Study On Bubble Growth Mechanism In Polymer Microcellutar Foaming

Microcellular foamed plastics, because of their low weight, good thermal and acousticinsulation properties and superior mechanical properties, are widely used in many fields,such as packaging material, automotive, aerospace, sports equipment, thermal insulationmaterials, biomedical materials and microelectronics. The bubble growth processes is thekey step to determine the final bubble structure, which is strongly related to the performanceof the foam. Knowing the mechanism of bubble growth phenomena in polymer foamingprocess is indispensable for controlling the cell morphology, optimizing the quality of thefoam and improving processing technologies utilized in the foaming industry.Based on the cell model, a mathematical model for bubble growth was established bysolving momentum equation, mass equation, diffusion equation and constitutive equation.The gas content distribution was calculated with the diffusion equation, and the rheologicalproperties of the polymer were described by the upper-convected Maxwell model. Thediffusion equation was discrete with the finite volume method. Assuming bubble growth isisothermal, the bubble growth process of PP/CO2system was simulated by handling thegoverning equation using MATLAB. The model is validated by comparing the simulationresults with experiment data obtained from visual observations of the PP/CO2batch foamingsystem. The bubble growth process was studied by analyzing the changes of gasconcentration and bubble growth rate. The driving force of bubble growth is gas diffusion.With the decrease of gas concentration gradient at bubble surface, the bubble growth rategets slowly. The effects of a series of parameters on bubble growth were investigated. Theincrease of initial bubble size slows down the early bubble growth, but has no influence onthe equilibrium size. The bubble, with a higher cell nucleus population density, grows up toequilibrium size more quickly and the equilibrium size is smaller. The increases of Henry’slaw constant and diffusion coefficient accelerate bubble growth, but relaxation time andsurface tension have little influence on bubble growth. Higher saturation pressure andpressure release rate facilitate bubble growth.Assuming the foaming polymer melt cooled at a set rate, a mathematical model fornon-isothermal bubble growth was established. The model took into account the effect of temperature on the rheological properties of the polymer-gas melt, Henry’s law constant,diffusion coefficient and surface tension. Several models were introduced to describe theproperties of the polymer-gas system as a function of the temperature. A PS/CO2system wasused herein as a case example to investigate the change of bubble growth behavior undernon-isothermal circumstances. The results demonstrate that the main reasons to hinderbubble growth are the increase of Henry’s law constant and the decrease of diffusioncoefficient during the cooling process. Although the viscosity increases greatly during thecooling process, the change of viscosity retard bubble growth only in the later stage. Theeffect of surface tension is negligible. The effect of cooling rate on bubble growth wasstudied by simulation and bath foam experiments. Increasing the cooling rate will slow downthe bubble growth rate and avoid cell collapse and coalescence due to excessive dilation.Increasing the cooling rate is an effective way to produce foamed plastics with higher bubbledensity and finer bubble size.