Thermoplastic carrying bag with binary polyolefin resin blend
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to thermoplastic carrying bags, commonly used as grocery sacks, shopping bags, etc. In particular, it relates to an improved &#34;undershirt&#34; type bag made of blown tubular film comprising blended polyolefin resins for improved strength and tear resistance. 
     2. Description of the Prior Art 
     Significant advances in thermoplastic film technology have made possible low cost blown tubular film, made with various olefinic polymers, out of which packaging materials are made. Thermoplastic bags, and in particular polyethylene bags, have in recent years gained prominence in the packaging of a wide variety of goods such as grocery items, dry goods and the like. Conventional low density polyethylenes (LDPE), made by high pressure radical polymerization methods, have been commercially available for many years and have been employed in blown films and shopping bags. These LDPE resins have a high molecular weight and are highly branched. One of the most common drawbacks of the employment of such LDPE grocery bags is their tendency to rupture under load stresses and, also, their fairly low puncture resistance. One solution is to increase the film gauge, but that would lead to an increase in product costs. 
     Development of low pressure polymerization processes, using stereo-specific catalysts, has permitted the manufacture of linear olefin homopolymers and interpolymers. High density polyethylene (HDPE) has been economically blended with LDPE to obtain advantageous film materials having a good balance of physical properties. The HDPE copolymers have a density greater than 0.94 and are commercially available as ethylene-alpha-olefin copolymers such as ethylene-octene or ethylene-hexene. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, it has been found that blended polyolefin resins containing linear low density polyethylene copolymers (LLDPE) are advantageous in the manufacture of thermoplastic films and bags. The blended polyolefin resins are particularly well suited for making seamless-wall handled strap bags from thin tubular film consisting essentially of a homogeneous binary blend of resins. The improved resin blend system employed in this invention contains a predominant amount of LDPE. This component is well known for thermoplastic undershirt-type handled carrying bags and is made by high pressure free-radical polymerization techniques to produce a highly-branched essentially homopolymeric macromolecular structure. Such LDPE resins have a density of less than 0.93 g/cc, preferably 0.915 to 0.925. 
     A minor amount of LLDPE constitutes the other resinous component, usually in the amount of 10 to 40 parts by weight per 100 parts of polyolefin resin. The LLDPE resin may comprise a copolymer of ethylene with at least one alpha-olefin having 4 to 12 carbon atoms. The amount of LLDPE is effective to increase film elongation at break and impart improved strength, impact resistance and tear properties. At least 10% LLDPE is required to obtain substantial benefit; however, a relatively small amount of this component can permit use of thinner gauge monolayer film for heat-sealed polyolefin bags. Advantageously, the blend contains about 10 to 40 wt. % LLDPE and 60 to 90 wt % ordinary branched LDPE. 
     Superior physical properties of blown film from this blend permits the fabrication of economical carrying bags from thinner films, resulting in substantial material savings. These and other features and advantages of the invention will be seen in the following description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevation view of one form of the bag structures of the present invention; 
     FIG. 2 is a perspective view of the bag illustrated in FIG. 1 in a partially open position; and 
     FIG. 3 is a front elevation view of another form of bags made according to the present invention. 
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     In FIGS. 1 to 3, typical handled carrying bag constructions 1 are shown. A blown tubular monolayer film is folded to form side pleats 2. 
     A transverse heat seal forms the bottom 3, and top seals 4 at each side in the pleated portion forms a pair of handles 5. A cutout portion 6 between the handles provides a bag mouth portion 7. 
     U.S. Pat. No. 3,867,083 (Herrington) describes the method and apparatus for preparing a continuous, seamless blown thermoplastic film tubing by extruding a melt of the thermoplastic through an annular orifice into a seamless cylindrical shape, inflating the film tube thus formed and cooling such inflated tubing. A typical undershirt bag structure is disclosed in U.S. Pat. No. 4,165,832 to Kuklies et al. The disclosure relates to thermoplastic bag structures which are characterized by having a pair of carrying handles which are formed integrally with the bag walls and extended upwardly from the open mouth portion of the bag. U.S. Pat. No. 4,062,170 to Orem, discloses an apparatus for dispensing such plastic handle bearing bags from a stack of bags and holding the dispensed bag in an open position for loading. These patents show the production and use of the present invention and are herein incorporated by reference. 
     Numerous techniques have been described in the prior art for the formation of thermoplastic polyolefin bags. In order to obtain the improvements in physical properties such as improved strength and tear resistance which are essential to a shopping bag, much of the prior art teaches the formation of multilayer laminar thermoplastic film. In bag construction, certain particularly desirable physical characteristics should be exhibited. The bag should have a relatively high tensile modulus and resistance to impact forces. It should also exhibit good elongation under stress with a high degree of tear resistance. These improved physical characteristics are achieved in this invention in a bag made of a single layer film. 
     In this description parts by weight and metric units are employed unless otherwise stated. The term &#34;density&#34; is used in ordinary metric fashion, equated to specific gravity or grams per cubic centimeter (g/cc). 
     The &#34;undershirt&#34; bag is made from an improved thermoplastic polyolefin film consisting essentially of a binary blend of about a major amount (e.g. 60 to 90 wt. %) of LDPE and a minor amount (e.g. 10 to 40 wt. %) of LLDPE. 
     The low density polyethylene (LDPE) is made by the conventional high pressure method and thus is highly branched. Advantageously, the LDPE has a density not greater than 0.93 and a fractional melt index range of 0.5 to 0.9, with a preferred melt index of 0.7. The preferred LDPE concentration is about 80 wt. %. The LDPE lends its favorable processing properties, which are advantageous for heat sealing. It also possesses excellent toughness, impact strength and tear strength. A suitable LDPE is made by Dow Chemical under the name &#34;Resin 682&#34;. Low density polyethylene &#34;Resin 682&#34; has a melt index of 0.7 and a density of 0.921. Other suitable materials include Dow 122 and a CaCO 3  -containing resin (Dow 123). 
     The linear low density polyethylene resins (LLDPE) are produced by newer low pressure methods, thus having less branching and more controlled molecular structure than the conventional high pressure LDPE resins. The LLDPE is a copolymer of polyethylene and at least one alpha-olefin, where said alpha-olefins have 4 to 12 carbon atoms. The preferred alpha-olefins are those with 4 to 8 carbon atoms such as butene-1, hexene-1, 4-methyl pentene, octene-1 and mixtures thereof. The LLDPE resins employed herein have a density not greater than 0.940, and preferably have a melt index range of 0.2 to 2. The preferred concentration of LLDPE in the bag blend is about 20 wt. %. Films made with LLDPE resins have significantly higher impact, tear, and tensile strength. Because of the improved physical properties, the film fabricator can either fabricate film having superior properties to conventional LDPE/HDPE blends, or can reduce film thickness to achieve comparable or even still superior film properties thus attaining significant savings in resin cost. 
     Suitable LLDPE resins are Dow Chemical&#39;s &#34;Dowlex 2045&#34;, which has a density of 0.920 and a melt index value of 1.0. Other commercially available resins such as Union Carbide GP-2 and Dowlex 2038 are useful, as are the Dow XO-61500 series of experimental resins. For example, Dow&#39;s XO-61500 LLDPE resin has a density of 0.934 and a melt index value of 1.0. These resins add to the tear strength, stiffness, and toughness of the bag. 
     EXAMPLES 
     Several blend formulations were tested against a control formulation containing no LLDPE. In all the formulations, including the control, the LDPE used is Dow&#39;s &#34;Resin 682&#34; LDPE. &#34;Dowlex 2045&#34; and &#34;Dowlex 2038&#34;, made by Dow Chemical Co., are used as the LLDPE components. Several bags with different nominal gauge films were made of each formulation according to Table 1. 
     
                       TABLE 1                                                     
______________________________________                                    
Example No.                                                               
           Gauge (mils)     Composition*                                  
______________________________________                                    
0          1.50             Control                                       
1          1.25             10% HD                                        
2          1.125            duPont F7810                                  
3          1.0                                                            
4          1.25             20% LLDPE                                     
5          1.125            Dowlex 2045                                   
6          1.0                                                            
7          1.25             30% LLDPE                                     
8          1.125            Dowlex 2045                                   
9          1.0                                                            
10         1.25             20% LLDPE                                     
11         1.125            Dowlex 2038                                   
12         1.0                                                            
13         1.25             30% LLDPE                                     
14         1.125            Dowlex 2038                                   
15         1.0                                                            
______________________________________                                    
 *All have masterbatch, which contains 50% pigment and 50% LDPE. Remainder
 of composition is LDPE.                                                  
 
    
     500 Bags of each of the different grocery sacks were subjected to stardardized simulation testing. This entailed the packaging of groceries into sacks making use of the dispenser system, and the transporting of those loaded grocery sacks by auto and by foot. An analysis of each bag was conducted as part of the work. 
     Historical data indicates that 75% of all customers transport groceries by automobile with the remaining 25% making their shopping trips on foot. Thus, the simulation assured this 75:25 ratio. Auto trips included carting a six (6) bag order to the car, driving a total of five miles and then noting any pertinent data about the bags. Walking trips included carrying a two (2) bag order for 150 yards and again noting pertinent bag data, studies of loaded bag weights indicate the average bag weighs 13-15 pounds; the simulation incorporated this data. Boxes were replaced frequently to maintain &#34;sharp&#34; corners representative of a normal environment during the bag usage. 
     Punctures are defined as rounded holes caused by cans and/or box corners. Can typically cause a puncture during loading, unloading, and/or bag placement in the auto; punctures from box corners are typically induced during the carrying phase. Tears/splits are defined as elongated holes and are most often induced by box corners during the loading operation. 
     Results of the simulation tests are summarized in Tables 2 and 3. The results clearly show the ability to reduce film gauge when linear low density polyethylene is used. With the addition of this component (LLDPE) one can make stronger thinner bags. Table 4 summarizes the properties of the bag films. The bags with LLDPE show improved elongation and better balanced properties than the ones without. 
     
                                           TABLE 2                                 
__________________________________________________________________________
15 Pound Load                                                             
               Example No.                                                
                                           9   10  12  13  15             
          Control                                                         
               1    3    4     6     7     1.0 1.25                       
                                                   1.0 1.25               
                                                           1.0            
          1.50 1.25 1.0  1.25  1.0   1.25  30% 20% 20% 30% 30%            
          10% HD                                                          
               10% HD                                                     
                    10% HD                                                
                         20% LLD                                          
                               20% LLD                                    
                                     30% LLD                              
                                           LLD LLD LLD LLD LLD            
__________________________________________________________________________
Number of Trips                                                           
Walk      55   62   66   62    63    59    54  62  61  40  56             
Drive     62   61   54   60    60    60    61  60  62  47  58             
Total     117  123  120  122   123   119   115 122 123 87  114            
Number of Bags                                                            
Walk      110  124  132  124   126   118   108 124 122 80  112            
Drive     372  366  324  360   360   360   366 360 372 282 348            
Total     482  490  456  484   486   478   474 484 494 362 460            
% of Incidence                                                            
Tears/Splits                                                              
          16   13   23   9     16    11    11  9   12  15  14             
Punctures 46   42   26   31    37    27    37  31  46  44  46             
__________________________________________________________________________
 
    
     
                       TABLE 3                                                     
______________________________________                                    
23 Pound Load                                                             
        Example No.                                                       
        0   1     3       4  6    7   9  10  12  13  15                   
______________________________________                                    
Number                                                                    
of Trips                                                                  
Walk      3     4     4    3   5   4   5   4   4   4   4                  
Drive     2     3     3    4   4   4   5   4   4   4   4                  
Total     5     7     7    7   9   8   10  8   8   8   8                  
Total Number                                                              
of Bags                                                                   
Walk      6     8     8    6   10  8   10  8   8   8   8                  
Drive     12    18    18   24  24  24  30  24  24  24  24                 
Total     18    26    26   30  34  32  40  32  32  32  32                 
% of Incidence                                                            
Tears/splits                                                              
          44    23    31   10  9   25  29  9   16  31  25                 
Punctures 56    81    115  43  24  53  43  38  50  56  50                 
Bottom Seal                                                               
Failure   --    --    4    --  5   --  --  --  --  --  --                 
Handle Failure                                                            
          --    --    3    --  --  --  --  --  --  --  --                 
______________________________________                                    
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
            Example No.                                                   
            0  1  2  3  4  5   6  7  8  9  10  11 12 13 14 15             
__________________________________________________________________________
Caliper (mils)                                                            
            1.46                                                          
               1.19                                                       
                  1.08                                                    
                     1.00                                                 
                        1.25                                              
                           1.16                                           
                               1.07                                       
                                  1.23                                    
                                     1.16                                 
                                        1.01                              
                                           1.26                           
                                               1.17                       
                                                  1.01                    
                                                     1.27                 
                                                        1.10              
                                                           .99            
yield (psi)                                                               
         MD 1210                                                          
               1238                                                       
                  1130                                                    
                     1079                                                 
                        1033                                              
                           1000                                           
                               1020                                       
                                  1082                                    
                                     1105                                 
                                        1182                              
                                           1443                           
                                               1222                       
                                                  1144                    
                                                     1160                 
                                                        1130              
                                                           1131           
         TD 1506                                                          
               1579                                                       
                  1585                                                    
                     1582                                                 
                        1377                                              
                           1459                                           
                               1458                                       
                                  1385                                    
                                     1380                                 
                                        1387                              
                                           1689                           
                                               1614                       
                                                  1596                    
                                                     1678                 
                                                        1607              
                                                           1781           
(p/x)    MD 1.84                                                          
               1.56                                                       
                  1.32                                                    
                     1.09                                                 
                        1.25                                              
                           1.09                                           
                               1.03                                       
                                  1.32                                    
                                     1.16                                 
                                        1.17                              
                                           1.76                           
                                               1.48                       
                                                  1.19                    
                                                     1.46                 
                                                        1.22              
                                                           1.21           
         TD 2.38                                                          
               1.99                                                       
                  1.68                                                    
                     1.65                                                 
                        1.68                                              
                           1.62                                           
                               1.40                                       
                                  1.69                                    
                                     1.49                                 
                                        1.47                              
                                           2.01                           
                                               1.84                       
                                                  1.66                    
                                                     1.98                 
                                                        1.80              
                                                           1.71           
ultimate (psi)                                                            
         MD 4118                                                          
               4460                                                       
                  4638                                                    
                     4673                                                 
                        4273                                              
                           4130                                           
                               4317                                       
                                  4467                                    
                                     4800                                 
                                        4616                              
                                           4404                           
                                               4231                       
                                                  4135                    
                                                     4288                 
                                                        4556              
                                                           4467           
         TD 2228                                                          
               2119                                                       
                  2226                                                    
                     2163                                                 
                        2418                                              
                           2232                                           
                               2292                                       
                                  2377                                    
                                     2546                                 
                                        2208                              
                                           2462                           
                                               2465                       
                                                  2269                    
                                                     2593                 
                                                        2482              
                                                           2354           
(p/x)    MD 6.26                                                          
               5.63                                                       
                  5.38                                                    
                     4.72                                                 
                        5.17                                              
                           4.51                                           
                               4.36                                       
                                  5.45                                    
                                     5.04                                 
                                        4.57                              
                                           5.47                           
                                               4.95                       
                                                  4.30                    
                                                     5.36                 
                                                        4.92              
                                                           4.78           
         TD 3.52                                                          
               2.67                                                       
                  2.36                                                    
                     2.12                                                 
                        2.95                                              
                           2.48                                           
                               2.20                                       
                                  2.90                                    
                                     2.75                                 
                                        2.34                              
                                           2.93                           
                                               2.81                       
                                                  2.36                    
                                                     3.06                 
                                                        2.78              
                                                           2.26           
Elongation (%)                                                            
         MD 292                                                           
               218                                                        
                  183                                                     
                     126                                                  
                        240                                               
                           212 180                                        
                                  298                                     
                                     280                                  
                                        238                               
                                           270 200                        
                                                  184                     
                                                     280                  
                                                        246               
                                                           192            
         TD 600                                                           
               527                                                        
                  500                                                     
                     502                                                  
                        570                                               
                           530 528                                        
                                  596                                     
                                     604                                  
                                        532                               
                                           600 596                        
                                                  592                     
                                                     592                  
                                                        588               
                                                           584            
Modulus (psi)                                                             
         MD 2.61                                                          
               2.84                                                       
                  2.80                                                    
                     2.78                                                 
                        2.47                                              
                           2.71                                           
                               2.68                                       
                                  2.43                                    
                                     2.43                                 
                                        2.65                              
                                           3.13                           
                                               2.81                       
                                                  3.17                    
                                                     3.12                 
                                                        3.37              
                                                           3.30           
X104     TD 3.12                                                          
               3.81                                                       
                  3.90                                                    
                     3.03                                                 
                        2.87                                              
                           3.10                                           
                               3.06                                       
                                  2.80                                    
                                     2.72                                 
                                        3.37                              
                                           4.06                           
                                               3.75                       
                                                  3.91                    
                                                     3.82                 
                                                        4.11              
                                                           4.55           
ELMENDORF                                                                 
         MD 103                                                           
               148                                                        
                  131                                                     
                     130                                                  
                        98 64  79 46 30 31 59  122                        
                                                  146                     
                                                     235                  
                                                        64 85             
(gm/mil) TD 190                                                           
               232                                                        
                  205                                                     
                     190                                                  
                        198                                               
                           193 196                                        
                                  231                                     
                                     253                                  
                                        304                               
                                           286 225                        
                                                  217                     
                                                     290                  
                                                        332               
                                                           313            
GMS      MD 157                                                           
               182                                                        
                  144                                                     
                     144                                                  
                        121                                               
                           74  80 50 45 32 74  141                        
                                                  150                     
                                                     294                  
                                                        77 80             
         TD 298                                                           
               285                                                        
                  230                                                     
                     198                                                  
                        23 224 198                                        
                                  294                                     
                                     294                                  
                                        304                               
                                           350 266                        
                                                  224                     
                                                     365                  
                                                        285               
                                                           326            
__________________________________________________________________________
 
    
     EXAMPLES 16 TO 27 
     Blown 40 u (1.75 mil) thick film was made by melt extruding (490° F. a blend of a Dowlex 2038 or 2045 LLDPE copolymer with Dow Resin 123 on Dow Resin 682 as the branched ethylene homopolymer. The film was extruded from a 2.5 inch diameter slit ring orifice die, having a die gap of 0.040 inch. An imposed shape cooling technique was used to force ambient air (72° F.) against the blower film, which has a blow up ratio of about 2:1. The extrusion speed was 154 ft/min., with a nip roll speed of 300 to 500 ft/min. The data in Table 5 show the film properties, as compared with a control film of 10% HDPE copolymer with 90% LDPE. 
     
                                           TABLE 5                                 
__________________________________________________________________________
                     Tensile Properties %                                 
                                    Elmendorf                             
Ex.                                                                       
   LLDPE     LDPE    Yield                                                
                         Ultimate                                         
                              Elongation                                  
                                    Tear (MD)                             
No.                                                                       
   Wt. %     wt. %   (psi)                                                
                         (psi)                                            
                              (%)   (grams)                               
__________________________________________________________________________
16 10% Dow 2038                                                           
             90% Dow* 123                                                 
                     1314                                                 
                         3714 312   106                                   
17 20% Dow 2038                                                           
             80% Dow* 123                                                 
                     1600                                                 
                         3886 344   135                                   
18 30% Dow 2038                                                           
             70% Dow* 123                                                 
                     1600                                                 
                         4343 414    82                                   
19 HDPE CONTROL                                                           
             90% Dow* 123                                                 
                     1486                                                 
                         4057 326    99                                   
20 10% Dow 2045                                                           
             90% Dow* 123                                                 
                     1257                                                 
                         3829 361   104                                   
21 20% Dow 2045                                                           
             80% Dow* 123                                                 
                     1257                                                 
                         4000 403   107                                   
22 10% Dow 2038                                                           
             90% Dow 682                                                  
                     1429                                                 
                         4057 435   146                                   
23 20% Dow 2038                                                           
             80% Dow 682                                                  
                     1600                                                 
                         4343 440   131                                   
24 30% Dow 2038                                                           
             70% Dow 682                                                  
                     1771                                                 
                         4514 455   112                                   
25 HDPE CONTROL                                                           
             90% Dow 682                                                  
                     1429                                                 
                         4171 410   132                                   
26 10% Dow 2045                                                           
             90% Dow 682                                                  
                     1486                                                 
                         4114 400   140                                   
27 20% Dow 2045                                                           
             80% Dow 682                                                  
                     1429                                                 
                         4286 450   124                                   
__________________________________________________________________________
 *Contains 2000 ppm CaCO.sub.3                                            
 
    
     It was found that a superior film could be obtained with the LLDPE/LDPE blend, having increased yield stress, ultimate tensile strength and elongation, compared with straight 100% LDPE base film by increasing the copolymer to 20 to 30 wt. %. When such large amounts of HDPE copolymer are used, film splittiness gives inferior carrying bag properties. 
     EXAMPLES 37 AND 38 
     A melt extruded blown film was made under conditions similar to the above examples, except that the blend consisted of 20 wt. % Dowlex 2038 LLDPE, 7% white pigmented master batch containing equal parts of TiO 2  and LDPE, and 73% Northern 941 LDPE resin. The line speed was adjusted to yield a film thickness of about 32 u (1.3 mil) and 35 u (1.6 mil) respectively, with cooled air ring imposed shape blowing. The control is substantially identical except that the copolymer is DuPont F7810 HDPE. The film properties are tabulated below in Tables 6 through 8. 
     
                       TABLE 6                                                     
______________________________________                                    
            Average      Average                                          
Sample      Caliper,     Total Energy,                                    
No.         Mils         in-lb  in-lb/mil                                 
______________________________________                                    
28          1.263        15.53  10.89                                     
29          1.657        19.89  11.32                                     
CONTROL     1.454        19.35  12.90                                     
CONTROL     1.721        23.37  13.46                                     
______________________________________                                    
 
    
     
                                           TABLE 7                                 
__________________________________________________________________________
          Caliper, Tear Length                                            
                            Tear Resistance,                              
Sample    Mils     cm       lb                                            
No.       High                                                            
             Low                                                          
                Avg.                                                      
                   High                                                   
                      Low                                                 
                         Avg.                                             
                            High                                          
                                Low                                       
                                   Avg.                                   
__________________________________________________________________________
28     MD 1.45                                                            
             1.30                                                         
                1.39                                                      
                   6.85                                                   
                      5.65                                                
                         6.05                                             
                            6.58                                          
                                5.79                                      
                                   6.28                                   
       TD 1.50                                                            
             1.25                                                         
                1.37                                                      
                   4.35                                                   
                      3.55                                                
                         3.80                                             
                            9.23                                          
                                7.92                                      
                                   8.76                                   
29     MD 1.85                                                            
             1.65                                                         
                1.76                                                      
                   5.60                                                   
                      4.25                                                
                         4.88                                             
                            8.06                                          
                                6.62                                      
                                   7.29                                   
       TD 1.80                                                            
             1.60                                                         
                1.71                                                      
                   3.65                                                   
                      3.05                                                
                         3.34                                             
                            10.41                                         
                                9.04                                      
                                   9.68                                   
CONTROL                                                                   
       MD 1.55                                                            
             1.35                                                         
                1.48                                                      
                   6.60                                                   
                      5.35                                                
                         6.02                                             
                            6.83                                          
                                5.93                                      
                                   6.30                                   
       TD 1.65                                                            
             1.35                                                         
                1.49                                                      
                   3.70                                                   
                      3.45                                                
                         3.52                                             
                            9.44                                          
                                8.94                                      
                                   9.30                                   
CONTROL                                                                   
       MD 1.85                                                            
             1.60                                                         
                1.74                                                      
                   5.80                                                   
                      4.90                                                
                         5.38                                             
                            7.26                                          
                                6.46                                      
                                   6.80                                   
       TD 1.80                                                            
             1.65                                                         
                1.72                                                      
                   3.35                                                   
                      2.95                                                
                         3.11                                             
                            10.69                                         
                                9.66                                      
                                   10.26                                  
__________________________________________________________________________
 
    
     
                                           TABLE 8                                 
__________________________________________________________________________
            Elastic        Tensile                                        
                                 Tensile                                  
                                       Tensile      Elmendorf             
            Modulus,                                                      
                    Stiffness,                                            
                           Yield,                                         
                                 Ultimate,                                
                                       Toughness,                         
                                              Elongation,                 
                                                    Tear, Film            
Example                                                                   
       Caliper,                                                           
            psi     lb/in  psi   psi   ft-lb/in.sup.3                     
                                              %     gm/mil.               
                                                          Density,        
No.    mils MD  TD  MD TD  MD TD MD TD MD  TD MD TD MD TD gm/cc           
__________________________________________________________________________
28     1.463                                                              
            43546                                                         
                58995                                                     
                    59.8                                                  
                       79.4                                               
                           1467                                           
                              1553                                        
                                 3997                                     
                                    3198                                  
                                       836 1275                           
                                              309                         
                                                 768                      
                                                    38 166                
                                                          .9528           
29     1.765                                                              
            39372                                                         
                51940                                                     
                    66.3                                                  
                       87.7                                               
                           1440                                           
                              1578                                        
                                 3733                                     
                                    3168                                  
                                       995 1287                           
                                              406                         
                                                 778                      
                                                    35 170                
                                                          .9513           
CONTROL                                                                   
       1.421                                                              
            45325                                                         
                65045                                                     
                    63.4                                                  
                       90.4                                               
                           1512                                           
                              1733                                        
                                 4109                                     
                                    3107                                  
                                       940 1345                           
                                              339                         
                                                 787                      
                                                    12 373                
                                                          .9492           
CONTROL                                                                   
       1.656                                                              
            44768                                                         
                64448                                                     
                    74.0                                                  
                       107.0                                              
                           1526                                           
                              1678                                        
                                 4086                                     
                                    3038                                  
                                       1099                               
                                           1301                           
                                              412                         
                                                 777                      
                                                    15 345                
                                                          .9469           
__________________________________________________________________________
 
    
     Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be resorted to, without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.