Rigid cellular polymers

The strong effect of density and polymer composition on compressive strength and modulus are illustrated in Tables 10.1 and 10.2 [8]. The cell shape or geometry also influences compressive properties [8,22,23,27,28]. In fact, the foam cell structure is controlled in some cases to optimize certain physical properties of rigid cellular polymers. In general, compressive strength and modulus of low-density foams may be expressed as 

Strengths and moduli of most polymers increase with decreasing temperature [29]. 

Tensile strength and modulus of rigid foams vary with density in much the same manner as the compressive strength and modulus [8,22,23]. 

The structural variables most important to the tensile properties are polymer composition, density, and cell shape [30]. 

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Fig. 3. Effect of density on compressive modulus of rigid cellular polymers. A, extmded polystyrene (131) B, expanded polystyrene (150) C-1, C-2, polyether polyurethane (151) D, phenol—formaldehyde (150) E, ebonite (150) E, urea—formaldehyde (150) G, poly(vinylchloride) (152). To convert...

Complete imidation will not occur but that which does will be accompanied by the formation of a cellular structure to produce a rigid cellular polymer. 

Figure 10.3 Effect of density on thermal conductivity of rigid cellular polymers. A, polystyrene [25] B, polystyrene [37] C, polyurethane-air [37] D, polyurethane-CFC 11 (CCI3F) [70] E, polyurethane [37] F, phenol-formaldehyde [37] G, ebonite [37]. To convert kg/m3 to Ib/ft3, multiply by 0.0624. Reproduced from F. O. Guenther, SPE Transactions, 2, 243 (1962), with permission from the society of Plastics Engineers, Brookfield, Connecticut, USA...

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. 

The group of rigid cellular polymers can be further subdivided according to whether they are used (1) for non-load-bearing applications, such as thermal insulation or as (2) load-bearing structural materials, which require high stiffness, strength and impact resistance. 

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