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

Abstract

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.

Influence of Molecular Weight on the Bead Foaming and Bead Fusion Behavior of Poly(butylene terephthalate) (PBT)

“…The different observations regarding the bead fusion potentials can be explained with the help of the previously carried out investigations for single beads (DSC, OCC, TMA, DMTA). Similar correlations have already been identified in previous studies . The comparison between the single beads showed that E-PBT Pocan 1100, 1300, and 1400 had the highest OCC (Figure ) and therefore the highest shrinkage (Figure ).…”

“…Similar correlations have already been identified in previous studies. 50 The comparison between the single beads showed that E-PBT Pocan 1100, 1300, and 1400 had the highest OCC (Figure 10) and therefore the highest shrinkage (Figure 12). Due to the shrinkage, there is no sufficient contact surface between the beads in the cavity during the steam chest molding process, so that no fusion can be realized for these beads.…”

Investigation of the Reaction Kinetics of Poly(butylene terephthalate) and Epoxide Chain Extender

“…Viscosity also has a significant effect on foam morphology, as already shown in earlier works. [11,15,48] The bead foams were produced as described in our previous study via a tandem foam extrusion line coupled on an underwater granulator using CO 2 as a blowing agent. [11] The influence of molecular weight and varied viscosity based on different chain architectures on foam density, cell density, and morphology was investigated.…”

“…The current state-of-the-art approach is the use of chemical modifications to overcome those issues, as it was shown in previous works. [11,15] Hence, the melt modification with CE during reactive extrusion processes is a promising approach to increase the molecular weight (branching and/or crosslinking) and consequently to change the rheological properties (i.e., increased shear viscosity and melt strength). [3,[16][17][18] Chain extenders are multifunctional additives that can react with the terminal functional groups of the polymers.…”

Extruded polylactide bead foams using anhydrous supercritical CO2 extrusion foaming

“…This was because using autoclave foaming technology, the polymer matrix foamed in a solid state, which was beneficial to obtain a cellular structure with a high closed cell content and post-molding. 36 However, this bead foam preparation process includes complex procedures, such as premixing, pelletizing, dryness, gas saturation, foaming, and so forth, which requires a longer time and higher costs. Different from EPLA-#0, the optical morphology and the cellular structures of ExPLA-#1 were bare (Figures 3b 1 and 4b-b 2 ) because it cut from foamed strips with a fixed morphology.…”