Nanoceramic and Polytetrafluoroethylene Polymer Composites Bull. Korean Chem. Soc. 2013, Vol. 34, No. 5 1345 http://dx.doi.org/10.5012/bkcs.2013.34.5.1345
Nanoceramic and Polytetrafluoroethylene Polymer Composites for Mechanical Seal Application at Low Temperature
A. A. Okhlopkova,†,‡,* S. A. Sleptsova,† G. N. Alexandrov,† A. E. Dedyukin,† Ee Le Shim,§ Dae-Yong Jeong,# and Jin-Ho Cho†,¶,*
†Department of Chemistry, North-Eastern Federal University, Yakutsk 677000, Russia. *E-mail: okhlopkova@yandex.ru
‡Institute of Oil and Gas Problem, SB-RSA, Yakutsk 677020, Russia
§Department of Physics, Myongji University, Kyunggido 449-728, Korea
#School of Materials Engineering, Inha University, Incheon 402-751, Korea
¶Department of Nanoscience and Engineering, Myongji University, Kyunggido 449-728, Korea. *E-mail: jinhcho@mju.ac.kr Received September 28, 2012, Accepted February 5, 2013
We investigated the tribochemical and wear properties of Polytetrafluoroethylene (PTFE) based polymer matrix composites with nanoceramic (NC) β-sialon, and Al2O3 particles for the mechanical seal applications at low temperature. SEM showed that NC particles were homogeneously distributed in the polymer matrix and initiated the formation of the supramolecular spherulites around NC. From the temperature stimulated depolarization (TSD) current results, it was analyzed that the surface charge on nanoceramic affected the formation of the spherulites structure. 2 wt % Al2O3 NC did not degrade the mechanical properties of PTFE so that composites showed the similar values of tensile strength, elongation at the rupture and friction coefficient as those of neat PTFE. However, the composite with 2 wt % Al2O3 NC revealed the improved wear resistance, wear rate of 0.4-1.2 mg/h at room temperature and 0.28 mg/h at −40 oC, respectively, while the neat PTFE the 70-75 mg/h at room temperature and 70.3 mg/h at −40 oC.
Key Words : Polytetrafluoroethylene, Nanocomposites, Wear resistance, Tribology, Seal
Introduction
Recent studies have shown that the polytetrafluoroethyl- ene (PTFE) is an effective material for tribological appli- cations because of its unique mechanical and frictional pro- perties, high chemical resistance, low coefficient of friction, and exceptional thermal stability.1,2 However, the application of PTFE for seal materials under mechanical vibration is limited due to the poor wear and abrasion resistance.1,2 Parti- cularly for lower temperature applications in the northern arctic areas including Siberia, seal materials lose their seal- ing properties due to the low frost resistance. The addition of suitable filler materials has been reported to significantly influence the tribological properties of PTFE and improve its wear resistance.3
As conventional fillers, glass fibers, graphite, and carbon fibers4 areexamined to increase wear resistance of PTFE.
However these fillers reduce the wear as well as the elasti- city and strength. Due to the growing demand for seal materials at low temperature, significant efforts are required to develop novel composite materials with increased wear resistance by adding one or more nonconventional fillers.5,6
Owing to the unusual structural characteristics and thermo- dynamic properties of nanoparticles, nanofillers could en- hance the operational characteristics of the composite syn- ergistically, even when added in small amounts.7 In this article, composites with PTFE polymer and ceramic nano- particles were fabricated and the effect of nanoparticles on microstructure and wear properties at low temperature were
investigated.
Experimental
PTFE was used as the polymer matrix. The rationale behind choosing PTFE stems from its unique properties that render it the only polymer that meets all the requirements for application in leak proofing in the northern arctic region where temperature is extremely low. Nanoceramic (NC) particles such as β-sialon obtained by using plasmochemical8 and Al2O3 mechanochemical9 synthesiswere used as fillers.
The average particle size of the NC fillers was 100 nm for β- sialon and 70 nm for Al2O3. Their relative surface area was about 40 5 m2/g and 30 5 m2/g respectively. The composites were fabricated by dry mixing the polymer with the filler using cold molding technology with subsequent heating.
The structure of the composites was characterized using a scanning electron microscope (SEM: JXA-50, JEOL). The low-temperature brittle spalls obtained at liquid nitrogen temperatures were used as specimens for microscopy.
Mass spectrometry was used to investigate the tribochemi- cal processes taking place in the composites during friction.
This tribochemical tests were performed on a quadrapole mass spectrometer (Finnigan MAT-4615) which was connect- ed to the friction machine I-2 (Russia). The wear products and formed gases from the wear of the composites have been investigated on the basis of a previously known methodo- logy.10 The energy of the ionizing electrons was 70 eV, the temperature of the ionizing boxes was maintained at 150 °C,
1346 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 5 A. A. Okhlopkova et al.
and the rate of heating of the sample was set to 50 °C/min.
To investigate the surface charge effect, thermally stimu- lated depolarization (TSD) current of the polymer compo- sites were recorded in accordance with Russian State standard method (GOST-25209-82)11 using an electrometer U5-9 when the samples were heated at a specific rate of heating in the range of 0.63-5.0 K/min in a measuring cell with aluminum electrodes.
The ultimate strength and relative elongation at rupture of the composite samples were determined by Russian State standard method (GOST-11262-80)12 using a mechanical tester (Instron, England). The cross-head velocity was set to 100 mm/min. The samples for strength and elongation were made in the form of scapula following the Russian State standard method.12 Friction coefficient was determined by Russian State standard method (GOST 11629-75)13 using a serial friction machine (SMT-2, Russia) according to the
“shaft-bushing” friction scheme, under a load of 0.45 MPa at the sliding speed of 0.39 m/s. The samples for tribological properties were prepared in the form of bushing.
Results and Discussion
The changes in the mechanical and tribomechanical pro- perties of all the evaluated polymer composite material (PCM) samples were analyzed with the structural trans- formations caused by the addition of the NC fillers. Figure 1 showed that the morphologies of a PCM with nanofillers were significantly different from that of neat PTFE.
The basic structural elements of neat PTFE (Fig. 1(a)) are distinctive band-like structures consisting of packs of lamellae. However, addition of active NC particles with relatively large surface areas caused significant changes in the mode of crystallization resulting in the formation of diverse supramolecular structural elements in PTFE (Fig.
1(b) and (c)). It is noticeable that the NC particles played a role as nuclei for crystallization. It is known that NC particles initiate the formation of internal structures with symmetrical polygonal shapes.1 The uniform morphology of PTFE indicated that the active Al2O3 nano particles were uniformly distributed and accelerated the formation of supramolecular spherulites. PCMs with such internal structures may be expected to exhibit the improved tribomechanical and de- formation properties.
To investigate the effect of surface charge of the fillers on the PCMs, we carried out TSD experiments. Figure 2 ex- hibits the variation of TSD currents with temperature changes for the NC, neat PTFE and PCM samples. While NC showed the particular peaks, neat PTFE did not generate the current for the temperature range. For example, in Figure 2(a)(1) nano size β-sialon had a broad peak in the 470-570 K, which indicated the presence of high-temperature traps for the charge carriers. This current is related with the relaxation of the polarization of β-sialon and was measured as 3.8 pA. In contrast, TSD spectrum of Al2O3 showed two TSD peaks at 430 and 510 K, respectively (Fig. 2(a), 2). Carrier relaxation at lower temperatures indicates the presence of traps in the
alumina-crystal structure capable of more easily capture the charge carriers. The polarization charge of the NC particles affected the PTFE polarization in PCM and charge adhesion to the binder. Consequently, the crystallization behavior of PTFE, microstructure in Figure 1 and wear properties were altered by the NC particlers.14-16 While the initial neat PTFE was electrically neutral (see Fig. 2(b)), samples containing 2 wt % NC particles (Fig. 2(c), curves 3 and 4) showed stable currents, and resembled conduction currents. However, stable current with significantly high values suggested that the TSD current may related to the relaxation of charge on the filler particles. Since the samples did not undergo electric polarization, the most probable origins of the currents could be the movement of the charge carriers that were released from the NC particles with during the heating process. It was Figure 1. SEM images of (a) neat PTFE, and PTFE-based com- posites containing 2 wt % (b) β-sialon and (c) Al2O3 NC filler.
Nanoceramic and Polytetrafluoroethylene Polymer Composites Bull. Korean Chem. Soc. 2013, Vol. 34, No. 5 1347
believed that the structuring of the polymer matrix occurs around NC under the impact of the field of electric charge on the NC surface.
It is known that tribochemical processes play an important role in polymer wear.18,19 Therefore, we investigated the relationship between the tribochemical behavior and wear resistance of PTFE/NC composites. For this experiment, the ratio of β-sialon nano particles in PTFE was fixed as 2 and 10 wt %, respectively, because these ratios have previously been characterized as critical amount of filler that induced significant changes to the relevant properties.3,17 As shown in Figure 3, we evaluated the characteristics of the tribo- chemical processes by measuring the destructions intensity of wear products for the increase of temperature.
The dependence of the destruction intensity of wear products on temperature indicated that as the temperature increases, the degree of destruction increases and thermal stability of the composite also decreased. The neat PTFE showed the stable low destruction intensity below 400 oC with the strong resistance to tribo destruction. However, at temperatures above 415 oC, we observed thermal degradation accompanied by isolation of TFE and its fragments (m/z = 100, 81, 50). At these temperatures, it was analyzed that typical products were fragments with m/z = 443, 431, 381, 331, 319. The difference in the mass values of these frag-
ments was calculated at 12 units of TFE. From this obser- vation, it may be inferred from splitting of chains at C-C bonds occurred. It is noteworthy that mass values of 50 units are the characteristic of -CF2- fragmentation.20-23
When comparing the neat PTFE of curve 1 with compo- sites with 2 wt % NC filler, composite showed lower de- struction intensity than neat PTFE below 250 oC and larger destruction intensity above 250 oC. The lower destruction intensity indicated that 2 wt % PCM had the strong wear resistance below 250 oC. However, PCM with 10 wt % NC showed the larger destruction intensity at the whole measur- ing temperature ranges. This result indicated that while 2 wt % NC filler enhanced the wear resistance but 10 wt % NC filler deteriorated the wear resistance. As opposed to the neat PTFE, PCM with NC fillers revealed two distinct areas on curves 2 and 3 in Figure 3. The first wide maximum near 340 oC occurs because of the isolation of low molecular weight chain fragment products at m/z = 431. Presence of the second peak and its multiplicity between 370 and 390 oC in curve 3 indicated the release of low molecular weight pro- ducts generated from wear. These two distinctive peaks indicated that crosslinked structures were formed owing to tribochemical reactions during the wear of PTFE/NC com- posites.20-22,24 These results showed that the adding of relatively small amounts (~2 wt %) of NC particles signifi- cantly changed the intensity of tribochemical processes and resulted in significant reduction of wear.
Table 1 summarized the mechanical and tribomechanical characteristics of the neat PTFE and composites. The optimal content of the fillers in the composite was found to be 2 wt %. NC did not degrade the mechanical properties of PTFE, such as the strength, elasticity, and friction coefficient.
Figure 2. TSD currents as a function of temperature for (a) β- sialon (1) and Al2O3 (2) NC particles, (b) neat PTFE, and (c) PCM samples consisting of PTFE with 2 wt % β-sialon (3) and PTFE with 2 wt % Al2O3 (4).
Figure 3. Destruction intensity of wear products for the temper- ature change. Curves marked by numbers 1, 2, and 3 denote the behaviors of neat PTFE, PTFE + 2 wt % of β-sialon, and PTFE with 10 wt % of β-sialon, respectively.
Table 1. Mechanical and tribomechanical properties of PTFE and PTFE/NC composites at room temperature
Sample Tensile strength
(MPa)
Relative elongation at rupture (%)
Wear rate
(mg/h) Friction coefficient
PTFE 20-22 300-320 70-75 0.04-0.20
PTFE+2 wt % Al2O3 20-25 300-320 0.4-1.2 0.18-0.20
PTFE+2 wt % β-sialon 18-22 310-330 5.0-5.6 0.20-0.22
1348 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 5 A. A. Okhlopkova et al.
However, 2 wt % Al2O3 nanoparticle decreased the wear rate of PTFE significantly. For example, while neat PTFE gave the wear rate of 70-75 mg/h, the PCM with 2 wt % Al2O3 showed 0.4-1.2 mg/h wear rate, which is almost 2 order smaller than the neat PTFE. This large increase of wear resistance might be related with the homogeneous formation of supramolecular spherulites in Figure 1(c). Table 2 showed the tribological properties measured at the low temperature of −40 oC. The PTFE containing 2 wt % Al2O3 also had the lower wear rate not sacrificing the tensile strength and elongation at the rupture.
Conclusions
PTFE based polymer matrix composites with NC β-sialon and Al2O3 particles were fabricated and their mechanical and wear properties were investigated for low temperature ap- plication. SEM revealed that NC was homogeneously di- stributed forming the supramolecular spherulites around NC.
TSD current results also supported that the surface charge of NC affected the formation of spherulite structure. Tribo- chemical tests showed that 2 wt % NC significantly increased the wear resistance. Compared with the neat PTFE, the com- posite with 2 wt % Al2O3 NC showed two orders of increase in wear resistance not sacrificing the tensile strength, elongation at the rupture and friction coefficient. PCM with NC would be a promising candidate for the mechanical seal at low temperature.
Acknowledgments. This research was supported by the Russian Foundation for Basic Research Grant (RFBR Grant 09-03-98504-p_East_a) and a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge and Economy, Republic of Korea.
One of the authors (D.-Y Jeong) thanks the financial support of an NRF funded by the Korea government (MEST, 2011- 001095).
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Table 2. Low temperature properties of PTFE and PTFE/Al2O3 composite at −40 °C
Sample
Tensile strength (MPa)
Relative elongation at
rupture (%)
Wear rate (mg/h)
PTFE 29.9 28.8 70.3
PTFE+2 wt % Al2O3 29.3 32.5 0.28
