Preparation Of All-cellulose Composites Based On Cellulose Dissolution Process Control

Natural cellulose is an inexhaustible and degradable polymer material.The rich hydrogen-bonds network structure in cellulose endows it with high axial stiffness,making it a promising candidate for applications in high-strength composites.However,the compatibility between cellulose and polymer matrix hinders the enhancement of cellulose in composite.In recent years,more and more attention has been paid to the all-cellulose?nano?composites?ACCs/ACNCs?.However,cellulose can not be processed using melting method,The modelling process of cellulose requires dissolution in certain solvents and then casting.The current techniques reported in literature are limited by their dissolution drawbacks,including complicated process,slow rate,limited ability and harsh conditions,which further jeopardize the industrial manufacture of ACCs/ACNCs.However,the new tetra-butylammonium hydroxide/dimethyl sulfoxide?TBAH/DMSO?solvent that developed earlier in our research group can significantly improve the dissolution rate and efficiency of cellulose.Moreover,the mechanical properties of ACCs/ACNCs are influenced by many factors.It is thus necessary to optimize the molding parameters of the regenerated cellulose film to obtain ACCs/ACNCs with excellent performances.In addition,nanocellulose has many preeminent characteristics and properties.So the preparation of ACNCs with nanoscale cellulose as filler is of significant research importance.Therefore,this thesis,takes nature cellulose as raw material and TBAH/DMSO aqueous solution as solvent,aiming at improving the mechanical properties of ACNCs.A series of research work are carried out to optimize the modelling technique of the regenerated cellulose film.Including the following:?1?The rice straw cellulose,cotton pulp,bamboo pulp and wood pulp are dissolved in TBAH/DMSO solvent,and the regenerated cellulose film was prepared by casting method,and the effects of different natural cellulose materials on the mechanical properties of the regenerated cellulose film were explored.It is found that the crystallinity of the films is mainly affected by the components of cellulose raw materials and forming process,which is not related to the crystalline structure of the raw materials,and the higher hemicellulose,lignin and ash content in natural cellulose will reduce the crystallization of the films.The mechanical properties of regenerated cellulose film were affected by many factors such as the degree of polymerization and?-cellulose content of the raw materials and the crystallinity of the films.It was found that the regenerated cellulose film prepared from cotton pulp cellulose has the best mechanical properties?137.05 MPa?.?2?The regenerated cellulose films were prepared by casting cellulose/TBAH/DMSO solution.The effects of gelation time on the structure and mechanical properties of regenerated cellulose films were studied.The mechanical properties test showed that the regenerated cellulose films with the gelation time of 3 h had the best mechanical properties?137.13 MPa?.Rheological tests,XRD and FTIR studies showed that the gel network structure of the physically crosslinked cellulose IV_II was formed during the gelation process.After which,a layer of cellulose II was regenerated in distilled water.The cellulose film produced regenerated directly in distilled water is with cellulose II crystalline,and the regenerated cellulose films,which coexist with cellulose II,cellulose IV_II and amorphous cellulose,were prepared after the gelation process.According to the characterization analysis of SEM and SAXS,it is found that when the cellulose solution is regenerated directly in the distilled water,rapid regeneration process occurs at this moment,causes defects that cannot be removed in the films,which reduce the mechanical properties of the films.After the gelation process,mild regeneration process,a compact,uniform,less defective and highly mechanical cellulose film was formed.However,the longer gelation time and the excessive production of cellulose IV_II can not build the cellulose II with strong and stable network structure,which made the mechanical properties of the regenerated cellulose film reduce.?3?In-situ self enhancement ACCs were prepared by regulating the multilevel structure of cellulose.The effect of the dissolution time of cotton pulp cellulose in the TBAH/DMSO solvent on the structure and mechanical properties of ACCs was studied.The results of mechanical properties test show that with the increase of dissolution time,the tensile strength of ACCs increases first and then decreases,and the optimum tensile strength?166.89 MPa?is obtained when the 5 wt%cotton pulp cellulose in TBAH/DMSO at room temperature dissolves 20 min.The results of XRD and SEM characterization showed that the prepared ACCs was composed of cellulose I,cellulose II,cellulose IV_II and amorphous.With the prolongation of the dissolution time,the cellulose I decreased gradually,and the cellulose II and cellulose IV_II increased gradually.When the dissolution time is increased,more cellulose is dissolved and regenerated to form the matrix,and the interfacial bonding between the undissolved reinforced cellulose and the matrix is enhanced and the tensile strength of the composite film increases.However,the undissolved cellulose I,which is enhanced in the film,is reduced,the enhancement effect is weakened,the tensile strength of the film is reduced.?4?The cellulose nanocrystals?CNC?with average diameter of 20.87±9.5 nm and average length of 194.61±101.75 nm were prepared by sulfuric acid hydrolysis of cotton pulp cellulose.The determined the amount of CNC/DMSO dispersions were added into the cellulose solution,and the ACNCs were prepared by pouring method.The effects of different CNC content on the mechanical properties of ACNCs were studied.It is found that when the content of CNC is less than 10 wt%,the mechanical properties of ACNCs can be significantly increased?139.81 MPa to 199 MPa?,but when the addition of CNC increases to 15,20 wt%,CNC is reaggreation,and the phase separation occurs,which reduces the tensile strength of the obtained ACNCs.