Towards sustainable light-weight materials: enhancing foamability of polypropylene in supercritical CO2 extrusion foaming via multifunctional additives

The supercritical CO2 (scCO2) foam extrusion process has gained significant attention in recent years as it combines the high-throughput feature of melt extrusion with the sustainability benefits and polymer plasticising effect provided by scCO2. Fabricating polypropylene (PP) foams through this process would allow PP foams, which have excellent mechanical and thermal properties that are highly beneficial for a wide range of applications, to be fabricated quickly and sustainably. However, scCO2 extrusion foaming of PP faces numerous issues such as the poor melt strength of PP, high diffusive gas loss of CO2 from PP foam during foaming, and poor CO2 nucleation rate in PP. These issues often cause the fabricated PP foam to possess low foamability, pronounced coalescence, low cell density, and large cell size. Consequently, the foam structure often has poor uniformity, which adversely affects the mechanical and energy absorption properties of PP foams. In recent years, the development of long-chain branched PP with high melt strength has resolved the melt strength issue of conventional PP. However, despite this advancement, the other mentioned issues of scCO2 extrusion foamed PP continue to pose significant challenges towards fabricating foams with desirable properties. While additives such as talc and organic surfactants have been studied to improve nucleation rate and reduce diffusive gas losses, they often result in a trade-off in other properties, such as lesser foamability and poorer mechanical properties, respectively. Although nano-additives, such as nano-clay and carbon nanotubes, that possess unique nano-sized related properties have the potential to overcome some of these issues, such work has been limited due to their high cost, dispersion issues, or production challenges. Hence, in this thesis, new economically feasible lightweight multifunctional additives that have the potential to be exploited by the scCO2 extrusion foaming process of PP and to be dispersed in PP have been identified or developed to study how they could resolve the aforementioned foaming issues while simultaneously reinforcing the scCO2 extrusion-foamed PP. Firstly, heptaisobutyl open-caged silsesquioxane (HOS) is studied as an economically feasible hybrid surfactant for PP foaming. When compounded into PP, it possesses excellent dispersion and during foaming, it can diffuse like a surfactant to the PP melt-CO2 interface to stabilise the cells in their growth stage and reinforce the cell surface upon foam solidification. The resulting PP/HOS foams exhibit a simultaneous enhancement in Expansion Ratio and compression strength. Secondly, the plasticising and solid dispersion properties of scCO2 during scCO2 foam extrusion have been leveraged to concurrently assist the dispersion of lignin in the PP foaming. This in situ dispersed lignin could act as a heterogeneous gas nucleating agent and a reinforcement for PP foam due to the cavity-rich surface of lignin and the rigid nature of its glassy state. The fabricated PP/lignin foams show significantly improved cell density and cell structure uniformity while possessing enhanced mechanical and energy absorption efficiency properties. As lignin is a waste material that is biodegradable, non-toxic, and lightweight, it is a low-cost non-toxic sustainable alternative to conventional nucleating agents such as talc. Finally, as nano-sized fillers often provide enhanced interfacial interactions with polymer matrices, lignin nanoparticles (LNP) fabricated using a rapid low-cost method are studied to determine if the high surface area of LNP can provide greater heterogeneous gas nucleation and if the enhanced interactions of LNP with the PP can lead to improved mechanical and energy absorption properties. To minimise the fusing of LNP into aggregates and to assist LNP dispersion, a very small amount of HOS is used as a compatibilizer. In the resulting nanocomposites, only HOS-compatibilized LNP (HLNP) could achieve good dispersion, which resulted in PP/HLNP foams with high cell density, good cell structure uniformity, and further enhanced mechanical and energy absorption properties. These findings thus provide scCO2 foam extrusion of PP with new multifunctional additives that could improve the foamability and mechanical properties while maintaining the cost-effectiveness and lightweight characteristic of PP foams.Doctor of Philosoph