Journal of Cellular Plastics

Novel isophorone diisocyanate-based flexible polyurethane foams were prepared by the one-step method in a computerized foam qualification system (FOAMAT). The experimental conditions to obtain this type of foams, in relation to the nature and concentration of catalysts as well as the reaction temperature, were established as no data were available in scientific literature. The chemical reactions occurring during the foam generation process were monitored in situ by attenuated total reflectance-FTIR spectroscopy. The kinetics of the foam generation was fitted to an nth order model and the data showed that the foaming process adjusted to a first-order kinetics. The physical changes as pressure, foam height, and dielectric polarization were monitored by the FOAM software (FOAMAT). According to these parameters, the foaming process was divided into four steps: bubble growth, bubble packing, cell opening, and final curing.

Representative volume elements of random equilateral Kelvin open-cell microstructures were modeled for the open-cell foam. We adopted the periodic boundary conditions developed in our previous research. The quasistatic compression properties of the representative volume elements in random Kelvin open-cell aluminum foam samples, both with different relative densities and different cross-sections of the beams in the structures were investigated. The results show that the features of the stress–strain curves in the representative volume elements with different relative densities and different cross-sections were similar, and the relationships between the yield strengths and relative densities of representative volume elements with four different cross-sections all agreed well with the quadratic power function. Among the representative volume elements with four different cross-sections, the yield strengths of the representative volume elements with a Plateau border cross-section were significantly larger than in representative volume elements with other cross-sections, while the yield strengths of representative volume elements with circular cross-sections were smaller than in representative volume elements with other cross-sections. This indicates that the simulation results of the compression strengths for open-cell foam in which the representative volume elements with circular cross-sections were employed are significantly smaller than their actual values. The main reason for this is that the moments of inertia in the Plateau border cross-sections are significantly greater than in the circular cross-sections of the same area. Our investigation results revealed that the compression responses of the representative volume elements for random equilateral Kelvin open-cell microstructures demonstrate isotropic behavior on the xoy plane, the yoz plane, and the xoz plane.

Expanded polyvinyl alcohol is regarded as excellent buffering and leak-proof packaging material of liquid products due to its characteristics such as good liquid absorption and liquid retention properties, good mechanical properties under dry condition, and good rebound resilience under wet condition. Through static compression experiment, this study analyzed the mechanical properties and energy absorption properties of expanded polyvinyl alcohol with different densities under different temperatures and relative humidity. The experimental results showed that the effect of ambient temperature and humidity on expanded polyvinyl alcohol performance was mainly to change its internal moisture. The initial elastic modulus, plateau stress, and energy absorption value per unit volume of expanded polyvinyl alcohol increased as the density increased, the relative humidity decreased, or temperature increased. The above research can provide reference for applications of expanded polyvinyl alcohol on buffering packaging in actual logistic environment.

Microcellular polystyrene foam sheets prepared by extrusion foaming with CO₂ as the blowing agent were found to exhibit a significant anisotropy in mechanical behavior which could not be explained solely by the shape anisotropy of the cells and/or the preferential orientation of polystyrene molecules in the machine direction. Surprisingly, samples allowed to fully relax were still found to maintain part of their mechanical anisotropy, which was attributed to the weakness of the cell walls perpendicular to machine direction.

It is critical to broaden the applications of poly(L-lactic acid) foams by improving heat resistance properties. The stereocomplex crystallites that are formed by melt blending of poly(L-lactic acid)/polylactide possess high melting point of about 220℃ and thus exhibit high heat resistance; therefore, the introduction of stereocomplex crystallites tends to improve the thermal stability of poly(lactic acid) foam. Unfortunately, using the solid-state foaming method, it was found that the expansion ratio of the obtained poly(lactic acid) foams was compromised with the value of 1.7 times once the stereocomplex crystallites were formed during the sample saturation stage. In this study, by applying a high compression molding temperature of 230℃, the as-prepared poly(L-lactic acid) and poly(L-lactic acid)/polylactide blends were amorphous. After being CO₂ saturated at a mild condition, the specimens were foamed at 90–160℃. The wide-angle X-ray diffraction profiles presented that the stereocomplex crystallites and PLA homocrystals were in-situ generated during the foaming process. It is observed that the in-situ formed stereocomplex crystallites could act as the physical cross-linking agent to stabilize the nucleated bubbles and suppress cell coalescence, resulting in the increased expansion ratio (with value of about 23.6–25.6 times) and cell density, especially at high foaming temperatures and extended foaming time. Furthermore, the in-situ formed stereocomplex crystallites during the foaming increased the heat resistance performance of poly(L-lactic acid) foams. This novel crystallization control method helps us to find a balance point in preparing poly(L-lactic acid) foam with high expansion ratio, well-defined cell structure and high heat resistance performance.