This review article aims to summarize the potential of using renewable natural resources, such as lignin and tannin, in the preparation of NIPUs for wood adhesives. Polyurethanes (PUs) are extremely versatile polymeric materials, which have been widely used in numerous applications, e.g., packaging, footwear, construction, the automotive industry, the lighting industry, insulation panels, bedding, furniture, metallurgy, sealants, coatings, foams, and wood adhesives. The isocyanate-based PUs exhibit strong adhesion properties, excellent flexibility, and durability, but they lack renewability. Therefore, this study focused on the development of non-isocyanate polyurethane lignin and tannin resins for wood adhesives. PUs are commercially synthesized using polyols and polyisocyanates. Isocyanates are toxic, costly, and not renewable; thus, a search of suitable alternatives in the synthesis of polyurethane resins is needed. The reaction with diamine compounds could result in NIPUs based on lignin and tannin. The research on bio-based components for PU synthesis confirmed that they have good characteristics as an alternative for the petroleum-based adhesives. The advantages of improved strength, low curing temperatures, shorter pressing times, and isocyanate-free properties were demonstrated by lignin- and tannin-based NIPUs. The elimination of isocyanate, associated with environmental and human health hazards, NIPU synthesis, and its properties and applications, including wood adhesives, are reported comprehensively in this paper. The future perspectives of NIPUs’ production and application were also outlined.

The efficient isolation process and understanding of lignin properties are essential to determine key features and insights for more effective lignin valorization as a renewable feedstock for the production of bio-based chemicals including wood adhesives. This study successfully used dilute acid precipitation to recover lignin from black liquor (BL) through a single-step and ethanol-fractionated-step, with a lignin recovery of ~35% and ~16%, respectively. The physical characteristics of lignin, i.e., its morphological structure, were evaluated by scanning electron microscopy (SEM). The chemical properties of the isolated lignin were characterized using comprehensive analytical techniques such as chemical composition, solubility test, morphological structure, Fourier-transform infrared spectroscopy (FTIR), 1H and 13C Nuclear Magnetic Resonance (NMR), elucidation structure by pyrolysis-gas chromatography-mass spectroscopy (Py-GCMS), and gel permeation chromatography (GPC). The fingerprint analysis by FTIR detected the unique peaks corresponding to lignin, such as C=C and C-O in aromatic rings, but no significant differences in the fingerprint result between both lignin. The 1H and 13C NMR showed unique signals related to functional groups in lignin molecules such as methoxy, aromatic protons, aldehyde, and carboxylic acid. The lower insoluble acid content of lignin derived from fractionated-step (69.94%) than single-step (77.45%) correlated to lignin yield, total phenolic content, solubility, thermal stability, and molecular distribution. It contradicted the syringyl/guaiacyl (S/G) units’ ratio where ethanol fractionation slightly increased syringyl unit content, increasing the S/G ratio. Hence, the fractionation step affected more rupture and pores on the lignin morphological surface than the ethanol-fractionated step. The interrelationships between these chemical and physicochemical as well as different isolation methods were investigated. The results obtained could enhance the wider industrial application of lignin in manufacturing wood-based composites with improved properties and lower environmental impact.

Lignin is the second most naturally abundant biopolymer in the cell wall of lignocellulosic compound (15-35%) after cellulose.Lignin can be generated in massive amounts as by-products in biorefineries and pulp and paper industries through differing processes. Most lignin is utilized as generating energy and has always been treated as waste. Due to the high amount of phenolic compounds in lignin, it is considered as a potential material for various polymers, building blocks, and biomaterials production. Even though lignin can be utilized in the form of isolated lignin directly, the modification of lignin can increase the wide range of lignin applications. Lignin-based copolymers and modified lignin show better miscibility with another polymeric matrix, outstanding to the enhanced performance of such lignin-based polymer composites.This article summarizes the properly updated information of lignin’s potential applications, such as bio-surfactant, active packaging, antimicrobial agent, and supercapacitor.Keywords: active packaging, antimicrobial agent, bio-surfactant, lignin, supercapacitor

In this study, technical lignin from black liquor was used as a pre-polymer for the preparation of bio-polyurethane (Bio-PU) resins. Briefly, the isolated lignin was fractionated using ethyl acetate (EtAc) and methanol (MeOH). The liquid fractions of lignin, such as lignin-EtAc (L-EtAc) and lignin-methanol (L-MeOH), were mixed with 10% of polymeric isocyanate (based on the weight of liquid fractions) to obtain Bio-PU resins. The isolated lignin, fractionated lignin, and lignin-derived Bio-PU resins were characterized using several techniques. The obtained Bio-PU resins were then used to modify ramie fibers using vacuum impregnation method. Fourier Transform Infrared (FTIR) spectroscopy, Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis (TGA) revealed that the isolated lignin had quite similar characteristics to the lignin standard. Fractionation of lignin with EtAc and MeOH altered its characteristics. FTIR, DSC, and TGA showed that solid fractions of lignin had similar characteristics to lignin standard and isolated lignin, while the liquid fractions had characteristics from lignin and the solvents. The absorption band of isocyanate (−N=C=O) groups was shifted to 2285 cm−1 from 2240 cm−1 owing to the reaction with the −OH groups in lignin, forming urethane (R−NH−C=O−R) groups at 1605 cm−1 in Bio-PU resins. Thermal properties of Bio-PU resins derived from L-EtAc exhibited greater endothermic reaction compared to Bio-PU-L-MeOH. As a result, the free −N=C=O groups in Bio-PU resins have reacted with –OH groups on the surface of ramie fibers and improved its thermal properties. Modification of ramie fibers with Bio-PU resins improved the fibers’ thermal stability by 15% using Bio-PU-LEtAc for 60 min of impregnation.Keywords: Bio-polyurethane resins, Impregnation, Lignin fractions, Ramie fibers, Thermal stability

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