Water-absorptivity and mechanical behaviors of PTFE/PA6 and PTFE/PA66 blends

   ZHAO Rong-guo (赵荣国) ^1,2, LUO Wen-bo (罗文波) ^1,2, XIAO Hua-ming (肖华明) ^1,3, WU Guo-zhong(吴国忠)³  

1. Institute of Fundamental Mechanics and Material Engineering, Xiangtan University, Xiantan 411105, China;

2. Key Laboratory for Advanced Materials and Rheological Properties of Ministry of Education, Xiangtan University, Xiangtan 411105, China;

3. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The effects of polytetrafluoroethylene (PTFE) content on water-absorptivity, tensile strength, flexural strength, and notched impact strength of polytetrafluoroethylene/polyamide 6 (PTFE/PA6) and polytetrafluoroethylene/polyamide 66 (PTFE/PA66) blends were investigated by water immersion test, uniaxial tensile test, three-point test, and Charpy impact fracture test. The water-absorptivity in the blend decreases with increasing PTFE content, which indicates that the PTFE phase restrains the polyamide phase from water absorption. For water-free blends, the addition of PTFE causes a reduction in tensile strength, while for water-absorbed PTFE/PA6 blends, the tensile strength increases with increasing PTFE. Simultaneously, the absorbed water improves the elongation, but results in a notable reduction in flexural strength of the blends. Although the addition of PTFE causes a reduction in notched impact strength of the blends, as compared to pure polyamide, the absorbed water has little effects on the notched impact strength of the blends. Finally, the effects of temperature and loading frequency on complex viscosity parameters of PTFE/PA6 and PTFE/PA66 melts were tested. It is found that the complex viscosity of PTFE/PA6 melt is reversed with increasing temperature and shear velocity, but that of PTFE/PA66 melt increases approximately in exponential form with increasing temperature. To fill polyamide with suitable mass percentage of PTFE can effectively reduce the viscosity of blend, and as a result, the molding and processing properties are improved.

Key words: PTFE/PA6 blends; PTFE/PA66 blends; water absorptivity; strength; complex viscosity

1 Introduction

Many researchers in the fields of material science and engineering have been focusing their eyes on the preparation and application of self-lubricating and wearable materials in the recent years [1,2]. Polyamide, as the one in the family of engineering plastics, has the advantages of high strength, excellent corrosion resistance, suitable wear resistance and favorable self-lubricating property, has been widely used in the engineering structures. However, the existence of abundant acylamino groups in the material results in some defects, such as strong water-absorptivity, unstability of structural sizes, highly shrinkage. Polytetrafluoroethylene(PTFE) is a self-lubricating material possessing low friction factor, excellent corrosion resistance and wide range of serviced temperature [3]. Due to the synergetic effect between PTFE and polyamide, PTFE improved polyamide is a class of materials resistant to wear, corrosion, solvent as oil or water, temperature, and have been producing in large quantities, such as PTFE/PA66(13%) produced by LNP Co, PA/PTFE(≤20%) and PA-11/PTFE produced by RTP Co. Commonly, polymeric materials are modified using the monomer copolymerization, the mechanical modification, the interpenetrating polymer network (IPN), or the chemical modification method. The mechanical modification method is often applied to prepare a copolymer for its simple technique, little investment to equipment, low cost and synergetic effect between two phases. The effects of PTFE content on water-absorptivity and mechanical behaviors, and the effect of temperature and loading frequency on complex viscosity parameters of PTFE/PA6 and PTFE/PA66 blends is investigated in the paper, and some interesting results are given for the purpose of providing a technical support on producing, processing and using such polymeric material.

2 Experimental

Polyamide using in this exoeriment was produced by Shijiazhuang Chemistry Fiber Inc, PA66(A45) was provided by RADICI (Italy), and PTFE whose particle-radium was 5 μm was supplied by Shanghai Nayuan Inc. The producing particle temperatures of PTFE/PA6 and PTFE/PA66 were 235-245 ℃ and 270-280 ℃ individually, and the PTFE contents were 3%, 8%, 15% for each blend. Firstly, the raw materials were placed in the electric drying oven with forced convection electrothermal blowing for 8 h, the drying temperature was 90 ℃, then the injection molding technique was applied to prepare the specimens, and the injection pressure was 5 MPa. For PTFE/PA6 blend, the temperatures in four zones were 235, 235, 225 and 220 ℃, respectively, and for PTFE/PA66 blend, the corresponding temperatures in four zones were 275, 270, 265 and 260 ℃, respectively.

The water absorptivity tests were implemented in group for PTFE/PA6 series and PTFE/PA66 one. Each group has six specimens. The specimens were dried in a vacuum drying oven for 24 h, the drying temperature was 90 ℃, the specimen was weighted as m₁ using an electronic balance. Following, the specimens were placed into the distilled water soaking for 24 h under a constant temperature of 20 ℃, after that, the beads on the specimen’s surface were absorbed using filter papers, and the specimen was weighted as m₂. Finally, the specimens were again placed in the vacuum drying oven for 24 h, and the dry temperature was 90 ℃ as well, the specimen was weighted as m₃. Therefore, the specimen’s water absorptivity was calculated as w=(m₂-m₃)/m₁.

The variations of water absorptivity with PTFE content for PTFE/PA6 and PTFE/PA66 blends are shown in Fig.1. It can be seen from Fig.1 that the water absorptivity decreases with increasing PTFE content for both PTFE/PA6 and PTFE/PA66 blend. For PA6 blends mixed with 3%, 8% and 15% PTFE, the reductions of 16.5%, 21.0% and 24.4% in water absorptivity are found comparing with the pure PA6. For PA66 blends with same PTFE contents, the reductions of 23.6%, 26.3% and 29.9% are observed comparing with the pure PA66. Although polyamide is a class of strong polar molecules possessing strong water absorptivity, after mixed with nonpolar PTFE, the water absorptivity of polyamide is restrained owing to the polar shielding action of PTFE. Simultaneously, PTFE particles increase the speed of nucleation and act as a nucleation accelerant in the process of polyamide crystallization, both crystal degree and crystal rate of polyamide are improved[4,5], and as a result, PTFE acts as a structural shield action to water absorptivity of polyamide.

Fig.1 Variation of water absorptivity with PTFE content

The uniaxial tensile experiments to specimens of water free and water absorbed PTFE/PA6 and PTFE/PA66 blends were carried out on a CSS-44020 Universal Electronic Test Machine, and the uniaxial tensile stress versus strain curves of water-free and water-absorbed blends under different PTFE contents are shown in Figs.2-5. For water-free PA6 blends with 3%, 8% and 15% PTFE, the reductions of 6.17%, 1.98% and 10.17% in tensile strength are observed comparing with the pure PA6 (Fig.2). For water-free PA66 blends with same PTFE contents, the reductions of 14.3%, 11.5% and 11.6% are discovered comparing with the pure PA66 (Fig.4), and an analogous result can be found for water-absorbed PTFE/PA66 blends as shown in Fig.5. But for water-absorbed PTFE/PA6 blends, the tensil

Fig.2 Uniaxial tensile stress vs strain curves of water-free PTFE/PA6 blends

Fig.3 Uniaxial tensile stress vs strain curves of water-absorbed PTFE/PA6 blends

Fig.4 Uniaxial tensile stress vs strain curves of water-free PTFE/PA66 blends

Fig.5 Uniaxial tensile stress vs. strain curves of  water-absorbed PTFE/PA66 blends

strengths increase with increasing PTFE content. The lower yield stresses are all about 44 MPa to water-free or water-absorbed PTFE/PA6, while the strains of water-absorbed PTFE/PA6 blends at lower yield point are larger than those of water-free PTFE/PA6 blends, as shown in Figs.2 and 3. A reduction of 55.1% in elongation at rupture point is found to water-absorbed pure PA6 comparing with water-free one, and to water-absorbed PTFE/PA6 (3/97) blend, a reduction of 70.1% in elongation at rupture point is found. But to PTFE/PA6 blends with 8% and 15% PTFE, the moisture content has a little effect to the elongation.

The relations between flexural strength and PTFE content of water-free and water-absorbed PTFE/PA6 and PTFE/PA66 blends are shown in Fig.6. It can be seen that the flexural strengths of water-absorbed PTFE/PA6 and PTFE/PA66 blends appear as a downtrend. For pure PA6 and PA6 with 3%, 8% and 15% PTFE, the reductions of 9.02%, 0.69%, 20.98% and 13.5% in flexural strength are investigated compared with the corresponding water-free polymeric materials. For pure PA66 and PA66 with 3%, 8% and 15% PTFE, the reductions of 20.65%, 18.48%, 17.85% and 16.33% are discovered compared with the corresponding water-free PA66 series. The reduction of flexural strength of water absorbed polymeric material likely arises from the permeation of water molecules into polymeric material’s molecular chains, which results in the reduction of compactness of molecular stacking, and the decrease of action force among polymeric material’s molecules.

Fig.6 Variation of flexural strength with PTFE content

The impact fracture tests to the freely supported beams with “U” notch at one side of specimen are implemented on a XJJ-5- (50) Impact Test Machine to investigate the effect of PTFE content and water absorptivity on impact strength of blends. The size of specimen is 120 mm×10 mm×4 mm, and the size of notch is 2 mm×2 mm. The experimental results of impact strength of PTFE/PA6 and PTFE/PA66 blends are shown in Figs.7 and 8.

It can be seen that despite a small mass percentage of PTFE, the impact strength has a dramatic reduction to both water-free and water-absorbed blends comparing

with that of pure polymeric matrix. For example, the impact strength of PA66 with 3% PTFE is about one-half

Fig.7 Variation of Charpy impact strength of PTFE/PA6 with PTFE content

Fig.8 Variation of Charpy impact strength of PTFE/PA66 with PTFE content

of that of pure PA66, and the impact strength of PA6 with 3% PTFE is only about one-fifth of that pure PA6. If we continue to fill PTFE into the polymeric matrix, we find that the PTFE content has a weak effect to the impact strength of blends. Simultaneously, it can be found from figures that for water-free and water- absorbed pure PA6 and PA66, the difference of impact strengths is large, but for water-free and water-absorbed PTFE/PA6 and PTFE/PA66 blends, the difference of impact strength is small.

Viscocity, which is usually adopted to describe the rheological properties of fluids and melts, is a kind of intrinsic impedance characterizing the resistance deformation of material. The viscosity of material is defined as the ratio of shear stress to shear strain rate under steady shear state, i.e., . If the viscocity is independent on shear strain rate, then the material appears as a linear property, we call this kind of material as Newtonian fluid. And if the viscocity is dependent on shear strain rate, then the material appears as a nonlinear property, we call this kind of material as non-Newtonian fluid. To polymeric fluid or melt, the correlation between viscocity and shear strain rate is shown as Fig.9. When the shear strain rate is big enough (≤a) or small enough (≥b), the material appears as a Newtonian fluid behavior. When , the relation of logη and is nonlinear, and the material appears as a Non-Newtonian fluid behavior. The viscosity decreases with increasing shear strain rate, which is called as pseudo-plastic behavior[6].

Fig.9 Sketch of viscosity vs shear rate curve

Due to the influence of material viscosity, the stress response is hysteretic to strain in the process of dynamic deformation of material, and the complex viscosity is obtained by dynamic modulus and angle frequency. In general, the relation between complex viscosity η* and steady shear viscosity η satisfies Cox-Merz relation or modified Cox-Merz relation[7],  or , where ω is the angle frequency.

The effects of temperature and loading frequency on complex viscosity parameters of PTFE/PA6 and PTFE/PA66 blends are tested using an Advanced Rheometrics Extension System(ARES). The complex viscosity measurement is carried out between two rotating plates whose diameter and space are 25 mm and 1.5 mm. and the notch’s size is 2 mm×2 mm. The stove chamber of ARES is filled with nitrogen in the process of test to prevent polyamide thermal oxidation decomposition. The frequency and temperature scanning tests are implemented under 2% strain amplitude to the same specimen. For the frequency scanning tests, the experimental temperature of PA6 series is set as 230 ℃, and the one of PA66 series is selected as 270 ℃, and the frequency range is 0.1-100 rad/s. For the temperature scanning tests, the experimemntal frequency is determined as 1.0 rad/s, the temperature ranges are 230-260 ℃ and 270-290 ℃ to PA6 series and PA66 ones. The complex viscosity vs temperature curves of PTFE/PA6 and PTFE/PA66 blends under frequency 1.0 rad/s are shown as Figs.10 and 11.

It can be found that the complex viscosity of PTFE/PA6 melt is reversed with ascending temperature, but the one of PTFE/PA66 melt increases approximately

Fig.10 Temperature dependence of complex viscosity of PTFE/PA6 melts

Fig.11 Temperature dependence of complex viscosity of PTFE/PA66 melts

in exponential form with increasing temperature. For the PTFE/PA6 blends, the flowing activation energy of molecular chains increases with ascending temperature, the free volume in melt increases, the action of hydrogen bond to PA6 main chain reduces, the orientations of molecular chain segments become easier, the internal friction between molecular chains reduces, so the viscositydecreases with increasing temperature. The phenomenon that viscosity increases with increasing temperature has been observed in PA1010/TPU blend[8] and some soliquids and liquid crystals[9,10]. Simultaneously, it can be seen from Figs.10 and 11 that the mass percentage of PTFE obviously has an influence to the viscosity of blend.

The complex viscosity vs frequency curves of PTFE/PA6 and PTFE/PA66 blends are shown in Figs.12 and 13.

It can be found that the complex viscosities of PTFE/PA6 and PTFE/PA66 melts are all reversed with increasing frequency, and appear as a pseudo-plastic fluid characteristic. Especially for pure PA66 melt, when the shear velocity increases from 0.1 rad/s to 10 rad/s, its

Fig.12 Complex viscosity of PTFE/PA6 vs frequency

Fig.13 Complex viscosity of PTFE/PA66 vs frequency

complex viscosity reduces from 2.2×10⁴ to 1.8×10² Pa·s. From the frequency scanning experiments to PTFE/PA6 melts under constant temperature 230 ℃, the complex viscosity of PTFE/PA6 melt firstly decreases, and then increases with the mass percentage of PTFE, and a similar result is obtained for PTFE/PA66 melt. To fill PA6 or PA66 with suitable mass percentage PTFE can effectively reduce the viscosity of blend melt, and as a result, the molding and processing properties are improved.

1) The PTFE phase restrains the polyamide phase from water absorption, which results in the water absorptivity of PTFE/PA6 and PTFE/PA66 blends increases with increasing PTFE.

2) For water-free blends, the increment of PTFE content causes a reduction in tensile strength, while for water-absorbed PTFE/PA6 blends, the tensile strength increases with increasing PTFE. The absorbed water improves the elongation at yield point, but results in a notable reduction in flexural strength of the blends.

3) Although the addition of PTFE causes a reduction in notched impact strength of the blends, as compared to pure polyamide, the absorbed water has little effects on the notched impact strength of the blends.

4) The complex viscosity of PTFE/PA6 melts is reversed with increasing temperature and shear velocity. To fill PA6 or PA66 with suitable mass percentage PTFE can effectively reduce the viscosity of blend.

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(Edited by LONG Huai-zhong)

Foundation item: Project(10572123) supported by the National Natural Science Foundation of China; Project(05JJ30014) supported by the Natural Science Foundation of Hunan Province, China; Project(05C100) supported by the Scientific Research Fund of Education Department of Hunan Province , China

  Corresponding author: ZHAO Rong-guo; Tel: +86-732-8293214; Fax: +86-732-8293240; E-mail: zhaorongguo@xtu.edu.cn