Issue 38/2014 - Research Progress on Synthetic Technology of Styrene-Butadiene Rubber

Soluble polystyrene-butadiene rubber (SSBR) is a random copolymer obtained by polymerization reaction of negative ion solution in aliphatic hydrocarbon organic solvents
with butadiene and styrene as polymeric monomers, organolithium compounds as initiators, Lewis base compounds such as etheramines or two polar compounds as regulators of polymerization chain microstructure at the same time 。 It has the advantages of wear resistance, cold resistance, low heat generation, low shrinkage, good color, less ash, high purity, fast vulcanization speed and low rolling resistance, excellent wet slip resistance and wear resistance, and has a wide range of applications
in the tire industry, especially in high-performance tires such as green tires, anti-slip tires and ultra-lightweight tires.
At present, the progress of SSBR synthesis technology in China is mainly reflected in initiators, regulators and modifications
.

1.
Initiator system

The commonly used initiator of SSBR is butyllithium (BuLi), and the new initiators currently researched and developed mainly include ether organolithium, amine organolithium, tin lithium, functionalized polylithium and complex organolithium
.
Among them, polylithium initiators and functionalized organolithium are the research directions
.
For example, the reaction of N,N-dimethylethylenediamine and 4-N,N-diethylaminobenzaldehyde to generate cycloammonia acetal is generated, and then treated with sec-butyllithium (s-BuLi) to obtain tertiary aminofunctionalized phenyl lithium
.
For example, with the reaction products of polycyclic aromatic hydrocarbons such as naphthalene, anthracene and biphenyl and lithium as initiators, conjugated diolefins or vinyl monomers are first polymerized in hydrocarbon solvents to form oligomers with active centers at both ends of the molecular chain, and then compounds that can react with active centers are added, and tin-containing polylithium initiators
are obtained after the reaction.

Kong Xinxin of Beijing University of Chemical Technology et al.
reacted trimethylchlorosilane with a double-ended active lithium initiator to prepare a silicon-containing organolithium initiator
.
The initiator is used to initiate the polymerization of butadiene and styrene, coupled with tin tetrachloride, to obtain a star styrene-butadiene rubber
with a silicone-containing group at its end.
This technology introduces silicon-containing special structural groups into the polymer chain ends, and further couples to form star polymers to reduce free ends and reduce rolling resistance, thereby improving the performance of
SSBR products.

Lv Wanshu et al.
of China Petroleum and Chemical Corporation used the initiation system of organolithium, alkyl sulfonate compounds and tetrahydrofurfuryl alcohol ethers to trigger the random copolymerization of conjugated olefins and monovinyl aromatic hydrocarbons, and polyfunctional coupling agents could be added for coupling reactions
in the later stage of polymerization.
The results show that the effective control
of SSBR styrene blocks can be realized in the range of vinyl structure with mass fraction of 15%~40% using this kind of initiation system.

Chen Bo et al.
of Beijing University of Chemical Technology used cyclohexane as solvent, and 1,1-diphenylethylene (DPE) reacted with n-butyllithium (n-BuLi) to obtain 1,1-diphenylhexyllithium (DPHL); Then, DPHL was used as initiator and tetrahydrofuran as structural regulator, and linear SSBR was synthesized by negative ion polymerization.
Finally, star SSBR
was prepared by coupling with SnCl4.
Sun Liang of Dalian University of Technology et al.
used cyclohexane as solvent, isoprene as solubilization, tetrahydrofuran as reaction rate regulator, and a certain proportion of divinylbenzene (DVB) and hexamethyleneiminolithium (LHMI) reacted under certain temperature conditions to prepare functionalized polylithium initiator
.
The polymers prepared with this new functionalized polylithium initiator have the characteristics of both star polymers and functionalized polymers, so they have excellent hysteresis loss and processability
.

2.
Regulator system

At present, alkyl lithium is widely used as an initiator in the world to prepare SSBR by adding polar structural regulators, and the key technology is the selection
of polar structural modifiers.
Choosing the right regulator can not only control the microstructure of the polymer, but also increase the polymerization reaction rate, thereby affecting the performance of
styrene-butadiene rubber.
This method is reliable, simple and easy to implement, and it is convenient to organize the production process
by the continuous method.
N,N,N',N,-TETRAMETHYLVINYLENEDIAMINE (TMEDA), ETHYLENE GLYCOL DIMETHYL ETHER (1G), DIETHYLENE GLYCOL DIMETHYL ETHER (2G) and tetrahydrofuran (THF) are commonly used regulators in the synthesis of SSBR
.
However, these traditional structural adjustment and adjustment efficiency is low, the dosage is large, it is not easy to recover, and it is easy to lead to the inactivation of the active chain and affect the quality of
rubber.
Therefore, the use of new structural conditioners is a hot spot
in research and development in recent years.

Gao Bibo of Beijing University of Chemical Technology et al.
used n-butyllithium as initiator, cyclohexane as solvent, sodium dodecylbenzenesulfonate (SDBS)/THF or SDBS/pentamethyldiethyltriamine (PMDETA) as binary composite structure regulator, and SiCl4, SnCl4 and DVB as coupling agent reaction system to synthesize coupled SSBR.

It was found that high coupling efficiency
could be obtained with this modulator.
When the vinyl content reaches 60%, the coupling efficiency can reach about
50%.

Shi Gongchang et al.
of Lanzhou Chemical Research Center of PetroChina prepared SSBR
by using n-butyllithium as initiator and cyclohexane as solvent, and SDBS/THF and Sdbs/PMDETA as composite regulators, respectively.
The results show that the composite regulator can increase the polymerization reaction rate, and evenly distribute the vinyl content and styrene content in the polymer chain, and the obtained product is completely irregular SSBR, and the relative molecular mass distribution is greater than 1.
6, which plays an important role
in improving the performance of SSBR.

Liu Lian et al.
of Dalian Maritime University used tetrahydrofurfuryl ethyl ether (ETE) as a structural regulator and n-butyllithium as an initiator to prepare SSBR
with high vinyl content.
The results show that changing the amount of regulator and initiating reaction temperature can control the change of vinyl content in the SSBR molecular chain segment between 10%~60%.
At 50 °C, the coupling efficiency of the active chain of butadiene and tin tetrachloride was always maintained above
60%.
Liao Mingyi et al.
of the school synthesized SSBR
by anionic high-temperature polymerization process using n-butyllithium as initiator, ether compound as regulator, cyclohexane/n-hexane as solvent.

3.
Modification technology

The modification of the end group functional group of the active chain of macromolecules is an effective means to
improve the comprehensive performance of SSBR.
After the introduction of certain polar groups into the SSBR molecular chain, its affinity with the active filler is strengthened, and the Payne effect of the self-accumulation of fillers such as carbon black in the rubber compound is reduced, thereby improving the hysteresis loss of SSBR
.
At present, the modification of SSBR mainly includes end-group coupling modification, end modification of nitrogenous compounds and modification of silicon-containing compounds
.
The aim is to improve product resilience and wet grip, while improving wear resistance and reducing rolling resistance
.
Among them, tin coupling SSBR can effectively reduce the concentration at the end of the long chain, promote the dispersion of carbon black, reduce internal friction and reduce heat generation during the mixing process.
In addition to high storage stability, silane-modified SSBR also has high strength and resilience of vulcanized rubber, which can be used to make white or light-colored passenger tires
.

Tong Yuanyuan of Beijing University of Chemical Technology and others first used lithium naphthalene to initiate the synthesis of bifunctional organolithium from butadiene, and then added quantitative tin tetrachloride to prepare polyfunctional organolithium, which was used as an initiator, cyclohexane as solvent, tetrahydrofuran as structural regulator, anionic polymerization prepared star-type SSBR with multiple active points, and then added lithium isopropanol oxide as a decoupling agent to reduce the viscosity of the system, and then added the end capping agent γ-chloropropyltrimethoxysilane for end-capping reaction to obtain a modified SSBR with trimethoxypropylsilane group
。 The results show that the viscosity of the polymerization system can be significantly reduced, and when the average molecular weight of SSBR is 9.
4×104, the molar ratio of the solver to the number of active centers is 1.
5, the molar ratio of the end capping agent to the number of active centers is 1.
5, and the end capping reaction is carried out for 60 minutes, the end capping rate increases from 10.
7% to 64.
1%
without decoupling direct capping 。 Compared with unterminated and unterminated low end rate SSBR vulcanized rubber, the carbon black-silica filler particles are more uniformly dispersed, and the tensile strength and 300% elongation stress are improved, the permanent deformation is significantly reduced, and it has the characteristics of high loss factor of
0 °C and low loss factor of 60 °C.

Ang Li of Beijing University of Chemical Technology et al.
first used bifunctional organolithium as initiator, cyclohexane as solvent, and tetrahydrofuran as regulator, and prepared SSBR with double-ended activity by negative ion polymerization, and then used tert-butyldiphenylchlorosilane as the end capping agent to modify
SSBR.
The results show that the termination with tert-butyldiphenylchlorosilane significantly improves the tensile strength and tearing elongation of SSBR, and reduces the dynamic compression temperature rise and rolling resistance
of SSBR.

Wang Lei et al.
of Beijing University of Chemical Technology prepared SSBR, large-volume functional group tert-butyldiphenylchlorosilane to prepare end-capped-modified SSBR (T-SSBR) and tin-coupled star SSBR
by anionic activity/controllable polymerization 。 The results show that compared with unmodified SSBR vulcanized rubber, T-SSBR and star SSBR vulcanized rubber have better carbon black dispersion, reach nanoscale dispersion of nanofiller, small hysteresis loss, good dynamic mechanical properties, tensile strength, tensile elongation, tear strength and springback value.
Compared with star SSBR vulcanizate, T-SSBR vulcanized rubber has smaller tensile strength, tensile elongation and tear strength, lower dynamic compression fatigue temperature rise, good wet slip resistance, but slightly lower
dynamic cutting resistance.

4.
Conclusion

In recent years, SSBR synthesis technology has been continuously improved, and a series of new SSBR grades with excellent comprehensive balance properties such as wet slip resistance, rolling resistance and wear resistance have
been developed by using polymer design and active chain end modification technology.
At the same time, with the formal implementation of the EU tire labeling regulations, the requirements for fuel efficiency, rolling noise and wet grip level are more stringent, and the status of "green environmental protection" in the tire industry will be more prominent, and SSBR will gradually become the development focus of
styrene-butadiene rubber 。 In the future, scientific research and development should be further strengthened to improve the ability to regulate vinyl content and styrene content in the process of SSBR polymerization; Develop a new and efficient initiation system with simple synthesis process, easy availability of raw materials, low price and easy adjustment of functionality; Develop a new regulator system with high efficiency, temperature resistance and no subsequent reaction; Improve the affinity of polymers and silica, solve the problem of interchange between products of different manufacturers, and form a core technology with independent intellectual property rights; Through the modification technology, we have developed new products with better performance and a wide range of uses, and improved the overall production level of
SSBR in China.