Effect of styrene content on mechanical and rheological behavior of styrene butadiene rubber

SBR 1500E (styrene of 23.5%) and SBR 1586 (styrene of 40.0%) were provided by PetroChina Lanzhou Petrochemical Company. Zinc oxide (ZnO) was purchased from Shijiazhuang Zhiyi Zinc Industry Co. Ltd, China. Stearic acid (SA) was acquired from Zhejiang Hengxiang Chemical Co. Ltd, China. Sulphur (S8) was obtained from Guiyang Hangyi Rubber and Plastic Accessories Co. Ltd, China. The accelerant, N-tert-butyl-2-benzothiazole sulfenamide (TBBS), was purchased from Yanggu Huatai Chemical Co. Ltd, China.

Sample formula: four kinds of SBR rubber respectively with styrene content of 23.5%, 28%, 34% and 40% were obtained by adjusting the mixing ratio of SBR1586 and SBR1500E, and the detailed formula for the four systems are listed in table 1. The samples are named as SBRxx, where the subscript 'xx' represents the percentage of styrene in the sample.

Mixing process: the raw materials were weighed according to the formula shown in table 1, then the plastic refining of raw rubber was taken place on an open mixer (XSS-300, Shanghai Kechuang Rubber and Plastic Machinery Equipment Co. Ltd, China). The roller distance was set as 1.1 mm and the roller temperature was set as 50 ± 5 °C, in order to ensure the rubber can wrap the roll, and the rubber was alternately made 3/4 cutters from both sides every 30 s. After homogenizing for 9 min, S8 and SA were evenly added into the rubber along the roller, and the rubber sheet was made 3/4 cutters on each side. After homogenizing for 12 min, ZnO and TBBS were evenly added into the rubber along the roller, and the rubber sheet was made 3/4 cutters on each side for 3 times. Then, the roller distance was set as 0.6 mm and the mixing rubber was vertically rolled for 6 times. At last, the roller distance was set as 2.0 mm, and the compounded rubber sheet can be obtained after being folded through the open mill for 4 times.

Vulcanization process: the compounded rubber was first placed on a flat surface for 24 h to relieve stress. And then the rubber sheet was cut into rectangle specimens with a size of 14 cm × 12 cm along the direction in which the sheet was taken from the open mill. Finally, the cut rubber sheet was cured at 151 °C for 40 min on a vulcanizing press.

2.3.1. Scanning electron microscope (SEM)

2.3.2. Testing of stress relaxation and vulcanization properties

2.3.3. Rheological measurements

2.3.4. Dynamic mechanical analysis (DMA)

2.3.5. Dynamic fatigue test

The vulcanized SBR was cut into the shape shown in figure 1 for fatigue testing. Pre-cuts of 2 mm were introduced at the centre of one edge of the specimens by a new razor blade, thus producing single-edge notched test pieces. The tensile fatigue cycle test was carried out on a dynamic fatigue tester (DF-8000-5, GO-TECH, China) in strain control mode and sinusoidal loading, with the specimen being pre-stretched for 5 mm. The circulating strain was ±5 mm, elongation was 67%, and the speed was 5 Hz. The length of the crack was measured by a magnifying glass (PEAK1983-15 X, PEAK, Japan) every 2500 fatigue cycles.

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2.3.6. Tearing energy and crack growth rate

Rivlin and Thomas [32] proposed that quasi-static crack propagation occurred above a certain critical tearing energy, independent of the specimen type, indicating that the critical tearing energy can be considered as an internal property of the rubber material. The tear energy (T) of single incision tensile specimen can be described as:

$$\begin{eqnarray*}&&T=2KU{c}_{0}\,K=\pi /\sqrt{\lambda }\end{eqnarray*}$$

Where K is a geometrical parameter. λ is the elongation ratio, λ = 1 + ε, where ε is the tensile strain. U is the strain energy density, which is the integral area of the tensile stress-strain curve after the Mullins effect is removed. c₀ is the crack length.

In the dynamic fatigue process, the crack and tear energy are constantly changing, so the concept of average tear energy is introduced. The average tear energy (Tav)for different loading times is calculated by the following equation:

$$\begin{eqnarray*}&&{T}_{{\rm{av}}}=\displaystyle \frac{{T}_{1}+{T}_{2}+\cdots +{T}_{n}}{n}\end{eqnarray*}$$

Where T1, T2, ..., Tn are the tear energy for every 2500 fatigue cycles.GJ Lake et al found that the relationship between the crack growth rate (dα/dN) and T of rubber followed a power law relationship (Equation 2) when the rubber sample was loaded with medium to high fatigue strain, that is:

$$\begin{eqnarray*}&&{\rm{d}}\alpha /{\rm{dN}}={{\rm{BT}}}^{\beta }\end{eqnarray*}$$

Where B and β are two parameters.

2.3.7. J-integral

J-integral value can be considered as the required energy to produce a new fractured surface per unit area, which is to characterize the energy release rate when the crack length (α) is extended from α to α + ∂α. The vulcanized sheet was cut into a specimen with a length of 150 mm, a width of 19 mm and a thickness of 2 mm. In order to eliminate 'Mullins effect', the specimen was cyclically stretched for 10 times at a tensile rate of 100 mm min⁻¹ and a strain of 80 mm on a tensile testing machine (Z005, Zwick, Germany) before the measurement. Then, 2 mm and 4 mm vertical edge incisions were cut respectively at the central and the edge of the specimen for the final tensile testing at a rate of 100 mm·min⁻¹. The J-integral value can be calculated as [33]:

$$\begin{eqnarray*}&&{\rm{J}}=-{\left(\displaystyle \frac{\partial U}{\partial A}\right)}_{{\rm{\Delta }}}=-\displaystyle \frac{1}{B}{\left(\displaystyle \frac{\partial U}{\partial \alpha }\right)}_{{\rm{\Delta }}}\end{eqnarray*}$$

Where U is the strain energy, which equals the integral area of the force-displacement curve at a constant displacement (∆). A is the area of the section, B and α are respectively the thickness and the crack length of the specimen.

2.3.8. Transmission electron microscopy (TEM)

Figure 2 shows the TEM images for SBR₄₀. For SBR₄₀, there is no styrene enrichment phase can be observed even though the content of styrene has reached 40%, which indicating that SBR₄₀ is still a random copolymer as a kind of rubber.

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DMA curves further reflect the chain structure of SBR sample. As shown in figure 3, peak of Tan δ shifts to the direction of high temperature with the increasing styrene content, which indicating that the higher styrene content dose SBR possesses, the higher glass-transition temperature (GTT) is produced. GTT of SBR_23.5 is −28 °C, while that of SBR₄₀ is 3 °C.

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Although styrene only owns a small polarity, its large volume and steric resistance make the butadiene chain segment not easy to move. With the increase of styrene content, the internal rotation steric resistance of the molecular chain increases, so that the GTT of SBR is increased. Furthermore, there is only one Tan δ peak for each of the four different SBR samples despite the increasing styrene content. The transition peak of styrene copolymer is not found at about 50 °C [34], further indicating that the four kinds of SBR samples are all styrene and butadiene random copolymers without obvious phase separation structure.

Figure 4 shows the vulcanization characteristics of SBR samples. With the increasing styrene content, the starting time (ts, acquired by the intersection of the bilateral tangent lines) of SBR vulcanization becomes more and more hysteretic. Furthermore, the maximum torque (MH) decreases, and the difference value between maximum and minimum torque (MH-ML) gradually becomes small, indicating the decline in crosslinking density of SBR [35]. The relative number of unsaturated double bonds in the rubber molecular chain (mainly distribute in butadiene segments) decreases with the rising content of styrene which have almost no unsaturated double bonds, resulting in the reduction of crosslinking points and the deceleration of torque growth during vulcanization, resulting in a low maximum torque. Before vulcanizing, SBR with high styrene content possesses a high minimum torque because there are lots of rigid styrene groups in the matrix. In order to reflect the butadiene crosslinking density more accurately, a characteristic numerical value φb = (MH-ML)/(1-percentage of styrene) is introduced, which stands for the crosslinking density per unit content of butadiene. As shown in table 2, the value of φb increases with the rise of styrene content, indicating that the augment of styrene dilutes the concentration of butadiene segments at the same amount of vulcanizing agent, although the total cross-linking density (reflected by the value of MH-ML) gets diminishing.

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Figure 5 displays the stress relaxation behavior of SBR samples. It can be found that the styrene content plays a significant role in the stress relaxation of SBR. At the same relaxation time, torque retention rate increases with the rise of styrene content, and SBR_23.5 has the shortest stress relaxation time. With the rise of styrene content, the steric hindrance increases correspondingly, which makes the conformational change of rubber macromolecules under the action of external forces more difficult and the relaxation time is prolonged. For the materials with similar elastic modulus, the longer relaxation time it owns, the higher elastic modulus will be represented in mechanical process.

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As shown in figure 6, with the increase of styrene content, the loss factor tan δ decreases gradually with the frequency (ω), especially for SBR₄₀, the tan δ value decreases greatly. In the low frequency region, SBR with higher styrene content possesses larger tan δ value. While in the high frequency region, the tan δ value is lower in SBR material with high styrene content (such as SBR₄₀). In the low-frequency region, the shear rate is slow, and the molecular chains in the SBR can change their conformations with the action of external force. For SBR with high styrene content (such as SBR₄₀), the tan δ value is higher owing to its more styrene groups, which make the butadiene segment has a larger deformation at the same deformation degree compared with that of SBR_23.5. However, in the high frequency region, the shear rate is faster, and the molecular chain in SBR has no time to change its conformation with the action of external force. Therefore, SBR with high styrene content possesses larger relative elastic modulus and lower tan δ value compared with those of SBR_23.5.

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In the small strain area, all SBR samples present a plateau region of storage modulus (G') Versus strains(γ) shown in figure 7, and the higher styrene content does SBR have, the higher platform region of G' can be observed. With the increasing strain, G' value begins to decline at a critical strain (γ_c), which can be attributed to the chain untangling of SBR. For SBR₄₀, the highest G' platform can be found owing to its high styrene content and chain rigidity. However, G' value of all the samples decreases sharply when the strain exceeds γ_c. With the increase of strain, the degree of untangling is higher, thus the G' value sharply decreases [36].

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J-integral is the energy value required to produce a new fractured surface per unit area, which represents the static crack growth. Its physical significance is to characterize the energy release rate when the crack length grows from α to α + ∂α.

As shown in figure 8, the J-integral value of SBR increases significantly with the rise of styrene content, indicating that SBR with high content styrene requires higher energy to produce new broken surface per unit area. For SBR₄₀, the J-integral exhibits a power function curve and much higher than that of other test groups. When the developing crack tip encount styrene segments in SBR, the crack development can be stopped or changed direction due to the great rigidity of styrene. Moreover, the increase of the crosslinking density of butadiene enhances the rigidity of butadiene segments, which can also delay the growth of cracks.

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Figure 9 shows the dynamic fatigue behaviors of SBR samples. As shown in figure 9(a), the dynamic fatigue of SBR is carried out in strain control mode. With the reciprocating stretch cycle going on, the initial crack (α) gradually becomes larger and larger (α + ∂α), and the SBR sample completely break at the end. The dynamic fatigue crack growth rate (dα/dN) and dynamic fatigue cycle follow a power function relationship reflected in figure 9(b). As the styrene content rising, the crack growth slows down, and the fatigue life of SBR increases gradually.

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The dynamic fatigue crack growth rate of SBR with low styrene content (such as SBR_23.5) is faster, and there is no stable crack development stage. The higher the styrene content is, the longer the stable crack development stage is. Furthermore, SBR with high styrene content (such as SBR₄₀) possesses more flat fatigue crack growth rate and longer fatigue life compared with those of SBR_23.5. Similarly, the relationship between dα/dN and tearing energy (T) also follows a power function as shown in figure 9(c). Under the same tearing energy, SBR with high styrene content owns slow dα/dN. Combined with the J-integral curve, it can be found that the higher the J-integral value is, the slower the dα/dN exhibits. Converting horizontal and vertical coordinates into logarithmic coordinates, it can be observed form figure 9(d) that the plots of SBR samples display a linear relationship which can be expressed by:

$$\begin{eqnarray*}&&{\rm{lg}}({\rm{d}}\alpha /dN)=\,{\rm{lg}}\,B+\beta \,{\rm{lg}}\,T\end{eqnarray*}$$

where β is the slope of the straight line, indicating the dependence of the dα/dN on the tear energy. The larger the value is, the larger the dα/dN varies with the change of tear energy. As shown, SBR₄₀ possesses the highest β value of 2.58, manifesting a high dependence between dα/dN and T. Both static crack growth and dynamic fatigue crack growth are related to the molecular structure of SBR. When the crack tip meets rigid styrene segments, they can prevent the development of the crack or change the developing direction of the crack. Therefore, the dynamic fatigue crack development of SBR can be delayed effectively by increasing the styrene content. On the other hand, butadiene is the flexible segments of SBR. The rising styrene content increases the crosslinking density of butadiene segments, which also increases the rigidity of SBR matrix, thus preventing the crack growth effectively.

Figure 10 displays SEM images for the fracture surface of SBR after fatiguing. With the rise of styrene content, the fracture surface becomes more and more rough. A relatively smooth fracture surface can be observed in the SEM image of SBR_23.5. Along the crack direction, SBR_23.5 exhibits a larger parabolic shape from the beginning, followed by a flat area, and the fracture edge is relatively neat. However, SBR with high content (SBR₄₀) shows rough fracture edge and rough fracture surface. The higher the styrene content is, the more uneven the surface will be, and after the initial smooth area, a smaller parabolic shape accompanied by uneven peaks and ravines can be observed clearly.

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Figure 11 shows the crack expansion and formation of parabolic morphology for SBR samples during dynamically fatiguing. The parabolic morphology is formed by the mergence of main crack with the secondary cracks in front of the main crack during their growth. For SBR with low styrene content (SBR_23.5), the secondary cracks propagate rapidly and merge with each other to form larger secondary cracks in the front of the main crack. When the main crack and the secondary crack meet, a large and neat parabolic morphology is formed. However, for SBR with high styrene content (SBR₄₀), the development of the secondary cracks is relatively slow owing to the high strength matrix. When the main crack meets the secondary crack, the formed parabolic shape is small, with rough edge and obvious tearing trace. When the main crack accelerates, the growth of main crack itself dominated.

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