US2346812A - Method of making coated paper - Google Patents

Description

Patented Apr. 18, 1944 unl're 2,346,812 Mm'non or MAKING coa'rnp PAPER.

Donald B. Bradner, Mount Healthy, Ohio, assignor to The Champion Paper and Fibre Company, V Hamilton, Ohio, a corporation of Ohio No Drawing. Application August 2, 1949, Serial N0. 350,129

'7 Claims.

This invention relates to a processof ak coated paper and it has particular reference to a process of making coated paper having a cast surface.

In U. S. Patent 1,719,166 there is described a process of making coated paper wherein an aqueous coating composition of pigment and adhesive is applied to a travelling web of paper and the coating is dried while in contact with a polished chromium plated drum or other solid finishin surface. Thus the coating has a surface which is a replica of the drum surface and, with a highly polished drum thecoating has the gloss of a costly highly glazed flint finished paper but is superior in smoothness, in printability and in other propert es. In the above designated patcut the surface so produced has been referred to as a cast surface and will be so designated in this application.

For manypurposes it is desirable that these cast surfaced coated papers have a suitable degree of waterproofness. For example, moisture from a finger print .will mar the appearance of a highly finished paper such as these to a much greater extent than with papers of 'low gloss, unless the coating is sufficiently waterproof. Also, for lithographic printing and for other pur- P ses a water-resisting coating is required.

The common practice of making coated paper manufactured by the usual method of festoon drying, water resistant by incorporating formaldehyde in the coating composition has been found disastrous when used in making cast surfaced coated paper, wherein an excess of coating composition is applied to the paper and the excess removed by means of a press roll pressing against the hot finishing drum. In such cases the excess coating which is removed fromthe paper and re-used, thickens by virtue of contact with the hot drum surface and in a short time the whole coating composition is worthless. Also, the hardening of such coatings on the press roll beyond the edges of the paper is ruinous to rubber press rolls. Two-step processes of waterproofing by first coating and drying the web and then exposing it to a waterproofing operation are timeconsuming and expensive. I

Also, in making cast surfaced coated paper as above described, it ishighly desirable that the coating composition have an exceptionally high solids content and yet remain relatively fluid in.

order to insure a high quality product at relatively high speeds.

One object of the present invention is to provide an improved method of making a waterp oof cast surfaced coated paper. "Another object is to provide a method of making a cast surfaced paper at relatively high speeds'without the necessity of the close operating control previously required. Another object is to provide an improved waterproof cast surfaced coated paper.

Briefly the cast surfaced coated'paper of the present invention is made by applying to paper a coating composition comprising pigment and a water soluble composition containing zinc for cadmium or copper), ammonia, and casein or other protein adhesive, and drying the coated paper, by'contact with a heated finishing surface.

In this way the coating takes on a cast surface and the heat drives oif ammonia to give decided dition as an insoluble aggregate or jel. The re- I moval of ammonia, as by heat, apparently decomposes the complex, allowing the formation of insoluble zinc caseinate which gives waterproofing properties to the coatedpaper. Whereas, the solutions of usual zinc salts, such as zinc chloride, zinc sulfate, etc. immediately precipitate casein as zinc caseinate and thus coagulate and render worthless the coating composition, when the zinc is in the form of a soluble complex zinc ammonia compound there is no precipitation of the zinc or casein at ordinary temperatures even on long standing.

' The'zinc ammonia casein composition may be prepared by mixing together zinc oxide or zinc hydroxide, a casein solution and ammonia water. The casein may be brought into solution by treatment in the usual manner-with alkaline compounds (sodium hydroxide, sodium: carbonate, etc.) but I prefer to solubilize it with ammonia water.

solution of zinc chloride is used for the preparation of the zinc ammonia complex, it is first converted into a complex zinc ammonium chloride by treating it with ammonium hydroxide sufficient to dissolve the precipitate of zinc'hydroxide which forms at first, and thereafter adding v the casein solution.

The coating compositions embraced in' the present invention ordinarily contain considerable proportions of mineral pigment such as for ex-, ample clay, calcium carbonate, etc. These may The Instead of using zinc oxide or hydroxide, other zinc compounds may be used. -Thus if aresistance and excellent printing properties.

be incorporated in any suitable manner. Thus a slurry or paste of the pigment may be made with water, and a solution of the zinc-ammoniacasein complex may be added thereto. Or if desired, dry-casein and zinc oxide may be mixed directly with the pigment and zinc ammonium caseinate formed in the mixture by incorporating ammonium hydroxide therewith.

The following example will illustrate the invention. Five hundred pounds of precipitated calcium carbonate was charged into a mixer and 125 pounds of water added and mixed. A slurry consisting of 3 pounds of zinc oxide and 30 pounds of water was next added and when this had been mixed in, 75 pounds of dry casein was added. Thereafter pounds of 28 ammonium hydroxide was added and the mixing continued for minutes. Then water was added to reduce the solids to about 55%. The coating now had the consistency of thick cream.

This coating was applied to a web of paper of a grade commonly used in making coated paper. A .large excess was applied by means of a roll dipping into a pan. The paper then passed into contact with a 12 foot diameter chromium plated drum travelling at 120 feet per minute. Initial contact of the coated paper with the drum was effected by means of a press roll which also served to remove excess coating material from the paper. A pool of coating formed in this nip, which, as it accumulated, dropped back into a tank and was delivered to the coating roll for reuse. The drum was internally heated to give a surface temperature of about 175 F. and drying was supplemented by a blast of hot air impinging theback (-uncoated) side of the paper as it travelled around the drum. Ammonia was driven out by means of the heat and the coating set up firmly and within a few seconds the coated paper was taken from the drum and wound into a roll. The paper had a smooth, brilliantly glazed surface, having a, high degree of water; was substantially proof against fi'nger marking and water failed to mar the finish. I

Coating compositions containing the zinc ammonia complex with casein are apparently thereby well buffered, as the addition of relatively enormous quantities of ammonia over that re- 'qu.ired to disperse the casein and form the amzinc on the weight of the casein gives very good results. Equivalent amounts of the corresponding copper and cadmium compounds may be used if desired, but zincis recommended because of its absence of color and lower cost.

In spite of the waterproofness of the dried coating, it is readily soluble in ammonia water.

This fact greatly facilitates the removal of dried coating from the machinery, and therein has a considerable practical advantage over the use of formaldehyde coatings.

The ink absorptivity of the coated paper made in accordance with the present invention may be varied considerably by a selection of the particular zinc compound that is used in preparing the zinc ammonia complex. Thus zinc oxide, zinc carbonate and zinc cyanide give relatively non-absorptive coatings whereas zinc chloride and zinc sulfate produce a relatively absorptive coating.

I claim: f

1. Process of making coated paper having a cast surface which comprises applying an aqueous coating composition comprising a solution of comprising pigment and a solution of casein, zinc 1 and ammonia to a web of .paper and drying the paper while in contact with a heated casting surface to produce a cast surface upon the coating.

3. Process of making substantially waterproof coated paper having a cast surface which comprises applying an aqeuous coating composition comprising mineral pigment and a water soluble casein-zinc-ammonia composition to a travelling web of paper, contacting the coated side of the paper while still wet with a solid, heated casting surface whereby ammonia is removed from the coating to produce zinc caseinate and removing the paper from said finishing surface after the coating has assumed a cast surface.

4. The process of claim 3 wherein the amount of zinc used is from about one percent to about six percent of the weight of the dry casein.

.5. Process of making waterproof coated paper which comprises mixing together mineral pigment, casein, ammonia, a zinc compound and water, applying said mixture to paper and drying said coated paper by contacting the coated side thereof with a heated casting surface whereby the coating is dried and ammonia is removed from the coating to produce a substantially water- I insoluble cast surfaced coating on the paper.

6. The process of claim 5 in which a relatively ink-absorptive coated paper is made and the zinc compound is a water-soluble zinc salt.

7. The process of claim 5 in which a coated paper of relatively low ink absorptivity is made in which the zinc compound is chosen from the class of zinc oxide, zinc carbonate and zinc cyanide.

DONALD B. BRADNER.