US2286259A - Manufacture of coated paper - Google Patents

Description

Patented J une 16,1942

MANUFACTURE OF COATED PAPER James H. Cagle, Aurora, 111., assignor to Glenn Davidson No Drawing. Application February 20, 1940,

Serial No. 319,915

Claims.

Coated papers for printing purposes and the like aremade according to a long established practice of applying to the surface of a sheet of paper, a coating consisting of a pigment, usually clay, and an aqueous solution of an adhesive, usually casein, dispersed by an alkali or an alkaline salt. The product of such an operation leaves a great deal .to be desired, particularly in the way of uniformity from one lot of coated paper to another 'and'even within a given single sheet. The reasons for this lack of uniformity are manifold and perhaps not completely understood. However, my work to be described hereinafter makes it possible to understand most of these factors in an empirical way at least.

If a sheet of body stock, i. e., a sheet of paper such as is ordinarily used in coating operations,

is held up to the light it will be readily noted that it is not of uniform opacity or density. Small areas which readily transmit light alternate with other small areas which are comparatively opaque. This means that under the conditions by which the sheet is formed on the paper machine, it is not practicable or possible in this grade of paper stock to secure an even lay and uniform density throughout the area of the sheet. If such a sheet is floated upon water, an aqueous alkali solution or an aqueous ink, it will be found that at first the aqueous solution does not penetrate through the sheet uniformly. That is, for

example, if the sheet is floated upon a black aqueous ink, when the ink first begins to show through the sheet it does not uniformly color the upper surface of the sheet but shows up as ,black patches of small area scattered here and there.

This would indicate conclusively that aqueous solutions do not uniformly penetrate into a sheet ,of coating stock but penetrate at a very much greater rate within certain small areas than within other adjacent small areas. This observed fact is connected with the observation that a sheet of coating stock held up to the light doesnot transmit light uniformly. In cases where I have found sheets in which the lack of uniform light transmission was pronounced, I have demonstrated this to be true by outlining the opaque areas with a lead pencil mark and then floating the sheet upon an aqueous ink. In this experiment I found the outlined areas to require the longest time for the penetration of the ink through the sheet.

Paper coaterspommonly use what is known as Case ink for testing the uniformity of .the ink receptivity of a sheet of coated paper. Case ink is a moderately heavy, oily paste containing an oil soluble dye. The test is made by applying a layer of ink thick enough to remain glossy for the period of the test to the surface of the coated sheet to .be tested, allowing it to remain for exactly two minutes and then completely removing the excessink with an absorbent cloth or paper. If the area covered by the ink is uniformly colored, the sheet is said to be satisfactory. If, on the other hand, this area appears very blotchy thev sheet is considered unsatisfactory for most printing work. The Case ink test gives a very good indication of the results to be ex pected under practical printing conditions.

If a sheet of coating stock is coated with a mixture of clay and casein, dispersed in' an aqueous alkaline solution and is carefully observed as the coating dries, it will be noted that at the time the "wet coating begins to lose its lustre, it does not .I have concluded that just as the ink penetrates certain small' areas of the coating stock more readily than it does certain other adjacent small areas, so does the casein solution. component of the wet coating. If this hypothesis is correct,

.there should be less casein associated with the clay within the areas which become dull first upon drying than within the other areas. I have confirmed this hypothesis as follows: I outlined with a fine lead pencil mark the areas which first lost their lustre upon drying of the coating and then, after. the sheet had completely dried and been calendered, I applied the Case ink test over these areas. that the areas which had beenoutlined showed a darker coloration with the Case ink than the adjacent areas. By making a series of sheets halving coatings wherein the casein ratio to the clay increases by successive steps, it can be readily demonstrated that the more casein in the clay, the lighter the coloration resulting from the Case ink test and vice versa. Thus it is demonstrated that the areas wherein the coating first becomes dull, contain less caseinthan the adjacent areas and that these areas primarily correversely, the areas of the body stockdirectly under the areas of the coating which first become dull In this experiment it was foundv as the coating dries must contain more casein than the surrounding areas of body stock.

I have carefully observed a very great number of pick tests made with printers ink on coated papers using a variable speed printing press. With this piece of equipment, the speed of printing can be so adjusted that any given sheet of coated paper will show a very small amount of pick, an intermediate amount, or a great deal of pick. These tests show that the break, rarely if ever, occurs in the casein-clay layer. It sometimes occurs at the interface between the caseinclay layer and the upper layer of fibres of the body stock, but by far, most commonly occurs between the uppermost fibres of the body stock and the remainder of the stock.

When the speed of the press is adjusted to produce an intermediate amount of pick, the pick occurs over small areas, more or less, surrounded byother areas which do not pick. In this way the relation of the areas which pick to those which do not, bear a strong resemblance to the areas which first become dull upon the drying of the coating in relation to those which retain their lustre. That is, in other words, when the coating has about half lost its lustre upon drying it presents a certain, typical mottled appearance. An intermediate amount of picking with printers ink presents an extremely similar mottled affect.

If a sheet of coating stock is merely dampened with water and allowed to dry, and then examined with a hand lens in comparison with a similar sheet not so treated, it is seen that the surface fibres have been disrupted from their original position and present a more or less fuzzy appearance. This is no doubt due to the swelling produced in these fibres by the wetting with water which dislodges them from their original position in the sheet.

Undoubtedly, a similar dislodgment of the surface fibres of the sheet occurs when the sheet is first wetted with an aqueous coating, for exame ple, of clay and casein. Undoubtedly, it is the function of part, of the casein which escapes from the clay coating to re-cement these fibres rigidly back into position. If the area of the body stock lying directly under a given loosened fibre, is capable of being permeated by casein readily, this re-cementing is readily accomplished. If, on the other hand, this area is not readily permeated by casein, then the re-cementing is not so successful. Accordingly, it follows that when the sheet is printed under conditions which tax its ability to withstand picking, the coating picks directly .over those areas which are not capable of being permeated with sufficient casein to re-cement the surface fibres which have being unavoidably disrupted by being wetted with the coating.

i To state the matter another way, the preponderately weak zone in most sheets of casein-clay coated paper, is the zone between the surface fibres of the body stock and the body stock proper. In coated paper produced under commercial conditions this zone may be moderately strong within certain areas, where the casein has been able to re-cement the surface fibres rigidly back into position but it is weak in other areas where this has not happened for reasons set forth above.

Further confirmatory evidence of the fact that casein leaves the coating and runs into the sheet more readily in the areas which are most readily permeated by alkaline solutions may be had in the following manner. Take two sheets, oneof sives.

which has been found by the above described float test to be readily wetted by alkaline solutions and another which has been found to be difiicult to wet by similar test. Apply to these sheets the same coating mix of clay and casein. After drying and calendering, make the Case ink test on the two sheets simultaneously. It will be found that the Case ink produces a much darker coloration on the coated sheet, the body stock of which was more easily wetted by alkaline solutions.

From all of the foregoing it may be deduced that the function of the casein in coating paper, is definitely three fold: (1) It binds the clay particles together. (2) It binds the clay-casein layer to the surface fibres of the body stock.

(3) It binds the surface fibres of the body stock which have been unavoidably disrupted by the wetting to the body stock proper. This conclusion is not altogether original with me, but has been set forth to a greater or lesser degree by authorities on the subject of paper coating as a result of research work they have done along entirely different lines of approach than my own. However, I feel justified in going one step farther than anyone else has done so far as I know. To the extent of pointing out that, under commercial conditions at least, the ability of casein to perform its third function, .over any given square inch of coated paper, due to the lack of uniformity of wetability by alkaline solutions of the body stock within that square inch, leaves a great deal 'to be desired. This results in a very undesirable tendency toward both mottled printing and a tendency to pick.

Over a period of several years a tremendous amount of research work has been carried on in an effort to supplant casein in coated papers with vegetable protein adhesives, such as, for example, soya bean flour, peanut flour, linseed flour, cotton-seed flour, lupine flour, corn gluten and wheat gluten. To date, this work has not been I productive of commercial success except in a few isolated instances. The prime difiiculty encountered has been that coated papers made using coatings of clay or other pigments and these vegetable proteins as, adhesives have shown entirely too great a tendency to pick.

I have found by treating casein-clay coated papers in comparison withvegetable-protein clay coated papers with Delafields haematoxylin solution (a histological stain) which stains proteins a deep purple, that while the casein definitely penetrates into the body stock, this is deflnitely not true for the vegetable protein adhe- That is, in the case of the casein-clay coated papers, the body stock as well as the casein-clay layer is definitely stained (although not uniformly so) by the haematoxylin solution. This is not true of the body stock coated with the vegetable-protein clay coating. I have further found that the predominantly weak zone in the vegetable-protein clay coated papers is the bond between the vegetable-protein clay coating and the surface fibres of the sheet. In other words, it would seem from this, safe to assume, that while the casein is able to escape from the caseinclay layer while the coating is still wet sufliciently to perform successfully function (2) above and more or less successfully function (3), on the other hand the vegetable-protein adhesive is so tightly held or absorbed by the clay that it cannot escape therefrom in sufficient quantities to even perform function. (2) to say nothing of function (3). If this assumption is correct, it

2,286,269 must logically follow that since all of the vegetable protein is remaining in the clay coating, the vegetable-protein clay coa ed sheet would not show the mottled effect upon drying .described above for casin-clay coated sheets and neither should it show the mottling experienced with casein in the Case ink tests. Both of thesethings have been found to be true by experiment.

Others have heretofore the difficulty? experienced in binding the vegetable-protein clay coating to the body stock by applying to the body stock prior to the applica-' tion of the vegetable-protein clay coating a, light coating of adhesive, notably starch. I have done proposed to remedy starches up to e12 %iand'hydrolyzed starch up to 30-40%. The consistency of these heavier soa great deal of experimental work with this propigments of high tintorial strength, applied them.

to body stocks and allowed to dry and then applied thereover vegetable-protein clay coating and made pick tests on the resulting sheets. Be-

cause the starch layer was highly colored I could I definitely locate the break in the pick test. I found this break to occur almost invariably between the starch layer and the vegetable-protein clay layer. Further, I repeated this experiment but did not allow the starch layer to dry. I applied over it within a fraction of a second after its application the vegetable-protein clay coating. I found that if this vegetable-protein clay coating was made by a, roller application and was not brushed or otherwise mechanically agitated, that the break upon picking occurred in the same place as was the case when the starch layer had been previously dried; namely between the starch layer and the vegetable-protein clay layer. I further found that if the vegetable-protein clay layer was mildly agitated by brushing, for example, while in the still wet stage, that the surface tension between the two wet layers was immediately broken and the two layers while still in the wet stage intermixed to a considerable extent. This occurs even though the brushing isvery mild and only the very upper surface of the vegetable-protein clay layer is touched with'the brush. -However,'in this case, when pick tests were made the break came between the starch layer and the surface fibre of the body stock not withstanding the fact that it could be easily seen that the inter-spaces between these surface fibres were, filled with the colored starch. That is, it seemed under these conditions that the vegetable-protein clay layer and the body stock both had a strong attraction for the starch. The result being that the starch dispersed in both directions away from the interface of the surface fibres of the body stock and the coating layer, leaving the interfacial bond between the coating layer and the body stock weak.

The foregoing experiments were made with starch solutions containing four pounds to six pounds starch per hundred pounds of water. Such starch solutions give (when unhydrolyzed starch is used) solutions of about the same consistency that is ordinarily used in surface sizing operations. I then tried very much heavier lutions bordered on what might be called pasty. This procedure, however, did not remedy the difficulty. By coloring the starch and making pick tests as previously described, I found that while the heavier starch paste did not markedly diffuse into the vegetable-protein clay layer, neither did it sufiiciently wet 'thebody stock toperform casein functions ,(2) and (3),, setforth above. That is, as before the break as shown by the pick test came .at the junction between the surface'v fibres of the body stock and the starch layer.

I have found that the foregoing difficulties can be corrected by applying to the sheet of body stock a light layer of adhesive, as for example, starch. Allowing this adhesive at least sufiicieut time to gel by loss of moisture to the sheet and to the air which requires thirty seconds or more,

or even'to dry. I then apply over this first lay-' er of adhesive 8, second layer of adhesive, as for example, starch and then. immediately apply thereover the main coating consisting of vegetable protein and pigment.

The first layer of adhesive disrupts, .by swell ing, the surface fibres ofthe sheet which must, necessarily, be disrupted by any aqueous coating operation. It permeates and fills the interspaces between the fibres in the more easily wetted areas,

completely, or nearly so, but considerably, less. completely in the more difiicultly wettedareas.

That is, the foregoing occurs before the application of the second coat of adhesive. When the second coat is applied over the dried or partly dried first coat, the first coat takes up moisture from the second coat and thus has a second opportunity to permeate the more 'difllcultly wetted areas. In this fay, over the entire surfaces of the sheet, in both easily and difiicultly wetted areas, an adhesive filled matrix is formed into which the surface fibres originally disrupted by swelling, can re-'cement themselves. Thus'the first two layers take care of casein function (3) in a fashion far superior to the way it is ordinarily accomplished in caseinpigment coating. v

In commercial coating operations using casein and pigment, the-casein has only one opportunity which, in most cases is probably less than thirty seconds in duration, to both escape from thepigment and permeate both the easily and the difiicultly wetted areas of the body stock. While, it is granted that the ability of casein to so escape from the pigment into the sheet is truly remarkable, complete success in this is just too muchto ask of it.

a In other words, in my process,- the surface fibres are relatively, completely cemented to the sheet and in this way form a sort of tough, continuous skin over the surface of the sheet as contrasted with-the more or less spotty or discontinuous skin formed by the surface fibres of the sheet when coated with casein-pigment coatings, due to the irregularity with which the casein is able to-escape from the coating into the sheet in the short space of time allowed under commercial conditions.

starch paste. This, I tried using unhydrolyzed 7 In connection with the'relation between my first two coats ofadhesive, it is well to remember that the first coat covers the'surface of the cellulose fibres of the sheet with adhesive and so presents an adhesive surface rather than a cellulose surface, to the second layer of adhesive. As a result, no diflic'ulty whatever results in getting the second layer of adhesive to bond to the first self well into the sheet, an extremely strong bond is obtained at this junction.

As set forth above, the main coating of vegev table-protein and pigment is applied almost instantly after the application of the second coat.

If this main coating is lightly brushed or otherwise similarly agitated, the surface tension between it and the immediately underlying wet layer is sufiiciently brokento allow the two layers to diffuse together slightly and a Very strong bond is thus formed at this junction. Thus, is I performed function (2) of the casein.

As stated before, casein-pigment coated sheets reflect in the Case ink test the easily and the difficultly wetted areas of the underlying body stock. They further reflect the same characteristics of the body stock in their tendency to pick when tested with printer's ink. These two shortcomescape from that layer into the more easily.

wetted areas of the sheet as casein does. It therefore, all-remains in the uppermost pigment bearing layer and since it is uniformly distributed throughout this layer, a surface is produced which is capable of uniform ink receptivity as shown by freedom from blotchiness in the Case ink test.

As a result of this technique, I make a coated sheet which is extremely resistant to picking and which absorbs ink as shown by the Case ink test, far more uniformly than corresponding sheets coated with casein and clay.

For my first coat of adhesive 1- may use a solution made by dissolving six pounds 'of starch in one hundred pounds of hot water. If, to this is added two or three pounds of soda ash some assistance is obtained in uniformly wetting diflicultly wetted sheets of body stock. In case it is desired to build up a heavy total coating weight on the sheet, say eighteen pounds or more per thirty three hundred square feet of paper stock, I may add pigment ranging in amounts from one to thirty pounds to theabove prepared adhesive. For my second coat of adhesive I may use a solution of hydrolyzed starch containing thirty pounds of starch per hundred pounds of solution. 'If desired, soda ash or similar alkali may also -be added to this. One alternative is to make up a mixture of this adhesive with a clay slip in such a way that the resulting solution contains about thirty parts of hydrolyzed starch, one hundred parts of dry clay and seventy parts of water.

The main coating of vegetable-protein and layer and since the first layer has imbedded it-' pigment may be made, for example, by dispersing one hundred pounds of soya flour in five hundred pounds of water with the assistance of five and a half pounds of caustic soda. This adhesive is then added to a clay slip in such a way that the final coating mix will contain, for example, twelve parts dry soya flour, one hundred parts dry clay and one hundred forty parts of Water.

By making suitable adjustments in concentration I may use less preferably in lieu of starch for the first two coats suchcommon adhesives as animal glue, casein, gum arable, dextrin, protein adhesive or silicate of soda.

Other modes of applyingthe principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims, or the equivalent of such, be employed.

1 therefore particularly point out and distinctly claim as my invention:

1. In a process-of making coated paper, supplying a vegetable proteinous finish coating of soya bean proteinous material out of contact with the cellulose of the paper by first applying to the cellulose a coating of starch, drying the same at least in part, then applying to this another coatingof starch, and before the latter is dry applying a top coating of soya bean proteinous material and finely divided mineral filler.

2. In a process of making coated paper, sup

plying a vegetable proteinous finish coating of soya bean proteinous material out of contact with thecellulose of the paper by first applying to the cellulose a coating of starch, drying the same at least in part, then applying to this another coating of starch, and before the latter is dry applying a top coating of vegetable proteinous material and finely divided mineral filler.

3. In a process of making coated paper, supplying a vegetable proteinous coating of soya bean proteinous material and finely divided mineral filler while insulating the same from contacting with the cellulose of the paper by interposing two preliminary layers of sizing material containing no vegetable proteinous material or cellulose.

4. In a process of making coated paper, supplying a finish coating of vegetable proteinous material and finely divided mineral filler while insulating the same from contacting with the cellulose of the paper by interposing a plurality of preliminary layers of sizing material containing no vegetableproteinous material or cellulose, the latterof said preliminary layers being still wet when receiving the outside vegetable proteinous coating.

5. In a process of making coated paper, supplying a finish coating. of vegetable proteinous material and finely divided mineral filler while insulating the same from contacting with the cellulose of the paper by interposing a plurality vof preliminary layers of sizing material containing no vegetable proteinous material or cellulose.

JAMES H. CAGLE.