expanded plastic[ik′spand·əd ′plas·tik]
a gas-filled plastic with a cellular internal structure that resembles solidified foam. Expanded plastic mostly consists of closed, discontinuous cavities whose walls are thin layers of polymer. Thus, it differs from porous plastic, which has a spongy structure; that is, it is permeated by a system of interconnecting pores. Many foamed substances largely consist of interconnecting pores, and the classification of expanded plastic as a separate category of gas-filled plastics on the basis of the presence of isolated cavities is arbitrary. It is more correct to use “expanded plastic” to refer to any gas-filled polymer that is obtained by foaming and subsequent hardening of the originally liquid or viscous mass.
|Table 1. Properties of some expanded plastics made in the USSR1|
|Brand||Apparent density (kg/m3)||Maximum working temperature (C)||Tensile strength (MN/m2)||Compression strength (MN/m2)||Dielectric loss angle tangent||Breakdown potential (kV/mm)||Water absorption (%)|
|1 The coefficients of thermal conductivity range from 0.096 to 0.180 kJ/m-hr-°K; the dielectric constants, from 1.1 to 1.6|
|Polyvinyl chloride ...........||PKhV–1||70–130||60||1.9–2.0||0.4–1||0.015||3.9||2.0–2.5|
|Organosilicon resin ..........||K–40||200–400||250–300||0.6||0.8–1.4||0.002||2.5||10|
Expanded plastics are made by dispersing a gas in a polymer intermediate, which can be a solution, melt, liquid oligomer, or suspension. Another method is to form the gas directly within the bulk of the product as the product hardens. Foaming is accomplished by various technological methods: (1) by mechanically mixing or bubbling the starting mixture in the presence of foaming agents; (2) by introducing substances that decompose to release gas or by introducing compounds that react to form gaseous products; (3) by saturating the starting mixture with a gas under pressure and subsequently lowering the pressure; and (4) by introducing a liquid that rapidly evaporates with increased temperature. Depending on the composition of the plastic and on the hardening conditions, most of the gas pockets within the finished product may be either open or closed.
Porous materials may also be obtained by washing a soluble filler from an entire polymer intermediate, by baking the powdered constituent materials in a polymer, or by forming condensation structures in solutions of polymers. Gas-filled plastics that are obtained by using hollow fillers, for example, polymers filled with gas that is contained in spherical microcapsules, have properties similar to those of expanded plastics.
Expanded plastics may be prepared from most synthetic and many natural polymers. Most of the industrially important ones are made from polystyrene, polyvinyl chloride, polyurethanes and polyethylene, as well as from resins that derive from phenol, epoxy, carbamide, and organosilicon compounds. Azo compounds, nitro compounds, and ammonium carbonate are among the substances that decompose to form a gas. Isopentane, methylene chloride, and freons are the low-boiling liquids that are used.
Industry produces rigid and elastic expanded plastics in which the cavities vary in diameter from 0.02 to 2 mm; sometimes the diameter is as large as 3–5 mm. The apparent density of these plastics, which are superior thermal and acoustic insulators, is very low, ranging from 0.02 to 0.5 g/cm3. The water resistance and mechanical and electrical properties depend on the chemical nature of the plastic and on the proportions of the admixtures in the polymer, as well as on the structural features of the finished product. The major properties of several expanded plastics that are produced in the USSR are presented in Table 1.
Expanded plastics are widely used in airplanes, ships, and chemical and transport machinery. They are also used in buildings and technical installations as thermal and acoustic insulators, in multi-ply products, and in various floating devices, for example, pontoons, light boats, buoys, and life jackets. Owing to their permeability to radio waves and their rather high dielectrical and water-insulating properties, expanded plastics are used in radio and electrical engineering. Other products that are made from expanded plastics include shock-absorber linings and various types of packing for optical instruments and electronic equipment. Elastic expanded plastics are used to make soft furniture and thermally insulated clothing.
REFERENCESRomanenkov, I. G. Fiziko-mekhanicheskie svoistva penistykh plastmass. Moscow, 1970.
Spravochnik po plasticheskim massam, vol. 2. Edited by M. I. Garbar [et al.]. Moscow, 1969. Page 155.
Entsiklopediia polimerov, vol. 2. Moscow, 1974. Page 549.
L. A. SHITS