US2395992A - Mineral-coated printing paper - Google Patents

Description

Patented Mar.,5 1946 MINERAL-COATED PRINTING PAPER John W. Clark, Westbrook, Maine, assignor to S. D. Warren Company, Boston, Mass., a corporation of Massachusetts No Drawing. Application April 1,1944,

Serial No. 529,179

2 Claims.

This invention relates to improved mineralcoated paper, and particularly to high-grade printing paper having high brightness, high gloss. and good affinity for printing ink. The new product embodies an unusual combination of qualities making for superior printing results; -it involves use of a coating of a composition not hitherto associated with glossy printing paper. The invention includes both process and product aspects.

For the reproduction of fine half-tone cuts, as by letter-press printing, a principal criterion is that the surface printed upon shall be very level. Supercalendered mineral-coated paper is the stock generally used for such printing purposes. By the term mineral-coated paper is meant paper, comprising a fibrous base, bearing on one or both surfaces a continuous layer of mineral coating composition which covers the fibers of the base and fills the hollows between the fibers on the surface of the base, so that when the paper is calendered it has a smooth, even and continuous surface (The hemistry of Pulp and Paper Making, Sutermeister, 3rd ed., 1941, Chapter XIII, page 360 et seq.) The mineral coating composition is applied to the web in the form of an aqueous suspension comprising a-water-dispersible adhesive component and a pigment component. Adhesives commonly used for this purpose include animal or vegetable proteins, starch, water-dispersible polyvinyl alcohol, gums, and the like; commonly used pigments are clay, satin white, calcium carbonate, blanc fixe, talc, titanium dioxide and the like. Any of the various suitable methods may be used to spread and smooth the coating composition on the web, after which the web is dried. Usually,

the coated and dried web is condensed and smoothed by being supercalendered in a stack of pressure rolls comprising alternately rolls having metallic surfaces and rolls having yielding surfaces.

Although the first requirement of a good printing surface for fine letter-press printing is undoubtedly flatness or levelness of surface, there background more striking, and helps make even mediocre printing crisp and efiective. On the other hand, a dingy or discolored'paper spoils the efi'ect of the finest printing. Numerical measurements for brightness are commonly made by means of the GeneraiElectric reflection meter. The best grades of supercalendered mineralcoated printing paper commonly have brightness value of the order of from 82 to 85.

, Shininess or glossiness of surface is, of course, not essential for good printing as such. Mineralcoated printing papers are generally divided into namely, dull-coated papers,, and glossy-coated papers. In general, good printing can be obtained on both classes of paper. Nevertheless, the greater part of the mineral-coated printing paper used is of the glossy-coated class; and in that class a high degree of gloss is considered a criterion of quality. It is true that high gloss in a sheet of paper makes for sparkle and brilliance in the finished printed page, and the trade demands high gloss in its high quality glossy-coated printing paper. A numerical value of gloss is commonly obtained by testing the paper on a Bausch & Lomb glossmeter. A glossmeter two classes:

reading of 65 is considered very good. Rarely, if

ever, is a glossmeter reading of 70 reachedby a supercalendered mineral-coated printing paper having the other requisites (above described) of correct quantity of ink to give satisfactory coverage, and is then immediately examined, it will be noted that the entire printed area glistens and looks wet. As the, glistening or wet appearance gradually disappears the print assumes the normal appearance of dry printed matter. At the moment when the appearance of wetness completely disappears th ink is said to be "set; it takes considerably longer, usually several hours, for the ink to dry hard by oxidation. But when it has'set it usually will not thereafter "offset onto another sheet of paper placed upon it.

It frequently happens, if the time required for the ink to set (i. e., the ink-setting time) on the paper is too long, that all areas of the printed ink do not set at the same time, but that glistening pools of ink continue to appear wet for a considerable time after adjacent printed areas have ceased to appear wet. When such pools have finally set-and it may at times take more than a minute for them to do soit is almost invariably found that the printed area exhibits a mottle which detracts considerably from the appearance of the printing. A paper giving such a result cannot be said to have good ink receptivity or aflinity. On the other hand, if the ink sets so quickly that there is no mottle apparent in the print after the ink is dry, the paper does have excellent ink receptivity. Obviously, it is impossible to set any absolute time limit under which mottling never occurs and above which it always appears. The best that can be said is that excellent affinity for or receptivity to printing-ink connotes freedom from mottle. with the ink setting in I a relatively short time. It seems to be true that in any case in which the ink sets in 40 seconds or less the print may be expected to be free from mottle. The paper of the present invention almost invariably has an ink-setting time of not over 40 seconds and prints made on it are free from mottle; that is to say, my paper has excellent receptivity to printing ink. In most cases the product of the invention has an ink-setting time of seconds or less, and in such cases may be considered to have exceptional ink receptivity.

From the preceding it will be seen that desirable qualities for high grade, glossy, mineral-coated printing paper include a high degress of levelness, glossiness, brightness, and quick ink-setting.

The gloss of the final surface of a supercalendered mineral-coated printing paper is dependent not only upon the degree of calendering and quantity of moisture present in the paper, but also upon the specific pigments present in the mineral coating. To obtain a glossymineral coating it heretofore has been generally considered necessary to have a considerable quantity of clay present in the coating, in order that supercalendering might give a surface of sufficient gloss. Very frequently satin white is used in addition to, and occasionally instead of, clay as the finish-promoting pigment of the coating composition. Naturally, minor amounts of various other pigments (e. g., blanc fixe, calcium carbonate. titanium pigments, etc.) frequently are incorporated into coating mixtures along with the clay for special purposes (e. g., to improve the color or to reduce greasiness of the surface, or for other reasons). It is generally conceded that, other things being equal, these latter pigments produce lower gloss in coatin s than do clay and/or satin white; they are used for other purposes than to promote gloss and are considered actually somewhat to reduce the gloss of the coatings in which they appear.

As has been stated previously, the gloss of a clay-coated paper depends in part upon the quantity of moisture it contains and upon the pressure to which it is subjected during the supercalendering operation. The greater the pressure applied, and/or the higher the moisture content of the coated paper at the time of 'calendering, the higher will be the gloss and the smoother will be the surface of the supercalendered paper. It is well known, however, that a point is reached beyond which pressure and/or water content can not be increased without seriously affecting the appearance of the finished sheet; that is, the surface becomes blackened or dingy. and the measured brightness'drops sharply. Thus it is seen that in any particular clay-coated paper, a compromise between gloss and brightness must be made; accordingly, the coated paper customarily is calendered only as severely as is possible without seriously aifecting the brightness. It is likewise found that increasing the 'severity of the calendering increases the ink-setting time of the paper; that is, it makes the ink set more slowly.

The product of my invention is a supercalendered mineral-coated paper having level surface, high gloss, high brightness, low ink-setting time, and generally fine printing characteristics. Furthermore, the pigment component of the coating of my new product consists entirely or substantially entirely, of calcium carbonate having certain definite characteristics.

Mineral coatings comprising calcium carbonate as the only or substantially only pigment are well known in the paper trade. For many years socalled dull-coated papers have been made with such coatings. Calcium carbonate has long been recognized as a pigment capable of preventing or decreasing glossiness in calendered coated paper.

It has now been found, however, that the mineral-coated paper product of this invention, having at least a 10 pound (per ream) mineral-coat in which the pigment is all or substantially all finely divided calcium carbonate can be supercalendered to give a gloss as high as, or in some cases even higher than, the gloss of first grade, glossy mineral-coated printing papers of commerce. This surprising result is made possible in part by the fact that it has been found that coatings comprising calcium carbonate as the sole pigment will withstand considerably more severe calendering than will coatings containing large quantities of clay and/or satin white. That is to say, in calendering calcium carbonate coatings, heavier pressures on the calender and/or higher moisture contents in the paper advantageously may be employed than are permissible in the case of clay-coated papers: pressures and/or moisture contents that blacken clay-coated paper to the extent cf making it valueless ai'e found to leave carbonate coatings withva high gloss and free from appreciable dinginess or blackening.

To illustrate the fact that carbonate-coated paper will endure more severe calendering than clay-coated paper, the following cases may be cited. Two lots of similar body stock were coated, one with a normally sized calcium carbonate coating composition, the other with a normally sized high grade clay-coating composition. Equal weights of coating-about 20 pounds per reamwere applied in both cases. The dried papers were then subjected to severe supercalendering under identical conditions. The calcium carbonate coated paper was not blackened, and yielded a fine product having a brightness of over 87%. The clay-coated sheet, however, was blackened so badly as to be practically worthless, and had a brightness of less than 70%.

Since the calcium carbonate-coated product of the invention stands severe calendering so well, it is advantageous to use in supercalendering such paper a supercalender whose yielding. rolls are harder than are the cotton rolls usually used for supercalendering coated printing papers. While a supercalender stack in which all the rolls are hard or metal rolls may be used with a measure of satisfaction for calendering, it is preferred that the supercalender used in the present process have alternate rolls. of somewhat yielding nature such as the paper rolls which have customarily been used for supercalendering uncoated paper but have previously been generally considered unsatisfactory for supercalendering mineral-coated printing paper.

In supercalendering papers having coatings comprising mixtures of calcium carbonate with clay or other pigments in considerable quantity it is noted that. calendering not severe enough to cause actual dinginess or blackening will often sheet and which also is reflected in the quality of the printing after the paper has been printed. The undesirable tendency towards mottling during calendering is greatly reduced or eliminated, however, when the pigment in the coating is substantially all calcium carbonate.

Many grades'of calcium carbonate may be used successfully according to the invention. Calcium carbonate 80% or more of which has a particle size of not over 2 microns equivalent spherical diameter can be used satisfactorily. Naturally, the better the grade of carbonate used the better the results that may be expected. For this reason the preferred carbonate is one having an average particle size of 1 micron or less. By equivalent spherical diameter is meant the diameter of a spherical particle having the .same settling rate as the actual particle under consideration.

It will be understood, of course, that to make a satisfactory coated printing paper, suilicient water-dispersible adhesive, e. g., casein, must be used with the pigment to make the coating adhere to the paper firmly enough so that the coating will not be lifted or picked from the paper by printing ink on the printing plate. The quancoated and dried weighed 80 pounds per ream (a tity of casein required to bind the pigment against same average particle size, may vary widely in casein requirement. Other things being equal, calcium carbonate having a casein requirement of not over that is, 25 parts by weight of casein to 100 parts by weight of calcium carbonate-is to be preferred for preparing the coated paper of the invention, while calcium carbonate of which the casein requirement lies between 6 and 16 usually gives the very best results. An exceptionally suitable calcium carbonate for the present use is the causticlzed calcium carbonate (i. e., the product resulting from causticization of soda ash with lime) having reduced adhesive requirement and improved gloss characteristics described by Alfred D. Wilson in U. S. Patent No. 2,345,311. Other methods of reducing the adhesive requirement of calcium carbonate pigment have been disclosed, e. g., in Jones et al. Patent No. 2.210.835.

Calcium carbonate coatings, normally sized,

vary considerably in appearance when s read upon a paper body-stock and dried, the variation probably depending upon both the particle size of the pigment and the adhesive requirement of the pigment. Some uncalendered coatings are very dull, while others exhibit a considerable sheen or gloss. Generally speaking, a glossmeter reading on the uncalendered coated paper of at least 'I is an indication that the sheet can be calendered to give the product of the present invention. A higher uncalendered gloss, say from 12 to 20 or over, is preferred.

Example 1 A web of paper body stock was coated on both sides with an aqueous suspension of coating composition essentiall consisting of finely divided calcium carbonate as the sole pigment and casein as the adhesive with 12 parts of casein used to each 100 of the calcium carbonate. The calcium ink-setting time was 12 seconds.

bonate with calcium hydroxide under known conditions of concentration, temperature, and agi- .-t'ation so that substantially all the precipitate to total dry casein 100 to 12. Likewise enough water was added to give the mixture a consistency suitable for spreading. The mixture was ground in a pebble-mill until smooth and then applied to paper body stock, the quantity of coating applied being 14 pounds dry weight per ream per side. The paper so coated and dried, but not calendered, had a gloss of 20 as measured on the Bausch and Lomb glossmeter. The paper web so ream being 500 sheets of the paper cut to 25" by 38"). when it went to the supercalender. It was steamed heavily on the supercalender and then subjected to heavy pressure of such a'degree that the coating would have been blackened badly if the coating had contained much clay. The paper did not blacken appreciably, however. The brightness of the supercalendered paper as measured on the G. E. reflectometer was the glossmeter reading was 67 When printed in solid black with Geographic Black-ink its ink- Example 2 Another paper web of about 40 pounds basis weight per ream 25" x 38"500, was coated on one side with 15 pounds, dry weight, of a coating consisting of calcium carbonate as the pigment and casein dissolved by alkali as the adhesive, the weight ratio of pigment to casein being to 11. The calcium carbonate pigment used in making up this coating composition had beenv made by causticizing sodium carbonate with slaked lime under controlled conditions to yield a product having an average particle size of about 1 micron; this very finely divided material, after having been washed substantially free from soluble alkali, had been treated, by the Wilson method, so to reduce its casein requirement that the weight ratio just recited gave a normal sizing. The glossmeter reading on the dried, coated, but uncalendered, paper was 15. The paper was moistened heavily with steam and then supercalendered under heavy pressure to such a degree that the coating would have been blackened badly if the coating had contained much clay. The brightness of the supercalendered paper, as measured on the General Electric reflectometer, was 86; the glossmeter reading was '72. When printed in solid black with Geographic Black" ink the The printing quality was excellent.

As one increases the weight of the coating,

from the lower limit'of 10 pounds per ream per side, there occurs a gradual improvement in the gloss of the final (supercalendered) product. That is to say, a 15-pound coating yields a somewhat glossier final product than does a 12-, or 10-. pound coating of the same composition. This difference in gloss is variable, tending to become carbonate was formed by reacting sodium car- I The paper contained 5% of moisture of the total, however, the properties of the product are practically unchanged from those of pure calcium carbonate, and-for the purpose of this invention calcium carbonate pigment containing up to 10% of magnesia compound as impurity is included Within the meaning of calcium carbonate as the latter expression is used in the specification and claims of this patent application.

Furthermore, it is found that dilution of the calcium carbonate pigment, used in coating the product of the invention, with up to 10% of other high grade coating pigments, such as, for example, high grade coating clay, satin white, and

titanium dioxide, does not appreciably affect the gloss and/or freedom from dinginess of the sucasein adhesive, in which the ratio of pigment to adhesive is at least 100 to 20, and preferably between 100 to 16 and 100 to 6. It will be seen that the novel product of the invention is a normally-sized, severely supercalendered, mineral-coated printing paper having a combination of properties not hitherto believed'possible to produce. Furthermore, the product has a gloss of at least 65 and preferably of at least '70. The

. preferred calcium carbonate coating of the prespercalendered product of the invention. Accordingly, up to 10% dilution with other pigments is permissible and such diluted calcium carbonate is understood to be included in the expression substantially all calcium carbonate as used herein.

Casein is the preferred adhesive for use under the invention; approximately identical results may be obtained, however, by use of other proteins such as animal glue, soy bean protein, or zein, or by the use of water-dispersible polyvinyl alcohol, or, in case water-resistance is not important, suitable starch adhesives such as oxidized or otherwise modified starch give satisfactory results.

The term brightness as used in the specification and claims refers to the value obtained by a measurement of the paper by means of a General Electric reflection meter by standard procedure.

"Gloss as used in the specification and claims refers to the property measured by a standard Bausch and Lomb glossmeter.

By "ink-setting time is meant the period of time in seconds between the time the paper is printed .by means of a solid plate with the proper quantity of ink to give good coverage, and the time the entire area of the printed surface ceases to glisten and look wet. Any of the standard black printing inks commonly used for printing glossy coated paper may be used. For instance, I normally use for my testing ink Geographic Black 148, made by Sigmund Ullman Co. of New York: some other inks thought to be practical equivalents as far as setting time goes are Wotta Black 448 made by E. J. Kelly Co. of Kalamazoo, Neo Black 286 made by Lewis Roberts, Inc. of Newark, Black G 2549N made by Geo, H. Morrill Co. of Boston, Black K7800 made by Martin Driscoll & Co. of Chicago, and Black New Star 37825 made by Braden Sutphin Ink Co. of Cleveland.

The mineral coating layer carried on the paper base ranges in weight from 10 to pounds per .25" x 38"-500 ream, and consists essentially of the above described finely divided calcium carbonate having reduced adhesive requirement and a water-dispersibleadhesive, preferably cut invention. Paper coated with the tinted carbonate composition and finished by severe supercalendering according to the procedure previously described will be foundto have all the desirable qualities of high gloss, excellent inkaflinity, and fine printing properties, but such coated paper will, of course, not have the extremely high brightness of untinted carbonate coated paper, since a colored coating naturally reflects less light than does a white coating. Nevertheless, such tinted calcium carbonate coated paper after severe supercalendering retains well the basic brightness it had before being calendered. That is, the paper may be severely supercalendered yet still be substantially free from dinginess; the muddy tone sometimes noticed in tinted clay-coated papers is not apparent in my tinted carbonate coated paper. I believe that a tinted coated printing paper of which the pigment content of the coating is at least calcium carbonate, which paper has a gloss of at least 65, excellent affinity for ink, excellent printing quality, and which is substantially free from dinginess, is a new and valuable product.

Severe Supercalendering is referred to in the foregoing description and appended claims as being such a supercalendering under such conditions of moisture content and heavy pressure as will cause objectionable blackening of a coated paper whose coating pigment is mainly, or solely, clay. To clarifythe meaning of severe supercalendering the following is given as an illustrative example of severe supercalendering such as was employed in the foregoing specific examples. Supercalendering was carried out on a 9-roll supercalender having alternate chilled iron and paper filled rolls. The paper filled rolls had a densityof 93 as measured by a Shore densometer, type D. The pressure on the paper per linear inch of nip was about 2000 pounds at the top nip increasing to about 2600 pounds at the bottom nip. The moisture content of the paper as a result of the heavy steam shower was about 7%.

This application contains subject-matter in common with, and is a continuationin-part of, my application Serial No. 413,040, filed September 30, 1941.

I claim:

1. Process of producing a mineral-coated printing paper of high gloss and high brightness and having excellent ink-receptivity, which comprises coating at least one side of a web of paper body stock with an aqueous slurry of mineralcoating composition consisting essentially of a water-dispersible adhesive-component and a pigment component substantially consisting of causticized calcium carbonate pigment having an average particle size of not to exceed 1 micron diameter and a casein requirement less than 25,

under conditions yielding a continuous layer of 2. Normally sized, mineral-coated paper having a gloss of at least 70, high brightness, and an ink-setting time not exceeding 40 seconds, said mineral coating being calendered and having a thickness equivalent to at least 10 but not more than 25 pounds per ream, the continuous layer of mineral coating of said mineral coated paper product substantially consisting ofwater-dispersible adhesive and causticized calcium carbonate pigment having an average particle size of not more than 1 micron diameter in which coating the weight ratio of pigment to adhesive is at least 100 to 20, said product being additionally characterized in that the paper has been coated and supercalendered by the process defined in claim 1.

JOHN W. CLARK.