Лако-красочные материалы — производство

The drawbacks inherent in the Pedersen and Shen procedures—high pressing temperatures and long pressing times or strong mineral acids—can be avoided if cross-linking of the lignin molecules is achieved by radical coupling instead of condensation reactions (Nimz et al. 1972 [2]). In this case, the formation of new carbon-carbon as well as carbon-oxygen bonds between two radicals is a very fast reaction with a low activation energy, which needs no external heating or mineral acids as catalyst. This means that the reaction is very specific, and side reactions such as decompositions and charring can be avoided, while linkages between wood and SSL may also occur.

The essential radicals are formed from phenolic groups in the lignosulfonate mole­cules by oxidation with hydrogen peroxide in the presence of a catalyst. Out of a number of catalysts, sulfur dioxide (SO2) has been proven to be the most effective [13]. A 50% calcium-based SSL containing about 1% SO2 at pH 2 reacts vehemently with a 35% hydrogen peroxide solution in a strongly exothermic reaction, forming an insoluble gel. The reaction time is less than 1 min but depends on the source and composition of SSL. At higher pH values, the reaction takes some minutes or needs heating up to about 70°C, but after reaching 70°C, the reaction is also very fast.

In has been reported [13] that the SSL containing the catalyst and the hydrogen peroxide solution have to be sprayed separately on the wood chips. Under certain condi­tions, the hydrogen peroxide can be mixed with the SSL and sprayed together on the wood chips, in a single operation [13]. Another possibility consists of adding half of the SSL as spray-dried powder, lowering the humidity of the blended chips to about 13%, which is the upper limit according to German standards. The powder may either be mixed together with the liquid SSL and the hydrogen peroxide or added separately after the wood chips have been sprayed with the mixture of SSL solution and hydrogen peroxide. The humidity of the wood chips can thus be adjusted to predetermined values. The pot life of the blended wood chips, which is the assembly time between spraying and pressing, would then be extended. Medium-density interior-grade particleboard can be obtained from wood chips with 20% SSL, based as dry material on dry wood chips, at pressing temperatures between 100 and 120°C under otherwise conventional manufacturing conditions for UF particleboard.

There are several reasons why this system has not found industrial favor: (1) the unfavorable situation due to the presence of a peroxide in wood panel plants, such as possible machinery corrosion, and other problems, and (2) the fact that the produced board is often relatively very soft immediately out of the particleboard press, rendering its early handling particularly problematic.