Evaluation of the Fusion Quality of Bead Foams Made from Polybutylene Terephthalate (E-PBT) Depending on the Processing Temperature

Increasing requirements are leading to new developments in bead foam materials. Engineering thermoplastics such as polybutylene terephthalate (PBT) outperform standard bead foams in thermal stability. In order to obtain molded parts, beads are fused together using steam chest molding. Classical theories for the fusion mechanism, explaining the molding of amorphous EPS expandable polystyrene) or semi-crystalline EPP (expanded polypropylene cannot be applied to E-PBT (expanded polybutylene terephthalate). In previous studies, sufficient time for polymer interdiffusion during molding is identified as crucial and requires adjusted crystallization kinetics. This study consequently examines to which extent the crystalline properties and the bead fusion behavior of E-PBT can be influenced by the bead foaming process. By varying the underwater granulation (UWG) water temperature, different cooling rates of the expanding melt are generated. The foamed beads show different cell morphologies, thermal and dynamic mechanical properties depending on the UWG water temperature. Those beads that show a pronounced shrinkage behavior in the thermomechanical analysis, caused by an increased open cell content and a pronounced cold-crystallization, exhibit a reduced bead fusion quality. The bead fusion quality is examined by the fracture surface. The shrinkage phenomenon causes a reduced bead to bead contact and partially separation between the beads.