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.
In-Situ Visualization of the Cell Formation Process of Foamed Polypropylene under Different Foaming Environments
In this paper, the dynamic foaming process of micro-foaming polypropylene (PP) in different foaming environments in real time was obtained via a visualization device. The relationship curve between cell number (n) and foaming time (t) was plotted, and then the nucleation kinetics of foam cells was analyzed. Results showed that the formation rate of cells changed obviously with the variation of melt temperature and the content of the foaming agent. The n-t curves presented a typical “S” shape, which indicated that the appearance of the cell number increased slowly in the initial foaming period, then increased rapidly in a short time, and finally maintained a certain value. When a certain pressure was applied to the PP melt, the external force had a great influence on the n-t curve. With the increasing external force, the rate of cell formation increased rapidly, and the shape of the n-t curve changed from “S” to “semi-S” without an obvious slow increase. The investigation of the n-t relationship in the PP dynamic foaming process under different foaming environments could provide effective bases for improving the foaming quality of injection molding foaming materials.
The Arab Spring: The End of Postcolonialism
* Introduction: The Arab Spring: The End of Postcoloniality * 1. Decentering the World: How the Arab Spring Unfolded * 2. Towards Liberation Geography * 3. A New Language of Revolt * 4. Discovering a New World * 5. From the Green Movement to the Jasmine Revolutions * 6. The center cannot hold * 7. The End of Postcolonialism * 8. Race, Gender, and Class in Transnational Revolutions * 9. Libya: The Crucible * 10. Delayed Defiance * Conclusion. The People Demand the Overthrow of the Regime