Polytetrafluoroethylene

from Wikipedia, the free encyclopedia
Structural formula
Structural formula of polytetrafluoroethylene
General
Surname Polytetrafluoroethylene
other names
  • Teflon
  • Polytetrafluoroethylene
  • PTFE
  • Xynflon
CAS number 9002-84-0
Monomer 1,1,2,2-tetrafluoroethylene ( IUPAC )
Molecular formula of the repeating unit C 2 F 4
Molar mass of the repeating unit 100.02 g mol −1
Type of polymer

Thermoplastic

Brief description

white odorless solid

properties
Physical state

firmly

density

2.2 g cm −3

Melting point

327 ° C

hardness

D55 (according to Shore)

modulus of elasticity

420 MPa

Poisson's number

0.46

Water absorption

<0.1%

Chemical resistance

very high, except for liquid sodium, highly fluorinated oils

Thermal conductivity

0.24 W / (m K)

Thermal expansion coefficient

130 · 10 −6 K −1

safety instructions
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Polytetrafluoroethylene ( abbreviation PTFE , occasionally also polytetrafluoroethene ) is an unbranched, linearly structured, partially crystalline polymer made from fluorine and carbon . Colloquially, this plastic is often referred to by the trade name Teflon from DuPont . Other frequently used trade names by other manufacturers of PTFE are Dyneon PTFE (formerly Hostaflon ) and Gore-Tex for PTFE membranes .

PTFE belongs to the class of polyhalogenolefins , which also includes PCTFE ( polychlorotrifluoroethylene ). It belongs to the group of thermoplastics , although it also has properties that require processing that is more typical for thermoset plastics.

history

Due to its low surface tension and good heat resistance, PTFE is used as a non-stick coating on frying pans and saucepans.

PTFE was discovered in 1938 by the chemist Roy Plunkett . When Plunkett was experimenting with tetrafluoroethylene (TFE) in search of refrigerants for refrigerators , he discovered “colorless crumbs” in his reaction vessel. Tetrafluoroethylene was polymerized into PTFE . After its discoverer, the manufacturing process still used today is called the Plunkett process . The polymerization is started at high pressure with peroxides. Roy Plunkett received on February 4, 1941, the US patent applied for on July 1, 1939 with the publication number US2230654 A on PTFE.

Initially, there was no technical use of PTFE because the production costs were too high and no application for the highly inert material was seen. In 1943, researchers in the Manhattan Project faced the problem of dealing with extremely corrosive uranium hexafluoride . For the first time, PTFE was used technically as a protection against corrosion in uranium enrichment . Later, the French chemist Marc Grégoire coated his fishing line with PTFE to make it easier to untangle. His wife Colette had the idea to coat pots and pans with it, for which she and Georgette Wamant received a patent in 1954.

Manufacturing

PTFE is produced from chloroform CHCl 3 by partial fluoridation , whereby chlorodifluoromethane CHClF 2 and tetrafluoroethylene C 2 F 4 are initially produced. Antimony (V) chloride fluoride (SbCl 4 F) acts as a catalyst .

Tetrafluoroethene is then subjected to radical polymerization under pressure . Depending on the conditions, there are different molecule and particle sizes:

Since this reaction is strongly exothermic and the monomer units decompose explosively at high temperatures, the polymerization is carried out in suspension . In addition, the instability of the monomer means that the polymer and monomer production facilities are in close proximity, since the monomer can only be transported to a very limited extent due to the risk of explosion.

properties

mechanical stabilization of fluoroplastics

PTFE is characterized by several special features:

  • PTFE is very inert. Even aggressive acids such as aqua regia cannot attack PTFE. The reason is, on the one hand, the particularly strong bond between the carbon and fluorine atoms , as fluorine is the element with the strongest electronegativity . Many substances fail to break the bonds and react chemically with PTFE. Furthermore, PTFE is kinetically inhibited by the compact shell made of fluorine atoms, which protects the carbon strand inside. Fine PTFE powder, however, is z. B. used as an oxidizing agent for metal powder in weapon applications.
  • It is extremely resistant to all bases , alcohols , ketones , gasoline , oils, etc .; it is only unstable to very strong reducing agents such as solutions of alkali metals (e.g. sodium ) in liquid ammonia or to very strong oxidizing agents such as elemental fluorine at higher temperatures; Operating temperature up to 260 ° C (at temperatures above 400 ° C highly toxic pyrolysis products such as fluorophosgene (COF 2 ) are released, which can lead to Teflon fever ); frost-resistant down to −270 ° C; can only be glued after pretreatment; Welding possible, but not common; slightly waxy surface (not as pronounced as with PE ); physiologically harmless.
  • PTFE has a very low coefficient of friction . PTFE slides on PTFE as well as wet ice slides on wet ice. In addition, the static friction is just as great as the sliding friction , so that the transition from standstill to movement takes place without jerk .
  • There are almost no materials that adhere to PTFE because the surface tension is extremely low. PTFE is difficult to wet and hardly sticky. The contact angle with water is 126 °.
  • Density : 2.1–2.3 g · cm −3 , Shore hardness D 50 to 72, ball indentation hardness : 23–32 N / mm 2 , tear strength: 22–40 N / mm 2
  • High thermal expansion (α in the range 20–100 ° C: ≈20 · 10 −5 K −1 ), phase transition from triclinic to hexagonal crystal lattice at 19 ° C with a change in volume.
  • Burning test : non-combustible; In a hot flame, decomposition takes place in red heat; this smell of salt - and hydrofluoric acid . There are also sources that prove that trifluoroacetic acid is produced, which humans can excrete, but not the plant world. The resulting vapors are poisonous and lead to polymer fever in humans .
  • Refractive index : PTFE has a very low refractive index of around 1.38.
  • Specific heat capacity : 0.96 J / (g · K).
  • Thermal conductivity : 0.25 W / (K m).
  • Permittivity : 2.1 (D150 at 10 3 Hz), dielectric loss factor: 0.3 · 10 −4 at 10 3 Hz, specific resistance: 10 18 Ω · cm.

PTFE compounds are also produced which are provided with fillers such as glass, carbon, graphite , molybdenum disulphide , bronze , organic fillers or V2A stainless steel . By compounding various properties can be changed.

Applications

PTFE tape according to DIN EN 751-3 for metal threaded connections

Because of its chemical inertness, PTFE is used as a coating where there are aggressive chemicals. Even when uranium was processed for the first atomic bombs ( Manhattan project ), the very reactive uranium hexafluoride was stored in PTFE-coated vessels.

The diverse and relatively simple possibilities of compounding enable special mixtures for numerous applications. In the field of sealing technology , PTFE is used as a basic compound in many applications. Especially in the area:

Industry and technology

Furthermore, PTFE is also used in chemical plant construction as a lining material for expansion joints , pipelines and columns . The most common form of processing for lining is the isostatic procedure. Here, PTFE is pressed against the walls of the unit to be lined under high pressure.

There are also many applications in the industrial sector as non-stick coatings, for example in molds in plastics processing.

In the field of high frequency technology , PTFE is a suitable insulating material due to its low dielectric constant and low losses . In the production of high-frequency circuit boards , the material, partially reinforced with glass fabric, serves as a dielectric base material.

In high voltage technology , PTFE is suitable as an electrical insulator (use in insulators) and as nozzle material in circuit breakers due to its high resistance to partial discharge and the low adhesion resistance of surface contamination .

Due to its low friction, PTFE is of interest as a dry lubricant ( solid lubricant ) and as a coating for bearings and seals.

The flame-retardant PTFE is also used in the construction industry to protect roofs or facades from the elements. In the field of architecture, glass fiber fabric is coated with PTFE in order to obtain weather and UV-resistant membranes. In the later processing, welding is mainly used, since sewing is problematic due to the low friction of the fibers. There are now also fabrics made entirely from PTFE. These have the advantage of being easier to handle and less risk of kinking. PTFE is also used as a sliding material in bridge bearing construction.

Medical applications

In medicine , PTFE is used for implants such as vascular prostheses , among other things . On the one hand, its chemical resistance ensures a long service life and good compatibility; on the other hand, the smooth surface reduces the formation of blood clots.

In dentistry , PTFE is used as a barrier membrane for building bones . The procedure is known as Guided Bone Regeneration (GBR) - controlled tissue regeneration. In the GBR procedure, the space that is to be filled with bone is additionally surrounded with a membrane. This has the task of preventing the surrounding cells of the surrounding soft tissue from growing too quickly into the cavity, as this forms faster than bone.

Because of this compatibility, it is also being used more and more as piercing jewelry - it should be ensured that the PTFE jewelry in question was manufactured for use in / on the body, as "industrial" PTFE always leads to chemical residues due to the Sintering can occur. By using piercing jewelry made of PTFE as the initial material, the healing times are significantly shorter than when using titanium . Because of its extremely high heat resistance, PTFE can, in contrast to other plastics, be steam-sterilized in an autoclave at 130 ° C. There are also implants for the face made of PTFE, which are used in plastic surgery.

Non-stick coatings for consumer goods

The best-known application is certainly the use as a non-stick coating in pans and pots. The coating itself adheres because the metal of the pan is roughened, which is done mechanically by sandblasting or chemically by acids. In the next step, the PTFE is applied with pressure and thus held in place by the countless small bumps in the pan. The bond is therefore mechanical and not chemical, which is why the surface is usually only slightly scratch-resistant. However, the top of the coating remains very smooth and prevents the food from sticking.

In the textile industry, the fabric is used to manufacture water-repellent fabrics such as Gore-Tex .

PTFE is also used for coatings on high-priced razor blades and high-quality garden cutting tools such as pruning shears or pruning shears. This considerably reduces the effort required to cut and increases the quality of the cut.

In high-quality computer mice , the "mouse feet" are also made of PTFE. This is intended to reduce the frictional resistance of the mouse and thus allow more comfortable guidance.

Coating the metal strings for stringed instruments such as guitars, bass guitars, mandolins and banjos increases the lifespan of the strings considerably because there is no corrosion from sweat and grease from the fingers.

Individual projectiles for handguns are coated with PTFE in order to be able to fire harder projectiles (e.g. made of brass) without excessive wear of the barrel . In this respect, PTFE is an alternative to molybdenum disulfide (MoS 2 ).

Expanded PTFE (ePTFE)

Expanded PTFE (ePTFE) is a specially processed form of polytetrafluoroethylene. During the manufacturing process, the PTFE molecular fibers are oriented, which results in improved strength and cold flow properties in the material compared to non-oriented PTFE. Stretched PTFE films (ePTFE) are also used in extremely thin layers under the trade name Gore-Tex as a so-called Gore-Tex laminate , whose membrane has fine pores that are still large enough to allow water vapor to pass through, but not water in liquid form. This can be used to make “breathable”, waterproof and windproof clothing (e.g. jackets, shoes and socks) that allow moisture to escape from the skin despite being highly impervious . In addition to its use in the textile industry, ePTFE is also used in the medical technology sector, e.g. B. used for stents or bypasses. As a sealing material, ePTFE is also used under the trade names FluorTex, KWO MultiTex and SoftFluor in the aerospace industry and, due to its unchanged high chemical resistance, also in the chemical and pharmaceutical industries.

Optical PTFE

For special metrological tasks, for example with the integrating sphere , PTFE in an optically pure white quality is used as a diffusely reflective coating. In optics, PTFE is used as a lens material because it is transparent in the far infrared range . But PTFE is also used as a coating for eyeglass lenses to make them easier to clean.

Export issues

In some countries, exports of chemical manufacturing equipment coated with PTFE are restricted. In the EU , this falls under Annex I to Regulation (EC) No. 428/2009 (Dual Use) at position 2B350 , sometimes with additional requirements, and the export of such goods is therefore subject to authorization in accordance with Article 3 of the Regulation. Since the preliminary products for chemical warfare agents and the warfare agents themselves (see e.g. sulfur mustard ) are partly highly corrosive , it is necessary to design the manufacturing facilities to be acid and alkali-proof. Valves, pipelines and containers completely coated on the inside with PTFE are necessary for the production of aggressive substances such as e.g. B. certain toxic gases. They are also used in seawater desalination plants , as the waste liquor produced is corrosive. The unauthorized export of such goods is a criminal offense under Section 17 (1) of the Foreign Trade Act .

Environmental and health issues

Environmental impact

In recent years, the production of fluoropolymers has come under fire because the perfluorooctanoic acid (PFOA) , which was previously used as a surfactant , has reproductive and PBT properties . The manufacture of Teflon creates long-lasting perfluorinated alkyl substances that can be detected in breast milk. How these substances affect health is unclear. Animal experiments do not allow any clear conclusions to be drawn because of the much longer retention time in the human organism. In the uterus , exposure to PFOA is associated with a decrease in birth weight . While the concentration of the known problematic substance perfluorooctanoic acid in human blood is falling in Germany, the concentrations of less known and studied polyfluorinated chemicals are increasing.

When perfluoropolymers are disposed of (incinerated), fluorine compounds, typically hydrofluoric acid and perfluorocarbons such as tetrafluoroethene or trifluoroacetic acid, are released into the environment . In 2010 a process for the recovery of fluoromonomers from PTFE was published. The University of Bayreuth and the Institut InVerTec e. V. and the company Dyneon GmbH. Dyneon is currently building a pilot plant for recycling 500 tonnes of perfluorinated polymers per year. The aim is to avoid the high environmental pollution caused by the incineration that has been common up to now.

Health risks

In connection with Teflon pans, there is a certain risk potential due to the possible formation of carcinogenic substances from the coating. The health-endangering fluorinated compounds occur from a temperature of approx. 202 ° C. However, the German Federal Institute for Risk Assessment only assumes a concentration that is toxic to humans from a pan temperature of 360 ° C. In order to avoid reaching these temperatures, it is therefore recommended not to empty coated pans for longer than three minutes. In the case of induction hotplates, the Federal Institute for Risk Assessment advises against heating empty pans, as this would cause them to reach critical temperatures too quickly. These recommendations only apply to empty pans, because the consumer is warned of overheating of the coating, for example when heating oil from a temperature of 270 ° C, due to smoke development.

Inhaling small amounts of PTFE vapors can lead to polymer fever , larger amounts are fatal. Birds are more sensitive to PTFE vapors: They can die from the small PTFE particles that are created when the material is heated to approx. 202 ° C. Scratches in the coating, as well as peeling coating particles, are regarded as harmless, since they are completely eliminated again.

See also

Web links

Commons : Polytetrafluoroethylene  - collection of pictures, videos and audio files

Individual evidence

  1. ↑ a b c d e Entry on polytetrafluoroethylene in the GESTIS substance database of the IFA , accessed on July 30, 2017(JavaScript required) .
  2. ↑ a b c d e f data sheet polytetrafluoroethylene at Kern, accessed on September 22, 2019.
  3. ↑ DuPont Teflon / PTFE Properties Handbook (PDF; 189 kB) .
  4. ↑ bürkert - Resistance table ( Memento from December 12, 2014 in the Internet Archive ) (PDF; 222 kB).
  5. ↑ PTFE fluoroplastics - properties and characteristics ( Memento of November 12, 2011 in the Internet Archive ) (PDF; 1.6 MB).
  6. ↑ Martin Schneider: From the atomic bomb to the frying pan . October 15, 2004. Archived from the original on September 28, 2007. Retrieved on August 20, 2010.
  7. ↑ Google patent search: Tetrafluoroethylene polymer US 2230654 A .
  8. ^ Richard Rhodes : The Making of the Atomic Bomb . Simon and Schuster, New York, New York 1986, ISBN 0-671-65719-4 , p. 494.
  9. ↑ Espacenet research: Method for coating containers and other articles and coated articles produced thereby. FR1137972. .
  10. ↑ a b Elringklinger plastic: PTFE - material characteristics ( Memento of December 3, 2013 in the Internet Archive ), accessed April 2, 2013.
  11. ↑ a b Elringklinger plastic: PTFE - thermal properties ( memento from April 29, 2013 in the web archive archive.today ), accessed April 2, 2013.
  12. ↑ David A. Ellis, Scott A. Mabury, Jonathan W. Martin, Derek CG Muir: Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the environment. In: Nature . 412, 321-324, doi : 10.1038 / 35085548 .
  13. ^ W. Temple, I. Edwards, S. Bell: "Poly" fume fever - two fatal cases. In: New Zealand Veterinary Journal. 33 (3), 1985, PMID 16031138 .
  14. ↑ PTFE Compounds (PDF; 531 kB), at hoefert.de, accessed on March 19, 2017.
  15. ↑ 3m: New design language in architecture - fluoropolymers protect roof fabrics . In: GAK 10/2011, pp. 602–604.
  16. ↑ P. Coulthard, M. Esposito et al. a .: Interventions for replacing missing teeth: bone augmentation techniques for dental implant treatment. In: Cochrane database of systematic reviews (online). Number 3, 2003, p. CD003607. doi : 10.1002 / 14651858.CD003607 . PMID 12917975 .
  17. ↑ Regulation (EC) No. 1334/2000 .
  18. ↑ Authority proves contamination of breast milk , aerztezeitung.de
  19. ↑ Environmental medical importance of perfluorinated surfactants , allum.de
  20. ↑ Julia Malits, Jan Blustein, Leonardo Trasande, Teresa M. Attina: Perfluorooctanoic acid and low birth weight: Estimates of US attributable burden and economic costs from 2003 through 2014. In: International Journal of Hygiene and Environmental Health. 221, 2018, p. 269, doi : 10.1016 / j.ijheh.2017.11.004 .
  21. ↑ Perfluorinated and polyfluorinated chemicals: Avoid pollution - protect the environment. Federal Environment Agency, July 2009, accessed on October 19, 2016 .
  22. ↑ Newsletter Perfluorinated Hydrocarbons (PFC) ( Memento from June 26, 2011 in the Internet Archive ) (PDF; 93 kB), on asstech.com.
  23. ↑ invertec-ev.de: Pilot project: Recycling of fluoropolymers (PTFE) .
  24. ↑ a b Federal Institute for Risk Assessment : Questions and answers on cookware and roasting utensils with non-stick coating ( memento of October 26, 2017 in the Internet Archive ).
  25. ↑ Health hazard «Teflon fever». In: Greenpeace magazine. Retrieved September 8, 2017 .
  26. ↑ Right? Scratched pans ( Memento from February 23, 2017 in the Internet Archive )