Abstract:
Rubber compositions are disclosed which exhibit enhanced dynamic modulus and tensile strength characteristics. A selected type of carbon black, resorcinsol and/or its precondensate and hexamethoxymethyl melamine are combined in specified amounts with a selected class of starting rubbers. The carbon black has specified nitrogen adsorption surface area, dibutyl phthalate adsorptivity and Dst mode of diameter.

Description:
This application is a continuation of application Ser. No. 07/281,738, filed Dec. 9, 1988, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to rubber compositions suitable for use particularly in automobile tires. 
     2. Description of the Prior Art 
     Rubber compositions of a high degree of hardness are in common use for belt and bead filler portions of automotive tires. To attain improved hardness, attempts have been made to blend a given starting rubber with large amounts of carbon black. Too much carbon black, say greater than 50 parts by weight based on 100 parts by weight of the starting rubber, however, will in most cases render the finished composition mechanically weak for example with respect to tensile strength on break. As is generally known in the art, this problem can be alleviated to some extent by the use of a low structure carbon black not abundant in dibutyl phthalate adsorptivity. To this end, N326 is oftentimes employed which is a carbon black of a lower structure type than a standard N330 type among high abrasion furnace blacks (HAF). Various N326 carbon blacks have been proposed as disclosed for instance in Japanese Laid-Open Publication Nos. 50-60904, 59-89339 and 60-4540. 
     Prior high-hardness rubber compositions leave their mechanical strength characteristics unsolved. These compositions when formulated to provide a dynamic modulus greater than 6.0 MPa involve too small tensile strength to meet the commercial requirements. 
     SUMMARY OF THE INVENTION 
     The present invention, therefore, seeks to provide new and improved rubber compositions which excel, in addition to increased dynamic modulus, in tensile strength on break among various important mechanical strength characteristics. The composition according to the invention is suitable particularly for use in steel cords and bead fillers of car tires. 
     As will be better understood from the following detailed description, the invention provides a rubber composition comprising (a) 100 parts by weight of a starting rubber, (b) 0.5 to 5 parts by weight of a resorcinol, a precondensate thereof or a combination thereof, (c) 1 to 5 parts by weight of a hexamethoxymethylmelamine and (d) 50 to 70 parts by weight of a carbon black having an N 2  SA of 90 to 100 m 2  /g as defined by nitrogen adsorption surface area, a DBP of 60 to 70 ml/100 g as defined by dibutyl phthalate adsorption and a ΔDst of 50 to 60 mμ as defined by the Dst mode of diameter, whereby the composition has an E&#39; of not less than 6.0 MPa as defined by dynamic modulus and determined at an initial strain of 10% and subsequently at a dynamic strain of 2%, at a frequency of 20 Hz and at a temperature of 20 ° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are graphic representations of the physical properties of different rubber compositions, showing the correlation between tensile strength on break (Tb) and dynamic modulus (E&#39;) converted by the method of least squares; and 
     FIG. 3 is a graphic representation exemplary of a size distribution curve of centrifugally classified carbon black aggregates for use in the determination of the Dst mode of diameter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Rubber compositions according to the present invention are essentially made up of (I) a selected type of carbon black, (II) a resorcinol and/or its precondensate and (III) a hexamethoxymethylmelamine combined with a selected class of starting rubbers. 
     Components (I) useful for the purpose of the invention include carbon blacks having certain physical characteristics determinable by the following methods. 
     Nitrogen Adsorption Surface Area (N 2  SA)(m 2  /g) 
     ASTM D-3037-78 is followed by &#34;Standard Method of Testing Carbon Black-Surface Area by Nitrogen Adsorption&#34;, Method C. 
     Dibutyl Phthalate Adsorption (DBP)(ml/100 g) 
     JIS K-6221 (1982) is followed by &#34;Method of Testing Carbon Black for Rubber&#34;, Section 6.1.2(1), Method A. 
     Dst Mode of Diameter and Aggregate Size Distribution (ΔDst)(mμ) 
     Centrifugal classification is effected with the use of a disc centrifuge manufactured by Joyce Loebl Co., England. A carbon black sample is dried as stipulated in JIS K-6221, 5th Method, followed by accurate weighing and dispersion in a 20% aqueous ethanol solution, thereby preparing a dispersion concentrated to 0.005% by weight. Complete dispersion is ultrasonically accomplished. The resulting dispersion is subjected to the above centrifuge set at 8,000 rpm. Into the centrifuge are put 10 ml of a spin liquid in distilled water, subsequently 0.5 ml of a buffer in 20% by volume of ethanol and finally 0.5 to 1.0 ml of the carbon black dispersion through a syringe. Centrifugation is initiated and a photoelectric recorder switched on to draw a distribution curve of aggregates as shown in FIG. 3. ΔDst is determined from the half value width of this curve. 
     Dynamic Modulus (E&#39;)(MPa) 
     A vulcanized rubber sheet is allowed to stretch at 10% initial strain and then at 2% dynamic strain, at 20 Hz and at 20° C. 
     According to one important aspect of the invention, component (I) should have a N 2  SA in the range of 90 to 100 m 2  /g, a DBP in the range of 60 to 70 ml/100 g and ΔDst in the range of 50 to 60 mμ. Improved dynamic modulus (E&#39;) and enhanced tensile strength on break (Tb) are governed by strict observance of component (I) within the above specified ranges. 
     The amount of component (I) to be used should be in the range of 50 to 70 parts, preferably 55 to 65 parts, based on 100 parts by weight of a given strating rubber. Component (I) if smaller amounts than 50 parts would not be ineffective in producing sufficient reinforcement and if larger amounts than 70 parts would not get fully compatible with the other components, leading to inadequate tensile strength. 
     Components (III) include a resorcinol, its precondensate and their combination, and they act to gain improved mechanical properties. This component should range in amount from 0.5 to 3 parts, preferably 1 to 2.5 parts, per 100 parts by weight of the starting rubber. Smaller amounts than 0.5 part would fail to give improved mechanical strength, whereas greater amounts than 3 parts would invite reduced tensile strength. 
     Components (II) are selected from hexamethoxymethylmelamines (HMMM) which are used to make the resulting composition highly adhesive to tire reinforcing materials such as steel cords in particular. The amount of component (III) to be added should preferably be between 1 and 5 parts per 100 parts by weight of the starting rubber. Too much component (III) would result in a rubber composition being physically poor. 
     Starting rubbers useful in the practice of the invention vary with the particular area of application of the composition, but include for example natural rubber (NR), polyisoprene rubber (IR), butyl rubber (IIR), acrylonitrile-butadienene copolymer rubber (NBR), styrene-butadiene copolymer rubber (SBR), butadiene rubber (BR) and their combinations. Preferred for use in automobile tires is NR or a blend of NR and IR. NR or NR--IR should be added in amounts more than 70% by weight of the total starting rubber. 
     Various other additives may be employed, if necessary, in which are included vulcanization accelerators, sulfur, antioxidants, zinc oxide, cobalt salts of organic acids, process oils and the like. 
     The following examples are provided to further illustrate the present invention, but should not be regarded as limiting the invention. 
     Different rubber compositions were prepared by blending Formulations A and B with varying amounts of four carbon blacks, i.e. N339, N330, N326 and inventive carbon black, as shown in Tables 1 and 2. Contained in Formulation B were resorcinol and HMMM. Formulation A was devoid of two such components with a large amount of sulfur used. 
     Vulcanization was effected at 160° C. for 15 minutes to form a rubber sheet which was subsequently measured for tensile strength (Tb) and dynamic modulus (E&#39;). 
     As seen from Table 3, Formulations A and B when combined with the test carbon blacks at five levels show Tb and E&#39; values that are rather irregular and hence difficult to put one above the other. To facilitate comparison, these values have been rearranged in accordance with the method of least squares as given in Table 4 in terms of the correlation coefficients of Tb and E&#39; converted from the regression equations. 
     The data in Table 5 demonstrate the Tb values of Formulations A and B determined relative to the E&#39; values converted above. Parentheses indicate the magnitude of Tb computed by taking N326 as an index of 100. The Tb values are also plotted against E&#39; in FIGS. 1 and 2. 
     The greater the dynamic modulus (E&#39;) is, the smaller tensile strength (Tb) whatever type of carbon black may be as is apparent from Table 5. Among N339, N330 and N326 commonly known in the art, N326 excels in Tb on a high E&#39; side, i.e. at from 9.0 to 12.0 MPa, in Formulation A and also over the entire E&#39; range, i.e. at from 5.0 to 12.0 MPa, in Formulation B. 
     The carbon black contemplated under the invention, given relatively high Tb at 5.0 and 6.0 MPa, shows no appreciable rise in Tb as E&#39; increases in the case of Formulation A. Surprisingly, the inventive carbon black, though slightly lower in Tb than N326 at 5.0 MPa, is highly capable of Tb improvement at from 6.0 to 12.0 MPa in Formulation B. The beneficial effects of the invention are attributable to the use of the unique carbon black combined with resorcinol and HMMM. 
     Although the invention has been shown and described in connection with certain preferred embodiments, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the scope of the appended claims. 
     
                       TABLE 1______________________________________component/additive           formulation A                       formulation B______________________________________NR              100.0       100.0ZnO             10.0        10.0antioxidant (6C)*           1.0         1.0cobalt naphthenate           2.0         2.0(10% Co content)process oil     2.0         2.0accelerator (OBS)           0.5         0.5insoluble sulfur (80%)           8.0         4.0resorcinol      --          2.0HMMM**          --          3.5carbon black    varying amount______________________________________ *N-phenyl-N&#39;-(1,3-dimethylbutyl)-p-phenylenediamine **hexamethoxymethylmelamine 
    
     
                       TABLE 2______________________________________        N.sub.2 SA                DBP         Dst  ΔDstcarbon black (m.sup.2 /g)                (ml/100 g)  (mμ)                                 (mμ)______________________________________N339         93      121         105  71Seast KHN330         79      101         111  66Seast 3N326         86       74          98  65Seast 300inventive    94       66          78  56carbon black______________________________________ Seast series: Tokai Carbon Co., Ltd. 
    
     
                       TABLE 3______________________________________  amount        formulation A formulation Bcarbon black    (phr)   Tb       E&#39;     Tb     E&#39;______________________________________N339     50      257      8.37   231    7.93    55      244      9.56   202    9.64    60      226      11.51  183    10.44    65      191      13.33  181    12.91    70      196      14.90  167    14.77N330     50      268      7.86   242    6.74    55      260      7.61   230    8.22    60      250      8.90   220    9.51    65      224      10.36  206    11.28    70      220      11.55  198    12.32N326     50      262      6.54   259    7.38    55      258      7.74   248    8.42    60      246      9.38   240    9.04    65      245      10.68  236    10.64    70      229      11.89  225    11.81inventive    50      268      6.68   269    6.54carbon black    55      262      8.13   261    8.08    60      253      9.59   251    9.69    65      246      10.76  256    10.66    70      230      12.60  249    12.52______________________________________ 
    
     
                       TABLE 4______________________________________                          correlation                          coefficientrun          regression equation                          (r)______________________________________formu- N339       Tb = 343.34 - 10.45 × (E&#39;)                              0.96lation N330       Tb = 358.69 - 12.35 × (E&#39;)                              0.97A     N326       Tb = 301.40 -  5.78 × (E&#39;)                              0.96 inventive  Tb = 312.89 -  6.40 × (E&#39;)                              0.99 carbon blackformu- N339       Tb = 285.42 - 8.32 × (E&#39;)                              0.91lation N330       Tb = 294.94 - 7.88 × (E&#39;)                              1.00B     N326       Tb = 308.03 - 7.02 × (E&#39;)                              0.97 inventive  Tb = 287.40 - 3.18 × (E&#39;)                              0.91 carbon black______________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________E&#39; (MPa)converted    5.0  6.0  7.0  8.0  9.0  10.0 11.0 12.0__________________________________________________________________________formu-N339    291 (107)         281 (105)              270 (103)                   260 (102)                        249 (100)                             239  (98)                                  228  (96)                                       218  (94)lation AN330    297 (109)         285 (107)              272 (104)                   260 (102)                        248 (100)                             235  (96)                                  223  (94)                                       221  (91)N326    273 (100)         267 (100)              261 (100)                   255 (100)                        249 (100)                             244 (100)                                  238 (100)                                       232 (100)inven-    281 (103)         274 (103)              268 (103)                   262 (103)                        255 (102)                             249 (102)                                  243 (102)                                       236 (102)tivecarbonblackformu-N339    244  (89)         236  (89)              227  (88)                   219  (87)                        211  (89)                             202  (85)                                  194  (84)                                       186  (83)lation BN330    256  (94)         248  (93)              240  (93)                   232  (92)                        224  (91)                             216  (91)                                  208  (90)                                       200  (89)N326    273 (100)         266 (100)              259 (100)                   252 (100)                        245 (100)                             238 (100)                                  231 (100)                                       224 (100)inven-    272 (100)         268 (101)              265 (102)                   262 (104)                        259 (106)                             256 (108)                                  252 (109)                                       249 (111)tivecarbonblack__________________________________________________________________________