Research On The Preparation And Modification Of Cellulose Nanocrystals And Its Application Of Reinforced Composites

As a novel biomass-based polymer material, cellulose nanocrystals (CNCs) is becoming one of the current research frontiers in many areas. In order to explore its application in these areas, we need to make a systematic study on its properties. Combined exceptional performance of water-borne polyurethane (WBPU) and polycaprolactone (PCL) in many fields, we composite cellulose nanocrystals with these two kind of polymers which can broaden the application areas of cellulose nanocrystals. Preparation and modification methods of cellulose nanocrystals are studied in this paper and we raised its potential applications by researching properties of its reinforced composites.The cellulose nanocrystals were prepared by acid hydrolysis of cellulose microcrystalline (MCC). We used single factor experiments-orthogonal experiment-single factor experiment method to optimize CNCs acid hydrolysis process conditions. The finally optimized conditions were:MCC:H2SO4=1:8.75g/mL, H2SO4concentration was63.5wt%, temperature was45℃, and acid hydrolysis time was110min, CNCs yield was37.09%in this condition. TEM images showed that the CNCs was in rod form with lengths of70-150nm and diameters of10-20nm in aqueous suspension, respectively. CNCs dispersed stably in aqueous dispersion with blue transparent, long-term placement at room temperature which is not available in MCC. XRD and TGA results show that compared with the MCC, the degree of crystallinity of CNCs increased, but its thermal stability decreased.As a chlorine-free polymer and coating agent, WBPU has a better anti-felting effect for wool fabrics. However, the higher concentration of WBPU was usually employed in practice in order to form a durable film and cover the scales of the surface of wool fibers. So that the WBPU concentration is high and the handle of finished fabric is hard, which limits its application. After the two aqueous suspensions were mixed homogeneously, cellulose nanocrystal reinforced polyurethane composite (nanocomposite) films were prepared and evaluated by means of transmission electron microscopy, scanning electron microscopy and dynamic mechanical analysis. Then the nanocrystal films were applied onto surfaces of wools by a pad-dry-cure process with nanocomposites containing different cellulose nanocrystal contents.DMA results showed that storage modulus and loss modulus of WBPU/CNCs composites films increased with CNCs content increasing. And because the CNs can improve the microphase separation between soft and hard segments in WBPU, Tg decreased with increasing of CNs content. Besides that, we studied viscoelastic properties of WBPU/CNCs composites films. In frequency sweep tests, the E1and E" were recorded and modeled by the Power Law equation. CNCs precipitation significantly increased the steady state viscous properties of cellulose nanocomposites films but decreased the dynamic viscoelastic properties of nanocomposites and frequency sensitivity. In the creep-recovery tests, the data were fitted to Burger’s model. The E2value and η1value in Burger’s model were increased by incorporating CNs. To the contrary, however, the relaxation time t2was decreased. It means CNs can improve WBPU’s creep resistance property. This result showed interactions existed between CNCs and WBPU once again. In additional, we also found that the frequency sensitivity and creep-Replies may exist between certain relationship.The anti-felting results indicated that with increasing cellulose nanocrystal content from0to1.0wt%, the area-shrinking rate of the treated wool fabrics was decreased from5.24%to0.70%, and the tensile strength of the fabric was increased by14.95%and decreased about45%use of waterborne polyurethane. The pure WBPU of concentration in50g/L covered surfaces of the fibers reveal a thin layer covering the scales of wool fibers, which improves the smoothness of the fibers. The WBPU coating the cellulose nanocrystals provided a relatively intact layer of coating covering the surfaces of the wool fibers, higher amounts nanoparticles absorbed on the surface of wool and, consequently, the ups and downs of the surface of wool covered completely which could further decrease Directional Frictional Effect (DEF) of wool fabrics and improve antifelting effect.Dispersed in organic solvent poorly is one of main problems of CNCs which restrict its application. So, we modified CNCs using a reverse micelle method and the chemical surface modification method in order to improve its dispersibility in organic solvents. At the same time, because PCL has poor mechanical properties and degradation for a long time, it can’t meet the bone defect repair and fracture fixation material mechanical requirements, we blended modified CNCs and PCL to form nano-composite films and studied these films’ performance.Reverse micelle has a polar "pool", we make use of this "pool" to solubilize concentrated CNCs aqueous dispersion in chloroform so that irreversible agglomeration was avoided which was caused by freeze-drying process. Firstly, the OP-7/n-octanol/chloroform reversed micelle system was chose by investigating the influences of the type and concentration of surfactants cosurfactant on the water solubilization of the reversed micelle system. The concentration of OP-7was0.02g/mL, octanol was cosurfactant and its mass ratio with OP-7was3:2, the solubilization Wo was237.36. Then, according to particle size test, we proposed CNCs in OP-7/n-octanol/chloroform reverse micelles state model:rod CNCs wrapped in rod reverse micelles. Finally, the XRD and mechanical test results show that with the increasing of RCNCs content, the degree of crystallinity, melting temperature of RCNCs/PCL composite material were decreased; With the increasing of the RCNCs, the degradation rate and the hydrophilic of RCNCs/PCL composite material increased.But, due to the limitation of the reversed micelle system’s solubilization, the adding amount of modified RCNCs was very small, no more than1.08%, so the chemical-in situ solvent exchange method was used to modify CNCs, choosing octadecyl isocyanate as chemical modification reagent. The CNCs chemical modification process was studied through the calculation of the graft rate of the product. By means of infrared spectrum analysis and static method, the properties of C18-CNCs were characterized. The results showed that C18-CNCs has good dispersion in chloroform. C18-CNCs/PCL composite material was prepared by compounding C18-CNCs and PCL. By means of Fourier transform infrared spectrum, X-ray diffraction, DSC test analysis, scanning electron microscopy (SEM) analysis, mechanical properties test and hydrophilic test, the properties of PCL/C18-CNCs composite material were characterized, and the content of C18-CNCs influence on PCL/C18-CNCs composite material performanee was also investigated. The results showed that with increasing of C18-CNCs, the tensile strength and the hydrophilic of PCL/C18-CNCs composite material increased, but the degree of crystallinity, glass transition temperature, melting temperature of PCL/C18-CNCs composite material first dropped down, when the content of C18-CNCs was6.0%, all reached the minimum value, with the content of C18-CNCs going on increasing, the degree of crystallinity, melting temperature of C18-CNCs/PCL composite material raised up. Scanning electron microscopy (SEM) analysis showed that, when the content of C18-CNCs was below6.0%, it can be evenly distributed on the surface of C18-CNCs/PCL composite membrane, when the content of C18-CNCs was more than6.0%, it gathered together on surface of C18-CNCs/PCL composite membrane.