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Gold is a chemical element with the symbol Au (from the Latin word aurum) and the atomic number 79. In its pure form, it is a bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal, a group 11 element, and one of the noble metals. It is one of the least reactive chemical elements ...
The golden ratio's negative −φ and reciprocal φ−1 are the two roots of the quadratic polynomial x2 + x − 1. The golden ratio is also an algebraic number and even an algebraic integer. It has minimal polynomial. This quadratic polynomial has two roots, and. The golden ratio is also closely related to the polynomial.
This is a list of the oldest living people who have been verified to be alive as of the dates of the cited supporting sources. It was estimated in 2015 that between 150 and 600 living people had reached the age of 110. The true number is uncertain, as not all supercentenarians are known to researchers at a given time, and some claims cannot be validated or are fraudulent.
- 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...
2006–present. Number built. 1,000+ [4] Developed from. Lockheed Martin X-35. The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, stealth multirole combat aircraft designed for air superiority and strike missions; it also has electronic warfare and intelligence, surveillance, and reconnaissance capabilities.
Vincent Leonard Price Jr. (May 27, 1911 – October 25, 1993) was an American actor, art historian, art collector, and gourmet cook. He appeared on stage, television, and radio, and in more than 100 films. Price has two stars on the Hollywood Walk of Fame, one for motion pictures and one for television.[1] Price's first film role was as a ...
Semaglutide is an antidiabetic medication used for the treatment of type 2 diabetes and an anti-obesity medication used for long-term weight management.[19][20][21] It is a peptide similar to the hormone glucagon-like peptide-1 (GLP-1), modified with a side chain.[22][23] It can be administered by subcutaneous injection or taken orally.[24][13 ...
