Study On The Changes Of Lignin Characteristics During Hot Pressing Of Molded Pulp

As a new type of plant fiber material forming technology that can be degraded,shaped and self glued,the technology of molded pulp has been paid more and more attention.Although a series of progress has been made in the study of the mechanism of the bonding strength of the fiber,it is still not perfect.In order to further confirm and improve the mechanism of the contribution of lignin components to the fiber-bonding strength of the molded fiber materials in plant fiber raw materials,this study first used chemical pulp(CP)with low content of lignin and high yield pulp(HYP)as the research object,through the comparison of the fiber-bonding strengths of the molded fiber materials produced by two objects under different working conditions,conforming the differences in the tensile strength of the molded materials produced by two raw materials from an engineering point of view.Based on the theory of non-adhesive gluing of wood based panel,analyzed the possibility of the positive influence of lignin on fiber-bonding strength under molded pulp working condition.On this basis,with the help of the research methods of classical lignin structure of the plant fiber chemistry,this paper studied the structure of lignin extracted from HYP by typical Bjorkman method before and after molding-hot-pressing process using relative gel permeation chromatography(GPC),Fourier transform infrared(FT-IR),two-dimensional heteronuclear signal quantum coherence(2D-HSQC)spectra and quantitative 31P-NMR and so on.At the same time,the Z-direction migration behavior of lignin in the hot-pressing process of molded fiber materials was studied by measuring the content of Z-direction lignin,XPS analysis,FT-IR analysis,contact angle measurement,and molecular weight analysis.The study was followed by some results.a)Under the background of hot-press drying,when the temperature was above 110℃,the tensile strength of HYP molded materials increased rapidly with the increase of drying temperature.At 190℃,the tensile strength was even higher than that of the chemical pulp of the same raw material,indicating that lignin had contributed positively to the fiber bonding strength during the molded pulp process.b)The molecular weight of lignin increased from 5216 g/mol to 6132 g/mol before and after hot pressing of molded HYP,indicating that lignin had polymerized during hot pressing.FT-IR analysis showed that the decrease of non-conjugated carbonyl groups and the increase of conjugated carbonyl groups in lignin which indicated that the breakage and formation of lignin chemical bonds coexisted in the hot pressing process.2D-HSQC analysis confirmed that the resinol structure(B)from HYP-MWL to MPB-MWL decreased from 8.66/1 OOAr(aromatic rings(Ar)as internal standard)to 6.68/100Ar,while the phenylcoumarans structure(C)increased from 15.45/100Ar to 20.88/100Ar.31P-NMR quantitative results showed that the structures of aliphatic hydroxyl groups,G type phenolic hydroxyl groups,and p-coumarates in MPB-MWL were lower than those in HYP-MWL.2D-HSQC and 31P-NMR analysis confirmed that depolymerization and polymerization coexisted,which were a pair of competitive reactions,but the extent of polymerization was greater than depolymerization.c)The lignin migrated along Z-direction in HYP molded fiber materials.The content of Klason lignin(30.67,34.82%)on the upper and lower surface of 190℃ molded pulp board was higher than that on 110℃ molded pulp board(30.13,31.36%).On the 190℃ molded pulp board,the O/C of the upper and lower surface layers was 0.42 and 0.36,respectively.The O/C on the upper and lower surface layers of the 110℃ molded pulp board were 0.50 and 0.48,respectively.The hot pressing conditions with higher temperature resulted in the decrease of O/C on surface layer.This value is closer to the theoretical value of lignin O/C 0.33.The aromatic ring stretching vibration peaks of lignin were detected at 110℃ molded pulp board and 190℃ molded pulp board.The corresponding displacement was 1423 cm-1,1510 cm-1and 1600 cm-1,and the peak intensity at the190℃ molded pulp board increased obviously.The contact angles of upper and lower surface of 190℃ molded pulp board are higher than that of 110 ℃ molded pulp board,and the improvement of the contact angle of the 190℃ molded pulp board surface is especially obvious.The weight average molecular weight(Mw=6132 g/mol)of MWL of 190℃ molded pulp board was significantly lower than that of 110℃molded pulp board(Mw=12258 g/mol).Depolymerization of lignin leaded to the decrease of molecular weight,which was conducive to the occurrence of lignin migration.Fiber screening confirmed that the degree of fiber separation on the lower surface of MPB was higher than that on the upper surface,which was an important reason for increasing lignin content on the lower surface of MPB.The completion of this study revealed for the first time the change rule of the lignin structure and Z-direction migration behavior during the process of preparing molded fiber materials with HYP as raw material.It provided a theoretical basis for the strength formation mechanism of molded fiber materials prepared from raw materials with high content of lignin represented by HYP,and laid the foundation for gradually building the formation theory of fiber bonding strength of molded fiber materials using the technology of molded pulp.