US3600215A

US3600215A - Cast coating process

Info

Publication number: US3600215A
Authority: US
Inventors: William C Mervine
Original assignee: Wyomissing Corp
Current assignee: Wyomissing Corp; Paper Corp of America
Priority date: 1968-04-16
Filing date: 1968-04-16
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17

Abstract

A METHOD OF CAST COATING PAPER WITH AN AQUEOUS MINERAL CAST-COATING COMPOSITION CONTAINING CASEIN OR SOY BEAN PROTEIN AS BINDER WHEREIN THE COMPOSITION HAS A PH ABOVE 5 AND BELOW 4 AND A SOLIDS CONTENT OF FROM ABOUT 45 TO ABOUT 60% AND WHEREIN THE WET COATING IS PARTIALLY DRIED, WHILE THE COATED PAPER WEB IS MOVING AND SUPPORTED ON ITS UNCOATED SIDE, BY DIRECTING A UNIFORM JET STREAM OF AIR ONTO THE WET COATING IN A DIRECTION TOWARD THE COATING BUT OPPOSED TO THE DIRECTION OF TRAVEL OF THE PAPER WEB.

Description

Aug. 17, 1971 w. c. MERVINE 3,600,215

CAST COATING PROCESS Original Filed July 6, 1967 FIGI.

/ HEATED CASTING DRUM COATED PAPER INVENTOR: WILLIAM C. MERVINE WWW ATTYS.

United States Patent ABSTRACT OF THE DISCLOSURE A method of cast coating paper With an aqueous mineral cast-coating composition containing casein or soy bean protein as binder wherein the composition has a pH above 5 and below 7 and a solids content of from about 45 to about 60% and wherein the wet coating is partially dried, while the coated paper web is moving and supported on its uncoated side, by directing a uniform jet stream of air onto the wet coating in a direction toward the coating but opposed to the direction of travel of the paper web.

The present invention relates to an improved method of making coated paper; and, more particularly, the invention relates to an improvement in the cast coating process of providing glossy mineral coatings on paper.

RELATIONSHIP TO OTHER APPLICATIONS This application is a continuation of application Ser. No. 651,625, filed July 6, 1967, now abandoned, which application Ser. No. 651,625 was a continuation of application Ser. No. 299,906, filed Aug. 5, 1963 (now abandoned).

BACKGROUND OF THE INVENTION US. Pat. No. 1,719,166 discloses the basic cast-coating process involving applying an aqueous mineral coating composition to a paper sheet or web, pressing the mineralcoated paper against a smooth, heated, highly polished surface, usually on a revolving drum, with the coating in a plastic condition against the surface of the drum, and removing water from the coating to set it to a non-plastic condition and thereafter removing the coated paper from the casting surface. The dried coating is supposed to possess a glossy finish as a substantial replica of the polished surface on which it is formed or cast. However, as is 'known, in actual practice various factors contribute to produce defects, such as pits, coating foam and craters, in the coating. To overcome this difficulty, it was proposed to apply an excess of the coating composition to the paper, running with an excess of the coating composition in the casting nip where the coated paper is pressed onto the casting surface, usually by means of a rubber nip roll, and relying on the pressure of the rubber nip roll to determine the weight of the ultimate coating. This proposal led to difficulties in keeping clean the surface of the casting drum beyond the edges of the paper, and in applying a predetermined and uniform weight of coating over the paper surface. It was also proposed to flood the nip, where the coated paper is pressed onto the casting drum, with water. This suggestion had certain disadvantages, principally in loss of coating composition washed away by the water. Moreover, cast coating processes are, in general, relatively slow because of the length of time required to remove water from the coating and paper. I believe that this is due to the fact that the coating composition is alkaline and that, under alkaline conditions, the protein binder does not easily release water. There has been a suggestion to gel the coating composition, after its application to the paper web and before pressing onto the finishing surface by passing the paper web with the coating composition thereon through an acid bath. This procedure not only involves extra steps but results in a coating which is not of the cast type in that it is not a mirror image of the surface against which it is finished. Thus, cast coating a paper Web continued to present problems.

It is the principal object of the present invention to provide an improved method of cast coating paper.

It is another object of the present invention to provide an improved method of cast coating paper wherein prior difliculties and limitations are overcome or reduced.

Another object of the present invention is to provide an improved method of cast coating paper wherein the coating is applied in advance of the casting drum, but wherein curling of the paper and pitting of the coating are overcome.

A further object of the present invention is to provide an improved method of cast coating paper wherein a high quality, smooth, glossy coating is provided.

Still another object of the present invention is to provide an improved method of cast coating paper wherein the above-mentioned high qualities are provided at substantially faster speeds.

Still another object of the present invention is to provide an improved method of cast coating wherein the paper web need not contain excessive filler or be precoated. I

Other objects will become apparent from a consideration of the following specification and the claims.

The improved method of the present invention comprises applying an aqueous mineral cast-coating composition having a pH above 5 and below 7, to a travelling web of paper; thereafter moving the coated paper Web in an upward direction and, while moving in an upward direction and supported on its uncoated side, directing a uniform jet stream of air onto the wet coating, said air stream having a width in the cross direction of the web substantially that of the coated paper web and being in a direction toward the coating but opposed to the direction of travel of the web; then pressing the coating, with the paper web, into non-slipping adherent contact with a travelling, heated, polished casting surface; drying the coating to a substantially non-plastic condition in contact with said casting surface, and removing the coated web from the surface.

The present invention will be more readily understood from a consideration of the drawings in which:

FIG. 1 is a side elevational schematic view of apparatus usable in accordance with the present method; and

FIG. 2 is a fragmentary and elevational view of a portion of the apparatus illustrated in FIG. 1, like numbers referring to the same members as shown in FIG. 1.

The present invention represents a significant improvement in the art of cast coating manifesting itself principally in the results obtained, namely lack of curl in the paper and'a high quality, gloss coating free from pits and other flaws, and in the fact that these results are obtained at appreciably higher speeds of operation. The exact reason for these results is not known; however, it is known that the combination of: (a) the coating composition having the specified pH (as distinguished from an alkaline composition), and (b) the jet air stream applied in the manner described, is necessary; one without the other not producing the same improved results. Furthermore, it is believed that curling of the paper was caused, at least in part, by tensions applied to the Web in the machine direction. For example, metering of the coating composition by moving the coated web in scraping engagement along a doctor roll or blade required tension in the moving web since the amount of coating remaining on the web after metering was governed by the pressure exerted between the web and the roll or knife. Metering according to the present method is accomplished by the air stream which does not produce tension in the paper web and requires only very light tensioning of the web as compared to the use of doctor roll or knife. While this accounts for reduction in curling of the paper, I believe that elimination of pitting is the result of the nature of the coating as it comes into contact with the casting surface which, in turn, is the result of the pH of the coating composition and the preliminary partial drying or dehydration of the coating by the air stream. Thus, at the pH specified, I am able to provide a high solids (45- 60% by weight) content in the coating composition while, nevertheless, having a relatively low viscosity (1001500 c.p.s. Brookfield at room temperature using a No. 3 spindle at 12 rpm). Some dehydration or drying of the coating is believed to be caused by the air stream. All these factors apparently combine to provide the superior final coating and to permit the use of higher operating speeds. By the use of the present method one is able to use a paper that is neither highly filled nor precoated in order to obtain the high quality coatings. Previously, highly filled or precoated paper had to be used to obtain the best coatings. Paper that is not highly filled or precoated is easier to work with since the moisture from the coating composition passes therethrough much more readily than is the case with certain highly filled paper or precoated paper. However certain advantages of the invention are realized even when highly filled or precoated paper is employed, and herein the terms paper, paper sheet and paper web, unless otherwise modified, will be understood to include highly filled paper and precoated paper.

Referring then to the coating composition, it will have water as its liquid vehicle; a binder comprising a hydrophylic protein adhesive, such as casein or soy bean protein; and the mineral pigment, such as fine, coating-grade clay, calcium carbonate, blanc fixe, satin white, titanium pigments, color pigments, and the like, and combinations thereof. Formaldehyde (including the conventional formaldehyde donors, like mono-methylol dimethyl hydantoin) may also be included in the coating composition to render the protein in the final coating water-resistant. In these respects, the present coating composition may be similar to those used in the past, and in which the preferred binder is casein and the preferred pigment is clay, with colored pigments when desired. Likewise, the composition may, in accordance with conventional practice, also contain a release agent which aids in separating the coated web from the casting drum surface, such as a fatty acid soap, like ammonium stearate, zinc stearate, and the like. The composition may also contain auxiliary or supplemental binders like acrylic resin, butadienestyrene latex, and the like, and a protein viscosity-reducing agent, like dicyanodiamide.

The coating composition of the present invention, however, will have a weakly acid pH; that is a pH above 5 and below 7, preferably between 5.4 and 6.6. This can be provided through the use of a weak organic acid, like formic, acetic, citric, and the like. Since the protein is normally initially solubilized by treatment with alkali, preferred practice involves markedly reducing the amount of alkali normally used in solubilizing the protein and adding sulficient weak acid subsequently to adjust the pH to within the stated range. Commercial aqueous formaldehyde solution (Formalin) is acid, containing formic acid, and may be relied upon to provide the desired pH. On the other hand, a weak acid as such may be used in place of or in conjunction with the Formalin to adjust the pH. When formaldehyde is omitted from the coating composition, the cast coated paper may subsequently be treated with a water proofing agent, such as a formaldehyde to provide enhanced water-resistance, if desired. Of course, water-resistance in the coating may be provided by means other than the use of formaldehyde, as by the use of water-resistant auxiliary or supplemental binders of the type mentioned above.

In the coating composition, the content of solids (binder and pigment, and release agent and/or viscosity reducing agent, when used) will generally be between about and about by weight, based on the total weight of the composition. As between the binder and pigment, the former will normally be present in a minor proportion, that is less than 50%, usually in an amount between about 10 and about 30%, by weight; and the pigment will normally be present in a major proportion, that is, more than 50%, usually in an amount between about and about 90%. Formaldehyde (as 37% formaldehyde solution) may be present in an amount between about 1.5 and about 21%, by weight, based on the dry weight of the protein binder.

Examples of coating compositions are as follows (parts are by weight):

Clay 220 Water 86 Zinc stearate paste (46.5% solids) 1.4 22 /2% casein solution (ammonia solubilized) 166.5 37% formaldehyde 5.5 Water added with 37% formaldehyde ll Styrene-butadiene latex, 4850% solids (Dow Latex 512K) 6.6 Acrylic resin, 38-40% solids (Rohm & Haas Rhoplex B-) 21.75 Dicyanodiamide 4.7

The solids content of this composition is about 52%, its pH is about 5.7-6, and its viscosity is between about 250 and about 1000 cps. Brookfield at room temperature using a No. 3 spindle at 12 r.p.m. The solids content and viscosity can be further adjusted by the addition of water.

Clay 220 Water 86 Zinc stearate paste (46.5% solids) 1.4 22 /2% casein solution (ammonia solubilized) 166.

Glacial acetic acid .35 Water with glacial acetic acid 6.7 Styrene-butadiene latex, 4850% solids 6.6 Acrylic resin, 38-40% solids 21.75 Dicyanodiamide 4.7

This composition has a pH of about 6. The addition thereto of 5.5 parts of 37% formaldehyde lowers the pH to 5.1-5.3. In this composition, formic acid or citric acid may be substituted for the acetic acid. In each of the above compositions, solubilized soya bean protein may be substituted for the casein, and calcium carbonate or other equivalent mineral pigment may be substituted at least in part for the clay.

The coating composition may be applied to the paper in any one of a wide variety of ways, although it is preferred to employ a roll coating method in which a revolving roll picks up the coating composition from a pool thereof and transfers it to one side of the travelling paper web. The application means will be adjusted to apply coating composition in excess of that ultimately desired.

The next main feature, as far as the present invention is concerned, is the direction of the jet air stream onto the freshly-applied, wet coating. For this operation, the coated paper web should be travelling in an upward direction, preferably substantially vertical, at the point where the jet air stream impinges upon it since it is a combination of gravity and the air stream which operates to meter the excess coating composition. Moreover, the uncoated side of the paper web should be supported at the site opposite the line of impingement of the air stream, as by means of a roll. The air stream may be provided by passing air through a so-called air knife or air doctor which possesses a wide shallow horizonal slit for directing a uniform jet transversely across the coating in its upward travel. The jet air stream, in the cross direction of the web, will be substantially as wide as the coated web, and preferably will have a width just slightly less than the width of the web. The jet air stream will be in a direction toward the wet coating but opposed to the direction of travel of the web. Thus, the angle (a in the drawings) formed by the plane of the web at the line of impingement (the tangent line if travelling about a roll) and the plane of the air stream, down stream in the direction of travel of the web from the line of impingement of the air stream, will be less than 90 so as to oppose the movement of translation of the composition with the movement of the paper web, and generally will be between about 40 and about 60" The pressure of the air stream will be moderately elevated, between about 0.5 and about 4 p.s.i. gauge, preferably between about 1 and about 3 p.s.i.g. being suitable.

After the wet coating has been subjected to the jet air stream, the coated web is pressed intimately against the moving casting surface, with the wet coating, in a highly plastic, ungelled condition, being against the casting surface. There should be no significant slippage between the coating and the casting surface, so that the linear speed of the casting surface should be the same as that of the coated web. The casting surface, which advantageously is the outer circumferential surface of a rotating hollow drum, will have a highly polished, smooth finish since its finish is duplicated by the outer surface of the coating which is formed thereon. Mirror finished chromium plated steel is highly suitable. The coated paper web may be pressed against the casting surface by a resilient or yielding member, like a rubber-covered roll.

The casting surface will be heated to drive water from the coating composition through the paper web and away from the exposed side. Steam may advantageously be used for this purpose, for example by being admitted to the interior of the casting drum. Since the object of the heated casting surface is to drive off water without deleteriously affecting the coating, the temperature of the casting surface should be above 150 F. but below 212 F. The temperature of the casting surface as well as the diameter of the casting drum, determines the speed of operation. With a 4' diameter chromium plated steel dnum having a surface temperature between about 170 and about 200 F. (the preferred temperature conditions), linear surface speeds of from about 80 to 1000 ft. per min. may be employed. The drying of the coating composition converts it to a relatively hard film which is bonded to the underlying paper :but does not adhere to the casting surface. While it is preferred that the coated -web reach its runtimate desired dryness on the casting surface, this is not altogether necessary so long as the coating has substantially set or dried to a non-plastic condition, that is to the extent it will retain the characteristics imparted to it by the casting surface. Therefore, the term drying as used herein and in the claims will be understood in this light.

Because the coating has been converted from a wet, highly plastic (ungelled) condition to a substantially dry non-plastic condition while in intimate contact with the casting surface, the outside surface of the coating (the cast surface) will acquire the same finish (a mirror image) as the casting surface. Thus, a mirror-finished chromium casting surface will produce a smooth, glossy coating.

Referring to the drawings, the paper web is led, by guide rolls 1, over variable speed coating roll 2 Coating roll 2 picks up, from pan 4, coating composition in excess of that ultimately desired. Substantially immediately after the coating composition is applied, the coated paper web is moved in a substantially vertical upward direction around roll 3 past the air-knife which directs jet air stream 5 onto the coating. The plane of the air stream will have a component opposing the forward movement of the coated paper web, thereby providing an angle a (between the plane of the air stream and the tangent line of the coated web at the point of intersection by the plane of the air stream) as defined above. As also shown in the drawing the plane of the coated web at the point of intersection of the plane of the air stream, while substantially vertical, is advantageously slightly less than from horizontal in the direction of the air stream. The width of the opening is substantially the same as, preferably slightly less than, the width of the coated paper web to insure that substantially all parts of the coating are contacted by the air stream but to avoid forcing coating composition beyond the edge of the paper web. This jet air stream, which is fed to the air-knife through conduit 12 from an outside source (not shown), meters off the excess coating composition which falls to collecting pan 1'1, and exerts an initial partial drying or dehydration of the coating. The coated paper web, after passing between variable speed smooth rolls 9 and guide roll 10, is then pressed against heated casting drum 7, by means of rubber-covered feed or nip roll 6. In its travel from feed or nip roll 6 to take-01f roll 8, moisture from the coating composition is forced through the paper web and away from the exposed surface as water vapor, until, by the time take-off roll 8 is reached, the coating is in a substantially dry, non-plastic condition and the coated web may be safely removed.

It will be noted from the foregoing that there is no excess coating composition on the paper web at the nip as the coated web is applied to the casting surface, nor is the coating, once the composition is applied to the paper web, contacted with further liquid, as by passing through a subsequent bath or by feeding water to the nip. Thus, the coating composition on the paper web at the time it contacts the casting surface is just that amount that is desired in the ultimate, final coating, and the moisture content of the coating composition at that point is as provided by the jet air stream. Moreover, at that point also, the viscosity of the coating composition is no less than, and may even be slightly greater than, that of the original coating composition as applied due to the partial drying by the jet air stream and the removal of water from the coating composition into the paper.

Modification is possible in the formulation of the coating composition as well as in the procedural steps and conditions without departing from the scope of the present invention.

I claim:

1. The improved method of cast coating paper which comprises applying an aqueous mineral cast-coating composition, having a pH above 5 and below 7, to a travelling web of paper, said composition having a solids content of from about 45 to about 60%, 'by weight, based on the total weight of the composition and containing, as binder, a protein selected from the group consisting of casein and soy bean protein; thereafter moving the coated paper web in an upward direction and, while moving in an up ward direction and supported on its uncoated side, directing a uniform jet stream of air onto the wet coating, said air stream having a width in the cross direction of the web substantially that of the coated paper web and being in a direction toward the coating but opposed to the direction of travel of the web, thereby partially drying said coating; then without contacting said coating with further liquid, pressing the coating, with the paper web, into non-slipping adherent contact with a travelling, heated polished casting surface; drying the coating to a substantially non-plastic condition in contact with said casting surface, and removing the coated web from the surface.

2. The method of claim 1 wherein the pressure of the air forming the jet stream is between about 0.5 and about 4 p.s.i. gauge.

3. The method of claim 2 wherein the pressure of the air forming the jet stream is between about 1 and about 3 p.s.i. gauge.

4. The 'method of claim 1 wherein the angle between the plane of the coated paper web, at the line of impingement of the air stream, and the plane of the air stream,

down stream in the direction of travel of the web is less than 90.

5. The method of claim 4 wherein said angle is between about 40 and about 60.

6. The method of claim. 1 wherein the plane of the coated paper web at the line of impingement of the air stream is substantially vertical.

7. The method of claim 1 wherein the viscosity of the aqueous mineral cast-coating composition is between about 100 and about 1500 cps. Brookfield at room temperature using a No. 3 spindle at 12 r.p.rn.

8. The method of claim 1 wherein the aqueous mineral cast-coating composition comprises a mineral pigment.

9. The method of claim 8 wherein said mineral pigment comprises clay.

10. The method of claim 9 wherein said protein comprises casein.

11. The method of claim 8 wherein, as between the binder and mineral pigment portions of the aqueous castcoating composition, the binder is present in a minor proportion, less than 50%, by weight, and the mineral pigment is present in a major proportion, greater than 50%, by weight.

12. The method of claim 11 wherein the binder is present in a proportion of between about and by weight, and the mineral pigment is present in a proportion of between about 70 and about 90%, by weight.

13. The method of claim 1 wherein a weak organic acid selected from the group consisting of formic, acetic and citric acids is present in said cast-coating composition in an amount providing said pH.

14. The improved method of cast-coating paper which comprises applying an aqueous mineral cast-coating composition, having a pH above 5 and below 7, to a travelling web of paper in an amount in excess of that desired in the coated product, said composition having a solids content of from about 45 to about 60%, by weight, based on the total weight of the composition and containing, as binder, a protein selected from the group consisting of casein and soy bean protein; thereafter moving the coated paper web in an upward direction and, while in a substantially vertical attitude and supported on its uncoated side, directing a uniform jet stream of air onto the web coating transversely thereacross, said air stream having a width in the cross direction of the web substantially that of the coated web and being in a direction toward the coating but opposed to the direction of travel of the web, to remove said excess coating composition from the web and partially dry said coating; then, without contacting said coating with further liquid, pressing the coating, with the paper web, into non-slipping adherent contact with a travelling, heated, polished casting surface; drying the coating to a substantially non-plastic condition in contact with said casting surface, and removing the coated web from said surface.

15. The improved method of cast-coating paper which comprises applying an aqueous mineral cast-coating composition, having a pH above 5 and below 7, to a travelling web of paper, said composition having a solids content of from about to about by weight, based on the total weight of the composition and containing, as binder, a protein selected from the group consisting of casein and soy bean protein; thereafter moving the coated paper web in an upward direction and, while in a substantially vertical attitude and supported on its uncoated side, directing a uniform jet stream of air onto the wet coating transversely thereacross, said air stream having a width in the cross direction of the web substantially that of the coated web and being in a direction toward the coating but opposed to the direction of travel of the web, thereby partially drying said coating; then, without contacting said coating with further liquid, pressing the coating, with the paper web into non-slipping adherent contact with a travelling, heated, polished casting surface; drying the coating to a substantially non-plastic condition in contact with said casting surface, and removing the coated web from said surface.

16. The method of claim 1 wherein the pH of the composition is between 5.4 and 6.6.

17. The method of claim 1 wherein the composition contains formalin in an amount providing said pH.

18. The method of claim 1 wherein the travelling web of paper employed is not precoated.

19. The method of claim 1 wherein the protein comprises casein; wherein the pH of the composition is between 5.4 and 6.6; wherein the pressure of the air forming the jet stream is between about 0.5 and about 4 psi gauge, and wherein the angle between the plane of the coated paper web, at the line of impingement of the air stream, and the plane of air stream, down stream in the direction of travel of the web is between about 40 and about 60.

20. The method of claim 1 wherein the composition also contains at least one auxiliary binder selected from the group consisting of styrene-butadiene latex and acrylic resin.

21. The method of claim 19 wherein the composition also contains at least one auxiliary binder selected from the group consisting of styrene-butadiene latex and acrylic resin.

References Cited UNITED STATES PATENTS 2,252,345 8/1941 Johnson. 2,678,890 5/1954 Leighton. 2,759,847 8/1956 Frost et a1. 2,849,334 8/1958 Hart. 2,919,205 12/ 1959 Hart. 2,996,405 8/ 1961 Van Campen. 3,115,438 12/1963 Randall. 3,186,861 6/1965 Smith.

MURRAY KATZ, Primary Examiner U.S. Cl. X.R.