WO2020002460A1

WO2020002460A1 - Method for producing a grafted rubber and tire comprising the grafted rubber

Info

Publication number: WO2020002460A1
Application number: PCT/EP2019/067061
Authority: WO
Inventors: Lehani VERWEY; Gyula Julius Vancso; Louis Reuvekamp; Steven M. Schultz
Original assignee: Apollo Tyres Global R&D B.V.
Priority date: 2018-06-29
Filing date: 2019-06-26
Publication date: 2020-01-02
Also published as: LU100868B1

Abstract

The present invention relates to a method for producing a grafted rubber, the method comprising the steps of: A) providing a rubber comprising at least one olefinic carbon-carbon-double bond, B) providing a polymer being capable of exhibiting lower critical solution temperature (LCST) properties and comprising a terminal functional thiol group capable of reacting with the olefinic carbon-carbon double bond of the rubber, wherein the polymer is obtained by the steps of: B1) reversible addition-fragmentation chain transfer (RAFT) polymerization of a monomer comprising at least one olefinic carbon-carbon-double bond in the presence of a thiocarbonylthio RAFT agent thereby forming a polymer comprising a terminal thiocarbonylthio group, and B2) cleaving the terminal thiocarbonylthio group to a thiol group thereby attaining the terminal thiol functionality of the polymer, and C) reacting the polymer with the rubber by a radical thiol-ene reaction thereby forming a grafted rubber, the grafted rubber comprising a backbone derived from the rubber and sidechains derived from the polymer. The polymer in a further step B3) is treated with a reducing agent before being used in the grafting reaction of step C).

Description

Method for producing a grafted rubber and tire comprising the grafted rubber The present invention relates to a method for producing a grafted rubber.

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. It is well known in rubber compounding that there is a trade-off between wet grip and rolling resistance. For low rolling resistance, the rubber must show elastic behavior and have low hysteresis. For proper contact with the road surface to deliver wet grip performance, the material must have high hysteresis and be non-elastic.

The classic approach of mixing well-known materials for tread compounding in order to obtain a tread with the best properties in both wet grip and rolling resistance will lead to a compromise between the two. The tread compound can be optimized to exhibit good wet performance by using high Tg polymers but it normally results in poor rolling resistance properties.

For balancing wet properties and rolling resistance at the same time, a further approach provides a thermoresponsive polymer incorporated into a rubber matrix. A thermoresponsive polymer, incorporated into a rubber matrix, that adapts its friction level based on road conditions would provide increased safety when needed while maintaining good rolling resistance. For example, when the tyre comes into contact with water, the thermoresponsive domains in the tread compound undergoes a structural change to become softer to improve wet grip performance. As the temperature increase, the thermoresponsive domains become stiffer to provide good rolling resistance. To develop a tread compound with improved wet grip and rolling resistance, the compound is expected to perform well in wide range of temperatures. In this case, a compound that exhibits a high tan d curve (dynamic mechanical properties) at 0 °C and low tan d at 70 °C.

In order to obtain a compound which can exhibit high tan d curve at 0 °C and low tan d at 70 °C, a material that responds differently at each temperature is required. A thermoresponsive polymer that can adapt its structure and properties to become softer and stiffer to benefit both wet grip and rolling resistance will create a decoupling between these two elements in tyre technology.

US 8,536,266 B2 describes a pneumatic tire having a tread, which comprises a rubber composition comprising a copolymer comprising a polymeric backbone chain derived from a monomer comprising at least one conjugated diene monomer and optionally at least one vinyl aromatic monomer; and polymeric sidechains bonded to the backbone chain. The sidechains comprise a polymer capable of exhibiting a lower critical solution temperature (LCST). The contact angle for a PNIPAM-functionalized styrene-butadiene rubber measured above the LCST was relatively constant with increasing amount of PNIPAM, indicating that the functionalized polymer was relatively hydrophobic above the LCST. The contact angle for the samples measured below the LCST however showed a significant decrease in contact angle with increasing amount of PNIPAM, indicating that the functionalized polymer becomes relatively hydrophilic below the LCST.

US 8,415,432 Bl describes a vulcanizable rubber composition comprising a diene based elastomer; and a compound of formula Q-[-X-S-R]_n (I) where Q is an n-valent organic group of 1 or more carbon atoms and optionally one or more of nitrogen, oxygen, sulfur, halogen, silicon, and phosphorus atoms; X is a divalent polymer group capable of exhibiting a lower critical solution temperature; R is independently a hydrogen atom or a monovalent group of formula -C(=S)-Z (II) where Z is a monovalent organic group of 1 or more carbon atoms and optionally one or more of nitrogen, oxygen, sulfur, halogen, silicon, and phosphorus atoms; and n is an integer greater than or equal to 2. Further described is a rubber composition comprising the reaction product of a diene based elastomer and a compound of formula I, and a pneumatic tire with a tread comprising the rubber composition.

US 8,883,884 B2 describes a pneumatic tire comprising at least one component, the at least one component comprising a polymer blend comprising a copolymer and an additional polymer, the copolymer comprising: a polymeric backbone chain derived from a monomer comprising at least one conjugated diene monomer and optionally at least one vinyl aromatic monomer; and polymeric sidechains bonded to the backbone chain, the sidechains comprising a polymer immiscible with the backbone; the additional polymer consisting of a polymer miscible with the polymeric sidechains.

Optimizing the tread compound for wet grip normally results in trade-off in rolling resistance performance. The present invention has the object to provide a method for producing a thermoresponsive rubber to be used in a tyre tread to serve well in very wide range of temperatures for both wet and rolling resistance.

This object is achieved by a method for producing a grafted rubber according to claim 1, a grafted rubber according to claim 9, and a tire according to claim 10. Advantageous embodiments are the subject of the dependent claims. They may be combined freely unless the context clearly indicates otherwise.

Accordingly, a method for producing a grafted rubber is provided, the method comprising the steps of:

A) providing a rubber component comprising at least one olefmic carbon-carbon-double bond,

B) providing a polymer being capable of exhibiting lower critical solution temperature (LCST) properties and comprising a terminal functional thiol group capable of reacting with the olefinic carbon-carbon double bond of the rubber,

wherein the polymer is obtained by the steps of:

Bl) reversible addition-fragmentation chain transfer (RAFT) polymerization of a monomer comprising at least one olefinic carbon-carbon-double bond in the presence of a thiocarbonylthio RAFT agent thereby forming a polymer comprising a terminal thiocarbonylthio group, and

B2) cleaving the terminal thiocarbonylthio group to a thiol group thereby attaining the terminal thiol functionality of the polymer, and

C) reacting the polymer with the rubber component by a radical thiol-ene reaction thereby forming a grafted rubber, the grafted rubber comprising a backbone derived from the rubber and sidechains derived from the polymer.

The polymer in a further step B3) is treated with a reducing agent before being used in the grafting reaction of step C).

It has surprisingly been found that a grafted SBR-PNIPAM rubber obtained by this method showed a decrease in contact angle at 25 °C with increase in PNIPAM, showing a increase in the hydrophilic nature of the rubber with increasing amount of LCST polymer. These enhanced rheological properties of the SBR-PNIPAM copolymer are assumed to be attributed to a more effective and homogenous grafting with PNIPAM due to the reduction step after aminolysis ensuring the PNIPAM chain ends having the thiol functionality to effectively couple to the vinyl groups. This thermoresponsive rubber offers vast potential for an improvement in wet grip and rolling resistance performance due to the alteration of the structure at different temperatures.

The rubber component may be selected from the group of styrene -butadiene rubber (SBR), solution polymerized styrene -butadiene rubber (SSBR), emulsion polymerized styrene -butadiene rubber (ESBR), styrene-isoprene-styrene (SIS) rubber, polyisoprene (IR) rubber, natural polybutadiene rubber (NR), synthetic polybutadiene rubber (BR) or a mixture thereof. Preferably, the rubber is manufactured by the solution process (SSBR or solution SBR). The rubber component comprises at least one olefinic carbon-carbon double bond. The vinyl groups along the backbone of the rubber component allow to covalently bind an LCST-capable polymer such as PNIPAM to.

The rubber component, preferably a solution polymerization prepared SSBR may have a styrene content in a range of > 5 weight- % to < 50 weight- %, preferably in a range of > 10 weight- % to < 35 weight-%, based on a total weight of 100 weight-%. The rubber component, preferably a SSBR may have a vinyl content in a range of > 30 weight-% to < 80 weight-%, preferably in a range of > 50 weight-% to < 70 weight-%, based on a total weight of 100 weight-%.

The rubber component may have a glass transition temperature of > -30 °C. The glass transition temperature T_g is measured by DSC, according to ISO 22768. This norm specifres a heating rate of 20 °C/min. Preferably the glass transition temperature T_g is > -25 °C.

Commercial available SSBR are S-SBR type sold by Sprintan under the Sprintan SLR name, e.g. Sprintan® SLR 4601. A commercially available BR is Neodymium Butadiene Rubber sold by Arlanxeo, e.g. BUNA® CB 24. The S-SBR can be conveniently prepared, for example, by anionic batch polymerisation.

The polymer being capable of exhibiting a lower critical solution temperature (LCST) may include homopolymers and copolymers of various monomers known to have LCST properties. By“capable of exhibiting a lower critical solution temperature (LCST)” it is meant that in the presence of water, the polymer associates with the water to form a water-swollen polymer phase, wherein the water- swollen polymer phase will show an LCST transition when heated from a temperature below the LCST to a temperature above the LCST. In other words, the polymer structure and properties are capable to change at different temperatures. The polymer is capable of exhibiting an LCST when the polymer exists as a side chain on the grafted rubber.

The LCST-capable polymer comprises a terminal functional thiol group. Such a terminal functional group is capable of reacting with an olefrnic carbon-carbon double bond of the rubber component by a radical thiol -ene reaction.

By the thiol-ene reaction the olefrnic bond present in the rubber is transformed into a thioether by reacting with the thiol group of the polymer. By this reaction a grafted rubber is formed, wherein the grafted rubber comprises a backbone derived from the rubber and sidechains derived from the polymer. To allow for the thiol-ene reaction a terminal thiol functionality of the LCST-capable polymer is required.

According to the method, a reducing reaction treating the polymer with a reducing agent before being used in the grafting reaction is incorporated. Without being bound to a specific theory, it is assumed that this reducing reaction provides that thiol groups eventually being consumed by undesired disulphide linking reactions are converted back to the thiol-terminal groups needed for the grafting.

In an embodiment of the method, the reducing agent is selected from the group of sodium borohydride (NaBfL), tributyl phosphine (PBm), tris(2-carboxyethyl)phosphine (TCEP), dithiothreitol, b-mercaptoethanol or a mixture thereof. The reducing agent preferably is tris(2- carboxyethyl)phosphine (TCEP).

In an embodiment of the method, the reducing agent is used in a molar ratio to the polymer in a range of > 5:1 to < 10:1. In a specific embodiment, tris(2-carboxyethyl)phosphine (TCEP) is used as the reducing agent in a molar ratio to the polymer in a range of > 5:1 to < 10:1.

The LCST-capable polymer may be selected from the group of acrylamides and substituted acrylamides, methacrylamides and substituted methacrylamides, acrylic acids and substituted acrylic acids, methacrylic acids and substituted methacrylic acids, caprolactams and substituted carbolactams, alkyl ethers and substituted alkyl ethers. In an embodiment, the polymer is selected from the group of poly(N-isopropylacrylamide), poly(N-cyclopropylacrylamide), poly(N,N- diethylacrylamide) and mixtures of these polymers. Preferably, the polymer is poly(N- isopropylacrylamide) (PNIPAM).

The LCST-capable polymer may exhibit a lower critical solution temperature (LCST) of > 0 °C to < 100 °C. Preferably, the polymer has an LCST of > 10 °C to < 40 °C. The LCST-capable polymer may have a glass transition temperature T_g of > 100 °C to < 150 °C, the glass transition temperatures being measured by differential scanning calorimetry (DSC) according to ISO 22768. Preferably, the glass transition temperature T_g is > 120 °C to < 130 °C.

The LCST-capable polymer according to the method is obtained by the steps of Bl) reversible addition-fragmentation chain transfer (RALT) polymerization of a monomer comprising an olefmic carbon-carbon-double bond in the presence of a thiocarbonylthio RALT agent, and B2) cleaving the terminal thiocarbonylthio group. The monomer may be selected from the group of N- isopropylacrylamide, N-cyclopropylacrylamide, N,N-diethylacrylamide and mixtures of these monomers. Preferably, the monomer is N-isopropylacrylamide.

In an embodiment of the method, the RAPT agent is selected from the group of dithioesters, trithiocarbonates, dithiocarbamates, and xanthates.

In an embodiment of the method, the RAPT agent is selected from the group of S-l-dodecyl-S’- (R,R’-dimethyl-R” -acetic acid)trithiocarbonate (DMP) and 4-cyano-4- dodecylsulfanylthiocarbonylsulfanyl-4-methyl butyric acid. A preferred RAPT agent is S-l- dodecyl-S’-(R,R’-dimethyl-R” -acetic acid)trithiocarbonate (DMP).

The RAPT polymerisation may be performed using a free-radical initiator, such as azobisisobutyonitrile (AIBN). In an embodiment of the method, the RAPT agent is used in a molar ratio to the free -radical initiator in a range of > 3:1 to < 5:1. Preferably, the RAPT agent/initiator ratio is kept constant, e.g. at 5:1. This may ensure only a negligible amount of chains is initiator- based.

Following the polymerisation, in step B2) the terminal thiocarbonylthio group is cleaved. Removal of the thiocarbonyl thio end-group may be carried out by a reduction in the presence of a nucleophile. Aminolysis may be provided by treating the polymer with hexylamine.

Thiocarbonylthio RAFT agents such as S-l-dodecyl-S’-(R,R’-dimethyl-R”-acetic acid)trithiocarbonate (DMP) are commercially available. Alternatively, the RAFT agent may be synthesized in a preceding reaction step of the method. In an embodiment of the method, DMP is synthesized by reacting 2-bromo-2-methylpropionic acid with dodecyl trithiocarbonate through a nucleophilic substitution. The dodecyl trithiocarbonate may be potassium trithiocarbonate. Potassium trithiocarbonate may be prepared by reacting potassium phosphate tribasic with dodecane thiol and carbon disulphide.

The present invention is further directed towards a method of preparing S-l-dodecyl-S’-(R,R’- dimethyl-R” -acetic acid)trithiocarbonate (DMP) by reacting 2-bromo-2-methylpropionic acid with potassium dodecyl trithiocarbonate through a nucleophilic substitution. The dodecyl trithiocarbonate may be potassium trithiocarbonate. Potassium trithiocarbonate may be prepared by reacting potassium phosphate tribasic with dodecane thiol and carbon disulphide.

The present invention also relates to a grafted rubber which is obtained by the method for producing a grafted rubber according to the invention. In the grafted rubber, the content of polymer is in a range of > 10 wt-% to < 15 wt-%, based on the total weight of the grafted rubber. Preferably, the content of polymer is in a range of > 12 wt-% to < 15 wt-%, based on the total weight of the grafted rubber. The grafted rubber may show a better nano-phase separation of the PNIPAM in the rubber compared to grafted rubber produced by a method without step B3). A better nano-phase separation of PNIPAM in the rubber matrix provides a softening and stiffening at different temperatures contributing to improved wet grip and rolling resistance properties of a tire.

Weight percent, weight-% or wt-%, are synonyms and are calculated on the basis of a total weight of 100 weight% of the respective object, if not otherwise stated. The total amount of all components of the respective object does not exceed 100 wt.-%.The present invention also relates to a tire comprising a tire tread, wherein the tire tread comprises a grafted rubber according to the invention.

Examples

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

FIG. 1 shows refractive index (RI) values and ultra violet (UV) values of Gel Permeation Chromatography (GPC) before (solid lines) and after (dotted lines) aminolysis of the PNIPAM in step 2.2 of Example 2.

FIG. 2 shows the atomic force microscopy micrographs of SBR on the left and SBR functionalised with 5 wt% and 12 wt% PNIPAM in the middle and to the right, respectively, as obtained of Example 3.

FIG. 3 shows the contact angle as a function of PNIPAM content of the PNIPAM functionalised SBR as obtained of Example 3.

FIG. 4 shows the thermal gravimetric analysis (TGA) of SBR and SBR functionalised with 12 wt% PNIPAM as obtained of Example 3. Example 1: Synthesis of S-l-Dodecyl-S’-(R,R’-dimethyl-R”-acetic acid)trithiocarbonate

In this example, the synthesis of the RAFT agent S-l-dodecyl-S’-(R,R’-dimethyl-R”-acetic acid)trithiocarbonate (DMP) is illustrated.

The synthesis of the RAFT agent DMP is shown in the Scheme 1 below:

Scheme 1 : Synthesis of RAFT agent DMP via a nucleophilic substitution reaction

16.6 g of Potassium phosphate tribasic (78.2 mmol) was dissolved in acetone in a 500 mL 3-neck round bottomed flask and stirred for 5 h to form a yellow suspension. 17.7 mL of Dodecane thiol (15.0 g, 74.2 mmol) was added dropwise and stirred for an additional hour. 9.1 mL of Carbon disulphide (11.5g, 151 mmol) was slowly added dropwise, and the solution was allowed to stir for 2 hours at 0 °C. 11.7g of 2-Bromo-2-methylpropionic acid (64.6 mmol) was added, and the solution was allowed to stir for 24 h at room temperature. The reaction mixture was filtered and concentrated. The residue was diluted with 10% HC1 solution and stirred overnight at room temp. The organic layer was extracted with hexane and dried over anhydrous MgS0₄. The solvent was removed under vacuum. The residue was diluted with 2-propanol and filtered. The filtrate was collected, concentrated and purified by recrystallisation in hexane twice to yield the product (14.13g, 60% yield) as a yellow solid. Nuclear Magnetic Resonance (' I I NMR, 400MHz) spectroscopy confirmed the successful synthesis of DMP.

Example 2 In this example, the preparation of poly-(N-isopropylacrylamide) (PNIPAM) is illustrated.

Step 2.1 : Polymerization of PNIPAM

The RAFT agent DMP as synthesised in Example 1 was employed to control the polymerisation of PNIPAM at 70 °C, using azobisisobutyonitrile (AIBN) as a thermal initiator. The RAFT agent/initiator (AIBN) ratio was kept constant at 5:1 to ensure only a negligible amount of chains is initiator-based. The RAFT polymerisation of PNIPAM is shown in the Scheme 2 below:

Scheme 2: RAFT polymerisation of PNIPAM

25 g of N-isopropylacrylamide (221 mmol), 0.041 g of AIBN (0.250 mmol), 0.824 g of DMP 1 (2.26 mmol) and 60 mP of dioxane were added to a 100 mP pear-shaped flask. The reaction mixture was degassed with argon for 90 minutes and placed in an oil bath at 70 °C for 20 hours. The solution was precipitated from hexane twice, and the polymer was dried under reduced pressure overnight at room temperature to yield a polymer product (21,3 g, 85% yield, Mn ~ 22 000 g/mol, D ~ 1.13). The molecular weight Mn was measured by gel permeation chromatograhy (GPC) in DMF/0.1M FiCl using PMMA as standard.

Step 2.2: Aminolysis of PNIPAM

The removal of the thiocarbonyl thio end-group on the PNIPAM was carried out by a reduction in the presence of nucleophiles, attaining thiol chain-end functionality. The PNIPAM polymer was treated with ten times excess hexylamine, and the reaction was performed in THF at room temperature. The aminolysis of PNIPAM is shown in the Scheme 3 below:

Scheme 3: Aminolysis of PNIPAM 21.3 g of PNIPAM (2.26 mmol RAFT ends) was dissolved in 60 mL THF in a 250 mL round bottom flask. 2.29 g of Hexylamine (22.6 mmol) and 0.92 g of tributylphosphine (4,52 mmol) in a molar ratio of 1:10:2 was placed in a dropping funnel with 5 mL THF and added drop- wise to the PNIPAM solution, at room temperature, over a period of 5 minutes. The solution was stirred overnight at room temperature for complete aminolysis of the RAFT end groups. The solution was precipitated in hexane twice. A white polymer was dried in vacuum at 40 °C for 24 hours (20.2 g, 95% yield).

Gel permeation chromatograhy (GPC) in DMF/0.1M LiCl using PMMA as standard was used to determine the cleavage of the RAFT agent to yield the thiol chain-end functionalised PNIPAM. The Figure 1 shows refractive index (RI) values and ultra violet (UV) values of Gel Permeation Chromatography (GPC) before aminolysis, shown as solid lines, and after reduction via aminolysis, shown as dotted lines. The UV signal at 320 nm, attributed to the presence of the thiocarbonyl thio moiety in PNIPAM showed strong absorbance after the polymerisation, which is an indication that the polymer chains did contain the RAFT-moiety as an end group. After aminolysis, the UV signal at 320 nm disappeared, which is indicative of a successful cleavage of the thiocarbonyl thio moiety and the formation of PNIPAM-SH polymer. The RI signal indicated a doubling in the molar mass distribution (MMD) after aminolysis. This is considered indicative for disulfide formation, by linking two chains together due to a disulfide bridge, resulting in a mixture of PNIPAM-S-S- PNIPAM and PNIPAM-SH after aminolysis. This effect is undesired since the thiol functionality is needed to ensure successful coupling to the rubber.

Step 2.3: Reduction of PNIPAM disulphides to thiol PNIPAM

To convert the disulphides to thiols an additional reduction step was introduced to ensure that all chains contain the right functionality to ensure successful grafting to the rubber. The the yielded mixture of PNIPAM-S-S-PNIPAM and PNIPAM-SH of step 2.2 was treated with tris(2- carboxyethyl)phosphine (TCEP) to ensure that all the disulphides were reduced back to thiol terminal polymer PNIPAM-SH. The reaction is illustrated in scheme 4:

Scheme 4: Reduction of disulphide to thiol terminal PNIPAM

20.2 g of the aminolysed mixture of PNIPAM-S-S-PNIPAM and PNIPAM-SH (2.00 mmol) of step

2.2 was dissolved in 30 mL DMF in a 250 mL round bottom flask. 5.7 g of Tris(2- carboxyethyl)phosphine (TCEP) (20.0 mmol) was placed in a dropping funnel with 5 mL DMF and added drop-wise to the PNIPAM mixture solution, in a molar ratio to the polymer of 10:1. The solution was stirred for 24h at room temperature and precipitated in diethyl ether to yield PNIPAM- SH (18.18 g, 90% yield).

Example 3 In this example, the functionalisation of a styrene -butadiene rubber with PNIPAM is illustrated. The PNIPAM-SH polymers are covalently bound to SBR via the vinyl groups as illustrated in the following Scheme 5 :

Scheme 5: Covalent linkage of PNIPAM-SH to SBR to obtain functionalized SBR-PNIPAM

20 g of SBR (Sprintan SLR 4601) was dissolved in 150 mL THF for 24h in a 500mL round bottom flask to ensure complete solvation of polymer chains. 7 g of PNIPAM-SH (0.70 mmol) of Example 2, step 2.3 and 0.3g of AIBN (1.83 mmol) was added to the solution and left to stir for 30 minutes. The reaction mixture was degassed with argon for 3 hours and placed in an oil bath at 60 °C for 6 hours. The solution was precipitated from methanol, and the rubber was dried under reduced pressure overnight at room temperature to yield the polymer product SBR-PNIPAM- 12wt% (23 g, 85% yield).

The synthesis of the SBR functionalized with PNIPAM is achieved by varying amount of the functionalization along the SBR backbone. The procedure was repeated using 3g of PNIPAM-SH to yield grafted SBR-PNIPAM containing 5 wt% of PNIPAM. I INMR was used to determine the successful grafting of the rubber with PNIPAM and the PNIPAM content on the SBR.

The rubber SBR, and the grafted rubbers comprising 5 wt% or 12 wt% PNIPAM, respectively, SBR-PNIPAM-5wt%, and SBR-PNIPAM-12wt%, was spin coated on a glass substrate and analysed with atomic force microscopy (AFM). The Figure 2 shows the atomic force microscopy micrographs of SBR and SBR functionalised with 5 wt% and 12 wt% PNIPAM as obtained of Example 3. The phase images of different functionalized SBR-PNIPAM samples show white domains, which are indicative of the rigid PNIPAM segments distributed in the soft SBR matrix, resulting in a nano-phase separation of the PNIPAM in SBR. As can be taken from the AFM images, the PNIPAM (white domains) are the same size and evenly distributed over the rubber. This indicates that the coupling step was regular along the SBR backbone. This is considered to be attributed to an effective and homogenous coupling of PNIPAM to SBR as a consequence of the reduction step 2.3 ensuring that all PNIPAM chain ends have the thiol functionality to effectively couple to the vinyl groups.

The LCST properties of the grafted rubber were confirmed by AFM with water at at 25 °C and 45 °C. In the presence of water a softening and hardening of PNIPAM segments in the rubber matrix was observed. This indicates that the thermoresponsive element was present in the grafted rubber.

The wettability of the functionalized SBR was determined by measuring the contact angle of water droplets on a glass plate coated with the functionalized rubbers comprising 5 wt% or 12 wt% PNIPAM, respectively. For measuring the contact angle, the functionalized SBR rubbers were dissolved in THF and spin-coated on a glass slide. After drying in vacuum the slides were placed under a needle and a water droplet was purged onto the rubber coated glass. The contact angle was determined by measurement of the inner angle between the droplet and the glass surface at 22 °C and 45 °C, as these temperatures are below and above the 32 °C LCST for PNIPAM. The Figure 3 shows the contact angle as a function of PNIPAM content of the PNIPAM functionalised SBR. The observed contact angle showed a stable contact angle at 45 °C with the increase in PNIPAM wt%. At 25 °C a decrease in contact angle was observed with increase in PNIPAM wt %. This shows the hydrophilic nature of the PNIPAM within the SBR matrix, as the PNIPAM domains increase the hydrophilic nature of the rubber with increasing amount of PNIPAM. This illustrates the LCST effect. Again, the enhanced rheological properties in the SBR-PNIPAM copolymer are assumed to be attributed to a more effective and homogenous grafting with reduced PNIPAM chain ends having improved thiol functionality.

Thermal gravimetrical analysis (TGA) was conducted to assess the thermal stability in view of a Wt% loss at temperature range of 0°C to 600 °C. The Figure 4 shows the thermal gravimetric analysis (TGA) of SBR and SBR functionalised with 12 wt% PNIPAM. The TGA curves show that the SBR-PNIPAM copolymer comprising 12 wt% PNIPAM is at high temperatures thermally as stable as the SBR.

Therefore, the reduction step after aminolysis provides for an effective and homogenous coupling of PNIPAM to SBR as a consequence of the reduction step ensuring the PNIPAM chain ends having the thiol functionality to effectively couple to the vinyl groups. The resulting SBR-PNIPAM material showed thermoresponsive features within the rubber matrix. In addition, the grafted SBR- PNIPAM rubber was thermally stable. This thermoresponsive rubber offers vast potential for an improvement in wet grip and rolling resistance performance due to the alteration of the structure at different temperatures.

Claims

Patent claims

1. Method for producing a grafted rubber, the method comprising the steps of:

B) providing a polymer being capable of exhibiting lower critical solution temperature (LCST) properties and comprising a terminal functional thiol group capable of reacting with the olefmic carbon-carbon double bond of the rubber, wherein the polymer is obtained by the steps of:

Bl) reversible addition-fragmentation chain transfer (RAFT) polymerization of a monomer comprising at least one olefmic carbon-carbon-double bond in the presence of a thiocarbonylthio RAFT agent thereby forming a polymer comprising a terminal thiocarbonylthio group, and

C) reacting the polymer with the rubber component by a radical thiol-ene reaction thereby forming a grafted rubber, the grafted rubber comprising a backbone derived from the rubber and sidechains derived from the polymer, characterised in that the polymer in a further step B3) is treated with a reducing agent before being used in the grafting reaction of step C).

2. The method according to claim 1, wherein the reducing agent is selected from the group of sodium borohydride (NaBFR), tributyl phosphine (PBm), tris(2-carboxyethyl)phosphine (TCEP), dithiothreitol, b-mercaptoethanol or a mixture thereof.

3. The method according to claim 1 or 2, wherein the reducing agent is used in a molar ratio to the polymer in a range of > 5:1 to < 10:1.

4. The method according to one of claims 1 to 3, wherein the polymer is selected from the group of poly(N-isopropylacrylamide), poly(N-cyclopropylacrylamide), poly(N,N-diethylacrylamide) and mixtures of these polymers.

5. The method according to one of claims 1 to 4, wherein the RAFT agent is selected from the group of dithioesters, trithiocarbonates, dithiocarbamates, and xanthates.

6. The method according to one of claims 1 to 5, wherein the RAFT agent is selected from the group of S-l-dodecyl-S’-(R,R’-dimethyl-R”-acetic acid)trithiocarbonate (DMP) and 4-cyano-4- dodecylsulfanylthiocarbonylsulfanyl-4-methyl butyric acid.

7. The method according to one of claims 1 to 6, wherein the RAFT agent is used in a molar ratio to a free -radical initiator in a range of > 3 : 1 to < 5 : 1.

8. The method according to claim 6, wherein S-l-dodecyl-S’-(R,R’-dimethyl-R” -acetic acid)trithiocarbonate (DMP) is synthesized by reacting 2-bromo-2-methylpropionic acid with dodecyl trithiocarbonate through a nucleophilic substitution.

9. A grafted rubber obtained by the method according to one of claims 1 to 8, wherein the content of polymer is in a range of > 10 wt-% to < 15 wt-%, based on the total weight of the grafted rubber.

10. A tire comprising a tire tread, characterised in that the tire tread comprises a grafted rubber according to claim 9.