Physical, mechanical and morphological properties of polymer composites manufactured from carbon nanotubes and wood flour

Composites: Part B 44 (2013) 750–755

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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb

Physical, mechanical and morphological properties of polymer composites manufactured from carbon nanotubes and wood flour Hamed Younesi Kordkheili a,⇑, Mohammad Farsi a, Zahra Rezazadeh b a b

Department of Wood and Paper Science and Technology, Sari Branch, Islamic Azad University, P.O. Box 48161-19318, Sari, Iran Department of Wood and Paper Science and Technology, Sari Agricultural Sciences and Natural Resources University, Iran

a r t i c l e

i n f o

Article history: Received 31 January 2012 Received in revised form 14 March 2012 Accepted 13 April 2012 Available online 21 April 2012 Keywords: A. Nanocomposites C. SWCTN D. MAPE B. Physical properties B. Mechanical properties

a b s t r a c t The objective of this investigation was to evaluate physical, mechanical and morphological properties of experimental polymer type panels made from single-wall carbon nanotube (SWCNT) and wood flour. The composites with different SWCNTs (0, 1, 2, 3 phc) and maleic anhydride grafted polyethylene (MAPE) (0 and 3 phc) contents were mixed by melt compounding in an internal mixer and then the composites manufactured by injection molding method. The mass ratio of the wood flour to LDPE was 50/50 (w/ w) in all compounds. Water absorption, thickness swelling, bending characteristics, impact strength and morphological properties of the manufactured composites were evaluated. Based on the findings in this work the water absorption and thickness swelling of the nanocomposites decreased with increasing with amount of the SWCNTs (from 1 to 3 phc) and MAPE (3 phc) in the panels. The mechanical properties of LDPE/wood-flour composites could be significantly enhanced with increased percentage of MAPE and SWCNTs content. Panels having 2 phc SWCNTs and 3 phc MAPE exhibited the highest impact strength value. Also Scanning Electron Microscope (SEM) micrographs showed that carbon nanotubes can fill the voids of wood plastic composites as well as addition of MAPE and SWCNTs enhanced interaction between the components. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, wood plastic composites (WPCs) are becoming more and more commonplace by the development of new production techniques and discovery of new modification methods. Compared to the traditional synthetic composites (such as carbon or glass fiber–polymer composites), WPCs present lower density, lower cost and they are biodegradable. However, WPCs exhibited weaker physical (higher water absorption and thickness swelling), mechanical (less flexural and tensile strength) as well as thermal properties compared with traditional synthetic composites. In recent years, there have been considerable efforts to decrease defects and develop natural fiber-reinforced polymer based composites for production of affordable structural units [1,2]. On the other hand, using of nano-materials such as nanoclay and carbon nanotubes is one of the newest methods to overcome negative effect of various composites. These improvements include high moduli, increased tensile strength and thermal stability, decrease in water absorbance and improve flammability properties [3,4]. So far several researchers focused on effect of nanoclay as nanofiller on WPCs [3,5] whereas about the influence of other nanofillers such ⇑ Corresponding author. E-mail addresses: [email protected] (H.Y. Kordkheili), [email protected] iausari.ac.ir (M. Farsi), [email protected] (Z. Rezazadeh). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.04.023

as carbon nanotube (CNT) on various properties of WPCs is not sufficient information. Today, because of CNTs unique functional properties such as high mechanical resistance, high water and chemical resistance, high electrical and thermal conductivity, they are widely being used as reinforcement in polymer, ceramic and cement based composites [4]. CNTs have two kinds, single cylindrical wall (SWCNTs) and multiple walls (MWCNTs). SWCNTs can be considered as simply a graphite-type sheet folded into a cylinder (Fig. 1), that has a large aspect ratio, often of the order of 1000– 25,00,000 [6]. Strengths of continuous SWCNTs are reported to be 100 times stronger than steel [7]. Results from researches by Loos et al. [8], Noguchi et al. [9], and Younesi et al. [4] show that adding CNTs as reinforcement to epoxy, aluminum and cement based nanocomposites increases their physical and mechanical properties [8,9,4]. Particularly, the mechanical properties of polymer/CNT composites as functions of CNT type, content, and processing parameters have been evaluated extensively [10,11]. Manchado et al. [12], indicate that incorporation of SWCNTs to plastic matrix increased crystallization of polypropylene. There are few researches which was introduced application of CNT to WPCs. Tavasoli Farsheh et al. [13] reported that by the addition of MWCNTs to rigid PVC/wood flour composite foams increased physical and mechanical properties of the composites. Currently there is no information on effect of single wall carbon nanotubes (SWCNTs) on physical and mechanical properties of polymer based panels

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2.2. Composite preparation Before preparation of the composites, SWCNTs was mixed with acetone in an ultrasonic generator to make them uniformly dispersed suspension so that the size of aggregated SWCNTs was minimized. After the sonication was carried out for 4 h, the acetone was allowed to evaporate. Then LDPE, oven dried wood flour and SWCNTs were weighed and bagged according to formulations given in Table 1. The mixing was carried out at 180 °C and 40 RPM for 10 min by a HAAKE internal mixer (HBI System 90, USA). The compounded materials were then ground using a pilot scale grinder (WIESER, WGLS 200/200 Model). From the compounds which had been granulated, specimens were injection molded by injection molder (Imen machine, Iran) at molding temperature of 180 °C, and the injection pressure was 3 MPa. The specimens were stored under controlled conditions (50% relative humidity and 23 °C) for at least 40 h prior to testing.

Fig. 1. Construction of single wall carbon nanotubes.

manufactured from SWCNTs and wood flour. Therefore, the objective of this study was investigate effect of single-wall carbon nanotubes as well as coupling agent (MAPE) used in experimental low density polyethylene (LDPE)-wood flour composites on their physical, mechanical and morphological properties.

2. Experimental 2.1. Materials Low Density Polyethylene (LDPE), (MFI = 0.51 g/10 min, density = 0.91 g/cm3) was supplied by Bandar Imam Petrochemical Company, Iran. Wood-flour was obtained from sawdust of Fagus orientalis was used as natural filler. The particle size of wood flour was 80 meshes. MAPE (MFI = 0.4 gr/10 min) provided by Kimia Javid Sepahan Company with trade name of KJS 111 was used as coupling agent. SWCNTs (outer diameter: 1–2 nm, length: 10 lm) were purchased from Research Institute of Petroleum Industry (RIPI), Iran. The SWNTs were prepared using a chemical vapor deposition (CVD) process, via methane as a carbon source, with a cobalt and molybdenum catalyst system and reaction temperature in the range 800–1000 °C. Purification of SWCNTs was performed by HCl and HNO3, respectively. The SWCNTs were washed out several times with deionized water until the pH value of the solution became neutral. The samples were then dried in oven. Raman spectra of the used SWCNTs are presented in Fig. 2.

raman intensity

1200

2.3.1. Water absorption and thickness swelling Water absorption and thickness swelling tests of the nanocomposites were performed according to ASTM D-7031–04 standard. Five specimens from each combination were taken and dried in an oven for 24 h at 100 ± 3 °C. The weight and thickness of dried specimens were measured at an accuracy of 0.001 g and 0.001 mm, respectively. The specimens were then immersed in distilled water for one week and kept at a temperature of 22 ± 2 °C. Weight and thicknesses of the specimens were measured after excessive water was removed from their surface. The value of the water absorption in percentage was calculated using the following equation:

WAðtÞ ¼

WðtÞ  W 0  100 W0

ð1Þ

where WA(t) is the water absorption (%) at time t, W0 is the oven dried weight and W (t) is the weight of specimen at a given immersion time t. Also the value of the thickness swelling in percentage was calculated using the following equation:

TSðtÞ ¼

TðtÞ  T 0  100 T0

ð2Þ

where TS(t) is the thickness swelling (%) at time t, T0 is the initial thickness of specimens, and T(t) is the thickness at time t.

2.3.2. Mechanical tests The flexural tests were measured according to the ASTM D790–03, using an Instron machine (Model 1186, England), The tests were performed at crosshead speeds of 5 mm/min. A Zwick impact tester (Model 5102, Germany) was used for the un-notched Izod impact test according to ASTM D256 standard. Five replications were tested for each treatment in both flexural and impact strength measurements.

Raman analysis

200

2.3. Measurements

2200

raman shift (cm-1) Fig. 2. Raman spectra of the used SWCNTs.

2.3.3. Scanning Electron Microscopy (SEM) The morphology of the composites was examined using a scanning electron microscope (XL 30) supplied by Philips Company Limited. The fracture surfaces of the specimens after impact test were sputter-coated with gold before analysis. All images were taken at an accelerating voltage of 17 kV.

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Table 1 Composition of evaluated formulations.

*

Number

Composite formula

LDPE (wt.%)

Wood flour (wt.%)

SWCNT (phc*)

Coupling agent (phc)

1 2 3 4 5 6 7 8

50 LDPE/50 WF/0 NT/0 M 50 LDPE/50 WF/0 NT/3 M 50 LDPE/50 WF/1 NT/0 M 50 LDPE/50 WF/1 NT/3 M 50 LDPE/50 WF/2 NT/0 M 50 LDPE/50 WF/2 NT/3 M 50 LDPE/50 WF/3 NT/0 M 50 LDPE/50 WF/3 NT/3 M

50 50 50 50 50 50 50 50

50 50 50 50 50 50 50 50

0 0 1 1 2 2 3 3

0 3 0 3 0 3 0 3

Per hundred compound (the weights of SWCNT and coupling agent were calculated based on 100% weight of compound).

10 0% MAPE

16

3% MAPE

12 8 4 0

control

1

2

Thickness swelling (%)

water absorption (%)

20

3

0% MAPE

8

3% MAPE

6 4 2 0

control

SWCNT (phc)

1

2

3

SWCNT (phc) Fig. 3. Effect of SWCNTs and MAPE content on water absorption of WPCs. Fig. 4. Effect of SWCNTs and MAPE content on thickness swelling of WPCs.

3. Results and discussion

Figs. 3 and 4 show the water absorption and thickness swelling content of the composites after one week immersion in distilled water, respectively. Because of constant wood flour content (50 wt.%) in all compounds, the different water absorption and thickness swelling values can be attributed to the role of MAPE and SWCNs. It can be seen that the composite without SWCNTs and MAPE exhibited the higher water absorption and thickness swelling values rather than those containing them. In constant level of SWCNTs content, the composites with 3 phc MAPE exhibited the least water absorption and thickness swelling values. This could be related to better adhesion between matrix and cellulosic materials by adding MAPE which caused decrease in the velocity of the diffusion processes (due to the existence of fewer gaps in the interfacial region and blocking hydroxyl groups by the coupling effect). Also chemically, coupling agent can form ester bonds between the anhydride carbonyl groups of MAPE and hydroxyl groups of the wood flours [14]. This hypothesis is confirmed by previous studies [15] that show anhydride moieties of functionalized polyolefin coupling agents entered into an esterification reaction with the surface hydroxyl groups of wood. Upon esterification, the exposed polyolefin chains diffuse into the polymer matrix phase and entangle with polymer chains during manufacturing of the composites [16]. Figs. 3 and 4 also indicated that in constant level of coupling agent, the composites containing SWCNs exhibited less water absorption and thickness swelling contents as compared with those made without them. According to Das et al. [14] results, initially, water saturates the cell wall of the fiber, and next water occupies void spaces. As the composite voids were filled with SWCNs, the penetration of water by the capillary action into the deeper parts of composite was prevented. This hypothesis confirmed by SEM photomicrograph (Fig. 8). Another reason for less water absorption and thickness swelling could be explained by the hydrophilic nature of the CNTs surface, which tends to immobilize some of the moisture, which inhibits the water permeation

in the polymer matrix. The water absorption test results showed that the composite having 3 phc MAPE and 3 phc SWCNTs exhibited the least water absorption and thickness swelling contents. Also Tavasoli Farsheh et al. [13] indicated that by the addition of MWCNTs to rigid PVC/wood flour composite foams decrease water absorption and thickness swelling of foamed samples. 3.2. Flexural behavior The flexural modulus and strength of the composites containing different contents of SWCNTs and MAPE are presented in Figs. 5 and 6, respectively. Fig. 4 indicated that addition of SWCNTs and MAPE increase flexural modulus of WPCs. As shown, maximum flexural modulus of nanocomposite was 4500 MPa for nanocomposites with 3 phc SWCNTs and 3 phc MAPE, while minimum flexural strength is approximately 1400 MPa for control samples (without MAPE and CNTs). In absence of MAPE flexural modulus of the samples with 1, 2 and 3 phc SWCNTs were 81, 106 and

5000 0% MAPE

Flexural modulus (MPa)

3.1. Water absorption and thickness swelling

4000

3% MAPE

3000

2000

1000

0

control

1

2

3

SWCNT (phc) Fig. 5. Flexural modulus of the LDPE/wood composites as a function of SWCNTs and MAPE.

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fracture toughness, hardness and strength in polymer matrices can be developed by adding carbon nanotubes in the samples [19]. Tavasoli Farsheh et al. [13] reported that compared to pure WPCs, addition of CNTs resulted in an increase of mechanical properties.

160

0% MAPE

140

3% MAPE

120 100

3.3. Un-notched impact strength

80 60 40 20 0

control

1

2

3

SWCNT (phc) Fig. 6. Flexural strength of the LDPE/wood composites as a function of SWCNTs and MAPE.

149% higher than control samples, respectively. Panels made with 3 phc MAPE with 1, 2 and 3 phc SWCNTs had flexural modulus of 59, 111 and 145% higher than the panels made without SWCNTs. The flexural modulus in composites is mainly function of the modulus of individual component [17]. Flexural modulus of SWCNTs (1TPa) was considerably higher than wood flour and LDPE, respectively. Hence it was expected that the composites with SWCNTs exhibited the higher flexural modulus value. In addition to, increased flexural properties for the composites with carbon nanotube can be attributed high aspect ratio of SWCNTs (typically higher than 1000:1 and as high as 2500,000:1) that transfer stress from polymer to the carbon nanotube [6]. Also Fig. 5 indicate that compared to composites without MAPE, in the presence of MAPE, flexural modulus was increased. So far several researches reported positive effect of compatibilizer on flexural modulus of WPCs [17,18]. Also the positive effect of SWCNTs and MAPE on flexural strength can be observed in Fig. 6. Greater flexural strength was achieved in the composites when carbon nanotubes were used in the manufacture of the composites increase. The strength of the composites depended on the properties of constituents and the interfacial interaction [4,17]. One of the most important parameters in fabricating carbon nanotube composites is tube dispersion in the matrix. Tube aggregation is harmful to physical and mechanical properties of the resultant nanocomposites. Using of a solvent such as acetone is the best method to prevent the nanotubes from aggregating together and improve good dispersion of CNTs in nanocomposites. This can be explained by the fact that acetone, as a solvent, dilutes the polymer and reduces its viscosity. Reducing the viscosity leads to an enhanced efficiency of dispersion with tip sonication. Because of carbon nanotube size, aspect ratios and high mechanical properties, SWCNTs can be well distributed in the composites and improve adhesion between the elements. SEM results of such type of samples was evaluated by Gojny et al. [10] and it was found that a strong bonding between the polymer and the carbon nanotubes. Also the results indicate that adding of 3 phc MAPE enhances the interface adhesion between wood flour and low density polyethylene. Compatibilizers can improve encapsulation of wood flour by the plastic, which consequently results in higher flexural strength. The samples containing 3 phc SWCNTs and 3 phc MAPE showed the highest flexural strength (about 156 MPa) among the studied composites. SEM results of the studied nanocomposites showed that MAPE and SWCNTs improve the interaction between the elements and the gaps in WPCs can fill with MAPE and SWCNTs (Fig. 8). Li et al. [19] indicated that carbon nanotubes had positive effect on flexural modulus and strength of the carbon nanotube reinforced polymer composites. Also several researchers showed significant improvements in

Fig. 7 shows the Izod impact strengths of the composites made with different content of SWCNTs and MAPE. In general, the energy required crack propagation was measured with un-notched impact strength. The impact strength test results show that in the absence of coupling agent, impact strength of composites containing 1, 2 and 3 phc of SWCNTs are respectively 74, 111 and 94% higher than control samples. In presence of 3 phc MAPE, impact strength of the nanocomposites with 1, 2 and 3 phc SWCNTs respectively 67, 93 and 71% is higher than control samples. These results demonstrated that adding 2 phc SWCNTs increase impact strengths of the composites. CNTs with bridge linkage mechanism prevent the spread of cracks and increase the impact strength of composites [15]. Fig. 7 generally showed that composites containing 3 phc MAPE exhibited more impact strength compared to samples without it. Impact strength show the strength of material against breakage and start cracking in the weakest point of the composite, which is the connecting point between lingocellulose material and polymer. Increasing impact strength of composites with addition of MAPE, is done probably by the improvement of fiber–polymer connection level that cause tension concentration in WPC and as a result cause an increase in impact strength. Reduction of impact strength with increasing SWCNTs from 2 to 3% can be related to increasing the probability of SWCNTs agglomeration that creates regions of stress concentrations that require less energy to elongate the crack propagation [20]. Generally the result of impact test showed that WPCs which contain 3 phc MAPE and 2 phc SWCNT have the more impact strength value. 3.4. Morphology characteristics SEM is an effective media for the morphological investigations of the composites. Through SEM study, the distribution and compatibility between the fillers and the matrix could be observed. Fig. 8a corresponds to WPC without SWCNTs and MAPE. As can be seen, there is some evidence of fiber pull out from matrix. Therefore when stress is applied it causes the fibers to be leave the matrix easily and cusses gaping holes. Fig. 8b is showing the position of nanoparticles in the composites with 1phc SWCNT and without MAPE at high magnification (30,000). It indicated that SWCNTs filled the voids of the wood plastic composites. The surface of nanocomposites containing 2 phc SWCNT and without MAPE analyzed by SEM is depicted in

2500 0% MAPE

Impact strength (J/m2)

Flexural strength (MPa)

180

2000

3% MAPE

1500 1000 500 0

control

1

2

3

SWCNT (phc) Fig. 7. Un-notched impact strength of the LDPE/wood composites as a function of SWCNTs and MAPE.

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Fig. 8. SEM photomicrographs of fractured samples of nanocomposites: 0 phc SWNCT/0 phc MAPE (a), 1 phc SWCNT/0 phc MAPE (b), 2 phc SWCNT/0 phc MAPE (c), 3 phcSWCNT/3 phc MAPE (d).

Fig. 8c. There are some cavities in WPC that can absorb water and/or reduce mechanical properties. This indicates that the level of interfacial bonding between the wood flour and LDPE in the composites without coupling agent is weak and the extent of improvement in the physical and mechanical properties is more prominent with MAPE. Fig. 8d is showing composites with 3 phc SWCNT and 3 phc MAPE. As can be seen, there is no separation of the fibers from the matrix and a very good interaction between the components can be inferred from the images. The strong adhesion that is observed at the interface has been already discussed in mechanical properties of the composites and is related to SWCN and MAPE, which encapsulated fibers in the matrix and cusses strong bonding. The significant decreasing in water absorption and thickness swelling of the blends including MAPE and SWCNTs were further supported by Fig. 8d, that when composite micro voids and the lumens of fibers were filled with SWCNT and MAPE, there is smooth and monotonous matrix without any holes and penetration of water into the deeper holes and cavities of composite is prevented.

4. Conclusions This study investigated effect of SWCNTs (as reinforcing agent) as well as MAPE (as coupling agent) on physical, mechanical and morphological properties of wood flour/LDPE composite. The physical and mechanical test results indicated that SWCNT loading and compatibilizing agent content significantly influences on properties of WPCs. Because of high water resistance nature of CNTs and filling the voids of the composites by the nanoparticles, with increasing the amount of SWCNTs up to 3 phc, water absorption and thickness swelling of the composites decreased. In addition, adding of MAPE improves physical properties of nanocomposites. High aspect ratio and large surface area of SWCNTs were causes the enhancement of flexural modulus and strength of the composites. The addition of MAPE had a positive effect on flexural properties, because it improves the interfacial bonding between the fiber and the matrix polymer. Also mechanical test results indicated that the composites containing 3 phc MAPE and 2 phc SWCNTs exhibited the highest unnotched impact strength value. Morphological study also showed that there are distinct cavities between the LDPE and wood flour, indicating poor adhesion, but a fewer pulled-out traces on the

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