US3485656A - Process for treating paperboard - Google Patents

Description

United States Patent M US. Cl. 117-60 6 Claims ABSTRACT OF THE DISCLOSURE A method for treating paperboard so as to produce a board which is substantially impervious to the penetration of molten wax applied thereto, the applied wax remaining on the surface of the board and thereby preventing water, used in solidifying the wax, from wetting and softening the thus-treated and waxed coated paperboard which involves first applying to the board an aqueous solution of borax, and then applying an aqueous solution of polyvinyl alcohol, followed by the steps of applying a coating of hot wax to the thus-treated paperboard and setting the Wax by chilling it with cold water.

This invention relates to the production of an improved Waxed carton which is particularly suitable for packaging frozen foods and the like.

In the production of wax-coated paperboard which is to be used for cartons and the like, it is customary to size the board with starch or similar materials, prior to wax application, in order to keep the molten Wax from penetrating into the paperboard. The Wax application is conventionally followed by the use of cooling water for setting the hot wax. If the molten wax has penetrated into the paperboard, and not formed a continuous film, then the cooling Water itself penetrates due to the vacuum created by the reduced temperature, thus Wetting,.and thereby softening the board, which is undesirable.

But the use of starch sizing alone does not satisfactorily prevent subsequently applied hot wax from penetrating through the said coating or sizing into the board. And, while it has been suggested in the art to coat paper with polyvinyl alcohol, or mixtures of polyvinyl alcohol, with coagulating agents including borates, in order to provide grease resistance, this technique likewise is not entirely satisfactory to effectively keep molten wax which is applied over such sizing or coating from penetrating into the paperboard.

It is a principal object of this invention to provide a method for treating paperboard so as to produce a board which is substantially impervious to the penetration of molten wax applied thereto, the applied Wax remaining on the surface of the board and thereby preventing water, used in solidifying the wax, from wetting and softening the thus treated and Wax-coated paperboard.

In the practice of our invention, as illustrated in the following description of an embodiment thereof, We utilize paperboard which has been previously starch treated in conventional manner at the size press and has a top surface which has been clay-coated in accordance with con ventional procedures so as to provide a printing surface. Such a board is first contacted with an aqueous solution of borax and then with an aqueous solution of polyvinyl alcohol. The polyvinyl alcohol-borax coating can be applied to either or to both sides, but in a preferred embodiment, is limited to the back, as contrasted to the claycoated side. When solutions of borax and polyvinyl alcohol come into contact, it appears that a reaction occurs which results in crosslinking borate ions and polyvinyl alcohol into a complex polymer, forming a rigid three-dimen- 3,485,656 Patented Dec. 23, 1969 sional gel. For this reaction to occur on paperboard surfaces to an extent or degree most suitable for the present invention, there should be provided a concentration of from about 0.15 to about 0.25 pound of borax (10 11101) per thousand square feet of paperboard, and a sufficient concentration of polyvinyl alcohol, preferably of the fully hydrolyzed type, to give continuity to the gel structure, that is to say, enough so that the reaction product fills the pores rather than coats the Walls. Borax is available as either 5-mol or 10-mol, designating whether 5 or 10 mols of Water of crystallization are combined with sodium tetraborate. On a chemically equivalent base (78.4 pounds of 5-mol:100 pounds of 10-mol). Either grade of borax may be used.

Polyvinyl alcohol is available in a range of polymers of varying solution viscosities and degree of hydrolysis, and is provided by hydrolyzing polyvinyl acetate polymers, the percentage of available acetate groups replaced by hydroxyl groups describing the degree of hydrolysis.

In general, the polyvinyl alcohol of highest viscosity and greatest degree of hydrolysis will be most efficient for the surface treatment of paperboard in accordance with my invention. High viscosity slows down polyvinyl alcohol penetration and a fully hydrolyzed polyvinyl alcohol reacts more extensively than a partially hydrolyzed polyvinyl alcohol. We have successfully used, as the polyvinyl component, Vinol No. 165, manufactured by the Air Reduction Company.

The F. viscosity of the polyvinyl alcohol may range from 2 up to 20,000 cps. depending on the grade and concentration of polyvinyl alcohol, but we prefer to utilize polyvinyl alcohol having a 95 F. viscosity of between about 20 and 200 cps. The viscosity can be kept at the desired level by controlling the pH of the solution, by use of phosphoric acid, for example, so that it is about 7 or below.

Excellent results are obtained using an aqueous solution of Technical Granular Borax ranging in concentration from about 0.5 to 20%, preferably 25-30%, and applying it to the surface of the paperboard by any conventional method, at the calenders. The treatment at the calenders is immediately followed by an application of an aqueous solution of polyvinyl alcohol, suitably by use of a trailing blade. The concentration of the polyvinyl alcohol may vary over a wide range, for instance, from about 0.5% to about 20%, depending on the amount of holdout desired; I have obtained very good results using a 2-3% aqueous solution. As an illustration of the effectiveness and efficiency of this invention, I have obtained a very satisfactory degree of Wax holdout plus grease resistance, using 0.06-0.20 pound of polyvinyl alcohol, and 0.15 to 0.25 pound of borax per thousand square feet of paperboard. The amount of polyvinyl alcohol on the board may vary from about 0.06 to 1.50 pounds per thousand square feet; and the borax may vary irom about 0.10 to about 2.0 pounds per thousand square eet.

While variations in application of the borax and of the polyvinyl alcohol solution will of course be apparent to those skilled in the art, we find that the borax solution is applied most satisfactorily by means of a calendar roll. Attempts to apply borax at the size press we have found result in the destruction of internal sizing because of excessive penetration of the borax solution. With internal sizing destroyed the paperboard is difiicult to dry and excessive amounts of subsequently applied calender solutions soak into the sheet. In accordance with this invention the borax solution is applied to paperboard which has been previously coated with starch, and/ or clay. The precoat (starch or clay over starch) cuts down penetration of the borax solution. The surface finish at the calender is also better than at the size press to further minimize borax solution penetration. The polyvinyl alcohol solution is applied to the board surface immediately after the calender application of the borax solution and while the borax solution is still wet, excess solution being removed preferably by means of a trailing blade, or by an air doctor or blade. Because the polyvinyl alcohol is applied while the borax solution is still wet, the polyvinyl alcohol mixes with the borax to cause gelling to take place on the paperboard surface and before it is permitted to migrate. There is thus produced a gel network on the board surface which bridges all large surface pores and provides a highly desirable level coating.

The thus treated paperboard may then be printed on its clay coated side and/or cut and passed through any conventional waxing equipment which requires either water baths or water curtains to chill and set the wax.

The outstanding characteristic of our coating lies in its ability to receive and maintain the Wax on the surface in a continuous film, thereby preventing negative pressures which are developed through expansion of water and gases by the heat of application (of the wax), from forcing water into the sheet resulting in a limp and inferior carton.

In the aforesaid description of our invention it will be understood that by wax we mean to include all waxes both natural, petroleum and synthetic waxes as are conventionally used in coating paperboard and which are susceptible to the above described coating procedures.

Other variations of our invention may include, for example, the utilization of paperboard which has been conventionally clay-coated on both sides of the board, as well as the above-described embodiment in which the board used has been clay-coated on only one side.

It will be understood that other changes may be made in the invention without departing from the spirit thereof, and it is accordingly not intended that the disclosed details of the invention shall be construed as limitations of the scope thereof.

We claim:

1. A process for treating a starch-sized paperboard which comprises, first applying to the board an aqueous solution of borax, then applying to the borax solution coated board While still wet with the borax solution, an

4 aqueous solution of polyvinyl alcohol, the amounts of borax and polyvinyl alcohol being effective to form a gelled reaction product, followed by the steps of applying a coating of hot wax to the said gelled reaction product on the thus treated paperboard, and setting the wax by chilling it with cold water.

2. A process in accordance with claim 1 wherein the amount of borax applied to the paperboard is from about 0.15 to about 0.25 pound of 10-mol borax per thousand square feet of paperboard.

3. A process in accordance with claim 1 wherein the solution of borax is used in a concentration ranging from about 05-20% and the concentration of the aqueous solution of polyvinyl alcohol is from about 0.5% to about 20%.

4. A process in accordance with claim 1 wherein one surface of the said board has been clay-coated and wherein the polyvinyl alcohol-borax wax treatment is applied to the opposite side of said board.

5. A process in accordance with claim 1 wherein one surface of the said board has been clay-coated and wherein the polyvinyl alcohol-borax wax treatment is applied to both sides of said board.

6. A process in accordance with claim 1 wherein the borax solution is applied by calendering.

References Cited UNITED STATES PATENTS 2,439,108 4/1948 Staehle 117-62.). 2,725,306 11/1955 Fisher et al. 117-92 X 2,927,865 3/1960 Lewis et al 117-92 X 2,949,382 8/1960 Dickerman et al. 117-84 X 3,112,214 11/1963 Roberts et al. 117-60 3,251,709 5/1966 Bonzagni.

3,285,771 11/ 1966 Dabroski.

WILLIAM D. MARTIN, Primary Examiner R. HUSACK, Assistant Examiner US. Cl. XLR.