Study On The Technology Of Molding And Drying For Spherical Nanocellulose Gel

Nanocellulose is a new type of biomass nano-material. It not only retains the basic properties of natural cellulose, but also has a large surface area, high strength, easy chemical modification, renewable and many other fine features, which shows great potential in the study of nano materials. Cellulose aerogel is known as a new generation of aerogel following organic and inorganic aerogel. It is the third generation of nascent material and has broad application prospects in the field of adsorption materials. In this paper, sulfuric acid hydrolysis method was adopted to get nanocellulose. The spherical nanocellulose aerogel prepared with physical gel molding in inorganic salt solution and supercritical drying or freeze drying had larger effective adsorption areas. Its internal network skeleton was made up of cellulose ? nanofibers, so it had more uniform pore structure. After amino modification treatment, the basic structure and adsorption properties of the spherical nanocellulose aerogel didn't change much and still accorded with the first class?characteristic of gas adsorption isotherm.The main conclusion of this paper was as follows.?1? Nanocellulose with sulfuric acid hydrolysis method still remained cellulose? structure. Its crystallinity index increased but the thermal stability decreased. The surface of nanocellulose could occur esterification reaction. The microstructure of nanocellulose showed a short rod-like structure and its fiber diameter mainly distributed in 20-30 nm.?2? The concentration of nano cellulose suspension affect ed the strength of wet gel and its density of internal porosity. When the concentration of nano cellulose suspension was at the same concentration, the higher the concentration of CaCl₂ salt solution, the more obvious the effect of gel molding. For the same concentration of CaCl₂ salt solution, the higher the concentration of nano cellulose suspension, the easier to form a complete structure of spherical nanocellulose wet gel. When the concentration of nanocellulose suspension was 1.5%-3.5% and the concentration of CaCl₂ salt solution was 0.25 mol/L?1.5%? and 0.20 mol/L?2.5% and 3.5%?, the result of gel molding was the best.?3? With the increase of supercritical CO₂ drying time and drying temperature, the drying contractibility rate of different concentrations of ethanol gel?1.5%, 2.5% and 3.5%? firstly decreased and then remained at a certain level. With the increase of drying pressure, the drying contractibility rate first decreased sharply and then the descend range slowed down gradually. The best drying time for supercritical drying was 90min?1.5%? and 120min?2.5% and 3.5%?. The best drying temperature was 45?. The best drying pressure was 12.0MPa?1.5%? and 11.0MPa?2.5% and 3.5%?. The drying contractibility rate of freeze drying decreased gradually with the increase of the concentration of nano cellulose and this rule was consistent with the contractibility rate measured by supercritical CO₂ drying.?4? The N₂ adsorption isotherm of different concentrations of aerogel with supercritical drying and freeze drying method accorded with the first class ?characteristic of gas adsorption isotherm, and the main adsorption was multimolecular layer. The adsorption hysteresis loop was close to hysteresis loop type ?, which showed that aerogel was rich in mesoporous structure.?5? Aerogel with supercritical drying method was a kind of porous material which mainly existed mesoporous structure, and also had a small amount of microporous structure. While aerogel with freeze drying method was a kind of porous material which mainly existed mesoporous structure. At various concentrations, the internal aperture of aerogel with freeze drying method was less than the aerogel with supercritical drying method on the whole. In the experimental range, the specific surface area, N₂ adsorption capacity and pore volume of 2.5% aerogel?supercritical drying? and 3.5% aerogel?freeze drying? were the largest.?6? Amination modification results show ed that with the increase of modification time and modification temperature, the nitrogen content of composite aerogel firstly increased and then decreased. But with the increase in the amount of modifier gradually, the nitrogen content increased gradually as well. After graft modification, the basic structure and adsorption properties of composite aerogel didn't change much and the N₂ adsorption isotherm also accorded with the first class?characteristic of gas adsorption isotherm. The pore size distribution of composite aerogel had changed, but it still mainly existed mesoporous structure. Compared with unmodified aerogel, the specific surface area of composite aerogel decreased, but the pore volume increased.?7? SEM observation analysis show ed that the interior of modified and unmodified aerogel with supercritical drying method was a three-dimensional network structure made of nanofibers connect ing together. But the interior of aerogel with freeze drying method showed a laminated structure.