Prozessführung und -optimierung der chemischen Schaumextrusion von PET und anschließender Verarbeitung im ISBM-Prozess
Research output: Thesis › Master's Thesis
The aim of this thesis was to produce a chemically foamed structure in Polyethylene terephthalate (PET) and to preserve this two-phase system over several process steps including the injection stretch blow molding (ISBM) process. Recycled PET flakes were used as a polymer matrix, whereby bulk shape and density, batch-to-batch fluctuations and thermal material history were showing a huge influence on processing. Chemical blowing agents (CBAs) from different material suppliers were used as additives. Since the thermal transition range of PET, compared to commonly used polymers for chemical foaming, are at high temperatures, only a limited number of chemical blowing agents are available, in addition exclusively with non-PET polymers as carriers. After preliminary tests with six blowing agents on a conventional single-screw extruder, the foam structures were examined using scanning electron microscopy (SEM), density measurements were carried out and the die swells were determined. By evaluating these parameters, a statistical design of experiments (DoE) could be drawn up to optimize the process settings in foam extrusion. The three best recipes were produced on a semi-industrial scale and part of the material was subjected to a solid state polycondensation (SSP) process to increase the intrinsic viscosity (IV) due to possible hydrolytic degradation by residual moisture or blowing agent reaction. The materials were then processed into preforms by injection molding. The process settings for this were also optimized with a DoE. As the final process step, the preforms were deformed into bottles by biaxial stretch blow molding. The foam structure of the produced preforms and bottles was examined under a light microscope, the densities of the bottles and the contact angles of the inner surface were determined, and the transmission and weight of the preforms were measured. Using all technical possibilities, bottles with a homogeneous cell structure along the stretched surfaces (excluding the neck) could be produced. Theoretically known influences on the foamed structure were confirmed experimentally both in the extrusion and in the injection molding process. A maximum density reduction of 40 % and a minimum average projected cell size of 1 700 µm2 with an elliptic shape could be achieved in the extruded strands. The cells in the preforms had a spherical shape and a lowest average projected cell size of 12 000 µm2. The cells in the bottles were deformed by biaxial stretching and had the lowest average projected cell size of 58 000 µm2. A relationship between opacity, weight and foaming of the preforms could not be established. Based on the successful implementation of chemical foaming, the more plant-intensive physical foaming can be used as an alternative to further refine the foam structure in this process as well as to decrease the projected cell size.
|Translated title of the contribution||Control and optimization of chemical foam extrusion process of PET and subsequent conversion via ISBM|
|Award date||20 Dec 2019|
|Publication status||Published - 2019|