FIELD OF THE INVENTION  
         [0001]    This invention relates to thin walled polyethylene containers. The containers are useful for packaging foods such as cottage cheese and ice cream.  
         BACKGROUND OF THE INVENTION  
         [0002]    Plastic food containers are ubiquitous items of commerce. Ideally, these containers should have thin walls (preferably from about 0.35 millimeters to 1.30 millimeters thick) in order to reduce the amount of plastic used to produce the container. However, the containers must also have strength at high temperatures (for example, to permit a container to be filled with ricotta cheese at temperatures over 80° C.) and at low temperatures (so as to withstand the impact when a filled ice cream container is dropped). Such “thinwalled” containers are typically prepared by injection molding.  
           [0003]    Injection molding equipment is widely available and is well described in the literature. The machinery is highly productive, with molding cycle times often being measured in seconds. These machines are also very expensive so there is a need to maximize productivity (i.e. minimize cycle times) in order to control overall production costs. Productivity may be influenced by the choice of plastic resin used in the process. In particular, a resin which flows well is desirable to reduce cycle times.  
           [0004]    Flow properties are typically influenced by molecular weight (with low molecular weight resin having superior flow properties in comparison to high molecular weight resin) and molecular weight distribution (with narrow molecular weight resins generally producing molded parts with reduced warpage in comparison to broad molecular weight distribution resins). Copolymer resins of similar molecular weight and molecular weight distribution generally have higher hexane extractables levels than homopolymer resins, making them less satisfactory for food applications.  
           [0005]    The strength of the finished product over a range of temperatures is also important. The strength of a finished product may often be increased by increasing the molecular weight of the resin used to prepare it, but this is done at the expense of machine productivity. Similarly, the use of a copolymer resin will often improve the impact strength and flexibility of a product in comparison to the use of homopolymer, but at the expense of extractables content. Thus, a suitable food container which is made at high “machine productivity” yet also demonstrates good strength properties at high and low temperatures would be a useful addition to the art.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a container having a nominal volume of from 100 mL to 12 L which is prepared by injection molding of ethylene copolymer resin, said container having a Vicat softening point of greater than 121° C. and an average test drop height point of greater than 2.5 feet as determined by ASTM D5276 wherein said ethylene copolymer resin is characterized by having:  
           [0007]    i) a density of from 0.950 g/cc to 0.955 g/cc;  
           [0008]    ii) a viscosity at 100,000 s−1 and 280° C. of less than 3.5 Pascal seconds;  
           [0009]    iii) a molecular weight distribution, Mw/Mn of from 2.2 to 2.8; and  
           [0010]    iv) a hexane extractables content of less than 0.5 weight %.  
           [0011]    Preferred containers also have a total impact energy required for base failure of greater than 0.2 foot-pounds at −20° C. as determined by Instrumented Impact Testing according to ASTM D3763 (with an instrument sold under the tradename “INSTRON-DYNATUP”).  
         DETAILED DESCRIPTION  
         [0012]    We have discovered that thinwalled polyethylene containers having a Vicat softening point of greater than 121° C. and an average test drop height of greater than 2.5 feet may be prepared from a linear polyethylene copolymer resin having all of the following essential characteristics:  
           [0013]    1) a density of from 0.950 to 0.955 g/cc;  
           [0014]    2) a melt index I 2 , of from 30 to 100 g/10 min as measured by ASTM D1238 at 190° C.;  
           [0015]    3) a molecular weight distribution (Mw/Mn) of from 2.2 to 2.8;  
           [0016]    4) an apparent viscosity at 100,000 s−1 and 280° C. of less than 3.5 Pascal seconds; and  
           [0017]    5) a hexane extractables content of less than 0.5 weight %. Each of these characteristics is described below.  
           [0018]    The density of a polyethylene copolymer is influenced by the molecular structure of the copolymer. “Linear” homopolymers of ethylene are rigid molecules that solidify as crystalline resins. Linear ethylene resins which also have a narrow molecular weight distribution (Mw/Mn, discussed below) are further characterized by having sharp (distinct) melting points, which is desirable for injection molding processes. However, the impact strength of such resins (especially at low temperatures) is poor. The density of a linear ethylene homopolymer having a narrow molecular weight distribution is generally greater than 0.958 grams per cubic centimeter (“g/cc”).  
           [0019]    The density of a linear ethylene polymer may be reduced by incorporating a comonomer (such as butene, hexene, or octene) into the polymer structure. The comonomers produce “branches” which inhibit crystal packing and the resulting copolymers generally display improved impact strengths in comparison to homopolymers. For example, flexible polyethylene films (not a part of this invention) are typically made from copolymers having more than 8 mole % comonomer and a density from about 0.905 to 0.935 g/cc.  
           [0020]    The copolymer used in this invention contains a comparatively small but critical amount of comonomer. The linear ethylene copolymers must have a density of from 0.950 to 0.955 g/cc. This very specific and narrow density range is essential in order to obtain high machine productivity and high strength containers. For the purpose of this invention, the density of the resin is determined according to ASTM standard test procedure D792.  
           [0021]    The melt index (I 2 , as determined by ASTM D1238) of the resins used to prepare the container of this invention must be from 30 to 100 g/10 min. The preferred melt index range is from 50 to 90 g/10 min. The melt index of a polyethylene copolymer resin is also established by the molecular structure. Molecular weight is particularly important and is inversely related to melt index I 2 . That is, an increase in molecular weight will generally reduce the ability of the copolymer to flow (and thus cause an decrease in I 2 ). High melt indices (lower molecular weights) are desirable to increase machine productivity but high molecular weight is desirable for strength.  
           [0022]    The ethylene copolymer resins used to prepare the containers of this invention are further characterized by having a molecular weight distribution (as determined by dividing the weight average molecular weight “Mw” by the number average molecular weight “Mn”) of from 2.2 to 2.8.  
           [0023]    Molecular weight determinations (Mw and Mn) are made by high temperature gel permeation chromatography (GPC) using techniques which are well known to those skilled in the art. It will be recognized by those skilled in the art that different GPC equipment and/or analytical techniques sometimes result in slightly different absolute values of weight average molecular weight (Mw) and number average molecular weight (Mn) for a given resin. Therefore, the resin used in this invention is defined by the ratio Mw/Mn.  
           [0024]    We have determined that resins having a Mw/Mn of from 2.2 to 2.8 (and the density, I 2 , viscosity characteristic and hexane extractables level specified for this invention) provide containers having excellent strength and allow very good productivity.  
           [0025]    The present containers are fabricated from ethylene copolymer resin which has apparent viscosity of less than 3.5 Pascal seconds when subjected to a shear rate of 100,000 s −1  at 280° C.  
           [0026]    We have determined that this viscosity range provides strong containers and high machine productivity. Lower viscosity resins typically produce containers having inferior strength properties. Viscosity is measured according to ASTM D3835.  
           [0027]    Finally, this invention uses a resin having a hexane extractables content (as determined by ASTM D5227) of less than 0.5 weight %.  
           [0028]    The containers of this invention must be made from ethylene copolymer resin which satisfies all of the above criteria. Such resin may be prepared using the polymerization catalyst and polymerization process which is described in U.S. Pat. No. 6,372,864 (Brown et al.). Further details of the invention are provided in the following non-limiting examples. 
       
    
    
     EXAMPLES  
       [0029]    Part 1: Test Procedures Used in the Examples  
         [0030]    1. “Instrumented Impact Testing” was completed using a commercially available instrument (sold under the tradename “INSTRON-DYNATUP”) according to ASTM D3763.  
         [0031]    2. Melt Index: 12 and 16 were determined according to ASTM D1238.  
         [0032]    3. Stress exponent is calculated by  
           log        (       I   6     /     I   2       )         log        (   3   )         .                         
 
         [0033]    4. Number average molecular weight (Mn), weight average molecular weight (Mw), z-average molecular weight (Mz) and polydispersity (calculated by Mw/Mn) were determined by high temperature Gel Permeation Chromatography (“GPC”).  
         [0034]    5. Flexural Secant Modulus and Flexural Tangent Modulus were determined according to ASTM D790.  
         [0035]    6. Elongation, Yield and Tensile Secant Modulus measurements were determined according to ASTM D636.  
         [0036]    7. Hexane Extractables were determined according to ASTM D5227.  
         [0037]    8. Densities were determined using the displacement method according to ASTM D792.  
         [0038]    9. “Drop Testing” was completed according to ASTM D5276.  
         [0039]    Part 2: Preparation of an Injection Molded Container  
         [0040]    For the resins in Example 1, containers were prepared using an injection molding apparatus sold under the tradename Husky LX 225 P60/60 E70. The mold used for these samples was a 4-cavity mold making containers with a nominal outside diameter of 4.35 inches (11.0 cm), a thickness of 0.025 inches (0.6 mm) and a volume of 750 mL. Details of the Husky LX 225 P60/60 E70 thin wall injection molding (TWIM) machine are below:  
                                             Husky X 225 P60/50 E70                                    Clamp:   250 tons           Plunger:    50 mm           Screw:    70 mm           Screw L/D Ratio:   25:1           Melt Channel Diameter:    8 mm                      
 
         [0041]    Conventional barrel temperatures for this apparatus typically range from 150 to 300° C. For the resins in Example 1, barrel temperatures ranged from 200 to 250° C., depending on the position in the barrel. Details on temperatures and other molding conditions are tabulated in Example 1.  
         [0042]    Part 3: Preparation of an Injection Molded Lid  
         [0043]    The machine sold under the tradename Husky LX 225 P60/60 E70 was also used for the resins in Example 2. The mold used for these samples was a 6-cavity mold making round lids for the containers produced in Example 1. The lids produced have a nominal outside diameter of 4.68 inches (11.9 cm) and a thickness of 0.04 inches (1.0 mm). Barrel temperatures were cooler than for the resins in Example 1, at 200 to 230° C. Details on temperatures and other molding conditions are tabulated in Example 2.  
       Example 1  
       [0044]    Inventive resins E1 and E2 were characterized and compared to three commercially available resins used in this application (Table 1). E1 is a higher molecular weight, broader molecular weight distribution resin while E2 provides the lowest molecular weight and narrowest molecular weight distribution of the five resins studied. The data in Table 1 were collected using conventional ASTM testing techniques on resin pellets and compression molded plaques.  
                                                                           TABLE 1                           Characterization of Experimental Container Resins E1 &amp; E2 vs.       Benchmarks*                Units   C1   E1   C2   C3   E2                        Density   g/cm 3     0.9493   0.9516   0.9513   0.9536   0.9517       I 2     g/10 min   56   69   73   86   95       I 6     g/10 min   265   268   280   323   352       Stress Exponent       1.43   1.24   1.23   1.21   1.19       I 21     g/10 min   836   838   772   805   834       Melt Flow Ratio       15   12   10.5   9.3   8.81       Viscosity @ 100000 sec −1  @ 250° C.   Pa-sec   3.6   3.9   4.2   3.8   3.9       Viscosity @ 100000 sec −1  @ 280° C.   Pa-sec   3.1   3.4   3.4   3.4   3.4       No. Ave. Mol. Wt. (Mn)   ×10 −3     10.3   13.1   9.8   10.4   13.9       Wt. Ave. Mol. Wt. (Mw)   ×10 −3     40.8   34.6   35.3   34.0   32.3       Z Ave. Mol. Wt. (Mz)   ×10 −3     152.5   75.8   77.2   70.4   59.7       Polydispersity Index       3.96   2.64   3.58   3.27   2.32       Hexane Extractables   %   0.81   0.24   0.78   0.70   0.29       Melting Point   ° C.   126.7   128.9   128.1   128.0   129.0       Crystallinity   %   71.7   75.9   69.3   69.0   81.4       Vicat Softening Point   ° C.   119   124   121   122   124       Shore D Hardness       66.4   67.2   66.3   66.1   67.2       Flex. Sec Modulus, 1%   MPa   934   1128   1177   1208   1161       Flex. Sec Modulus, 2%   MPa   809   994   1024   1058   1010       Flexural Strength   MPa   27.9   35.2   33.1   35.6   35.5       Yield Elongation   %   6   15   7   8   11       Yield Strength   MPa   23.5   26.8   25.3   27.8   26.3       Ultimate Elongation   %   7   24   7   8   12       Ultimate Strength   MPa   23.7   25   25.3   27.8   26.3       Tensile Impact   ft-lb/in 2     9.39   38   23.5   21.7   22.6       Whiteness Index       79.21   91.3   87.58   90   91.38       Yellowness Index       −3.31   −7.23   −6.22   −6.56   −7.04                                                  
 
         [0045]    The data in Table 1 show that the experimental resins provide by far the lowest hexane extractable content, making them suitable for food applications. Their higher crystallinity, Vicat softening point, Shore D hardness and Flexural Modulus suggest their suitability for higher temperature filling and capping operations, (e.g. ricotta cheese). This data set also shows that the experimental resins should provide equivalent toughness and better color in comparison to incumbent products used in this market.  
         [0046]    Container products were produced using the five resins in Table 1. They were produced on the Husky injection molding unit described above using the conditions listed in Table 2.  
                                                                           TABLE 2                           Husky Injection Molding Machine Settings and Variables for Molding       Container Resins                Units   C1   E1   C2   C3   E2                        Resin Specs                               MI   g/10 min   56   69   73   86   95       Density   g/cm 3     0.9493   0.9516   0.9513   0.9536   0.9517       S. Ex.       1.43   1.24   1.23   1.21   1.19       M/C Settings       Fill pressure   %   78   78   78   78   78       High Speed enable start   mm   70   70   70   70   70       High Speed enable stop   mm   30   32   30   34   36       Pullback   mm   12   12   12   12   25       Gate heat   % on   75   75   75   75   50       Barrel temperature Zone 1   ° C.   200   200   200   200   200       Barrel temperature Zone 2   ° C.   210   210   210   210   210       Barrel temperature Zone 3   ° C.   220   220   220   220   220       Barrel temperature Zone 4   ° C.   230   230   230   230   230       Barrel temperature Zone 5   ° C.   250   250   250   250   250       Variables       Shot weight   g   104.09   104.36   104.25   104.47   104.77       Cycle time   sec   5.78   5.80   5.88   5.81   5.80       Injection time   sec   0.36   0.39   0.41   0.39   0.40       Screw run time   sec   2.11   2.03   2.03   2.06   2.07       Screw back pres   psi   245   245   248   254   248       Ext. drive pres   psi   1059   1115   1131   1085   1045       Max. inj. Pres   psi   2236   2219   2230   2217   2205       Hold pressure Zone 1   psi   1088   1087   1087   1088   1088       Hold pressure Zone 2   psi   635   636   637   631   630       Hold pressure Zone 3   psi   301   303   304   302   302       Barrel temperature Zone 1   ° C.   200   200   200   197   200       Barrel temperature Zone 2   ° C.   211   211   211   208   211       Barrel temperature Zone 3   ° C.   221   221   221   221   221       Barrel temperature Zone 4   ° C.   230   230   230   230   230       Barrel temperature Zone 5   ° C.   251   251   251   251   251                  
 
         [0047]    In a conventional injection molding cycle, the molten resin is injected into a closed mold which is water cooled. It is desirable to maximize the productivity of these expensive machines, while also reducing energy requirements. In order to achieve this, the resin must have excellent rheological properties so that the resin flows sufficiently to completely fill the mold.  
         [0048]    Table 2 provides data which show that the resin E2 from Example 1 requires lower pressure to mold a part. As a result, the barrel temperatures may be lowered in order to reduce energy consumption while maintaining cycle time. Conversely, temperatures could be maintained with a reduced cycle time, thus increasing the molding unit&#39;s unit productivity.  
         [0049]    Conventional resins used in thin wall injection molding (TWIM) container applications are typically of medium to high density and also exhibit higher molecular weight than resins used in thin wall injection molding (TWIM) lid applications. The typical tradeoff in these applications is that if a stiffer product is desired, density is increased at the expense of product toughness. Similarly, if better product toughness is desired, the density of the resin is reduced somewhat and molecular weight of the resin is also increased, lowering the melt index and making the resin more difficult to process.  
         [0050]    Extensive physical testing of the containers yielded the data in Table 3. It is clear that in general, the superior properties of the experimental resins predicted in Table 1 follow through to the injection molded parts. What is surprising is that the experimental resins, (while providing equivalent stiffness, as indicated by the retention of density for various positions on the part relative to the maximum density available, i.e. pellet density), also provide enhanced toughness, both at low and ambient temperature. This “decoupling” of the stiffness/toughness balance appears to apply at both lower and higher melt index. This is illustrated by the part drop test data, as defined by ASTM D5276. It shows that the experimental resins provide a pass at nearly twice the height of the incumbent resins.  
                                                                                                                                                                                                                     TABLE 3                           Injection Molded Containers                Units   C1   E1   C2   C3   E2                        Pellet   Density   g/cm 3     0.9493   0.9516   0.9513   0.9536   0.9517           Melt Index I 2     g/10 min   56   69   73   86   95           Melt Index I 6     g/10 min   265   268   280   323   352           Stress Exponent       1.43   1.24   1.23   1.21   1.19       Part   Density - gate   g/cm 3     0.941   0.9429   0.9424   0.9428   0.943           mid floor   g/cm 3     0.9399   0.9419   0.9411   0.9412   0.942           step   g/cm 3     0.94   0.9421   0.9413   0.9414   0.9421           skirt   g/cm 3     0.9405   0.9427   0.9412   0.943   0.9428           Melt Index I 2     g/10 min   55   71   70   81   93           Melt Index I 6     g/10 min   266   281   269   296   328           Stress Exponent       1.44   1.25   1.23   1.18   1.15            Tensile Properties            MD   Elong. at Yield   %   17   14   17   17   14           Yield Strength   MPa   18   21.1   19.9   19.9   21.5           Ultimate Elong.   %   650   1093   1138   391   1077           Ultimate Strength   MPa   18.8   19.7   18.8   13.9   16.9       TD   Elong. At Yield   %   15   12   15   16   13           Yield Strength   MPa   10.8   13.2   11.6   12   12.9           Ultimate Elong.   %   185   423   337   197   325           Ultimate Strength   MPa   10.8   13.2   11.6   12   12.9            Impact T sting                               Max. Load @ 23° C. on wall   lb   121   118   122   119   117       Total Energy @ 23° C. on wall   ft-lb   2.85   3.59   2.04   1.82   3.06       Max. Load @ −20° C. on wall   lb   165   153   159   151   148       Total Energy @ −20° C. on wall   ft-lb   2.84   2.44   2.52   2.05   3.25       Max. Load @ 23° C. on bottom   lb   14   11   12   12   24       Total Energy @ 23° C.   ft-lb   0.51   0.42   0.4   0.42   0.46       on bottom       Max. Load @ −20° C.   lb   19   10   15   13   30       on bottom       Total Energy @ −20° C.   ft-lb   0.11   0.31   0.11   0.16   0.23       on bottom       Initial Tear Resistance            MD   Load At Max.   N   66.4   68.3   72.7   61   54.5           Stress At Max.   N/mm   103.1   107.2   112.9   95.5   89.5           % Strain At Max.   %   16.7   4.5   6.6   4.3   2.5       TD   Load At Max.   N   89   94   95.2   82.6   64.5           Stress At Max.   N/mm   139.1   148.1   153.6   126.1   105           % Strain At Max.   %   66.4   68.7   74.1   38.9   5.5           Whiteness Index (part)       77.58   88.84   86.57   88.32   87.26           Yellowness Index (part)       −4.76   −8.3   −8.82   −9.76   −7.89            Part Drop Test                               (Bruceton Staircase)       Ave. Pass Drop Height   ft   1.6   2.7   1.5   1.3   2.6       Max Pass Height   ft   3   5   3   3   5       Min Pass Height   ft   1   1   1   1   1       Part Shrinkage, 72 hours   %   2.15   1.82   2.12   2.11   1.80                  
 
       Example 2  
       [0051]    Parallel to Example 1, Table 4 provides characterization results of experimental resins E3 and E4 in comparison to four competitive grades in the TWIM lid market. In similar fashion to the container resins, the experimental lid resins have significantly lower extractables content making them well suited for food applications. They also provide equivalent crystallinity at a lower melting point along with a higher Vicat softening point temperature and equivalent Shore D hardness. This combination of properties suggests lids produced from these resins would be suitable for hot fill applications, such as those described above for the experimental container resins. They also appear to have equivalent or slightly better toughness and equivalent color properties.  
                                                                                   TABLE 4                           Characterization of Experimental Lid Resins E3 &amp; E4 vs.       Benchmarks*                Units   C4   C5   C6   E3   C7   E4                        Density (g/cm 3 )       0.9311   0.9319   0.9354   0.9324   0.9308   0.9321       I 2     g/10 min   117   118   132   150   156   168       I 6     g/10 min   454   458   525   535   600   670       Stress Exponent       1.24   1.24   1.28   1.16   1.23   1.26       I 21     g/10 min   665   844   820   840   845   846       Melt Flow Ratio       5.7   7.2   6.1   5.57   5.4   5.06       Viscosity @ 100000 sec −1  @ 230° C.   Pa-sec   3.6   3.5   2.9   3.7   3.3   2.8       Viscosity @ 100000 sec −1  @ 250° C.   Pa-sec   3.2   3   2.7   3.3   2.8   2.6       No. Ave. Mol. Wt. (Mn)   ×10 −3     10.0   9.1   8.3   10.6   10.5   9.1       Wt. Ave. Mol. Wt. (Mw)   ×10 −3     30.0   29.7   30.7   28.6   28.4   29.2       Z Ave. Mol. Wt. (Mz)   ×10 −3     60.6   60.4   74.3   51.2   55.9   67.3       Polydispersity Index       3.00   3.27   3.72   2.70   2.70   3.20       Hexane Extractables   wt %   3.50   3.27   4.49   0.87   2.26   1.30       Melting Point   ° C.   122.2   123.6   125.8   119.5   124.0   119.0       Crystallinity   %   44.0   52.4   56.6   63.7   55.9   56.8       Vicat Softening Point   ° C.   90   87   96   104   96   101       Shore D Hardness       57.1   59.5   60.3   59.6   59.6   60.2       Flex. Sec Modulus, 1%   MPa   475   627   631   569   498   534       Flex. Sec Modulus, 2%   MPa   444   577   580   513   464   486       Flex. Strength   MPa   17.1   21.3   21   20.5   17.7   19.9       Yield Elongation   %   11   10   11   18   13   17       Yield Strength   MPa   15   15.7   16.6   16.6   15   16.2       Ultimate Elongation   %   40   36   76   54   47   46       Ultimate Strength   MPa   12.9   12.7   11.1   9.3   12.9   12.6       Tensile Impact   ft-lb/in 2     34.9   37.8   40.1   49.8   42.8   43.6       Whiteness Index       80.08   87.25   90.29   84.17   78.29   85.15       Yellowness Index       −9.05   −10.15   −10.72   −9.31   −8.22   −9.98                                                          
 
         [0052]    Lid products were produced using the six resins in Table 4. They were produced on the Husky injection molding unit mentioned above under the conditions listed in Table 5. These data indicate that the experimental resins process very similarly to the incumbent resins. In addition, the resin E4 requires lower pressure to mold a part. As a result, the barrel temperatures may be lowered in order to reduce energy consumption while maintaining cycle time, or cycle time reduced at the same temperature.  
                                                                                   TABLE 5                           Husky Injection Molding Machine Settings and Variables for Molding       Lid Resins                Units   C4   C5   C6   E3   C7   E4                        Resin Specs                                   MI   g/10 min   117   118   132   150   156   168       Density   g/cm 3     0.9311   0.9319   0.9354   0.9324   0.9308   0.9321       S. Ex.       1.24   1.24   1.28   1.16   1.23   1.26       M/C Settings       Fill pressure   %   65   55   55   50   55   50       Pullback   mm   0   10   10   10   10   10       Hold pressure Zone 1   %   20   20   20   20   20   20       Hold pressure Zone 2   %   15   15   15   15   15   15       Hold pressure Zone 3   %   10   10   10   10   10   10       Barrel temperature Zone 1   ° C.   200   200   200   200   200   200       Barrel temperature Zone 2   ° C.   210   210   210   210   210   210       Barrel temperature Zone 3   ° C.   220   220   220   220   220   220       Barrel temperature Zone 4   ° C.   230   230   230   230   230   230       Barrel temperature Zone 5   ° C.   230   230   230   230   230   230       Variables       Shot weight   g   55.85   55.73   55.73   55.74   55.73   55.80       Cycle time   sec   4.82   4.84   4.83   4.84   4.83   4.81       Injection time   sec   0.37   0.37   0.36   0.38   0.37   0.36       Screw run time   sec   1.40   1.32   1.40   1.40   1.44   1.56       Screw back pres   psi   257   255   258   255   257   255       Ext. drive pres   psi   867   888   827   882   818   767       Max. inj. Pres   psi   851   845   778   832   770   725       Hold pressure z. 1   psi   545   426   426   376   422   375       Hold pressure z. 2   psi   271   271   271   270   270   271       Hold pressure z. 3   psi   220   221   223   220   220   221       Barrel temperature Zone 1   ° C.   200   197   199   197   200   200       Barrel temperature Zone 2   ° C.   209   207   209   208   211   211       Barrel temperature Zone 3   ° C.   220   219   219   220   221   221       Barrel temperature Zone 4   ° C.   230   227   228   229   230   230       Barrel temperature Zone 5   ° C.   230   229   229   231   230   230                  
 
         [0053]    Extensive physical testing of the lids yielded the data in Table 6. These data show that the experimental resins E3 and E4 retain their stiffness properties and provide excellent toughness. Additionally, these experimental resins provide vastly superior clarity. This clarity is apparent for the two experimental resins based on testing using ASTM D1003 (Table 6). Thus, text placed a short distance behind lids made from any of the incumbent resins is not even discernible, let alone legible, yet can be clearly read when placed a similar distance behind a lid made from the either of the experimental resins. At smaller distances, such as might occur in packaging a product like yogurt or coffee with a printed foil seal beneath the lid, this effect is less dramatic. However, the improved clarity would allow a customer to more easily read the label and thus make the product more attractive.  
                                                                                                                                                                                                           TABLE 6                           Injection Molded Lids                Units   C4   C5   C5   E3   C7   E4                        Pellet   Density   g/cm 3     0.9311   0.9319   0.9354   0.9324   0.9308   0.9321           Melt Index I 2     g/10 min   117   118   132   150   156   168           Melt Index I 6     g/10 min   454   458   525   535   600   670           Stress Exponent       1.24   1.24   1.28   1.16   1.23   1.26       Part   Density - gate   g/cm 3     0.9267   0.9269   0.9264   0.9276   0.9259   0.9274           mid floor   g/cm 3     0.9256   0.9264   0.9256   0.9267   0.9253   0.9265           step   g/cm 3     0.9254   0.926   0.9254   0.9265   0.9249   0.9265           skirt   g/cm 3     0.9257   0.9268   0.9261   0.9276   0.9258   0.9271           Melt Index I 2     g/10 min   118   114   129   148   152   171           Melt Index I 6     g/10 min   458   444   515   534   571   676           Stress Exponent       1.24   1.24   1.26   1.17   1.21   1.25            Tensile Properties            MD   Elong. at Yield   %   23   22   21   19   24   20           Yield Strength   MPa   10.4   11.2   12.4   12   10.6   11.6           Ultimate Elong.   %   238   209   318   337   287   312           Ultimate Strength   MPa   9   9.4   9.6   9.8   8.9   9.6       TD   Elong. at Yield   %   21   20   20   20   22   20           Yield Strength   MPa   10.8   11.5   12   11.9   10.2   11.9           Ultimate Elong.   %   94   149   469   103   141   234           Ultimate Strength   MPa   9.8   8.6   8.8   8.6   8.4   9            Impact Testing                                   Max. Load @ 23° C. on Gate   lb   99   97   105   107   101   105       Total Energy @ 23° C. on Gate   ft-lb   3.06   3.07   3.27   3.19   3.14   3.2       Max. Load @ −20° C. on Gate   lb   149   144   151   103   114   152       Total Energy @ −20° C. on Gate   ft-lb   4.9   5.13   5.23   2.86   4.17   5.4       Max. Load @ 23° C. off Gate   lb   93   92   87   94   100   93       Total Energy @ 23° C. off Gate   ft-lb   2.62   2.74   2.7   2.92   3.04   2.82       Max. Load @ −20° C. off Gate   lb   141   153   145   149   160   130       Total Energy @ −20° C. off Gate   ft-lb   4.61   5.64   4.95   5.41   5.65   5.14       Initial Tear Resistance            MD   Load At Max.   N   55.3   57.7   63   65.6   57.4   61.2           Stress At Max.   N/mm   77.8   81.1   89.3   92.2   80.6   86.1           % Strain At Max.   %   17.3   23.2   41.3   15.9   42.1   16.8       TD   Load At Max.   N   53.3   54.8   61.1   61.8   55.8   59.6           Stress At Max.   N/mm   79.5   81.1   89.7   92.1   82.6   90.2           % Strain At Max.   %   44.2   32.8   37   35.9   62.2   26.4           Whiteness Index WI, (part)       72.49   75.26   81.2   77.36   74.52   75.64           Yellowness Index, YI (part)       −15.94   −12.76   −15.56   −8.62   −13.69   −8.57           Gloss   %   54   54   54   55   54   55           Haze   %   87.3   93.9   94.5   78   90.9   81.1           Clarity   %   13   20   7   98   15   98           Part Shrinkage, 96 hours   %   1.82   1.82   1.87   1.78   1.81   1.79
Thin walled polyethylene containers are suitable for the packaging of foods such as cottage cheese, ricotta cheese and ice cream. The containers have a higher softening point (which permits the containers to be "hot filled") and high impact strength at low temperature (which is useful when a container of ice cream is dropped).