CA2475750A1 - Hot cup made from an insulating paperboard - Google Patents

Abstract

An insulating paperboard contains at least one layer of cellulose fibers. The one layer is at least partially composed of bulky fibers. The paperboard is sufficiently insulated to provide a hot water .DELTA.T across the paperboard of at least 0.7°C ~ 2.3°C per 0.1 mm of caliper. The paperboard may be embossed to decrease surface transmission of heat. A hot cup may be produced from the insulating paperboard.

Description

HOT CUP MADE FROM AN INSULATING PAPERBOARD
FIELD OF THE INVENTION
The present invention pertains to hot cups, and more particularly to hot cups made of an insulating paperboard that includes bulky fibers.
lp BACKGROUND OF THE IIWENTION
Hot foods, particularly hot liquids, are commonly served and consumed in disposable containers. These containers are made from a variety of materials including paperboard and foamed polymeric sheet material. One of the least expensive sources of paperboard material is cellulose fibers. Cellulose fibers are employed to produce excellent paperboards for the production of hot cups, paper plates, and other food and beverage containers. Conventional paperboard produced from cellulosic fibers, however, is relatively dense, and therefore, transmits heat more readily than, for example, foamed polymeric sheet material. Thus, hot liquids are typically served in double cups or in cups containing multiple plies of conventional paperboard.
It is desirable to possess an insulating paperboard produced from cellulosic material that has good insulating characteristics, that will allow the user to sense that food in the container is warm or hot and at the same time will allow the consumer of the food or beverage in the container to hold the container for a lengthy period of time without the sensation of excessive temperature. It is further desirable to provide an insulating paperboard that can be tailored to provide a variety of insulating characteristics so that the temperature drop across the paperboard can be adjusted for a particular end use.
SUMMARY OF THE INVENTION
The present invention provides a hot cup made from an insulating paperboard.
The hot cup comprises a side wall and a bottom wall. The side wall is composed of an insulating paperboard having at least one layer of cellulose fibers. At least some of the cellulose fibers in the paperboard layer are bulky fibers. Bulky fibers may be produced by intrafiber crosslinking. The paperboard is sufficiently insulating to provide a hot water DT across the paperboard of at least 0.?°C ~ 2.3°C per 0.1 mm of caliper.
BRIEF DESCRIPTION OF THE DRAWINGS
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:
FIGURE 1 is a schematic cross-sectional view of a two-ply paperboard constructed in accordance with the present invention;
FIGURE 2 is an isometric view of a hot cup made from the paperboard similar to that shown in FIGURE 1 with a portion cut away; and FIGURE 3 is an enlarged cross-sectional view of a portion of the paperboard used to make the hot cup shown in FIGURE 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODlIVViENT
Referring to FIGURE l, the substrate 10 for the insulating paperboard 12 of the present invention is produced in a conventional manner from readily available fibers such as cellulosic fibers. The paperboard of the present invention can be made in a single-ply, a two-ply construction, or a mufti-ply construction, as desired. While the paperboard of the present invention may employ synthetic fibers as set forth above, it is most preferred that paperboard comprise all or substantially all of the cellulosic fibers.
The distinguishing characteristic of the present invention is that at least one ply 14 of the paperboard, whether a single-ply or a multiple-ply structure, contains bulky fibers.
The bulky fzbers increase the bulk density of the paperboard and. thus the insulating characteristics. As used herein, bulky fibers are kinked, twisted, cuxly, 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 20.0 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 bulky 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 bulky 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. Yn a most preferred form, the paperboard of the present invention exhibits a hot water DT of 10°C -~
2.3°C at a caliper of 0.64 rnm and a hot water ~T of 14°C ~ 2.3°C at a caliper of 1.25 mm. The relationship of hot water DT 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 DT (as defined below) of 0.7°C t 2.3°C per 0.1 rnm of caliper, and most preferably a hot water ~T 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 bulky 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.
S 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. Tt is preferred, however, that a mufti-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 mufti-ply product can be the same or different.
The paperboard of the present invention can be formed using conventional papermaking machines including, fox example, Rotoformer, Fourdrinier, inclined wire Delta former, and twin-wire forming machines.
i5 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 bulky fibers and another stratum enriched with non-bulky fibers. For example, one surface of the paperboard may be enriched with bulky fibers to enhance that surface's bulk and the other surface enriched with non-crosslinked fibers to provide a smooth, denser, less porous surface.
As stated, it is preferred and most economical to produce a paperboard that is homogeneous in composition. The bulky fibers are uniformly intermixed with the regular cellulosic fibers. For example, in the headbox furnish it is preferred that the bulky 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-bulky fibers while the second ply may contain from 25% to 100% bulky fibers and preferably from 30% to 70% bulky fibers. In a three-ply layer, for example, the bottom and top layers may comprise 100% of non-bulky fibers while the middle layer contains from about 25% to about 100% and preferably from about 30% to about 70% of bulky fibers.
When bulky 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 4T 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 single-ply layer for a hot cup, it is preferred that a fine pattern of indentations be embossed into 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 surface 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.
Referring to FIGURE 2, 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 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. As shown in FIGURES 2 and 3, a hot cup type container produced in accordance with the present invention may comprise one or more plies 22 and 24, one of which, in this instance 24, contains bulky fibers. In this embodiment the bulky fibers are in the interior ply 24. A
_$_ liquid impervious backing 26 is preferably 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 bulky f bers, Although available from other sources, nonbulky 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 laiown 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, PL416, 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 banding agents include these 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, NJ; latex; wet strength resins, such as polyamide-epichlorohydrin resin (e.g., KYMENE 557LX, Hercules, Inc., Wilmington, DE), and polyacrylamide resin (see, e.g., U.S. Patent No. 3,556,932 and also the commercially available polyacrylamide marketed by American Cyanamid Co., Stanford, CT, under the trade name PAREZ

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 arid Paper Industry (New York, 1965).

Other suitable binding agents include starch, modified starch, polyvinyl alcohol, polyvinyl acetate, polyethylenelacrylic acid copolymer, acrylic acid polymers, polyacrylate, polyacrylamide, polyamine, guar gum, oxidized polyethylene, polyvinyl chloride, polyvinyl chloridelacrylic 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 bulky 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 areas, such as methylolated areas, methylolated cyclic areas, methylolated lower alkyl cyclic areas, methylolated dihydroxy cyclic areas, dihydroxy cyclic areas, and lower alkyl substituted cyclic areas. Specific urea-based crosslinking agents include dimethyldihydraxy urea (DMDHU, 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethyloidihydroxy-ethylene urea (DMDHEU, 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea (DMU, his[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 CZ-Cg dialdehydes (e.g., glyoxal), C2-Cg dialdehyde acid analogs having at least one aldehyde group, and oligomers of these aldehyde and dialdehyde acid analogs, as described. in U.S_ Patent 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. Patent Nas. 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.
Patent 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 areas, for example, U.S. Patent No.4,968,774, and . glyoxal/cyclic urea adducts as described in U.S. Patent 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. PolycarboxyIic acid cro~sslinking agents (e.g., citric acid, propane tricarboxylic acid; and butane tetracarboxylic acid) and catalysts are described in U.S. Patent Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209; and. 5,221,285.
The use of C2-Cg polycarboxylic acids that contain at least three carboxyl groups (e.g., citric acid and oxydisuccinic acid) as crosslinking agents is described in U.S. Patent 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. Patent 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. Patent Nos.5,549,791 and 5,998,511. Polymaleic acid crosslinking agents are described in U.S. Patent No. 5,998,511 and U.S. Application Serial 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, malefic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, poiymethylvinylether-co-maleate copolymer, polymethylvinyiether-co-itaconate copolymer, copolymers of acrylic acid, and copolymers of malefic acid. Other suitable crosslinking agents are described in U.S. Patent 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 _g_ about 6% icy weight based on the total weight of fibers. Mixtures or blends of crosslinking agents and catalysts can also be used.
EXAMPLES
A variety of test methods are utilized in the following examples. Hot water is determined in a simulated tester that models the heat transfer through a paper cup. A box of plexiglass measuring 12.1 cm by 12.1 cm by 12.1 cm has a sample opening of 8.9 cm by 8.9 cm. The box is insulated with 2.54 cm thick polystyrene foam. A sample of paperboard is laminated with a sheet of polyethylene using a hot air gun to adhere the polyethylene to the surface of the paperboard. Alternatively, the polyethylene may be extruded onto the surface of the board. Hot Water at a temperature of 87.8°C is poured into the box, a small stir bar inserted, and the polyethylene coated face of the sample is placed into the apparatus. The box is then turned 90° to the horizontal plane so that the water is in full contact with the sample and placed on a stir plate to permit stirring during the measurement phase. Five thermocouple microprobes are taped to the outside of the paperboard surface with conducting tape. A data logger records the temperature of the inside water temperature and the outside surface temperature from which the temperature drop (hot water DT) can be calculated. When the water temperature reaches 82.2°C, an infrared pyrometer with a 0.93 emissivity is aimed at the outside of the sample and the IR
radiation measured. This IR gun is used to correlate the thermocouple accuracy.
Durameter tests were conducted in accordance with ASTM method D2240-91.
This ASTM method is for robber, cellular materials, elastomeric materials, thermoplastic materials, and hard plastics.

A plurality of lab scale samples were produced on a pilot scale on a Delta Former, an inclined wire twinhead former. Both single-ply and two-ply samples were produced.
The single-ply samples contained varying weight percentages of bulky fibers.
In the two-ply samples, varying levels of bulky fiber were used in the base (bottom) Layer. The nonbulky fiber was a cellulose softwood pine that was refined to 400 Canadian standard freeness (CSF). The bulky fiber employed was a fiber crosslinked with malic acid. The crosslinked cellulose fiber was crosslinked with a crosslinking agent. The pH
of the system was adjusted to 8 with caustic. 20 g/kg of cooked cationic potato starch (Sta-Lok 400 available from Staley Manufacturing Company), 2 glkg to 3 g/kg of AKD
(alkyl ketene dimer) for water repellency, 5 g/kg to 7.5 g/kg Kymene, and 0 g/kg to 20 glkg of uncooked cationic potato starch were added to the machine chest. See Table lA
below.
Blends of crosslinked fiber and pine were lightly deflaked prior to board formation. The paperboard made was sized with an ethylated starch (~taley starch, Ethylx 2065) at the size press. Various samples were produced and are set Earth in Table 1B below.

'fable lA
Sample No. AKD Level Kymene Level Uncooked Starch Level glkg lk Ik 702P 3 7.5 0 7028 3 7.5 20 702S 3 7.5 20 Table 1B
Sample Base Nominal Top Nominal Actual Actual Actual No. Ply Base Ply Top Ply Board Board Board HBA Ply C-Fine Weight Weight CaliperDensity % Weight % g/m2 m2 mm g/cc m2 702P 50% 350 N/A 0 379 1.20 0.32 7028 50% 350 NIA 0 427 1.22 0.35 702S 50% 275 10010 75 396 1.03 0.38 802D 60% 450 N/A 0 439 1.22 0.361 802E 60% 350 100% 75 437 1.16 0.378 8026 50% 325 100% 75 405 0.95 0.427 802H 50% 275 100% 75 313 0.73 0.428 802I 40% 325 100% 75 412 0.90 0.457 802J 40% 325 N/A 0 436 0.99 0.439 The insulating characteristics of each of the samples produced in accordance with Example 1 were measured using the hot water OT method described above. 1n addition, samples of the paperboards 702P, 7028, and 7025 were pressed to varying calipers on a flat press. The caliper of the original boards as well as the pressed paperboards were measured along with their corresponding temperature drops. Those results are set forth in Table 2.

Table 2 Experimental Pressure Board Hot Water Board 0702H kg/cm2 Caliper OTC

Sam le (mm) 0702P 0 1.21 14 0702P 57 0.'98 13 0702P 85 0.92 13 0702P 114 0.81 12 0702P 171 0.73 12 - . _ .

07028 ~ 117 13 07028 57 0.77 11 07028 85 0.70 10 07028 114 0.67 11 07028 171 0.64 10 0702S 0 1.06 14 07025 85 0.80 12 07025 114 0.77 11 0702S 171 0.69 10 0802D 0 1.22 25 0802E 0 1.I6 14 08026 0 0.95 11 0802H 0 0.73 10 0802I 0 _ 0.90 9 0802) 0 ~ 0.99 1l Samples of paperboards 802E, 8026, and 8021 were tested for hardness and embossability using the Durometer testing method set forth above. In addition, a standard hot cup paperboard sheet containing no bulky fiber was also tested.
The results of the durometer testing are set forth in Table 3 below.
T able 3 Durometer ID

Board ID % HBA Type A: PTC Type D: Shore Mode1306L #62126 802E 60% 81 34 8026 50% 88 40 802I 40% 90 44 Standard Pa erboard 0% 96 60 The reduced hardness of the paperboard made in accordance with the present invention clearly indicates that the paperboard is more easily embossable than standard paperboard with no bulky fiber.

Three samples of the paperboards 802E, 8026, and 802I were subjected to pressure in a press, and thereafter, the caliper was measured and the percent caliper change calculated. Each of the boards was compared with a standard hot cup paperboard containing no bulky fiber. The results are shown in Table 4.
Table 4 k /cm2 0 90 226 316 Board call r, % HBA
ID mm 802E 1.10 0.82 0.58 0.54 60%a 8026 1.07 0.81 0.57 0.52 50%

802I 0.91 0.77 0.64 0.61 40%

Standard 0.45 0.45 0.44 0.40 0%
Board Board call er % HBA
ID chan a 802E 0% 25% 48% 51% 60%

8026 0% 25% 47% 51 % 50%

802I 0% 16% 29% 33% 40%

Standard 0% 0% 3% 11% 0%
Board The compressibility, and thus ernbossability, of paperboard made in accordance with the present invention is clearly superior to that of standard paperboard.
The foregoing invention has been described in conjunction with a preferred embodiment and various alterations and variations thereof. One of ordinary skill will be able to substitute equivalents in the disclosed invention without departing from the broad concepts imparted herein. It is therefor intended that the present invention be limited only by the definition contained in the appended claims.
_l~_

Claims (10)

1. A container made from an insulating paperboard comprising:
a sidewall and a bottom wall, said sidewall comprising an insulating paperboard having at least one layer of cellulose fibers, at least some of the cellulose fibers being bulky fibers, said paperboard being sufficiently insulating to provide a hot water .DELTA.T
across said paperboard of at least 0.7°C ~ 2.3°C per 0.1 mm of caliper.

2. The container of Claim l, wherein said paperboard has a density of less than 0.5 g/cc.

3. The container of Claim 2, wherein said paperboard has a basis weight of from 200 gsm to 500 gsm.

4. The container of Claim 2, wherein said paperboard has a basis weight greater than or equal to 250 gsm.

5. The container of Claim 3, wherein the caliper of said paperboard is greater than or equal to 0.5 mm.

6. The container of Claim 4, wherein said paperboard has a hot water .DELTA.T
of 9°C ~ 2.3°C at a caliper of 0.6 mm and a hot water .DELTA.T of 14°C ~ 2.3°C at a caliper of 1.25 mm, said hot water .DELTA.T being a substantially linear progression relative to caliper in the temperature range from 9°C to 14°C.

7. The container of Claim 6, wherein said linear progression extends below a .DELTA.T of 9°C.

8. The container of Claim 7, wherein said linear progression extends above a .DELTA.T of 14°C.

9. The container of Claim 1, wherein said paperboard is at least a two-ply board, at least one ply containing said bulky fibers.

10. The container of Claim 1 comprising a hot cup.