Cured fibers coated with adhesion promoting additives for ethylene-propylene-nonconjugated diene elastomers
BACKGROUND OF THE INVENTION 
     The present invention relates to the addition of adhesion promoting additives to ethylene-propylene-nonconjugated diene (EPDM) elastomers. 
     Adhesion of EPDM elastomers to fibers or fabrics has been very difficult for those skilled in the art. Among the methods used for improving such adhesion are pretreating the fabric with a special latex or a chemically reactive substance, chemically modifying the elastomer, and adding an ingredient directly to an elastomer composition that will render it more adherent upon curing the composition onto a fabric substrate. 
     Various adhesion promoting additives useful in the last method are disclosed in Belgian Pat. No. 778,284, U.S.S.R. Pat. No. 275,372, French Pat. No. 1,439,245, Rumanian Pat. No. 192,329, and U.S. Pat. No. 2,844,502. 
     It has been found that an acid of the formula A--COOH) x , or an anhydride thereof, wherein x and A are as defined below, renders ethylene-propylene-nonconjugated diene (EPDM) elastomers more adherent to fibers that have been treated with a resorcinolformaldehyde-latex (RFL) dip when said elastomers are cured directly onto said fibers. 
     In a preferred embodiment of the present invention, a cobalt or copper salt, for example, cobalt naphthenate, is added along with the aforementioned adhesion promoting compounds to further improve adhesion. While cobalt naphthenate is known to improve metal adhesion of elastomeric stocks, no mention has ever been made of its use in fabric adhesion, by itself or in conjunction with other compounds. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of bonding an EPDM elastomer to fibers that have been treated with an RFL dip, comprising preparing a composition comprising said elastomer and 0.5 to 5.0 parts per hundred, by weight, based on the weight of said elastomer, of sulfur, 0.1 to 3.5 parts per hundred, by weight, based on the weight of said elastomer, of accelerator, and 1.0 to 5.0 parts per hundred, by weight, based on the weight of said elastomer, of an additive which is an acid of the formula A--COOH) x , or an anhydride thereof, wherein x is an interger of 1 or 2, wherein the acid groups are vicinal if x is 2, and A is phenyl, C 2  -C 10  alkylene, C 2  -C 10  alkenylene, C 6  -C 10  bridged or nonbridged cycloalkylene or cycloalkenylene, with the proviso that A is C 1  -C 6  alkyl if x is 1, and then applying the resulting mixture to said fibers and curing the resulting combination. 
     The present invention also relates to a composition of matter comprising an EPDM elastomer, 0.5 to 5.0 parts per hundred, by weight, based on the weight of said elastomer, of sulfur, 0.1 to 3.5 parts by weight, based on the weight of said elastomer, of accelerator, and 1.0 to 5.0 parts by weight, based on the weight of said elastomer, of an additive selected from an acid of the formula A--COOH) x , or an anhydride thereof, wherein x and A are as defined above. 
     The present invention also relates to a cured composite comprising a first layer of a plurality of fibers which are coated with the dried residue of a RFL dip adhered to a second layer comprising an EPDM elastomer in admixture with 0.5 to 5.0 parts per hundred, based on the weight of said elastomer of sulfur, 0.1 to 3.5 parts per hundred, by weight, based on the weight of said elastomer, of accelerator, and 1.0 to 5.0 parts by weight, based on the weight of said elastomer, of an additive selected from an acid of the formula A--COOH) x , or an anhydride thereof, wherein x and A are as defined above. 
     The preferred additives used in the present invention are 5-norbornene-2,3-dicarboxylic anhydride, 5-norbornene-2,3-dicarboxylic acid and cis-4-cyclohexene-1,2-dicarboxylic anhydride. 
     In a most preferred embodiment of the present invention, the additive is 5-norbornene-2,3-dicarboxylic anhydride and 0.1 to 1.0 grams of cobalt or copper per hundred grams of EPDM elastomer composition are used. The cobalt or copper is added as an organic or inorganic salt. For example, 3.0 grams of cobalt naphthenate would yield the required 0.42 grams of cobalt. The preferred salts are cobalt naphthenate, cobalt acetate, cobalt stearate, cobalt sulfate, cobalt 2-ethylhexonate, cobalt oleate, cobalt phosphate, cobalt resinate, and copper naphthenate. Although both cobalt and copper salts further increase the adhesion when 5-norbornene-2,3-dicarboxylic anhydride is used according to the present invention, only in the case of cobalt salts is it observed that there is an increased amount of rubber remaining attached to the fabric in the stock failure test described in the examples. Cobalt salts are therefore more preferred than copper salts as additives. 
     The EPDM of the present invention preferably comprises from 30 to 80 percent by weight ethylene, from 20 to 70 percent by weight propylene, and from 2 to 25 percent by weight of a nonconjugated diene. Nonlimiting examples of suitable dienes are dicyclopentadiene, methylidene norbornene, ethylidene norbornene, and 1,4-hexadiene. The norbornenes are preferred dienes, and 5-ethylidene-2-norbornene is most preferred. 
     The sulfur that is required in the present invention may be elemental sulfur or may be provided by a sulfur donor system, a nonlimiting example being dipentamethylene thiuram hexasulfide which is 35% available sulfur. Among the forms of elemental sulfur that may be used are those that are referred to in the art as soluble sulfur and insoluble sulfur (polymerized sulfur). 
     The fabric adhesion achieved with the present invention is dependent upon the cure rate. The longer the cure rate, the better will be the adhesion. The use of thiazoles, which are slower curing, generally results in better adhesion than the use of thiurams or carbamates. 
     The accelerator that is required in the present invention is thus preferably a thiazole or thiazole salt, nonlimiting examples being N-t-butyl-2-benzothiazole, N-cyclohexyl-2-benzothiazole, 2-(4-morpholinylmercapto)benzothiazole, 2-mercaptobenzothiazole and its zinc salt, 2,2&#39;-dibenzothiazyl disulfide, and thiocarbamyl sulfenamide. The most preferred accelerator is N-t-butyl-2-benzothiazole. 
     The accelerator used may also be a mixture of two or more accelerators, for example, a thiuram or carbamate plus a thiazole. 
     Cure temperature and cure time are selected to yield optimum fabric adhesion. Generally, cure temperatures of 275° F. to 400° F. and cure times of 5 to 120 minutes are preferred. Most preferred are cure temperatures of 325° F. to 350° F. and cure times of 7 to 25 minutes. 
     The additives of the present invention may be added to the elastomer alone, before the addition of any other materials, or they may be added to a compounded stock that also contains filler and/or curatives. Fillers and other compounding ingredients can be varied to meet the needs of various applications, for examples, wire and cable, conveyor belting, tires, etc. The addition of the additives to the elastomer or the elastomer composition may be performed in standard rubber compounding equipment such as Banbury mixer or a mill. The additives may be added at normal rubber processing temperatures, generally 100° F. to 375° F. 
     The fibers that are used in the present invention are those fibers that can be successfully treated with an RFL dip system. The RFL dip may be preceded by another dip, nonlimiting examples being an isocyanate dip or a polyepoxy dip. Nonlimiting examples of such fibers are rayon, nylon and polyester. The fibers may take a variety of forms, for example, yarn, cord or fabric. The preferred latex used in the dip system is butadiene/styrene/vinyl pryidine. 
     In the examples below, the fabric treating solution is an RFL dip. The RFL dip was prepared using the following: 
     
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                          Parts                                           
______________________________________                                    
Butadiene/styrene/vinyl pyridine                                          
(70/16/14) copolymer rubber latex,                                        
40% solids (Pyratex [trademark]                                           
5360)                     100.0                                           
                          Parts                                           
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Resorcinol-formaldehyde resin                                             
ball and ring softening point                                             
102-112° C., 75% solids (Koppers                                   
[trademark] R2170)        11.4                                            
Ammonium hydroxide (28%)  3.1                                             
Distilled water           85.5                                            
                          100.0                                           
                          Parts                                           
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Formaldehyde (37%)        6.5                                             
Distilled water           93.5                                            
                          100.0                                           
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     31.1 grams of solution B was added to 40.4 grams of solution A with rapid stirring. 28.5 grams of solution C was then added to the mixture of A and B. The resulting solution was adjusted to a pH of 9.0 by the addition of ammonium hydroxide (28%). After the dip was brushed onto the fabric, the treated fabric was oven dried for three minutes at 400° F. The amount of resin retained by the fabric was 5.5 percent by weight of the fabric. 
     The EPDM elastomer compositions containing sulfur, accelerator, and adhesion promoters in the following examples were prepared by mixing the elastomer with compounding ingredients in a Brabender Prep Center [trademark] equipped with a cam-style mixing head. The EPDM composition was blended for five minutes at 100° C. using a mixing speed of 50 RPM. Thereafter, the stock was refined by passing it eight times through a mill at room temperature. The stock removed from the mill had a 40-gauge thickness. 
     Adhesion and stock failure test pads evaluated in the examples were prepared in a 5-inch×6-inch×0.25-inch stainless steel mold. A 5-inch×6-inch piece of treated fabric was placed on each side of a 5-inch or 6-inch×40-gauge test stock. A 5-inch×6-inch×100 gauge backing stock was placed on each side of the fabric. The backing stock was an EPDM also containing a sulfur curative system. Aluminum foil spacers were spaced between the fabric and test stock at one end of the pad so that the backing-fabric composite could be peeled back after cure and inserted in the jaws of a tensile tester. The pad was cured in the mold under 10,000 psi gauge pressure. The cured pad was cut into one-inch strips and tested on a tensile tester at a separation rate of two inches per minute. The test strips were tested at room temperature after aging 24 hours at room temperature. Adhesion results are reported as average pounds per linear inch (pli). 
     Stock failure results are reported as a visual evaluation of the percent stock remaining attached to the surface of the RFL treated fabric. 
     Test pads were prepared as described above using the indicated treated fabric and compounded stock. The stock was cured 30 minutes at 350° F. The physical properties and adhesion results of the cured stock are given in the examples. 
    
    
     EXAMPLES 
     The EPDM elastomer compositions shown in Tables I-VII were compounded as described above. The following recipe was used for the Examples shown in Tables I, II, III, IV, V, and VI: 
     
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Ingredients          Parts                                                
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EPDM                 100                                                  
FEF Carbon Black     40                                                   
Paraffinic oil       15                                                   
Zinc Oxide            5                                                   
Stearic Acid          1                                                   
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     The Mooney Viscosity (ML-4) of the EPDM starting materials of the Examples shown in Tables I-IV and VI at 257° F. is 30, and the diene used in the EPDM as above was 5-ethylidene-2-norbornene having an ethylene/propylene weight ratio of 57/43 and an Iodine Number of 10. 
     The EPDM elastomer compositions shown in Table V were also compounded as described above. However, the EPDM polymers of Table V had the following properties: 
     
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            Example 23 Example 24                                         
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Termonomer    DCPD         1,4-Hexadiene                                  
E/P Ratio     54/46        56/44                                          
Iodine No.    10            8                                             
Mooney Viscosity                                                          
ML-4 at 257° F.                                                    
              60           60                                             
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     The EPDM elastomer compositions shown in Table VII were compounded as described previously according to the following recipe: 
     
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Ingredients          Parts                                                
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EPDM                 100                                                  
FEF Carbon Black     40                                                   
Paraffinic Oil       10                                                   
Zinc Oxide            5                                                   
Stearic Acid          1                                                   
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     The Mooney Viscosity (ML-4) of the EPDM starting material at 257° F. was 50. The diene used in the EPDM was 5-ethylidene-2-norbornene having an ethylene/propylene weight ratio of 57/43 and Iodine Number of 21. 
     The levels of the constituents in Examples 1-32, other than those listed above, are in parts per hundred (phr) by weight, based on the weight of elastomer. 
     The fabric employed in the Examples was a plain weave nylon, 808/1 denier treated with the RFL solution. The cobalt naphthenate used in the Examples had a molecular weight of 419 and contained 14% cobalt. 
     EXAMPLES 1-8 (TABLE I) 
     Example 1 illustrates the most preferred embodiment of the present invention. Excellent adhesion is obtained as evidenced by the amount of pull necessary to separate the test strips. Moreover, the addition of cobalt naphthenate gives 100% stock failure; i.e., even after the test strips have been separated, 100% of the elastomer composition prepared according to the present invention remains attached to the fabric. 
     Example 2 illustrates that 5-norbornene-2,3-dicarboxylic anhydride will give good adhesion even in the absence of copper or cobalt. This is shown by the fact that the amount of force necessary to separate the test strips is high. Although a surprising improvement in adhesion is obtained, percent stock failure is poor. This means that when separation finally occurs, the rubber pulls away from the fabric completely. It is for this reason that the further addition of cobalt or copper is most preferred. 
     Example 3 which is outside the scope of the present invention is presented for comparative purposes. 
     Examples 4 and 5 illustrate that various levels of 5-norbornene-2,3-dicarboxylic anhydride can be used to achieve acceptable levels of adhesion. 
     Examples 6, 7 and 8 illustrate that various levels of cobalt naphthenate (expressed as grams of cobalt per hundred grams of EPDM elastomer) achieved acceptable adhesion and improved stock failure. 
     EXAMPLES 9-12 (TABLE II) 
     Examples 9 and 10 illustrate that different typs of cyclic dicarboxylic acids or anhydrides thereof can be used in place of 5-norbornene-2,3-dicarboxylic anhydride and good adhesion obtained. 
     Examples 11 and 12 illustrate that metal salts other than cobalt napthenate can be used in combination with 5-norbornene-2,3-dicarboxylic anhydride to improve adhesion. Cobalt or copper levels are expressed as described above. 
     EXAMPLES 13-17 (TABLE III) 
     Example 13 illustrates that good adhesion is still obtained even when the level of accelerator is increased. 
     Example 14 illustrates that an accelerator is necessary for good adhesion. 
     Examples 15, 16 and 17 illustrate that accelerators other than N-t-butyl-2-benzothiazole can be used and good adhesion obtained. 
     EXAMPLES 18-22 (TABLE IV) 
     Examples 18 and 19 illustrate that various levels of sulfur can be used and good adhesion obtained. 
     Examples 20, 21 and 22 illustrate that sulfur in various forms can be used, including sulfur donor systems or insoluble (polymerized) sulfur. Good adhesion and stock failure values are obtained using either system. 
     EXAMPLES 23 AND 24 (TABLE V) 
     Both examples 23 and 24 illustrate that the adhesion of the EPDM to RFL treated fabric is increased by the use of the method of the present invention. 
     TABLE VI 
     EXAMPLES 25-26 
     The results shown in Table VI illustrate that other anhydrides including succinic and phthlate anhydride give good levels of adhesion. They also show that other compounding ingredients may be used without affecting the level of adhesion. 
     TABLE VII 
     EXAMPLES 27-32 
     The results shown in Table VII illustrate that other anhydrides and acids including maleic anhydride or maleic acid, phthalic acid, adipic acid, and acetic anhydride or acetic acid give good levels of adhesion as well as showing that a polyester fabric that has been pretreated with an isocyanate dip prior to the RFL dip can be used. 
     
                       TABLE I                                                     
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Examples 1-8                                                              
Ingredients                                                               
(phr)    1      2      3    4    5    6    7    8                         
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N-t-butyl-2-                                                              
         1.0    1.0    1.0  1.0  1.0  1.0  1.0  1.0                       
benzothiazole                                                             
Sulfur   1.5    1.5    1.5  1.5  1.5  1.5  1.5  1.5                       
5-Norbor-                                                                 
         3.0    3.0    --   1.5  5.0  3.0  3.0  3.0                       
nene-2,3-                                                                 
dicarboxylic                                                              
anhydride                                                                 
Cobalt    0.42  --     --   --   --   0.18 0.54 0.78                      
naphthenate                                                               
(as cobalt                                                                
(phr))                                                                    
Adhesion, pli                                                             
          97     45      8   36   30   62   73   43                       
Stock     100     0      0    0    0   20   20   20                       
failure, %                                                                
Physical                                                                  
Properties                                                                
Tensile, psi                                                              
         2340   2910   2240 2840 2900 2410 2550 2070                      
Modulus,  740    610   1120  980  830  910  590  540                      
300%, psi                                                                 
Elongation                                                                
          640    840    470  560  670  800  770  760                      
at break, %                                                               
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                       TABLE II                                                    
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Examples 9-12                                                             
Ingredients (phr) 9       10     11    12                                 
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N-t-butyl-2-benzothiazole                                                 
                  1.0     1.0    1.0   1.0                                
Sulfur            1.5     1.5    1.5   1.5                                
5-Norbornene-2,3-dicarboxylic                                             
                  --      --     3.0   3.0                                
anhydride                                                                 
5-Norbornene-2,3-dicarboxylic                                             
                  3.0     --     --    --                                 
acid                                                                      
cis-4-cyclohexene-1,2-dicarboxylic                                        
                  --      3.0    --    --                                 
anhydride                                                                 
Cobalt sulfate (as cobalt (phr))                                          
                  --      --      0.84 --                                 
Copper naphthenate                                                        
                  --      --     --     0.44                              
(as copper (phr))                                                         
Adhesion, pli.     39      27     81    53                                
Stock failure, %   0       40     20    0                                 
Physical Properties                                                       
Tensile, psi      2750    3250   2370  2570                               
Modulus, 300%, psi                                                        
                  940     1190   640   740                                
Elongation at break, %                                                    
                  600      560   740   660                                
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                       TABLE III                                                   
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Examples 13-17                                                            
Ingredients (phr)                                                         
             13      14      15    16    17                               
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N-t-butyl-2- 2.0     --      --    --    --                               
benzothiazole                                                             
Sulfur       1.5     1.5     1.5   1.5   1.5                              
2-mercapto-  --      --      1.5   --    --                               
benzothiazole                                                             
2,2&#39;-dibenzothiazyl                                                       
             --      --      --    1.0   --                               
disulfide                                                                 
Thiocarbamyl --      --      --    --    2.0                              
sulfenamide                                                               
5-Norbornene-2,3-                                                         
             3.0     3.0     3.0   3.0   3.0                              
dicarboxylic anhydride                                                    
Cobalt naphthenate                                                        
              0.42    0.42    0.42  0.42  0.42                            
(as cobalt (phr))                                                         
Adhesion, pli                                                             
              63      16      88    82    69                              
Stock failure, %                                                          
              20      0      100    0     0                               
Physical Properties                                                       
Tensile, psi 2260    2550    2100  2370  2000                             
Modulus, 300%, psi                                                        
             560     590     510   600   700                              
Elongation   860     770     860   750   600                              
at break, %                                                               
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                       TABLE IV                                                    
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Examples 18-22                                                            
Ingredients (phr)                                                         
                18     19      20   21    22                              
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N-t-butyl-2-benzothiazole                                                 
                1.0    1.0     1.0  1.0   1.0                             
Sulfur          1.0    2.5     --   --    --                              
Dipentamethylene thiuram                                                  
                --     --      3.0  --    --                              
hexasulfide                                                               
Insoluble sulfur                                                          
                --     --      --   3.0   5.0                             
(polymerized sulfur)                                                      
5-Norbornene-2,3-                                                         
                3.0    3.0     3.0  3.0   3.0                             
dicarboxylic anhydride                                                    
Cobalt naphthenate                                                        
                 0.42   0.42    0.42                                      
                                     0.42  0.42                           
(as cobalt (phr))                                                         
Adhesion, pli    41     37      64   85    62                             
Stock failure, %                                                          
                 20     20      100 100    100                            
Physical Properties                                                       
Tensile, psi    1370   1950    1150 1610  1430                            
Modulus, 300%, psi                                                        
                 300   940      270 810   1070                            
Elongation      1000   520     1000 520    390                            
at break, %                                                               
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                       TABLE V                                                     
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Examples 23-24                                                            
Ingredients (phr)      23       24                                        
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EPDM (DCPD)            100      --                                        
EPDM (1,4-hexadiene)   --       100                                       
N-t-butyl-2-benzothiazole                                                 
                       1.0      1.0                                       
Sulfur                 1.5      1.5                                       
5-Norbornene-2,3-dicarboxylic anhydride                                   
                       3.0      3.0                                       
Adhesion, pli           38       29                                       
Stock failure, %        0        0                                        
Physical Properties                                                       
Tensile, psi           2110     3020                                      
Modulus, 300%, psi     480      410                                       
Elongation at break, % 740      940                                       
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                       TABLE VI                                                    
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Examples 25-26                                                            
Ingredients (phr)  25         26                                          
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N-t-butyl-2-benzothiazole                                                 
                   0.8        0.8                                         
Sulfur             1.5        1.5                                         
Diphenyl guanidine 0.3        0.3                                         
Succinic anhydride 3.0        --                                          
phtalic anhydride  --         3.0                                         
Adhesion, pli       31         29                                         
Stock failure, %    20          0                                         
Physical Properties                                                       
Tensile, psi       2670       2700                                        
Modulus, 300%, psi 1370       1200                                        
Elongation at break, %                                                    
                    460        490                                        
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                       TABLE VII                                                   
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Examples 27-32                                                            
Ingredients (phr)                                                         
               27     28     29   30   31   32                            
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N-t-butyl-2-benzothiazole                                                 
               0.8    0.8    0.8  0.8  0.8  0.8                           
Sulfur          1.25   1.25   1.25                                        
                                   1.25                                   
                                        1.25                              
                                             1.25                         
Diphenyl guanidine                                                        
               0.3    0.3    0.3  0.3  0.3  0.3                           
Maleic anhydride                                                          
               5.0    --     --   --   --   --                            
Maleic acid    --     5.0    --   --   --   --                            
Phthalic acid  --     --     5.0  --   --   --                            
Adipic acid    --     --     --   5.0  --   --                            
Acetic anhydride                                                          
               --     --     --   --   5.0  --                            
Acetic acid    --     --     --   --   --   1.8                           
Adhesion, pli   43     40     50   38   45   43                           
Stock failure, %                                                          
                80     30     20   20    0    0                           
Physical Properties                                                       
Tensile, psi   2050   1890   3000 1320 3390 3690                          
Modulus, 300%, psi                                                        
                400    440   1120  480  910 1160                          
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