Carboxymethylation Modification Of Cellulose Nanofibril And Its Application In Paper Coatings

Cellulose nanofibril(CNF),as a natural biomass nanomaterial,has the characteristics of large aspect ratio and specific surface area,high hydrophilicity,good thickening,excellent dispersion stability,high Young’s modulus,high strength,and good biocompatibility,which makes it hold great potential in the field of water-based coatings,particularly paper coatings.However,a large number of hydroxyl groups was contained in the surface of CNF,which form the strong inter-and intra-molecular hydrogen bonds of cellulose,leading to the occurrence of agglomeration.Meanwhile,the agglomerated CNF was difficult to be re-dispersed by physical methods,which greatly limits the applications of CNF.Therefore,chemical modification of CNF to prevent agglomeration and increase its redispersibility has become a key technology to realize its functional application.In this work,based on the physical and chemical properties of CNF itself,CNF was chemically modified to obtain carboxymethylated cellulose nanofibrils(CMCNF)with different degrees of substitution.Subsequently,CMCNF was used as an additive to prepare paper coatings.Finally,the surface coating process was successfully performed to prepare coated paper,and the effect of CMCNF on the structure and performance of coated paper was systematically discussed.Firstly,the carboxymethylation modification of CNF was carried out by twice alkalization method to obtain CMCNF with different degrees of substitution.By discussing the amount of sodium hydroxide,alkalization temperature,alkalization time,chloroacetic acid dosage,etherification temperature and etherification time,a suitable preparation process for CMCNF was obtained.Based on suitable process conditions,the degree of substitution(DS)of the prepared CMCNF can reach 0.47.Zeta potential analysis showed that compared with unmodified CNF,the absolute value of the zeta potential of modified CMCNF was increased,indicating that the dispersion of CMCNF suspension after carboxymethylation modification was more stable.FT-IR analysis revealed that cellulose had the chemical reaction with chloroacetic acid,and the partial hydroxyl hydrogen on the cellulose molecules was successfully replaced by carboxymethyl.XRD analysis results showed that the crystallinity of CMCNF decreased obviously due to etherification substitution,but the crystal structure was not significantly changed.TG analysis results showed that the initial thermal decomposition temperature of CMCNF was lower than that of CNF,but it still had high thermal stability.Secondly,paper coatings were prepared by adding calcium carbonate and ultrafine kaolin as the main pigments,polyvinyl alcohol as the main adhesive,sodium hexametaphosphate as the dispersant,and CMCNF with different substitution degrees and additives.The influence of CMCNF on the rheological properties and water retention of paper coatings was highlighted.The steady shear rheological results revealed that each coating system exhibited non-Newtonian fluid behavior.With the increase of CMCNF substitution degree and content,the viscosity of papermaking coatings was improved.The dynamic rheological results showed that with the increase of angular frequency,each coating system first exhibited the properties of an elastic solid and then gradually tended to a viscous fluid.The paper coatings with higher substitution degree or higher content of CMCNF showed higher viscoelasticity macroscopically.Meanwhile,the water retention performance of paper coatings was greatly improved with the increase of CMCNF substitution degree.Specially,when the degree of substitution of CMCNF was 0.47,the water retention value was only 195.0 g/m2,which showed an increase of 74.43% in water retention properties for paper coatings.Finally,the paper coating containing CMCNF was coated on the base paper based on the surface coating method.Meanwhile,the micro morphology,tensile properties,folding resistance and barrier properties(waterproof,oil,and gas)of the coated paper was systematically studied.The results showed that the substitution degree and addition of CMCNF had an important influence on the relevant properties of coated paper.As the degree of substitution of CMCNF increased,the tensile strength,folding resistance,and barrier properties of coated paper were all improved.Particularly,compared with CMCNF(DS=0)coated paper,the air barrier properties of coated paper with CMCNF(DS=0.47)increased by 61.60%.Compared to coated paper without CMCNF,the air barrier performance of coated paper with 0.20 wt% CMCNF increased by 66.61%.