Rigid paperboard container

Abstract: A container (10) formed from a unitary paperboard blank having a substantially planar inner region (12), a sidewall region (11) and a rim region (13) formed about the periphery thereof is disclosed. The sidewall region (11) includes a generally annular region (14) flaring upwardly and outwardly from a periphery of the planar inner region (12) and a first frusto-conical region (16) adjoining the annular region (14) with the frusto-conical region (16) sloping outwardly and upwardly from the annular region (14). The rim region (13) includes an outwardly flaring arcuate annular region (18) adjoining an outer periphery of the first frusto-conical region (16), and a second frusto-conical region (24) extending substantially tangentially from the arcuate annular region (18). The second frusto-conical region (24) extends outwardly and downwardly at an angle of about 6° to about 12° and preferably about 6°-10.5° relative to the plane defined by the planar inner region (12). The rim of the container may further include an outwardly and downwardly flaring frusto-conical lip (26) adjoining an outer periphery of the second frusto-conical region (24) in order to aid in grasping of the paperboard container (10) by the consumer. Additionally, a plurality of radially extending mutually spaced pleats (50) are also formed in the rim region (13) and are internally bonded with portions of the rim region (13) during formation of the paperboard container (10) by a die press. During the fabrication of the paperboard material for forming the aforementioned container (10), the paperboard material is impregnated with a sizing adhesive equivalent to at least 6 pounds of starch per 3,000 ft² ream of paperboard material such that the tensile strength of the board is increased, and such that during the press forming of the paperboard containers between respective dies, the pleats (50) adhere to respective portions of the pleats (50) and rim, thereby maintaining the integrity of the pleats (50) during use of the container (10) by the consumer by resisting the opening of the pleats (50).

Description: Technical Field of the Invention The present invention relates generally to the formation of disposable containers, or paperboard containers. In particular aspects, the present invention relates to the formation of pressed paperboard plates or trays having a high resistance to bending. Background of the Invention Formed fiber containers, such as paper plates and trays, are commonly produced either by molding fibers from a pulp slurry into the desired form of the container or by pressing a paperboard blank between forming dies into the desired shape. The molded pulp articles, after drying, are fairly strong and rigid but generally have rough surface characteristics and are not usually coated so that they are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers, on the other hand, can be decorated and coated with a liquid-proof coating before being stamped by the forming dies into the desired shape. Pressed paperboard containers generally cost less and require less storage space than the molded pulp articles. Large numbers of paper plates and similar products are produced by each of these methods every year at relatively low unit cost. These products come in many different shapes, rectangular or polygonal as well as round, and in multicompartment configurations. Pressed paperboard containers tend to have somewhat less strength and rigidity than do comparable containers made by the pulp molding processes. Much of the strength and resistance to bending of a plate-like container made by either process lies in the sidewall and rim areas which surround the center or bottom portion of the container. When in use, such containers are supported by the rim and sidewall while the weight held by the container is located on the bottom portion. Thus, the rim and sidewall generally are placed in tension and flexure when the container is being used. In plate-like structures made by the pulp molding process, the sidewall and overturned rim of the plate are unitary, cohesive fibrous structures which have good resistance to bending as long as they are not damaged or split. Because the rim and sidewall of the pulp molded containers are of a cohesive, unitary structure, they may be placed under considerable tension and flexure without failing. In contrast, when a container is made by pressing a paperboard blank, the flat blank must be distorted and changed in area in order to form the blank into the desired three dimensional shape. This necessary distortion results in seams or pleats in the sidewall and rim, the areas of the container which are reduced in press forming the container. These seams or pleats constitute material fault lines in the sidewall and rim areas about which such containers bend more readily than do containers having unflawed sidewalls and rims. Moreover, such seams or pleats have a tendency to return to their original shape - flat. The necessary location of these pleats in the sidewall and rim of pressed paperboard containers places the greatest weakness in the area requiring the greatest strength. Such containers have been unable to support loads comparable to pulp molded containers since, when in use, the greater the load is, the higher the stress imposed on the rim and sidewall. Imposing tension, flexure or torsion on pleats merely enhances their tendency to open. Accordingly, known pressed paperboard containers have significantly less load carrying ability than do pulp molded containers. Being less costly than an equivalent pulp molded plate, a pressed paperboard plate with comparable strength and rigidity would have significant commercial value. Many efforts have been made to strengthen pressed paperboard containers while accommodating the necessary reduction in area at the sidewalls and rims. Blanks from which paperboard containers are pressed have been provided with score lines at their periphery to eliminate the random creation of seams or pleats. The score lines define the locations of the seams or pleats. Score lines, sometimes in conjunction with special die shapes, have been used to create flutes or corrugations in the sidewall and rim for aesthetic and structural purposes. The additional cost and complexity of dies used to create flutes or corrugations in the sidewall of such containers presents a cost disadvantage which may not be entirely justified. Whether the area reduction of the sidewall and rim is accommodated by pleats, seams, flutes or corrugations, the basic difficulty has been that under limited stress the paperboard will tend to return to its original shape. To overcome this tendency, it has been suggested that the rim be subjected to various strengthening techniques. The earliest efforts comprised the addition of several thicknesses of paperboard at the rim. This container, however, required additional manufacturing steps and increased the cost and required storage space of the containers. More recently, as disclosed in commonly assigned U.S. Patent No. 4,609,140 issued to Van Handel et al., improved rigidity in a pressed paperboard container has been achieved by application of pressure and temperature to the rim of the container while applying substantially no pressure to the sidewall and bottom wall. In particular, the container had a generally planar bottom wall, a sidewall upwardly rising from the bottom wall periphery and an overturned rim extending from the sidewall periphery. During integrally press-forming of the container, substantially no pressure was applied to the bottom and sidewalls and pressure was applied to the overturned rim. The amount of pressure imposed on the rim was approximately 1379-1724 kPa (200-250 psi) and gradually increased from the juncture of the rim and sidewall to the peripheral edge of the rim. The pleats formed in the rim were compressed to the thickness of the rim while the pleats formed in the sidewall were not subjected to any significant pressure. The container thus formed provided a significant improvement over prior paperboard containers. Additionally, commonly assigned U.S. Patent No. 4,606,496 issued to Marx et al. discloses an improvement over that set forth hereinabove. Therein, a container is disclosed which includes densified regions radially extending through and circumferentially spaced about annular sections of the rim. The densified regions are formed from pleats including at least three layers of paperboard which are created during press forming of the blank into a container, the rim region being subjected to sufficient pressure to form the pleats into cohesive fibrous structures having a density substantially greater than the density of the area of the rim adjacent the pleats while still being of substantially the same thickness. While the paperboard container formed in accordance with that process set forth in U.S. Patent No. 4.606.496 exhibits a greater rigidity than that of the remaining prior art, the containers do not have the strength of pulp molded containers. Thus, there remains a need for yet a stronger, more stable pressed paperboard container which will reliably resist bending when food is placed thereon by the consumer. Further, there is clearly a need for a stronger, more rigid paperboard container which resists bending and which can be manufactured in a cost effective manner. Summary of the Invention A primary object of the preferred embodiment of the present invention is to overcome the aforementioned shortcomings associated with prior rigid paperboard containers. A further object thereof is to provide a paperboard container in the form of a paper plate having a rim and sidewall configuration which better withstands a load force placed thereon when being held by the consumer with either one or two hands. Yet another object thereof is to provide a paperboard container having a plurality of opening-resistant radially extending pleats mutually spaced about a sidewall and rim portion of the container for enhancing the rigidity thereof, so that the paperboard container has a substantially equivalent strength and rigidity to a pleatless container, preferably having rigidity of at least 80% of that of a pleatless container. Another object thereof is to provide a paperboard material which when formed into the aforementioned paperboard container aids in the completion and maintenance of the pleats formed in the rim and sidewall portion thereof. The term "pleat" as used in this specification includes a seam, and thus "pleating" includes any working of a blank such that adjacent parts overlap whether by folding or by the excision of portions of material and the subsequent overlapping of portions bordering the excisions. According to one preferred embodiment, a disposable container comprises: a substantially planar inner region; a sidewall flaring upwardly and outwardly from the inner region; a rim region extending outwardly from the top of the sidewall, the rim region extending outwardly and downwardly at an angle between about 6° to about 12° relative to the plane of the inner region; and a lip portion extending outwardly and downwardly from the rim portion at an angle greater than about 45° to said plane. In another preferred embodiment, a container, having a finished diameter D, formed from a blank of fibrous material, comprises: a substantially planar inner region; a sidewall region including; a generally annular region flaring upwardly and outwardly from a periphery of said planar inner region and a first frusto-conical region adjoining said annular region, said first frusto-conical region sloping outwardly and upwardly from said annular region; and a rim region including an outwardly flaring arcuate annular region adjoining an outer periphery of said first frusto-conical region having a first portion thereof extending generally upwardly from said first frusto-conical region and a second portion thereof flaring generally downwardly at an angle between about 6° to about 12° relative to a plane defined by said planar inner region, a second frusto-conical region extending downwardly and outwardly from said second portion of said arcuate annular region at an angle of about 6° to about 12° relative to a plane defined by said planar inner region and a lip portion extending outwardly and downwardly from said second frusto-conical region at an angle greater than 45° with respect to said plane. The first frusto-conical region may extend at an angle from about 55° to about 70° relative to the plane defined by said substantially planar inner region. The first frusto-conical region may have a length greater than about 0.015D. The outwardly flaring arcuate annular region may include a radius of curvature between about 0.015D and about 0.030D. Said second portion of said outwardly flaring arcuate annular region may flare generally downwardly at an angle of approximately 6° to about 10.5°. Said second frusto-conical region may extend downwardly at an angle of approximately 6° to 10.5°, for example 10.5°. Said lip portion may include an outwardly and downwardly flaring frusto conical portion adjoining an outer-periphery of said second frusto-conical region, said lip having a length of at least 0.005D. Said lip may extend downwardly at an angle between about 15° to about 30° relative to a central axis of said planar inner region, and preferably at about 22.5°. A plurality of pleats may extend in said sidewall region and optionally also in the rim region, a stiffening means being provided for stiffening said container by increasing the resistance of said pleats to opening. Thus a plurality of radially extending mutually spaced pleats may be formed in the sidewall and rim region and be pressed closed during formation of the paperboard container by a die press. By radially extending pleats, it is meant any pleats having a sufficient radial component that a sufficient reduction in area be accomplished in the portions of the board at the outer periphery of the blank. Other objects of the preferred embodiments include improvements in the material from which the containers are made and in the bonding of pleated portions thereof whereby to improve the bending resistance of the containers. According to claim 1, the invention provides a container formed of pleated paperboard material, having a rim portion and a depressed base portion joined by a pleated portion, characterised in that the paperboard material comprises sizing adhesive equivalent to at least 2.7 kg (6lbs) of starch per 279 m 2 (3000ft 2 ) ream which bonds overlapping surfaces of the pleats together. Thus, during the fabrication of the paperboard material for forming the aforementioned container or subsequently thereto, the paperboard material may be impregnated with sizing adhesive in an amount equivalent to at least 2.7kg (6 pounds) (for example 2.7kg to 9.1kg (6lb to 20lb)) of starch per 279 m 2 (3000 ft 2 ) ream of paperboard material. Preferably the paperboard material is impregnated with the equivalent of between 4.1kg to 9.1kg (9 and 20lbs), for example approximately 5.9kg (13 pounds) of starch per 279 m 2 (3000 ft 2 ) of paperboard material. Surprisingly, the effect of increased size adhesive weight causes an unexpected increase in the ability of the pleats to resist opening when subjected to stresses during use. In many cases, pleat bonding can be improved to such a degree that the resulting container will have a strength substantially equivalent to that of an equivalent pleatless container, and often having over 80% or 90% of the strength of an equivalent pleatless container. Said container has at least 80% and preferably 90% of the strength of a pleatless container having the same composition and geometric configuration. Thus, the load required to produce a deflection of 1.27cm (0.5 inch) in accordance with the SSI rigidity test may be at least 80% and preferably 90% of the load required to produce that deflection in an equivalent pleatless container having the same composition and geometric configuration. It is further believed that the increased concentration of sizing adhesive also results in improved (stiffer) containers independently of the pleat-bonding effect by increasing the tensile strength of the paperboard material. Further preferred embodiments of the invention are defined by the features of the dependent claims. A concentration (lbs/3000 ft 2 ream) of sizing adhesive is "equivalent" to a given concentration of starch if otherwise identical paperboard materials, one comprising that sizing adhesive and the other starch in their respective concentrations have substantially identical stiffness. These as well as additional advantages of the present invention will become more apparent from the following "Detailed Description of the Preferred Embodiment" and the accompanying drawings, which description and drawings are by way of example only. Brief Description of the Drawings Figure 1 is a perspective view of a section of a plate-like container formed in accordance with the present invention; Figure 2 is a radial cross-section of the container illustrated in Figure 1; Figure 3 is a graphic representation of the radial cross-sectional shape of the container formed in accordance with the present invention; Figure 4 is a plan view of a blank for forming the plate-like container in accordance with the present invention; Figure 5 is a cross-sectional view of a pleat taken along line V-V before pressing of the pleat against the respective portion of the rim of the container; Figures 6 to 8 are graphic illustrations of the rigidity of the paperboard container formed in accordance with the present invention as compared to prior containers, and Figures 9 to 12 illustrate the improvement in rigidity resulting from the use of high levels of sizing adhesive in accordance with the present invention. Detailed Description of the Preferred Embodiment Reference will now be made in detail to the preferred embodiment of the present invention, an example of which is set forth in the above noted drawings. In accordance with the present invention, the paperboard container comprises a substantially planar inner region, a sidewall region and a rim region adjoined to and extending about the periphery of the planar inner region. The container of the present invention may be formed into a plate or bowl thus having a circular configuration or such container may be square or rectangular in shape having annular corners, such as a tray. Further, additional shapes are contemplated including compartmented trays and plates as well as oval platters. In each contemplated embodiment, all corners are rounded or curved with the preferred embodiment of the present invention being depicted in Figure 1. As is illustrated in Figure 1, the container 10 includes a substantially planar inner region 12 which is surrounded about its periphery by a sidewall region 11 and adjoining rim region 13. The sidewall region 11 includes a generally annular region 14 which flares upwardly and outwardly from a periphery of the planar inner region 12 and a first frusto-conical region 16 which slopes upwardly and outwardly from a periphery of the annular region 14. In the rim region 13, outwardly flaring arcuate annular region 18 adjoining an outer periphery of the first frusto-conical region is provided with: first portion 20 thereof extending generally upwardly form the first frusto-conical region 16; and second portion 22 thereof flaring generally downwardly outwardly at an angle relative to the plane defined by the planar inner region 12. Second frusto-conical region 24 extends substantially tangentially from the second portion 22 of the arcuate annular region 18 and thus extends downwardly and outwardly from the second portion 22 of the arcuate annular region 18 at an angle relative to the plane defined by the planar inner region 12. Adjoining the outer periphery of the second frusto-conical region 24 is an outwardly and downwardly flaring frusto-conical lip 26 which both adds significantly to strength and aids the consumer in grasping the periphery of the container 10. The phantom lines 28, 30, 32, 34 and 36 illustrated in Figure 1 are provided for ease in identifying the various structural segments of the container and do not represent lines actually appearing on the container. Moreover, the phantom lines do not represent actual demarcations between the sections in that, as explained in greater detail hereinbelow, the size relationships between the segments may vary without departing from the spirit and scope of the present invention. Additionally, as is illustrated in Figure 2, the container is formed from an integral and unitary paperboard blank. Referring to Figure 3, the particular configuration of the sidewall and rim portions will be explained in greater detail. Figure 3 is a diagrammatic view of the cross-section of the rigid paperboard container illustrated in Figure 1 beginning from the center line of the container. The profile shown is of the underside of the container, and the dimensions quoted relate to that surface. Throughout the following description, each of the dimensions are referenced with respect to a given diameter D which in accordance with the present invention as illustrated in Figure 3 is approximately 22.2cm (8.75 inches). However, the particular diameter of the paperboard container is not a critical limitation and is only set forth herein by way of example. It is the relationship between the various portions of the rim configuration which are important. The planar inner region 12 in accordance with the illustrated embodiment has a radius X1 which is equal to approximately 0.3D to 0.4D and preferably 0.348D. Adjoining an outer periphery of the planar inner region 12 is a sidewall portion 11 including annular region 14 having a radius of curvature equal to approximately 0.05D to 0.06D and preferably 0.0572D with the center point thereof being positioned a distance Y1 from the planar inner region 12. Included angle 37 of the annular region 14 is from about 40° to about 70° and preferably about 60 to 65° or approximately 62°. Adjoining the periphery of the annular region 14 is the first frusto-conical region 16 which slopes upwardly at an angle A1 with respect to the vertical from about 20° to about 35° and preferably about 25°-30° or approximately 27.5°. Additionally, the frusto-conical region 16 is of a length greater than about 0.015D, preferably from about .025D to .05D and more preferably approximately .036D. Further, adjoining the first frusto-conical region 16 is the arcuate annular region 18 which includes a radius of curvature in the range of .015D to .03D and preferably approximately .024D with the center point thereof being positioned a distance Y2 from the planar inner region 12. The included angle 39 of the arcuate annular region 18 may range from about 61° to about 82° and is preferably 66° to 77° or about 73°. The second portion 22 of the arcuate annular region 18, that is the distal portion of the arcuate annular region 18, is positioned such that a line tangent to the curvature of the arcuate annular region 18 at the second portion 22 slopes downwardly and outwardly at an angle of approximately 6° to 12° and preferably approximately 10.5°. With the present invention, it is critical that the combination of the annular region 14 and arcuate annular region 18 combine to position the second portion 22 of the arcuate annular region 18 in the manner set forth herein above. That is, the included angle 37 of the annular region 14 when combined with the included angle 39 of the arcuate annular region 18 with the first frusto-conical region 16 spanning therebetween, positions the second portion 22 of the arcuate annular region 18 in a manner such that second frusto-conical region 24, which extends substantially tangentially from the distal end of the second portion 22 of the arcuate annular region 18 extends outwardly and downwardly at an angle of about 6° to 12° and preferably about 10.5°. The second frusto-conical region 24 is of a length in a range from about .03D to about .05D and is preferably .04D. Because the second frusto-conical region 24 extends substantially tangentially from the second portion 22 of the arcuate annular region 18, the second frusto-conical region 24 extends outwardly and downwardly at an angle in the range from approximately 6° to 12° and preferably extends at an angle A3 of approximately 6° - 10.5° with respect to a horizontal plane formed by the planar inner region 12. Adjoining an outer periphery of the second frusto-conical region 24 is the lip 26 which is in the form of yet another frusto-conical region which extends outwardly and downwardly from the second frusto-conical region 24. The lip 26 is of a length of at least .005D and is preferably approximately .010D. Further, the lip 26 extends at an angle A2 of no more than 45° from vertical, preferably approximately 15° to 30° with respect to the vertical plane and more preferably approximately 22.5°. At the transition between the second frusto-conical region 24 and the lip 26 is a transition region 38. The transition region 38 includes a radius of curvature R3 which is in the range of about .008D and .01D and is preferably approximately .0092D with the center point thereof being positioned a distance Y3 from the planar inner region 12. Additionally, the transition region 38 has an included angle A4 of approximately 48° to 70° and preferably approximately 57°. Referring now to Figure 4, the blank 40 is of the type generally used to form circular containers such as plates and bowls. Preferably the blank includes a plurality of radially extending score lines 42 mutually spaced about the periphery of the blank 40. The score lines define locations at which pleats are created in the sidewall and rim portions during the forming the container. The number of score lines 42 may vary between 10 and 100 for a circular container depending on the rigidity desired and on the radius and height of the container. The pleats formed by the radial score lines 42 may begin in the generally annular region 14 and extend through the first frusto-conical region 16 of the formed container 10 to the outer periphery of the container. The paperboard blank is formed into a rigid paperboard container in accordance with that process set forth in commonly assigned U.S. Patent No. 4,606,496, the contents of which are incorporated herein by reference. When forming the container, the paperboard blank is press formed between male and female die surfaces which define the shape and thickness of the container. Preferably, at least one die surface is heated so as to maintain a temperature during press forming of the blank of 93.3°C to 204.4°C (200 °F to 400 °F). Referring now to Figure 5, Figure 5 is a cross-section taken along line V-V of Figure 1 and represents formation of a pleat in sidewall 11 and rim 13 of container 10. Upon initiation of the forming process, pleats 50 pass through the configuration illustrated schematically in Figure 5. As can be seen from Figure 5, during formation of the container, three overlapping layers 52, 54, and 56 of paperboard material fold into an upside down "Ω" (upper case omega) centered about each of score lines 42, thereby forming incipient pleats centered on each of score lines 42. When subjected to pressure, heat and moisture during the formation of paperboard container 10; layers 52, 54, and 56 of the paperboard material created by the score lines are forced into close proximity to one another and laminae within the board adjacent to the pleats slip and deform relative to each other. In one embodiment of the invention, the boardstock used contains only the usual amounts of starch typically used in platestock of about 1.4kg (3 lbs) per 279m 2 (3000 sq.ft.) ream. In such cases as shown in Figures 6-8, the plate formed will exhibit greater rigidity than plates having known prior art configurations wherein the second frusto-conical region 24 is substantially horizontal. In accordance with a preferred embodiment of the subject invention, the paperboard stock material is impregnated with sizing adhesives in an amount equivalent to form about at least 2.7 to about 9.1 kg (6 to about 20 lb)/ream of starch per 279m 2 (3,000 sq.ft.) ream of paperboard materal, preferably in an amount equivalent to 5.9 kg (13 pounds) of starch per 279m 2 (3,000 sq.ft) ream of paperboard material. Normal practice in formulation of paperboard for containers has generally been to limit the amount of starch used to about 1.4kg to 2.3kg (3 to 5 lbs) because bending stiffness at a given basis weight is thought to pass through a maximum in the vicinity of this region. Such sizing adhesives are preferably applied to the paperboard material at the size press and forced into the paperboard during the formation of the paperboard stock material. The sizing adhesive is preferably applied to both surfaces of the paperboard material. Sizing adhesive may be incorporated into the containers of the present invention by any convenient method which serves to impregnate portions of the board in the vicinity of the pleats with enough sizing adhesive to increase the resistance of the pleats to pleat opening above the resistance to pleat opening experienced in standard grades of boardstock wherein approximately 1.4 to 2.3kg (3-5 lbs)/ream of starch are incorporated. It is believed that the sizing adhesive internally locks the deformed laminae into the desired configuration. As mentioned, the most preferred method of adding sizing adhesive is to add it to the board during manufacture as it passes through the size press. Although the board may be thoroughly permeated with sizing adhesive, it does not appear necessary to do so. It is, however, desirable to achieve a substantial degree of penetration into the board as the available evidence suggests that the major contribution arises from the improvement in internal bonding within the board forming the pleats. Nevertheless, we have found that significant improvement in plate rigidity can result when sizing adhesive is merely "painted" onto the pleats on the non-food-contact side of the plate and cured. Accordingly, during the formation of the paperboard container 10 by the male and female dies, temperature, pressure and moisture are controlled in the region of pleats 50 such that pleats 50 become much more resistant to opening than they would be if board having normal levels of starch were used. Ideally the amount of sizing adhesive will be sufficient that pleat bonding can be improved to such a degree that the resulting container will have a strength substantially equivalent to that of an equivalent pleatless container, preferably having over 80% or 90% of the strength of an equivalent pleatless container. It appears that several effects may contribute to making pleats so much more resistant to opening. Microscopic analysis of formed pleats suggest that one of the primary benefits is probably bonding which is internal to the board that is, between the plies of the board rather than between adjacent surfaces of the board. On the food contact surface of the container, it appears that the presence of coating pigment and polymer may interfere with bonding between adjoining sections of the surface of the board but, on the other surface, bonding between adjoining portions of the board surfaces in the pleat may be rather more significant. The sizing adhesive also seems to increase the tensile stiffness of the board (while slightly sacrificing bending stiffness) in the sections between the pleats. Whilst some evidence suggests that this effect may be secondary in comparison to the effect of internal bonding upon resistance to opening of the pleats, the mechanism by which the increased size adhesive concentration is beneficial is not known for certain. However, it nevertheless is clear that increasing the content of sizing adhesive provides a significant improvement over containers made from standard (1.4 to 2.3 kg (3 to 5lb) of starch/ream) of boardstock of the same basis weight. By impregnating the paperboard stock material with a sufficient amount of a size adhesive as referred to herein above, the basis weight of the paperboard material may be reduced while achieving the same rigidity as with the prior art paperboard containers formed from heavier board. Alternatively, the same basis as weight of paperboard material may be used and thus a significantly greater rigidity would be achieved in accordance with the present invention. With the present invention for many paperboard products, a basis weight reduction of approximately 22% at constant rigidity may be achieved. That is, with the previous paperboard containers a basis weight of approximately 117 kg (258 lbs) per 279 m 2 (3000 sq ft) ream would be required to meet rigidity standards while in accordance with the present invention, a basis weight of approximately 91.7 kg (202 lbs) per 279m 2 (3000 sq ft) ream is required. The sizing adhesives suitable for use in the present invention include those materials commonly applied to paperboard which serve to stiffen the board in the pleat regions subsequent to pleat forming. Suitable adhesives seem to serve to lock the displaced laminate into the desired configuration and reduce the tendency of the board or ply separate in the pleat regions subsequent to pleat forming. Typical sizing adhesives include polyvinyl alcohol, carboxymethyl cellulose, naturally occurring gums, sodium silicate, polyvinyl acetate, styrene butadiene, starches, and the like as well as combinations of the foregoing. For economic reasons, starches are the preferred sizing adhesives for use in connection with the present invention. For materials other than starches, the proper amount of sizing adhesive may be determined by production of trial plates and comparing resistance to pleat opening and rigidity of these plates to those incorporating starches in the suggested ranges. Numerous plates having configurations set forth in Table I were manufactured in both pilot and commercial scale plate forming presses. Prior Art Horizontal Flange Prior Art 3° Negative Flange Present Invention 8° Negative Flange R1/D 0.057 0.057 0.057 X1/D 0.349 0.349 0.349 Y1/D 0.057 0.057 0.057 R2/D 0.027 0.026 0.024 X2/D 0.440 0.438 0.437 Y2/D 0.051 0.051 0.052 R3/D 0.008 0.009 0.009 X3/D 0.480 0.479 0.478 Y3/D 0.068 0.065 0.059 Y4/D 0.062 0.059 0.054 A1 27.5 27.5 27.5 A2 22.5 22.5 22.5 A3 2.5 5.5 10.5 Referring now to Figures 6-8, deflection of pressed paperboard plates having an 8° negative flange angle in accordance with the present invention was compared to prior art paperboard plates, the overall geometric configurations being as set forth in Table 1. In each of Figures 6-8, data received during tests conducted on the paperboard container in accordance with the present invention is graphically illustrated by the solid line A while that data recorded with respect to prior art paperboard containers is set forth by the interrupted line B. As can be seen from each of Figures 6-8, the paperboard container formed in accordance with the present invention exhibits a consistently higher rigidity than that of the prior art containers which were of substantially the same geometric configuration except that the second frusto-conical region 24 was substantially horizontal. In all known prior art containers, the frusto-conical region flared outwardly and downwardly at an angle of less than about 5°. The data set forth in Figures 6-8 was obtained using paperboard material having a starch content of approximately 1.4 to 2.3kg (3-5 lbs) per 279m 2 (3,000 sq.ft)/ream and thus represents only the contribution to improved rigidity resulting from the geometric configuration. The basis weight of the paperstock material used in forming the plates used in obtaining the data illustrated in Figure 6 was 77.6kg (171 lbs)/ream, while the basis weight of the paperboard stock used in obtaining the data illustrated in Figures 7 and 8 were 108.1kg (238 lbs)/ ream and 93.5kg (206 lbs)/ream respectively. Additional tests were conducted to measure the rigidity of various plates under simulated use conditions in which the plates are held either with one hand (to simulate going through a buffet line for example) or two (to simulate removal from a microwave or sitting with the plate on the knees). The results of these tests are set forth in Tables II and III from which it can be readily appreciated that increasing the negative flange angle to 8° from the prior art of 3° yields a surprising increase in rigidity. SSI RIGIDITY (ONE HAND) GRAMS PER 1.27cm (1/2 INCH) flange angle 99.9kg/ream (220 (lbs/ream)) 106.7kg/ream (235 (lbs/ream) 113.5kg/ream (250 (lbs/ream) -3° 385g/1.27cm(0.5") 407g/1.27cm(0.5") 437g/1.27cm(0.5") -8° 397g/1.27cm(0.5") 479g/1.27cm(0.5") 482g/1.27cm(0.5") RIGIDITY (TWO HAND) g per 0.76cm (0.3 in) flange angle 99.9kg/ream (220 lb/ream) 106.7kg/ream (235 lb/ream) -3° 664g/0.76cm(0.3") 721g/0.76cm(0.3") -8° 760g/0.76cm(0.3") 804g/0.76cm(0.3") Figures 9-12 illustrate the improved rigidity due to the improved pleat bonding resulting from increased levels of sizing adhesives. Forming efficiency is illustrated for 22.3cm (8 3/4 inch) plates having the approximate configurations set forth in Table I and which were formed from 49.9kg (220 lbs) per ream boardstock incroporating various amounts of oxidized corn starch as the sizing adhesive, the amount of starch added varying between 0 and 9.1kg (20 lbs) per ream as indicated in the figures. In Figure 9, the forming efficiency of plates formed as described herein as a function of the amount of starch applied in the size press are plotted. By forming efficiency it is meant the ratio between the actual strength of the plate and the calculated strength that a pleatless plate, or a plate with perfectly bonded pleats, having the physical properties of the unsized board, would have as calculated by finite element analysis. It can be seen that the forming efficiency of plates having normal amounts of starch, say up to 1.4 to 1.8kg (3 or 4 lbs)/ream, is under 80%; but that size press weights over about 2.7kg (6 lbs)/ream produce farming efficiencies approaching and even in excess of 90%. It should be noted that Figure 9 combines data obtained from both commercial and pilot scale equipment, the results from commercial scale equipment being indicated by the solid line and the pilot scale by the interrupted line. Experience shows that results obtained on the pilot scale equipment at lower size press weights are comparable to those obtained on commercial scale equipment, but that at higher weights, the pilot scale equipment produces plate forming efficiencies which are low. Thus the forming efficiencies in excess of 90% as indicated in Figure 9 may be the result of the effect of the sizing adhesive on the tensile stiffness properties of the board, forming efficiency being calculated on the basis of properties of unsized board before forming. Press drying effects may also lead to improvements in the properties of the board. Figure 10 reports the tensile stiffness of the board in pounds per inch of width as a function of the amount of sizing adhesive applied. It can be seen that actual tensile stiffness of the board is increased by less than about 20% as the amount of sizing adhesive is increased from 0 to about 9.1kg (20 lbs)/ream. Figure 11 illustrates the effect of the amount of sizing adhesive applied on plate rigidity in grams measured in accordance with the Single Service Institute procedure for measurement of plate rigidity. It shows as well the relative contribution made by better pleat bonding as compared to the calculated contribution due to increased tensile stiffness of the boardstock. Figure 11 dramatically illustrates the point that the increase in plate rigidity cannot be explained based upon the change in the strength properties of the boardstock due to the increased amount of sizing adhesives used in the practice of the present invention. Figure 12 illustrates the calculated percentage increase in plate rigidity attributable to the effect of the amount of sizing adhesive applied on the tensile properties of the board neglecting the improvement in pleat bonding as compared to the percentage increase in plate rigidity attributable to the effect of improved pleat bonding and thus shows the relative contribution made by better pleat bonding as compared to the calculated contribution due to increased tensile stiffness of the board as determined using a finite element technique. The calculated effect of each was based upon correlation derived from a large number of experiments in which the effects of variation in each were observed.

Claims: A disposable container (10) formed of pleated paperboard material, having a rim portion (13) and a depressed base portion (12), characterised in that the paperboard material comprises sizing adhesive equivalent to at least 2.7 kg (6lbs) of starch per 557m 2 (3000ft 2 ) ream, the sizing adhesive bonding the pleated material. A container as defined in claim 1, characterized in that said container is a plate, bowl, tray or other disposable shallow food service container integrally formed of paperboard stock material and comprising: a substantially planer inner region (12); a sidewall region (11) flaring upwardly and outwardly from a periphery of said planer inner region; an outwardly flaring rim region (13); and a plurality of substantially radially extending pleats (50) formed in and extending through said sidewall and said rim region; A container as defined in any of Claims 1 or 2, wherein the sizing adhesive is equivalent to between 2.7 kg and 9.1 kg (6 and 20lbs) of starch per 557m 2 (3000ft 2 ) ream. A container as claimed in Claim 2: wherein the rim portion (13) extends outwardly from the top of the sidewall region (11), the rim region extending outwardly and downwardly at an angle between about 6° to about 12° relative to the plane of the base portion; and a lip portion (26) extends outwardly and downwardly from the rim portion at an angle greater than about 45° to said plane. A container as defined in Claim 4, wherein the sidewall region (11) is joined to the base portion and to the rim portion by respective blending portions. A container, as claimed in Claim 2 wherein the sidewall region comprises: a generally annular region (14) flaring upwardly and outwardly from a periphery of said planar inner region and a first frusto-conical region (16) adjoining said annular region, said first frusto-conical region sloping outwardly and upwardly from said annular region; the rim region including an outwardly flaring arcuate annular region (18) adjoining an outer periphery of said first frusto-conical region (16) having a first portion (20) thereof extending generally upwardly from said first frusto-conical region and a second portion (22) thereof flaring generally downwardly at an angle between about 6° to about 12° relative to a plane defined by said planar inner region, a second frusto-conical (24) region extending downwardly and outwardly from said second portion (22) of said arcuate annular region at an angle of about 6° to about 12° relative to a plane defined by said planar inner region, and a lip portion (26) extending outwardly and downwardly from said second frusto-conical region at an angle greater than 45° with respect to said plane. A container as defined in Claim 6, wherein said second portion (22) of said outwardly flaring arcuate annular region flares generally downwardly at an angle of between approximately 6° and approximately 10.5.°

Inventors: CHESHIRE JAMES O, GARNS DENNY, LITTLEJOHN MARK B, SANDSTROM ERLAND R, CHESHIRE, JAMES O., GARNS, DENNY, LITTLEJOHN, MARK B., SANDSTROM, ERLAND R.
Applicants: FORT JAMES CORP, FORT JAMES CORPORATION
Application year: 1993
Main classification: B65D1
IPC codes: A47G19, B65D1