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The methylolation of softwood and hardwood lignosulfonates was studied. Six different lignosulfonate samples (three from hardwoods and three from softwoods) were characterized in order to assess their suitability for methylolation. The techniques employed in characterization were UV/vis spectroscopy, Fourier transform infrared (FTIR), and 1 H‐nuclear magnetic resonance ( 1 H‐NMR). The best properties were shown by softwood ammonium lignosulfonate (LAS), which was used to optimize the operation conditions to promote the Lederer–Manasse reaction. The methylolation variables studied were the sodium hydroxide‐to‐lignin molar ratio, the formaldehyde‐to‐lignin molar ratio, time, and temperature. The reaction was followed by the changes in the concentration of free formaldehyde. IR and NMR spectra of nonmodified and modified samples were used to study the structural changes. Under optimum operation conditions, softwood lignosulfonates showed higher reactivity toward formaldehyde than did hardwood lignosulfonates.
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Alonso et al. (2001) studied this question.
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