Лако-красочные материалы — производство

According to its structure as a polyphenol, lignin as an adhesive should be similar to phenol-formaldehyde (PF) resins. This is true for native lignin in wood; to transfer them into insoluble resins, technical lignins (lignosulfonate and black liquor) have to be additionally cross-linked. However, condensation reactions in lignin by heat or mineral acids cannot be as effective as in synthetic PF resins, due to the lower number of free positions in the aromatic nuclei of lignin and their considerably lower reactivity than in PF resins. First, there is only 0.5 of a free 5-position (ortho to the phenolic groups) per C9 unit; positions 6 and 2 are less reactive. Second, there is less than one benzyl alcohol or ether group per C9 unit in lignin, whereas in synthetic PF resins up to three methylol groups can be introduced into one phenolic ring. Finally, the aromatics in lignin are considerably less reactive toward hydroxybenzyl alcohol groups than is phenol, due to the presence of methoxy or methoxy-equivalent groups rather than hydroxy groups on the lignin aromatic rings. For these reasons, lignin in technical spent liquors cannot be as effectively cross-linked as synthetic PF resins. At least higher press temperatures at longer heating times or higher acid concentrations are necessary.

Quite a number of patents have been pending during the past four decades dealing with lignin as adhesive for particleboard, plywood, and fiberboard in the absence of conventional PF or urea-formaldehyde (UF) adhesives [2]. Besides lignin, in most cases additional cross-linking agents for lignin are necessary, such as epoxides, polyisocyanates, polyols, polyacrylamides, polyethyleneimine, aldehydes, maleic anhydride, amines, pro­teins, melamine, hydrazine, and so on. So far, these procedures, for different reasons, have not led to any major practical application. Very few procedures, such as those of Pelikan et al. (1954), Pedersen and Jul-Rasmussen (1963), Shen (1973), Nimz et al. (1972), and others, use lignosulfonates or SSL without integrated cross-linking chemicals [2]. The patent of Pelikan et al. describes ways of using lignin as an adhesive for floor layers by cross-linking it oxidatively with chromium trioxide. The mechanism of cross-linking is the same as with hydrogen peroxide (Nimz, 1972) [2], but it is much less effective. Its applic­ability to particleboards has been tried, but the boards exhibited low tensile strengths and disintegrated in water at 20° C in less than 2h.