US2186957A - Coating method - Google Patents

Description

Patented Jan. 16, 1940 UNITED STATES PATENT OFFICE 2,186,957 I COATING METHOD William RQCollings and John J. Hoeltzel, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing. Application April 13, 1938,

Serial No. 201,753 1 1 Claim.

This invention relates to a method for uniformly coating paper and other fibrous materials, particularly embossed paper and the like.

In coating embossed paper and the llke'with 5 solutions of film-forming materials for the purpose of protecting the paper and for decorative eifect, it has been found desirable to employ the film-formingsolution in a comparatively narrow range of viscosity, as is pointed out in our concurrently filed co-pending application-Serial No.

201,742. When the film-forming solution is too fiuid,,it is found to be readily absorbed by the paper and does not deposit a glossyfilm. If, on the other hand, the coating solution is too viscous, its 1; application to an uneven surface, such as that of embossed paper, results in entrapping air bubbles in the depressions of the surface to be coated. The high viscosity of the so-deposited film prevents the air bubbles from rising to the surface prior to hardening of the film on the material.

When embossed paper, textiles, and similar materials, the surface of which is rough and uneven are to be coated, it is desirable to provide the entire surface with a uniform thin film of the protecting material. In order that the film be not discontinuous, the aforesaid desired results are] most economically obtained by extruding or castingv a thin web of the protecting material in the form of a solution having a high solids content,

and hence a high viscosity, on to the fibrous sheet.

It is accordingly an object of the present invention to provide a method'whereby a. uniform continuous protective coating may be applied to a fibrous material, such as embossed paper or textiles, from a high viscosity solution of ,such coating material.

We have found that the foregoing and other objects may be accomplished and that entrap- 40 ment of air bubbles beneath the coating film may be avoided, while at the same time providinga means whereby the composition is caused to penetrate the porous surface sufficiently to provide good adhesion between the material and the protective film, by spreading a layer of a viscous solution of the coating composition on a porous fibrous sheet, while subjecting the sheet to reduced pressure or vacuum. Such reduced pressure may be applied simultaneously with, or imgo mediately after, the application of the coating film to the sheet. Such procedure results in removing air bubbles from between the newly deposited film and the fibrous sheet, and draws a certain amount of the coating solution to a sufficient depth into the pores of the fibrous material to insure good penetration and adhesion of the coating composition to the treated surface. The method may be made continuous by drawing the sheet of fibrous material past the points at whcih the solution and the suction are applied.

Our method is particularly advantageous when employed in coating heavy paper stocks, such .as are used for the covers of catalogues, magazines,

. books, etc., since it provides a means of uniformly coating the paper while holding the same flat during the coating process. The deposition of a uniform coating obviates the heretofore existing difficulty of Warping inherent in old coating methods which deposited films of uneven thickness. I 15 Since the coating deposited by the method of r the. present invention is uniform, it emphasizes the decorative appearance of rough-textured or embossed papers by providing such surfaces with a uniform gloss which follows the contours of the 20 paper. Thus the normal shadow lines in an embossed paper stand out more clearly after application of a coating by means of the present method than before treatment. Modes of application not involving the herein-described vacuum 25 I method have almost invariably resulted in filling the depressions of embossed paper and the like with a thick deposit of the coating composition, while at the same time leaving the high spots of the paper surface only very thinly covered. 30 Such prior methods have had the effect of impairing rather' than enhancing the decorative effect of an embossed paper.

The herein-described method may be satisfac torily employed with coating compositions of 35 relatively high viscosity'as compared with those which are described in our aforesaid co-pending application. In fact, it is possible to produce a continuous and uniform coating according to the present method by employing a solution, the viscosity of which may vary from 50 to as high as 1000 seconds or more when tested by the fallingball method at 25" Q. (AJS, T. M. D301-33).

, In carrying out the method of the invention the coating composition in the form of a solution .of high solids content is preferably applied to a running sheet or web of fibrous material, which is supported horizontally. The application of such solution may be done in various ways For example, the web of fibrous material may be passed between rollers, at least one of which dips in a bath of the solution in the known manner. Another way is 'to extrude a film of the viscous solution upon the moving sheet from a hopper providedwith a transverse slot, the length of 56 Y which is the same as the width of the sheet to be coated. In a further modification the coating composition may be poured or dropped on or otherwise applied to the stock and evenly distributed by means of a doctor blade. The coating composition is ordinarily applied to the upper surface of the sheet just prior to applying the suction to the under surface of the sheet. This may be done by passing the sheet horizontally over a perforated plate or table which is connected to a vacuum pump, the degree of suction being adjusted to the material being coated and v the rate of travel of the sheet over the vacuum element. The moving sheet is then passed through a drier to remove the remaining solvent from the coating composition deposited thereon.

If desired, the coating composition and suction may be applied to the same surface of the sheet. thus making possible the use of a vacuum coating process for successively covering both surfaces of a fibrous material. We have'observed that when paper is drawn past a vacuum slot, the effective suction extends at least 0.25 inch beyond the slot. -Hence, if suction is applied by means of a perforated or slitted orifice at a point about 0.25 inch or less from the point at which the coating composition is being deposited on a fibrous material, and if the fibrous material is conveyed away from the point at which suction is applied past that at which the coating composiion is deposited, the same results are obtained as though the suction had been applied beneath the fibrous sheet while depositing the coating composition from above.

Suitable coating compositions, which we have employed and found satisfactory, are solutions of the cellulose derivatives, including both esters and ethcrs thereof. Examples of such materials are cellulose nitrate, cellulose acetate, cellulose aceto-butyrate, ethyl cellulose, ethyl-propyl cellulose, benzyl cellulose, benzyl-ethyl cellulose, and the like. Such compositions are applied in the form of a substantially clear solution having a viscosity in the range from about to about 1000 seconds, or higher, and preferably from about 350 to about 400 seconds, as determined at 25 C. by the falling-ball method. In the case of the water-insoluble cellulose ethers in the usual solvents for these materials, this viscosity range represents concentrations varying from about 10 to about per cent. By using a cellulose derivative which has a low intrinsic viscosity, it is possible to produce a low viscosity solution having a high solids content. Conversely, where the cellulose derivative has a high intrinsic viscosity, high viscosity solutions may have a low solids content.

The following examples define compositions which we have employed satisfactorily according to the herein-described method:

Example 1 4'7 grams of a cellulose acetate containing 40 per cent acetyl'groups is mixed with 20.1 grams of triphenyl'phosphate as a plasticizer and dissolved in 142 grams of a solvent consisting of 50 per cent acetone and 50 per cent dioxane, by

volume. The viscosity of the resulting solution is 370 seconds.

Example 2 45 grams of nitrocellulose (6 second R. S.) and 19.3 grams of dibutyl phthalate are dissolved in 135 grams of a solvent mixture consisting of 20 per cent butyl acetate, 10 per cent butyl alcohol. 10 per cent ethyl acetate, 10 per cent ethyl alcohol, 25 per cent toluene, and 25 per cent "troluoil, by volume. (Troluoil is a petroleum hydrocarbon distillate having an evaporation rate equivalent to that of toluene.) The viscosity of the resulting solution is about 350 seconds.

Example 3 grams of a medium viscosity benzyl cellulose and 23 grams of triphenyl phosphate is dissolved in 120 grams of a solvent consisting of 50 per cent toluene, 20 per'cent ethanol, and 30 per cent butyl acetate, by volume. This composition has a viscosity of 445 seconds.

Example 4 A white pigmented ethyl cellulose composition having a viscosity of about 250 to 300 seconds'was prepared as follows: Equal parts of ethyl cellulose and titanium oxide chips were mixed and dissolved to form a 30 per cent solution in a mixture of parts of toluene and 20 parts of per cent ethanol, by volume. A plasticizer, monophenyl-di-orthoxenyl phosphate, and a small amount of stearic acid were added and the resulting composition adjusted to the desired viscosity by the addition of a suitable amount of 30 per cent solution of a "70 centipoise" ethyl cellulose in an 80:20 toluenezethanol mixture. The solids in the final composition had the following approximate analysis:

Per cent Titanium oxide 8.8 Plasticizer 24.5 Ethyl cellulose 64.7 Stearic acid 2.0

tour of the surface variations of the paper, which consists in spreading a layer of a cellulose derivative solution, having a falling ball viscosity at 25 C. of between 350 and 400 seconds, upon a horizontally moving fiat sheet of embossed coverstock,

and immediately subjecting the embossed sheet to suction applied to the undersurface thereof,

whereby there is produced a uniformly coated,

non-warping paper having a uniform high gloss.

WILLIAM R.COLLINGS. JOHN J. HOELTZEL..