IMPROVEMENTS IN OR RELATING TO PRINTING

Abstract: A water based screen printing ink that is at least partially curable by irradiation and comprises an emulsion of a resin dispersed in water.

Description: IMPROVEMENTS IN OR RELATING TO PRINTING This invention relates to printing and in particular to screen printing (commonly known as silk screen printing). As is well known, the silk screen printing process involves the use of a screen of silk, polyester, nylon, metal gauze or any other suitable mesh-like or perforated material. For convenience (and for consistency with the terminology historically employed in this art) the term "silk screen printing" will be used herein and it will be appreciated that no restriction to "silk" material is thereby intended. Although silk screen printing is a very old technique, it still has numerous problems and/or limitations associated with it. For example, where several large-size multi-colour sheets are to be silk screen printed for, say, a poster to be pasted up on a hoarding, large quantities of specialised, screen printing inks are needed which are expensive. The printing inks required for screen printing are quite different from those to be used for other forms of printing. The screen printing inks being generally used at present are organic solvent-based and usually contain ethyl or nitro cellulose resins. They are dried in warm air tunnels by driving off the organic solvents, and are thus potentially hazardous to the printing operatives. Another problem which arises with such organic solvent-based screen printing inks, relates to solvent loss from the ink on the printing screen whereby the ink increases in viscosity and after a certain period of time will not properly pass through the open areas of the printing mesh. This "drying in" results in loss of printed image and is described as screen stability, i.e. inks with poor screen stability "dry in" rapidly. Recent developments over the last ten years have seen the introduction of Ultra-Violet-curing screen process inks. SUBSTITUTE SHEET Here the formulations use raw materials which do not increase in their viscosity on the printing screen but instead dry by exposure to intense Ultra Violet light. This means that this type of ink has almost unlimited screen stability compared to non-UV curing, organic solvent-based, screen printing inks. However, such UV-curing inks are expensive, and leave a very thick print coat on the paper surface - particularly where multi-colour, e.g. 4-colour, silk screen printing is effected. Such thick or heavy print images on the surface of a sheet of poster paper often necessitate that for mounting the poster to its hoarding, a special adhesive - which is other than an inexpensive starch-based adhesive - must be used if efficient bonding is to be achieved. A similar problem often arises with the older type of organic solvent-based silk screen printing inks. This adhesion problem is common where the individual sheets overlap, e.g. at overlapping marginal edges of adjacent poster sheets. The back of the top sheet does not have sufficient adhesion to the ink surface printed on the sheet beneath. When the poster paste dries out, the top sheet of the overlap often curls up and away from the sheet underneath. The problem is particularly severe when the paper is completely covered with several layers of ink. Posters printed with offset litho inks have less tendency to curl when they dry out compared to those produced with screen printing inks. As the screen printing process achieves its high quality print appearance by a much higher ink deposit, the curl problem is pronounced with screen printing inks. Therefore the adhesives used with screen printed posters need to have better adhesion properties to prevent overlap curl. Mixtures of one or more starch adhesives and one or more polyvinyl alchol (P.V.A.) adhesives were investigated by the applicants, and these were found to give a much improved adhesion at the overlapping edges of superimposed sheets as compared to a 100% starch-based adhesive product. However, mixtures of starch and P.V.A. adhesives have the disadvantage of being more expensive and the resultant improved adhesion makes the removal of the total poster from the hoarding more difficult. Thus they are not likely to find favour with bill-posters, i.e. those responsible for puting up the posters on the hoardings. It is therefore considered desirable to reduce, if not wholly obviate, some or all of the above-mentioned and/or other disadvantages associated with or relating to prior art screen printing methods and inks. According to this-invention there is provided a printing ink for silk screen printing, characterised in that the screen printing ink is a water-based ink that is at least partially curable by irradiation and comprises an emulsion of a resin dispersed in water. The term "irradiation" as used herein in relation to curing refers to eloctro-magnetic radiations having wavelengths shorter than those of the visible spectrum, and the term thus includes UV (ultra-violet) and electron beam radiations. Preferably the emulsion has said resin wholly (or at least substantially) in dispersed phase and wholly (or at least substantially) insoluble in the water, the water being the continuous phase of the emulsion. Advantageously, said resin comprises 30% to 60% (preferably 50%) of the emulsion. Preferably said ' resin comprises an unsaturated acrylate resin, e.g. polyester acrylate resin. It is envisaged that an epoxy acrylate resin and/or a urethane acrylate resin might additionally or alternatively be employed. SUBSTITUTESHEET Preferably the water-based screen printing ink has a composition which comprises one or more photoinitiators and is at least partially curable by irradiation comprising electromagnetic radiations in the ultra-violet light waveband. Alternatively, or additionally, the screen printing ink may be at least partially curable by irradiation with an electron beam. Where the screen printing ink is curable solely by irradiation withTan electron beam, then photoinitiator(s) may be omitted from the printing ink composition. Preferably, and in either case, the water-based printing ink composition is subjected to heat or to thermal radiation, e.g. is subjected to electromagnetic radiations in the infra-red light waveband, prior to curing. The following relates to the production of individual silk screen printed sheets and their treatment for application onto hoarding sites in accord with embodiments of the present invention. As well as single sheet posters, the embodiments are concerned with the usual combinations of multiple sheet posters (e.g. "16 sheet", "48 sheet" or "96 sheet" posters) for pasting up onto large hoardings. The screen printing method used in these embodiments may be for the production of four colour halftone prints as well as continuous line and tone colour prints. The following embodiments of this invention concern the screen printing ink used to print the sheets. The screen printing ink in each of these embodiments comprises an emulsion of a resin dispersed in water, i.e. is water miscible, and dries by exposure to Ultra -Violet radiation. The emulsion has said resin wholly (or at least substantially) in dispersed phase and wholly (or at least SUBSTITUTESHEET substantially) insoluble in the water, the water being the continuous phase of the emulsion. Thus the screen printing ink contains water and is water thinnable to a considerable extent, but the water is to be regarded as a carrier or diluent (rendering the ink aqueously reducible in viscosity ) rather than as a solvent. Preferably the water-based screen ink contains at least 40% water. In a first embodiment of this invention the printing ink coπr..osition comprises: an Ultra Violet curing resin emulsified with water, e.g. an equal part of water; a photoinitiator and an aqueous pigment dispersion. In the emulsion the water is the continuous phase and the resin (amounting to 50% of the emulsion) is the dispersed phase. One specific formulation of this is given in Example 1. Example 1 Parts by weight Emulsion Laromer* PE 55W* 1 ) 95.3 Darocur* 1173 (2) Photoinitiator 2.0 Aquadisperse* Phthalo Blue GB EP (3) 2.7 A composition as formulated in Example 1 was printed through a screen made from 165 threads per cm polyester mesh half- calendered. The calendered side was positioned face down towards the substrate, ie. M.G. (machine glazed) poster paper. The stencil was made from direct emulsion using a half-tone positive. The print was producer, on an S.P.S. cylinder action screen printing press. The ink was dried through a two lamp S.P.S. U.V. dryer (rated at 300 watt per inch). The resultant dried print was a good half tone image of high definition. However this initial formulation, when tested, gave poor screen stability. This was' improved in a, second embodiment by the addition of n-vinyl pyrrolidone - a solvent miscible with water which also takes part in the curing reaction. A SUBSTITUTESHEET 00961 _ fi _ -6- specific formulation of this second embodiment, which likewise had water as the continuous phase and had the resin (amounting to 50% of the emulsion) as the dispersed phase, is given in Example 2. Example 2 Parts by weight Laromer* PE 55W (1 } ' 85.3 Darocur* 1173 (2) 2.0 Aquadisperse* Phthalo Blue GB EP {3) 2.7 n-vinyl pyrrolidone (4) 10.0 A composition as formulated in Example 2 was printed and dried in the same manner as for Example 1. The resultant dried print was a good half tone image of high definition. Due to the relatively high price of this water-miscible solvent, i.e. the n-vinyl pyrrolidone, compared to the other materials used in the formulation, the final material cost was considered uneconomic. To offset this, a calcium carbonate extender was introduced into the composition to obtain a third embodiment with an acceptable raw material cost. A mixture of photoinitiators was also used in place of the single type. A specific formulation of this third embodiment, which likewise had water as the continuous phase and had the resin (amounting to 50% of the emulsion) as the dispersed phase, is given in Example 3. Example 3 Parts by weight Emulsion Laromer* PE 55W* 1 * 63.3 Aquadisperse* Phthalo Blue GB EP ' 2.7 N Vinyl Pyrrolidone* 4 5.0 Omyalite* 95T Extender 5 ' 20.0 Photoinitiator Irgacure* 651 * ' -- 6.0 Photoinitiator Quantacure* ITX* ' 3.0 SUBSTITUTE SHEET A composition formulated as in Example 3 was printed and dried in the same manner as for Example 1. The resultant dried print was a good half tone image of high definition. Samples of four colours Trichromatic yellow, magenta, cyan and black were made for container storage stability trials. After four weeks storage, the magenta and black inks had gelled whereas the yellow and cyan were unchanged. Further work was carried out to improve storage stability.. In order to retain screen stability and an economic raw matrerra1 cost, a fourth embodiment was devised in which, they; π_-v nyL- pyrrolidone and the photoinitiator mixture.-were replaced by a solvent, ethoxy-propanol and a single, initiator compound rather than the mixture of photoinitiators. The calcium carbonate extender was also omitted as this was not now required to obtain an acceptable raw material cost. A thickening agent was introduced to maintain the ink at a printable viscosity. Again the emulsion had water as the continuous phase and the resin as the dispersed phase. Slight improvements in screen stability were obtained with this embodiment by the introduction of small amounts of a humectant/surfactant compound and of a silicone emulsion lubricant. A specific formulation of this fourth embodiment is provided in Example 4. Example 4 Parts by weight Laromer* PE 55W 1 75.2 Darocur* 1173 (2) 2.9 Ethoxy Propanol* 8) 9.7 Curlout Plus** 9) 5.8 Alcoprint* PTF* 10) 0.5 Alcoprint* PHL* 11 } 1.0 Alcoprint* PSJ 12 1.9 Aquadisperse* Phthalo Blue GB EP 3) -.3.0 A composition formulated as in Example 4 was printed and dried in the same manner as for Example 1. The resultant dried print was a good half tone image of high definition. The storage stability of trichromatic yello., magenta, cyan and black printing inks tilth this latest formulation //ere then retested and found to be satisfactory. To provide a completely aqueous-based composition avoiding the use of any inflammable solvents, a fifth embodiment as devised in .hich the ethoxy propanol //as replaced by a mixture of Abater and an acrylic monomer - Tripropylene glycol diacrylate (T.P.G.D.A.) - for maintaining the appropriate dispersion of the resin solids. A specific formulation of this fifth embodiment is provided in Example 5. Example 5 Parts by .eight Laromer* PE 55W* 1 ) 61.6 Darocur* 1173* 2) 3.1 Acrylic Monomer T.P.G.D.A. ' 7.7 Water 15.4 Curlout Plus** 9) 5.7 Alcoprint* PTF (10) 0.5 Alcoprint* PHL* 11 1.0 Alcoprint* PSJ* 12) 2.0 Aquadisperse* Phthalo Blue GB EP* 3 * 3.0 A composition formulated as in Example 5 /.as printed and dried in the same manner as for Example 1. The resultant dried print //as a good half tone image of high definition. All the above-mentioned tests /ere carried out using 60" x 40" (1.5m x 1m) machine glazed (M.G.) Poster Paper. It *as found that .hen heavy deposits of colour vere used across the //hole area of the paper, a high degree of curl at the SUBSTITUTE SHEET edges of the paper //as experienced. This //as overcome (in accord //ith embodiments of the invention of our co-pending UK Patent Applications Nos. 8618595 and 8619245) by providing for the first colour to be printed do/n onto the paper to be a screen printing ink producing minimal paper curl (e.g. enabling the paper sheet to be re-fed through the printing machine), and for the or each susequent printing thereon to be /ith a /ater-based UV curing ink. The screen printing ink producing minimal curl can be a conventional non-aqueous UV curing ink or a conventional solvent-based printing ink, or an aqueous ink of lo/ /ater content (e.g. less than 20%). The above-described ater-based inks of Examples 1 to 5, /hen cured only by U.V. radiation, /ere found to have a /et rub-resistance not adequate for all purposes, and to have a variable gloss level. Experiments /ere carried out /ith infra red heating just before the UV curing //as effected. This technique produced prints /ith good /et rub-resistance and /ith a substantially uniform level of gloss. It is thought that this as due to the removal of excess /ater by the infra-red heating. It //as also found that more efficient /ater removal could be achieved by using a jet air dryer to force cold air over the print. It is considered that excess ater might be removed, additionally or alternatively, by micro/ave or radio frequency drying. Tests //ere conducted in pasting up the individual sheets to produce hoarding posters, and starch based pastes (obtained from Wilsons Adhesives and Morris Greenhaugh) //ere used for these tests. The surface of the prints produced by the above ink Examples //as found to accept the starch pastes used by bill-posters more readily than prints printed /ith the solvent-based inks //hich are currently in general use. Prints produced //ith the //ater-based inks .of Examples 1 to 5, /hen dried only by UV radiation, sho/ed excellent adhesion properties ith the starch adhesives. These prints SUBSTITUTESHEET when dried both by infra-red and by UV radiation showed a reduction in adhesion properties but this was still an improvement on that attainable with multi-colour prints produced with the standard organic solvent based screen printing inks. Never-theless, by subjecting the prints to corona discharge, this reduction in adhesion properties was overcome and prints with good gloss and wet-rub resistance were still obtained. In a preferred example, prints dried by infra-red and UV radiation were treated to 73 dynes/cm using a Sherman Treaters Corona Discharge Machine. These prints showed the same excellent adhesion properties that were obtained by just drying with UV radiation. Notes * means Trade Mark 1. Laromer* PE 55W is a 50% emulsion of a polyester acrylate in water obtainable from B.A.S.F. 2. Darocur* 1173 is a Hydroxy-2-methyl-l-propan-l-one obtainable from E. Merck 3. Aquadisperse* Phthalo Blue GB EP is an aqueous dispersion of pigment Blue 15 obtainable from Tennant- K.V.K. 4. n-vinyl pyrrolidone is a UV reactive solvent obtainable from B.A.S.F. 5. Omyalite* 95T is a calcium carbonate extender obtainable from Croxton & Garry. 6. Irgacure* 651 is Benzil Dimethyl Ketal- obtainable from Ciba Geigy. SUBSTITUTESHEET 7. Quantacure* ITX is _so|»xopyLthioxanthone obtainable from Ward Blenkinsop. 8. Ethoxy Pτtvpylene Glycol Ether obtainable from B.P. Chemicals. 9. Curlout* Plus is a modified acrylic solution polymer obtainable front American Carbonyl Inc. 10. Alcoprint* PTF is an acrylic copolymer in mineral oil thickner obtainable from Allied Colloids. 11. Alcoprint* PHL is a humectant and surfactant blend obtainable from Allied Colloids. 12. Alcoprint* PSJ is a silicone emulsion softner/lubricant obtainable from Allied Colloids. 13. T.P.G.D.A. is a Tripropylene Glycol Diacrylate obtainable from Degussa.

Claims: CLAIMS 1. A printing ink for silk screen printing, characterised in that the screen printing ink is a water-based ink that is at least partially curable by irradiation and comprises an emulsion of a resin disposed in water. 2. A screen printing ink according to Claim 1 characterised in that the emulsion has said resin at least substantially in dispersed phase and at least substantially insoluble in the water, the water being the continuous phase of the emulsion. 3. A screen printing ink according to Claim 1 or Claim 2, characterised in that said resin comprises 30% to 60% of the emulsion. 4. A screen printing ink according to any preceding Claim, characterised in that said resin is 50% of the emulsion. 5. A screen printing ink according to any preceding Claim, characterised in that said resin comprises an unsaturated acrylate resin. 6. A screen printing ink according to any preceding Claim, characterised in that said resin comprises a polyester acrylate resin. 7. A screen printing ink according to any preceding Claim, comprising one or more photoinitiators and being at least partially curable by irradiation comprising electromagnetic radiations in the ultra-violet light waveband. 8. A screen printing ink according to any one of Claims 1 to 6, characterised in that said ink is at least partially curable by irradiation with an Electron beam. 9. A printing ink according substantially to that of Example 1. 10. A printing ink according substantially to that of Example 2. 11. A printing ink according substantially to that of Example 3. 12. A printing ink according substantially to that of Example 4. 13. A printing ink according substantially to that of Example 5.

Inventors: BARROW LASCELLE AUGUSTUS, DIX BARRY MARTIN, PATEL DAKSHESH SHASHIKANT, SMALL RICHARD FREDERICK, BARROW, LASCELLE, AUGUSTUS, DIX, BARRY, MARTIN, PATEL, DAKSHESH, SHASHIKANT, SMALL, RICHARD, FREDERICK
Applicants: SMALL PROD LTD, SMALL PRODUCTS LTD
Application year: 1987
Main classification: C09D11
IPC codes: C09D11