Synthesis Mechanism of an Environment-Friendly Sodium Lignosulfonate/Chitosan Medium-Density Fiberboard Adhesive and Response of Bonding Performance to Synthesis Mechanism

Abstract

Environment-friendly medium-density fiberboards (MDFs) prepared using sodium lignosulfonate/chitosan adhesives (L/C) show potential in environment-friendly wood-based panel application. However, the synthesis mechanism of this adhesive and the relationships between synthesis mechanism and bonding performance were not discussed in depth. Herein, the synthesis mechanism of L/C was explored in detail based on characterizations of L/C with different mass ratios of sodium lignosulfonate to chitosan by Fourier-transform infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction. For L/C with different mass ratios of sodium lignosulfonate to chitosan, the corresponding bonding performance was also determined based on characterizations of mechanical and dimensional performance of MDFs. Results showed a 3D network structure of L/C formed through the hydrogen linkages among hydroxyl groups in sodium lignosulfonate and hydroxyl and amino groups in chitosan, amide linkages resulted from reaction between carbonyl groups in sodium lignosulfonate and amino groups in chitosan, and sulfonamide linkages originated from reaction between sulfonic groups in sodium lignosulfonate and amino groups in chitosan. The mechanical performance of MDF was closely related to the 3D network and amino groups of L/C, while the dimensional performance of MDF was negatively affected by sodium lignosulfonate. The MDFs with 1:3 and 1:2 mass ratios of sodium lignosulfonate to chitosan showed superior mechanical properties and comparable dimensional performance with a commercial panel.

Effect of sodium lignosulfonate on bonding strength and chemical structure of a lignosulfonate/chitosan-glutaraldehyde medium-density fiberboard adhesive

“…Compared with the FTIR spectrum of C-G, a new peak appeared in the FTIR spectra of L/C-G at 2967 cm −1 with the gradual increase in the mass ratio of sodium lignosulfonate to chitosan, corresponding to the CH stretching vibration in sodium lignosulfonate. At the same time, the peak at 2931 cm −1 corresponds to the increase in CH stretching vibration, and the peak at 2866 cm −1 corresponds to the decrease in CH stretching vibration, consistent with the change trend in the FTIR spectra of sodium lignosulfonate/chitosan adhesives [14]. The peak at 1552 cm −1 in the FTIR spectrum of C-G corresponds to the overlapping of C-N in amide II band in chitosan and C = C resulting from the aldol reaction of glutaraldehyde [20].…”

“…These results show that the effect of sodium lignosulfonate in L/C-G on the mechanical properties of MDFs was not completely the same as that in L/C. Moreover, for L/C, the 24h TS grew consistently with the increase in the mass ratio of sodium lignosulfonate to chitosan [14]. Similar to L/C-G, the 24h TS first decreased and then increased with the increase in the mass ratio of sodium lignosulfonate to chitosan.…”

“…The peak at 1552 cm −1 in the FTIR spectrum of C-G corresponds to the overlapping of C-N in amide II band in chitosan and C = C resulting from the aldol reaction of glutaraldehyde [20]. This peak gradually shifted to higher wavenumbers with the increase in the mass ratio of sodium lignosulfonate to chitosan in L/C-G and finally moved to 1572 cm −1 in the FTIR spectrum of L/C-G 5, because this peak was finally overlapped with the peak of stretching vibration of aromatic ring in sodium lignosulfonate [14]. Figures 2 and 3 show that the intensities and positions of some peaks changed simply owing to the improvement of sodium lignosulfonate content in L/C-G, while the intensities and positions of the other peaks changed owing to the reaction between the growing sodium lignosulfonate and chitosan or glutaraldehyde.…”

“…Notably, in sodium lignosulfonate/chitosan adhesives, the mechanical properties immediately decreased when the mass ratio of sodium lignosulfonate to chitosan was larger than 1:3 [14]. However, for L/C-G, the mechanical properties did not fall immediately when the mass ratio of sodium lignosulfonate to chitosan was in excess of 1:3.…”

“…For the FTIR spectrum of L/C-G with a mass ratio of sodium lignosulfonate to chitosan of 0, namely, for the FTIR spectrum of C-G, the main peaks were assigned as follows [7]: 3214 cm −1 (overlapping of O-H stretching vibration and N-H stretching vibration in original chitosan caused by glutaraldehyde crosslinking), 2931 cm −1 and 2866 cm −1 (C-H symmetric stretching vibration), 1708 cm −1 (C = O stretching vibration), 1652 cm −1 (stretching vibration of C = N resulting from the reaction between the amino groups in chitosan and aldehyde groups in glutaraldehyde), 1552 cm −1 (overlapping of C-N in amide II band in chitosan and C = C resulting from the aldol reaction of glutaraldehyde), 1255 cm −1 (bending vibration of NH in amide III band in chitosan), 1030 cm −1 (C-O-C stretching vibrations), and 892 cm −1 (characteristic peak of β-1,4-glycosidic bond). For the FTIR spectrum of sodium lignosulfonate, the characteristic peaks were assigned as follows: 3259 cm −1 (-OH stretching vibrations), 2967 cm −1 and 2931 cm −1 (-CH symmetric stretching vibrations), 1585 cm −1 and 1413 cm −1 (stretching vibrations of aromatic skeleton), 1119 cm −1 , 878 cm −1 , and 770 cm −1 (bending vibrations of guaiacyl unit), 1046 cm −1 (S = O symmetric stretching of the -SO 3 groups), and 973 cm −1 (out-plane bending vibrations of aromatic skeleton) [14].…”