Recently, functional applications of thermoplastic foams have received extensive attention from the research and materials communities, focusing on their various applications, key challenges, material systems designs, processing methods, and cellular structure characteristics needed for specific functional applications. This review paper starts with consideration of the microcellular foaming mechanism and basic concepts of microcellular foam processing, followed by polymer modification methods, and crucial factors that determine the performance of thermoplastic foams. Special emphasis has been placed on the synergies between foaming and reinforcements, including functional fillers and polymer blends; improvements in homogeneous, functional properties by achieving uniform cell structure and cell dispersion in polymer systems; and comparison of melt processing and solvent-based methods. Then, a wide array of advanced functional applications for foams-such as lightweight applications, heat and sound insulation, electromagnetic shielding, tissue engineering, oil spill cleanup, shape memory, and flexible materials-will be presented. In particular, the relationships between cellular structure and anticipated properties-including mechanical, barrier, dielectric, biomedical, and other properties required in advanced functional applications-will be discussed. Finally, we will outline a future perspective of lightweight and functional foams and suggest recommended future work regarding functional microcellular foams.
Cell structure is a key factor that determines the final properties of microcellular polylactide (PLA) product. In the mold opening process, adjusting the rate of mold opening can effectively control cell structure. PLA and PLA composites with a void fraction as high as 50% were fabricated using the mold opening technique. The effects of mold opening rate and the addition of nanoclay on the cell structure, mechanical properties, and surface quality of microcellular PLA and PLA composites samples were investigated. The results showed that finer cell structure was received in the microcellular PLA samples and the surface quality was improved effectively when decreasing the rate of mold opening. The effect of mold opening rate on the foaming behavior of microcellular PLA-nanoclay was the same as that of microcellular PLA. The addition of 5 wt % nanoclay significantly improved the foaming properties, such as cell density, cell size, and structural uniformity, which consequently enhanced the mechanical properties of foams and the surface quality.
Human milk fat-style structured triacylglycerols were produced from microalgal oil in a continuous microfluidic reactor packed with immobilized lipase for the first time. A remarkably high conversion efficiency was demonstrated in the microreactor with reaction time being reduced by 8 times, Michaelis constant decreased 10 times, the lipase reuse times increased 2.25-fold compared to those in a batch reactor. In addition, the content of palmitic acid at sn-2 position (89.0%) and polyunsaturated fatty acids at sn-1, 3 positions (81.3%) are slightly improved compared to the product in a batch reactor. The increase of melting points (1.7°C) and decrease of crystallizing point (3°C) implied higher quality product was produced using the microfluidic technology. The main cost can be reduced from $212.3 to $14.6 per batch with the microreactor. Overall, the microfluidic bioconversion technology is promising for modified functional lipids production allowing for cost-effective approach to produce high-value microalgal coproducts.
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