CA1098483A - Dry-release transfers - Google Patents

Abstract

ABSTRACT
A dry release transfer is disclosed in which the design layer is formed by printing one or more inks onto a carrier sheet, at least one of the inks being a photopolymerisable composition, and subjecting the design layer to ultra-violet radiation or an electron beam discharge in order to polymerise the photopoly-merisable ink. By suitable selection of polymerisable components of the ink so that the photopolymerised ink has a high Young's modulus, a stress-resisting design layer is produced which will release readily from the carrier on mechanically stressing the carrier, e.g. by rubbing lightly with a ball-point pen.

Description

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~IFLD OF ~ ENTION
~ his invention relates to dry release trans~ers and to a method of producing such transfers.
DESCRIPTIO~T O~ TH~ PRIOR ART
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Dr~ release transfers comprise a carrier sheet (alternatively ter~ed a support sheet) with one or ~ore designs printed on one surface of the carrier sheet so that a selected design can be physically transferred as a dry in~ layer to receiving substrate and adhered thereto lO by an adhesive. Such translers are termed ~'drv releasel' because the release of the designs from the oarrier sheet does not require the application of a liquid.
.~ Dry transfers are usually produced ~lith a pressure sensitive adhesive so that on application of pressure to 15 the back surface of the carrier sheet over the desi~n ~.rhile placed in contact with a receiving substra~e ~he desi~
. adheres to the substrate so that the carrier sheet can be peeled awa~ to leave the design transferred and adhered ~o the substrate.
Two types of such dry transfers havin~ differen~
transfers mechanisms have been previously described in the art in British patents 959,67O and 1,491,678 and both of these rely on the use of a low-tack sçnsitive adhesive in which the adhesive may overlap the desi~-n to avoid the 25 difficulty of printing an adhesive layer in exact re~ister - . . . ' .

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1~8~33 with the design. This ~dhesive has hig~her adhesion to the carrier sheet than the receivin~ substrate so that when the carrier sheet is peeled ~way the adhesive outside the design area is retained on the carrier sheet and the adhesive tears or shears around the edges of the design to permit physical transfer of the design.
Strong forces are exerted on the design layer during trans~er and these are increased when an overlapping adhesive is used. ~hese forces frequently cause the design to break so that only a part transfers and alternatively the design transfers in a distorted for~ often with visible cracks.
British Patent 1,~91,678 describes a method of reducing the edge adhesion of the design and weakenin~ the adhesive layer strength at the design edges and this somewhat reduces the risk of breaking the design during transfer.
However the design layers of such prior transfers have all been produced by screenprint~ng a solvent-based 20 screen ink based on cellulose nitrate as the film forming polymer and the concentration oP the poly~er in the formulae given for these inks is very low, e.g.22-27~ by wei~ht.
Since this polymer is the only film-forming component of the inks, the resalt of this is to give a dry ink film after 25 evaporation of the solvents which is extre~ely thin, .. : .
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generally 5 micrometres and only about 60% of this is polymer.
This polymer thickness is totally inadequate to produce a transfer which is essentially unbreakable under normal transfer conditions and in fact even in skilled hands transfers fre~
quently break during transfer.
The applied transfers have very poor scratch or abrasion resistance and this has greatly restricted their field of application, for example, they are unsuitable for marking or decoration of equipment and components, packaging applications and numerous outdoor uses.
Moreover, cellulose nitrate is an extremely inflammable polymer and the transfers are hazardous when used for childrens toys and games, home decor, and skin trans~ers.
If a coarse screen mesh is used having less than 90 meshes per centimetre and having a higher open area percentage in order to increase ink thickness, print quality becomes worse and ink drying time increased and this increases the size of the drying plant which is costly and already occupies about 75% of the area of the printing plant.
A further problem exists in these dry transfers in that print quality and geometrical accuracy are already inadequate due to imperfect print edges and line width variations which are partly caused by the normal screenprinting mesh of 90 mesh per centimetre and also by evaporation o~ the ink solvent during prin~ing causing sd~r~ 5-....

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partial clogging of the screen mesh apertures. -If a ~iner mesh is used to improve print quality,the dry film thickness falls below 5 micrometres and has a totally inadequate film strength and mesh clogging becomes worse.
Print quality can be numerically expressed by the maximum number of lines per millimetre which are resolved in the print having lines and spaces of e~ual width. Generally dry transfer materials have a resolution of only up to 5 lines per millimetre.
In addition, all the transfer mechanisms hitherto known do not give adequate control of transfer properties and frequently lead to failure to transfer or accidental or unwanted transfer.
SUMMARY OF THE INVENTION
All of these problems of prior art transfers are overcome in accordance with the invention by a dry release transfer which comprises a carrier substrate and a flexible design layer releasably adhered thereto, the design layer comprising a flexible solid polymer produced by photopolymerisation of a viscous liquid ink comprising photopolymerisable, ethylenically unsaturated material, the photopolymerisation having been effected by exposure of the entire liquid ink layer to actinic radiation or to electron beam radiation, whereby the liquid ink layer is converted rapidly to the solid state.
The above transfer may be produced by a method of producing a dry release transfer which comprises the ~ollowing steps:
(a) printing a design layer on a carrier sheet using at least one liquid photopolymerisable ink which contains not more than 20~ by weight of volatile solvent, the ink being a fast polymerisable ink and comprising one or more ethylenically unsaturated monomers or prepoly-mers, and tb) exposing the printed sheets to actinic radiation or 30 to electron beam radiation to cause photopolymerisation of the design layer.

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' :: ' ~ ' '` ,.. ~' : ' ' ~, ,, 10~8~83 The term "design" includes all manner of pictures, decorations, pictorial games and toys, education, uniform colour areas, advertising, ~arkings and typographical characters such as alphabets of various lettering styles and sizes, numerals, symbols such as electronic, architec-tural, chemical engineering and mathematical symbols, various ~e~tures, titles, logos and text matter all of which may be single-coloured or multicoloured.
The term design layer includes all those layers wllich are physically released from the carrier sheet by the application of external force to the carrier sheet and inc-udes a single colour desi~n layer, ~ultiple colour layers, clear la~er and adhesive la~er is present, all of which are released as a single composite layer. Examples of sin~le colour design layer plus adhesive layer are transfer sheets used for small designs such as sheets of ; letters or numerals. T~hen the colour design is large or comple~ or is multicolour produced by halftone printin~, a clear or coloured overall layer is printed to extend 20 o~er the whole of the colour desi~n components so that these physically co-release together and ¢an be transPerred in one piece in their printed spatial relationship.
In this specification "photopolymerised" means polymerised b~ actinic radiation or by electron beam dis-25 charge. Actinic radiation includes ultra-violet and visible . ,"' .
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lOq~483 -radiation, as well as other electromagnetic radiation capable of activating poly~erisation. Ultra-violet radiation requires the presence of photoinitiators in the p~otopolymerisable ink but electron beam radiation does not.
DETAILED DESCRIPTION OF ~HE INVENTION
The photopolymerised layer should not be brittle and a minimum elongation at breakpoint should be 0.5~ and preferably in the range of 2-15~ depending on the design size and shape and the flatness of the receiving substrate.
It has been found that reduction or physical breaking of the adhesive bonding between the carrier sheet and the photopolymerîsed design layer is dependent on the che~ical properties of the carrier sheet and design layer, the stress-trans~itting properties of the carrier sheet and 15 stress-resisting properties of the design layer and on any pre-stressing of the design layer. All of these properties are readily controlled in the invention so that physical release or transfer of the design can be accurately and r~liably predetermined by selection of carrier sheet and 20 design layer materials.
~ he carrier sheet and design layer inks are selected 50 that no chemical reaction occurs between these to form strong and irreversible bonds. ~or example, there should be no strong solvent action of the design la~er inks 071 the 2S carrier sheet~ There should also be no covalent chemical . ,, , , . - - .
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. , - I 10 84~l3 bonding between the carrier sheet and the design laver produced by co-poly~erisation during photopolymerisation of the liquid inks. Only weak physico-che~ical bonds should exist between carrier sheet and the design layer 5 in contact with it.
It has been found that to break these physico-che~ical bonds during transfer, the photopolymerised desi~n la~er should have stress-resisting properties so that when an external force is applied to the carrier sheet this is 10 resisted by the design layer and this stresses the adhesive bond causing failure and physical release of the design ,-layer. Two factors deter~ine the stress resisting proper~ies of the design layer; its thickness and its stiffness and 'the latter is convenientl~ expressed by Young's modulus.
15 Stress-resisting properties are approximately proportional to the cube of layer thickness and directly proportional to Young's modulus.
Stress-transmitting properties of the carrier sheet should be such that the carrier sheet thickne~s and Young 9 20 modulus should not be too high otherwise, the ~aterial will be so stiff that external forces will not be transmitted to the adhesive bonds between design and carrier and ~ould not be too low and co~pliant so that again no stress will ,-be trans~itted to the adhesive bond.
~enerally plastic films and cellulosic based sheets _ g _ . .

109~4~3 _ and combinations thereof in the thickness ran~,e of 20-150 micrometres have the required mechanical properties r~e~
i used with an appropriate stress-resisting photopol~merisable layer. A suitable combination must be determined by simple experiment in which the external force is a~plied and physical release or transfer properties are assessed.
~he practice of the invention allows a photopol~er-! ised layer to be selected which will provide the correc~
, release and transfer properties and the t~Jo basic para~eters are layer thickness and Young's modulus. ~ayer thic~ness ; . is readily controlled by the printing process and the nu~ber of ink lagers that are applied and Young's modulus l can be controlled by the crosslink densit~ of the photo-il pol~merised layer, lS The inherent flexibility of the molecules which form the photopolymerised layer also affect layer stress-t resisting properties and elongation properties but with given materials having adequate elongation, crosslink d~nsity determines very precisel~ the stress-resisting properties.
In one e~bodiment of the invention, physical release of the design layer occurs to such an extent that it is clearly visible as lightening of the colour of the design ' due to an air film enterin~ between transferable layer i 25 and carrier sheet~ This is an impor~ant aid to reliable ~'' ~ - 10 -. ; ~

10~84~33 trans~er ~hich ensures that release is complete and guarantees freedom fro~ fracture of the desi~n. Such visible release is termed herein 'pre-release' since it can be produced without adhesi~e assistance for example without adhesive layer i~ present in the ~ssembly being in contact with the receiving substrate.
In a further e~bodi~ent of the invention, release of the design layer during transfer is assisted by pre-stressing of the adhesive bonds between design layer and carrier sheet~ Such pre-stressing may be chemical or physical in nature. Physical pre-stressing is carried out for example by the shrinkage of the design layer during photopolymerisation. Suitable shrinkage can range from O.5S~ - 12~ and is partly dependent on crosslink density, the higher the crosslinking density~ the higher the shrinkage, Shrinkage is resisted bg the carrier sheet so the net effect is to place the adhesive bonds in a state of strain so that the application of only a small external force i9 required to physically release the design layer~
20 Physical pre-stressing can occur to such an extent that spontaneous release occurs on photopolymerisation and therefore the composition of the photopoly~erisable ink is selected to pro~uce a degree of pre-stressing which is less than this.
Che~ical pre-stressing is carried out by the action ~8~E~3 _ of an adhesi~e layer on a photopol~merised design layer whereby a solvent or other liquid in the sdhesive layer causes the design layer to swell. Since lateral swelling is resisted by .the carrier sheet this a~ain places the adhesive bonds in a s-tate of strain and bond stren~h is usually permanently reduced so that even after evaporation : of volatile liquid the design layer has reduced adhesion to the carrier sheet.
~y control of pre-stressing and of the stress-resisting properties of the design layer, transfer sheetscan be prepared with accurately pre-determined release characteristics and in which release is produced hg a small external force ~hich is desirable for easy and fast transfer properties.
1~ A further advantage of the photopolymerised design layers of the invention is that photopolymerisable inks are free from volatile materials or contain only a minor proportion of these so that screen mesh clo~gi~gcause~ by evaporation on the printing screen cannot occur and ~ery 20 high and consistent print quality is obtained and this is una~fected by temperature variations in the printinO
environment.
Ultra fine screenprinting meshes may be used without mesh cloggirgand meshes as fine as 220 meshes per centimetre

2~ using monofilament polyamide and 180 meshes per centimetre - 12 _ ~: .
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10"8483 using monofilament polyester can be used and print resolution of 12 lines per millimetre can be obtained.
Much higher values of dry ink layer thickness are therefore readily obtained because there is a little or : ~ no loss o~ volatile materials t~lhen the liquid ink is photopoly~erised and a desi~n layer thickness in the range : of 8-50 ~icrometres thickness is obtained by selecting the appropriate screen mesh. For single layer designs such as lettering nu~erals and symbol sheets for graphic artists and desi~ners a layer thickness of 10-12 micrometres is preferred.
Any ~eans may be used to attach the design layer to the receiving substrate including, mechanical fixing, electrostatic, magnetic, air pressure, suction and adhesives.
Adhesives include:
no-tack, low-tack and high-tac~ pressure sensi~ive, heat-fix, solvent-fix and water-fix, liauid polymerising adhesives, ~ sel~-seal adhe~ives, photopolymerising pressure sensitive adhesives, adhesive receivin6 substrates, delayed tack heat-fix adhesives, encapsulated adhesives~
: . and these may be printed in register with the design layer, 25 or overlap the design layer and shear during transfer ,' - 13 - -:

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according to known mechanisms. Adhesives of the kind described in British Patent Specification No. 1,491,678 may be employed.
Because of the improved release characteristics of photopolymerised inks compared with prior art transfer inks, much wider variety of carrier sheets may be used in the invention. These comprise plastic films and cellulosic sheets and combinations of these. Plastic films include polyethylene, polypropylene, polystyrene, polystyrene-butadiene, polyvinyl chloride, copolymers of vinyl-chloride and vinylacetate, polyesters and cellulose acetate. Such plastics may have a further coated layer giving better release properties. Cellulosic materials include glassine, greaseproof and vegetable parchment papers in which the porosity of the cellulosic material has been reduced or eliminated.
Cellulosic sheets may be coated, laminated or impregnated with a plastic film or polymer such as polyethylene extrusion coated paper, polypropylene laminated paper and amino-formaldehyde polymer impregnated paper.

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~ 3 Release coatings may also be applied tb the carrier sheet surface such as silicones and Werner chromiu~ complexes.
~ ight transmitting carrier sheets are generally preferred to assist in positioning the transfer on the S receiving substrate.
Photopolymerisable design layers are applied by all printing, painting and coating processes which employ liquid inks such as screen, litho, le~ter-press, gravure, flexo, brush, spray, roller and the like. l~en the application lO ~ethod applies a layer which is too thin for stress-resisting properties, ~ultiple layers are applied with intermediate exposure to photopolymerising radiation to build up the correct layer thickness.
~he external force that release the design layer lS may consist of any mech2nical means. For example, the strokes of a ball-pen, pencil or stylu5 a?plied with a force of fro~ e.g. 5O-500 gra~s, or a bending, twisting or stretching force applied to the carrier sheet. Alterna~iYel~
t~e design layer can be transferred by a direct tenslle 20 pull or a peelin~ force applied for example by adhering the design la~er to a receiving substrate and then peelin~-off the carrier sheet.
In a multila~er transfer of the invention at least-one of the layers is produced by photopolymerisatiQn.
25 Other layers may be produced with either photopoly~erisable ~ - 15 -.

, ~6~!98g~3 inks or by conventional inks which are dried by the appropriate method. Non-photopolymerisable layers may be applied before or after ~he photopolymerisable layer. For example a clear overall photopolymerisable layer may be applied to the carrier sheet by screenprintin~ and a~ter photopol~merisation a coloured design layer is applied b~
printing with conventional evaporation drying so]vent-based inks or oxidation drying inks overprinted onto the clear layer by screen or litho printing respectively.
Alternatively a design layer or layers in conventional inks may be applied first to the carrier sheet and after drving are then overprinted with a clear or coloured overall stress-resisting photopolymerisable layer which ~rhen photo-polymerised can be physically released and transferred and 1~ carries with it all the design co~ponents in their original printed spatial relationship. ~his procedure is very convenient when the design is a four colour halftone picture or consists of much fine line detail.
Photopolymerisation is produced by brie~ e~Yposura o~ eth~lenically unsaturated materials to actinic radi~tion such as ultra violet radiation or a mixture of ultra violet and visible radiation or accelerated electron beam radiation. Ultra ~iolet radiation o~.higll intensity is conveniently produced by ~edium pressure mercury vapour 25 discharge lamps operated at 80 watts per centimetre or more ~i - 16 -lQ~84~3 .

in fused silica or quertz tubes. Other useful sources of intense ultra violet light are xenon discharge lamps and xenon flash lamps and swirl flow plasma radiation arcs.
Crosslinl~ density is ~ainly determined b~ the number of photopolymerisable ethylenicall~ unsaturQted groups per ~olecule of the materials used in the liquid ink, termed functionalit~. One eth~lenic group per molecule cannot crosslink and gives a soft and very extensible la~er with inadequate Young's modulus. ~wo ethylenic groups per molecule generally gives a suitable ~alue and three ethylenic groups gives high values which mav lead to spontaneous release. A mixture of materials with one, two and three ethylenic groups is a useful means -of achieving crosslink density which will then be an average value, The ~ono-eth~lenic material can be compared to a plasticiser in conventional inks, the di-ethylenic material provides the main co~ponent and the tri-ethylenic material is added to increase the stress-resisting properties to precisely the desired value.
Elongation properties are achieved by usin~ flexlble chemical groups in the photopolymerisable materials such as polyalkyl, polyether and polyester groups, combined with control of the crosslinX density.
Another important advanta~e of photopolymerisation in operation of the invention is that very rast ink "dr~ing"

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is obt~ined. It is ve~y desirable to use fast photo-polymerising materials in order to reduce the e~posure ti~e to the actinic radiation since the radiation frequently has an infra-red component which causes heating of the 5 carrier sheet and can cause distortion or shrinkage with excessive e~posure~
Ver~ fast photopolymer~sing inks are obtained by photoinitiated vinyl addi-tion polymerisation of mono~ers and prepolymers containing terminal or pendant acryloyl 10 or methacryloyl groups: CH2=CR-CO- where R is ~ or CT~3 -respectivel~. ~he acr~loyl group is fas-ter polymerising than the ~ethacryloyl group and reference below to acryloyl groups includes methacryloyl groups.
To obtain excellent printability the liquid ink 15 must possess correct viscosity and tack values and these can be readily achieved together with all the other requîrements described above by controllin~ the molecular `-weight and composition of the photopolymerisable materials. ', C~nveniently a material of high viscosity is used in -20 admixture with a liquid of lower viscosity.
~ ow viscosity and liquid photopolymerisable materialsare mono~er5~ that is ~aterials which do not contain polymeric groups in the molecule and ~suitable ~aterials ~-are ac~ylate esters of ~ono, di, tri and tetrahydric 25 alcohols. Monomers are preferred which have very lo~r 1(~ 83 _ . volatility and low skin and eye irritancy and these properties are ~enerally obtained with monomers of hi,~er ~olecular weight, Acrvlate es~ers of the following . alcohols are suitable and are given by way of exaM~le:
5 Monohydric alcohols: ? phenoxDethanol, 2 phenox~rethox~ethanol . and hydro~enated derivatives, Dihydric alcohols:: tripropylene gl~col, bisphenol A, hydrogenated bisphenol A and hydroxethyl ethers and hydroxypolyethoxyethers of bisphenol A and h~drogenated ~isphenol A.
~rihydric alcohols: trimethylolpropane ~etrahydric alcohols: pentaerythritol Polyhydric alcohols: dipentaerythritol All hydroxyl ~roups may be esterfied or one or more IS groups ma~ be left unesterfied to provide materials with controlled,hydrophilic - lyophilic balance for offset ',, litho inks. ~ree'hydroxyl groups may be further reacted or partially reacted with isocyanates to produce urethanes.
High viscosity is readily obtained by photopolymer-isable prepolymers in which there is a polymeric com~onent in the molecule. ~hese materials ran~e from highly ~iscous . liauids to solids and have molecular weight range of about 250-5000. The terminal or pendan~ acryloyl ~roups can be incorporated in polymeric components such as a polyurethane~
polyepoxide, polyether, polyester ar.d polyaminoformaldehyde polDmers .
19 'I

~ ~ , . , lo~s4sa Preferably 2-6 acryloyl groups are incolporated in the polymer nolecule and t'~lls can be carried out for example bv reacting acrylic acid or acryloyl chloride with a polymer or polymerisable material containing free S hydroxyl groups. Alternatively such groups can be in-corporated by reaction of a hydroxy alkly acrylate with a polymer or polymerisable material containirg isocyanate, epoxide, carboxylic acid, anhydride or a~inoformaldehyde groups.
~ o~ example an acrylated epoxy prepolymer is prepared by reacting bisphenol A polyglycidyl e~er with terminal epoxide groups with acr~lic acid which open the oxirane ring and the hydroxyl groups so produced can be further reacted with acryloyl chloride to introduce additional acryloyl groups.
Acrylated urethaneprepolymers are prepared for example by reacting hydro~y~ropyl acrylate with hexamsthylene di-isocyanate or polyisocyanates, Alternatively, acrvvloyl polyether urethanes and acryloyl 2a polyester urethanes are prepared by reactin~ sn excess of a di- or polyisocyanate with a polyether or pol~es~er having ~ree hydrox~l groups and then reacting this polymer with a hydroxylalkyl acr~late.
To obtain the correct balance of properties more than one monomer and more than one prepolymer may be used . - ~0 -,,-10C~8~B3 in the inks. One or more photoini-tiators are dissolveQ
or dispersed in the unsaturated materials at a concen~ration of 0.01 - 30~ and more usually 1-10~ based on the weight of unsaturated material to photoinitiate polymerisation when using ultra violet radiation or ultra violet plus visible radiation. Photoinitiators are not required w~en high energy accelerated electron beam radiation is used.
The following are examples of photoinitiators:
Ketones and derivatives such as benzophenone, 4,4'dimethyl-aminobenzophenone, acetophenone, 2,2 diethoxy-acetophone, halogenated benzophenone, benzil, benzil di-~ethylacetal. Acrylions and derivatives such as oen~oir., benzil dimeth~lacetate and benzoin isopropyl ether. Thio ~ompounds such as thioxanthone, 2 chlorothioxanthone, benzoyl diphenyl sulphide, polynuclear quinones and derivatives such as benzoquinone, chloroanthraquinone~
Chlorinated hydrocarbons such as hexachlorethane and ~iazo co~pounds including fluoroborate salt of di~onium compounds.
~he effect of photoinitiators may be accelerated by a tertiary amine such as eth~l dimeth~la~i~obenzoate or an amino acrylate polymer.
Other types of unsaturated monomers and polymers can be added to the main photopolymer~isable materials listed above to participate in the photopolymerisation such as N-vinylpyrrolidone, vinyl acetate~ allyl and .
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cinnamyl esters, acryla~ide derivatives such as (~-isobutoxyme~h~l) acr~lamide, triallylcyanurate.
Unsaturated polyesters include maleate, fumarate, itaconate and citraconate esters of glycols.
Non-reactive polymers can also be dissolved or dispersed in the main photopolymerisable ~aterials such as a high acid value polyester to give alkali solubility to the photopolymerised transferable layer, or dispersed finely po~dered polyvinylchloride or vinyl chloride-acetate copolymer which solvate durin~ photopolymerisation to increase ~ilm strength and flexibility.
Finally, Yarious o~her additives m2y be added to the inks such as pig~ents, fillers, flow a~_ents, waxes which are well ~no~m to persons skilled in the art of lS printing inks.
Photopolymerisation can be subject to inhibition by atmospheric oxy~en which effects mainly the outer surface of the design layer. ~his can lead to a reduction in film strength with thin design layers and oxyGen inhibition is prevented in the invention by v0ry hi~h intensity focussed radiation using an elliptical reflector and by the use of poly-acryloyl unsaturated materials plus the ~ost efficient photoinitiators and accelerators.
If necessary photopolymerisation may also be carried out in a nitrogen atmosphere or by placing a transparent plastic '~:, ; ,.: , . ~,, .

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~ 33 film over the liquid ink durin~ ex~osure, both of ~rhich reduce access by atmospheric oxygen.
Mos~ carrier sheets readily trans~it long wavelength ultra violet radiation such as 36~ n~ and polyethylene carrier sheets readily transmit also the shorter wavelengths of 254 and 310 nm. Consequently photopolymerisation can be carried out by reverse exposure that is by passing the radiation throu~h the carrier sheet. This has the advantage that the most highly polymerised layer will then be 10 adaacent to the release layer where the effect of a high Young's modulus is ~ost pronounced. h~nen using inks with a high optical density such as a black ink with a density of 2.0 or ~ore it is useful to use both reverse and direct exposure simultaneously or successively.
In an alternative embodiment of the invention, oxygen inhibition of the transferrable layer is deliheratel~
arranged by selection of suitable acryloyl unsaturated materials, photoinitiators and control of radiation intensit~
to reduce the rate of photoinitiation to cause adhesiveness 20 and tackiness in the outer surface of the photopolymerised transferable layer by formation of soft or tacky low ~olecular weight poly~er species. By this ~eans an extra adhesi~e layer is avoided and of course this 'self-adhesive' surface is in perfect re~ister with the transferable layer.
~uch surface adhesiveness is particularly easily - ~ , , 1~ 3 __ achieved by reverse exposure tha-t is by,passing radi~tion throug'h the carrier sheet rather than by the normal direct exposure. The adhesiveness of a self-adhesive la~er is increased ~hen the outer surface is produced so as to have a high gloss since this increases the contact area to receiving substrates.
Such sel~-adhesive trans~ers are particularly useful where an easily removable adhesive bond is required such as le~ter and symbol sheets for graphic artists arld 1~ for homa decor of walls and furniture.
Photopolymerised ink layers when p~e-released from the carrier sheet can be of sufficient stiff~ess to be ha dled and used like a piece of plastic film or label.
The transfer can be transferred to a substrate and moved 15 about on its surface into an exact position and later the transfer can be adhered or removed and reused if required.
~ he control of viscosit~ and tack of the liquid photopolymerisable inks can also be carried out bv applyin~
the inks at elevated te~perature or by the addition of a 20 minor proportion~ for example less than 20,aJ volatile organic solvent. When such solvent is used it should have a low evaporatio~ rate of less than 5 and preferably less than 1 with reference to n-but~l acetate as ~OO as determined on the Shell ~hin ~ilm Evaporometer at 9O~ evaporation point, 2~ 'rhis avoids screen clogging in screenprinting with ver~ fine . . .
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10984~13 screen ~eshes.
Transfer lettering and symbol sheets used by graphic artists and designers requires a black photopolymeris2ble ink which has a high optical density for exa~ple 2.0 or higher. Slo~ photopolymerisation is usually exhibited by such blacX inXs and reverse and direct exposure to radiation is advantageously used simultaneously or successively to ca~3e adequate photopolymerisation of the ink film, particularly at the carrier sheet interface.
The mos~ ef~icient photoinitiators and acceleratcrs are required which include benzil dimethyl ketal, and an intima~e mixture of benzophenone and 4.4' - aimethylamino-benzo~henone, prepared by melting the constituents together, -cooling and grinding, and thioxanthone derivatives such as 15 ~eth~l- or chlorothioxanthone. A tertiary amine is included such as 4- N-dimethylamino eth~lbenzoate and all these photoinitiators can also be used in admixture.
Carbon black pigments cause a particularly low rate of photopolymerisation and this can be overcome by replacing 20 all or part o~ carbon black with black metal oxide such as iron oxide, very finely divided metal powders such as aluminium powder and a mixture o~ coloured pigments which do not substantially reduce the rate of photopolymerisation such as ultramarine blue pigment and yellow and magenta 25 pigments which ha~e good transmission of the photopolymerisin radiation, ` - 25 - ... .
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Photopolymerised dr~ transfers o the invention can be used for decoration nnd marking of ceramics, vitreous enamels, ~lass and similar substrates b~ incorp-ora-tion of frits, po~dered glazes and inor~anic pigmen~s 5 in the photopolymeri~abl~ ink medium and after printing~
photo~oly~erisation and appIication of a pressure sensitive or other adhesive, the design la~er is transferred to the substrate which is fired to burn away the organic constit-uents and fuse the frits, glazes and pi~ments onto or into 10 the substrate.
Anatase and rutile titanium dioxide pi~ments also reduce the rate of photopolymerisation when used in hi~h concentration and all or part of these are re~laced b~ zinc sulphide, barium sulphate~ lithopone or antimon~ o~ide 15 ~igments. Photoinitiators effective in white in~s include benzil dimethyl ketal and homologues, and benzoyl derivatives of diphenyl sulphide, dimethylanthraquinone, chlorinated ketones and thioxanthone derivatives in low concentra~ion -. ?
to-avoid yellowing.
~he effect of pigments on rate o~ photopolymerisation s most pronounced when these absorb the actinic radiation such as ultra violet radiation. When accelerated electron ea~ radiation is used the ef~ect o~ pigments is minimal.
The following ~xamples, in which parts are ~ wei~ht, re given to illustrate the invention and the manner in ,.

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10"8483 . which it may be carried into effect:-Exam~le 1 The following black photopolymerisable screen ink was printed throu~h 2 pIzin weave monofila~ent polyamide 5 mesh having 180 meshes per centi~etre and a filament diameter of 30 micrometrast using an indirect photostencil: :
~n~redients Parts 1. Urethane acrylate prepolymer 36 2. 2-Phenoxyethyl acrylate 9 l~ 3. Tripropylene glycol diacrylate 15 4. ~rim~thylolpropane triacr~late 8 . 5. Benzophenone 4 .. . 6. 4,4'-dimethyla~inobenzophenone 5 . . 7. ~enzildimethyl ketal 3.85 lS 8. Black iron oxide 10 9. Ultramarine Blue 14 ., Item 1 is a highly viscous prepol~mer ~vin~ an average o~ three aceyloyl groups per ~olecule and i9 Drepared from hexamethylene di-isocyanate, and a linear aliphatic polyester with free hydroxyl groups and hydro-. . ~7propylacrylate as described. ~his prepolymer is dissol~ed in monomers 2 and 3, and mono~er l~ is added to the finished '.' .
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1~ 3 ink progressively until the required le~el of release properties are obtained. Items 5, 6 and 7 are photo-initiators and Items 8 and 9 give a blue-toned black print.
A design was ~rinted consisting of alphabets of S let~ering plus à revolving power chart and printing carried out on blown high density polyethylene film which as translucent with a semi-gloss finish and a thickness of 100 micrometres. Photopolymerisation was carried out by exposure to two tubular medium pressure mercury vapour 10 lamps at 80 watts per centimetre and housed in elliptical polished aluminiu~ reflectors and the printed sheets were conveyed through the focussed radiation at 30 metres per minute.
Excellent print quality was obtained with a resolving 15 power of 10 line pairs per millimetre and the design la~er is physically released by applying strokes of a ball-pen or similar stylus with a force of 100 grams as shQwn by lightening of colour. 'he design layer had a thickness o~ 12 micrometres, elongation at breakpoint of 4-5~ and 20 good optical density, Exam~le 2 . I
A black photopolymerisable ink has the following composition and was prepared by dispersion on a triple roll mill:

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. .. . --, lQ'~ 3 _ Ingredients Parts . 1. Urethane acrylate prepol~mer 40 2. Di-acrylate ester of di-hydroxy-ethyl ether of bisphenol A36 S ~. Monoacrylate ester of mono-hydrox-. yethyl ether of bisphenol A 8 4. Carbon black 3.8 5. ~enzil dimetheyl ketal 4 6. ~enzophenone 5.7 7. Methylthioxanthone 0.5 8. 4-Dimethylaminoethylbenzoate2 .. 100 ~his was screenprinted through a 140 ~es~ per IS centi~etre monofilament polyester mesh onto an extruded ~ilm of polystyrene-butadiene of 120 micrometres thic~ness and photopolymerised as in ~xample 1 to give a high optical density black print with a thickness of 16 micrometres . which is physically released by light s~ylus action~ The liquid ink is based on monomers 2 and 3 of hi~h molecular weight having extremely low volatility and very low skin irritancy and are essentially non-toxic.

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109~4i 3~1 . ~xamnle 3 A white photopoly~erisable screen ink has the following co~position and was prepared by dispersion on a triple roll mill:
Ingredients Parts . Urethane acrylate prepoly~er ~5 2-Phenoxyethyl acrylate 9 ~ripropylene glycol diacrylate 16 Benzophenone 4 BenziIdi~ethylketal 4 Anatase titanium dioxide 15 ~ithopone 17 lS This was printed as in Example 2 and photopolymerisea as in Exa~ple 1 to give a design la~er which is readily physically released by light stylus action.
Exam~le 4 ~ The following low-tack pressure sensitive adhesive 20 was screenprinted using 120 mesh per centimetre screen on any of the photopol~merised inks o~ Examples 1-3 so as to overlap the inks on the adjacent carrier sheet:

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34~3 In~redients Parts Polyvinyl isobutyl ether 10 Polyvinyl ethyl ether high m.wt 3 Polyvinyl octadecyl ether 2 Aerogel silica 10-12 milli~icron 5 ~thylene glycol ~ono isopropyl ether10 Aliphatic hydrocarbon solvent 70 A~ter drylng for 25 seconds on a conveyorised drier having air jets impinging on the sheet at a te~perature of 60C, the dry sheets were cooled and interleaved with silicone coated release paper.
The ink design could be readily transferred to ~arious receiving surfaces such as paper, plastic, glass, metal etc,, by placing the adhesive surface of the design ;`
in contact with the receiving surface and applyin~ strokes of a st~lus to the carrier sheet over the design, Release of the design was clearly visible by 20 ightenin~ of colour and the carrier sheet could then be peeled away to leave the design trans~erred and adhered to the receiving substrate and the overlap part of the adhesive remained on the carrier sheet by shear of the adhesive around the design edges.

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~ss4~a -~xample 5 The carrier sheet of ~xample 2, having a semi-matt extruded finish was printed at 4000 sheets per hour on a

4~colour offset litho press using the following four S photopoly~erisable process inks. The-inks were photo-polymerised at the delivery of the press by exposure to ultra violet radiation from two medium press~lre mercury vapour lamps:
Yellow In~redients Parts Colour Index Pigment Yellow 13 15 Acrylated epoxy prepolymer 20 Pentaerythritol triacrylate . phenyl carbamate 60 Benzil dimethyl acetal 3.5 2~2-Diethoxyaceophenone 1.5 __ .

~ he yellow pigment is dispersed in the mixture of the ethylenically unsatured material on a triple roll mill and the photoinitiators are added as a dispersio~
in the remai~der of the material in subdued light.
Ma,~neta ~ his was prepared in the same way as the yellow ink but replacing the yellow pigment with 18 parts of Colour Index Pigment Red 57.

4~

. ~
~ his was prepared in the sa~e way as the yello~l in~
except that the yellow pigment was replaced with 16 parts of Colour Index Pigment ~lue 15. .
S Black . ,.
~ his was prepared in the same way as the yellow ink except that the yellow pi~ment was replaced with 18 parts of carbon black and 1 part of Colour Index Pig~ent 31ue 15.
~he ink5 were printed in the above sequence and 10 tack-graded by addition of a small quantity of trimethylol prop~ne tri-acrylate and photopolymerised as in Example 1.
The colour designs were overprinted by screenprinting ..
using a 77 ~esh per centimetre screen with the following clear photopoly~erisable screen ink to overlap all the colour (printing) and which ~-as cured by exposure at 30 metres per minute to ultra violet radiation from two tubular ~edium pressure mercury vapour lamps operated at 80 watts per centimetre to give a cross-linked layer of high Young.'s modulus and a thickness of 25 micro~etres;
20 Clear Ink In~redients Parts 1. Acrylated urethane prepolymer 52 '.
2, 2-Phenoxyethyl acrylate ~6 3. Tripropylene glycol diacrylate 15 4. Benzophenone

5. Benzil dimethylacetal . 3 .. , :.:

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loqs4a3 . Thc low molecular weight monomers (2 and 3) can be replaced by the high molecular weight monomers (2 and 3) of Example 2.
~ tylus action caused ph~sical release of the clear photopolymerised layer which carried with it the entire . litho printed colour desi~n.
Various adhesives were overprinted on the clear pho~opolymerised layer i~cluding a high-tack pressure sensitive adhesive based on crepe rubber tackified with resin ester gum, a spirit-fix adhesive based on an oil modified pol~amide resin and a heat-fix adhesive based on pol~vinyl acetate.
Example 6 ~he transfer sheets of Example 5 with photopoly~er-lS ised 4-colour hal~-tone litho printing and photopol~merise~

overall clear screen printed layer but without adhesive ~I
were overprin~ed with the white photopolymerisable screen ;.
ink o~ Example ~ which was printed so as to underlap the . clear layer by 1 mm all round (i,e. fall short of the extent of the clear layer b~ 1mm) and photopoly~erised as in Example 1.
The following photopolymerisable ~ressure sensitive adhesive was overprinted using the same stencil as used for the clear layer and photopolymerised as in Example 1.
.

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_ InGredients Parts Ac~ylated urethane prepolymer 33 Mellitic anhydride-diethylenegl~-ycol polyester 23 2-Phenoxyethyl acrylate 16 Tripropylene ~lycol diacry]ate 19 Benzophenone 4 Benzil dimethyl ketal 4 The mellitic anhydri.de-diethylene glycol polyester is a solid saturated polyester with high acid value which is dissolved in the liquid monomers and a little 2-butoxy- 1 ethanol solvent is added to adjust ViscQsity. The -`
adhesive photopolymerises to a layer with a pressure IS sensitive adhesive surface and light stylus action causes transfer of all the layers simultaneously and the colour design has good colour contrast even on dark recei~ing surfaces.
Example 7 ~he photopolymerisable litho inks of Example 5 can be replaced by the following conventional litho inXs which dry by oxidation and after these inks are thorou~hly dry the photopolymerisable clear screen ink of Exa~ple 5 is overprinted and photopolymerised as in ~xample 5.

`' lOq8483 - -Yell.ow In~redien~,s Parts Colour Index Pi~ent Yello-rr 13 14~ong oil linseed alkyd 35 Phenolic modi~ied wood oil alkgd 35 Distillate b.pt. 225-26~C 13.5 12~ cobalt octoate (drier) 10~ manganese siccatol (drier~ 1 Methyl ethyl ketoxime (antioxidant) 0.5 100 .0 . - . .
~he yellow pigment was dispersed in the long oil linseed alkyd on a hydraulic tri~le roll mill to a value gauge of 6 on a Hegman/. ~he ink was finally thinned with 15-20,~ of distillate to give an in~ viscosity of 15 poises.

IS Ma~neta This was prepared similarl~ by replacing the yelloN
pigment ~ith 18 parts of Colour Index Pigment Red 57.
Cyan . ~ . .
~ his was prepared in the same manner as the yellow ink by replacing the yellow pigment with 16 parts of Colour Index ~igment Blue 15.

Bl2ck -;~
Carbon black 18 parts, toned with one part of Colour Index Pig~ent Blue 15 was used as the pigment in place of the yellow pigment in the above formulation to ~5 produce a black ink .

~Q~84B3 The use of the transfers according to the inver.-tion . is illustrated b~ the accompan~ing drawings in which:-Figure 1 is magnified section through one e~bodiment of transfer sheet;
Figure 2 is a similar section through a second embodiment of transfer sheet;
Figure 3 is magnified sectional view of a third e~bodiment showing the release of the design on applic~tion of stylus pressure to the carrier sheet, ~igure 4 is a ~agnified view of the embodiment of Figure 1 sho~ing the effect of pulling the carrier sheet around a sma~ dia~eter rod. -Referring to Figure 1, a carrier sheet 1 is printed -with a plurality of designs 2 (only one of which is shown~.
IS Because the design layer is pre-stressed (by shrinkage during photopolymerisation of the photopol~erised i~
and/or by shrinkage during drying or curing of the non-photopolymerised ink component), the bond 3 between the carrier sheet 1 and the design 2 is already weakened. ~hus release of the design la~er on application of an external force is faci~tated since a smaller disrupting force is necessary than would otherwise be required to break the bond.
~igure 2 shows a similar transfer to that shown in Figure 1, the difference being that the desi~n la~er ~ 37 -~ . . .
, ' lOq848~

consists of a stress-resisting photo~o]ymerised white backing layer 2 which increases the contrast of the hal~-tone colour dots 4 on coloured receiving substra~es.
The colour dots were applied to the carrier sheet prior to the la~er 2.
~ igure 3 shows a ~odification o~ the transfer ho~
in Flgure 1 and the mechanism of release. A photopolymer-ised ink design layer 2 is printed onto a carrier sheet 1.
The difference between this embodiment and that shol~n in 1~ ~igure 1 is that the e~bodir3ent includes an adhesive layer 3 which overlaps the edges of the de~ign layer 2. Release of the design layer may be achievsd by Application of local pressure 6 to the back of the carrier in the region of the design with a stylus 79 Because of pre-stressing lS of the design layer 2, the adhesive bond at 8 is weaXened before application of the external force and the release initiated b~ the stylus pressure extends over a wider area than the stylus tip diameter. Air has entered at 9 between the design layer and the carrier sheet, ~igure ~ illustrates the manner in which a design la~er of the kind shown in Fi~ure 1 could be released from a carrier sheet. ~he carrier sheet 1 is conducted around a small radius rod 7. In order that release can be achieved in this way, a design layer of considerable thickness and high Young's ~odulus would be required so that it would resist the bonding force 6 aDplied by conductinga the sheet around rod 7, _ 38 -:: , . . :
.:

Claims (36)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A dry release transfer which comprises a carrier substrate and a flexible design layer releasably adhered thereto, said design layer comprising a flexible solid polymer produced by photopolymerisation of a viscous liquid ink comprising photo-polymerisable, ethylenically unsaturated material, said photo-polymerisation having been effected by exposure of the entire liquid ink layer to actinic radiation or to electron beam radiation whereby the liquid ink layer is converted rapidly to the solid state.

2. A transfer according to claim 1 wherein the poly-merisable, ethylenically unsaturated material contains acryloyl or methacryloyl groups.

3. A transfer according to claim 2 wherein the liquid ink is a photopolymerisable composition comprising prepolymers and/
or monomers containing ethylenically unsaturated groups, at least some of said groups being pendant or terminal acryloyl or metha-cryloyl groups.

4. A transfer according to claim 2 wherein the liquid ink contains at least one photopolymerisable component containing at least 2 acryloyl or methacryloyl groups per molecule, whereby the design layer produced on photopolymerisation is cross-linked.

5. A transfer according to claim 3 wherein the liquid ink contains at least one photopolymerisable component containing at least 2 acryloyl or methacryloyl groups per molecule, whereby the design layer produced on photopolymerisation is cross-linked.

6. A transfer according to claim 4 or 5 wherein the liquid ink contains a photopolymerisable monomer component which contains about 2 acryloyl groups per molecule.

7. A transfer according to claim 2 wherein the photo-polymerisable, ethylenically unsaturated material is a blend of monomer and prepolymer, wherein a substantial proportion of the monomer has about 2 acryloyl groups per molecule and the pre-polymer has about 2 to 6 acryloyl groups per molecule, whereby on photopolymerisation a cross-linked and flexible design layer is rapidly produced.

8. A transfer according to claim 1 wherein the flexible design layer adjacent to the carrier substrate possesses stress-resisting properties which resist deformation by a mechanical disrupting force, such as a stylus, applied to the back surface of the carrier substrate in the region of the design layer, whereby the design layer is physically released from the carrier sheet without fracture of the design layer.

9. A transfer according to claim 1 wherein the design layer consists of a plurality of layers at least one of which is photopolymerised.

10. A transfer according to claim 9 wherein all of the individual layers making up the design layer are photopoly-merised.

11. A transfer according to claim 9 or claim 10 wherein at least one of the photopolymerised layers is cross-linked.

12. A transfer according to claim 1 in which said liquid ink contains a photoinitiator and is photopolymerised by exposure to ultra-violet light.

13. A transfer according to claim 1 in which the design layer is coloured by pigments or dyes which do not inhibit fast photopolymerisation when exposed to actinic radiation.

14. A transfer according to claim 13 wherein the pigment includes one or both of zinc sulfide and barium sulfate.

15. A transfer according to claim 1 wherein the design layer is from 8 to 50 micrometres thick.

16. A transfer according to claim 1 wherein the design layer is pre-stressed by photopolymerisation.

17. A transfer according to claim 16 wherein pre-stressing of the ink layer arises by cross-linking during the photopolymerisation.

18. A transfer according to claim 1 wherein the design layer is pre-stressed by chemical treatment of the design layer after photopolymerisation.

19. A transfer according to claim 1 wherein the design layer bears a superficial layer of pressure-sensitive adhesive.

20. A dry release transfer which comprises:
(a) a flexible light transmitting carrier sheet, (b) a design layer releasably adhered thereto, said design layer comprising a flexible, solid cross-linked polymer produced by photopolymerisation of a viscous liquid ink containing not more than 20% of volatile solvent printed on the carrier sheet, (c) said ink prior to photopolymerisation comprising one or more ethylenically unsaturated monomers and/or prepolymers, (d) said photopolymerisation having been effected by exposure of the entire liquid ink layer to actinic radiation or to electron beam radiation whereby the liquid layer is rapidly converted to a flexible, cross-linked solid design layer, said design layer possessing stress-resisting properties which resist deformation by a mechanical disrupting force, such as a stylus, applied to the carrier sheet and enables the design layer to be released from the carrier sheet without fracturing the design layer.

21. A transfer according to claim 20 wherein at least one of the unsaturated monomers or prepolymers contains pendant or terminal acryloyl or methacryloyl groups.

22. A transfer according to claim 21 wherein the ink includes a prepolymer which is an acrylated or methacrylated urethane prepolymer which contains about 2 to 6 acryloyl or metha-cryloyl groups per molecule.

23. A transfer according to claim 21 wherein the ink contains a monomer which is a mono- or polyacrylate ester.

24. A transfer according to claim 22 wherein the ink contains a monomer which is a mono- or polyacrylate ester.

25. A transfer according to any one of claims 21 to 23 wherein the density of cross-linking in the photopolymerised design layer is such that physical pre-stressing of adhesive bonds between the design layer and the carrier sheet occurs and enabling the design layer to be released more readily by application of stylus pressure to the carrier sheet in the region of the design.

26. A transfer according to claim 24 wherein the density of cross-linking in the photopolymerised design layer is such that physical pre-stressing of adhesive bonds between the design layer and the carrier sheet occurs and enabling the design layer to be released more readily by application of stylus pressure to the carrier sheet in the region of the design.

27. A method of producing a dry release transfer which comprises the following steps:
(a) printing a design layer on a carrier sheet using at least one liquid photopolymerisable ink which contains not more than 20% by weight of volatile solvent, said ink being a fast polymerisable ink and comprising one or more ethylenically unsaturated monomers or prepolymers, and (b) exposing the printed sheets to actinic radiation or to electron beam radiation to cause photopolymerisation of the design layer.

28. A method according to claim 27 wherein the outer surface of the design layer and the surface in contact with the carrier sheet are simultaneously exposed to actinic radiation.

29. A method according to claim 27 wherein the surface of the design layer in contact with the carrier sheet is exposed to actinic radiation through the carrier sheet, with or without simultaneous exposure of the outer surface of the design layer.

30. A method according to any one of claims 27 to 29 wherein the printed sheets are passed under a stationary source of actinic radiation.

31. A method according to claim 27 wherein a plurality of photopolymerisable inks are printed sequentially on the carrier sheet to form a multilayer design, each ink being at least partially cured before application of the succeeding ink.

32. A method according to claim 27 wherein access to the ink by atmospheric oxygen is restricted during photopoly-merisation.

33. A method according to claim 27 wherein the photo-polymerisable ink comprises a blend of a high viscosity liquid or solid photopolymerisable prepolymer and a low viscosity liquid monomer or low molecular weight prepolymer.

34. A method according to claim 33 wherein the high viscosity liquid or solid prepolymer is an acrylated urethane prepolymer having a molecular weight of between 250 and 5000.

35. A method according to claim 33 or 34 in which the low viscosity liquid monomer or prepolymer is an ester of acrylic or methacrylic acid and a mono- or polyhydric alcohol.

36. A method according to claim 27 wherein the design layer is printed in step (a) at a thickness of from 8 to 50 micrometres.