US3257226A

US3257226A - Wax coating method and apparatus

Info

Publication number: US3257226A
Application number: US236373A
Authority: US
Inventors: Herman L Thwaites
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1962-11-08
Filing date: 1962-11-08
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21

Description

June 2l, 1966 H. L. THWAn'Es 3,257,226

WAX COATING METHOD AND APPARATUS Filed NOV. 8, 1.962

HERMAN L.THWAITES INVENTOR BY M PATENT ATTORNEY UnitedStates Patent O 3,257,226 WAX COATING METH D AND APPARATUS Herman L. Thwaites, Clark, NJ., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Nov. 8, 1962, Ser. No. 236,373 6 Claims. (Cl. 117-64) This invention relates to a method and apparatus for coating a succession of paperboard sheet materials with a wax composition. Particularly, the invention relates to a method and apparatus for obtaining a wax composition coating of improved smoothness and gloss on both surfaces of paperboard carton blanks.

`Pa-perboard sheet materials, e.g. paperboard carton blanks, are commonly coated on both surfaces with petroleum-derived hydrocarbon waxes and wax compositions to impart waterproofness and grease resistance, particularly for use in foodstuff packaging. One commonly accepted procedure for coating paperboard materials involves, for example, passing them between heated coating rolls which dip into a bath of molten wax composition. This process applies a relatively small amount of wax to the carton and, due to the rapid removal of heat from the wax by the cooling action of the carton, a portion ofthe wax is solidified as a surface Yfilm on the carton, while a further portion of wax remains fluid long enough to penetrate intol the inner structure of the paperboard. The coated carton blanks may then be allowed to cool naturally, or they may be plunged into a cold water bath to solidify the remaining wax composition. While such procedures generally produce paperboard cartons which are quite satisfactory for many uses, there are certain drawbacks which limit their acceptability. One drawback is the production of a dull, waxy surface appearance which is frequently marred by surface irregularities, pimples, smears, so-called crows-feet, watermarks, and

what might be termed a serpentine appearance. Addi-` tionally, because of the unevenness of the surface coating, the cooling of the wax, even when accomplished by immediate plunging into cold water, is quite commonly non-uniform and causes the formation of haze, thereby further detracting from the appearance of the coated carton. Furthermore, this uneven surface coating also presents a non-uniform moisture barrier which is undesirable for many purposes due to the possibility of failure of the paperboard. There is thus a constant demand by the packaging industry for coated cartons of improved appearance and protective quality for the purpose of improving customer acceptance of the packaged product.

The present invention, therefore, generally provides a process and apparatus for coating paperboard sheet materials, e.g. paperboard carton blanks, with a wax composi-` 'tion in such manner as to provide improved apperance and protective quality. It is an object of the invention to provide a means for coating the top and bottom surfaces of a successionr of paperboard sheet materials with wax composition in a single-pass multistage operation. It is a further object of the invention to obtain a glossy wax composition coating of uniform thickness in order to ensure uniformand thorough cooling and to thereby prevent the aforementioned haze formation, as well as to eliminate the various undesirable surface irregularities which detract from the appearance of the final coated product.

Additionally, it is very often desirable to utilize a paperboard material having a different wax composition coating oneach surface. For example, it may be advantageous to utilize a paperboard carton having an interior coating resistant to the contents of the carton, and an exterior coating resistant to' a particular outside environment. Accordingly, it is a still further object of 3,257,225 Patented June 21, 1966 ice rollers, with the application of the molten wax composition to each surface being performed at different stages of a single-pass operation. The applied films of wax composition are then separately contacted by reverse rollers which serve to smooth and reduce them to a uniform thickness. Immediately after the reverse roller smoothing operation performed on each surface, the molten film of wax composition is rapidly chilled to a temperature substantially below the melting point of the wax composition in order to form a solidified coating.

One important feature of the invention is the provision of reverse smoothing rollers, rotating in contact with the molten wax films, in a direction opposite to the travel of the paperboard sheets. Another important feature of the invention is the separate treatment of each of the two surfaces of the paperboard sheets while utilizing a single- -pass throughput of the paperboard sheets. This separate sheet may be coated with wax composition while the' bottom surface is being simultaneously cooled. Additionally, because of this independent treatment, different wax compositions may be readily applied to opposite surfaces. Thus, a convenient means for coating both surfaces of a succession of paperboard sheets is provided with a minimum of expended space, time, and material.

The apparatus employed preferably includes alseries of at least four closely spaced and Yaligned sets of substantially parallel, rotatable, cylindrical rollers through which a series of paperboard sheets is successively passed. The two rollers in each set are suitably mounted for the desired direction' of rotation. Additionally, means are provided for maintaining the surface of each roller at a predetermined temperature. A final cooling Water bath or the like is also provided which receives and cools the coated paperboard sheets.

The first set of cylindrical rollers includes a bottom coating roller which dips into a bath of molten wax and Vapplies a molten wax film to the bottom surface of each paperboard sheets. The second set of rollers includes a bottom reverse smoothing roller rotating in a direction opposite to the travel of the paperboard sheets, which roller serves to smooth the bottom wax film and to reduce and regulate it to a uniform thickness; and a top roller rotating in the direction of travel of the paperboard sheets. The third set of rollers includes a bottom cooling roller which cools and solidifies th-e bottom wax film; and a top coating rollers, to which is applied a stream of molten wax composition, thereby serving to apply a top film of molten wax to the top surface of each paperboard sheet. The fourth set of cylindrical rolls includes a top reverse smoothing roller, rotating in a direction opposite to the direction of travel of the paperboard sheets, which roller serves to smooth the top wax lfilm and to reduce and regulate it to a uniformV thickness; and a bottom cooling roller which further -serves to cool the bottom wax film. Finally, a cooling bath is provided through which the paperboard sheets are immediately passed after the fourth set of cylindrical rollers, which bath serves to chill the sheets and to solidify the top Wax composition film. Any other conventional cooling means may, of course, be employed as a substitute for the cooling bath, e.g. mult1- ple water showers, etc.

It should be noted that although the reverse smoothing rollers rotate in a direction opposite to the travel of the paperboard sheets, the sheets are maintained in their forwardly-moving direction of travel. This is accomplished by either or both of the following factors. Firstly, while a reverse roller is contacting a fluid or slipping surface due to the molten wax film, the other roller of the same pair, which may be termed a pull roller, is contacting a dry or fixed surface. Thus, because of this difference in frictional resistance, the sheets are propelled in a forward direction. Secondly, if necessary, the speed of a pull roller may be adjusted to a somewhat higher value than the speed of the corresponding reverse roller, to thereby maintain the` forwardlymoving direction. The difference between the rotational speeds of the pull roller and reverse roller of the same pairof rollers is commonly termed the wipe ratio, which ratio may vary, for example, from a-bout 0.5 :1 to about 1.521, depending upon the particular conditions involved. It is apparent, therefore, that a reverse roller may rotate at a lower, equal, or higher rotational speed than the corresponding pull roller, with the only requirement being that the paperboard sheets lbe propelled in a forward direction.

In this fashion, a ysuccession of separate paperboard sheets, e.g. carton blanks, are conveniently coated with either the same or different wax composition on both top and bottom surfaces, with a smooth wax coating of uniform thickness being provided. Additionally, the quick chilling of each surface film prevents the formation of substantial haze, thereby producing a desirable glossy appearance.

It should be noted that it would be impractical to simultaneously treat both surfaces of a succession of paperboard sheets in the manner contemplated by the present invention, and still provide a continuous multistage, single-pass operation. A distinction should therefore be made between the coating of a succession of individual paper-board sheet materials, e.g. carton blanks, and the coating of a continuous strip of paper stock and the like, commonly referred to as web coating. By way of illustration, a reverse rolling operation could easily be performed simultaneously `on both surfaces of a continuous strip of paper stock, since the strip could be pulled through the reverse rollers by a forwardly rotating roller suitably positioned with respect to the two reverse rollers. However, such arrangement would not be readily adaptable to the coating of a succession of individual paperboard sheets since the continuous f-orward motion of the sheets could not be maintained, due to the rotation of the reverse rollers in a direction opposite to the travel of the sheets and the absence of a pulling or pushing. force. The present invention, however, ensures a continuous an-d constant forward motion of each paperboard sheet through the apparatus =by a separate reverse roller operation on each surface, which operation is performed at different time intervals, as opposed to simultaneously. 'Additionally, there is a minimum of expended time and space since one stage of -the multistage operation may be performed on one surface while another stage is simultaneously being performed on the second surface.

The invention will be more fully understood from the following detailed disclosure, and from the accompanying drawing which is a fragmentary and somewhat schematic longitudinal section of the apparatus.

As shown in the drawing, the apparatus comprises at least four sets of rollers that are generally similar to those employed for a variety of purposes in different industrial arts. To the extent that the structure of the apparatus may follow well-known design principles, many of the details of the apparatus vhave `been omitted for simplicity. For example, it is readily appar-ent that each pair of rollers must vbe suitably positioned and spaced for receiving a paperboard sheet therebetween while at the same time assuring a rolling contacting surface on both the top and `bottomsurface of each sheet. It will be evident that the spacing between the rollers will depend upon the thickness of the paperboard sheets since the pressure exerted by the rollers must be sufficient to propel the paperboard sheets forward and yet insufficient to cause deformation of the sheets. The various rotational speed rotios and exerted pressures lbetween the rollers will 'be readily determined by vwell-known procedures depending upon the particular coating application desired.

Referring to the drawing, unwaxed paperboard sheets 10 (eg. carton blanks) are continuously fed to the apparatus by means of conveyor belt 11 passing over rollers 11/1 and 11b. The sheets may be fed to conveyor belt 11 by any conventional feeding device. The sheets are successively passed to the first pair of the series of pairs of cylindrical rollers, which pairs are mounted in horizontal alignment and define a horizontal path for successive paperboard sheets. The rollers are journaled in a frame (not shown) and driven by conventional means, with provision being made for speed adjustments.

The first pair of

rollers

12 and 13,'includes a bottom coating roller 12, vwhich dips into tank 16 in contact with molten wax composition 15. Roller 12 is engaged by a parallel and axially coextensive doctor roller 14, which is suitably mounted for free rotation. The pressure of the doctor roller 14 against the coating roller 12 is made adjustable in any known manner to control the thickness of the film of moltenwax composition on the moving surface of the coating roller 12. The molten wax composition 15 is maintained at a predetermined temperature above the melting point of the waxA composition by conventional heating means. The top roller 13, of the first pair of rollers, serves as a companion to the bottom coating roller 12, in order to propel the paperboard sheets in a forward direction.

The second pair of

rollers

17 and 18, includes a bottom reverse smoothing roller 18, which rotates in a direction fopposite to the direction of travel of the paperboard sheets as indicated by the arrow affixed thereto. Roller 18 is also preferably engaged by doctor roller 19, to remove or to regulate the amount of excess molten wax composition left on roller 18. Doctor roller 19 may be replaced with a doctor blade which would serve the same function. Roller 18 thereby serves to smooth, reduce and regulate the lower surface Afilm of wax composition (applied by coating roller 12) to a uniform thickness. The top roller 17, of the second pair of rollers, is similar in function to roller 13, and provides a means for propelling the paperboard sheets in a forward direction as previously described.

The third pair of

rollers

20 and 21, includes a top coating roller 20 and a bottom cooling roller 21. Coating roller 20 is engaged by doctor roller 22, which is similar to

doctor rollers

14 and 19. Conduit 24 is provided for feeding molten wax composition at a controlled rate to coating roller 20, and is so positioned that it terminates directly above the converging surfaces of the oppositely rotating rollers 2t) and 22. The pressure of doctor roller 22 is again made adjustable in any well-known manner to thereby control the thickness of the molten wax film on the downwardly moving surface of the coating roller 20. Any excess wax composition flows over the ends of the rollers to a conduit (not shown) leading back to the wax feed tank (not shown). In this fashion, coating roller 2l) serves to coat the top surface of each paperboard sheet with a top film of molten wax composition. The other member of the third set of roller pairs is cooling roller 21, which is maintained at a predetermined temperature substantially below the melting point of the wax composition by suitablemeans hereinafter to be described, and which serves to chill and solidify the bottom film of wax composition. Also provided is scraper blade 23, which is suitably mounted-for engaging roller 21 to thereby remove any adherent solidied wax composition. The resulting wax scrapings drop to a suitable catch-pan (not shown). It will be readily apparent that the coating operation performed by roller on the top surface of each sheet is performed simultaneously withthe cooling and solidifying operation performed on the bottom surface of each sheet.

The fourth pair of

rollers

26 and 27, includes a top reverse smoothing roller 26, which rotates in a direction opposite to the direction of travel of the paperboard sheets and which is engaged by doctor roller 28. Reverse smoothing roller 26 serves to smooth and reduce the applied u'pper film of wax composition to a uniform thickness in a manner similar to roller 18. The bottom roller 27, of the fourth pair of rollers, serves as a propelling means `for the paperboard sheets in a manner analogous to roller 17. Also preferably included is scraper blade 29,

which serves to remove any excess solidified wax composition adhering to roller 27. Additionally, roller 27 is preferably maintained at the same predetermined temperature as roller 21 by means hereinafter to be described.

Finally, after emergence from the final pair of

rollers

26 and 27, the paperboard sheetsare propelled into cooling bath 30 containing cooling Water 31, and conveyed by conveyor belts 32,'which in turn are propelled and directed by rollers 33. Conveyor belt 32A merely serves as a guide belt for the sheets. Cooling bath 30 serves -bath to chill the entire paperboard sheet and to solidify the top film of molten wax composition. As previously mentioned, a series of .water showers may also be utilized for this purpose. Upon emergence from cooling bath 30'the paperboard sheets may be conveyed by optional

auxiliary rollers

34, 35, 36 and 37, to iinal storage at 38.

Rollers

36 and 37 serve as squeeze rollers to remove excess cooling waterfrom the surfaces of the paperboard sheets. Additionally, any convenient means may be optionally included for further drying of the sheets, e.g. heated air blasts, etc.

Another optional variation contemplated is the provision of a post heat zone prior to the water cooling step. For example, upon emergence from

rollers

26 and 27, the sheets may be propelled by a moving belt under a radiant heater to thereby reheat the surfaces of the sheets. Such reheating sometimes proves beneficial in providing additional smoothness and glossiness.

6 top lm on the upper surface-of the paperboard sheets, in place of cooling bath 30.

In summary, the present invention is concerned with 4performing the three distinct stages of coating, reverse smoothing, and cooling on each of the two surfaces of a succession of paperboard sheets while, at the same time, maintaining the forwardly-moving direction of travel of the sheets through the apparatus in order to achieve a continuous coating operation. Any of a number of arrangements of rollers may be utilized, with the arrangement shown in the drawing being particularly preferred. A As mentioned in the above description, it is desirable and sometimes necessary to maintain the various rollers at predetermined temperatures. In particular, the

coating rollers

12 and 20, and the

reverse smoothing rollers

18 and 26, should preferably be maintained at about the temperature of the-molten Wax composition. Additionally, it will be preferred, but not necessary, to maintain

rollers

13 and 17 at the same predetermined temperature, i.e. the temperature of themolten Wax composition, for the purpose of preheating the upper surface of the paperboard sheets. Finally, cooling roller 21, and optionally roller 27, are maintained at a suitable predetermined temperature substantially below the melting point of the wax composition. Maintenance -of the various rollers at the above-mentioned temperatures is most easily accomplished by passing streams of a suitable heat exchange fluid (e.g. oil, water, etc.), maintained at the desired temperature, continuously through the rollers in question. It will be noted, as shown in the drawing, that each roller is proy vided with a hollow interior, through which the heat exchange fluid may be circulated by conventional means.

In general, the present invention is .adaptable for use with any conventional wax composition. Specifically, however, certain improved wax compositions have recently been discovered which have been found to provide higher wet strength and antirupture properties than conventional Wax blends. While these compositions are not intended to be a subject of the present invention, they will be briefly described for purposes of completeness.

One particularly useful wax composition, designated composition A, will comprise (l) iat least 60 wt. percent, e.g. 75 Wt. percent, of crystalline refined, substantially oilfree parain wax having a melting point of about 120 to 160 F., e.g. 130 to 155 F.; (2) up to 30 wt. percent, e.g.

. about 5 to 15 wt. percent, of a microcrystalline wax hav- It will thus be readily apparent that the apparatus comprises a convenient means for coating both surfaces of a series of paperboard sheets in a single pass while at the same time providing a means for smoothing and reducing the appliedl coating films to a uniform thickness. As mentioned, two different wax compositions may be applied to the opposite surfaces of the sheets. Additionally, the provision of a means for quickly chilling and solidifying the applied films immediately after the smoothing operation minimizes haze formation. The apparatus is particularly suitable for the coating of separate paperboardy sheet materials since the forward motion of said materials is maintained While the reverse roller smoothing operation is performed. `Moreover, since the coating, smoothing, Iand cooling operations on each separate surface kare performed independently from the same operasurface, two or more reverse rollers may be desirable for 4 some purposes, such as, for example, where the wax composition is unusually viscous. variation, 'another pair of rollers could be provided after

rollers

26 and 27, with the top member serving to cool the Additionally, as an obvious previously described method and apparatus.

ing a melting point above F.; (3) up to 8 wt. percent,

' e.g. 1 to 4 wt, percent, of a polyolen, eg., polyethylene having a molecular weight in the range of 1|500 to 25,000; and (4) about l to 25 wt. percent, e.g. 5 to 10 wt. percent, of a solid polymer resin having a softening point of at least 158 F., e.g. 207 to 218 F., and a molecular weight of about 1000 to 1200, said resin being a petroleum polymer or styrene polymer resin. i

Another useful composition, designated composition B, will comprise the same ingredients as the above composition With the exclusion of the polyolen. Thus, a typical composition will comprise at least 4 wt. percent of the paran wax, up to 30 wt. percent, of the microcrystalline wax and about l0 to 30 wt. percent of the solid petroleum polymer or styrene polymer resin.

In conjunction with the use of the above wax compositions, certain operating conditions have been found to be particularly useful for the formation of a high quality coated paperboard. Thus, for example, the temperature of the molten wax composition should preferably be in the range of 220 to 320 F. Additionally, the preferred temperature ranges for the various stages 4are as follows:

coating rollers and reverse rollers, 220 to 320 F.; cool- I ing rollers, 50 to 100 F.; cooling bath or showers, 30 to 50 F.

The following example is presented to indicate typical operating conditions for the coating of Iboth surfaces of a Vsuccession of paperboard sheets in accordance with the Flat paperboard sheets having dimensions 111/2 x 11%," x 18 mil (thickness) are passed through `the above-described apparatus at a rate of about 200 sheets per minute. Molten wax composition at a temperature of about 275 F. is applied by coating

rollers

12 and 20.

Rollers

12, 13, 17, 18, and 26 are maintained ata temperature of about 275 F. by recirculation of hot oil through the hollow inner portion of each of said rollers as previously described, the oil being suitably maintained at a temperature slightly above 275

F. Rollers

21 and 27 are maintained at a temperature of about 90 F., by means of recirculating 45 F. water maintained at sufiicientflow rate. Cooling water 31 is maintained at a temperature of about 40 to 45 F. The four pairs of rollers are suitably positioned with respect to each other such that the lapsed time. interval between a reverse smoothing operation and a cooling operation is no greater than about 0.3 second per board to thereby minimize haze formation. Typical operating rotational speeds for 6" diameter rollers are as follows: coating rollers, 128 r.p.m.; reverse smoothing rollers, 140 rpm.; cooling rollers, 128 r.p.m.

It is obvious from the above that many modifications are possible without departing from the spirit of the presentpinvention. Accordingly, the invention is not to be construed as limited to the particular deails disclosed herein for illustrative purposes except as specifically set forth in the appended claims.

What is claimed is:

1. An apparat-us for applying a lwax composition coating of uniform thickness to top and bottom surfaces of a succession of paperboard .sheets comprising:

(A) a bath of molten -wax composition;

(1B) at least four pairs of substantially parallel, ro-

tating, cylindrical rollers adapted to receive said paperboard sheets therebetween, e-ach pair of said rol-1ers consisting o-f a top roller and a bottom roller,

(l) a first pair of said rollers rotating in the direction of travel of said sheets and including a bottom coating roller dipping into said bath of molten wax composition for applying -albottom Lfi'lm of `said wax composition to the bottom surfaces of said sheets;

(2) a seoond pair of said rollers including -a bottom reverse roller rotating `in a direction opposite to the ltravel of sa-id sheets to thereby smooth and reduce said bottom film to a uniform thickness, and a top rol-ler rotating in the direction of travel of said sheets to thereby. maintain the travel of said sheets in a 'forwardly-moving direction;

(3) a (third pair of said rollers 'including a top coating roller [for applying a top film of said molten wax composition to the top surfaces of said sheets, and a `bottom cooling roller maintained at a temperature below the melting point of said waX composition to lthereby solidify said bottom film, both of said rollers rotating in the direct-ion of travel of said sheets;

(t4) fa fourth pair of said rollers including a top reverse roller rotating in a direction opposite to the travel of said sheets to :thereby smooth and reduce said top film tto .a uniform thickness, and a bottom roller rot-ating in the direction of travel of said sheets to thereby maintain the travel of said sheets in a forwardly-moving direotion;

(C) -a conduit positioned `above said top coating roller i for applying molten wax composition thereto;

(D) doc-tor rollers engaging said coating rollers;

(E) means for heating each of said coating rollers'and each of said reverse rollers;

(F) means for cooling said bottom cooling roller to a temperature below the melting point of said wax composition;

(G) a cooling bath maintained at a temperature below the melting point of said wax composition; and

(H) means for .passing said sheets ythrough said cooling bath after their exit from said fourth pair of rollers.

2. A method of obtaining a wax composition coating of uniform thickness on ythe 'top and bottom surfaces of a succession of paperboard sheets, which comprises advancing said sheets in a forward direction while performin-g in sequence the steps of:

(a) applying a first film of molten wax composition to one of said surfaces;

(b) contacting said first film with a first heated rolling surface moving in a direction opposite to the travel of said sheets, tto thereby smooth and reduce said first film to a uniform thickness;

(c) cooling said first fillm to a temperature below the melting point of said waX composition to thereby solidify said first film;

(d) simultaneously with said cooling of said first film, applying a second film of molten wax composition to the other of said surfaces;

(e) contacting :said second film with a second heated rolling surface moving in a direction opposite to the travel of said sheets, `to thereby smooth and reduce said second film to a uniform thickness; and

(f) cooling said second fil-m to a temperature below the melting point of said wax composition to thereby solidify said second film.

3. A method according to claim 2, wherein said first and second films are applied by means of heated rolling surfaces having said molten wax composition applied thereto.

4. A method according to claim 2, wherein said cooling of said first film is accomplished by contacting said first film with a rolling surface maintained at a temperature below the melting point of said wax composition.

S. A method according to claim 2, wherein said cooling of said second film is accomplished by contacting said sheets with 'water maintained at a temperature below the melting point o-f said wax composition.

`6. An apparatus for applying a wax composition coating of uniform thickness to the top and bottom surfaces o-f a succession of paperboard sheets, which comprises, in combination:

(a) at least four pairs of substantially parallel, rotatable, cylindrical rollers adapted to receive said paperboard sheets therebetween, each pair of said rollers consisting of a top roller and a bottom roller;

(l) a first pair of said rollers rotatable in the direction of travel of said sheets, including a first coating roller, for applying a first film o-f molten wax composition to one surface of a paperboard sheet;

(2) a second pair of said rollers comprising a roller rotatable in the direction of travel of said paperboard sheets and a first reverse roller rotatable in a direction opposite to the travel of said paperboard sheets, said first reverse roller being positioned on the saine side with respect to top and bottom as said first coating roller;

(3) -a third pai-r of said rollers, rotatable in the direction of travel of said sheets, comprising a second coating roller for applying a second film of said. molten wax composition to the other surface of said paperboard sheet and a cooling roller opposite said second coating roller;

(4) -a Ifourth pair of said `rollers comprising a roller rotatable in the direction of travel of said paperboard sheets and a second reverse roller rotatable in a direction opposite to the travelof said paperboard sheets, said second `reverse roller being positioned on the same side with respect to top and bottom as s-aid second coating roller;

(5) said reverse rollers functioning to smooth and reduce the respective films to uniform thickness;

(b) means `for heating each of sai-d coating rollers and each of said reverse rollers;

(c) means 4for cooling said cooling roller to a temper- `ature .below the melting point of said wax composition; and

(d) means for cool-ing said second -lm to a temperature `below the melting point of said Wax composition after a papenboard sheet vhas passed through said fourth pair of rollers;

whereby each off .said applied lms may be maintained in a molten state during application and reverse rolling stages and immediately cooled after a reverse rolling sta-ge, and whereby application of said second film and References Cited by the Examiner UNITED STATES PATENTS Blum.

Labornbarde.

Sup'ligeau et al.

Yates 118-59 X Blackmore 118-261 X Curler et al 1418-69 Pierce et al 117-111 Labomlbarde 117-111 Labombarde 117-111 X RICHARD D. NEVIUS, Primary Examiner.

cooling of said rst lmrcan'ibe performed simultaneously. 15 J. A. HAUG, J. P. MCINTOSH, Assistant Examiners.

Claims

2. A METHOD OF OBTAINING A WAX COMPOSITION COATING OF UNIFORM THICKNESS ON THE TOP AND BOTTOM SURFACES OF A SUCCESSION OF PAPERBOARD SHEETS, WHICH COMPRISES ADVANCING SAID SHEETS IN A FORWARD DIRECTION WHILE PERFORMING IN SEQUENCE THE STEPS OF: (A) APPLYING A FIRST FILM OF MOLTEN WAX COMPOSITION TO ONE OF SAID SURFACES; (B) CONTACTING SAID FIRST FILM WITH A FIRST HEATED ROLLING SURFACE MOVING IN A DIRECTION OPPOSITE TO THE TRAVEL OF SAID SHEETS, TO THEREBY SMOOTH AND REDUCE SAID FIRST FILM TO A UNIFORM THICKNESS; (C) COOLING SAID FIRST FILM TO A TEMPERATURE BELOW THE