US 2685571 A
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Patented Aug. 3, 1954 UNITED STATES hdlENT OFFICE MINERAL-COATED PAPER AND COMPOSI- TION THEREFOR John C. Stinchficld and Frank Kaulakis, Westbrook, Maine, assignors to S. D. Warren Company, Boston, chusetts Mass., a corporation of Massa- No Drawing. Application February 15, 1949, Serial No. 76,671
position is applied to the web in the form of an, 7
aqueous suspension comprising a Water-dispersible adhesive component and a pigment component. Adhesives commonly used for this purpose include animal or vegetable proteins, starch products, water-dispersible polyvinyl alcohol, gums, and the like, and recently including synthetic rubber latices and similar emulsions of synthetic elastomers as disclosed in the copending application Serial No. 641,662 of John C. Stinchfield and Frank Kaulakis, of which this is a continuation-in-part. Commonly used pigments are clay, satin White, calcium carbonate, blanc fixe, talc, lithopone, titanium dioxide, and the like. Any of various known suitable methods may be used to spread and smooth the coating composition on the paper Web, after which the web is dried. Usually the coated and dried web is condensed and smoothed by being supercalendered.
It is primarily the pigment content of the coating composition which provides the desirable printing qualities of the coating, whereas the chief function of the adhesive is to bind the mineral pigment to the paper to such an extent that it will not be removed by the pull of the printing ink during the printing operation. It may be said that in general the adhesive in a mineral-coating composition adds little to the printing quality of the coating produced but on the contrary actually detracts from the good qualities of the pigment component. Accordingly, it is customary practice in manufacturing coated printing paper to use the minimum quantity of adhesive which will permit satisfactory performance of the paper in its intended use.
Mineral-coated paper intended for letterpress printing is made with a smaller proportion of adhesive than is necessary in mineral-coated paper intended for lithographic printing, because letterpress printing ink is considerably less tacky than lithographic printing ink. Consequently, coated letterpress printing paper normally cannot be printed on an offset lithographic press because the coating is too weak to withstand the pull of the lithographic ink. 0n the other hand, coated lithographic printing paper, if printed on a letterpress, will in general give results far inferior to those that can be obtained by use of good coated letterpress printing paper, the poor results being due primarily to the higher adhesive content of the coating on the litho graphic printing paper. Thus it is apparent that coated lithographic printing paper diiiers from coated letterpress printing paper in adhesive content.
Coated lithographic printing paper has likewise to some extent diliered from letterpress printing paper in pigment content. For instance, calcium carbonate is widely used in mineral-coated paper used for letterpress printing. That material possesses many qualities making it a very desirable paper-coating pigment: it is of excellent color; it takes ink exceptionally well; the, better grades can be calendered to a high gloss; and it is readily available at very reasonable cost.
In the case of mineral-coated papers intended to be printed by the lithographic or offset printing process, however, in which the paper is wet by a so-called fountainor dampening-solution-usually acidic-during the printing operation, it has been the general belief that calcium carbonate and other alkaline earth metal carbonate are objectionable for use. Reference is made to pages 34 and 35 of the booklet entitled Papers for Ofiset Lithography, Shop Manual No. 25, issued by the Lithographic Technical Foundation, Inc, in 1946.
Nevertheless, because of its recognized good qualities, calcium carbonate has been experimented with in coatings for lithographic papers, not always with satisfactory results, as indicated on page 28 of the booklet Pigments for Paper Coating published in 1948 by the Technical Association of the Pulp and Paper Industry.
It is apparent, therefore, that it is greatly to be desired to be able to use calcium carbonate in the coating of lithographic printing paper and still have the paper perfectly satisfactory for lithographic use. The present invention provides an excellent mineral coated lithographic printing paper the coating of which contains a substantial proportion of alkaline earth metal carbonate, and a process for producing such coated paper.
In lithographic printing, a plate bearing a water-repellent image is wet first with. a Wetout or dampening fluid, usually a slightly acidi- 3 fied aqueous solution of glycerine or similar substance which wets the non-imaged area of the plate, and then with greasy lithographic ink which wets the image but is repelled by the aqueous solution on the unimaged areas; the inked plate when brought into contact with another ink-receptive surface transfers thereto ink in a pattern reverse to that on the plate and at the same time transfers aqueous solution to the blank or clear areas of the surface. The surface so printed upon may be a paper sheet, but in most cases is a rubber offset blanket which in turn transfers the image in original form to the final paper sheet and at the same time transf ers aqueous solution to the sheet in the unprinted areas. As the printing occurs both the ink film and the water film actually split, part of each adhering to the paper and part to the printing blanket, or to the plate if no offset blanket is used.
It is apparent that the time of contact between the paper being printed and the offset blanket is very short, only a small fraction of a second. Nevertheless during that very short interval calcium carbonate of the coating layer may be dissolved by the acid lithographic solution which is neutralized thereby, the casein or other adhesive may be loosened, and when the aqueous film splits some of the neutralized solution and some of the loosened particles may be left on the blanket and in turn transferred back to the plate and thence to the water-fountain or in some cases to the ink-fountain. Of course only minute quantities of such material are transferred at a time, so that short runs of lithographic paper having a calcium carbonate-containing coating may occasionally be made without noticeable trouble. In long runs, however, trouble usually develops, whence comes the general belief that calcium carbonate should not be used in coating lithographic printing paper.
It must be borne in mind that the actual time of contact between paper and lithographic blanket or plate is extremely short. If the pigment particles can be protected from the action of the aqueous lithographic solution during that brief period of contact obviously then the solution left on the blanket cannot be contaminated by products resulting from dissolution of the calcium carbonate. It makes no particular difference whether or not the protection of the pigment particles continues after the paper leaves the printing blanket. If the protection continues, well and good; if it fails after a few seconds no appreciable harm will be done to the printed paper sheet.
It has now been found that calcium carbonate pigment in coatings of mineral-coated lithographic printing papers can be protected against the deleterious effect of aqueous lithographic solution during the period in which the paper is in contact with the printing surface of the lithographic printing press in the course of the printing operation. This desirable end is accomplished by including in the mineral-coating composition to be applied to the paper a sufficient quantity, amounting toat least 25% of the total weight of the adhesive present, of a water-insoluble elastic substance. The said elastic substance possibly acts by forming a film around each particle which is at least transiently impermeable to water or aqueous lithographic solution. On the other hand, the coating containing the clastic substance is definitely less wettable by aqueous liquids than is a coating containing only hydrophilic adhesive, as is readily shown by measurement of the contact angle made by a drop of water placed thereon. Hence it seems likely that the poorer wettability of the coated surface engendered by the presence therein of the watersoluble elastic material is effective in preventing reaction of the lithographic solution with the calcium carbonate pigment during the brief period of contact between the paper and the lithographic printing surface.
Whatever the mechanism may be by which the water-insoluble elastic substance acts, experience shows that the elastic substance included in a mineral-coating containing calcium carbonate or other alkaline earth metal carbonate substantially prevents, or at least materially reduces, deleterious effect of usual lithographic solutions upon said carbonate during the period in which the coated printing paper is in contact with the lithographic printing blanket or plate in the printing operation on a lithographic or offset printing press.
In carrying out the invention a mineral-coating composition may be prepared in customary fashion, comprising an aqueous suspension of finely divided pigment at least 20 percent of which is pigmentary alkaline earth metal carbonate, and suificient adhesive to bind the pigment to paper body stock with sufficient strength to withstand picking on a lithographic oifset printing press. Into the coating composition there is incorporated the water-insoluble elastic material, in the form of a primary emulsion thereof. Since elastomers deposited from primary emulsions make excellent adhesives for mineral-coating compositions, the preferred practice is to replace part or all of the usual hydrophilic adhesive with the elastomeric adhesive in primary emulsion. The coating can be applied to conventional paper raw-stock in known fash-, ion and the coated paper will then be dried and calendered.
A very satisfactory elastomeric material to add to calcium carbonate-containing coating compositions for use in making mineral-coated lithographic printing paper, as well as one that is comparatively cheap and readily available, is a primary aqueous emulsion of copolymerized butadiene and styrene. Such emulsion of butadiene-styrene copolymers are commercially available with considerable choice as to the butadiene-styrene ratio, :25, 60:40, and 50:50 being common ratios, and all of these are satisfactory for use. Other very satisfactory primary aqueous emulsions are those of butadiene-acrylonitrile copolymers, polychloroprene, methylacrylate-acrylonitrile copolymers and similar mers or coploymers of acrylates and methacrylates.
The term synthetic elastomer as used herein denotes a synthetic polymerized organic substance characterized by being of a definite elastic nature, and has the scope attributed to the term elastomers at page 942 of Industrial and Engineering Chemistry, vol. 31 (1939), pages 941 et seq., in an article by Harry L. Fisher. Most compounds of this nature form a continuous coherent film structure from primary emulsions thereof when the water phase is lost by evaporation, whether at room temperature or by forced drying. Synthetic elastomers of the inherently elastic type are typified by those mentioned in the preceding paragraph; elastomers of this type are the ones preferredfor use in practicing the present invention.
p lv- The term primary aqueous emulsion as used herein denotes an aqueous dispersion in which the polymer is formed in situ, by polymerization or copolymerization of a previously dispersed substance or substances of a lower molecular weight, e. g., monomers or dimers. Emulsions made by dispersing in water substances already fully polymerized before dispersion are less desirable than primary emulsions because their adhesive value is so much less.
Any paper raw stock may be used which is sufficiently strong to be printed upon without splitting under the pull of lithographic ink. The coating composition may be applied by conventional coating machines on one or both sides of the paper raw-stock, from 3 to 5 pounds, by weight, of the composition per 1000 square feet of surface being quantities commonly applied. The coated paper is dried and smoothed by supercalendering.
Typical coating compositions satisfactory for use in practising the invention are shown in the examples given below. In every case the pigments before use were prepared in an aqueous suspension in deflocculated condition. The elastomer used was in every case a commercial latex in the form of a primary aqueous emulsion of the elastomer. When other adhesive was used, that adhesive was dispersed in water before addition to the mixture. The quantities given are dry weights.
Examplel Parts High quality paper coating clay 65 Fine particle calcium carbonate 35 Butadiene-styrene copolymer (60:40 ratio) 6 Ammonium caseinate 12 Water to make solids content 50 per cent.
Example 2 Parts Fine particle calcium carbonate 100 Butadiene-styrene copolymer 14 Ammonium caseinate 5 Water to make solids content 48 per cent.
(This coating gave a finished product of very high gloss as well as excellent lithographic printing quality.)
Example 3 Parts Pulverized limestone, finest grade 100 Styrene-isoprene copolymer Water to make solids content 56 per cent.
(This coating gave a finished product of very low gloss and mediocre lithographic printing quality. This particular pigment could be bound satisfactorily for letterpress printing with 6 parts of adhesive.)
Water to make solids content 50 per cent.
. Example 6 Parts Paper coating clay 50 Fine particle calcium carbonate 50 Butadiene-acrylonitrile copolymer 12 Ammonium caseinate 6 Water to make solids content 50 percent.
Example 7 Parts Paper coating clay 40 Fine particle calcium carbonate 60 Elastic copolymer of methylacrylate and acrylonitrile 12 Ammonium caseinate 4 Water to make solids content 51 per cent.
Example 8 Parts Paper coating clay l6 Fine particle calcium carbonate 50' Yellow iron pigment 4 I-Iansa yellow pigment 0.12 Polychloroprene 12 Ammonium caseinate 6 Water to make solids content 50 per cent.
' Example 9 Parts Paper coating clay 50 Calcium carbonate-magnesium basic carbonate 50 Butadiene-styrene copolymer 20 Ammonium caseinate l0 Waterto make solids content 44 per cent.
Example 10 Parts Paper coating clay 45 Fine particle calcium carbonate 55 Butadiene-styrene copolymer (60:40) 23 Water to make solids content 52 per cent.
Example 11 Parts Paper coating clay 60 Fine particle calcium carbonate 40 Elastic copolymer of methylacrylate and acrylonitrile 19 Water to make solids content 49 per cent.
Example 12 Parts Paper coating clay 50 Fine particle calcium carbonate 50 Butadiene-acrylonitrile copolymer 22 Water to make solids content 50 Per cent.
1. A mineral-coating composition being an aqueous slurry consisting essentially of a finely divided mineral pigment component, at least 20% of which is pigmentary calcium carbonate, and a dispersed adhesive component in an amount between 10 and 30 parts by weight per parts of pigment component, said adhesive component containing a substantial quantity of a rubbery polymer in the form of a primary emulsion directly produced by the emulsion polymerization of a composition comprising a conjugated diene monomer, the quantity of said rubbery polymer present in the composition being from 10 to 44 parts by weight per each 100 parts of the pigmentary calcium carbonate present.
2. Mineral-coated lithographic printing paper comprising a paper base and on at least one side thereof a mineral coating comprising a mineral pigment component, at least 20% of which is pigmentary calcium carbonate, and an adhesive component, said adhesive component being present in an amount between and 30- parts by weight per 100 parts of pigment component and containing a substantial quantity of a rubbery polymer said polymer having been deposited on the paper from a primary emulsion directly produced by emulsion polymerization of a composition comprising a conjugated diene monomer, the quantity of said rubbery polymer present being from 10 to 44 parts by weight per each 100 parts by weight of the pigmentary calcium carbonate present and sufiicient to protect the latter from deleterious action of lithographic solution during lithographic printing operations, said mineral-coated lithographic printing paper being produced by applying to a paper base a coating of the composition defined in claim 1 and drying the coating.
3. The mineral-coating composition defined in claim 1, in which the calcium carbonate content amounts to from about 35% to about 60 of the total pigment component, and in which the total adhesive component amounts to between and parts per each 100 parts of the total pigment component.
4. The mineral-coating composition defined in claim 3, in which the pigment component consists of clay and calcium carbonate, and in which the adhesive component is a mixture of about equal parts of a rubbery copolymer of butadiene and styrene and an adhesive of the group consisting of starch and casein.
5. A mineral-coated lithographic printing paper as defined in claim 2, in which the calcium carbonate content amounts to from about to about of the total pigment component, and in which the total adhesive component amounts to between 15 and 20 parts per each parts of the total pigment component.
6. A mineral-coated lithographic printing paper as defined in claim 5, in which the pigment' component consists of clay and calcium carbonate, and in which the adhesive component is a mixture of about equal parts of a rubbery copolymer of butadiene and styrene and an adhesive of the group consisting of starch and casein.
References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,938,731 Tschunkur Dec. 12, 1933 1,982,018 Owen Nov. 27, 1934 2,214,565 Montgomery et a1. Sept. 10, 1940 2,287,348 Hayden June 23, 1942 2,364,847 mers Dec. 12, 1944 2,370,057 Mack Feb. 20, 1945 2,389,796 Mack Nov. 27, 1945 2,395,992 Clark Mar. 5, 1946 2,416,232 Soday Feb. 18, 194'? 2,444,994 Duggan June 29, 1948 2,465,295 Strauss Mar. 22, 1949 2,537,114 Young et a1 Jan. 9, 1951 2,583,274 Niles Jan. 22, 1952 OTHER REFERENCES Fisher, Ind. Eng. Chem. 61, August 1939, pp. 941-945.