Abstract:
In accordance with the present invention there is provided a vulcanizable halogenated butyl rubber compound having an improved degree of scorch resistance without causing unreasonably long cure time. The compound is characterized in that it comprises a retarder conforming structurally to ##STR1## wherein X is a C 1  -C 6  alkyl and 
     n is an integer from 0 to 4. 
     The physical properties of the compound are substantially unchanged by the incorporation of the retarder.

Description:
FIELD OF THE INVENTION 
     The invention relates to the stabilization of compounds based on halogenated elastomers including bromo butyl rubber and chloro butyl rubber; more specifically the invention relates to retarding the cure of these rubbers in order to prevent premature vulcanization (scorch). 
     BACKGROUND OF THE INVENTION 
     Halogenated elastomers, chloro and bromo butyl rubbers are widely used in the preparation of mechanical goods and of inner liners of tires. These rubbers are characterized by their excellent chemical resistance, and low gas permeability. A salient drawback of compounds based on these rubbers is their inherent susceptibility to premature vulcanization, i.e. scorching during processing and/or storage. Premature vulcanization causes the compounds to become tough and unworkable. Commonly used cure systems for these elastomers therefore include retarders. 
     In U.S. Pat. No. 3,865,763 certain boron compounds were disclosed to be effective by retarding the scorch of halogenated butyl rubbers. In U.S. Pat. No. 3,880,821 there were disclosed certain free acids or the magnesium, aluminum, calcium, and barium salts of an oligomer of linoleic acid to be suitable scorch retarding agents in halogenated butyl rubbers. In U.S. Pat. No. 4,128,510 it was disclosed that certain derivatives of 2,5-dimercapto-1,3,4-thiadiazole used in the presence of certain basic materials are suitable in promoting crosslinking without sticking or premature curing of certain saturated and unsaturated halogen containing polymers. These halogen containing polymers include chloro butyl rubber and bromo butyl rubber. U.S. Pat. No. 4,410,704 disclosed N-substituted benzothiazole-2-sulfonamide to be useful in preventing scorching in halogenated butyl rubbers. In U.S. Pat. No. 4,569,958 there were disclosed compounds containing halogenated butyl rubbers which have been rendered scorch resistant upon the addition of a di or tri-thiol-s-triazine compound and at least one carboxylic acid compound. A cure system used in crosslinking halogenated butyl rubbers disclosed in U.S. Pat. No. 4,514,396 uses a thiourea derivative as a crosslinking agent, an inorganic base as an acid exceptor and a thiuran sulfide as an accelerator. 
     In addition to the above, dibenzothiazyl disulfide (MBTS) tetramethylthiuran disulfide (TMTD) and N-cyclohexylbenzothiazyl-2-sulfenamide (CBS) have been disclosed to be useful as retardants. For instance, in a zinc oxide/sulfur cure system MBTS is often added. This system is not entirely satisfactory, especially for bromo butyl rubbers, because it results in an unfavorable cure rate and a reduction in the level of the mechanical properties after aging. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a vulcanizable halogenated butyl rubber compound having an improved degree of scorch resistance without the accompanying unreasonably long cure time which is characterized in that it incorporates a retarder conforming structurally to ##STR2## wherein X denotes a C 1  -C 6  alkyl and 
     n denotes an integer of from 0 to 4. 
     The retarder of the invention has no significant adverse effect on the physical properties of the elastomer. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The term halogenated butyl rubber refers to a vulcanizable rubbery copolymer containing from about 85 to about 99.5% C 4  -C 8  isoolefin and 0.5 to 15% conjugated C 4  -C 8  diolefin. In particular, the halogenated rubber is a copolymer of isobutylene and isoprene prepared from about 99.5% to about 98.5% isobutylene and from about 1.5 to about 4.5% isoprene. The halogenation of the copolymers is carried out by well known processes and result in a halogenated content of about 0.5 to about 15 weight percent. Most preferably the halogenated rubber is a brominated or chlorinated butyl rubber containing from about 1.5 to 2.5 weight percent of bromine or chlorine. The retarder of the present invention conforms to ##STR3## wherein X is a C 1  -C 6  alkyl radical and 
     n is an integer of from 0 to 4. Benzotriazole and tolyltriazole are the preferred species. The retarders are readily synthesized in accordance with known procedures. 
     The preferred vulcanization retarder of the present invention is a blend of 4- and 5-methylbenzotriazole (tolytriazole) conforming to the formula ##STR4## It is commercially available under the trademark Vulkalent™ from Mobay Corporation and it is characterized in that its density is approximately 1.2. gm/cm 3  and its melting point is about 181° F. (83° C.) It is further characterized in that it is soluble in acetone, ethyl acetate, ethanol, methylethylketone and methylene chloride and is sparingly soluble in toluene and insoluble in hexane and water. Tolyltriazole has been previously recognized as a prevulcanization retarder in sulfur modified polychloroprene rubbers. It is also effective in nitrile rubber compounds provided MBTS/sulfur curing systems are used. 
     The triazole of the invention is easily incorporated in rubber compounds. The addition of about 0.1 to 5.0, preferably 0.2 to 2 phr (parts per one hundred parts of rubber) of the triazole in 100 parts by weight of the elastomer was found to impart scorch safety to the compound. Significantly, rubber goods containing triazole were found to be generally color stable and to have their mechanical properties practically unaffected by the addition of the retarder. 
    
    
     EXAMPLES 
     Experimental 
     In evaluating the rubber compounds the following standards were used. 
     ML1+4&#39; @ 100° C.--compound viscosity before vulcanization per ASTM D-1646. 
     MS5 @ 135° C. (min)--scorch resistance at 135° C. in minutes per ASTM D-1646. 
     RHEOMETER @ 177° C.--(per ASTM D-2084). 
     T2=time (minutes) until a 2 lb-in in rise from minimum. 
     T90=time (minutes) until the compound reaches 90% of total cure. 
     Physical properties per ASTM D-412. 
     Generally, a system should have high &#34;MS5 @ 135° C.&#34; values, i.e. long time to premature vulcanization, along with a minimum change in its T90, Rheometer maximum torque and physical properties. The physical properties were determined using specimens which were cured under the noted conditions. 
     In the following experiments the noted additives are: 
     Carbon Black N-660--moderately reinforcing carbon black; 
     N-550--moderate to high reinforcing carbon black; 
     N-330 and N-339--highly reinforcing carbon black; 
     Sunpar 2280 is paraffinic oil--a plasticizer; 
     Maglite D--magnesium oxide; 
     Vulkacit DM/C--mercapto benzothiazole disulfide (MBTS); 
     Vultac 5--alkyl phenol disulfide; 
     Vulkalent™--tolytriazole; 
     Vulkanox DDA--styrenated diphenyl amine; 
     DOP--di-octyl phthalate (a plasticizer); 
     Vulkacit Thiuram/C--tetramethyl thiuram disulfide TMTD; a cure accelerator; 
     Vulkacit MOZ/LG and Vulkacit MOZ/SG-2--(morpholinothio) benzothiazole (MBS), a cure accelerator; 
     Sundex 8125--aromatic oil, used as a plasticizer. 
     Sunolite 240--an ozone protective wax; 
     Vulkanox 4020--N-(1,3-dimethylbutyl)N&#39;-phenyl-p-phenylenediamine (an antiozonant); 
     Vulkanox HS/LG--2,2,4-trimethyl-1,2-dihydroquinoline (polymerized); an antioxidant; 
     Crystex (80%)--insoluble sulfur, a curing agent. 
     The tabulated amounts are in phr (per 100 parts of rubber). 
     In SBR compounds (styrene-butadiene copolymer rubber) the tolyltriazole of the invention does slow the scorching but only when present at twice the level of the commonly used retarder--cyclohexyl-thio-phthalimide, CTP--and even then, at a much reduced effectiveness. 
     The compounds shown below also contained conventional, known additives as follows: Sundex 8125--16 phr; carbon black N-339--65 phr; zinc oxide--5 phr; stearic acid--2 phr; Sunolite 240--1 phr; Vulkanox 4020--2 phr and Vulkanox HS/LG--1.5 phr. None of these is believed to be critical in relation to the demonstrated relative efficacy of the retarders. 
     
         ______________________________________FORMULATION        CONTROL    1       2______________________________________SBR                100        100     100Insoluble Sulfur   1.88       1.88    1.88MBS                1.5        1.5     1.5Tolyltriazole      --         1       --CTP                --         --      0.5PropertiesML1 + 4&#39; @ 100° C.              61         60      60MS5 @ 121° C. (min)              76.8       83.2    147.2RHEOMETER @ 160° C.T2 (min)           7.8        7.5     11.2T90 (min)          19.0       18.6    24.2Torque, Min. (in-lb)              10         10      9Torque, Max. (in-lb)              69         63      64PHYSICAL PROPERTIES(Cured for 90 minutes at 160° C.)Hardness, Shore A  70         67      68Tensile Strength, psi              3110       3030    3135Elongation (%)     510        585     545100% Modulus, psi  380        320     350300% Modulus, psi  1765       1445    1635______________________________________ 
    
     Also in natural rubber compounds the retarder of the invention is less effective then CTP which is the commercial standard for these compounds. The compound presented below contained in addition to the tabulated components the following additives: carbon black (N-330)--55 phr; zinc oxide--5 phr; Sundex 8125--3 phr; stearic acid--2.5 phr; Sunolite 240--1 phr; Vulkanox 4020--2 phr; Vulkanox HS/LG-1.5 phr; none of which is believed to be critical in demonstrating the relative efficacy of the retarders. 
     
         ______________________________________FORMULATION       CONTROL    1       2______________________________________Natural rubber (SMR-L)             100        100      100Insoluble Sulfur  2.75       2.75     2.75MSB               0.5        0.5      0.5Tolyltriazole     --         1        --CTP               --         --       0.5COMPOUND PROPERTIESML1 + 4&#39; @ 100° C.             74         76       72MS5 @ 121° C. (min)             31.0       43.5     84.8RHEOMETER @ 150° C.T2 (min)          4.6        5.0      11.5T90 (min)         18.8       22.4     26.5Torque, Min. (in-lb)             15         16       15Torque, Max. (in-lb)             68         62       66PHYSICAL PROPERTIESCured at 150° C. for             20 min     25 min   30 minHardness, Shore A 65         62       60Tensile Strength, psi             3335       3330     3345Elongation, (%)   490        530      495100% Modulus, psi 380        330      360300% Modulus, psi 1895       1640     1835______________________________________ 
    
     The following demonstrates the efficacy of the retarder of the invention in bromobutyl rubber, using a ZnO/Sulfur cure system. The Table contains a comparison between the retarder of the invention on the one hand and CTP and MBTS (mercapto benzothiazole disulfide) on the other. 
     Each of the compositions contained also the following: 65 phr of carbon black (N-660); 3 phr of zinc oxide; 0.5 phr of sulfur; 1 phr--stearic acid; 12 phr--Sunpar 2280 and 0.5 phr--Maglite D, which are believed to have no effect on the demonstrated relative efficacy of the retarders. 
     
         ______________________________________            CON-FORMULATION      TROL    1      2    3    4______________________________________Bromobutyl Rubber            100     100    100  100  100MBTS             --      --     --   1    --Tolyltriazole    --      0.5    1    --   --CTP              --      --     --   --   0.5COMPOUND PROPERTIESML1 + 4&#39; @ 100° C.            64      68     64   64   66MS5 @ 135° C. (min)            8.9     11.5   12.0 14.4 8.4RHEOMETER @ 177° C.(60 min chart)T2 (min)         4.0     4.4    5.4  6.0  3.5T90 (min)        26.3    23.9   27.5 33.0 30.5Torque, Min. (lb-in)            15      14     14   15   14Torque, Max. (lb-in)            54      55     54   49   50PHYSICAL PROPERTIESCure time at 177° C.            23      21     21   24   25(min):Hardness, Shore A            59      59     58   58   57Tensile strength,            2045    2045   2070 1930 2055psiElongation, (%)  380     410    420  495  455100% Modulus, psi            330     350    335  285  300300% Modulus, psi            1665    1575   1610 1335 1445______________________________________ 
    
     A corresponding showing in the context of chlorobutyl rubber is shown below. This system too used a ZnO/Sulfur cure system 
     The compounds also contained 65 phr of carbon black; 3 phr of zinc oxide; 0.5 phr of sulfur; 1 phr of stearic acid and 12 phr of Sunpar 2280 which are believed to have no criticality in the showing of the relative efficacy of the retarder of the invention. 
     
         ______________________________________FORMULATION      CONTROL    1      2    3______________________________________Chlorobutyl Rubber            100        100    100  100Tolyltriazole    --         0.5    --   --Magnesium Oxide  --         --     0.5  --CTP              --         --     --   0.5COMPOUND PROPERTIESML1 + 4&#39; @ 100° C.            60         58     58   58MS5 @ 135° C. (min)            21.2       63.2   63.5 35.0RHEOMETER @ 177° C.T2 (min)         2.0        3.1    6.2  2.5T90 (min)        6.6        13.0   29.0 17.5Torque, Min. (lb-in)            11         10     11   10Torque, Max. (lb-in)            32         33     36   32PHYSICAL PROPERTIES(Cured to T90 @ 177° C.)Hardness, Shore A            52         53     54   53Tensile Strength, psi            1620       1755   1700 1690Elongation (%)   520        570    440  580100% Modulus, psi            195        205    250  190300% Modulus, psi            925        900    1130 845______________________________________ 
    
     In the following Tables there is demonstrated the efficacy of tolyltriazole as a retarding agent in variously cured chlorobutyl rubbers. The Tables also contain a comparison between the retarder of the invention and other known retarders. The cure system in the Table below is ZnO/alkylphenol disulfide/sulfur. 
     The compositions also contained 65 phr of carbon black; 3 phr of zinc oxide; 0.5 phr of sulfur and 1 phr of Vultac 5; 1 phr of stearic acid and 12 phr of Sunpar 2280 which are believed to have no criticality in the present context. 
     
         ______________________________________            CON-FORMULATION      TROL    1      2    3    4______________________________________Chlorobutyl Rubber            100     100    100  100  100MBTS             --      1      --   --   --Magnesum Oxide   --      --     0.5  --   --Tolyltriazole    --      --     --   0.5  1COMPOUND PROPERTIESML1 + 4&#39; @ 100° C.            58      59     60   58   59MS5 @ 135° C. (min)            5.2     9.5    9.5  10.0 12.5RHEOMETER @ 177° C.T2 (min)         1.4     1.9    2.8  2.0  2.4T90 (min)        9.2     9.0    20.6 11.4 13.0Torque, Min. (lb-in)            11      11     11   11   11Torque, Max. (lb-in)            46      43     51   47   47PHYSICAL PROPERTIES(Cured to T90 @ 177° C.)Hardness, Shore A            56      58     58   56   58Tensile Strength,            1885    1735   1855 1880 1865psiElongation (%)   450     550    385  485  455100% Modulus, psi            300     270    370  265  310300% Modulus, psi            1365    1100   1560 1240 1330______________________________________ 
    
     A similar comparison is presented below for chlorobutyl rubber where the cure system is ZnO/Sulfur/MBTS. 
     The compositions also contain 65 phr of carbon black (N-660); 3 phr of zinc oxide; 0.5 phr of sulfur; 1 phr of MBTS; 1 phr of stearic acid and 12 phr of Sunpar 2280 which re believed to have no criticality in the context of the invention. 
     
         ______________________________________FORMULATION      CONTROL    1      2    3______________________________________Chlorobutyl Rubber            100        100    100  100Magnesium Oxide  --         0.5    --   --Tolyltriazole    --         --     0.5  1COMPOUND PROPERTIESML1 + 4&#39; @ 100° C.            55         57     56   55MS5 @ 135° C. (min)            18.8       56.8   35.3 61.5RHEOMETER @ 177° C.T2 (min)         2.5        4.5    3.3  4.7T80 (min)        6.6        11.6   7.4  9.2T90 (min)        13.1       13.6   10.0 10.6Torque, Min. (lb-in)            11         11     11   10Torque, Max. (lb-in)            33         36     33   33PHYSICAL PROPERTIES(cured to T90 @ 177° C.)Hardness, Shore A            53         54     55   55Tensile Strength, psi            1600       1695   1610 1675Elongation, (%)  610        555    650  640100% Modulus, psi            185        190    180  185300% Modulus, psi            810        905    780  790______________________________________ 
    
     In the following Table the comparison is based on chlorobutyl rubber where the cure system is ZnO/Sulfur/alkylphenol disulfide/MBTS. 
     The compositions also contain 65 phr of carbon black (N-660); 3 phr of zinc oxide; 0.5 phr of sulfur; 1 phr of alkyl phenol disulfide; 1 phr of MBTS; 1 phr of stearic acid and 12 phr of Sunpar 2280 which are believed to have no criticality in the context of the invention. 
     
         ______________________________________FORMULATION      CONTROL    1      2    3______________________________________Chlorobutyl Rubber            100        100    100  100Magnesium Oxide  --         0.5    --   --Tolyltriazole    --         --     0.5  --CTP              --         --     --   0.5COMPOUND PROPERTIESML1 + 4&#39; @ 100° C.            58         58     58   59MS5 @ 135° C. (min)            10.2       15.0   13.0 11.9RHEOMETER @ 177° C.T2 (min)         1.8        2.7    2.2  2.1T90 (min)        7.0        13.0   7.6  12.0Torque, Min. (lb-in)            12         12     11   10Torque, Mix. (lb-in)            44         49     43   39PHYSICAL PROPERTIES(Cured to T90 at 177° C.)Hardness, Shore A            57         55     57   57Tensile Strength, psi            1690       1785   1710 1640Elongation (%)   620        510    585  600100% Modulus, psi            230        285    240  245300% Modulus, psi            975        1215   1005 935______________________________________ 
    
     The following table summarizes the evaluation of the properties of a chlorobutyl rubber compound which contains no retarder to similar compounds which contain either benzotriazole or tolyltriazole. The compound described below contained also 1 phr of stearic acid, 65 phr of carbon black (N-660); 12 phr of Sunpar 2280; 3 phr of zinc oxide; and 0.5 phr of sulfur. 
     
         ______________________________________FORMULATION        1         2      3______________________________________Chlorobutyl rubber, phr              100       100    100Tolyltriazole      --        0.5    --Benzotriazole      --        --     0.5COMPOUND PROPERTIESML 1 + 4&#39; @ 100° C.              56        57     56MS 5 @ 135° C.(min)              29.0      60.5   62.0RHEOMETER @ 177° C.T2 (min)           2.0       3.1    3.1T90 (min)          8.1       10.4   11.4MIN. TORQUE (lb-in)              10        10     10MAX. TORQUE (lb-in)              31        32     35PHYSICAL PROPERTIESCURED T90 @ 177 C.HARDNESS (SHORE A) 51        51     54TENSILE (psi)      1565      1590   1680ELONGATION (%)     535       545    480100% MOD. (psi)    175       195    230300% MOD. (psi)    810       845    1045______________________________________ 
    
     The above data shows that the scorch resistance of halogenated butyl rubber is significantly improved by the practice of the present invention. The incorporation of the claimed triazoles in halogenated butyl rubber in sufficient amount to reduce scorch does not adversely effect the mechanical properties of the compounds. 
     The following demonstrates that the addition of tolyltriazole retarder to bromobutyl rubber or to chlorobutyl rubber, does not adversely affect the heat aging or flex cracking of these rubbers. The bromobutyl rubber compound contained in the following components (the amounts are noted per 100 weight of rubber--phr); stearic acid--1 phr; N660 carbon black--65 phr; Sunpar 2280--12 phr; Maglite D--0.5 phr; Zinc oxide--3 phr; spider sulfur--0.5 phr. The chlorobutyl rubber contained the following: Stearic acid--1 phr; N660 carbon black--65 phr; Sunpar 2280--12 phr; Zinc oxide--3 phr; Sulfur spider--0.5 phr. 
     The Tables below show a comparison between the properties of these rubber systems containing tolyltriazole and similar systems containing other, known retarders. Table A is directed to bromobutyl rubber and Table B is directed to chlorobutyl rubber. 
     
                       TABLE A______________________________________Tolyltriazole    --     0.5    1.0  --   --MBTS             --     --     --   1.0  --CTP              --     --     --   --   0.5Physical Properties(T90 @ 177° C.)Hardness (Shore A)            59     59     58   58   57Tensile Strength (psi)            2045   2045   2070 1930 2055Elongation (%)   380    410    420  495  445300% Modulus (psi)            1665   1575   1610 1335 1445Hot Air Aging: 3d* @ 135° C.Hardness Change  0      -1     -1   +4   +2(Shore A)Tensile Retention (%)            69     68     65   82   68Elongation Retention (%)            111    113    119  91   96300% Mod. Retention (%)            68     65     61   93   77Hot Air Aging: 7d* @ 100° C.Hardness Change  +2     +2     +4   +3   +5(Shore A)Tensile Retention (%)            88     86     84   89   85Elongation Retention (%)            81     83     80   75   74300% Mod. Retention (%)            105    102    104  116  116DeMattia - Crack Growth**(% Cracking) Unaged (kc***)1                9      9      9    7    86                27     18     22   13   2010               37     34     30   20   2520               50     50     43   30   4040               84     80     72   60   65100              100    100    98   90   100Hot Air: 7d @ 100° C. (kc)1                14     10     12   12   126                43     27     33   37   3110               59     36     43   49   3720               84     53     65   73   5440               96     73     86   92   7370               100    87     98   100  82______________________________________ *d-days **per ASTM D813 ***kilocycles 
    
     
                       TABLE B______________________________________MBTS                 1.0    1.0    1.0  1.0Tolyltriazole        --     0.5    1.0  --Magnesium Oxide      --     --     --   0.5Physical Properties (T90 @ 177° C.)Hardness (Shore A)   53     55     55   54Tensile Strength (psi)                1600   1610   1645 1695Elongation (%)       610    650    640  555300% Modulus (psi)   810    780    790  905Hot Air Aging: 5d* @ 135° C.Hardness Change (Shore A)                +4     +3     +5   +6Tensile Retention (%)                79     79     78   82Elongation Retention (%)                64     61     64   68300% Mod. Retention (%)                128    135    134  133Hot Air Aging: 7d* @ 100° C.Hardness Change (Shore A)                +2     +2     +3   +6Tensile Retention (%)                99     99     91   99Elongation Retention (%)                77     75     70   75300% Mod. Retention (%)                128    131    133  143DeMattia - Crack Growth**(% Cracking)Unaged (kc)***10                   10     8      9    1050                   20     10     11   14100                  35     15     14   28200                  52     22     26   40400                  74     42     42   58500                  90     52     52   66Hot Air: 7d* @ 100° C.(kc)10                   15     11     16   1450                   38     20     34   35100                  56     38     52   57200                  78     69     74   86250                  89     75     84   92______________________________________ *d-days **per ASTM D813 ***kilocycles 
    
     In the preferred embodiment of the invention, the triazole is added to the halogenated butyl rubber by mixing in a suitable mixer. Such mixing may be carried out simultaneously with the other conventional additives or it can be incorporated in the rubber either before, during or after the addition of the normal compounding ingredients. The triazole should be introduced into the compound at an effective amount to achieve the desired stabilization. 
     Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.