14 августа, 2015 
Malyar Ecole Nationale Superieure des Technologies et Industries du Bois, Universite de Nancy I, Epinal, France
The occurrence of lignin as a waste product in pulp mills has made it an attractive raw material for adhesives ever since the beginning of the sulfite pulping of wood. The first patents dealing with the application of spent sulfite liquor (SSL) as an adhesive for paper, wood, and other lignocellulosic materials date back to the end of the nineteenth century [1], and since then have shown an ever-growing number. On the other hand, technical utilization of lignin on a large scale is still at a very low level for the amount produced worldwide. Presently, most of the spent liquors in pulp mills are burned. Only about 20% is used for various purposes, such as dispersants, oil-well-drilling muds, pelletizing materials, molding stabilizers, and concrete grinding additives.
As a major wood component, native lignin is neither hygroscopic nor soluble in water. However, during technical sulfite pulping, lignin becomes soluble in water, due to partial degradation and introduction of sulfonic acid groups (-SO3H). In applying SSL as an adhesive, it has to be converted to an insoluble state during the curing period. Cross-linking in lignin can be achieved either by condensation or by radical coupling reactions. A great number of patents have become known during the past four decades [2] dealing with the application of SSL as a wood adhesive, in which the lignin is cross-linked by condensation reactions. However, either high temperatures and long heating times or mineral acids are required for these condensation reactions, which cause structural changes or charring in the wood particles. Recently, cross-linking of the ligno — sulfonate molecules by radical combinations, which avoids mineral acids and high temperatures, has been developed, but this presents disadvantages as well, since the use of peroxides is not favored in wood processing plants, for a variety of reasons. The use of lignin by polycondensation reactions with formaldehyde also presents the disadvantage of slower pressing time in their application to panel products.
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