Abstract:

Objective New generation of cellulose aerogel has become a research hotspot in recent years because of its wide source of raw materials, good biocompatibility, low density and low thermal conductivity, and has been widely used in civil, military, aerospace and other fields. However, cellulose-based aerogel has some disadvantages such as poor skeleton strength, brittleness, which seriously limit its development and application in the field of thermal insulation. Therefore, a cellulose-based aerogel with high strength and high compressive resilience was studied by introducing silane coupling agent to covalent crosslinking with cellulose.

Method A cellular-network aerogels with cellular-network structure was constructed by freeze-drying by using in situ covalent cross-linking of silane coupling agent 1,2-di (trimethoxysilyl) ethane (BTMSE) and cellulose nanofibers (CNF) to form strong interfacial interaction. By adjusting the content of silane coupling agent, the thermal insulation aerogel with high strength and good compressive resilience was obtained. The relationship between the amount of BTMSE added and the mechanical properties and thermal conductivity of aerogel was studied. The microstructure, chemical structure and thermal insulation properties of aerogel were analyzed.

Results Chemical bonding was considered first in studying the forming mechanism of CNF/BTMSE aerogel. Under acidic conditions, BTMSE was hydrolyzed to form reactive silanol, and covalently was polycopated with the hydroxyl group on cellulose to form Si—O—C bond, which acted as the cross-linking point between CNF. Chemical bonding enabled cross-linking and entangling among CNF to form network structure. The second was the investigation of the temperature-induced effect. In the process of low temperature freezing, water continuously formed ice crystals, and layered ice crystals gradually grew and squeezed nanofibers, so that the nanofibers gathered among the ice crystals, and the fibers were tightly stacked and intertwined to form a three-dimensional network structure. Finally, after freeze drying, the ice crystals were directly sublimated to form a honeycomb cell structure. Due to chemical crosslinking with siloxane, the CNF/BTMSE aerogel demonstrated a more regular pore structure. After BTMSE modification, the pore size of cellulose aerogel showed a decreasing trend, proving the formation of crosslinking network. Infrared spectroscopy and XPS spectroscopy confirmed the successful introduction of silane coupling agents in 3^# aerogel (the mass ratio of CNF and BTMSE is 2∶3) and the covalent force with the hydroxyl group on cellulose. Under 60% compression strain, the strength of CNF aerogel was 13.1 kPa, and the strength of 3^# aerogel was 34.8 kPa, and the deformation recovery rate was 97% after the external force was removed, indicating good resilience. In addition to higher compressive stress and resilience, the aerogel modified by silane coupling agent also showed excellent cyclic compressibility resistance. After 200 cycles of cyclic compression, the aerogel still maintained 90.4% of its initial height, and the strain loss was less than 10%. The regular pore structure formed by silane modification and the mesopole formed by crosslinked network make the aerogel demonstrated low thermal conductivity. The thermal conductivity of 3^# aerogel was 31.90 mW/(m·K), representing good thermal insulation stability, and the thermal conductivity increase was kept below 1% after 60% compression strain, which is well below the 20% increase in CNF aerogels. The 3^# aerogel produced a temperature difference of about 70 ℃ on a 130 ℃ platform, showing good thermal insulation performance.

Conclusion High strength and superelastic cellulose based aerogel materials were prepared by in situ covalent crosslinking and freeze-induced assembly. It improves the problems that the structure of pure cellulose aerogel with poor resilience is easy to collapse and the thermal insulation performance is decreased in real environment. It has great application value in flexible thermal insulation field.

Key words: silane coupling agent, cellulose aerogel, thermal conductivity, mechanical strength, compressive resilience