In particular, this invention concerns an aqueous radiation-curable composition that can be used as a base coat or varnish which exhibits good adhesion to electrostatic offset inks.

In electrostatic offset printing an image is formed by directing one or more pigmented inks or toners on to a selectively charged drum to form an image which is then transferred, either directly or via an intermediate member, on to a substrate.

Electrostatic offset printers are made and sold by, for example, Indigo NV and Xeikon SA.

Inks used in electrostatic offset printing have difficulty adhering to certain substrates such as plastics. This can be overcome by modifying the surface of the substrate.

The surface can be modified by the application of a preliminary base coat comprising, for example, a solvent-based resin such as, for example, a polyamide, a butadiene resin or an ethylene-acrylic acid resin (see, for example, JP 10076744).

When a solvent-based resin is used, the solvent needs to be evaporated before printing, which can be slow, hazardous and offensive, and is environmentally unfriendly. The use of water as solvent avoids these drawbacks, but restricts the range of resins that can be used. In particular without some curing or hardening process in addition to loss of water, the compositions do not produce coatings of high strength or toughness.

WO 98/49604 discloses a copolymer receptor layer which is applied by extrusion to a polyvinyl chloride backing to improve the abrasion resistance of a printed image.

The copolymer receptor layer can also be used to improve print quality on a polycarbonate backing layer. In both of these applications, we are told that the copolymer receptor layer adheres to the polyvinyl chloride backing layer in the absence of UV radiation.

One aim of the present invention is to provide a composition for use as a base coat or varnish that can be used on difficult substrates, such as plastic substrates, which are printed using, for example, electrostatic offset printing.

Another aim of the present invention is to provide a composition for a base coat or varnish which dries by exposure to ultra-violet radiation rather than by evaporation of solvent.

A further aim of the present invention is to provide a water-based composition for use as a base coat or varnish.

A further aim of the present invention is to provide a composition for a base coat or varnish which exhibits good adhesion to the surface of a compact disc.

A further aim of the present invention is for the composition to form a good bond with electrostatic offset inks.

In accordance with the present invention there is provided a composition for use as a base coat or varnish, the composition comprising: (i) one or more ethylenically unsaturated monomers, oligomers or prepolymers that are capable of being photopolymerized ; (ii) a photoinitiator that is capable of initiating the polymerization of the ethylenically unsaturated monomers (i) when irradiated with ultra-violet light; (iii) a water-soluble inorganic alkaline salt; (iv) an inorganic transfer agent; and (v) water.

In accordance with the present invention there is also provided a method for preparing a substrate for printing, the method comprising the steps of : applying a base coat to the substrate before printing, the base coat comprising the composition defined above; and exposing the base coat to ultra-violet radiation.

There is also provided a method for varnishing a printed substrate, the method comprising the steps of : applying a varnish to a printed substrate, the varnish comprising the composition defined above; and exposing the varnish to ultra-violet radiation.

In accordance with the present invention there is also provided use of the composition defined above as a base coat or varnish for printing.

The composition of the present invention is preferably substantially free of volatile (organic) solvents (other than water) which must be evaporated in use. Preferably the composition is substantially free of"aggressive"amine compounds and particularly preferably substantially free of amines, especially when the composition is applied to polycarbonate substrates. By"aggressive"amines, the applicant means amines which are water soluble and remain unreacted in the composition after curing. This is disadvantageous, especially when the composition is applied to polycarbonate substrates, since any subsequent contact with water may damage the base coat or varnish layer.

The composition may be applied to, for example, a polycarbonate compact disc by various printing or coating methods, and, after hardening by ultra-violet irradiation, gives a tough layer to which an electrostatic offset ink adheres well. In another embodiment of the invention the composition is applied as a varnish on to a printed substrate, for example, a printed polycarbonate compact disc, by various printing or coating methods to give after hardening by ultra-violet radiation a tough resistant layer.

The base coat may be applied to a substrate using, for example, screen process printing, flexography, curtain coating or roller coating.

In use, the base coat is applied to a substrate and then irradiated using ultra-violet light, typically of wavelength between 250 and 450 nanometres. Preferred substrates are plastics articles, especially spin-coated polycarbonate compact discs

or DVDs.

The ethylenically unsaturated monomer, oligomer or prepolymer (i) is preferably either an ester of acrylic or methacrylic acid, such as, for example, octyl acrylate, decyl acrylate, lauryl acrylate, phenoxyethyl acrylate, hexanediol diacrylate, tri (propylene glycol) triacrylate, trimethylolpropane triacrylate, polyethylene glycol diacrylate, hydroxyethyl methacrylate ; or an N-vinylamide such as, for example, N- vinylcaprolactam or N-vinylformamide. Other useful monomers, oligomers or prepolymers are acryloylmorpholine, the acrylate esters of the ethoxylated or propoxylated derivatives of di-, tri-or tetrahydric aliphatic alcohols, and the acrylate or methacrylate esters of epoxy, urethane, melamine or polyester resins or their ethoxylated or propoxylated derivatives. Monomers, oligomers or prepolymers which are soluble or dispersible in water are preferred. Mixtures of these materials may be used. The monomer, oligomer or prepolymer is preferably present in an amount from 10 to 80%, more preferably 23 to 60%, by weight of the composition.

The photoinitiator (ii) is of the type known to produce free radicals when irradiated with ultra-violet light. Photoinitiators of this type are, for example, benzophenone, 1-hydroxycyclohexyl phenyl ketone, isopropylthioxanthone, 2-hydroxy-2-methyl-1- <BR> <BR> phenylpropan-1-one,2-benzyl-2-dimethylamino- (4-morpholinophenyl) butan-1-one, benzil dimethylketal, bis (2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, or a mixture thereof.

Photoinitiators of this class are commercially available, for example, under the trade names Darocur, Irgacure (from Ciba), Genocure (from Rahn AG), Lucirin (from BASF). Colourless or nearly colourless materials are preferred. The photoinitiator is preferably present in a total amount from 0.5 to 10%, more preferably from 2.5 to 8%, by weight of the composition.

The water-soluble alkaline salt (iii) is one which gives the composition a pH between 7 and 14, preferably between 8 and 10. We have found that compositions of this pH exhibit good adhesion to substrates and to ink layers, especially those which contain polymers bearing carboxylic groups. The salt (iii) may be an alkaline

metal, an alkaline earth metal, an ammonium or a zirconium salt or combinations thereof. The counter ion may be hydroxide, carbonate or bicarbonate. Specific examples include sodium hydroxide, potassium hydroxide, sodium carbonate etc. A preferred salt is ammonium zirconyl carbonate, where it is possible that the presence of zirconium gives improved adhesion to substrates or ink layers.

The inorganic transfer agent (iv) is preferably silica or modified silica or mixtures thereof. Suitable silicas are Aerosil 200 (available from Degussa) and Cab-o-sil TS610 (available from Cabot). We have found that a suitable inorganic transfer agent has the combined advantages of giving a desirable"structure"to the compositions, i. e. resistance to flow under gravity without excessive viscosity at higher shear, and rendering the final cured base coat particularly receptive to electrostatic offset ink. The inorganic transfer agent (iv) is preferably present in an amount from 0.5 to 15%, more preferably from 2 to 4%, by weight of the composition.

Optionally, other additives of known types may also be present such as, for example, passive resins (i. e. resins that are not cured by ultra-violet light) to stabilise the viscosity of the composition and improve the adhesion or other physical properties of the cured layer; dyes or pigments, especially white pigment to form an opaque background for the electrostatic offset print; synergists for the photoinitiator; stabilisers to prevent deterioration of the composition to heat or on storage; flow and slip aids; biocides; identifying tracers; defoamers ; and surfactants.

The components of the compositions are preferably selected so that they are compatible with each other and provide a mixture stable on storage, with the required rheological and other properties. In an alternative embodiment of the compositions they are supplied in two portions which are mixed shortly before use.

The compositions are preferably formulated to give a rheological profile suitable for application by printing, especially by screen-printing. For this purpose their strength, as measured as the viscosity at 25 °C on a Brookfield RVDT digital viscometer at 5 r. p. m using a No. 6 spindle, preferably lies between 100 and 1000

poise, but more preferably between 250 and 750 poise, and their viscosity under higher shear, as measured on the Rotothinner (supplied by Sheen Instruments), preferably lies between 5 and 65 poise but more preferably lies between 30 and 50 poise.

The substrate may be foil, paper or board, or a plastics material such as, for example, polycarbonate, or poly (vinyl chloride). Preferred is polycarbonate such as is used in the production of compact discs. The substrate may be printed with the composition using a screen stencil or by any other appropriate printing or coating method, and then hardened by exposure to ultra-violet light. Typical exposures are at a speed of 27-50 metres per minute on a Svecia u. v. dryer, or at up to 56 impressions per minute with two lamps at 100% power on a Kammann K15 printer.

After curing, the substrate bearing the base coat is especially suitable for receiving electrostatic offset ink. Alternatively the composition may be applied to a compact disc already printed with an electrostatic offset ink and hardened by exposure to ultra-violet light as described above.

The invention will now be described with reference to the following Examples, in which all parts are by weight: Example 1 (In this process the temperature was not allowed to exceed 50°C.) The following components were mixed in a stainless steel pot using a Torrance stirrer for 15 minutes until homogeneous: Actilane SP061 (acrylate monomer from Akcros) 23.76 parts Triacrylate of ethoxylated tris (methylol) propane 5.94 parts Tripropylene glycol triacrylate 3.74 parts Ebecryl 140 (acrylate monomer from UCB) 4.65 parts Genorad 16 (stabiliser from Lambson) 0.99 parts Igepal CO-897 (surfactant from Caldic) 0.99 parts

Ammonium zirconium carbonate 12.0 parts Florstab UV-5 (stabiliser from Kromachem) 0.05 parts The following were then added in the order given with continued stirring: Lucirin TPO (photoinitiator from BASF) 1.42 parts Darocur 1173 (photoinitiator from Ciba) 3.48 parts Deionised water 2.18 parts Bentone EW (gellant from Elementis 0.50 parts Polyethylene glycol 1500 0.99 parts Aerosil 200 (silica from Degussa) 3.5 parts Mistron Monomix (talc from Luzenac) 14.32 parts The stirring was continued until the product was nib-free when measured on a Hegman gauge. The product had a viscosity of 420 poise at 25°C when measured on a Brookfield RVDT viscometer at 25°C at 5 r. p. m. using a no. 6 spindle, and 47 poise at 25°C on a Rotothinner. The pH of the product was 8.2.

When printed as a primer on a polycarbonate substrate and exposed to ultra-violet light, it formed an opaque white base-coat, which was suitable to receive a coloured image printed by the electrostatic offset process.

Example 2 Two-pack colourless base-coat Part A (In this process the temperature was not allowed to exceed 50°C.) The following components were mixed in a stainless steel pot using a Torrance stirrer for 15 minutes until homogeneous: Actilane SP061 (acrylate monomer from Akcros) 23.76 parts Triacrylate of ethoxylated tris (methylol) propane 5.94 parts Tripropylene glycol diacrylate 3.74 parts

Ebecryl 140 (acrylate monomer from UCB) 4.65 parts Genorad 16 (stabiliser from Lambson) 0.99 parts Igepal CO-897 (surfactant from Caldic) 0.99 parts Florstab UV-5 (stabiliser from 0.05 parts The following were then added and with continued stirring until the solids had dissolved: Lucirin TPO 1.42 parts Darocur 1173 3.48 parts Deionised water 22.18 parts Finally the following were added with continued stirring until the mixture was nib- free when measured on a Hegman gauge: Bentone EW 0.5 parts Polyethylene glycol 1500 0.99 parts Aerosil 200 3.5 parts Mistron Monomix 14.32 parts Part B Sodium hydroxide, 10% solution in water 12.0 parts Before use parts A and B were mixed using a low-speed stirrer to give a composition having a pH of 13.2. The composition was printed and exposed as in Example 1 to give a basecoat suitable to receive an image printed on an Indigo electrostatic offset printer.