Лако-красочные материалы — производство

According to Forss and Fuhrmann (1972) [5,18], the amount of lignin in lignin-PF adhe­sives for particleboard, plywood, and fiberboard can be increased to 40 to 70% if a high — molecular-weight fraction (MW > 5000) of either lignosulfonate or black liquor, obtained from alkaline pulping of wood, is applied. Fractionation of SSL or black liquor can be achieved by ultrafiltration [5,18]. According to the authors, the higher effectivity of high — molecular-weight lignin molecules is due to their higher level of cross-linking, which requires less PF for the formation of an insoluble copolymer than do low-molecular — weight lignin molecules. However, bearing in mind the findings of Shen [8] that low — molecular-weight ammonium-based SSL is more effective, Forss and Fuhrmann appear not to have checked the influence of inorganic salts in SSL or black liquor that are separated off during ultrafiltration [2]. Forss and Fuhrmann assume that condensates between smaller lignin molecules and PF ‘‘are unable to contribute to the three-dimen­sional network’’ [5], which is unlikely because low-molecular-weight lignin molecules are more reactive than are high-molecular-weight molecules.

In the manufacture of particleboard either high-frequency heating or combined contact high-frequency heating has been applied. In the latter case, the press platen tem­perature has been 180°C. German standard requirements for weather-resistant particle­boards were met at pressing times between 10 and 12s/mm and 8 to 12% adhesive, based on dry wood particles. One advantage inherent in the fractionation by ultrafiltration is that the lignin becomes more uniform and less dependent on variations in pulping condi­tions and wood source, which sometimes cause serious problems in the application of technical lignins. Full-scale plywood mill tests, some of them running continuously for several weeks, appear to have been performed in two Finnish plywood mills. Again, this procedure does not appear to be in operation anymore.