A Polymer Visualization System with Accurate Heating and Cooling Control and High-Speed Imaging
A visualization system to observe crystal and bubble formation in polymers under high temperature and pressure has been developed. Using this system, polymer can be subjected to a programmable thermal treatment to simulate the process in high pressure differential scanning calorimetry (HPDSC). With a high-temperature/high-pressure view-cell unit, this system enables in situ observation of crystal formation in semi-crystalline polymers to complement thermal analyses with HPDSC. The high-speed recording capability of the camera not only allows detailed recording of crystal formation, it also enables in situ capture of plastic foaming processes with a high temporal resolution. To demonstrate the system’s capability, crystal formation and foaming processes of polypropylene/carbon dioxide systems were examined. It was observed that crystals nucleated and grew into spherulites, and they grew at faster rates as temperature decreased. This observation agrees with the crystallinity measurement obtained with the HPDSC. Cell nucleation first occurred at crystals’ boundaries due to CO2 exclusion from crystal growth fronts. Subsequently, cells were nucleated around the existing ones due to tensile stresses generated in the constrained amorphous regions between networks of crystals.
Study of the bubble nucleation and growth mechanisms in high-pressure foam injection molding through in-situ visualization
Abstract Although major efforts have been made to uncover the complex mechanisms of bubble nucleation and dynamics in foam injection molding, most theories and interpretations still require additional experimental verification. An innovative visualization mold was employed with which the mechanisms of bubble nucleation and growth in high-pressure foam injection molding were investigated. For the first time, the development of the cell structure in high-pressure foam injection molding was comprehensively explored and experimentally verified via in-situ visualization using polystyrene and supercritical carbon dioxide system. Two bubble-nucleation mechanisms were observed, namely, the nucleation due to a pressure drop at the gate during filling, and the nucleation due to melt shrinkage in the cavity after filling. It was the cavity pressure that determined which bubble-nucleation mechanism was dominated between these two in high-pressure foam injection molding. The final cellular structure and morphology of the foamed parts were, in turn, determined by the governing bubble-nucleation mechanism. It was concluded that a melt packing pressure is required to re-dissolve the bubbles nucleated at the gate back into the melt, and that a larger packing pressure was needed with a higher cell density by increasing the injection speed, the gate resistance, or the blowing-agent content. Because of the high resistance in the narrow cavity, the bubbles within the cavity were not pressurized uniformly, when the packing pressure was applied. A higher cell density increased the compressibility of the two-phase gas–melt mixture, aggravating the non-uniformity of the pressure in the cavity. Consequently, the bubbles far from the gate could not be pressurized immediately, and thereby could not dissolve into the polymer melt quickly. Additionally, the lower temperature of the melt far from the gate further delayed the permeation of gas into the polymer.
Crystal polymorphism of poly(l-lactic acid) and its influence on thermal properties
Abstract The influence of crystal polymorphism on the thermal properties of poly( l -lactic acid) (PLLA) is discussed in this contribution. Crystallization of PLLA at high temperatures yields the stable α form, whereas at low temperatures the metastable α′ modification develops, which is characterized by slightly larger lattice dimensions compared to the α counterpart, and by some degree of conformational disorder. Quantitative analysis with conventional and temperature-modulated calorimetry revealed a three-phase structure of PLLA composed of a crystal phase and two amorphous fractions with different mobility, for all the analyzed thermal histories. A higher coupling of the amorphous chain segments with the crystal phase was found in the presence of α crystals, probably due to the slightly larger lattice dimensions and the looser arrangements of PLLA chains in the α′ structure. Some peculiarities in the thermal behavior were rationalized, like an unusual frequency-dependence of the reversing apparent heat capacity upon the solid–solid transition from the α′ to the α crystals. Devitrification of the rigid amorphous segments seems also to be differently affected by the coupled crystal structure for the two analyzed crystal modifications of PLLA.