Polyphenyl ether

Phenyl ether polymers are a class of polymers that contain a phenoxy and or a thiophenoxy group as the repeating group. Commercial phenyl ether polymers belong to two chemical classes: polyphenyl ethers, or PPEs, and polyphenylene oxides, or PPOs. The phenoxy groups in the former class of polymers do not contain any substituents but those in the latter class contain 2 to 4 alkyl groups on the phenyl ring. The structure of an oxygen-containing PPE is provided in Figure 1 and that of a 2, 6-xylenol derived PPO is shown in Figure 2.

Polyphenyl ethers (PPEs)

Proper name for a phenyl ether polymer is poly(phenyl ether) or polyphenyl polyether, but the name polyphenyl ether is widely accepted. Polyphenyl ethers (PPEs) are obtained by the reaction of an alkali metal phenate with a halogenated benzene, in the presence of a copper catalyst (Ullman Ether Synthesis) ["The Ullmann Ether Condensation,” by A A Moroz and Mark S Shvartsberg, 1974, Russ. Chem. Rev. 43 (8), 679-689] . Figure 3 provides the Ullman Ether Reaction Scheme.

PPEs of up to 6 phenyl rings, both oxy and thio ethers, are commercially available. See Table 1 [SANTOLUBES LLC Product Brochure] . They are characterized by indicating the substitution pattern of each ring, followed by the number of phenyl rings and the number of ether linkages. Thus, the structure in Figure 1 with n equal to 1 is identified as pmp5P4E, indicating para, meta, para substitution of the three middle rings, a total of 5 rings, and 4 ether linkages. Meta substitution of the aryl rings in these materials is most common and often desired. Longer chain analogues with up to 10 benzene rings are also known.

The simplest member of the phenyl ether family is diphenyl ether (DPE), also called diphenyl oxide, the structure of which is provided in Figure 4. Low molecular weight polyphenyl ethers and thioethers are used in a variety of applications and include high vacuum devices, optics, electronics, and in high-temperature and radiation-resistant fluids and greases. Figure 5 show the structure of the sulfur analogue of 3-R polyphenyl ether shown in Figure 3.

Electronic Connector Lubricants

Previously we stated that 5R4E PPE has a surface tension of 49.9 dynes/cm, which is amongst the highest in pure organic liquids. Because of this, this PPE and the other PPEs do not effectively wet metal surfaces. This property is useful when migration of a lubricant from one part of the equipment to another part must be avoided, such as in certain electronic devices. A thin film of polyphenyl ether on a surface is not a thin contiguous film as one would envision, but rather comprises tiny droplets. This PPE property tends to keep the film stationary, or at least to cause it to remain in the area where the lubrication is needed, rather than migrating away by spreading and forming a new surface. As a result, contamination of other components and equipment, which do not require a lubricant, is avoided. The high surface tension of PPEs, therefore, makes them useful in lubricating electronic contacts. Polyphenyl ether lubricants have a 30-year history of commercial service for connectors with precious and base metal contacts in telecom, automotive, aerospace, instrumentation and general-purpose applications [Using Lubricants to Avoid Failures in Medical Electronic Connectors," by Sibtain Hamid in Medical Electronics Manufacturing, Spring 2004 and SANTOLUBES Brochure on Stationary lubricants prevent connector failures] [SANTOLUBES Brochure on Stationary lubricants prevent connector failures] . In addition to maintaining the current flow and providing long-term lubrication, PPEs offer protection to connectors against aggressive acidic and oxidative environments. By providing a protective surface film, polyphenyl ethers not only protect connectors against corrosion but also against vibration-related wear and abrasion that leads to fretting wear. The devices that benefit from the specialized properties of PPEs include cell phones, printers and a variety of other electronic appliances. The protection lasts for decades or for the life of the equipment. SantoLubes of Saint Charles MO supplies PPEs for electronic applications both in concentrated form and in diluted form to facilitate application.

Use in Optics

Polyphenyl ethers (PPEs) possess good optical clarity, a high refractive index, and other beneficial optical properties. Because of these, PPEs have the ability to meet the rigorous performance demands of signal processing in advanced photonics systems. Optical clarity of PPEs resembles that of the other optical polymers, that is, they have refractive indices of between 1.5 and 1.7 and provide good propagation of light between approximately 400 nm and 1700 nm. Close refractive index (RI) matching between materials is important for proper propagation of light through them. Because of the ease of RI matching, PPEs are used in many optical devices as optical fluids. Extreme resistance to ionizing radiation gives PPEs an added advantage in the manufacture of solar cells and solid-state UV/blue emitters and telecommunication equipment made from high-index glasses and semiconductors.

High-Temperature and Radiation-Resistant Lubricants

PPEs, being of excellent thermo-oxidative stability and radiation resistance, have found extensive use in high temperature applications that also require radiation resistance. In addition, PPEs demonstrate better wear control and load-carrying ability than mineral oils, especially when used in bearings.

As noted earlier, PPEs were developed for use in jet engines that involved high speed-related frictional temperatures of as high as 320°C. While the use of PPEs in lubricating jet engines has somewhat subsided due to their higher cost, they are still used in some aerospace applications. PPEs are also used as base fluids for radiation-resistant greases used in nuclear power plant mechanisms. PPEs and their derivatives have also found use as vapor phase lubricants in gas turbines and custom bearings, and wherever extreme environmental conditions exist. Vapor phase lubrication is achieved by heating the liquid lubricant above its boiling point. The resultant vapors are then transported to the hot bearing surface. If the temperatures of the bearing surface are below the lubricant’s boiling point, the vapors condense to provide liquid lubrication. Polyphenyl ether technology can also provide superior fire safety and fatigue life, depending on the specific bearing design. In this application, PPEs have the advantage of providing lubrication both as a liquid at low temperatures and as a vapor at temperatures above 600°F (316°C). Due to the low volatility and excellent high-temperature thermo-oxidative stability, PPEs have also found use as a lubricant for chains used in and around kilns, metal fabrication plants, and glass molding and manufacturing equipment. In these high temperature applications, PPEs do not form any sludge and hard deposits. The low soft carbon residue that is left behind is removed easily by wiping. PPEs low volatility, low flammability, and good thermodynamic properties make them ideally suited for use as heat transfer fluids and in heat sink applications as well [Hamid, S. and Burian, S. A., “Polyphenyl Ether Lubricants,” published in Synthetics, Mineral Oils, and Bio-based Lubricants: Chemistry and Technology, Leslie R. Rudnick Editor, pp. 175-199, Taylor and Francis Publisher] .

Polyphenylene oxides (PPOs)

These polymers are made through oxidative coupling of substituted phenol in the presence of oxygen and copper and amine containing catalysts, such as Cuprous Bromide and pyridine. See Figure 2 for the PPO structure. PPO polymers can be classified as plastic resins. They and their composites with polystyrene, glass, and Nylon^® are used as high-strength, moisture-resistant engineering plastics in a number of industries, including computer, telecommunication, and automotive parts. PPOs are marketed by General Electric Co. under the trademarked name of Noryl^® [ [http://www.geglobalresearch.com/cooltechnologies/pdf/2002grc087.pdf 2002grc087 High Heat PPO.: 13C and 31P NMR Methods for Characterizing End Groups and Chain Structures in Poly(2,6-dimethyl-1,4-phenylene oxide)/Poly(2,3,6-trimethyl-1,4-phenylene oxide) Copolymers ] ] .

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