Preparation of microcellular injection molded foams with high performance

Abstract

Microcellular plastic foams are applied in a wide range of applications due to their enhanced toughness, good sound insulation properties and electrical properties. It is well known that cell structure would strongly influence the properties of polymer foams. However, the precise effect of cell size on properties under a constant void fraction is still inconclusive. To achieve foams with high performance, the effect of cell structure on the impact strength, electrical properties and sound insulation properties was studied systematically in this research.
(a) Effect of cell structure on impact strength:
First, the cell structure and impact strength of poly(lactic acid)/ poly(butylene adipate-co-terephthalate) (PLA/PBAT) blend foams under different content of compatibilizer were compared. The obtained PLA blend foams displayed a super high impact strength of 49.1 kJ/m², 9.3 and 6.4 times that of the unmodified PLA/PBAT blend and its corresponding foam, respectively. It proved that enhancing interfacial adhesion and reducing cell size could improve the impact strength of PLA/PBAT foams.
Then, to explore the relationship between cell size and impact toughness, PLA/PBAT foams with different cell sizes but a constant void fraction were prepared. When the cell size was smaller than a critical value, the foam would be tough; when the cell size was greater than the critical value, the foam would be brittle. In other words, there was a brittle-tough transition at the critical cell size. For foams with cell sizes below the critical value, the proximity of the cells facilitated a robust interaction of stress fields generated by adjacent cells, coupled with the beneficial role of rubber particles in hindering the progression of cell-induced crazes to cracks, absorbing significant energy. For foams with cell sizes above the critical value, however, the cell-induced crazes could directly develop into cracks due to the negligible interaction, leading to a low impact strength.
Finally, the combined effects of the PLA crystallinity and cell size on the impact strength of PLA/PBAT foams were studied. The PLA crystallinity was tailored by carbon dioxide (CO₂) treatment and cell size was well controlled by changing processing parameters. As expected, the brittle-tough transition can be found by reducing cell size regardless of the degree of PLA crystallinity. When the cell size was above 12 μm, the impact strength was almost unchanged by varying cell size, since those foams were all fractured through crazing. When cell size was below 12 μm, the impact strength was significantly enhanced by improving the PLA crystallinity due to the transformation of the fracture mechanism from multiple crazing to shear yielding.
(b) Effect of cell structure on electrical properties:
Polypropylene/carbon nanostructure (PP/CNS) nanocomposite foams with a fixed void fraction were produced using core-back foaming injection molding (FIM) technique, and their cell sizes were well-controlled by modifying the nitrogen (N₂) content. The results revealed that as the cell size increased from 71 to 317 μm, the EC and EMI shielding effectiveness (SE) increased from 1.43×10^-3 to 5.07×10^-3 S/cm and from 48.5 to 59.2 dB, respectively. The enhanced EC was attributed to the slightly aligned CNS and the shorter actual conductive paths.
(c) Effect of cell structure on sound insulation properties:
PP/mica composite foams with different void fractions (VF) and cell sizes were obtained using core-back FIM. The results showed that increasing the VF (the sample thickness increased but the sample density remained constant) could enhance the sound transmission loss (STL) from 16.8 dB for PP solid to 29.9 dB for PP foam with 75% VF. In addition, reducing the cell size and adding mica could also improve the STL due to the increase in reflection and scattering of sound waves in cells.
In summary, it has been demonstrated that polymer foams with high performance in terms of impact toughness, electrical properties and soundproofing properties can be achieved by tailoring cell size. This study provided a deeper understanding of the cell structure-properties relationship for polymer foams.

Date of Award	Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Xiaosu Yi (Supervisor), Ping Cui (Supervisor) & Wenge Zheng (Supervisor)