Surface Tension Measurement of High Density Polyethylene and Its Clay Nanocomposites in Supercritical Nitrogen

Surface Tension;Polymer Foaming;Chemical Engineering

Wei, Hua

University of Waterloo

关键词: Surface Tension;    Polymer Foaming;    Chemical Engineering;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/4551/1/thesis%20final%20final.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

Surface tension of a polymer melt in a supercritical fluid is a principal factor indetermining cell nucleation and growth in polymer microcellular foaming.Previous work has presented the surface tension of the amorphous polymer, polystyrene(PS), in supercritical CO2 determined by Axisymmetric Drop Shape Analysis-Profile(ADSA-P), together with theoretical calculations for a corresponding system. Thedependences of the surface tension on temperature, pressure and polymer molecular weightwere discussed and the physical mechanisms for three main experimental trends wereexplained using Self Consistent Field Theory (SCFT).This thesis introduces recent work on the surface tension measurement of the crystallinepolymer, high density polyethylene (HDPE), in supercritical N2 under various temperaturesand pressures. The surface tension was determined by ADSA-P and the results werecompared with those of the amorphous polymer PS. The dependence of the surface tensionon temperature and pressure, at temperatures above the HDPE melting point, ~125°C, wasfound to be similar to that of PS; that is, the surface tension decreased with increasingtemperature and pressure. Below 125°C and above 100°C, HDPE underwent a process ofcrystallization, where the surface tension dependence on temperature was different from thatabove the melting point, i.e., decreased with decreasing temperature. Differential ScanningCalorimetry (DSC) characterization of the polymer was carried out to reveal the process ofHDPE crystallization and relate this to the surface tension behavior. It was found that theamount of the decrease in surface tension was related to the rate of temperature change and hence the extent of polymer crystallization.In the second part of the thesis, surface tension dependences on temperature, pressureand clay concentrations were studied for HDPE nano-clay composites (HNC) and comparedwith pure HDPE. It was found the trends with temperature and pressure were the same withPS in CO2 and HDPE in N2; that is, the surface tension decreased with increasingtemperature and pressure. In all nanocomposite samples, the surface tension decreasedcompared with pure HDPE. This could be a good explanation for the better polymer foamingquality with the addition of clay in the polymer. A minimum surface tension was found withthe sample at ~3% concentration of clay. The degree of crystallinity of HNC was analyzedby Differential Scanning Calorimetry (DSC) at different clay concentrations. A minimumzcrystallinity was also found at the clay concentration of 3%. The possible relationshipbetween surface tension and polymer crystallinity was discussed.