Patent Application
20080156857
The present invention relates to paper cups, more particularly to insulated paper cups, and most particularly to a method for forming blanks therefor and for producing paper cups from the blanks.
Insulating paperboard is used for paper cups in applications where the cups are utilized to serve hot liquids. A number of ways to enhance the insulating characteristics of paperboards from which the hot cups are made have been developed. One such paperboard is disclosed in U.S. Pat. No. 7,056,563, issued Jun. 6, 2006, to Donald D. Halabisky and assigned to the Weyerhaeuser Company of Federal Way, Wash. The insulating paperboard of the '563 patent comprises at least one layer having cross-linked fiber therein to enhance the thickness and thus the insulating characteristics of the paperboard.
When paper cups are manufactured, they are manufactured from a single blank which is overlapped along its edge portions and sealed together. In addition, the top portions of the paper cup are curled outwardly and then inwardly to form a lip on the cup. When thicker paperboards are employed, the overlapping edge seam becomes bulky. In addition, the lip has a larger diameter than when conventional paperboard is utilized
The present invention provides a blank for producing a container such as a paper cup from insulated paperboard, a method of forming the blank into the paper cup, and the paper cup itself. The blank for the container comprises a paperboard blank having a predetermined thickness. The blank also has side edges, a top edge, and a bottom edge. The blank, adjacent at least one of the side edges and/or the top edge, is compressed to a thickness less than the predetermined thickness of the paperboard blank itself.
The method of forming the container from a paperboard blank comprises forming and cutting a paperboard blank from a sheet of paperboard having a predetermined thickness. The blank has side edges, a top edge, and a bottom edge. Thereafter, the blank is preferably compressed adjacent the top edge to form a strip of paperboard having a thickness less than the predetermined thickness. In its preferred form, the container is formed from a blank in which the blank is also compressed adjacent the side edges of the container to form compressed strips that when overlapped have a total thickness less than twice the predetermined thickness. In addition, the lip of the container is created from a strip of compressed paperboard to provide a final lip having a lesser diameter than would be created from the paperboard of predetermined thickness.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
Referring to
At least one ply of the paperboard, whether a single-ply or a multiple-ply structure, contains cross-linked fibers. The cross-linked fibers increase the bulk density of the paperboard and thus the insulating characteristics. As used herein, cross-linked fibers are kinked, twisted, curly, cellulosic fibers. It is preferred, however, that the fibers be produced by intrafiber crosslinking of the cellulosic fibers as described in more detail below.
Paperboard of the present invention may have a broad set of characteristics. For example, its basis weight can range from 200 gsm to 500 gsm, more preferably, from 250 gsm to 400 gsm. Most preferably, the basis weight of the paperboard is equal to or greater than 250 gsm. To achieve the insulating characteristics of the present invention, it is preferred that the paperboard has a density of less than 0.5 g/cc, more preferably, from 0.3 g/cc to 0.45 g/cc, and most preferably, from 0.35 g/cc to 0.40 g/cc.
When at least one ply of the paperboard contains cross-linked fibers in accordance with the present invention, advantageous temperature drop characteristics can be achieved. These temperature drop characteristics can be achieved by altering the amount of cross-linked fiber introduced into the paperboard, by adjusting the basis weight of the paperboard, by adjusting the caliper of the paperboard after it has been produced by running it, for example, through nip rolls, and of course, by varying the number and thickness of additional plies incorporated in the paperboard structure. It is preferred that this paperboard have a caliper greater than or equal to 0.5 mm, a basis weight equal to or greater than 250 gsm, and a density less than 0.5 g/cc. In a most preferred form, the paperboard of the present invention exhibits a hot water ΔT of 10° C.±2.3° C. at a caliper of 0.64 mm and a hot water ΔT of 14° C.±2.3° C. at a caliper of 1.25 mm. The relationship of hot water ΔT to thickness is a linear one between the calipers of 0.6 mm and 1.25 mm and continues to be linear with a reduction in the caliper below 0.6 mm or an increase above 1.25 mm. Stated another way, a paperboard constructed in accordance with the present invention having a caliper of 0.3 mm or greater will exhibit a hot water ΔT (as defined in U.S. Pat. No. 7,056,563) of 0.7° C.±2.3° C. per 0.1 mm of caliper, and most preferably a hot water ΔT of 0.7° C.±2.0° C.
The paperboard of the invention can be a single-ply product. When a single-ply product is employed, the low density characteristics of the paperboard of the present invention allow the manufacture of a thicker paperboard at a reasonable basis weight. To achieve the same insulating characteristics with a normal paperboard, the normal paperboard thickness would have to be doubled relative to that of the present invention. Using the cross-linked fibers of the present invention, an insulating paperboard having the same basis weight as a normal paperboard can be made. This effectively allows the manufacture of insulating paperboard on existing paperboard machines with minor modifications and minor losses in productivity. Moreover, a one-ply paperboard has the advantage that the whole structure is at a low density. Furthermore, as will be described later, the low density paperboard of the present invention is easily embossable.
Alternatively, the paperboard of the invention can be multi-ply product, and include two, three, or more plies. Paperboard that includes more than a single-ply can be made by combining the plies either before or after drying. It is preferred, however, that a multi-ply paperboard be made by using multiple headboxes arranged sequentially in a wet-forming process, or by a baffled headbox having the capacity of receiving and then laying multiple pulp furnishes. The individual plies of a multi-ply product can be the same or different.
The paperboard of the present invention can be formed using conventional papermaking machines including, for example, Rotofolmer, Fourdrinier, inclined wire Delta former, and twin-wire forming machines.
When a single-ply paperboard is used in accordance with the present invention, it is preferably homogeneous in composition. The single ply, however, may be stratified with respect to composition and have one stratum enriched with cross-linked fibers and another stratum enriched with non-cross-linked fibers. For example, one surface of the paperboard may be enriched with cross-linked fibers to enhance that surface's bulk and the other surface enriched with non-crosslinked fibers to provide a smooth, denser, less porous surface.
It is preferred that a single ply paperboard be homogeneous in composition. The cross-linked fibers are uniformly intermixed with the regular cellulosic fibers. For example, in the headbox furnish it is preferred that the cross-linked fibers present in the insulating ply or layer be present in an amount from about 25% to about 100%, and more preferably from about 30% to about 70%. In a two-ply structure, for example, the first ply may contain 100% non-cross-linked fibers while the second ply may contain from 25% to 100% cross-linked fibers and preferably from 30% to 70% cross-linked fibers. In a three-ply layer, for example, the bottom and top layers may comprise 100% of non-cross-linked fibers while the middle layer contains from about 25% to about 100% and preferably from about 30% to about 70% of cross-linked fibers.
When cross-linked fibers are used in paperboard in accordance with the present invention, it has been found that the paperboard exiting the papermaking machine can be compressed to varying degrees to adjust the temperature drop characteristics across the paperboard. In accordance with the present invention, the paperboard once leaving the papermaking machine may be compressed or reduced in caliper by up to 50%, and more preferably, from 15% to 25%. This adjustment in the caliper of the paperboard made in accordance with the present invention allows the hot water ΔT to be varied as desired. This same result can be achieved by lowering the basis weight of the paperboard.
In addition, the paperboard of the present invention can be embossed with a variety of conventional embossing rollers to produce a paperboard that has a tactile sense to the user quite different from that of the conventional paperboard. An embossed surface not only provides a better gripping surface, but also provides an actual and perceived reduction in the heat transfer from the surface of the paperboard to a person touching the exterior of the paperboard. Flat embossed cauls may also be used to form an embossed pattern on the paperboard. Any of a variety of embossed patterns can be employed. However, when the paperboard is to be employed as a hot cup or other container, it is preferred that a fine pattern of indentations be embossed into the outer surface of the cup so as in essence to provide a multiplicity of small surface indents that effectively reduce the contact surface area for a person touching the surface of the paperboard. This is especially effective when the paperboard is used in a hot cup or other container that is held by a person for any period of time. The reduction in contact area reduces the amount of heat transferred to the person's fingers and thus reduces the sensation of excessive temperature. For example, the number of bumps and depressions in a one centimeter square surface of paperboard might comprise a 6 by 6 array.
The paperboard of the present invention can be utilized to make a variety of structures, particularly containers, in which it is desired to have insulating characteristics. One of the most common of these containers is the ubiquitous hot cup utilized for hot beverages such as coffee, tea, and the like. Other insulating containers such as a noodle cup, a soup cup, or the ordinary paper plate can also incorporate the paperboard of the present invention. Also; carry-out containers conventionally produced of paperboard or of foam material can also employ the paperboard of the present invention. A hot cup type container produced in accordance with the present invention may comprise one or more plies, one of which contains cross-linked fibers. In one embodiment the cross-linked fibers may be in the interior ply. A liquid impervious backing may be laminated to the interior ply. The backing may comprise, for example, a variety of thermoplastic materials, such as polyethylene. It is preferred that the paperboard used in the bottom of the cup contain no cross-linked fibers.
Although available from other sources, noncross-linked cellulosic fibers usable in the present invention are derived primarily from wood pulp. Suitable wood pulp fibers for use with the invention can be obtained from well-known chemical processes such as the kraft and sulfite processes, with or without subsequent bleaching. Pulp fibers can also be processed by thermomechanical, chemithermomechanical methods, or combinations thereof. The preferred pulp fiber is produced by chemical methods. Groundwood fibers, recycled or secondary wood pulp fibers, and bleached and unbleached wood pulp fibers can be used. Softwoods and hardwoods can be used. Details of the selection of wood pulp fibers are well known to those skilled in the art. These fibers are commercially available from a number of companies, including Weyerhaeuser Company, the assignee of the present invention. For example, suitable cellulose fibers produced from southern pine that are usable with the present invention are available from Weyerhaeuser Company under the designations CF416, NF405, FR516, and NB416.
In addition to fibrous materials, the paperboard of the invention may optionally include a binding agent. Suitable binding agents are soluble in, dispersible in, or form a suspension in water. Suitable binding agents include those agents commonly used in the paper industry to impart wet and dry tensile and tearing strength to such products. Suitable wet strength agents include cationic modified starch having nitrogen-containing groups (e.g., amino groups), such as those available from National Starch and Chemical Corp., Bridgewater, N.J.; latex; wet strength resins, such as polyamide-epichlorohydrin resin (e.g., KYMENE 557LX, Hercules, Inc., Wilmington, Del.), and polyacrylamide resin (see, e.g., U.S. Pat. No. 3,556,932 and also the commercially available polyacrylamide marketed by American Cyanamid Co., Stanford, Conn., under the trade name PAREZ 631 NC); urea formaldehyde and melamine formaldehyde resins; and polyethylenimine resins. A general discussion on wet strength resins utilized in the paper field, and generally applicable in the present invention, can be found in TAPPI monograph series No. 29, “Wet Strength in Paper and Paperboard”, Technical Association of the Pulp and Paper Industry (New York, 1965).
Other suitable binding agents include starch, modified starch, polyvinyl alcohol, polyvinyl acetate, polyethylene/acrylic acid copolymer, acrylic acid polymers, polyacrylate, polyacrylamide, polyamine, guar gum, oxidized polyethylene, polyvinyl chloride, polyvinyl chloride/acrylic acid copolymers, acrylonitrile/butadiene/styrene copolymers, and polyacrylonitrile. Many of these will be formed into latex polymers for dispersion or suspension in water.
The preferred cross-linked fibers for use in the invention are crosslinked cellulosic fibers. Any one of a number of crosslinking agents and crosslinking catalysts, if necessary, can be used to provide the crosslinked fibers to be included in the layer. The following is a representative list of useful crosslinking agents and catalysts. Each of the patents noted below is expressly incorporated herein by reference in its entirety.
Suitable urea-based crosslinking agents include substituted ureas, such as methylolated ureas, methylolated cyclic ureas, methylolated tower alkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas. Specific urea-based crosslinking agents include dimethyldihydroxy urea (DMDHU, 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethyloldihydroxyethylene urea (DMDHEU, 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea (DMU, bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU, 4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU, 1,3-dihydroxymethyl-2-imidazolidinone), and dimethyldihydroxyethylene urea (DMeDHEU or DDI, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).
Suitable crosslinking agents include dialdehydes such as C2-C8 dialdehydes (e.g., glyoxal), C2-C8 dialdehyde acid analogs having at least one aldehyde group, and oligomers of these aldehyde and dialdehyde acid analogs, as described in U.S. Pat. Nos. 4,822,453; 4,888,093; 4,889,595; 4,889,596; 4,889,597; and 4,898,642. Other suitable dialdehyde crosslinking agents include those described in U.S. Pat. Nos. 4,853,086; 4,900,324; and 5,843,061. Other suitable crosslinking agents include aldehyde and urea-based formaldehyde addition products. See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533; 3,932,209; 4,035,147; 3,756,913; 4,689,118; 4,822,453; 3,440,135; 4,935,022; 3,819,470; and 3,658,613. Suitable crosslinking agents may also include glyoxal adducts of ureas, for example, U.S. Pat. No. 4,968,774, and glyoxal/cyclic urea adducts as described in U.S. Pat. Nos. 4,285,690; 4,332,586; 4,396,391; 4,455,416; and 4,505,712.
Other suitable crosslinking agents include carboxylic acid crosslinking agents such as polycarboxylic acids. Polycarboxylic acid crosslinking agents (e.g., citric acid, propane tricarboxylic acid, and butane tetracarboxylic acid) and catalysts are described in U.S. Pat. Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209; and 5,221,285. The use of C2-C9 polycarboxylic acids that contain at least three carboxyl groups (e.g., citric acid and oxydisuccinic acid) as crosslinking agents is described in U.S. Pat. Nos. 5,137,537; 5,183,707, 5,190,563; 5,562,740; and 5,873,979.
Polymeric polycarboxylic acids are also suitable crosslinking agents. Suitable polymeric polycarboxylic acid crosslinking agents are described in U.S. Pat. Nos. 4,391,878; 4,420,368; 4,431,481; 5,049,235; 5,160,789; 5,442,899; 5,698,074; 5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981,739. Polyacrylic acid and related copolymers as crosslinking agents are described U.S. Pat. Nos. 5,549,791 and 5,998,511. Polymaleic acid crosslinking agents are described in U.S. Pat. No. 5,998,511 and U.S. application Ser. No. 09/886,821.
Specific suitable polycarboxylic acid crosslinking agents include citric acid, tartaric acid, malic acid, succinic acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, polymethylvinylether-co-maleate copolymer, polymethylvinylether-co-itaconate copolymer, copolymers of acrylic acid, and copolymers of maleic acid. Other suitable crosslinking agents are described in U.S. Pat. Nos. 5,225,047; 5,366,591; 5,556,976; and 5,536,369.
Suitable crosslinking catalysts can include acidic salts, such as ammonium chloride, ammonium sulfate, aluminum chloride, magnesium chloride, magnesium nitrate, and alkali metal salts of phosphorous-containing acids. In one embodiment, the crosslinking catalyst is sodium hypophosphite.
The crosslinking agent is applied to the cellulosic fibers as they are being produced in an amount sufficient to effect intrafiber crosslinking. The amount applied to the cellulosic fibers may be from about 1% to about 25% by weight based on the total weight of fibers. In one embodiment, crosslinking agent in an amount from about 4% to about 6% by weight based on the total weight of fibers. Mixtures or blends of crosslinking agents and catalysts can also be used.
Still referring to
In accordance with the present invention, the strips 12a and 14a along at least one of the edge portions 12 and 14, and the strip 16a adjacent the upper edge 16 are compressed to a thickness less than the original thickness of the paperboard blank. Usually these compressed strips are on the order of 5 to 9 mm wide. Referring conjunctively to
Because the paperboard blank is produced from a single or multiple ply paperboard in which at least one of the plies contains a cross-linked fiber such as the fiber described above, the strip 14a can be easily compressed to a depth of on the order of 40-60% of the original thickness T. It has been found, however, that the paperboard made from cross-linked fibers tend to rebound somewhat so that when the material is compressed to a thickness originally on the order of 40-60% of the original thickness T, the paperboard with rebound to a thickness on the order of 70-80% of the original thickness T. The amount of rebound, of course, depends upon the amount of cross-linked fiber in the paperboard, with the amount of rebound lessening with a lesser amount of cross-linked fiber in the paperboard.
A typical 3-ply paperboard, in which the mid-ply has on the order of 35-45% cross-linked fiber, based on the total dry weight of the board, will have a thickness on the order of 0.89 mm. When the material is compressed to approximately 0.46 mm, the material will rebound to a thickness of on the order of 0.56-0.76 mm, and usually to about 0.66 mm. This will result in an overall thickness reduction of about 25% when compared to the original thickness T of the paperboard.
The blank shown in
Referring to
Referring to
The following example is intended to illustrate the compressibility of a paperboard having at least one ply containing cross-linked fibers, such as polyacrylic or citric acid cross-linked cellulose fiber available from The Weyerhaeuser Company. In accordance with the test procedure, two three-ply paperboard samples, A and B, were produced in a conventional manner, as described above. The composition of the paperboards A and B are set forth in Table 1, below.
The noncross-linked fibers in the pulp are refined to the stated Canadian Standard Freeness (CSF). The weight percentages are based on the total dry fiber weight of the board. The polyvinyl alcohol (PVOH) (Celvol 165 SF from Celanese Corporation, Dallas, Tex.) is added in the weight percentage based on the dry fiber weight of the midply. In addition, samples of each of the paperboards A and B were made varying amounts of additives, as set forth in Table 2. Aquapel is a trademark of Hercules Incorporated for as sizing agent. Hercobond is a trademark of Hercules Incorporated for anionic polyacrylamide retention aid. It was found that varying the additives had very little effect on the final compressibility and resiliency of the paperboard blank after compressing.
Multiple specimens of paperboards A and B were made by cutting samples into 10.1 cm. by 20.3 cm. rectangles. Compression tests were run on strips 2.54 cm. wide adjacent the longitudinal edges of the test specimens. Caliper test points were marked at spaced locations along each of the strips. One third of the samples for each of the paperboards A and B were conditioned at 20%, 50%, and 65% relative humidity for a minimum of 24 hours. The initial calipers were measured on the strips on each of the samples. The samples were then placed in position on the bottom bar of a platen at ambient temperature with the top ply facing upwardly. A press bar was then heated during multiple runs to predetermined temperatures of about 25.5° C., about 139.1° C., and about 188.8° C. Compression forces of 4922 kPa, 9852 kPa, 17724 kPa, and 24618 kPa were used. A hot press bar, approximately 2.54 cm. wide, was pressed down onto the paperboard lying on the bottom bar. Multiple specimens were then compressed at each of the different temperatures, pressures, and different relative humidities. It was found that higher compression forces, higher temperatures and higher relative humidities led to slightly higher compressibility, resulting in a lesser thickness. The higher temperatures, pressures, and relative humidities provided a final thickness that was about 5-10% less than compression at the lower temperatures, pressures, and relative humidities.
A standard paperboard containing no cross-linked fiber was similarly compressed as a control. The standard paperboard had an initial thickness of 0.46 mm, while the average thicknesses of the paperboard A was about 0.90 mm, and of paperboard B about 0.87 mm. The minimum caliper at full compression was measured, as well as the caliper 1 minute, 30 minutes, and 60 minutes after the compression bar was released. The results of the trials were averaged and are set forth in
It can be seen by reference to
While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
