Production of porous cellulose aerogel fibers by an extrusion process

Kurzfassung

The preparation, production and properties of light weight, porous cellulose aerogels in the form of thin extruded fibers is compared to monolithic pieces. The cellulose aerogels were synthesized from microcrystalline cellulose in a hydrated calciumthiocyanate salt melt, which upon cooling forms a gel at around 80 °C. Twin screw extrusion experiments were performed systematically yielding thin and wet cellulose filaments. Washing and coagulation of the wet gels in ethanol was followed by supercritical drying with CO2 yielding cellulose aerogel filaments. These were characterized with regard to envelope density, nitrogen adsorption-desorption (BET) analysis, thermal conductivity measurements, tensile and compression tests and scanning electron microscopy (SEM). The microstructure can be described as an open porous network of nano-fibrils with pore sizes ranging from 10 to 100 nm and fibril diameters of around 10 to 25 nm. The densities of supercritically dried (SCD) cellulose aerogels were in the range of 0.009–0.137 g/cm3 and the BET specific surface areas (SSABET) were between 120 and 230 m2/g. The cellulose aerogel possessed thermal conductivities from 0.04 to 0.075 W/m.K and compressive moduli up to 16.2 MPa. The tensile strength of aerogel filaments increases with the increasing cellulose amount in the spin dope. Extruded cellulose aerogel filaments show a dependency of their specific surface area on the extrusion temperature: the higher the spinning temperature the higher the surface area.