US2819184A - Production of cast surfaced coated paper - Google Patents

Description

PRODUCTION OF CAST SURFACE!) COATED No Drawing. Application October 22, 1953 Serial No. 387,804

4 Claims. (Cl. 117-64) This invention relates to a process for the production of mineral-coated paper having a high gloss of the kind commonly called cast-surfaced coated paper. By mineral-coated paper we mean paper bearing a coating consisting essentially of a mineral pigment and a hydrophilic adhesive.

Cast-surfaced mineral-coated paper was first described in Bradner Patent No. 1,719,166, issued July 2, 1929. Since that time several minor variations in procedure have been made public, all of which yield products generally similar in appearance to that described in said Bradner patent. All known methods of producing castsurfaced coated paper are similar to the extent that in all cases a coated surface of the paper is set while in adherent contact with a finishing surface such as a polished chromium-plated cylinder or drum, from which the coated surface separates with a finish substantially mirroring the finishing surface.

A difficulty often encountered in the production of cast-surfaced coated paper is sticking of the set coated surface to the finishing surface, resulting in marring of the coated surface or, in cases of extensive sticking, even complete spoilage of the coated product. U. S. Patent No. 2,568,288 to Montgomery discloses a method of preventing sticking. According to this patent there is maintained on the finishing surface an invisible film, substantially less than 2000 Angstrom units in thickness, of an oil or other fatty material, such substances being designated as oleaginous film-forming material. The film has to be maintained at a critical thickness not great enough to prevent adhesion of the wet coated surface to the finishing surface but still thick enough to prevent sticking of the dry coated surface to the finishing surface. Since the paper removes part of the oleaginous film from the finishing surface it is necessary to replenish the oleaginous film. The preferred method of maintaining the film is to include in the aqueous mineral coating composition with which the paper is coated a quantity of the oleaginous release agent generally amounting to between 0.1 and 1.0% of the dry weight of the coating. Enough of the oleaginous material is adsorbed on the finishing surface from the wet coating composition to make up for the quantity which the dried coating takes away and thus the oleaginous film is maintained.

it is apparent that for optimum results the process of Patent 2,568,288 requires very close and constant observation with adjustment of the oleaginous ingredient in the coating composition to maintain the film at the desired thickness. Any increase in the thickness of the oleaginous film over the minimum required for satisfactory release causes a corresponding diminution in the gloss of the finished coated paper. Even a moderate increase in the thickness of the oleaginous film may cause a mottled appearance of the finished coated paper and may leave sufficient grease on the surface of the coated paper both to make the paper distasteful to handle and to produce scumming on a printing press when the paper is printed.

It is an object of this invention to provide a non-oleates Patent Patented Jan. 7, 1958 agiuous release agent to facilitate the separation of castsurfaced coated paper from the finishing surface against which it is dried.

Another object is to provide a release agent for cast surfaced coated paper which when used in excess of the minimum quantity required for release does not cause diminution of gloss of the finished coated paper.

Another object is to provide a release agent for use in making cast-surfaced coated paper which release agent substantially all leaves the finishing surface and becomes or remains a part of the cast surface of the coated paper.

Another object is to provide a method of imparting various desirable functional qualities to cast-surfaced coated paper without detracting from the glossiness thereof.

The preferred release agents for use according to the invention are normally solid resinous materials which are soluble, either molecularly or colloidally, in aqueous 'media which contain dissolved alkali or other agent to solvate this resinous material. Such resinous release agents are particularly adapted for use in producing castsurfaced coated paper by the modern processes in which a layer of coating already formed on a paper base is brought into contact with a finishing surface while both the layer of coating and the finishing surface are bathed in aqueous medium.

The use of such an aqueous bath is an exceedingly important development in the art of producing cast-surfaced coated papers and is disclosed in Patent No. 2,678,890 and also in Patent No. 2,759,847. According to the disclosure in Patent No. 2,678,890 a freshly applied but already formed layer of coating on a paper web is brought, before it has dried, into contact with a heated finishing surface, e. g. a polished chromium plated drum, while the line of contact i. e. the nip between the coated surface and the finishing surface is kept flooded to a depth of A to 2 inches with aqueous medium substantially free from suspended solids. The resulting product is indistinguishable from prior art cast-surfaced coated paper, but the simplicity and cleanliness of operation of the process appear to render prior-art methods obsolete. Patent No. 2,759,847 discloses a process by which paper is coated, dried, and super-calendered, and then the densified supercalendered surface is passed through a pool of aqueous medium substantially free from suspended solids and into contact with a heated finishing surface, against which it is dried as in Patent No. 2,678,890. Visually the product yielded by this process appears identical with prior art cast-surfaced coated papers but the product, due to its having been previously supercalendered, possesses other desirable properties not obtainable previously in cast-surfaced coated papers.

To practice the present invention in conjunction with the two previously described modern methods of producing cast-surfaced coated paper requires merely that resinous material be dissolved in the pool of aqueous moistening liquid which is used to wet the coated surface just before it makes contact with the finishing drum. The present invention is, however, not restricted to the presence of the resinous material in the pool of moistening liquid. Resin solution may be applied to the coated surface, as for example as a spray, before the paper enters the pool in which case the pool may either contain resin or be free of resin. The invention is useful in any process for producing cast-surfaced coated paper in which the surface of the coating can be wet with aqueous solution of resinous material prior to contact of said coating with the finishing surface.

It will be seen that the resinous release agents of the present invention are as efiicacious in promoting clean release of dry cast-surfaced coated paper from a casting or finishing surface as are the oleaginous release agents of the prior art but it is believed that the resinous release agents must effect release by a mechanism which is different from that of the greasy agents. Oleaginous release agents, as is well known, must, for best results, be used in amount not far above the minimum quantity which will effect release, since only a slight increase over the minimum quantity will result in a decrease in gloss of the dried coated surface, and a considerable increase may not only entirely prevent formation of a cast surface but if continued in use may actually build up into a visible film of grease on the finishing surface. Quite contrariwise in the case of resinous release agents according to the invention, use of a large excess of the resinous agent over the min imum quantity required to give release has no deleterious effect upon the finish of the cast-surfaced coated product. For instance in a case where a one-half percent aqueous solution of resinous material was found adequate to produce perfect release of a cast-surfaced coated paper when the solution was fed into the pressure-nip, it was found to be possible to increase the concentration of resinous material in the aqueous solution to 10% without decreasing the finish or gloss of the cast-surfaced product and continual use did not cause any visible film of resin to build up on the casting surface.

For practical purposes it can be said that when resinous release agents are used according to the invention substantially all of the resinous release agent is removed from the finishing or casting surface by the coated paper, so that a cast surface results regardless of the quantity of resinous release agent used provided there is present at least the minimum quantity necessary to ensure release. Hence there is no need for the continual adjustment of release agent that harasses an operator using the oleaginous release agent of the prior art. Furthermore, enough of tr e resinous material may be used to impart other special qualities to the coated paper as may be desired. For example, a. 5 percent aqueous ammoniacal solution of shellac fed to the pool thereof formed in the nip of the casting surface and the coated paper not only facilitates release of the dry coated paper but also greatly improves the resulting cast-surfaced coating in its ability to hold up glossy printing-ink, a desirable property not usually found in cast-surfaced coated papers. Again if a 6 to 3 percent aqueous ammoniacal solution of Manila copal is fed to the entering nip between the coated surface and the casting drum at a rate sufficient to maintain a pool thereof in the nip, the dried coated surface not only releases perfectiy with a high gloss but it is also substantially Water-repellent.

The reason is not known with certainty for the surprising difference in behavior of resinous release agents when contrasted with the oleaginous release agents of the prior art. Part of the explanation may lie in the good COl'lC 've strength possessed by resinous materials in contradistinction to the exceedingly low degree of cohesive ness found in film of oleaginous grease or oil. it is believed that olcaginous agents give release because the film splits, part remaining on the finishing surface and part adhering to the coated paper, so that when a thick oleagincus film is used both the finishing surface and the coated paper may appear dull after parting occurs. On the other hand when resinous release agents are used it is thought that parting occurs between the finishing surface and the resinous material, so that none or practically none or" the resinous material remains on the finish ing surface and the released coated paper always has a finish resembling that of the finishing surface.

Resins suitable for use under the invention are resins which are soluble in water containing ammonia, caustic soda, or similar alkali. Rosin, shellac and Manila copal are examples of such resins. Synthetic resins soluble in the presence of alkali: are also usable. An example of such a synthetic resin, is. synthetics A-56, an adduct of maleic anhydride and pentaerythritol ester of rosin sold by Hercules Powder Company. synthetics A-56 is soluble in aqueous ammonia and in aqueous caustic soda. it is a desirable non-oleaginous release agent which is effective in promoting release of dried cast-surfaced coated paper from a finishing surface when applied in aqueous solution to the surface of the coating prior to contact of the latter with said finishing surface.

The absolute minimum of resinous material required to effect release of the dry coated paper surface from the finishing surface may vary depending upon several factors, including the specific resinous material, the particular coating composition, and the particular finishing surface being used. Generally speaking, application to the surface of the coated paper of an aqueous solution conraining from 0.25 to 0.50 percent of resinous release agent in quantity suflicient to form a continuous film of solution on said surface is adequate to ensure satisfactory release of the dry coated surface from the finishing surface. Since the presence of a large excess of resinous release agent has no deleterious effect on the finish of the coated paper product, however, it is convenient to apply to the coated surface of the paper an aqueous solution containing from 1 to 2 percent of resinous release agent in order to provide a substantial factor of safety and thus free the operator from one source of worry which has hitherto invariably plagued operators when using the oleaginous release agents of the prior art.

When aqueous solutions of resinous materials are used to impart functional qualities to the coated paper in addition to acting as release agents, the usable concentration is limited chiefly by the viscosity of the resulting solution. Generally speaking, it is not desirable to use solutions having a viscosity greater than the viscosity of glycerine. Hence, it is rarely advisable to use concentrations of resinous materials greater than 20 to 25 percent. In the case of some resinous materials it may be impossible to reach a concentration as high as 20 percent without making the solution too viscous for satisfactory use.

The quantity of resinous release agent deposited per unit of area of the coated paper is, as will be apparent from the foregoing description, quite variable. If all other conditions which might result in variation of the quantity of release agent deposited on the paper surface are disregarded or considered to be non-variable it appears that the amount of release agent deposited per unit of area may vary in the same ratio as the concentration of the solution applied i. e. from .25 to 25. The principal variable factors aside from the concentration of the solution which may result in variation of the amount of release agent deposited are (1) the viscosity of the solution of the release agent (2) the pressure at the nip (3) the smoothness or texture of the coated surface being treated and (4) the density of the release agent. We have calculated that the weight of release agent deposited on the cast-surface coated paper may vary within the range from 0.01 to several pounds per 1080 square feet.

It has been indicated above t.at the resin solution may be applied to the paper coating by any means which will deposit a film thereof on said surface, for instance by spraying or the resin solution may be applied by passing the coated paper into contact with the finishing surface in such a way that a V shaped pocket is formed by the paper and the finishing surfce adjacent the nip and this pocket is kept flooded With resin solution to a suitable depth e. g. V4 to 2 inches. The resin solution may be applied to the paper in advance of the nip in quantity sufficient to flood the nip with resin solution or resin solution may be applied to the paper in advance of the nip in quantity only sufficient to form a film on the surface thereof but insufficient to flood the ni in this event the nip may remain free of any pool of liquid. or it may he flooded with resin solution fer} thereto directly, or it may be flooded with water or aqueous liquid free of resin. For the purpose of producing a cast surface of the highest quality in addition to applying a superficial coating of resin to the coated paper, it is preferred to employ a flooded nip i. e. a nip which is flooded with water or resin solution or any of the other aqueous liquid which is free of suspended solids. The flooded nip is in fact practically essential when the coated paper to which the resin solution is applied is one which has been dried and calendered and is dry at the point at which the resin solution is applied because the aqueous liquid in the nip serves to rewet and replasticize the surface of the coating and enable it to be deformed sufliciently by contact with the finishing surface to yield in effect a cast surface. The flooded nip may however be not used if suflicient resin solution is applied to the coated paper ahead of the nip to moisten and plasticize the surface thereof.

We claim:

1. Process for cast-coating paper which comprises applying to a paper base a layer of aqueous coating composition comprising finely divided mineral pigment and hydrophilic adhesive, wetting the surface of the layer of coating with a release agent comprising an alkaline aqueous solution of a normally solid resin, thereafter pressing the coated surface while still in a wet and plastic condition into adherent contact with a heated finishing surface, and drying the coating in contact with said finishing surface until it no longer adheres thereto.

2. Process as defined in claim 1 in which the aqueous solution is a solution of a resinate of an alkali metal.

3. Process as defined in claim 1 in which the aqueous solution is a solution of an ammonium resinate.

4. Process as defined in claim 1 in which the aqueous solution contains from 0.25 to 25% of the normally solid resin.

References Cited in the file of this patent UNITED STATES PATENTS 1,719,166 Brodner July 2, 1929 2,331,922 Montgomery Oct. 19, 1943 2,524,915 Lippirt Oct. 10, 1950 2,656,286 Fisher Oct. 20, 1953 FOREIGN PATENTS 611,317 Great Britain Oct. 28, 1948

Claims (1)

1. PROCESS FOR CAST-COATING PAPER WHICH COMPRISES APPLYING TO A PAPER BASE A LAYER OF AQUEOUS COATING COMPOSITION COMPRISING FINELY DIVIDED MINERAL PIGMENT AND HYDROPHILIC ADHESIVE, WETTING THE SURFACE OF THE LAYER OF COATING WITH A RELEASE AGENT COMPRISING AN ALKALINE AQUEOUS SOLUTION OF A NORMALLY SOLID RESIN, THEREAFTER PRESSING THE COATED SURFACE WHILE STILL IN A WET AND PLASTIC CONDITION INTO ADHERENT CONTACT WITH A HEATED FINISHING SURFACE, AND DRYING THE COATING IN CONTACT WITH SAID FINISHING SURFACE UNTIL IT NO LONGER ADHERES THERETO.