The extracted lignin with ethanol fractionation is rich in phenolic aromatic rings, suggesting that it is a potential feedstock for preparing phenolic resins in the replacement of phenol presenting an environmental and economical process [66]. The synthesis of lignin-formaldehyde resins involves primarily a hydroxymethy — lation step. Lignin extracted from sugarcane bagasse had a large amount of active centers toward formaldehyde as compared to that from wood due to its higher proportion of H unit which was easily attacked by electrophilic groups [67].

Lignin extracted from white pine with ethanol/water fractionation was used to synthesize phenol-formaldehyde resol resins [68]. Under the optimal conditions, i. e., ethanol concentration 50%, reaction temperature 180°C, reaction time 4 h, lignin was extracted with a yield of 26% and a purity of around 83%. The obtained lignin showed a wide molecular weight distribution: Mw 1150, Mn 537 and polydispersity 2.14. The lignin fraction was used to replace phenol for the synthesis of bio-based phenol-formaldehyde resol resins. By substitution of phe­nol with the pine lignins at various ratios ranging from 25 to 75%, a series of dark — brown viscous resol-type phenolic resins were prepared. The solid concentrations and viscosities of these bio-based resins could be adjusted readily by controlling their water contents. The obtained lignin-phenol-formaldehyde resols solidified upon heating with main exothermic peaks at 150-175°C, and secondary peaks at 135-145°C, depending on the lignin content in the resin formula. When the phenol substitution ratio was lower than 50%, the thermal cure of lignin-phenol-formaldehyde resols proceeded at lower temperatures than that of the corresponding phenol — formaldehyde resol. The introduction of lignin in the resin formula decreased the thermal stability, leading to a lowered decomposition temperature and a reduced amount of carbon residue at elevated temperatures. However, the thermal stability was improved by purifying the lignin feedstock (to remove aliphatic sugars and increase aromatic structures) before the resin synthesis.

The ethanol lignin extracted from bagasse was subjected to purification including cyclohexane/ethanol extraction and acid precipitation. Then the lignin fraction was further hydroxymethylated and used to prepare lignin-phenol-formaldehyde resins [69]. With increased lignin content from 10 to 40%, the Tg of the resins increased from 120 to 150°C, and the rate of cure and the heat of reaction also increased. The negative surface charges resulting from the interaction between the substrate and the lignin-PF resins can reduce the contact angle; therefore, the film prepared from lignin-PF resins was good water-barrier coatings and used as cardboard substrates.

Sugarcane lignin released from Dehini rapid hydrolysis (using ethanol cata­lyzed with diluted sulfuric acid) was used to prepare lignin-formaldehyde resins and lignocellulosic fiber-reinforced composites [70]. The presence of lignin in both fiber and matrix greatly improved the adhesion at the fiber-matrix interface. The increased affinity improved the load transference performance from the matrix to the fiber, leading to good impact strength of the bio-based composites.

Antioxidant is a potential application of lignin. Research on lignin model compounds indicates that ortho-disubstituted phenolic groups are essential for antioxidant activity [71, 72]. The radical scavenging ability of lignin is decided by the ability to form a phenoxyl radical (i. e., hydrogen atom abstraction) as well as the stability of the phenoxyl radical. In lignin, ortho substituents such as methoxyl groups can stabilize phenoxyl radicals by resonance as well as hindering them from propagation. Conjugated double bonds can provide additional stabilization of the phenoxyl radicals through extended delocalization. Lignin was extracted with ethanol/water from hybrid poplar under various conditions, and the yield of the extracted lignin and the antioxidant activity were evaluated [73]. In general, the lignin prepared at elevated temperature, extended reaction time, increased catalyst and diluted ethanol shows high antioxidation activity due to more phenolic hydroxyl groups, low molecular weight and narrow polydispersity of the lignin. Under the optimal conditions, i. e., 190°C, 70 min, 1.4% H2SO4 and 60% ethanol, lignin yield was achieved at 20.1% with a high radical scavenging index of 56.4. Ethanol/water lignin extracted from Miscanthus sinensis with specific molecular weight was separated by ultra-filtration, and its antioxidant capacity was investigated [74]. The data indicated that even though phenolic content was the major factor that determined the antioxidation activity, the molecular weight and purity of the lignin were also contributors. Compared to the crude lignin, the resulted ultra-filtrated lignin exhibited higher antioxidation capacity due to its narrow molecular weight distribution and lower carbohydrate contamination.

Ethanol lignin has potential to sorb metals due to the richness in metal-binding functional groups including carboxylic and phenolic groups [75]. Lignin extracted with ethanol/water catalyzed by dilute sulfuric acid was used as an adsorbent for removal of copper (II) from CuSO4 aqueous solution [76]. It was found that the maximum removal of Cu (II) ions was achieved to *41% by using the organosolv lignin in 10 min at 20°C when the initial concentration of CuSO4 was 3 x In addition, the absorbed lignin can be recovered using HCl in a contact time of 10 min. In a comparative study, the organosolv lignin and kraft lignin from both softwood and hardwood were used to sorb Cu and Cd [77]. The conditions covered a range of pH (2-6.5), ionic strength (0.0001-0.1 M) and initial metal concen­tration (1-25 mg Me (II)/l). The results indicated all sorbents exhibited a prefer­ence for Cu over Cd, and kraft lignins showed higher sorption capacity and faster uptake rate. However, the absorption capacities of the lignin-based sorbents were lower than those reported such as chitosan, green alga. Therefore, further modi­fication of the organosolv lignin is necessary to achieve a higher metal sorption capacity thus can be used commercially.

Ethanol lignin was used as filler in printing ink vehicles and paints [78]. The lignin extracted by Alcell process with a lower molecular weight (Mn 700, Mw 1,700) can significantly improve the properties of the viscous media used for offset inks and paints with respect to tack and misting reduction. The addition of the lignin resulted in a brown coloration in these liquids, but did not bring about fundamental modification of their other basic physical and chemical properties. Therefore, no negative effects were produced for their most applications.

Selective hydrogenolysis is one effective way that can decrease the degree of polymerization while increasing the H/C ratio and lowering the O/C ratio of lignin, thus can convert it from a low grade fuel into potential fuel precursors or other value-added chemicals [79]. In a typical reaction catalyzed with RuCl2(PPh3)3 [80], the solubility of ethanol lignin in DMSO increased from 59.1 to 96.4% with increase in temperature from 50 to 175°C. The hydrogenolysis mechanism was mainly selective cleavage of aryl-O-aryl and aryl-O-aliphatic linkages, which was demonstrated by 31P NMR spectroscopy.