An Approach for Preparation of Excellent Antiwear PTFE Nanocomposites by Filling As-prepared Carbon Nanotubes/Nanorods (CNT/CNR) Mixed Nano-Carbon Material

Cheng Zhilin; Cao Baochong; Liu Zan; Qin Dunzhong,2; Zhu Aiping,3

(1. School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002;2. Jiangsu Sinvochem Co., Ltd., Yangzhou 225002; 3. Zhenjiang High Technology Research Institute of Yangzhou University, Zhenjiang 212000)

Abstract: The one-dimensional carbon nanotubes/nanorods (CNT/CNR) mixed nano-carbon material was successfully prepared by halloysite nanotubes (HNTs) as the template for the first time, in which CNT was formed through PVA modification in internal surface of HNTs and CNR was obtained by nanocasting PVA in hollow nanostructure of HNTs. The CNT of the mixture with flexible structure has ca. 20 nm in pore diameter and ca. 500 nm in length, whereas the CNR with hard and solid structure shows ca. 30 nm in diameter and ca. 2 μm in length. For application as fillers, the CNT/CNR mixed nano-material is used to reinforce the properties of polytetrafluoroethylene (PTFE). The mechanical and tribological properties of PTFE nanocomposites were intensively examined by a series of testing. The ring-on-ring counterface was used to evaluate the tribological behavior of the nanocomposites. The results showed that the volume wear rate of the CNT/CNR-reinforced PTFE nanocomposite after being filled with 0.3% of CNT/CNR was only 1/700 of that of the pure PTFE under a load of 200 N and a rotary speed of 200 r/min, while other mechanical and tribological performance was comparable to the performance of pure PTFE, which exhibited a desirable application prospect.

Key words: carbon materials; nanocomposite; polytetrafluoroethylene; mechanical properties; tribological properties

1 Introduction

In recent years, carbon nanotubes (CNT) and carbon nanorods (CNR) have been attracting intensive interests because of their desirable physical, chemical and electronic properties. Thus far, CNT and CNR have been prepared by many approaches. The preparation of the former can be achieved by the electric arc discharge method[1], the chemical vapor deposition[2], the laser ablation, etc.[3]However, the latter can be obtained by the electric arc discharge method[4], the chemical vapor deposition[5], the template method[6], the electron beam-induced route[7], the reduction of carbon bisulfide[8], the catalytic copyrolysis method[9], etc.

More recently, carbon nanorods（CNR）were successfully prepared by a facile and efficient nanocasting method, which was related to the formation of composites containing carbon precursors and template[10-13]. By virtue of this method, we recently developed a novel and facile route to prepare CNT or CNR by HNTs template, in which CNT was achieved by PVA modification of internal surface of HNTs, and CNR was gained by PVA nanocasted in the hollow nanostructure of HNTs[14-15]. Thus far, the as-prepared CNT/CNR mixed nano-carbon material has not been reported.

As naturally formed over millions of years, halloysite nanotubes (HNTs) are unique and tremendous nanomaterials consisting of double layers of aluminum, silicon,hydrogen and oxygen. HNTs are nanosized hollow tubes with 30—50 nm in diameter and 0.5—1.2 μm in length.Hence, it can become a promising candidate for nanoconfined synthesis for nanomaterials due to its natural hollow tubular nanostructure[8].

Nano-scale materials have attracted extensive interests from the researchers due to their unique characteristics,including large surface area, high surface reactivity, and dramatic improvement in the wear behavior of the polymer composites. Since carbon nanotubes (CNTs) and carbon nanorods (CNRs) are mechanically very strong and have a high aspect ratio, they would be expected to significantly improve the tribological properties of PTFE-based composites. Chen, et al.[16]studied the tribological behavior of the CNT-filled PTFE composites. The results showed that the CNTs could significantly improve the wear resistance of PTFE, and the friction coefficient of the composites was obviously decreased. The wear rate of the CNTs/PTFE composite with 20% (volume fraction)of CNTs filling was only 1/290 of that of pure PTFE.Vailet, et al.[17]reported the friction and wear behaviors of the single-walled CNTs/PTFE nanocomposites. The ultimate engineering stress was improved by approximately 50%, and the engineering strain increased two orders of magnitude. The wear resistance of nanocomposites was improved by 21 times whereas the friction coefficient was also increased by about 50%. To our knowledge,the native stiffness of CNR is better than CNT, but the native toughness of CNT is superior to CNR. Hence, the improvement in both the stiffness and the toughness is desirable for the polymer–matrix composites. In fact, the addition of inorganic fillers in polymer usually leads to the decrease of some mechanical properties even though some targeted reinforcement has been better achieved in many researches[18-23]. To solve this problem, the mixed filler with different distinctive properties could provide a desirable solution for overall embodying the reinforcing performance.

Scheme 1 The schematic route for obtaining the as-prepared CNT/CNR mixed nano-carbon material based on HNTs template

Herein, we developed an approach for preparing the CNT/CNR mixed nano-carbon material based HNTs template. As the filler, the nano-carbon material was used to reinforce the properties of PTFE. The mechanical performance and wear resistance of the PTFE nanocomposites were intensively investigated.

2 Experimental

2.1 Preparation of CNTs/CNRs mixed nano-carbon material

One-dimensional CNT/CNR mixed nano-carbon materials were synthesized through the HNTs template method,which was achieved by polyvinyl alcohol (PVA) molecules in the internal surface of HNTs and nanocasting in the hollow nanotubular structure of HNTs, where PVA acted as the carbon source[11-12]. To obtain the CNT/CNR mixed nano-carbon material, we adjusted the PVA concentration and preparation procedures in this work.

Before using, HNTs (200―500 nm in length and 15―25 nm in pore diameter, purchased from the Yangzhou Xigema New Material Co., Ltd.) were calcined at 550 °C for 6 h in air (marked as HNTs-550). Then, after 1.5 g of the HNTs-550 were dried at 100 °C for 24 h in a drying oven, the aqueous solution containing 10% of PVA was successively drop-wise added into the dried HNT till adsorption saturation without the surplus liquid on the surface of HNTs powder. At the end of reaction, the above sample was dried at 50 °C for 12 h. Subsequently,the PVA-HNTs nanocomposites (PVA-HNTs) were carbonized at 700 °C for 3 h under N2atmosphere in a tube furnace. Finally, the HNTs template was etched by using a 40% HF solution for the removal of the template.The surplus insoluble solid was filtered and washed to obtain the CNT/CNR mixed nano-carbon material (CNT/CNR). The preparation process is shown in Scheme 1.

2.2 Preparation of as-prepared CNTs/CNRs-reinforced PTFE nanocomposites

PTFE (purchased from the Shanghai 3F New Material Co., Ltd.) and the as-prepared CNT/CNR mixed nano-carbon material were mixed mechanically. The specimens were prepared via compression molding under a pressure of 10 MPa at room temperature. Then, the specimens were sintered at 375 °C and held for 2 h. Finally, the samples were cooled down to the ambient temperature. The mass fraction of the as-prepared CNTs/CNRs in the nanocomposite ranged from 0.1% to 0.2%, 0.3%, 1%, 2%, and 3% (by weight), which corresponded to PTFE-1, PTFE-2,PTFE-3, PTFE-4, PTFE-5, and PTFE-6, respectively.

2.3 Mechanical and tribological measurement

The tensile testing was carried out by a WDW-5 tensile machine (Shanghai Longhua). The size of samples was set at 42.8 mm×5.9 mm×0.8 mm. Each specimen was tested for five times to acquire the mean value. The thickness of each specimen was the average of five measurements taken along the gauge length with a digital micrometer. The tensile speed was 50 mm/min. The tribological experiments of all composites were carried out with a circular motion under dry sliding conditions via the ring-on-ring friction configuration. The testing was carried out using a multifunctional coefficient of friction tester (MMW-1, Jinan Chenda Co., Ltd.). The contact schematic diagram of frictional parts adopted the ring-on-ring contacting mode[19].

The friction coefficient was obtained by the computer automatically. The wear rateK(cm3/h) was calculated according to the following equation：

where dVand dtare the volume loss and the sliding time,respectively; Δmis the mass loss in mg, andρwas the density of PTFE composites(g/cm3).

3 Results and Discussion

As shown in Figure 1, the feature peaks at between 2θ=10° and 2θ=25° for HNTs had hardly disappeared after calcination at 550 °C, suggesting that the crystalline structure of HNTs could be destroyed at that temperature[24]. Furthermore, the PVA-HNTs showed a characteristic peak at 2θ=19°, which is originated from PVA. It indicates that PVA has been successfully nanocasted in the nanoporous channels of HNTs. Through carbonization at higher temperature, the sample recovers to the similar characteristic peaks like those of the HNTs-550. Following the removal of HNTs, the resulting residue has the feature peaks at 2θ=25°and 44o, which can be indexed to (002) and (101) diffraction planes corresponding to the hexagonal graphite phase[25]. No impurity is observed in the XRD pattern.

Figure 1 The XRD patterns of (a) pristine HNTs, (b) HNTs-550, (c) PVA, (d) PVA-HNTs, (e) C-nanocasted HNTs, and (f)as-prepared CNT/CNR

Figure 2 presents the TEM and SEM images of the asprepared CNT/CNR mixed nano-carbon material. It can be seen that the SEM images of the CNT/CNR mixed nano-carbon material display an abundant nanotubular structure, indicating that we have successfully prepared the nanotubular carbon material by using HNTs as the template and PVA as the carbon source. The TEM image (Figure 2-B) definitely affirms that they are of the nanotubular structure. In order to convince whether the sample is a nanotube or a nanorod, further magnification and high resolution of TEM images were adopted.Judging from these images (Figure 2-C and Figure 2-D),we can distinguish the structural features of CNT and CNR, suggesting that the nanotubular structured carbon material consists of a mixture of CNT and CNR. The CNT with a flexible structure in the mixture has ca. 20 nm in pore diameter and ca. 500 nm in length, whereas the CNR with a hard and solid structure shows ca. 30 nm in diameter and ca. 2 μm in length. These results distinctly demonstrate that the CNT/CNR mixed nano-carbon material can be successfully obtained by using HNTs as the template for the first time.

Figure 2 SEM (A) and TEM images (B, C, D) of the asprepared CNT/CNR mixed nano-carbon material (inset:high resolution TEM images)

Figure 3 shows the XRD patterns of CNT/CNR, PTFE and PTFE-3 nanocomposites. By contrast, we definitely corroborate that the filling of CNT/CNR in PTFE polymer matrix is uniform and stable. Figure 4A shows the tensile strength and elongation at break of the PTFE nanocomposites with different mass fractions of CNT/CNR. The tensile strength and fracture elongation of pure PTFE are 28.0 MPa and 490%, respectively. At a 0.3% mass fraction of the CNT/CNR, the tensile strength and elongation at break of the PTFE nanocomposite are increased to 36.2 MPa and 521%, respectively. Thereafter,the tensile strength and elongation at break of the PTFE nanocomposites are decreased with the increase of the mass fraction of the CNT/CNR. Figure 4B illustrates the Young’s modulus of the PTFE nanocomposites with various mass fractions of the CNT/CNR. In contrast to pure PTFE, the Young’s modulus of the PTFE nanocomposite is significantly increased from 310 MPa for pure PTFE up to 584 MPa for PTFE filled with 1% of CNT/CNR, which is 1.9 times greater than that of pure PTFE. Figure 4C shows the dry friction coefficient of the PTFE nanocomposites with different mass fractions of the CNT/ CNR. At the start of sliding, during the so-called run-in period, all materials rubbed against a #45 carbon steel specimen exhibited a relatively high coefficient of friction, followed by a steady-state sliding motion until the coefficient of friction remained unchanged due to the formation of the steady transfer films on the counterface during the repetitive sliding action. It is well known that the transfer film formed on a counterface during sliding plays an important role in the tribological behavior of polymers. The steady-state friction coefficient of pure PTFE after undergoing the run-in-period is about 0.152.After being filled with CNT/CNR, the friction coefficient of the PTFE nanocomposites thereupon rises with the increase of the content of filling material. At a 0.3%of filling material, the friction coefficient of the PTFE nanocomposite is 0.167, which is slightly larger than that of pure PTFE.

Figure 3 The XRD patterns of CNT/CNR (a), PTFE (b),and PTFE-3 nanocomposite (c)

Figure 4 The tensile strength and elongation at break (A),Young’s modulus (B) of various CNT/CNR mass fractions in PTFE and the friction coefficients depending on friction time (C) of PTFE(a), PTFE-1(b), PTFE-2(c), PTFE-3(d),PTFE-4(e); PTFE-5(f) PTFE-6(g)

Figure 5 displays the friction coefficient and volume wear rate of the PTFE nanocomposites varying with the load and the rotary speed. It can be concluded that the friction coefficient of the PTFE nanocomposites shows a declining trend with an increasing load, whereas the volume wear rate is increased thereupon. Additionally,with the increase of the CNT/CNR filling content, the wear rates of the PTFE nanocomposites is decreased because of the CNT/CNR mixed nano-carbon material with high strength and strong interaction vested in the matrix material. Conversely, the effect of the load and rotary speed on the friction coefficient of the PTFE nanocomposites at higher filling contents is weakened. At a filling content of 0.3% and under a load of 200 N and a rotary speed of 200 r/min, the volume wear rate of the PTFE nanocomposite is only 1/700 of the wear rate of pure PTFE.

Figure 5 The average friction coefficients (A, C) and volume wear loss (B, D) of PTFE and PTFE nanocomposites at different loads (A, B) and rotary speeds (C, D)a— PTFE; b— PTFE-1; c— PTFE-2; d— PTFE-3;e— PTFE-4; f— PTFE-5; g— PTFE-6.

Figure 6 The actual samples and 3D laser scanning microscope photos of PTFE and PTFE nanocomposite

Figure 6 represents the actual morphology and worn surface photos of PTFE nanocomposite filled with CNT/CNR. After compression molding, the PTFE nanocomposite shows the black color due to the existence of carbon material while retaining good polymer strength (Figure 6A and 6B). After being tested by the ring-on-ring configuration, the wear rate of neat PTFE is more serious than the PTFE nanocomposite,which can verify that the filling with the CNT/CNT mixed nano-carbon material can obviously promote the wear resistance of PTFE. A crystal of PTFE has a ribbon like structure and a smooth surface consisting of macromolecular chains. A single crystal is hexagonal and 20—50 nm thick. Because the outer diameter of the CNT/CNR is 20—30 nm, i.e., being of the same order of magnitude as the dimension of PTFE single crystals,the CNT/CNR can be incorporated with the crystals of PTFE. And because of their stronger mechanical properties and higher aspect ratio than that of graphite fibers, the CNT/CNR nanomaterials are able to reinforce PTFE composites significantly, thus preventing the crystal structure of PTFE from being destroyed during the friction test. By contrast, the crystals of PTFE are drawn out from a PTFE surface by shear during a friction test[26]. The CNT/CNR can effectively impede such drawing-out of PTFE crystals to improve the wear resistance of PTFE significantly.

4 Conclusions

In a word, we have successfully fabricated the CNT/CNR mixed nano-carbon material by means of the HNTs template method. In the course of application, we extensively investigated its performance for reinforcement of PTFE. Consequently, the CNT/CNR-reinforced PTFE nanocomposites exhibited an excellent wear resistance,while retaining the good mechanical properties and low friction coefficient. This fact suggested that it was promising in the practical application.

Acknowledgment:This work was funded by the Talent Introduction Fund of Yangzhou University (2012), the Zhenjiang High Technology Research Institute of Yangzhou University(2017), the Key Research Project-Industry Foresight and General Key Technology of Yangzhou (YZ2015020), the Innovative Talent Program of Green Yang Golden Phoenix (yzlyjfjh2015CX073),the Yangzhou Social Development Project (YZ2016072), the Jiangsu Province Six Talent Peaks Project (2014-XCL-013),and the Jiangsu Industrial-Academic-Research Prospective Joint Project (BY2016069-02). The authors also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015B112) for the financial support. The data of this paper are originated from the Test Center of Yangzhou University.