US2237068A - Method for making coated paper - Google Patents

Description

April 1, 1941,

D. B. BRADNER METHOD FOR MAKING COATED PAPER Filed Aug. 27, 1958 Patented Apr. 1, 1941 METHOD FOR MAKING COATED PAPER 4 Donald B. Bradner, Hamilton, Ohio, assignor to The Champion Paper and Fibre Company, Hamilton, Ohio, a corporation of Ohio Application August 21, 1938, Serial ndzznoss (c1. sac-'40) 4 Claims.

This invention relates to methods for making coated paper and particularly for making coatedpaper on a paper machine.

It has been the almost universal practice in the manufacture of coated paper to make the paper raw stock on a paper machine, wind it into rolls, and then as a separate operation to apply the coating composition, such as, for example, clay, calcium carbonate, blanc fine, satin white, and the like and adhesive such as, for example, casein, starch, etc., in aqueous suspension to one or both surfaces of the web by means of rolls, brushes, etc. Such coated paper is usually dried on iestoons and then supercalendered. For a great many years attempts have been made to coat the paper while it was still on the paper machine, thereby eii'ecting a saving in time, labor, and capitalinvestment. So far as I am aware, however, none of these processes has given a product which ranked in quality with coated paper made by the two-step process.

I have succeeded in making coated paper directly on the paper machine, of a quality closely approximating, if not equalling, the quality of coated paper made on a coating machine from the same grade of raw stock and coating compo-.

sition and festooning in the commonly practiced method. In some grades the product is indistinguishable from the usual coated paper, and in somerespects, as for example printing qualities, the product is frequently superior to a corresponding grade of the usual coated paper.

In my copending application Serial No. 212,877, filed June 10, 1938, I have described a method of coating paper which may be advantageously carried out during the process of the paper manu facture on a paper making machine having the usual arrangement of screen, press rolls and drying cylinders.

The novel process to which my said copending application is directed is based on the discovery that upon the applicationof a suitable coating composition to the paper web in its passage through the paper machine a filter cake is formed of the coating composition immediately adjacent the surface of the paper web by the absorption dried by contact with the steam heated cylindrical dryers commonly used in paper machines,

without sticking to or being marred by said I dryers.

The step of removing the excess coating com-- position from the paper web without impairing the filter cake described in my said copending application is accomplished by drawing the coated side of the web after the formation of the filter cake across a rounded scraper bar, or reversely rotating scraper roll, the pressure of the web against the scraper or roll being determined solely by the tension 01' the web. That is to say, there is no fixed roll, belt, or other backing for the web as it passes over the scraper, so that the paper rides freely over the scraper which wipes ofi the liquid coating composition and smoothes the surface of the filter cake.

The present invention has for its object the provision of an alternative method for removing the excess fluid coating composition above the filter cake.

As described in my above designated application, the apparatus for applying the coating composition may be installed on the paper machine to coat either the wire side of the web, or the felt side, or both. Although the coating may be applied to either side of the web at any point in the process of manufacture after the web is formed, the apparatus for coating the wire side of the web may be mounted between the last press roller and the first drying cylinder, while for coating the felt side it is desirable that the web be at least partially dried before the coating is applied, all as fully described in my above mentioned pending application. For the present application I have selected for illustration an arrangement of the coating apparatus in the paper machine to coat the wire side of the web at a point in advance of the first dryer section, but it will be understood that this apparatus may be installed to apply the coating to either or both sides of the web and at any other stage in the manufacture of the paper.

The invention will be further described by referring to the accompanying drawing in which:

Fig. 1 represents more or less diagrammatically one form of apparatus embodying the invention; and

Fig. 2 is a diagrammatic sectional view of a web of paper showing the filter cake and the removal of the excess coating.

In Fig. 1 the paper web is indicated by the numeral ii. It is shown passing through a pair of press rolls i2, I2 which may be the final press rolls of the wet press section of the paper machine. It is then carried around carrier rolls I3, I 3, I3 to coating applying roll I which dips into coating l5 which is circulated and maintained at constant level in the pan I6 by constant inflow through a supply pipe I! and overflow through a return pipe l8, which, by means of a suitable pump, not shown, take the coating composition from and return it to a supply tank, not shown. The-applicator roll I rotates, advantageously, in,a direction opposite to the travel of the paper thereover, and carries coating material from pan l6 and wipes it onto paper weband thereby applies coating in considerably greater quantity than is desired on the finished paper. The paper web then.tr'avels around roll 2| with the freshly coated side away from the roll. At a point where the web is firmly supported on roll 2|, and at a distance from the applicator roll to allow the filter cake to form, is located a yielding or flexible wiper .22 which is carried on a rigid supporting member 23. The wiper 22 is advantageously made or natural or synthetic rubber or other suitable material of a thickness and density which permit it to yield by compression or flexure or a combination of the two before it will dig into the filter cake and roughen its surface or dislodge an appreciable amount of the solid material therefrom.

The coating composition applied in this man- I ner is usually an aqueous suspension of finely divided mineral filler such as clay, calcium carbonate, satin white, etc., with an adhesive such as casein, starch, or the like. As the composition is-applied by roll I4 the water in. the composition begins to be absorbed by .the ,paperweb,

ing composition. As here shown, 0. indicates the paper web with itsrough and porous surface. Upon the application to this surface of the coating composition, there is a rapid absorption of water from the applied coating immediately adjacent the paper web, forming a layer of filter cake as illustrated at b. The absorption of the water by the paper is much more rapid than the equalization of the water content in the coatin' layer throughout its total thickness, hence th water content of the coating composition in the outer portion of the applied layer is altered but The line of demarcation between the filter cake and by the time the web reaches wiper 22, Fig. 1,

a substantial percentage of the water in the coating composition immediately adjacent the surface of thepaper web has been absorbed by the paper, thereby forming a filter cake of the deand the fluid coating composition does not appear to be'sharp, and between the layers 1) and 0 there appears'to be a transition zone which may be represented as a third layer 11. The yielding scraper 22 (see also Fig. 1) is sufiiciently resilient to project through fluid layer 0 and into layer cl, exert pressure on the filter cake b and hold the paper web a in contact with the surface of roll 2| as shown. The scraper 22 is not so rigid as to dig through the layer it into the filter cake I) and cause it to be scratched, cracked or broken loose from the paper. At least a part of the layer it is apparently removed by the scraper '22 and returned to the pan IE, but it must be borne in mind that the combined thickness of layers b and d is, in practice, less than one thousandth of an inch, so that the composi- .tion and even the existence of the layer it is largely speculative. Regardless of theory, however, a remarkably uniform coated paper is produced by my method.

While my new process is most advantageously carried out on the paper machine as herein desired thickness, with the surplus coating composition forming a more fluid overlying layer.

In order that the time allowed for filter cake formation, between the time'of application of coating and the removal of thestill fluid excess, may be regulated, the wiper 22 and its support 23 may be mounted on arms 24 so that it may swing around the axis of roll 2| to any position required, where it may be locked by known means, not shown. The coating material wiped from the paper web is directed back into pan l6 by a suitable apron. 25. r

The paper web I carrying the desired amount of coating passes off of roll 2| and aro ddrying cylinders 3| and 32 with the coated side out,

before the coated side contacts the dryer 33, by which time it is sufliciently hard that it willnot adhere to or be marred by the surface of dryer drum 33. From thispoint it passes over a sumcient number of dryers to accomplish the necessary drying. It may or may not, as desired, be

subsequently coated on the-other side or have an great harm in disturbing the smoothness or flrmness of the coating.

. Fig. 2 shows diagrammatically what probably takes place during the removal of the excess coatscribed, it may also be employed for coating prefabricated paper as it leaves the paper machine by the simple expedient of running the web' tln*ough the coating apparatus and then through an additional bank of drying cylinders. Also, of

course, the paper may be wound into rolls in the usual manner and then subsequently coated by my new process and dried on internally heated .drying cylinders, or other known types of dryers, as a separate operation. Nor is the process nec-' essarily limited to the use of aqueous coating compositions; non-aqueous coating compositions may also be used.

No hard and fast rule can be laid down for 4 making up coating compositions to be used in the practiceof the present invention. Generally speaking, it is desirable to lower the water con tent of the coating composition as far as possible while still leaving it sufliciently fluid to be himdled by the applying apparatus. The amount of water required is not critical, as variation in the amount of water in the composition may be compensated for by increasing or decreasing the timepermitted for the formation of the filter cake. In the apparatus shown in the drawing, this increase and decrease in the time may be very readily effected by adjusting the wiper todifference in the two processes is that in the process of my pending application the penetration of the scraper to the surface of the filter cake and not into it is obtained by properly regulating the tension on the web of paper, while by the present process the proper penetration is secured by the flexibility of the scraper and is independent of the tension on the web. The process of this application is therefore particularly suited for coating webs under circumstances which make it dit- -ficult to properly regulate the tension on the web.

The term wiping as used in the appended claims should be understood as designating a rubbing or wiping action exerted on the coated surface of the web by a solid as distinguished from a fluid wiping medium, all as more particularly hereinbefore described.

I claim:

1. Process of making coated paper which comprises applying an excess of fluent coating composition, comprising a liquid vehicle containing pigment in suspension and adhesive, to a travel ling web of paper which is absorptive to the liquid of the coating composition to a degree to form on thesurface of the webs. layer of firm filter cake from the pigment in the coating composition adjacent the web while the overlying coating composition remains fluent, and then after the formation of the layer of filter cake, yieldingly wiping off substantially all said fluent coating composition'without removing substantially any of the filter cake, and firmly supporting the web on its other side at a location where the wiping is accomplished and thereafter drying the coated paper.

2. Process of making coated paper on a paper machine which comprises applying an excess of fluent coating composition, comprising an aqueous liquid vehicle containing pigment in suspension and adhesive, to a travelling web of paper which is absorptive to the liquid the coating composition to a degree to form on the surface or the web a layer of firm filter cake from the pigment in the coating composition adjacent the web while the overlying coating composition remains fluent, then after the formation of the layer of filter cake yieldingly wiping oi! substantially all said fluent coating composition without removing substantially any of the filter cake, and firmly supporting the web on its other side at a location where the wiping is accomplished, and thereafter contacting the coated surface with the surface of a heated dryer.

3. Method of coating paper on a paper machine which is characterized by the application of considerably more fluent coating material comprising adhesive and pigment in an aqueous vehicle than is desired on the finished paper, the formation on the surface of the flbrous web of a layer of firm filter cake corresponding to the quantity of coating desired on the finished paper, and the removal of substantially all thefluentcoating material above the filter cake by means of a wiper more yielding than the filter cake, said wiper acting at a location where the other side of the web is firmly supported.

4. The process of making coated paper in a paper machine which comprises applying an excess of fluent coating composition containing adhesive and pigment in aqueous suspension to a partially dewatered travelling web of paper, at a location in the paper machine where it is sufliciently absorptive-to liquidot the coating composition to form on the web a firm filter cake from the solids oi the coating composition adjacent the web while the overlying coating composition remains fluent, then wiping oi! said fluent coating composition approximately at the surface of the filter cake by means of a wiper more yielding than the filter cake, said wiper acting at a location where the other side'ot the web is firmly supported, anddrying the coated web in the paper machine.

. DONALD B. BRADNER.

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