Foaming of Low Density Polyethylene with Carbon Dioxide Based on Its in Situ Crystallization Behavior Characterized by High-Pressure Rheometer

Refined Spherulites of PP Induced by Supercritical N2 and Graphite Nanosheet and Foaming Performance

The isothermal crystallization properties of polypropylene/graphite nanosheet (PP/GN) nanocomposites under supercritical N₂ were systematically studied by a self-made in situ high-pressure microscope system. The results showed that the GN caused irregular lamellar crystals to form within the spherulites due to its effect on heterogeneous nucleation. It was found that the grain growth rate exhibits a decreasing and then increasing trend with the enhancement of N₂ pressure. Using the secondary nucleation model, the secondary nucleation rate for spherulites of PP/GN nanocomposites was investigated from an energy perspective. The increase in free energy introduced by the desorbed N₂ is the essential reason for the increase in the secondary nucleation rate. The results from the secondary nucleation model were consistent with those acquired through isothermal crystallization experiments, suggesting that the model can accurately predict the grain growth rate of PP/GN nanocomposites under supercritical N₂ conditions. Furthermore, these nanocomposites demonstrated good foam behavior under supercritical N₂.

Insight into the Influence of Properties of Poly(Ethylene-co-octene) with Different Chain Structures on Their Cell Morphology and Dimensional Stability Foamed by Supercritical CO2

Poly(ethylene-co-octene) (POE) elastomers with different copolymer compositions and molecular weight exhibit quite distinctive foaming behaviors and dimensional stability using supercritical carbon dioxide (CO₂) as a blowing agent. As the octene content decreases from 16.54% to 4.48% with constant melting index of 1, both the melting point and crystallinity of POE increase, due to the increase in fraction of ethylene homo-polymerization segment. the foaming window of POE moves to a narrow higher temperature zone from 20–50 °C to 90–110 °C under 11 Mpa CO₂ pressure, and CO₂ solubility as well as CO₂ desorption rate decrease, so that the average cell diameter becomes larger. POE foams with higher octene content have more serious shrinkage problem due to lower compression modulus, weaker crystal structure and higher CO₂ permeability. As POE molecular weight increases at similar octene content, there is little effect on crystallization and CO₂ diffusion behavior, the foaming window becomes wider and cell density increases, mainly owing to higher polymer melt strength, the volume shrinkage ratio of their foams is less than 20% because of similar higher polymer modulus. In addition, when the initiate expansion ratio is over 17 times, POE foams with longer and thinner cell wall structures are more prone to shrinkage and recovery during aging process, due to more bending deformation and less compression deformation.