Applications PTFE dispersions

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. 

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... 

Table 5.11. PTFE Dispersion Application Categories Based on Fabrication Processing...

Many applications of dispersions require a number of properties in the end use which can be achieved by the addition of fillers, pigments, leveling enhancement additives, flow improvement additives, and other additives. For example, cold flow (creep) properties of the coating can be reduced by the addition of fillers such as fiberglass. Additives should be mixed only by mild stirring to avoid coagulation of PTFE. 

A convenient way to apply PTFE dispersions to surfaces is by use of aqueous dispersion coatings. For this a dispersion as provided by the manufacturer maybe used. There are also many coating formulations that can be purchased for use in such applications as cookware and bakeware. Coating is done as dip coating and, with thickened dispersions, as roller coating. 

For some applications the powder is suspended in an aqueous medium or a solvent with the help of emnlsifying agents and then sprayed onto the substrate. The powder is also used as a filler to prepare sprayable compositions of PTFE dispersions, which then can be used to coat various substrates (45). 

Chemical Applications. The chemical processing industry uses large amounts of granular and fine powder PTFE. Soft packing appHcations are manufactured from dispersions, and hard packings are molded or machined from stocks and shapes made from granular resin. 

PTFE is used for lining chutes and coating other metal objects where low coefficients of friction or non-adhesive characteristics are required. Because of its excellent flexing resistance, inner linings made from dispersion polymer are used in flexible steam hose. A variety of mouldings are used in aircraft and missiles and also in other applications where use at elevated temperatures is required. 

Porous Membrane DS Devices. The applicability of a simple tubular DS based on a porous hydrophobic PTFE membrane tube was demonstrated for the collection of S02 (dilute H202 was used as the scrubber liquid, and conductometric detection was used) (46). The parameters of available tubular membranes that are important in determining the overall behavior of such a device include the following First, the fractional surface porosity, which is typically between 0.4 and 0.7 and represents the probability of an analyte gas molecule entering a pore in the event of a collision with the wall. Second, wall thickness, which is typically between 25 and 1000 xm and determines, together with the pore tortuosity (a measure of how convoluted the path is from one side of the membrane to the other), the overall diffusion distance from one side of the wall to the other. If uptake probability at the air-liquid interface in the pore is not the controlling factor, then items 1 and 2 together determine the collection efficiency. The transport of the analyte gas molecule takes place within the pores, in the gas phase. This process is far faster than the situation with a hydrophilic membrane the relaxation time is well below 100 ms, and the overall response time may in fact be determined by liquid-phase diffusion in the boundary layer within the lumen of the membrane tube, by liquid-phase dispersion within the... 

PTFE aqueous dispersions are made by the polymerization process used to make fine powders. Raw dispersions are polymerized to different particle sizes.24 The optimum particle size for most applications is about 0.2 pm. The dispersion from the autoclave is stabilized by the addition of nonionic or anionic surfactants, followed by concentration to a solids content of 60 to 65% by electrodecantation, evaporation, or thermal concentration.25 After further modification with chemical additives, the commercial product is sold with a polymer content of about 60% by weight, viscosity of several centipoise, and specific gravity around 1.5. The processing characteristics of the dispersion depends on the conditions for the polymerization and the type and amounts of the chemical additives contained in it. 

---