Electrical properties of molded plastic or formed plastic asbestos-based products: This article describes the electrical properties of asbestos-based molded or formed products.

This article series about asbestos plastics & molded materials describes the history, manufacturing process & uses of asbestos plastics and molded materials such as asbestos reinforced handles, the Vanguard rocked nose cone, automobile parts & housings, electronic equipment (radar scanner), asbestos-filled Teflon, rocket motor parts, plastic drop tanks for the Hawker Sea Hawk, and hundreds of other products.

NEMA [Electrical] Properties of Asbestos-Reinforced Plastic Products

The National Electric Manufacturers' Association (NEMA) has standard specifications for lam- m ated thermosetting asbestos (Grade A and AA) sheets, tubes and rods used in the manufacture of electrical ap- paratus and supplies. The maximum NEMA properties are low in comparison to those of the more recently developed asbestos felts and sheet materials or higher asbestos content fabrics treated by improved methods.

Grade A sheets (asbestos paper base) are identified as being more resistant to flame and slightly more resistant to heat than other laminated grades because of high inorganic content. They are not recommended for primary insulation for electrical applications involving commercial power frequencies at voltages in excess of 250 V. Grade AA sheets (asbestos fabric base) are more resistant to heat and stronger and tougher than Grade A. It is not recommended for primary insulation for electrical applications at any voltage.

Electrical Properties of Asbestos-Reinforced Plastics & Molded Products

Asbestos filled phenolic and urea formaldehyde resins are used in different electrical applications, particularly where high heat resistance is desired. Asbestos filled polyester and diallylphthalate premix compounds are readily used in electrical applications.

Asbestos is used as a filler and/or reinforcement with the various resins in order to produce different electrical properties. See Figure 9.4 below.

Figure 9.4. High-speed radar scanner manufactured from high-pressure asbestos-phenolic plastic. Applications are ground control approach, airfield radar, sector surveillance for harbor control, marine navigation and coastal defense. - Courtesy The Bristol Aeroplane Co. Ltd.

[Click to enlarge any image]

The asbestos moldings offer advantages such as availability on a mass production scale of complicated shapes and sizes normally difficult and expensive to fabricate in other dielectric materials; moldings can very readily be made with metal inserts; moldings can be made with an exceptionally high degree of accuracy; and variations in the molding compound can be made with various grades and amount of asbestos filler.

Silicone-asbestos molding compounds have been prepared with such special fillers as titinates in order to produce electrical transmission properties for radar which permit no electrical change for temperatures of 500°F and higher.

The electrical properties can be made to vary by changing the proportions of asbestos, silicone and filler. With the proper proportions, the electrical thickness remains constant with change in temperature.

Data have been reported on high temperature properties of asbestos filled "Teflon." * In the electronic industry, "Teflon" in the pure state is recognized for its excellent dielectric properties. This fact coupled with low water absorption, heat resistance to 300°C and a low coefficient of friction makes "Teflon" an attractive material in high temperature or missile radome applications.

The aforementioned attributes of this insulating material are overshadowed by mechanical drawbacks which are amplified at increased temperatures. Most undesirable of these properties is Teflon's extremely high coefficient of linear expansion and low resistance to deformation under load. By combining Teflon with asbestos, it was found that thermal expansion and heat of distortion were greatly improved.

* Marshall, R. F. and Ritt, P. E., "High Temperature Properties of filled Teflon," The Institute of Radio Engineers, University of Pennsylvania, Phila., Pa. (June 4-5, 1957).

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