Foamed plastics classification

One ASTM test procedure has suggested (24) that foamed plastics be classified as either rigid or flexible, a flexible foam being one that does not mpture when a 20 x 2.5 x 2.5 cm piece is wrapped around a 2.5 cm mandrel at a uniform rate of 1 lap/5 s at 15—25°C. Rigid foams are those that do mpture under this test. This classification is used in this article. 

On the other hand, syntactic materials may also be thought of as reinforced or filled plastics, with the gas-containing particles being the reinforcing component. This classification is also justifiable in view of the manufacturing technology. The matrix is not foamed chemically, but is filled mechanically with the hollow spheres. Syntactic foamed plastics are thus called physical foams 6,7). 

Shutov, F. A The gas-structure element and a new classification of plastic foams. All-Union Conference on Gas-Filled Polymers, Vladimir/USSR, 1974 (in Russian)... 

Table 1 shows a classification of thermosetting foams. Among the foams listed in this table, polyurethane foams have the largest market share in the thermosetting-plastic-foam market. 

Isocyanates, n.o.s. or Isocyanate Solution, n.o.s. These include a number of chemical products used in the manufacture of plastic foams, synthetic rubber, etc. Some are sufficiently toxic or lachrymatory to need classification as toxic substances, particularly isocyanates in pure form. Others may need to be classified as flammable liquids, depending on their characteristics, and a number may not be subject to these Instructions. ICAO A2... 

A rigid foam is defined as one in which the polymer matrix exists in the crystalline state or, if amorphous, is below its Tg. Following from this, a flexible cellular polymer is a system in which the matrix polymer is above its Tg. According to this classification, most polyolefins, polystyrene, phenolic, polyycarbonate, polyphenylene oxide, and some polyurethane foams are rigid, whereas rubber foams, elastomeric polyurethanes, certain polyolefins, and plasticized PVC are flexible. Intermediate between these two extremes is a class of polymer foams known as semirigid. Their stress-strain behavior is, however, closer to that of flexible systems than to that exhibited by rigid cellular polymers. 

Classification according to the pressure used, the continuity of the process (discontinuous, semicontinuous, continuous, or automated), or the degree of finish (intermediate component, i.e., section, or finished product, e.g., foam) is also possible (see Plastics Handbook or BS, ASTM, or DIN standards.)... 

Air classification also uses density as the characteristic by which different plastics may be separated. A blast of air is blown through a steady stream of falling ground, mixed plastics. The dense plastics continue to fall while expanded foams and other lighter materials are blown into a separate collector. This technique is typically used to separate plastics with significantly different densities whereas liquid based density separations may be tuned more tightly. 

Many resins can be foamed by a variety of processes. Each method of foam manufacturing involves the steps of cell initiation, growth, and stabilization. The usual method for classification of foaming methods is based on the cell growth and stabilization. There are three general methods for producing foam from plastics mechanical, chemical, and physical. 

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