US3326708A - Method for producing a heat sealable paperboard article - Google Patents

Description

ERBOARD ARTICLE 2 Sheets-Sheet 1 s mm a W. W Q8 Q; e p gas? M a U; a P Q p i s I mwwl R. F. HAWKINS METHOD FOR PRODUCING A HEAT SBALABLE PAPERBOARD ARTICLE June 20, 1967 Sheets INVENTOR. PasserF/iiqwzrnv:

Sheet 2 Original Filed March 28, 1963 United States Patent 2 Claims. (Cl. 117--10) This is a division of application Ser. No. 268,787, filed Mar. 28, 1963, and now abandoned.

This invention relates to flexible sheet material, preferably paperboard, which is adapted to be folded and sealed to itself through the intermediate of a heat sealable composition to form a container, carton or the like. The invention is particularly directed to apparatus and method of producing a coated heat sealable paperboard container blank.

The method and the composition hereof are applicable to the coating of a variety of flexible sheet materials, but the present invention contemplates primarily the coating of flexible paperboard container blanks. Paperboard as employed herein shall be understood to include but not limited to solid board of a caliper, i.e. thickness, ranging from about 8 to about 30 points (1 point0.001 inch or 1 mil). The paperboard employed for butter, oleomargarine and frozen food cartons for example, to which the present invention is particularly applicable, is a solid bleached sulfate board of a caliper ranging between about 12 and 16 points. Boxes or cartons of this type bear a wax coating on one or both inner and outer surfaces to reduce or effectively eliminate moisture vapor transmission. The present invention is directed to a paperboard container blank with an overall outer wax coated surface and an inner surface which is also preferably overall coated, but at least coated in sealing areas, with a heat sealable wax-containing composition. Heretofore, the paperboard container blanks have been formed into a box or carton and sealed at one or more overlapping portions with either a cold liquid adhesive or through the intermediate of a heat sealing material, that is to say an adhesive which is thermoplastic.

In the cold method a liquid glue is spot applied to one or both overlapping laps or portions of the waxed blank at the filling or packaging machine, and thereafter, heat is applied to the outer surface of the container in the glue areas. This heat melts the wax coating in the areas where the glue has been applied and allows the wax to flow into the paperboard thus presenting a cellulose fiber surface to which the glue may readily adhere. This cold method of course requires application of the glue in the plant where the containers are shaped and filled.

In order to simplify sealing for the packager, paper converters have resorted to heat sealable adhesives or bonding material, which are spot applied to the blank after it is Wax coated. Sealing is then effected simply by application of heat and pressure to the joint or closure areas of the filled container. Pressure is of course required with both cold and heat sealable adhesives to overcome the tendency of the scored yet relatively stiff paperboard to snap back, and to maintain the glue in contact with both surfaces to be joined for the required setting time.

Any spot application of adhesive, whether it be a cold glue or a heat sealable composition, is difficult because of the underlying wax coating which also tends to interfere with a uniformly satisfactory seal. Also, a special attachment is required adjacent the waxer or the packaging machine to apply the adhesive. Furthermore, application of adhesive to only selected areas of-the blank requires precise timing, and where a timed operation is necessary the waxing or packaging can not be effected with maximum efficiency.

It is an object of the present invention to provide a flexible sheet material container blank which meets all of the requirements of a normal wax coated blank, but which does not require special application of a heat sealable or cold adhesive to the portions thereof to be sealed.

It is a further object of this invention to provide a method for producing a wax coated container blank which is heat sealable as formed.

It is still a further object of the present invention to provide a novel wax-containing heat sealable coating composition having a fast set-up time and a high set-up temperature.

It is yet another object of this invention to provide a coating composition which when applied to paperboard container blanks exhibits excellent blocking resistance, low slip resistance, good seal strength, seals at a low temperature, has a high set-up temperature and is of a viscosity which allows coating application on conventional equipment and enables good control of the wax weight on the blank. Further objects Will be apparent from the following detailed description of the invention and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

In the accompanying drawings:

FIG. 1 is a schematic representation of a conventional waxer to which is attached a device for carrying out the method of this invention;

FIG. 2 is a perspective view of a portion of a paperboard container blank of the present invention;

FIG. 3 is a greatly enlarged perspective sectional elevation of a portion of a container blank in accordance with this invention illustrating the configuration of the coating on the inner side of the container;

FIG. 4 is a view similar to FIG. 3, but illustrating a typical pattern on the inner surface;

FIG. 5 is a perspective view of a portion of a blank in accordance with one embodiment of the present invention illustrating a scored end flap;

FIG. 6 is an enlarged section taken on the lines 6-6 of FIG. 5;

FIG. 7 is a perspective view of a portion of a oontainer blank in accordance with this invention illustrating opposed overlapping flaps prior to closing, and

FIG. 8 is a view similar to FIG. 7 of a blank in the process of being closed, with one flap overlying the other and about to be heat sealed thereto.

In accordance with the present invention, I provide a paperboard container blank of the type adapted to be sealed along at least one overlapping portion which has a heat sealable wax-containing composition completely and continuously coating the upper or outer surface of the blank presenting a smooth and/ or glossy finish. The inner surface of the blank, at least at one or more portions thereof which are adapted to be heat sealed by means of the aforesaid composition and preferably the entire inner surface, is coated with the same composition and presents a continuous coating, at least in said sealing portion or portions which is formed with slight protuberances thereover.

Referring to FIGS. 2 and 3, it will be noted that the upper or outer surface 10 of blank 9 is smooth. In a preferred embodiment this surface 10 by reason of the characteristics of the heat sealable wax coating composition thereon, presents a high gloss. The lower or inner surface 11 of blank 9 is rough or irregular. This surface is best illustrated in FIG. 3 where it can be seen that the irregularities are made up of a plurality of slight protuberances 12. These protuberances or bulges extend outwardly from the base coating 13. Bulges 12 are in fact drawn out from the continuous wax composition coating 13 in a manner hereinafter described. These protuberances or bulges are closely adjacent and of varied cross-section caused by the occasional merging of bodies of wax being drawn out from the base coating 13. This occasional merging of protuberances 12 results from the perferred manner in which the present products are produced and is also a function of pressure. It is not essential to this invention that bulges 12 present different cross-sections. It is, however, critically important that the protuberances at least be closely adjacent one another and be disposed in a more or less random fashion. Furthermore, it is essential that the protuberances extend at least about 0.4 point, and preferably at least about 0.5 point above the base coating 13 in order to achieve all of the benefits possible in accordance with this aspect of the present invention. In this connection, it is important to note that protuberances 12 regardless of cross-sectional area are of substantially the same height as shown at the edges of FIG. 3.

Protuberances 12 differ appreciably in their random distribution and configuration from spots or line projections of heat sealable material which may be applied to a surface by means of a rotogravure or other printing roll. Rotogravure printing produces a plurality of uniformly disposed upstanding projections which do not gradually merge into the base coating, if indeed such a coating is present.

In the embodiment illustrated in FIG. 4 protuberances 12 are present only in pattern areas 14, although the intermediate areas of the inner surface of the blank bear the continuous wax composition as at 13. An adjacent line pattern is shown but it is to be understood that any suitable pattern may be employed. Regardless of the pattern, the characteristics of the surface in the pattern areas is as described in connection with FIG. 3.

In the preferred container blank of this invention the paperboard is overall coated on both inner and outer surfaces with the same heat sealable wax-containing composition. If desired, however, protuberances 12 may only be provided on the inner surface portions which are to be heat sealed. Generally this will not be the practice because creation of the protuberances only in selected areas of the blank presents a time control and register problem, thus reducing production speed. Likewise, the pattern effect illustrated in FIG. 4 involves additional special attention and will generally not be employed except possibly with high caliper board.

It is also within the contemplation of this invention to provide wax on the inner surface only in pattern areas. That is to say, wax is deposited only in certain areas and not intermediate these areas. In the pattern areas the coating is in the form illustrated in FIG. 3. Such a technique reduces the quantity of wax on the inner surface of the blank and may be resorted to with heavy board stock.

The same wax-containing composition on both inner and outer surfaces of the blank enables a uniform seal at the lap joints, primarily because the composition as it is applied to each surface bonds directly to the cellulose fibers of the board. When the container is formed and closed and heat and pressure are applied in overlapping areas, only fusion of the coating is required to produce an integral seal. In effect, the coating bonds to itself producing at least a laminate-type seal. However, when the proper heat sealable wax-containing compositions are employed a fiber tearing rather than a wax tearing bond is achieved.

The protuberances on the inner surfaces of the coated blank serve a number of purposes. They very substantially improve the blocking characteristics of the wax coating by allowing only the raised ends of the protuberances to touch the smooth outer surface of the carton immediately below. Thus, the blanks will not stick together when stacked for extended periods under conditions of high ambient temperature and humidity. While blocking resistance is a function of the particular wax coating composition employed and the weight of this material on the blank, the protuberances on the inner surface very definitely improve blocking characteristics. The combination of the coating compositions of this invention and the configuration on the inner surface of the blank completely eliminate blocking.

The protuberances also improve the heat sealing operation. For example, referring to FIGS. 7 and 8 when the flap 21 presenting a glossy surface 10, is folded down and the flap 22 is folded up over flap 21 as shown in FIG. 8 just prior to application of a heating element to the joint, the ends of bulges 12 on flap 22 contact the smooth outer surface 10 of flap 21. The contact area between flaps is thus greatly reduced as compared with a smooth inner surface, and accordingly the unit pressure between flaps is very greatly increased. This increased pressure facilitates lamination of inner and outer surface coatings. Protuberances 12 when rendered plastic by heat from the sealing means, blend directly into the smooth coating on the outer surface without striking into the paperboard. The protuberances appreciably reduce heat sealing time, i.e., they hasten set up. They also reduce somewhat the temperature required for sealing and, surprisingly, produce a more uniform seal than is possible with a smooth Wax coating on both surfaces. It is also quite surprising that a lower weight wax coating with protuberances seals more effectively than a much heavier smooth wax coating.

The protuberances range in height from about 0.4 to 2 mils above the continuous wax layer on the inner surface, depending upon the weight, i.e. thickness, of wax on that surface. As a general proposition the lighter the wax coating, the lower the height of the protuberances, and vice versa. In a particularly preferred embodiment employing 10-16 point board stock the bulge height is between about 0.7 and 1.2 mils with wax weights between about 6 and 8.5 pounds per ream.

The coating composition on the outer surface of the blank is, as indicated above, of substantially uniform thickness. The weight of wax may vary with 8-30 point board stock from about one-half to eight pounds per ream (3,000 square feet). On the inner surface the wax coating will range between about one and twelve pounds per ream. As a practical matter, only with the very heaviest board or for particular purposes would the outer surface wax coating exceed about five pounds per ream. On the inner side the practical upper limit with boards of intermediate caliber is about eight to nine pounds per ream. It is desirable, and in accordance with the method of the present invention quite possible, to maintain a given weight of wax on the outer surface while varying the weight on the inner surface depending upon the caliper of the paperboard. For example, 10-12 point board will have a given wax weight on the outer surface and a somewhat greater weight on the inner surface. With board calipering about 16-18 points, an entirely satisfactory product is obtained if the weight of wax on the outer surface is the same as for 10-12 point stock, but the wax coating on the inner surface is increased substantially. Since the inner surface is the primary source of the heat sealable composition substantially more material is required in this area for heavier board, by reason of its greater tendency to snap or spring back during sealing. As a practical matter, it can be said that the coating on the inner side of the blank will range from about 1 to about 3 times the weight of said coating composition on the outer surface. Preferably, the ratio ranges from about 1:15 to 1:25.

In a further embodiment of the invention selected overlapping areas of the blank may be scored to present raised portions on the inner surface to facilitate opening of the sealed container. For example, the butter or oleomargarine container blank of FIG. 5 may have an end flap 17 scored to present raised lines 18 which are shown in greater detail in FIG. 6, along with their corresponding depressions 18a in the outer surface. This scoring is desirably accomplished at the time the blank is cut and formed, but definitely before the wax coating is applied. It will be noted in FIG. 8 that the protuberances 12 occur over the flap in random fashion both on the raised portions 18 and in the spaces between them. With this technique sealing occurs primarily along lines 18 rather than over the entire surface of the flap. In the embodiment illustrated the score lines are parallel and extend over the entire length of flap 17, but this is a matter of choice. The scoring may take virtually any form, as arcuate, intersecting or randomly disposed lines for example.

Referring now to the method of the present invention, the'paperboard container blanks may be coated on conventional waxing equipment which applies the heat sealable wax containing composition in the form of a hot melt to both sides of the blank preferably simultaneously. A typical waxer is illustrated schematically in FIG. 1 and includes a trough containing the hot liquid heat sealable wax coating composition 31. A plurality of applicator rolls 32, which are preferably fabric covered, carry the hot wax melt to one surface of the container blanks 9. A plurality of upper rolls 33, also desirably fabric covered, contact the upper surface of the blanks. Hot liquid heat scalable wax containing coating composition in header box 35, the same composition as that in trough 30, is distributed over rolls 33 by means of perforated manifolds 34. Suitable heating means are of course provided to maintain the wax composition in trough 30 and header box 35 fluid at all times. The waxer is operated above about 200 F. and preferably between about 210 and 250 F. The cut and scored container blanks 9 are fed periodically to the waxer by means not shown. The waxer illustrated includes a pair of opposed rolls 3'7 and 38 which are referred to in the art as the wax nip. These rolls serve to further assure uniform application of the coating and to meter excess wax from the blanks.

Now is accordance with this invention and while the wax composition is still fluid and well above its congcaling point, the coating on the surface of the blank which is to be the outer surface of the container is smoothed and reduced to the desired weight, i.e. thickness, and the composition on the opposite or inner surface of the blank is also metered and'so drawn away from said latter surface as to form the above-described protuberances or bulges over this surface. Following these treatments the blank is promptly chilled to below its congealing point to preserve the protuberances so formed and itssmooth and uniform outer surface. Chilling may be accomplished with a cool gas or liquid. Preferably the blank is immersed in water maintained well below the congealing point of the wax composition.

A particularly preferred device for carrying out these post waxing operations is illustrated generally at the broken away box in FIG. 1. Perhaps the most convenient but certainly not the only means for smoothing or leveling and metering excess wax from the upper surface of the blank is a doctor knife 40. The knife is suitably heated as by steam box 41 in line 42 to well above the congealing point of the wax composition. The pitch and height of this blade are variable, by means not shown, to control thickness and smoothness of the outer wax coating. The drawing operation on the opposite or inner surface of the blank may be accomplished by means of a pile fabric covered heated roll 43, which in the apparatus illustrated is positioned opposite doctor blade 40. Roll 43 is backed up in a conventional manner by metal roll 44 which is actuated by a pair of double acting pressure. cylinders 46 and 46. By'controllingfluid pressure to the cylinders back up roll 44 is caused to move up or down as indicated by the arrows at each end thereof, thus exerting increased or decreased pressure against fabric covered roll 43. The pressure applied by the back up roll affects the quantity of wax on roll 43, which in turn controls the quantity of wax composition metered from the inner side of the blank. The greater the pressure applied to roll 43 the more wax removed from the inner surface of the blank. This excess wax is transferred from roll 43 to the back up roll and removed therefrom by a doctor blade 47. The greater the wax weight the higher the protuberances.

In the smoothing and texturing device illustrated in FIG. 1 the pressure applied to roll 43 has virtually no effect upon the quantity of wax on the upper surface of the blank beyond the doctor blade, by reason of the inherent stiffness of the paperboard blank passing between the roller and the doctor. Roll 43 of course serves as a comparatively solid back up member for the doctor. The wax weight on the upper surface is controlled by the blade characteristics, its height and pitch or angular disposition to the blank.

While fabric roll 43 serves to control the weight of wax on the inner surface of the blank, it also functions to produce the irregular inner surface. The surface coating of roll 43 must be resilient and soft and flexible enough so as not to mark through the blank and affect its smooth upper surface. The flexibility and resilience of the roll coating enable easy removal of excess wax therefrom in the nip between roll 43 and 44. The coating has a regular surface of upstanding fibers and is often referred to in the papermaking art as a felt, but it is rather a pile "as distinguished from a matted fabric. More correctly the roll is covered with a pile carpet. The upstanding fibers or pile penetrate the fluid wax coating on the inner surface of the blank and are then withdrawn as the roller turns and the blank moves on. During withdrawal the wax is drawn away from the surface of the blank. That is to say, a typical unstanding fiber on the surface of the roll 43 penetrates the wax coating and is then withdrawn bringing with it the wax adhering to this fiber. If the viscosity of the wax composition is sufliciently high, the protuberances so formed will remain at or substantially at the height above the surface which it reaches as the fiber is completely withdrawn. If the viscosity of the wax coating composition is too low the still fluid drawn out protuberance will flow back into and merge With the continuous base coating of wax on the inner surface before the blank can be suitably chilled to preserve the drawn out distortion. Theoccasion-al merging of protuberances which is apparent in FIG. 3 is occasioned by two or more closely adjacent upstanding fibers on roll 43 simultaneously drawing a relatively large mass of wax from the inner coating, or these fibers draw out a plurality of bodies of wax to a point where they coalesce.

I have found that the viscosity of the wax coating composition must be at least 40 cps, at 220 F. in order to produce protuberances of sufficient height to achieve the objects of the invention. The viscosity of the coating composition may range up to 400 cps. at 220 F. In this range the coating thickness on both surfaces of the blank can easily be controlled.

The smoothing and drawing operations in accordance with the present method are of coursecarried out well above the congealing point of the coating composition, and preferably at or above about 200 FfDesirably the doctor and fabric roller are about the same temperature as the Waxing rolls.

In accordance with the present method the outer or upper surface smoothing and the inner or lower surface drawing are preferably accomplished simultaneously in the manner illustrated and just described. However, the present concept contemplates independent upper surface smoothing and lower surface drawing. However, at the present time simultaneous operation appears to provide the most convenient method.

While the invention has thus far been described as providing a smooth and/or glossy upper surface, it will be readily apparent that the wax coating on this surface may in fact be discontinuous or may be textured. In-line texturing may conveniently be achieved at the doctor, but discontinuous application of wax will of course have to be accomplished on the waxer since it is difficult to completely doctor off in only selected areas while producing a uniform wax thickness in other areas of the surface. On the other hand, it is not too inconvenient to provide protuberances on the inner surface of the blank only in pattern areas. This is accomplished by arranging the fabric on roll 43 in a suitable pattern. The intermediate or nonpattern areas of the roll must of course be so constructed as to efficiently meter excess wax from the non-pattern areas, and yet retain a continuous wax coat.

The heat sealable wax-containing coating composition must have a viscosity above about 40 cps. at 220 P. not only to enable drawing of the coating into bulges of the required height on the surface of the blank, but also to enable accurate control of the Weight of coating on the blank. If the viscosity is below the above minimum the coating levels entirely too much, and the strength of heat sealed joints is not suflicient. The upper viscosity limit is about 400 cps. at 220 F. Above this it is more difficult to apply the composition on the waxer and to satisfactorily meter the composition from both the outer and the inner surfaces of the blank during smoothing and drawing. Also the drawing roll tends to clog with the more viscous material, which impairs removal of excess wax from this roll. Preferably viscosity should not exceed about 150-200 cps. at 220 F., and is most practicably about 50-100 cps.

It will be noted that the protuberance height is directly related to the wax weight on the back or inner surface of the blank, and with the present compositions this height ranges between about 0.4 and 2 points above the continuous wax coating on this surface, with at least 0.4 and preferably 0.5 point protuberances required to achieve the objects of this invention. Heavier coatings on'the back enable the production of higher bulges with more merging. On the other hand, with lighter coating the protuberances are of much more uniform cross-section and are fairly discrete.

As a general proposition a given wax weight is chosen for the upper or outer surface of the blank and as a practical matter this weight is adhered to as closely as possible. The inner surface wax coating is varied depending upon the caliper of the paperboard. For example, with a board calipering 16-18 points, the outer surface coating is desirably the same weight as for 12 point stock, or about 3 pounds per ream. However, the coating on the inner surface will be increased from about 6 to about 8 or more pounds per ream with this heavier stock.

Referring again to the present method, the more wax applied to the inner surface of the blank, the higher will be the protuberances on the back up to a point where the pile fabric roller becomes saturated with the Wax composition. When this happens the roller produces a leveling effect on the under side which is overcome by increasing pressure on the roller.

As mentioned earlier, the wax weight on the upper surface is virtually independent of the pressure applied to the fabric covered roller in the arrangement of FIG. 1. This is clearly illustrated in the following example employing the composition comprising: microcrystalline wax (M.P. 160-l80 F.); 2% polyethylene (M.W. circa 2000); 6869% intermediate paraffin wax containing 64-72% N-paraffins; viscosity of the blend: 60- 85 cps. at 200 F.; congealing point: 153-155 F.; 14- 15 ethylene-vinyl acetate copolyrner containing 27- 29% by weight vinyl acetate (Du Pont Elvax-220), 12- 16 point paperboard and the apparatu of FIG. 1 with the doctor blade at a preset height and angle.

Gauge pressure: front/back 90 3.5/2.3 80 3.4/2.8 3.5/3.3 60 3.3/4.3 50 3.5/5.5 40 3.5/6.8 30 3.6/7.9 20 3.6/8.0

For this intermediate weight paperboard the lower pressures enable the retention of entirely too much wax on the back or inner surface. As a general proposition blanks of this weight treated below about 60 p.s.i.g. have the desired coating weight. Note that only the back coating varies with the fabric roll pressure. The number of pro tuberances in a given area is greater as the pressure increases and accordingly the wax weight decreases. Also, for a given roll pressure and thus a given wax weight, protuberance height increases as viscosity of the coating increases.

It will be understood that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

I claim:

1. A method of producing a heat sealable paperboard container blank, which comprises applying a hot melt of a heat-sealable wax containing coating composition to both surfaces of the blank, said wax-containing composition having a viscosity above about 40 cps. at 220 degrees F., then while maintaining the coating composition in fluid form leveling the coating on one surface of the blank and drawing the coating on the other surface away from said other surface to form a plurality of adjacent slight protuberances in the coating thereon, and chilling the so treated blank, said step of leveling the coating on one surface of the blank and drawing the coating on the other surface comprising passing the so coated blank between a doctor knife and a pilefabric covered roll while applying heat and pressure to the blank.

2. A method of producing a heat sealable paperboard container blank according to claim 1 wherein said pile fabric covered roll has a preselected pattern of upstanding, resilient, soft, flexible pile fabric thereon, and while pressing said roll against said blank obtaining a pattern of projections of said coating composition on the surface of said blank which is approximately the mirror image of the pattern.

References Cited UNITED STATES PATENTS 1,681,350 8/1928 La Bombarde 11768 1,741,698 12/1929 Hampson 118-102 X 3,174,889 3/1965 Anderson et a1 156-254 WILLIAM D. MARTIN, Primdry Examiner.

H. W. MYLIUS, M. LUSIGNAN, Assistant Examiners.

Claims (1)

1. A METHOD OF PRODUCING A HEAT SEALABLE PAPERBOARD CONTAINER BLANK, WHICH COMPRISES APPLYING A HOT MELT OF A HEAT-SEALABLE WAX CONTAINING COATING COMPOSITION TO BOTH SURFACES OF THE BLANK, SAID WAX-CONTAINING COMPOSITION HAVING A VISCOSITY ABOVE ABOUT 40 CPS. AT 220 DEGREES F., THEN WHILE MAINTAINING THE COATING COMPOSITION IN FLUID FORM LEVELING THE COATING ON ONE SURFACE OF THE BLANK AND DRAWING THE COATING ON THE OTHER SURFACE AWAY FROM SAID OTHER SURFACE TO FORM A PLURALITY OF ADJACENT SLIGHT PROTUBERANCES IN THE COATING THEREON, AND CHILING THE SO TREATED BLANK, SAID STEP OF LEAVING THE COATING ON ONE SURFACE OF THE BLANK AND DRAWING THE COATING ON THE OTHER SURFACE COMPRISING PASSING THE SO COATED BLANK BETWEEN A DOCTOR KNIFE AND A PILE FABRIC COVERED ROLL WHILE APPLYING HEAT AND PRESSURE TO THE BLANK.

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