What is PTFE?
What is PTFE?
Polytetrafluoroethylene or PTFE is the commonly used versatile, high-performance fluoropolymer. It is made up of carbon and fluorine atoms. The chemical structure of PTFE [CF2-CF2]n is like that of polyethylene (PE). The hydrogen atoms in PE are completely replaced by fluorine. Hence it is referred to as perfluoro polymer. However, it is important to note that in practice PTFE and PE are prepared and used in totally different ways.
Molecular Structure of PTFE
It is the size of a fluorine atom that forms a uniform and continuous sheath around carbon-carbon bonds. This uniform fluorine sheath:
- Imparts good chemical resistance and stability to the molecule.
- Provides electrical inertness to the molecule.
The fluorine content in PTFE is theoretically 76% and it has 95% crystallinity.
PTFE was first discovered “accidentally” in 1938 by Dr. Plunkett at DuPont. After that PTFE was made commercially available in 1947 with the trademark “Teflon™” from Chemours. It was the discovery of PTFE that accelerated the development of the other fluoropolymers.
How is PTFE made?
How is PTFE made?
PTFE is a linear polymer of tetrafluoroethylene (TFE). It is manufactured by a free-radical polymerization mechanism in an aqueous media via the addition polymerization of TFE in a batch process.What are the properties of PTFE?
What are the properties of PTFE?
The basic properties of PTFE which make it an interesting material with high commercial value are:
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Density: It has a density in the range of 2.1 - 2.3 g/cm3 and melt viscosity in the range of 1-10 GPa per second.
Chemically resistant: The exceptions include molten alkali metals, gaseous fluorine at high temperatures and pressures, and a few organic halogenated compounds such as chlorine trifluoride (ClF3) and oxygen difluoride (OF2). View PTFE Grades With Good Chemical Resistance »
Mechanical properties: Properties of PTFE are generally inferior to engineering plastics at room temperature. Compounding with fillers has been the strategy to overcome this shortage. PTFE has useful mechanical properties in its use temperature range. These properties are also affected by processing variables such a preform pressure, sintering temperature, cooling rate, etc. Polymer variable such as molar mass, particle size, particle size distribution poses a significant impact on mechanical properties.
Electrical properties: High insulation resistance, low dielectric constant, and low dielectric constant of 2.0 due to the highly symmetric structure of the macromolecules.
Thermal properties: PTFE exhibits high thermal stability without obvious degradation below 440°C. These materials can becontinuously used below 260°C.
Radiation resistance: PTFE is attacked by radiation, and degradation in the air begins at a dose of 0.02 Mrad.
These properties come from the special electronic structure of the fluorine atom, the stable carbon-fluorine covalent bonding, and the unique intramolecular and intermolecular interactions between the fluorinated polymer segments and the main chains.
| Property | Value |
| Melting Temperature (°C) | 317-337 |
| Tensile Modulus (MPa) | 550 |
| Elongation at Break (%) | 300-550 |
| Dielectric strength (kV/mm) | 19.7 |
| Dielectric Constant | 2.0 |
| Dynamic Co-efficient of Friction | 0.04 |
| Surface Energy (Dynes/g) | 18 |
| Appl. Temperature (°C) | 260 |
| Refractive Index | 1.35 |
| Get more information about polymer properties here » | |
What is the impact of fillers on PTFE properties?
What is the impact of fillers on PTFE properties?
The addition of fillers or additives results in the following changes in PTFE:- They can enhance its mechanical properties, particularly creep and wear rate.
- They provide excellent properties of PTFE at low and high temperatures.
- They increase the porosity of PTFE compounds.
- The dielectric strength decreases while the dielectric constant and dissipation factor increase.
- The chemical properties will depend on the type of filler used.
- They impart a change in the electrical and thermal conductivity of PTFE.
Up to 40% by volume of filler can be added to the PTFE without complete loss of physical properties. The impact of fillers below 5% is low.
Some common fillers are glass fiber, carbon, carbon fiber, steel, bronze, graphite, etc.
- Glass fiber has a positive impact on the creep performance of PTFE. The fiber reduces its low and high temperatures. Glass-filled compounds perform well in oxidizing environments. They further improve the wear characteristics of PTFE.
- Carbon reduces creep, increases hardness, and elevates the thermal conductivity of PTFE. When combined with graphite, the wear resistance of carbon-filled compounds can be improved. They are suitable for non-lubricated applications such as piston rings in compressor cylinders. Further, graphite imparts excellent wear properties to PTFE. Graphite-filled PTFE has an extremely low coefficient of friction.
- Carbon fiber lowers creep, increases flex and compressive modulus, and raises hardness. Unlike glass fibers, carbon fibers are inert to hydrofluoric acid and strong bases. Carbon fiber PTFE compounds have low coefficient of thermal expansion and high thermal conductivity. These compounds are ideal for automotive parts in shock absorbers, water pumps, etc.
- Bronze-filled PTFE compounds have high thermal and electrical conductivity. Hence, these compounds are suitable for applications where a part is subjected to load in extreme temperatures.
Other fillers: Calcium fluoride, Alumina, Mica, and polymeric fillers.
How PTFE is processed?
How PTFE is processed?
PTFE has a very high-melt viscosity and a high-melting temperature. This is due to the rigid polymeric chain structure. This in turn makes processing difficult by extrusion and injection molding. Processing technologies have more similarities to powder metallurgy than those of traditional processing.- Sintering, pressing, ram or paste extrusion, compression molding or isotactic molding, machining, hot stamping, and extrusion of pre-sintered powders on special machines.
- Paste extrusion in which PTFE blends with a hydrocarbon, before molding a preform. This continuously fabricates PTFE into tubes, tapes, and wire insulation. The hydrocarbon vaporizes before the part is sintered.
- Dispersion – metal coatings, coatings, pulverization, impregnation, cast for thin films, and fiber spinning.
- [Operating range (-270°C) -200°C to 260°C (280°C)]
The properties of the PTFE products are strongly dependent on the processing procedure. These include:
- Particle size
- Sintering temperature
- Processing pressure
Other fluoropolymers are needed for specific applications where PTFE is not completely suitable. This led to a search for melt-processable fluoropolymers and the development of other members of the family.
What are the physical forms of PTFE available?
What are the physical forms of PTFE available?
PTFE is available in granular, fine powder and water-based dispersion forms.- The granular PTFE resin is produced by suspension polymerization in an aqueous medium with little or no dispersing agent. Granular PTFE resins are mainly used for molding (compression and isostatic) and ram extrusion.
- The fine PTFE powder is prepared by controlled emulsion polymerization, and the products are white, small-sized particles. Fine PTFE powders can be processed into thin sections by paste extrusion or used as additives to increase wear resistance or frictional property of other materials.
- PTFE dispersions are prepared by the aqueous polymerization using more dispersing agents with agitation. Dispersions are used for coatings and film casting.
What are the main applications of PTFE?
What are the main applications of PTFE?
One of the common applications of this polymer is non-stick coating in kitchen cookware (pans, baking trays, etc.). PTFE is used as a cost-effective solution for oil & gas, chemical processing, industrial, electrical/electronic, and construction sectors.Applications of Polytetrafluoroethylene (PTFE)
Where else can you find PTFE? View key applications of this polymer here »
What are the limitations of PTFE?
What are the limitations of PTFE?
The conventional PTFE has some limitations in its applications, such as:- Impossibility of using conventional molten-state processing methods and difficulty and cost of the suitable specific methods
- Sensitivity to creep and abrasion
- Significant dimensional variation around glass transition temperature (19°C)
- Difficulties of joining
- Corrosive and prone to toxic fumes
- Low radiation resistance
What are the commercially available PTFE grades?
