Cellulose aerogels cross-linked with glyoxal

Abstract

This study explores the preparation of cellulose aerogels from microcrystalline cellulose and comparing structural modification with and without glyoxal as a cross-linker. The cellulose wet gels were synthesized by dissolving microcrystalline cellulose (1, 3 and 5 wt.%) in a calciumthiocyanate salt melt hydrate-Ca(SCN)2.4H2O at 110°C. Wet gels were washed and regenerated in an ethanol bath. The aerogels were dried supercritically with carbon dioxide (scCO2). Crosslinking with an aqueous 40 wt.% glyoxal solution was performed using aluminum sulfate as a catalyst at various concentrations of glyoxal with respect to cellulose in the ethanol regeneration bath. The morphology and structure of cellulose aerogels were characterized using scanning electron microscopy (SEM), nitrogen adsorption/desorption analysis and pore size distribution (BET) and thermogravimetric analysis (TGA). Mechanical behavior of cellulose aerogels was investigated in compression tests. Thermal conductivity of cellulose aerogels was determined at atmospheric pressure and room temperature. The envelope density of the samples varied between 20-140 g/cm3. Nitrogen adsorption analysis gave specific surface areas of 150-297 m2/g. The microstructure can be described as an open porous nano-felt with pore sizes ranging from 10 to 100 nm and fiber diameters of around 10-20 nm (Figure 1). Thermal conductivity of cellulose aerogels varies linearly with cellulose concentration from 0.04 to 0.075 W/m.K. The mechanical behavior is typical for a porous material: after onset of deformation there is a long plateau at almost constant stress. The aerogels exhibit a Poisson ratio of around zero. The Young’s modulus is 8.4 MPa for the 3 % cellulose aerogel and cross-linking of cellulose aerogels with glyoxal improves the compression strength of cellulose aerogel by approximately 50 %.