Journal of Cellular Plastics

Carbon nanofibers (CNFs) are used as both the nucleants and the reinforcements for polystyrene (PS) foams in both batch and continuous extrusion processes with CO₂ as the blowing agent. The inclusion of CNFs exhibits substantial impact on the morphology and properties of PS foams. The presence of CNFs results in a decrease in the cell size and an increase in the cell density. Macroscopic strength enhancement of PS foams due to the incorporation of CNFs is experimentally observed. These results are qualitatively explained via a series of nanoscaled observations. Through scanning electron microscopy (SEM), the alignment of CNFs and the polymer-sheathing phenomenon are identified. The protective CNF layers around the cell wall and the substantial polymer-fiber interactions resulted in the enhancement of foam strengths. Thermal properties (heat conductivity, infrared transmission, and thermal expansion coefficient) of the PS foams can also be influenced by CNFs. Intensive shear force exerted by the twin-screw extruder broke the fibers in lengths. The impact of the fiber length on the foam structures (cell size and cell density) is also studied.

Use of mixtures of blowing agents in thermoplastic foam extrusion has been an industrial practice for a long time. However, it has gained renewed interest in the past few years due to the introduction of difficult-to-process alternative gases, targeted as potential replacement for the banned ozone-depleting blowing agents. Reasons for blending physical foaming agents (PFA) are numerous. The incentives may be economical, environmental, or technical. With respect to that latter factor, blending suitable PFAs is often regarded as providing a better control of processing conditions. For example, a specific PFA could be selected for its inflation performance and blended with other co-blowing agents chosen for their stabilizing role. Although a considerable amount of work has been done in that area, very little information has been disclosed in open literature.

Carbon dioxide (CO₂) has been reported as an interesting candidate for low-density polystyrene (PS) foaming, although the required concentrations are associated with high processing pressures due to the low solubility of the gas. Thus, stable processing conditions are difficult to achieve. This work studies the effect of blending CO₂ with ethanol (EtOH) as a co-blowing agent for PS foaming. Extrusion foaming performance of this mixture is discussed, with respect to its solubility (i.e., degassing conditions) and rheological behavior. The function of each blowing agent during the process is analyzed with respect to the plasticization, nucleation, expansion, and stabilization phases. Attention is also paid to the interaction involving the two PFA components.

The feasibility of applying the single-charge rotational foam molding processing principle to the fabrication of integral skin polypropylene (PP) foams comprising a PP solid skin and a PP foamed core is investigated in this study. A systematic process interruption and sample evaluation approach was used to quantify the experimental results and explore possibilities for improving the process control strategies to ultimately achieve a desired homogeneity and thickness uniformity of the solid PP skin layer that would be fully encapsulating the PP foamed core of a desired cell population density and average cell size. The experimental results revealed that this is quite a challenging task, not only because of the well-known intrinsically poor foaming nature of PP due to its low melt strength at elevated temperatures, but also because, in single-charge rotational molding, the processing parameters are often conflicting with each other and therefore, have to be optimized within a very narrow processing window. However, simultaneous, single-charge, quality PP integral skin and foamed PP core formation in rotational foam molding is feasible. Optimizing the heating profile, heating rate, heating time, and the mold rotational speed as well as careful selection of PP resins (or resin blends), chemical blowing agents (CBA), and their composition formulations is strongly recommended.

This work examines the foam density of polystyrene (PS) blended with low density polyethylene (LDPE) at various ratios, and the thermal conductivity of vacuum insulation panels (VIPs) using PS/PE with filler foam as core materials. The experimental results indicate that LDPE strongly influences the foam density of PS/PE foam and the content of open-cells in PS/PE with filler foam. A higher content of open-cells in PS/PE with filler foam can lead to a lower thermal conductivity of VIP.

An investigation of the foaming behavior of polybutylene terephthalate (PBT) resins in extrusion is performed. Commercial grades of PBT with different molecular weights and rheological characteristics are chosen. PBTs with a high temperature chemical blowing agent are extruded under different material and operational conditions (e.g., amount of blowing agent, set temperature at the die, screw RPM, etc.) to allow the understanding of the effects of those conditions on product characteristics. The foamed extrudates are analyzed for density, morphology, and crystallinity. It is shown that the foaming behavior and the foam quality of PBT are functions of the characteristic properties of the resins including rheological and crystallization behavior.