PTFE as a material

Thanks to its chemical structure, the molecule polytetrafluoroethylene exhibits remarkable and unique properties unusual in plastics. The carbon chain is completely shielded by the fluorine atoms surrounding it.

• PTFE does not readily react with other chemicals
  An attack of chemicals on the carbon chain is not possible due the shielding effect of the fluorine atoms. Given the low polarizability of halogen atoms, this is tantamount to an “armor” protecting the carbon backbone against exter-nal influences. The C-F bond is the strongest single bond formed by carbon (very polar bond, strong attraction forces between the chains). This extremely tight C-F bond can be attacked only by very few, particularly aggressive substances such as for example alkali metals or elemental fluorine.
• other substances do not adhere or only very slightly to PTFE
  The forces acting between the polymer chains are considerably smaller than in other plastics. This results from the high binding energy between the fluorine and carbon atoms as well as from the low polarizability of the fluorine atoms.
• the material with the lowest coefficient of friction
  The small forces acting between the polymer chains lead to PTFE having the lowest coefficient of friction of all solid materials. Since the coefficients of static and kinetic friction are the same, there is no “stick-slip” effect.
• advantageous electrical values
  The rigid, alternately arranged CF2 groups coupled with a negligible electron mobility within the molecule yield the extremely advantageous electrical values.

The source materials for the manufacture of PTFE are fluorite (CaF2) and sulfuric acid (H2SO4). After a run through different chemical processing steps, tetrafluoroethylene is formed (TFE). The polymerization of tetrafluoroethylene (TFE) to PTFE then takes place in a reactor at temperatures between 20 to 90 °C and pressures of 2 to 15 bar.

Press and sintering technology

The semi-finished products are manufactured with appropriate allowances. These allowances are set such that as little waste as possible is produced during machining while at the same time ensuring that the finished part contour can be manufactured without problems. PTFE CC GmbH has a narrowly graded tools park to achieve an optimal ratio between the dimensions of the semi-finished product and that of the finished component. Due to the varying materialrelated shrinkage that occurs (there are marked differences between pure PTFE, modified PTFE and PTFE com-pounds), slight variances in the dimensions of the semi-finished products are possible even when working with the same tool dimensions. For this reason, we do not work with semi-finished listings, but adapt the dimensions of the semi-finished product to your finished product dimensions.

PTFE owes its extremely high melt viscosity (approx. 1011 to 1012 Pa x s) to a high molecular weight. Therefore, even if PTFE from a purely chemical point of view actually belongs to the thermoplastics product group, it is in reality a sintered material that cannot be processed using the typical thermoplastic processing techniques such as injection molding or blow molding. It also cannot be welded using standard thermoplastic welding methods. With special press and sintering techniques, simple geometric semi-finished forms such as solid rods, hollow rods and sheets can be manufactured. The PTFE material is mechanically compacted at forming pressures between 120 and 700 bar. The further processing of these blanks, also called “green compacts”, then follows in special sintering furnaces. In these furnaces, the material is then finished sintered under precisely defined sintering programs which depend on the material and fillers and also the wall thickness. The sintering temperature lies between 360 and 380 °C. When the crystallization melting point is reached at about 342 °C, the PTFE turns into an amorphous state and the previously compressed powder particles sinter together to form a homogenous structure. Even after the melting or gel point is reached, the sintering of the pressed parts is “free-form”, since the gel stability of PTFE is very high given the high molecular weight.

Semi-finished parts are manufactured by extrusion

During a process called RAM extrusion, a periodically acting punch presses the PTFE powder batchwise through a heated tube. The pressing force required for the compression is realized by wall friction or, additionally, by a brake. Several heat zones along the extrusion tube and a special temperature profile ensure that the PTFE rod is sintered through fully. The design is generally subject to the shape of the extrusion cylinder. For example, when a tube is manufactured, a mandrel is used to form the inner diameter. Using RAM extrusion, rod-shaped semi-finished prod-ucts (solid rods, hollow rods, tubes) are manufactured quasi-continuously and economically.

Owing to the strength of the fluorine-carbon bond and the almost complete shielding of the carbon chain by the fluorine atoms, PTFE has a very good and universal resistance against chemicals. This makes a chemical resistance list, as required for other plastics, unnecessary.

Exceptions here are:

• Hydrofluorocarbons: they lead to swelling, which after longer contact becomes irreversible.
• Dissolved or molten alkali metals: eliminate fluorine, destroy the polymer.
• Halogens, elemental fluorine, chlorine trifluoride: in some instances, strong chemical reactions at high tempera-tures causing material damage.
• Nitrating acid (mixture of conc. sulfuric and nitric acid): at temperatures of > 100 °C gradual material degradation.
• Certain monomers, e.g. styrene, butadiene, acrylonitrile: can infiltrate the surface, causing swelling and/or destruc-tion of the polymer.

The chemical stability of PTFE compounds depends on the type and amount of the fillers used. A general statement as for unfilled PTFE is unfortunately not possible here.

Pure PTFE and modified pure PTFE do not belong to the radiation resistant plastics. They are particularly sensitive to ionized radiation in the presence of oxygen. The primary step in reaction to the energy influence is not the creation of radicals but unsaturated fragments. The main changes PTFE goes through under radiation are gas development, production of fluoride ions, weight loss, change in density and crystallinity as well as in the mechanical properties.

• Resistance to gamma radiation: 2.0E+03 (gamma radiation dose in Joule/kg)
• According to DIN 25493, a maximum dose of 10² Gy is recommended for PTFE.