US2997406A - Method and apparatus for cast-coating paper - Google Patents

Description

Aug. 22, 1961 E. w. FREEMAN ET AL 2,997,406

METHOD AND APPARATUS FOR CAST-COATING PAPER Filed June 5, 1957 ATTORNEY United States Patent 2,997,406 METHOD AND APPARATUS FOR CAST- COATING PAPER Eben W. Freeman, Portland, Lindsay 0. Golf, Westbrook, and Robert C. Rose, South Portland, Maine, assignors to S. D. Warren Company, Boston, Mass.,

a corporation of Massachusetts Filed June 3, 1957, Ser. No. 663,234 4 Claims. (Cl. 117-64) This invention relates to a method of and apparatus for treating paper to avoid stretching the paper web when it is passed between a rigid roll or drum and a yielding roll such as a rubber or rubber covered roll. When a paper web is passed between a rigid roll and a yielding roll under sufficient pressure between the rolls to cause substantial deformation of the yielding roll there is a tendency to stretch the web. This is objectionable not only in that it tends to weaken the web and change its dimensions but the web may even be torn or creped and especially in the case of coated paper strains may be developed which cause the paper to warp or curl objectionably. The harmful efl ects produced depend upon several factors such as whether the paper web is wet or dry, its strength and thickness, the amount of deformation at the roll nip etc.

The invention is particularly applicable in the finishing of cast-coated paper and will be fully described in connection therewith.

In the manufacture of drum-finished coated paper such as the paper commonly designated as cast-surface mineralcoated paper it is customary to press against a moving, heated, highly polished metal surface, such as the surface of a chromium plated drum or roll, a layer of wet mineral-coating composition supported by a Web of paper, the pressure being applied by means of a flexible rubbercovered roll acting on the reverse or uncoated side of the paper web. The term rubber as used in rubbercovered roll is intended to include both natural and synthetic rubber and similar el-astomers, usually reinforced with carbon black or other powder filler. The rubbercovered roll and the polished metal roll have their axes parallel and horizontal so that when the rolls are brought together they make contact in a rectangular area the long axis of which is a straight horizontal line. As pressure is applied between the rolls the metal roll slightly sinks into the surface of the rubber-covered roll to form therein an indentation or concave depression so that the contact between the two rolls is not a mere line of insignificant width but is a bite or nip of considerable width, say from /s inch to 2 inches or more depending upon the magnitude of the pressure, the diameter of the rolls, and the hardness of the rubber-covered roll.

It is apparent that since in a rotating roll all points on its surface must move with the same angular velocity it follows that as the radius of the roll is shortened by compression the linear velocity at the point of compression may be expected to be correspondingly decreased. On this basis, as a point on the soft roll surface enters a compression nip it would be expected to show a slight decrease in speed graduallyto the middle of the nip and then to show a corresponding slight increase in speed during the remainder of the nip. In the past it has been believed by some people that the slight shortening of the radius of the pressure-roll has been the cause of the development of curling tendency that frequently results from passage of paper through such pressure-nips in certain coating operations, and particularly in cast-coating operations.

It has now been proved that the development of curling tendency by passage through a pressure nip of the type described above is due not to the change in radius of the pressure roll under compression, but to a wholly diiferent and hitherto unrecognized cause. As a matter of fact the eifect of the radius change is exceedingly slight in most cases, but its action, such as it is, is to oppose or decrease the elfect of the newly recognized major cause of curl development in the pressure nip.

Ourl is particularly troublesome in the case of drumfinished coated paper because after the wet coated paper has been pressed against the metal finishing surface it is allowed to remain in contact therewith until it has become substantially dry. No further finishing treatment is customarily given the drum-finished paper subsequent to the drying thereof; so there is no opportunity to alleviate the curl after the drying process as there is in the case of other coated papers which are subjected to calendering operation after they have been dried. This is not to intimate that exceedingly bad curl can be wholly removed or controlled by a calendering treatment, but it can at least be somewhat lessened thereby.

In the production of cast-surface coated paper, especially light weight coated paper having only one cast surface, there has always existed a serious problem in respect to the tendency of the finished paper to curl rather than to lie fiat as desired. Many expedients have been suggested for removing or reducing the curl in such paper. In some cases such palliative treatments have been successful in reducing curl appreciably; in other cases they have failed to result in salable paper.

Curl in paper usually is due to strains or distortions introduced during the manufacture or processing of the paper Web. Whereas practice in the past has been to relieve those strains insofar as possible or to compensate for them, the present invention is concerned with seeking out the cause of the strains and preventing their formation in the first place.

It has been found that when paper having a wet coating is passed under pressure between a metal roll and a flexible rubber-covered roll the paper may be permanently stretched or elongated to a measurable extentin extreme cases as much as 5% or more. It has likewise been found that as the amount of stretch or elongation of the paper increases, the greater becomes the tendency to curl and the more difficult it is to remove the curl.

The major cause of stretch in roll-coated paper generally and in cast-surface coated paper particularly has now been found to be the stretch that takes place in the surface of the rubber-covered pressure-roll or backingroll when it passes through a pressure-nip. When a web of paper is passed through a pressure-nip between a metal roll and a roll with a soft rubber surface, because of the high coefiicient of friction between the web and the rubber the paper tends to cling to the rubber surface. Consequently if the rubber surface stretches the paper is likewise stretched or elongated to some extent thereby. The considerable degree to which stretch occurs in the surface of a rubber roll has apparently hitherto gone unnoticed in connection with strains and distortions in a paper web, and particularly by those who have mistakenly attributed curl in paper to strains caused by the changing radiusof the roll under pressure.

It is common to speak of a rubber-covered roll as being compressed in a pressure-nip. It should be borne in mind, however, that solid elastic rubber, whether pure gum rubber or rubber reinforced by powder filler, is not to any appreciable degree truly compressible. That is to say, pressure causes only an exceedingly slight diminution in the volume of the rubber but instead causes a considerable displacement of the rubber in its position in space. In this respect rubber behaves like a liquid, which is substantially non-compressible but whose shape is dependent upon whatever forces act thereon. A layer of rubber forming the surface of a rubber-covered pressure-roll therefore is substantially unchanged in volume when pressed against another roll although its shape may be changed considerably. It is apparent that if, as a rubber-covered roll is turning, the thickness or cross sectional area of the rubber layer is decreased as it passes through the constriction of a pressure-nip, then the velocity of the layer must proportionately increase during passage through the constriction. If the thickness of the layer is decreased by one-half in the constriction, then obviously the corresponding velocity of the layer within the constriction must be doubled.

It is apparent that all portions of the elastic layer under constriction cannot move with the same angular velocity. Obviously the inner side of the layer which is firmly attached to the solid metal core of the roll cannot vary in angular velocity. At a distance from the core, however, the extreme elasticity inherent in the rubber permits the layer to act somewhat like a liquid so that the velocity in that part of the layer is considerably increased during passage through the nip constriction. Friction against the surface of the contacting drum very likely acts on the outer surface of the rubber layer to prevent the velocity thereof reaching that of the interior of the layer. But nevertheless the change in velocity of the elastic surface is sufficient to stretch paper passed under pressure between said elastic surface and a metal roll surface, and paper so stretched acquires a magnified tendency to curl objectionably.

Having ascertained that a major cause of curling trouble in coated paper and particularly in drum-finished coated paper is the strain induced by the elastic stretch of the surface of the rubber-covered backing-roll in a pressure nip, it became apparentthat the curl could be prevented, diminished, or controlled by control of the elastic stretch in the surface bearing against the back of the paper in the pressure-nip. There are several ways for reducing elastic stretch in the backing surface.

Since the decrease in the cross-sectional area of the rubberlayer in the pressure nip is proportional to the depth of impression of the mating roll it is apparent that decreasing the applied pressure will decrease the indentation in the rubber layer and correspondingly decrease its tendency to stretch. Likewise increasingthe hardness of the rubber layers used will, for a given .applied pressure, decrease the depth of indentation and so correspondingly decrease the tendency of the rubber to stretch. Both of these practices, however, are of limited use in most cases, especially in the field of drum-finishing coated papers, because in the newer high-speed processes for producing such paper both considerable pressure and a considerable width of the pressure nip or bite are desirable.

Another method of decreasing the depth of indentation of the rubber layer is to increase the diameter of the roll. Then for a given pressure the width of nip is increased and the change in cross sectional area is decreased. Increasing the thickness of the layer of rubber on the pressure-roll is helpful, since the percentage change in cross sectional area caused by any particular indentation is of course related to the thickness of the rubber layer. Thus a thick rubber layer, say of 5 inches, is affected much less and causes less stretch and resultant curl in paper treated thereby than is the case with a thin rubber layer, say one of only one inch in thickness.

However, the most generally satisfactory results in preventing or controlling curl inpaper passed through a pressure nip of appreciable width are achieved by using against the back of the paper web a surface which itself does not stretch appreciably in the nip.

It has been found thatapaper web bearing on one side a layer of .wet aqueous coating composition may be pressed with its wet coated face into contact with a metal roll through ,a. nip of appreciablewidthby means of a yielding-surface roll which transmits pressure to the back of said paper web throughasurface which is .relativelyor substantially, non-stretching and that thereby substantially no stretch and little tendency to curl will be induced in the paper web. A satisfactory means for exerting pressure on the web to accomplish the desired results is a specially designed roll having a rigid core, as of metal, over the core a layer of substantial thickness, preferably at least about f/z an inch, of soft, stretchable rubber or equivalent, and on the outside a thin, substantially non-stretching layer such as a layer of fabric or cord which is impregnated with or embedded in rubber which is harder and less stretchable than the rubber of. the inner layer.

When such a roll passes through and is distorted in a pressure-nip, the stretch or velocity change is confined substantially wholly to the inner portion composed of soft stretchable rubber; the dimensions of the reinforced surface layer do not appreciably change. Consequently paper passing through such a pressure nip is not appreciably stretched and little or no tendency towards curlingis induced by the action of the pressure nip.

It is realized that no roll can be prepared having a surface which is absolutely non-stretchable. By substantially non-stretchable as used herein is meant having a relatively very small degree of stretch in comparison with the stretch of elastic'rubber.

Instead 'of using 'a' pressure-roll of the construction described, a substantially non-stretching endless belt may be inserted between the paper-web and an ordinary soft rubber-covered pressure-roll as the paper passes through a pressure-nip. Thisprocedure gives equally good results in regard to freedom from stretch and curl in the treated paperweb. Such an endless belt, however, is somewhat more difiicult to prepare and keep in condition than is the previously described yielding roll with a nonstretching surface. Less desirably the endless belt can be replaced by a non-stretching web of finite length.

A substantially non-stretching but yielding backing surface is advantageous for use in any coating process where paper is passed through a pressure-nip. Its advantages are especially apparent in thefield of drum-finished or cast-coated paper because in that field it is customary to use backing rolls which are softer, and with resultant nip widths which are greater, than those generally used in other coating processes.

Paper is made by filtration of a very dilute suspension in water of cellulose fibers, in particular cellulose fibers derived from woodepulp, upon a moving foraminous surface or screen. The water escapes through the screen, leaving a layer of felted fibers on the surface of the screen. The individual fibers are frornl to 5 millimeters in length, depending upon the particular tree from which they are obtained' In spite of their shortlength, the fibers are relatively long and slender in comparison with their breadth which isonly =a smallpercentage of their'length. In consequence the fibers as they settleout on the moving :sereentend to become oriented with their long dimension parallel to the direction of travel of the screen. The paper web so formcdtherefore has many more ifibe'rs jlying substantially parallel to, its length than lie substantially atiright anglestoiitslength. Paper is said to have a grain in the direction of travel of the screen upon which i-twas formedf'and its cross-grain direction is across or perpendicular to the direction of travel of. the :screen.

Paper is very sensitive to the moisture in the surrounding. atmosphere, readily. absorbing moisture as the relative humidity rises iand'likewise giving up moisture as .the relativehumidity falls. Paper changes in dimensions as its moisturec'onte'nt varies. As water is absorbed, paper expands; as water evaporates, paper contracts. Because of the'jorientation of :fibers in paper, the change in dimension greater in the cross-grain direction than it is in the screen direction or direction of the grain. The cross-grain change in dimension due to humidity change lmaybe 2 01",3 times as great as in the machine-direction. When. layer. of coating composition is applied tonne side of such a web of paper, and the paper is thereafter subjected to atmospheres of varying relative humidity it is understandable that if the coating layer and the paper web change dimensions at different rates with changes in the relative humidity then there may develop a tendency for the coated sheet to curl somewhat. Normally, coated paper is processed so that it will lie flat in an atmosphere having a relative humidity of between 50% and 60%. Such one-side coated paper is generally expected to show no appreciable tendency to curl at relative humidities be tween about 30% and about 80%. At greater extremes of relative humidity such paper may be expected to show some tendency to curl, but the curl, because of the grain of the paper, should be parallel to the grain of the paper. That is, since the dimension change is greater across the width of the web, the edges tend to curl toward the middle of the web to form a cylinder having its axis parallel to the direction of screen travel.

When a web of paper has been stretched and elongated as described above, it is found that it becomes more sensitive to humidity changes. Thus, for example, a coatedone-side paper which has not been stretched may lie reasonably fl-at at all relative humidities between 30% and 70% but if stretched moderately during its processing it may lie fiat only between relative humidites of 40% and 60%. If the paper is elongated still more, the tendency to curl becomes more pronounced and surprisingly it may be found that the paper tends to curl toward the uncoated side at both high and low relative humidities. Moreover, if elongation is pronounced the paper may tend to develop also a cross-grain curl, with its axis at right angles to the grain. In such a case the paper becomes almost unmanageable when it has been cut into sheets.

In one particular instance a paper web of the type customarily used as bodystock for coated paper, being made from a moderately beaten cellulosic fibrous furnish containing a moderate amount of sizing agent, said web weighing about 40 pounds per 500 sheet ream 25 x 38 inches in size, was cast-coated on one side with about 18 pounds dry weight, per ream of a typical clay and adhesive coating. Conditions at the pressure-nip on the casting drum were varied to produce various degrees of elongation in the paper web, the stretch varying from substantially zero percent to over 4 percent. In the region from substantially zero percent to 0.25% stretch the sheets showed very little tendency to curl over a wide range of relative humidity, and there was no perceptible difierence among the samples. In the range from 0.25% to 0.75% stretch, the curling tendency increased gradually but did not become especially objectionable. Above 0.75 stretch the curl became objectionable. At about 1.5% stretch the paper became definitely unsalable because of crankiness caused by curling. At about 4% stretch the paper actually rolled up into a tight cylinder having its axis at 45 degrees to the grain.

The preceding data are not given to establish definite limits, but merely to show what happened with one specific example; It is known that the degree of curl at any degree of stretch will also depend, among other conditions, upon the weight of the paper base, the degree of hydration of the fibers therein, and the weight and composition of the coating layer. If stretch can be avoided, however, the eifects of the other variables are greatly reduced and usually become insignificant.

The present invention controls the stretch or distortion in a paper web which is normally imparted to said paper web by passage through a pressure nip having a surface of stretchable elastic substance, and thereby is effective in controlling curl in the paper web. In general it is desirable to control the stretch of the paper to less than 0.75%. Although theoretically it might be preferable to avoid all stretch in the paper, actually controlling the stretch in the region of 0.25 is entirely satisfactory.

The invention will be further described and illustrated in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic side elevation of the. casting drum and pressure roller for the production of cast coated P p FIG. 2 is an enlarged cross section of the pressure roller of FIG. 1,

FIG. 3 is a diagrammatic side elevation of the casting drum and pressure roller with an endless belt surrounding the pressure roller,

. FIG. 4 is-an enlarged cross section of the pressure roller of FIG. 3, and

FIG. '5 is a cross section of the belt of FIG. 3.

Referring to FIGS. 1 and 2, 1 is the paper web coming from a source such as a coating machine (not shown). The ,web passes on to the pressure roller 2, then downwardly through the nip between the pressure roller 2 and the casting roller or drum 3 and around the casting drum 3 to the take-01f roller 4. This is the conventional arrangement of the rollers in cast coating paper and the rollers are conventional excepting with respect to the structure of the pressure roller 2. This roller, as stated above, has a flexible core or support for the outer surface layer. The specific embodiment illustrated consists of a rigid core 5 of metal, e.g. iron or steel, surrounded by a relatively thin layer 6 say A inch thick of hard vulcanized rubber which in turn is surrounded by a layer 7, say /8 inch thick, of relatively soft elastic rubber having a P and J plastometer value of about 50 and an outer layer 8 about A inch thick. The inner layer 6 of hard rubber serves only to ensure a good bonding between the surface of the metal core 5 and the soft rubber layer 7. The outer layer 8 is designed to be flexible but relatively non-stretchable and is made of rubber having a P and J plastometer value of about 35 with fabric or cords embedded therein, say linen cords of 0.023 inch diameter laid side by side at a spacing of 12 per inch and extending around the roller in planes which are substantially perpendicular to the axis of the roller. The outer layer 8 is similar in structure to the well known cord or fabric reinforced rubber belts.

The P and J plastometer values referred to above are the distances measured in 10 micron units which a /8 steel ball will sink into the surface of a body of the rubber under a load of 1 kilogram.

The roller described above is about 15 inches in diameter. As has been explained the pressure roller may be of any suitable size within reasonable limits, say 48 inches diameter and may have any desired construction so long as it meets the basic requirements that it shall have a substantially non-stretchable flexible surface layer supported by an elastic layer. The outer layer 8 should be sutficiently thin to be quite flexible and sufiiciently thick to be substantially non-stretchable and to permit a reasonable amount. of wear before the surface layer of rubber wears off and exposes the embedded cord or fabric. The layer 7 may vary in thickness depending upon the size of the roller, its elasticity, the pressure to be applied to the rollers and the desired width of the nip between the rollers. In this respect the pressure roller may be designed and constructed in accordance with the teachings of the prior art, the only novelty of our pressure. roller being, as stated above, that it has a flexible, non-stretchable smooth surface or surface layer. The layer 6 serves merely to provide a bond between the solid metal core of the roller and the elastic layer 7 and may be omitted or substituted by other means of securing the elastic layer 7 to the metal core. The casting drum or roller 3 is the usual well known construction e.g. a metal drum having a polished chromium surface. The roller 4 is the conventional take-off roller. The paper web 1 and the coating thereon may be the conventional paper and coatings used in making cast coated paper. In fact the whole apparatus illustrated in FIG. 1 and the process carried out therein are well known with the single exception that the pressure roller has a flexible non-stretchable surface layer surrounding and covering the usual elastic layer.

Referring to FIGS. 3, 4 and 5 the parts illustrated are conventional with the exception that the flexible, nonstretchable belt is interposed between the casting drum 3 and the pressure roller 2a. Here the pressure roller 2a also is conventional, consisting of a metal core 5, a bonding layer 6 of hard rubber and an elastic layer 7 of soft rubber. In this figure We have shown a roller 11 for delivering the coated paper web 1 to the nip between the casting drum 3 and the pressure roller 2a. The take-off roller 4 is the same as in FIG. 1. The two rollers 12 and 13 serve to support the endless belt 10 which may be of the usual construction of rubber coated fabric or cord reinforced belts. Such belts generally are designed to have a smooth surface and to be substantially non-stretchable. As shown in FIG. 5 the belt consists of longitudinally extending parallel cords laid side by side, such as linen cords 14 of 0.023 inch diameter, spaced 12 to the inch, embedded in a wear resistant, flexible rubber matrix 15.

a The structure of the layer 8 in FIGS. 1 and 2 is similar to the structure of the belt 10 as shown in FIG. 5.

As is apparent the belt '10 may be replaced by a strip of finite length which may be equal to the length of the paper web to be treated or may be shorter and may be run through the nip repeatedly. Illustration of this obvi ous modification has been omitted as being unnecessary.

We wish to point out that any of the known methods of cast coating may be followed. In this respect our method does not differ from the known methods. For instance the paper web may be coated and delivered directly, while the coating is still wet and plastic, to the nip between the casting druinor the pressure roller. Or the coating may be applied and dried and supercalendered if desired and then remoistened to a plastic state and then delivered into the nip between the casting drum and the pressure roller.

Any paper web suitable for cast coating may be used. As has been pointed out above our invention is especially useful in the manufacture of cast coated paper which has a tendency to curl objectionably i.e. lightweight paper but is not limited to such paper. Conventional paper coating compositions, suitable for east coating and conventional weights thereof may be used.

Our invention is not strictly limited to cast coating in any limited sense or meaning of this term. Cast coating is sometimes understood to mean a coating process in which the coating on the paper web spontaneously adheres to the surface of the casting drum until it becomes non-adhesive thereto due to solidification as a result of drying out or cooling and then spontaneously releases from said surface. Our invention is applicable in any process in which a plastic coating on a paper web is pressed against a finishing surface and hardened in contact with said surface and then separated from said surface.

Our invention not only serves to avoid or reduce the stretch imparted to the paper web in the conventional cast coating process and thus to reduce the tendency of the paper to curl but also by avoiding stretching the paper web" avoids weakening the web and further by avoiding relative movement between the casting surface and the surface of the coating gives a more nearly perfeet cast surface. wrinkling and creping of the paper web are avoided.

We claim:

1. Apparatus for simultaneously drying and drum finishing coated paper comprising: means for coating paper with an aqueous coating which is relatively plastic when a predetermined quantity of moisture is present in said coating; a finishing drum; means for heating said drum; means for applying said paper to said drum with said coated side in contact with the surface of said drum and while said predetermined quantity of moisture is present in said coating; and means for pressingsaid paper against said drum and at least in part drying the same in contact therewith by holding the same against said drum for a substantial interval of time without simultaneously stretching the same substantially comprising a backing roll mounted for rotation in unison with and in pressure contact against said drum, an elastic resilient layer on said backing roll, andthe surface of said resilient layer being substantially non-stretchable While permitting said layer to yield under said pressure.

2. The apparatus defined in claim 1 further characterized by a textile reinforcement in the surface of said elastic resilient layer.

3. A process for simultaneously drying and drum finishing paper comprising the steps of: applying an aqueous coating containing a water plasticizable adhesive to a paper base; applying said paper to a heated, convex, cylindrical finishing surface with the coated side toward the finishing surface and with sufiicient water in said coating to render said adhesive substantially plastic; pressing said coated paper against said finishing surface for a substantial length of time and across a substantial area by running it through a hip created by an elastic resilient backing roll pressing against said finishing drum; and simultaneously with said pressing step preventing said paper from stretching by applying a pressure to hold the paper against the cylindrical surface over a substantial area and advancing the back surface of the paper with the applied pressure at a uniform speed and tension through said area.

4. The process of simultaneously dryingand drum finishing paper defined in claim 3 further characterized by bringing said coating into relatively moist adhesive contact with said finishing drum, and holding the same in said contact until said adhesive is substantially dry.

References Cited in the tile of this patent UNITED STATES PATENTS 2,312,853 Toland et al. Mar. 2, 1943 2,316,202 Warner Apr. 13, 1943 2,324,631 May July 20, 1943 2,442,443 Swallow June 1, 1948 2,597,858 Freelander May 27, 1952 FOREIGN PATENTS 527,207 Canada July 3, 1956

Claims (1)

1. APPARATUS FOR SIMULTANEOUSLY DRYING AND DRUM FINISHING COATED PAPER COMPRISING: MEANS FOR COATING PAPER WITH AN AQUEOUS COATING WHICH IS RELATIVELY PLASTIC WHEN A PREDETERMINED QUANTITY OF MOISTURE IS PRESENT IN SAID COATING, A FINISHING DRUM, MEANS FOR HEATING SAID DRUM, MEANS FOR APPLYING SAID PAPER TO SAID DRUM WITH SAID COATED SIDE IN CONTACT WITH THE SURFACE OF SAID DRUM AND WHILE SAID PREDETERMINED QUANTITY OF MOISTURE IS PRESENT IN SAID COATING, AND MEANS FOR PRESSING SAID PAPER AGAINST SAID DRUM AND AT LEAST IN PART DRYING THE SAME IN CONTACT THEREWITH BY HOLDING THE SAME AGAINST SAID DRUM FOR A SUBSTANTIAL INTERVAL OF TIME WITHOUT SIMULTANEOUSLY STRETCHING THE SAME SUBSTANTIALLY COMPRISING A BACKING ROLL MOUNTED FOR ROTATION IN UNISON WITH AND IN PRESSURE CONTACT AGAINST SAID DRUM, AN ELASTIC RESILIENT LAYER ON SAID BACKING ROLL, AND THE SURFACE OF SAID RESILIENT LAYER BEING SUBSTANTIALLY NON-STRETCHABLE WHILE PERMITTING SAID LAYER TO YIELD UNDER SAID PRESSURE.

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