Tensile creep behavior and structures of sisal fiber reinforced styrene butadiene thermoplastic elastomer/polystyrene composites

Abstract

Styrene butadiene thermoplastic elastomer (SBS)/polystyrene (PS)/sisal fiber (SF) composites were prepared by melt-blending method. The tensile creep behavior of SBS/PS/SF composites was studied and fitted by four viscoelastic models including Findley, Burger, generalized Kelvin-Voigt and Maxwell models. The effects of content and surface properties of SF treated by alkali and silane coupling agent on the creep behavior, structures and thermal properties of the composites under the action of tensile stress were investigated. The results indicated that the generalized Kelvin-Voigt model and generalized Maxwell model could best fit the tensile creep behavior of SBS/PS/SF composites. The creep resistance and thermal stability of the composites improved with the increases of SF content and tensile stress. The interfacial properties of fiber and matrix were enhanced after SF treated by silane, which would be beneficial to the improvements of the creep resistance and thermal stability. The glass transition temperature of the composite increased with the increase of SF content, but decreased after the action of the tensile stress.

Keywords

Styrene-butadiene-styrene sisal fiber tensile creep thermal stability glass transition temperature

Introduction

Styrene-butadiene-styrene (SBS) block copolymer is a kind of typical thermoplastic elastomer, which has a lot of advantages such as the convenient processing, excellent mechanical properties, and recyclability comparing to the traditional vulcanized rubbers. Therefore, SBS has been widely used in many applications like tyre, conveyor belt, seal, gasket and rubber pipe. However, SBS materials also have some shortcomings, such as low creep resistance and poor heat resistance, which limited the further application of SBS materials.

Polystyrene (PS) has higher tensile strength, easy processing, small molding shrinkage and good dimensional stability. In addition, PS has good compatibility with SBS, which could improve the processing performance, tensile strength and aging resistance of SBS. In recent years, natural fibers could be used as reinforcing and toughening materials to improve the performance of polymer materials. Sisal fiber (SF) as an important natural fiber, has outstanding advantages such as low cost, strong tensile, high toughness and good wear resistance, which has been applied as reinforced filler to improve the mechanical and thermal properties of polymers.^1,2 In addition, the interfacial interaction between fiber and polymer would affect the properties of fiber reinforced polymer composites. The enhancement of the adhesion force between fiber and polymer, would be beneficial to the increases of the mechanical properties and rigidity of the composite material. The surface treatment of fiber could reduce the content of OH groups on the surface, and increase the interface between fiber and matrix.³ The impurities on the surface of fiber were removed after the alkali treatment, and the surface became smooth after the fiber treated by silane coupling agent.⁴ Subramanya et al.⁵ conducted the alkali treatment on banana fiber could remove impurities on the surface of banana fiber, and improve the interaction between fiber and cassava starch. Therefore, the surface property of sisal fiber could play an important role in the mechanical and thermal properties of polymer composite and need further research.

Some elastomer products were subjected to the loadings in the usage, resulting in the breaking and failure of materials. The tensile stress, deformation, load rate and time would greatly affect the properties of polymer composites. The elastic and viscous response of composites would occur simultaneously in the process of creep, relaxation and dynamic mechanical loading, which is called viscoelastic response.⁶ Creep phenomenon happen that the deformation of materials increases with the time under the action of constant mechanical load. Creep behavior could reflect the dimensional stability and long-term load capacity of the material,⁷ which could be analyzed from the constitutive models including micromechanics and macromechanics methods. The micromechanics method was analyzed from the molecular level, while the micromechanics method was considered through the macro level from the mathematical equations related to stress and strain.⁸ Martins et al.⁹ found that Burgers model fairly well represented creep under linear viscoelasticity, but non-linear model should be used at higher stress levels. Xu et al.¹⁰ studied the creep changes of bagasse/polyvinyl chloride (PVC) composites, bagasse/high density polyethylene (HDPE) composites, and industrial wood/HDPE composites, and fitted the creep behavior of these composites by different models. They found that Burgers model, Findley power law model, and two-parameter power law model could all fit the creep data. The parameters of Burgers model and two-parameter power law model were more consistent in similar materials.

Different materials under the various state would be analyzed according to the different mathematical models, and the appropriate models were applied to analyze and simulate the change of materials. SBS materials has been used in some products for bearing the action of tension, however, there is still lack of research on the tensile creep behavior of SBS composites under the effect of tensile creep. In this paper, the effects of SF content and surface properties of SF on the creep behavior, creep prediction and structures of SBS/PS/SF composites were analyzed. The creep behavior of SBS/PS/SF composites was predicted by four viscoelastic models including Findley, Burger, generalized Kelvin-Voigt and Maxwell models, in order to obtain the most suitable mathematical model for better predicting the creep behavior of this materials. Prediction method is a useful alternative or complement experimental testing, which could lead to a significant reduction in cost and still maintain the reliability of the data obtained. In addition, the effects of SF content, surface treatment and tensile creep on the thermal stability and structure of the composite was also analyzed, in order to explore the relationships of structure and properties under the action of tensile creep. Therefore, it is of theoretical and practical significance to study the structural and properties changes of SBS under load for the exploration of material failure mechanism, and the improvement of material properties for further application of SBS materials.

Experiment

Materials

SBS (YH-796) was produced by Baling Petrochemical Branch of Sinopec. PS (GP5250) was obtained from Taiwan Chemical Fiber Co. SF was industrial grade and purchased from Guangxi Agricultural Reclamation Group. Silane coupling agent (AR, KH550) was procured from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide (AR) was obtained from Xilong Scientific Co., Ltd.

Sample preparation

Alkali-treated SF

As shown in Figure 1(A), the raw sisal fiber was long and the surface was covered with many impurities. The long sisal fiber was cut into about 5 mm, and soaked in 10% mass fraction NaOH solution for 4 h, which was washed to neutral, and dried in an oven at 80°C.

Figure 1.

Schematic diagram of preparation process of SBS/PS/SF composite (A) natural sisal fiber (B) Short sisal fiber (C) SBS pellets (D) PS pellets (E) Twin-screw extruder (F) SBS/PS/SF pellets (G) injection molding machine (H) standard dumbbell specimen (I) Tensile creep by universal test machine.

Silane -treated SF

Silane coupling agent (KH550) was used to treat SF with 2 wt % weight base and mixed with 99.7% ethanol to prepare a solution with 10% mass fraction. The alkali-treated SF was soaked for 1 h and dried in a natural environment, then continually dried in an oven at 80°C. Thus, SF was modified by silane coupling agent, as shown in Figure 1(B).

Preparation of SBS/PS/SF composites

SBS pellet (Figure 1(C)), PS pellet (Figure 1(D)) and modified SF were weighed according to the weight ratio in Table 1. They were mixed through a high-speed mixer, and then extruded by a co-direction twin-screw extruder (KTE20/500-4-40, Nanjing Kolko extrusion equipment Co., LTD) (Figure 1(E)), and prepared the SBS/PS/SF composite (Figure 1(F)). The temperature of the extruder was set in the range of 180-200°C. The extruded granules were dried in an oven at 70°C for 4 h. Finally, the injection machine (HTF90W1, Ningbo Haitian Plastic Machine Group Co. LTD) (Figure 1(G)) was operated in the range temperature of 200-230°C and pressure 68-80 MPa, and the dumbbell type standard samples were obtained (Figure 1(H)).

Table 1.

Formula of SBS/PS/SF composite.

Sample no.	SBS/phr	PS/phr	Alkali-treated SF/phr	Silane -treated SF/phr
0 SF	70	30	0	0
5 SF	70	30	5	5
10 SF	70	30	10	10
20 SF	70	30	20	20
30 SF	70	30	30	30

Characterization

Creep test

The creep test was carried out according to testing method for creep properties of fiber reinforced plastics (GB/T 41061-2021). A universal testing machine (AGS-X, Shimadzu Corporation) was used to apply a constant stress on the standard dumbbell sample with a rate of 5 mm/min at room temperature loading to 6.5 and 9 MPa, respectively, holding it for 5 min, as shown in Figure 1(H).

Scanning electron microscope (SEM)

The fracture morphology was characterized by scanning electron microscope (EVO18, Carl ZEISS). The fracture surface of sample was treated by spraying-gold, and the sample was performed at 5 kV accelerating voltage.

Thermogravimetric analysis (TG)

The thermal decomposition experiment was carried out by thermogravimetric analyzer (Discovery, TA Instruments). The sample weighing 5-8 mg was heated from 40 to 600°C at 20°C/min rate in high-purity nitrogen.

Fourier transform infrared spectroscopy (FTIR)

The groups and structural information of sample were tested by FTIR spectrometer (IS10, Thermo Scientific) in the range of 500-4000 cm ⁻¹ at 4 cm ⁻¹ resolution under the attenuated total reflection (ATR) mode.

Static thermo-mechanical analysis

The static thermo-mechanical analysis was performed by the thermomechanical analyzer (XWJ-500B, Chengde Kecheng Testing Machine Co., LTD). The sample with the dimension of 4.5 mm × 4.5 mm × 6 mm was heated from room temperature to 130°C at 10°C/min heating rate in compression mode.

Creep model

Findley model

Findley and Lai¹¹ found that the creep behavior and the recovery after creep under the combinations of tensile and torsion could be described mathematically. This mathematical method could be used to predict creep behavior under different stress states, as shown in equation (1).

ε (t) = ε_{0} + A t^{n}

(1)

Where

ε (t)

is the time−varying creep strain,

ε_{0}

is the instantaneous elastic strain, A is the amplitude of the instantaneous creep strain, and n is a constant.

Burger model

Burger model is a classical linear viscoelastic model, composing of Maxwell and Kelvin-Voigt model, which is mainly used to express the viscoelastic behavior of polymers.⁹ The details are shown in equation (2).

ε (t) = \frac{σ_{0}}{E_{1}} + \frac{σ_{0}}{E_{2}} [1 - \exp (- t \frac{E_{2}}{η_{2}})] + t \frac{σ_{0}}{η_{1}}

(2)

Where

σ

represents the applied stress, t represents the creep time, E₁ and E₂ represent the elastic modulus,

η_{1}

and

η_{2}

is the viscoelasticity of the composite.

Generalized Kelvin-Voigt model

Generalized Kelvin-Voigt model is a phenomenological model commonly used to describe the mechanical behavior of viscoelastic solids. The model consists of an elastic element and N viscoelastic elements in series.¹² The details are shown in equation (3).

D (t) = D_{0} + \sum_{k = 1}^{n} D_{k} (1 - e^{- t / τ_{k}})

(3)

Where

D_{0} =

1/E₀ is the creep compliance of the first spring,

D_{k} =

1/E_k is the creep compliance of the spring,

τ_{k} =

η_k/E_k is the retardation time for the k.

Generalized Maxwell model

Generalized Maxwell method is modeled by connecting elastic elements in series with a set of viscous elements.¹³ The Prony series is a finite representation of the exponential sum over time. The relaxation modulus E_(t) was characterized, and the equation in the form of Prony series was obtained, as shown in equation 4.

E (t) = σ (t) / ε_{0} = E_{\infty} + \sum_{k = 1}^{n} E_{k} e^{- t / τ_{k}}

(4)

Where

E_{k}

corresponds to the relaxation modulus of each Maxwell element,

τ_{k} =

η_k/E_k is the relaxation time of element k,

E_{\infty}

represents the relaxation modulus of the entire system for t = ∞.

Results and discussions

Creep behavior analysis

The tensile creep curve is composed of the typical primary creep stage, steady-state creep stage and the accelerated creep stage. In the first stage, the material deforms at a gradually increasing rate. In the second stage, there is a steady state creep stage, where the material deforms at a constant rate. In the third stage, creep occurs at an increasing rate until the fracture of the material in the end.² Among them, instantaneous deformation, primary creep and secondary creep all occur, but tertiary creep only occurs when the stress value is large or at this stress level for a long time. The creep curves of SBS/PS/SF composites and fitted by four models were shown in Figure 2. With the increase of time, the strain of sample increases, indicating the primary creep stage and steady-state creep stage. With the increase of SF content, the strains of SBS/PS/SF composites are 14.27%, 11.13%, 10.95%, 8.39% and 8.22%, respectively, showing a gradually decreasing trend, and the range of steady-state creep zone increases. With the increase of SF content, the creep resistance of composite was improved.¹⁴ The addition of SF could inhibit the movement of SBS long molecular chains and hinder the deformation of composites.¹⁵

Figure 2.

Creep curves of SBS/PS/SF composites with different SF content under 6.5MPa tensile stress were fitted by four models (SF treated by silane) (A) Findley model (B) Burger model (C) Generalized Kelvin-Voigt model (D) Generalized Maxwell model.

The regression coefficients (R²) of four models predicting creep behavior were shown in Figure 3. Findley model fitted the creep the fractional derivative curve of composites, and R² was more than 0.9. Burger model was used to fit the creep curves of composite materials, and R² was much less than 0.9. The Burger model is not suitable to describe the creep process of composite materials, but is more suitable for linear viscoelastic materials. The generalized Kelvin-Voigt model and the generalized Maxwell model were used to fit the creep curves of the composites, and the R² was above 0.99, indicating good fitting effect. Comparing the R² of four models of composite materials at the different SF content, the fitting correlation decreased with the increase of SF content.

Figure 3.

Regression coefficients of four models under 6.5MPa tensile stress in predicting creep behavior.

The creep behavior of fiber reinforced composites is affected by the matrix, the elastic and fracture behavior of fiber, the geometry and arrangement of fiber and the fiber-matrix interface properties.¹⁶ Figure 4 demonstrated the creep curves of composites with the different SF surface properties fitting by four models. The strain of composites containing SF treated by silane coupling agent is 10.95%, which decreased by 6.42% compared with that of SF treated by alkali. This is because SF composites treated with silane have better interface performance between SF and matrix, which could inhibit the motion of SBS molecules, leading to the enhancement of creep resistance and dimensional stability.¹⁷

Figure 4.

The creep curves of composite with SF surface properties under 6.5MPa tensile stress were fitted by four models (10 SF). (A) Findley model (B) Burger model (C) Generalized Kelvin-Voigt model (D) Generalized Maxwell model.

The corresponding R² of four models of the composites were shown in Figure 5. The Findley model was used to fit the creep curve of the composite material, and the fitting effect was good. The fitting correlation was 0.95349 after SF treated by alkali, but decreased to 0.93957 after SF treated by silane. Burger model was used to fit the creep curve of the composite material, and the fitting correlation was low, indicating that the Burger model was not suitable for describing the creep process of the composite material. Generalized Kelvin-Voigt model and generalized Maxwell model were used to fit the creep curve of composite materials, and the both were very suitable for fitting creep behavior of SBS/PS/SF composites.

Figure 5.

Regression coefficients of four models with different SF surface properties in predicting creep behavior.

The creep behavior of fiber reinforced composites is also related to the applied stress, which would affect the interface of fiber and matrix.¹⁰ Figure 6 indicated the creep curves of composites under different stresses. In Figure 6(A), the maximum strain of the composite material under 6.5 MPa stress is 10.95%. With the stress increases to 9 MPa, the maximum strain is 73.65%. With the increase of stress, the strain of composite material increases and relatively large deformation occurs. This might be the polymer chain could move and orient along the loading direction under low stress. With the increase of applied stress, the tensile expansion of the chain would gradually become slow, and the polymer molecules would slip between chains. In addition, the interaction between SF and the polymer matrix also limits the stretching, expansion and sliding of the polymer chain. When the applied stress at the interface between SF and the matrix exceeds the interface bond stress, the composite material would undergo larger deformation.¹⁸ Meanwhile, R² of composites under different stress fitted by four models in Figure 7. The generalized Kelvin-Voigt model and the generalized Maxwell model have the best fitting effect, while Burger model has poor fitting effect and is not suitable for the fitting of this composite material.

Figure 6.

Creep curves of composites with different stresses fitted by different models (10 SF treated by silane). (A) 6.5 MPa (B) 9 MPa.

Figure 7.

Regression coefficients of four models under different stress in predicting creep behavior (10 SF treated by silane).

SEM analysis

SEM fracture morphology of SBS/PS/SF composite was shown in Figure 8. In Figure 8(A), the interface of SF and matrix with SF treated by alkali was not well, and some cavities were formed between the two interfaces. The interface of composites containing SF treated by silane becomes better (Figure 8(B)), which could increase the absorbed energy in the plastic deformation zone. When SF content increases (Figure 8(C)), SF could form more plastic deformation zone to absorb energy.

Figure 8.

SEM of fracture morphology of SBS/PS/SF composites (A) 10 alkali treated SF -0 MPa (B) 10 silane treated SF -0 MPa (C) 30 silane treated SF -0 MPa (D) 10 alkali treated SF -9 MPa (E) 10 silane treated SF -9 MPa (F) 30 silane treated SF -9 MPa.

When the tensile creep was applied, the composite containing 10 phr SF treated by alkali indicated that SF was separated from the matrix and pulled out (Figure 8(D)). The fracture surface of the composite containing SF treated by silane presented that SF was embedded into the matrix, and less SF was pulled out. In Figure 8(E) and (F), the matrix had obvious orientation and deformation under tensile action, and the plastic deformation zone appeared in the matrix material, which reduced the damage of external force to the SBS composite and improved the creep resistance of SBS composite.

The mechanical properties of fiber-reinforced composites are related with the strength and modulus of fiber, the strength and toughness of matrix and the fiber-matrix interface properties.¹⁹ When SBS and PS are blended, PS molecular chain and two-phase continuous structure of polystyrene and polybutadiene blocks in SBS are interlaced and interlocked.^20,21 The impurities on SF surface were removed after SF was treated by alkali, but there were still a lot of hydroxyl groups in SF. While when SF was treated by silane, the hydrolyzed silanol group could directly condensation with the hydroxyl group of SF. The polymerized silane long hydrophobic polymer chain could be connected to the matrix by van der Waals' gravitational force, promoting the interface bonding between SF and the matrix.^22,23

FTIR analysis

FTIR curves of SBS/PS/SF composites were demonstrated in Figure 9. In Figure 9(A), the bands at 698 and 752 cm⁻¹ are related to the out-of-plane bending of the C-H group in the styrene aromatic ring, and the deformation vibration of the C-H group in the aromatic ring. The bands at 910, 968 and 1040 cm⁻¹, respectively, which are attributed to the vibration of the benzene ring and the torsion of –CH₂. The band at 1440 cm⁻¹ is related to the stretching vibration of d (CH) in CH₂=CH– of polybutadiene. The stretch vibration absorption peak of conjugated C=C bond is right on 1650 cm⁻¹. 2850, 2920 and 3010 cm⁻¹ can be attributed to the stretching vibration and asymmetric stretching vibration of the CH₂ group in the polybutadiene and the stretching vibration of the CH group in the aromatic ring, respectively.²⁴

Figure 9.

FTIR curves of SBS/PS/SF composites (A) and local enlarged view of FTIR curves (B).

With the increase of SF content from 0 to 10 phr, the absorption peak at 3020 cm⁻¹ drops to 3010 cm⁻¹, indicating that the change of hydrogen bond of composites. Comparing with SF treated by alkali, the absorption intensity of each absorption peak of silane-treated fibers decreased. The number of hydroxyl groups in SF is large, easily forming hydrogen bonds and aggregating, which would make the poor compatibility between SF and polymer matrix. After SF was treated by silane coupling agent, the number of hydroxyl groups on the surface of SF decreased and the polarity was weakened.

In Figure 9(B), the band at 1750 cm⁻¹ is assigned to the stretching vibration of hemicellulose C=O in sisal fiber. The intensity of the absorption peak was weakened after SF treated by silane, indicating that hemicellulose and lignin components in sisal fiber were removed by silane coupling agent treatment. Different absorption peaks appear at 1155 and 1550 cm⁻¹, which are mainly due to the bending vibrations of Si–O–Si and NH₂ of the silane reagent.²⁵ The absorption peaks at these two positions indicated that the silane coupling agent had been successfully grafted onto the sisal fiber. In addition, with the application of stress 0 MPa, 6.5 MPa, and 9 MPa on the composites, the absorption intensity of each absorption peak of the composite does not change significantly, indicating that the stress application does not change the group of the composites.

Thermogravimetric analysis

TG and DTG curves of SBS/PS/SF composites were shown in Figure 10, and the data were listed in Table 2. From Figure 10(A), before 300°C, the mass loss was caused by evaporation of water and decomposition, and the mass loss increased gradually with the increase of SF content. Thermal degradation of SF has three stages^22,26: the volatilization of inherent water is below 100°C, and the decomposition of fiber components such as hemicellulose, lignin and cellulose between 210 and 450°C. According to the DTG curve, the composite material has two stages of degradation peak. The degradation peak of SF in the first stage was attributed to the degradation of hemicellulose, lignin and cellulose. At 300 -500°C, the material is greatly decomposed, and the decomposition peak of the second stage is the decomposition of the composite material. Meanwhile, with the increase of SF content, the temperature of the second degradation peak increased from 482.46 to 488.68°C, indicating that the heat resistance of the composite increased with the increase of SF content.

Figure 10.

TG and DTG curves of SBS/PS/SF composite (A) different SF content (SF treated by silane) (B) different SF surface properties (10 SF) (C) different stresses (10 SF treated by silane).

Table 2.

TG and DTG data of SBS/PS/SF composite.

SF treatment	Silane							Alkali
Stress/MPa	0					6.5	9	0
SF content/phr	0	5	10	20	30	10	10	10
T_i/°C	444.3	452.31	451.69	448.17	448.99	450.11	452.84	449.44
T_5%/°C	409.9	381.22	372.22	352.94	347.79	374.04	380.27	370.47
T_50%/°C	477.06	478.44	477.63	477.69	477.59	477.13	479	476.72
T_t/°C	502.4	505.36	504.48	505.53	506.73	505.11	506.12	503.18
T₁/°C	—	379.65	385.16	386.94	387.61	387.98	389.73	387 .06
T₂/°C	482.46	482.61	486.07	488.34	488.68	486.23	487.97	485.16

T_i is epitaxial initial decomposition temperature.

T_5% is weight loss 5% temperature.

T_50% is weight loss 50% temperature.

T_t is epitaxial termination decomposition temperature.

T₁ is the maximum decomposition rate at first stage.

T₂ is the maximum decomposition rate at second stage.

The TG and DTG curves of composites containing SF treated by alkali and silane were shown in Figure 10(B). From Table 2, comparing with the alkali-treated SF composite, the second-stage degradation temperature of the silicane treated SF composite rose from 485.16 to 486.07°C, and the epitaxial initial decomposition temperature and epitaxial termination decomposition temperature increased by 2.25°C and 1.3°C, respectively. This is mainly because when SF is treated with silane, the covalent bonds are generated between silane coupling agent and SF through condensation reaction, and the interface binding force is stronger than that of alkali treating SF through van der Waals attraction.²⁷ Asim et al.²⁸ showed that the thermal degradation temperature of the treated fiber composite was increased because the silane treated fiber was coated with silane. Panaitescu et al.²⁹ found that the thermal stability of composites was improved by surface of fiber with silane coupling agent, proving that the composites treated with silane have better thermal stability. The interfacial bonding between SF and SBS could increase temperature of the breaking ang decomposition of macromolecular chains, which could enhance heat stability of SBS.

Figure 10(C) presented the effect of different stresses on the thermogravimetric curves of the composites, and data were also shown in Table 2. When the stress increased from 0 MPa to 9 MPa, the first-stage degradation temperature of the composites increased slightly from thermal degradation temperature, and the main degradation peak temperature shifted to a higher temperature. Under the action of tensile, the polymer molecular chain would be stretched along the direction of the force, and the molecular chain could orient along the tensile direction with the increase of stress. When the molecular chain is highly oriented along the tensile direction, the polymer would form the order structure.^30,31 During the stretching process of styrene block copolymer, only the rubber phase was oriented, while the PS phase was not oriented.^32,33 Khiêm et al.³⁴ found that strain induced crystallization occurs in natural rubber filled natural rubber under uniaxial and biaxial loadings. Long et al.³⁵ found that polyolefin elastomer (POE) blends had high crystallization orientation in the crystallization process under uniaxial tension, and thus had high mechanical properties and orientation. After crystallization, the thermal stability of the polymer would be improved. Khawas et al.³⁶ prepared nanocellulose with different crystallinity, and the results showed that the higher the crystallinity of nanocellulose, the better thermal stability. Gond et al.³⁷ found that the thermal stability of polylactic acid was enhanced by the addition of nanocellulose. Therefore, the creep tension could improve the stability of composite materials.

Thermal mechanical analysis

The static thermo-mechanical analyzer was applied to analyze the change of deformation with temperature.³⁸ The polymers could happen the change from glassy state to high-elastic state with the increase of temperature.³⁹ Therefore, the glass transition temperature (T_g) could be characterized according to the deformation displacement (ΔL) with the increase of temperature, as shown in Figure 11 and Table 3. In Figure 11(A), with the increase of SF content from 0 to 30 phr, T_g of the composites increased from 82.5 to 89.3°C, increasing by 7.61%. The addition of fiber would reduce the free volume of SBS, and restrict the movement of polymer molecular chain segment,^40,41 resulting in the increase of T_g. This phenomena became more obvious with the increase of SF content.

Figure 11.

Thermal mechanical curves of SBS/PS/SF composite (A) different SF content (SF treated by silane) (B) different SF surface properties (10 SF) (C) different stresses (10 SF treated by silane).

Table 3.

Glass transition temperature of SBS/PS/SF composite.

SF treatment	Silane							Alkali
Stress/MPa	0					6.5	9	0
SF content/phr	0	5	10	20	30	10	10	10
T_g/°C	82.5	83.8	84.6	87.8	89.3	82.5	82.2	83.8

Figure 11(B) indicated the thermal mechanical curves of composite with different SF surface properties, and the data were also shown in Table 3. T_g of the composite containing SF treated by silane coupling agent was higher than that of alkali. With the increase of the interaction between filler and polymer matrix, the free volume fraction of polymer molecular chain decreases, which would be favorable to enhance T_g of the composite material.^42,43 Therefore, the composites containing SF treated by silane slightly increased, because SF has stronger interfacial binding with polymer matrix, and the movement of polymer molecular chain became more difficult.

Figure 11(C) showed the thermal mechanical curves of composite under different stresses. T_g of the composite decreases gradually with the increase of tensile stress. The tensile stress could affect the free volume, the motion and the orientation of SBS molecules. When the tensile stress is applied to the composite material, the polymer molecular chain would be stretched, untangled and oriented, promoting the increase of molecular mobility and thus reducing T_g of polymer materials.

The schematic representations for structures of SBS/PS/SF composite under tensile action were demonstrated in Figure 12. SBS includes polystyrene molecular chain and polybutadiene molecular chain. Polystyrene could provide SBS rigidity and strength as a rigid chain, and polybutadiene provides SBS flexibility as a flexible chain. The dispersion of SF and the interaction between SF and SBS molecular chain could affect the mobility of SBS molecular chain, thus changing the mechanical properties and heat resistance of SBS composites. With the increase of SF content, the creep resistance, thermal degradation temperature and glass transition temperature of SBS composites increase, which are consistent with the above discussion results. Because SF is hydrophilic after alkali treatment (Figure 12(A)), while SBS is hydrophobic, the compatibility and interfacial adhesion between SF and SBS are poor. When SF is treated with silane coupling agent (Figure 12(B)), SF could interact well with the matrix and enhance the bonding between SF and SBS. Therefore, comparing with the SF treated by alkali, it can significantly improve the creep resistance, thermal degradation temperature and glass transition temperature of SBS composites. The fiber treated by silane coupling agent have better fiber-matrix adhesion due to the coupling reaction between KH550 and the matrix and fiber, which could be beneficial to the improvement in the properties of the composites.^44,45 The composites treated with silane have better interface properties and smaller voids than those treated with alkali. The good interface performance of SF treated by silane could increase the plastic deformation zone. When SBS composite is subjected to external force, SF could form a plastic deformation zone to absorb energy, reducing the damage of external force on SBS composite.

Figure 12.

Schematic representations for structures of SBS/PS/SF composite under tensile action (A) SF treated by alkali (B) SF treated by silane (C) motions of SBS, PS molecules and SF under the tensile stress (D) orientation of SBS, PS molecules and SF under higher tensile stress (E) pulling out of SF from matrix.

When the composites were subjected to tensile action, SBS, PS molecules and SF of composites undergo the movement, orientation, deformation and failure. The fiber pull-out process is divided into three stages: elastic deformation, interface debonding and interface friction.^46-48 When the tensile stress was applied on the composites, the fiber could be oriented and deform along the direction of the force ((Figure 12(C) and (D)), and form the plastic deformation zone to absorb energy. When the stress is greater than the interface adhesion force, the interface begins to debond. When the interface completely debonded, the fiber is gradually pulled out from the matrix under the action of interfacial friction ((Figure 12(E)).

Due to the poor interface adhesion between alkali treated SF and SBS, the conduction and dispersion of energy at the interface were poor when the sample was subjected to tensile action, easily forming interface voids. If the interface between SF treated by silane and SBS was well bonded, when the sample was subjected to tensile action, the energy was well transmitted and dispersed at the interface, and the SF would be not easily pulled out from the matrix.

Conclusions

The effects of content and surface properties of SF on the creep behavior of SBS/PS/SF composites under the action of tensile stress were investigated. The creep behavior of SBS/PS/SF composites was fitted by Findley, Burger, generalized Kelvin-Voigt and generalized Maxwell models. The generalized Kelvin-Voigt and generalized Maxwell models were suitable for describing the creep process of the material due to near 1of fitting correlation.

With the increase of SF content, the creep resistance of SBS/PS/SF composite was enhanced. The SF treated with silane had better improvement in the creep resistance of the composite. The surface of SF treated with silane coupling agent had good adhesion with the matrix, which was not easy to fall off from the matrix. With the increase of SF content, the thermal degradation temperature and T_g of the composite increased, which improved the heat resistance of the composite. The silane treated SF composite had better thermal stability and higher T_g. The thermal stability of composite material improved after the composite undergoing the tensile stress, but there was a slight decrease in T_g.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Guangxi Natural Science Foundation of China (Grant No. 2020GXNSFAA159113).

ORCID iD

Yuxin Liu

References

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