Upgrading of kraft lignin with phosphoric acid purification: A promising route for the synthesis of renewable epoxy resins

Obtaining bioproducts of industrial interest from kraft lignin not only adds value to this residual biomass while reducing environmental impact, but also fosters the circular economy and promotes sustainable development. However, its use without the removal of ash, metals, sulfur, and nitrogen implies a reduction in the final product properties, limiting its applications. This study aims to present a simple and efficient route for the production of purified kraft lignin (LKP) with low ash 0.51% and total sulfur 0.74% content. It is produced from impure kraft lignin (LKI), with high ash 9.74% and total sulfur 1.39% content which was treated with hot H3PO4/H2O solution (dilute acid purification), aiming at the synthesis of lignin-based epoxy resins. Elemental Analysis of carbon (C), hydrogen (H), and nitrogen (N) (wt%); ash (wt%); total sulfur (wt%); moisture contents (wt%); X-ray Fluorescence; X-ray Diffraction; Infrared Spectroscopy; Thermogravimetric Analysis; and Differential Scanning Calorimetry showed that LKP improved its structural, functional; and chemical properties, particularly thermal properties: 17% increase initial decomposition temperature (Tonset), 18% decrease final decomposition temperature (Tendset), 14% increase maximum decomposition temperature (Tdmax.), 35% increase carbonaceous residue (CR) and 8% increase in the value of glass transition temperature (Tg). The limiting oxygen index (LOI) calculated for this sample was around 34%, thus configuring a "self-extinguishing" material. Moreover, its high phenolic and aliphatic hydroxyl content 8.02 mmol L−1, high aromaticity (H/C = 1.11), high weight average molar mass (Mw̅≅1900g mol−1), and ash and total sulfur low contents justify its use in the synthesis of lignin-based epoxy resins. A sample of LKP was tested in a classic epoxidation reaction to obtain the prepolymer denominated of lignin-based epoxy resin (E-LKP), whose epoxy equivalent weight (EEW) was around 3500 g mol−1, which is similar to that of the high molar mass epoxy resin, D.E.R. 668-20 2000, commercialized by Dow Chemical Company®. E-LKP was polymerized with a conventional curing agent (polyetheramine) to produce the cured lignin-based epoxy resin (R-ELKP) with good physical-chemical properties. The high glass transition temperature (Tg2 = 133 ºC), a good limiting oxygen index (LOI = 27%), and excellent aesthetic characteristics (homogeneity and high brightness) stand out. These promising results suggest that it is possible to develop cured lignin-based epoxy resin with properties comparable to cured petroleum-based epoxy resin. The use of purified kraft lignin to obtain materials (prepolymers and polymers epoxy, among others) will also favor the environment, developing greener and renewable products.