No abstract

Medium to low density thermoplastic nanofoams have previously been produced using nanoparticles as nucleating center. Here we show that by designing the molecular structure of the polymer matrix to achieve high CO 2 solubility while controlling the glass transition temperature, it is possible to produce nanofoams with cell nucleation densities as high as 10 16 /cm 3 without introducing nucleation aids. This was achieved by maximizing foam expansion without uncontrolled cell ripening for a series of acrylic copolymers, which were foamed under a set of standard conditions. To predict the role of foaming conditions on foam characteristics, a theoretical foaming model was built to simulate cell nucleation, growth and foam stabilization. Experimental or predicted properties of the polymer/carbon dioxide mixture were used as inputs. Despite simplifying assumptions, such as the use of classical nucleation equations, the semi-quantitative model provides insight into the foam expansion behavior and validates experimental observations.

A numerical model is developed to simulate the simultaneous bubble nucleation and growth during depressurization of thermoplastic polymers saturated with supercritical blowing agents. Of particular importance is the ability of the model to predict the formation of nano-cellular foams, including the cell size distribution within the foam, based on the specific process conditions and polymer properties. Additionally the model differentiates between the "Free" and "Limited" expansion phases in the growth of a single bubble. Classical nucleation theory is used to predict nucleation rate and the popular "Influence Volume Approach" is used to determine the end of nucleation phase. By solving the mass, momentum and species conservation equations for each bubble, the model is capable of predicting bubble size distribution and bulk porosity.