GB2142279A - Dry transfer materials - Google Patents

Abstract

Dry transfer materials are obtained by printing on to a carrier sheet a non-solvent drying, polymerisable or crosslinkable ink comprising a cationically polymerisable or crosslinkable organic material and a cationic initiator therefor. The printed indicium or indicia exposed to radiation to cure the ink and then, if appropriate, adhesive is applied thereover.

Description

SPECIFICATION Dry transfer materials This invention relates to dry transfer materials and to a method of manufacturing them.

Dry transfer systems have been known for many years. Usually the dry transfer material consists of a carrier film, e.g. a polymer film or a release coated paper, on which are printed, usually screen printed, one or several indicia and a pressure-sensitive adhesive which may be applied over the whole surface or in register with the indicia.

Transfer of the indicia from the dry transfer material to the chosen receptor surface may be promoted either by a "tug-of-war" process in which the adhesion of the indicia to the carrier is lower than its adhesion to the receptor surface or by dynamic pressure as described in British Patent Specifications 954459 and 959670 in which the indicia-to-carrier adhesion is weakened by the "stretch-release" phenomenon.

An important problem in the production of dry transfer materials is the provision of satisfactory indicia. The indicia necessarily must transfer bodily and so have adequate film strength. It is also desirable that the indicia should be highly resistant to scratching and abrasion. Further of course the indicia must readily release from the carrier sheet.

Heretofore in the production of dry transfer materials, indicia have generally been formed of printing inks which are essentially pigmented solutions of a polymer in an evaporable solvent, the dry indicia being formed simply by evaporation of the solvent from the ink layer applied, e.g. by screen printing, to the carrier. Such inks are well known but it is found in practice that the dried ink films obtained generally have a lower mechanical strength than is desirable, so that the distortion of the indicia (e.g. by bending the carrier sheet on which the indicia are printed) may cause the ink film to crack, and the scratch and abrasion resistance is usually less than desired.Some improvement in film strength can often be achieved by increasing film thickness but this usually brings about concomitant disadvantages, e.g. a tendency to a lower degree of definition in the print using such conventional printing inks.

In order to improve scratch and abrasion resistance, British Patent Specification 1 324796 proposes transfer materials in which the indicia are formed of or include a urethane polymer formed in situ. These transfer materials are manufactured by applying to a carrier sheet one or more indicia in a printing ink which comprises solvent and a precursor for a urethane polymer, preferably a moisture-cure urethane polymer. The ink is caused or allowed to dry and then the urethane polymer is caused or allowed to form in situ thereby to produce a dry and non-tacky surface to the indicia. Thereafter a coating of an adhesive is applied to cover at least the indicia.

Further British Patent Specification 1 437326 proposes transfer materials wherein the indicia have high scratch and abrasion resistance in which the indicia are formed of or include epoxy polymer formed in situ. Thus these transfer materials of British Patent Specification 1437326 are made by applying to a carrier sheet one or more indicia in a printing ink which comprises solvent and a precursor for an epoxy polymer, allowing the ink to dry and the formation of the epoxy polymer in situ thereby to produce a dry and non-tacky surface to the indicia and thereafter applying a coating of an adhesive to cover at least the indicia.

While the transfer materials of British Patent Specifications 1 324796 and 1437326 have much improved scratch and abrasion resistance compared with conventional dry transfer materials and can have acceptable film strength, their manufacturing time is of the same order as that of conventional dry transfer materials. Indeed it may even be longer since, in addition to solvent evaporation to dry the ink, curing of the urethane polymer or epoxy resin is also necessary.

British Patent Specification 1 580076 describes the production of dry transfer materials by printing using a viscous liquid ink comprising photopolymerisable, ethylenically unsaturated material. The printed ink is then exposed to active radiation whereby the ethylenically unsaturated material is caused to photopolymerise by a free radical mechanism. With this process there is substantially no need for a separate drying stage and accordingly dry transfer production can be speeded up. Furthermore the photopolymerised final product can have good scratch and abrasion resistance. However the free radical systems to which this reference relates are subject to oxygen inhibition.This can have an important and disadvantageous effect in the case of such thin films as are applied in the printing step of British Patent Specification 1 580076 which have a high availability to oxygen. Moreover, though substantially quicker than other processes of dry transfer production, the free radical system of British Patent Specification 1 580076 requires comparatively long exposure times to ensure sufficient photopolymerisation of inks.

In addition U.S. Patent Specification 429114 describes the preparation of dry transfer sheets using a composite material comprising a cover sheet and a base sheet having therebetween a photopolymerisable layer comprising a cationically polymerisable organic material, a cationic polymerisation initiator therefor and, optionally, a thermoplastic hydrocarbon resin. This compo site is imagewise exposed to radiation whereby, in the image areas, the photopolymerisable layer is caused to polymerise cationically. The material is developed by peeling apart the two sheets with the unexposed areas adhering to the cover sheet and the exposed areas adhering to the base sheet. The cover sheet is then re-exposed to provide the final dry transfer material.

While the method of U.S. Patent Specification 4291114 may be useful for providing dry transfer materials at small volume, it is totally unsuitable and too complex a system for the large volume production of conventional sheet transfer materials. Moreover the resolution obtained with the process of U.S. Patent Specification 4291114 is insufficient for many uses.

According to the present invention, there is provided a dry transfer material which comprises a carrier sheet and one or more indicia thereon, the indicium or indicia having been applied to the carrier sheet by printing in a non-solvent drying, polymerisable or crosslinkable ink comprising a cationically polymerisable or crosslinkable organic material and a cationic initiator therefor and exposing the printed indicium or indicia to radiation to cause polymerisation or crosslinking in the ink, the indicium or indicia having an adhesive surface or the dry transfer material further comprising a layer of adhesive covering at least the indicium or indicia.

The present invention also provides a method of manufacturing a dry transfer material which method comprises applying to a carrier sheet by printing one or more indicia in a non-solvent drying, polymerisable or crosslinkable ink comprising a cationically polymerisable or crosslinkable organic material and a cationic initiator therefor, exposing the printed indicium or indicia to radiation to cause polymerisation or crosslinking in the ink, and, if necessary or desired, applying adhesive to cover at least the indicium or indicia.

The dry transfer material according to the present invention has good scratch and abrasion resistance and high film strength. Further the dry transfer materials are suitable to be produced in fine detail and can be produced in large volume at high speed. Not only is the ink used according to the present invention non-solvent drying so there is no need for a separate ink drying step in the manufacturing method but also the inks used according to the present invention are not susceptible to oxygen inhibition and have dark reaction. This means that only very short exposure times are required according to the present invention to obtain polymerisation or crosslinking in the ink. This reaction is not subject to inhibition by oxygen in the environment and further the reaction continues in the ink film after the exposure.Materials according to the present invention can for example be rolled up when touch dry and the curing reaction will continue in the roll. If desired this dark reaction may be further speeded up by heating the material after the exposure step.

The carrier sheet used according to the present invention may be of any convenient flexible material. A wide variety of such carrier sheets is already known and used in the manufacture of various dry transfer materials. Papers, including treated papers e.g. polymer coated or release coated papers, may be used. However, to obtain accurate positioning of the dry transfer material on the receptor surface, it is desirable that the carrier sheet should be light transmitting i.e.

transparent or translucent, so that the receptor surface is visible through the dry transfer at the moment of transfer. Suitable such carrier sheets include transparent or translucent plastics films e.g. polyethylene, polystyrene or polyethylene terephthalate. Again of course if desired such plastics film may be surface treated or coated appropriately to provide the required release properties.

The ink used according to the present invention is-non-solvent drying. Thus the ink is one which is not reliant on solvent evaporation for its drying. The ink may contain a small percentage of solvent, generally at most 20% by weight. Indeed some small solvent content may be inevitable since the ink ingredients may be provided commercially in the presence of solvent. Furthermore it may be desirable to add solvent to obtain the desired viscosity for printing. Any solvent used in substantial amount though will be non-volatile. - Residual solvent may be retained in the final indicia and act as a plasticiser.

The indicia are printed on to the carrier sheet using a non-solvent drying ink. Any suitable printing method can be used e.g. letterpress, lithography, gravure. Generally the ink will be applied by screen printing in conventional manner. However, owing to the fact that ink solvent is not evaporated from the system, in contrast to current methods, it is desirable that the deposit of ink is less. This may be achieved by the use of a fine screen mesh.

The inks used according to the present invention may also contain, in addition to the cationically polymerisable or crosslinkable organic material and cationic initiator therefor, pigment or dye, polymeric binder and other conventional additives.

Preferably the ink according to the present invention contains, as cationically polymerisable or crosslinking organic material, a monomeric, oligomeric or polymeric epoxy material containing one or more epoxy functional groups e.g. a bis-phenol-A(4,4'-isopropylidenediphenol) and epichlorohydrin reaction product or the reaction product of a low molecular weight phenolformaldehyde resin with epichlorohydrin, alone or in combination with an epoxy containing compound as a reactive diluent. Other polymeric materials containing terminal or pendant epoxy groups which are cationically curable may also be used e.g. epoxy silane resins, epoxy polyurethanes and epoxy-polyesters having terminal epoxy groups. Suitable epoxy materials are mentioned in U.S. Patent Specification 4108747.Most preferably the epoxy material used is a liquid epoxy resin e.g. an epoxy novolac or a cycloaliphatic epoxy resin or a cycloaliphatic epoxide.

Other cationically polymerisable or crosslinkable materials may also be used including vinyl organic monomers, vinyl organic prepolymers, cyclic organic ethers, cyclic organic sulphides, cyclic amines and organo silican cyclics as described in U.S. Patent Specification 4108747 and thermosetting formaldehyde condensation resins comprising thermosetting organic condensation resins of formaldehyde and urea, thiourea, phenol or melamine as described in U.S. Patent Specification 4102687.

The cationic initiator used according to the present invention can be for example a triarylsulphonium or diarylsulphonium salt e.g. triphenylsulphonium hexafluorophosphate, or an aryldiazonium, diaryliodonium or triarylselenonium compound. Also suitable as cationic photoinitiators are Brnsted acids including HBF4, HAsF6, HSbF6 which are active initiators for the ring opening polymerisations of epoxides. Cationic initiators which may be used are described in the aforesaid U.S. Patent Specifications 4108747 and 4102687.

For example when triarylsulphonium salts (Ar3S+X-) are exposed to radiation of 200-300 no they undergo photolysis which results in a homolytic rupture of one of the carbon-sulphur bonds. The mechanism for this photolysis is believed to be as follows:

Interaction of the radicalcation Ar2S + with the hydrogen-containing compound YH in Step c results in the release of a proton in Step d and the formation of the acid HX.

The printed indicia are exposed to radiation to cause polymerisation or crosslinking in the ink.

Most preferably UV radiation is used to cure the ink in this way. However other forms of radiation, including electron beam, can be used to initiate reaction.

The final indicia must of course be transferable from the carrier sheet to a receptor surface.

Preferably the materials are so formulated that they operate by the "stretch-release" technique described in British Patent Specifications 954459 and 959670. In addition to ensure transferability it may be necessary to apply an adhesive coating to the cured indicia. Such an adhesive coating can be applied as an overall adhesive layer but will at least be applied over the indicia.

Alternatively the ink composition of the indicia may have adhesive ingredients in which case a separate coating may not be necessary.

The invention is further illustrated in the following Examples.

EXAMPLE 1 A dry transfer ink was made up as follows: Parts by weight Epoxy novolac resin (DEM 438 ex Dow Chemical) 23 Cyclo aliphatic epoxy resin (CY 179 cox Ciba-Geigy) 68 Carbon black (Printex 2/20- ex Degussa) 1.8 These ingredients were mixed in a triple roll mill and then there were added: Initiator (50% triphenylphos phonium hexafluorophosphate [(PH3S) +PH6--ex 3M and 50% aromatic solvent) 4.5 Levelling agent (Perenol F3 ex Henkel) 2.7 100.0 This ink was then applied, by screen printing using a 150T mesh, on to untreated high density polyethylene film and then cured using a Colordry U.V. unit operating with two U.V.

lamps rated at 79 watts per linear centimetre using a throughput speed of 6.1 metres per minute.

An adhesive consisting of the following ingredients: Parts by weight Polybutene (NAMW 0400) 6.9 Polyisobutylene (VAMW 380,000) 3.2 Polyethylene wax (NAMW 2000, melting point 104 to 108"C 4.9 Oleophilic grade silica 4.0 Aliphatic hydrocarbon solvent (boiling range 144 to 160"C) 71.0 Oxitol 9.0 Oleamide 1.5 was applied as an overall layer. [The abbreviation NAMW is used for number average molecular weight and VAMW for viscosity average molecular weight.] There was thus obtained a dry transfer material from which the indicia transferred well on to a receptor surface.

EXAMPLE 2 A dry transfer ink was made up using the following ingredients.

Parts by weight 3,4-Epoxy-cyclohexyl-methyl 3,4-epoxy-cyclohexane carboxylate (ERL 4221-ex Union Carbide) 33.0 Liquid epoxy resin (CY 208 ex Ciba-Geigy) 49.6 Furnace black (Special Black 25 ex Degussaoil absorption 85%, pH 3.5, average particle size 56 nm) 6.6 Initiator (triarylsulphonium hexafluorophosphate dispersed 50/50 by weight in aromatic solvent-FC 508 ex 3M) 8.3 Flow aid 2.5 100.0 The flow aid used consisted of: Parts by weight Butyl polyacrylate (Acronal 4F ex B.A.S.F.) 25 2-Ethyl-hexyl acrylate, low molecular weight polymer (Modaflow ex Monsanto) 25 CY 179 50 This ink was then applied, by screen printing using a 150T mesh, on to untreated high density polyethylene film and then cured as in Example 1.

Adhesive as used in Example 1 was applied then as an overall layer.

There was thus obtained a dry transfer material from which the indicia transferred well on to a receptor surface.

Claims (8)

1, A dry transfer material which comprises a carrier sheet and one or more indicia thereon, the indicium or indicia having been applied to the carrier sheet by printing in a non-solvent drying, polymerisable or crosslinkable ink comprising a cationically polymerisable or crosslinkable organic material and a cationic initiator therefor and exposing the printed indicium or indicia to radiation to cause polymerisation or crosslinking in the ink, the indicium or indicia having an adhesive surface or the dry transfer material further comprising a layer of adhesive covering at least the indicium or indicia.

2. A dry transfer material according to claim 1 wherein the cationically polymerisable or crosslinkable organic material is a monomeric, oligomeric or polymeric epoxy material containing one or more epoxy functional groups.

3. A dry transfer material according to claim 1 wherein the cationically polymerisable or crosslinkable organic material is a liquid epoxy novolac or cycloaliphatic epoxy resin.

4. A dry transfer material according to any one of claims 1 to 3 wherein the cationic initiator is'a"'triarylsulphonium or diarylsulphonium salt.

5. A dry transfer material substantially as described in any one of the Examples.

6. A method of manufacturing a dry transfer material which method comprises applying to a carrier sheet by printing one or more indicia in a non-solvent drying polymerisable or crosslinkable ink comprising a cationically polymerisable or crosslinkable organic material and a cationic initiator therefor, exposing the printed indicium or indicia to radiation to cause polymerisation or crosslinking in the ink, and, if necessary or desired, applying adhesive to cover at least the indicium or indicia.

7. A method according to claim 6 wherein the printed indicium or indicia are exposed to U.V. radiation.

8. A dry transfer material obtained by the process claimed in claim 6 or 7.