Extrusion machine:What is PTFE (Polytetrafluoroethylene)
PTFE estrusion machine
PTFE polytetrafluoroethylene is a high-performance, multifunctional fluoropolymer composed of carbon and fluorine atoms. PTFE is one such fluoropolymer whose discovery has revolutionized the fluoropolymer group forever and paved the way for a wide variety of applications.
One of the common applications for PTFE materials includes non-stick coatings for kitchen cookware. Because of its non-reactive nature and the strength of the carbon-fluorine bond, it is commonly used in the manufacture of pipes and vessels for reactive and corrosive chemicals.
How is PTFE made?
The manufacturing process of PTFE is very similar to any other polymer. It is made by free radical polymerization technology in an aqueous medium using TFE addition polymerization in an intermittent process.
The chemical structure of PTFE is same to that of polyethylene; the only major difference is that the hydrogen atoms are completely replaced by fluorine. However, the preparation methods of PE and PTFE are very different.
The size of fluorine atoms is large, it forms a uniform and continuous sheath layer around the carbon-carbon bond, thus providing the molecule with good chemical resistance, electrical inertness and stability.
Interesting history of PTFE –
Like many other great discoveries, PTFE was discovered by accident. Roy J. Plunkett first discovered it in 1938 while working for Dupont in New Jersey. Plunkett’s original goal was to create a new chlorofluorocarbon refrigerant. However, the tetrafluoroethylene gas in the pressure bottle stopped flowing before the weight of the bottle reached the “empty” signal.
But when Plunkett measured the amount of gas used by weighing the bottles, he began interested in the source of the weight and eventually decided to take the bottles apart.
He discovered that the interior of the bottle was coated with a white waxy smooth material. It was later named polymerized perfluoroethylene, and the iron inside the container acted as a catalyst.
The new material was patented by Kinetic Chemical Company in 1941 as a new fluorinated plastic, and the Teflon trademark was registered in 1945. In 1961, the first American-made PTFE-coated pan was marketed in the U.S. with the name “The Happy Pan.” Since then, nonstick cookware has become one of the most common household products produced by thousands of manufacturers worldwide.
Characteristics and properties of PTFE –
PTFE has three main forms – granules, water-based dispersions and fine powders.
Granules PTFE materials are produced by suspension polymerization in aqueous media with little or no dispersant. Granules PTFE materials are mainly used in compression, isostatic pressing and plunger extrusion methods.
Water-based PTFE dispersions use the same aqueous polymerization, with more dispersant and mixing. Water-based dispersions are mainly used in coatings and film casting methods.
Fine powder PTFE is made by controlled emulsion polymerization of small white particles. Fine PTFE powders can be processed into flakes by paste extrusion or additives to improve wear resistance.
Other notable properties of PTFE include excellent resistance to high and low temperatures, electrical insulation, chemical inertness, low coefficient of friction (0.08 static and 0.01 dynamic), and non-stick properties over a wide temperature range (-260 to 260°C).
PTFE is one of the most reliable materials in terms of chemical resistance. It will only be attacked by molten alkali metals, organic halides such as chlorine trifluoride (ClF3) and oxygen difluoride (OF2), and gaseous fluorine at high temperatures.
PTFE also has impressive mechanical properties, but is not as good as other engineering plastics at room temperature. The addition of fillers has proven to be a successful method of overcoming this obstacle. Within its normal temperature range, PTFE exhibits a number of useful mechanical properties. These properties are also hindered by processing variables such as sintering temperature, preform pressure, cooling rate, etc. Polymer properties such as molar mass particle size and particle size distribution can negatively affect the mechanical properties.
PTFE has outstanding electrical insulation properties, low dielectric constant and insulation withstand voltage. The very low dielectric constant (2.0) is the result of the complex symmetrical structure of the macromolecule.
PTFE materials also show good thermal properties, without significant degradation below 440 °C.
It is also subject to airborne degradation and radiation attack, starting from a dose of 0.02 Mrad.
Traditional PTFE materials also have some drawbacks. As the following:
Creep and wear sensitive
It cannot be processed by molten state machining methods, and suitable methods can often be unconventional and extended.
Difficulties in joining
High dimensional change near the glass transition temperature.
Low radiation resistance
Corrosive and prone to produce toxic fumes.
Advantages of adding fillers –
The filler/additive is critical to increase the porosity of the PTFE compound and therefore affects the electrical properties – it reduces the dielectric strength while increasing the dielectric constant and dissipation factor.
Fillers can improve the performance of PTFE at high and low temperatures.
The change in chemical properties depends to a large extent on the type of additive used, however. In general, it also leaves positive results.
Usually, fluorinated thermoplastics are used in high performance applications with high temperature resistance, high purity, low temperature, chemical inertness, non-stick and self-lubricating properties. The following are some of the most common usage of PTFE:
Engineering – bearings, non-stick surfaces, valve seats, plugs, fittings, valves and pump parts.
Medical – Heart patches, cardiovascular grafts, ligament replacements.
Chemical industry – coating of pumps, diaphragms, impellers, heat exchangers, autoclaves, reaction vessels, storage tanks, containers, etc.
Automotive – valve stem seals, shaft seals, gaskets, O-rings, fuel hose liners, power steering, transmissions, etc.
Electrical and electronics – flexible printed circuit boards, electrical insulation, etc.
Best technology for processing PTFE –
The rigid polymer chain structure of PTFE makes it extremely difficult to process by conventional methods such as injection molding and extrusion. Nevertheless, its extremely high melt viscosity and high melting temperature do not help. The ideal processing technology for handling powder metallurgy is well suited for PTFE.
Sintering, compression molding, pressing, stamping or paste extrusion, hot stamping, machining, and extrusion of pre-sintered powders on special machines.
Paste extrusion mixes PTFE with hydrocarbons used to make tape, tubing and wire insulation from it. The hydrocarbons evaporate before the parts are sintered.
Operating range -200°C to 260°C.
Frequently Asked Questions –
1、Does PTFE cause cancer?
PTFE has been shown to be toxic to human health because it contains a carcinogenic substance called PFOA. However, there is no need to worry because PTFE non-stick coatings no longer contain this substance.
2. What is PTFE pipe used for?
Plastic pipe/sheet extrusion machine: PTFE pipeis most commonly used as laboratory tubes, where chemical resistance and purity are most important. PTFE has an extremely low coefficient of friction and is known to be one of the most “slippery” substances known.
3. What kind of plastic is PTFE?
PTFE is a thermoplastic polymer that is a fluoropolymer consisting of carbon and fluorine atoms.
4. What is the use of PTFE sheet?
PTFE sheet is used in a variety of applications such as PTFE encapsulated gaskets and PTFE packing. It has excellent resistance to gases, water, chemicals, fuels and oils.