**1. Introduction**

Polymeric foams are widely used in different fields due to their lightness, reduced thermal conductivity, high-energy absorption, and excellent strength/weight ratio. Application area of the polymer foams has high variety such as transportation, bedding, carpet underlay, textile, toys, sport instruments, insulation appliances, and construction, biomedical, and automotive

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

**Figure 1.** Illustration of polymer foam cellular structures (a) closed cell type (b) open cell type.

sectors [1–5]. A polymer foam is basically a polymer-and-gas mixture, which gives the material a microcellular structure. Polymer foams can be flexible or rigid due to their cell geometry such as open cells or closed cells (**Figure 1**). If the gas pores roughly spherical and separated from each other by the polymer matrix, then this type is called closed cell structure. On the contrary, if the pores are interconnected to each other to some extent which provides passage of fluid through the foam, then this is called open cell structure. A close cell structure is a good candidate to be a life jacket material, while an open cell structure would be waterlogged. The open cell foams are for bedding, acoustical insulation car seating, and furniture, while the closed cell foams are suitable for thermal insulation, and they are generally rigid, which makes them a preferable lightweight material for automotive and aerospace [6–9].

The development of polymeric foams started with the macrocellular polystyrene foams having cell size above 100 μm in the 1930s [10]. Developments continued for providing finer cells, and solid-state batch foaming method was applied and foam cells less than 100 μ in diameter were obtained in the 1980s. Since then, polymer foam processing and shaping methods have developed speedily. Besides polystyrene foams, polyurethane has become popular. However, in this work, the most used thermoplastic foams with closed cell structure are focused. The cell generation, cell size and density, mechanical properties, and shaping processes of thermoplastic foams are given in detail. The effect of nanoparticle addition is also discussed in generation of multifunctional materials, polymer nanocomposite foams.
