Rubber compositions

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rubber composition for tires, hoses, conveyor belts and the like and which has an improved reinforcing property, particularly improved heat generation and processability.
2. Related Art Statement
Carbon blacks such as ISAF or the like, possessing highly reinforcing ability have formerly been used for rubber compositions in treads of tires for trucks and buses, and the like in which considerably high wear resistance is required. In recent years, as the demand for economy has become greater in the market, it is important to attain higher wear resistance from the standpoint of the determination of product value.
In order to improve wear resistance, the compounding recipe is varied, for instance, the compounding ratio of carbon black is increased or the amount of a softener such as a process oil is decreased, or, alternatively, SAF grade carbon black having an excellent reinforcing ability is used instead of ISAF grade carbon black.
However, if the compounding ratio of carbon black is increased, the wear resistance is indeed improved within a certain range of the compounding ratio. But, as the compounding ratio increases, demerits become greater. For example, with an increase in the compounding ratio, the effect obtained gradually decreases, and the wear resistance of the rubber composition lowers.
On the other hand, if the ratio of the softening agent such as process oil is decreased, tacking of the kneaded rubber composition becomes poorer, although the wear resistance is improved. In addition, the viscosity of the rubber increases. Thereby, operability is deteriorated to a large extent.
Further, when the SAF grade carbon black is used, operability becomes poorer due to increased viscosity, although wear resistance is improved.
Under the circumstances, although various investigations have been made with respect to extension of the rubber-kneading time period and production techniques, sufficient effects could not be obtained prior to the present invention.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a rubber composition which has low heat generation, high reinforcing ability, and particularly high wear resistance without deteriorating operability due to increased viscosity.
The present inventors have repeatedly made strenuous studies and investigations to solve the above-mentioned problems, and they have reached the conclusion that ISAF grade carbon black and SAF grade carbon black which satisfy specified characteristic requirements are effective for solving the above problems. The inventors have accomplished the invention based on this conclusion. As the ISAF grade carbon and the SAF grade carbon, N110 (SAF) and N220 (ISAF) may be recited.
That is, the present invention relates to the rubber composition comprising natural rubber and/or a diene based synthetic rubber and carbon black, wherein the carbon black has a characteristic that a line-width .DELTA.H (mT) in an electron spin resonance spectrum (ESR) satisfies .DELTA.H (mT).gtoreq..sub.e 0.11T-7.56+5 in which mT and T are militesla and a toluene discoloration (%) falling in a range of 100-60%, respectively, and that an N.sub.2 SA value (a nitrogen-adsorbing specific surface area) (m.sup.2 /g) is not less than 114 m.sup.2 /g but not more than 250 m.sup.2 /g. The line-width in the ESR is defined as .DELTA.H which is a distance between peaks in the ESR spectrum.
As examples of the diene based synthetic rubber which is one of the rubber components in the present invention, a synthesized polyisoprene rubber, a polybutadiene rubber, a styrene butadiene rubber, etc. may be recited. Each of these rubbers may be used singly or in combination with other.
The carbon black used in the present invention is novel, and can be produced by a tunnel-type reactor including a combustion section 1, a reaction zone 2 and a choke section 3 between the combustion section and the reaction zone as shown in FIG. 2a. The combustion section and the reaction zone have larger and smaller diameters, respectively, and the chock section has a diameter smaller than that of the reaction zone. Hydrocarbon feed stock, fuel and air are introduced through lines 4, 5, and 6, respectively and injected into the combustion zone through an injection assembly 7. A quench liquid such as water is sprayed into the reaction zone at two points (sprayers) 8 and 9 spaced from the influent end toward the effluent end, and quenching is controlled such that an amount of the quench liquid sprayed through the upstream point 8 is made greater than that of the quench liquid sprayed through the downstream point 9. In the quenching, the quench liquid is sprayed into the reaction zone through the sprayers 8, 9 in a direction opposite to the combustion stream.
The carbon black according to the present invention is preferably used in an amount of 30-80 PHR.
These and other objects, features and advantages of the invention will be appreciated upon reading of the invention when taken in conjunction with the attached drawing, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains without departing from the spirit of the invention or the scope of claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWING
For a better understanding of the invention, reference is made to the attached drawings, wherein:
FIG. 1 is a diagram showing the ESR spectrum; and
FIGS. 2a and 2b are sectional views of a carbon black-manufacturing equipment to be used for the production of carbon black for the present invention and conventional carbon black, respectively.

DETAILED DESCRIPTION OF THE INVENTION
In the following, carbon black as the main ingredient of the present invention will be explained.
First, carbon black is required to have the characteristic that the line-width .DELTA.H (mT) in the ESR satisfies .DELTA.H (mT).gtoreq..sub.e 0.11T-7.56+5. This is for the following reasons:
It is said that as the firing temperature and time increase, the line-width of the fired product in the ESR measurement is greater. In such a case, the toluene discoloration T approaches 100%. In general, carbon black having a low toluene discoloration has excellent affinity with a polymer, and attains both high reinforcing ability and low heat generation as the physical properties of the rubber composition. However, since the graphitization degree of such a carbon black is low, this carbon black itself has a defect in the reinforcement. That is, when T is smaller than 60%, grains themselves of the carbon black become brittle to in turn adversely affect the rubber-reinforcing ability. The carbon black according to the present invention is a carbon black capable of overcoming the above-mentioned contradictory properties. This carbon black has excellent affinity with the polymer at its surface, and also has the structure in which graphitization proceeds. The requirement to be satisfied by the carbon black is the relationship given above. T is preferably not more than 95%, which has empirically determined.
The carbon black to be used in the present invention is required to have an N.sub.2 SA value of not less than 114 m.sup.2 /g but not more than 250 m.sup.2 /g. If it is less than 114 m.sup.2 /g, sufficient wear resistance cannot be obtained. On the other hand, if it is more than 250 m.sup.2 /g, kneading becomes impossible.
Next, it is preferable that the carbon black according to the present invention has a CTAB value (cetyltrimethyl ammonium bromide-absorbed amount) of not less than 105 m.sup.2 /g. This is because if the CTAB value is less than 105 m.sup.2 /g, the carbon black may have poorer reinforcing ability as compared with currently used carbon blacks.
The carbon black according to the present invention preferably has a .sub.24 M.sub.4 DBP oil-adsorbed amount (an amount of dibutylphthalate adsorbed onto carbon black after it is compressed at four times at 24,000 psi) of not less than 85 ml/100 g. This is because if the value is less than 85 ml/100 g, the rubber composition added with the carbon black has poorer wear resistance, particularly at a high severity zone.
Further, in the carbon black according to the present invention, the N.sub.2 SA value preferably differs from the CTAB value by not more than 10 m.sup.2 /g. The reason is that if the difference is greater than 10 m.sup.2 /g, the number of fine pores in the surface of the carbon black increases so that the viscosity of the compounded rubber increases, the reinforcing ability is deteriorated due to poor dispersion, and in turn the wear resistance becomes insufficient.
As a matter of course, compounding agents ordinarily used in the rubber industry, such as a reinforcing filler, a vulcanizating agent, a vulcanization accelerator, a vulcanization accelerator aid, and an antioxidant may appropriately be added.
In the following, the present invention will be explained in more detail with reference to Examples and Comparative Examples.
First, methods for measuring the characteristic values of carbon black and those for measuring physical properties of the rubber will be explained.
(A) Methods for measuring the characte carbon black
The toluene discoloration was measured according to JIS K 6221-1982.
The toluene discoloration-measuring process in ASTM D 1618-83, is basically as follows:
2.00.+-.0.01 g of a dried sample pulverized in a mortar is precisely taken in a 100 ml Erlenmayer flask (JIS 3503), 20 ml toluene is added thereto, and a ground stopper is fitted to the flask. Immediately thereafter, the content is vigorously shaken by hand or a machine for 60-110 seconds. (In the present invention, the shaking is effected for 60 seconds). Immediately after the shaking, the content is placed onto a filter paper-fitted glass funnel preliminarily set in another Erlenmayer flask. Immediately after the filtration, the funnel is removed, and a stopper is fitted to the flask.
After an absorbing cell is washed with a part of the filtrate three times, the cell is filled with the filtrate, and transmittance of the filtrate is measured at a wavelength of 420.+-.5 nm by a photoelectric photometer or a spectrometer. Measurement results are shown by %. In every measurement, toluene having been preliminarily filtered is used, and adjustment is performed to make its transmittance equal to 100 at 420.+-.5 nm.
The CTAB value was measured according to ASTM D 3765-85.
The .sub.24 M.sub.4 DBP oil-adsorbed amount was measured according to ASTM D 3493-84.
The N.sub.2 SA value was measured according to ASTM D-3037-84B.
An electron spin resonance apparatus JES-RE 2X (JEOL, Ltd.) was used as the ESR, and measurements were effected under the following conditions.
______________________________________Measurement temperature 25.degree. C.Power 1 mWCenter field 331 mTSweep width 150 mT (331 .+-. 75 mT)Sweep time 12 minField modulation 100 KHzField modulation width 2 mTTime constant 1 sec______________________________________
As mentioned before, the line-width is the ESR was defined as .DELTA.H which was taken as a difference between peaks in the ESH spectrum (see FIG. 1).
(B) Method for measuring rubber properties
The Mooney viscosity was measured according to JIS 6300-1974. The wear resistance index representing wear resistance was obtained by measuring the worn loss amount with use of a Lambourn tester and effecting calculation according to the following expression. ##EQU1##
The tan .delta. value representing the heat generating characteristic was measured with the use of a viscoelastic spectrometer manufactured by Iwamoto Seisakusho Corporation under conditions that tensile dynamic strain, frequency, and temperature were 1%, 50 Hz, and 25.degree. C., respectively. Slab sheets having the thickness of about 2 mm and the width of 5 mm were used as test pieces, and a span between holding points of the test pieces and an initial load were 2 cm and 100 g, respectively. The lower the tan .delta., the lower the heat generation.
By using the carbon black-producing equipment shown in FIGS. 2a and 2b, carbon blacks according to the present invention and a conventional method were produced, respectively. In the present invention, it is important to spray the quench liquid into the reaction zone through the sprayers at two locations spaced in a flowing direction, in a direction opposite to the combustion stream. If the quench liquid is sprayed in the same direction as the combustion stream, contacting efficiency between the combustion stream and the quench liquid decreases, so that the discoloration value increases, and the line width becomes smaller. In FIG. 2b, the same reference numerals denote the same constituent parts as in FIG. 2a. FIG. 2b differs from FIG. 2a only in that the quench liquid is sprayed into the reaction section through a single position 8. In the apparatuses of FIGS. 2a and 2b, the combustion section, the chock section and the reaction section had the dimensions, 700 mm in diameter .times.450 mm in length, 110 mm in diameter .times.150 mm in length, and 220 mm in diameter .times.8,000 mm in length, respectively.
Producing conditions are given in Table 1 together with line-width in ESR (mT), Toluene discoloration T (%) and N.sub.2 SA (m.sup.2 /g), and physical and chemical properties of hydrocarbon feed stock used are given in Table 2.
TABLE 1__________________________________________________________________________ Compar- Compar- Compar- ative ative ative Example Example Example Example Example Example Example A B C D E F G__________________________________________________________________________Line-width in ESR (mT) 29.0 37.5 22.3 22.0 24.5 31.1 37.5Line-width in ESR (mT) (calculated) 30.0 36.2 12.5 27.4 8.5 32.9 26.5Toluene discoloration T (%) 98 100 87.1 97 80 99 96.6N.sub.2 SA 126 114 116 130 126 148 142Total amount of air used 5000 5000 5000 5000 5000 5000 5000(temperature 500.degree. C.) (kg/h)Amount of hydrocarbon feed stock introduced 938 950 948 950 955 845 840(temperature 150.degree. C.) (l/h)Amount of fuel introduced (LPG) (kg/h) 165 165 165 250 250 300 300Quenching first quenching locations 6000 5250 5250 3000 2250 1350 1200locations (mm) second quenching location -- 5850 5850 -- 3150 -- 1500Amount of quench- first quenching locations 1000 75 110 1000 200 1000 67ing water (l/h) second quenching location -- 925 880 -- 800 -- 933__________________________________________________________________________ Compar- Compar- ative ative Example Example Example Example Example Example H I J K L M__________________________________________________________________________Line-width in ESR (mT) 27.8 24.1 32.0 20.8 20.0 21.0Line-width in ESR (mT) (calculated) 30.0 9.9 32.9 9.9 12.5 30.0Toluene discoloration T (%) 98 83.1 99 83.1 87.1 98N.sub.2 SA 146 145 173 157 164 93Total amount of air used 5000 5000 5000 5000 5000 5000(temperature 500.degree. C.) (kg/h)Amount of hydrocarbon feed stock introduced 840 845 676 703 730 980(temperature 150.degree. C.) (l/h)Amount of fuel introduced (LPG) (kg/h) 300 300 180 250 180 180Quenching first quenching locations 1200 1200 1100 1200 1050 6300locations (mm) second quenching location 1500 1500 -- 1500 1250 --Amount of quench- first quenching locations 67 108 1000 120 100 1000ing water (l/h) second quenching location 933 892 -- 880 900 --__________________________________________________________________________
TABLE 2______________________________________Specific gravity 1.1310 Distillation(JIS K 2249) (15/4.degree. C.) characteristics (.degree.C.)Kinematic viscosity 16.8 Initial distillation(JIS K 2283) (cSt AT 50.degree. C.) point 202Content of water 0.3 (Initial boiling point)(JIS K 2275) (%) 10 vol % 239Residual carbon 9.5 30 vol % 292(JIS K 2275) (%) 50 vol % 339Sulfur content 0.5(JIS K 2275) (%)Amount of carbon (wt %) 90.2Amount of water (wt %) 5.4BMCI 160______________________________________ *BMCI: Bureau of Mines Correlation Index
Rubber composition having the respective compounding recipes shown in below Table 3 were separately kneaded by a Nanbury mixer using carbon blacks having different properties, and their Mooney viscosities were measured. Further, the wear resistance indexes and the tan .delta. value of vulcanization obtained at 145.degree. C. for 30 minutes were evaluated by measurement. Results are shown in Table 4.
TABLE 3______________________________________Compounding recipe of test pieces RatioIngredients Parts by weight______________________________________Polymer 100Carbon black 50Stearic acid 3.0Zinc oxide 4.0Antioxidant *1 1.0Vulcanization accelerator *2 0.5Sulfur 2.5______________________________________ *1 Nphenyl-Nisopropyl-p-phenyldiamine *2 Noxydiethylene-2-benzothiazole sulfenamide
TABLE 4__________________________________________________________________________ Comparative Comparative Comparative Comparative Example Example Example Example Example Example Example 1 1 2 2 3 3 4__________________________________________________________________________Characteristics Kind A B C M D E Fof carbon black Line-width 29.0 37.5 22.3 21.0 22.0 24.5 31.1 in ESR (mT) Toluene 98 100 87.1 98 97 80 99 discolora- tion T (%) e.sup.0.11T-7.56 +5 30.0 36.2 12.5 30.0 27.4 8.5 32.9 N.sub.2 SA (m.sup.2 /g) 126 114 116 93 130 126 148 CTAB (m.sup.2 /g) 115 114 115 90 122 120 135 .sub.24 M.sub.4 DB) 98 97 96 100 98 98 96 (ml/100 g)) N.sub.2 SA-CTAB 11 0 1 3 8 6 13 (m.sup.2 /g)Characteristics Natural 100 100 100 100 100 100 100of rubber rubber SBR 1500 *1 -- -- -- -- -- -- -- Mooney 67 60 60 61 69 64 83 viscosity Wear 100 106 112 88 100 112 100 resistance index *2 Tan .delta. 100 93 91 89 100 89 100 (indicated by index) *3__________________________________________________________________________ Comparative Comparative Example Example Example Example Example Example Example 4 5 5 6 7 6 8__________________________________________________________________________Characteristics Kind G H I J K I Jof carbon black Line-width 37.5 24.1 32.0 20.8 20.0 32.0 20.8 in ESR (mT) Toluene 96.6 83.1 99 83.1 87.1 99 83.1 discolora- tion T (%) e.sup.0.11T-7.56 +5 26.5 9.9 32.9 9.9 12.5 32.9 9.9 N.sub.2 SA (m.sup.2 /g) 142 145 173 157 164 173 157 CTAB (m.sup.2 /g) 133 137 157 156 158 157 156 .sub.24 M.sub.4 DB) 91 96 102 104 97 102 104 (ml/100 g)) N.sub.2 SA-CTAB 9 8 16 1 6 16 1 (m.sup.2 /g)Characteristics Natural 100 100 100 100 100 50 50of rubber rubber SBR 1500 *1 -- -- -- -- -- 50 50 Mooney 78 77 104 82 95 95 89 viscosity Wear 107 117 100 119 116 100 111 resistance index *2 Tan .delta. 93 91 100 92 96 100 95 (indicated by index) *3__________________________________________________________________________ *1: Styrene butadiene copolymeric rubber made by Japan Synthetic Rubber Co., Ltd. *2: The larger the value, the better the characteristic. *3: the smaller the value, the better the characteristic.
From the results in Table 4, it is seen that the rubber compositions using the carbon black Satisfying the characteristic requirements according to the present invention (Examples 1-8) have smaller Mooney viscosity of their unvulcanized compositions and excellent wear resistance and heat generation properties of their vulcanizates as compared with those not satisfying these requirements. According to the present invention, processability, wear resistance and heat generation property are all improved.
As having been explained above, in the rubber composition of the present invention in which the carbon black satisfying the specific requirements is compounded into the diene based rubber, the Mooney viscosity of the unvulcanized product, as one of important factors in processability, can be suppressed to a low level with the greatly improved wear resistance and the heat generation property by using the carbon black as the reinforcing agent. Therefore, when the rubber composition according to the present invention can be applied to treads for various tires, and to rubber articles such as conveyor belts or hoses, particularly the wear resistance and heat generation property are improved. Thus, the invention can contribute to the improved performances of the articles. In addition, since the Mooney viscosity of the unvulcanized rubber composition is smaller and the processability is excellent as mentioned above, the merits are great in the production of the articles.

Number	Name	Date
4075160	Mills et al.	Feb 1978
4228143	Cheng et al.	Oct 1980
4503027	Cheng	Mar 1985
4548980	Nagata	Oct 1985

Rubber compositions

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