Freeze-casting porous PTFE foam via constant temperature cold source

Porous PTFE foam with unique microstructure fabricated by freeze-casting may provide many potential applications in various fields. In this report, freeze-casting was applied to manufacture porous PTFE foam. The microstructure of the green bodies affected by the freezing temperature, PVA concentration and PTFE concentration was investigated. SEM, the actual temperature change curve, Low-temperature DSC and viscosity were characterized to investigate the microstructure forming process. The green bodies formed in freeze-casting were found bimodal in pore distribution. The underlying mechanism was further discussed by Oswald ripening which may be caused by long crystallization time. Cavities were also formed on the pore walls of the green bodies. That may be caused by PVA gelation during the growth of ice crystals. The orientated pore can be observed in the green bodies clearly. However, the pore orientations become less regular as the number of loaded particles being reduced. The explanation for this is that longer crystallization time may also lead to parts of crystallization occurring far from the original generated ice-front. Undergoing sintering process, the microstructure of the sintered sample had an incredible transformation from green body. Louvered shape of the pore wall with worm-like pore arms appeared, and pore shapes were transformed from oval to polygon. We present the schematic of the microstructure evolution of PTFE foam also.