PTFE Dispersions

PTFE resin dispersions are milky-white liquids, with viscosity of approximately 20 cP and pH about 10. The resin contained therein has the characteristics of fine powders, that is, a high sensitivity to shear. 

The relationship between solids content and gravity is approximately linear and can be expressed by the equation [5]  

Correlation between Solids Content of PTFE Dispersion and Its Specific Gravity 

Solids Content, % Specific Gravity Amount of Solids, g/L 

The viscosity of PTFE dispersions increases proportionally with increasing solids content up to about 30% to 35% solids, and beyond that the increase is much more rapid. Dispersions with surfactants exhibit the same pattern, but the rate of increase is faster and depends on the type and amount of surfactant [5]. 

---

The discovery of PTFE (1) in 1938 opened the commercial field of perfluoropolymers. Initial production of PTFE was directed toward the World War II effort, and commercial production was delayed by Du Pont until 1947. Commercial PTFE is manufactured by two different polymerization techniques that result in two different types of chemically identical polymer. Suspension polymerization produces a granular resin, and emulsion polymerization produces the coagulated dispersion that is often referred to as a fine powder or PTFE dispersion. 

In a typical process a preform billet is produced by compacting a mixture of 83 parts PTFE dispersion polymer and 17 parts of petroleum ether (100-120°C fraction). This is then extmded using a vertical ram extruder. The extrudate is subsequently heated in an oven at about 105°C to remove the lubricant, this being followed by sintering at about 380°C. By this process it is possible to produce thin-walled tube with excellent flexing fatigue resistance and to coat wire with very thin coatings or polymer. 

The resistance of PTFE to creep can be improved by blending in up to 25% of glass or asbestos fibre using PTFE dispersions as mentioned in the previous section. By the same technique alumina, silica and lithia may be incorporated to... 

See also Teflon PTFE-based ionomers, 14 481 PTFE dispersion, 18 288. See also Polytetrafluoroethylene (PTFE)... 

Emulsion polymerization a sufficient amount of dispersing agent and mild agitation is employed. This produces small colloidal particles dispersed in the aqueous reaction medium. In this procedure, called aqueous dispersion polymerization, precipitation of the resin particles is avoided. The coagulated dispersion produced by emulsion polymerization is often called a fine-powder or PTFE dispersion. 

The major utility of PTFE dispersions is that they allow processing of PTFE resin, which cannot be processed as ordinary polymeric melt, because of its extraordinarily high melt viscosity, or as solution, because it is insoluble. Thus, PTFE dispersions can be used to coat fabrics and yams, impregnate fibers, nonwoven fabrics, and other porous structures to produce antistick and low-friction coatings on metals and other substrates and to produce cast films. 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be heated to about 290°C (555°F) for several minutes to remove the surfactant. Lower temperatures and longer times are used if the substrate cannot tolerate such a high temperature. In some cases, the impregnated material is calendered or compressed in a mold to compact the PTFE resin and to hold it in place. 

FIGURE 6.3 Detail of coating of glass fabric by PTFE dispersion using smooth bars. 

Another application of PTFE dispersions is the preparation of a variety of compositions with other materials, such as mineral fillers, other polymers in powdered form by co-coagulation. The dispersion of the other component is blended with the PTFE dispersion and the blend is then coagulated. The resulting composition can be processed by extrusion with lubricants (see processing of fine powders) or by compression molding.16... 

Aqueous dispersions of these two melt-processible perfluoropolymers are processed in a way similar to PTFE dispersion. FEP dispersions can be used for coating fabrics, metals, and polyimide films. They are very well suited for bonding seals and bearings from PTFE to metallic and nonmetallic components and as nonstick and low-friction coatings for metals.16 FEP can be fused completely into a continuous film in approximately 1 min at 400°C (752°F) or 40 min at 290°C (554°F).17 PFA is used to coat various surfaces, including glass fabric, glass, and metals. 

Teflon PTFE, Dispersion Properties and Processing Techniques, Bulletin No. X-50G, E. I. du Pont de Nemours Co., Wilmington, DE, Publication E-55541-2, p. 2. 

At about the same time as the papers by Speerschneider and Li, Symons [4,9] described the growth of lamellar single crystals from dispersed PTFE dispersion particles (Fig. 4) and we described thin lamellae on the free surfaces of bulk dispersion PTFE samples (Fig. 5) [4], both prepared by slow cooling fol-... 

Fig. 5 Lamellae on a free surface of compacted PTFE dispersion particles sintered at 380 °C followed by slow cooling. (Similar to Fig. IV-67in Ref. [4])...

The morphology of nascent PTFE dispersion also remains unclear. Several authors, during the next decade of PTFE morphology research, suggested standard-size PTFE dispersion particles (ellipsoidal particles with dimensions... 

Fig. 11 Nascent radiation initiated PTFE dispersion particles. The polymerization conditions were (a) 0% emulsifier, 90 min, (b) 0.5% emulsifier, 60 min, (c) 1% emulsifier, 60 min, (d) 2% emulsifier, 90 min, all for essentially the same radiation dose rate at 25 °C in water (with 1.3% hexadecane and ammonium perfluorooctanoate emulsifier) at 30-kg/cm2 pressure. The measured molecular weights, the corresponding extended chain lengths (eel) the and dimensional characteristics are, respectively, (a) Mn = 230 x 104, edn = 6.0 pm, particle volume approximately 7 x 108 Da (b) Mn = 50 x 104, eel = 1.3 pm, rod diameter approximately 100 molecules (c) Mn = 20 x 104, ed = 0.52 pm rod length (d) Mn=2xl04, ed = 520A. (Reprinted from Ref. [14] with permission from Wiley-Interscience)...

Figure 14a is a typical TEM micrograph of Pt/C shadowed standard-size PTFE dispersion particles, (here sample G) with dark-field images of unshadowed particles in the insets typical BFDC micrographs of several standard-size DuPont dispersion particles are shown in Fig. 14b and c. The dark-field micrographs, taken with 100 reflections, are similar to the bright-held ones except... 

The ability to dr aw approximately 100 - A diameter fibrils from PTFE dispersion particles, either by fracture after cold compaction [34] or by uniaxial or biaxial expansion of sheets or tubes after paste extrusion (extrusion of mixtures of the dispersion particles and lubricants such as mineral spirits) and lubricant removal [35], was demonstrated a number of years ago. The former process results in the development of a myriad of fibrils spanning the gap between the fracture faces these were utilized for ED characterization of the PTFE conformation and crystal packing. 

With fluorosulfonate ionomers other than Nation, the specifications for the GDL has to be changed. More recently, Nishikawa et al. [79] developed GDLs for novel organic-inorganic hybrid electrolytes. They mixed Triton and PTFE dispersions with carbon black and coated the mixture on to carbon paper that had been wet-proofed with FEP copolymer. A nanohybrid electrolyte was prepared by a sol-gel method and formed over the GDL. Addition of uncatalyzed carbon black to the cathode GDL was found to enhance the performance at high current densities. 

---