Patent Abstract:
A thermoformed disposable food container having a wall caliper of from about 10 to about 80 mils consisting essentially of from about 30 to about 80 percent by weight of a matrix polymer composition consisting predominantly of a polypropylene polymer and optionally including a polyethylene polymer, from about 10 to about 50 percent mica, from about 2.5 to about 25 percent calcium carbonate, and up to about 5 weight percent titanium dioxide, exhibits enhanced rigidity when the calcium carbonate has a mean particle size of less than about 8 microns. The extrudable compositions are likewise useful for film, sheet and injection molding applications.

Full Description:
CLAIM FOR PRIORITY 
     This non-provisional application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/366,790, of the same title, filed Mar. 22, 2002. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to disposable plates, bowls, platters and the like and more particularly to plastic articles of this class produced by thermoforming. The articles of the invention are thermoformed from extruded plastic sheet stock which is mineral-filled with a combination of mica and calcium carbonate. 
     BACKGROUND 
     Disposable food containers are well known in the art. Typically, such containers are made from paper or plastic. 
     Pressed paperboard containers may be made as noted in one or more of U.S. Pat. No. 4,606,496 entitled “Rigid Paperboard Container” of R. P. Marx et al; U.S. Pat. No. 4,609,140 entitled “Rigid Paperboard Container and Method and Apparatus for Producing Same” of G. J. Van Handel et al; U.S. Pat. No. 4,721,499 entitled “Method of Producing a Rigid Paperboard Container” of R. P. Marx et al; U.S. Pat. No. 4,721,500 entitled “Method of Forming a Rigid Paper-Board Container” of G. J. Van Handel et al; and U.S. Pat. No. 5,203,491 entitled “Bake-In Press-Formed Container” of R. P. Marx et al. Equipment and methods for making paperboard containers are also disclosed in U.S. Pat. No. 4,781,566 entitled “Apparatus and Related Method for Aligning Irregular Blanks Relative to a Die Half” of A. F. Rossi et al; U.S. Pat. No. 4,832,676 entitled “Method and Apparatus for Forming Paperboard Containers” of A. D. Johns et al; and U.S. Pat. No. 5,249,946 entitled “Plate Forming Die Set” of R. P. Marx et al. 
     Thermoformed plastic containers, particularly polypropylene mica-filled containers with a micronodular surface are disclosed in U.S. Pat. No. 6,100,512 to Neculescu et al. Such containers have the advantages that they are durable and may be washed and re-used if so desired and are microwaveable. The disclosure of the foregoing patents is incorporated by reference. 
     A drawback of plastic thermoformed containers is that they tend to be more costly than their paper counterparts due, in part, to material costs. More rigid materials can be used more sparingly and are thus highly desirable in the field. One way to increase rigidity of polypropylene containers is to use a filler such as mica as disclosed in the &#39;512 patent. However, mica tends to interact with polypropylene to produce undesirable odors, believed to be caused by certain organic ketone compounds generated during melt-processing of the material. The generation of odors is minimized by including a basic inorganic compound, such as calcium carbonate in the composition. 
     It has been unexpectedly found that fine grades of calcium carbonate can greatly increase the rigidity of polypropylene containers filled with calcium carbonate and mica as described hereinafter. 
     SUMMARY OF INVENTION 
     Disposable polypropylene containers made from extruded polypropylene mica and calcium carbonate filled sheet exhibits enhanced rigidity when fine grades of calcium carbonate are used. An added advantage is that the sheet filled with finer calcium carbonate exhibits less die lip buildup as it is produced. For 11″ plates, normalized rigidity was observed to increase from 11 g/g to 12.8 g/g when the mean particle size of calcium carbonate used was changed from 12 to 6 microns. A further increase to 13.63 g/g of normalized rigidity was achieved when calcium carbonate with a mean particle size of 1 micron was used. 
     There is thus provided in accordance with the present invention a thermoformed disposable food container having a wall caliper of from about 10 to about 80 mils consisting essentially of from about 30 to about 80 percent by weight of a matrix polymer composition consisting predominantly of a polypropylene polymer and optionally including a polyethylene polymer, from about 10 to about 50 percent mica, from about 2.5 to about 25 percent calcium carbonate, and up to about 5 weight percent titanium dioxide, wherein the calcium carbonate has a mean particle size of less than about 8 microns. More preferably the calcium carbonate has a mean particle size of 6 microns or less; yet more preferably less than about 5 microns, and still more preferably the calcium carbonate has a mean particle size of less than about 3 or 2.5 microns. In some preferred embodiments, the calcium carbonate has a mean particle size of about 1 micron or less. 
     Particularly preferred embodiments include those wherein the mean particle size of the calcium carbonate is about 6 microns; those in which the mean particle size of the calcium carbonate is about 3 microns; and those wherein the mean particle size of the calcium carbonate is about 1 micron. 
     In other aspects of the invention, there are provided extrudable and injection moldable compositions having the same components in like proportions as the disposable food containers. These compositions may be in the form of pellets, for example, or extruded into sheet or film form or injection molded into useful articles. Here again, the compositions thus have a calcium carbonate content of from about 2.5 weight percent to about 25 weight percent calcium carbonate and so forth. Particularly preferred compositions include those wherein the calcium carbonate has a mean particle size of 6 microns; those wherein the calcium carbonate has a mean particle size of 3 microns; and those wherein the calcium carbonate has a mean particle size of about 1 micron. 
     Typically, the calcium carbonate is present in an amount of from about 5 to about 15 percent by weight of the container, whereas mica is present in an amount of from about 20 to about 40 percent by weight of the container. The matrix polymer composition generally consists of a polypropylene polymer and a polyethylene polymer in preferred cases. The polyethylene polymer may be present in an amount of from about 1 to about 15 percent by weight of the container, typically present in an amount of from 2.5 to about 7.5 percent by weight. A preferred polyethylene polymer is HDPE. 
     The polypropylene polymer is typically present in an amount of from about 40 to about 60 percent by weight of the container, and may be isotactic polypropylene. Optionally included is titanium dioxide typically in an amount of from about 0.5 to about 4 percent by weight of the container. 
     The containers may have a wall caliper of from about 10 to about 50 mils, typically from about 15 to about 25 mils. 
     These and other aspects of the invention will be further appreciated from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention is described in detail below with reference to the various drawings. In the drawings: 
         FIG. 1  is a view in perspective of a plate constructed in accordance with the present invention; 
         FIG. 2  is a view in cross-section and elevation of the plate of  FIG. 1  illustrating the profile of the plate; 
         FIG. 3  is a schematic diagram illustrating the profile of the plate of  FIGS. 1 and 2 . 
         FIG. 4  is a plot of SSI Rigidity versus product weight for 11″ mica-filled polypropylene plates containing 1 micron and 12 micron mean particle size calcium carbonate showing numerous runs; and 
         FIG. 5  is a plot of average SSI Rigidity versus average product weight for 11″ mica-filled polypropylene plates containing particles from different lots of 1 micron mean particle size calcium carbonate and 12 micron mean particle size calcium carbonate. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is described in detail below with reference to the figures. Such description is for purposes of illustration only and is not limitative of the invention in any way. 
     Numerous modifications within the spirit and scope of the present invention, set forth in the appended claims, will be readily apparent to those of skill in the art. 
     Generally speaking, the present invention is directed to the discovery that using calcium carbonate having a mean particle size of less than 12 microns is beneficial when making polypropylene food containers from polypropylene sheet filled with mica and calcium carbonate. The smaller particle size calcium carbonate has a beneficial effect on container rigidity making it possible to use less material for a given container and also appears to reduce die lip build up during sheet extrusion. A composition with six (6) micron mean size calcium carbonate extruded into sheet extruded well with less die lip build up (about {fraction (5/16)}″ vs. about ⅜″) than a composition with twelve (12) micron mean particle size calcium carbonate. Still less die lip build up (about {fraction (3/16)}″) was observed when a calcium carbonate having a mean particle size of one (1) micron was used in corresponding compositions. 
     Containers formed from the sheet had the properties summarized in Table 1 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 General Observations 
               
               
                   
               
             
             
               
                 Properties of 11″ thermoformed plates made from sheet composed of: 
               
             
          
           
               
                   
                 Polypropylene 
                 52% 
               
               
                   
                 Mica 
                 30% 
               
               
                   
                 Calcium Carbonate 
                 10% 
               
               
                   
                 HDPE 
                  5% 
               
               
                   
                 TIO 2  + color 
                  3% 
               
               
                   
                   
               
             
          
           
               
                   
                 Case 1 
                 Case 2 
                 Case 3 
               
               
                   
                   
               
             
          
           
               
                   
                 Product Weight 
                 33.4 
                 32.5 
                 33.15 
               
               
                   
                 Calcium Carbonate 
                 12 
                 6 
                 1 
               
               
                   
                 Particle Size (Microns) 
               
               
                   
                 GM SSI Rigidity 
                 367 
                 416 
                 452 
               
               
                   
                 Normalized Rigidity g/g 
                 11.0 
                 12.8 
                 13.63 
               
               
                   
                   
               
             
          
         
       
     
     As can be seen, product rigidity increases markedly as the particle size of the calcium carbonate is reduced. This discovery makes it possible to make a more rigid product with the same amount of material or maintain a target rigidity while reducing material consumption. 
     The foregoing is described and illustrated further below. 
     Test Methods, Definitions and Materials 
     SSI Rigidity is measured with the Single Service Institute Plate Rigidity Tester of the type originally available through Single Service Institute, 1025 Connecticut Ave., N.W., Washington, D.C. The SSI Rigidity test apparatus has been manufactured and sold through Sherwood Tool, Inc. Kensington, Conn. This test is designed to measure the rigidity (i.e., resistance to buckling and bending) of paper and plastic plates, bowls, dishes, and trays by measuring the force required to deflect the rim of these products a distance of 0.5 inch while the product is supported at its geometric center. Specifically, the plate specimen is restrained by an adjustable bar on one side and is center supported. The rim or flange side opposite to the restrained side is subjected to 0.5 inch deflection by means of a motorized cam assembly equipped with a load cell, and the force (grams) is recorded. The test simulates in many respects the performance of a container as it is held in the hand of a consumer, supporting the weight of the container&#39;s contents. SSI Rigidity is expressed as grams per 0.5 inch deflection. A higher SSI value is desirable since this indicates a more rigid product. All measurements were done at standard TAPPI conditions for paperboard testing, 72° F. and 50% relative humidity. Geometric mean averages for the machine direction (MD) and cross machine direction (CD) are reported herein. 
     The particular apparatus employed for SSI Rigidity measurements was a Model No. ML-4431-2 SSI Rigidity tester as modified by Georgia Pacific Corporation, National Quality Assurance Lab, Lehigh Valley Plant, Easton, Pa. 18040 using a Chatillon gauge available from Chatillon, Force Measurements Division, P.O. Box 35668, Greensboro, N.C. 27425-5668. 
     Unless otherwise specified, the following terms have the following meanings: 
     “Rigidity” refers to SSI Rigidity (kilograms or grams/0.5 inches). 
     “Sheet”, “sheet stock” and the like refers to both a web or roll of material and to material that is cut into sheet form for processing. 
     Particle size refers to mean particle size. 
     Mean particle size of a particulate material such as calcium carbonate is the particle diameter as to which 50 percent by weight of the particles of the particulate material have a smaller diameter. This quantity may be determined by any suitable technique. 
     Unless otherwise indicated, “mil”, “mils” and like terminology refers to thousandths of an inch and dimensions appear in inches. Likewise, caliper is the thickness of material and is expressed in mils unless otherwise specified. 
     The term major component, predominant component and the like refers to a component making up at least about 50% of a composition or that class of compound in the composition by weight as the context indicates; for example, a filler is the predominant filler in a filled plastic composition if it makes up more than about 50% by weight of the filler in the composition based on the combined weight of fillers in the composition, and a resin is the predominant resin in a composition if it makes up more than 50 percent of the resin in the composition. 
     Basis weights appear in lbs per 3000 square foot ream unless otherwise indicated. 
     Percents refer to weight percents. 
     Polypropylene polymers which are suitable are preferably selected from the group consisting of isotactic polypropylene, and copolymers of propylene and ethylene wherein the ethylene moiety is less than about 10% of the units making up the polymer, and mixtures thereof. Generally, such polymers have a melt flow index from about 0.3 to about 4, but most preferably the polymer is isotactic polypropylene with a melt-flow index of about 1.5. 
     A polyethylene polymer or component may be any suitable polyethylene such as HDPE, LDPE, MDPE, LLDPE or mixtures thereof and may be melt-blended with polypropylene if so desired. The various polyethylene polymers referred to herein are described at length in the  Encyclopedia of polymer Science  &amp;  Engineering  (2d Ed.), Vol. 6; pp: 383-522, Wiley 1986; the disclosure of which is incorporated herein by reference. HDPE refers to high density polyethylene which is substantially linear and has a density of generally greater that 0.94 up to about 0.97 g/cc. LDPE refers to low density polyethylene which is characterized by relatively long chain branching and a density of about 0.912 to about 0.925 g/cc. LLDPE or linear low density polyethylene is characterized by short chain branching and a density of from about 0.92 to about 0.94 g/cc. Finally, intermediate density polyethylene (MDPE) is characterized by relatively low branching and a density of from about 0.925 to about 0.94 g/cc. 
     “Thermoforming”, “thermoformed” and like terminology is given its ordinary meaning. In the simplest form, thermoforming is the draping of a softened sheet over a shaped mold. In the more advanced form, thermoforming is the automatic high speed positioning of a sheet having an accurately controlled temperature into a pneumatically actuated forming station whereby the article&#39;s shape is defined by the mold, followed by trimming and regrind collection as is well known in the art. Still other alternative arrangements include the use of drape, vacuum, pressure, free blowing, matched die, billow drape, vacuum snap-back, billow vacuum, plug assist vacuum, reverse draw with plug assist, pressure bubble immersion, trapped sheet, slip, diaphragm, twin-sheet cut sheet, twin-sheet roll-fed forming or any suitable combinations of the above. Details are provided in J. L. Throne&#39;s book,  Thermoforming , published in 1987 by Coulthard. Pages 21 through 29 of that book are incorporated herein by reference. Suitable alternate arrangements also include a pillow forming technique which creates a positive air pressure between two heat softened sheets to inflate them against a clamped male/female mold system to produce a hollow product. Metal molds are etched with patterns ranging from fine to coarse in order to simulate a natural or grain like texturized look. Suitable formed articles are trimmed in line with a cutting die and regrind is optionally reused since the material is thermoplastic in nature. Other arrangements for productivity enhancements include the simultaneous forming of multiple articles with multiple dies in order to maximize throughput and minimize scrap. In some preferred embodiments, the melt-compounded composition from which the articles are made may include polypropylene and optionally further includes a polyethylene component and titanium dioxide. Suitable materials and techniques for fabricating the disposable containers of the present invention from thermoplastic materials appear in U.S. Pat. No. 6,211,501 to McCarthy et al. as well as U.S. Pat. No. 6,211,500 to Cochran II et al. the disclosures of which are incorporated herein by reference. 
     Preferred Embodiments 
     In general, products of the invention are made by first extruding a polypropylene sheet of suitable composition as described in the &#39;500 and &#39;501 patents followed by thermoforming the sheet as is also described in the &#39;500 and &#39;501 patents. A suitable container shape is that described in U.S. Co-Pending application Ser. No. 09/603,579, filed Jun. 26, 2000, entitled “Smooth Profiled Food Service Articles”. These plates have the characteristics seen in  FIGS. 1-3  below and in Tables 2-4. 
     Illustrated in  FIGS. 1 through 3 , there is a plate  10  which includes a planar center  12  which, in turn, includes an outer peripheral surface  14 . This center region  12  may have a slight convex crown to improve plate stability during use. The planar center  12  forms a bottom for the plate  10 . An outwardly projecting sidewall  16  includes a first rim portion  18  which is joined to the outer peripheral surface  14  of the planar center  12 . A second rim portion  20  is joined to the first rim portion  18 . The first rim portion  18  and the second rim portion  20  form the outwardly projecting sidewall  16  which forms the sidewall of the plate  10 . A rim  22  includes a third rim portion  24  which is joined to the second rim portion  20  of the outwardly projecting sidewall  16 . A fourth rim portion  26  is joined to the third rim portion  24 . The fourth rim portion  26  forms the outer edge of the plate  10 . 
       FIG. 3  illustrates a partial cross-sectional view of a plate, diameter D, according to the present invention. The plate  10  defines a center line  34 . A base or bottom-forming portion  30  extends from the center line  34  to an outer peripheral portion  32 . 
     From the center line  34  a predetermined distance X 12  extends toward the outer peripheral surface forming portion  32 . A distance Y 12  extends a predetermined distance from the base or bottom-forming portion  30  upwardly therefrom. A radius R 12  extends from the intersection point of the distance X 12  and Y 12  to form a first rim portion  36  of the outwardly projecting sidewall  35 . The first rim portion  36  is defined by an arc A 12  which extends from a substantially vertical line defined at an outer peripheral point  37  to a fixed point  40 . The arc A 12  may be approximately 60°. 
     A distance X 22  extends from the center line  34  to a predetermined point. A distance Y 22  extends from the base or bottom-forming portion  30  of the plate  10  downwardly a predetermined distance. A radius R 22  extends from the intersection of the lines X 22  and Y 22  to define the radius of curvature of a second rim portion  38  of the sidewall  35 . The radius R 22  extends from the first fixed point  40  to the second fixed point  42  through an arc A 22 . The arc A 22  may be approximately 4°. 
     A distance X 32  extends from the center line  34  to a predetermined distance. A distance Y 32  extends from the base or bottom-forming section  30  of the plate  10  to project upwardly a predetermined distance. A radius R 32  extends from the intersection of the lines X 32  and Y 32  which is the radius of the third rim portion  44  of the rim  46 . The radius R 32  extends from the second fixed point  42  to a third fixed point  48 . An arc A 32  is formed between the second fixed point  44  and the third fixed point  48  to extend a predetermined distance. The arc A 32  may be approximately 55°. 
     A distance X 42  extends a predetermined distance from the center line  34 . Similarly, a distance Y 42  extends from the base or bottom-forming section  30  of the plate  10  to project upwardly. A radius R 42  extends from the intersection of the lines X 42  and Y 42  to define the radius of curvature of a fourth rim portion  47  of the rim  46 . An arc A 42  is formed between the third fixed point  48  and a fourth fixed point  50  at diameter D from the center line. The arc A 42  may be approximately 60°. A section disposed at  50  forms the outer edge of the plate. 
     The article made according to the present invention may have any particular size or shape. In various embodiments of the present invention the container may be a 9″ or 11″ plate with profile coordinates as illustrated in  FIGS. 1 through 3  having the dimensions, angles, or relative dimensions enumerated in Tables 2 through 4. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Dimensions and Angles For 9″ Plate 
               
             
          
           
               
                   
                 DIMENSION and ANGLES 
                 VALUE (inches or degrees) 
               
               
                   
                   
               
               
                   
                 R12 
                 0.537 
               
               
                   
                 X12 
                 3.156 
               
               
                   
                 Y12 
                 0.537 
               
               
                   
                 R22 
                 2.057 
               
               
                   
                 X22 
                 5.402 
               
               
                   
                 Y22 
                 0.760 
               
               
                   
                 R32 
                 0.564 
               
               
                   
                 X32 
                 4.167 
               
               
                   
                 Y32 
                 0.079 
               
               
                   
                 R42 
                 0.385 
               
               
                   
                 X42 
                 4.167 
               
               
                   
                 Y42 
                 0.258 
               
               
                   
                 A12 
                 60.00°  
               
               
                   
                 A22 
                 4.19° 
               
               
                   
                 A32 
                 55.81°  
               
               
                   
                 A42 
                 60.00°  
               
               
                   
                 D 
                 9.00  
               
               
                   
                 BOTTOM CONVEX CROWN 
                 0.06  
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Dimensions and Angles For 11′ PLATE 
               
             
          
           
               
                   
                 DIMENSION/ANGLES 
                 VALUE (inches or degrees) 
               
               
                   
                   
               
               
                   
                 R12 
                 0.656 
               
               
                   
                 X12 
                 3.857 
               
               
                   
                 Y12 
                 0.656 
               
               
                   
                 R22 
                 2.514 
               
               
                   
                 X22 
                 6.602 
               
               
                   
                 Y22 
                 0.929 
               
               
                   
                 R32 
                 0.689 
               
               
                   
                 X32 
                 5.093 
               
               
                   
                 Y32 
                 0.097 
               
               
                   
                 R42 
                 0.470 
               
               
                   
                 X42 
                 5.093 
               
               
                   
                 Y42 
                 0.315 
               
               
                   
                 A12 
                 60.00°  
               
               
                   
                 A22 
                 4.19° 
               
               
                   
                 A32 
                 55.81°  
               
               
                   
                 A42 
                 60.00°  
               
               
                   
                 D 
                 11.00  
               
               
                   
                 BOTTOM CONVEX CROWN 
                 0.06  
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Dimensions For 9″ and 11″ PLATES 
               
             
          
           
               
                 DIMENSION 
                   
               
               
                 RATIO OR 
                 VALUES (Dimensionless or degrees) 
               
             
          
           
               
                 ANGLE 
                 PREFERRED 
                 MINIMUM 
                 MAXIMUM 
               
               
                   
               
               
                 R12/D 
                 0.060 
                 0.045 
                 0.075 
               
               
                 X12/D 
                 0.351 
                 0.280 
                 0.420 
               
               
                 Y12/D 
                 0.060 
                 0.045 
                 0.075 
               
               
                 R22/D 
                 0.228 
                 0.180 
                 0.275 
               
               
                 X22/D 
                 0.600 
                 0.480 
                 0.720 
               
               
                 Y22/D 
                 0.084 
                 0.065 
                 0.100 
               
               
                 R32/D 
                 0.063 
                 0.050 
                 0.075 
               
               
                 X32/D 
                 0.463 
                 0.370 
                 0.555 
               
               
                 Y32/D 
                 0.009 
                 0.007 
                 0.011 
               
               
                 R42/D 
                 0.043 
                 0.034 
                 0.052 
               
               
                 X42/D 
                 0.463 
                 0.370 
                 0.555 
               
               
                 Y42/D 
                 0.029 
                 0.023 
                 0.035 
               
               
                 A12 
                 60.00°  
                 55.00°  
                 75.00°  
               
               
                 A22 
                 4.19° 
                 1.00° 
                 10.00°  
               
               
                 A32 
                 55.81°  
                 45.00°  
                 75.00°  
               
               
                 A42 
                 60.00°  
                 45.00°  
                 75.00°  
               
               
                   
               
             
          
         
       
     
     Salient features of the plate illustrated in  FIGS. 1 through 3  generally include a substantially planar center portion (which may be crowned as noted above and illustrated throughout the various figures) with four adjacent rim portions extending outwardly therefrom, each rim portion defining a radius of curvature as set forth above and further noted below. The first rim portion extends outwardly from the planar center portion and is convex upwardly as shown. There is defined by the plate a first arc A 12  with a first radius of curvature R 12  wherein the arc has a length S 1 . A second rim portion is joined to the first rim portion and is downwardly convex, defining a second arc A 22 , with a radius of curvature R 22  and a length S 2 . A third, downwardly convex, rim portion is joined to the second rim portion and defines another arc A 32 . There is defined a third radius of curvature R 32  and a third arc length S 3 . A tangent to the third arc at the upper portion thereof is substantially parallel to the planer center portion as shown in  FIG. 2. A  fourth rim portion is joined to the third rim portion, which is also downwardly convex. The fourth rim portion defines a fourth arc A 42  with a length S 4 , with a radius of curvature R 42 . 
     The length of the second arc, S 2  is generally less the length of the fourth arc S 4 , which, in turn, is less than the length S 1  of the first arc A 12 . The radius of curvature R 42  of the fourth arc is less than the radius of curvature R 32  of the third rim portion, which in turn, is less than radius of curvature R 22  of the second rim portion. The angle of the first arc, A 12  is generally greater that about 55 degrees, while, the angle of the third arc, A 32  is generally greater than about 45 degrees as is set forth in the foregoing tables. The angle of the fourth arc A 42  is generally less than about 75 degrees and more preferably is about 60 degrees. 
     Typically, the length S 1  of arc A 12  is equivalent to the length S 3  of arc A 32  and R 12  of the first rim portion is equivalent in length to the radius of curvature R 32  of the third rim portion. 
     Generally speaking, the height of the center of curvature of the first arc (that is the origin of ray R 12 ) above the central planar portion is substantially less than, perhaps twenty five percent or so less than, the distance that the center of curvature of the second rim portion (the origin of ray R 22 ) is below the central planar portion. In other words, the length Y 12  is about 0.75 times or less the length Y 22 . 
     So also, the horizontal displacement of the center of curvature of the second rim portion from the center of curvature of the first rim portion is at least about twice the length of the first radius of curvature R 12 . The height of the center of curvature of the third rim portion above the central planar portion is generally less than the height of the center of curvature of the fourth rim portion above the plane of the central planar portion. The horizontal displacement of the center of curvature of the second rim portion is generally outwardly disposed from the center of curvature of the third and fourth rim portions. A further noteworthy feature of the plate of  FIGS. 1 through 3  is that the height of the center of curvature of the third rim portion above the planar central portion is less than about 0.3 times the radius of curvature R 42  of the fourth rim portion; while the height of the center of curvature of the fourth rim portion above the plane of the central portion is at least about 0.4 times the first radius of curvature R 12 . 
     Specific Examples 
     A series of 11″ plates described generally above were thermoformed from extruded sheet having the following composition: 
                                                           Component   Wt. Percent                                        Polypropylene   52           Mica   30           Calcium Carbonate   10           HDPE   5           TiO 2  + color   3                        
Basis weights of sheet material used were from 255 to 315 lbs/3000 square foot ream and 3 different types of particulate calcium carbonate were used: 12 micron mean particle size material, 1 micron mean particle size material (Lot A) and another 1 micron mean particle size material (Lot B). The one micron material is available from Imerys as supermite calcium carbonate. Six micron material, available from Omya, called Omya 5, has an average particle size of about 6 microns and may likewise be employed. The mica may have a mean particle size of 50 microns or so.
 
     Results are summarized in Table 5 below and appear graphically in  FIGS. 4 and 5  which are plots of SSI Rigidity versus product weights, that is, the weight of the plate. As can be seen, products made with the 1 micron calcium carbonate exhibited consistently higher rigidity levels at all weights, whereas products with the Lot A 1 micron mean size material exhibited a remarkable increase in rigidity at all weights tested. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 SSI Rigidity for 11″ Thermoformed Mica/Calcium Carbonate-Filled 
               
               
                 Polypropylene Plates 
               
             
          
           
               
                   
                   
                   
                 Average 
                   
               
               
                   
                 Nominal 
                 Mean CaCO 3   
                 Product 
                 Average SSI 
               
               
                 Example 
                 Basis 
                 Particle Size 
                 Weight 
                 GM Rigidity 
               
               
                 Series 
                 Weight (lbs) 
                 (Microns)/Lot 
                 (grams) 
                 (grams) 
               
               
                   
               
             
          
           
               
                 A 
                 315 
                 12 
                 33.4 
                 367 
               
               
                 1 
                 315 
                 1/A 
                 33.15 
                 452 
               
               
                 2 
                 315 
                 1/B 
                 33.7 
                 409 
               
               
                 B 
                 295 
                 12 
                 31.5 
                 330 
               
               
                 3 
                 295 
                 1/B 
                 31 
                 337 
               
               
                 C 
                 275 
                 12 
                 28.9 
                 270 
               
               
                 4 
                 275 
                 1/B 
                 28.6 
                 286 
               
               
                 D 
                 255 
                 12 
                 27.1 
                 237 
               
               
                 5 
                 255 
                 1/A 
                 26.6 
                 280 
               
               
                   
               
             
          
         
       
     
     The invention has been described in detail hereinabove in connection with numerous embodiments. That discussion is not intended to limit in any way the scope of the present invention which is defined in the appended claims. It will be readily appreciated by one of skill in the art that the particular embodiments illustrated may be scaled up or down in size with the relative proportions shown herein or that product shapes such as square or rectangular with rounded corners, triangular, multi-sided, oval platters, polygonal platters with rounded corners and the like may be formed in accordance with the present invention. Typical products include plates, bowls, trays, deep dish containers, platters and so forth.

Technology Classification (CPC): 8