Applications insulating materials

Polyethylene is the most common thermoplastic polymer with multiple applications insulating materials in electrical cables, pipes, packaging (bags, film, containers, a. s. o.), medical devices and other fields because of its good chemical and stress-crack resistance besides high strength. 

An Assessment of Thermal Insulation Materials and Systemsfor Building Applications, DOE Report, BNE-S0862 UC-9Sd, U.S. Dept, of Energy, Washington, D.C., 1978 R. P. Tye and D. L. McElroy, eds., ASTM STP 718, Thermal Insulation Peformance, American Society for Testing and Materials, Philadelphia, Pa., 1980, pp. 9—26. 

Applications. The high heat tolerance and good salt compatibiUty of welan gum indicate its potential for use as an additive in several aspects of oil and natural gas recovery. Welan also has suspension properties superior to xanthan gum, which is desirable in oil-field drilling operations and hydraulic fracturing projects. It is compatible with ethylene glycol, and a welan—ethylene glycol composition that forms a viscous material useful in the formulation of insulating materials has been described (244). 

In 1993, worldwide consumption of phenoHc resins exceeded 3 x 10 t slightly less than half of the total volume was produced in the United States (73). The largest-volume appHcation is in plywood adhesives, an area that accounts for ca 49% of U.S. consumption (Table 11). During the early 1980s, the volume of this apphcation more than doubled as mills converted from urea—formaldehyde (UF) to phenol—formaldehyde adhesives because of the release of formaldehyde from UF products. Other wood bonding applications account for another 15% of the volume. The next largest-volume application is insulation material at 12%. 

The use of ABS has in recent years met considerable competition on two fronts, particularly in automotive applications. For lower cost applications, where demands of finish and heat resistance are not too severe, blends of polypropylene and ethylene-propylene rubbers have found application (see Chapters 11 and 31). On the other hand, where enhanced heat resistance and surface hardness are required in conjunction with excellent impact properties, polycarbonate-ABS alloys (see Section 20.8) have found many applications. These materials have also replaced ABS in a number of electrical fittings and housings for business and domestic applications. Where improved heat distortion temperature and good electrical insulation properties (including tracking resistance) are important, then ABS may be replaced by poly(butylene terephthalate). 

Electrical Properties Traditionally plastics have established themselves in applications which require electrical insulation. PlFt and polyethylene are among the best insulating materials available. The material properties which are particularly relevant to electrical insulation are dielectric strength, resistance and tracking. 

The toughness of a material is a design driver in many structures subjected to impact loading. For those materials that must function under a wide range of temperatures, the temperature dependence of the various material properties is often of primary concern. Other structures are subjected to wear or corrosion, so the resistance of a material to those attacks is an important part of the material choice. Thermal and electrical conductivity can be design drivers for some applications, so materials with proper ranges of behavior for those factors must be chosen. Similarly, the acoustical and thermal insulation characteristics of materials often dictate the choice of materials. 

In order to perform effectively as an insulant a material must restrict heat flow by any (and preferably) all three methods of heat transfer. Most insulating materials adequately reduce conduction and convection elements by the cellular structure of the material. The radiation component is decreased by absorption into the body of the insulant and is further reduced by the application of bright foil outer facing to the product. 

The application of insulating materials is a specialist trade and justifies careful supervision and inspection. 

The conduction through residual gases can be reduced by the application of porous structures. The convection within a single pore is minimal if pore sizes are small. In small pores the temperature difference at the walls of the pore are negligible and no convection occurs. The convection is further reduced by the evacuation of the thermal insulating material. 

Other thermal insulation materials are known besides the three described above. These materials are compared in Table 19, which shows that the thermal conductivity can be reduced by a factor of 2-A by evacuation in the evacuated state it is below lOmWirT1 K l for all materials. The lowest conductivity is shown for foil insulations, but this requires an extremely low pressure of 10 5 mbar which is difficult to obtain in practical applications which ought to last for ten years. 

The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are important properties for applications of plastics insulating materials. The volume resistivity is the electrical resistance of the material measured in ohms as though the material was a conductor. Insulators will not sustain an indefinitely high voltage as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompanied by a physical breakdown of the insulator. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC-TRICAL/ELECTR ONICS PRODUCT). 

Surface resistivity is the ratio of the potential gradient parallel to the current along its surface to the current per unit width of the surface. Knowing the volume and surface resistivity of an insulating material makes it possible to design an insulator for a specific application. 

Polymers containing each of these configurations are known, the most common being the cis- A and the 1,4-isomers. The first of these, poly(c/ -l,4-isoprene), is the macromolecular constituent of natural rubber the second is the material known as gutta percha. The latter, unlike natural rubber, has no elastomeric properties, but has a leathery texture. It has been used for diverse applications such as golf-ball covers and as an insulating material for the trans-Atlantic cables of the late nineteenth century. 

Current market trends indicate a gaining momentum of corrosion and temperature-resistant MEMS devices for the use in mass market applications. Silicon-on-insulator material properties make way for a whole new range of uses and applications while being available in sufficiently large quantities to satisfy large volume production requirements. A breakthrough of these devices can be expected through automotive applications, if they can meet the price pressure. 

Polystyrene can be moulded, cast or extruded in sheets, rods and tubes. It can be easily pigmented. It finds wide, application in making household goods, lenses, plastic moulds, toys, wood laminates, films, battery boxes and electrical components. Expanded polystyrene is used as thermal insulating material and packaging. It is also used in lifejackets and floats. 

The use of silicone elastomers for high-voltage applications (insulators and cable accessories) requires special formulations. Unusually, stringent requirements for these materials must be met.509,510 Fluorosilicone rubbers, which offer some unique combinations of properties (e.g., chemical resistance and higher temperature stability), have attracted considerable attention and have been reviewed in recent publications.511,512 It was noted that a modification of perfluoroether elastomers with silicone elastomer via hydrosilylation reaction opens the possibility of novel applications.5... 

---