Abstract
Expandable polystyrene (EPS) and expanded polypropylene (EPP) dominate the bead foam market. As the low thermal performance of EPS and EPP limits application at elevated temperatures novel solutions such as expanded polybutylene terephthalate (E-PBT) are gaining importance. To produce parts, individual beads are typically molded by hot steam. While molding of EPP is well-understood and related to two distinct melting temperatures, the mechanisms of E-PBT are different. E-PBT shows only one melting peak and can surprisingly only be molded when adding chain extender (CE). This publication therefore aims to understand the impact of thermal properties of E-PBT on its molding behavior. Detailed differential scanning calorimetry was performed on neat and chain extended E-PBT. The crystallinity of the outer layer and center of the bead was similar. Thus, a former hypothesis that a completely amorphous bead layer enables molding, was discarded. However, the incorporation of CE remarkably reduces the crystallization and re-crystallization rate. As a consequence, the time available for interdiffusion of chains across neighboring beads increases and facilitates crystallization across the bead interface. For E-PBT bead foams, it is concluded that sufficient time for polymer interdiffusion during molding is crucial and requires adjusted crystallization kinetics.
Highlights
In contrast to foam extrusion and foam injection molding, the bead foaming technology is a manufacturing process which involves molding and sintering of tiny foamed beads into plastic foam components
The most important representatives of bead foams in terms of volume are expandable polystyrene (EPS) [5,6], which is widely used in packaging, heat and sound insulation, expanded polyethylene (EPE) [7] and expanded polypropylene (EPP) [8,9], which is used in applications such as packaging, cushioning, exterior and interior automotive parts and many other commodity applications
The study of the outer layer of the manufactured polybutylene terephthalate (PBT) bead foams forms the basis for understanding the processes that occur during the molding of the beads
Summary
In contrast to foam extrusion and foam injection molding, the bead foaming technology is a manufacturing process which involves molding and sintering of tiny foamed beads into plastic foam components In this aspect, the bead foaming procedure is consi-dered to be a highly promising technology, which allows both the high foam expansion of extrusion foaming and the geometrical complexity of the resulting part in the foam injection molding process [1,2,3,4]. The most important representatives of bead foams in terms of volume are expandable polystyrene (EPS) [5,6], which is widely used in packaging, heat and sound insulation, expanded polyethylene (EPE) [7] and expanded polypropylene (EPP) [8,9], which is used in applications such as packaging, cushioning, exterior and interior automotive parts and many other commodity applications. The first commercial approaches for the realization of bead foams with higher continuous service temperatures are already available, including PMI (Evonik AG) [17], PET (Armacell S.A.) [18] and PA (BASF SE, Asahi Kasei K.K.) [19,20] PESU (BASF SE) [21]
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