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Worldwide distribution of country calling codes. Regions are coloured by first digit. Country calling codes, country dial-in codes, international subscriber dialing (ISD) codes, or most commonly, telephone country codes are telephone number prefixes for reaching telephone subscribers in foreign countries or areas via international telecommunication networks.
Tencent Holdings Ltd. (Chinese: 腾讯; pinyin: Téngxùn) is a Chinese multinational technology conglomerate and holding company headquartered in Shenzhen. It is one of the highest grossing multimedia companies in the world based on revenue. It is also the world's largest company in the video game industry based on its equity investments ...
268 7xx. 10. NANP member. Argentina. +54. 9/15. 10. All carriers: Claro, Movistar, Personal, Tuenti. 15 before the local number but after long distance area code for national calls (0 11 15 xxxx-xxxx) and 9 placed after the international access code excluding the 15 for international calls (+54 9 11 xxxx-xxxx).
- History
- Production
- Properties
- Processing
- Applications and Uses
- Safety
- Ecotoxicity
- Similar Polymers
- See Also
- Further Reading
Polytetrafluoroethylene (PTFE) was accidentally discovered in 1938 by Roy J. Plunkett while he was working in New Jersey for DuPont. As Plunkett attempted to make a new chlorofluorocarbon refrigerant, the tetrafluoroethylene gas in its pressure bottle stopped flowing before the bottle's weight had dropped to the point signaling "empty". Since Plunk...
PTFE is produced by free-radical polymerization of tetrafluoroethylene.The net equation is 1. n F2C=CF2 → −(F2C−CF2)n− Because tetrafluoroethylene can explosively decompose to tetrafluoromethane (CF4) and carbon, a special apparatus is required for the polymerization to prevent hot spots that might initiate this dangerous side reaction. The process...
PTFE is a thermoplastic polymer, which is a white solid at room temperature, with a density of about 2200 kg/m3 and a melting point of 600 K (327 °C; 620 °F). It maintains high strength, toughness and self-lubrication at low temperatures down to 5 K (−268.15 °C; −450.67 °F), and good flexibility at temperatures above 194 K (−79 °C; −110 °F). PTFE g...
Processing PTFE can be difficult and expensive, because the high melting temperature, 327 °C (621 °F), is above the decomposition temperature. Even when molten, PTFE does not flow due to its exceedingly high melt-viscosity. The viscosity and melting point can be decreased by inclusion of small amount of comonomers such as perfluoro (propylvinyl eth...
Wire insulation, electronics
The major application of PTFE, consuming about 50% of production, is for the insulation of wiring in aerospace and computer applications (e.g. hookup wire, coaxial cables). This application exploits the fact that PTFE has excellent dielectric properties, specifically low group velocity dispersion, especially at high radio frequencies, making it suitable for use as an excellent insulator in connector assemblies and cables, and in printed circuit boards used at microwave frequencies. Combined w...
Bearings seals
In industrial applications, owing to its low friction, PTFE is used for plain bearings, gears, slide plates, seals, gaskets, bushings, and more applications with sliding action of parts, where it outperforms acetal and nylon.
Electrets
Its extremely high bulk resistivity makes it an ideal material for fabricating long-life electrets, the electrostatic analogues of permanent magnets.
While PTFE is stable at lower temperatures, it begins to deteriorate at temperatures of about 260 °C (500 °F), it decomposes above 350 °C (662 °F), and pyrolysis occurs at temperatures above 400 °C (752 °F). The main decomposition products are fluorocarbon gases and a sublimate, including tetrafluoroethylene (TFE) and difluorocarbene radicals(RCF2)...
Trifluoroacetate
Sodium trifluoroacetate and the similar compound sodium chlorodifluoroacetate can both be generated when PTFE undergoes thermolysis, as well as producing longer chain polyfluoro- and/or polychlorofluoro- (C3-C14) carboxylic acids which may be equally persistent. These products can accumulate in evaporative wetlandsand have been found in the roots and seeds of wetland plant species, but has not been observed to have an adverse impact on plant health or germination success.
PFOA
Perfluorooctanoic acid (PFOA, or C8) has been used as a surfactant in the emulsion polymerizationof PTFE, although several manufacturers have entirely discontinued its use. PFOA persists indefinitely in the environment. PFOA has been detected in the blood of many individuals of the general US population in the low and sub-parts per billion range, and levels are higher in chemical plant employees and surrounding subpopulations. PFOA and perfluorooctanesulfonic acid (PFOS) have been estimated t...
GenX
As a result of the lawsuits concerning the PFOA class-action lawsuit, DuPont began to use GenX, a similarly fluorinated compound, as a replacement for perfluorooctanoic acid in the manufacture of fluoropolymers, such as Teflon-brand PTFE.However, in lab tests on rats, GenX has been shown to cause many of the same health problems as PFOA. The chemicals are manufactured by Chemours, a corporate spin-off of DuPont, in Fayetteville, North Carolina. Fayetteville Works was the site where DuPont beg...
The Teflon trade name is also used for other polymers with similar compositions: 1. Perfluoroalkoxy alkane(PFA) 2. Fluorinated ethylene propylene(FEP) These retain the useful PTFE properties of low friction and nonreactivity, but are also more easily formable. For example, FEP is softer than PTFE and melts at 533 K (260 °C; 500 °F); it is also high...
Ellis, D.A.; Mabury, S.A.; Martin, J.W.; Muir, D.C.G.; Mabury, S.A.; Martin, J.W.; Muir, D.C.G. (2001). "Thermolysis of fluoropolymers as a potential source of halogenated organic acids in the envi...
The Philippines is generally mountainous; uplands make up 65 percent of the country's total land area. [53]: 38 [201] The Philippines is an archipelagoof about 7,641 islands,[202][203]covering a total area (including inland bodies of water) of about 300,000 square kilometers (115,831 sq mi).
American Airlines is a major airline in the United States headquartered in Fort Worth, Texas, within the Dallas–Fort Worth metroplex. It is the largest airline in the world when measured by scheduled passengers carried, revenue passenger mile. American, together with its regional partners and affiliates, operates an extensive international ...
