US2532140A

US2532140A - Process of coating paper

Info

Publication number: US2532140A
Application number: US511754A
Authority: US
Inventors: William H Barrett; Bernard K Asdell
Original assignee: Minnesota and Ontario Paper Co
Current assignee: Minnesota and Ontario Paper Co
Priority date: 1943-11-24
Filing date: 1943-11-24
Publication date: 1950-11-28
Anticipated expiration: 1967-11-28

Description

Nov. 28, 1950 w, BARRETT ET AL 7 2,532,140

PROCESS OF COATING PAPER Filed Nov. 24, 1943 Fig-i Q 23 Q Q \2 ,r l c 27 26 a 25 g 22 0 23d a '0 \7 22a l3 9 28a a |9a 30 30a g 44 F R fZ 39 InvenEor-s: William H.Bc1r'r-etf After-neg.

Bernard KAsc lelL Patented Nov. 28, 1950 PROCESS OF COATING PAPER William H. Barrett and Bernard K. Asdell, In-

ternational Falls, Minn., assignors to Minnesota and Ontario Paper Company, Minneapolis,

Minn.

Application November 24, 1943 Serial No. 511,754

This invention is concerned with a papercOating process to produce a coating having a superior surface in respect to flatness, smoothness, freedom from imperfections, etc. The raw paper stock, in web form, may be of any suitable kind, and the ingredients contained in the coating applied thereto may be chosen from a number of different materials. The coating mixture may include for example, a mineral pigment, such as clay, calcium carbonate, blanc fixe, zinc sulphide, titanium dioxide, and the like, and a binder of starch, protein, casein, glue or other suitable material, all contained in an aqueous dispersion.

In the manufacture of paper for use as high grade printing paper, it is a customary procedure to apply to a paper web (base-stock) a 'layeror layers of an aqueous suspension of mineral pigment and adhesive, generally organic adhesive; the web is then dried and finished in any manner desired. Paper so'made is generally referred to asmineral coated paper.

The coatingof paper with suspensions of mineral pigments has presented serious problems for the reasons that such suspensions commonly used "in the paper art do notof themselves smooth out when applied with even slight irregularities. Heretofore when the paper wascoated by a direct application of coating liquid there was formed, unless special precautions were taken, ripples or irregularities of the coating material on the paper which, when dried, produce streaks. Therefore, in much of the prior art practice, provision is made for smoothing out the ripples or other irregularities formed upon the application, either by the use of brushes, doctor blade or of smoothing or traveling rolls. Such aids, however, have the drawback that they are cumbersome and are responsible for acomparatively low rate of production.

In view-of these di'fliculties, the present process proposes to apply a predetermined or measured amount of coating material upon the traveling web of paper to be coated in such manner as to produce an even coating inthe first instance and thus obviate the use of subsequent smoothing devices.

It is one of the principal objects of this invention to provide a method which will produce a coating of'mineral or other pigments having in a high degree the qualities of smoothness, and uniformity, and Lin-addition, having the advantanges of high output and reliability of operatime.

Another object of the invention relates to the 5 Claims. (Cl. 117-403) use oi a coating color having high solid contents to a fast moving web of paper: In general the finer the particle size of the mineral pigment, the better the color and finishing qualities. Clays, for example, from different sources even in the same particle size group vary in degree of surface activity. standpoint of high speed coating operations because it afiects'the degree of hydration in water, starch and the like slurries, and is a controlling factor in flow properties which a given coating composition will have.

All starch-mineral coating colors show thixotropic properties. With any given mineral coating composition these properties become more pronounced with increasing solids contents. In order to avoid confusing the terms used in describing coating colors flow properties are hereinafter defined:

Thixotropy is a reversible cso'lgel structure shown by many colloidal suspensions of high solid content. When a thixotropi'c solution is agitated, the absolute viscosity becomes lower as the speed of agitationincreases. When theagitation ceases the solution will again gradually set to a'flrm jell.

Dilatancy is the opposite of thixotropy. As the agitation speed increases, the absolute viscosity increases. Unlike thixotropy, no time factor is involved in dilatancy.

Viscosity .is the internal resistance of a fluid to flow. The absolute unit of viscosity is poise;

Fluid-ity is the reciprocal of viscosity.

Plasticity is the rate of decrease of viscosity with increasing rates of flow. A true plastic shows a definite yield point which is not :the case with most coated material. The term pseudoplastic is sometimes used.

Another important phenomenon of trixotropic solutions is that as the viscosity of the solution decreases by agitation, the rate at which the solution'will. give 11p its water to an absorbingunedium such as a web 'of paper increases. This is a very important factor in paper coatin for :it determines the rate at which the coating solution will wet the paper at the moment of contacuantl this in turn influences the character of the coat ing film. For example, it has been found in-our coating'process that if the coatinggives up water to the paper "fast enough so that it has become set (that is it is no longer a fluid film) before leaving the nip of the applicator roll, 'the coating film will be rougher than if it is still fluid, This is due to the fact that as the film breaksaway from "the applicator roll, it is pulled "up into This is important from the i 53; spherical droplets because of the forces of surface tension and cohesion. If the film is still fluid when leaving this zone, these spheres flow together into a smooth surface. If the film is no longer fluid, the small spheres become permanently set and a rough film results and poor fiber coverage also.

It has been found that as the color density increases the surface sheen also increases, and better fiber coverage results. It is highly desirable that the color density be controlled to obtain the desired coating.

Mineral-adhesive coating compositions are thixotropic to a degree, and this property is quite significant in the present method of coating. Advantages are obtained by applying the coating at high solids contents. This raises the question as to how high solids content may go with a given mineral-adhesive combination. The answer appears to be that the solids content should not exceed the point where the coating begins to show a'pronounced degree of dilatancy at high rates of flow. This point can be determined for any given composition by plotting rates of flow against force or viscosities against rates of flow.

The doctor roll is driven through suitable medium such as a variable speed Reeves drive so that the speed ratio between it and the applicator roll can be varied. The variable speed gives a great flexibility in control over the coating weight. For example, with a starch clay coating of 65% total solids and with a constant pressure on the doctor roll there is obtained a variation in coating weight from 12 pounds at a speed ratio of 1111.2 to four pounds at 1:192 at a constant machine speed.

It has been found that when colors of high solids are used a very thin film of coating passes under the doctor blade and dries before coming in contact with the applicator roll. This causes markings in the coating film on the applicator roll which will be transferred to the paper. This difficulty can be overcome by placing the doctor blade as near the applicator roll nip as possible and keeping the doctor roll wet.

It has been found that in order to obtain the smoothest coating film it is necessary to keep the nip between the doctor roll and the applicator roll flooded with a large excess of color. A number of ways may be used to accomplish this and one way is disclosed in the accompanying drawings. Air bubbles may accumulate in the color fountain between the doctor roll and the applicator roll. These bubbles may pass through the nip and cause skipped areas in the coating and the elimination of these bubbles is essential to a satisfactory coating.

If drier wrinkles are permitted to form there will appear in the finished sheet parallel lines of variable ink absorption. This difiiculty can be overcome by the use of a felt that comes into contact with the coated surface of the web of paper as soon as possible after the coating has been applied without marring the coated surface. The coated paper web wrinkles on the drier surface due to uneven drying and the use of a felt which holds the coated web in close contact with the drier drum surface causes a redistribution and equalizing of the moisture in the the drying coated web and substantially uniform drying and this results in the elimination of drier wrinkles. The use of a felt also permits higher drying rates with reduced drying temperature.

Figure l discloses diagrammatic illustration of an apparatus for carrying out the new method of coating, and

Figure 2 is a side view in elevation of a coating fountain and a coating backing roll.

Referring to the drawing in detail where l0 indicates a web of paper coming from a suitable source of supply (not shown). The web passes over roll ll and roll I! and then into contact with the large diameter heated drier drum I 8. The paper is coated preferably adjacent the point of contact with the surface of the drier surface. The paper web travels over a major portion of the drier drum surface. A suitable coating unit indicated at 9 applies the coating to the fast moving web of paper. The web leaves the surface of the drier drum at roll 23.

A felt 22 contacts the coated surface of the paper for a predetermined arc of the surface drier drum and the arc of contact is varied as the speed of the drier drum varies, the type of coating employed, the temperature of the drier drum, etc. The felt leaves the surface of the coated paper at 2! and the paper and felt 22 travel together from roll 23 over roll 24 to roll 25. The felt then passes over rolls 21, 28, and 29 and 35! back to the surface of the drier drum.

The web of paper with one side coated passes over roll l2 and then over roll [3. The web contacts the surface of the heated drier drum |8a and travels over the major surface of the drier drum. The web is coated by a suitably constructed coating unit indicated generally at [9a adjacent the point where the web first contacts the drier drum. It is to be understood that the coating may be applied just prior to or at the time the web contacts the surface of the drier drum. The coated web leaves the drum at roll 20a and then passes over

rolls

23a, 26, 3|, 32, and 33. The paper after passing over these last mentioned rols then may be subjected to other processing or may be wound into rolls on a suitably constructed wind up apparatus (not shown). The felt 22a leaves the roll surface at roll 21a and passes over

rolls

23a, 26, 3|, 28a, 29a and 35a where it comes into contact with the coated side of the paper web on the drier drum surface. The amount of contact of the felt with the coated paper on the drier drum is varied by adjusting the position of

rolls

28a and 29a, and 30a.

In Figure 2 of the drawings is shown a coating unit and a heated drier drum backing roll. A resilient surfaced applicator roll l8 contacts the surface of the web of paper on the drier drum and the speed of the applicator roll is preferably about the speed of travel of the paper web. In the fountain 37 is mounted a rotatable doctor roll 35 which controls the amount and the smoothness of the coating on the applicator roll. A flexible seal 36 contacts the surface of the applicator roll adjacent the nip between the doctor roll 35 and the applicator roll 34.

Secured to the machine frame so at 39 is applicator roll supporting frame 38 and hingedly or pivotally connected to frame 38 is doctor roll support 42, at the upper end of member :%2 is secured member 43 and means :54 for varying and controlling the pressure applied by the applicator roll 34. For varying and controlling the pressure between applicator roll 34 and the heated drier drum surface is member 45 and pressure varying means 46. The member 45 is connected to member 38 at one end and to machine frame member 47 at the other end.

The coating material is supplied to container 49 and withdrawn from the container through pipe. fiorby pump 52 and: is .forced' into. the: ileum.

tam;- er through pipe 56. The pipe- 56.. enters" fountain preferably nearthe topof the fountain; Aw surplus of coating material; is. maintained atthe nip. between the rolls. 35 and 34.. A surplus of coating material. fiowsover the. top of thezback of the fountain into container 49 and: fiowssover the incline plate 48. positioned in the container;

inclined plate aids in eliminating entrained: air inithe coating materials.

To clean the: surface. of the: doctor roll= a resilient. doctor blade 54: contacts the. surface: of the. doctor roll. 35. provided so that the pressure between the doctor blade and the doctor roll can be controlled. The

doctor blade is carried by member 53 which is attached to. doctor roll supporting frame 4'2. Some of the factors which determine the amountxof; coating applied to a traveling web: of paper:

Variable machine speed Constants:

Color solids Doctor roll pressure; Doctor r011 speedratio. Doctor r011 speed;

Ratio 4:1

Machine Coating Speed, Wgt, Lbs. F. P. M. per Ream Variable doctor roll speediratio Constants:

Coating solids Doctor roll pressure Machine speed A pressure varying: means: 551

Variable color solids;

Constants:

Machine speed Doctor roll pressure Doctor r011 speed ratio Per Cent Coating Wgt. SolidsIn Color Lbs. per Ream The present process has beenccarri'ed out very" successfully Wherethe applicator roll is. backed by a drier rollzoi' 8 feet indiameter but it is to. be understood: that the drier roll employed may vary."

considerably in diameter from that indicated;

The effect of the nip pressure betweenthedrier:

roll and the. applicatorroll" controls the ty-peof coating applied to the. moving paper web: The 4 density of: the coating: material may v-ary oven a considerable 1 range, but. highly successful coating Ratio between Surface speed i l .Sum of Doctor Roll and ztigrar g lalhcator roll and doc DOCtOI RollSpeed F.P..M. Applicator .Ronspeeds Coatmg Wgt. 1n lbs/ream 625 F. P. M. 492 F. M. P. 625 F. M P. 492 F. P. M. 625 F. P M 27 tooth gear 36 tooth gear 27 tooth gear 36 tooth gear 27 tooth gear C has been obtained where the solids contents of Variable doctor To pressure the coatmg falls Within the range of about 61% Constants:

Machine speed Doctor roll speed ratio Coating solids to about 72% solids and the paper web was traveling at speed of about 800 feet per minute to about i 1290 feet per minute.

One important advantage resulting from the high speeds at which webs can be coated with a coating material of high solids content is that paper webs may be coated with a uniform, smooth and selected weight of coating material as they are delivered from a paper making machine substantially as part of the paper making operation or as a separate operation it is preferred that the coating color be applied to the paper when sub- The additions of stantially dry, that is, having a moisture content from about 2% to about 10%.

It will be understood that the term color as used in this specification and claims, includes any of the coating materials usually employed for coating purposes.

Various changes from the detailed description and specific steps of the method will occur to those skilled in the art Without departing from the spirit ofthe invention or the scope of the appended claims.

What we claim:

1. A method of coating a continuously advancing web of paper which comprises, depositing a controlled and a Substantially uniform amount of mineral pigment coating material of high solids contents upon and evenly spread across the width of a rotating resilient surface, rotating said surface in contact with a supported surface of a web of paper, and in the same direction of movement with said Web, and at a speed substantially the same as the translatory speed of the said web to apply a uniform coating to said web surface, and subsequently contacting the coated surface of said web with a movin porous surface after the coating material has received a preliminary set.

, 2. A method of coating a traveling web of paper,

which'comprises applying a coating composition comprising mineral pigments to a cylindrical surface having resilient properties, removing a portion of said coat ng so applied by a rotatable surface whereby said cylindrical surface has thereon a uniform amount of coating composition which is less than the amount initially present on said surface, then contacting said cylindrical surface with a moving web of paper, drawing the web over the major portion of a heated drier drum surface, and contacting the coated surface of the web of paper while on the dried surface with a moving felt surface after the coating has received a preliminary set.

3. A method of applying mineral coating of a solids content of about 61% to about 72% to a fast moving Web of paper having a moisture content of about 2% to about 10%, comprising applying the mineral coating to a rotatable resilient surface, transferring the coating on the resilient surface to the Web of paper, carrying the web of paper in contact with a heated supporting surface for a portion of the surface of the said supportingsurface, and redistributing the moisture content of the coated web of paper while on the said supporting surface by means of a moving felt after the coating has received 'a preliminary set.

4. The method of applying mineral coating of high solids contents to a fast moving web of paper, which comprises applying a wet mineral coating in a smooth film to a face of the paper Web, moving the web forwardly on a round heated supporting surface, and bringing a moving felt into contact with the coated surface of the web on said supporting surface after the coating material has received a preliminary set.

5. A method of coating a fast moving web of paper comprising, applying mineral coating material of solids content in excess of sixty per cent to a web of paper, carrying the web of paper into contact with a heated cylindrical surface for a major portion of said surface and contacting the coated surface of the paper during its travel over a portion of a cylindrical drying surface with a moving porous surface for redistributing the moisture contents of the coated web after the coating has received a prel'minary set.

WILLIAM H. BARRETT. BERNARD K. ASDELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,866,689 Wilshire July 12, 1932 1,933,963 Bradner Nov. 7, 1933 2,133,933 Daley Oct. 18, 1938 2,204,600 Kabela et al June 18, 1940 2,267,470 Kabela et al so. 23, 1941 2,293,690 Harrigan Aug. 18, 1942 2,304,819 Grupe Dec. 15, 1942 2,313,497 Adrian Mar. 9, 1943 2,331,922 Montgomery Oct. 19, 1943 2,337,013 Bradner et a1 Dec. 14, 1943 FOREIGN PATENTS Number Country Date 320,477 Great Britain 1929

Claims

3. A METHOD OF APPLYING MINERAL COATING OF A SOLIDS CONTENT OF ABOUT 61% TO ABOUT 72% TO A FAST MOVING WEB OF PAPER HAVING A MOISTURE CONTENT OF ABOUT 2% TO ABOUT 10%, COMPRISING APPLYING THE MINERAL COATING TO A ROTATABLE RESILIENT SURFACE, TRANSFERRING THE COATING ON THE RESILIENT SURFACE TO THE WEB OF PAPER, CARRYING THE WEB OF PAPER IN CONTACT WITH A HEATED SUPPORTING SURFACE FOR A PORTION OF THE SURFACE OF THE SAID SUPPORTING SURFACE, AND REDISTRIBUTING THE MOISTURE CONTENT OF THE COATED WEB OF PAPER WHILE ON THE SAID SUPPORTING SURFACE BY MEANS OF A MOVING FELT AFTER THE COATING HAS RECEIVED A PRELIMINARY SET.