Abstract
Amino resin precursors prepared by the addition of a new, colourless, non-volatile and non-toxic aldehyde, dimethoxyethanal (DME), to urea gave resins for boards that while able to harden were underperforming due to the lower reactivity of DME in relation to formaldehyde. Urea reacts with one and even two molecules of DME to form UDME and U(DME)2 (called DU) but the subsequent cross-linking reaction to form bridges between two ureas, although existing as observed by CP MAS 13C NMR, was too slow at temperatures lower than 140 °C to be of significance for wood panel adhesives. However, addition of 20% isocyanate (pMDI) contributed to cross-linking of DU by its reaction with pMDI to also form urethane bridges, their existence being confirmed by CP-MAS 13C NMR. The adhesive resins so formed had excellent performance, were colourless, and produced boards that satisfied well the requirements of the relevant norms for interior panels (EN 120 and EN 312). The results were good enough to decrease the proportion of pMDI to 14% at pressing times starting to be of significance for industrial panel products. Formaldehyde emission, by perforator method was down exclusively to the formaldehyde produced by heating the wood chips. The panel emission was sufficiently low to even satisfy the relevant F**** JIS A 5908 Japanese standard (JIS A 5908, 1994 ). These adhesives are colourless, as UF resins.
Zusammenfassung
Durch Zugabe von Dimethoxyethanal (DME), einem neuen, farblosen, nicht flüchtigen und ungiftigen Aldehyd, zu Harnstoff, wurden Vorstufen von Aminoharz hergestellt. Dies ergab Harze, die zwar härteten, jedoch aufgrund der im Vergleich zu Formaldehyd geringeren Reaktivität von DME die Erwartungen nicht ganz erfüllten. Harnstoff reagiert mit einem oder auch zwei DME-Molekülen und bildet UDME und U(DME)2 (als DU bezeichnet). Die nachfolgende Vernetzungsreaktion zur Bildung von Brücken zwischen zwei Harnstoffen war zwar, wie anhand CP-MAS 13CNMR gezeigt wurde, vorhanden, bei Temperaturen unter 140 °C war sie jedoch zu langsam, um die Anforderungen an Klebstoffe für Holzwerkstoffe zu erfüllen. Mittels CP-MAS 13CNMR konnte nachgewiesen werden, dass durch Zugabe von 20% Isocyanat (pMDI) die Vernetzung von DU durch Reaktion mit pMDI durch zusätzliche Bildung von Urethan-Brücken verbessert wurde. Die so hergestellten Harze zeigten ausgezeichnete Eigenschaften, sie waren farblos und ergaben Platten, die die Anforderungen der entsprechenden Normen für Platten im Innenbereich (EN 120 und 312) erfüllten. Die Ergebnisse waren so gut, dass der pMDI-Anteil auf 14% verringert werden konnte bei Presszeiten, die für eine industrielle Produktion noch akzeptabel sind. Die mittels Perforatormethode ermittelte Formaldehydemission hing ausschließlich von dem beim Erhitzen der Holzspäne entstandenen Formaldehyd ab. Die Plattenemission war so niedrig, dass sie sogar die Anforderungen der entsprechenden japanischen Norm F**** JIS A 5908 erfüllte. Diese Klebstoffe sind wie die UF-Harze farblos.
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Despres, A., Pizzi, A., Vu, C. et al. Colourless formaldehyde-free urea resin adhesives for wood panels . Eur. J. Wood Prod. 68, 13–20 (2010). https://doi.org/10.1007/s00107-009-0344-y
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DOI: https://doi.org/10.1007/s00107-009-0344-y