RUBBER COMPOSITION

Abstract

A rubber composition is provided. The rubber composition is based at least on an elastomer matrix comprising natural rubber and a functional diene elastomer, the functional diene elastomer bearing an SiOR function, R being hydrogen or a carbon-based group, the carbon black having a BET specific surface area of greater than 130 m.sup.2/g, the dispersion of the carbon black in the elastomer matrix having a Z value of greater than 80. The composition has an improved compromise between the hysteresis properties and the properties at break.

Claims

1. A rubber composition based at least on an elastomer matrix comprising natural rubber and a functional diene elastomer, on a reinforcing filler comprising a carbon black and a silica, on a coupling agent for binding the silica to the elastomer matrix and on a crosslinking system, the functional diene elastomer being a diene elastomer that bears an SiOR function, R being hydrogen or a carbon-based group, the carbon black having a BET specific surface area of greater than 130 m.sup.2/g, the dispersion of the carbon black in the elastomer matrix having a Z value of greater than 80.

2. A rubber composition according to claim 1, in which the contents of carbon black and of silica are respectively between 5 and 25 phr and between 10 and 30 phr.

3. A rubber composition according to claim 1, in which the content of reinforcing filler is between 30 and 55 phr.

4. A rubber composition according to claim 1, in which the diene elastomer is a styrene/butadiene copolymer.

5. A rubber composition according to claim 1, in which the functional diene elastomer bears a silanol function, preferably at the chain end, or SiOR function with R being methyl or ethyl.

6. A rubber composition according to claim 1, in which the content of natural rubber is from 55 to 85 phr and the content of functional diene elastomer is from 15 to 45 phr.

7. A rubber composition according to claim 1, in which the crosslinking system is a vulcanization system.

8. A rubber composition according to claim 1, which additionally comprises an agent for covering the silica.

9. A process for preparing a rubber composition defined according to claim 1, which comprises the following steps: a. a first masterbatch comprising the natural rubber and the carbon black dispersed with a Z value greater than or equal to 90 is prepared, b. the silica and the functional diene elastomer are added to the first masterbatch, c. the combined mixture is mixed by thermomechanical kneading.

10. A process according to claim 9, in which the silica and the functional diene elastomer are added to the first masterbatch in the form of a second masterbatch.

11. A process according to claim 10, in which the second masterbatch additionally contains the coupling agent and optionally an agent for covering the silica.

12. A process according to claim 11, in which the content of covering agent is between 0 and 5 phr.

13. A process according to claim 10, in which the preparation of the second masterbatch is followed, before step c, by a thermomechanical kneading until a maximum temperature between 140? C. and 180? C. is reached.

14. A process according to claim 9, in which the first masterbatch is prepared by liquid-phase mixing starting from a natural rubber latex and an aqueous dispersion of carbon black.

15. A process according to claim 14, in which the first masterbatch is prepared according to the following steps: a. a first continuous stream of a natural rubber latex is fed to a mixing zone of a coagulation reactor defining an elongated coagulation zone extending between the mixing zone and an outlet, b. the mixing zone of the coagulation reactor is fed with a second continuous stream of a fluid comprising carbon black under pressure in order to form a mixture with the natural rubber latex by mixing the first stream and the second stream in the mixing zone sufficiently energetically to coagulate the natural rubber latex with the carbon black prior to the outlet, said mixture flowing as a continuous stream towards the outlet zone and said filler being capable of coagulating the elastomer latex, c. the coagulum obtained previously is recovered at the outlet of the reactor in the form of a continuous stream and it is dried in order to recover the masterbatch.

16. A process according to claim 9, which additionally comprises the following steps: adding, during a first non-productive stage, to the first masterbatch, the functional diene elastomer and the silica, by kneading thermomechanically until a maximum temperature of between 130? C. and 200? C. is reached, cooling the combined mixture to a temperature below 100? C., subsequently incorporating a crosslinking system, kneading everything up to a maximum temperature below 120? C.

17. A tire tread which comprises a rubber composition defined according to claim 1 or capable of being obtained by the process defined according to the following steps: a. a first masterbatch comprising the natural rubber and the carbon black dispersed with a Z value greater than or equal to 90 is prepared, b. the silica and the functional diene elastomer are added to the first masterbatch, c. the combined mixture is mixed by thermomechanical kneading.

18. A tire which comprises a rubber composition defined according to claim 1 or capable of being obtained by the process defined according to the following steps: a. a first masterbatch comprising the natural rubber and the carbon black dispersed with a Z value greater than or equal to 90 is prepared, b. the silica and the functional diene elastomer are added to the first masterbatch, c. the combined mixture is mixed by thermomechanical kneading.

19. A tire according to claim 18, intended to be fitted to vehicles bearing heavy loads and running on off-road surfaces.

20. A tire according to claim 18, wherein the composition is in its tread.

Description

II. EXEMPLARY EMBODIMENTS

II.1Measurements and Tests Used:

II.1.1Properties at Break:

[0103] The tests make it possible to determine the elasticity stresses and the properties at break; those carried out on cured mixtures are carried out in accordance with standard AFNOR-NF-T46-002 of September 1988.

[0104] At a temperature of 60? C.-2? C., and under standard hygrometry conditions (50-5% relative humidity), according to French standard NF T 40-101 (December 1979), the tensile strengths (in MPa) and the elongations at break (in %) are measured, the energy at break (breaking energy) being the product of the tensile strength and the elongation at break.

II.1.2Dynamic Properties:

[0105] The dynamic property tan(d)max is measured on a viscosity analyser (Metravib VA4000), according to standard ASTM D 5992-96. The response of a sample of vulcanized composition (cylindrical test specimen with a thickness of 4 mm and a cross section of 400 mm.sup.2), subjected to a simple alternating sinusoidal shear stress, at a frequency of 10 Hz and at a temperature of 100? C., according to standard ASTM D 1349-99, is recorded. A strain amplitude sweep is carried out from 0.1% to 50% (outward cycle) and then from 50% to 0.1% (return cycle). The result made use of is the loss factor (tan d). The maximum value of tan d observed (tan(d)max) between the values at 0.1% and at 50% strain (Payne effect) is shown for the return cycle.

II.2Preparation of the Rubber Compositions:

[0106] This section relates to the preparation of the compositions T1, T2, T3, T1-F, M2 and M3 for illustrating embodiments of the invention, and also to the preparation of the masterbatches used in the preparation of these same rubber compositions T2, T3, M2 and M3.

[0107] Independently of their production process, the formulation of all the rubber compositions T1, T2, T3, T1-F, M2 and M3 is the following: [0108] NR: 55 phr [0109] SBR: 45 phr [0110] Carbon black: 20 phr [0111] Silica: 25 phr [0112] Silane: 1.9 phr [0113] DPG: 0.3 phr [0114] Antioxidant: 2.5 phr [0115] Antiozone wax: 1 phr [0116] Stearic acid: 2.5 phr [0117] ZnO: 2.7 phr [0118] CBS 1.4 phr [0119] Sulfur: 1 phr.

II.2.1Preparation of the Masterbatch of Natural Rubber and Carbon Black (MB-NR):

[0120] The masterbatch of natural rubber and carbon black (MB-NR) used in the compositions T2, T3, M2 and M3 is produced in the liquid phase according to the process described in U.S. Pat. No. 6,048,923.

[0121] Thus, the masterbatch is prepared, according to the protocol explained in detail in the aforementioned patent, from carbon black N134 sold by Cabot Corporation, and natural rubber field latex originating from Malaysia and having a rubber solids content of 28% and an ammonia content of 0.3%.

[0122] The masterbatch MB-NR of natural rubber and carbon black is thus obtained in which the content of carbon black is 50 phr.

TABLE-US-00001 Masterbatch MB-NR NR 100 N134 50

II.2.2Preparation of the Masterbatches of Diene Elastomer and of Silica (MB-SBR1 and MB-SBR2):

[0123] The masterbatches of SBR and of silica (MB-SBR) contain not only the diene elastomer and the silica, but also the silane and the DPG, in the contents indicated below in phr. The silica and the silane are the same as those used in the other rubber compositions T1, T3, T1-F and M3. The silica is a silica with a BET of 120 m.sup.2/g, the coupling agent is the Si69 TESPT silane, the secondary accelerator is diphenylguanidine (DPG).

TABLE-US-00002 Masterbatch MB-SBR SBR 100 Silica 45 Silane 3.4 DPG 0.6

[0124] The contents of silica, silane and DPG in the MB-SBR masterbatches are adjusted so that their contents in the rubber compositions T2 and M2 containing natural rubber, carbon black and MB-SBR masterbatch are identical to the other rubber compositions T1, T3, T1-F and M3.

[0125] The MB-SBR masterbatches differ from one another by the presence or absence of a function on the SBR diene elastomer: the diene elastomer used in the MB-SBR1 masterbatch is a non-functional SBR (SBR1), the diene elastomer used in the MB-SBR2 masterbatch is a functional SBR in which more than 70% of the chains bear a silanol function at the chain end (SBR2). The MB-SBR1 masterbatch is used in the composition T2, the MB-SBR2 masterbatch in the composition M2.

[0126] The MB-SBR masterbatches are prepared in a mixer, filled to 70% and the initial vessel temperature of which is around 90? C., by incorporating the silica, the silane and the DPG into the diene elastomer by thermomechanical kneading until a maximum dropping temperature of between 140 and 180? C. is reached.

II.2.3Preparation of the Rubber Compositions T1, T2, T3, T1-F, M2 and M3:

[0127] The rubber compositions differ by the presence or absence of an SiOR function on the SBR diene elastomer, by their preparation process which may or may not involve the use of a masterbatch of natural rubber and carbon black and of a masterbatch of SBR and silica.

[0128] II.2.3.1Preparation of the Rubber Compositions T1 and T1-F:

[0129] The rubber compositions T1 and T1-F are rubber compositions prepared in a conventional manner, since their preparation does not require the preparation of a masterbatch of natural rubber and carbon black. The composition T1-F differs from T1 in that the SBR contained in T1-F is an SBR in which more than 70% of the chains bear a silanol function at the chain end. The contents of natural rubber and of SBR in T1 and T1-F are indicated in phr below:

TABLE-US-00003 Composition T1 T1-F NR (1) 55 55 SBR1 (2) 45 SBR2 (3) 45 (1) natural rubber; (2) SBR with 26% of styrene and 24% of 1,2- units of the butadiene part; (3) SBR with 26% of styrene units and 24% of 1,2- units of the butadiene part, in which more than 70% of the chains bear a silanol function at the end of the elastomer chain.

[0130] The natural rubber in solid form, the carbon black, the SBR, the silica, the silane, the DPG and the various other ingredients, with the exception of the vulcanization system, are introduced into an internal mixer which is 70% filled and which has an initial vessel temperature of approximately 90? C. Thermomechanical working (non-productive phase) is then performed in one step (total kneading time equal to about 5 min), until a maximum dropping temperature of about 165? C. is reached.

[0131] The mixture thus obtained is recovered and cooled and then the vulcanization system (sulfur and sulfenamide accelerator) is added on an external mixer (homofinisher) at 70? C., everything being mixed (productive phase) for approximately 5 to 6 min.

[0132] The compositions thus obtained are subsequently calendered, either in the form of slabs (thickness of 2 to 3 mm) or of thin sheets of rubber, for the measurement of their physical or mechanical properties, or in the form of profiled elements which can be used directly, after cutting and/or assembling to the desired dimensions, for example as semi-finished products for tires, in particular as tire treads.

[0133] II.2.3.2Preparation of the Rubber Compositions T3 and M3:

[0134] The compositions T3 and M3 both contain a masterbatch of natural rubber and carbon black; they differ in that the SBR in M3 bears an SiOR function (R being H), which is not the case for the SBR in T3. The contents of natural rubber and of SBR alone (i.e. not in masterbatch form) and of masterbatches in T3 and M3 are indicated in phr below.

TABLE-US-00004 Composition T3 M3 NR (1) 15 15 SBR1 (2) 45 SBR2 (3) 45 MB-NR (4) 60 60 (1) natural rubber; (2) SBR with 26% of styrene and 24% of 1,2- units of the butadiene part; (3) SBR with 26% of styrene units and 24% of 1,2- units of the butadiene part, of which more than 70% of the chains bear a silanol function at the end of the elastomer chain; (4) masterbatch mentioned in section II.2.1

[0135] The same process as for the preparation of T1 and T1-F is followed apart from the fact that the natural rubber and the carbon black are introduced together in the form of the MB-NR masterbatch described in section II.2.1.

[0136] II.2.3.3Preparation of the Rubber Compositions T2 and M2:

[0137] The compositions T2 and M2 both contain a masterbatch of natural rubber and carbon black (MB-NR) and a masterbatch of SBR and silica; they differ in that the SBR in M2 bears an SiOR function (R being H), which is not the case for the SBR in T2. The contents of natural rubber alone and of masterbatches in T2 and M2 are indicated in phr below.

TABLE-US-00005 Composition T2 M2 NR (1) 15 15 MB-NR (4) 60 60 MB-SBR1 (5) 72 MB-SBR2 (6) 72 (1) natural rubber; (4) masterbatch mentioned in section II.2.1; (5) and (6) masterbatches mentioned in section II.2.2.

[0138] The same process as for the preparation of T3 and M3 is followed apart from the fact that the SBR, the silica and the DPG are introduced together in the form of the masterbatch MB-SBR1 (for T2) or MB-SBR2 (for M2).

II.3Results:

[0139] The properties of the rubber compositions prepared appear in Table 1.

[0140] The rubber compositions T1, T2 and T3 are not in accordance with the invention for at least the following reason: they do not contain a functional diene elastomer bearing an SiOR function. The rubber composition T1-F comprising a functional diene elastomer bearing a silanol function is not in accordance with the invention, since the Z value is not greater than 80.

[0141] The compositions M2 and M3 are in accordance with embodiments of the invention: the diene elastomer bears a silanol function and the Z value is greater than 80.

TABLE-US-00006 TABLE 1 Composition T1 T2 T3 T1-F M2 M3 Z value 74 86 84 54 86 83 Energy at break 104 98 85 61 94 84 60? C. (MJ) Tand(max) 0.078 0.079 0.086 0.066 0.063 0.073 100? C.

[0142] It is observed that the rubber compositions M2 and M3 in accordance with embodiments of the invention have the best compromise of hysteresis and energy at break properties.

[0143] The compositions made from a masterbatch of natural rubber and carbon black all have a good dispersion of the carbon black, since the corresponding Z values are greater than 80.

[0144] Although the composition T1-F is the composition with the lowest hysteresis (tandof 0.066), it is also the one that has the worst properties at break (energy at break being only 61 J). The compositions T1, T2 and T3 for which the energy varies from 85 to 104 J are among those which have the best properties at break, but they also have the highest hysteresis (tandof at least 0.078).

[0145] The improvement in the compromise observed with the compositions M2 and M3 is clearly due to the coexistence in the rubber composition of a diene elastomer bearing an SiOR function and of a Z value of greater than 80. Indeed, no improvement in the compromise is observed in the case of the compositions T2 and T3 having a Z value of greater than 80 free of diene elastomers bearing an SiOR function. Quite the opposite, for these compositions T2 and T3 not only is a reduction in the properties at break observed, but also an increase in the hysteresis. Specifically, the energy at break values for T2 and T3 are respectively 98 and 85 versus 104 for T1, whereas the t and values are 0.079 for T2 and 0.086 for T3 versus 0.078 for T1. A good dispersion of the carbon black, in the absence of diene elastomer bearing an SiOR function, in a rubber composition reinforced by a silica and a carbon black, is insufficient to improve the compromise.

[0146] In summary, it has surprisingly been discovered that a rubber composition reinforced by a silica and a carbon black with a Z value of greater than 80 based on natural rubber and on a diene elastomer bearing an SiOR function made it possible to obtain the best compromise between the hysteresis properties and the properties at break.