Fluoropolymer
Fluoropolymers, or fluorine plastics are polymers in which a large part of most or even all of the hydrogens are replaced by fluorine otherwise contained.
Fluoropolymers are characterized, inter alia, to by high chemical and temperature resistance. The mechanical properties, however, are rather moderate. The greatest economic importance has 60-70 % market share polytetrafluoroethylene (PTFE ), which is marketed under the trade name Teflon ®. Another important fluoropolymers are tetrafluoroethylene -hexafluoropropylene copolymer ( FEP), and polychlorotrifluoroethylene (PCTFE). All thermoplastic fluoropolymers together account for 30 % market share of total fluorine polymers. Generally, the rule is that the temperature and chemical resistance, the better the higher the fluorine content. Here is the 1938 discovered polytetrafluoroethene still unsurpassed.
- 3.1 PTFE
- 3.2 Thermoplastic fluoropolymers
General
A division of the fluoropolymers is possible in those that are not processable in the melt (English non -melt processible Fluoroplastics such as PTFE), those which are melt processable (English Melt processible Fluoroplastics, thermoplastic fluoropolymers or fluorothermoplasts ), fluorine rubber and fluoroelastomers.
Fluoropolymers are often made solely in the main chain of carbon atoms, but there are also those with heteroatoms, such as Polyfluorsiloxane and polyfluoroalkoxyphosphazenes.
Properties
Polytetrafluoroethylene (PTFE ) is not only quantitatively the most important fluoropolymer, the properties of all other fluoropolymers can be explained to him, since PTFE is used as a regular body for most other fluoropolymers.
Chemical Properties
The mechanical properties of a material are based on the chemical properties. Polytetrafluoroethene is despite the large electronegativity difference between carbon and fluorine, a non-polar polymer because cancel the spatially opposite dipoles in their effect. The chemical characteristics are also determined by the strong carbon-fluorine bond, which is difficult to be broken. Furthermore, provision unbranched chain molecule with a very high degree of polymerization (strong linearity) to high crystallinity. The molecular structure of PTFE is completely symmetrical; since the fluorine atoms (relative to hydrogen in polyethene ) also are relatively large, they shield and force the molecular chain, the carbon skeleton in a helical structure.
Mechanical properties
In general, at low polarity strength, stiffness and hardness as well as thermal expansion are low, the insulation capacity, however, is large. This is also the case of PTFE. Due to the high crystallinity, a high softening temperature and an extremely high melt viscosity and a high resistance to heat and chemicals. Act simultaneously as little intermolecular forces, it results in low strength and stiffness and low surface adhesion.
Properties of thermoplastic fluoropolymers
The processing properties of PTFE are bad: Due to the high viscosity of the melt PTFE can not be melt processed. However, since thermoplastic processibility is desirable, this is achieved by modifications of the monomer tetrafluoroethylene by fluorine atoms are replaced by chlorine, as polychlorotrifluoroethylene or trifluoromethyl, as tetrafluoroethylene - hexafluoropropylene copolymer, leading to disturbances in the regular molecular structure. Although these polymers are thermoplastic fluoropolymers, but their chemical and heat resistance is inferior by this modification.
Use and processing
Fluorine polymers have high chemical and thermal stability, good electrical insulation properties, excellent weatherability, anti-adhesive behavior and are non-flammable. Furthermore, they are characterized by a good notch toughness, and resistance at high temperatures. From the anti-adhesive behavior is followed by a low wettability and good sliding properties. Finally, fluoropolymers are physiologically harmless.
Because of these properties, fluoropolymers be counted among the high-performance plastics, even if especially the elastic modulus and the tensile strength of thermoplastic fluoropolymers is closer to those of standard plastics. A disadvantage is the high cost and the difficulty in processing fluoropolymers.
PTFE
PTFE has among all materials, the largest chemical resistance, and there is high temperature resistant, non-combustible and a good electrical insulator. PTFE is used in a range of -270 to 280 ° C. Above the melting temperature of PTFE not use, but only softens at about 400 ° C decomposition takes place. PTFE is not thermoplastic and can not be processed accordingly ( the high viscosity in the melt). It is white and flexible ( has a low strength and hardness) and is very slippery. PTFE is used in a laboratory environment, for high temperature insulation and high voltage engineering. Foamed PTFE has a microporous structure and is water repellent and permeable to air and is under the name of Gore -Tex ® use as clothing. PTFE tends to creep under load; However, the can be counteracted by fillers. Because PTFE is very non-polar, it is anti-adhesive, and has excellent sliding properties. Due to the strong carbon- fluorine bond, it is also very resistant to weathering and UV resistant.
Since PTFE above the high melting temperature is very viscous, it can not be processed by conventional methods. PTFE is therefore either of the two processes in the form of pastes or powders by pressing and sintering, as well as a combination thereof. Machining is avoided due to the high material costs.
Thermoplastic fluoropolymers
There exist in addition to the two shown here fluoropolymers tetrafluoroethylene -hexafluoropropylene copolymer ( FEP), and polychlorotrifluoroethylene (PCTFE ), a number of other thermoplastic fluoropolymers. However, these polymers are not as durable as PTFE; PCTFE is attacked by chlorinated hydrocarbons and esters, FEP of chlorinated hydrocarbons. The thermoplastics can be processed by conventional methods such as extrusion and injection molding. Presentation and application of these polymers is analogous to PTFE.
Economic Importance
Today's world demand for fluoropolymers is about 100,000 tons per year ( 2004). Fluoropolymers are available with 10 - very expensive, due to the expensive manufacturing process 35 € / kg compared to standard plastics. Basis for fluorine production is the mineral fluorite, which is sufficiently available worldwide. Important areas can be found in the construction of chemical plants and equipment, mechanical engineering and electrical engineering.