Effect of alkali content and synthesis duration on the properties of lignin-phenol-formaldehyde resins prepared with technical lignosulfonates

The structure and properties of lignin-phenol-formaldehyde resins prepared using technical sodium lignosulfonates were researched. At the first stage, highly alkaline resins were synthesized by replacing up to 40% of phenol with lignosulfonates; at the second stage, medium alkaline resins were synthesized with 30% phenol substituted by lignosulfonates, varying the duration of the polycondensation stage from 15 to 35 min. Based on the results of ¹³C NMR spectroscopy method it was suggested that highly alkaline resins contain predominantly phenol-formaldehyde oligomers of low molecular weight; the interaction of formaldehyde with lignosulfonates occurs mainly at the first stage of synthesis (at 55–60 °C). Highly alkaline lignin-phenolformaldehyde resins with 20% phenol replaced by lignosulfonates provide the highest physical and mechanical properties among all highly alkaline resins. However, boards manufactured using highly alkaline lignin-phenol-formaldehyde are significantly inferior to boards manufactured using industrial phenol-formaldehyde resin SFZh3014. Reducing the alkali content and increasing the duration of resin holding at 96– 98 °C (second stage) allows to significantly increase the molecular weight of phenolformaldehyde oligomers and improve the properties of lignin-phenol-formaldehyde resins-based particleboard. Phenol-formaldehyde oligomers of medium alkaline resins have a relatively high molecular weight. Particleboards based on medium alkaline resins synthesized with a second stage duration of 25 min meet all the requirements of the state standard. However, boards based on medium alkaline lignin-phenolformaldehyde resins are still inferior to boards based on SFZh-3014.

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