Polypropylene (PP) is an outstanding material for polymeric foams due to its favorable mechanical and chemical properties. However, its low melt strength and fast crystallization result in unfavorable foaming properties. Long-chain branching of PP is regarded as a game changer in foaming due to the introduction of strain hardening, which stabilizes the foam morphology. In this work, a thorough characterization with respect to rheology and crystallization characteristics of a linear PP, a PP/PE-block co-polymer, and a long-chain branched PP are conducted. Using these results, the processing window in foam-extrusion trials with CO₂ and finally the foam properties are explained. Although only LCB-PP exhibits strain hardening, it neither provide the broadest foaming window nor the best foam quality. Therefore, multiwave experiments were conducted to study the gelation due to crystallization and its influence on foaming. Here, linear PP exhibited a gel-like behavior over a broad time frame, whereas the other two froze quickly. Thus, apart from strain hardening, the crystallization behavior/crystallization kinetics is of utmost importance for foaming in terms of a broad processing window, low-density, and good morphology. Therefore, the question arises, whether strain hardening is really essential for low density foams with a good cellular morphology.

The expanded polypropylene beads with microcellular structure were prepared by an autoclave-based batch foaming process using non-supercritical CO₂ as the foaming agent. Herein, smaller polypropylene micropellets (∼0.6 mm) were obtained from three kinds of polypropylene resin by underwater micro-pelletizer system. The results from differential scanning calorimetry indicated that the double melting peaks of beads moved to higher temperature with increasing saturation temperature and pressure. The kind of comonomer did not exert noticeable influence on the double melting behaviors. The X-ray diffraction of expanded polypropylene bead revealed that characteristic peaks of α-type crystal did not change compared with that of original polypropylene micropellets. The relationship between cell morphology and saturation temperature/pressure of expanded polypropylene beads are discussed preliminarily by scanning electron microscope.

Through soil incubation experiments, the effects of aged PBAT + PLA (polybutylene adipate terephthalate + polylactic acid) film fragments were analyzed. Surface characteristics and chemical structures of the films changed significantly after one (T2) and two years (T1) of aging compared to new films (T3). Both new and aged fragments reduced soil pH, altered enzyme activities, and influenced dissolved organic matter (DOM) fluorescence. Alkaline phosphatase activity declined by 33.2%, 23.8%, and 11.6% for T1, T2, and T3, respectively, while urease and sucrase activities increased in a time-dependent manner. The degree of soil humification rose by 66.4%, 60.4%, 49.3%, and 88.6% for T1, T2, T3, and T4, respectively, compared to the control (CK). Aged films exhibited stronger DOM fluorescence intensity than new films. Tessier extraction analysis revealed a decrease in exchangeable Cd by 22.9%, 13.1%, and 10.2% for T1, T2, and T3, respectively, while organically bound Cu increased. Correlation analysis indicated a significant positive relationship between soil humification and heavy metal bioavailability. These findings provide insight into the ecological effects of biodegradable agricultural films, offering a theoretical foundation for assessing their environmental risks and safety.

Our experiment revealed that the addition of Janus nanosheets to polypropylene (PP) has a significant impact on the viscoelasticity of the composite system. Specifically, when 0.10 wt% of Janus nanosheets were added, the complex viscosity of the composite system increased. However, when we added less than 0.05 wt% of Janus nanosheets, there was a reduction in complex viscosity, which is known as the non-Einstein phenomenon. The Cole–Cole plot showed that the nanosheet network structure did not have a significant effect on the viscosity of the composite system. Additionally, we used carbon dioxide as a foaming agent to autoclave foaming using modified PP from Janus nanosheets, and the results demonstrated that increasing the number of Janus nanosheets decreased the apparent density and strengthened the cell structure of foaming beads, resulting in improved closed porosity.