Low-density porous materials,
otherwise known as foams, were first developed in the United State and Europe
in the mid-to-late 1930’s. The term polymer foams or cellular polymers
refer to a two-phase gas-solid system in which the solid polymer is continuous
and the gaseous cells are dispersed throughout the solid. In most cases,
the plastic represents only a minor portion of the volume of the composite
system, but contributes largely to its properties and utility. Other term used
interchangeably for polymer foams are plastic foams, cellular plastics, foamed
plastics, expanded polymers, and plastic sponges. The ratio of the gas-filler
to the solid plastic matrix determines the density of the polymeric foams. In
actual practice the foam density vary anywhere from the unusual seen range of
10-200 kg/m3 or to almost as high as that of the solid polymer
itself.
Foams may be organic, inorganic,
or metalo-organic in nature. There are two main type of polymeric foams:
thermoplastics and thermoset foams. Thermoplastic foams can be reprocessed and
recycled, while thermosets foams are intractable since they are usually
crosslinked. Within these classes, the
polymeric foams are further classified as rigid, semi-rigid, or flexible. A
rigid foam is defined as one in which the polymer matrix exist 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 the crystalline Tm or above its Tg. According to
this classification, most polyolefin, polystyrene, phenolic, polycarbonate,
polyphenylene oxide, and some polyurethanes, certain polyolefins and
plasticized PVC are flexible. Intermediate between these two extremes is a
class of polymer foams known as semi-rigid.