Another drawback is that not all polymers can be synthesised via suspension-polymerisation. They are not allowed to change the polymerisation process and the interfacial tension between water and styrene. Problems arise with additivation, since the additives have to meet high requirements. After this process the beads are sieved to get several fractions with a narrow size distribution and coated with antistatic agents to prevent agglomeration. This process consists of two steps, namely the polymerisation where the granules are formed and the addition of pentane and/or other blowing agents, which diffuse into the granules. The most often used method to produce high quantities of expandable beads of polystyrene is the suspension-polymerisation with a blowing agent. An overview of the possible methods for producing polymer bead foams is given in Fig. Expanded beads are produced from semi-crystalline thermoplastics, since the presence of crystalline domains prevents the storage of a blowing agent inside the solid bead. Ef fi cient transportation of the unfoamed blowing agent impregnated polymer material and control of density by the part- manufacturer are main advantages compared to expanded beads.
pre-expansion step) before the actual welding-process (i.e. pentane) is trapped and the impregnated polymer granules are expanded in a separate step (i.e. Expandable beads are polymer granules in which a blowing agent (e.g. The fi rst approach can be applied for amorphous thermoplastic resins only, like polystyrene, since only they retain a blowing agent in solid state (temperature below T G ). In principle, two approaches for the production of foamed bead exist: a) the creation of expandable beads, which must be pre- expanded and b) the production of already expanded beads. Effects countering cell stabilisation are the creation of crevices by large and fast deformations and rupture of cell walls.
Strain-hardening can be induced by long chain branching, the introduction of high-aspect-ratio nano-additives or by blends having a fi bril morphology.
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