Foams from engineering thermoplastics like poly(butylene terephthalate) (PBT) are a new generation of polymer foams and, probably, the future for lightweight, insulation and damping materials. By means of foam extrusion or bead foaming, it is possible to achieve low-to-medium density foams (< 500 kg/m3). However, foam extrusion of PBT is quite challenging due to its low melt strength and drawability combined with a small temperature-processing window, which is a characteristic of semi-crystalline thermoplastics. This work proves that the problem of cell coalescence and insufficient cell stabilisation can be reduced by choosing the right material and processing parameters in foam extrusion with underwater pelletizing. Therefore, expanded PBT beads could be realised for the first time using CO2 as supercritical blowing agent. To obtain spherical low-density PBT beads with a homogenous foam structure, different process parameters were systematically studied with two different commercial extrusion grades and different blowing agent concentrations. The influence of water pressure and cutting speed of the underwater pelletizer on foam morphology of E-PBT and bead structure was studied. It was shown that using a polymer grade with a sufficiently high-melt viscosity helps to reduce cell coalescence. The lowest achieved density was 230 kg/m3. An increase of the blowing agent concentration did not help in reducing the density. The melting range was investigated by differential scanning calorimetry and yielded reasonable moulding temperatures of 205–215 ℃. This corresponds to steam pressures of 17–21 bar in a steam-moulding machine.

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