| Enzymatic hydrolysis (EH) lignin is a new kind lignin derived from bio-ethanol production residue which takes lignocellulosic resources as the raw material. EH lignin presents relative high chemical activity with abundant functional groups, such as benzene rings, phenol hydroxyls, ether linkages and so on. So it can be used to prepare kinds of lignin based bio-materials. Phenol-formaldehyde (PF) resin is a widely used wood adhesive for the industrial production of exterior grade plywoods and particle boards due to its high bonding strength and thermal stability and excellent resistance to water. In the meaning time, PF resin is quite expensive and its raw material derived from non-renewable petroleum resources. Those disadvantages have limited its application. The utilization of EH lignin in PF resin by partially replacing phenol, will not only reduce the costs of resin and the dependence on non-renewable fossil resources, but also make the utilization of industrial wastes more efficient and promote the development of bio-ethanol industry.In this paper, the raw material (bio-ethanol production residue) was purified firstly by using a novel solvent "furfuryl alcohol". Then the pretreated EH lignin was phenolated under alkaline conditions, and the synthesis of lignin-modified PF (LPF) was conducted consequently. By using single factor experimental method, the effects of time, temperature and liquid ratio (mass ratio of solvent/raw material) on the lignin yield were investigated in the purification of lignin; The influences of time, temperature and dosage of NaOH (molar ratio of phenolics/NaOH) on the phenolation of lignin were studied in the phenolation process. And the impacts of lignin substitution rate on the properties of LPF resin were explored.For the purification of the raw material in furfuryl alcohol, the increasing of temperature or liquid ratio would improve the lignin yield, while the prolonging of time almost made no difference on the lignin yield. Considering both of the economic factors and the lignin yield, the technological conditions of purification process of lignin in furfuryl alcohol was confirmed as:time of15min, temperature of140℃, mass ratio (solvent/raw material) of5:1. FTIR, XRD and XPS analysis showed that furfuryl alcohol was selective on the dissolvation of lignin. The ash and the carbohydrates remained in the raw material can be removed as residue in the purification process. The lignin content of furfuryl alcohol pretreated product was measured to be92%-96%, so the furfuryl alcohol-lignin product can be regarded as the pure lignin product.Compared to the alkali-solution and acid-isolation method, dioxane and ethanol extraction method, furfuryl alcohol extraction showed higher yield of lignin. Element analysis indicated that furfuryl alcohol-lignin lignin had the closest C. H.O. N element content with alkaline-lignin, which revealed that the lignin product obtained by furfuryl alcohol extraction had almost the same organic components with the pure lignin. Based on the FTIR, GPC and1H-NMR technicals on the structural analysis of lignin products, furfuryl alcohol-lignin had more guaiacol units. So it was more reactive than the alkaline-lignin and dioxane-lignin as guaiacol units are more active than syringyl units. The structural traits were beneficial for the application of EH lignin in PF resin.For the phenolation of lignin under alkaline conditions, the optimizational parameters were determined as:time of1h, temperature of90℃, molar ratio (phenolics/NaOH) of1:0.6. Phenolation of lignin promoted the rupture of ester linkages, and the partial shedding of methoxyl groups. It would improve the chemical reactivity of lignin, and made the polymerization reaction between lignin and formaldehyde easier.When the lignin substitution rate was in the range of10%-50%, the bonding strength of lignin-modified PF resin (LPF) was superior compared to PF resin, while the lignin replacement rate reached60%, the bonding strength of LPF was lower than PF resin, but still met the requirement of national standard I for exterior plywoods. In the meaning time, the introduction of lignin could significantly reduce the free phenol content of PF resin, friendly to the environment. But when the lignin substitution was higher than20%, the free formaldehyde content of LPF was higher than the national standard of0.3%, which needs further improvement. In order to explore the effect of purification process on the replacement rate of lignin in PF resin’s preparation, the raw material-modified PF resin (RPF) was synthesized for comparison. With the same lignin/raw material substitution, the bonding strength of LPF resin was superior with respect to the RPF resin. The purification process of lignin in furfuryl alcohol could double the substitution rate of lignin in PF resin’s preparation. SEM analysis showed that the lower bonding strength of RPF resin was partially due to its rough internal structure. |
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