PREPARATION AND PROPERTIES OF NANOFIBRILLATED CELLULOSE BASED AEROGEL WITH PARTICULAR REFERENCE TO THERMAL SUPER INSULATION APPLICATION

Abstract

Thermal insulation is one of the most important tools to save energy, thus reducing greenhouse gas emissions. Researchers are working on active and passive energy-saving techniques. From this concept, the research was motivated to synthesize a material that will be abundantly available and does not harm the environment during processing and installation. Nanotechnological utilization of biomaterial such as wood for advanced superinsulation is an intense area of current research. The key challenges of insulation such as thermal resistance, low density, high porosity, and temperature effect on the mechanical properties have been discussed in the thesis. Low density and high strength nanofibrillated cellulose (NFC) aerogel based on pinewood were prepared by the freeze-drying method. The study was focused on reducing the thermal conductivity of the prepared aerogel along with the improvement in mechanical strength. Synthesized NFC aerogel has demonstrated high porosity (99.4%) and ultra-low density (8.1 kg/m3). Morphological analysis of aerogel by FESEM (Field emission scanning electron microscope) confirmed nano-dimensional diameter of cellulosic fibers and pore size distribution of aerogel in the range of 2-50 nm. X-ray microtomography confirmed the three-dimensional, monolithic and porous structure. The mechanical and thermal transport properties of aerogel have been tailored via controlling the concentration of NFC in the hydrogel. The synthesized aerogel acts as a thermal insulator with thermal conductivity of 25.5 mW/m K at 1.00 wt% of aerogel, which is near the thermal conductivity of air in ambient conditions. NFC aerogel would be a candidate for practical applications such as heat insulators, kinetic energy absorbers, and energy-efficient buildings. Afterward, flame retardancy and thermal insulating properties of sepiolite clay and NFC-based aerogel have been discussed. Due to the fragile characteristic of sepiolite-based aerogel, nanofibrillated cellulose has been introduced into the clay to make it a monolithic composite aerogel. The prepared hybrid aerogel has been modified with a hydrophobic precursor, i.e., Methyltrimethoxysilane (MTMS). The prepared specimens have shown the typical properties of aerogels like low density (11.5 kg/m3 to 32.5 kg/m3), high porosity (99.2 to 98.7%), and good dimensional stability. Vertical burning test along with V-0 performance, horizontal burning test, and flame penetration have confirmed its thermal shielding and flame retardancy. Apart from that, the aerogel had chances to improve its mechanical and thermal properties. For this specific requisite, many literature surveys were done based on several types of aerogel devoted to thermo-mechanical properties. The improvement was found by the in-situ synthesis of NFC / polymethylsilsesquioxane (PMSQ) aerogel. The NFC-PMSQ aerogels were prepared by optimizing precursors, surfactants, and base catalysts. The rheological premonitory of hydrogel has been performed to predict the physical properties (i.e., density, mechanical properties) of aerogel. Van-Gurp Palmen's plot of hydrogel has represented a relationship between complex modulus and phase angle. This study establishes that hydrogel's premonitory analysis could compare aerogel's physical properties without drying and further analysis. The chemical structure of the aerogel was studied by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Solidstate nuclear magnetic resonance (NMR). X-ray microtomographic analysis (XMT) confirmed the homogeneous and monolithic structure of aerogel. The lowest thermal conductivity was achieved as 23.21 mW/m K with V-0 and HBF rating through UL-94 test. The thermal performance of aerogels was cross-verified through modelling and simulation in the COMSOL multiphysics platform. The mechanical properties of aerogel were evaluated by monolithic compression test in axial and radial compression test up to 90% strain, cyclic compression loading-unloading-reloading test, and flexural test. Further, the present work emphasizes the particular application i.e., building insulation. The NFC-sepiolite-PMSQ aerogel was synthesized with improved mechanical, thermal insulation, flame resistance, etc. Conformal coating through PLA was done on the aerogel, which assisted in drastically enhancement in the aerogel properties. The aerogel holds a combination of properties such as ultralow thermal conductivity (18 mW/mK), high mechanical properties (600 kPa) at different climatic conditions, high flame retardancy and low density, etc. For a clear visualization of real field application, a small prototype of the house was prepared in the laboratory. The house was simulated for real field application. There was a 11ºC temperature difference in the middle of the room after using aerogel insulation for a long duration of time. Hence, we can save energy for cooling the room using aerogel on the exposed surface. As well as the prepared material will also protect from the environmental burden. Overall, our prepared nanofibrillated cellulose based composite has shown a candidate material for building thermal insulation.