Enhancing interfacial interaction and mechanical properties of styrene-butadiene rubber composites via silica-supported vulcanization accelerator

Composites: Part A 96 (2017) 129–136

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Enhancing interfacial interaction and mechanical properties of styrene-butadiene rubber composites via silica-supported vulcanization accelerator Bangchao Zhong, Zhixin Jia ⇑, Dechao Hu, Yuanfang Luo, Demin Jia, Fang Liu School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 26 August 2016 Received in revised form 6 February 2017 Accepted 12 February 2017 Available online 16 February 2017 Keywords: A. Polymer-matrix composites A. Particle-reinforcement B. Interface/interphase B. Mechanical properties

a b s t r a c t To modify the filler surface and simultaneously elude issues related to the physical loss by migration of rubber additives, the concept of ‘‘supported rubber additives” was proposed and vulcanization accelerator 2-benzothiazolethiol (M) was chemically grafted onto the surface of silane modified silica (m-silica) to prepare silica-supported vulcanization accelerator (silica-s-M). Silica-s-M could be homogeneously dispersed into styrene-butadiene rubber (SBR). Besides, the interfacial interaction between silica-s-M and SBR was significantly enhanced, which was confirmed by the constrained rubber chains approaching the filler surface. Consequently, silica-s-M effectively reduced the activation energy of vulcanization and SBR/silica-s-M composites showed much better mechanical properties than SBR/m-silica and SBR/silica composites containing equivalent accelerator component. From this work, it is envisioned that this methodology for the surface treatment of silica to prepare supported accelerator may be extended for other nanofillers and functional rubber additives, which may be promising for the preparation of highperformance and functional rubber/nanofiller composites. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction As one of the most important polymer materials, rubber exhibits remarkable properties, especially high elasticity, which enables rubber to become a non-replaceable material and find use in a multitude of applications. However, most of the synthetic rubbers, such as styrene-butadiene rubber (SBR), are mechanical weak and nanofillers are filled to get improved strength and controlled cost of rubber composites [1–5]. Among fillers for rubber, silica is attracting a great deal of attention and gaining increasing importance as non-black reinforcing filler. In particular, SBR/silica composites is one of the most important material systems in green tire, for the use of silica in tire results in lower rolling resistance and consequently fuel savings compared to carbon black [6]. It has been proved that homogenous dispersion and strong interfacial interaction are essential conditions to make the best of the extraordinary properties of nanofillers for reinforcement of rubber [7,8]. Nevertheless, due to a high density of silanol groups on silica surface, there will be a great chance to form hydrogen-bond interactions between silica particles. Therefore,

⇑ Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.compositesa.2017.02.016 1359-835X/Ó 2017 Elsevier Ltd. All rights reserved.

silica particles tend to exist as agglomerates and obtaining homogeneous dispersions in rubber is still a big challenge. To make matter worse, the interfacial interaction between hydrophilic silica and hydrophobic rubber is generally poor, leading to the greatly deteriorated performances of rubber/silica composites. Because of these issues, large experimental efforts are devoted to preparing high performance rubber/silica composites with uniform filler dispersion and strong filler-rubber interfacial interaction. In particular, surface treatment of silica is a promising and effective approach. For example, Mathew et al. [9] found that the surface treatment of silica with plasma could improve the filler dispersion and filler-rubber interaction, leading to high comprehensive properties of SBR composites, while Liu et al. [10] used a novel method to investigate the modification of silica with bis(3-triethoxysilyl propyl)tetrasulfide and the results revealed that the nanoparticle size decreased and the agglomeration trend of silica weakened after modification. So far, silane coupling agents have been the most commonly used surface modifier of silica [11–13]. To the best of the authors’ knowledge, vulcanization accelerators have not been used for silica surface modification except by our group. It is known that traditional rubber additives (i.e. accelerator, antioxidant) are always organic molecules with low molecular weight. In addition to the poor compatibility with rubber, rubber

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additives often migrate and diffuse toward the surface of rubber vulcanizates in an event called ‘‘blooming” [14,15]. ‘‘Blooming” causes defect of microstructure, loss of efficiency of rubber additives and contamination of environment, which highly impedes the applications of rubber composites. In this work, to achieve the homogeneous dispersion of silica, enhance the silica-SBR interfacial interaction and elude issues related to ‘‘blooming” of rubber additives, the concept of ‘‘supported rubber additives” was proposed and conventional vulcanization accelerator 2-benzothiazolethiol (M) was reacted with silanized silica (m-silica) to prepared silica-supported accelerator (silica-s-M). The effects of silica-s-M on the filler dispersion, filler-rubber interfacial interaction, curing kinetics and mechanical properties of SBR composites were fully investigated. 2. Experimental section 2.1. Materials

silica-s-M), which was varied to prepare three kinds of SBR composites (SBR/silica, SBR/m-silica and SBR/silica-s-M) with the increment of filler content from 0 (unfilled SBR) to 50 phr. The other ingredients in the rubber compounds were (in phr): rubber, 100; stearic acid, 2.0; zinc oxide, 5.0; antioxidant, 2.0; sulfur, 1.6. The total content of vulcanization accelerator consisted of CZ and M with the weight ratio of 3/2 was fixed at 2 phr. The quantity of supported M determined by the residue weight after being heated to 700 °C in thermogravimetric analysis (TGA) for silica-sM must be calculated as part of the accelerator content to ensure that all of the compounds contained equivalent accelerator component. To obtain unfilled SBR vulcanizate and SBR composites, the compounds which were mixed for 8 min at room temperature in an open mill in advance were vulcanized in an electrically heated press at 160 °C for the optimum cure time (t90). 2.4. Characterization

Silica was kindly provided by Huiming Chemical Co., Ltd., China. The primary silica particle was of 10–20 nm diameter and 200 m2/ g to 220 m2/g surface area. c-chloropropyltriethoxysilane (CTS) was from Wanda Chemical Co., Ltd., China. SBR (1502) was produced by Sinopec Group, China. Sulfur, stearic acid, zinc oxide, Ncyclohexyl-2-benzothiazole sulfenamide (CZ), accelerator M and N-1, 3-dimethylbutyl-N0 -phenyl-p-phenylenediamine (antioxidant) were industrial grade products. Absolute ethanol and sodium hydroxide (NaOH) were analytical grade. 2.2. Synthesis of silica-s-M The preparation processing of silica-s-M is illustrated in Fig. 1. The detailed procedures were as follows: firstly, 5.0 g of silica was dispersed in 350 mL of absolute ethanol, and then 2.5 g of CTS was dropped into the suspension. The mixture was stirred for 24 h at 50 °C. The product was then washed with absolute ethanol to remove the ungrafted CTS and dried in a vacuum oven to constant weight to prepare silane coupling agent modified silica (msilica). Secondly, 1.6 g of M and 0.4 g of NaOH were dissolved in 300 mL of absolute ethanol under ultrasound, and then m-silica was further added. After stirring for 8 h at 70 °C under nitrogen atmosphere, the suspension was filtered and washed with absolute ethanol and deionized water. To obtain silica-s-M, the product was dried under vacuum condition at 50 °C to constant weight. 2.3. Preparation of unfilled SBR and SBR composites For a better comparison, all compounds were based on the same composition, except for the content of filler (silica, m-silica and

Fourier transform infrared spectrum (FTIR) was recorded by a Bruker Vector 33 with KBr pellets in the range of 4000– 400 cm1. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra (DLD) with an aluminum (mono) Ka source (1486.6 eV). To ensure the accuracy of FTIR and XPS results, m-silica and silica-s-M samples were extracted in advance by a Soxhlet extractor for 72 h with hot ethanol to get rid of the unreacted CTS and M. TGA was conducted on a NETZSCH TG209F1 under nitrogen from 30 °C to 700 °C at the rate of 10 °C/ min. Scanning electron microscopy (SEM) was performed by a ZEISS Merlin. Dynamic mechanical analysis (DMA) was performed using a TA DMA Q800 instrument using a tensile mode with a dynamic strain of 0.5%. The samples were heated from 80 °C to 70 °C at the rate of 3 °C/min. The frequency was fixed at 5 Hz. The thermal behaviors of unfilled SBR and SBR composites in glass transition were recorded by a NETZSCH DSC 204 F. Samples were scanned from 80 °C to 20 °C at a rate of 10 °C/min under nitrogen. The heat capacity step DCpn and weight fraction of constrained polymer layer vim [16] of SBR composites were calculated according to the following equations:

DC pn ¼ DC P =ð1  wÞ

ð1Þ

vim ¼ ðDC p0  DC pn Þ=DC p0

ð2Þ

DCp is the heat capacity jump at glass transition temperature (Tg), while DCpn is normalized to the rubber weight fraction. w is the weight fraction of filler. DCp0 is the heat capacity jump at Tg of the unfilled rubber matrix. vim is the weight fraction of constrained rubber layer.

Fig. 1. Synthesis route of silica-s-M.

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Tensile and tear tests were performed using an UCAN UT-2060 following ASTM D 412 and ASTM D 624, respectively. A rubber processibility analyzer (Alpha, RPA 2000) was introduced to record the dependence of the storage modulus on the strains of the uncured compounds. The vulcanization parameters of SBR compounds were determined using an UCAN UR-2030 moving-die rheometer at 160 °C. The vulcanization rate (k) of the SBR compounds is calculated by the following equation [17]:

k¼

100 t90  t s2

ð3Þ

The activation energy (Ea) of vulcanization for SBR compounds was calculated based on the Arrhenius equation:

ln k ¼ 

Ea þ ln A RT

ð4Þ

ts2 is scorch time of SBR compounds. R is the general constant in the gaseous state. T and A are the vulcanization temperature and pre-exponential factor, respectively.

Fig. 2. FTIR spectra of silica, m-silica and silica-s-M.

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3. Results and discussion 3.1. Characterization of silica-s-M Silica-s-M was characterized by FTIR, XPS and TGA. Fig. 2 shows the FTIR spectra of silica, m-silica and silica-s-M. Compared to silica, the characteristic bands ranging from 2980 cm1 to 2936 cm1 attributing to CH3 and CH2 vibrations are visible in m-silica, indicating that silane coupling agent has been grafted onto the surface of silica. Because some of the oxyethyl groups of the grafted silane coupling agent was not hydrolyzed, the CH3 asymmetric stretching at 2873 cm1 is still visible in the spectrum of m-silica. In silica-sM, in addition to the vibrations of silane molecules, the characteristic absorption of M attributing to ortho-bisubstitutional benzene at 757 cm1 is noticeable. Considering the fact that scarcely any unreacted M molecules were left after being Soxhlet extracted sufficiently, it is demonstrated that M has been chemically bonded onto the surface of silica successfully through the chemical linkages provided by silane coupling agents. The XPS spectra of m-silica and silica-s-M are shown in Fig. 3. In m-silica of Fig. 3a, the peak of Cl 2p at 199.6 eV denotes that silane coupling agent has been grafted onto the surface of silica. Compared with the spectrum of m-silica, the peaks at 162.8 eV corresponding to S 2p in the spectrum of silica-s-M can be observed. Besides, the intensity of Cl 2p in silica-s-M is much weaker than that of m-silica due to the consumption of ACl groups, which confirms the chemical reaction between the reactive accelerator and the ACl groups on the m-silica surface. As shown in the high resolution spectrum of S element in Fig. 3b, the peak of S 2p was divided into two peaks with about the same peak area at 163.4 eV and 161.5 eV. It means that there are two kinds of chemical structures for S element on the surface of silica-s-M, which agrees well with the two S atoms in the structure of the supported M molecule. However, because of the low N content of M on the surface of silica-s-M and the low molecular weight of N atom, the peak of N in the XPS spectrum of silica-s-M was not detected. The TGA curves of silica, m-silica and silica-s-M are shown in Fig. 4. Silica loses more weight before 100 °C than m-silica and silica-s-M due to the volatilization of adsorbed water on the hydrophilic surface before organic modification. At high temperature above 100 °C, silica-s-M demonstrates worse thermal stability than silica and m-silica. This behavior is ascribed to the decomposition of silane and M on the surface of silica-s-M. Based on the weight loses of m-silica and silica-s-M from 110 °C to

Fig. 3. XPS spectra of (a) m-silica and silica-s-M and (b) silica-s-M in the S 2p region.

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silica-s-M shows a more uniform dispersion compared to msilica, for there is ‘‘large blank area” (no filler dispersed as indicated by the red circle in Fig. 5b) in SBR/m-silica composites. Predictably, when external stress is applied into the composites, the ‘‘blank area” and agglomerates will act as stress concentration, causing the low strength of the composites. 3.3. Dependence of storage modulus for SBR compounds on strain

Fig. 4. TGA curves of silica, m-silica and silica-s-M.

700 °C, the loading of M on the surface of silica-s-M is 1.7 wt%, corresponding to 0.1 mmol M per 1 g of silica-s-M.

3.2. Morphologies of SBR composites Fig. 5 compares the dispersion of silica, m-silica and silica-s-M in SBR with the filler content of 50 phr. From Fig. 5a, the unmodified silica particles are nonuniformly dispersed in SBR in the form of large agglomerates. After surface modification by silane and vulcanization accelerator, m-silica and silica-s-M disperse in SBR matrix with much smaller particle size than silica as shown in Fig. 5b and c, respectively. Nevertheless, it is worth noting that

Due to the inextensibility of rigid particles, reinforcing fillers, such as carbon black and silica, increase the modulus of the rubber compounds. This hydrodynamic effect is strain independent and the nonlinear rheological amplitude dependency is generally known as the Payne effect [18]. The weaker Payne effect indicates the higher uniformity of filler dispersion. The changes of the storage modulus (G0 ) of the unvulcanizated SBR compounds with the strain amplitude were measured to analyze the filler dispersion. Fig. 6a and b shows the curves of strain-dependant G0 of the unvulcanizated compounds with the filler content of 40 phr and 50 phr, respectively. As clearly seen in Fig. 6, obvious Payne effect occurs in SBR/silica compounds at any level of filler content. However, surface modification of filler weakens the Payne effect of the compounds. Payne effect is the weakest when the compounds are filled with silica-s-M, revealing the highest uniformity of filler dispersion in SBR/silica-s-M compounds [19]. 3.4. Analysis of constrained rubber approaching the filler surface Due to the characteristic long-chain morphology, polymer molecule is sensitive to the environment change (especially temperature change) [20]. Therefore, DCpn is an important parameter providing information about the structural changes of polymer chain [21]. Fig. 7 presents the DSC curves of unfilled SBR and SBR

Fig. 5. SEM images of (a) SBR/silica; (b) SBR/ m-silica; (c) SBR/silica-s-M.

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Fig. 6. Strain dependence of storage modulus (G0 ) of the uncured SBR compounds with the filler content of 40 phr (a) and 50 phr (b) on strain.

Fig. 7. DSC curves of the glass transition region of SBR nanocomposites.

composites with the filler content of 50 phr in the glass transition region. The value of DCpn at glass transition is proportional to the number of internal degree of freedom of molecular motion and represents the change in polymer chain mobility in the composites directly [22,23]. From the glass transition parameters also listed in Fig. 7, the value of DCpn for SBR/silica-s-M is the lowest among the composites, suggesting that the amount of rubber confined between silica-s-M particle galleries is the highest, which is also proved by the highest values of the weight fraction of constrained layer (vim). As the amount of constrained rubber is not due to a diffuse hardening of the rubber, the amount of constrained rubber will not change before and after vulcanization [24]. DMA was also carried out to evaluate the constrained rubber approaching filler. Basically, a depression in loss factor (tan d) height indicates reduction number of mobile polymer chains during the glass transition and the relative peak height is proportional to the volume of the constrained polymer chains [25]. Fig. 8 shows the temperature dependency of tan d for SBR composites with the filler content of 50 phr. Compared to the peak heights of tan d for SBR/silica composites, those for SBR composites with modified filler are lower. SBR/silica-s-M composites shows further decreased peak height of tan d compared to SBR/m-silica composites, indicating that more rubber chains are constrained approaching silica-sM than those approaching m-silica. This result is agreed well with

Fig. 8. Tan d versus temperature for SBR nanocomposites.

the DSC result and further illustrates that the interfacial interactions in SBR/silica-s-M composites are stronger than those in SBR/m-silica and SBR/silica composites. The strong interfacial interactions in SBR/silica-s-M composites may be explained by the chemical bonding between filler and rubber chains due to the participation of vulcanization accelerator in the vulcanization of rubber chains. As shown in Fig. 9, during the curing process, the supported M molecules can react with stearic acid (RCOOH), ZnO and sulfur to form organometallic sulfur complexes on the surface of silica-s-M. The organometallic sulfur complexes then react with rubber chains through the rearrangement and breakage of sulfur bonds. Therefore, silica particles grafted with these complexes are participated in the crosslinking reaction and chemically grafted to rubber chains, significantly enhancing the filler-rubber interaction in SBR/silica-s-M composites. 3.5. Activation energy of vulcanization reaction of SBR compounds The activation energy (Ea) is directly related to the level of difficulty of rubber vulcanization reaction. The higher the Ea is, the more difficult the reaction is. To calculate the Ea of vulcanization reaction, SBR compounds were vulcanizated at different temperature (140 °C, 150 °C, 160 °C and 170 °C). The values of vulcanization rate k at different temperature are tabulated in Table 1.

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Fig. 9. Proposed interfacial chemical reaction in SBR/silica-s-M nanocomposites during curing.

Table 1 Vulcanization rate of SBR compounds at different temperature and activation energy of vulcanization.

k140a k150 k160 k170 Ea (kJ mol1) a

Unfilled SBR

SBR/silica

SBR/m-silica

SBR/silica-s-M

4.66 8.25 14.13 26.93 88.20

1.22 2.69 5.52 9.55 105.00

0.85 1.54 3.12 7.52 110.03

2.12 3.71 8.32 13.27 96.04

k represents the vulcanization rate, while the subscript is the value of vulcanization temperature (°C).

The plots of ln k versus 1000/T of SBR compounds are linear as shown in Fig. 10, suggesting that the values of Ea calculated by the Arrhenius equation are accurate and reliable. From the Ea values of SBR compounds also listed in Table 1, the Ea value (88.20 kJ mol1) of the unfilled SBR compounds is the lowest, which can be interpreted by the diffusion of the vulcanization additives in the rubber matrix [26]. As for the unfilled SBR compounds without filler forming barrier, the vulcanization additives can diffuse freely to implement the vulcanization process, thus low activation energy is needed. On the other hand, for the filled SBR compounds, the vulcanization additives are isolated by the filler particles and have to go through tortuous paths to form polysulfide species and crosslinking points, leading to high activation energy. Interestingly, there is also obvious difference in the Ea values among the filled compounds (SBR/m-silica: 110.03 kJ mol1; SBR/silica: 1 105.00 kJ mol ; SBR/silica-s-M: 96.04 kJ mol1). The difference of the Ea values for the filled SBR compounds is attributed to the hard

diffusion of the vulcanization additives in the constrained rubber layer. In SBR/m-silica compounds, the better dispersed filler particles without large agglomeration possess larger specific surface area, leading to more constrained rubber chains approaching their surface than those approaching the agglomerated silica in SBR/ silica compounds. It is commonly admitted that the mobility of the constrained rubber is significantly restricted [27–29]. Consequently, the vulcanization additives, especially M molecules with severe steric hindrance, are further blocked to diffuse in the constrained rubber layer, leading to the higher activation energy of SBR/m-silica compounds than that of SBR/silica compounds. However, as for SBR/silica-s-M compounds, the supported M molecules are located exactly at the filler-rubber interface, where the constrained rubber is located, corresponding to higher concentration of vulcanization additives. Thus, the collision chances between vulcanization additives in the constrained rubber of SBR/silica-s-M compounds is more, effectively reducing Ea value [30].

3.6. Mechanical properties of unfilled SBR and SBR composites

Fig. 10. Plots of ln k versus 1000/T for unfilled SBR and filled SBR compounds.

The enhanced interfacial interaction, together with improved dispersion of silica-s-M will have a significant impact on the mechanical properties of SBR composites. Fig. 11 shows the dependences of tensile strength, tear strength, modulus (always estimated by the stress at 300%) and elongation at break on filler content. In all of the SBR composites, the tensile strength, tear strength and modulus are continually improved with the filler content increasing, and the composites with modified filler (m-silica and silica-s-M) generally give higher values than SBR/silica composites at the same filler content. As expected, the tensile strength, tear strength and modulus of SBR/silica-s-M composites are further enhanced compared to those of SBR/m-silica composites and the difference is particularly magnified when the filler content is increased to 30 phr, indicating a better reinforcing effect of silicas-M than m-silica. For example, taking the value of SBR/silica composites with the filler content of 40 phr as a reference, the increment of tensile strength for SBR/silica-s-M composites is 52.5%,

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Fig. 11. Mechanical properties of the unfilled SBR and SBR nanocomposites: (a) tensile strength and tear strength; (b) stress at 300% and elongation at break.

Fig. 12. Mooney-Rivlin plots of the unfilled SBR and SBR nanocomposites (the insets are true stress-strain curves).

which is more than twice as that of SBR/m-silica composites (25.8%). Nevertheless, owing to the enhanced interfacial interaction between organic modified silica and rubber chains, the slippage of rubber chains on the surface of the modified silica during stretch is effectively constrained. That is the cause of the lower elongations at break of SBR composites with modified silica than those of SBR composites with untreated silica. To better understand these tensile results, the data are recast in the form of a reduced stress (r⁄) against the reciprocal of the extension ratio (k1) using Mooney-Rivlin equation. The equation is listed as follows:

r ¼ r=ðk  k2 Þ ¼ 2C 1 þ 2C 2 k1 :

ð5Þ

r is the stress and 2C1 and 2C2 are constants that are independent of k. Fig. 12 compares the Mooney-Rivlin plots of SBR composites with the filler content of 50 phr (the insets are the true stressstrain curves). As can be seen in Fig. 12, upturn appears at high strain, which mainly reflect the presence of shorter network chains due to the rubber chain bounded to the filler [31]. The upturn at high strain of SBR/silica-s-M composites begins at the lower strain compared to those of SBR/m-silica and SBR/silica composites, giving another apparent evidence that the interfacial interaction between silica-s-M and SBR is much stronger.

4. Conclusion Silica-supported vulcanization accelerator (silica-s-M) was prepared by the reaction of vulcanization accelerator M with silane modified silica (m-silica). Silica-s-M could be uniformly dispersed in SBR matrix with much more constrained rubber chains approaching its surface than those approaching m-silica, which indicated a stronger interfacial interaction between silica-s-M and rubber. It was found that silica-s-M reduced the activation energy of vulcanization and SBR composites with silica-s-M showed exceptional mechanical properties compared to SBR composites with m-silica or pristine silica while containing equivalent accelerator component. Basically, this work offers a new general design strategy for the filler surface modification with rubber additives and may open up new opportunities to prepare high performance rubber composites.

Acknowledgments This work was supported by the 973 Program, Grant No. 2015CB654700 (2015654703), the National Natural Science Foundation of China (51573051) and the Special Fund for Agro-scientific

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Research in the Public Interest from the Ministry of Agriculture, China (201403066).

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