Journal of Cellular Plastics

Integral skin foam (designated ISF) consisting of core (cellular foam) and solid skin is produced in one operation. Since the recommendation of Montreal Protocol in 1987 to phase out five CFCs, the three major possibilities for producing ISF studied in the past include replacement of CFC with HCFC, HFC, or water. Recently, the use of HCFC-141b for blowing agent was also phased out at the end of 2003, and HFCs have been criticized for their high Global Warming Potential (GWP). However, it is difficult to produce a skin layer using water as the blowing agent because carbon dioxide (CO₂), which is generated by an isocyanate—water reaction is unlikely to condense at the mold surface. We have developed a new technology to produce all-water-blown ISF. A novel polypropylene glycol, which contains an extremely small amount of byproducts (monool) and has narrow molecular weight distribution compared with conventional polypropylene glycols was applied to this new system. In case of applying this novel polypropylene glycol in all-water-blown ISF system, it is easy to control the balance between the gelling reaction and blowing reaction of ISF. Therefore, we have been able to obtain a tight skin layer like HFC-blown ISF.

Poly(dimethylsiloxane) (PDMS) was blended with two different types of polypropylene (PP). The blends were subsequently batch-foamed with supercritical CO₂ at a series of temperatures that varied by a narrow increment of 2°C to investigate the effect of the foaming temperature on foaming. In the case of the random copolymer PP, it was found that the cell density of the blends containing PDMS increased significantly and good cell structures could be obtained across a wide temperature spectrum. PDMS typically generated high CO₂ concentration and low surface tension, which positively impacted the cell nucleation. In the case of linear homopolymer PP, the addition of PDMS did not result in any pronounced improvement to cell morphology; however, at very low temperatures, much lower than the melting point, a few very small cells appeared. In both experiments, the addition of maleic anhydride grafted PP (PP-g-MAH) as a compatibilizer promoted the dispersion of PDMS and yielded a better cell morphology within a specific temperature range. Moreover, the presence of a compatibilizer enhanced the melt strength, which in turn served to broaden the processing window.

Several efforts have been made to predict the mechanical properties of structural foams, mainly under tensile and flexural loading modes. The prediction of the mechanical behavior of foams subjected to impact loads remains, however, an unfulfilled challenge. Several investigations have studied the variation of impact properties of structural foams. Both the studied materials, the testing methods employed and the techniques used for sample preparation, vary broadly. Some studies regarding the relation between density and energy absorption capabilities have been conclusive. The influence of cell size and skin thickness have also been analyzed. So far, results show that the foam structure adds a number of complexities to the limited characterization capabilities of conventional impact methods. This study aims to contribute to the understanding of the failure behavior of structural foams. The influence of three different factors will be addressed: the type of imposed loading, through variations in the test method used; the nature of the studied base material (semicrystalline vs. amorphous), and the influence of the foam morphology.

Open-celled solid-state foams are expected to have many applications, such as filtration, biochemical sensing, and tissue engineering scaffolding. For bio-related applications, the solid-state foaming process has a unique advantage, i.e., the process does not involve any organic solvent that could leave residues harmful to biological cells. The disadvantage, however, is that solid-state foams are mostly close-celled and do not allow biological cells or fluids to permeate through. This article presents a parametric study on a novel fabrication method to create open-celled solid-state foams using ultrasound. Biodegradable polymer was foamed in a solid-state foaming process. Ultrasound was then applied to break the pore walls. The parameters examined in this study included the pore size, ultrasound power, ultrasound frequency, and water temperature. The inter-pore connectivity was verified using permeability measurements, dye diffusion, and a degradability test. The permeability values were further used to analyze the effects of the experimental parameters. In the analysis a logistic regression model was first used to ensure sample integrity after the ultrasound treatment. A linear regression was then conducted to determine the significance of process variables. It was found that the pore size was the most significant factor affecting the ultrasound effectiveness. The bigger the pores are, the higher the permeability could be obtained with ultrasound. In order to increase the permeability of the ultrasound treated samples, high ultrasound power, high water temperature, and low ultrasound frequency should be used.