Ideal surface geometries of nucleating agents to enhance cell nucleation in polymeric foaming processes

Abstract

Ideal nucleating agents are expected to improve the cell morphology of plastics foams (i.e., increasing the cell density, reducing the cell size, and narrowing cell-size distribution) by providing heterogeneous nucleation sites. A nucleating agent's surface geometry is one factor that governs its nucleating power. This paper discusses the surface geometry of an ideal nucleating agent. On the basis of computer simulations of a batch foaming process using polystyrene and carbon dioxide, we found that nucleating agents having numerous crevices with small semiconical angles are the most desirable for polymeric foaming processes.

Origins of the failure of classical nucleation theory for nanocellular polymer foams

“…We found similar disagreements for other parameter values. Given that polymer foaming is thought to be dominated by heterogenous nucleation [17,18] in part because CNT predicts a negligible rate for homogenous nucleation, our SCFT results provide circumstantial evidence that there may be much more homogenous nucleation taking place in polymers foams than previously thought. Our results also suggest that heterogenous nucleation rates will be different than predicted by some models [17,18].…”

“…Evans and Oxtoby have studied non-polymeric systems using classical density functional theory and they too find orders of magnitude deviations from CNT -see their figure 4 [19]. In polymer foam systems, heterogenous nucleation is thought to dominate bubble nucleation because CNT predicts such a small rate for homogenous nucleation [17,18]. The above results indicate that homogenous nucleation may be taking place at rates previously thought to be consistent only with heterogenous nucleation.…”

“…To confirm this, more quantitative predictions would be required -see Conclusions and Outlook section 4. In any case, the above results should also modify heterogenous nucleation rate predictions since these rates are often based on modifications of the free energy barrier height used in CNT for homogenous nucleation [17,18].…”

“…Although heterogenous nucleation is a much more important factor determining nucleation rates in polymer foams than homogenous nucleation [17,18], we will examine only the latter, following the philosophy that homogenous nucleation is an appropriate place to start when testing CNT predictions given that CNT, in its simplest form, is a theory for homogenous nucleation [7,19,20]. Furthermore, many heterogenous nucleation theories are modifications of homogenous nucleation quantities [17,18], so corrections to the latter will affect the former. In section 3, we show an example of the differences between critical radii, nucleation barriers and nucleation rates predicted by SCFT and CNT.…”

In situ fibrillation of CO2-philic polymers: Sustainable route to polymer foams in a continuous process

“…Assuming that the polymer/gas solution that forms prior to bubble nucleation is a dilute polymer/gas solution, the expressions for the free energy needed for bubble nucleation (DG) and R cr are given by [13,14,33,34]:…”