Styrene–Butadiene

Jan 23, 2017 - The low surface energy of silicone rubber (SiR) makes it difficult to blend SiR with conventional rubbers such as solution styrene−bu...
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Structure and Properties of Silicone Rubber/Styrene−Butadiene Rubber Blends with in Situ Interface Coupling by Thiol-ene Click Reaction Zheng Sun,† Qiang Huang,§ Youzhi Wang,§ Liqun Zhang,†,‡ and Youping Wu*,†,‡ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Beijing Engineering Research Center of Advanced Elastomers, Beijing University of Chemical Technology, Beijing 100029, China § Chengdu Guibao Science and Technology Co., Ltd., Chengdu 610041, China ‡

ABSTRACT: The low surface energy of silicone rubber (SiR) makes it difficult to blend SiR with conventional rubbers such as solution styrene−butadiene rubber (SSBR) and natural rubber. In this study, to enhance the interface interaction between SiR and SSBR, trimethylolpropane tris(3-mercaptopropionate) (TMPMP) was carefully chosen to couple SiR and SSBR by in situ thiol-ene click reactions between mercapto groups of TMPMP and vinyl groups of SiR and SSBR. The reaction of TMPMP with SiR and SSBR was characterized via torque−time curves, element analysis, and the dissolving−swelling test. Accordingly, a two-step strategy was proposed to prepare SiR/SSBR composites. Transmission electron microscopy indicated that the two-step strategy reduced the phase domain size of the blend. The composite exhibited higher mechanical properties and lower hysteresis. The results would extend SiR applications.

1. INTRODUCTION Silicone rubber (SiR) is not dependent on the ever-reducing petroleum sources. Due to the high flexibility and high bond energy of its Si−O−Si backbone, SiR has many valuable properties such as extremely low glass translation temperature (Tg), great resilience, and outstanding heat resistance.1 However, the low cohesion energy density makes the mechanical strength of SiR weak. Besides, the unique structure of SiR makes it incompatible with other conventional rubbers such as styrene−butadiene rubber and natural rubber.2 These two drawbacks greatly limit SiR applications. Solution styrene−butadiene rubber (SSBR) is synthesized by anionic polymerization to meet the requirement of tread compounds. It has been widely used in passenger tires to achieve high wet-skid resistance and low rolling resistance.3 At the same time, cis-butadiene rubber (cis-BR), with high flexibility and low glass transition temperature (Tg), is applied to blend with SSBR to reduce the rolling resistance and abrasion.4,5 As mentioned above, SiR is a kind of nonpetrochemical rubber and has better flexibility than cis-BR. Therefore, the aim of our study is to apply SiR in tread compounds to reduce the rolling resistance and abrasion. Due to the large difference in surface energy of SiR and SSBR, the key to prepare SiR/SSBR blends is how to improve their interfacial interactions. In the literature, the simplest method of improving blend compatibility is to add a compatibilizer; for example, fluorosilicone rubber as an interfacial compatibilizer was introduced into the SiR/poly(vinylidene fluoride) blend.6 In addition, the thiol-ene click © 2017 American Chemical Society

reactions possess high efficiency, selectivity, and insensitivity to water and oxygen.7,8 Until now, the thiol-ene click reactions were widely used to improve blend compatibility. For acrylonitrile−butadiene rubber (NBR) and fluorosilicon rubber (FSR), the mercapto telechelic polymethyltrifluoropropylsilicone was first specially synthesized and then reacted with NBR by thiol-ene click reaction.9 Soares et al. reported a series of works about using mercapto-modified polymers as reactive compatibilizers.10−18 However, the above-mentioned ways are involved in preparation of mercapto-modified polymers, and the process is relatively complicated and costly for manufacture. In this study, the trifunctional thiol trimethylolpropane tris(3-mercaptopropionate) (TMPMP) was chosen to couple SiR and SSBR via in situ interface coupling technique during the mixing process. This solvent-free strategy can effectively circumvent the complexity of directly introducing mercapto groups in polymer molecules. Considering the different reactivity of TMPMP with the SiR and SSBR, a two-step strategy was carried out: the TMPMP reacted with SiR in first step, and then the compound reacted with SSBR. Blends prepared by one-step strategy and without TMPMP were also prepared for comparison. The results indicate that SiR should be applied in tread compounds. Received: Revised: Accepted: Published: 1471

October 26, 2016 January 19, 2017 January 23, 2017 January 23, 2017 DOI: 10.1021/acs.iecr.6b04146 Ind. Eng. Chem. Res. 2017, 56, 1471−1477

Article

Industrial & Engineering Chemistry Research

Figure 1. Molecular structures of (a) SiR, (b) SSBR, and (c) TMPMP. swollen sample was dried in vacuum oven at 50 °C for 24 h. The swelling ratio (SR) was calculated by m SR = 2 × 100% m3 (1)

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial SiR 110-2 (vinyl content of 0.15 mol %) was purchased from Chengrand Research Institute of Chemical Industry Co., Ltd., China. Commercial SSBR 2466 (styrene content of 23.5 wt % and vinyl content of 41.3 wt %) was purchased from Taiwan Synthetic Rubber Co., Ltd. TMPMP was the product of Sigma-Aldrich Co., LLC. Fumed silica AEROSIL 200 and precipitated silica ULTRASIL VN3 were purchased from Evonik Industries AG. All of the other ingredients, such as 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (DBPMH), bis[3-(triethoxysilyl)propyl]-tetrasulfide (TESPT), and stearic acid (SA), were all commercially available industrial products. The molecular structures of SiR, SSBR, and TMPMP are shown in Figure 1. 2.2. Characterization of the Reactivity of TMPMP with SiR and SSBR. To investigate the reactivity of TMPMP with SiR and SSBR, six compounds were prepared according to formulas in Table 1

and the percentage of cross-linked rubber (PCR) was calculated by m PCR = 3 × 100% m1 (2) where m1 and m2 are the weight of the sample before and after extraction, respectively, and m3 is the dry weight of the swollen sample. 2.4. Preparation of Silica/SiR Masterbatches. Due to the large differences in viscosity of SiR and SSBR, silica/SiR masterbatches were prepared for matching the viscosity of SSBR. Formulas of SiR masterbatches are shown in Table 2. The masterbatches were prepared

Table 2. Formulas of silica/SiR Masterbatches

Table 1. Formulas of Compounds for the Characterization of the Reactivity of TMPMP with SiR and SSBR

contents of masterbatches (phr)

contents of mixtures (phra)

ingredients SiR SSBR precipitated silica TMPMP DBPMH a

SSBR T=0 D = 0.1 100 0 30 0 0.1

SSBR T=1 D=0 100 0 30 1 0

SSBR T=1 D = 0.1 100 0 30 1 0.1

SiR T=0 D = 0.1 0 100 30 0 0.1

SiR T=1 D=0 0 100 30 1 0

SiR T=1 D = 0.1 0 100 30

ingredients

blank

one-step

two-step

SiR fumed silica TMPMP DBPMH

20 6 0 0

20 6 0.8 0

20 6 0.8 0.06

on two-roll mill at room temperature. For the masterbatch of the twostep process, after the ingredients were well-dispersed, the compound was heated in 170 °C for 15 min (heat treatment) to ensure the reaction between TMPMP and SiR, and the silica/SiR masterbatch was obtained. For comparison, the masterbatches of the blank and the one-step process without heat treatment were also prepared. 2.5. Preparation of SiR/SSBR Composites. Formulas of SiR/ SSBR composites were shown in Table 3. The compounds were prepared as follows. First, the above prepared silica/SiR masterbatch, silica, TESPT, SA, and hydroxyl silicone oil were dispersed in the SSBR on a two-roll mill at room temperature. Second, the compounds were sheared for 8 min on another two-roll mill at 150 °C to ensure the reactions between TMPMP and SSBR and the reactions of TESPT

1 0.1

phr: parts per hundred parts of rubbers.

on a two-roll mill at room temperature. Torque−time curves of the compounds were measured via MR-C3 rotorless rheometer (Beijing Ruidayuchen Instrument Co., Ltd., China) at 170 °C. 2.3. Characterization of Interface Coupling Reaction of the TMPMP Modified SiR (T-SiR)/SSBR Mixture. SiR, TMPMP, and DBPMH were first mixed at the weight ratio of 100/4/0.3 on two-roll mill at room temperature; then, the mixture was heated to 170 °C for 15 min to ensure the reactions between the mercapto groups of TMPMP and vinyl groups of SiR, and the TMPMP modified SiR (TSiR) was obtained. T-SiR was also slightly cross-linked by the side reactions. After that, the T-SiR was extracted by tetrahydrofuran for 24 h to remove the unreacted TMPMP. Then, the T-SiR gel was dried in vacuum oven at 50 °C for 24 h. At last, the dried T-SiR was mixed with SSBR at the mass ratio of 2/8 on two-roll mill at room temperature, and the T-SiR/SSBR blend was obtained. The sulfur element concentration of the dried T-SiR sample was measured by vario EL cube elemental analysis instrument (Elementar Analysensysteme GmbH) to characterize the reaction degree. Torque−time curves of the T-SiR/SSBR blend and pure SSBR were measured by MR-C3 rotorless rheometer (Beijing Ruidayuchen instrument Co., Ltd., China) at 170 °C to investigate the interface coupling reaction. The sample of the T-SiR/SSBR blend after torque−time curve measurement was further put through a dissolving−swelling test to determine the reaction degree. First, the sample was placed in toluene at 30 °C for 96 h to extract the un-cross-linked rubber, and then the

Table 3. Formulas of SiR/SSBR Composites contents of compounds (phr) ingredients

blank

one-step

two-step

masterbatchesa SSBR fumed silica precipitated silica TESPT hydroxyl silicone oil SA 4010NAb RDc DBPMH

26 80 24 30 4.8 2.5 1 1.5 1.5 1.3

26.8 80 24 30 4.8 2.5 1 1.5 1.5 1.3

26.86 80 24 30 4.8 2.5 1 1.5 1.5 1.3

a

Prepared as the formulas were in Table 2. bN-Isopropyl-N′-phenyl-pphenylenediamine. cPoly(1,2-dihydro-2,2,4-trimethylquinoline). 1472

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Figure 2. Torque vs time curves of (a) SSBR and (b) SiR with TMPMP and DBPMH independent or synergistic.

Figure 3. Reaction formulas of (a) SSBR cross-linked by deficient TMPMP and (b) SiR modified by excessive TMPMP. modified silica. Last, the compounds were cooled to room temperature, and the DBPMH, 4010NA, and RD were well-dispersed in the compounds. The compounds were vulcanized at 170 °C for 16 min to prepare SiR/SSBR composites. 2.6. Characterization and Measurement of SiR/SSBR Composites. The vulcanization characteristics of the compounds were measured by an MR-C3 rotorless rheometer (Beijing Ruidayuchen Instrument Co., Ltd., China) at 170 °C. The microstructure morphology of the composites was observed via a Tecnai G220 transmission electron microscope (TEM) (FEI Co., United States) with an accelerating voltage of 200 kV. The samples for TEM observations were cut by an ultramicrotome under liquid nitrogen cooling. According to the large difference in vinyl group concentrations between SiR and SSBR, the OsO4 stained samples were used to study the rubber phase separation. The unstained samples were used to observe the dispersion of silica. The tensile properties were measured using an electronic tensile machine (Shenzhen SANS Test Machine Co., Ltd., China) at a crosshead speed of 500 mm/min according to standards ISO 37:2011. The shore A hardness was measured according to ISO 868:2003. The dynamic mechanical properties were measured using a VA 3000 dynamic mechanical analyzer (DMA) (Metravib Co., Ltd., France). The specimens were tested under the tensile mode, strain amplitude of 0.1%, frequency of 10 Hz, and temperature range from −40 to 80 °C at a heating rate of 3 °C/min.

3. RESULTS AND DISCUSSION 3.1. Reactions of TMPMP with SSBR and SiR. The thiolene click reactions can be triggered by heat or UV light.6 DBPMH was used as the thermal initiator to trigger the

Figure 4. Torque vs time curves of the T-SiR/SSBR mixture and the pure SSBR. 1473

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Figure 5. Schematic diagram of the two-step interface coupling reactions.

curves are shown in Figure 2. For SSBR in Figure 2a, 0.1 phr of DBPMH cross-linked SSBR evidently, and 1 phr TMPMP could also slightly cross-link SSBR without initiator. This was due to the high reactivity of the trifunctional thiol in TMPMP and the high vinyl content of SSBR (41.3 wt %). The ratio of mercapto groups of TMPMP and vinyl groups of SSBR is low at 1:101. When TMPMP and DBPMH were used together in SSBR, a synergistic effect was observed. For SiR in Figure 2b, 0.1 phr DBPMH could cross-link SiR, whereas 1 phr TMPMP could not cross-link the SiR. This was due to the low vinyl concentration of SiR (0.15 mol %). The mole ratio of TMPMP molecule to vinyl groups is high at 1.24:1, and the amount of mercapto groups in TMPMP was largely excessive: one TMPMP molecule only reacted with one vinyl group of SiR theoretically, with the redundant mercapto groups as side groups of the SiR molecules. Moreover, when 1 phr TMPMP and 0.1 phr DBPMH were used together in SiR, the torque showed a trend similar to that of the SiR with 1 phr TMPMP, indicating that TMPMP also existed mostly as side groups of SiR, although the slight cross-linking was not evident in torque rising in Figure 2b. In Figure 3 a, SSBR was cross-linked by deficient TMPMP relative to the amount of vinyl groups in SSBR, while the reaction of SiR modified by excessive TMPMP is shown in Figure 3b. To confirm that the mercapto groups of TMPMP were successfully reacted with the vinyl groups of SiR, the slightly cross-linked T-SiR with the weight ratio of SiR/TMPMP/ DBPMH 100/4/0.3 was extracted by tetrahydrofuran for 24 h to remove the unreacted TMPMP. The mole ratio of TMPMP molecules to vinyl groups was 4.96:1 to ensure excessive mercapto groups. The Fourier transform infrared (FTIR) and Raman spectra of the extracted T-SiR were measured, but no significant signals of S−H or S−C bonds were found, owing to the very low concentration. Further, elemental analysis was conducted, and the results showed that the sulfur concentration of extracted T-SiR was 0.20 wt %; the corresponding TMPMP amount was 2.08 × 10−3 mol/100 g, and the vinyl group amount in the SiR was 2.03 × 10−3 mol/100 g. The mole ratio between TMPMP and vinyl groups was 1.02:1, indicating that the vinyl groups were fully reacted and that T-SiR was successfully formed with dangled mercapto groups. To further confirm that the dangled mercapto groups of TSiR can react with the vinyl groups of SSBR, the above extracted T-SiR was mixed with SSBR in a weight ratio of 20/ 80. The torque vs time curves of the mixture and pure SSBR are shown in Figure 4. In contrast to the constant torque of pure

Figure 6. Torque vs time curves of SiR/SSBR compounds.

Table 4. Static Mechanical Properties of the Three Composites samples

blank

one-step

two-step

tensile strength (MPa) elongation at break (%) modulus at 100% (MPa) modulus at 200% (MPa) shore A hardness

14.7 368 1.9 5.5 63

14.1 264 2.6 8.4 63

15.6 290 2.5 7.8 64

Figure 7. Loss factor tan δ vs temperature curves of the composites.

reaction of TMPMP with the vinyl groups of SSBR or SiR. Meanwhile, DBPMH can vulcanize the rubber independently. Considering the large difference in vinyl concentration of SSBR and SiR, we measured the torque vs time of SSBR or SiR with TMPMP and DBPMH independently or synergistically. The 1474

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Figure 8. Stained microstructure morphology images of SiR/SSBR blends: (a and b) blank and (c and d) two-step interface coupling strategy.

Figure 9. Unstained microstructure morphology images of SiR/SSBR blends: (a) blank and (b) two-step interface coupling strategy.

rate was 712%. This indicated that dangled mercapto groups of T-SiR reacted with vinyl groups of SSBR. On the basis of the above results, we proposed the two-step interface coupling strategy to improve the compatibility of SiR and SSBR, and the schematic diagram is shown in Figure 5. The first step was that TMPMP reacted with SiR (4/100 in weight ratio) under 170 °C to obtain T-SiR; the second step was that the dangled mercapto groups of T-SiR in situ reacted with SSBR under shear and heat, forming the chemical bonds between SiR and SSBR. 3.2. Vulcanization Characteristics of Compounds. The curing curves of the SiR/SSBR compounds prepared by different compounding strategies are shown in Figure 6. For the compound prepared by the one-step strategy, the torque was the highest, which was attributed to the fact that TMPMP reacted with SSBR preferably during dynamic heat treatment because of the higher vinyl content of SSBR. For the compound prepared by the two-step strategy, the torque was lower than that of one-step compound. The reason was that TMPMP first reacted with SiR and then reacted with SSBR, leading to the reduced cross-linking efficiency of TMPMP. The torque of the

Figure 10. Backside illuminated images of SiR/SSBR blends: (a) blank and (b) two-step interface coupling strategy.

SSBR, the torque of the T-SiR/SSBR mixture increased, indicating that the interface coupling reaction between T-SiR and SSBR occurred. Furthermore, after torque−time curve measurement, the dissolving−swelling test was conducted on the compound, and the results demonstrated that the percentage of cross-linked rubber was 94.8% and the swelling 1475

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Industrial & Engineering Chemistry Research

4. CONCLUSIONS We used the trifunctional thiol TMPMP to couple the molecules of SSBR and SiR at the interface through thiol-ene click reactions during the high-temperature mixing. Because of the rare concentration of vinyl groups in SiR, we proposed a two-step interface coupling strategy which made the TMPMP react with SiR first. By addition of excessive TMPMP into SiR, the mercapto modified SiR was successfully synthesized in the first step. Then, the dangled mercapto groups reacted with SSBR in the second step. The phase separation morphology confirmed that the in situ coupling could reduce the domain size of SiR. The hysteresis tan δ of the composites was also reduced due to the lower interface friction. This in situ interface coupling technique would make the SiR widely used, partly replacing the petrochemical rubber in the tire industry and other fields.

blank compound without TMPMP was the lowest, as expected. In addition, the similar torque rising trend of the three compounds suggested that the TMPMP was fully reacted during heat treatment. 3.3. Static Mechanical Properties of the Composites. The static mechanical properties of the three composites are shown in Table 4. Compared with the blank composite, both the one-step and two-step composites showed an increased modulus, and the one-step composite showed a higher rise in modulus. This was consistent with the torque results of the compounds. Whereas the tensile strength and elongation at break of the one-step composite decreased, indicating the increasing of stress concentration, which was introduced by the tendency of TMPMP to cross-link the SSBR rather than couple the SiR with the SSBR. However, the two-step composite showed an increased tensile strength and tensile modulus, indicating that the interface interaction between SiR and SSBR were successfully enhanced by the coupling reaction. 3.4. Dynamic Mechanical Properties of the Composites. The viscoelastic properties of the composites were measured to investigate the interface interaction of the blends. The temperature sweep was carried out, and the curves are shown in Figure 7. The glass transition peak of SSBR showed no significant differences among the composites, suggesting that the thermodynamic immiscibility of SiR and SSBR could hardly be changed by the interface coupling. However, the onestep and two-step composites had tan δ at high temperature lower than that of the blank composite. This phenomenon was ascribed to the stronger interface interaction, which decreased the interface friction. The dynamic mechanical properties could also reflect the potential of this interface coupling technique in the tire industry. The two-step composite showed lower tan δ at 60 °C and higher tan δ at 0 °C compared to those of the blank, which correlated to lower rolling resistance and higher wet-skid resistance when the compound was used as treads. 3.5. Microstructure Morphology of the Composites. The phase separation morphology is an important factor in determining the properties of the polymer blends. TEM was used to evaluate the phase separation morphology of the blends. The SSBR in the blends was stained by OsO4 to make a contrast with the unstained SiR. Figures 8a and b showed the morphology of the blend without interface coupling. The phase separation was clear, and the domain scale was over 100 nm, suggesting that the low surface energy of SiR led to the aggregation of SiR. Figures 8c and d showed the morphology of the two-step composite. The SiR domain was smaller and dispersed better in SSBR matrix. The misty interface suggested that the SiR molecules were coupled with the SSBR molecules during the in situ interface coupling process. The in situ formed copolymers of SSBR and SiR also played a role in reducing the surface tension and then the domain size. Meanwhile, the unstained TEM images in Figure 9 showed the dispersion of silica in the two-step composite (b) improved over that of the blank (a). The silica always tended to concentrate in one rubber phase due to the stronger filler−rubber interaction. When the dispersion of rubber was improved, the dispersion of silica was improved as well. From a macroscopic view, Figure 10 showed the backside illuminated image under the same lighting and exposure condition. Consistent with Figures 8 and 9, the two-step composite slice was more transparent than the blank, suggesting better dispersion of SiR and silica.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +86-10-64442621; Fax: +86-10- 64456158. ORCID

Youping Wu: 0000-0001-6723-7043 Notes

The authors declare no competing financial interest.

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REFERENCES

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