Freeze drying known as lyophilisation or cryodesiccation, is a low temperature dehydration process which involves freezing the product, lowering pressure removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat. Freeze drying results in a high quality product because of the low temperature used in processing; the original shape of the product is maintained and quality of the rehydrated product is excellent. Primary applications of freeze drying include biological and food processing and preservation; the first application of the freeze-drying process were in the Andes where Indigenous people would take low-land tubers up to high altitudes and leave them to freeze. The potatoes were squashed to facilitate the water loss but when left to freeze at night and exposed through the days, the mash would lose its water while the nutrients were preserved through the lyophilization process; this resulted in a product called chuño, a long shelf life food eaten well past its seasonal availability and stored for emergency rations, if needed.
Freeze drying as an industrial process began in as early as 1890 by Richard Altmann who devised a method to freeze dry tissues, but went unnoticed until the 1930s. In 1909, Shackell independently created the vacuum chamber by using an electrical pump. However, no further data on freeze drying was documented until Tival in 1927 and Elser in 1934 had patented freeze drying systems with improvements to freezing and condenser steps. A significant turning point for freeze drying occurred during World War II. Blood plasma and penicillin were needed to treat the wounded in the field, because of the lack of simultaneous refrigeration and transport, many serum supplies were spoiling before reaching their intended recipients; the freeze-drying process was developed as a commercial technique that enabled blood plasma and penicillin to be rendered chemically stable and viable without having to be refrigerated. In the 1950s-60s, freeze drying began to be viewed for its multi-purpose application to both pharmaceuticals and food processing.
Freeze-drying products became a major commodity for military food rations. What began for astronaut crews as tubed meals and freeze-dried snacks that were difficult to rehydrate, they are now able to enjoy warm hot meals while in space by improving the easability for rehydrating freeze-dried meals with water; as technology and food processing improved NASA looked for ways to provide a complete nutrient profile while reducing crumbs, disease-producing bacteria, toxins. The complete nutrient profile was improved with the addition of an algae-based vegetable-like oil to add polyunsaturated fatty acids. Polyunsaturated fatty acids are beneficial in mental and vision development, as it remains stable after space travel can provide astronauts with its added benefits; the crumb problem was solved with the addition of a gelatin coating on the foods to lock in and prevent crumbs. Disease-producing bacteria and toxins were reduced by quality control and the development of the Hazard Analysis Critical Control Point plan, used today to evaluate food material before and after processing.
With the combination of these 3 things, NASA could provide safe and wholesome foods to their crews while in space in a freeze-dried meal source. Military rations have come a long way from being served spoiled pork and corn meal to beefsteak with mushroom gravy. How rations are chosen and developed are based on acceptance, wholesomeness, producibility and sanitation. Additional requirements for rations include a minimum shelf life of 3 years, be deliverable by air, consumable in worldwide environments, provide a complete nutritional profile; the new tray rations, improved upon by increasing acceptable items and provide high quality meals while in the field. Freeze-dried coffee was incorporated by replacing spray-dried coffee within the meal, ready-to-eat category. There are four stages in the complete freeze drying process: pretreatment, primary drying, secondary drying. Pretreatment includes any method of treating the product prior to freezing; this may include concentrating the product, formulation revision, decreasing a high-vapor-pressure solvent, or increasing the surface area.
Food pieces are IQF treated to make it free flowing prior to freeze drying. In many instances the decision to pretreat a product is based on theoretical knowledge of freeze-drying and its requirements, or is demanded by cycle time or product quality considerations. During the freezing stage, the material is cooled below its triple point, the lowest temperature at which the solid and gas phases of the material can coexist; this ensures. To facilitate faster and more efficient freeze drying, larger ice crystals are preferable; the large ice crystals forms a network within the product which promotes faster removal of water vapor during sublimation. To produce larger crystals, the product should be frozen or can be cycled up and down in temperature in a process called annealing; the freezing phase is the most critical in the whole freeze-drying process, as the freezing method can impact the speed of reconstitution, duration of freeze-drying cycle, product stability, appropriate crystallization. Amorphous materials do not have a eutectic point, but they do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primar
A surface condenser is a used term for a water-cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is used. An air-cooled condenser is however more expensive and cannot achieve as low a steam turbine exhaust pressure as a water-cooled surface condenser. Surface condensers are used in applications and industries other than the condensing of steam turbine exhaust in power plants. In thermal power plants, the purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency, to convert the turbine exhaust steam into pure water so that it may be reused in the steam generator or boiler as boiler feed water; the steam turbine. The difference between the heat of steam per unit mass at the inlet to the turbine and the heat of steam per unit mass at the outlet from the turbine represents the heat, converted to mechanical power.
Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in the turbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power. Most of the heat liberated due to condensation of the exhaust steam is carried away by the cooling medium used by the surface condenser; the adjacent diagram depicts a typical water-cooled surface condenser as used in power stations to condense the exhaust steam from a steam turbine driving an electrical generator as well in other applications. There are many fabrication design variations depending on the manufacturer, the size of the steam turbine, other site-specific conditions; the shell contains the heat exchanger tubes. The shell is fabricated from carbon steel plates and is stiffened as needed to provide rigidity for the shell.
When required by the selected design, intermediate plates are installed to serve as baffle plates that provide the desired flow path of the condensing steam. The plates provide support that help prevent sagging of long tube lengths. At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs, a sump is provided. Condensate is pumped from the hotwell for reuse as boiler feedwater. For most water-cooled surface condensers, the shell is under vacuum during normal operating conditions. For water-cooled surface condensers, the shell's internal vacuum is most supplied by and maintained by an external steam jet ejector system; such an ejector system uses steam as the motive fluid to remove any non-condensible gases that may be present in the surface condenser. The Venturi effect, a particular case of Bernoulli's principle, applies to the operation of steam jet ejectors. Motor driven mechanical vacuum pumps, such as the liquid ring type, are popular for this service.
At each end of the shell, a sheet of sufficient thickness made of stainless steel is provided, with holes for the tubes to be inserted and rolled. The inlet end of each tube is bellmouthed for streamlined entry of water; this is to avoid eddies at the inlet of each tube giving rise to erosion, to reduce flow friction. Some makers recommend plastic inserts at the entry of tubes to avoid eddies eroding the inlet end. In smaller units some manufacturers use ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes some designs have expansion joint between the shell and the tube sheet allowing the latter to move longitudinally. In smaller units some sag is given to the tubes to take care of tube expansion with both end water boxes fixed rigidly to the shell; the tubes are made of stainless steel, copper alloys such as brass or bronze, cupro nickel, or titanium depending on several selection criteria. The use of copper bearing alloys such as brass or cupro nickel is rare in new plants, due to environmental concerns of toxic copper alloys.
Depending on the steam cycle water treatment for the boiler, it may be desirable to avoid tube materials containing copper. Titanium condenser tubes are the best technical choice, however the use of titanium condenser tubes has been eliminated by the sharp increases in the costs for this material; the tube lengths range to about 85 ft for modern power plants, depending on the size of the condenser. The size chosen is based on transportability from the manufacturers’ site and ease of erection at the installation site; the outer diameter of condenser tubes ranges from 3/4 inch to 1-1/4 inch, based on condenser cooling water friction considerations and overall condenser size. The tube sheet at each end with tube ends rolled, for each end of the condenser is closed by a fabricated box cover known as a waterbox, with flanged connection to the tube sheet or condenser shell; the waterbox is provided with man holes on hinged covers to allow inspection and cleaning. These waterboxes on inlet side will have flanged connections for cooling water inlet butterfly valves, small vent pipe with hand valve for air venting at higher level, hand operated drain valve at bottom to drain the waterbox for maintenance.
On the outlet waterbox the cooling water connection will have large flanges, butter
Nature is a British multidisciplinary scientific journal, first published on 4 November 1869. It is one of the most recognizable scientific journals in the world, was ranked the world's most cited scientific journal by the Science Edition of the 2010 Journal Citation Reports and is ascribed an impact factor of 40.137, making it one of the world's top academic journals. It is one of the few remaining academic journals that publishes original research across a wide range of scientific fields. Research scientists are the primary audience for the journal, but summaries and accompanying articles are intended to make many of the most important papers understandable to scientists in other fields and the educated public. Towards the front of each issue are editorials and feature articles on issues of general interest to scientists, including current affairs, science funding, scientific ethics and research breakthroughs. There are sections on books and short science fiction stories; the remainder of the journal consists of research papers, which are dense and technical.
Because of strict limits on the length of papers the printed text is a summary of the work in question with many details relegated to accompanying supplementary material on the journal's website. There are many fields of research in which important new advances and original research are published as either articles or letters in Nature; the papers that have been published in this journal are internationally acclaimed for maintaining high research standards. Fewer than 8% of submitted papers are accepted for publication. In 2007 Nature received the Prince of Asturias Award for Humanity; the enormous progress in science and mathematics during the 19th century was recorded in journals written in German or French, as well as in English. Britain underwent enormous technological and industrial changes and advances in the latter half of the 19th century. In English the most respected scientific journals of this time were the refereed journals of the Royal Society, which had published many of the great works from Isaac Newton, Michael Faraday through to early works from Charles Darwin.
In addition, during this period, the number of popular science periodicals doubled from the 1850s to the 1860s. According to the editors of these popular science magazines, the publications were designed to serve as "organs of science", in essence, a means of connecting the public to the scientific world. Nature, first created in 1869, was not the first magazine of its kind in Britain. One journal to precede Nature was Recreative Science: A Record and Remembrancer of Intellectual Observation, created in 1859, began as a natural history magazine and progressed to include more physical observational science and technical subjects and less natural history; the journal's name changed from its original title to Intellectual Observer: A Review of Natural History, Microscopic Research, Recreative Science and later to the Student and Intellectual Observer of Science and Art. While Recreative Science had attempted to include more physical sciences such as astronomy and archaeology, the Intellectual Observer broadened itself further to include literature and art as well.
Similar to Recreative Science was the scientific journal Popular Science Review, created in 1862, which covered different fields of science by creating subsections titled "Scientific Summary" or "Quarterly Retrospect", with book reviews and commentary on the latest scientific works and publications. Two other journals produced in England prior to the development of Nature were the Quarterly Journal of Science and Scientific Opinion, established in 1864 and 1868, respectively; the journal most related to Nature in its editorship and format was The Reader, created in 1864. These similar journals all failed; the Popular Science Review survived longest, lasting 20 years and ending its publication in 1881. The Quarterly Journal, after undergoing a number of editorial changes, ceased publication in 1885; the Reader terminated in 1867, Scientific Opinion lasted a mere 2 years, until June 1870. Not long after the conclusion of The Reader, a former editor, Norman Lockyer, decided to create a new scientific journal titled Nature, taking its name from a line by William Wordsworth: "To the solid ground of nature trusts the Mind that builds for aye".
First owned and published by Alexander Macmillan, Nature was similar to its predecessors in its attempt to "provide cultivated readers with an accessible forum for reading about advances in scientific knowledge." Janet Browne has proposed that "far more than any other science journal of the period, Nature was conceived and raised to serve polemic purpose." Many of the early editions of Nature consisted of articles written by members of a group that called itself the X Club, a group of scientists known for having liberal and somewhat controversial scientific beliefs relative to the time period. Initiated by Thomas Henry Huxley, the group consisted of such important scientists as Joseph Dalton Hooker, Herbert Spencer, John Tyndall, along with another five scientists and mathematicians, it was in part its scientific liberality that made Nature a longer-lasti
A calutron is a mass spectrometer designed and used for separating the isotopes of uranium. It was developed by Ernest Lawrence during the Manhattan Project and was based on his earlier invention, the cyclotron, its name was derived from California University Cyclotron, in tribute to Lawrence's institution, the University of California, where it was invented. Calutrons were used in the industrial-scale Y-12 uranium enrichment plant at the Clinton Engineer Works in Oak Ridge, Tennessee; the enriched uranium produced was used in the Little Boy atomic bomb, detonated over Hiroshima on 6 August 1945. The calutron is a type of sector mass spectrometer, an instrument in which a sample is ionized and accelerated by electric fields and deflected by magnetic fields; the ions collide with a plate and produce a measurable electric current. Since the ions of the different isotopes have the same electric charge but different masses, the heavier isotopes are deflected less by the magnetic field, causing the beam of particles to separate out into several beams by mass, striking the plate at different locations.
The mass of the ions can be calculated according to the strength of the field and the charge of the ions. During World War II, calutrons were developed to use this principle to obtain substantial quantities of high-purity uranium-235, by taking advantage of the small mass difference between uranium isotopes. Electromagnetic separation for uranium enrichment was abandoned in the post-war period in favor of the more complicated, but more efficient, gaseous diffusion method. Although most of the calutrons of the Manhattan Project were dismantled at the end of the war, some remained in use to produce isotopically enriched samples of occurring elements for military and medical purposes. News of the discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, its theoretical explanation by Lise Meitner and Otto Frisch, was brought to the United States by Niels Bohr. Based on his liquid drop model of the nucleus, he theorized that it was the uranium-235 isotope and not the more abundant uranium-238, responsible for fission with thermal neutrons.
To verify this Alfred O. C. Nier at the University of Minnesota used a mass spectrometer to create a microscopic amount of enriched uranium-235 in April 1940. John R. Dunning, Aristid von Grosse and Eugene T. Booth were able to confirm that Bohr was correct. Leo Szilard and Walter Zinn soon confirmed that more than one neutron was released per fission, which made it certain that a nuclear chain reaction could be initiated, therefore that the development of an atomic bomb was a theoretical possibility. There were fears that a German atomic bomb project would develop one first among scientists who were refugees from Nazi Germany and other fascist countries. At the University of Birmingham in Britain, the Australian physicist Mark Oliphant assigned two refugee physicists—Otto Frisch and Rudolf Peierls—the task of investigating the feasibility of an atomic bomb because their status as enemy aliens precluded their working on secret projects like radar, their March 1940 Frisch–Peierls memorandum indicated that the critical mass of uranium-235 was within an order of magnitude of 10 kg, small enough to be carried by a bomber of the day.
The British Maud Committee unanimously recommended pursuing the development of an atomic bomb. Britain had offered to give the United States access to its scientific research, so the Tizard Mission's John Cockcroft briefed American scientists on British developments, he discovered that the American project was smaller than the British, not as far advanced. A disappointed Oliphant flew to the United States to speak to the American scientists; these included Ernest Lawrence at the University of California's Radiation Laboratory in Berkeley. The two men had met before the war, were friends. Lawrence was sufficiently impressed to commence his own research into uranium. Uranium-235 makes up only about 0.72% of natural uranium, so the separation factor of any uranium enrichment process needs to be higher than 1250 to produce 90% uranium-235 from natural uranium. The Maud Committee had recommended that this be done by a process of gaseous diffusion, but Oliphant had pioneered another technique in 1934: electromagnetic separation.
This was the process. The principle of electromagnetic separation is that charged ions are deflected by a magnetic field, lighter ones are deflected more than heavy ones; the reason the Maud Committee, its American counterpart, the S-1 Section of the Office of Scientific Research and Development, had passed over the electromagnetic method was that while the mass spectrometer was capable of separating isotopes, it produced low yields. The reason for this was the so-called space-charge limitation. Positive ions have positive charge, so they tend to repel each other, which causes the beam to scatter. Drawing on his experience with the precise control of charged-particle beams from his work with his invention, the cyclotron, Lawrence suspected that the air molecules in the vacuum chamber would neutralize the ions, create a focused beam. Oliphant inspired Lawrence to convert his old 37-inch cyclotron into a giant mass spectrometer for isotope separation; the 37-inch cyclotron at Berkeley was dismantled on 24 November 1941, its magnet used to create the first calutron.
Its name came from cyclotron. The work was funded by the Radiation Laboratory from its own resources, with a $5,000 grant from the Research Corporation. In December Lawrence received a $400,000 grant from the S-1 Uranium Committee; the calutron consisted of an ion source, in the form of a box with a
In vacuum applications, a cold trap is a device that condenses all vapors except the permanent gases into a liquid or solid. The most common objective is to prevent vapors being evacuated from an experiment from entering a vacuum pump where they would condense and contaminate it. Large cold traps are necessary when removing large amounts of liquid as in freeze drying. Cold traps refer to the application of cooled surfaces or baffles to prevent oil vapours from flowing from a pump and into a chamber. In such a case, a baffle or a section of pipe containing a number of cooled vanes, will be attached to the inlet of an existing pumping system. By cooling the baffle, either with a cryogen such as liquid nitrogen, or by use of an electrically driven Peltier element, oil vapour molecules that strike the baffle vanes will condense and thus be removed from the pumped cavity. Pumps that use oil either as their working fluid, or as their lubricant, are the sources of contamination in vacuum systems. Placing a cold trap at the mouth of such a pump lowers the risk that oil vapours will backstream into the cavity.
Cold traps can be used for experiments involving vacuum lines such as small-scale low temperature distillations/condensations. This is accomplished through the use of a coolant such as liquid nitrogen or a freezing mixture of dry ice in acetone or a similar solvent with a low melting point; when performed on a larger scale, this technique is called freeze-drying, the cold trap is referred to as the condenser. Cold traps are used in cryopump systems to generate hard vacua by condensing the major constituents of the atmosphere into their liquid or solid forms. Care should be taken. Liquid oxygen is explosive, this is true if the trap has been used to trap solvent. Liquid oxygen can be condensed into a cold trap if a pump has sucked air through the trap when the trap is cold, e.g. when cooled with liquid nitrogen. Besides oxygen, many hazardous gases emitted in reactions, e.g. sulfur dioxide, condense into cold traps. Cold traps consist of two parts: The bottom is a large, thick round tube with ground-glass joints, the second is a cap with ground-glass connections.
The length of the tube is selected so that, when assembled, the total reached is about half the length of the tube. Cold traps should be assembled such that the down tube is connected to the source of gas whilst the cap is connected to the source of vacuum. Reversing this, connecting the down tube to the source of vacuum, places the inlet of the vacuum directly above the condensate, increasing the chances of vapour phase condensate moving up the down tube or, should the trap begin to fill to an appreciable volume, liquid phase condensate being pulled into the pump. Sublimation Cold finger
Supersonic travel is a rate of travel of an object that exceeds the speed of sound. For objects traveling in dry air of a temperature of 20 °C at sea level, this speed is 344 m/s, 1,125 ft/s, 768 mph, 667 knots, or 1,235 km/h. Speeds greater than five times the speed of sound are referred to as hypersonic. Flights during which only some parts of the air surrounding an object, such as the ends of rotor blades, reach supersonic speeds are called transonic; this occurs somewhere between Mach 0.8 and Mach 1.2. Sounds are traveling vibrations in the form of pressure waves in an elastic medium. In gases, sound travels longitudinally at different speeds depending on the molecular mass and temperature of the gas, pressure has little effect. Since air temperature and composition varies with altitude, Mach numbers for aircraft may change despite a constant travel speed. In water at room temperature supersonic speed can be considered as any speed greater than 1,440 m/s. In solids, sound waves can be polarized longitudinally or transversely and have higher velocities.
Supersonic fracture is crack motion faster than the speed of sound in a brittle material. At the beginning of the 20th century, the term "supersonic" was used as an adjective to describe sound whose frequency is above the range of normal human hearing; the modern term for this meaning is "ultrasonic". The tip of a bullwhip is thought to be the first man-made object to break the sound barrier, resulting in the telltale "crack"; the wave motion traveling through the bullwhip is what makes it capable of achieving supersonic speeds. Most modern fighter aircraft are supersonic aircraft, but there have been supersonic passenger aircraft, namely Concorde and the Tupolev Tu-144. Both these passenger aircraft and some modern fighters are capable of supercruise, a condition of sustained supersonic flight without the use of an afterburner. Due to its ability to supercruise for several hours and the high frequency of flight over several decades, Concorde spent more time flying supersonically than all other aircraft combined by a considerable margin.
Since Concorde's final retirement flight on November 26, 2003, there are no supersonic passenger aircraft left in service. Some large bombers, such as the Tupolev Tu-160 and Rockwell B-1 Lancer are supersonic-capable. Most modern firearm bullets are supersonic, with rifle projectiles travelling at speeds approaching and in some cases well exceeding Mach 3. Most spacecraft, most notably the Space Shuttle are supersonic at least during portions of their reentry, though the effects on the spacecraft are reduced by low air densities. During ascent, launch vehicles avoid going supersonic below 30 km to reduce air drag. Note that the speed of sound decreases somewhat with altitude, due to lower temperatures found there. At higher altitudes the temperature starts increasing, with the corresponding increase in the speed of sound; when an inflated balloon is burst, the torn pieces of latex contract at supersonic speed, which contributes to the sharp and loud popping noise. To date, only one land vehicle has travelled at supersonic speed.
It is ThrustSSC, driven by Andy Green, which holds the world land speed record, having achieved an average speed on its bi-directional run of 1,228 km/h in the Black Rock Desert on 15 October 1997. Richard Noble, Andy Green and a team of engineers are planning to break this record in 2019 at Hakskeen Pan in South Africa with the Bloodhound SSC hybrid jet- and rocket-propelled car. Supersonic aerodynamics is simpler than subsonic aerodynamics because the airsheets at different points along the plane cannot affect each other. Supersonic jets and rocket vehicles require several times greater thrust to push through the extra aerodynamic drag experienced within the transonic region. At these speeds aerospace engineers can guide air around the fuselage of the aircraft without producing new shock waves, but any change in cross area farther down the vehicle leads to shock waves along the body. Designers use the Supersonic area rule and the Whitcomb area rule to minimize sudden changes in size. However, in practical applications, a supersonic aircraft must operate stably in both subsonic and supersonic profiles, hence aerodynamic design is more complex.
One problem with sustained supersonic flight is the generation of heat in flight. At high speeds aerodynamic heating can occur, so an aircraft must be designed to operate and function under high temperatures. Duralumin, the traditional aircraft material, starts to lose strength and go into plastic deformation at low temperatures, is unsuitable for continuous use at speeds above Mach 2.2 to 2.4. Materials such as titanium and stainless steel allow operations at much higher temperatures. For example, the Lockheed SR-71 Blackbird jet could fly continuously at Mach 3.1 which could lead to temperatures on some parts of the aircraft getting above 315 °C. Another area of concern for sustained high-speed flight is engine operation. Jet engines create thrust by increasing the temperature of the air they ingest, as the aircraft speeds up, friction and compression heat this air before it reaches the engines; the maximum allowable temperature of the exhaust is determined by the materials in the turbine at the rear of the engine, so as the aircraft speeds up, the difference in intake and exhaust temperature that the engine can create decreases, the thrust along with it.
Air cooling the turbine area to allow operations at higher temperatures was a key solution, one that continued to improve through the 1950s and on to this day. Intake design
Phenyl ether polymers are a class of polymers that contain a phenoxy or a thiophenoxy group as the repeating group in ether linkages. Commercial phenyl ether polymers belong to two chemical classes: polyphenyl ethers and polyphenylene oxides; the phenoxy groups in the former class of polymers do not contain any substituents whereas 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. Either class can have the oxygen atoms attached at various positions around the rings; the proper name for a phenyl ether polymer is poly or polyphenyl polyether, but the name polyphenyl ether is accepted. Polyphenyl ethers are obtained by repeated application of the Ullmann Ether Synthesis: reaction of an alkali-metal phenate with a halogenated benzene catalyzed by copper. PPEs of up to 6 phenyl rings, both oxy and thio ethers, are commercially available. See Table 1.
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, para substitution of the three middle rings, a total of 5 rings, 4 ether linkages. Meta substitution of the aryl rings in these materials is most common and desired. Longer chain analogues with up to 10 benzene rings are known; the simplest member of the phenyl ether family is diphenyl ether called diphenyl oxide, the structure of, provided in Figure 4. Low molecular weight polyphenyl ethers and thioethers are used in a variety of applications, include high-vacuum devices, electronics, in high-temperature and radiation-resistant fluids and greases. Figure 5 shows the structure of the sulfur analogue of 3-R polyphenyl ether shown in Figure 3. Typical physical properties of polyphenyl ethers are provided in Table 2. Physical properties of a particular PPE depend upon the number of aromatic rings, their substitution pattern, whether it is an ether or a thioether.
In the case of products of mixed structures, properties are hard to predict from only the structural features. The important attributes of PPEs include their thermal and oxidative stability and stability in the presence of ionizing radiation. PPEs have the disadvantage of having somewhat high pour points. For example, PPEs that contain two and three benzene rings are solids at room temperatures; the melting points of the ordinarily solid PPEs are lowered if they contain more m-phenylene rings, alkyl groups, or are mixtures of isomers. PPEs that contain only o- and p-substituted rings have the highest melting points. PPEs have good oxidation stability. With respect to volatilities, p-derivatives have the lowest volatilities, the o-derivatives have the highest volatilities; the opposite is true for flash points and fire points. Spontaneous ignition temperatures of polyphenyl ethers lie between 550 and 595 °C, alkyl substitution reduces this value by ~50 °C. PPEs are compatible with most metals and elastomers that are used in high-temperature applications.
They swell common seal materials. Oxidation stability of un-substituted PPEs is quite good because they lack oxidizable carbon-hydrogen bonds. Thermal decomposition temperature, as measured by the isoteniscope procedure, is between 440 and 465 °C. Ionizing radiation affects all organic compounds, causing a change in their properties because radiation disrupts covalent bonds that are most prevalent in organic compounds. One result of ionization is that the organic molecules disproportionate to form smaller hydrocarbon molecules as well as larger hydrocarbons molecules; this is reflected by increased evaporation loss, lowering of the flash and fire points, increased viscosity. Other chemical reactions caused by radiation include isomerization; the former leads to increased acidity and coke formation. PPEs have high radiation resistance. Of all classes of synthetic lubricants the polyphenyl ethers are the most radiation resistant. Excellent radiation stability of PPEs can be ascribed to the limited number of ionizable carbon-carbon and carbon-hydrogen bonds.
In one study, the performance of PPE under the influence of 1x1011 ergs/gram of radiation at 99 °C was compared with synthetic ester, synthetic hydrocarbon, silicone fluids. PPE showed a viscosity increase of only 35%, while all other fluids showed a viscosity increase of 1700% and gelled. Further tests have shown PPEs to be resistant to gamma and associated neutron radiation dosages of 1x1010 erg/g at temperatures up to 315 °C. PPEs have high surface tension; the surface tension of the commercially available 5R4E is 49.9 dynes/cm, one of the highest in pure organic liquids. This property is useful in applications where migration of the lubricant into the surrounding environment must be avoided. While PPEs were developed for use in extreme environments that were experienced in aerospace applications, they are now used in other applications requiring low volatility and excellent thermo-oxidative and ionizing radiation stability; such applications include use as diffusion pump fluids. In addition, because of e