Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore

Supercritical CO2 Foaming of Polypropylene Nanocomposites: Surfactant and Reinforcing Effects of Silsesquioxane-based Hybrid Janus Molecules

Keen Hoe Ho1,3, Martin Van Meur2, Lu Xuehong1,a and Lau Soo Khim3,b

1School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

2Institute of Sustainability for Chemicals, Energy and Environment, 1 Pesek Rd, Singapore 627833

3Singapore Institute of Manufacturing Technology, 73 Nanyang Dr, Singapore 637662


The development of high melt-strength (HMS) long-chain-branched polypropylene (PP) has enhanced foamability of PP by extrusion foaming process, even with supercritical CO2 (scCO2) as blowing agent. However, numerous issues still persist in scCO2-foaming of HMS PP that tend to limit such foams to having lower expansion ratio (ER), higher foam density, and poorer compression strength (CS). One of such issue is the high diffusion rate of CO2 through PP. This causes high diffusive gas losses which results in reduced ER and higher foam density. Even if ER can be improved by optimizing process parameters or by introducing organic surfactants such as glycerol monostearate, the increased void fraction would generally produce foams with lower CS. Moreover, organic surfactants may also pose degradation concerns during application, especially when exposed to UV. Another issue encountered is the poor nucleation rate of CO2 in PP, which results in low cell density and large cell size. This would cause poor homogeneity in the PP foam's cell structure, which also leads to CS reduction. Although nucleating agents such as Talc could be introduced to achieve high cell density, small cell size, and hence a higher compression strength, it is often associated with ER reduction due to increase diffusive gas losses via the talc-polymer interface. In this study, we have demonstrated a novel approach of incorporating HMS PP with a low-cost hybrid Janus molecule surfactant, heptaisobutyl open-cage silsesquioxane (HOS), and achieved PP foams with simultaneous enhancement in ER and CS. Since HOS' organic-side consists of isobutyl groups that are compatible with PP while the inorganic-side consists of a partially condensed silsesquioxane cage that has good affinity with CO2, HOS could act as a surfactant that can diffuse to the PP-CO2 interface during foaming to stabilise the cells during their growth stage while reinforcing the cell surface with its rigid silsesquioxane structure upon solidification. Our results show that with 1 wt% HOS, the PP/HOS nanocomposite foam could offer 38 % ER improvement, 23 % foam density reduction, and 20 % CS increase when compared to neat PP foam. Using X-ray Photoelectron Spectroscopy characterization, this surfactant effect of HOS has also been validated. In addition, HOS could also provide further energy savings during processing by acting as a lubricant to reduce PP's shear viscosity without deteriorating its extensional viscosity and melt strength. This hybrid surfactant approach hence opens a new avenue for simultaneously enhancing the ER and CS of polymer foams in a simple and economically viable way and has the potential to be applied or further developed for other polymer foams.

Keywords: Supercritical CO2, Foaming, Polypropylene, Surfactant, Reinforcement

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