Preparation And Modification Of Nanocellulose And Its Application In Waterborne Coatings

Nanocellulose has shown good application prospects in many fields due to its excellent performance and natural degradability.With the continuous improvement of human awareness of environmental protection,waterborne coatings are gradually replacing solvent-based coatings as an important part of the coatings market.However,waterborne coatings still have some problems such as low mechanical properties and poor storage stability,which brings new opportunities for the application of nanocellulose.At present,the research on the application of nanocellulose in waterborne coatings is mainly focused on cellulose nanocrystals(CNC),but rarely involves nanofibrillated cellulose(NFC).Although NFC not only has the advantages of CNC,but also has the characteristics of large specific surface area and high aspect ratio,NFC is prone to flocculation in aqueous media,which affects its application effect.Therefore,the application of NFC in waterborne coatings needs to be further studied.Waterborne polyurethane(WPU)and waterborne acrylic acid(WPAA)are commonly used as filmforming agents in waterborne coatings,which play a decisive role in the basic performance of the coating.In view of this,this paper introduced NFC or silylation-modified NFC into WPU or WPAA through physical blending,and prepared composite films through casting and drying.The effect of NFC addition and modification on the performance of WPU film or WPAA film were analyzed,and the strengthening mechanism of NFC on the films was preliminarily proposed.Finally,the improvement effect of NFC on the storage stability performance of waterborne coatings was studied.The main research contents and conclusions of the paper are as follows:(1)The effect of NFC addition on the physical properties of the composite films was investigated.NFC/WPU and NFC/WPAA composite films with NFC contents of 0.1%,0.3%,0.6%,0.9% and 1.2% were prepared by physically blending of NFC with WPU and WPAA,respectively,followed by casting and drying.The effect of NFC addition on the physical properties of the composite films was investigated.The results showed that when the NFC addition was 0.3 wt%,the tensile strength of NFC/WPU and NFC/WPAA composite films increased by 108.15% and 48.57%,respectively,and the elongation at break decreased by 17.27% and 9.75%,respectively,compared with the pure film without NFC.In addition,the addition of NFC reduced the water contact angle of the composite films.(2)Different silane coupling agents modified NFC and their effects on the properties of WPAA film were investigated.Three silane coupling agentsγ-aminopropyltriethoxysilane(KH550),γ-(2,3-epoxypropoxy)propyltrimethoxysilane(KH560),and γ-(methacryloyloxy)propyltrimethoxysilane(KH570)were used to modify NFC respectively,and the silylated samples NFC-550,NFC-560,and NFC-570 were obtained.Then the composite films WPAA-550,WPAA-560 and WPAA-570 were obtained by physical blending of the modified NFC with WPAA followed by casting method.The effects of different silane coupling agent modifications on the performance of NFC and WPAA films were investigated.The results showed that compared with other samples,NFC-570 has maller particle size distribution,the highest absolute value of zeta potential and larger yield stress value,and its suspension stability in aqueous media is the best.However,the composite film WPAA-560 has the best mechanical properties and hydrophobicity.Compared to the pure WPAA film,the tensile strength of the composite film with the addition of 0.3 wt%NFC-560 increased by 131.9%,and the water contact angle increased from85.2°to 101.6°.(3)The difference between NFC and HEC in improving the storage stability of waterborne coatings at room temperature and 45 ℃ was discussed,and the improvement mechanism was analyzed.NFC was prepared by mechanical dissociation method and added to waterborne coatings,and compared with hydroxyethyl cellulose(HEC),a common thickener for coatings.The results showed that adding of 0.33 wt% NFC(based on waterborne coatings)could make the waterborne coatings have good storage stability at room temperature,but adding of 1.33 wt% HEC(based on waterborne coatings)could have the same improvement effect.In addition,HEC improves the storage stability of coatings by increasing the viscosity of coatings.The thickening effect is greatly influenced by temperature.When the ambient temperature increases,its improvement effect will be significantly reduced.NFC improves the suspension of coatings through network entanglement of the fibers,and its improvement effect is not affected by temperature,so it could still make the coatings have good storage stability at higher temperatures.