US2651580A - Mineral coated paper and coating composition therefor - Google Patents

Description

Patented Sept. 8, 1953 MINERAL COATED PAPERAND. COATING COMPOSITION THEREEQR Arthur Reilly, Westbrook; Maine, assignor to S. D. Warren Company, Boston, MESS.) acme-- poration of Massachusetts N o Drawinm. Application, September. 1,. 1949, Serial No. 113,683

This invention relates to mineral-coated printing paper especially adapted for being printed upon by the lithographic or offset printing process and a coating composition therefor;

Mineral-coated paper is the most suitable-medium now known for the. reproduction. of halftone cuts by any of the. conventional printing processes; Such paper is made by applying to a paper body stock, which may or may not contain mineral filler, an aqueous coating composition containing .a major proportion of finely divided mineral matter, or pigment, and. a minor proportion of a hydrophilic adhesive material. The so-coated surface is then dried and smoothed to a flat and level condition. In most cases the smoothing is accomplished by means of a super-calender, but it may be done by other means, as bydrying the coated surface in contact with a polished chromium surface.

The surface of mineral-coated paper, owing to the small proportion of adhesive to mineral mat"- ter therein, contains countless capillary'or interstitial spaces between the individual pigment particles. Printing ink is readily transferred to the mineral-coated surface where it adheres well. The vehicle of the printing-ink penetrates into the interstices readily and the ink sets so quickly that as a result freshly printed sheets-can be superposed without serious offset: of the ink from one sheet to another.

In the lithographic or offset printing process, printing is done from a printing surface (usually a rubber blanket) which carries ink in some areas and water or aqueous'lithographic solution in other areas. Normally part. of both ink and lithographic solution is transferred: to the printed. paper surface whereupon the aqueous solution quickly penetrates intothe interstitial spaces of the coating. Such absorption of, water or aqueous fluid causes trouble from curling of the paper sheet, and if the sheet is being printed in different colors the. resulting expansion may cause very serious trouble. from mis-register.v of successive prints.

A purpose of the present invention is to provide a mineral-coated paper suitable to be printed by lithography which tends to be waterrepellent and so does not take up appreciable quantities of. aqueous fluid from. a. lithographic printing surface. Water-repellency of the type indicated. is entirely different from. the so-called water-proofness usually impartedto. the. adhesive used in customary mineral-coated lithoslve in the lithographic fluid it has long been customary to Waterproof the adhesive used in such papers. For example, formaldehyde has been used to render casein coatings insoluble, and resins such as ester gum, melamine-aldehyde resin, and urea-formaldehyde resin have been used to insolubilize starch adhesive in such coatlngs. Such prior-art papers, however, have not been water repellent, but on the contrary they have been wet by and have absorbed aqueous lithographic solutions. As a matter of fact, it has been believed necessary to have the paper surface wettable by aqueous lithographic fluid in order to achieve good lithographic printing. Consequently all prior-art lithographic printing papers have to some extent exhibited the troubles from curling and mis-register mentioned above. In contr-adistinction to such prior papers, the paper of the present invention actually is difficult to wet with water, and its use 'to a large degree cuts down troubles from paper curling" and mis-register.

'Itis, of course, possible to render a mineralcoated surface water-repellent by applying,

thereto a continuous film of water-repellent material, e. g. a coating of lacquer. A surface so treated, however, no longer retains its desirable printing characteristics. are sealed off by the continuous film, the printing-ink vehicle no longer can penetrate. As a result, difficulties arise in relation to ink-drying and offsetting.

According to the present invention, on the' contrary, the capillaries are not sealed off and printing-ink vehicle can readily penetrate into the capillaries but the pigment particles forming the walls of the capillary spaces are rendered water repellent but not ink-repellent. The water-repellency of the pigment particles need be, and usually probably is, only transient. Mementarily, at least, the angle of contact which the aqueous lithographic fluid makes with the said pigment particles is so high that substantially no aqueous lithographic fluid can enter the capillaries. Moreover the aqueous lithographic fi uid tends to be repelled from the surface of the paper over. its entire area. Consequently there is little tendency for aqueous fluid to be transferred-to the paper-surface from the moist 1itho graphic printing surface, any quantity transferred being only a small fraction of the quantity which would be. transferred under the same circumstances to a surface readily wettable b the aqueous fluid.

The. desirable purposes of the invention are Since the capillaries accomplished by depositing in situ on the individual pigment particles of the coating layer while the coating is being dried on the paper surface, a small but effective quantity of the decomposition product of ammonium resinate. Ammonium resinate is a well-known sizing material useful in the beater-sizing of paper. It has also been used as a surface size for paper (as disclosed in De Cew Patent No. 1,223,480 and my Patent No. 2,192,488). Ammonium resinate when freshly prepared gives a transparent solution in water. Upon aging the solution becomes milky, doubtless owing to liberation of some free resin which is kept dispersed by the dissolved ammonium resinate. Such milky solutions are satisfactory for use under the invention. When dried the ammonium resinate decomposes, leaving resin substantially free from any alkali.

The purpose of beater-sizing and surface-sizing paper is to render the paper sufficiently proof or resistant to aqueous writing-ink so that it can be written on with pen and ink. In such cases the effect of the rosin size is to prevent absorption of aqueous ink by the normally absorptive cellulose fibers. In the case of the mineral-coated paper of the present invention, however, the cellulosic paper base is completely covered by the superposed mineral coating so that no cellulose fibers are exposed to be wet by fluid applied to the surface. In this case the effect of the residue from the decomposition of ammonium resinate obviously is not to prevent absorption of aqueous fluid by the cellulose, nor is it to prevent absorption of the fluid by the pigment particles which are non-absorptive per se. Instead the effect is to prevent wetting and penetration of the aqueous fluid between the pigment particles. Consequently when a mineral pigment layer is laid down from an aqueous coating composition containing dissolved ammonium resinate the resulting entire pigment coated surface acquires a considerable degree of water-repellency, at least for as long a time as the paper is to be in contact with the lithographic printing surface or offset blanket. Since such contact ordinarily is in the order of only one second or less, it is apparent that a transient water-repellency will sufiice. Naturally, a longer period of water-repellency is satisfactory, but is not necessary under the invention.

The effect of the water-repellency produced in the coating by practice of the invention becomes plainly evident when the coated paper is printed on an offset printing press. As has been mentioned before, in customary offset printing, aqueous lithographic fluid is supplied to the printing plate which transfers part of the fluid to an offset blanket which in turn transfers part of the fluid to the paper surface that is being printed. At the same time, a small part of the aqueous fluid is lost by evaporation from the moving plate and blanket. Normally, aqueous fluid is applied to the plate by passage of a roller thereover before each printing impression is made. The fluid-applying roller is supplied in turn, usually through an intermediate series of rolls, from a fountain-roll which dips in a supply of aqueous fluid and which turns slightly with each revolution of the printing cylinder. The fountain-roll is turned by a pawl which engages a selected number of notches on the driven end of the roll. The roll can be turned a distance corresponding to one notch or, on most offset presses, a distance corresponding to as many as eight or ten notches, the quantity of fluid taken up by the system being directly proportional to the number of notches the fountain-roll is turned.

BLANK EXAMPLE A paper web of 43 pounds, per 500 sheet ream 25" x 38" in size, was coated on each side with 7 pounds dry weight, per side, of a coating composition comprising the following constituents:

FormulaA 100 parts by weight fine coating clay .25 part by weight tetra sodium pyrophosphate (a dispersing agent) 35 parts by weight low viscosity heat converted starch (adhesive) 1.75 parts by weight dimethylol urea (to insolubilize the starch) 1.5 parts by weight 60-40 styrene butadiene copolymer (added in emulsified form to flexibilize the coating) 1 part dry milk (to stabilize the copolymer emulsion) I Water to give a solids content of 45 per cent.

The coated paper was dried and supercalendered. It was then cut into sheets and printed on a commercial rotary offset printing press. It was found that for satisfactory operation the fountain-roll had to be set at 5 notches to supply sufficient fluid to keep the plate clean.

EXAMPLE 1 A similar paper web was prepared in exactly the same way except that 0.75 part, by weight, of ammonium resinate size was included in the coating composition of Formula A. The weight of dry coating applied was the same as in the first case, and the coated web was dried and supercalendered as before. In this case, however, when the paper was printed on the same offset press it was found that the fountain-roll set at one notch supplied all the lithographic fluid required. In fact, at the one notch setting more fluid was supplied than was actually needed the adjustment of the fountain was not sufliciently delicate to cut down the fluid to the minimum quantity at which the paper could be printed successfully. This remarkable fact indicates that the paper of the invention is markedly different from previous lithographic printing papers in that it can be printed with use of less fountain solution than the minimum quantity present-day offset presses are built to supply. It appears that this new kind of paper will make possible the use of offset printing presses having considerably finer adjustments than present-day presses.

Other examples of useful coating compositions containing ammonium resinate size follow:

EXAMPLE 2 parts by weight fine coating clay 15 parts by weight fine calcium carbonate 0.3 part by weight sodium pyrophosphate 33 parts by weight dextrinized starch 1.7 parts by weight dimethylol urea 1 part by weight ammonium resinate size Water to make solids content about 46 per cent.

EXAMPLE 3 parts by weight fine coating clay 10 parts by weight titanium dioxide 23 parts by weight casein (solvated by ammonia) 0.5 part by weight ammonium resinate size Water to make solids content about 42 per cent.

Ammonium resinate is an ammonium soap of a natural resin which has a high acid value and which is dissolved and neutralized in aqueous ammoniacal solution. Manila copal may be dissolved in ammoniacal solution to make such ammonium resinate. Generally, however, ordinary pine rosin is preferred, because of its lower price. Any of the common grades of rosin may be used, as well as hydrogenated, oxidized, or more or less polymerized rosin. Aqueous solutions of ammonium resinate are conveniently prepared by stirring finely powdered resin into an aqueous solution of ammonia and then heating the mixture until solution takes place.

Ammonium resinate forms a viscous solution in water at ordinary temperatures if the concentration is above about 6 or 7 percent. When it is dried by evaporation the compound decomposes, ammonia being liberated and the resin is precipitated in insoluble and water-repellent form. When a thin film of the resinate surrounding a pigment particle is'dried the resin precipitated on the particle is in very finely divided form.

Ammonium resinate may be included in any mineral-coating composition suitable for use in producing mineral-coated paper suitable to be printed by the offset lithographic printing process. Pigments commonly used in such compositions include clay, blanc fixe, satin white, and titanium dioxide; common adhesives include casein, insolubilized starch, and synthetic rubber latex. Enough adhesive must be used, of course, to bind the pigment so that the coating will not fail while being printed on the offset press. The water-repellency imparted by the ammonium resinate makes possible the use of calcium carbonate pigment in the coating although heretofore calcium carbonate has been considered unsuitable for use in coatings of paper intended for offset printing. Likewise it makes it feasible to use starch adhesive without taking auxiliary steps to insolubilize the starch.

It is found that ammonium resinate size included in a mineral coating in quantity as little as 0.25 per cent resin, based on the pigment content, will give an appreciable degree of water-repellency to the dried coating. More size gives greater repellency, of course. Quantities greater than 5 per cent of resin based upon the weight of the pigment ordinarily cause too great discoloration, however, and usually it is preferred not to use much over 2 percent of resin based upon the weight of the pigment in the coating composition. Usually 1 per cent of resin based on the weight of pigment will give a sufficient degree of transient water-repellency.

I claim:

1. A fluid mineral paper coating composition comprising an aqueous medium having dispersed therein a major proportion of a finely divided mineral pigment and a minor proportion of a hydrophilic adhesive, the ratio of pigment to adhesive being such that a dried coating formed from said composition will contain capillary interstices and pores between the pigment particles and that the coating will not fail while being printed, said coating composition containing also an ammonium soap of a natural resin having a high acid value, the weight of the resin in said soap amounting to from .25 to 5 percent of the weight of said pigment.

2. A fluid mineral paper coating composition as defined in claim 1 in which the adhesive is casein.

3. A fiuid mineral paper coating composition as defined in claim 1 in which the adhesive is starch.

4. A fluid mineral paper coating composition as defined in claim 1 in which the adhesive is starch and the pigment is calcium carbonate.

5. Mineral coated printing paper as defined in claim 12 in which the adhesive is casein.

6. Mineral coated printing paper as defined in claim 12 in which the adhesive is starch.

7. Mineral coated printing paper as defined in claim 12 in which the adhesive is starch and the pigmentary material is calcium carbonate.

8. A fluid mineral coating composition as defined in claim 1 in which the natural resin is pine rosin.

9. A fluid mineral coating composition as defined in claim 1 in which the natural resin is Manila copal.

10. Mineral-coated printing paper as defined in claim 12 in which the natural resin is pine rosin.

11. Mineral-coated printing paper as defined in claim 12 in which the natural resin is Manila copal.

12. Mineral-coated printing paper consisting essentially of a paper base having a surface coating thereon suitable for being printed upon by lithography, said coating being the solid evaporation residue of the fluid mineral paper coating composition defined in claim 1.

ARTHUR REILLY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,223,480 De Cew Apr. 24, 1917 2,192,488 Reilly Mar. 5, 1940 2,210,835 Jones et a1. Aug. 6, 1940 2,288,476 Mashburn June 30, 1942 2,354,318 Hughes et a1. July 25, 1944 2,360,828 Craig Oct. 24, 1944 2,399,748 Luettgen May 7, 1946 FOREIGN PATENTS Number Country Date 19,324 Great Britain of 1898 OTHER REFERENCES The Dictionary of Paper, publ. by the American Paper & Pulp Assoc., page 318.

Claims (1)

1. A FLUID MINERAL PAPER COATING COMPOSITION COMPRISING AN AQUEOUS MEDIUM HAVING DISPERSED THEREIN A MAJOR PROPORTION OF A CINELY DIVIDED MINERAL PIGMENT AND A MINOR PROPORTION OF A HYDROPHILIC ADHESIVE, THE RATIO OF PIGMENT TO ADHESIVE BEING SUCH THAT A DRIED COATING FORMED FROM SAID COMPOSITION WILL CONTAIN CAPILLARY INTERSTICES AND PORES BETWEEN THE PIGMENT PARTICLES AND THAT THE COATING WILL NOT FAIL WHILE BEING PRINTED, SAID COATING COMPOSITION CONTAINING ALSO AN AMMONIUM SOAP OF A NATURAL RESIN HAVING A HIGH ACID VALUE, THE WEIGHT OF THE RESIN IN SAID SOAP AMOUNTING TO FROM .25 TO 5 PERCENT OF THE WEIGHT OF SAID PIGMENT.