WATER INSULATION

For insulation applications, plastics are mostly used. For water insulation (waterproofing), PU, PVC, thermoplastic elastomers, silicones, various rubbers, acrylics, and even PE are mainly used. For heat insulation applications, PVC, EPS, PU foams are used directly (for roofs and walls), as well as in sandwich panels. In addition, UPVC-hollow panels are being used as interior cladding to provide multi-purpose insulation (for water, heat, noise), specifically for pipes, wall insulation, and so on. 

Modified bitumen can be used in the whole range of bitumen works, namely, for producing bituminous mixtures (hot to cold), in surface dressings, surface sealing, water insulation and so on. Modified bitumens may be employed directly or in the form of cut-backs or emulsions, or blended with, for example, natural asphalt. 

The addition of fillers to bitumen does not modify the properties of the bitumen. However, it is as an old and very economical way to increase the stability and stiffness of the asphalt (bituminous mixture) so as to improve resistance to deformation. Today, with the development of other modifiers and additives, their use is limited to the production of bituminous materials for water insulation, acting as a substance to increase volume. 

Figure 6.16 Sealing IDA packaging with water-insulating splicing epoxy. (See insert for...

One of the most challenging environments that insulating materials experience is a combination of dirt and water. Insulators subjected to this combination in the presence of an electric field may fail by tracking or erosion in the vicinity of the electrodes. To determine the relative resistance of insulators to dust and fog, three parallel strip electrodes of brass or copper are placed in contact with the surface of an insulator. The outer electrodes are grounded, and the central one is attached to an alternating current supply. The surface of the insulator is coated with a layer of synthetic dust about 0.020 in. thick, and the surface is evenly wetted with a fine water spray emitted by a fog nozzle. When the central electrode is energized, arcs form between it and the adjacent electrodes. Power is maintained until the insulator fails, either by the formation of a track that conducts current or by erosion beneath the electrodes that punctures the specimen. A minimum of three specimens should be tested and the average time to failure and type of failure reported. The results of this test should be used for comparative purposes only. 

One may now consider how changes can be made in a system across an adiabatic wall. The first law of thermodynamics can now be stated as another generalization of experimental observation, but in an unfamiliar form the M/ork required to transform an adiabatic (thermally insulated) system, from a completely specified initial state to a completely specifiedfinal state is independent of the source of the work (mechanical, electrical, etc.) and independent of the nature of the adiabatic path. This is exactly what Joule observed the same amount of work, mechanical or electrical, was always required to bring an adiabatically enclosed volume of water from one temperature 0 to another 02. 

Guokenberger R, Fleim M, Cevo G, Knapp FI F, Wiegrabe W and Flillebrand A 1994 Soanning tunnelling miorosoopy of insulators and biologioal speoimens based on lateral oonduotivity of ultrathin water films Science 266 1538... 

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

A modified Hahn condenser, a form of partial condenser, is illustrated in Fig. VI, 12, 1 it is best constructed of Pyrex glass. The dimensions given are only approximate and may be varied slightly. The inside clearance should be approximately 0-3 cm. a water jacket should be fitted over the central portion of the side arm by moans of rubber stoppers. Alternatively, the side arm may have a length of about 10 cm. and a condenser fitted to this in the usual manner. An approximately 1 cm. layer of absolute alcohol is placed in the inner condensing tube and the top of the tube is connected to a reflux condenser. The outside of the condenser below the side arm should be insulated with asbestos cloth or paper. The refluxing mixture boils the ethyl alcohol in the inner tube, most of the isopropyl alcohol is returned to the flask and the acetone distils over. 

Ethylene. Under the influence of pressure and a catalyst, ethylene yields a white, tough but flexible waxy sohd, known as Polythene. Polyethylene possesses excellent electric insulation properties and high water resistance it has a low specific gravity and a low softening point (about 110°). The chemical inertness oi Polythene has found application in the manufacture of many items of apparatus for the laboratory. It is a useful lubricant for ground glass connexions, particularly at relatively high temperatures. 

The silicone oils and silicone resins find application as (i) lubricants (their change of viscosity with temperature is small), (ii) hydraulic fluids (they are unusually compressible), (iii) dielectric fluids, (iv) for the pro duction of water-repellant surfaces, and (v) in the electrical industry (because of their high insulating properties). 

Structural Insulation Boards. Stmctural iasulation boards are made by a process similar to that used for iasulation board, with the exception of another additive which provides additional weight, strength, and water resistance. The additive is normally asphalt, which is added ia a... 

The Bathythermograph. The thermistor sensing probe of a disposable bathythermograph is coated with parylene. This instmment is used to chart the ocean water temperature as a function of depth. Parylene provides the needed insulation resistance and is thin and uniform enough to permit a rapid and accurate response to the temperature of the surrounding salt water (64). 

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

Polyimide. Polyimide is a biaxiaHy oriented high performance film that is tough, flexible, and temperature- and combustion-resistant. Its room temperature properties compare to poly(ethylene terephthalate), but it retains these good characteristics at temperatures above 400°C. Its electrical resistance is good and it is dimensionally stable. The principal detriment is fairly high moisture absorbance. The main uses are for electrical insulation, particularly where high temperatures are prevalent or ionizing radiation is a problem. The films may be coated to reduce water absorption and enhance... 

Electrical Properties. Polytetrafluoroethylene is an excellent electrical insulator because of its mechanical strength and chemical and thermal stabihty as well as excellent electrical properties (Table 6). It does not absorb water and volume resistivity remains unchanged even after prolonged soaking. The dielectric constant remains constant at 2.1 for a temperature range of —40 to 250°C and a frequency range of 5 Hz to 10 GHz. 

Electrica.1 Properties. Because of excellent electrical properties, FEP is a valuable and versatile electrical insulator. Within the recommended service temperature range, PTFE and EEP have identical properties as electrical insulators. Volume resistivity, which is >10 H/cm, remains unchanged even after prolonged soaking in water surface resistivity is >10 H/sq. 

Commercial Construction. The same attributes desirable on residential constmction appHcations hold for commercial constmction as weU but insulation quaHty, permanence, moisture insensitivity, and resistance to free2e—thaw cycling in the presence of water are of greater significance. For this reason ceUular plastics have greater appHcation here. Both polystyrene and polyurethane foams are highly desirable roof insulations in commercial as in residential constmction. 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

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