Abstract:
Vulcanizable rubber compounds useful for preparing pneumatic tires in which an unoxidized petroleum pitch serves to replace partially or totally the aromatic extender oil content customarily included in such compounds.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 828,301 filed Aug. 29, 1977 now abandoned, the latter being a continuation of application Ser. No. 509,172 filed Sept. 25, 1974, now abandoned, which in turn is a continuation of application Ser. No. 342,831 filed Mar. 19, 1973, now adandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to vulcanizable rubber compounds and the cured products thereof characterized in containing a petroleum pitch as a plasticizing additive. 
     2. Prior Art 
     In modern rubber technology, plasticizers in the form of extender oils are widely used in formulating resilient type rubber compositions. These oils, when incorporated into a rubber composition, presumedly lubricate the molecules of rubber so that they slide easily on each other. Consequently, the rubber composition can be mixed and extruded through processing apparatus without rupturing of the individual rubber molecules and consequent degradation of desired rubber properties. U.S. Pat. No. 2,964,083, for example, discloses compositions of synthetic rubber containing preferably liquid or oily plasticizers, reinforced with carbon black and additionally containing the usual additives. The plasticizers disclosed generally are derived from the refining of petroleum and represent petroleum fractions obtained by distillation or extraction before these fractions have been subjected to any sort of decomposition by pyrolysis or other forms of cracking. The types of rubber in which plasticizers or extender oils are used include the styrene-butadiene, polybutadiene, polyisoprene, ethylene-propylene terpolymer and natural rubbers with the styrene-butadiene type predominating. 
     For practical and economic reasons rubber compounders have continued to increase the concentrations of petroleum extender oils in their rubber formulations, and the result has been improved processing characteristics of the rubber and enhanced mechanical performance of pneumatic tires and other fabricated rubber articles. There have been some indications, however, that the beneficial effects of oil extenders are not permanent. For example, increased wear, decreased adhesion and tread separation accompanying the prolonged service use of tires may result from the migration of diffusion of extender oil from a region of higher concentration to an adjacent region of lower concentration. The oil concentration gradients between different sections in a tire are designed to obtain the optimum balance of elastomer properties in the tire as a complete entity. Studies with laboratory-simulated tires containing radio-active-tagged extender oils have further corroborated the phenomenon of extender oil migration. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, petroleum pitch is incorporated into a rubber composition as a total or partial replacement of the aromatic extender oil normally included in such compositions. The word &#34;pitch&#34;, in this context, accordingly designates an unoxidized petroleum pitch and excludes other types of pitch such as coal tar or oxidized petroleum pitches. The pitch functions primarily as a plasticizer although it imparts other beneficial properties. The pitch advantageously exhibits less tendency to migrate from an area of great concentration to a region of less concentration in a rubber article such as a tire thus reducing the problems from extender oil migration. The compositions of this invention may contain in addition to a requisite amount of sulfur for vulcanization purposes other components ordinarily used in rubber compounding such as carbon black, zinc oxide, stearic acid as well as a curing agent and an oxidation stabilizer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally any of the natural or synthetic types of rubber polymers presently used in commercial rubber compounding may be used in our invention. Generally we prefer the styrene-butadiene, polybutadiene, polysisoprene, ethylene-propylene terpolymer and natural rubbers and mixtures of these. A preferred rubber is the styrene-butadiene type. The proportion of synthetic or natural rubber polymer in the final composition will, of course, be determined by the end use of the composition and the amount of carbon black and other enumerated additives included in the composition. 
     The Pitch 
     The unoxidized petroleum pitches useful in the practice of this invention are characterized in remaining rigid at temperatures closely approaching their melting points. The preferred procedure for preparing the thermal petroleum pitches uses as starting material, a clarified slurry oil or cycle oil from which substantially all paraffins have been removed in fluid catalytic cracking. Where the fluid catayltic cracking is not sufficiently severe to remove substantially all paraffins from the slurry oil or cycle oil, they may be extracted with furfural. In either case, the resultant starting material is a highly aromatic oil boiling at about 700° to 850° F. This oil is thermally cracked at elevated temperatures and pressures for a time sufficient to produce a thermally cracked petroleum pitch with a softening point of about 150° to about 240° F. although pitches of softening points as low as 100° F. and as high as 350° F. can be produced. The manufacture of unoxidized thermal petroleum pitches is described in U.S. Pat. Nos. 2,768,119 and 3,140,248. Representative of the foregoing pitches useful in the practice of this invention are characterized in terms of their more important physical properties in Table 1 set forth below. 
     As indicated hereinabove the gist of the present invention resides in the use of the aforementioned pitches as a partial or total replacement of the aromatic extender oils conventionally used for plasticizing purposes in formulating resilient carbon black reinforced rubber products. In this connection, the extender oil is customarily employed in the amount of about 21 to 42 parts by weight per 100 parts of the elastomer component. Accordingly, useful amounts of pitch range from 10 to 100% of the amount of extender oil that would normally be used in a particular formulation. 
     
                                           Table I__________________________________________________________________________Test         Test Method                Pitch A                     Pitch B                          Pitch C                               Pitch D                                    Pitch E__________________________________________________________________________Softening Point, °F., R &amp; B        ASTM D-2398                173  174  168.5                               166  240Density, G/cc        Mettler 1.192                     1.205                          1.160                               1.178                                    1.216Mod. Con. Carbon Wt. %        ASTM D-2416                37.8 43.8 39.6 36.9 51Flash, COC, °F.        ASTM D-92                540  495  545  535  595Pen. 100/5/77        ASTM D-5                0    0    0    0    0.sup.2Pen. 200/60/115        ASTM D-5                128  47   208  --   --Pen. 50/60/15        ASTM D-5                40   37   68   --   --Sulfur, Wt. %        ASTM D-1552                2.73 1.47 2.01 0.95 1.5Benzene Ins., Wt. %        ASTM D-2317                0.80 11.7 1.9  2.3  5.0Quinoline Insol., Wt. %        ASTM D-2318                0.11 .sup.1                          Nil  Nil  NilViscosity, CPS 350        Brookfield                40   70   45   32   --(#2 Spindle, 325        Brookfield                60   110  82.5 65   --30 RPM) 300  Brookfield                140  230  165  135  --__________________________________________________________________________ .sup.1 Quality of solids content too great to permit filtering .sup.2 Penetration, 200/60/77 
    
     METHOD OF MIXING 
     Ordinarily rubber is supplied to the manufacturer in bales of coagulated latex commonly known as masterbatch. The masterbatch material may have some plasticiser oil already present in it as well as other components. The technique of adding the petroleum pitch of our composition is similar to the technique now used in incorporating into the masterbatch additives such as carbon black, zinc oxide, silica, antioxidants, and curing agents. The masterbatch material is placed in a mixing machine such as a roll mill, Gordon plasticator or a Banbury mixer. To the masterbatch is added the desired amount of petroleum pitch which preferably is in a flaked or other particulated form. Other desired additives such as carbon black are added and the combined materials are mixed for a predetermined period in the mixing apparatus. The resulting composition is then processed further through extruding apparatus or other fabricating machines. 
     EXAMPLE 
     In laboratory tests rubber samples of standard compositions were made using both styrene-butadiene rubber and a blend of styrene and butadiene rubbers. Standard additives including carbon black, zinc oxide, oil, stearic acid, accelerators, and antioxidants were also added in a constant ratio by weight of additive to rubber. The composition of the standard rubber samples is presented in Table II. To the standard compositions of Table II various weight percent of two commercial extender oils and two thermal petroleum pitches were added. 
     In Table II and in the other data following the designation SBR indicates a rubber consisting completely of styrene-butadiene polymer, and the designation SBR/BR indicates a blend of 50 percent styrene-butadiene polymer and 40 percent styrene-butadiene polymer. 
     
                       Table II______________________________________Standard Rubber Composition           Parts by weight per 100           parts by weight of polymer______________________________________Polymer (SBR or SBR/BR)             100Carbon Black      50Zinc Oxide        3Oil               42Stearic Acid      1.5Sulfur            1.75*Accelerators      1.13&#34;Santocure&#34;      0.85&#34;DPG&#34;      0.25Antioxidant (&#34;Flexzone&#34; 7L)             0.50             199.88______________________________________ *The sulfur can be varied up to 3 parts per 100 parts of the elastomer 
    
     To each of these standard samples various weight percentages of two commercial extender oils and two thermal petroleum pitches having softening points of 170° F. and 240° F. were added. The two commerical extender oils designated A and B in the following data were ASTM Type 102 and ASTM Type 101 respectively. Each of the compositions thus formulated was submitted to a series of tests as indicated below in Table III. These tests are standard throughout the rubber compounding industry and so are not described further as to the procedures involved. Test results are as shown in Table III. 
     
                                           Table III__________________________________________________________________________Standard  Rubber Composition Tested              SBR             SBR/BR (60/40)Type of carbon black used in standardcomposition        N-660           N-330Type of commercial extender or pitch                      Pitch                          Pitch       Pitch                                          Pitch     Pitch                                                       Pitchadded              A   B   170 240 A   B   170 240 A  B  170                                                       240Percent by weight of composition ofextender or pitch added              16              21              16Cure               90%* × 1.2 at 320° F.                              90%* × 1.2 at 305° F.Modulus (Load in psi to produce 300%elongation)        580 590 680 630 390 380 520 900 650                                                 600                                                    680                                                       980Tensile strength, psi              2460                  2590                      2710                          2420                              2200                                  2630                                      2880                                          2800                                              2380                                                 3030                                                    3100                                                       3200Percent elongation at breaking load              860 890 920 790 860 940 1000+                                          820 630                                                 830                                                    880Hardness (Shore A Durometer)              48  51  55  60  46  47  56  63  50 52 58Firestone Running Temperature, °F.              217 222 249 264 216 230 274 306 210                                                 226                                                    250Mooney, 4ML, 212° F.              28  32  43  52  25  27  44  66  32 36 51Rebound, percent (Goodyear)              59.5                  55.5                      56.4                          56.4                              59.5                                  55.9                                      51.0                                          57.4                                              61.0                                                 57.4                                                    54.7Standard Rubber Composition Tested               SBR/BR (60/40)Type of carbon black used in standardcomposition         N-330                N-660Type of commercial extender or pitch                          Pitch                               Pitch           Pitch                                                    Pitchadded               A    B     170  240  A     B    170  240Percent by weight of composition ofextender or pitch added               10                   16Cure                90%* × 1.2 at 305° F.Modulus (Load in psi to produce 300%elongation)         1000 1020  910  1000 520   590  760  950Tensile strength, psi               2900 3130  3400 3390 1790  2200 2220 2420Percent elongation at breaking load               590  640   780  720  700   820  810  800Hardness (Shore A Durometer)               58   59    61   63   50    51   56   62Firestone Running Temperature, °F.               211  226   244  251  186   202  227  240Mooney, 4ML, 212° F.               43   49    59   66   28    33   46   57Rebound, percent (Goodyear)               61.5 60    57.9 51.0 65.7  63.6 64.1 62.0Standard Rubber Composition Tested               SBRType of carbon black used in standardcomposition         N-330Type of commercial extender or pitch                          Pitch                               Pitch           Pitch                                                    Pitchadded               A    B     170  240  A     B    170  240Percent by weight of composition ofextender or pitch added               21                   10Cure                90%* × 1.2 at 320° F.Modulus (Load in psi to produce 300%               440  400   540  810  1020  1100 1120 1190elongation)Tensile strength, psi               2490 2740  2950 3010 3200  3710 3780 3550Percent elongation at breaking load               730  880   890  820  600   700  720  700Hardness (Shore A Durometer)               46   47    52   62   56    59   60   62Firestone Running Temperature, °F.               238  251   282  325  245   248  255  264Mooney, 4ML, 212° F.               21   --    --   56   39    43   53   57Rebound, percent (Goodyear)               46.9 49.6  51.5 48.7 56.4  54.4 54.9 53.5__________________________________________________________________________ *Montsanto Rheometer Cure 
    
     From the data in Table III it is readily apparent that petroleum pitch imparts to rubber compositions the properties necessary to manufactuers of rubber articles, particularly tires. Petroleum pitch maintains and improves the modulus of rubber compositions in comparison to the modulus of rubber compositions containing equivalent amounts of extender oils. The hardness of pitch-rubber compositions is greater than that of rubber compositions containing equivalent amounts of commerical extender oils. This is a particularly unexpected and beneficial result in view of the present practice of incorporating special additives to improve the hardness of rubber compositions. These hardness-enhancing additives are relatively expensive in comparison to other materials used in rubber compounding. Conversely petroleum pitch affords an inexpensive means of enhancing hardness. The presently used hardness-enhancing additives often have the adverse effect of decreasing the tensile strength and elongation properties of rubber compositions. The tensile strength and elongation properties of the petroleum pitch-rubber compositions of our invention in contrast show improved tensile strength and elongation. 
     It will be noted from the data in Table III that rebound values also are not adversely affected by incorporating petroleum pitch. This is another unexpected result since normally the addition of a highly aromatic material such as petroleum pitch would be expected to depress rebound values. Hardness does not vary appreciably with small changes in pitch concentration whereas similar variarions in concentrations of the presently used extender oils does notably decrease hardness. 
     The Mooney values of petroleum pitch-rubber compositions are increased more in comparison to the Mooney values of rubber compositions containing equivalent concentrations of present-day commercial extender oils. This property enables the rubber compounder to utilize a rubber polymer having a lower original Mooney value than is now possible in formulating a rubber composition having a desired higher Mooney value.