Process of offset lithographic printing

Abstract: Aqueous polymer emulsions are used in paper coating prior to applying inks to the paper surface. This invention provides an emulsion comprising a copolymer of vinyl alkanoate, alkylene and vinyl versatate. This emulsion, when used for coating paper subjected to offset lithographic printing, provides improved wet pick strength.

Description: Field of the Invention: This invention relates to a process of offset lithographic printing wherein, prior to printing, the paper is coated with a pigmented composition comprising an aqueous emulsion containing copolymers of vinyl alkanoate and alkylene monomer in which vinyl alkanoate is the major component. Background of the Invention: Paper coating compositions usually comprise an aqueous polymer binder emulsion, a pigment and optionally other additives common in the technology. EP-A-0295727 (Shell) describes polymer emulsions containing vinyl acetate, ethylene and versatates (Veova 9®) for use in paint compositions to give non tacky surfaces which are alkali resistant. These benefits are demonstrated in comparison with Veova® 10. The description suggests the emulsions are usable in paper coating and other fields, but there is no disclosure of the wet pick benefit identified in the present application. EP-A-0172354 (Air Products) describes vinyl alkanoate/Ethylene emulsions for paper coating which provide enhanced dry pick strength and gloss. The additional monomers are unsaturated acids/esters and dicyclopenta-dienyl acrylate. Vinyl versatates are quoted as examples of the alkanoates but there is no suggestion to use them as a third monomer with VA/E. VA/E is stated to lack sufficient wet pick resistance for offset printing. GB-A-1144316 (Dunlop) describes alkali-soluble polymer emulsions formed from quaternary alpha carbon carboxylic, vinyl carboxylate and unsaturated carboxylic monomers. pH and viscosity stability derive from the tertiary monomer. Coating compositions will contain a pigment, for example clay, which is compounded with the latex binder and used in coating a cellulosic web for example a paper or paperboard web. The characteristics of the latex binder component are significant in the ease of preparation of the paper coating composition, its application and on the qualities of the coated product. The coating composition will be coated on to a paper substrate, dried and calendered. Usually the composition will be applied at a level to give a loading of about 18 to about 24 g/m² solids on one surface. The copolymer particles will bind those of the pigment filler to provide cohesive strength and contribute to the adhesion between the coating and paper substrate. The coating, which will usually have a thickness of about 3 µm (microns) to about 10 µm (microns) imparts gloss and whiteness to the paper and provides a suitable surface for receiving ink. When the coating is subjected to successive printing operations, for example in offset lithography, the coating must provide acceptable wet and dry pick strengths. When the first colour is printed any pull by the tacky ink giving failure of the coating is termed 'dry pick'. Failure when the coated paper passes to the second inking stage, when ink is applied to prewetted areas, is termed 'wet pick'. Subsequent inking steps can also give rise to wet pick failure. General description of the Invention: The paper coating composition used in the process of the invention comprises, in addition to a pigment, an aqueous copolymer emulsion having a solids content from about 20% to about 70% by weight and comprising i) from about 60%, preferably from about 70%, to about 90% by weight of a vinyl C1 to C4 alkanoate, ii) from about 5% to about 20% of a C2 to C4 alkylene monomer, and iii) from about 5% to about 20% by weight, preferably from about 8% to about 15% by weight, of vinyl esters having the general formula R₁R₂R₃ CCOOCHCH₂ wherein R₁R₂ and R₃ are each alkyl groups having at least one carbon atom and R₁+R₂+R₃ is from 6 to 9. This product has good rheology and dry pick strength and enhanced wet pick strength which is attributable to the presence of vinyl ester (iii) monomers. There is a general requirement in paper coating to improve these characteristics. The invention pertains to a process for offset lithographic printing of paper wherein, prior to printing, the paper is coated with a pigmented paper coating composition comprising, i) based upon 100 parts of pigment, and of ii) from about 3 parts to about 30 parts by dry weight of an aqueous emulsion as defined above. In the emulsions prepared for use in paper coatings the polymerisation system will be substantially free of protective colloids. Colloidal materials may be added subsequently to the paper coating composition. Although vinyl acetate is the preferred vinyl alkanoate monomer because of its availability, cost and known reactivity, other vinyl esters within the class defined are usable in particular vinyl formate, propionate, butyrate and isobutyrate. The preferred alkylene is ethylene but other ethylenic hydrocarbons, for example propylene, butylene and isobutene are usable. Optionally the copolymer may contain minor monomer components added to provide specific benefits, examples are sodium vinyl sulphonate, acrylic acid, methacrylic acid, acrylamide, hydroxy functional acrylates, vinyl silanes and vinyl halides. A favoured comonomer is a polyethylencially unsaturated compound selected from triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl fumarate, divinyl benzene and diallyl phthalate. Methods for preparing the copolymer emulsions of the invention are well characterised in the literature. Polymer Synthesis (vols I and III) by Sandler & Karo (Academic Press 1974) and Preparative Methods of Polymer Chemistry (2nd Ed) by Sorenson and Campbell (Interscience 1968) provide preparative information. Methoden der Organischen Chemie (Houben-Weyi) Band XIV published by George Thieme Verlag Stuttgart (1961) also provides preparative descriptions. Preferably at least one surfactant used in the polymerisation process contains a C14 to C20 alkenylene moiety and examples are an oleyl propanol amide sulphosuccinate obtainable from Witco of USA under the trade name Emcol K8300® and the potassium salt of the sulphonation product of oleic acid obtainable from Lankro Chemicals of Manchester England under the trade name Lankropol OPA®. The paper coating composition comprises a pigment, for example clays, hydrated silica clays, and other conventional ingredients. The clays used include Kaolin group clays and hydrated silica clays, specific clays are disclosed in "Kaolin Clays and their Industrial Uses" by J. M. Huber Corp. (1949) New York chapters 10 to 16. Other pigments, for example calcium carbonate, titanium dioxide, blanc fixe, lithopone and zinc sulphide may be used in addition to clay. In general the paper coating composition will comprise Component Parts by weight Pigment 100 dispersing agent eg. sodium pyrophosphate 0.1 to 0.5 latex (on solids basis) 3 to 30 Cobinder eg. starch or casein 0 to 25 Defoamer, eg. a hydrocarbon oil 0 to 0.2 Water to provide desired solids level. The coating compositions produced with the aid of the latexes of the invention will be applied to fibrous paper webs using conventional means for example trailing blade coaters, air knife coaters and roll coaters. Test Methods: i) Dry pick strength values (cm/sec) were measured using a coating composition on Reeds Aircoat woodfree paper using an IGT Dynamic Pick Tester (AC2 Model) with 25 Kg load and low viscosity ink. The composition tested contained: Pigment SPS clay to 100 parts Emulsion under test 24 parts of 50% solids emulsion Water retention aid Finnfix FF5® 0.5 parts Dispersant Dispex N40® 0.3 parts Sodium hydroxide 0.15 parts Water to give 45% solids ii) The wet pick strength was measured with the apparatus of the dry pick method. 0.3 ml of ink was applied to each roller and distributed for 15 minutes. A test strip of paper coated with the test composition to a level of 20g/m² (solids) was placed in the device. The inked roller was contacted with the test strip and adjusted to a printing force of 35 Kg. A drop of tap water was placed on the test strip immediately in the path of the roller so ink is applied over a layer of water spread by progress of the roller. The degree of resistance to pick is judged at a qualitative level i.e. good - fair - bad. Specific description of the invention: Examples of the invention will now be given to illustrate but not limit the invention. In the examples several functional monomers and surfactants were used and these are represented by abbreviations and letters in the table of results in order to simplify the presentations. Functional monomers: A172 is vinyl tris trimethoxy silane obtainable from Union Carbide of USA. DAM is diallyl maleate. EHM is ethyl hydrogen maleate SVS is sodium vinyl sulphonate Surfactants: A is sodium nonylphenol 20 EO sulphate obtainable from Lankro Chemicals of Manchester England as Perlankrol RN75®. B is sodium oleyl propanol amide sulphosuccinate obtainable from Witco of USA as Emcol K8300®. C is sodium dodecyl benzene sulphonate obtainable from Lankro Chemicals as Arylan SC15®. D is disodium salt of an ethoxylated alcohol half ester of sulphosuccinic acid obtainable from Cyanamid of Wayne New Jersey USA as Aerosol A102®. Example I A surfactant solution (initial charge) comprising 37% aq. solution of Emcol K8300® (74.2g), 30% aq solution of Aerosol A102® (53.2g) and ammonium persulphate (4.3g) dissolved in deionised water (999g) was prepared and charged to a 4 litre pressure vessel. The contents are heated to 68°C. At 68°C the reactor was purged twice with nitrogen, once with ethylene and pressurised with ethylene to 31 bar (450 lbs/sqin) gauge with stirring. The temperature was adjusted to 76°C while 10% of the liquid monomer phase was pumped to the reactor. The continuous addition of persulphate solution (5.3g in 300g of deionised water) was begun and the reaction mass was held at 76°C for 45 minutes. At the end of this period, the continuous addition of the remainder (90%) of the monomer phase was started. The persulphate solution was added over 6.5 hours and the monomer phase over 5.0 hours. The pressure was maintained at 31 bar (450 lbs/sq in) gauge until the desired quantity of ethylene had been reacted, this was after four hours from start of continuous persulphate additon. After completion of the addition of monomer, the reaction mass was held at 80°C for 45 mins, at the end of which the addition of persulphate solution was stopped. The contents of the reactor was cooled to 40°C, followed by pumping of finishing off stage (4.6g of each in 70g of deionised water) to the reactor, in separate streams, over 30 minutes. The contents of the reactor were then cooled to 25°C and discharged to a degassing tank. Ammonia solution was used to adjust the pH of the emulsion latex to 8. The emulsion had a solids content of 51.6% by wt. FORMULA Initial Charge: Parts per hundred monomer distilled water 64.06 Emcol K8300® 4.75 Aerosol A102® 3.41 Ammonium Persulphate 0.28 Liquid monomers: Vinyl Acetate (VA) 74.95 Veova 9® (VV9) 9.93 Silane A172 0.49 Diallylmaleate 0.19 Ethylene (E) 14.41 Distilled Water 0.19 Ammonium Persulphate 0.34 Distilled Water 4.48 TBH 0.30 Distilled Water 4.48 Sodium Metabisulphite 0.30 182.56 The result of the dry and wet pick tests on a coating composition containing the resulting emulsion are given in Table I. Examples II to XII Additional examples were performed using the process steps of Example I but varying the quantities of the base monomers, i.e. vinyl acetate, VeoVa 9® and ethylene, the functional monomers and the stabilising surfactants. These emulsions were also tested for wet and dry pick strengths. The emulsion products had solids contents comparable to that obtained in Example I. The applicants have found the addition of a vinyl ester of the class exemplified by VeoVa 9® to a vinyl acetate/ethylene emulsion polymer system improves the wet pick resistance of a paper coating composition when applied to a paper in a process for offset lithographic printing.

Claims: A process of offset lithographic printing of paper wherein, prior to printing, the paper is coated with a pigmented paper coating composition comprising, i) based upon 100 parts of pigment, and of ii) from about 3 parts to about 30 parts by dry weight of an aqueous copolymer emulsion having a solids content of from about 20% to about 70% by weight and comprising iii) from about 60% to about 90% by weight of a vinyl C1 to C4 alkanoate, iv) from about 5% to about 20% of a C2 to C4 alkylene monomer, and v) from about 5% to about 20% by weight, preferably from about 8% to 15% by weight, of vinyl esters having the general formula R₁R₂R₃ CCOOCHCH₂ wherein R₁ R₂ and R₃ are each alkyl groups having at least one carbon atom and the sum of the carbon atoms in R₁+R₂+R₃ is from 6 to 9. A process as claimed in claim 1 wherein the pigmented paper coating composition further comprises from about 0.1 to about 0.5 parts of dispersing agent, from about 0 to about 2.5 parts of cobinder, from about 0 to about 0.2 parts of defoamer, and water to appropriate level. A process as claimed in claim 1 wherein the composition is applied at a level to give about 18 g/m² to about 24 g/m² dry solids

Inventors: BOODAGHIANS RAZMIK BAGHDASSAR-, FARMER DAVID BRETLAND, PHILLIPPS ROY GORDON, BOODAGHIANS, RAZMIK BAGHDASSAR-ZADEH, FARMER, DAVID BRETLAND, PHILLIPPS, ROY GORDON
Applicants: UNILEVER NV, UNILEVER PLC, UNILEVER N.V.
Application year: 1990
Main classification: B41M1
IPC codes: D21H19