Лако-красочные материалы — производство

Lignins are large three-dimensional polymers produced by all vascular terrestrial plants; they are second only to cellulose in natural abundance and are essentially the ‘‘natural glue’’ that holds plant fibers together. Lignins are phenolic materials. They are primarily obtained as a byproduct in wood pulping processes with estimates exceeding 75 million tonnes per annum. Therefore great interest exists for possible applications. Lignins of very different chemical composition and possible applications in the wood-based panels indus­try (adhesives, additive for part replacement of adhesives, raw material for synthetic resins) have been described in a large number of papers and patents. Research into lignin-based adhesives dates back more than 100 years with many separate examples of resins involving lignin being cited. In reality, existing applications are very rare. No industrial use as a pure adhesive for wood is currently known despite the fact that con­siderable research has been directed toward producing wood adhesives from lignins. By themselves lignins offer no advantages in terms of chemical reactivity, product quality, or

color when compared to conventional wood adhesives. The greatest disadvantages of lignins in their application as adhesives are (i) their low reactivity and, therefore, slow hardening compared to phenol due to the lower number of reactive sites in the molecule, causing increased press times, and (ii) the concern over the chemical variation of the feedstock. The chemical structure of lignin is very complex with the added difficulty that unlike tannin the individual molecules are not fixed to any particular structure, there­fore no true generic molecule exists for lignin from softwood, hardwood, or cereals. Lignosulfonates can be added to synthetic glue resins as extenders (by partial replacement of resin). The partial replacement of phenol during the cooking procedure of PF resins has no real industrial importance.

1. Use of Lignins as Adhesives Without Adding Other Synthetic Resins The application of lignins as adhesives is, in principle, possible. Initial attempts needed very long press times due to the low reactivity of lignin (Pedersen process) [317,318]. This process was a condensation under strong acidic conditions, which led to considerable corrosion problems in the plant [318]. The particles are sprayed with spent sulfite liquor (pH 3 to 4) and pressed at 180° C. After this step the boards are tempered in an autoclave under pressure at 170-200°C, whereby the sulfite liquor becomes insoluble by splitting off water and SO2. Shen [319-321], Shen and Fung [323] as well as Shen et al. [322,324] modified this process by spraying the particles with spent sulfite liquor containing sulfuric acid and pressing them at temperatures well above 210°C.

Nimz [317,325] describes the crosslinking of lignin after an oxidation of the phenolic ring in the lignin molecule using H2O2 in the presence of a catalyst, especially SO2 [326]. This leads to the formation of phenoxy radicals and with this to radical coupling (but not to a condensation reaction), whereby inter- and intramolecular C-C bonds are formed.

This reaction does not necessarily need heat or acidic conditions, but is accelerated by higher temperatures (maximum 70°C) as well as lower pHs. In this way the disadvantages of the processes mentioned above (high press temperatures, long press times, use of strong acids) can be avoided [317,326]. An oxidative activation of the lignin also can be achieved by biochemical means, e. g., by adding enzymes (phenoloxidase laccase) to the spent sulfite liquor, whereby a polymerization via a radical mechanism is initiated. The enzymes are obtained from nutrient solutions of white fungi [327]. The two-component adhesive is prepared by mixing the lignin with the enzyme solution (after filtration of the mycelium). At the beginning of the press cycle the enzyme still works, since it is stable up to a temperature of 65° C. If this temperature is surpassed, the enzyme is deactivated. At such time, however, the number of quinone methides is already high enough to initiate a crosslinking reaction [86,87,327-334]. This system, however, is not capable of keeping up with the demands of modern day panel hot press times. To achieve viability this problem was solved by the addition of a smaller than usual amount of isocyanate adhesives. The use of an adhesive thus denies this system any advantage [335]; the enzymatic approach alone only achieves results and pressing times comparable to those of nonenzyme treated nonglued hardboard, a long-existing process and product.