In today’s highly competitive, global marketplace, companies are moving into higher value-added chemicals and materials. Further, environmental regulations are becoming ever more stringent as governments at the national, state and local levels are requiring that industries reduce waste and emissions. Thus, new technologies for capturing those wastes and emissions continue to evolve. In this competitive environment, the filtration design engineer or filter end user will require materials with increasing performance and durability. This shows that polytetrafluoroethylene (PTFE) is well suited to meet many of these demands and help the industry to meet its customer and environmental obligations.

PTFE OVERVIEW

PTFE was discovered by Dr. Roy Plunkett at DuPont in 1938. Its use was limited to wartime applications until after World War II, when it was first introduced under the trademark Teflon®. Today, there are many other suppliers for this material with annual worldwide usage in the range of 60,000 tons/year [1]. The unique properties of PTFE accrue from its structure shown in Figure 1 (above) [2], where the large fluorine atoms form a sheath around the carbon backbone, protecting it from attack by any organic, acid, or base solvent, except at elevated temperatures. This sheath also imparts a low surface energy, resulting in a very low coefficient of friction (< 0.1) and nonstick properties [3]. Finally, this low surface energy renders PTFE both hydrophobic and oleophobic, so neither aqueous nor oily materials adhere to it. Thus, it is easy to mechanically remove any dust or filter cake which builds up on its surface.

PTFE is insoluble in all solvents below 300°C and has a useful property range from -260°C to +260°C. It is inert to UV radiation and has a V-0 flammability rating, which recognizes its extremely high Limited Oxygen Index of 95%, which exceeds that of any other polymer [4].

Table I, extracted from reference 5, compares the chemical resistance of PTFE vs. other common engineering polymers used in fiber form. It is seen that PTFE is the only polymer showing a “Good” rating all the way across.

Table II presents the heat resistance of PTFE and other engineering polymers as a function of their “continuous use temperature,” which is the maximum temperature that they can withstand under zero load. This is a relative measure of how a polymer will perform at higher temperatures. Here one can see that PTFE has excellent heat resistance, which should make it suitable for many hot stack gas applications, and especially those with acid or base concentrations in the flue gas.

While PTFE is semicrystalline, melting in the range of 320°C–345°C (depending upon its heat history), it cannot be melt processed, as it begins to decompose prior to melting. Thus, PTFE is processed into film and fiber form primarily using two processes. In the first, the PTFE fine powder is combined with cellulosic binders in an aqueous mixture that is pressured through fine holes into a hot gas atmosphere, which drives off the water. The fiber is then sintered to consolidate its structure and drive off the cellulosic binders [10]. This method allows for production of multifilament yarns and staple fibers similar to what are obtained from melt processable polymers.

In a second process, the PTFE reactor powder is combined with a lubricating oil to form a paste that is solid-phase extruded through a film die at temperatures near room temperature. The film is calendered, then subsequently stretched, either biaxially (i.e., stretched in both the machine direction and perpendicular to the machine direction) or uniaxially (machine direction only) to form an “expanded” film or “membrane.” These expanded PTFE (or ePTFE) films are quite strong and have a pore size which can be engineered to pass vapors and gases, while blocking water droplets (first patented by W. L. Gore in the early ‘70’s [11] and marketed as “Gore-Tex”). Much ePTFE membrane is used in the filtration industry.

Another useful form of ePTFE is produced by highly orienting the extruded film in one direction, then slitting it into monofilaments between 400 and 1,200+ denier (where denier is the weight of a 9,000 m length of the fiber in grams). Figure 2 (above) shows a cone of PTFE slit monofilament.

These monofilaments are more easily used, especially for sewing, when they are twisted into threads as shown in Figure 3 (above).

PTFE monofilament or thread is suitable for filter manufacture either as:

  1. A sewing thread for sewing filter bags of PTFE or other high performance fibers used in hot gas filtration, e.g., bag house filters located in the stacks of coal-fired boilers, cement plants, etc.
  2. As a woven scrim for needle punching PTFE or other high-performance fibers to form the fabrics used in bag house or other filter formats.
  3. A filament for wound-core filters for corrosive liquid filtration applications.

All of these applications exploit the superior chemical resistance and excellent heat resistance of PTFE.

Figure 4 (above) presents the relative cost of PTFE vs. some other common engineering plastics, derived from cost/unit volumes reported in Reference 11 , with PET’s cost/unit mass defined as 1.0. As the figure shows, the higher cost of PTFE vs. other more common engineering plastics restricts its application to those end uses where its low coefficient of friction, high UV resistance, excellent biocompatibility, unsurpassed chemical resistance and high heat and flame resistance provide a competitive advantage.

For ePTFE, the room temperature creep resistance of PTFE can be an issue in part because of its excellent ductility at cryogenic temperatures. Creep resistance can be improved somewhat with comonomer addition, the so-called modified PTFE resins. With sintered PTFE, fillers can be added which substantially reduce the creep resistance, but these fillers may compromise COF or chemical resistance, while greatly reducing drawability. Finally, despite its low COF, PTFE tends to demonstrate lower abrasion resistance relative to what is seen with a PET or Nylon, although these cannot tolerate the same severity of chemical and temperature environments.

CONCLUSION

PTFE has been in the market for more than sixty years, but it is still finding new applications at the frontiers of the chemical and combustion industries, where its unique mix of very low and high service temperatures, chemical resistance, and low coefficient of friction should make it one of the first materials to be considered in any air or liquid filter application.

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