WO2019141667A1

WO2019141667A1 - Rubber composition for tyres with good wet grip and winter properties by tailoring phase morphology

Info

Publication number: WO2019141667A1
Application number: PCT/EP2019/050900
Authority: WO
Inventors: Swarupini GANESAN; Sander MARINUS; Mohamed THARIK; Hendrik Stevens; Louis Reuvekamp
Original assignee: Apollo Tyres Global R&D B.V.
Priority date: 2018-01-17
Filing date: 2019-01-15
Publication date: 2019-07-25
Also published as: GB201800760D0

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. In a cross-linkable rubber composition the cross-linkable rubber composition comprises, per hundred parts by weight of rubber (phr): ≥ 25 phr of a first rubber, the first rubber being a solution polymerized styrene-butadiene rubber (SSBR) comprising an alkoxysilane group and a primary amino group; ≥ 40 phr of a second rubber; and ≥ 1 phr of an aliphatic or aromatic resin. The first rubber has a glass transition temperature T_g of ≥ −30°C to ≤ 0°C, the second rubber has a glass transition temperature T_g of ≥ −110°C to ≤ −60°C and the resin has a glass transition temperature T_g of ≥ 30°C.

Description

Rubber composition for tyres with good wet grip and winter properties by tailoring phase morphology

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.

Tread rubber is one of the important portions of a pneumatic tyre which contributes enormously to the overall performance of a tyre. A tyre has to perform well in severe weather conditions and it has to exhibit a variety of performances such as wet grip, abrasion resistance and low rolling resistance. In addition, further requirements for all-season and winter tyres are required, such as retaining good performance on snow and ice.

A tread compound can be optimized to exhibit good winter performance by using low T_g polymers but it normally results in poor wet grip properties. On the other hand, tuning the rubber compound by using high T_g polymers or performance resins to improve wet grip properties possibly leads to impairment in winter performance and/or rolling resistance. In order to obtain good winter tyres, all properties need to be improved simultaneously.

US 2013/267640 Al discloses a snow tyre with an improved grip on wet ground which includes a tread formed of a rubber composition. The rubber composition includes: 20 to 100 phr of a first diene elastomer bearing at least one SiOR function, with R being hydrogen or a hydrocarbon radical, and with phr referring to parts by weight per hundred parts of elastomer; 100 to 160 phr of a reinforcing inorganic filler; and a plasticizing system. The plasticizing system includes: a content A of between 10 and 60 phr of a hydrocarbon resin having a T_g above 20 °C; and a content B of between 10 and 60 phr of a liquid plasticizing agent. A total content A+B is greater than 50 phr. Optionally, the rubber composition also includes 0 to 80 phr of a second diene elastomer.

US 2010/186868 Al is directed to a pneumatic tire having a component comprising a vulcanizable rubber composition comprising, based on 100 parts by weight of elastomer (phr), (A) from about 60 to about 90 phr of a solution polymerized styrene -butadiene rubber functionalized with an alkoxysilane group and a primary amines; (B) from about 40 to about 10 phr of polybutadiene having a microstructure comprised of about 96 to about 99 percent cis l,4-isomeric units, about 0.1 to about 1 percent trans l,4-isomeric units and from about 1 to about 3 percent vinyl 1,2 -isomeric units; a number average molecular weight (M„) in a range of from about 75,000 to about 150,000 and a heterogeneity index (M_w/M„) in a range of from about 3/1 to about 5/1; and (C) from about 50 to about 150 phr of silica. EP 3 031 621 Al concerns a pneumatic tire having a tread comprising a rubber composition comprising, based on 100 parts by weight of elastomer (phr), from 50 to 90 phr of a solution polymerized styrene -butadiene rubber having a glass transition temperature (T_g) ranging from -65 °C to -55 °C and being functionalized with an alkoxy silane group and at least one functional group selected from the group consisting of primary amines and thiols; from 50 to 10 phr of polybutadiene having a cis 1,4 content greater than 95 percent and a T_g ranging from -80 to -110 °C; and from 30 to 80 phr of a combination of a resin having a T_g of at least 30 °C and an oil, wherein the weight ratio of resin to oil is greater than 1.

Optimizing the tread compound for wet grip normally results in trade-off in winter performance. The present invention has the object to at least partially overcome the drawbacks and in particular to provide a composition for a tyre tread which can serve well in wide range of temperatures and conditions - both winter and wet.

This object is achieved by a cross-linkable rubber composition according to claim 1, a cross-linked rubber composition according to claim 9, a method according to claim 14 and a tyre according to claim 15. Advantageous embodiments are the subject of dependent claims. They may be combined freely unless the context clearly indicates otherwise.

Hence, a cross-linkable rubber composition is provided, the cross-linkable rubber composition comprising, per hundred parts by weight of rubber (phr):

> 25 phr of a first rubber, the first rubber being a solution polymerized styrene-butadiene rubber (SSBR) comprising an alkoxy silane group and a primary amino group;

> 40 phr of a second rubber; and

> 1 phr of an aliphatic or aromatic resin.

The first rubber has a glass transition temperature T_g of > -30 °C to < 0 °C, the second rubber has a glass transition temperature T_g of > -110 °C to < -60 °C and the resin has a glass transition temperature T_g of > 30 °C. The glass transition temperatures T_g are measured by differential scanning calorimetry (DSC) according to ISO 22768. This norm specifies a heating rate of 20 °C/min.

It will be understood that in formulations discussed in connection with the present invention the phr amount of all rubber components adds up to 100. Furthermore, a mixture of rubbers which satisfies the definition of the first or the second rubber according to the invention is also regarded as the first or second rubber, respectively. The cross-linkable rubber composition according to the invention comprises cross-linkable groups in the individual rubber components. They may be cross-linked (cured, vulcanised) by methods known to a skilled person in the rubber technology field.

The first rubber is a SSBR which comprises alkoxysilane groups such as -Si(OR)3 with each R independently being an alkyl rest. Preferred are trimethoxysilane groups and triethoxysilane groups. Furthermore, this SSBR also comprises primary amino groups -NFF. Such dual- functionalised SSBRs are commercially available and may be synthesised from unfunctionalised SSBRs by methods known in the art such as hydrosilylation with H-Si(OR)3 compounds, thiol-ene- coupling using aminothiol compounds and hydrosilylation with H-Si(OR)2-Linker-NH2 compounds.

For example, and as taught in US 7,342,070, the first mbber may be of the formula (I) or (II):

(R¹ - NH₂)n

Pk - Si - (OR²)m

R²4-(n+m+k) I wherein P is a (co)polymer chain of a conjugated diolefm or a conjugated diolefm and an aromatic vinyl compound, R¹ is an alkylene group having 1 to 12 carbon atoms, R² and R³ are each independently an alkyl group having 1 to 20 carbon atoms, an allyl group or an aryl group, n is an integer of 1 or 2, m is an integer of 1 or 2, and k is an integer of 1 or 2, with the proviso that n+m+k is an integer of 3 or 4,

(NH₂— R¹— P)j— Si— (OR²)h

R³4-(_j+h) II wherein P, R¹, R² and R³ have the same definitions as give for the above-mentioned formula I, j is an integer of 1 to 3, and h is an integer of 1 to 3, with the provision that j+h is an integer of 2 to 4.

The second rubber is preferably a polybutadiene rubber which has been obtained under nickel or neodymium catalysis.

If desired, further SSBRs and other rubbers not falling under the definition of the first and second rubbers can be present. However, it is preferred that the rubber composition according to the invention does not contain rubbers not falling under the definition of the first and second rubbers. The resins may be aromatic resins or aliphatic resins. Examples for aromatic resins include C5/C9 aromatic hydrocarbon resin. Aliphatic resins are resins without aromatic structure such as polyterpene resins, or aliphatic hydrocarbon resins such as C5 or DCPD (dicyclopentadiene) resins. The resin may be one of these resins or a combination thereof. Without wishing to be bound by theory it is believed that the first rubber with its comparatively high T_g and the second rubber with its comparatively low T_g are only partially miscible, thereby leading to a broad tan delta curve in the cured rubber and offering a good balance between wet grip and winter performance of a tyre employing such a material. The presence of the resin with its comparatively high T_g further broadens the tan delta curve of the cured rubber. Preferably the first rubber has a glass transition temperature T_g of > -28 °C to < -20 °C, the second rubber has a glass transition temperature T_g of > - 110 °C to < -80 °C and the resin has a glass transition temperature T_g of > 60 °C to < 90 °C.

The cross -linkable rubber compositions may be sulfur-vulcanizable and/or peroxide -vulcanizable. Other vulcanization systems may also be used. If desired, additives can be added. Examples of usual additives are stabilizers, antioxidants, lubricants, fillers, dyes, pigments, flame retardants, conductive fibres and reinforcing fibres. The filler can be carbon black, silica or a combination of both. The total amount of filler in the rubber composition is preferably > 50 phr to < 140 phr, more preferably > 65 phr to < 130 phr.

If desired, the cross-linkable rubber composition can also comprise a coupling agent. Suitable coupling agents comprise silane compounds. Particularly suitable silane compounds comprise di- and tetrasulphides and mercaptosilanes. It is possible for the rubber composition to be provided with a conductive filler to make it at least partially conductive.

In an embodiment of the rubber composition the composition comprises:

> 25 phr to < 60 phr (preferably > 30 phr to < 50 phr) of the first rubber; > 40 phr to < 75 (preferably > 40 phr to < 70 phr) phr of the second rubber; and

> 1 phr to < 80 phr (preferably > 20 phr to < 35 phr) of the resin (preferably an aliphatic resin).

In another embodiment of the rubber composition in the first rubber > 75 mol-% (preferably > 90 mol-% to < 100 mol-%), as determined by nuclear magnetic resonance (NMR) spectroscopy, of the alkoxysilane groups and the primary amino groups are located at the chain ends of the rubber polymer chains. In another embodiment of the rubber composition the first rubber comprises > 25% to < 35% (preferably > 27 to < 29%), as determined by nuclear magnetic resonance (NMR) spectroscopy, of styrenic groups.

In another embodiment of the rubber composition the second rubber is a butadiene rubber (BR) with a cis group content, as determined by infrared spectroscopy (IR), of > 90%. Preferably the cis group content is > 93%. The cis content of the polybutadiene rubber is usually provided by the supplier and may be determined with FTIR. The method is based on the calculation of the ratio between the intensity of the bands attributable to the l,4-trans and 1, 2-vinyl isomers and a reference band (internal standard) falling at 1312 cm ¹ (L. J. Bellamy, The Infrared Spectra of Complex Molecules, Vol. 1 Third Edition, Chapman and Hall). The 1,4-cis content is determined by the difference from 100. Sample preparation is performed on a polybutadiene film, obtained by starting from a solution, evaporated on a KBr window.

In another embodiment of the rubber composition the resin (preferably an aliphatic resin) has a molecular weight Mw of > 500 to < 4000 g/mol. Preferred is a molecular weight Mw of > 800 to < 2500 g/mol and more preferred > 1000 to < 2200 g/mol.

In another embodiment of the rubber composition the resin comprises a polyterpene resin, C5 resin, C9 resin or DCPD resin. A preferred polyterpene resin has a Mw of > 800 g/mol to < 1200 g/mol. A preferred C5 resin has a Mw of > 2000 to < 4000 g/mol. The polyterpene resin is a resin obtained by polymerizing a terpene compound, or a hydrogenated product of the resin. The terpene compound is a hydrocarbon represented by (CsHs), or an oxygenous derivative thereof, whose basic structure is any of terpenes classified into monoterpenes (C10H16), sesquiterpenes (C15H24), diterpenes (C20H32), and the like. Examples of the compound include a-pinene, b-pinene, dipentene, limonene, myrcene, allo-ocimene, ocimene, a-phellandrene, a-terpinene, g-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, a-terpineol, b-terpineol, and g-terpineol. Examples of the polyterpene resins include terpene resins formed from the terpene compounds described above, such as a-pinene resin, b-pinene resin, limonene resin, dipentene resin, or b- pinene/limonene resin, as well as hydrogenated terpene resins prepared by hydrogenating any of the terpene resins. Preferably non-hydrogenated terpene resins are used; most preferably resins comprising limonene as a co-monomer are used. In another embodiment the rubber composition further comprises a polymer which is liquid at 20 °C and which has a glass transition temperature T_g of > -100 °C to < -70 °C, the glass transition temperatures being measured by differential scanning calorimetry (DSC) according to ISO 22768. Examples for such polymers include Liquid Isoprene Rubber (LIR) and Liquid Butadiene Rubber (LBR). These liquid rubbers may be functionalised by copolymerisation with other monomers.

The present invention is further directed towards a cross-linked rubber composition which is obtained by cross-linking a rubber composition according to the invention.

In an embodiment the cross-linked rubber composition has a tan delta at 0 °C of > 0.2 to < 0.3 (determined from dynamic mechanical analysis (DMA) measurements according to ISO 4664-1, frequency 10 Hz, 0.1% dynamic strain) and a tan delta at 70 °C of > 0.1 to < 0.2 (determined from DMA measurements according to ISO 4664-1, frequency 10 Hz, 6 % dynamic strain). Preferably the tan delta at 0 °C is > 0.21 to < 0.29 and the tan delta at 70 °C is > 0.11 to < 0.20.

In another embodiment of the cross-linked rubber composition its tan delta curve (determined from dynamic mechanical analysis (DMA) measurements according to ISO 4664-1, frequency 10 Hz, 0.1% dynamic strain) has a full width at half maximum (FWHM) of > 65 °C. Preferably this width is > 65 °C to < 100 °C.

In another embodiment the cross-linked rubber composition has a glass transition temperature T_g (determined as the maximum of tan delta from dynamic mechanical analysis (DMA) measurements according to ISO 4664-1, frequency 10 Hz, 0.1 % dynamic strain) of > -40 °C to < -20 °C (preferably > -37 °C to < -22 °C).

In another embodiment the cross-linked rubber composition has a rebound at 23 °C, determined according to ISO 4662, of > 20 % to < 30% (preferably > 21 to < 28%).

The present invention also relates to a method of preparing a tyre, comprising the steps of: - providing a tyre assembly comprising a rubber composition according to the invention;

- cross-linking at least the rubber composition according to the invention in the tyre assembly.

The present invention also encompasses a tyre comprising a tyre tread, wherein the tyre tread comprises a cross-linked rubber composition according to the invention.

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

FIG. 1 shows temperature -dependent tan delta curves for compositions Cl and C2

FIG. 2 shows temperature -dependent tan delta curves for compositions C3 and II

FIG 3 shows temperature -dependent tan delta curves for compositions 12, 13, C4 and C5 To develop a tread compound with improved winter performance and wet grip, the compound is expected to perform well in wide range of temperatures. In this case, a compound which exhibits a broad tan delta curve (dynamic mechanical properties) is of great interest. Such a broad tan delta curve implies heterogeneity of the polymer network or polymer chains with wide distribution of segmental motions at different temperatures. As a measure for the broadness of the tan delta curve, the full width at half maximum (FWHM) of the tan delta curves was determined. This also translates to the ability of the vulcanised rubber to perform in a broad temperature range.

Higher tan delta values at 0 °C indicate better wet grip, lower tan delta values at 70 °C indicate lower rolling resistance of the vulcanised rubber. The rebound at 23 °C is also an indicator for wet grip properties of the vulcanised rubber. Lower rebound value at 23 °C relates to better wet grip.

In accordance with the preceding, cross-linkable rubber compositions were prepared as described in the examples 1 to 3 and cross-linked. Materials mentioned in the tables were:

BR: Butadiene Rubber, (Nd catalyst) with 98% cis content and a glass transition temperature of about -105 °C

BR-OE: Butadiene rubber (Nd catalyst) with 94% cis content and extended with 37.5 phr TDAE oil and a glass transition temperature around -100 °C.

Resin: a polyterpene resin with a molecular weight of 1090 g/mol, a softening point of 125 ° C, and a glass transition temperature of 73 °C. Example 1: compositions Cl (comparative) and C2 (comparative)

Cross -linkable rubber compositions were prepared according to the following table:

After vulcanisation the following properties were determined:

In this experiment, lower loading (20 phr) of SSBR having a T_g of -28°C functionalized with alkoxysilane and primary amines is compared against SSBR having a T_g of -25°C functionalized with alkoxysilane and thiol groups. The tan d curves of these two compounds (FIG. 1) are comparable and there is no significant change in the compound properties. Therefore, a content of SSBR 3 (which corresponds the first rubber according to the claims) which is lower than specified in the claims does not provide any beneficial effects. Example 2: compositions C3 (comparative) and II (inventive)

After vulcanisation the following properties were determined:

The comparative example C3 contains 50 phr of BR and 50 phr of SSBR functionalized with alkoxysilane and thiol groups having a T_g of -25 °C. In the inventive example II, the aforementioned functionalized SSBR is replaced with 50 phr of SSBR functionalized with alkoxysilane and primary amines having a T_g of -28 °C. Although the T_g and microstructures of these two functionalized SSBRs are comparable, surprisingly, II reveals a higher T_g (-23 °C) compared to C3. In addition, II also exhibits a slightly broader tan delta curve (FIG. 2) compared to C3 which is evident from the FWHM range. It is believed that functionalized SSBR containing alkoxysilane and primary amines has lower degree of miscibility with BR compared to the other functionalized SSBR containing alkoxysilane and thiol groups. Example 3: compositions 12 and 13 (inventive) and C4 and C5 (comparison)

After vulcanisation the following properties were determined:

In case of comparative example C4, the polymer blend containing SSBR functionalized with alkoxysilane and thiol groups is adjusted to meet the same T_g as 12. The inventive example 12 contains SSBR functionalized with alkoxysilane and primary amines. C4 and 12 exhibit similar wet grip indicator tan d at 0 °C but there is a significant difference concerning the broadness of the tan delta curve (FIG. 3). Because of the reduced or partial miscibility of functionalized SSBR (alkoxysilane and primary amine) with BR, 12 displays broader tan delta curve thereby eliminating the trade-off between wet grip and winter properties.

The polyterpene resin (20 phr) originally present in the recipe of 12 is completely replaced with processing oil. Upon removal of resin, C5 displays lower T_g as expected but the tan delta curve is not as broad as 12. It is believed that polyterpene resin has a higher affinity to the functionalized SSBR which helps in additional broadening of the tan delta curve. The compound recipe containing the combination of SSBR functionalized with alkoxysilane and primary amines, resin and low T_g polymers (T_g ranging from -60 to -110°C), as described in 12 and 13, offers the possibility to improve both wet grip and winter performance.

Claims

Patent claims

1. A cross-linkable rubber composition, the cross-linkable rubber composition comprising, per hundred parts by weight of rubber (phr):

> 25 phr of a first rubber, the first rubber being a solution polymerized styrene -butadiene rubber (SSBR) comprising an alkoxy silane group and a primary amino group;

> 40 phr of a second rubber; and

> 1 phr of an aliphatic or aromatic resin; characterised in that the first rubber has a glass transition temperature T_g of > -30 °C to < 0 °C, the second rubber has a glass transition temperature T_g of > -110 °C to < -60 °C and the resin has a glass transition temperature T_g of > 30 °C, the glass transition temperatures T_g being measured by differential scanning calorimetry (DSC) according to ISO 22768.

2. The rubber composition according to claim 1, wherein the composition comprises: > 25 phr to < 60 phr of the first rubber;

> 40 phr to < 75 phr of the second rubber; and

> 1 phr to < 80 phr of the resin.

3. The rubber composition according to one of the preceding claims, wherein in the first rubber > 75 mol-%, as determined by nuclear magnetic resonance (NMR) spectroscopy, of the alkoxysilane groups and the primary amino groups are located at the chain ends of the rubber polymer chains.

4. The rubber composition according to one of the preceding claims, wherein the first rubber comprises > 25% to < 35%, as determined by nuclear magnetic resonance (NMR) spectroscopy, of styrenic groups.

5. The rubber composition according to one of the preceding claims, wherein the second rubber is a butadiene rubber (BR) with a cis group content, as determined by infrared spectroscopy (IR), of >

90%.

6. The rubber composition according to one of the preceding claims, wherein the resin has a molecular weight Mw of > 500 to < 4000 g/mol.

7. The rubber composition according to one of the preceding claims, wherein the resin comprises a polyterpene resin, C5 resin, C9 resin or DCPD resin.

8. The rubber composition according to one of the preceding claims, further comprising a polymer which is liquid at 20 °C and which has a glass transition temperature T_g of > -100 °C to < -70 °C, the glass transition temperatures being measured by differential scanning calorimetry (DSC) according to ISO 22768.

9. A cross-linked rubber composition, characterised in that it is obtained by cross-linking a rubber composition according to one of claims 1 to 8.

10. The cross-linked rubber composition according to claim 9 with a tan delta at 0 °C of > 0.2 to < 0.3 (determined from dynamic mechanical analysis (DMA) measurements according to ISO 4664- 1, frequency 10 Hz, 0.1% dynamic strain) and a tan delta at 70 °C of > 0.1 to < 0.2 (determined from DMA measurements according to ISO 4664-1, frequency 10 Hz, 6 % dynamic strain).

11. The cross-linked rubber composition according to claim 9 or 10, wherein its tan delta curve

(determined from dynamic mechanical analysis (DMA) measurements according to ISO 4664-1, frequency 10 Hz, 0.1% dynamic strain) has a full width at half maximum of > 65 °C.

12. The cross-linked rubber composition according to one of claims 9 to 11, having a glass transition temperature T_g (determined as the maximum of tan delta from dynamic mechanical analysis (DMA) measurements according to ISO 4664-1, frequency 10 Hz, 0.1 % dynamic strain) of > -40 °C to < -20 °C.

13. The cross-linked rubber composition according to one of claims 9 to 12, having a rebound at 23 °C, determined according to ISO 4662, of > 20 % to < 30%.

14. A method of preparing a tyre, comprising the steps of: - providing a tyre assembly comprising a rubber composition according to one of claims 1 to 8;

- cross-linking at least the rubber composition according to one of claims 1 to 8 in the tyre assembly.

15. A tyre comprising a tyre tread, characterised in that the tyre tread comprises a cross-linked rubber composition according to one of claims 9 to 13.