US1838358A - Process for making coated paper - Google Patents

Description

I DCQZQ, 1931. i BBRADNER 1,838,358

PROCESS FOR MAKING COATED PAPER Filed Feb. 11, 1929 3 Sheets-Sheet 1 gwoento DOM-M Bradncr I D. B. BRADNER PROCESS FOR MAKING COATED PAPER Dec. 29, 1931 s Sheets- Sheet 2 Filed Feb. 11, 1929 gwuewto:

Dona/J Bradmzr D. B. BRADNER 1,

PROCESS FOR MAKING COATED PAPER Paper- \zaeed FZ oer/M/a.

Donald Brain" gwuento attomup.

Patented Dec. 29, 1931 DONALD B. BRADNER, OF HAMILTON,

OHIO, ASSIGNOR TO THE CHAMPION COATED PAPER COMPANY, OF HAMILTON, OHIO, A CORPORATI ON OF OHIO PROCESS FOR MAKING COATED PAPER Application filed February 11, 1929. Serial No. 339,165.

This invention relates to an improved process for making mineral coated paper.

By the expression mineral coated paper is meant a paper (including cardboard) raw 5 stock or base, which has been coated with an aqueous suspension of mineral pigment and adhesive. The mineral pigment may be, for

example, clay, satin white, calcium carbonate and the like; the adhesive may be, for ex ample, casein, starch and the like.

Paper may be coated on one side or on both sides. \Vhen only one side of the paper is to be coated, the common practice is to apply aqueous coating mixture to the travelling web of paper by means of a rotating brush,

the brush picking up the coating mixture from a roll rotating partially submerged in a pan containing the coating mixture. When coating both sides of the sheet simul 33 tancously, the usual procedureis to flow the coating mixture onto the sheet as the latter passes between two rotating squeeze rolls. The lower squeeze roll dips into a pan containing coating mixture supplied from the stream of coating mixture flowing over the edges of the paper in front. of the top squeeze roll. Although I have described the usual practice of applying the coating to the paper in the coated paper industry, a number of other means have been proposed, but all the methods, as far as I am aware (except the one described in my copending application Serial No. 259,262, filed March 5, 1928), and its continuationinpart, Serial No. 290,093, filed July 3, 1928, consist in first applying a desired weight of coating to the paper, and then, as a separate operation, spreading this coating into a more or less uniform layer before drying, this spreading being accom- J plished by means of brushes, rolls, or equivalent means. A secondary spreading operation such as that just mentioned is open to a number of objections, among which may be mentioned the tendency of the spreading means to mark the paper, especially when a fairly viscous coating mixture is employed. Thus coated paper is frequently marred by brush marks or by ridges from smoothing rolls.

In my copending application Serial No.

of, Serial No. 290,093, filed July 3, 1928, I

have described a novel coating apparatus and method of coating, characterized by applying a coating mixture to a roll, smoothing the coatingmlxture onthe roll, and then transferring the smoothed layer of coating onto the paper, the roll rotating in the opposite direc tion to the travel of the paper. The said method includes the optional step of conditioning or pro-wetting the paper web whereby to displace any air which otherwise might become entrapped in the paper by the film ofcoatingthcreafter formed thereon and thus cause skips in the finished coating. It is there described that this conditioning or pre-wetting may be effected by treating the paper web with a wetting agent which may be a portion of the aqueous coating composition, water alone, and adhesive solution, a solution of a dyestuff or steam, and thereafter forming the film of coating on the so-wetted web. Vhcre a liquid wetting agent is employed, the wetting step may be efl'ected by passing the uncoated web surface in contact with a surface, such as the surface of a roll, carrying a film of the wetting agent.

The present invention accomplishes the same results as the novel roll coater described in the above mentioned application, namely:

1. It applies to the paper a coating which is so smooth and uniform that it requires no subsequent spreading or brushing before drymg. q

2. It permits the application of coatings containing less water than is customarily required, thus reducing the amount of drying necessary and also improving the quality of the coated paper.

3. It permits the application of heavier or lighter coatings than is normally possible with the customary coating machine.

The present invention, in addition to accomplishing the above mentioned results, provides the following further advantages:

1. It provides a more positive method of coating the web.

2. It permits the use of coating mixtures of an even higher viscosity.

wrinkl described with reference to the accompanying drawings, of which:

Fig. 1 diagrammatically represents,in elevation, one form of apparatus adapted perform the process of the invention;

Fig. 1a represents a detail of Fig. '1 in plan view;

Fig. 2 represents a modified form of appaiatus adapted to perform the process of the invention;

Figs. 3, 4, and 5 are diagrammatic representations of the phenomena occurring in the coating of paper; and

Fig. 6 is a graphical representation of the critical conditions'associated with the phenomena represented in Figs. 3, 4 and 5.

Referring to Fig; 1, 1 isa web of paper being unwound from the roll of paper 2. The roll of paper 2 is mounted on the shaft 3 and is equipped with a brake 4 of standard construction well known in the industry, whose purpose is to regulate the tension on the paper. 5 is a pull roll rotating in the direction indicated by the arrow. The paper passes partly around the pull roll 5 and through the nip or opening between the pull roll 5 and the doctor roll 6. The paper then asses back around the fly rolls 77 and ally out to a standard drying system not shown. Pull roll 5 is a metal roll and it should be smooth, round, straight and com centric with its journal 9. The doctor roll 6 rotates in the direction indicated by the arrow, which it wilhbe noted is contrary to the direction of the travel of the paper at the narrowest point of the ni It is made of metal and should be smoot round, straight and concentric with its journal 10, and should not be absorbent to aqueous coatingmixtures. The surface of the roll should be free from such grooves, scratches, and other blemishes as would show up on the coated paper. The doctor roll 6 is provided with a wi r 8. The wiper 8 may be of rubber, felt, soft metal, or other suitable material, the function of the wiper being to remove practically all the coating from the doctor roll so that the surface of the doctor roll is substantially free of coating mixture as it approaches the nip between rolls 5 and 6. More than one wiper in series may be used if desired. The coating mixture is supplied to the machine through the pipe 11, and under the proper conditions of operation it forms a pool or layer at 12 between the two rolls 5 and 6..

The wiper 8 also helps to maintain the pool of 12. End plates (not shown) to form a dam to restrain the coating from-flowing over the edges of therolls, and also to help maintain the aforesaid pool of coating, may be rovided if desired.

The distance between the two rolls 5 and 6 can be varied by means of the adjusting screws 13 and 14 contacting with the bearings 28 and 29 of each roll. This adjusting equipment provides a means of regulating the weight'of coating to be applied. Rolls 5 and 6 are driven through the belts 15 and 16, connecting pulleys 17 and 18 with pulleys 30 and 31, respectively. As indicated in the plan view, Fig. 1a, the pulleys 17 and 18 are attached to the cone pulleys 19 and 20, re-

spectively, by means of the shafts 21 and 22, and the cone pulleys are connected together by the 'belt 23. The belt 23 may be shifted along the cone pulleys 19 and 20 by. means of the belt shifter 24, thus providing for variably controlling the relative speeds of pull roll 5 and doctor roll 6. Pulley 19 is driven through shaft 21 by the source of power 25.

In order to obtain a satisfactory coating by means of this machine, it is necessary to operate it under certain redetermined conditions. I have carefulFy investigated the various factors affecting the operation of the machine, and by controlling these factors in the manner hereinafter described, I- have Succeeded in making mineral coated paper superior to mineral coated paper made on the customary brush coating machine, and free from many of the fauls and difficulties in-- herent in existing coating methods.

' In discussing the various factors involved,

it will be advantageous to examine closely the phenomena which occur in applying a. coatmg by this method.

It is well known that when an aqueous coating mixture is applied to paper by means of squeeze rolls rotating with the paper, the coating is not uniformly distributed on the paper but is concentrated in longitudinal ridges which require an elaborate brushing operation to-spread them out into an even layer.

-Lfineral coated paper having this same ridged appearance is produced on my machine when the surface speed of the doctor roll is too slow relative to the paper speed. However, as the speed of the doctor roll is increased, these ridges gradually become smaller, and finally at a speed characteristic for each coating mixture, they entirely disappear and the coating on the sheet becomes smooth. If the speed of the doctor roll is still further increased, a speed is finally reached at which'the coating on the sheet assumes a wavy or rippled appearance. These ripples may be heavy and broad, or small and numerous, depending on the speed of the doctor roll and the character of the coating mixture. As the doctor roll speed is still further in: creased, the ripples become smaller and closer together, eventually being no longer separately distin shable, but the coating is more or less froti which produces large numbers of pin holies on drylng. A coated sheet on which ri ples or ridges exist requires a subsequent grushing or other smoothing operation, but a sheet coated at the correct doctor roll speed is very smooth and requires no additional smoothin before drying.

The formation of ridges, smooth paper and ripples, depending on t e speed of the doctor roll, may be explained as follows: Under the conditions at which ridges are formed, that is, at very low doctor roll speeds, it is reasonable to believe that the pool or .film of coating mixture takes the position indicated in I Fig. 3. Gravity tends to cause the coating to flow downward through the nip and this tendency is of course greater as the hydrostatic head of coating mixture is increased. Increasing the hydrostatic head of coatin mixture requires an increase in doctor rol speed for the formation of smooth coating, the amount of the increase depending on the particular coating mixture. The slower the speed of the doctor roll relative to the paper tively large quantity of coating passes through the nip and is deposited on the paper. Under these conditions the coating on the paper has a wavy or rippled appearance. ,At very high doctor speeds it may be assumed that the coating breaks through the air film with such frequency that the individual ripples are no longer separately distinguishable, but that the air drawn in and mixed with the coating causes froth.

Intermediate between the conditions favorable to the formation of ridged coating and a rippled coating, a certain balance is established between the forces tending to cause the coating to flow through the nip and the opposite force exerted by the doctor roll. Such a condition is indicated diagrammatically in Fig. 4, where the film of coating is seen to divide in the general neighborhood of the narrowest point of the nip. This represents the condition suitable for the production of a smooth uniform coating.

In other words, as the speed of the doctor roll relative to the paper speed is increased, the coating on the paper is first ridged, then smooth, then rippled, and finally smooth but frothy. Obviously these effects may be accomplished by maintaining the paper at a constant speed and increasing the doctor roll speed, or by maintaining the doctor roll at a,

constant speed and decreasing the paper speed. i p

The transition from one condition of coating to another is usually a very gradual one.

For example, starting with a heavily ridged coating and increasing the doctor roll speed, the ridges become progressively thinner and fainter, gradually fading out until they are no longer discernible to even an experienced, observer. Notwithstanding this-difiiculty in.

determining the exact transition points, the results obtained on repeated experiments have been remarkably reproducible, indicating that control over the various factors involved can be readily obtained.

I have conducted many hundreds of tests with my new method of coating paper, using all the various kinds of coatings commonly employed in the manufacture of mineral coated paper and a wide variety of raw stocks, and I have discovered that the'appearance of ridges at low doctor roll speeds, ripples at high doctor roll speeds, and a zone of smooth coating between these two rangesis a general phenomenon. It is not .to be inferred. however, that these ranges are numericallythe same for all kinds of mineral coating mixtures. i

To illustrate in detail the conditions under which ridged, rippled and smooth coating is produced by my process, I will describe a series of coating experiments using machines of the types shown in Figs. 1 and 2. The pull roll and doctor roll were each steel rolls approximately 5 inches in diameter. Paper coating speeds ranging through the customary speeds used in coating mills and up to speeds much higher than ordinarilv practiced were employed in these tests. The results with four representative coating mixtures (A. B. C, and D) will be given.-

. Coating A was a casein-clay-water mixture of rather high viscosity. Stormer viscosimeter gave 36 sec. Sp. or. 1.38. Total solids 48%.

Coating B wasidentical with coating A, except that it had a lower viscosity due to a larger percentage of water. Stormer viscosimeter gave 25 sec. Sp. Gr. 1.34. Total solids 44.5%.

Coating C was a starch-clay-water mixture containing also a small amount of satin white. Stormer viscosimeter gave 15 sec. Sp. Gr. 1.26; Total solids 40%.

Coating D was a casein-clay-satin white water mixture with a viscosity (Stormer) of 15 sec. Sp. Gr. 1.16. Total solids 28%.

Coatings B, C and D, were coatings similar or identical with coatings commonly used with the usual type-of brush coaters in coating mills. Coating A had a viscosity too high for successful operation with the usual brush coaters. Coatings B, C, and D, moreover are below the line i. du'ced at values above the line typical odf moderate grade, lovlv grade, and

e coat'- respective y. ranges ofridcoat' ,smoothcoating, and rip led coatin with ese four coatingsarein lcatedin .6. Inthisfigure, doctor roll speeds are p otted algainst paper Considering, for examp e,co'at A, Azrepnlidgilced at all v ues ripp e coatmg; are pro- V'A, and smooth coa are produced'in the zone be-. tween linesand A'A. The u per limit of rippled coatings has not been p otted as it was not determined in man of the tests. 1 Just to indicate the magnitu e of this limit, it ma be stated that for coating A at a aper speed of 120 ft. per minute,'ri ples 'sappeared between doctor roll spec of 900 and 1400 ft. per minute. For coating C the limit was-somewhat lower; at a aper s d of 200 ft. per minute, rip les ha just ppeared at a doctor speed 500 ft. perminute.

It willbe noted that there is considerable variation in the zones of smooth coating for the various coating mixtures. There is a parently a greater variation between t e smooth coating zone for the starch coating and the casein coatings than exists among the various caseiii coatin In fact, the zone of ridges is not ve di erent for all three casein coatings. In a cases the zone of smooth coating is wider at higher paper-speeds, and it becomes narrower atlow paper speeds.

In addition to the four coatlngs discussed 3" above, a large number of tests have been made with other coatings consisting of aqueous suspensionsof mineral pigment and adhesive. In all cases the zone of smooth coating is more or less wedge shaped, widening out at highera doctor roll speeds. The viscosity of the coating does notseem to exert any marked effect until a ve watery condition of the coating is reached. v At these ver low viscosities,

- higher speeds of doctor rol are required for 46 a correspondin paper speed before smooth coatings are tained. Likewise, a much higher doctor roll speed is generally required for the formation of" a rippled coating with very low viscosity coatings. It should be to inted out, however, that these extremely Doctor rolls andpull rolls of the same and diflerent diameters have been tried out but without appreciable efiects on the zone of production of smooth paper.

It is essential, however, that the opening a between the rolls should not vary appreciably during rotation of the rolls, as permits alternatel more and less coating to passgiving the pa r a ladder-marked appearance. preciably across the Ian this would cause a heavier weight of coating on one side of the paper than the other.

A wide range of w hts of coating deposited on the paper has covered, rang; mg from weights of 5 unds up to poun on each side. (Weig t of coating refers to the dry weight deposited on a ream of 500 sheets of paper 25 x 38 inches.) The weight of coating applied does not have a marked effect on the rid -ripple range of coatings.

The weight 0 coatmg applied is most conveniently controlled by ad'usting the opening between the two rolls. owever, as thedoce opening must not vary aptor roll speed increases there is a gradual decrease in the weight of smooth coating :1 plied for any given opening between roll; Also, the weight of coating increases some; what at higher paper speed, other things being equal. Of course, t e percent of solids in a coatin afiects the we 7 considerab y. As indicating the magnitude of the values under discussion, it may be stated that with coating D applied to a pa r .003 inches thick, a nip opening of .006 inc es gave a coating weighing 14 lbs. at a paper speed of 200 ft. per minute and a doctor roll speed of ft. per minute.

The method of in herein described is not limited to any particular kind of raw stock. Excellent results have been obtained with the usual coating stock which is moderpf the rolls, since t of coating very longer or shorter at the edge than in the middle), etc.-

In case a paper is used which is diflicult to wet with ueous coating mixture the device shown in ig. 2 has roved useful. Fig. 2 shows diagrammati y an alternate arrangement for supplying coating mixture to the nip between pull roll 5 and doctor roll 6. The coating mixture 12 is delivered through the pipe 11 behind the guard plate 26. This plate almost touches the paper on the'pull roll, so that the coating forms a seal between to edge of the guard plate and the paper.

It apparent that a number of modifications in the above described process and apparatus may be made without departing from the scope of my invention. Thus, while I have mentioned in describing the apparatus a pull roll and a doctor roll made of metal, other materials may be used for this purpose such as, for example, rubber or bakelite. Likewise instead of a doctor roll, I may employ a belt which may be, for example, of metal, and which moves in a direction opposite to the travel of the paper at a predetermined uniform speed relative 'to the paper to produce a coated paper free from a ridged or rippled appearance. Also, instead of using stationary blades to clean the coating from the doctor member, the wiper means may be in the form of a, rubber covered press roll rotating with the doctor member. Re-

gardless of the particular form of the wiper, the cleaning action accomplished thereby mechanically removes the coatin from the doctor member, leaving not more t an a very thin film of coating on said doctor member.

In'the preferred embodiment of the apparatus I have described the paper being carried by a pull roll. In some cases it may be advisable to carry the paper on a travelling belt or apron during its passage under the doctor roll. Another form of support for the paper as it approaches the doctor roll surface may be astationary slightly arched surface 'over which the paper slides, the arched form tendin to hold the paper out flat.

Furthermore, it is not essential that the coating mixture be applied to the paper immediately before the doctor member contacts with the resulting coating layer. Thus, coating mixture in excess of that finally desired on the sheet may be applied to the paper by any suitable means and the paper then con-.

ducted to the doctormember While the coating is still wet. The doctor member removes the excess coating mixture from the paper, which excess may be returned to the main coating mixture supply in any suitable manner.

It will be evident from the foregoing description that the resent invention broadly comprises a metho of making mineral coated paper wherein aqueous coating composition is-aplplied to a moving web of paper in excess and the coating on the paper is contacted with a doctor member, the surface of which is continuously cleaned by mechanical means and 'moves in the opposite direction to the travel of the paper, at a predetermined distance from the paper and at a predetermined speed,

to remove the excess of coating from the paper and to leave the coatin remainin on the paper substantially free rom a ri ed or rippled appearance, and o the esired weight.

By the term doctor member I mean a roll I e quantity desired on the finished paper,

therefrom and is moving in theopposite direction to the travel of the paper at a speed which will leave the coating remaining on the paper without apparent ridges or ripples, continuously cleaning the surface of said doctor member, and thereafter drying the coated paper.

2. Process of making mineral coated pa per which, comprises applying a substantial excess of aqueous coating mixture to a travelling web of paper, removing said excess except the desired weight of coating from said paper by means of a doctor member whose surface at its point of closest approach to said paperis spaced a predetermined distance therefrom and is movm in the opposite direction to the travel of t e paper at a predetermined speed above that at which ridges are produced on the coated paper and belowthat at which ripples are produced on the coated paper, continuously cleaning the surface of said doctor member, and thereafter drying the coated paper.

3. Process of making'mineralcoated pa per which comprises applying a substantial excess of aqueous coating mixture to a travelsurface and a doctor member, removing said excess except the desired welght of coating from the said paper by means of the doctor member whose surface at its point of closest approach to said paper is spaced a predetermined distance therefrom and is moving in the opposite direction to the travel of the paper and at a predetermined speed above t at at which ridges are produced on the coated paper and below that at which ripples are produced on the coated pa er, continuously cleaning the surface of sai doctor member, and thereafter drying the coated paper.

4. Process of making mineral coated aper as defined in claim 1, characterized in t at the web of paper is pre-wetted.

' In testimony whereof, I aflix my signature.

DONALD B. BRADNER.

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