Abstract:
Rubber compositions are disclosed for use as inner liners in automobile tires. Sulfur-vulcanizable, halogenated butyl rubbers are combined with 2-(4&#39;-morpholinodithio)benzothiazole, whereby the resulting composition is rendered highly tacky and satisfactorily processable.

Description:
This application is a continuation of application Ser. No. 07/152,823 filed Feb. 5, 1988 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to rubber compositions suitable for use particularly as inner linings in automobile tires. 
     2. Description of the Prior Art 
     Halogenated butyl rubbers are known in common use as inner liners for automotive tires. These rubbers are acceleratively sulfur-vulcanizable with use of certain accelerating agents. Dibenzothiazyl disulfide (DM) has been proposed to this end as disclosed for instance in Japanese Patent Laid-Open Publication Nos. 57-172945, 57-195760 and 59-89208. Japanese Patent Laid-Open Publication No. 59-120501 is directed to incorporating tetramethylthiuram disulfide (TT). A still another accelerated system of sulfur vulcanization is that two different accelerators are combined as taught in Japanese Patent Publication No. 57-49405 which involves the use of TT/DM or alkylphenol disulfide (APDS)/DM. Japanese Patent Laid-Open Publication No. 55-80446 discloses that halogenated butyl rubbers may be used to improve durability of tire sidewalls. 
     DM would often make the resulting rubber composition readily susceptible to blooming and hence less tacky in an unvulcanized state, and therefore, hardly processable. Blooming is the more frequent the higher the processing temperature. TT would be ineffective for improving tack strength, whereas APDS would be less resistant to scorch usually taken to mean premature vulcanization. Moreover, TT and APDS would lead to excessive crosslinking, resulting in a vulcanizate of inadequate elongation. 
     SUMMARY OF THE INVENTION 
     It is the primary object of the present invention to provide an improved rubber composition for use in the inner lining of an automobile tire which excels in tackiness prior to vulcanization without involving objectionable blooming, thus contributing to the provision of tires of enhanced physical characteristics. 
     The rubber composition contemplated under the invention is applicable, where desired, to tire sidewalls so as to attain improved durability. 
     More specifically, such rubber composition comprises a starting rubber vulcanizable with sulfur, the starting rubber comprising a halogenated butyl rubber, and as a vulcanization accelerator 2-(4&#39;-morpholinodithio)-benzothiazole. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Starting rubbers eligible for the purpose of the present invention are sulfur-vulcanizable, halogenated butyl rubbers, examples of which include chlorinated butyl rubber, brominated butyl rubber and the like. The starting rubber may be blended with a diene rubber such for example as ethylene/butadiene copolymer rubber, totalling at 100 parts by weight. 
     According to an important aspect of the invention, 2-(4&#39;-morpholinodithio)benzothiazole (MDB) should be used as a vulcanization accelerator. When it is desirable to further improve vulcanizate quality, co-accelerators may be added which include tetramethylthiuram disulfide and alkylphenol disulfide for their freedom from blooming. No particular restriction is imposed upon the amount of MDB to be used which however may be feasibly in the range of 0.2 to 2.0 parts by weight based on 100 parts by weight of the starting rubber. 
     In Table 1 there is shown a typical recipe for the rubber composition according to the invention. 
     
                       TABLE 1______________________________________Component        Weight Part______________________________________halogenated rubber             30-100diene rubber     70-0sulfur           0.2-2.0MDB              0.2-2.0______________________________________ 
    
     Various other additives may be employed which include reinforcing agents, fillers, tackifiers, softeners, vulcanizing activators and the like. 
     The following examples are provided for a better understanding of the present invention. All formulations are in part by weight in these examples. 
    
    
     EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLES 1 TO 10 
     Different rubber compositions were prepared as shown in Table 2. All these compositions were examined for their physical properties with the results tabulated. 
     Tackiness and blooming were determined under the following conditions. 
     Tackiness 
     A vulcanized test composition was rolled at from 105° to 100° C. into a rubber sheet which was thereafter exposed to accelerated blooming on a stitcher. Tack strength was measured at an interval of 4, 24 and 72 hours and at a peel speed of 125 cm/minute with the sheet contact-bonded at 500 g for 3 seconds with use of a tester tradenamed Picma-Tack Tester II, Toyo Seiki Seisakusho, Ltd. 
     Blooming 
     Analysis was made of the infrared spectrum (ATR-IR) on a rubber sheet prepared as in the tackiness test and disposed for 72 hours. Bloomed material was inspected and identified. 
     As appears clear from Examples 1 to 6, the rubber compositions according to the invention are all free from blooming and satisfactory in respect of tackiness and other qualities tested. 
     DM has been found to invite blooming as is apparent from Comparative Examples 1, 3, 5, 7 and 9. TT when used alone or combined with DM, Comparative Examples 2 and 3, was insufficient in tackiness. APDS, either alone or in combination with DM, showed premature vulcanization as evidenced by Comparative Examples 4 and 5. The use of TT and APDS, Comparative Examples 2 to 5, was inadequate in elongation due to overcuring. Controls using only NOBS, Comparative Examples 8 and 10, failed to improve scorch resistance. 
     Having thus described the invention, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the appended claims. 
     
                                           TABLE 2__________________________________________________________________________                           Comparative ExampleFormulation  Comparative Example 1                    Example 1                           2     3     4     5     6__________________________________________________________________________Br-IIR       100         100    80    80    80    80    80SMR-20                          20    20    20    20    20GPF carbon   65          65     65    65    65    65    65stearic acid  1           1      1     1     1     1     1tackifier     4           4      4     4     4     4     4FR-120*.sup.1        10          10     10    10    10    10    10paraffinic oil         6           6      6     6     6     6     6zinc oxide    5           5      5     5     5     5     5TT                               1     1APDS                                         1     1DM            1                        1           1     1NOBS*.sup.2MDB                       1sulfur         0.5         0.5    0.5   0.5   0.5   0.5   0.5scorch time    26.2        28.3   10.3                                   12.2                                         5.8    6.8                                                     9.7@ 125° C., t.sub.5 (min)tack strength (g)after 4 hr   1080        1100   780   750   800   720   750after 24 hr  960         1140   680   510   720   640   600after 72 hr  910         1170   580   420   740   570   550blooming     positive    negative                           negative                                 positive                                       negative                                             positive                                                   positiveafter 72 hr  (DM)                     (DM)        (DM)  (DM)tensile quality160° C. × 15&#39; curing300% stress (kg/cm)        44          50     65    60    69    66    48strength (kg/cm)        86          98     108   96    104   104   100elongation   790         750    590   620   510   570   690hardness     54          53     55    57    55    57    51160° C. × 15&#39; curing__________________________________________________________________________      Examples       Comparative Examples                                       Comparative ExamplesFormulation      2    3    4    7     8     Example 5                                       9     10    Example__________________________________________________________________________                                                   6Br-IIR     80   80   80   60    60    60    40    40    40SMR-20     20   20   20   40    40    40    60    60    60GPF carbon 65   65   65   65    65    65    65    65    65stearic acid       1    1    1    1     1     1     1     1     1tackifier   4    4    4    4     4     4     4     4     4FR-120*.sup.1      10   10   10   10    10    10    10    10    10paraffinic oil       6    6    6    6     6     6     6     6     6zinc oxide  5    5    5    5     5     5     5     5     5TT               1APDS                  1DM                         1                 1NOBS*.sup.2                      1                 1MDB         1    1    1                1                 1sulfur       0.5             0.5                  0.5                      1     1     1      1.5   1.5   1.5scorch time        11.6             10.9                  6.1                       7.8   6.4   9.0   8.0   5.4   9.8@ 125° C., t.sub.5 (min)tack strength (g)after 4 hr 750  750  810  830   1430  1270  420   920   890after 24 hr      720  720  800  780   1210  1130  270   910   880after 72 hr      740  660  730  610   1080  1060  250   870   850blooming   negative           negative                negative                     positive                           negative                                 negative                                       positive                                             negative                                                   negativeafter 72 hr               (DM)              (DM)tensile quality160° C. × 15&#39; curing300% stress (kg/cm)      54   65   67   65    52    64    74    80    78strength (kg/cm)      105  103  102  119   98    115   132   130   137elongation 620  590  550  580   570   570   550   510   540hardness   52   57   56   52    50    52    54    57    55160° C. × 15&#39; curing__________________________________________________________________________ *.sup.1 oxidized petroleum pitch, Fuji Kosan Co., Ltd. *.sup. 2 N-oxydiethylene-2-benzothiazyl sulfenamide