Mechanism Of Bamboo Cellulose Nanocrystal Prepared With Extremely Low Acid Hydrolysis And Its Redispersibility Through Alcohol Precipitation Of Hydrolyzed Sugars

Nowadays,energy and environment are the two hot issues of common concern.Cellulose is one of the most abundant,biodegradable and sustainable natural polymers on the earth.Cellulose nanocrystal(CNC)is cellulose crystallites with at least one dimension less than 100nm.CNC not only has the inherent characteristics of natural cellulose,but also exhibits appealing properties such as exceptional mechanical properties,high specific surface area,high aspect ratio and low density.Due to these features,CNC has good prospects for applications.But CNC full potential is yet to be fulfilled yet due to bottlenecks in the production:the recycling the strong acid catalyst is difficult,and purification steps are cumbersome,particularly with lengthy dialysis.Furthermore,the redispersion of CNC is also a formidable challenge.Owing to the large surface area and abundant surface hydroxyl groups on the surface of CNC,the aggregation phenomena occurred through the hydrogen bonds between the CNC particles during the drying process,which cannot be redispersed to its original nano-size by physical or chemical methods.In this work,advanced production process for CNC with pollution-free and preparation process of water-redispersible CNC are investigated.The deformation of bleached kraft bamboo pulp(BKBP)cellulose and its influence on the extremely low acid(ELA)hydrolysis are also studied.In the first part,CNC was produced from bleached bamboo kraft pulp(BKBP)via ELA hydrolysis process.It was found that the CNC yield was strongly affected by the operating pressure,the concentration of hydrochloric acid(HCl),as well as temperature.Among the studied variables that affected the CNC yields,the operating pressure was the most effective parameter.This was because that much more intermolecular and intermolecular hydrogen bond was formed when the BKBP fiber was pressed.And some small crystal region combined,thus,the yield of CNC was increased along with the increase of operating pressure.Rod-like CNC was obtained with width between 50 nm and 90 nm and length between 500 nm and 700 nm.The fourier transform-infrared spectroscopy(FTIR)and X-ray diffraction(XRD)measurements show that the resulted CNC maintained cellulose I structure.Compared with conventional CNC obtained by strong sulfuric acid(H2SO4)hydrolysis,the CNC prepared by ELA hydrolysis process exhibited compact and ordered crystalline structure,and higher crystal size,aspect ratio and thermal stability.BKBP fiber was quickly broken during extremely low acid hydrolysis process,resulting in shorter fiber fragments during the initial 40 min of hydrolysis.Polarized light microscopy images and SEM images showed a relationship between dislocation and fiber breaking.In combination,this data strongly implicated dislocation sites as the first location of fiber breaking during ELA hydrolysis.The value of CNC’s CI was less than 100%,which may be due to partial crystalline region damaged during ELA hydrolysis.The elementary fibrils in bamboo CNC may be observed by spherical aberration corrected transmission electron microscope(AC-TEM).The crystalline region in elementary fibrils were made up of molecular chains that diverge from the fibril at various positions throughout its length.In the second part,we proved the possibility to use ELA hydrolysis combined with ultrasonic pretreatment to prepare CNC with smaller size from BKBP.The results indicate the successful preparation of rod-like CNC with length of 50-250nm and hollow-type spherical CNC with the diameter of 50nm.The resulting CNC had high crystallinity(77.6%).Thermogravimetric analysis reveals that the resulting CNC had good thermal stability.The FT-IR and XRD measurements show that the resulting CNC had cellulosic structure(cellulose I type).The hollow-type spherical CNC may be created by a self-assembly process from CNC.The combination of ultrasonic treatment and ELA hydrolysis process may provide a new,efficient,and environmentally benign way to produce CNC from BKBP.In the third part,a novel approach for the preparation of water-redispersion CNC was demonstrated.The CNC was isolated from BKBP by ELA hydrolysis process.The composition of spent liquor concluded:oligosaccharide,glucose,xylose,acetic acid,formic and furfural.The hydrolyzed sugars had an oval-shaped structure and was non-crystalline structure.When alcohol solvent was added to filtrate,which contained CNC and spent liquor,the hydrolyzed sugars will deposit on surface of CNC,the redispersion of CNC coated with hydrolyzed sugars closely resembles the never dried sample.The redispersion concentrations and moisture content in the dried CNC were all found to influence redispersibility.Mild sonication was generally enough to recover measured particle sizes identical to those in the never-dried CNC.A threshold moisture content of 20 wt%was found,above which dried CNC was fully dispersible in water.The properties of never-dried CNC and redispersed CNC were also investigated.The experimental data indicate that the chemical composition,crystal structure and thermal stability of two samples were not changed.When alcohol solvent wss added to suspension,which contained sulfated CNC(s-CNC)and glucose,the glucose will deposit on surface of s-CNC,the redispersion of s-CNC coated with glucose closely resembles the never dried sample.The properties of never-dried s-CNC and redispersed s-CNC were also investigated.The experimental data indicate that the chemical composition,crystal structure and thermal stability of two samples were not changed.Above all,a simple and environmentally friendly preparation method and mechanism of CNC was first investigated.And then the redispersibility of CNC coated with hydrolyzed sugars based on alcohol precipitation process was studied.the relationship between fiber dislocations and BKBP fiber breaking point during extremely low acid hydrolysis of BKBP was also discussed.The acid concentration of the resulting CNC was reduced from 64%to 0.1%.This preparation method was suit for mass product with lower pollution.The dislocation sites in BKBP fiber were firstly degraded during extremely low acid hydrolysis process.The new morphology of CNC was found.The present work also explored an alternative that prevents the irreversible agglomeration of CNCs during their oven-drying through alcohol precipitation of hydrolyzed sugars.