Лако-красочные материалы — производство

When lignosulfonate is treated with strong mineral acids at elevated temperatures or heated at temperatures above 180° C, condensation reactions leading to diphenylmethanes and sulfones take place. The reactivity of lignosulfonates depends to some extent on the cation. Of the four lignosulfonates obtained technically, the calcium-based exhibit the lowest and the ammonium-based the highest reactivity; the sodium and magnesium lignosulfonates show a medium reactivity.

Hydrobenzyl alcohol groups as well as sulfonic acid groups on the carbon a to the aromatic rings of some of the phenylpropane units of the random polymer react in the presence of strong mineral acids with the aromatic nuclei of other phenylpropane units. This reaction, leading to diphenylmethanes, is of the same type as the formation of phenolic resins from phenol and formaldehyde. Lignin also reacts with formaldehyde and can be cross-linked by it in the same manner as that of synthetic polyphenolic resins.

A. Cross-Linking by Oxidative Coupling [2]

Lignosulfonic acid in technical SSL contains about 0.4 of a free phenolic hydroxy group per C9 unit. Therefore, like the formation of lignin in plants, cross-linking of lignosulfo — nate is possible by oxidative coupling. Oxidants such as hydrogen peroxide, and catalysts such as sulfur dioxide or potassium ferricyanide, are most effective. Treatment of a 50% technical SSL with this redox system leads to a very vigorous exothermic reaction under evaporation of water. The yield of the resin under certain conditions exceeds 70%, indi­cating that some carbohydrates must also have been enclosed in the resin. The advantage of this type of cross-linking compared with condensation reactions is that it needs neither mineral acids nor high temperatures, due to the recombination of radicals, for which the activation energy is very low. The strongly exothermic reaction causes a uniform tempera­ture profile during pressing of particleboard without external heat.