Preparation and mechanical properties of micro- and nano-sized SiC/fluoroelastomer composites

658 Vol.28 No.4 SHUAI Zhijun et al: Preparation and Mechanical Properties of Micro- and...

Preparation and Mechanical Properties of Micro- and Nano-sized SiC/Fluoroelastomer Composites

SHUAI Zhijun1, LIU Zhigang1*, WANG Donghua1, ZHOU Pan1, LI Wanyou1, QIAO Yingjie2, LIU Ruiliang2, ZHOU Shi2

(1.College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, China; 2.College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China)

Abstract: Micro- and nano-sized SiC/fl uoroelastomer (FKM) composites were prepared by a mechanical mixing method. These composites were fi rst characterized by a rotorless rheometer. Then the effects of micro- and nano-sized SiC on hardness, static and dynamic mechanical properties of the composites were investigated. The increasing amount of the SiC fi ller increased the curing effi ciency of the biphenyl curing system, which was evident from the rheometric properties of the resulting composites. The tensile properties of composite increased with the increasing of micro- and nano-sized SiC content. When the micro- and nano-sized SiC content was higher than 20 phr, the composites showed almost unchanged tensile properties. The increasing of the tensile property was mainly attributed to the well dispersed micro- and nano-sized SiC particles characterized by SEM images. Compared to pure FKM, the composites exhibited a higher glass transition temperature and lower tan δ peak value.

Key words: fl uoroelastomer (FKM); micro- and nano-sized SiC; composites; mechanical properties

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2013(Received: Dec. 12, 2012; Accepted: Jan. 8, 2013)

SHUAI Zhijun(率志君): Lecturer: Ph D Candidate; E-mail: szj9831223 @yahoo.com.cn

*Correspondent author: LIU Zhigang(刘志刚):Prof.;E-mail: [email protected]

Funded by the National Natural Science Foundation of China (No.50979016)

DOI 10.1007/s11595-013-0747-9

1 Introduction

Elastomeric materials are usually reinforced with carbon black or silica, and well dispersed fillers in rubbers could obtain beneficial mechanical and physical properties[1]. Although carbon black is effective reinforcing fi ller for rubber, the carbon black reinforced rubber is always black in color, which limits its applications in medical, sports and domestic products[2]. In recent years, common or nano-sized clay mineral and carbon nano-tubes/polymer composites have attracted the interest of researchers around the world in both industry and science[3-5]. However, introducing better linkages between the nanoparticles and the polymer-matrix is still a challenge for specific composite fabrication. Thus, surface functionalization

of nanoparticles with a surfactant or a coupling agent is important not only to stabilize the nanoparticles during processing but also to render them compatible with the polymer-matrix[6].

Fluoroelastomers (FKM) have been widely used as high performance sealing materials in the car and aerospace technology, petrochemical industry, semiconductor manufacturing, nuclear plants and so on, due to their good thermal and fluid resistance characteristics[7-9]. However, because of the fluorine rubber chemical structural characteristics, there does not exist chemical reaction between the fluorine rubber matrix and the vast majority of filler, which result in a lower the interfacial adhesion strength between the fluorine rubber are now commonly used filler. Moreover, in the process of fluorine rubber strengthening, there exist sticky mold, hot tearing, poor filling flow and so on[10]. Hence, choosing a suitable fi ller of the fl uorine rubber reinforcement has been the diffi culty of the most of the researchers.

The interest in ceramic particle reinforced composites has begun a decade ago, but until recently, ceramic particles/polymer composites become attracted more and more researcher’s interesting[11, 12], which

Journal of Wuhan University of Technology-Mater. Sci. Ed. Aug.2013 659

are mainly concentrated on a polymer reinforced with highly thermally conductive but electrically insulating fillers such as aluminum nitride (AlN), boron nitride (BN), silicon carbide (SiC), and alumina (Al2O3). As a well known ceramic material, silicon carbide (SiC) has unique physical properties such as superior resistance to chemicals and high-temperature, high-electron mobility, good thermal conductivity and excellent mechanical properties, which permit possible various applications in some harsh environments[13]. Recently, SiC particles as the reinforcement phase for polymers have become attracted some researcher’ interesting[5,14,15].

Although SiC have been widely used with different kinds of composites, very little work has been done on incorporating the micro- and nano-sized SiC powder in rubber, the studies about the microstructure and properties of the FKM fi lled with different content of micro-and nano-sized SiC particles are even more less[14,15]. The present investigation was aimed at developing micro- and nano-sized SiC/fl uoroelastomer (FKM) composites using silane modifi ed SiC ceramic particles. The micro- and nano-sized SiC/FKM composites were prepared by a mechanical mixing process and characterized by a rotorless rheometer and scanning electron microscope (SEM). The effects of the modifi ed micro- and nano-sized SiC particles on the hardness, static and dynamic mechanical properties of FKM were studied.

2 Experimental

2.1 Materials

Fluoroelastomer gum was purchased from Shanghai 3F New Materials Co., Ltd. (China). Other materials were commercial products. The micro- and

nano-sized SiC powders were purchased from Henan Xinhai silicon carbide abrasive Co., Ltd. (China). Before adding, the powders were modified by silane coupling agent KH-550 (Beijing shenda fi ne chemical Co., Ltd. China). The surface morphology of the powder is shown in Fig.1. It can be seen that the original SiC particles are shown in the irregular shape of fl ake, block, etc, and the massive particles and fi ne particles are easy to form larger aggregates. However, after being modifi ed by KH-550, the SiC powders are in shape and no obvious agglomeration. 2.2 Preparation of the composites

Micro-and nano-sized SiC/FKM composites were prepared in an open two-roll mill. The modifi ed micro- and nano-sized SiC powder was mixed with the FKM gum. Then, the vulcanizing agents were added.

Recipes of micro- and nano-sized SiC/FKM composites: FKM gum: 100 parts per hundreds in volume (phr); MgO: 3 phr; Ca(OH)2: 6 phr; BPP (Benzyltriphenylphosphonium Chloride): 0.5 phr; Biphenyl-AF: 2 phr. The amounts of the modified micro- and nano-sized SiC powder added to FKM gum are shown in Table 1.2.3 Characterization

For the determination of the vulcanization behaviour, a rotorless rheometer (MDR-100E, Liyuan Chemical Equipment Co., Ltd., China) was used. Experiments were carried out at 170 ℃ for 30 min; the moving die frequency and oscillation amplitude were 1.66 Hz and ±0.5o, respectively. The sample volume used for each testing was from 3 to 5 cm3. All samples were conditioned at room temperature before tested.

The composite morphology and the dispersion of the micro- and nano-sized SiC were observed by scanning electron microscopy (SEM, Hitachi S-4800) at 15 kV after gold coating (Fine Coat Jeol Ion Sputter JFC-1100) the cryogenically fractured tensile specimens.

Hardness was determined by a Shore A durometer (LX-A) according to ASTM D-2240 at 20 ℃. Five determinations of hardness at fi ve different positions on the specimen were taken and averaged.

The mechanical properties were measured on an electronic universal testing machine (Instron 4505, Instron Company, America) at a crosshead speed of 500


mm/min and room temperature. An average of three test results was chosen for analysis.

Dynamic mechanical thermal analysis (DMTA) spectra was determined using a dynamic mechanical analyzer (Q 800, TA Company, America) in the temperature range from 50 ℃ to 100 ℃ at a heating rate of 3 ℃/min. The samples were analyzed in tension mode at a constant frequency of 1 Hz. The loss factor (tan δ) was measured variation in the storage modulus (E′), loss modulus (E′′) and loss tangent (tan δ) as a function of temperature was measured and chosen for all the samples under identical conditions. The temperature corresponding to the peak in tanδ vs temperature plot was taken as the glass-rubber transition temperature (Tg).

3 Results and discussion

3.1 Rheological characterization

Fig.2 gives the rheometric curves of pure FKM and SiC/FKM composites and the rheometric parameters are collected in Table 2. It is evident from Fig.2 that an excellent cross-linking of the fl uoroelastomers is taking place for all the composites. The maximum rheometric torque (Fmax) of the composites increases with the increasing of SiC loading (Table 2, Fig.2). The previous study showed that the Fmax value mainly depended on the extent of cross-linking in the polymer matrix[16]. Then, increase of Fmax indicates that incorporation of micro- and nano-sized SiC enhances the crosslink density of the rubber matrix.

Moreover, with the loading of SiC, the values of T90 (optimum curing time, the time required to reach 90% crosslinking) and T2 (scorch time) first slightly increase and then decrease until up to 25 phr (Table 2). The high value of cure time demonstrates the clogging effect on the curing reaction during the course of vulcanization at elevated temperature[17]. The addition of micro- and nano-sized SiC also enhances the cure rate index of the composites except FKM-25, that is to say, the curing activities of the composites increase with the content of SiC. Such an activation effect was reported for some other rubber composites[18]. So, under the present test conditions, the addition of SiC powder lower than 25 phr could accelerate the vulcanization process, and the micro- and nano-sized SiC particles act as a vulcanizate accelerator.3.2 Static mechanical properties

The hardness and tensile properties of the pure FKM and SiC/FKM composites with different contents of micro- and nano-sized SiC powders are summarized


in Table 3. It can be obviously seen that the hardness of the composites increases with the loading of the SiC fi ller. The high hardness of the composites could be due to the dispersion strengthening of the micro- and nano-sized SiC particles.

Fig.3 shows the typical tensile stress-strain curves of the pure FKM and composites with different contents of SiC powders. Combined with the data shown in Table 3, it can be seen that the addition of micro- and nano-sized SiC particles obviously increase the strength and the modulus together with an enhanced toughness.

As compared with the pure FKM, the addition of 10 phr SiC particles increases the Young’s modulus and strength by a factor of 95 % and 34 %, respectively, though the elongation is lost about 13%. For the composites reinforced with 20 phr SiC particles, there is an increase of about 1.45 times and 3.2 times in the modulus and strength as compared with the pure FKM. When the SiC content is higher than 20 phr (such as 25 phr), the tensile properties of the composites do not increase significantly with the SiC content. The improved mechanical properties after the addition of micro- and nano-sized SiC could be mainly contributed to the following reasons[10]: higher stress bearing capability and effi ciency of dispersed micro- and nano-sized SiC with high aspect ratio; stronger interactions between modifi ed micro- and nano-sized SiC and FKM chains associated with a larger contact surface.3.3 Tensile fracture surfaces of the composite

The tensile fracture surfaces of the composite samples with different particle loading were studied by SEM, as shown in Fig.4. It can be seen from Fig.4(a) that the tensile fracture surface is smooth. The image with high-magnifi cation inserted in Fig.4(a) of the pure FKM show some void/holes in the FKM matrix. This

could be due to some large particles peeling from the FKM matrix.

Figs.4(b)-4(d) show the tensile fracture surfaces of the composites with 5%, 20% and 25% micro- and nano-sized SiC. It can be seen that the roughness of the SiC/FKM composite increases with the loading of micro-and nano-sized SiC. There is no obvious agglomerated particles observed in the fracture surface, which indicates that the fracture is not due to the agglomerated particles, which often serves as microparticle like defects within poor composites and has a deleterious effect on the mechanical properties[15]. However, there is no obvious void/holes shown in the images with high-magnification inserted in Figs.4(b) -4(d), which indicates that there is a strong chemical interaction between the micro- and nano-sized particles and FKM matrix.

The strong interfacial interactions between the nanoparticles and the FKM matrix thus have an important effect on the effective transfer of the local stress. The extremely higher specific surface area of nanoscale particles together with the strong interfacial chemical bonding between the FKM matrix and the reinforcing nanoparticles effectively facilitate the local stress transfer into the tougher silicon carbide nanoparticles[15]. In addition, according to Pasbakhsh et al[19, 20], a composite material with a rougher fracture surface and more matrix tearing required higher energy to failure, which contributes to the higher tensile strength and Eb. Thus, a much higher tensile strength could be obtained compared with the cured pure FKM.3.4 Dynamic mechanical properties of the

composites


DMA spectra in dynamic scans could be used to analyze the storage modulus, loss modulus, loss factor (tan δ), glass transition temperature (Tg) and other minor intensity relaxation phenomena. In the present study, the DMA curves of vulcanized composites with different content of SiC are shown in Figs.5-7.

Fig.5 shows the effect of SiC loading on the storage modulus of the composites. The storage modulus of SiC/FKM composites increases obviously after addition of micro-and nano-sized SiC, which gives a clear indication of the increasing stiffness of composite. That is because the small particle size, higher specifi c surface area of the modifi ed micro- and nano-sized particles improve the interaction between fi ller and FKM matrix.

Fig.6 and Fig.7 displays the effect of SiC addition on the loss modulus, Tg and tan δ of the composites. From Fig. 7, it can be seen that Tg of pure FKM is 4.57 ℃, and increases to 5.44 ℃ for the composite with 20 phr SiC. This phenomenon is attributed to the decreased mobility of rubber molecules in the presence of the modified micro- and nano-sized SiC. In the

rubbery region (35 ℃-90 ℃), the amplitude of the loss factor of the composite is higher than that of pure FKM, indicating a stronger filler network and larger interfacial hysteresis confi nement of rubber molecules by modifi ed micro- and nano-sized SiC particles[21].

In addition, tan δ can be related to the elasticity and damping properties of composite materials. The highly elastic composite materials usually show a lower tan δ and damping properties[20, 22]. In the present test condition, a decrease of the tan δ value at peak indicates decreasing damping properties and an increasing of the elasticity of the composite material.

4 Conclusions

Micro- and nano-sized SiC/FKM composites were prepared by mechanical mixing method. Increasing the amount of the SiC fi ller lower than 15 phr can increase the curing efficiency of the biphenyl curing system. The tensile properties of composite increased with the increasing of the modifi ed micro- and nano-sized SiC content, when the micro- and nano-sized SiC content was higher than 20 phr, the composites showed almost unchanged tensile properties. Compared to pure FKM, the composites exhibited a higher glass transition temperature and lower tan δ peak value.

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Preparation and mechanical properties of micro- and nano-sized SiC/fluoroelastomer composites