Polymers plastic foams

A cellular plastic has been defined as a plastic the apparent density of which is decreased substantially by the presence of numerous cells disposed throughout its mass (21). In this article the terms cellular plastic, foamed plastic, expanded plastic, and plastic foam are used interchangeably to denote all two-phase gas—soHd systems in which the soHd is continuous and composed of a synthetic polymer or mbber. 

The most important stmctural variables are again polymer composition, density, and ceU size and shape. Stmctural foams have relatively high densities (typically >300 kg/m ) and ceU stmctures similar to those in Figure 2d which are primarily comprised of holes in contrast to a pentagonal dodecahedron type of ceU stmcture in low density plastic foams. Since stmctural foams are generally not uniform in ceU stmcture, they exhibit considerable variation in properties with particle geometry (103). 

Thermal Conductivity. More information is available relating thermal conductivity to stmctural variables of cellular polymers than for any other property. Several papers have discussed the relation of the thermal conductivity of heterogeneous materials in general (187,188) and of plastic foams in particular (132,143,151,189—191) with the characteristic stmctural variables of the systems. 

Flammability. The results of small-scale laboratory tests of plastic foams have been recognized as not predictive of their tme behavior in other fire situations (205). Work aimed at developing tests to evaluate the performance of plastic foams in actual fire situations continues. All plastic foams are combustible, some burning more readily than others when exposed to fire. Some additives (131,135), when added in small quantities to the polymer, markedly improve the behavior of the foam in the presence of small fire sources. Plastic foams must be used properly following the manufacturers recommendations and any appHcable regulations. 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Miscellaneous Properties. The acoustical properties of polymers are altered considerably by their fabrication into a ceUular stmcture. Sound transmission is altered only slightly because it depends predominandy on the density of the barrier (in this case, the polymer phase). CeUular polymers by themselves are, therefore, very poor materials for reducing sound transmission. They are, however, quite effective in absorbing sound waves of certain frequencies (150) materials with open ceUs on the surface are particulady effective. The combination of other advantageous physical properties with fair acoustical properties has led to the use of several different types of plastic foams in sound-absorbing constmctions (215,216). The sound absorption of a number of ceUular polymers has been reported (21,150,215,217). 

FRISCH, K. c., and saunders, j. h. (Eds.), Plastic Foams Part I, Dekker, New York (1972) GEUSKENS, G. (Ed.), Degradation and Stabilisation of Polymers, Applied Science, London (1975) HAWKINS, E. L. (Ed.), Polymer Stabilisation, Wiley-Interscience, New York (1972)... 

The opacity of plastic foams, and polymers with scratched surfaces, is also governed by Fresnel s law. The n value of the gas which occupies the scratch indentation is much lower than that of the polymer. Light may be directed through rods of transparent polymers, such as PMMA. This effect may be enhanced when the rod or filament is coated with a polymer with a different refractive index, such as polytetrafluoroethylene (ptfe). Optical fibers utilize this principle. 

FOAMED PLASTICS. Foamed polymers, otherwise known as cellular polymers or polymeric foams, or expanded plastics have been important to human life since primitive people began to use wood, a cellular form of the polymer cellulose. Cellular polymers have been commercially accepted in a wide variety of applications since the 1940s. The total usage of foamed plastics in the United Stales has risen from 441 x 10 i in 1967 lo a projected 2.8xIO6 i in 1995. 

Comfort cushioning is the largest single application of cellular polymers flexible foams are the principal contributors to this field. However, the rapid growth rate of structural, packaging, and insulation applications has brought their volume over that of flexible loams during the past Tew years. Table 5 shows U.S, consumption of foamed plastics by resin and market,... 

Interestingly, a temperature increase lowers the strength of a novolac syntactic foam with carbon filler less than it reduces the strength of the unfilled foam (Fig. 16)39). This is undoubtably because there is destructive thermal oxidation in the plastic foamed by gas, due to the oxygen in the gas. Thermal oxidation in syntactic foams is much lower, because the microsphere shell forms a protective barrier between the gas within the sphere and the polymer matrix. 

An examination of the experimental findings and the calculation model shows that the deformability of a syntactic foam depends mainly on the elastic properties of the polymer matrix, whereas the filler concentration mainly affects its compressibility. In fact, monolithic (unfilled) samples do deform elastically at the start of the compression curve, but when the material is deformed further, the forced elasticity limit is reached (Fig. 21). Thus, the nominal ultimate strength for non-brittle failure is determined by the fact that the forced elastic limit is reached, and not because the adhesive ties have lost their stability (as it is the case with light plastic foams) 8 10). 

A plastic foam is a heterogeneous blend of a polymer with a gas. The gas cells are between 1 mm and 0.1 mm. Foams are made from thermoplasts, thermosets and rubbers. In all these cases the foam structure is generated in the fluid condition with thermoplasts it is fixed by solidification, with thermosets and rubbers by the curing or vulcanisation reaction. 

The thermal conductivity of cellular polymers has been thoroughly studied in heterogeneous materials [33,34] and plastic foams [25,35-38]. 

Rodents chew through cellular polymers but do not ingest the foam as a foodstuff. The resistance to rot, mildew, and fungi is related to moisture absorption [64]. Therefore, open-cell foams support such growth better than closed-cell foams. High humidity and temperature are necessary for the growth of microbes on any plastic foam. 

At present, hundreds of various elastic and rigid gas-filled materials used literally in all branches of industry are produced on the basis of reactive oligonwrs and high polymers. The production of these materials is rapidly expanding. Thus, in 1970 the world output of plastic foams was 2 million tons, in 1975 3.5 million tons, 1980 it will be 5-6 million tons, and in 1985 about 20% (6% in 1975) of all plastic materials will be gas-filled ... 

In the last decade several novel types of gas-filled polymers appeared which belong to the second generation integral (structural) plastic foams syntactic foams reinforced polymer foams multilayer foams (foamed laminates), metallized plastic foams mineral and metallic foamed materials obtained on the basis of foamed polymers laminated constructions on the basis of foamed polymers and monolithic (unfoamed) plastics, metals, paper, leather, etc. 

In the author s opinion, the analysis of the problems of chemistry, technology and properties of plastic foams should be carried out both from the viewpoint of the chemistry and physics of ojndensed systems, including the physics of polymers. 

A more profound understanding of the behavior of plastic foams under various exploitation conditions will make it possible to estimate service conditions and also to determine the precise and most rational functional application of a given foamed polymer. Unfortunately, some foamed materials are still erroneously rejected altogether, only because their properties once did not fit specific service conditions. 

Another study of foamed polymers uses modern physicochemical methods of structure elucidation of gas-filled polymers and more accurate mathematical descriptioa These studies have considerably contributed to the understanding of the general character of the spatial structure of plastic foams. Thus, quantitative estimations of the effect of each morphological parameter (specific gravity, size and shape of ceUs, type of communication between ceUs, cell distributbn in the bulk, etc.), on the properties of a given material could be made. 

In Eq. (14) the index is in the preexponential factor and con quently affects t>w less than the volume fraction dp which is in the exponent. This leads to a very important conclusion it is not the volume fraction of open cells, as usually considered, but the volume fraction of polymer and the type of packing of cells (through the value 7) which primarUy determine the extent of equilibrium moisture absorption of plastic foams. 

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