Structure and properties of polytetrafluoroethylene (PTFE) fibers

2.1 Materials

The PTFE fibers used in this study were provided by Changzhou XCF Polymer Materials Co., Ltd. (Jiangsu, PR China). The fibers were obtained by extruding a mixture of PTFE fine powder and lubricants into a rod, followed by rolling into a film and stretching uniaxially, then tearing the PTFE films into filaments and cutting the filaments into staple fibers according to the required length.

2.2 Characterization of PTFE fibers

3.1 Morphological structure

Longitudinal and transverse cross section SEM images of PTFE fibers are presented in Figure 1. The longitudinal surface of the fiber was full of grooves, which would largely increase the specific area of the fiber. The cross sectional figures of the fibers were sheet-like, irregular with rough edges due to the method of tearing stretched uniaxially films into fibers. The rough surface could increase cohesion among the fibers in the yarns.

Figure 1:

SEM images of PTFE fiber, indicating (A) longitudinal surface morphology and (B) cross sectional morphology.

3.2 FT-IR analysis

Figure 2 showed the infrared spectrum of PTFE fibers, three apparent peaks can be seen at 1207, 1151 and 636 cm^-1, respectively. The 1207 and 1151 cm^-l bands, which were considered to be insensitive to crystallinity (19), were assigned as the symmetric and asymmetric CF₂ and CC stretching vibrations (20), (21). The doublet at 638–626 cm^-1 appeared in the spectrum of PTFE film (22) was not found in that of PTFE fibers except a peak at 636 cm^-l. From previous research, we learned that the band at 636 cm^-1 was assigned to a regular helix and the band at 626 cm^-1 was associated with the helix-reversal defect accompanying the conformations transition (23). Biswas and Vijayan stated in a review that the molecular conformations of the PTFE chain involved 13₆ helix, 15₇ helix, disordered 15₇ helix, etc, i.e. 15₇ helix meant that 15 -CF2- units were present in seven turns of the chain, and the thermally activated structural transformations of PTFE chain (from 13₆ helix to 15₇ helix) would happen when the temperature was increased to 19°C (4). Therefore we speculated that there were no helix-reversal defects in the fibers and the conformations transition of the fibers did not happen, however, whether it was 13₆ helix or not was still an intriguing issue.

Figure 2:

FT-IR spectrum of PTFE fibers.

3.3 Fiber fineness

The linear density of PTFE fibers ranged from 2.09 dtex to 11.50 dtex and the average value was 5.07 dtex. The fineness distribution of PTFE fibers are shown in Figure 3(A). The distribution was approximately partial normal distribution and the linear density under 5 dtex accounted for 60%. This would bring difficulties for spinning and weaving of PTFE fibers, so most of PTFE fibers have been used in non-woven in the past years. During the fineness test, some fibers had branches or were not completely separated, which may make the test complicated.

Figure 3:

(A) Fineness distribution of PTFE fibers, (B) Typical tensile curve of PTFE fiber.

3.4 Tensile properties

As shown in Table 1, the average strength of PTFE fibers (1.37 cN/dtex) was calculated. Both orientation and degree of crystallinity of fibers were high, but the strength of fibers was low. As shown in Figure 3(B), there were no jitter at the end of load-elongation curve, so we considered that low strength may be not ascribed to non-simultaneous breaking of fibril bundles, but due to weak links between crystal networks.

3.5 Thermal properties

TG and DSC curve of PTFE fibers are depicted in Figure 4. The melting point (T_m), crystallization temperature (T_c) and the decomposition temperature (T_d) were noted on the DSC and TG curves. The T_d was defined as the point at which the mass loss was 5%. As shown in Figure 4, the T_m of PTFE fiber was 329.1°C, which was consistent with that of PTFE film (329°C). The T_d of PTFE fiber was 508.6°C, which was a little lower than that of PTFE film (525°C) (24). These data indicated that PTFE fibers still possessed the excellent thermal stability although the degree of crystallinity of fibers (shown in Table 1) was reduced compared with that of PTFE films. No glass transition temperature can be detected on the DSC curve due to the still high degree of crystallinity.

Figure 4:

TG analysis and DSC curve of PTFE fibers.

3.6 Surface properties

Table 1 summarizes the results of surface properties. The water contact angle and the shape of the water droplets on the surface of the fibers layer are shown in Table 1 and Figure 5, respectively. The water contact angle was around 120°, which indicated that it was a hydrophobic surface, and could be used as a self-cleaning surface with some treatments. Friction coefficient of fibers (0.17–0.19) was lower than that of hydrocarbon polymers, but a little higher than that of PTFE films (0.14) (20) because of the rough surface full of grooves.

Figure 5:

Water contact angles of PTFE fibers.