oxide, zinc sulphide or barium sulphate, of which titanium dioxide is preferred. A combination of any two or more of these colorants may be used, if desired, in order to achieve particular effects.

These formulations may also contain extenders and fillers in order to further improve the behaviour and appearance of the ink both during and after print processing.

Typical generic examples of preferred fillers and extenders are synthetic and natural grades of chalk, talc, kaolin and other aluminium hydroxides and silicates. They are known commercially under many trade names, examples of which are Calcigloss, Microtalc, Speswhite, Spacerite and Zeolex.

These formulations may also be modified with small amounts of a structuring additive, such as fumed silica or activated bentonite, in order to produce the preferred rheological behaviour on the screen printing presses. Examples of such materials are Aerosil from Degussa, Bentone SDI, Bentone SD2, Bentone 34 ex Elementis and Tixogel DS, Tixogel VPA, Tixogel MP100 from Sud Chemie.

The various components of the composition of the present invention may be present in a wide range of quantities, depending on the desired physical and chemical properties of the final cured coating. However, in general, we prefer to employ from 1 to 60, more preferably from 20 to 40, % by weight of the radiation-curable prepolymer; from 5 to 60, more preferably from 20 to 40, % by weight of the radiation-curable monomer(s), from 0 to 15, more preferably from 0 to 8, % by weight of the photoinitiator, from 0 to 5, preferably from 0 to 3, % by weight of the levelling additive, from 1 to 60, preferably from 7 to 50, % by weight of the pigment and/or filler and/or extender (if used) and from 0.1 to 8, preferably from 0.2 to 5, % by weight of the structuring additive (if used) The compositions of the present invention may be prepared by simple mixing and dispersion techniques as are well known in the art, and may be applied to the optical disc by any well known printing technique, e.g. screen printing, spin coating, pad printing or flexographic printing, preferably screen printing. The whole of one surface of the optical disc may be coated, or only a portion may be coated. The coating may then be cured, using any well known technique of radiation curing.

The invention is further illustrated by the following non-limiting Examples.

EXAMPLE 1

The following screen ink composition was prepared by first premixing the materials and then grinding the resultant mixture on a triple roll mill until a grind of <12 microns (tim) was achieved.

Epoxy Acrylate -Ebecryl 3701 ex Cytec 28.0 N-vinylcaprolactam ex BASF 12.9 Monomer -Ebecryl 110 ex Cytec 4.7 Lucirin TPO ex Ciba 4.0 Stabiliser 0.2 BYKA5O1 exBYKChemie 1.1 Titanium Dioxide White Pigment, anatase 49.1 100.0 The resulting screen ink composition was printed through a 150-31 mesh onto an optical media substrate (CDR, DVD-R) and cured using a medium pressure mercury lamp (80Wcm5. A 4 colour high definition test image was ink jet printed on top of the cured coating using a commercial UV ink jet ink through a XAAR Omnijet 318 piezo DOD print head and UV cured. The resultant prints were examined for image quality and durability. The results are shown in the following Table 1.

EXAMPLE 2

The following screen ink composition was prepared by first premixing the materials and then grinding the resultant mixture on a triple roll mill until a grind of<12 microns (tm) was achieved.

Epoxy Acrylate -Ebecryl 3701 ex Cytec 29.4 N-vinylcaprolactam ex BASF 13.7 Monomer -Ebecryl 110 ex Cytec 7.7 Lucirin TPO ex Ciba 3.9 Stabiliser 0.2 BYK A501 ex BYK Chemie 1.0 Titanium Dioxide White Pigment, anatase 39.2 Silica 4.9 100.0 The resulting screen ink composition was printed through a 150-31 mesh onto an optical media substrate (CDR, DVD-R) and cured using a medium pressure mercury lamp (8OWcm'). A 4 colour high definition test image was ink jet printed on top of the cured coating using a commercial UV ink jet ink through a XAAR Omnijet 318 piezo DOD print head and UV cured. The resultant prints were examined for image quality and durability. The results are shown in the following Table 1.

EXAMPLE 3

The following screen ink composition was prepared by first premixing the materials and then grinding the resultant mixture on a triple roll mill until a grind of <12 microns (tm) was achieved.

Epoxy Acrylate -Ebecryl 605/20 ex Cytec 30.7 N-vinylcaprolactam ex BASF 14.2 Monomer -Ebecryl 110 ex Cytec 7.9 Lucirin TPO ex Ciba 4.1 Stabiliser 0.2 BYK A501 ex BYK Chemie 1.0 Titanium Dioxide White Pigment, anatase 40.9 Silica 1.0 100.0 The resulting screen ink composition was printed through a 150-31 mesh onto an optical media substrate (CDR, DVD-R) and cured using a medium pressure mercury lamp (8OWcm-1). A 4 colour high definition test image was ink jet printed on top of the cured coating using a commercial UV ink jet ink through a XAAR Omnijet 318 piezo DOD print head and UV cured. The resultant prints were examined for image quality and durability. The results are shown in the following Table 1.

EXAMPLE 4 (Comparative) This Example uses a silicone antifoaming agent in place of the silicone-free levelling agent of the present invention.

The following screen ink composition was prepared by first premixing the materials and then grinding the resultant mixture on a triple roll mill until a grind of <12 microns Q.tm) was achieved.

Epoxy Acrylate -Ebecryl 3701 ex Cytec 24.2 N-vinylcaprolactam ex BASF 12.5 Monomer -Ebecryl 110 ex Cytec 12.2 Lucirin TPO ex Ciba 3.5 Stabiliser 0.2 Silicone antifoam 0.7 Wetting Agent, Zonyl FSN 100 0.5 Titanium Dioxide White Pigment, anatase 46.0 Silica 0.2 100.0 The resulting screen ink composition was printed through a 150-31 mesh onto an optical media substrate (CDR, DVD-R) and cured using a medium pressure mercury lamp (8OWcm-l). A 4 colour high definition test image was ink jet printed on top of the cured coating using a commercial UV ink jet ink through a XAAR Omnijet 318 piezo DOD print head and UV cured. The resultant prints were examined for image quality and durability. The results are shown in the following Table 1.

EXAMPLE 5

Urethane Acrylate -CN982/388 cx Cray Valley 29.5 N-vinylcaprolactam ex BASF 6.9 Monomer -Sartomer 238 ex Cray Valley 9.9 Monomer -Sartomer 256 ex Cray Valley 5.0 Irgacure 907 1.0 Lucirin TPO ex Ciba 3.8 Stabilisers 0.4 BYK A501 cx BYK Chemie 1.1 Titanium Dioxide White Pigment, rutile 41.7 Silica 0.7 100.0 The resulting screen ink composition was printed through a 150-31 mesh onto an optical media substrate (CDR, DVD-R) and cured using a medium pressure mercury lamp (8OWcm-1). A 4 colour high definition test image was ink jet printed on top of the cured coating using a commercial UV ink jet ink through a XAAR Omnijet 318 piezo DOD print head and UV cured. The resultant prints were examined for image quality and durability. The results are shown in the following Table 1.

Table 1

Image Quality Durability Water Colour Resistance Development/ Overall Tape Scratch (Spot test/Wet Receptive Coating Wetting Impact Appearance Resistance Resistance rub) Poor, de-Undecorated CD, spin coated wetting Very Little Patchy Poor/patchy Fair, marking Fair Poor, de-Undecorated DVD wetting Very Little Patchy Poor Poor Fair CDR decorated with SOHO inkjet Good Excellent Good Patchy/Good Poor Very Poor Poor, de-DVD-S 102 Std DVD White ex. Sun Chemical wetting Very Little Patchy Good Fair, marking Good Poor, de-Seridisc Flash White UR024 ex Sericol wetting Very Little Patchy Good Fair, marking Good Ultradisc TJVOD 173 Opaque White ex Marabu Reasonable Fair Patchy Good Fair, marking Good CDRJPW Ink Jet Printable White ex. Sun Chemical* Reasonable Fair Fair Poor Soft, marking Very Poor Example 1 Good Excellent Very Good Good Good Good Example 2 Good ExcellentVery Good Good Good Good Example 3 Good Excellent Very Good Good Fair, marking Good Poor, de-Example 4 wetting None Very Patchy Good Good, marking Good Example 5 Poor Poor Patchy Good Good, marking Good

Claims

CLAIMS: 1. An optical disc having an optically readable side and a

printed side, the printed side bearing a radiation-cured receptive coating formed by radiation curing a composition comprising a prepolymer, at least one monomer, and a levelling additive, said composition being free from any surface-modifying additives that would interfere with printability, and the cured composition being water-insoluble and non-absorbing to water.

2. An optical disc according to Claim 1, in which said levelling additive is a polymeric acrylate, a mineral oil blend, a low temperature wax dispersion or a paraffinic wax dispersion, and is free of silicone.

3. A composition according to Claim 1 or Claim 2, in which said prepolymer is an epoxy acrylate, an acrylated oil, a urethane acrylate, a polyester acrylate, a polyether acrylate, an acrylated amine, an acrylic saturated resin, an acrylic acrylate, a cycloaliphatic epoxide, a phenol epoxy novolac or an acrylated epoxidised soya bean oil.

4. A composition according to Claim 3, in which said prepolymer is an epoxy acrylate.

5. A composition according to any one of the preceding Claims, in which said prepolymer has a surface tension of at least 35 mN per m (dynes per cm2).

6. A composition according to Claim 5, in which said prepolymer has a surface tension of at least 38 mN per m (dynes per cm2).

7. A composition according to any one of the preceding Claims, in which said monomer is an acrylate, a methacrylate, a vinyl compound, or a compound capable of polymerising by a ring-opening mechanism.

8. A composition according to any one of the preceding Claims, in which said monomer has a surface tension of at least 32 mN per m (dynes per cm2).

9. A composition according to Claim 8, in which said monomer has a surface tension of at least 35 rnN per m (dynes per cm2).

10. A composition according to any one of the preceding Claims, additionally comprising a photoinitiator.

11. A method of preparing an optical disc having an optically readable side and a printed side, in which a composition comprising a prepolymer, at least one monomer and a levelling additive, and, where curing is by ultraviolet energy, a photoinitiator, said composition being free from any surface-modifying additives that would interfere with printability, and the cured composition being water-insoluble and non-absorbing to water, is applied to the printed side and is radiation-cured.

12. A method according to Claim 11, in which the composition is as claimed in any one of Claims 2 to 10.

13. A method according to Claim 11 or Claim 12, in which the coating composition is applied to the optical disc by screen printing, spin coating, pad printing or flexographic printing.

14. A process in which a radiation-curable jet ink is jet printed onto the printed side of an optical disc according to any one of Claims 1 to 10, and is then radiation-cured.