US2772184A

US2772184A - Paper coating

Info

Publication number: US2772184A
Application number: US381256A
Authority: US
Inventors: Dexter L Wolfe; Leroy W Hoel
Original assignee: Kimberly Clark Corp
Current assignee: Kimberly Clark Corp
Priority date: 1953-09-21
Filing date: 1953-09-21
Publication date: 1956-11-27
Anticipated expiration: 1973-11-27

Description

Nov. 27, 1956 D. L. WOLFE ETAL 2,772,184

PAPER COATING Filed Sept. 21, 1955 APP| Y APPLY COATING IMMOBILIZING DRY MIXTURE AGENT T l 1 PAPER PREss [12. 1

PAPER MACHINE PAPER TO MACHINE DRYER INVENTORS DEXTER L, WOLFE BY LEROY W. HOEL ,dawfixam ma ATTORNEYS United States Patent Ofiice 2,772,184 Patented Nov. 27, 1956 PAPER COATING Dexter L. Wolfe and Leroy w. Hoe], Appleton, Wis., I assignors, by mesne assignments, to Kimberly-Clark Corporation, a corporation of Delaware Application September 21, 1953, Serial No. 381,256

11 Claims. (Cl. 117-64) The present invention is directed to an improved process for coating paper, and is particularly directed to the coating of paper suitable for use in printing by the oifset, letterpress, roto-gravure and other printing processes.

Papers used for printing purposes are of various types and have characteristics designed to suit the conditions of the 'printing process orprocesse's employed. For certain purposes, it is desirable to use a paper which is'c'oated with a pigment material such as clay or other mineral filler,

-the pigment being bonded to the surface of the base Web as might be desired. As a result, most of the paper produced in this mannerhas to be subjected to a very'intense supercalendering operation subsequent to the application of the coating. Even then, the quality of the paper leaves much to be desired. Furthermore, the roll coating process is limited in its application to relatively thin coatings, e. g., less than about 12 lbs per side per ream of 3300 sq. ft per single application if excessive roughness is to be avoided. 7

Another process previously employed is the so-called cast coating procedure, in which a suitable coating layer is applied to a sheet by means of a roll or other applicator and the coated sheet is then pressed into contact with a smooth surfaced roll or casting drum. The coated sheet adheres very tightly to the roll or drum and normally cannot be removed from the roll until it has dried. This process produces very high grade coated paper, although it is very difiicult to apply a coating to more than one side of the sheet by means of this process, and in no case can both sides be coated simultaneously.

Considerable care'must be exercised in removing the dried sheet from the drying or-casting surface in order to avoid cracking the coating on the paper and -to facilitate this removal, it is common practice to apply a' parting agent to the surface of the drying roll before the coated sheet is applied to the roll. To our knowledge, no single 2 i J L parting agent has proven satisfactory in all respectsand much experimental work is still being carried out to .develop suitable parting agents. In addition, the very nature of the process makes this procedure tnost suitable for the application of rather heavy coatings, on the order of 15 lbs. per side per ream.

The major drawback, however, to the use of the cast coating process is the slow processing speed characteristic of this process. The relatively long time required for drying the coated sheet against the surface of the drying roll, of itself, makes the cast coating procedure a timeconsuming and expensive process;

Attempts have been made to overcome this inherent difliculty of the cast-coating process. For example, in one proposed variation, thecoating is applied 'to' the sheet, and the coated sheet is then dried by passage through an air dryer or other mechanism. Subsequently, the dried coating is remoistened, and'applied .to the surface of a casting drum, Whereon it is redried, similar to the conventional. cast coating process. The supposed advantage of this variation is a reduction in the amount of-moisture that must be evaporated from the sheet while it is in crease in the speed, of operation of the equipment. The

amount of moisture added during this remoistening-operation is supposed to be just sufificient to render the coating semi-plastic, but not fluent, during the drying of the coating on the casting surface. In practice, the process. gives rise to other difiiculties, due primarily to the considerable lapse of time'betw'een application of the coatingto the sheet and the forming and setting of that coating into the desired smooth surface. As previously mentioned, the least desirable characteristic of the high speed; pressure roll coating process'is the quality of the coated surface produced? -We*have observed that the roughness or lack of smoothness (usually called tracking or Worminess) on the surface of the coating as it comes from the coating press is due to 'the fact that the surface portion of the coating layer adheres to the applicator roll. This causes the coating to split on the applicator roll with part of the coating remaining on the paper and a part remaining on the roll. A pattern of tracks is thereby formed on the outer'surface of he coat ing and will beretained on the paper, the coarseness and character of the pattern being determined by the rheological behaviour of the coating mixture and various other factors. For example, an increase in coating Weight usuallytend to produce a greaterdegree ofworminess or tracking,

Furthermore, because of the inherent roughness and ing a plastometer reading of one hundred twentyrfive. Roll 13 may be a twenty-five inch diameter, roll, and it maybe of metal or of a resilient material. If a resilient roll is used, it may constitute a rubber covered roll having a plastometer reading of ninety. All plastometer readings herein are P. & J. 4 inch ball values.

The coating is applied to the surface of the applicator roll 12 by means of a metering roll 14 and a nip metering roll 16 between which rolls a pond of coating material 17 is continuously maintained. The outer roll 14 should be resilient, and it may comprise a ten inch diameter rubber covered roll (plastometer reading of one hundred), while the nip metering roll 16 may be a six inch diameter chromium-plated, steel roll. The

rolls

12 and 13 are driven to provide a linear web speed in the approximate range of 400-1200 feet per minute; these values, of course, are not limiting, but merely representative of the speeds which can be conveniently employed in the practice of our process.

In the illustrated structure, after the web leaves the coating nip, it passes over a small diameter, reverse turning roll 18 which may be on the order of two or more inches in diameter, or a supported smoothing bar, which may be of the order of one-fourth inch in diameter. Next, the coated web passes into a reaction nip between a backing roll 19 and a reaction roll 21, at least one of which should be a resilient roll. In one particularly satisfactory embodiment of the invention, the backing roll was a twenty-five inch, resilient, roll having a surface plastometer reading of forty-five, while the reaction roll 21 was a twenty-five inch chromium-plated, steel roll. A metering roll 22, of resilient construction and having a plastometer reading of about thirty-five at its surface, was arranged to apply a pond 23 of the setting composition to the reaction roll 21 so as to form a film of the setting composition on the surface of the roll 21.

' As this film contacts the surface of the previouslyapplied fluent coating, the reaction commences. The Web speed is such that the application of the film to the coated web and the pressure exerted at the nip between the rolls 19 and 21 is of very short duration. As a result, the reaction proceeds to a point where the wet, plastic coating is rendered nontacky and hardened at least at the surface, the extent to which the hardening proceeds inwardly of the coating being dependent to a large extent on the thickness of the coating. When coating materials of the types to be hereinafter described are used, it appears that 'the reaction time involved in achieving a non-tacky, hardened surface does not exceed a fraction of a second, and preferably should not exceed 0.05 second. In other words, the coating should become sufficiently nonadherent within this short time interval to be readily releasable from the roll without drying. The coating may, of course, be held in contact with the roll surface for a longer period, if desired.

The nip pressures between the

rolls

14, 16 and 12, may be approximately 60 pounds per linear inch; the nippressures between the roll 12 and roll 13 may be approximately 50 pounds per linear inch and the nip pressure between rolls 19 and 21 may be approximately 80 pounds per linear inch.

The smoothing means, such as the roll 18 should be such as to make the coating on the paper as smooth as possible before the coated paper enters the reaction nip. At the nip, the pressure between the roll 19 and the roll 21 provides a very important additional smoothing at the time the reaction is taking place. In fact, without this additional smoothing effect, the coating, as applied by the usual pressure roll coating mechanism, may retain defects which will not be entirely erased during the reaction smoothing in most instances.

If, however, the coating is sufiiciently smooth at the time of its initial application, which is the case with certain coating methods, theintermediate smoothing may where the coating rolls are operated to give a linear velocity of 465 feet per minute, the rolls in the reactor train should be operated to yield a linear speed of about 470 feet per minute. After the smooth, nonadherent coated web leaves the surface of the reaction roll 21, it may be passed immediately to conventional driers where the moisture content of the coated Web is reduced to the desired value without danger of having the coating adhere to guide rolls or drier rolls.

An apparatus for coating both sides of. the paper web is illustrated in Figure 3. Similarly to the previously described apparatus, this mechanism may be used in conjunction with a paper machine schematically illustrated at 26 in an on-the-machine coater or with. an unwind stand in an otf-the-machine coater. A paper web 27 from either of these sources is passed into a coating nip between'a pair of applicator rolls 28 and 29. A metering train consisting of a metering roll- 31 and a nip metering roll 32 applies a coating composition from a pond 33 locatedbetween the rolls 31 .and 32 to the surface of the lower applicator roll 28. Similarly, a train of rolls 34, 35.and 36 applies a coating composition from a pond 37 to, the surface of the upper applicator roll 29.

The paper web after leaving the coating nip has its coating smoothed by a pair of smoothing rolls-or bars-38 and 39 located on opposite sides ofthe web. Next, the coated Web is passed into the reaction nip between apair of

rolls

41 and 42, which are driven at a slightly higher velocity than the applicator rolls 28 and, 29,- to allowfor the slight expansion of the web Which occurs during the coating operation. A metering roll 43 applies a solution of the immobilizing agent from a pond 44 to the surface of the roll 41 in the form of a thin film. Similarly, a second metering roll 46 applies an immobilizing composition from the pond 47 to the surface of the roll 42. As in the previously described embodiment, a the web velocity and the roll velocities are adjusted so that the coated surface is in contact with the immobilizing or reacting agent applied by the surface of the

rolls

41 and 42 for a period of time sufiicient to produce the desired hardened, surface condition, which in the case of the materials to be mentioned, is substantially less than one second, and preferably not in excess of about 0.05 second. The ability of the present invention to produce a condition of immobility and non-adherence in the coating in a very short interval of time is of especial importance in the simultaneous smoothing of both sides of the sheet.

After the reaction, the nonadherent, coated web is then conducted to suitable drying equipment. At leastvone of the rolls in each of the main roll combinations 2829 and 41-42 should be resilient. Rollcombinations structurally similar to the combinations 1213 and 1921 have been used with good results. In general, it will be understood that changes in the main roll combination 2829 and 41-42 may require complementary changes in the cooperating

roll combinations

3132, 343536, 43 and 46. Desirable arrangements and combinations are disclosed in U. S. Patent No. 2,606,520 to Hoel.

After drying, the coatings produced by our process can be super-calendered by relatively light pressures of about 500 lbs. per linear inch (applied to the-super-calendar stack) to produce coated papers having a dull finish and excellent printing properties. Because the coatingscan be finished such relatively light pressures, the present invention makes possible the inclusion of asuper-calendar assemblyas part of the same machine includes the coating mechanism, where previously, the super-calendar was necessarily a separate assembly. Q

The present invention is applicable to the coating of a wide variety of base sheets of varying composition and surface characteristics. 'Forexample, we have used successfully basesheets containing 50-55% groundwood with thebalance chemical pulp,-s'uch as bleached kraft or bleached sulphite. A particularly satisfactory coated sheethas been made by use of abase sheet containing 40 percent bleached groundwood pulp and 60 percent 1 chemical pulp consisting of one-half bleached kraft pulp and one-half bleached sulfite'pulp. Very high quality coated paper has been made by the use of base sheets containing 100 percent chemical pulp, of which 70 percent has consisted ofbleached, short-fiber kraft. Regardless of the type of base sheet used to achievethe best coating results, the base sheet should have as smooth a surface as possible before entering the coating equipment and when used with coating compositions as hereinafter described; should preferably have a moisture content of about 10 percent on a bone dry basis, although satisfactory. results can be achieved with sheets of higher moisture contents.

The invention lends itself especially to multiple layer coating operations. To illustrate, a first or base coating or coatings of any desired weight can be applied by conventional procedures or by the process of the present invention, and this can be followed by an overlying or final coating according to this invention. Such combinations produce particularly high grade finished coatings. As an example of the procedure, a base coating of 5-15 pounds per side, per ream of 3300 square feet, has been applied to a suitable base sheet followed by an overlying, final coating of 5-15 pounds per sideto produce a total coat weightof from about 10 to 30 pounds per side.

The various important advantages of the present invention can be realized in the application ofvarious types of coatings, and the invention can be practiced by the use of various types of immobilizing agents. The primary requirement which dictates the, selection of suitable coating compositions and suitable immobilizing agents is the requirement that the coating composition include a compound which can be rapidly immobilized into a nonadherent or at least a non-ofisetting condition by an ingredient present in the material applied at'the reaction mp. r

As will be more evident from the following description,

. the word immobilizing as used in this specification should be interpreted to include various types of chemical and physical reactions. For example, one of the most suitable coating compositions useful for *our process contains a proteinaceous adhesive such as casein or soya protein in the coating composition. These proteinaceous materials in alkaline solution react very readily with compounds such as papermakers alum (Al2( SOr)s-18H2O) or mixtures of papermakers alum and formaldehyde, in

solution, to form a hardened surface film which resists offsetting onto the surface of the reaction roll as the sheet is removed from contact with that roll. The term immobilizing when applied to the treatment of such materials and to cellulose derivatives would therefore relate V primarily to a chemical reaction.

for practicing the present 1 invention, either alone or in combination with others and immobilizing agents which are appropriate to each of the classes is giyen belovvr- Reactive Materials 7 lmmobilizingagents I. Proteina'ceous adhesives: e A; Casein in alkaline soluon. B. Soya proteln'in alkaline solution.

0. Animal glues.

rivatives:

se. B. Oarboxy methyl methyl cellulose. III. Alkali soluble cellulose derivatives:

' A. Hydroxy ethyl cellulose.

B. Oxidized cellulose. IV. Latex of elastomers:

A. Polychloroprene.

. Esters of polyacrylic acid. Polystyrene hutadiene copolymers. Isoprene 'isobutylenc copolymers. Polyaerylonitrile butadiene copolymers. F. Natural rubber. V. Emulsions of compounds:

A. Polyvinyl acetate emulsions. B. Polyvinyl chloride-acetate copolymers. C. Polyvinylidene chloridevinyl chloride copolymers. VI. Sodium alginate.

VII. Sodium salt of styrene- II. Water soluble cellulose demaleic anhydride copolymers.

I. Bivalent and trivalent ions minum),

papermakers alum.

III. Acidic salts (particularly alum and ammonium sulfate).

iron).

V. Electrolytes such as NazCOa.

VI. Biyalent and trivalent ions (particularly calcium and aluminum VII. Bivalent and trivalent ions (particularly aluminum and iron) (particularly copper zinc aluformaldehyde plus II. Bivalent and trivalent ions (particularly aluminum and A. Caitrbo'xy methyl cellulead).

W. Bivalent and trivalent 1o... (particularly aluminum and The reactive materials listed above, as well as others which fall within the general classes set forth, vary in 7 their effectiveness. Thus, carboxy methyl cellulose provides an excellent coating compound for the manufacture of printing papers. Hydroxy ethyl cellulose, in contrast, while satisfactory for use in some coating compounds, may not be satisfactory. for other types, because of the relatively high concentration of alkali necessary to :dissolve this material.

The alkali-soluble cellulose derivatives cannot be used with certain of the latices. Similarly, water soluble cellulose derivatives, such as carboxy-methyl cellulose, and

water-soluble alginate, such as sodium alginate, may be' degraded under the conditions necessary to maintain alkali-soluble cellulose derivatives .in solution. Hence, these materials would not usuallybe used with the alkalisoluble cellulose derivatives. Starch is'also degraded in alkaline solutions, and hence starch is notsuitable as an extender for alkali-soluble cellulose derivatives.

In this connection, it will be understood that the various reactive binder materials listed above, while capable of being used alone, will most commonly be used in combination with each other or with an extender such as starch. Usually, because of cost consideration, it is desirable to use the smallest possible amount of reactive material or materials. The minimum amount that can-be used successfully will vary somewhat, depending on the characteristics of that material and the operating conditions. The required flow, leveling and setting properties of the coating will also limit the maximum amount of j reactive binder material that will be used.

should contain not less than about 5 parts of the reactive binder material per 100 parts of mineral. It-will be understood, as previously pointed out, that more than one of the reactive materials may be used, andthat in instances where cost is important, it will be found advantageous to include starch as an extender material in the coating composition. If more than one reactable binder is used in any given coating composition, the individual amounts of each binder can be reduced over that" required if the binders are used alone. Carboxy methyl cellulose, as above noted, is particularly effective as a reactive material and can be used in smaller amounts than the proteinaceous binders or latices, and when used alone or in combination with other reactive materials, will reduce the total minimum amount of reactive binder required.-

When using a proteinaceous adhesive, satisfactory coating compositions have been made by the combination of 100 parts mineral with from 12 to 16 parts by weight of the proteinaceous adhesive. If starch is added to the mixture, the amount of proteinaceous adhesive can be reduced to a value within the range of from about 8 to' 10 parts, the total of the starch and the proteinaceous adhesive being in the range of from about 14 to 20 parts by weight to 100 parts by weight of mineral. When a latex is added, in addition to the starch, about 3 to 10 parts of latex solids may be employed and the amount of starch and protein may be correspondingly decreased so that the total content of proteinaceous adhesive, starch and latex is in the range of from 14 to 20 parts by weight per 100 parts of mineral. Satisfactory coating compositions can also be made by combination of proteinaceous adhesive and latex without starch. For roll coating operations with such combinations, a minimum of about '6 parts of the proteinaceous adhesive per 100 parts of mineral should be used with a minimum of from 8 to 14 parts of latex.

Starch as used herein is intended to refer to the modified starches commonly employed in paper coatings. Chemically modified or enzyme modified starches are typical.

When using a water-soluble cellulose derivative binding material, which is another of the particularly preferred reactive binder materials, satisfactory coating compositrons will result from the use of mixtures containing 100 parts mineral and from aboutl4 to 18 parts of carboxy methyl cellulose. Because of costs and flow property cons derations, it is generally preferred to use carboxy methyl cellulose in combination with starch or with starch and one or more of the other reactive binder materials, such as latex. When used with starch alone, it will generally be found satisfactory to provide from 2 to 6 parts by weight of carboxy methyl cellulose and from 10 to 14 parts by weight of starch to each 100 parts of mineral. Sansfactory coating compositions using carboxy methyl cellulose with starch and latex have been made by combining from 1 to 5 parts carboxy methyl cellulose with 4 to 14 parts of starch and from 3 to 10 parts latex per 100 parts 'of mineral, all on a weight basis. Carboxy methyl cellulose may also be used with latex alone. Suchcompositions will generally include from 4 to '6 parts carboxy methyl cellulose and from 10 to 14'p'arts latex per 100 parts of mineral. v

In connection with the foregoing general examples,- it

"should be kept in mind that the maximum economical binder content is usually from to 20 percent by weight based on the mineral content of the composition.

Th'e'rather substantial number of available reactive materials makes possible a considerable variation in the coating compositions and large numbers of these, in addition to those specifically set forth above, will be apparent to those skilled in the art.

A choice of a' particular immobilizing agent for a given compound will be based upon various considerations. For example, copper salts or iron salts would not be employed where it is desired'to avoid any coloring in the set coating.

The best over-all immobilizing agent which we have found in our work is papermakers alum (A1z(SO4)s-18 H20) and the term alum as used in this specification and claims is intended to mean that compound. This compound not only has the ability to precipitate or coagulate binders of various types into a nonadherent form, but when used as an immobilizing agent where calcium carbonate is present in the filler, the alum provides a very definite brightening effect to the'applied coating. This brightening effect has important commercial utility, and is one of the more unexpected advantages of the invention.

For most types of coating applied in weight of 2 to 20' pounds per side per ream of 3300 sq. ft., about 0.1 to 1 pound of alum per dried ream of paper will be sufficient to set the coating to the extent required. The same amount of immobilizing agent is appropriate for agents other than alum. The immobilizing solutions may also include such auxiliary materials as surface active agents or acids.

Our experience has indicated that a small excess of the immobilizing agent should be applied to the reaction roll in order to achieve th 'best results. That is, a sufficient amount of the agent should be applied to the roll to leave a film of the agent on the roll after the reacted coating is stripped from the roll. On the other hand, the excess should not be so large as to form a visible bead of the immobilizing agent in advance of the pressure nip, as premature reaction will occur in the coating and proper smoothing is not likely to be achieved.

In practicing this process, there should be a reasonable balancing and correlation of the coating film thickness with reference to the absorbency characteristics of the base sheet and to the flow properties of the coating suspension. The application of too thick a film by the coating apparatus may lead to surface rupture of the coating at the reaction nip and difiiculty in releasing the coated paper from the reaction roll. If too thin a film is applied, the coating will be insufficient to fill in all the depressions in the Web, with the result that the smoothness will not be as high as desirable.

The best coating mixtures have been found to contain about 35 to 60 percent by Weight of solids, the major portion of the solids constituting coating clay, or a mixture of clay and calcium carbonate and the remainder of the solids constituting the'binder material, which may include the reactable binder alone or in combination with a non-reactable binder such as starch. The minimum amount of binder is about 10 percent, based on the weight of the mineral pigment, and the usual amount is from about 14 to 20 percent. The ability to use such high solids content coatings is a distinct advantage for this process, as it decreases the drying load, and thereby permits faster coating speeds.

Control of the viscosity of the coating solution is also important to achieve best results. For pressure roll coating processes, we prefer to employ coating suspensions having a MacMichael viscosity of from about 40 to (measured with a No. 26 wire at 40 R. P. M. and 70 F.). Coating suspensions with higher viscosities are more diflicult to level, and those with lower viscosities are more liable to rupture in the nip. Generally higher values of viscosity can be handled satisfactorily when the coating applied is to b relatively heavy, and the lower viscosities are more appropriate for light weight coatings.

Illustrative coating mixtures suitable for use in the practice of the invention are given in the following specific examples:

Example I The coating mixture had the following compositions:

12 parts starch 12 parts by weight of a latex of a styrene-butadiene copolymer having a solids content of 48 per cent, representing 6 per cent of the solids in the coating mixture.

The clay was dispersed in a water solution with 0.3 percent solium hexametaphosphate and 0.1 percent of sodium hydroxide and the carboxy methyl cellulose and latex added to produce a coating mixture having a final solids content of 50 percent and a MacMichael viscosity of 70.

For-the above described coating mixture, the immobilizing agent employed consisted of a solution of papermakers alum having a specific gravity of 1.18 at 25 C. About 0.5 lb. of alum per ream of paper was employed to immobilize the coating.

Example II In each of the succeeding formulae, the pigment filler consisted of approximately 67 percent coating clay and 33 percent calcium carbonate. The pigments were dispersed in a water solution containing 0.3 percent of sodium hexametaphosphate and 0.2 percent sodium hydroxide. The composition of binders which were employed with this pigment fillerv are listed below, the parts t r (K) 6 parts sodium alginate 12 parts starch v 1 part sodium alginate 12 parts starchv 4 parts latex ofstyrene-butadiene copolymer.

(M) 1 part sodium algmate 10 parts starch V 6 parts plasticized polyvinylidene chloride-vinyl chloride copolymer I t t (N) a a 3 parts sodiuni salt of styrene-maleic anhydridecopolymer I r a 7 parts soya'protein' 8 parts starch In each case, the coatings were effectively reacted into a hardened, non-offsetting, non-tacky surface film by the application of about 0.5 lb. of alum perream of 3300 sq. ft. r r 1 Example 111 In this formulation, the paper was coated with'a pigment filler and a binder identical in composition to that shown in section E of the Example II. The coating was effectively immobilized by the application of 0.5 lb. of

by weight being based upon 100 parts of the mixture of clay and calcium carbonate.

3 parts soya protein 12 parts latex of styrene-butadiene copolymer (50 percent solids) 15 parts soya protein 7 parts soya protem 7 parts starch 3 parts latex of styrene-butadiene copolymer 6 parts low viscosity carboxy methyl cellulose 2 parts medium viscosity carboxy methyl cellulose 11 parts starch 4 parts latex of styrene-butadiene copolymer 10 parts latex of styrene-butadiene copolymer 7 6 parts starch 7 parts soya protein 7 parts starch 3 parts latex of polyacrylonitrile butadiene copolymer 10 parts soya protein or casein 3 parts polyvinylidene chloride-vinyl chloride copolymer 3 parts styrene-butadiene copolymer zinc chloride per ream in aqueous solution to the coating and after drying, the coating had an excellent smoothness and a level surface. 7

Example IV The coating in this instance was identical with that described in section L of Example 11. This coating was eflectively immobolized by the application of 0.5 lb. of calcium chloride per ream of coated paper to produce an excellent printing surface after drying of the paper web and its coating.

Example V parts clay dispersed in water containing 5 percent NaOH 8 parts hydroxyethyl cellulose (degree of substitution .35 to .4) dissolved in water containing 5 percent NaOH Thiscoating was effectively immobolized by the application of' 0.5 lb. of alum per ream of coated paper.

Example VI A good barrier coating for the manufacture of greaseproof paper was prepared by coating 21 base Web with a composition containing 30 percent by weight ortho protein, 30 percent starch, and 40 percent of a latex of a styrene-butadiene copolymer. This coating was immobilized by the application of an aqueous solution of alum in amountssufl'icient to provide 0.5 lb of alum per ream of paper. was produced on the paper. V

The flow properties of coatings compounded as described in the foregoing can be readily adjusted to suit the type of coating mechanism employed. For, example, if a pressure roll coater is used, a relatively high solids, high viscosity coating would be compounded. If an air knife or reverse roll coater is to be used, 'a lower solids, lowviscosity type of coating would be made. The wide variety of reactable binders which this process can use therefore makes possible a wide range of coating flow properties. a 7

Minor ingredients such as eveners, softening agents, wetting agentsand dye stufis can be added to any of the above coatings, as desired.

The present invention makes possible the production of high quality coatings of improved brightness, opacity, and bulk at low supercalendering pressuresat speeds which could never previously be employed in the production of Upon drying, at smooth, level coating 7 coated paper of Comparable quality. The process of the present invention provides a coated paper of excellent printing quality, suitable for use in the offset, letterpress, and rotogravure processes.

This application is a continuation-in-part of our copending application, Serial No. 202,923, entitled Paper Coating, filed on December 27, 1950, and now abandoned.

It will be evident that various modifications can be made to the processes described above without depart ing from the scope of the present invention.

'We claim:

1. The process of coating paper which comprises continuously moving a web of paper, continuously applying to at least one surface of said web a coating composition containing from 35 to 60 percent by weight of solids and including a pigment and a hardenable binder, and relatively soon thereafter and without applying any intermediate'drying step so that at least the outermost region of said coating remains in a wet, mobile, moldable state capable of offsetting upon a surface brought to bear upon the outer surface of said coating, contacting the outermost surface of the coating on said Web with a pressure member, continuously applying a regulated amount of a liquid immobilizing agent to the surface of said pressure member before it is brought into contact with said coating, said liquid immobilizing agent being capable of almost instantaneously converting the outermost region of said binder to a non-tacky, hardened state, to almost instantaneously convert and render non-adherent and smooth a layer of said coating composition at the outermost surface thereof when said pressure member contacts said coating, said regulated amount of liquid immobilizing agent being insufiicient to form a visible bead of the immobilizing agent at the point at which said pressure member first contacts said coating, withdrawing said coated Web from pressure contact with said pressure member to produce a paper Web which is provided with a smooth, continuous, level coating, the regulated amount of said liquid immobilizing agent being such that a film of such agent is left on said pressure member after said converted coating is withdrawn from said pressure member, and then drying the web and coating without further smoothing during the drying operation.

2. A method according to claim 1 wherein said reactive binder includes an alkaline solution of soya protein.

3. A method according to claim 1 wherein said reactive binder includes carboxy methyl cellulose.

4. A method according to claim 1 wherein said reactive binder includes a copolymer of polystyrene and butadiene;

5. A method according to claim 1 wherein said reactive binder includes sodium alginate.

6. A method according to claim 1 wherein said reactive binder includes a sodium salt of a styrenemaleic anhydride copolymer.

7. The process of coating paper which comprises continuously moving a web of paper, continuously applying to one surface of said web a coating composition containing from 35 to 60 percent by weight of solids and including a pigment and a hardenable binder, and relatively soon thereafter and without applying any intermediate drying step so that at least the outermost region of said coating remains in a wet, mobile, moldable state capable of offsetting upon a surface brought to bear upon the outer surface of said coating, contacting the outermost surface of the coating on said web with a rotating pressure member, continuously applying a regulated amount of a liquid immobilizing agent to the surface of said rotating pressure member before it is brought into contact with said coating, said liquid immobilizing agent being capable of almost instantaneously converting the outermost region of said binder to a non-tacky, hardened state to almost instantaneously convert and render non-adherent and smooth a layer of said coating composition at-the outermost surface'thereof when said pressure member contacts said coating, said regulated amount of liquid immobilizing agent being insufiicient to form a visible bead of the immobilizing agent at the point at which said pressure member first contacts said coating, maintaining said pressure member in engagement with said coating for a period of time at least equal to the period of time required to render said coating nonadherent to and non-offsetting on the surface of said pressure member, withdrawing said coated web from pressure contact with said pressure member to produce a paper web which is provided with a smooth, continuous, level'coating, the regulated amount of said liquid immo bilizing agent being such that a film of such agent'is left on said pressure member after said converted coating is withdrawn from said pressure member, and then drying the web and coating Without further smoothing during the drying operation.

8. The process of coating paper which comprises continuously moving a Web of paper, continuously applying to one surface of said web a coating composition containing from 35 to 60 percent by weight of solids and including a pigment filler and a hardenable binder composition, and relatively soon thereafter and Without applying any intermediate drying step so that at least the outermost region of said coating remains in a wet, mobile, moldable state capable of offsetting upon a surface brought to bear upon the outer surface of said coating, smoothing the coating on said Web of paper and then contacting the outermost surface of the coating with a rotating pressure member, continuously applying a regulated amount of a liquid immobilizing agent to the surface of said rotating pressure member before it is brought into contact with said coating, said liquid immobilizing agent being capable of almost instantaneously converting the outermost region of said binder composition to a non-tacky, hardened state to almost instantaneously convert and render nonadherent and smooth a layer of said coating composition at the outermost surface thereof when said pressure member contacts said coating, said regulated amount of liquid immobilizing agent being insufiicient to form a visible head of the immobilizing agent at the point at which said pressure member first contacts said coating, maintaining said pressure member in engagement with said coating for a period of time at least equal to the period of time required to render said coating non-adherent to and non-offsetting on the outer surface of said pressure member, withdrawing said coated web from pressure contact with said pressure member to produce a paper web which is provided with a smooth, continuous, level coating, the regulated amount of said liquid immobilizing agent being such that a film of such agent is left on said pressure member after said converted coating is withdrawn from said pressure member, and then drying the web and coating without further smoothing during the drying operation.

9. The process of coating paper which comprises continuously moving a web of paper, continuously applying to at least one surface of said web a hardenable coating composition containing from 35 to 60 percent by weight of solids, and relatively soon thereafter and without applying any intermediate drying step so that at least the outermost region of said coating remains in a wet, mobile, moldable state capable of offsetting upon a surface brought to bear upon the outer surface of said coating, passing said web between a pair of press rolls, continuously applying a regulated amount of a liquid immobilizing agent to the roll surface which contacts the coated surface of said web of paper before said roll surface is brought into contact with said coating, said liquid immobilizing agent being capable of almost instantaneously converting the outermost region of said coating composition to a non tacky, hardened state to almost instantaneously convert and render non-adherent a layer of said coating composi tion at the outermost surface thereof as said Web passes through said press rolls, said press rolls engaging said coated web with suflicient pressure to smooth the coated surface as said immobilizing agent contacts said mobile, moldable coating composition, said regulated amount of liquid immobilizing agent being insuflicient to form a visible bead of the immobilizing agent in advance of the pressure nip of said press rolls, withdrawing said coated web from pressure contact with said press rolls to produce a paper web which is provided with a smooth, continuous, level coating, the regulated amount of said liquid immobilizing agent beingsuch that a film of such agent is left on the press roll which contacts said coated surface after said converted coating is withdrawn from said roll, and then drying the web and coating without further smoothing during the drying operation.

10. The process of coating paper which comprises continuously moving a web of paper, continuously applying to at least one surface of said web, a layer of fluid coating material containing from 35 to 60 percent by weight of solids and including a pigment filler and a hardenable binder composition, and relatively soon thereafter and without applying any intermediate drying step so that at least the outermost region of said layer of fluid coating material remains in a wet, mobile, moldable state capable of oflsetting upon a surface brought to bear upon the outer surface of said coating, passing said web into the nip between a pair of press rolls, continuously applying a regulated amount of a fluid reactive hardening material to the roll surface which contacts the coated surface of said Web of paper before said roll surface is brought into contact with said coating, said fluid reactive hardening material being capable of almost instantaneously converting the outermost region of said coating material to a non-tacky, hardened state to almost instantaneously convert and produce a hardened, non-adherent surface condition at the outer surface of said coating during the passage of said web through said press rolls, said press rolls engaging said coated web with suflicient pressure to smooth the surface of said coating as said reactive material contacts said mobile, moldable coating material, said regulated amount of reactive material being insuflicient to form :a visible bead of the reactive material in advance of the pressure nip of said press rolls, withdrawing said coated web from pressure contact with said press rolls to produce a paper web which is provided with a smooth, continuous, level coating, the regulated amount of said reactive material being such that a film of such reactive material is left on the press roll which contacts said coated surface after the converted coating is withdrawn'from said roll, and then drying the web and coating without further smoothing during the drying operation. 7

11. The process of coating paper which comprises con tinuously moving a web of paper, continuously applying to at least one surface of said Web a coating composition '16 containing from 35 to per cent by weight of solids and including calcium carbonate and a hardenable binder composition, moving the coated paper without applying any intermediate drying step and While the outermost region of said coating remains in a wet, mobile, moldable state capable of offsetting upon a surface brought to bear upon the outer surface of said coating, into the nip between a pair of rotating press rolls, continuously ap plying a regulated amount of an aqueous solution of (alum to the roll surface which contacts the coated sur: face of said web of paper, before said roll surface is brought into contact with said coating, said aqueous solution of alum being capable of almost instantaneously converting the outermost region of said coating composition to a non-tacky, hardened state to almost instantaneously convert and render non-adherent a layer of said coating composition at the outermost surface thereof and to react with the calcium carbonate in said surface layer to effect an increase in the surface brightness of the coated web as said web passes through said press rolls, said press rolls engaging said coated web with suflicient pressure to smooth the coated surface and thereby pro duce during the time that said coated web is in the nip between the press rolls a smooth, hardened film resulting from the almost instantaneous reaction between said reactive material and said aqueous solution of alum, said hardened film preventing adherence and offsetting of the coating onto the pressure roll surface which contacts the coating during the pressure molding operation, said regulated amount of alum solution being insuflicient to form a visible bead of the alum solution in advancerof the pressure nip of said press rolls, withdrawing said coated web from pressure contact with said press rolls to produce a paper web which is provided with a smooth, continuous, level coating, the regulated amount of said alum solution being such that a film of said alum solution is left on the press roll which contacts said coated surface after the converted coating is withdrawn fromtsaid roll,

and then drying the web and coating without further,

smoothing during the drying operation.

References Cited in the file of this patent UNITED STATES PATENTS Trosset et al. Dec. 28, 1954

Claims

1. THE PROCESS OF COATING PAPER WHICH COMPRISES CONTINUOUSLY MOVING A WEB OF PAPER, CONTINUOUSLY APPLYING TO AT LEAST ONE SURFACE OF SAID WEB A COATING COMPOSITION CONTAINING FROM 35 TO 60 PERCENT BY WEIGHT OF SOLIDS AND INCLUDING A PIGMENT AND A HARDENABLE BINDER, AND RELATIVELY SOON THEREAFTER AND WITHOUT APPLYING ANY INTERMEDIATE DRYING STEP SO THAT AT LEAST THE OUTERMOST REGION OF SAID COATING REMAINS IN A WET, MOBILE, MOLDABLE STATE CAPABLE OF OFFSETTING UPON A SURFACE BROUGHT TO BEAR UPON THE OUTER SURFACE OF SAID COATING, CONTACTING THE OUTERMOST SURFACE OF THE COATING ON SAID WEB WITH A PRESSURE MEMBER, CONTINUOUSLY APPLYING A REGUALTED AMOUNT OF A LIQUID IMMOBILIZING AGENT TO THE SURFACE OF SAID PRESSURE MEMBER BEFORE IT IS BROUGHT INTO CONTACT WITH SAID COATING, SAID LIQUID IMMOBILIZING AGENT BEING CAPABLE OF ALMOST INSTANTEOUSLY CONVERTING THE OUTERMOST REGION OF SAID BINDER TO A NON-TACKY, HARDENED STATE, TO ALMOST INSTANTANEOUSLY CONVERT AND RENDER NON-ADHERENT AND SMOOTH A LAYER OF SAID COATING COMPOSITION AT THE OUTERMOST SURFACE THEREOF WHEN SAID PRESSURE MEMBER CONTACTS SAID COATING, SAID REGULATED AMOUNT OF LIQUID IMMOBILIZING AGENT BEING INSUFFICIENT TO FORM A VISIBLE BEAD OF THE IMMOBILIZING AGENT AT THE POINT AT WHICH SAID PRESSURE MEMBER FIRST CONTACTS SAID COATING, WITHDRAWING SAID COATING WEB FROM PRESSURE CONTACT WITH SAID PRESSURE MEMBER TO PRODUCE A PAPER WEB WHICH IS PROVIDED WITH A SMOOTH, CONTINUOUS, LEVEL COATING, THE REGULATED AMOUNT OF SAID LIQUID IMMOBILIZING AGENT BEING SUCH THAT FILM OF SUCH AGENT IS LEFT ON SAID PRESSURE MEMBER AFTER SAID CONVERTED COATING IS WITHDRAWN FROM SAID PRESSURE MEMBER, AND THEN DRYING THE WEB AND COATING WITHOUT FURTHER SMOOTHING DURING THE DRYING OPERATION.