US2700621A - Manufacture of coated paper and product - Google Patents

Description

United States Patent Ofi 2,700,621 Patented Jan. 25, 1955 ice MANUFACTURE OF COATED PAPER AND PRODUCT Hellmuth C. Schwalbe, Robert C. Hydell, and James J. OConnor, Chillicothe, Ohio, assignors to The Mead Corporation, Dayton, Ohio, a corporation of Ohio No Drawing. Application January 6, 1950, Serial No. 137,278

7 Claims. (Cl. 11765) This invention relates to the manufacture of coated paper and more particularly to mineral coated paper adapted to have surface characteristics suitable for receiving fine printed impressions such as are used in 'high quality magazine printing of fine half-tone and multicolor reproductions.

Generally in the manufacture of coated paper of the character described a coating material comprising a mineral pigment and an adhesive or binding material in Water suspension is applied to the surface of the paper by suitable coating apparatus. The pending application of OConnor, Savage and Schwalbe, Serial No. 774,294,

now Patent No. 2,565,260, shows roll coating apparatus with which a suitable finish imparting surface coating may be applied to paper to form a satisfactory smooth surface thereon with characteristics adapted for high quality and exacting printing requirements demanded for the faithful reproduction of fine half-tone and multicolor illustrations in magazines and the like but utilizing coating material comprising generally a highly thixotropic aqueous suspension with relatively high solids content of a mineral pigment and adhesive material such as v casein or modified starch.

As attempts are made to increase the speed of operation of such roll coating apparatus, it is desirable to apply coating material from a suspension the solids content of which is as great as may be practicable toachieve a finished coated product with suitably formed surface characteristics. That is to say, with such high speed roll coating apparatus, the application of desirably heavy coat weights of the order of ten pounds or more per side per ream of five hundred sheets 25 x 38 inches will be facilitated if the mineral-adhesive coating suspensions applied to the paper have solids contents of substantially more than of the order of As the speed of travel of the paper web through the coating apparatus is increased, a greater quantity of coating suspension must be introduced to the paper web per unit of time in order to apply a desirably heavy coat Weight. If it is attempted, however, to increase the solids content of such mineraladhesive suspensions as heretofore used, such coating suspenslons rapldly become more and more viscous until a they reach a state wherein a further increase in solids content of but a few per cent will produce such a substantial and drastic increase in the viscosity of the suspension that regular roll coating apparatus will not adequately apply and meter and acteristics to the coated paper in high speed commercial operations with such roll coating apparatus and techniques. One solution of this problemis shown in the above mentioned copending application of applicants assignee where it was the "characteristics of the coating suspension it could be made highly thixotropic so that, notwithstanding the greatly increased viscosities resulting from increasing the solids content substantially above what had been considered practicable, working in shear, the highly thixotropic characteristic of the coating suspension renders it temporarily less viscous for a sufiicient time to permit its ready and satisfactory application to the paper. suspensions of this character, however, such a drastic rise in viscosity may occur when relatively high solids contents are increased even a few per cent, e. g., in the range of 56% to 60% solids, to an extent that above such heretofore workable Even with coating material uniformly distribute the coating material to give suitably smooth surface charfound that by properly controlling i sions become so viscous and substantially nonflowable, even though highly thixotropic, that it is impracticable to get sufficient working in shear within the limitations of the power and strength of the roll coating apparatus to reduce the viscosity sufiiciently to be satisfactorily fiowable and readily measured and applied to the paper to give desired results.

It has now been discovered, however, that, by the present invention, a suitable coating material suspension may be readily applied to paper, using existing high speed roll coating apparatus, with solids content as high as approximately 65% to 80% or more yet with initial viscosities (as determined with a MacMichael viscosimeter with a cup diameter of 2%", a bob diameter of 1 and thickness of A" and clearance from the bottom of at 40 R. P. M.) of as low as 5 to 20 poises or well within the range of practicable coating material viscosity limits of such existing high speed commercial roll coating apparatus. This advantageous coating operation is accomplished hereunder by using materials such as urea and starch adhesive materials hereinafter described in combination with suitable additional components such as highly dispersed mineral pigments and nonbulking lubricants which give relatively reduced viscosity so that the relatively high solids content colors have such normal viscosity and flowabilityas to be readily usable in such roll coating apparatus.

A principal object of this invention is to provide for the manufacture of coated paper of the character described using finely divided finish imparting mineral coatings which may be applied to the paper in the form of aqueous suspension with high solids content yet low MacMichael viscosities.

Another object of this invention is to provide a method of coating paper of the character described using coating material suspensions which possess substantially higher solids content than has heretofore been possible with other satisfactory coating materials of comparably low MacMichael viscosity.

Still another object of this invention is to provide a method of coating paper of the character described using coating material suspensions which possess substantially lower initial MacMichael viscosities than has heretofore been possible for satisfactory coating materials of comparably high solids content.

A further object of this invention is to provide a coating material suitable for application to paper by high speed roll coating apparatus and techniques to produce a heavier coat weight applied to such paper than has heretofore been possible on existing high speed apparatus andwith superior smooth surface characteristics adapted for fine high quality printing.

A further object of this invention is to provide a coating material of the character described which will resist excessive adsorption into the paper sheet or web to which it is applied notwithstanding a low initial MacMichael viscosity so that substantially all of the coating material may be retained upon the surface of the paper to perform its smooth surface providing function thereon.

Another object of this invention is to provide a suitable coating material of the character described adaptable for convenient and economic use and application with existing paper coating apparatusand methods within the intended range of operationthereof.

A still further object of thisinvention is to provide a coated paper product of the character described to which has been imparted a desirably smooth surface notwithstanding the application of a heavy coat weight of high solids coating material compositions.

Still another object of thisinvention is to provide a coating material of the character described such that, when applied to the paper and dried, the applied surface coating will possess suitable plastic characteristics to admit of readily obtaining thereon or imparting thereto extremely smooth surface characteristics upon supercalendering of the applied coating.

Still another object of this invention is to provide a paper coating process of the character described in which the variable characteristics of viscosity of the coating suspension, readily practicable solids content thereof, and applied coat weight and wet fihn thickness may be so =3) coordinated one to the other as to arrive at desirably optimum combinations of such variable characteristics for coating materials of difierent compositions and conforming to the requirements of paper coating applications, apparatus and problems.

Still a further object of this invention is to provide for use in a paper coating process of the character described an improved coating material suspension in which the properties of viscosity and solids content are so adjusted to give very low or the lowest possible low shear viscosity concurrently with maximum mineral and adhesive coating solids content with a substantially low high shear viscosity.

Other objects and advantages of this invention will be apparent from the following description and the appended c aims.

The high solids contents of coating material suspensions used in accordance with this invention facilitate the manufacture of coated paper with desired surface characteristics by resisting excessive penetration of the coating suspension into the paper so that substantially all of the mineral coating material remains effectively upon the surface of the paper to provide the desired smooth mineral coating thereon; yet the low viscosities of these coating suspensions greatly facilitate the pumping and other handling and introduction of the coating material suspension to the coating apparatus as well as facilitating the smooth application of a fluid coating film to the paper without the undesirable formation of irregular surface characteristics obtained from excessively viscous coating material. Furthermore coating material suspensions of this invention possess what might be termed a self-plasticizing action, as hereinafter described, whereby coated paper produced in accordance with this invention admits of much more satisfactory surface smoothing as by supercalendering even after the application and drying of the coated material. In contrast to relatively high solids and highly thixotropic coating suspensions as heretofore used, the viscosities of the coating material suspensions of this invention vary within a comparatively narrow range for a given solids content range under normal working conditions well within applying and handling limits of high speed roll coating apparatus, such as is described in the above mentioned copending application of applicants assignee, when used in its normal and intended manner.

As illustrative of this astonishing advance embodied in this invention, it should be noted that coating material compositions hereof, as hereinafter specified. having a solids content of as high as 75.5 and MacMichael viscosities of as low as 19 poises have been applied with existing roll coating apparatus to obtain a coated paper product carrying a coat weight of as much as seventeen pounds per ream per side with a desirable surface smoothness comparable to that heretofore obtainable on high speed roll coating apparatus with but ten pounds or less of coating per side-an increase of as much as 70% in weight of coating of satisfactory high quality characteristics over procedures heretofore used.

While this invention has so far been found of greatest commercial advantage when the solids content of the coating material suspension is in the high solids range of 65% to 70% solids or even higher, nevertheless in the lower ranges, such as 50% ing difficulties for certain desired coated products have heretofore limited satisfactory use of nonthixotropic suspensions, more desirable and satisfactory heavily coated paper is secured since lower viscosity of the coating suspensions embodying this invention will also permit of more uniformly applied and more readily distributed and smoother surface coatings. That is to say, with coating compositions as heretofore used, the viscosity of the coating suspension so increased (in certain ranges of solids content) that a point was reached at which the coating suspension was so viscous that its application to the paper by the coating applying rolls of the roll coating apparatus .resulted in the formation of irregular surface characteristics such as so-called piling or worming evidenced by the formation .of sharp spike-like peaks and sharp crater-like valleys in the coating surface.

In'this invention .it has been discovered that high solids content, low viscosity coating material suspensions may be suitably applied using roll coating apparatus vsuch .as is shown and described in the above mentioned pending application of applicants assignee. One such coating composition with which satisfactory results have been obtained may contain the following formulation:

4 parts Stayco M 8 parts Sweetose 15 parts calcium carbonate 73 parts S. M. C. Clay 1 part Calgon 0.25 part calcium stearate 10 parts urea The above coating composition may be admixed with Water to provide an aqueous coating material suspension having total solids content of 7.5% and, when so admixed to give this extremely high solids content, the MacMichael viscosity at R. P. M. of the coating composition suspension will be 19 poises. Such a suspension may be applied to a raw stock paper web comprising essentially 30% pine pulp, 31% sulphite pulp, 27% soda pulp and 12% sulphite broke at web speeds of approximately 600 feet per minute to give a coat weight of approximately 17 pounds per ream on one side with sati sfa'ctory surface smoothness characteristics using roll coating apparatus of the character described with a metering pressure of 200 pounds per linear inch.

In the formula set out above, the S. M. C. Clay is a finely divided mineral clay pigment suitably processed for use in the paper industry and manufactured by Georgia Kaolin Co. Both such finely divided clays and the calcium carbonate noted are selected for inclusion in the coating compositions of this invention because their highly dispersed or greatly deflocculated nature diminishes'as much as practicable'the contribution made by the pigment suspension to the overall viscosity of the coating composition. Calgon is a commonly accepted trade name for a sodium polyphosphate developed for water conditioning. In water solution it disperses finely divided metal oxides and salts, and inhibits crystallization of slightly soluble combinations such as calcium to solids, where operatv carbonate, and is used in this composition as a dispersing agent. Calcium stearate is here used as a lubricant. This particular lubricant is selected as being a so-called nonbulking lubricant instead of the more generally used viscosity increasing soap lubricant in order to further minimize the viscosity contributions of the various components of the coating suspension to the viscosity of the whole suspension. Stayco M is a trade name for a thin boiling oxidized starch adhesive material made by the Staley Manufacturing Company, and Sweetose" is a syrupy starch product converted or modified as hereinafter described to a greater degree than the Stayco M referred to and is here used as part of the adhesive component of the coating material.

The terms modified or converted starch indicate very generally a starch colloidal suspension or sol which has been oxidized, as by reaction with s are such suitable chemical as sodium hypochlorite, or has been acted. upon by such liquefying enzymes as, for example alpha amylase by methods of starch adhesive material manufacture welllgnown in the coated paper industry. Starch adhesive materials may be so modified or converted to various degrees by controlling the severity or completeness of such converting or modifying treatment. The extent or severity of such converting treatment tends to modify considerably both the viscosity of the starch sol and its adhesive properties, a greater extent of conversion tending to decrease both the viscosity and the adhesive strength.

As indicated in the above formulation. the properties of the adhesive strength and viscosity of the starch com ponent of the coating compositions are controlled by admixing a slightly converted and relatively adhesive starch such as the Stayco M with a highly converted, highly fiuid starch product of lower adhesive strength such as the Sweetose so that the combination thereof will give a starch adhesive material component of adequately high adhesive strength yet adequately loW viscosity. Satisfactory results have been obtained with the above formulation using as the less highly converted starch sol one which may have a so-called high shear viscosity (as measured as disclosed in the patent to Gricsheirner No. 2,392,662) of centipoises at 14 F. when the starch solids content is approximately 31%, and. as the fluid starch component, astarch ,sol with a high shear viscosity of 15 ccntipoises at F. with starch solids content of approximately 31%. Therefore, as in the coating com position set forth above, satisfactory results have been obtained by using a combination of fluidstarch" with a somewhat less converted or binding material which will contribute as little as possible to the over-all viscosity of the whole coating material suspension and yet will provide adequate adhesive or binding properties to cause the mineral pigment in the coating material to adhere satisfactorily to the surface of the paper to be coated.

The astonishing combination of such heretofore believed conflicting properties as such usable'high solids content ranges with such conveniently low MacMichael viscosities is attributable in part, it is believed, to the inclusion in coating composition of this invention of the component urea set forth above in combination with the use of a highly converted or fluid starch adhesive material. The actual mechanics whereby the coating compositions set forth herein achieve their phenomenal results when used with this invention, however, are not now completely understood.

The fluidity of mineral-starch coating suspensions may be increased (i.e. the viscosity decreased) by subjecting the starch adhesive material to converting operations as noted above, and that such converting of the starch will also decrease the adhesive properties of the starch as noted. It is believed that, within limits, the increased solids content of these coating suspensions inhibits the penetration of the fluid suspension into the fibrous paper sheet so that, with high solids content suspensions, more adhesive material remains effectively in and upon the surface zone of the paper to accomplish the pigment binding function. Consequently, notwithstanding the decreased adhesive properties of more highly converted starch, actually less starch adhesive material is needed in the original suspension to hold a given quantity of pigment upon the surface of the paper with compositions of this invention.

Furthermore it is believed that the inclusion of urea in such a coating material suspension causes some physical, chemical or mechanical combination or interaction to occur between the urea and the starch adhesive material whereby the viscosity of the suspension is lowered perhaps because the extremely hydrophilic starch sol is made to release effectively more water for dilution or viscosity decreasing action. The decreased viscosity effect of converting the starch itself may indeed be partially due to the action of such converting treatment tending to break down many of the entangled branches and side chains of the highly complicated starch molecules so that the highly converted starch apparently has less water withholding characteristics, so to speak, so that a smaller amount of water may be used in the coating suspension to provide adequate flowability. Such an effect is substantially increased by the use of urea so that more of the water present in the coating suspension is available for dilution or viscosity decreasing action.

Also the inclusion of urea in such starch-mineral-water suspensions has the effect of actually, increasing the amount of solid coating material left deposited upon the paper surface after the application and drying of the coating suspension while actually decreasing the normal initial viscosity of the original aqueous suspension yet without appreciably increasing the volume of diluent or the liquid phase of the coating suspension. This peculiar effect has the double advantage of making usefully avail able a greater quantity of ultimately solid materialto produce the finished coating at greatly decreased viscosities while yet avoiding disadvantages heretofore attended upon the use of highly fluid coating materials with high speed roll coating apparatus of the character described. That is to say the penetrating into or absorption by the paper of the highly fluid coating suspension is inhibited by the high solids content thereof as well as the tendency heretofore of highly fluid suspensions to splash or be cast off the coating applying rolls of the apparatus by the action of centrifugal force to fall upon the already coated paper web and form irregularities thereon. I

It is pointed out, however, that whether or not the above hypotheses be correct is not necessarily a determining factor in practicing this invention. It has been discovered, in any event, that a smaller percentage of water will produce added fluidity without highly thixotropic characteristics for a given high solids contentif a coating material suspension is used in accordance with this starch to provide an adhesive invention containing urea and that the advantageous combination of high solids content with low viscosity may be further enhanced by also using in such coating material suspensions a highly converted or fluid starch adhesive material. Consequently, it is now no longer necessary to depend 'upon highly thixotropic coating suspensions to obtain desirably high solids contents for the high speed commercial application of desirably heavy coat weights with suitably formed surface characteristics utilizing existing roll coating apparatus operated in its normal and intended manner. Thus, with these coating compositions containing urea, the mineral-starch suspension is no more dilute (in the sense that the ratio of water to suspended pigment is no greater) when urea is included in the coating composition, yet the presence of urea yields a greater quantity of solids than-has heretofore been obtainable from a given amount of coating suspension of comparably low viscosity as heretofore used.

Accordingly, for satisfactory high speed coating results with heavy coat weights, coating material compositions may satisfactorily be produced in accordance with this invention so that extremely high solids contents are obtainable before the point at which the viscosities of coating suspensions undergo the sudden and drastic increase referred to above.

Additional coating composition formulations embodying this invention may be produced as set forth below:

Example 1 11 parts Stayco M 12 parts calcium carbonate 77 parts 81 Huber clay 0.5 part pyro phosphate 0.25 part calcium stearate 10.0 parts urea The above formula when admixed with water to give a coating composition suspension of 65.0% solids had a MacMichael viscosity of 40 R. P. M. of 32.2 poises. It was applied toa raw stock paper comprising 30% pine pulp, 31% sulphite pulp, 27% soda pulp, and 12% sulphite broke at a speed of 625 feet per minute on conventional apparatus to give a coat weight of 14.4 pounds on one side.

- Example 2 7.0 parts Stayco M 4.0 parts Sweetose 10 parts calcium carbonate 79 parts S. M. C. Clay 1.0 part Calgon 0.25 pa-rt calcium stearate 10 parts urea Satisfactory results have been obtained from the above composition when admixed with Water to give a coating material suspension having a solids content of 70.8% and a MacMichael viscosity at 40 R. P. M. of 24 poises. Such composition, when coated on a paper raw stock comprising approximately the same composition as set forth for Example 1 above at approximately 600 feet per minute produced a coat weight of 11 pounds for one side of eX- tremely'satisfactory smoothness even before calendering.

Quite apart from the unusually high solids content obtainable with the coating compositions of this invention, the lower viscosities available as Well as the fact that coating compositions of this invention are quite readily calendered with supercalendering after having been dried upon the paper all combine to make the compositions of this invention useful for attaining highly satisfactory coating results even when such'unusually high solids content are not desired or required. In this connection it should be noted that it has also been discovered that the solids material deposited upon the surface of the paper sheet'or web coated according to this invention and after the Water has been removed as by drying possesses the characteristic .of apparently being somewhat plasticized so that it is readily adaptable to the smoothing act-ion of a supercalender. Although the exact mechanics of this phenomenon are not understood, it is believed that the inclusion of urea in the coating compositions of this inventron tends to impart plasticity to the dried coating so that smoothing and finishing action of the supercalender Such coating composition,

In this'connection it may be well to note that coating material compositions formulated in accordance with this invention may be thixotropic to a certain degree. That is to say, although their viscosities may not undergo the wide variations common to highly thixotropic colors as heretofore used, some such thixotropic variation, although to a substantially small degree, may be present. Although any thixotr-opy which may be present in the formulations of this invention may well be utilized as desired, the characteristics and surprising results obtainable from coating materials of this invention render it no longer necessary to depend upon nicely controlled thixotropy in order to obtain satisfactory coating results with high solids content mineral-starch coating suspensions.

If however, the coating applying apparatus to be used is so constructed as to take advantage of or to be able to utilize thixotropic coating compositions, coating suspensions embodying this invention may be formulated for highly satisfactory use on such apparatus. One such example of a coating composition embodying this invention and formulated for use upon high speed coating apparatus designed to utilize highly thixotropic colors is as follows:

Example 3 10.5 parts starch (converted to 90 centipoises high shear viscosity at 140 F. and 31% solids) 10.0 parts calcium carbonate 74.5 parts S. M. C. Clay 5.0 parts urea 0.8 part Calgon 0.7 part soap 0.1 part soda ash when admixed with water to form a coating composition suspension having 70% solids content, exhibits an initial MacMichael viscosity of from 75 to 90 poises. Such a suspension has been satisfactorily applied to a paper raw stock comprising 14.5% sulphite pulp, 35.5% bleached kraft pulp, 50.0% groundwood at a speed of 950 feet per minute to give a coat weight of 14 pounds on one side and 15.5 pounds on the other side.

Another such coating composition embodying this invention is formulated as follows:

Example 4 11.5 parts starch (converted to 174 centopoises high shear viscosity at 140 F. and 31% solids) 26.5 parts calcium carbonate 67 parts Stellar Clay (manufactured by Edgar Brothers Company) 0.5 part soap 0.5 part pyro phosphate 10.0 parts urea Such formulation, when admixed with water to give a coating composition suspension having 58.5 to 60.9% solids, has a viscosity of approximately 79 poises and produces satisfactory results with coat weights from 19.5 to 21.9 pounds. Such application should be contrasted with coat weights obtained of less than 17 pounds using colors not formulated according to this invention wherein the highest solids contents available was some lower than that given above to produce comparable finished coated products on the same high speed apparatus.

It will be noted in the coating compositions above set forth that the quantity of mineral pigment utilized in each case remains approximately of the same order of magnitude. The actual quantity of adhesive necessary to cause adhesion and cohesion and binding to the surface of the paper of such quantities of mineral pigmented material depends upon the degree of conversion which such adhesive has undergone as well as the characteristic desired to be imparted to coated paper.

The specific ranges of amounts of components in the i above formulations such as dispersing agents and lubricants, etc., are quite readily determined according to principles well-known in the art of coating paper. It should be noted, however, as pointed out above, that the viscosity of the final complete coating composition suspension will not be increased in a non'bulking lubricant such as calcium stearate or castor oil and hexylene glycol be used insteadof soap. Coating compositions embodying this invention may contain urea in an amount from approximately 3% to 20%: of the total solids of the coating com position, although urea in amounts of from 5% to 12% are preferred and have been found satisfactory to give highly satisfactory results in high speed commercml manufacture. Satisfactory pigment-adhesive ratios according to this invention include from to 95 parts by weight of pigment to 8 to 15 parts by weight of starch, with ratios in the range of to parts by weight pigment to 10 to 13 parts by weight of starch being preferred. Satisfactory results have been obtained with compositions according to this invention having a low shear viscosity of less than poises MacMichael with about 15 to 40 poises being preferred. These compositions also include the starch converted as noted above to have a high shear viscosity corresponding to 35 to 175 centipoises measured at F. and at starch solids content of approximately 31%.

Furthermore highly satisfactory results have been obtained using coating compositions and methods embodying this invention in coating applying operations in which extremely high solids or extremely heavy coat weights were not required. The coating compositions of this invention were found highly satisfactory not only because of the lower viscosities obtainable even at such lower solids content but also because of the readily smoothable characteristic of these coating compositions upon being supercalendered as has been set forth above. One such coating composition of somewhat lower solids content is as follows:

Example 5 12.75 parts starch (converted to centipoises high shear viscosity at 140 F. and 31% solids) 30.25 parts Huber 81a Clay 26.5 parts calcium carbonate 30.5 parts Stellar Clay 0.5 part soap 0.5 part Calgon 0.5 part soda ash 8.3 parts urea Such a color composition, when admixed with water to give a. color composition suspension having 56.6% solids, at a viscosity of 31.0 poises, was applied in regular high speed commercial use to give a coat weight of an average of 10.5 pounds per side. The effect of using a lower percentage of urea is indicated additionally by the Example 3 above wherein the utilization of 5% urea enabled the production of a coating color composition containing 70% solids yet did not lower the MacMichael viscosity below 75 to 90 poises.

It should be noted that viscosity values given hereinbefore have been obtained using a so-called MacMichael viscosimeter at 40 R. P. M. Such viscosity readings may be generally categorized as low shear viscosities. It is convenient in carrying out coating procedures embodying this invention to consider also so-called high shear viscosity of the coating suspension. Such viscosity determinations may be measured while the coating suspension under consideration is being Worked in high shear or undergoing high shearing forces and such high shear viscosities may be satisfactorily determined by the use of the viscosimeter disclosed in the Patent No. 2,392,662 to Griesheimer.

It has been found that, as the solids content of starchmineral coating suspensions is increased, the variations in low shear viscosity are not necessarily parallel to the variations in high shear viscosity. This is particularly true if the coating composition suspension exhibits pronounced characteristics of thixotropy. With coating procedures embodying this invention, however, satisfactory comparative determinations of coating composition suspension results may be conveniently considered in terms of the high shear viscosity characteristics.

It has been found, for example, that the high shear viscosity of starch-mineral coating composition suspensions increase proportionately to increases in solids content over a certain range of solids contents but that a point may be reached where high shear viscosity undergoes a very drastic and sudden increase with a very small increase incoating composition solids. Thus the high shear viscosity of a starch-mineral coating composition suspension in a certainrange of solids content may suddenly increase quite drastically by. as much as 25% to 30% when the solids contents are. increased by less than 1%. For satisfactory finished results with coating apparatus of the character described, the effective practicable limit on solids content is marked in various types of starch-mineral coating composition suspensions by the point at which this drastic and sud-den rise occurs in high shear viscosity. As has been pointed out before, this effective limit with coating compositions embodying this invention may be as high as 75 or 80% solids. a

In attempting to formulate coating compositlon suspensions embodying this invention, therefore, this relationship of solids content and low and'high shear viscosities may conveniently be considered. That is to say, the compositions may be formulated to obtain the highest solids content below that range of solids content at which occurs the drastic and sudden increase in high shear viscosity referred to above, and also so that the low shear or MacMichael viscosity of the coating composition suspension is sufliciently low to be readily handled, controlled, and applied by the coating apparatus. It is believed that the low shear or MacMichael viscosity characteristic indicates to a large measu're the smoothness characteristics obtainable on the applied coating composition; that the solids content, as has been noted, contributes to the obtaining of desirably heavy coat weights at desirably high speeds as well as avoiding the problems of excessive penetration of the coating into the paper sheet; and that the high shear viscosity controls the ease of measuring, and applying the coating composition suspension to the paper.

For satisfactory high speed coating results with desirably smooth heavy coat weights using apparatus of the character described, coating material compositions may be satisfactorily produced and applied embodying this invention and having viscosities which .fall below the point of sudden drastic increase in viscosity referred to above yet having such extremely high solids content by considering the correlation of the above factors. Thus, after the amount of mineral pigment material desired for application to the surface of the paper has been determined according to principles well-known and understood in the art of high quality coated paper manufacture, the amount of starch adhesive material in combination with a viscosity reducing agent such as urea and converted to a degree necessary to produce a coating material suspension of satisfactory viscosity characteristics may be conveniently determined within the following ranges.

That is to say, satisfactory coating material composition for application according to this invention may be such that the various components such as converted starch, urea, nonbulking lubricants, and highly dispersed pigments will all cooperate so that the ratio of the high shear viscosity of the coating composition suspension to the solids content thereof falls within the ranges of from 4 to 1.5 and the ratio of the low shear or MacMichael viscosity of the coating composition suspension to the high shear viscosity thereof falls within the range of from 75 to 5. Thus it is possible to select and correlate for economical reasons or otherwise such variable factors as the extent of starch conversion, the amount of starch adhesive material needed to cause satisfactory binding of the desired quantity of pigment to the paper surface, the amount of urea and the amount of water necessary to give a coating suspension of the desired characteristics for use on apparatus of the character described to produce a coated paper product with a desirably heavy coat weight of' satisfactory surface smoothness characteristics.

While the processes and products herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise processes and products, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. In a method for coating a paper web to provide a surface adapted to receive printing impressions and utilizing roll coating apparatus of the character described, the steps of preparing an aqueous starch adhesive material converted to have a high shear viscosity corresponding to 35 to 175 centipoises measured at 140 F. and at a starch solids content of 31%, admixing with said adhesive material a deflocculated mineral coating pigment and urea as a viscosity reducing agent for said adhesive in the ratio of 80 to 95 parts by weight pigment to 3 to parts by weight urea and 8 to 15 parts by weight starch, including in said admixture a lubricant and dispersant for said pigment and water to form an aqueous coating suspension having a total solids content of about 60% to 80% and a low shear viscosity of less than 100 poises MacMichael, supplying MacMichael, supplying saidsuspension to the rolls of said apparatus, metering and conditioning said suspension with high shear working in said rolls, applying said suspension to the surface of a moving web. of paper as an evenly distributed'a'nd blended coating thereon, maintain ing during said-metering and-applying steps the low' and high shear viscosities and solidscontent of said suspen sion such that the ratio of the high shear viscosity to the solids content falls within the range of 4 to 1.5 and the ratio of'low shear viscosity to high shear viscosity falls within the range of 75 to 5, and drying said coating on said Web for the product-ion of smooth surface characteristics thereon.

2. For use in a method of coating printing paper including a supercalendering step and utilizing roll coating apparatus of the character position comprising 80 to parts by weight of mineral coating pigment, 8 to 15 parts by weight of starch converted to an aqueous adhesive sol having a highshear viscosity of 35 to 175 centipoises measured at140" F. and at 31% starchsolids content, 3 to 20 parts by weight of urea as a viscosity reducing and plasticizing agent, minor portions of a dispersant and a lubricating agent for said pigment, and water providing in said composition prior to application to paper a total solids content of about 60% to 80% and a low shear MacMichael viscosity of less than about poises, said composition being further characterized in that the ratio of the high shear viscosity to the total solids content of said composition prior to application to paper falls within the range of 4 to 1.5 while the ratio of low shear viscosity to high shear viscosity falls within the range of 75 to 5 and said urea is'present in amount efliecting a and smoothing of said coating composition in said supercalendering step after drying on said paper. 1

3. In a method of coating'paper to provide smooth surface characteristics adapted to receiving printing impressions and utilizing roll coating apparatus of the character described, the steps of supplying to the rolls of said apparatus an aqueous coating suspension comprising mineral coating pigment and a fluid starch adhesive material in the ratio of 80 to 95 parts by weight of pigment to 8 to 15 parts by weight of starch admixed with water to give a total solids content of about 60% to 80%, said suspension including 3 to 20 parts by weight of urea providing a low shear viscosity of less than 100 poises MacMichael, conditioning and metering and working said suspension in said rolls, applying said suspension to the surface of a traveling web of paper as a uniformly distributed and blended coating thereon, maintaining the rheological characteristics of said suspension during said metering and applying steps such that the ratio of high shear viscosity to total solids content of said suspension falls within the range of 4 to 1.5 and the ratio of low shear viscosity to high shear viscosity within the range of 75 to 5, drying said applied coating on said web, and supercalendering said dried coating on said web, said urea eifecting hygroscopic plasticizing of said coating during said supercalendering step producing said smooth surface characteristics.

4. A coated paper product possessing extremely smooth surface characteristics adapted for receiving fine printing lmpressions, comprising paper carrying on the surface thereof a mineral pigmented coating in excess of 7 pounds of coating per side per ream of 500 sheets 25" by 38", said coating including mineral coating pigment and converted starch adhesive and urea in the ratio of 80 to 95 parts by Weight pigment to 3 to 20 parts by weight urea and 8 to 15 parts by weight starch.

5. In a method for coating a paper web to provide a surface adapted to receive printing impressions and utilizing roll coating apparatus of the character described, the steps of preparing an aqueous starch adhesive material converted to have a high shear viscosity corresponding to 35 to 175 centipoises measured at F. and at a starch solids content of 31%, admixing with said adhesive material a deflocculated mineral coating pigment and urea as a viscosity reducing agent for said adhesive in the ratio of 85 to 95 parts by weight pigment to 5 to 15 parts by weight urea and 10 to 13 parts by weight starch, including in said admixture a lubricant and dispersant for said pigment and water to form an aqueous coating sus pension having a total solids content of about 65% to 75% and a low shear viscosity of about 15 to 40 poises said suspension to the rolls of described, a paper coating com substantial plasticizing said apparatus, metering and conditioning said suspension with high .shear working in said rolls, applying said suspension to the surface of a moving web of paper as an evenly distributed and blended coating thereon, maintaining during said metering and applying steps the low and high shear viscosities and solids content of said suspension such that the ratio of the high shear viscosity to the solids content falls within the range of 4 to 1.5 and the ratio of loW shear viscosity to high shear viscosity falls within the range of 75 to 5, and drying said coating on said web for the production of smooth surface characteristics thereon.

6. In a method of coating paper to provide smooth surface characteristics adapted to receive printing impressions :and utilizing roll coating apparatus of the character described, the steps of supplying to the rolls of said appa ratus an aqueous coating suspension comprising mineral coating pigmentand a fluid starch adhesive material in the ratio of 85 to 95 parts by weight pigment to to 13 parts by weight starch admixed with water to give .a total solids content of about 65% to 75 said suspension including 5 to 15 parts by weight urea providing a low shear viscos'ity of about 15 to 40 porses lviacMichael, conditioning and metering and working said suspension 1n said rolls, applying said suspension to the surface of a traveling web of paper as a uniformly distributed and blended coating thereon, maintaining the rheological characteristics of said suspension during said metering and applying steps such that the ratio of high shear viscosity to total solids content of said suspension falls within the range of 4 to 1.5 and the ratio of low shear viscosity to high shear viscosity within the range of to 5, drying said applied coating on said web, and supercalendering said dried .coating on said web, said urea effecting hygroscopic plasticizing of said coating during said supercalendering step producing said smooth surface characteristics.

7. For use in a method of coating printing paper including a supercalend'ering step and utilizing roll coating apparatus of the character described, a paper coating composition comprising to parts by weight of mineral coating pigment, 10 to 13 parts by Weight of starch converted to an aqueous adhesive sol having a high shear viscosity of 35 to 175 centipoises measured at F. and at 31% starch solids content, 5 to 15 parts by Weight of urea as a viscosity reducing and plasticizing agent, minor portions of a dispersant and a lubricating agent for said pigment, and water providing in said composition prior to application to paper a total solids content of about 65% to 75 and a low shear MacMichael viscosity of about 15 to 40 poises, said composition being further characterized in that the ratio of the high shear viscosity to the total solids content of said composition prior to application to paper falls within the range of 4 to 1.5 while the ratio of low shear viscosity to high shear viscosity falls within the range of 75 to 5 and said urea is present in amount effecting a substantial plasticizing and smoothing of said coating composition in said supercalendering step after dry ing on said paper.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Gregory: Uses and Applications of Chemicals and Related Materials, Rheinhold, 1939, page 629.

Claims (1)

1. 3. IN A METHOD OF COATING PAPER TO PROVIDE SMOOTH SURFACE CHARACTERISTICS ADAPTED TO RECEIVING PRINTING IMPRESSIONS AND UTILIZING ROLL COATING APPARATUS OF THE CHARACTER DESCRIBED, THE STEPS OF SUPPLYING TO THE ROLLS OF SAID APPARATUS AN EQUEOUS COATING SUSPENSION COMPRISING MINERAL COATING PIGMENT AND FLUID STARCH ADHESIVE MATERIAL IN THE RATIO OF 80 TO 95 PARTS BY WEIGHT OF PIGMENT 8 TO 15 PARTS BY WEIGHT OF STARCH ADMIXED WITH WATER TO GIVE A TOTAL SOLIDS CONTENT OF ABOUT 60% TO 80%, SAID SUSPENSION INCLUDING 3 TO 20 PARTS BY WEIGHT OF UREA PROVIDING A LOW SHEAR VISCOSITY OF LESS THAN 100 POISES MACMICHAEL, CONDITIONING AND METERING AND WORKING SAID SUSPENSION IN SAID ROLLS, APPLYING SAID SUSPENSION TO THE SURFACE OF A TRAVELING WEB OF PAPER AS A UNIFORMLY DIS TRIBUTED AND BLENDED COATING THEREON, MAINTAINING THE RHEOLOGICAL CHARACTERISTICS OF SAID SUSPENSION DURING SAID METERING AND APPLYING STEPS SUCH THAT THE RATIO OF HIGH SHEAR VISCOSITY TO TOTAL SOLIDS CONTENT OF SAID SUSPENSION FALLS WITHIN THE RANGE OF 4 TO 1.5 AND THE RATIO OF LOW SHEAR VISCOSITY TO HIGH SHEAR VISCOSITY WITHIN THE RANGE OF 75 TO 5, DRYING SAID APPLIED COATING ON SAID WEB, AND SUPERCALENDERING SAID DRIED COATING ON SAID WEB, SAID UREA EFFECTING HYGROSCOPIC PLASTICIZING OF SAID COATING DURING SAID SUPERCALENDERING STEP PRODUCING SAID SMOOTH SURFACE CHARACTERISTICS.