Study On The Preparation And Properties Of Cellulose Nanofibrils-Silica Composite Aerogels

Aerogels are well recognized for their low density,high porosity,high surface area,low thermal conductivity,low dielectric constant and unique structure,which can be applied in a wide variety of areas.Traditional silica aerogels haves some weaknesses,such as low strength and great brittleness.Cellulose aerogels combine the important properties of conventional silica aerogels and polymer aerogels with their own excellent biocompatibility and biodegradability,which have gained great attention of new materials explorers.The Due to the remarkable mechanical properties,cellulose nanofibrils(CNF)is a promising potential structural reinforcement material for silica aerogel.The CNF and silica composite aerogel could improve the mechanical properties of silica aerogel.And at the same time,the silica could also improve the thermolability and hygroscopicity.The preparation of composite aerogel is of great significance in widening the development and application of aerogel in various fields.In this work,the CNF-silica composite aerogels were prepared through the elaboration of silica particles interpenetrated with a nanocellulosic network structure by immersion of the CNF aerogels into a silica sol in in-situ two-step sol-gel process.The effects of preparation process parameters(CNF concentration,pH value of condensation process,TEOS concentration and immersion time)on the properties and structure of composite aerogels using single factor experiment were investigated.A response surface methodology(RSM)was applied to optimize the preparation process with the target being to synthesize a desired aerogel with stable microstructure and good properties.The improvement and modification mechanism of the preparation process parameters in improving the mechanical properties,thermal stability and hydrophobicity of composite aerogel were analyzed.The main conclusions are as follow:(1)The analysis of the effect of CNF concentration on the properties and structure of CNF-silica composite aerogels indicates that in comparison with silica aerogel,the compressive properties of all composite aerogels prepared under different CNF concentration(3wt%,4wt%,5wt%,6wt%,and 7wt%)have been improved.The composite aerogel prepared with 6wt%CNF concentration had the highest compressive properties,the modulus and strength were 12.43MPa and 2.59MPa,which were 26?69 and 16?55 times higher of those of silica aerogel,respectively.With increasing CNF concentration,the silica content of composite aerogel increased first and then decreased.The skeleton of CNF aerogel could not bear enough silica particles in a lower CNF concentration.While in higher CNF concentration,the heterogeneous distribution of CNF would lower its deposition capacity.When the CNF concentration lower than 5wt%,the composite aerogel could represent clear fiber skeleton;higher CNF concentration could cause great agglomeration of silica particles,the distribution of two matrixes was heterogeneous.The composite aerogel prepared at 5wt%have homogeneous micro structure and good comprehensive properties.(2)The analysis of the effect of pH value of condensation process on the properties and structure of CNF-silica composite aerogels indicates that the pH value of condensation process had great influence on the condensation and yielding of silica particles in the CNF matrix.The higher pH value,the condensation would process faster and yield more silica particles.But at higher the pH value,the composite aerogel would yield more silica,which caused a degradation of the mechanical properties and the surface of the composite aerogel was beginning to crack.The performance of composite aerogel was more like to silica aerogel.The composite aerogels have improved mechanical properties compared with silica aerogel.In comparison with silica aerogel,the compressvie modulus of composite aerogel prepared at the pH=10 had increased from 0.18MPa to 5.42MPa,the compress vie strength increased from 0.047MPa to 1.38MPa.The onset proylysis temperature of composite aerogel was 31.7? higher than CNF aerogel.The composite aerogel also had better hydrophobicity,the contact angle was far beyond 90°.(3)The analysis of the effect of TEOS concentration of the properties and structure of CNF-silica composite aerogels indicates that the TEOS concentration would affect the condensation process.When the basic solution reacted with higher TEOS concentration hydrolysis solution,the silica particles would experience fast condensation,and forming a layered structure:a layer with larger silica content wrapped around the core layer with relatively less silica content.In other words,the composite aerogel seems to have exhibited a more unstable structure and heterogeneous silica dispersion,which may weaken the functional properties of the composite aerogel.The composite aerogel prepared at 2.5mol/L TEOS concentration possesses comparative mechanical properties of CNF aerogel(compressive modulus 5.91MPa vs.5.94MPa,compressvie strength 1.38MPa vs.1.44MPa).And it also had better thermal stability and hydrophobic characteristics.(4)The analysis of the effect of immersion time of the properties and structure of CNF-silica composite aerogels indicates that the immersion time have great influence on the structure of composite aerogel and the distribution of silica particles in the CNF matrix.Shorter immersion time could not complete the condensation process.While longer immersion time would appear silica agglomeration coating on the surface of CNF aerogel,which would block the immersion process and cause uneven silica dispersion,and further influence the structure and properties of composite aerogel.The composite aerogel prepared in 10min immersion have uniform microstructure,better mechanical properties,larger specific area,and still have high porosity(93.62%).This composite aerogel was also hydrophobic(contact angle reach to 152.1°)and low density(0.1 g·cm-3).(5)To study the effects of the preparation parameters(CNF concentration,TEOS concentration,pH value of condensation process and immersion)on the compressive modulus and compressive strength of CNF-silica aerogels,a response surface methodology(RSM),the Box-Behnken design(BBD)was applied in this case.The results show that the density can be seen as a function of the silica content as the two had a positive dependent relationship.The mechanical properties of composite aerogels also have close relation with silica content:the composite aerogel with 40?50wt%silica content displayed a significant improvement of mechanical properties.After optimization,the optimal preparation process was identified:immersing the solid content 5wt%CNF aerogel into the 2.5mol/L TEOS solution for 10min,adjusting the pH value of the condensation environment to 10.This is just the center sample,which also confirmed the rationality of model.When comparing with CNF aerogel and silica aerogel,the compressive modulus and strength of the target composite aerogel was 6.57MPa and 1.37MPa,which are 13?36 and 8?30 times of silica aerogel,respectively.The density of the composite aerogel was only 0.117g·cm-3.The target composite aerogel also had low thermal conductivity(0.0226W·m-1·K-1)and super-hydrophobicity(contact angle 152.1°).The preparation of composite aerogel not only ameliorate the poor mechanical properties and brittle innate nature of silica aerogel,but also improve the thermolability and hydrophilicity of CNF aerogel.