Physico-chemical characterization of lignins from different sources for use in phenol–formaldehyde resin synthesis

During the last decades lignin has been investigated as a promising natural alternative to petrochemicals in phenol–formaldehyde (PF) resin production, due to their structural similarity. Physico-chemical characterization of three types of lignin, namely kraft pine lignin (L1), soda–anthraquinone flax lignin (L2), and ethanol–water wild tamarind lignin (L3) has been evaluated to determine which one is the most suitable chemical structure for above purpose. Characterization has been performed using Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance spectrometry (1H NMR) to analyse the chemical structure, gel permeation chromatography (GPC) for determining molecular weight (MW) and molecular weight distribution (MWD), differential scanning calorimetry (DSC) to measure the glass transition temperature and thermogravimetric analysis (TGA) to follow the thermal degradation. Both structural and thermal characteristics suggest that kraft pine lignin (L1) would be a better phenol (P) substitute in the synthesis of lignin–phenol–formaldehyde (LPF) resins, as it presents higher amounts of activated free ring positions, higher MW and higher thermal decomposition temperature.