Aerogels based on biodegradable polymers and clay

Resumen

Foam-like aerogels based on biodegradable polymers and sodium montmorillonite (Na+-MMT) clay were prepared through an environmentally friendly freeze-drying process. Both synthesized and bio-based polymers were utilized in this thesis, including polyvinyl alcohol (PVOH), carboxylmethylcellulose (CMC), xanthan gum, agar, Arabic gum and starch. The morphologies of aerogels were characterized using scanning electron microscopy. The mechanical properties investigation included compression and impact tests. Porosities and solid densities were measured using a helium pycnometer while the pore size distribution was determined by automated mercury porosimeters. Most of polymer-clay aerogels exhibited porous and layered structures that were formed via ice templating. However, high viscosity of the precursor solution may break the layered architecture by retarding the formation of ice crystals (e.g. 2.5 wt% agar aqueous solution). The structures as well as the properties of aerogels were mainly influenced by polymer/clay proportion. Polymer molecules play a role of glue linking the clay nanoparticles, improving the structural integrity and hence the mechanical performance of the aerogels. On the other hand, clay platelets serve as a physical barrier that increases the heat endurance. Recycled cellulose fibers (RCF) that were isolated from waste paper pulp were also used to prepare bio-based aerogels. Adding another biopolymer CMC into RCF aerogels, the resultant RCF-CMC composite aerogels showed different microstructures and enhanced mechanical properties. Physical blending and chemical crosslinking were used to tailor the mechanical properties of xanthan gum/clay aerogels and starch/clay aerogels, respectively. Blending agar with xanthan gum in aqueous solution, the resultant aerogels displayed a significant improvement in mechanical properties compared with those containing a single biopolymer. Moreover, they exhibited tunable microstructures and mechanical properties by changing agar/xanthan gum ratio in the aerogels. As to starch/clay aerogels, the incorporation of glutaraldehyde enhanced the structural integrity and mechanical properties of the aerogels through crosslinking reaction between glutaraldehyde and starch molecules, which was proved by Fourier-Transform infrared (FT-IR) spectroscopy analysis. The evaluation of the flammability of aerogels was conducted with a cone colorimeter while the thermal stability was obtained from the results of thermogravimetric analysis. In regard to PVOH-clay aerogel, different types of flame retardant fillers, such as aluminum trihydroxide (ALH), ammonium polyphosphate (APP), silica gel and potassium carbonate, were adopted to modify their flame retardant properties. The results showed that ALH addition enhanced the flame retardancy as well as mechanical properties. For RCF-CMC aerogels, APP and clay played a synergetic effect on the flame retardancy and thermal stability.