Influence of resin content and pulping temperature on the formaldehyde release from medium density fibreboards (MDF)
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Increasing the binder content from 12 % to 20 % in medium density fibreboards (MDF) using a slightly melamine modified (3 % melamine) UF-resin of a molar ratio F:U of 1:1 causes a decrease in the formaldehyde release from the boards prepared under the same conditions. The formaldehyde emanation was measured using the perforator (EN120) and the flask method (EN 717-3) after a reaction period of 3 h and 24 h. The results apply for boards made from thermo-mechanical pulps (TMP) prepared by pulping wood at 140 °C and 175 °C.
The pulping temperature of wood also impacts the formaldehyde release from MDF prepared therefrom. Ascending the pulping temperature from 140 °C to 175 °C has a positive, but less significant influence on the formaldehyde release compared to that of increasing the binder content from 12 % to 20 %. Insofar, it becomes evident from the results that the conclusions drawn in the literature pertaining to the influence of resin content in case of high molar ratio resins do not necessarily apply to low molar ratio resins.
KeywordsMelamine Formaldehyde Emanation Urea Formaldehyde Binder Content Resin Content
Einfluss des Bindemittelgehalts und der Aufschlusstemperatur auf die Formaldehydabgabe von mitteldichten Faserplatten (MDF)
Die Erhöhung des Gehalts an mit Melamin (3 %) modifiziertem UF-Harz mit dem Molverhältnis F:U 1:1 von 12 % auf 20 % in MDF führt unter sonst vergleichbaren Herstellungsbedingungen der MDF zu einer Verminderung der Formaldehydabgabe der hergestellten Platten, ermittelt nach dem Perforatorverfahren (EN 120) und der Flaschenmethode (EN 717-3) nach einer Reaktionsdauer von 3 h und 24 h. Die Anhebung der Aufschlusstemperatur von 140 °C auf 175 °C bei der Herstellung des thermo-mechanischen Holzstoffes (TMP) für die Plattenherstellung hat einen schwachen positiven Einfluss auf die Formaldehydabgabe, ermittelt nach den beiden oben erwähnten Verfahren. Dieser Effekt bleibt hinter dem der Erhöhung des Bindemittelgehalts von 12 % auf 20 % zurück. Insofern lassen die Ergebnisse deutlich werden, dass die in der Literatur veröffentlichten Ergebnisse bezüglich des Einflusses des Bindemittelaufwandes auf die Formaldehydabgabe im Falle von Harzen mit hohen Molverhältnissen (F:U) nicht zwangsläufig auf die der Harze mit sehr niedrigem Molverhältnis übertragbar sind.
Urea formaldehyde resins (UF-resins) are the main binders used in the wood-based panels industry. Recently, increased awareness of ecology and clean indoor requirement boosted research activity to reduce the formaldehyde emanation from wood-based composites and resulted in the development of UF-resins of extremely low molar ratio (F:U) to be used as a bonding agent. To meet the tight specifications of different regulations enforced in several countries like F**** in Japan (JIS A 1460 2001) UF-resins of molar ratio F:U 1:1 or even much less are used today as adhesives in the industry. Decreasing the molar ratio of formaldehyde to urea (F:U) below a certain limit leads, however, to a significant deterioration in the properties of the resin itself (low reactivity, lower moisture resistance) and in the physical-mechanical properties of the boards (high thickness swelling, low internal bond strength) bonded therewith (Sundin et al. 1987). To offset the negative impact of low molar ratio resins on the board properties, the level of resin loading in the boards is generally increased.
The influence of resin content in particleboards has been studied by Petersen et al. (1973) using resins of molar ratio between F:U 1.6 and 1.4. The results clearly indicate that by using high molar ratio resins of F:U 1.6:1 increasing the binder content between 5 % and 12 % leads to a remarkably higher perforator value; by using F:U resins of 1.4:1 the influence of resin level, however, was found to be too insignificant over the range of binder content between 5 % and 12 %. Efforts were in vain to find literature systematically covering the influence of resin content on the formaldehyde release in particle- or fibreboards bonded with very low formaldehyde UF-resins of molar ratio F:U of 1:1 or lower.
2 Experimental work
The influence of resin content in medium density fibreboards (MDF) was evaluated on 16 mm-boards with a density of 0.78 g/cm3 made from thermo-mechanical pulps (TMP) prepared from beech wood at two different pulping temperatures (140 °C and 175 °C) using two resin levels (12 % and 20 % solids based on dry fibres). The boards were made using the blow-line technique with a UF-resin of molar ratio 1:1 containing about 3 % melamine. No hardener was used to cure the resin. The pressing temperature was 190 °C and the pressing time was 24 s/mm. After conditioning for three weeks at 20 °C and 65 % rel. humidity the formaldehyde release of the boards was assessed using the perforator technique (EN 120) and the flask method (EN 717-3) after a reaction time of 3 h and 24 h. The measured perforator values were also corrected to a moisture content of 6.5 % according to EN 312 using the empirical formula developed by Jann and Deppe (1990).
3 Results and discussion
The formaldehyde release of medium density fibreboards (MDF) bonded with low molar ratio (F:NH2) urea-formaldehyde resins (UF-resins) decreases significantly by increasing the binder content from 12 % to 20 %. Increasing the pulping temperature of wood from 140 °C to 175 °C reduces also the emanation of formaldehyde from the boards. The liberation of formaldehyde was measured by the flask method (EN 717-3) and the perforator technique (EN 120).
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- JIS A 1460 (2001) Building boards determination of formaldehyde emission—desiccator method. Japanese Standards Association (JSA) Google Scholar
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