US2636838A

US2636838A - Method of producing a flat moistureproof paper lamination

Info

Publication number: US2636838A
Application number: US741239A
Authority: US
Inventors: Russell H Savage
Original assignee: Mead Corp
Current assignee: Mead Corp
Priority date: 1947-04-14
Filing date: 1947-04-14
Publication date: 1953-04-28
Anticipated expiration: 1970-04-28

Description

April 28, 1953 R. H. SAVAGE METHOD OF PRODUCING A FLAT MOISTUREPROOF PAPER LAMINATION Filed April 14, 1947 R m M N m .m WW Wm M m u v. B u m: m9 08 WH Nam mm L mm Q m n a R Q n N is 0 mm Om Mm ow m a v 4% m ml moistureproofing of the paper is accomplished by the application of wax, such wax coating being applied to the uncoated side of coated-oneside paper for this purpose and as hereinafter described as a laminating or bonding layer. With the waxes ordinarily used in the waxing of papers, an uncovered or untreated waxed surface could be too slippery when cold and objectionably tacky when hot. A stack of such sheets could be unpredictably difficult if not impossible to handle since the individual sheets under certain temperature and pressure conditions would tend to stick together and form an unmanageable block of paper and, under other temperature conditions might slip over each other and over the press handling mechanisms so easily that a slight pressure would cause the whole stock to slide apart and scatter about the work room, making it quite diflicult to keep the sheets stacked properly for transporting them, and difficult or impossible to handle them properly on printing or folding machines. Furthermore the usual waxes are quite penetrative when melted and would penetrate or strike through the ordinary coated-one-side paper and spoil the appearance and printability of the mineral coated surface unless specially prevented from doing so.

According to this invention it has been found that a satisfactory moisture resistant coated paper, which avoids the above difficulties, may

be produced by forming a film of molten wax on a carrier sheet of wax resistant paper, cooling the wax below its applied molten temperature to an adequately non-flowing but heatsoftened tacky condition, and laminating the wax-coated carrier sheet and the uncoated side of the coated-one-side paper to provide an interposed bonding wax film adequate to form an interlayer which is sufficiently moisture resistant to be generally termed moistureproof. It is important to so control the operation that the carrier sheet is not penetrated by the wax, in order to avoid the above mentioned difficulties in the transportation and use of sheets having waxy surfaces. It is usually desirable for the carrier sheet to be thin so that the laminated product will be thin enough to be handled comparably to suitably usable unlaminated coated-one-side paper. As the exposed surface of the waxing backing sheet or web should have such slip free surface characteristics that it may be handled like other used printing paper sheets, it has been found that the wax coating should be prevented from striking through and herewith that this can be accomplished with ordinary uncoated, machine finished, paper by applying a wax resistant size to the paper to be coated, and by causing the wax coating to assume a non-flowing condition, as by proper temperature control, immediately after the coating is applied to the paper.

In attempting to produce such a laminated sheet or web which will lie flat in use, substantial difiiculties arise due to the dissimilarities of coated-one-side paper and said sized paper. These dissimilarities are increased or made more unpredictable by the effect of the hot wax in driving off moisture from the sized paper. This tends to produce unbalanced stress relations between the sides of the waxed paper and also between the webs which make up the finished lamination. This has presented great difficulties as resultant uncoordinated curling of each web of the laminated papers may occur and the effect maybe additive or subtractive or more or less neutralizing each other. I have found that this will cause unpredictable curling effects unless the laminating conditions are carefully controlled, and that such control is provided by the practicing of this invention, in which by regulating the relative moisture contents of the webs, the over-all and localized tensions on the webs as they are laminated, the laminating pressures and the relative driving speeds effective on web surfaces, such are avoided.

In the making of the laminated webs according to this invention to produce commercially satisfactory products the laminating material should be of such character that under the temperatures, heat and other conditions encountered in the manufacturing of envelopes or like products, or in the use thereof, the bending, folding, scoring, sealing, etc., of the products can be satisfactorily and effectively performed without disrupting the product as a whole or any of its constituent material, including the laminating material or adhesive, or without adversely affecting them for the intended use. For lower temperatures, sharp folds or creases, and less disruptive or adverse effects, so-called microcrystalline waxes have given quite satisfactory results. Microcrystalline waxes of varying characteristics are used, and generally speaking they are more flexible, less likely to break or crack, and more satisfactory as a laminating adhesive than the larger crystalline form of paraffin wax. Various laminating materials of varying characteristics are natural products in commercial use, and paraffin and microcrystalline Waxes of varying characteristics are also produced in the petroleum and other industries, and similar or equivalent resinous adhesive or laminating materials likewise are found in nature and are produced synthetically. The characteristics of those which are readily available are usually readily ascertainable, or specified, and also they are made or modified to meet desired specifications. While so-called microcrystalline waxes have been found quite satisfactory in making the illustrative laminated paper for the manufacture of envelopes, as herein described, other materials of equivalent characteristics in laminating operations and subsequent manufacturing and usage may be used, and the precise specific characteristics of the wax or laminating material as herein referred to are not necessary to be more specifically stated.

Referring to Fig. 1, which illustrates a preferred apparatus embodying and for practicing the invention, the installation is shown somewhat diagrammatically, in connection with a paper making machine and arranged, as so illustrated, for applying a wax resistant size to the travelling paper Web, as in a tub sizing operation during the production of paper on a paper machine. The size is applied at a point about two-thirds of the way through the drier section of the conventional paper machine, one section of drying rolls being indicated generally at In and another at H. A web of paper l2 passes from the drying rolls l0 around a guide roll I3, through the nip of a pair of size press rolls M and I5, which apply the size to the paper, over guide rolls l6 and I1 and to drying rolls ll proceeding thence through the drying and calendaring operations usually performed on a conventional paper making machine. Roll .13 is spring mounted to maintain proper tension on the paper web l2 as it passes through the size press rolls. Roll 16 has grooves "cut in its surface which spiral outwardly from the center and function to keep the paper web stretched transversely. Such a roll is commonly called a worm roll. Roll I1 is also a worm roll, and is spring mounted to keep a uniform tension on the paper web.

" The size press roll l5 dips in a pool of size maintained in tank I8, carrying a film of size on its surface and applying it to the paper web passing through the nip of the two size press rolls. Roll l5 carries an excess of size to the nip and the pressure between the rolls meters rolls in distributing the size uniformly across the paper web, it may be desirable to provide a spreader 2| between guide rolls [6 and ll.

A preferred formula for wax resistant size is 250 parts of methyl cellulose of medium low viscosity, 700 parts of an oxidized starch such as Stayco, 440 parts of a dextrose corn sugar such as Sweetose as a plasticizer, and sufficient water to make a solution of about 8% solids content. The weight of the finished sized paper may desirably be about 26 pounds for 3,000

square feet, and conditions of application should be controlled so that about 1 to 1 /2 pounds of size (dry weight) are applied per 3,000 square feet. Application of such size will usually increase the resistance of the paper by about four times in the standard greaseresistance test using mineral oil at 100 F.

Referring to Fig. 2, paper carrying a wax resistant size, prepared as described above, is unwound from the roll 3n'supported on a conventional unwind stand 3|. The web of paper 32 is led from roll 30 at a speed of about 300 feet per minute around idler guide rolls 33 and 33a, over idler guide roll 34, around a driven web feeding roll 35, around an idler roll 36, and over a waxapplying roll 37. Braking means 38 are pro-. vided to act on the roll 30 at the unwind stand to assist in maintaining proper tension on the paper web. The means shown comprise the brake band 39 frictionally engaging the brake drum 40 attached to the roll shaft, one end of the brake means, indicated diagrammatically at 45 in Fig. 2, providing for variable speed control, and 'a surface such as rubber is provided onthe roll to grip the paper web. The tension on the paper web passing through the'subsequent operations is principally controlled by the speed of this roll 35.

Wax-applying roll 3'! is made of metal, such as chilled iron or steel, and dips in a pool of molten wax 45 contained in tank 41. A heater,

indicated diagrammatically at 48, is provided to melt the wax. This heater is preferably electrical, and is'provided with suitable. temperature face.

controls to maintain the waxat a constanttein perat'ure. .The roll 3'! is driven by motor 49 through driving means 49a providing for a vari able speed control which may be varied independently of the speed of roll 35, and is driven at a surface speed which is controlled with relation to the speed of travel of the paper web so thata slight excess of molten wax is picked up by roll3l and applied to the paper web as the web passes over its surface. A doctor 50 is provided to smooth the wax on the paper and doctor off the excess, which excess wax drops back into the tank 41. This doctor is shown in more detail in Fig. 3. It comprises a bar 5!, which may have a diameter of about /4, and which has fine wire 52 wrapped in a close spiral around its sur- The bar 5| is supported by bearings such as 53, and is provided with the sprocket wheel 54 at one end which is slowly rotated by sprocket chain driving means 55 connected to motor 56 through a suitable speed reducing mechanism. The diameter of the wire 52 determines the thickness of the film of wax left on the paper web 32, since substantially the only wax passing the doctor is the wax which passes through the spaces between the wires. The wax, being molten, flows together again after passing the doctor to form a uniform even film. The

rolls

35, 36 and 31,

the motors 4t, t9 and 56, the tank 41, and the' doctor bar 50 may all be suitably supported by end pieces 50, located on each side of the paper web and supported by a pedestal 6|, which also supports the heater 43.

The molten wax would soon penetrate the paper web if the wax were not quickly cooled below its melting point, even though the paper has been sized to greatly retard the rate of penetration. In order to cool the wax a stream of air is blown onto the paper through the duct indicated at in Fig. 2. This duct extends across the width of the paper web and may conveniently be connected into the ventilating system usually provided in paper mills. The air is directed onto the uncoated side of the paper web so that there will be less chance for particles of dirt to be caught in the hot wax. The paper is guided from the wax-applying roll 31 into proper position just beneath the duct 65 by passing beneath a guide r011 66 and then passes beneath a guide bar 68. The wax is only cooled sufliciently to solidify it to a non-flowing condition'and is left in a heat-softened tacky condition so that it may be readily bonded to the coated-one-side paper.

' The wax applied to the sized paper is preferably a wax derived from petroleum which is commonly designated as microcrystalline wax. Microcrystalline wax has the desirable property of being soft enough to be flexible at room temperature and yet has a suificiently high melting point so that it will not be unduly softened under any normal conditions of use to which laminations of the type disclosed herein are subjected. Preferably a microcrystalline wax is used which has a so-called melting point, as defined by standard test methods, of about to F. The wax is applied to the paper web at a temperature about 5 to 10 F. higher than its melting point.

After being cooled, the wax-coated paper web passes directly from guide bar 68 through a pair of laminating press rolls, the upper roll being indicated at 10 and the lower roll at H. These rolls may conveniently be about 16" in diameter. The upper roll 10 should be provided with an 'tion of the rubber cover.

acne-sea elastically yielding surface, such as a rubber.- cover about 50-75 Pusey and Jones hardness, and the lower roll II should have an unyielding. surface, such as chilled iron or steel. The rolls l0 and ll may suitably be journaled in end pieces 12, located on each side of the paper web and supported. on a pedestal l3. Ihe rolls are. individually driven by variable speed motors I4 and 15, motor '14 driving the upper roll through driving means 11 and motor 15 driving. the lower roll through driving means 18. A. girder 19 provides a support for these motors.

A roll of coated-one-side paper is indicated at 80.. This may suitably weigh about 53 pounds per ream of x 38" sheets, of which weight 8 pounds may be the mineral coating. This coated paper may be produced as described in the above mentioned patent to Morris et aL, No. 2,314,797. The roll of paper is supported on a conventional unwind stand 81 provided with wheels 82 which roll on tracks 83,, so that the stand 8| may be rolled to one side of the apparatus for placing new rolls of paper in position. The web 84 of. coated-one-side paper is led. from the roll 80 around a guide roll 85 and guide bar 86 and onto. the surface of the laminating roll H, and travels with its coated. surface in contact with roll ll into the nip with the upper laminating roll lil. Guide bar 86 exerts a drag on the paper to feed the web to the roll ll under sufi'icient tension so. that it will lie smoothly on the. roll surface. An additional control. on the tension is provided by braking means 88 on the unwind stand 8|, which is similar in construction and operation to braking means 38 on unwind stand 3|. The wax-coated, sized paper web 32. is led into the nip with the uncoated side in contact with roll 10 so that the heatsoftened wax coating is. applied to the uncoated side of the. coated-one-side paper.

The laminating nip between rolls l8 and H functions to cause the wax coating on the sized paper to conform to surface irregularities of. the coated-one-side paper and to cause the wax to penetrate between the surface fibers of the paper sufiiciently to produce a firm bond over the entire surface. This is accomplished by providing a yielding rubber cover on roll H3 which will conform. to irregularities in the thickness of the wax-coated sized paper,,and by providing sufficient pressure between the rolls to cause the rubber cover to be deformed and. provide a nip of. considerable area. The weight of roll Til will be sufficient for this purpose if it is substantially solid and made of iron or steel with the excep- One or both of the rolls l0 and TI may be slightly crowned, i. e., the diameter increased toward the middle, and to the extent desired to offset" axial bending resulting from the pressure or weight of the rolls, so that the pressure can. be: kept uniform along the nip.

In practice it has been found that the pressure between the rolls must be carefully con-- trolled to produce a lamination which will lie fiat in use. When the pressure was above a proper value it was observed that the lamination would curl transversely, the sides of the laminathe rolls than at the other end, one side of the lamination would curl upwardly and the other side of the lamination would curl downwardly.

"To provide close control of the pressure, each end of the upper laminating roll-1c is journaled in adjustable bearings provided with an adjust.- able screw means, indicated at. 90, for moving each end of the roll up or down. The screw means are adjusted during operation until the lamination produced is flat in a transverse direction.

The tendency toward curling as above described is believed to. result from a stretching of the lamination iii-the nip of the laminating rolls. The rubber cover of the upper roll 10 is deformed in. a circumferential direction by the pressure applied by the lower roll H. If the pressure is. not equally distributed along the nip the rubber will be deformed more where the pressure is greater. The lamination passing through the nip will be gripped. more firmly by the. rubber than by the metal surface of roll H and will. be stretched as the rubber is deformed. If the pressure is not equal along the nip between the rolls. the lamination will. be stretched to a. greater extent where the pressure is greater, causing the curl described. Thus, in the case of rolls which are crowned to compensate for bending, when the pressure between the rolls is correctly correlated with the crown. provided on one or both of rolls [0 and H, the crown will just ofiset bending of the rolls and the pressure between the rolls will be equal along the nip. At lesser pressures the rolls will not bend as much as is provided for by the crown and the pressure between the rolls will be greater in the middle. than at the ends. When the pressure is greater than the correct pressure, the rolls will be. bent too much for the crown provided and the pressure between the rolls will be greater at the ends than in the middle.

In order to produce. alamination which is free from curl in a longitudinal direction it has been found that the tension on the paper webs passing to the laminating nip between rolls 7'0 andv ll should be carefully controlled, and the relative speeds of rolls l6 and. H should also be carefully controlled. During. the time that longitudinal tension is applied to a paper web, the web is stretched. longitudinally by an amount dependent upon the amount of tension. When the tension is removed an elastic. recovery takes place, the

web shrinking longitudinally to regain its original dimension at least in part.. If, for example, the upper sized. web 32 is stretched more than the lower mineral coated web 34 as the webs are laminated together, web 32 will subsequently tend to shrink more than web 84', tending to cause the lamination to curl upwardly in a longitudinal direction toward the web 32. Such a curl may be corrected by reducing the tension on the web which is being stretched too much, or by increasing the tension on the other web. The tension on web 32 is controlled by the speed of the feeding roll 35 relative to. the speed of the laminating rolls, and by the amount of drag exerted by the braking means on

rolls

30 and 36. The tension on. web 114 is controlled by the amount of drag exerted by the braking means on r011 80. The speeds of laminating rolls l9 and H relative to each other provide an additional control of the amount of stretch given the paper webs at the time they are laminated together. In practice, the above mentioned controls should be maintained in proper correlated relationship to avoid such. an unequal stretching of the. one web with respect to the other web that the lamination would tend to curl longitudinally.

In order to produce a lamination; which will 9: lie flat in use it is desirable that the two webs, as they are brought together to be laminated, be at approximately the same moisture content. If two webs having different moisture contents are laminated together and then come to equilibrium at the same moisture content, the web originally having a higher moisture content will shrink and the web having a lower moisture content will expand, causing the lamination to curl in the direction of the Web which initially had the higher moisture content. The heat of the hot wax applied to the sized paper web 32, and the cooling air blown on the web through the duct 65, drive off moisture from this web. Therefore the moisture content of the sized paper as supplied to the wax coating step should be higher than the moisture content of the coated-one-side paper as supplied to the laminating step. This.

could be provided for by accurately controlling the moisture contents of rolls of paper to be used so that the moisture content of the roll of sized paper 30 is just enough more than the roll of coated-one-side paper 80 to offset moisture losses caused by the operation of applying and cooling the wax coating. However, this would present diificulties in the ordinary processes of manufacture, since both types of papers are ordinarily produced to have a moisture content of the order of about with a variation in moisture content of as much as 1% from roll to roll.

It has proven to be expedient to increase the moisture content of the sized paper as necessary at the laminating machine by adding moisture to it immediately before the wax coating operation. Steam is supplied through a pipe 5H pro vided with a narrow slot extending the width of the paper web. A galvanized sheet iron box 92 forms a chamber through which the paper web passes in contact with the steam. This box is of slightly greater width than the width of the paper web and is made sufficiently high for the paper to have time to take up the required amount of moisture by the time the paper web has passed through the box and out of a slot 93 at the top of the box. By controlling the flow of steam the required amount of moisture may be added to the paper web to produce a lamination which will be free of curl in use. Under the conditions of operation described it has been found that an increase of approximately 2% in the moisture content of the paper web will offset the loss of moisture caused by the hot wax and the cooling air, but control of the flow of steam may bejudged satisfactorily by experience and observation of the lamination being produced.

From the laminating press rolls I0 and II the lamination passes over idler guide rolls 95 and 96 to a pair of metal cooling rolls 9!- and 98. The cooling rolls are spaced apart slightly and geared together by gears I00 and Illl to rotate at the same speed, the lower roll 91 being rotated at the speed of the lamination by motor I02 through driving means I03. The rolls are about 24 inches 'in diameter, are hollow and are arranged so that cooling water may be circulated through them to keep the surface at about room temperature or a little below. The lamination passes first around most of the surface of cooling roll 91 and then the opposite face of the lamination passes most of the way around the surface of cooling r011 93. The lamination is thus cooled, first from one side and then from the other side, until the wax interlayer is cooled to about room temperature. At this temperature the wax is in a condition which will hold the 10 two paper webs firmly bonded together through normal handling conditions.

The laminated web continues on to a sheet cutter, shown diagrammatically in Fig. 2a, which cuts the web into sheets of the required size. A suitable number of spaced apart idler guide rolls I05, I06, I01, I08 and I09 conduct the web to slitters, indicated at H0, in proper position for slitting, and the web then passes through the nip between driven rolls H5 and H6, which feed the web through the cutter at the proper speed and maintain the web under proper tension for slitting. The rolls I I5 and I I6 are driven by a variable speed motor II'I through driving means I IS, the speed of the motor being controlled by an automatic tension control indicated at I20. The tension control comprises an idler roll I 2I mounted on the end of a pivoted arm I22, the arm being connected to a rheostat controlling the speed of the motor II I in proper manner for movement of the end of the arm supporting the idler roll I2I to effect regulation of the speed of the motor. The idler roll I2I is arranged to press on the laminated web after the web has passed over guide roll I01 and before the web passes over guide roll I08, the arrangement being such that the web is under proper tension when there is a slight sag between rolls I07 and I08, due to the pressure of roll I2I, and any increase or decrease in the amount of sag will cause the roll I2I to be lowered or raised, which in turn causes the supporting arm I22 to regulate the rheostat as necessary to increaseor decrease the speed of rolls II5-I I6 and correct the web tension.

The slitters IIO are of the usual construction comprising rotary knives adapted to trim the edges of the web and may, in addition, slit the web longitudinally into required widths, as is understood in the industry. The web then passes from rolls II5--I I6 to a revolving knife cutter, indicated at I25, which cuts the web laterally between a revolving knife and a fixed knife bar to produce sheets. The length of the sheets is varied as required by adjusting the speed of revolution of the revolving knife relative to the speed of the web, and for this purpose the revolving knife is driven by motor II'I through driving means I26 providing variable speed control. The cut sheets fall onto lay boy tapes I30 which convey the sheets to a receiving box I3I where the sheets are stacked. The tapes are driven and supported by rolls I32, I33, I34 and I35, which are rotated by the motor II'I acting through a driving means I36.

A wind-up stand I40 is also shown in Fig. 2, and may be :used to wind the laminated web into a roll when it is desired to cut the web into sheets at some later time. However, winding into a roll has been found to stretch the lamination unevenly unless the lamination has an extremely uniform caliper, the uneven stretching causing waviness or tendency to curl in the finished sheets. .It is preferable, therefore, to produce the lamination and cut it into sheets in one continuous operation as described.

In accordance with this invention means are provided for continuously applying a moisture: proof coating to vcoated-one-side paper on a thin backing sheet o that the moistureproof coated,- one-side paper may be handled in the usual manner in such operations as printing and forming. A process has been provided by which the lamination of coated-one-side paper and the protecting backing sheet may be produced which will lie flat in use and be free from tendency to curl. Additionally the lamination will have an appearance and feel and ease of handling comparable to that of a single unlaminated sheet of coated-one-side paper, because the exposed surface of the backing sheet will have a surface similar to that of the uncoated surface of the coated-one-side paper, and the thickness of the lamination will not be objectionably greater than that of unlaminated coated-one-side paper.

While the process and product herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to this precise process and product, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. A method of producing a fiat moistureproof paper lamination having a mineralcoating on one side thereof suitable to receive high'quality printing which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a paper web, passing said web through a moisture laden atmosphere for a limited time for increasing the moisture content of said sized paper in excess of the moisture content of a coated-one-side paper Web by an amount sufflcient to offset loss of moisture from said sized paper in subsequent coating operations, applying a coating of molten microcrystalline wax to one side of said sized paper, immediately cooling said coating to a heat-softening condition below the'melting point of said wax and eliminating moisture therefrom, pressing the wax-coated 4.

side of said sized paper onto the uncoated side of said coated-one-side paper under controlled uniform transverse pressure and uniform longitudinal tension producing a balanced stressed relation between said sized paper and said coated-one-side paper, and cooling said wax to produce a flat, evenly bonded lamination and the moisture contents of said webs being in such equilibrium that changes in atmospheric humidity produce substantially coextensive dimensional changes in both said webs.

2. A method of moistureproofing a sheet of paper having a mineral coating on one side thereof without interfering with its utility for such operations as printing and folding which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a web of paper, passing said web into contact with steam for a limited time increasing the moisture content to a value greater than the moisture content of a web of coated-one-side paper, applying a uniform coating of molten .microcrystalline wax to said sized paper, immediately cooling said coating to a heat-softened condition below the melting point of said wax and eliminating moisture from said sized web, said size and said cooling of the wax preventing said wax from striking through said paper, applying the heat-softened wax-coated surface of said sized paper web to the uncoated surface of said coated-one-side paper Web while maintaining controlled uniform stressproducing longitudinal tension on said webs and uniform pressure across said webs, and while maintaining said moisture content of the webs and transverse and longitudinal tension applied thereto so regulated that a balanced stress relation between the webs is produced, and cooling saidwax to produce a fiat, evenly bonded lamination and the moisture contents of said Webs being in such equilibrium that changes in atmospheric 12 humidity produce substantially coextensive dimensional changes in both said webs.

3. A method of producing a flat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing and having an ease of handling in printing and forming operations comparable to that of a single unlaminated sheet of coated-one-side paper which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a thin sheet of paper and passing said sheet through a moisture laden atmosphere for a limited time increasing the moisture content of said sheet to a value greater than a sheet of coated-one-side paper, applying a uniform coating of molten wax to one side of said sized sheet, partially cooling said coating sufiiciently to prevent the wax from striking through said sheet but leaving said wax in a heat-softened condition and eliminating moisture from said Waxcoated sheet, bringing the uncoated surface of said sheet of coated-one-side paper having a moisture content substantially equal to the then moisture content of said wax-coated sheet against the heat-softened wax coating on said sheet, pressing said sheets together under controlled pressure across said sheets with controlled uniform stress-producing tension longitudinally of said sheets and cooling said wax to produce a flat, evenly bonded lamination and the moisture contents of said sheets being in such equilibrium that changes in atmospheric humidity produce substantially coextensive dimensional changes in both said sheets.

4. A method of producing a flat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing and having an ease of handling in printing and forming operations comparable to that of a single unlaminated sheet of coated-one-side paper which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a thin sheet of paper and passing said sheet through a moisture laden atmosphere for v a limited time increasing the moisture content of said sheet to a value greater than a sheet of coated-one-side paper, applying an excess of molten microcrystalline wax to said sized sheet, doctoring off excess wax to produce an even coating of predetermined thickness on said sheet, partially cooling said coating sufficiently to prevent the wax from striking through said sheet but leaving said wax in a heat-softened condition and eliminating moisture from said wax coated sheet, bringing the uncoated surface of said sheet of coated-one-side paper having a moisture content substantially equal to the then moisture content of said wax-coated sheet against the heat-softened wax coating on said sheet, pressing said sheets together between a yielding elastic surface and an unyielding surface with controlled pressure across said sheets and con-- trolled uniform stress-producing longitudinal tension, and cooling said wax to produce a flat, evenly bonded lamination.

5. A method of continuously producing a flat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing and having an ease of handling in printing and forming operations comparable to a single unlaminated sheet of coatedone-side paper which comprises the steps of continuously feeding to coating and laminating operations, a thin paperweb which has been sized so that it resists the penetration of wax, said sized webduring said operations being passed through a moisture laden atmosphere for a limited time increasing the moisture content thereof to a value greater than a web of coated-one-side paper, continuously applying a coating of molten microcrystalline wax to one side of said web, partially cooling said coating sufficiently to prevent the wax from striking through said web but leaving said wax in a heat-softened condition and eliminating moisture from said wax-coated web, continuously feeding said coated-one-side paper web having a moisture content substantially equal to the then moisture content of said wax-coated web onto said wax-coated web with the uncoated side next to said heat-softened wax coating, continuously rolling said webs together under controlled pressure across said webs while applying controlled tension longitudinally of said webs, and cooling said wax to produce a flat, evenly bonded lamination free of curl-producing unbalanced stresses.

6. A method of producing a flat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a thin paper web, passing said web through a moisture laden atmosphere for a limited time for increasing the moisture content of said sized paper in excess of the moisture content of a coated-one-side paper web by an amount of about 2%, applying a coating of molten microcrystalline wax to one side of said sized paper, immediately cooling said coating to a heat-softened condition below the melting point of said wax, pressing the wax-coated side of said sized paper onto the uncoated side of said coated-oneside paper under controlled pressure across said webs while applying controlled longitudinal tension to said sized sheet, and cooling said wax to produce a fiat, evenly bonded lamination free of curl-producing unbalanced stresses.

7. A method of producing a fiat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a thin paper web, passing said web through a moisture laden atmosphere for a limited time causing the moisture content of said sized paper to exceed the moisture content of a coated-oneside paper web by an amount of about 2%, applying a coating of molten microcrystalline wax to one side of said sized paper, immediately cooling said coating to a heat-softened condition below the meltin point of said wax, pressing the waxcoated side of said sized paper onto the uncoated side of said coated-one-side paper by passing said papers through the nip of a pair of laminating press rolls, the roll pressing on said sized sheet having a yielding elastic surface and the roll pressing on said coated-one-side paper having an unyielding surface, maintaining substantially uniform pressure between said rolls across said web to prevent curling in a transverse direction in the finished lamination and maintaining both said webs passing through said nip under substantially uniform tension to produce equivalent stresses therein for preventing curling in a longitudinal direction in the finished lamination.

8. A method of continuously producing a flat moistureproof paper lamination having a mineral coatin on one side thereof suitable to receive high quality printing which comprises the steps of applying a wax resistant size to a thin paper web, advancing said sized paper web in a continuous and progressive fashion under controlled uniform tension, applying steam to said advancing web to raise the moisture content thereof to exceed the moisture content of a coated-on-oneside paper web by an amount of about 2% suffrcient to offset loss of moisture in the subsequent coating operation, applying a coating of molten microcrystalline wax to one side of said advancing web, cooling said coating to a heat-softened condition below the melting point of said wax, passing said advancing web under controlled uniform tension through the nip of a pair of rotating press rolls, the roll in contact with the uncoated side of said sized web having a yielding elastic surface and the other roll having an unyielding surface, continuously and progressively feeding said coated-one-side paper web through said nip with the uncoated side in contact with the heatsoftened wax coating of said sized web, applying sufiicient pressure between said press rolls to cause said yielding surface to be deformed at the nip and provide a nip of considerable area, maintaining substantially evenly distributed and uniform pressure along the nip to prevent curling of the finished lamination, and cooling said wax to produce a flat, evenly bonded lamination.

9. A method of producing a flat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing and having an ease of handling in printing and forming operations comparable to that of a single unlaminated sheet of coatedone-side paper which comprises the steps, performed in a continuous and progressive manner, of applying a wax resistant size to a thin web of paper and passing said web through a moisture laden atmosphere for a limited time increasing the moisture content of said sheet to a value greater than a web of coated-one-side paper, applying a uniform coating of molten wax to one side of said sized web, partially cooling said coating sufficiently to prevent the wax from striking through said web but leaving said wax in a heat-softened condition and eliminating moisture from said wax-coated web, bringing the uncoated surface of said web of coated-oneside paper having a moisture content substantially equal to the then moisture content of said wax-coated web, pressing said webs together between a yielding elastic surface and an unyielding surface with controlled uniform pressure across said webs while maintaining both of said webs under controlled uniform tension, cooling said wax to produce a fiat, evenly bonded lamination, and cutting said lamination into sheets.

10. A method of producing a flat moistureproof paper lamination having a mineral coating on one side thereof suitable to receive high quality printing and having an ease of handling in printing and forming operations comparable to that of a single unlaminated sheet of coatedone-side paper which comprises the steps, performed in a continuous manner, of applying a wax resistant size to a thin web of paper and passing said. web through a moisture laden atmosphere for a limited time increasing the moisture content of said web to a value, applying a uniform coating of molten wax to one side of said sized web, partially cooling said coating sufficiently to prevent the wax from striking through said web but leaving said wax in a heatsoftened condition, bringing the uncoated surface of a web of coated-one-side paper having a moisture content substantially equal to the RUSSELL H. SAVAGE.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Hill Nov. 6, 1923 Number Number Name 1 Date 1,866,689 Van Hamm-Wilshire July 12, 1932 v203 1,035 Dreymann Feb. 18, 1936 2,031,036 Dreymann Feb. 18, 1936 2,158,754 Hodgdon May 16, 1939 2,366,277 Robertson Oct. 10, 1944 2,402,903 Massey et a1 June 25, 1946 2,430,459 Farrell et a] Nov. 11, 1947 OTHER REFERENCES Microcrystalline Waxes and the Paper Industry by John C. Dean, published in Paper Trade Journal, vol. 120, No 7, February 15, 1945; pages 28-32.

Claims

2. A METHOD OF MOISTUREPROOFING A SHEET OF PAPER HAVING A MINERAL COATING ON ONE SIDE THEREOF WITHOUT INTERFERING WITH ITS UTILITY FOR SUCH OPERATIONS AS PRINTING AND FOLDING WHICH COMPRISES THE STEPS, PERFORMED IN A CONTINUOUS MANNER, OF APPLYING A WAX RESISTANT SIZE TO A WEB OF PAPER, PASSING SAID WEB INTO CONTACT WITH STEAM FOR A LIMITED TIME INCREASING THE MOISTURE CONTENT TO A VALUE GREATER THAN THE MOISTURE CNTENT OF A WEB OF COATED-ONE-SIDE PAPER, APPLYING A UNIFORM COATING OF MOLTEN MICROCRYSTALLINE WAX TO SAID SIZED PAPER, IMMEDIATELY COOLING SAID COATING TO A HEAT-SOFTENED CONDITION BELOW THE MELTING POINT OF SAID WAX AND ELIMINATING MOISTURE FROM SAID SIZED WEB, SAID SIZE AND SAID COOL-ROM STRIKING ING OF THE WAX PREVENTING SAID WAX FROM STRIKING THROUGH SAID PAPER, APPLYING THE HEAT-SOFTENED