US2321244A - Coating of paper - Google Patents

Description

June 8, 1943.

F. G. RAWLING- COATING OF PAPER Filed March 13, 1940 SUPER/"P0550 M4722 F was CQATYNG COLOE SUPPLY PAIER WEB SY/VEEES/S OCCl/k HEEE mvEN-roR fikA/vas GTE/mans 'f'ToRNEY Patented June 8, 1943 UNITED STATES PATENT oFFieE COATING OF PAPER Francis G. Bawling, Piedmont, W. Va., assignor to West Virginia Pulp and Paper Company, New York, N. Y., a corporation of Delaware Application March 13, 1940, Serial No. 323,680

2 Claims. 117- 65) My present invention relates to the art of coating paper and the like. Compositions for the coating of paper commonly consist of a finely divided mineral pigment or filler, e. g., clay, chalk, calcium sulflte, titanium oxide, satin white, together with an adhesive such as casein or other protein, cooked-starch, the whole being ground and suspended in water. Such compositions are characterized by a lack of fiowability whereby in the absence of special mechanical expedients the .layer of coating deposited on the paper, as by means of rolls, is" not smooth but contains irregularitios in depth due to the formation of ripples or ridges. For this reason the art has been much occupied with the devising of special means for applying a smooth coating or for smoothing out the coating after it is applied. The reason for the formation of ripples and ridges in the coating when applied, for instance, by squeeze rolls is due to the property of the coating composition of adhering to both the surface of the roll and the surface of the paper; and as the paper leaves the nip of the rolls the coating is split into two films. The parting of the films takes place irregularly, so that the coating is applied to the paper in ridges of varying thickness. These ridges may vary in width from 0.02 inch .to 0.25

inch, depending upon the nature of the coating composition. A number of mechanical devices such as scraper blades, spinning rolls, brushes or air blasts have been employed to smooth out these ridges. The use of such expedients can 'be avoided only if the film has the property of leaving the roll completely. This property, closely related to the syneresis exhibited by certain colloidal solutions, which exude water readily, leaving the general mass of the colloidal structure substantially unchanged, except for a reduction in volume, is not possessed in a suificient degree by the ordinary coating compositions.

It is a principal object of my invention to so modify coating compositions of standard type as to enable syneresis to occur. This I accomplish by incorporating in the coating composition of the type commonly employed and in which syneresis does not occur, a small quantity of an easily breakable emulsion of an oleaginous substance or mixture of substances. Such an emulsion, to be suitable, is breakable by rubbing or pressure in the operation of coating, whereby an oily film is maintained upon the applicator roll, such that on leaving the nip, the colloidal composition. now in the form of a coating on the paper, tends to bleed water rather than split the coating film. The water leaving the coating composition at the point of contact with the oily film in the medium which splits, leaving part of the water on the roll, where it can be felt by hand and part on the coating. The film of water on the roll prevents the surface of the latter from drying and no deposits of coating color form on the roll, so that cleaning devices such as doctor blades or sprays are not required, the roll being quite clean after several hours operation.

In the drawing the single figure, which is selfexplanatory, shows diagrammatically a pair of squeeze rolls the lower of which is rubber coated, and a web of paper being passed therebetween and receiving a coating of "color" as above described. The drawing also shows diagrammatically the occurring syneresis and the superimposed oil and water films on the applicator roll.-

nauba wax are melted together, whereupon 3 pounds of hot water containing 0.015 pound of' caustic. soda are added with thorough stirring followed by the addition of 50 pounds oi a 4% solution of starch. The emulsion so formed is only moderately stable and shows a tendency to break and form small agglomerates ofoily material on being rubbed. The amount of the emulsion so formed can thenbe added to a quantity of a coating composition of a standard type, as for example gallons of a composition containing 200 pounds of clay, 42 pounds of cooked starch (such as oxidized starch known in the trade as superfllm, Hercules gum, stayco, etc).

The coating composition so formed may then be colloid and the roll surface as the paper leaves the nip. More than this will serve no useful purpose. Thus a quantity ten times as great as given in the aboveexample will result in the formation of a thick layer of waxy material on the applicator roll, with numerous small lumps floating in the coating color, necessitating frequent cleaning. It is immaterial what type of i action of the caustic soda.

mineral pigment with which the easily breakable emulsion is employed, i. e., whether clay, chalk, calcium sulfite, titanium oxide, etc., so long as the emulsion is broken at the nip of the squeeze rolls. The composition of the emulsion for the described purpose may be varied considerably, as will be evident to those skilled in the art. In the example above given, sodium stearate is the emulsifying agent, part of the stearic'acid having been transformed into sodium stearate by the The remainder ofthe stearic acid (which has both oleophilic and hydrophilic groups in the molecule) dissolves in thelubricating oil, a hydrocarbon comprising the internal phase of the emulsion, thereby increasing the stability. A decrease in the amount of stearic acid will thus render the emulsion less stable. The use of diglycol stearate, for example (a known emulsifying agent), usually results in an emulsion which is not quite stable enough, whereas the use of larger amounts of sodium stearate, or preferably ammonium stearate and ammonia, would increase the stability of the emulsion so that it would not break' in the nip. Further stability is also had by the addition of a sodium salt of casein or other protein to the emulsion. A hydrocarbon oil as the chief component of the internal phase of the emulsion is chosen because it is liquid at room temperature and readily spreads to form a thin film when the emulsion is broken. The presence of the wax makes the oily film stronger and more stable on the surface of the applicator roll so that pressure will not readily force it off the surface. It could be omitted, with corresponding loss in efiiciency. In place of carnauba wax, other waxes or wax-like compounds such as candelilla, bayberry, halow ax, cumar, which are soluble in light lubricating oil, may be used. A non oil-soluble resin such as copal is not suitable because of its oil insolubility.

The proper functioning of the emulsion is indicated by the presence of an oily film on the applicator roll showing an overlie of moisture which is readily detected by the hand.

The use of the breakable emulsion does not preclude the simultaneous use of emulsions of a more stable kind which may be added to control the gloss and ink reception of the coated paper, so long as these emulsions do not break in the nip of the rolls.

The press or squeeze rolls employed may consist of hard or soft rubber or metal or may be a combination of these types, the only requirement being that the surface of the applicator roll be such as will enable the oily film to attach itself.

I claim:

1. In the art of coating paper and the like with a coating composition comprising an aqueous suspension of mineral pigments and an adhesive by the use of an applicator roll to which said aqueous. suspension is continuously fed and by which said composition is spread out into a film upon the paper undergoing coating by the aid of a cooperating roll between which and said applicator roll the paper is squeezed, the method of preventing splitting of the applied coating film and adherence thereof to the applicator roll after said coating has been applied to the paper, which consists in incorporating into said coating composition an oleaginous emulsion having an oil and water phase and containing a hydrocarbon oil, a.

fatty acid, a soap, and starch, which emulsion is breakable by the physical rubbing incident to the coating operation above described, whereby in said operation a portion of the oil phase adheres to said applicator roll as an oil film thereby causing an adherence to said applicator roll of a water film derived from the surface of the coating material applied to the paper, whereby the coating material is caused to adhere to the paper and not to the applicator roll.

2. A coating composition for coating paper and the like, comprising an aqueous suspension of mineral pigments and an adhesive containing a relatively small quantity of an emulsion in water of an oleaginous material, which emulsion is capable of being broken by rubbing, said emulsion containing a fatty acid, a hydrocarbon oil, a soap, and starch,

FRANCIS G. RAWLING.

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