**2. Filler grade PTFE**

PTFE undergoes creep (deformation under loading) which can be reduced with the help of high shear modulus fillers such as glass. Fillers hinder the relative movement of the PTFE molecules past one another and in this way reduce creep or deformation of the parts, reduce the wear rate of parts used in dynamic applications, and reduce the coefficient of thermal expansion. Other popular fillers used along with PTFE include carbon (improved thermal conductivity and low deformation under load), graphite (improved lubrication), bronze or stainless steel (excellent wear resistance) etc. Since PTFE powder is hydrophobic (it floats in water as seen in Fig.3) and does not flow freely, mixing it will free flowing fillers is a major task. One has to use a cryogenic medium such as liquid nitrogen to remove the electrostatic forces that hold the PTFE powder together. This technique is being used to manufacture thermoluminescent material filled PTFE discs (1:3 weight ratio) which are used for personnel radiation monitoring in India and elsewhere [2]. After radiation exposures, these discs are usually heated to 300°C during luminescence measurements and PTFE is the suitable binder for such applications. Organic liquids such as ethanol can also be used to mix free flowing fillers with non free flow PTFE since they wet PTFE powder unlike water medium. Alternately, one could use mechanical shearing force to separate the PTFE particles. The last choice is industrially viable and hence was adopted by us for manufacturing filler grade PTFE powders (Figs.4 and 5). An overview of different fillers used along with end use can be had from the brochures supplied by Dupont and other PTFE manufacturers.

**Figure 3.** PTFE is hydrophobic – water repellant

PTFE repels everything and hence no molecules can stick to the PTFE surface which makes it slippery (Fig.2). Ice is the only material that is slicker than PTFE. A thin PTFE coating over metal cooking pans makes them nonsticky with food items. PTFE can withstand a wide range of temperature (-184°C to 260°C) and is used in cold as well hot environments. It is hydrophobic (water repellant) and hence is resistant to weathering. It has fantastic chemical resistance and superb electrical insulation properties. It is the only plastic which can withstand temperatures up to 300°C. On heating to temperatures above 400°C, PTFE disintegrates with the production of carbon. Above 500°C, when heated in air, PTFE disappears altogether due to the production and escape of carbon and fluorine in the form of CO2 and fluorine gases into the atmosphere [1]. PTFE is insoluble in common solvents and is resistant to nearly all acidic and alkaline

**Figure 1.** Typical interwinded PTFE polymer. Due to strong intermolecular forces, the polymer chains are tangled. Due

to its chemical inertness, PTFE cannot be cross-linked.

170 Sintering Techniques of Materials

**Figure 2.** The molecular structure of PTFE

**Figure 4.** A blue pigment (3%) mixed non free flow PTFE powder.

**Figure 5.** Sintered rods made from blue pigmented PTFE, glass mixed PTFE and carbon mixed PTFE
