Polytetrafluoroethylene deformation

Samples of polytetrafluoroethylene having very high degrees of crystallinity are characterized by a very low yield strain and exhibit lead-like behavior after deformation. In contrast with other crystalline polymers the yield stress does not increase with increasing crystallinity, but remains almost constant at room temperature. At lower temperatures the yield stress decreases with increasing crystallinity (Riley). 

The rheology of lubricated polytetrafluoroethylene compositions was studied by Lewis and Winchester. The mechanism appeared to be a combination of permanent and elastic deformations in the region just before the orifice of the die in the extruder. As a result of permanent deformation, the polymer particles are partially transformed into long fibers. The relative amounts of permanent and recoverable deformation were related to the rate and temperature of extrusion and the geometry of the extruder. Plastic deformation is favored by extruding at temperatures above the 19 and 30° transitions (Snelling and Lontz). 

Let us consider one final example the application of atomic force microscopy (AFM) relating to nanoscale scratch and indentation tests on short carbon-fibre-reinforced PEEK/polytetrafluoroethylene (PTFE) composite blends (Han et al, 1999). In the scratch test, the tip was moved across the surface at constant velocity and fixed applied force to produce grooves with nanometre scale dimensions on the PEEK matrix surfaces. The grooves consisted of a central trough with pile-ups on each side. These grooves provide information about the deformation mechanisms and scratch resistance of the individual phases. In the nanoscale, indentation and... 

Deformation under load of all filled polytetrafluoroethylene compounds decreases in comparison to unfilled resin, as seen in Table 3.13. Combinations of carbon and graphite reduce deformation the most at room and at elevated temperatures. The next effective filler in reducing deformation under load is bronze at 60% by weight. Hardness is increased by the addition of additives, particularly bronze, carbon, and graphite (Table 3.14). 

This property is an important consideration in the design of parts from polytetrafluoroethylene. PTFE deforms substantially overtime when it is subjected to load. Metals similarly deform at elevated temperatures. Creep is defined as the total deformation under stress after a period of time, beyond the instantaneous deformation upon load application. Significant variables that affect creep are load, time under load, and temperature. Creep data under various conditions in tensile, compressive, and torsional modes can be found in Figs. 3.12 through 3.19. 

Resin manufacturers have long recognized the excessive deformation of polytetrafluoroethylene in applications where parts such as gaskets and seals experience high pressures. Copolymers oftetrafluo-roethylene with small amoimts of other fluorinated monomer are known as Modified PTFE resins and have been reported to exhibit reduced deformation under load. Examples of the properties of some of the commercial products can be seen in Tables 3.22-3.24 and Figs. 3.20 and 3.21. Significant reduction in deformation under load can be achieved, particularly at elevated temperatures and pressures. 

Major applications of unsintered polytetrafluoroethylene are as tape in thread sealing and wrapping electrical cables, and as rod and tape in packings. Important properties of PTFE like chemical resistance, broad service temperature, low friction, flexibility, high machine direction strength, and deformability in the cross direction make unsintered fine powder PTFE ideal for these applications. 

Crushing. Byrskii et al. (8) used a vibration ball mill to study the volatile compounds released from a series of crushed polymers. They chose this method to intensify the mechanical degradation process and thus to increase the rate of volatile evolution from the polymer. For polymers such as polyethylene (PE) and polytetrafluoroethylene (FIFE), compound evolution rates are low from other methods of mechanical deformation hence, it is difficult to obtain mass spectra. Byl skii et al. used this technique to successfully obtain mass spectra of compounds from PE and PTFE and demonstrated the feasibility of the vibration ball mill for performing kinetic studies of mechanical degradation as a function of the amplitude of the vibration and duration of the grinding. 

Figure 10.3.18 illustrates the structure of the electrode/membrane system where the membrane (e.g., Nafion 900 Series) is reinforced by a nonconductive polytetrafluoroethylene (PTFE) fabric. Note that the current flux deforms around the PTFE reinforcement, leading to an increase in the voltage drop through the membrane. Therefore, the arrangement of the PTFE reinforcement must be optimized for mechanical strength and voltage drop. 

Evans and collaborators [59] have shown how an anisotropic microstructure consisting of nodules and fibrils can be produced in polytetrafluoroethylene (PTFE) that gives rise to a very large negative Poisson s ratio. Figure 7.22 is a schematic representation of the deformation of microporous PTFE. 

Figure 7.22 A schematic representation of the structural changes observed in microporous polytetrafluoroethylene undergoing tensile loading in the x direction (a) initial dense micro-structure (b) tension in fibrils causing transverse displacement of anisotropic nodal particles with lateral expansion (c) rotation of nodes producing further lateral expansion (d) fully expanded structure prior to further, plastic deformation due to node break-up. (Reproduced with permission Ifom Evans and Caddock, J. Phys. D Appl. Phys., 22, 1883 (1989)).

Fluoropolymers have outstanding chemical resistance, low coefficient of friction, low dielectric constant, high purity, and broad use temperatures. Most of these properties are enhanced with an increase in the fluorine content of the polymers. For example, polytetrafluoroethylene, which contains four fluorine atoms per repeat unit, has superior properties compared to polyvinylidene fluoride, which has two fluorine atoms for each repeat unit. Generally, these plastics are mechanically weaker than engineering polymers. Their relatively low values of tensile strength, deformation under load or creep, and wear rate require the use of fillers and special design strategies. 

Fig. 5.44 A replica of a fracture surface of polytetrafluoroethylene after compressive deformation in the vertical direction.

All plastics or elastomers except one become brittle at low temperatures. Polytetrafluoroethylene is unique in that it can still be deformed plastically to a small degree at 4 K. Plastics and elastomers do not respond to stress as do metals. The less cross-linked elastomers yield by uncoiling their long-chain molecules and by sliding over one another. The thermal energy of the material at room temperature facilitates this motion. 

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