Der Konstukteur 9/ 2004 Authors: Dipl.- Ing (FH) Walter Schuhmacher und Dr. Michael Schlipf, ElringKlinger Kunststofftechnik GmbH, Heidenheim - BietigheimBissingen

Tailor-Made Compounds Based on PTFE and Modified PTFE

Figure 2: Comparison between abrasion and wear (in cm³/Nm)

Figure 3: Comparison between static friction coefficients

Figre 4: Cold flow of PTFEITMF compounds may be varied by different fillers

Products made from polytetrafluoroethylene (PTFE/TFM) and PTFE/TFM compounds are used in virtually all fields of engineering - in the automotive industry, in plant and mechanical engineering as well as in the electrical industry and in medical technology. These compounds can be adapted individually to suit the specific application profile.

Continuing to rank at the top of fully fluorinated materials is PTFE, a partially crystalline compound made from tetrafluoroethylene (TFE). It has a melting temperature of 327 °C. Its very high molecular weight of up to 10⁸ g/mol, however, results in such a high melting viscosity that processing requires special pressing and sintering techniques or so-called paste extrusion technology. PTFE’s most important properties include: virtually unlimited chemical resistance; operating temperatures ranging from –250°C to +250°C; no water absorption; no cracking; no aging and high levels of anti-adhesive properties. In addition, PTFE has the lowest friction coefficient of all solid substances and excellent dielectric properties. Co-polymerization with a small quantity of a perfluorinated modifier, perfluoropropylvinylether (PPVE), and reduction of the molecular weight results in a new product that continues to be processed according to the same methods typically used for PTFE but offers further improvements of its properties profile. In particular, such improvements are reduced cold flow and lower permeation as well as reduced pore volume and a lower stretch-void-index(SVI), which indicates the compound’s tendency to develop pores when being stretched.

The Role of Fillers

Processing of PTFE and TFM-PTFE involves a sintering step, during which a maximum temperature of app. 370 to 380 °C is reached. This means that only highly temperature-resistant fillers may be used.  Typical anorganic fillers are glass fibers, carbon (powder/fibers), graphite, bronze, molybdenum disulfide, steel powder, aluminum oxide, calcium fluoride and mica. Organic fillers with high temperature resistance are polyimide P1, polyphenylenesulfide PPS, polyamideimide PAl, polyetheretherketone PEEK, polyphenylenesulfone PPSO₂ aromatic polyester and aramide.

Fillers in PTFE compounds primarily serve to reduce abrasion in dynamic applications involving contact with a mating surface (figure 2) and to reduce the cold flow typically encountered with PTFE under pressure loads (figure 4). Since compounds normally have a higher friction coefficient than PTFE or TFM-PTFE unfilled (figure 3), the addition of an ‘internal lubricant’ to reduce friction is often required. Typical anorganic fillers meeting this requirement are graphite and molybdenum disulfide. Because organic fillers lead to a clearly lower increase of the friction coefficient than anorganic fillers, organically filled PTFE compounds usually do not require any lubricating additives.

Looking at the fillers alone reveals both synergistic and counter-acting effects. When including the PTFE/TFM matrix (table) into these considerations, the inherent correlations become even more complex.

Two Case Studies

Seat rings and sealing elements for shut-off valves in the chemical industry: to ensure reliable sealing of the shut-off valve a compound with low cold flow properties and resistance to as many chemicals as possible is required. In case of high switching cycle rates, high abrasion resistance is added to these requirements .

Influences	Mechanical Property	Cold Flow	Friction Coeffi-cient	Abrasion	Chemical Resis-tance	Expansion Coefficient	Thermal Conductivity
Matrix PTFE => TFM
Füllstoff Anorg./org.
Table: Influences of fillers on properties profile

Low cold flow can be achieved by using modified PTFE in conjunction with a high filler content. The higher the filler content, the higher the switching forces as a result of the increased friction coefficient and the lower the abrasion rate of the sealing element. The latter has a positive effect on service life. Typically, the contact surface used in these valves/fittings is stainless steel. Fillers, however, limit the nearly universal chemical resistance of the matrix material, increase the permeation rate through the material and are inhibitive or prohibitive for applications requiring special approvals, such as BGVV or FDA.

Consequently, for field applications of this type, modified PTFE, TFM-PTFE, which is only reinforced with a filler if required by the number of switching cycles and the resulting abrasion resistance, are increasingly being used successfully. Fillers of choice may be: glass fibers or carbon.

Sealing ring used in automotive applications, i.e. aluminum components: typical requirements profiles include extremely long service life, minimum friction forces, virtually no wear of the seal itself as well as the soft contact surface. Minimum wear combined with long service life can only be achieved by the use of fillers. The decision as to whether to use PTFE or modified PTFE is of secondary importance. To achieve the required protection of the soft contact surface, practically none of the anorganic fillers are suitable. Instead, either an organic high-performance polymer or carbon fiber must be chosen. Additionally, when combining various organic fillers, it is advantageous to have one of the components act as a stabilizer against deformation and abrasion while the other serves to reduce the friction coefficient. Typical product examples include:  

• Shaft seals with PTFE sealing lip for mechanical engineering (general machine construction) and automotive engineering
• Spring-energized seals and memory packings for hydraulic and pneumatic sealing applications
• PTFE as a membrane and bellows material for the chemical industry, painting technology, the pharmaceutical and food processing industries
• Casings for pistons, rollers and heating elements for abrasion and corrosion protection
• Insulation material for the electronics and electrical industry
• Hoses for the chemical industry, painting and medical technologies
• Isostatically pressed molded shapes
• Laminates and sheets

Approvals

PTFE and PTFE compound applications in the medical, pharmaceutical and food processing industries are subject to exacting hygienic requirements. They must be suitable for sterilization as well as being physiologically harmless. Unfilled PTFE is physiologically neutral, thus easily complying with the requirements pertaining to BGVV and FDA approvals. Many of our compounds, as well, have been awarded BGW-approval and/or FDA-conformance. Highly positive in this context is PTFE’s resistance to hot vapor, which means that PTFE parts are extremely well suited for sterilization.

Particularly important for medical technology applications are biocompatibility tests, specifically cyto-toxicity tests and tests for contact-allergenic properties (sensitization). Several PTFE types offered by Klinger Kunststofftechnik have been awarded such approvals.