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.
Methodology for describing characteristic foam parameters during chemical foaming of Low-density Polyethylene
The micro-cellular foaming of two Low-density Polyethylene (LDPE) composites as the temperature increased from 140 °C to 160 °C was investigated through visualization technology with the XTL-550E OM (optical Microscopy), with which, in situ observations during heating were carried out. The results demonstrated that the relationships of N-t and D-t followed the Boltzmann and Logistic functions with higher correction coefficients beyond 0.97 through non-linear fitting. Also, the three stages (Less-foamed, Fine-foamed and Over-foamed) were obtained by the first and second derivatives of the fitting curve: (1) the first stage was the less-foamed with the rate and driving force for the cell formation as well as the growth increase with temperature; (2) the second stage was the fine-foamed with competitive interaction between the formation rate and the growth rate; (3) the third stage was the over-foamed with the formation rate and driving force at an almost zero value. Moreover, the demarcation points of the three stages by the derivatives were almost consistent with the results through the fitting equation (t0.5 and tmax) and experimentation. By contrast, certain differences existed for the three stages in the two LDPE composites. These were attributed to the heterogeneous nucleation and strengthening towards the LDPE matrix of the nano-Montmorillonite (nano-OMMT).
Effects of the mineral phase and valence of vanadium on vanadium extraction from stone coal
The influence of roasting on the leaching rate and valence of vanadium was evaluated during vanadium extraction from stone coal. Vanadium in stone coal is hard to be leached and the leaching rate is less than 10% when the raw ore is leached by 4 mol/L H2SO4 at 90°C for 2 h. After the sample is roasted at 900°C for 2 h, the leaching rate of vanadium reaches the maximum, and more than 70% of vanadium can be leached. The crystal of vanadium-bearing mica minerals decomposes and the content of V(V) increases with the rise of roasting temperature from 600 to 900°C, therefore the leaching rate of vanadium increases significantly with the decomposition of the mica minerals. Some new phases, anorthite for example, form when the roasting temperature reaches 1000°C. A part of vanadium may be enwrapped in the sintered materials and newly formed phases, which may impede the oxidation of low valent vanadium and make the leaching rate of vanadium drop dramatically. The leaching rate of vanadium is not only determined by the valence state of vanadium but also controlled by the decomposition of vanadium-bearing minerals and the existence state of vanadium to a large extent.