Zhimei Xu

Microcellular Foaming of Polypropylene/Clay Nanocomposites with Supercritical Carbon Dioxide

The solid-state foaming of two series of polypropylene (PP)/clay nanocomposites based on two different types of organoclays were studied. The wide-angle X-ray diffraction results showed that a supercritical carbon dioxide (scCO 2) saturation process could lead to the further exfoliation of clay when the initial interlayer spacing was larger enough. It was found that the incorporation of clays exerted significant impacts on cell nucleation and growth and the effect of clays was determined by their dispersion state. However, the influence of clay on the foaming of the PP/clay nanocomposites became marginal at a higher saturation pressure and a lower foaming temperature.

Effects of Blend Morphology on the Foaming of Polypropylene/ Low-density Polyethylene Blends during a Batch Foaming Process

The effect of blend morphology on the foaming of polypropylene (PP)/low-density polyethylene (LDPE) blends with different blend ratios has been studied in this work. The results showed that the cell morphology of a foamed blend was significantly impacted by the blend morphology and foaming conditions. When the foaming temperature was close to the melting temperature of LDPE, cells were nucleated at the interfaces of PP droplets and LDPE matrix in the 10/90 and 25/75 PP/LDPE blends and grew in the LDPE matrix. Also, cells could only grow in the LDPE phase in the 90/10 and 75/25 PP/LDPE blends and consequently their sizes were determined by the sizes of the LDPE droplets. However, when the foaming temperature was close to the melting temperature of PP, cells with bimodal cell size distributions were generated in the pure PP, 90/10 and 75/25 PP/LDPE foams. In particular, fine open-cell structure was formed in the co-continuous 50/50 PP/LDPE blend.

Extrusion foaming of poly(ethylene terephthalate) with carbon dioxide based on rheology analysis

Although low density poly(ethylene terephthalate) foams take advantage of the combination of good performances and lightness and have a wide range of engineering applications, extrusion foaming of the poly(ethylene terephthalate) is still faced with many challenges, especially the inadequate rheological characteristics. In this work, the melt strength of four types of poly(ethylene terephthalate) was compared on the basis of the analysis on the dynamic complex viscosity, storage and loss modulus, relaxation spectrum, loss factor and extensional viscosity using rheometer. It was found that the two long chain branching poly(ethylene terephthalates) owned overall higher melt viscosity and greater elasticity than the other two linear ones. Furthermore, only one long chain branching poly(ethylene terephthalate), i.e. PET4, succeeded in the extensional viscosity characterization and showed the phenomenon of strain hardening. Thereafter, PET4 was successfully applied for the extrusion foaming with CO2 as the blowing agent. The effects of the foaming temperature, screw revolution speed, gas input and addition of nucleation agent were carefully investigated on the extrusion foaming and the resulted poly(ethylene terephthalate) foams properties. Under the optimization condition, the extruded poly(ethylene terephthalate) foams were produced with the volume expansion ratio of 9.5, average cell size of 265 µm, cell density of 4.58 × 105 cells/cm3 and a narrow cell size distribution.