Materials Letters 62 (2008) 4380–4382

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Materials Letters

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Mechanical strength improvement of polypropylene threads modified by PVA/CNTcomposite coatings

Bo Zhao ⁎, Jing Wang, Zijiong Li, Ping Liu, Da Chen, Yafei ZhangNational Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education,Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, Shanghai 200030, PR China

⁎ Corresponding author. Tel.: +86 21 6293 2017 816.E-mail address: phyzhaobo@sjtu.edu.cn (B. Zhao).

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.07.037

a b s t r a c t

a r t i c l e i n f o

Article history:

Poly (vinyl alcohol)/carbon Received 29 July 2007Accepted 24 July 2008Available online 26 July 2008

Keywords:Carbon nanotubeComposite materialsPolymers

nanotube (PVA/CNT) composite was coated on the surface of polypropylenethread for toughness enhancement. Multiwall carbon nanotubes (MWNTs) were treated in acid and alkali toget water-soluble nanotubes, and then embedded into poly (vinyl alcohol) (PVA) matrix, resulting inpolymer–carbon composite with homogeneous nanotube dispersion. The stress–strain measurements showthat the tensile strength and toughness of the PVA/CNT coated thread increased by 117% and 560%,respectively. These results are supportive of good interfacial bonding between the carbon nanotubes (CNTs)and polymer matrix.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) have received much attention because oftheir unique mechanical, thermal and electrical properties since theirdiscovery by Iijima in 1991 [1]. Theoretical and experimental studieshave shown that CNTs possess a Young's modulus of 1 TPa and atensile strength of 63 GPa [2,3]. As a result, the CNT-based composites,which may be one of the most promising applications, have beenintensively studied using different matrix materials, polymers [4,5],ceramics [6,7] and metals [8]. In the fabrication of high-qualitypolymer–carbon nanocomposites with predictable and optimalperformance, a homogeneous dispersion of the CNTs in the polymermatrix is required. Some impressive results of strength and toughnessincrease have been obtained for poly (vinyl alcohol) (PVA) compositefibers, where great care was taken to achieve a good dispersion [9].

In this paper, we report a simple method for preparing high tensilestrength and toughness thread. The thread is obtained by coating thepolypropylene thread with the PVA/CNTs composite solutions. Highsoluble CNTs are used to facilitate their dispersion in the PVA matrix,improving the homogeneity of the composite solution. The solubilityof the functionalized CNTs and the mechanical properties of thecoated thread are characterized and discussed.

2. Experimental

The polymer matrix PVA is 95% hydrolyzed with an averagemolecular weight 60,000. The multiwall carbon nanotubes (MWNTs)produced by chemical vapor deposition have a specific diameter of 10–

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30 nm and the purity is greater than 95%. The CNTs were firstly treatedwith mixed acid and then sodium hydroxide (NaOH). In a typicalexperiment, 1 g of pristine CNT was added to 60 ml of mixed acid(HNO3:H2SO4 H2SO4=3:1) in a round-bottomed flask, and refluxed at90 °C for 70 min. After centrifugation, the mixture was washed withdeionizedwater until the PH value reached neutral. The collectionwasdried and designated as CNT-COOH. 0.5 g of CNT-COOH was added in10 mol/l NaOH solution and stirred at 90 °C for 5 h. The mixture wasthen filtered through a polyvinylidene fluoride (PVDF) filter with thepore size of 1.2 μm. The collectionwaswashedwith deionizedwater toremove all of the NaOH residues. Then, the collection was dried undervacuum at 60 °C and designated as CNT-COONa.

PVA/CNT solutions were prepared using pristine and treated CNTs.In a typical procedure, 15 g of PVA and 0.5 g of CNTs were dissolved in100 ml of deionized water. The mixture was ultrasonicated for 15 minand stirred at 90 °C for 6 h to get a homogenous solution. In order tostudy the effects of CNTs on the homogenization of PVA/CNTcomposites, some of the as-prepared PVA/CNT solutions were pouredinto a Teflon mould, where the water evaporated to leave a free-standing film. The enhanced tensile strength threads were obtainedby coating the polypropylene threads with the PVA/CNT solutions.Prior to the coating, the polypropylene threads should be boiled inwater to enhance the surfacewettability. Then, the threads were driedfor testing. Scanning Electron Microscope (SEM, FEI SIRION 200)measurements were conducted to investigate the morphology of thePVA/CNT composite. Tensile tests were carried out on a universalmaterial testing system (CMT 6404).

3. Results and discussion

To achieve full reinforcing potential of CNTs, they must be well dispersed andexhibit good interfacial strength with the polymer matrix. The solubility

Fig.1. The photographs of the CNT-COOH (a) and CNT-COONa (b) solutions undisturbed for7 days.

Fig. 2. SEM images of fracture surface of PVA/CNT composite films with CNT-COOH(a) and CNT-COONa (b). The inset is the magnified image.

Fig. 3. Typical stress–strain curves for original polypropylene (a) and polypropylenecoated with PVA (b), PVA/CNT-COOH (c) and PVA/CNT-COONa (d).

4381B. Zhao et al. / Materials Letters 62 (2008) 4380–4382

measurements were carried out by dissolving 0.002 g of pristine CNT, CNT-COOHand CNT-COONa in 10 ml of deionized water, respectively. Then, keep thesolutions undisturbed for 7 days for measurements. The photographs of the CNT-COOH (same to the pristine CNT) and CNT-COONa solutions are shown in Fig. 1.The pristine CNT and CNT-COOH solutions are easily precipitated, while the CNT-COONa solution is very stable. There is no agglomeration or sediment even forseveral weeks. Polydispersity index (PDI) is used to characterize the uniformityof the particle diameter. In general, the smaller the PDI value, the better thesolution's dispersion [10]. The PDIs are measured by high performance particlesizer. The values of the pristine CNT, CNT-COOH and CNT-COONa are 0.477, 0.405and 0.249, respectively. That is to say, the CNT bundles are deagglomeratedin certain degree after treatment. The well dispersion may be caused by thehigh ionization of the CNT-COONa in water. The surfaces of CNTs possessnegative charge, which made them repulse each other because of the coulombinteraction.

The fracture morphologies of PVA/CNT composites films were examined in orderto determine the quality of the internal structure. As shown in Fig. 2 (a), there aremany agglomerates and bundles in the composite films with CNT-COOH, and thatthese bundles do not disperse evenly through the polymer matrix. However, theCNT-COONa disperses well through the PVA matrix and exhibits an excellentwetting by the matrix. Some CNTs are pulled out of the polymer, which indicatesthat they can form interfacial bonding with matrix and transfer some load frommatrix.

The typical stress–strain curves for the thread coated with PVA, PVA/CNT-COOH and PVA/CNT-COONa are given in Fig. 3 and their mechanical properties arelisted in Table 1. Results show that the mechanical properties of the thread coatedwith PVA/CNT-COOH, and PVA/CNT-COONa are significantly improved. Comparedto the original polypropylene thread, tensile strength of the thread coated withPVA/CNT-COONa increases by 117% from 51 MPa to 124 MPa, and the toughnessincrease by 560% from 6.27 MPa to 41.32 MPa. These results may be explained bythe effective load–transfer of the CNT-COONa. The well dispersed CNT-COONa bondstrongly with PVA matrix, which is similar to the reinforced bar in the concrete.While in the thread coated with PVA/CNT-COOH, nanotubes tend to agglomerate,

which may become defects in the polymers. So, the enhancement was limited inthis condition.

4. Conclusions

Coated the PVA/CNT composites on polypropylene thread, hightensile strength and toughness polymer thread could be obtained.Water soluble MWNTs were used to get homogeneous PVA/CNTsolutions. The MWNTs were treated firstly by strong mixed acid andthen sodium hydroxide. The results show that the well dispersion of

Table 1Tensile properties of the polypropylene thread coated with PVA/CNTs solutions

Sample Young's modulus(MPa)

Tensile strength(MPa)

Toughness(MPa)

Original polypropylene 223 51 6.27PVA 264 67 17.38PVA/CNT-COOH 283 65 11.55PVA/CNT-COONa 484 124 41.32

4382 B. Zhao et al. / Materials Letters 62 (2008) 4380–4382

MWNTs in PVA solution and a strong interfacial bonding with thematrix are response for the significant mechanical enhancements inthe coated thread.

Acknowledgements

This work is supported by Shanghai Science and Technology GrantNo:0552nm033 and National Basic Research Program of China

No.2006CB300406. The authors would like to thank Mr. Xialin Caofor providing the technological helps.

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