The Preparation Of Cellulose Nanofibers And Study On The Crystal Form Transformation

Cellulose exists in the form of crystalline nanofibers in nature. There are five differentpolymorphs of crystalline cellulose (I, II, III, IV and X), of which cellulose II is the most stablestructure. So far, investigations dealing with CNFs have focused mainly on the cellulose Icrystal structure; very few studies have dealt with the production and characterization of CNFswith the cellulose II crystal form. The principal objective of this study was to explore thepreparation and understand the unique property of cellulose I and cellulose II nanofibers basedon the crystal form transformation technique; to explore the effect of ethylenediamine／alkalitreatment on the crystal form, morphology and performance of cellulose nanofibers; to look foran effective method of preparing cellulose nanofibers with a high aspect ratio based on biomassmaterial and high-performance nanocomposites.(1) The preparation of cellulose I nanofibers (CNF-I): First, ground wood powder waspurified by series of chemical treatments. The resulting purified pulp was passed once through agrinder to isolate the nanofibers, which showed a uniform diameter of approximately15nm, aswell as a high aspect ratio. Here we refer it as CNF-I. The CNF-I sheet exhibited a good tensileproperty, with a tensile strength of210MPa, a tensile modulus of11.8GPa and a fracture strainof7.5%. The CNF-I sheet showed a very low thermal expansion coefficient of9ppm/K.The preparation of cellulose II nanofibers (CNF-II): Well-dispersed CNF-II with high purityof92%and uniform width of15–40nm were isolated from wood and compared to CNF-I. First,ground wood powder was purified by series of chemical treatments. The resulting purified pulpwas treated with17.5wt%sodium hydroxide (NaOH) solution to mercerize the cellulose. Themercerized pulp was further mechanically nanofibrillated to isolate the nanofibers. X-raydiffraction patterns revealed that the purified pulp had been transformed into the cellulose IIcrystal structure after treatment with17.5wt%NaOH, and the cellulose II polymorph wasretained after nanofibrillation. The CNF-II sheet exhibited a decrease in Young’s modulus (8.6GPa) and an increase in fracture strain (13.6%) compared to the values for a cellulose Inanofiber sheet (11.8GPa and7.5%, respectively), which translated into improved toughness.The CNF-II sheet also showed a very low thermal expansion coefficient of15.9ppm/K.Thermogravimetric analysis indicated that the CNF-II sheet had better thermal stability than theCNF-I sheet, which was likely due to the stronger interaction of the hydroxyls in cellulose IIcrystal structure, as well as the higher purity of the CNF-II.(2) So far, there’s few studies dealt with the reactive behavior between cellulose nanofibersand amine, such as ethylenediamine (EDA). This study investigated the effect of EDA treatment on the crystal structure, morphology and performance of the CNF. The results showed that thecrystal form transformation (e.g.: from CNF-I to CNF-II, and from CNF-I to CNF-IIII) of thenanofibrillated cellulose occurred more easily when compared with the untreated cellulose, asthe frequency and duration of the treatment were both reduced. This may be due to the ultra-finenanostructure, as well as the high surface area of the CNF which make EDA molecules easier toenter the crystalline regions of the cellulose, prompting its crystalline transformation more likelyto occur. Moreover, an interdigitation and aggregation of the CNF occur in the conversion fromCNF-I to CNF-II polymorph, which provides a proof for the preparation of high-performanceCNF hydrogels. FE-SEM revealed that the CNF-II gel showed an entangled network stucture,resulting in an increase of the fiber width and increase in the mesh size. The CNF-II gelexhibited good tensile property, with a tensile strength of3.1MPa.(3) Preparation of CNF from the waste paper pulp and increase its value. The hornificationof the recycling waste paper makes mechanical fibrillation become more difficult. The methodsof isolating cellulose nanofibers from dry paper pulps (including waste corrugated paper, wasteprinting paper and dry bleaching kraft paper) were explored, and the effect of EDApre-treatment on the mechanical fibrillation of dry kraft paper pulp was studied. First, ultralongcellulose nano bers with extremely high aspect ratio were successfully manufactured fromwaste corrugated paper pulp and printing paper pulp through a series of chemical treatmentscombined with grinding, ultrasonication, and centrifugation. SEM images revealed that theprepared cellulose nanofibers exhibited a uniform width ranging from30to100nm. Thenanopaper produced by filtration and drying presented high tensile properties, with a tensilestrength of135MPa and a tensile modulus of6.67GPa, which was approximately10timeshigher than the untreated waste corrugated paper. The obtained nanopaper also exhibited hightransmittance of85.2%and low thermal expansion of16.2ppm/K. Compared to the wastecorrugated paper, the nanopaper prepared from the waste printed paper showed lower tensileproperty. The yield of the cellulose nanofibers prepared from the above chemical andmechanical (three step method: grinding, ultrasonication, and centrifugation) method was about23.5%because of the centrifugation step.In order to improve the mechanical fibrillation efficiency on dry paper, the effect of the EDApretreatment on the mechnical fibrillation of dry paper was explored. Dry bleached kraft paperwas selected as the raw material.75%EDA was used to swell the dry kraft paper, thennanofibers were obtained by the following mechanical treatment (two step method: gringing andhomogenization, without centrifugation). The results showed that the water retention value ofthe EDA pretreated paper was20%higher than the untreated paper, resulting in a reducedhornification of the paper and an increased efficiency and productivity of the nanofibers. Theobtained nanofibers showed a uniform distribution and a width of15-40nm. The resultingnanopaper exhibited a high tensile strength of195Mpa and a tensile modulus of10GPa. (4) Investigation on the composites of PVA/cellulose nanofibers using mixing method: CNFwere obtained by only one pass through the grinder treatment. CNF/PVA composite wasprepared by mixing CNF suspension and PVA aqueous solution and then dried. FE-SEMshowed that CNF and PVA mixed very uniformly, they entangled and interpenetrated with eachother and then three-dimensional network structure was formed. The coefficient of thermalexpansions (CTE) of CNF/PVA was decreased by30%compared to that of the pure PVA. Thetensile strength (110MPa) and the Young’s modulus (6.6GPa) was improved by50%compared to that of the pure PVA.Investigation on composite film of PVA/cellulose nanofiber and the effect of mercerizationusing impregnation method: Cellulose nanofiber (CNF) was obtained by only one pass throughthe grinder treatment. CNF film (CNF-I film) was prepared by filtration, and cellulose IInanofiber (CNF–II) film was prepared by further mercerization. PVA/CNF-I and PVA/CNF-IIcomposite films were obtained by impregnation method. FE-SEM observation showed that thePVA/CNF composite film showed good interfacial bonding between the CNF and PVA. Thetensile strength and Young’s modulus of PVA/CNF-I was greatly improved, which was abouttwice as high as the pure PVA. After mercerization, the elongation at break of PVA/CNF-II was70%higher than that of the PVA/CNF-I, corresponding to the higher toughness. The CTE ofPVA/CNF-I was decreased to20.5ppm/K, which was similar to the CTE of aluminum,corresponding to the better dimensional stability, which could broaden its processing andapplications field.