Chinese Journal of Polymer Science Vol. 27, No. 2, (2009), 267−274

Chinese Journal of Polymer Science ©2009 World Scientific

MECHANICAL PROPERTIES OF SMC WHISKER REINFORCED HIGH DENSITY POLYETHYLENE COMPOSITES* Bao-feng Pan, Nan-ying Ning, Jun Liu, Lan-ying Bai and Qiang Fu** College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China Abstract A new type of SiO2-MgO-CaO (SMC) whisker was used to modify high density polyethylene (HDPE). The melting behavior and crystallinity were investigated by differential scanning calorimetry (DSC). The dispersion of whiskers and interfacial adhesion in the prepared HDPE/SMC whisker composites were investigated by scanning electron microscopy (SEM). The mechanical properties were evaluated by mechanical tests and dynamic mechanical analysis (DMA). DSC data indicated that the melting temperature and the crystallinity of HDPE/SMC whisker composites kept almost constant with the addition of SMC whisker. SEM results revealed that SMC whisker could be well-dispersed in HDPE matrix and had a good interfacial interaction with HDPE, even up to a high content of 30 wt%. Mechanical tests showed that the tensile strength, tensile modulus and flexural strength of HDPE were significantly improved by the addition of SMC whisker. DMA results indicated a much enhanced storage modulus, loss modulus and loss factor. By comparing the literature data from other fillerreinforced HDPE, our work suggested that SMC whisker could be one of the best reinforcing agents for the modification of HDPE. Keywords: SMC whisker; Polyethylene; Mechanical properties.

INTRODUCTION Polyethylene (PE) is the most widely used family of synthetic polymers today because of its low cost and its easy processability. However, its poor mechanical properties (such as tensile strength, tensile modulus and flexural strength) restrict its application. Fibers are considered to be effective reinforcing fillers for PE. In the past several decades, various kinds of fibers (such as natural-fiber, glass fiber, keratin feather fiber, metallic fibres et al.) have been widely used to improve the mechanical properties of PE[1−7]. Whiskers, with large length/diameter ratio, are fiber-shaped single crystals. Owing to their small diameters, whiskers are nearly free of internal defects and yield a strength close to the maximum theoretical value. In recent years, a lot of works have been done on the preparation of various polymer/inorganic whiskers (such as aluminium borate whisker and potassium titanate whisker) composites[8−16]. And it was found that whiskers could have much higher specific strength than short glass or carbon fibers and can reinforce polymers more effectively[17−19]. Therefore, whiskers are considered to be an attractive alternative to short glass or carbon fibers for reinforcing polymers. For example, Zhuang et al. observed that the tensile strength and the tensile modulus of the potassium titanate whiskers (PTW)/poly(ether ether ketone) (PEEK) composites increase with the increase of PTW content[8]. Sriupayo et al. had prepared the α-chitin whisker-reinforced PVA nanocomposite films by solution-casting technique and found that the tensile strength increased from 55.5 MPa for pure PVA film to 83.3 MPa for the nanocomposite films containing 2.96 wt% α-chitin whisker[9]. In this work, a new type of SiO2-MgO-CaO (SMC) whisker was used to modify high density polyethylene *

This work was financially supported by the Natural Science Foundation of China (No. 50533050) and the Special Funds for Major State Basic Research Projects of China (No. 2003CB615600). ** Corresponding author: Qiang Fu (傅强), E-mail:[email protected] Received January 24, 2008; Revised March 20, 2008; Accepted April 7, 2008

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(HDPE). SMC whisker was first surface treated by silico-hydride coupling agent, then melt mixed with HDPE, and the blends were injection molded into dumbbell-shaped specimens. We focus our attention on the mechanical properties of the composites. The experimental results showed that tensile strength, tensile modulus and flexural strength were significantly improved by the addition of SMC whiskers, suggesting that the SMC whisker has a very good reinforcing effect on the mechanical properties of HDPE, which can be attributed to the good dispersion of SMC whiskers and strong interfacial interaction between SMC whiskers and HDPE. EXPERIMENTAL Materials HDPE (5000S, MFI = 1.1) was supplied by Lanzhou Petrochemical Co. Ltd., China. SMC whiskers (shown in Fig. 1) were produced in our laboratory. It is a kind of light yellow crystal material with a density of 3.0 g/cm3, diameter ranging from 0.2 μm to 2 μm, and length in the range of 5−50 μm. Silico-hydride KH-500, the coupling agent for SMC whisker treatment, was supplied by Chenguang Research Institute of Chemical Industry (Chengdu, China).

Fig. 1 SEM photo of SMC whisker

Sample Preparation and Measurements SMC whisker was first treated for 10 min with 10 wt% KH-550 solution in a high-speed mixer (the rotation speed was about 1000 r/m). After that, the mixture was exposed in air for 5 h and then was dried at 80°C for 6 h. The ultimate powder was used as inorganic filler for HDPE. HDPE and SMC whiskers were melt-mixed using a TSSJ-25 twin screw extruder. The temperature of the extruder was maintained at 160°C, 180°C, 180°C, 190°C, 190°C and 180°C from hopper to die, and the screw speed was 110 r/m. Pure HDPE was also passed through extruder under the same conditions to serve as a reference. The palletized granules were dried for 20 h under 80°C and then injection molded under the temperature of 200°C. The ultimate HDPE/SMC whisker (with different modified SMC whisker content of 5 wt%−30 wt%) samples were termed as SH composites, as listed in Table 1. Differential Scanning Calorimetry (DSC) A Perkin-Elmer Pyris-1 DSC with nitrogen as purge gas was used to investigate the crystallization behavior of pure HDPE and SH composites. The non-isothermal crystallization behavior of the specimens was investigated by heating the samples (about 5 mg) to 200°C at a heating rate of 10 K/min. The melting temperatures (Tm) were determined from the maxima of the fusion peaks. The crystallinity (Xc) of HDPE in the composites was calculated by the equation Xc = ΔHi/(φi × ΔHim), where ΔHi is the enthalpy of fusion of SH composites, directly obtained by DSC, and φi is the mass fraction of HDPE in the blends. ΔHim, the enthalpy of fusion of 100% crystalline polymer is 293 J/g for HDPE[20]. Scanning Electron Microscopy (SEM) The morphologies of SMC whiskers and SH composites were examined under an acceleration voltage of 20 kV with a JEOL JSM-5900 LV for SEM experiments. Before being examined, the specimens of SH composites were cryogenically fractured parallel to the flow direction in liquid nitrogen (−170°C), then the fractured surface was coated with a thin layer of gold.

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Mechanical Properties Standard tensile tests were performed using an Instron universal tensile testing machine with a cross-head speed of 50 mm/min at room temperature (25 ± 2)°C. The width and thickness of the dumbbell shaped specimens were about 10.20 mm and 4.24 mm, respectively. The tensile strength at yield was determined according to ASTM D412-87 standard. In the flexural experiment, the test speed was 2 mm/min and the specimens were prepared according to ASTM D-790. For impact strength measurements, a notch with 45° was made by machine, and the remaining width of the specimens was 8.0 mm. The notched Izod impact strength of the specimens was tested with a VJ-40 Izod machine, according to GB/T 1834-1996 standard. The values of all the mechanical parameters were calculated as averages over 5 specimens for each composition. Dynamic Mechanical Analysis (DMA)Dynamic mechanical analysis (DMA) was performed using a DMAQ800 apparatus supplied by TA Instruments. The experiments were conducted in a single cantilever mode under nitrogen conditions at a frequency of 1 Hz. The specimens with a dimension of 9.35 mm × 9.8 mm × 4 mm were heated from −100°C to 125°C at a heating rate of 3 K/min. The storage moduli, loss moduli and tanδ were recorded as a function of temperature. RUSULTS The Dispersion of Whiskers and Interfacial Adhesion in SH Composites It is well known that the dispersion of any fillers in polymer matrix is a key factor for the reinforcement of mechanical properties of polymer composites. Good dispersion can be achieved by surface modification of the filler and appropriate processing conditions. In this work, the dispersion of SMC whiskers in HDPE matrix and their interfacial interaction with HDPE can be obtained by the SEM photos as shown in Fig. 2. One can see that the modified SMC whiskers are well-dispersed in HDPE matrix for all the composites containing 10 wt%, 20 wt% and 30 wt% SMC whiskers, as shown in Fig. 2(a), Fig. 2(b) and Fig. 2(c). This can be ascribed to the good interfacial adhesion of SMC whiskers with PE matrix. To clearly see the interfacial adhesion between whiskers and the PE matrix, the SEM photograph with high magnification is shown in Fig. 2(d). It can be observed that the interface between PE and whiskers is blurred, and no obvious cavities can be seen. One can observe clearly that the SMC whisker is attached with a layer of PE matrix. In our previous work, we have observed a hybrid shish-kebab structure in the injection molded bar of PE/SMC whisker composites, in which the SMC whisker serves as the shish and PE lamella serves as kebab[21]. This indicates that interfacial interaction

Fig. 2 SEM photos of fracture surface of SH composites in the direction parallel to flow direction: (a) 10SH, (b) 20SH, (c) 30SH and (d) 10SH (high magnification)

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between the SMC whisker and PE matrix is really good. In a word, the modified SMC whisker can be well dispersed in PE matrix and the interfacial adhesion is strong. Mechanical Properties Table 1 shows the mechanical properties of HDPE and SH composites. It can be seen that the tensile strength, tensile modulus and the flexural strength of HDPE are obviously improved by the presence of SMC whiskers, which increase with the increase of SMC whisker content. For example, the tensile strength of pure HDPE is about 24 MPa, which increases to 28.1 MPa and 39.5 for 15SH and 30SH, respectively. This result suggests that SMC whiskers have significant reinforcing effect on the mechanical properties of HDPE. It should be noted that a decreased impact strength and elongation of HDPE by adding SMC whiskers are also observed from Table 1. For comparison, the data from literature for other HDPE composites is presented in Table 2. As it can be seen, among the fillers used, including twaron fiber, fruit bunch, lignocellulosic fiber, CaCO3, HA whisker, magnesium whisker and SMC whisker, SMC whisker shows the best reinforcing efficiency for HDPE. At the filler content of about 30 wt%, the tensile strength for most HDPE composites is below 30 MPa, while the tensile strength for SMC whisker reinforced HDPE is about 40 MPa. Sample code DPE 5SH 10SH 15SH 20SH 25SH 30SH

Component SMC HDPE 0 100 5 95 10 90 15 85 20 80 25 75 30 70

Table 1. Mechanical properties of SH composites Flexural Tensile strength Tensile modulus Elongation strength (MPa) (MPa) (MPa) (%) 24.26 322 604 17.91 24.92 533 862 18.75 26.84 780 774 22.56 28.16 907 663 23.86 35.08 1297 50 25.61 38.40 1540 68 32.13 39.59 1580 57 35.21

Impact strength (kJ/m2) 15.25 6.50 5.15 4.70 4.95 5.90 6.40

One may argue that the good dispersion and strong interfacial interaction between SMC whisker and HDPE should result in an increase of impact strength, why a decreased impact strength of HDPE by adding of SMC whiskers is observed? It has been reported that the impact properties of whisker modified polymer composites vary with the different filler sorts and interfacial behavior. Whiskers could improve or reduce the impact strength of polymers depending on process conditions[22, 23]. For the acicular-shape or rod-shape whiskers treated properly by surface coupling modifiers, an improved strength and toughness is found as the tight and flexible layer in the interface of polymer with whisker is formed[24]. However, If the interfacial strength of polymer/whisker composites is very strong, the tip of crack can not develop via the whisker surface during impact fracture deformation, which may result in a reduction of toughness obviously[25]. In our system, there exists strong interface interplay in HDPE/SMC whisker composites after SMC is treated by siliane coupling agent, and a reduced chain mobility in the interface is expected. When the composites are suffered from the impact effects, the macromolecular chains of polyethylene move hardly, and the shear yield behavior of composite is quite difficult to take place for the strong interfacial interaction. Thus a reduced impact strength is observed even the interfacial interaction is strong. Table 2. Tensile properties of the neat HDPE and the HDPE/filler composites Sample Tensile strength (MPa) Tensile modulus (MPa) HDPE 24.00 320 75 wt% HDPE + 25 wt% Twaron fiber[26] 29.76 1567 70 wt% HDPE + 30 wt% fruit bunch[27] 14.00 290 70 wt% HDPE + 30 wt% lignocellulosic fiber[28] 37.90 − 70 wt% HDPE + 30 wt% CaCO3[29] 24.00 850 70 vol% HDPE + 30 vol% HA whisker[30] 28.00 − 70 wt% HDPE + 30 wt% Magnesium whisker[31] 21.50 − 70 wt% HDPE + 30 wt% SMC whisker 39.59 1580

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Dynamic Mechanical Analysis (DMA) The enhancement of mechanical properties can be further evaluated by DMA. The storage modulus, loss modulus and loss factor (tanδ) versus temperature are ploted in Fig. 3. It can be seen from Fig. 3(a) that the storage modulus is significantly enhanced by the presence of SMC whiskers, and it increases with the content of whisker increasing in the temperature range investigated. This indicates that the stiffness of SH composites is improved with the addition of SMC whiskers.

Fig. 3 (a) Storage modulus versus temperature of pure HDPE and SH composites; (b) Loss modulus versus temperature of pure HDPE and SH composites; (c) Loss factor versus temperature of pure HDPE and SH composites

It is widely accepted that the increase of the interfacial interaction between polymer matrix and filler can result in an increase of the damping force for the motion of macromolecules, which can lead to the increase of the loss modulus and the loss factor of composites. The stronger interfacial interaction, the higher the loss modulus and the tanδ of composites[32, 33]. The loss modulus and loss factor (tanδ) versus temperature are shown in Fig. 3(b) and Fig. 3(c), respectively. It can be observed that the loss modulus and the tanδ of SH composites are higher than those of pure HDPE over the entire temperature range of the study, especially for the 30SH composite. This suggests that the loss modulus and the loss factor of the composites increase with the addition of SMC whiskers. It is due to a good interfacial interaction between the SMC whiskers and HDPE matrix. In a word, the DMA results indicate that the interfacial interaction between the modified SMC whiskers and HDPE matrix is really good, and the addition of SMC whiskers can significantly reinforce HDPE composites. These results are in good agreement with the above SEM results and mechanical property results.

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DISCUSSION Analysis of the Relative Modulus and Strength Many equations and models have been used to evaluate the modulus and tensile strength of filled materials. Among the most prominent are Halpin-Tsai equations, Tsai-Pagano equations and Nicolais-Nicodemo model[34−37]. For short-fiber-reinforced oriented composites, the Halpin-Tsai equation is the most widely used, which is shown as follows: Ec E / E −1 E / E −1 Vf ) /(1 − f m Vf ) = (1 + ξ f m Em Ef / E m + ξ Ef / Em + ξ

(1)

Where Ec is the modulus of the composite, Em is the modulus of the polymer matrix, Vf is the volume fraction of fiber filler, Ef is the modulus of the reinforcing material. For SMC whisker, Ef = 280 GPa. ξ is a constant parameter accounting for the reinforcing effect of the fiber. If the short fiber has no reinforcing effect on the modulus of composites, ξ = 0; if it has very good reinforcing effect, ξ = 2(l/d) (l is the mean length of fiber, d is the mean diameter of fiber). For the SH composites, ξ = 2(l/d) = 72 indicates the best reinforcing effect that SMC whisker could achieve. Halpin-Tsai equation is used to evaluate the modulus of our SH composites and the result is shown in Fig. 4. A linear fit of the data with ξ = 35 is obtained. This result indicates that SMC whisker is a quite good reinforcing agent for the modification of HDPE.

Fig. 4 Relative tensile modulus (Ec/Em) versus Vf (%) for HDPE/whisker composites

Effects of SMC Whisker on the Mechanical Properties of HDPE It is well-known that the reinforcing effects of a kind of filler on the mechanical properties of polymer composites depend strongly on its shape, particles size, aggregate size, aspect ratio, surface characteristics and degree of dispersion and the interfacial interfacial interaction between filler and matrix[38−41]. It also depends on the change of crystallinity of a polymer matrix due to the addition of a filler. To find out the possible effect of crystallinity on the mechanical properties of HDPE/SMC whiker composites, the melting behavior and crystallinity of pure HDPE and SH composites were analyzed by DSC, and the result is presented in Fig. 5 and summarized in Table 3. Obviously, no significant change in the full width of the melting peaks was detected among the samples. One can also see that the melting temperature of all the composites is almost the same (around 129°C). The crystallinity of the SH composites is found more or less the same among the samples within the experiment error (around 50%). Therefore, these are not the reasons for the reinforcement of the mechanical properties of composites. On the other hand, from the SEM result, we can find that SMC whiskers are dispersed in HDPE matrix uniformly for all the composition prepared, and the interfacial interaction between HDPE matrix and SMC whiskers is strong, as shown in Fig. 2(d). And also the length to diameter ratio of SMC whiskers is quite large (in the range of 25 to 50). Thus the good dispersion and the strong interaction as well as

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the large aspect ratio could be the main reasons that SMC whisker has a very good reinforcing effect on the mechanical properties of HDPE.

Fig. 5 Non-isothermal melting traces of SH composites and pure HDPE at a heating rate of 10 K/min

Sample code

Table 3. Melting behavior and crystallinity of SH composites Component weight (g) Heat of fusion, Melting temperature, Tm (°C) ΔH (J/g) HDPE SMC

Crystallinity (%)

HDPE

100

0

133.2

128.8

48.8

10SH

90

10

135.1

128.8

49.5

20SH

80

20

139.9

128.8

51.3

30SH

70

30

127.9

128.8

46.9

CONCLUSIONS The mechanical properties and dynamic mechanical behavior of SMC/HDPE composites were investigated. It is shown that the tensile strength, tensile modulus and flexural strength were significantly improved by the addition of SMC whiskers. DMA results indicated that the addition of SMC whiskers can significantly reinforce the modulus of HDPE composites. SEM results revealed that SMC whiskers could be well-dispersed in HDPE matrix and had a good interfacial interaction with HDPE, even up to a high content of 30 wt%. The improvement of mechanical properties of composites can mainly due to the good dispersion of whiskers and strong interfacial interaction between SMC whiskers and HDPE matrix, as well as the large aspect ratio of SMC whiskers.

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