A continuous extrusion process for the manufacture of low-density microcellular polymers is presented. Microcellular polymers are foamed plastics characterized by a cell density greater than lo9 cells/cm3 and a fully grown cell size on the order of 10 pm. F'revious research on continuous processing of microcellular polymers has focused on control of microcell nucleation in extrusion. This paper presents an effective means for control of cell growth to achieve a desired expansion ratio with CO, as a blowing agent in microcellular foam processing. Also, a strategy to prevent deterioration of the cell-population density via cell coalescence during expansion is presented. Promotion of a desired volume expansion ratio and prevention of cell coalescence in microcellular foam processing were experimentally verified. By tailoring the extrusion processing parameters, microcellular HIPS foams with a cell density of 1Olo cells/cm3 and a controlled expansion ratio in the range of 1.5 to 23 were obtained.
The conditions are examined under which a single bubble and a number of bubbles are in equilibrium within a closed volume of liquid that is maintained at constant temperature and pressure. It is predicted that depending on the amount of gas present in the volume, there may be no equilibrium state for the bubble or bubbles, one equilibrium size, or two possible equilibrium sizes. In the latter case, it is also predicted that the equilibrium state corresponding to the larger bubble size is a stable equilibrium state. This is in contrast to the case of an unbounded volume of liquid where there is the possibility of only one equilibrium state for a bubble, and this state is unstable. The predicted stability for a bubble in a closed volume was examined experimentally, and agreement was found between the measurement and the prediction. A striking result is the reduction in the stable equilibrium size with the number of bubbles present. In particular, micron-sized bubbles can be shown to be in stable equilibrium under the constraint of a closed volume, and for reasonable conditions of liquid temperature and pressure.
Electrochemical reduction of oxygen has been studied in detail employing rotating disk and ring‐disk electrode techniques in aqueous KOH solutions of various concentrations at various temperatures. Analysis of voltammetric results recorded at the rotating disk electrode (RDE) indicates that there are two clearly defined Tafel regions of low (60 mV/decade) and high false(200∼300 normalmV/normaldecadefalse) Tafel slopes. In general, electrode kinetics improves and Tafel slopes of the low‐slope region decreases slightly as the basicity of the electrolyte solution increases. The rotating ring‐disk electrode (RRDE) results were analyzed according to a simple model for oxygen reduction, first proposed by Damjanovic et al. The model is in agreement with experimental results at lower concentrations of KOH . Rate constants for oxygen reduction directly to H2O and H2O2 , and for H2O2 to H2O were calculated at four different disk electrode potentials. The rate constant of direct reduction of oxygen to H2O increases with the overpotential, but the plots of rate constants for oxygen reduction to H2O2 and for H2O2 reduction vs. potential pass through a maximum at about 0.85V vs. DHE. All these rate constants are shown to be slightly dependent on temperature.
The conditions are considered under which heterogeneous bubble nucleation takes place in a conical pit in the boundary of a constant size volume containing a liquid-gas solution, and the size to which the nucleate bubble grows is predicted. Four possible equilibrium states are found for the nucleate bubble: two unstable, one metastable, and one stable. The unstable state corresponding to the smallest equilibrium size is the one that acts as the threshold size that must be exceeded in the nucleation event. The metastable and second unstable state are encountered as the nucleate bubble emerges from the conical pit and the stable state corresponds to the largest equilibrium size. It arises from the bubble being in a finite volume with fixed mass. The pressure produced in the volume by the growth of the bubble depends on the final state it attains (i.e., either the metastable or stable state). The theoretical expressions obtained from the analysis are applied to predict the conditions under which bubble nucleation and growth take place within a type of bone cell, and the results are used to explain certain types of damage to the bone of animals undergoing decompression from high pressures that have been previously reported in the literature. The expressions are also applied to predict the superheat necessary to produce bubble emergence from a conical pit and are shown to be in agreement with results previously reported. PACS numbers: 64.80. -v, 05.90. + m, 87.70. -k 1833 J.
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