This thesis highlights a comprehensive research for the cell nucleation, growth and coarsening mechanisms during plastic foaming processes. Enforced environmental regulations have forced the plastic foam industry to adopt alternative blowing agents (e.g., carbon dioxide, nitrogen, argon and helium). Nevertheless, the low solubilities and high diffusivities of these viable alternatives have made the production of foamed plastics to be non-trivial. Since the controls of the cell nucleation, growth and coarsening phenomena, and ultimately the cellular morphology, involve delicate thermodynamic, kinetic, and rheological mechanisms, the production of plastics foams with customized cell morphology have been challenging. In light of this, the aforementioned phenomena were investigated through a series of theoretical studies, computer simulations, and experimental investigations. Firstly, the effects of processing conditions on the cell nucleation phenomena were studied through the in-situ visualization of various batch foaming experiments. Most importantly, these investigations have led to the identification of a new heterogeneous nucleation mechanism to explain the inorganic fillers-enhanced nucleation dynamics. Secondly, a simulation scheme to precisely simulate the bubble growth behaviors, a modified heterogeneous nucleation theory to estimate the cell nucleation rate, and an integrated model to simultaneously simulate cell nucleation and growth processes were developed. Consequently, through the simulations of the cell nucleation, growth, and coarsening dynamics, this research has advanced the understanding of the underlying sciences that govern these different physical phenomena during plastic foaming. Furthermore, the impacts of various commonly adopted approximations or assumptions were studied. The end results have provided useful guidelines to conduct computer simulation on plastic foaming processes. Finally, an experimental research on foaming with blowing agent blends served as a case example to demonstrate how the elucidation of the mechanisms of various foaming phenomena would aid in the development of novel processing strategies to enhance the control of cellular structures in plastic foams.

Traditional blowing agents (e.g., hydrochlorofluorocarbons) in plastic foaming processes has been phasing out due to environmental regulations. Plastic foaming industry is forced to employ greener alternatives (e.g., carbon dioxide, nitrogen), but their foaming processes are technologically challenging. Moreover, to improve the competitiveness of the foaming industry, it is imperative to develop a new generation of value-added plastic foams with cell structures that can be tailored to different applications. In this context, the objective of this thesis is to achieve a thorough understanding on cell nucleation and growth phenomena that determine cell structures in plastic foaming processes. The core research strategy is to develop innovative visualization systems to capture and study these phenomena. A system with accurate heating and cooling control has been developed to observe and study crystallization-induced foaming behaviors of polymers under static conditions. The cell nucleation and initial growth behavior of polymers blown with different blowing agents (nitrogen, argon and helium, and carbon dioxide-nitrogen mixtures) have also been investigated in great detail. Furthermore, two innovative systems have been developed to simulate the dynamic conditions in industrial foaming processes: one system captures a foaming process under an easily adjustable and uniform extensional strain in a high temperature and pressure environment, while the other achieves the same target, but with shear strain. Using these systems, the extensional and shear effects on bubble nucleation and initial growth processes has been investigated independently in an isolated manner, which has never been achieved previously. The effectiveness of cell nucleating agents has also been evaluated under dynamic conditions, which have led to the identification of new foaming mechanisms based on polymer-chain alignment and generation of microvoids under stress. Knowledge generated from these researches and the wide range of future studies made possible by the visualization systems will be valuable to the development of innovative plastic foaming technologies and foams.

Traditional blowing agents (e.g., hydrochlorofluorocarbons) in plastic foaming processes has been phasing out due to environmental regulations. Plastic foaming industry is forced to employ greener alternatives (e.g., carbon dioxide, nitrogen), but their foaming processes are technologically challenging. Moreover, to improve the competitiveness of the foaming industry, it is imperative to develop a new generation of value-added plastic foams with cell structures that can be tailored to different applications. In this context, the objective of this thesis is to achieve a thorough understanding on cell nucleation and growth phenomena that determine cell structures in plastic foaming processes. The core research strategy is to develop innovative visualization systems to capture and study these phenomena. A system with accurate heating and cooling control has been developed to observe and study crystallization-induced foaming behaviors of polymers under static conditions. The cell nucleation and initial growth behavior of polymers blown with different blowing agents (nitrogen, argon and helium, and carbon dioxide-nitrogen mixtures) have also been investigated in great detail. Furthermore, two innovative systems have been developed to simulate the dynamic conditions in industrial foaming processes: one system captures a foaming process under an easily adjustable and uniform extensional strain in a high temperature and pressure environment, while the other achieves the same target, but with shear strain. Using these systems, the extensional and shear effects on bubble nucleation and initial growth processes has been investigated independently in an isolated manner, which has never been achieved previously. The effectiveness of cell nucleating agents has also been evaluated under dynamic conditions, which have led to the identification of new foaming mechanisms based on polymer-chain alignment and generation of microvoids under stress. Knowledge generated from these researches and the wide range of future studies made possible by the visualization systems will be valuable to the development of innovative plastic foaming technologies and foams.

This thesis highlights a comprehensive research for the cell nucleation, growth and coarsening mechanisms during plastic foaming processes. Enforced environmental regulations have forced the plastic foam industry to adopt alternative blowing agents (e.g., carbon dioxide, nitrogen, argon and helium). Nevertheless, the low solubilities and high diffusivities of these viable alternatives have made the production of foamed plastics to be non-trivial. Since the controls of the cell nucleation, growth and coarsening phenomena, and ultimately the cellular morphology, involve delicate thermodynamic, kinetic, and rheological mechanisms, the production of plastics foams with customized cell morphology have been challenging. In light of this, the aforementioned phenomena were investigated through a series of theoretical studies, computer simulations, and experimental investigations. Firstly, the effects of processing conditions on the cell nucleation phenomena were studied through the in-situ visualization of various batch foaming experiments. Most importantly, these investigations have led to the identification of a new heterogeneous nucleation mechanism to explain the inorganic fillers-enhanced nucleation dynamics. Secondly, a simulation scheme to precisely simulate the bubble growth behaviors, a modified heterogeneous nucleation theory to estimate the cell nucleation rate, and an integrated model to simultaneously simulate cell nucleation and growth processes were developed. Consequently, through the simulations of the cell nucleation, growth, and coarsening dynamics, this research has advanced the understanding of the underlying sciences that govern these different physical phenomena during plastic foaming. Furthermore, the impacts of various commonly adopted approximations or assumptions were studied. The end results have provided useful guidelines to conduct computer simulation on plastic foaming processes. Finally, an experimental research on foaming with blowing agent blends served as a case example to demonstrate how the elucidation of the mechanisms of various foaming phenomena would aid in the development of novel processing strategies to enhance the control of cellular structures in plastic foams.