RUBBER COMPOSITION FOR TYRES WITH GOOD ROLLING RESISTANCE AND WET GRIP PROPERTIES

Abstract

The present invention relates to a cross-linkable rubber composition, a cross-linked rubber composition obtained by cross-linking such a rubber composition, a method of preparing a tyre and a tyre. A cross-linkable rubber composition comprises, per hundred parts by weight of rubber (phr): ≧1 phr to ≦99 phr of a styrene-butadiene rubber (SBR) component; ≧1 phr to ≦99 phr of a polybutadiene rubber (BR) component; ≧5 phr to ≦35 phr of a polyterpene resin component;

The polybutadiene rubber has a branching index G of ≧0.7 to ≦0.85 and a polydispersity index Mw/Mn of ≧3.5 and ≦4.5 and the polyterpene resin has a molecular weight Mw of ≧900 to ≦1200.

Claims

1. A cross-linkable rubber composition, the cross-linkable rubber composition comprising, per hundred parts by weight of rubber (phr): ≧1 phr to ≦99 phr of a styrene-butadiene rubber (SBR) component; ≧1 phr to ≦99 phr of a polybutadiene rubber (BR) component; ≧5 phr to ≦35 phr of a polyterpene resin component; wherein the polybutadiene rubber has a branching index G of ≧0.7 to ≦0.85 and a polydispersity index Mw/Mn of ≧3.5 and ≦4.5 and the polyterpene resin has a molecular weight Mw of ≧900 to ≦1200 g/mol.

2. The rubber composition according to claim 1, wherein the polyterpene resin component comprises a polylimonene resin.

3. The rubber composition according to claim 1, wherein the polybutadiene rubber component comprises a polybutadiene rubber which has been obtained under nickel catalysis.

4. The rubber composition according to claim 1, wherein the polybutadiene rubber component comprises a polybutadiene rubber with a cis group content of ≧90%.

5. The rubber composition according to claim 1, wherein the styrene-butadiene rubber component comprises a first styrene-butadiene rubber and a second styrene-butadiene rubber which is different from the first styrene-butadiene rubber.

6. The rubber composition according to claim 5, wherein the first styrene-butadiene rubber is a functionalized styrene-butadiene rubber and the second styrene-butadiene rubber is an unfunctionalised styrene-butadiene rubber.

7. The rubber composition according to claim 6 comprising ≧20 phr to ≦70 phr of the first styrene-butadiene rubber component, ≧20 phr to ≦70 phr of the second styrene-butadiene rubber component, ≧20 phr to ≦70 phr of the polybutadiene rubber component and ≧5 phr to ≦35 phr of the polyterpene resin component.

8. The rubber composition according to claim 1 comprising a filler component.

9. A cross-linked rubber composition obtained by cross-linking the rubber composition according to claim 1.

10. The cross-linked rubber composition according to claim 9 with a tan δ at 0° C. of ≧0.4 to ≦0.8 and a tan δ at 70° C. of ≧0.07 to ≦0.2.

11. The cross-linked rubber composition according to claim 9 with a glass transition temperature Tg of ≧−15° C. to ≦−5° C.

12. A method of preparing a tyre, comprising the steps of: providing a tyre assembly comprising a rubber composition according to claim 1; cross-linking at least the rubber composition in the tyre assembly.

13. A tyre comprising a tyre tread, wherein the tyre tread comprises a cross-linked rubber composition according to claim 9.

Description

[0041] The present invention will be further described with reference to the following figures and examples without wishing to be limited by them.

[0042] FIG. 1 shows the resin uptake for high molecular weight resins in example 1.

[0043] FIG. 2 shows the resin uptake for low molecular weight resins in example 1.

[0044] FIG. 3 shows DMA curves from example 2 at a temperature range from −80° C. to 20° C.

[0045] FIG. 4 shows DMA curves from example 2 at a temperature range from 20 C to 80° C.

[0046] FIG. 5 shows DMA curves from example 3 at a temperature range from −80° C. to 20° C.

[0047] FIG. 6 shows DMA curves from example 3 at a temperature range from 20° C. to 80° C.

EXAMPLE 1

[0048] The solubility of several resins into a high-branched BR/SBR and a low-branched BR/SBR mix, as described in the table below, were studied following a procedure described in the PhD thesis of Guo, R. “Improved properties of dissimilar rubber-rubber blends using plasma polymer encapsulated curatives. 2009, University of Twente: Enschede, the Netherlands, page 42-44. http://dx.doi.org/10.3990/1.9789036529143.”

[0049] Small vulcanized rubber sheets were prepared according to the table below and placed in a glass bottle containing a resin in such a fashion that all sides were packed with the resin. The bottle was subsequently placed in an oven at 60° C. The sample weights were measured after careful cleaning of the sheet. The amount of weight increase over time corresponds with the solubility of the resin into the rubber mix.

TABLE-US-00001 high branched BR mixture low branched BR mixture Component amount (phr) amount (phr) SBR 70 70 BR (Ni cat) 30 — BR (Nd cat) — 30 ZnO 3 3 Stearic acid 1 1 Sulfur 0.25 0.25 TBBS 0.25 0.25 Total 104.5 104.5

[0050] The SBR rubber used was BUNA VSL 5025-OHM, a solution SBR with a vinyl content of 50% and a styrene content of 25%.

[0051] The polybutadiene rubber obtained under nickel catalysis had a polydispersity index Mw/Mn of 4.2 and a branching index G of 0.73. In contrast, the polybutadiene rubber obtained under neodymium catalysis had a polydispersity index Mw/Mn of 2.4 and a branching index G of 0.86.

[0052] The results of a first set of experiments are depicted in FIG. 1. The weight increase of the vulcanized rubber sheets was measured over the course of 30 days. The resins used in this first series of experiments were characterised as high molecular weight resins. In particular, the resins were:

[0053] High MW polyterpene resin: Sylvares TR 7125 having a Mw of 1090 g/mol

[0054] High MW AMS (alpha-methyl styrene) resin: Sylvares SA 140 having a Mw of 4019 g/mol

[0055] High MW rosin ester: Sylvacote 7003 having a Mw of 9614 g/mol

[0056] The results of a second set of experiments are depicted in FIG. 2. The weight increase of the vulcanized rubber sheets was measured over the course of 20 days. The resins used in this second series of experiments were characterised as low molecular weight resins. In particular, the resins were:

[0057] a low MW polyterpene resin having a Mw of 392 g/mol

[0058] a low MW rosin ester having a Mw of 430 g/mol

[0059] From the results in this example it is clear that all low MW resins diffuse quickly in both the high-branched and low-branched BR/SBR mixture. No clear differences between low MW resin types could be observed.

[0060] The high MW resins, on the other hand, show a significant different behaviour for high MW polyterpene at the one hand, and high MW AMS and rosin ester at the other hand. The AMS and rosin ester are taken up much slower in the mixture, and a small difference between high branched and low branched BR mixture is only for the AMS resin is only apparent after 10 days. The polyterpene is taken up faster in the mixture in both the high branched and low branched BR mixture, however, the uptake is about twice as fast in the low branched BR mixture and apparent from day 1.

EXAMPLE 2

[0061] Samples with a high MW polyterpene resin were mixed in various amounts in a standard recipe containing SBR and either high (Ni catalysed) or low (Nd catalysed) branched BR and vulcanized to study the DMA curve according to ISO 6721. The recipe (amounts stated are given in phr) and glass transition temperatures Tg are given in the table below and the obtained graph is shown in FIG. 3.

[0062] The SBR rubber used was BUNA VSL 5025-OHM, a solution SBR with a vinyl content of 50% and a styrene content of 25%.

[0063] The polybutadiene rubber obtained under nickel catalysis (Ni BR) had a polydispersity index Mw/Mn of 4.2 and a branching index G of 0.73. In contrast, the polybutadiene rubber obtained under neodymium catalysis (Nd BR) had a polydispersity index Mw/Mn of 2.4 and a branching index G of 0.86

[0064] The resin Sylvares TR 7125 is a polylimonene resin with a molecular weight of about 1090 g/mol, a softening point of about 125° C., and a glass transition temperature of about 73° C.

[0065] A clear difference in Tg was observed for the mixture containing high branched Ni BR and the mixture containing low branched Nd BR when adding 15 or 30 phr of resin in the mixture. Additional amounts (from 15 phr to 30 phr) of the Sylvares TR 7125 resin in combination with the Ni BR gave a significantly higher shift in Tg than in the Nd BR sample.

TABLE-US-00002 Comparative Comparative Sample 1 Sample 2 Sample 1 Sample 2 S SBR* 70 70 70 70 Nd BR 30 30 0 0 Ni BR 0 0 30 30 Sylvares TR 7125 15 30 15 30 Tg −9.2 −4.3 −10.1 −0.9 *In addition to the components listed in the table, all samples contained a vulcanization package of 1.5 phr sulphur, 1.5 phr TBBS (N-Tert-Butyl-2 benzothiazole sulfenamide), 3 phr ZnO and 1 phr stearic acid.

[0066] This difference can be related to the mixing behaviour of the compounds. In the comparative samples a larger amount of Sylvares TR 7125 will be mixed in the Nd BR phase, thereby minimizing a Tg shift at lower temperatures. The high Tg shift in sample 2 suggests that most of the Sylvares TR 7125 is mixed in the SBR phase. This Tg shift is expected to have a positive effect on the wet grip of the tire.

[0067] Without being bound to theory, it is believed that the high MW polyterpene resin is less miscible with a high branched BR (Ni BR) than a low branched BR (Nd BR). In the first case the polyterpene is more likely to end up in the SBR phase, causing a Tg shift and a higher tan δ at 0° C. than in the latter case. In general Nd BR has better rolling resistance properties than Ni BR, this is also apparent from the tan δ at 70° C. in FIG. 3. However, the difference between Ni BR and Nd BR tan δ at 70° C. is reduced upon the addition of more resin.

[0068] Overall, the slow migration in BR suggests a higher tendency of the high MW polyterpene to mix with the SBR phase, thereby increasing the wet grip properties, without significantly influencing the rolling resistance properties.

EXAMPLE 3

[0069] Two polyterpene resins were investigated for their effect on the tan δ on a cured rubber formulation with 70 phr of the SBR used in example 2 and 30 phr of the Ni BR used in example 2. The measured graphs are given in FIG. 4.

[0070] As already mentioned, the resin Sylvares TR 7125 is a polylimonene resin with a molecular weight of about 1090 g/mol, a softening point of about 125° C., and a glass transition temperature of about 73° C. Sylvares TR 5147 is a polylimonene resin having a molecular weight of about 945 g/mol, a softening point of about 120° C. and has a glass transition temperature of about 71° C.

[0071] The results are given in the following table. Tan δ values are normalized to 100 for the rubber not containing any polyterpene resin.

TABLE-US-00003 no added resin Sylvares TR 5147 Sylvares TR 7125 tan δ (0° C.) 100 123 122 4 phr resin tan δ (70° C.) 100 131 127 4 phr resin tan δ (0° C.) 100 196 177 20 phr resin tan δ (70° C.) 100 123 119 20 phr resin

[0072] Therefore, a high MW polyterpene combined with a high branched BR and SBR results in an excellent balance of tyre tread properties, as apparent from the high tan δ at 0° C. and low tan δ at 70° C. values.