Synthesis and characterization of lignin–melamine–formaldehyde resin

Lignin–melamine–formaldehyde (LMF) resin was prepared by three steps: (i) tosylation of lignin, (ii) synthesis of lignin-melamine (LM) copolymer, and (iii) formation of methylol LM. The synthesized resins were characterized by Fourier transform infrared (FTIR) spectroscopy and phosphorous 31 nuclear magnetic resonance analysis. The curing parameters of LMF resin were determined by differential scanning calorimetry (DSC) and thermal gravimetric analysis. The yield of tosylation is 80%. The FTIR spectrum of tosylated lignin shows the presence of two new bands at 1171 and 1370 cm⁻¹. The formation of the LM was demonstrated by the disappearance of both bands and appearance of the absorbances at 3115, 3312, 3415, and 3470 cm⁻¹ corresponding to the stretching vibrations of primary and secondary amine. The peaks observed at 147.0 and 148.5 ppm are attributed to the new aliphatic hydroxyl groups formed by the methylolation of LM. One exothermic peak was observed in the DSC analysis indicating a one cross-linking reaction.