Thermoplastics (PTFE and others)

PTFE (polytetrafluorethylene) – filled and unfilled – is an unbranched, linear, semi-crystalline polymer of fluorine and carbon.

PTFE belongs to the class of polyolefins, which also includes PCTFE (polychlorotrifluoroethylene). Even though it exhibits properties that would tend to require processing typical of thermosets, it is in fact a thermoplastic.

PTFE is characterised by several features:

PTFE is highly chemically inert. Even aggressive acids such as aqua regia have no effect on it. The reason for this is the especially strong bond between the carbon and fluorine atoms. Therefore, many substances are unable to break the bonds and react with PTFE chemically. PTFE is extremely resistant to all bases, alcohols, ketones, petroleum fuels, oils, etc. It is only attacked by very strong reducing agents such as solutions of alkali metals (e.g. sodium) in liquid ammonia or very strong oxidising agents such as elemental fluorine at higher temperatures. It can be used in temperatures up to 260 °C and remains stable down to -200 °C. PTFE has a slightly waxy surface (not as pronounced as PE) and is physiologically harmless. PTFE has a very low coefficient of friction. PTFE slides on PTFE to just about the same extent as wet ice does on wet ice. In addition, its static friction is equal to its dynamic friction, so there is no abrupt jolt at the transition from the motionless to the moving state. The stick-slip phenomenon is practically absent with sealing elements made from PTFE (+ compounds). As the surface tension of PTFE is very low, virtually no materials adhere to it. PTFE is difficult to wet and almost impossible to glue. For density, hardness and tear strength, please refer to the data sheets for the individual materials. High thermal expansion, phase change from triclinic (three non-orthogonal vectors – no axis of symmetry) to hexagonal (six-fold axis of symmetry or inversion axis) crystal lattice @ 19 °C with change in volume.

A few parameters of virgin PTFE:

Refractive index: PTFE has a very low refractive index of about 1.38. Specific heat capacity: 0.96 J/(gK) Thermal conductivity: 0.25 W/(Km) Permittivity: 2.1 (D150 @ 103 Hz); dielectric constant: 0.3x10-4 @ 103 Hz; bulk resistivity: 1018 Ωcm

Polyoxymethylene (POM, also known as polyacetal, polyformaldehyde or just acetal) is a high-molecular-weight thermoplastic. Owing to its high stiffness, low coefficient of friction and excellent dimensional and thermal stability, it is ideal for precision-engineered components.

Polyamides (PA) are linear polymers with the same amide group repeated regularly along the main chain. Polyamides exhibit outstanding strength and toughness. They resist organic solvents well but are easily attacked by acids and oxidising chemicals. Polyamides react to the moisture in their surroundings by absorbing or releasing water. This absorption of water essentially depends on the amide group concentration. In air, PA6G absorbs approx. 2.5–3.5% water.