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Preparation of Carbon Nanotubes/Epoxy Resin Composites by Using Hollow Glass Beads as the Carrier.

Byline: XIANG-FENG WU, YONG-KE ZHAO, DAN ZHANG, TIE-BING CHEN AND LI-YA MA

Summary: Hollow glass beads had been utilized as the carrier to assist dispersion of carbon nanotubes in epoxy resin. Hollow glass beads were firstly aminated with g-aminopropyl- triethoxysilane, sencondly reacted with carboxyl-functionalized carbon nanotubes via an amidation reaction and susequently mixed with epoxy resin and hardener. The experimental results showed that carbon nanotubes could be loaded on the surfaces of hollow glass beads and approximately a monolayer of carbon nanotubes was formed when the weight ratio of hollow glass beads to carbon nanotubes was 100:5. Moreover, the dispersity of carbon nanotubes in the matrix was improved as compared to the control samples prepared by using a conventional mixing method.

Keywords: Carbon nanotubes; Epoxy matrix composites; Hollow glass beads.

Introduction

In recent years, attentions have been paid to carbon nanotubes (CNTs) for their special structures, excellent mechanical properties and outstanding electrical conductivity [1-5]. Therefore, CNTs are widely used in polymer materials, especially in epoxy-matrix composites [6-10]. It is generally accepted that a small amount of CNTs (typically less than 5 wt%) can obviously improve the mechanical and electrical properties of epoxy resins. However, if CNTs are utilized in epoxy resin, proper dispersion has to be guaranteed, because CNTs are very easy to agglomerate and entangle together due to their nanosize and high aspect ratio [11-13]. To solve this problem, researchers have developed many good methods. The typical ones are sonication dispersion [14], vacuum-assisted resin-transfer-mold [15], electro-deposition [16], magnetic force [17], layer- by-layer technology [18] and so on.

Because hollow glass beads (HGB) possess the characters of low density, isotropy and so on, they were broadly used to fabricate syntactic foams [19-23]. The most important clue suggested to us is that using HGB as the carrier of CNTs can not only decrease the density of the composites, but also improve the dispersity of CNTs in the epoxy resin. Moreover, to our knowledge, there is no study so far on using carrier method to improve the dispersity of CNTs in the matrix . In this work, the surfaces of HGB was firstly aminated with g- aminopropyltriethoxysilane to be amino- functionalized HGB (HGB-NH2), subsequently reacted with short carboxyl-functionalized CNTs (CNTs-COOH) and then mixed with the matrix (Previous results showed that the short CNTs were more easy to be loaded on HGB than those the CNTswith relatively high aspect ratio [24]. Therefore, in this work, the short CNTs were used).

The morphology, composition, structure of the as- prepared HGB-CNTs (CNTs were loaded on HGB) and the dispersity of CNTs in the matrix were evaluated.

Results and Discussion

Morphology of HGB-CNTs

It is found that the amount of CNTs on the surfaces of HGB increased as increase of the weight ratio of HGB-NH2 to CNTs-COOH (the weight ratio of HGB-NH2 to CNTs-COOH is denoted as WHGB-NH2:WCNTs-COOH below). Clearly, when the WHGB-NH2:WCNTs-COOH was 100:1 there were few CNTs on the surfaces of HGB (Fig. 1 (a)). Moreover, there was approximately a monolayer of CNTs on HGB when the WHGB-NH2:WCNTs-COOH was 100:5 .

However, there were many CNTs aggregations on HGB when the WHGB-NH2:WCNTs-COOH continued to increase . Therefore, the best WHGB- NH2:WCNTs-COOH was 100:5. This phenomenon could be explained by the fact that -NH2 groups could react with equal molar ratio of -COOH groups. Furthermore, CNTs would easily aggregate if they were superfluous. In order to identify the composition of the as-prepared HGB-CNTs, energy dispersion X-ray spectroscopy (EDS) was used, and the experimental results showed that there was C on HGB, indicating that the particles with relatively high aspect ratio were CNTs. Moreover, there was Au which was caused by the sputtered gold, because discharge effects should be eliminated before the samples were observed. In addition, O, Na, Si and Ca were contributed by HGB.

XRD Analysis

Two typical peaks of CNTs can be observed at 2th = 25.8 deg and 42.9 deg in Fig. 3 (a). Moreover, there was a broad peak which is close to 25.8 deg in Fig.3 (b). This was contributed by the amorphous structure of HGB [25]. In addition, there was also a peak close to 42.9 deg. This was contributed by the CNTs of HGB-CNTs. The result was consistent with the results of EDS and field emission scanning electron microscopy (FE-SEM).

Morphology of Fracture Surfaces

There are obvious CNTs aggregations in the matrix. This was normal because CNTs were nano-sized and possessed high specific surfaces energy, which usually makes CNTs easily aggregated in the epoxy resin. However, in Fig. 4 (b), it shows that CNTs on the HGB were relatively even distributed (as shown as the black arrows, the part of HGB was removed). This result showed that HGB, as the carrier, could improve the dispersity of CNTs in the matrix.

Morphology of Surfaces of the Samples

The surfaces of the epoxy /HGB samples was white. However, the surfaces of the epoxy/HGB-CNTs and the control samples (Fig. 5 (b and c)) were obviously dark. The difference between b and c was that there were some "black stripes" on the surfaces of c, while that of b was more homogeneous. This might be caused by the fact that HGB were isotropy, and CNTs could be well- dispersed in the matrix when they employed HGB as the carrier. However, if the CNTs were directly added in matrix (without the initial step of loading CNTs on the HGB), they were easily aggregated. Therefore,there were "black stripes" on the surfaces of the control samples.

Experimental

HGB (trademark VS5500, density 0.38 g/cm3, size range from 15~80 (mu)m and average size 40 (mu)m, 3M Corp., USA) were first added into alcohol/water/g-aminopropyltriethoxysilane mixture (the volume ratio of the alcohol to water was 9:1 and the g-aminopropyltriethoxysilane concentration was 1 v/v%). Subsequently, HGB were collected and dried at 115 for 4 h, yielding HGB-NH2.

Secondly, a given amount of CNTs-COOH (outer diameter ~20 nm, length 50~250 nm and - COOH layer thicknes and content 2.0 wt.%, Chengdu Organic Chemicals Corp, China) were well-dispersed in N, N-dimethyl formamide (DMF, Tianjin Kewei Chemicals Corp., China) with the use of ultrasonics. Subsequently, CNTs-COOH, DMF and the as-prepared HGB-NH2 were mixed in a glass flask at 145 for 8 h (WHGB-NH2:WCNT-COOH = 100:0, 100:1, 100:3, 100:5 and 100:7). Then, HGB- CNTs were collected and washed with alcohol several times to remove impurities.

Thirdly, the as-prepared HGB-CNTs were well-mixed with bisphenol-A epoxy resin (trademark Airstone 760E, the epoxy value is 174.4, Dow Corp., USA) and hardener (trademark Airstone 766H, Dow Corp., USA) by using strong mechanical stir. The weight ratio of the epoxy resin to the hardener was 100:30, and the as-prepared HGB-CNTs loading was 30 wt.% based on the weight of matrix. Subsequently, the mixture (epoxy/HGB-CNTs composites) was degassed in a vacuum oven for 10 min and casted into a mould at 65 for curing 6 h. The control samples (epoxy/HGB/CNTs composites, the same mass ratio as compared to the epoxy/HGB-CNTs composites) were prepared by using a conventional mxing method (without the initial step of loading CNTs on the HGB), and carried out via the same mixing, degassing and curing processes..

The impact fracture surfacess of the composites were characterized by FE-SEM (Model: Hitachi S-4800, Japan) and EDS (Model: EDAX Genesis, USA). The surfaces of the samples were carried out by a camera (Model: Canon ixus 95, Japan). The FT-IR spectrums of CNTs-COOH, the as- prepared HGB-NH2 and HGB-CNTs were performed by a Fourier Transform Infrared Spectroscopy (FT-IR, Model: Shimadzu FTIR-8400S, Japan). The structural identification of CNTs-COOH and the as- prepared HGB-CNTs was characterized by X-Ray Diffraction (XRD, Model: Rigaku D/max 2500v, Japan).

Conclusions

In sum, a carrier method for assisting the dispersion of CNTs in the epoxy was presented. CNTs were initially loaded on the surfaces of HGB via the amidation reaction, and subsequently mixed with the epoxy and hardener. FT-IR showed that - NH2 groups on the HGB reacted with -COOH groups on the CNTs. FE-SEM showed that CNTs were loaded on HGB and approximately a monolayer of CNTs was formed when the WHGB-NH2 :WCNTs-COOH was 100:5. In addition, both microphotograph and macrophotograph of the samples showed that the dispersity of CNTs in the matrix was improved.

Acknowledgement

The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (No. 02490220).

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Article Details
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Author:Xiang-Feng Wu; Yong-Ke Zhao; Dan Zhang; Tie-Bing Chen; Li-Ya Ma
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Geographic Code:9CHIN
Date:Dec 31, 2012
Words:1898
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