Nano-modification Of Lignin Phenolic Resin Composite Adhesives And Their Properties

As the rapid development of China's wood processing industry,the demand for wood adhesives has been increased.Phenolic resin is currently one of the most common wood adhesives in China,occupying a large share of the wood adhesive market.However,the main raw materials of phenolic resin adhesives,phenol and formaldehyde,are both refined from petroleum products.Phenol has a strong corrosive effect on the skin and mucous membranes.Formaldehyde is a potential 2-A carcinogen.In view of the increasing depletion of petroleum resources and the increasing emphasis on people's health,the research of wood adhesives based on natural renewable materials has become research hotspot.In this thesis,in order to synthesize a type of green and renewable lignin phenolic resin composite adhesives?LPF?,nanolignin was firstly prepared by acid precipitation,and the structure and reactivity of lignin before and after nano-modification were investigated;then,nanolignin was used as a raw material instead of phenol to prepare nanolignin phenolic resin composite adhesives?NLPF?,the preparation process was optimized,and the performances of NLPF were characterized;finally,in order to solve the poor thermal stability of the LPF,inorganic nano-oxide particles of the LPF was investigated to improve the characteristics of thermal stability,which might lay a foundation for the industrial production and large-scale utilization of LPF resins.Nanolignin was prepared by the acid precipitation method and the preparation process was optimized.When the concentration of the alkali lignin/glycol solution was 3 wt%,the concentration of hydrochloric acid was 0.025 mol/L,the volume of hydrochloric acid was 5wt%,and the addition rate of hydrochloric acid was dropped at 4 d/min,the particle size of nanolignin would reach the minimum,which was at 343 nm.The structure and reactivity of the nanolignin and alkali lignin were characterized by FTIR,BET,Zeta potential and GPC test,etc.The results showed that the structure of the two lignins before and after nano-modification were basically the same,and there were more-OH functional groups in nanolignin than in alkali lignin;the specific surface area and pore volumes of nanolignin were5.38 m²/g and 0.0092 cm³/g,respectively,which were about 10 times and 11 times of alkali lignin,and the porosity of nanolignin?6.91 nm?was smaller than that of alkali lignin?8.51nm?;the Zeta potential of nanolignin was-49.50 mV,which was much lower than alkali lignin?-17.80 mV?;and the number-average molecular weight and weight-average molecular weight of the nanolignin are 907 g/mol and 1241 g/mol,respectively,which are smaller than the alkali lignin's 1609 g/mol and 2706 g/mol.Much more-OH based structures,larger specific surface area,denser and more complex pore structures in nanolignin than alkali lignin,and lower Zeta potential and molecular weight,which demonstrated that nanolignin was more reactive than ordinary alkali lignin,and could be used to substitute phenol to react with formaldehyde to prepare phenolic resin.NLPF resin was prepared by replacing partial phenol with nanolignin,and the preparation conditions were optimized,when the material ratio of formaldehyde to nanolignin/phenol was 1.8:1,the reaction time was 4 h,the reaction temperature was at 85?,the amount of NaOH was 10 wt%,and the urea amount was 4 wt%,the lowest free formaldehyde content of the prepared NLPF have.Then,nanolignin was used to substitute different proportions of phenol?0-60%?to synthesize NLPF,their physical properties,bonding strength,formaldehyde emission,and other properties were compared and studied.The results showed that when the substitution rate of nanolignin for phenol is less than 40%,all the indicators of the prepared NLPF resins adhesive met the national standards;when the substitution rate was over 40%,only the free formaldehyde could not meet national standard.The bonding strength of 40%NLPF resin?1.30±0.08 MPa?was 1.85 times of national level I plywood?0.7 MPa?;plywood produced by 40%NLPF had the formaldehyde emission of 0.40mg/L,which meet national E₀ level standards and it could be used in indoor and outdoor environment.Last but not least,the curing behavior and thermal stability of NLPF and LPF,whose substitution rate were the same,were investigated and compared.The results showed that the curing temperature of 40%NLPF was 145.4?,which was significantly lower than that of 40%LPF?186.2??.The reducing of curing temperature will lead to a reduction in hot pressing temperature and the costs of the manufacturing process of the artificial board.Thermal stability analysis showed that the introduction of more reactive nanolignin will promote a higher degree of polymerization of NLPF resin,which further increases the thermal stability of NLPF,resulting in more weight residues at high temperatures,with residual rate increasing from 39.99%?40%LPF?to 45.01%?40%NLPF?,at 800?.NanoAl₂O₃,nanoSiO₂ and nanoTiO₂ were selected as three kinds of nano-modifiers to modify and enhance the thermal stability of LPF resins.The results showed that the thermal stability of LPF resins were all increased significantly after modifying by three nano-modifiers,at 800?.Using 3%nanoSiO₂ as modifier,the weight residual rate of LPF resin could reach 64.42%;the weight residual ratio of LPF resin became 62.43%and 61.16%,when using 4%nanoTiO₂ and 2%nanoAl₂O₃ as modifiers,respectively.Compared with ordinary LPF resin?44.51%?,the weight residue ratios increased by 1.45,1.40,and 1.37 times,respectively,indicating that the thermal stability modification effect of the three nano-modifiers on LPF resin was nanoSiO₂>nanoTiO₂>nanoAl₂O_3.The nano-modifiers nearly had no effect on the physical properties such as viscosity,pH,and solid content of LPF resin,and some other performances such as the bonding strength and formaldehyde emission of the modified LPF resins still met national standards.