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This research presents the foaming behaviors of linear polypropylene (PP) and PP/clay nanocomposites blown with supercritical CO 2 . The cell nucleation and expansion behaviors of the linear PP and PP-based nanocomposites at various clay contents during extrusion foaming are studied. The experimental results indicate that the nano-particles have a positive impact on improving the cell morphology, the cell density and the expansion ratio of the linear PP foams.

Intercalated and exfoliated low-density polyethylene (LDPE)/clay nanocomposites were prepared by melt blending with and without a maleated polyethylene (PE-g-MAn) as the coupling agent. Their morphology was examined and confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of clay content and dispersion on the cell morphology of nanocomposite foams during extrusion foaming process were also thoroughly investigated, especially with a small amount of clay of 0.05-1.0 wt%. This research shows the optimum clay content for achieving microcellular PE/clay nanocomposite foams blown with supercritical CO 2 . It is found that < 0.1 wt% of clay addition can produce the microcellular foam structure with a cell density of > 10 9 cells/cm 3 and a cell size of $ 5 mm.

It is shown that Axisymmetric Drop Shape Analysis (ADSA) is well‐suited for the study of polymer melt surface tensions. The technique is not restricted to equilibrium surface (interfacial) tensions; it is also suitable for measuring the time dependence (or kinetics) of surface tension of polymer melts. Results for three polymers, polypropylene, polyethylene, and polystyrene, at temperatures above 170°C are reported. Contrary to the well‐known decrease of surface tension in low molecular weight surfactant solutions as a result of equilibration, an increase in the melt surface tension is observed under isothermal conditions.

A magnetic suspension balance was employed in experiments to measure gas solubility in the polymer melts. The gas solubilities of CO 2 and N 2 in both linear and branched Polypropylene (PP) were investigated. The swollen volumes predicted by the Sanchez-Lacombe equation of state (EOS) and Simha-Somcynsky EOS were applied to incorporate the buoyancy effect, which is essential for the accurate measurement of solubility data. The effects of the branched structure on the swollen volume and gas solubility were discussed. It was observed that the long chain branched PP exhibited less expandability than the linear PP, due to the entangled molecular chain structure. Therefore, the total amount of gas that was able to dissolve into the long chain branched PP turned out to be less.

ABSTRACT:The main benefits of incorporating wood fibers (WF) in plastics are the increased stiffness and lowered cost of the resulting composites. However, these improvements are usually accompanied by a reduction in the ductility and impact resistance. These shortcomings can be removed by effectively foaming and incorporating a finecelled structure in these composites. The volatiles released from WF during processing are known to deteriorate the cell structure. The maximum processing temperature, which affects the amount of volatiles released by the WF during extrusion of fine-celled plastic/WF composite, affects the cell morphology. This study was undertaken to identify the critical temperature above which the cellular structure of WF composite foams is significantly deteriorated. To clearly identify the effects of the volatiles generated from WF on the cellular morphology, neither a chemical blowing agent nor a physical blowing agent was used in the foam processing. The experimental results show that regardless of the drying method, the highest processing temperature of plastic/WF composites should be minimized, preferably below 170°C, to avoid the adverse effects of the volatiles generated from the WF during processing. A method of estimating the emissions from WF during extrusion processing by using the TGA data is also proposed.

Nucleating agents have long been employed in polymeric foaming processes to promote cell nucleation, increase cell density, and improve cell uniformity. This improvement in foam morphology is usually considered to result from the enhanced heterogeneous nucleation caused by the lower free energy barrier for cell nucleation. However, less is known about the underlying mechanisms of nucleating-agent-enhanced nucleation. In the polymer foaming process, pressure is a critical parameter that affects the degree of supersaturation of gas within a polymer-gas solution. In most previous theoretical studies on cell nucleation, a uniform pressure was assumed throughout the solution. Although this assumption may be acceptable when no particles have been added, its validity is questionable when nucleating agents are present. It has been speculated that growing cells that have already been nucleated generate local flow fields that induce tensile stresses around nearby particles, resulting in local pressure fluctuations. The discontinuity at the interface between a nucleating agent particle and the surrounding polymer melt yields local pressure and stress fields around the particle that are different from those in the bulk, which may enhance it as a potential heterogeneous nucleation site. This paper presents a numerical analysis to investigate the pressure profile in the vicinity of nucleating agents and provides new information about the underlying mechanism that promotes cell nucleation in the presence of nucleating agents.

This article investigates the effects of nanoparticles on cell morphology and foam expansion in the extrusion foaming of metallocene polyethylene/wood fiber nanocomposites with a chemical blowing agent. The results indicate that the addition of clay generally reduces the cell size, increases the cell density, and facilitates foam expansion. Furthermore, the foam material with added clay shows good char formation when it is burned.