Nanocrystalline Cellulose Modified Melamine-formaldehyde Resin And Preparation Of Nanocomposites

Cellulose Nanofibers(CNF) has low density, high modulus, large aspect ratio, large surface area and other outstanding characteristics like thermal stability, electrical insulation properties, biodegradable and renewable features, which showed great potential in nanocomposites reinforced studies. Belonging to and sharing common characteristics with CNF, nanocrystalline cellulose(NCC) was widely used as enhanced green nanocomposites because of its simple and fast preparation process and formed hydrogen bonds by adjacent strong interaction, woving into a certain rigidity of three-dimensional mesh, which was able to be used to enhance the modified physical and chemical properties of synthetic resin or adhesive. At present, not only MF resin widely is used in surface decoration of wood-based panels and modification of wood and composite fields, but also MF resin can be used for tableware and household items etc. In order to further improve the utility value of MF resin and supply experimental and theoretical basis for its performance improvement and new uses. In this paper, microcrystalline cellulose(MCC) was utilized as raw material. The mechanical method and sulfuric acid hydrolysis were respectively adopted to get the microfibrillated cellulose(MFC) and NCC. Modern analysis techniques were also used to characterize their morphology as well as physical and chemical properties. NCC was used in the synthesis of crosslinked copolymer resin to modify melamine-formaldehyde resin(MF). The penetration of NCC modified MF in the wood cell walls and cell cavities was studied and the enhancement modification mechanism of NCC modified MF resins was also discussed. NCC modified MF resin reinforced nanocomposite films were prepared by resin impregnating method and the surface wettability, transmittance characteristics and tensile mechanical properties of the prepared films were also explored. The main conclusion of this paper is as follows.(1) MCC, whose amorphous area was damaged by acid catalysis, was stripped to get nano-single crystal fragment of NCC. NCC suspension liquid has a better dispersion and stability. Their transmittances differed at different concentrations, which have a better transmittance when the concentration was below 1.0wt%. Mechanical treatment and sulfuric acid hydrolysis both reduced its thermal stability, and under the action of strong acids, surface hydroxyl groups of cellulose can be substituted by nucleophile groups, which produced cellulose sulfate, but this esterification was weak.(2) NCC modified MF resin, reducing the content of free formaldehyde in the MF resin system and extending resin storage life. When the added content of NCC was 0.2% of solid resin, the shear strength of monogolian oak wood bonded by this MF resin could increase by more than 46%. DSC analysis showed that, first endothermic peak indicated the condensation reacting of hydroxyl methylmelamine. With increased NCC added volume, peak temperature dropped from 137℃ to 122℃. Second endothermic peak temperature appeared between 150℃ and 160℃, which was due to the heat absorbance of condensed water in the condensation action. The relative stiffness value of pure MF resin was 273 GPa, while 0.1% NCC was added, the stiffness value of modified MF resin(0.1NCC-MF) increased by 76%, to a value of 480 GPa。(3) With the increase in added content of NCC and glue volume, the penetrating depth of NCC modified MF adhesives on the surface of wood increased, which in radial direction was significantly larger than what of tangential direction. While NCC modified MF resin penetrated the cell wall, the infiltration volume of MF resin decreased slowly when the cell wall infiltration thickness increased. Roman spectroscopic analysis showed that O-H scaling peaks of cell wall hydroxyl within the interface disappeared, suggesting that cellulose hydroxyl exposed on the surface of the cell walls damaged, which may result in crosslinking reactions with free formaldehyde in MF resins or methylol melamine, generating ether bonds.(4) With the increase of impregnated volume of MF resin, the NCC/MF nanocomposite films had an increased thickness, decreased density and reduced thickness swelling rate, which was effected by strong hydrogen bonding interaction between NCC and MF resin resin matrix. Using the LW/AB method, the membrane surface free energy, Lifshitz-van der waals component and the acid-based component could be calculated, which decreased by the sequence of MFC membrane, NCC membrane and NCC/MF membrane. Compared with MFC, NCC was reacted with sulphuric acid hydrolysis, some cellulose chain on the surface dehydrating glucose unit, which resulted in the decrease in the wettability. Compared with pure MF resin film, light transmittance of NCC/MF nanocomposite films declined and was gradually decreased with reductions in MF resin.The decline in the light transmittance was related to the different index of refraction of light scattering of composite matrix and reinforcement phase of the composite. NCC has significantly enhanced impact on MF resin. When content of NCC was 72%. NCC/MF nanocomposite membrane had a tensile strength of 79 MPa, a fracture elongation rate of 1.92% and a Young modulus of 4.84 GPa, which respectively increased 5.1 times, 2.1 times and 0.95 times when compared to pure MF resin films.