TY - THES

T1 - Morphology and fracture mechanical behavior of Polyoxymethylene platelets produced by injection molding under industrial-near processing conditions

AU - Helfer, Eric

N1 - embargoed until null

PY - 2020

Y1 - 2020

N2 - Polyoxymethylene (POM) is an engineering thermoplastic with a high degree of crystallinity and excellent mechanical properties. It is used in a wide field of industry sectors that require good mechanical properties. For polymers, the mechanical properties are amongst others determined by the morphology of the material. These can be influenced by various aspects like additives and especially by the processing. In this thesis, injection molded platelets were analyzed to find a correlation between process settings, morphology and fracture mechanical properties. The production of the platelets was carried out following a design of experiments (DOE), where mold temperature, injection speed and packing pressure were varied. To analyze the morphological state in the specimens, small angle X ray scattering (SAXS) and wide angle X ray diffraction (WAXD) as well as polarized light microscopy (PLM) were used. The fracture mechanical analysis included monotonic fracture tests as well as cyclic fatigue fracture tests. For these tests, so called micro single-edge notched tension (µ SENT) specimens, which were obtained from the injection molded platelets, were used. Additionally to all tests, filling simulations in MoldFlow were carried out. MoldFlow allows to model the crystallization in polymers during injection molding. To asses these results, they were compared to the results obtained by the X-ray measurements and by PLM. Specimens produced with a higher mold temperature showed an increase in the crystalline fraction and the lamellar thickness while a decrease in the fracture toughness occurred. The fatigue behavior did not seem to be influenced much by the mold temperature. Higher injection speeds led to higher oriented samples. These samples showed an improvement in both the monotonic and fatigue fracture properties in the range examined. The packing pressure did not show any significant influence on both the morphology and the fracture mechanical properties. It is assumed that the effect of the holding pressure in the very small platelets is rather low, especially in the area away from the sprue. This leads to the conclusion that the orientation of the lamellae and crystals improved the fracture mechanical behavior for the resin used in this thesis. With the PLM, it was difficult to accurately determine the transition between the different layers in the injection molded samples. Hence, further investigation with etching in combination with Scanning Electron Microscopy is recommended. The simulations provided information about the different crystal sizes of the various process conditions. However, the individual spherulites were too small to be quantified by PLM and therefore could not be compared to the results from the simulations. Furthermore, the simulations gave information about the orientation of the crystals. The values obtained could not be directly compared to the values obtained by the X ray measurements, because the reference axis was different and the X-ray measurements only took into account a single crystal plane.

AB - Polyoxymethylene (POM) is an engineering thermoplastic with a high degree of crystallinity and excellent mechanical properties. It is used in a wide field of industry sectors that require good mechanical properties. For polymers, the mechanical properties are amongst others determined by the morphology of the material. These can be influenced by various aspects like additives and especially by the processing. In this thesis, injection molded platelets were analyzed to find a correlation between process settings, morphology and fracture mechanical properties. The production of the platelets was carried out following a design of experiments (DOE), where mold temperature, injection speed and packing pressure were varied. To analyze the morphological state in the specimens, small angle X ray scattering (SAXS) and wide angle X ray diffraction (WAXD) as well as polarized light microscopy (PLM) were used. The fracture mechanical analysis included monotonic fracture tests as well as cyclic fatigue fracture tests. For these tests, so called micro single-edge notched tension (µ SENT) specimens, which were obtained from the injection molded platelets, were used. Additionally to all tests, filling simulations in MoldFlow were carried out. MoldFlow allows to model the crystallization in polymers during injection molding. To asses these results, they were compared to the results obtained by the X-ray measurements and by PLM. Specimens produced with a higher mold temperature showed an increase in the crystalline fraction and the lamellar thickness while a decrease in the fracture toughness occurred. The fatigue behavior did not seem to be influenced much by the mold temperature. Higher injection speeds led to higher oriented samples. These samples showed an improvement in both the monotonic and fatigue fracture properties in the range examined. The packing pressure did not show any significant influence on both the morphology and the fracture mechanical properties. It is assumed that the effect of the holding pressure in the very small platelets is rather low, especially in the area away from the sprue. This leads to the conclusion that the orientation of the lamellae and crystals improved the fracture mechanical behavior for the resin used in this thesis. With the PLM, it was difficult to accurately determine the transition between the different layers in the injection molded samples. Hence, further investigation with etching in combination with Scanning Electron Microscopy is recommended. The simulations provided information about the different crystal sizes of the various process conditions. However, the individual spherulites were too small to be quantified by PLM and therefore could not be compared to the results from the simulations. Furthermore, the simulations gave information about the orientation of the crystals. The values obtained could not be directly compared to the values obtained by the X ray measurements, because the reference axis was different and the X-ray measurements only took into account a single crystal plane.

KW - Polyoxymethylen

KW - SAXS und WAXD

KW - Bruchmechanik

KW - Spritzgießen

KW - Polyoxymethylene

KW - SAXS & WAXD

KW - fracture mechanics

KW - injection molding

M3 - Master's Thesis

ER -