Electrical insulators foams

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Electrical Insulation. The substitution of a gas for part of a soHd polymer usuaUy results in large changes in the electrical properties of the resulting material. The dielectric constant, dissipation factor, and dielectric strength are aU generaUy lowered in amounts roughly proportional to the amount of gas in the foam. 

In the special case of pipelines operating at relatively high temperatures such as for the transmission of heavy fuel oil at up to 85°C, heat insulation and electrical insulation are provided by up to 50 mm of foam-expanded polyurethane. As a further insurance against penetration of water, and to prevent mechanical damage, outer coatings of polyethylene (5 mm), butyl laminate tape (0-8 mm) or coal-tar enamel reinforced with glass fibre (2-5 mm) have been used. 

The foam detector based on the resistance method acts on the conductivity of the probe. The length of the probe is coated with some electrically insulating material, leaving the tip... 

Plastic also refers to a material that has a physical characteristic such as plasticity and toughness. The general term commodity plastic, engineering plastic, advanced plastic, advanced reinforced plastic, or advanced plastic composite is used to indicate different performance materials. These terms and others will be reviewed latter in this chapter. Plastics are made into specialty products that have developed into major markets. An example is plastic foams that can provide flexibility to rigidity as well as other desired properties (heat and electrical insulation, toughness, filtration, etc.). 

For certain products, skill is required to estimate a product s performance under steady-state heat-flow conditions, especially those made of RPs (Fig. 7-19). The method and repeatability of the processing technique can have a significant effect. In general, thermal conductivity is low for plastics and the plastic s structure does not alter its value significantly. To increase it the usual approach is to add metallic fillers, glass fibers, or electrically insulating fillers such as alumina. Foaming can be used to decrease thermal conductivity. 

Poly(aryloxyphosphazene) elastomers can be cured with peroxides, sulfur and radiation. The resulting vulcanizates are resistant to attack by moisture and oils and have been found to have desirable characteristics for electrical insulation applications where fire safety is a concern (Table II) (12). Fire resistant, low smoke, closed cell foams with excellent properties (Table III) have also been developed from poly(aryloxyphosphazene) elastomers (13). Applications for these foams, which can be produced as either slabstock or tube stock, are being developed for military, aerospace and commercial uses. (See Table II and III.)... 

Epoxy resins find a large number of uses because of their remarkable chemical resistance and good adhesion. Epoxy resins are excellent structural adhesives. When properly cured, epoxy resins can yield very tough materials. They are used in industrial floorings, foams, potting materials for electrical insulations, etc. One of the principal constituents in many of the Fibre-reinforced plastics (FRP) is an epoxy polymer. 

Heat stabilizers - [HEAT STABILIZERS] (Vol 12) -m electric insulation [INSULATION, ELECTRIC] (Vol 14) -hydrazine derivatives as [HYDRAZINE AND ITS DERIVATIVES] (Vol 13) -lead compounds [LEAD COMPOUNDS - LEAD SALTS] (Vol 15) -metal soaps [DRIERS AND METALLIC SOAPS] (Vol 8) -for PVC foam [FOAMED PLASTICS] (Vol 11)... 

An unusually wide variety of products and usages are possible with plasticized vinyls. Typical applications include floor and wall coverings, boots, rainwear, jackets, upholstery, garden hose, electrical insulation, film and sheeting, foams and many others. 

The dielectric properties of the syntactic foams used for electric insulation (SPAB-1 and ENS-6T) are very good. Even an increase in temperature in humid environments does not raise their s or tan 8 values by more than 20 %. The electric resistance of a SPAB-1 sample did not exceed 400-500 MOhm even after 24 hours in an environment of 95 % ( 3 %) relative humidity u. 

In the aerospace industry, too, there is an increasing demand for high-performance electrical insulation with a high degree of tire safety and light weight. Here the trend is for foamed fluoroplastics. FEP is attractive for this purpose. Fuel-tank seals are made predominantly from special perfluoroelastomers. 

For practical calculations of durability and for the determination of application conditions it should be taken into account that these materials are composed of a three-phase structure (gas - solid - liquid). A certain amount of a liquid phase, due to the condensation of water vapor in the air, is practically always present inside plastic foams. The presence of the liquid phase plays a decisive role in mass, gas and heat transfer and sharply reduces the heat and electrical insulation properties of polymeric foams (see Chap. 6). 

Freely foamed oligoester plastics are being increasingly used as light fillers in load-carrying structures, as heat, sound and electrical insulating and buoyant materials. This type of gas-filled material finds application in building, furniture and other industries. 

Electroconductivity a is one of the most important characteristics of foams. For solid foams it determines the reliabflity of a foam as an electrical insulator, for liquid foams it serves as a technological parameter which is used to monitor foaming and hardening kinetics of gas-filled polymers. 

Since the electroconductivity of pure water is by several orders of m itude hi er than those of polymers, even smaU amounts of moisture markedly reduce electrical insulation properties of foams. For this reason, measuren nt of dielectric properties is a precise, rapid and non-destructive method of monitoring the kinetics and level of moisture absorption. Thus, the establishment of correlations between dielectric properties and the hygroscopicity of plastic foams makes it possible to solve two practical problems how moisture affects dielectric properties and how to determine non-electrical properties by electrical measurements ... 

Shutov, F. A. On the use of electrical insulation from phenolic foams in high humidity media. All-Union Seminar on Plastic Foams, Leningrad 1975 (in Russian)... 

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