Aramid floc reinforcement of rubber using particular adhesive
This invention relates to the reinforcement of rubber by means of aramid fibers. More particularly it relates to the use of chopped aramid fibers, i.e. discrete discontinuous relatively short fibers, in conjunction with an adhesion system to get a higher reinforcement of the rubber. In addition, this invention relates to the use of a floc of aramid fibers in tires to improve the penetration resistance. 
     In certain rubber articles designed to withstand considerable stresses in use, the rubber is reinforced with comparatively inextensible materials. Thus rubber hose, belts, and tires are commonly reinforced with filamentary textiles in the form of yarns, cords or fabric. In such articles, it is important that the textile reinforcing material be firmly adhered to the rubber and remain effectively adhered even after the article has been subjected to repeated varying strains in use, because any separation and relative movement of the rubber and textile parts leads to abrasion between these parts and consequent failure. 
     While the adhesion system herein has been used for other textiles (see U.S. Pat. No. 3,256,137), and floc has been used as a reinforcing agent for tires (see U.S. Pat. No. 3,057,389), and aramid fibers are known for possible use as a tire cord (see U.S. Pat. No. 3,393,210), the combinaton, when the aramid is in the form of a chopped fiber floc, produces a rubber with greatly increased compressive modulus as compared to the normal additive results which would be expected. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to produce a reinforced elastomer blend, i.e. rubber, especially for use in tires, having a greatly increased compressive modulus. 
     It is a further object to provide a method of reinforcing any of the rubbery portions of a tire, especially the tread and breaker plies. 
     It is a further object to adhere a floc of chopped aramid textile fibers to rubber so as to reinforce the rubber. 
    
    
     FIG. 1 is a graph of the compression load results of Examples 1 and 2, wherein the compression modulus for various fibers both with and without the adhesive system is shown. It is readily apparent that the aramid floc plus the adhesive system shows synergistic results. 
     FIG. 2 is another graph of the results of Examples 1 and 2. It shows that the normal result when various flocs are added with the adhesive system is merely additive, i.e. the actual compression modulus is generally the sum of the base rubber modulus plus the increases due to the floc and to the adhesive system. Whereas there is a synergistic combination when aramid floc is used in conjunction with the adhesive system. 
    
    
     The rubber used in the present invention for making the reinforced rubbers for use in tire manufacture and for other purposes may be natural (Hevea) rubber or conjugated diolefine polymer synthetic rubber or mixtures of any of them including their reclaims. Such conjugated diolefine polymer synthetic rubbers are polymers of butadienes-1,3, e.g. butadiene-1,3, isoprene, 2,3-dimethylbutadiene-1,3, and polymers of mixtures thereof and copolymers of mixtures of one or more such butadienes-1,3, with one or more other polymerizable compounds which are capable of forming rubber copolymers with butadienes-1,3, for example, up to 60% by weight of such mixture of one or more monoethylenic compounds which contain a 
     
         CH.sub.2 =C&lt; 
    
     group where at least one of the disconnected valences is attached to an electro-negative group, that is, a group which substantially increases the electrical dissymmetry or polar character of the molecule. Examples of compounds which contain a CH 2  =C&lt; group and are copolymerizable with butadienes-1,3 are aryl olefines, such as styrene, vinyl toluene, alpha methyl styrene, chlorostyrene, dichlorostyrene, vinyl naphthalene; the alpha methylene carboxylic acids and their esters, nitriles and amides, such as acrylic acid, methyl acrylate, methyl methacrylate, acrylonitrile, methacrylonitrile, methacrylamide; vinyl pyridines, such as 2-vinyl pyridine, 2-methyl-5-vinyl pyridine; methyl vinyl ketone. Examples of such conjugated diolefine polymer synthetic rubbers are polybutadiene, polyisoprene, butadiene-styrene copolymers (SBR) and butadiene-acrylonitrile copolymers. The rubber may also contain conventional compounding and vulcanizing ingredients such as carbon black, rubber processing or softening oils which may be added as such or may be present from oil-extended rubbers, antioxidants, sulfur, zinc oxide and accelerators. 
     The aramid fibers used in the present invention are commercially available in yarn form under the trademarks &#34;Fiber B&#34;, &#34;Kevlar&#34;, &#34;DP-01&#34;, and &#34;Nomex&#34;, as well as others. Aramid is the generic name for fibers made from the condensation product of isophthalic or terephthalic acid and m- or p-phenylenediamine. &#34;Fiber B&#34; and &#34;Kevlar&#34; are generally understood to be products of the condensation of terephthalic acid and p-phenylenedianine while &#34;Nomex&#34; is understod to be a product of the condensation of isophthalic acid and m-phenylenediamine. Aramid is defined as &#34;a manufactured fiber in which the fiber-forming substance is a long-chain synthetic aromatic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic linkges.&#34; Further details of the fiber are shown in U.S. Pat. No. 3,393,210, incorporated herein by reference. 
     The aramid fibers which are useable herein are bare (so-called grey) and then choppped into short fibers so as to form the floc. 
     The grey aramid fibers are used in the form of a floc (flock) which is short lengths of fibers randomly disposed (oriented). The grey fibers more readily separate into individual discrete filaments when incorporated into the rubber mixture. The filaments may be of any length, up to about 5 inches in length, preferably about 1/8 (0.125) to 3 inches, most preferably about 1/4 to 1 inch, though the actual size is not critical to the advantages noted herein. 
     The adhesion system useable herein is disclosed in U.S. Pat. No. 3,256,137 and comprises a mixture of a 1-aza-3,7 -dioxabicyclo [3.3.0]-octane or derivative thereof and resorcinol or other meta disubstituted benzene in which each of the substitutents is an OH, NH 2  or OCOCH 3  radical (e.g. m-aminophenol, m-phenylenediamine, resorcinol monoacetate or resorcinol diacetate), or 1,5-naphthalenediol or a fusible partially reacted resorcinol-formaldehyde resin (condensate of resorcinol and formaldehyde in the ratio of 1 mole of resorcinol to about 0.5 to 0.8 mole of formaldehyde). Such fusible partially reacted resorcinolformaldehyde resins may be prepared in known manner by heating a concentrated aqueous solution of the resorcinol and formaldehyde, with or without a catalyst such as oxalic acid. The Ball and Ring softening point (ASTM: E28-58T) of such fusible partially reacted resins will generally be from about 60° C. to 120°C. The 1-aza-3,7-dioxabicyclo[3.3.0]octane and resorcinol, or partially reacted resorcinol-formaldehyde resin, or such other material, react on heating, as in the vulcanization of the rubber to form an infusible resin. 
     The 1-aza-3,7-dioxabicyclo[3.3.0]octanes that may be used are compounds having the general formula ##EQU1## where R is hydrogen, methyl, ethyl, n-propyl, isopropyl, methylol, beta-hydroxyethyl, acetoxymethyl or methoxymethyl. 
     These compounds are described in the paper &#34;Some New Derivatives of Amino Hydroxy Compounds&#34; by Murray Senkus, in Jour. Amer. Chem. Soc. 67 1515-1519 (1945) and in U.S. Pat. No. 2,448,980 to William B. Johnston and, except where R is acetoxymethyl or methoxymethyl are made by the reaction of two moles of formaldehyde or paraformaldehyde with 1 mole of the selected 2-amino-1,3-propanediol. For example, 1-aza-3,7-dioxabicyclo[3.3.0]octane is made by reating 1 mole of 2-amino-1,3-propanediol with 2 moles of paraformaldehyde; 1-aza-5-methyl-3,7-dioxabicyclo[3.3.0] octane is made by reacting 1 mole of 2-amino-2-methyl-1,3-propanediol with 2 moles of paraformaldehyde; 1-aza-5-methylol-3,7-dioxabicyclo[3.3.0]octane the preferred compound is made by reacting 1 mole of 2-amino-2-methylol-1,3-propanediol with 2 moles of paraformaldehyde, etc. The 1-aza-5-acetoxymethyl-3,7-dioxabicyclo[3.3.0] octane is made by reacting 1-aza-5-methylol-3,7-dioxabicyclo[3.3.0] octane with acetic anhydride in the presence of pyridine. The 1-aza-5-methoxymethyl-3,7-dioxabicycol[3.3.0]octane is made by reacting the 1-aza-5-methylol-3,7-dioxabicyclo [ 3.3.0]octane with dimethyl sulfate in a strongly basic alkalimetal hydroxide solution. 
     The components of the present invention, i.e. the solid rubber, the aramid floc, the adhesion system and any other conventional compounding and vulcanizing ingredients, may be combined by means of any conventional equipment, such as a mill or a Banbury mixer, which provides a thorough mixing action. The aramid floc is used in about 0.5 to 50 parts per 100 parts of rubber, preferably 1 to 20 parts. The ratio of the two adhesion system components is not critical and generally will be from one-half to two moles of the 1-aza-3,7-dioxabicyclo[3.3.0] octane per mole of the resorcinol, or resorcinol-formaldehyde condensate or other resin component. The amount of adhesion components mixed with the rubber is not critical. Generally, 0.5 to 8 parts, and preferably 1 to 4 parts of the two components (total) per 100 parts of the rubber will be mixed in the rubber compound. 
     The following examples illustrate the invention. All parts and percentages referred to herein are by weight unless otherwise indicated. 
     EXAMPLE 1 
     A rubber compound masterbatch was prepared with the following ingredients: 50 parts natural rubber, 20 parts cis-polybutadiene, 41.25 parts oil extended styrene-butadiene rubber (23.5% styrene), 13.75 parts petroleum distillate oil, 50 parts carbon black, 5 parts activated zinc oxide, 1 part stearic acid, 0.5 parts Betanox Special (antioxidant of Uniroyal, Inc.) 1 part cyclohexylbenzothiazole disulfate, 0.2 parts diphenyl guanidene, and 3 parts sulfur. 
     The masterbatch was then used to make up six blends as shown in Table I having (1) no floc, (2) aramid (Kevlar) floc, (3) polyester floc, (4) nylon floc, (5) rayon floc, and (6) glass fiber (RICS) floc, with the flocs being added in equal volume loadings. All of the flocs were nominally 1/2 inch in length i.e. 1/4 to 1 inch fibers. 
     The blends were each cured for 60 minutes at 293°F. and physical properties were determined with the results as shown in Table I. For the compression modulus tests the samples were cured for 79 minutes at 293°F. 
     
                                           Table I                                 
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            A871MB                                                        
                  A871   A872   A873   A874   A875                        
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Ingredients                                                               
Masterbatch 185.70                                                        
                  185.70 185.70 185.70 185.70 185.70                      
Aramid            4.00                                                    
Polyester                4.00                                             
Nylon                           3.30                                      
Rayon                                  4.22                               
Glass fibers                                  6.60                        
Physical Properties                                                       
Mill Grain                                                                
Modulus-100%      1050   775    250    275    700                         
    200%          1225   1000   775    775    1050                        
    300%          --     1500   1350   1350   --                          
Tensile           1575   1875   2150   1800   1450                        
% Elongation      245    380    440    385    285                         
Shore A Hardness  73     67     62     63     67                          
Cross Grain                                                               
Modulus-100%      425    350    225    225    475                         
    200%          850    800    650    650    850                         
    300%          --     1325   1175   1200   --                          
Tensile           1100   1625   1950   1675   1550                        
% Elongation      265    360    440    380    320                         
Compression Modulus (Compression Strain, lbs. force)                      
10% Compression                                                           
            117   159    160    123    130    166                         
15% Compression                                                           
            173   261    236    183    202    264                         
20% Compression                                                           
            239   400    322    258    267    362                         
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     EXAMPLE 2 
     The procedure of Example 1 was repeated using the same masterbatch and flocs as indicated, but further incorporating in each of the blends before curing, an example of the adhesion system usable herein. 
     1.50 parts of resorcinol monoacetate (RMA) and 1.75 parts of 1-aza-5-methylol-3,7-dioxabicyclo[3.3.0] octane(GDUE). 
     The samples were then cured as in Example 1 and physical properties were determined with the results as shown in Table II. 
     
                                           Table II                                
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            A871MB                                                        
                  A876   A877   A878   A879   A880                        
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Ingredients                                                               
Masterbatch 185.70                                                        
                  185.70 185.70 185.70 185.70 185.70                      
RMA         1.50  1.50   1.50   1.50   1.50   1.50                        
GDUE        1.75  1.75   1.75   1.75   1.75   1.75                        
Aramid            4.00                                                    
Polyester                4.00                                             
Nylon                           3.30                                      
Rayon                                  4.22                               
Glass fibers                                  6.60                        
Physical Properties                                                       
Mill Grain                                                                
Modulus-100%      1325   775    625    575    950                         
    200%          --     1175   1250   1075   1300                        
     300%         --     2025   1775   1700   --                          
Tensile           1400   2125   1875   1800   1650                        
% Elongation      150    310    325    320    260                         
Shore A Hardness  73     69     65     66     70                          
Cross Grain                                                               
Modulus-100%      625    400    425    400    550                         
    200%          1150   875    1050   875    1000                        
    300%          --     1500   1675   1525   --                          
Tensile           1275   1850   2000   1700   1550                        
% Elongation      235    350    355    335    280                         
Compression Modulus (Compression Strain, lbs. force)                      
10% Compression                                                           
            143   266    229    150    160    200                         
15% Compression                                                           
            213   519    330    225    235    307                         
20% Compression                                                           
            283   764    442    313    327    416                         
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     A comparison of the results of Examples 1 and 2, as is shown visually by FIGS. 1 and 2 shows that the use of either a floc or the adhesion system alone will result in a higher compression modulus and lower elongaton, i.e. a stiffer rubber. When both a floc and the adhesion system are used together, the results show that for polyester, nylon, rayon, or glass fiber the results are essentially only additive, i.e. the overall increase is approximately the sum of the two increases. However, for aramid (Kevlar) floc, the combination shows synergistic results much greater than the mere sum of the two effects. 
     EXAMPLE 3 
     In order to determine what effect the spin finish, which is normally present on the aramid fibers when purchased, has on the present invention, the chopped aramid and some chopped nylon with a similar finish were both treated to remove the spin finish. Each group of fibers (about 1/2 inch in length) were treated by first washing with petroleum ether and then ethanol. The fibers were then dried in a vacuum oven overnight at 60°F. This is in accord with ASTM Procedure D2257-69. 
     The fibers treated as above were then incorporated into the masterbatch of Example 1 with the adhesion system of Example 2. Test samples were prepared as in the previous Example and the compression modulus results were as follows: 
     
                       Compression Modulus Lbs. Force                              
              10%     15%      20%                                        
______________________________________                                    
Aramid, treated as above                                                  
                514       894      1212                                   
Aramid, untreated                                                         
                266       519      764                                    
Nylon, treated as above                                                   
                158       237      327                                    
Nylon, untreated                                                          
                150       225      313                                    
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     Accordingly, it can be seen that removal of the spin finish greatly improves the results with aramid floc, while having almost no effect on nylon floc. 
     EXAMPLE 4 
     The procedures of the previous Examples were repeated using an all natural rubber based blend with HAF-carbon black therein. The masterbatch composition was as follows: 100 parts natural rubber, 0.10 parts zinc-2-benzamid thiophenate peptizer, 45 parts HAD carbon black, 5 parts activated zinc oxide, 2.50 parts stearic acid, 6 parts pine tar oil, 1.50 parts N-alkyl, naphthenyl-p-phenylene diamine, 0.50 parts phenol-formaldehyde tackifying resin, 0150 parts N-t-butyl-2-benzothiazole sulfenamide, 0.10 parts PVI retarder, and 2.65 parts flowers of sulfur. 
     To the above masterbatch was then added the respective flocs in the amounts shown previously in Example 1, the samples milled and cured and compression modulus tested as previously. The results were: 
     
         Compression Modulus                                                       
Compression 10%        15%        20%                                     
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Compression 10%        15%        20%                                     
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No floc     118        179        235                                     
Aramid      233        372        516                                     
Polyester   203        302        389                                     
Nylon       164        224        300                                     
Rayon       163        233        302                                     
Glass fibers                                                              
            209        302        402                                     
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     To mixtures of the above were added adhesion system chemicals as in Example 2 (with respect to Example 1). 1.50 parts of resorcinol monacetate and 1.75 parts of 1-aza-5-methylol-3,7-dioxabicyclo[3.3.0]octane were added and the results were as follows: 
     
         Compression Modulus                                                       
Compression Strain   10%     15%     20%                                  
______________________________________                                    
Adhesion system plus no floc                                              
                     206     295     377                                  
Adhesion system plus Aramid                                               
                     371     608     857                                  
Adhesion system plus Polyester                                            
                     283     377     500                                  
Adhesion system plus Nylon                                                
                     208     284     388                                  
Adhesion system plus Rayon                                                
                     206     291     377                                  
Adhesion system plus Glass fibers                                         
                     232     322     427                                  
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     The synergistic combination of the adhesion system and aramid floc results in greatly increased compression moduli whereas only slight increases are noted with the other chopped fibers. 
     EXAMPLE 5 
     The procedures of Examples 1 and 2 were repeated using a natural rubber based stock with hydrated silica as the primary filler. The flocs and adhesion systems (both with parts used), and results were as follows: 
     
                    A    B      C       D    E     F                               
______________________________________                                    
Floc                                                                      
 Aramid      --     4.00   --    --   4.00  --                            
 Nylon       --     --     4.00  --   --    4.00                          
Adhesion system                                                           
 RMA         --     --     --    1.50 1.50  1.50                          
 GDUE        --     --     --    1.75 1.75  1.75                          
Compression Modulus                                                       
Compression Strain                                                        
10%          152    160    126   260   480  256                           
15%          222    286    200   370   790  404                           
20%          308    426    280   480  1040  559                           
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     These results again show that there is synergism between the aramid floc and the adhesion system. 
     EXAMPLE 6 
     To show that the synergism of the adhesion system with the aramid floc does not repeat for aramid cord (though there is enhanced cord adhesion), H-adhesive strength was determined according to ASTM D2138-72. Samples were prepared using the all natural rubber masterbatch of Example 4 with and without the adhesion system and nylon, rayon, polyester, and aramid cords. The samples used were cured for 45 minutes at 293°F. and tested at 250°F. 
     The results were as follows: 
     
                   Pull (lbs.)                                                     
          No Adhesion System                                              
                       Adhesion System                                    
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Nylon results                                                             
            4.2            17.6                                           
Rayon results                                                             
            3.1            15.3                                           
Polyester results                                                         
            4.2            5.1                                            
Aramid results                                                            
            6.1            16.1                                           
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     Thus, the percent improvement for aramid cord was actually less than for nylon and rayon.