GB2142576A - Manufacture of 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 polymerisable or crosslinkable organic material and a thermally activated initiator therefor. The printed indicium or indicia heated to cure the ink and then, if appropriate, adhesive is applied thereover.

Description

SPECIFICATION Manufacture of dry transfer materials This invention relates to a method of manufacturing dry transfer materials.

Dry transfer materials 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 inidicia and a pressure-sensitive adhesive which may be applied over the whole surface or in register with the indicia.

Transfer of the inidicia from the 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.

Heretofore in the production of dry transfer materials, indicia may 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.

Thus in this conventional production method there is a drying stage for evaporation of solvent, which delays and can add to the expense of production. Further 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.

British Patent Specification 1580076 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 wherby the ethylenically unsaturated material is caused to photopolymerise.

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 production method of British Patent Specification 1580076 can require the use of special equipment to provide the required radiation to photopolymerise the ink, e.g. a U.V. light or electron beam source and this may involve an expensive purchase.

According to the present invention, there is provided a method of manufacturing a dry transfer material which method comprises applying to a carrier sheet one or more indicia in a non-solvent drying, polymerisable or crosslinkable ink comprising a polymerisable or crosslinkable organic material and a thermally activated initiator therefor, heating the indicium or indicia applied to cause activation of the initiator and thus polymerisation or cross-linking in the ink, and, if necessary or desired, applying adhesive to cover at least the indicium or indicia.

The method of the present invention enables dry transfer materials having good scratch and abrasion resistance and high film strength to be produced in large volume at high speed using heating to initiate the crosslinking or polymerisation of the ink. The ink used according to the present invention is non-solvent drying so there is no need for a separate ink drying step in the manufacturing method.

The carrier sheet used according to the present invention may be of any conventional 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. of 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 precentage 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 conveniently 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 inkwill 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 ink used according to the present invention contains a polymerisable or crosslinkable organic material and thermally activated initiator therefor. The polymerisable or crosslinkable organic material may be any suitable monomeric, oligomeric or polymeric material which can be polymerised or crosslinked by a free radical mechanism. Acrylates may suitably be used.

The initiator used is one which is relatively stable at room temperature but which, under the influence of heat, decomposes with the generation of free radicals e.g. a peroxide or hydroperoxide. For economical reasons it is naturally desired that the decomposition temperature is not too high. In addition of course there is a danger of damage to the dry transfer material itself if very high temperatures are used. Thus while on the one hand the decomposition temperature of any peroxide used should preferably not be too low to give the ink sufficient shelf life it should also not be impractically high. It may be desirable to include an accelerator e.g. an amine for the decomposition of the peroxide effectively to reduce the amount of heat required.

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

The indicia applied to the carrier sheet are heated to cause polymerisation or crosslinking in the ink. The heat may be externally applied or may be generated internally in the ink by exothermic reaction.

The final inidicia 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 inidicia. 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 Urethane acrylate (Lankro 614, ex Diamond Shamrock 56.9 Tri-functional acrylate (Oligomer OTA 40, ex U.C.B.) 23.7 Mono-functional acrylate (Synacure 3165, ex Cray Valley products) 14.6 Cumene hydroperoxide 4.8 N,N-dimethyl-p-toluidine 0.1 This ink was then applied, by screen printing using a 95S mesh, on to matt polyethylene terephthalate (Melinex ex l.C.I.) under subdued lighting and then placed in an oven at 1 10'C and kept in the dark for 11/2 hours. The printed inidicia had cured to a solid tack-free film form which was releasable from the polyethylene terephthalate carrier.

An adhesive consisting of: Parts by weight Polybutene (NAMW 0400) 6.9 Polyisobutylene (VAMW 380,000) 3.2 Oleophilic grade silica 4.0 Aliphatic hydrocarbon solvent (boiling range 144 to 160 C) 71.0 Oxitol 9.0 was applied at the rate of 2 g.s.m. as an overall layer over the cured indicia. [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 as follows: Parts by weight Urethane acrylate (Lankro 614) 100 N,N-dimethyl aniline 0.2 Benzoyl peroxide/dibutyl phthalate in 50:50 by weight mixture 1.0 Trimethylol propane triacrylate 30.0 This ink was then applied, by screen printing using a 95S mesh, on to untreated high density polyethylene.

The printed indicia cured fairly rapidly at room temperature; a significant exothermic reaction was observed to occur.

Adhesive as used in Example 1 was applied at the rate of 2 g.s.m. as an overall layer over the cured indicia.

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

Claims (5)

1. A method of manufacturing a dry transfer material which method comprises applying to a carrier sheet one or more indicia in a non-solvent drying polymerisable or crosslinkable ink comprising a polymerisable or crosslinkable organic material and a thermally activated initiator therefor, heating the indicium or indicia applied to cause activation of the initiator and thus polymerisation or crosslinking in the ink, and if necessary or desired, applying adhesive to cover at least the indicium or indicia.

2. A method according to claim 1 wherein the polymerisable of crosslinkable organic material is an acrylate.

3. A method according to claim 1 or claim 2 wherein the initiator is a peroxide or hydroperoxide.

4. A method for manufacturing a dry transfer material substantially as described in Example 1 or 2.

5. A dry transfer material obtained by the method claimed in any one of claims 1 to 4.