Improvement Of High Temperature Sulfonation Of Wheat Straw Alkali Lignin And Its Precipitation Mechanism On Fiber Surface

China is a big country with annual huge yield of wheat straw alkali lignin. The usage ofthese byproducts in paper industry mostly is of low-value, and to find a way for theirhigh-value ulilization is very meaningful. The traditional high temperature sophonation ofalkali lignin was difficult to expand and promote due to its own defects. Meanwhile, therewere significant precipitation of lignin during pulping and washing process, so that it isimportant to explore the mechanism of lignin precipitating on cellulose surface. So our workon the one hand is aimed at improving the traditional high temperature sophonation of alkalilignin and its real-time monitoring, in order to improving its application. On the other hand,by analyzing the respective precipitation situation of lignin model compound and wheat strawalkali lignin, the phenomenon of lignin precipitating on cellulose surface can be explained andprecipitation discipline can be revealed, which would, as we wish, provide the theoreticalsupport for the following research and application at control mechanisms to ligninprecipitation on cellulose surface.Firstly, wheat straw alkali lignin as raw materials sulphonated with sulphonating agentNa2SO3under a mild condition at100¹40℃, its products purified through ion exchangeresin. The occurrence of sulphonation can be proved by the characteristic absorption ofsulphonic acid group at1044cm^-1in FTIR spectra of alkali lignin and its sophonated productlignosulphonate. According to the UV-vis spectra, some of alkali lignin is soluble in aqueoussolution, but with constant and small content, which means after deduced by the absorption ofdissolved alkali lignin, the concentration of lignosulphonate in solution after reaction can bedirectly calculated by a standard curve of lignosulphonate. Since UV spectrophotometry isfeasible for monitoring the sulphonation process, the optimal condition for high temperaturesophonation of alkali lignin can be confirmed by single-factor experiments as pH4.0, Na2SO3to be30%of the dry weight of alkali lignin, reaction temperature125℃and3hours. Inaddition, the largest concentration of lignin sulfonate can be gained under experimentalconditions was12.38g/L. The reaction temperature was much lower than that in traditionalhigh temperature sophonation, which means a lot to the present situation of energy shortage.What is more, lignosulphonate can notably lower the surface tension of the aqueous solution, and be able to act as a surfactant owing to its remarkable surface activity.The self assembly of guaiacol oligomers as lignin model compounds on the surface ofcellulosic fiber was reported, which was synthesized via Fe （III）-meditated coupling. The IRspectra of guaiacol and its oligomers indicated aggregation between guaiacol monomers, andwere helpful to explain the formation mechanism of guaiacol oligomers. The molecularweight of prepared oligomers was in range of800-1200g/mol determined byMALDI-TOF-MS, which means6-10guaiacol units aggregated. The self-assembly ofguaiacol oligomer on cellulosic fiber surface was compared in two approaches and themorphology of fiber was observed by atomic force microscopy. Th he approach by absorbingguaiacol on fiber with subsequently polymerization produced a layer of particles, but the sizeof particles was not well distributed. The second approach by simultaneous synthesis andself-assembly on cellulosic fiber can give a thin uniformly layer of nano-particles （thicknessabout20nm）, which was formed by association of oligomer of guaiacol. From images ofAFM we can find that the substrate surface was almost completely covered by grain-likeoligomer particles （diameter10-50nm）. The formation of such layer on the surface of fibercan result in significant increase of roughness and hydrophobicity. This work may provide anew method for modification of fibers and preparation of cell wall-like composites.In order to further explore the mechanism and influencing factors of alkali ligninprecipitating on cellulose surface, precipitation of lignin at a specific concentration withincreasing time was analysed by UV spectra. The result indicated that continuousprecipitation took place on the fiber surface of filter pulp with its precipitating rate slow downduring the whole2hours and concentration of alkali lignin decreased by15.7%. Moreover,the AFM figures showed that there were grain-like particles of lignin on the fiber surface offilter pulp and smaller roughness than that of blank control, similar with the self assembly oflignin model compounds on cellulosic fiber. According to the O/C ratio from XPS anddynamic contact angle tests of the samples, there were other evidences that lignin content onthe fiber surface gradually increased with the increasing initial concentration of alkali lignin.So did there hydrophobicity.