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The paper addresses the role of the main components of wood cellulose, hemicellulose, and lignin as well as of wood extractives as a potential source of formaldehyde. Lignin seems to have a higher emission potential than cellulose and hemicellulose. Moreover, the results reveal that on the one hand extractives release formaldehyde and on the other hand that certain wood extractives react with formaldehyde and hence act as a formaldehyde scavenger. Formaldehyde emanates from wood at temperatures as low as 40°C. Higher temperatures increase the formaldehyde emission tremendously. Therefore, thermo-mechanical pulping enhances formaldehyde release exorbitantely. Differences in the formaldehyde emission between wood species are discussed and related to differences in their chemical composition. Formaldehydfreisetzung aus HolzDie Hauptbestandteile von Holz, Cellulose, Hemicellulose und Lignin, sowie Extraktstoffe des Holzes wurden hinsichtlich ihres mo Èglichen Beitrags zur Formaldehydabgabe untersucht. Lignin scheint ein ho Èheres Formaldehydabgabepotential aufzuweisen als Cellulose oder Hemicellulose. Die Ergebnisse zeigen, dass bestimmte Holzextraktstoffe zum einen zur Formaldehydbildung beitragen ko Ènnen und zum anderen als Formaldehydfa Ènger fungieren. Aus den untersuchten Ho Èlzern wurde Formaldehyd bereits bei Temperaturen von 40°C freigesetzt. Ho Èhere Temperaturen lassen die Formaldehydbildung stark ansteigen. Dementsprechend erho Èht der thermo-mechanische Holzaufschluû die Formaldehydabgabe der hergestellten Fasern in groûem Ausmaû. Unterschiede in der Formaldehydabgabe verschiedener Holzarten werden aufgezeigt und in Beziehung zu der chemischen Zusammensetzung gebracht.

Wood contains primary extractives, which are present in all woods, and secondary extractives, which are confined in certain wood species. Extractives in wood play a major role in wood-bonding processes, as they can contribute to or determine the bonding relevant properties of wood such as acidity and wettability. Therefore, extractives play an immanent role in bonding of wood chips and wood fibres with common synthetic adhesives such as urea-formaldehyde-resins (UF-resins) and phenol-formaldehyde-resins (PF-resins). Extractives of high acidity accelerate the curing of acid curing UF-resins and decelerate bonding with alkaline hardening PF-resins. Water-soluble extractives like free sugars are detrimental for bonding of wood with cement. Polyphenolic extractives (tannins) can be used as a binder in the wood-based industry. Additionally, extractives in wood can react with formaldehyde and reduce the formaldehyde emission of wood-based panels. Moreover, some wood extractives are volatile organic compounds (VOC) and insofar also relevant to the emission of VOC from wood and wood-based panels.

In the recent past, increasing attention has been paid to the use of natural adhesives as a binder in the wood-based panel industry. Nowadays, two German companies are using Quebracho tannin and Wattle tannin for the production of particleboards and medium density ®berboards (MDF), respectively. To date no serious attention has been given to bond ®berboards using tannin derived from spruce bark. Results of research work on the use of spruce tannin as a binder for medium density ®berboards carried out in the last two years are presented. The results reveal that spruce tannin can be used to partially substitute Quebracho tannin in MDF up to about 60%. Such boards still meet the speci®cation of moisture resistant MDF (according to EN 319 and EN 321). Moreover, spruce tannin can be used to 100% as a binder for the production of MDF for indoor use (type MDF EN 622-5). Fichten-Tannin alsKleber fu Èr Spanplatten und MDF Seit kurzem wird natu Èrlichen Klebern als Bindemittel fu Èr Spanplatten auch in der Industrie erho Èhte Aufmerksamkeit gewidmet. Zur Zeit verwenden zwei deutsche Firmen Quebracho-und Akazientannin fu Èr die Produktion von Spanplatten und MDF. Bisher wurden Tannine aus Fichtenrinde hierfu Èr noch nicht ernsthaft in Betracht gezogen. Die vorliegende Arbeit stellt Ergebnisse eines Forschungsprojektes zur Verwendung von Fichtentannin fu Èr MDF vor. Mit Fichtentannin kann bis zu 60% des Quebrachotannins in MDF ersetzt werden. Solche Platten erfu Èllen die Richtwerte der EN 319 und 321 fu Èr die Feuchteresistenz. Daru Èberhinaus kann bei Platten fu Èr Innenverwendung (Typ EN 622-5) bis zu 100% Fichtentannin verwendet werden. IntroductionUtilization of bark is one of the utmost pressing problems facing the wood industry. The quantities of bark available are substantial, since approximately 10±15% of the volume of every log consists of bark. In Germany, about 2 million tons of bark are produced annually. Dealing with such enormous quantities of bark has created thorny disposal problems, particularly with the outcome of more stringent environmental regulations regarding air and water pollution enacted in the recent past. It has become imperative that the industry no longer considers bark as a waste. Bark is a low cost material with a limited market for diverse applications such as a high-quality mulch or as energy source. However, the current uses do not require bark in suf®cient quantities to consume the supply available.Bark and heartwood of limited wood species contain large quantities of water soluble extractives of polyphenolic nature suitable as a starting material for the production of adhesives. The extractives are mainly concentrated in the inner layer of the bark (Fig 1). Figure 2 shows the extractable amounts by water at about 100°C from different barks and wood species. In general the hot water extract of bark or heartwood comprise about 60±80% of polyphenolic tannin polymers as measured by the so called``hide powder'' method. The remainder is a mixture of many polymeric and monomeric compounds, such...

Emission of volatile organic compounds (VOC) from wood during processing to pelletssignificance of wood extractives. Cellulose, hemicelluloses and lignin are the main components of the cell wall in wood. Moreover, wood contains different amounts of extractives, depending on the wood species. Extractives in softwoods play an important role during processing of wood to different wood products. This work studies the influence of storage on the change of extractives as well as on the emission of monoterpenes (primary emission) and aliphatic aldehydes, as degradation products of fats and fatty acids (secondary emission). In case of pine wood storage for 2 days at 80uC proved to be more efficient in the reduction of emissions of terpenes and aldehydes than 4 weeks at 40uC. Quantitative differences between pine (Pinus sylvestris) and spruce (Picea abies) wood chips in the emission were evaluated and discussed. Pinewood emits about 10 times higher amounts of terpenes and aldehydes than spruce wood. The results indicate that the intense of change in primary and secondary emissions due to storage depends on the wood species as well as on the boundary conditions under which storage is carried out. In general, storage at high temperature (80uC) is much more effective than at low temperature (40uC). The results also reveal that removal of wood extractives from pine wood by extraction with ethanol-cyclohexane or treatment with sodium hydroxide decreases the emission of primary and secondary volatile organic compounds drastically. Nevertheless, after such treatments minute amounts of volatile organic compounds are still detectable.

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The work aimed to study the influence of replacing virgin wood fibres by recycled fibres, made from recovered medium-density fibreboards (MDF) by thermo-mechanical pulping, on the physical-mechanical properties and formaldehyde release of urea-formaldehyde (UF)-bonded MDF. It was found that replacement of 33% virgin fibres by recycled fibres decreased significantly the thickness swelling and water absorption of the UF-bonded MDF and mitigated the formaldehyde release. However, supplanting 33% of virgin fibres by recycled fibres had a negative impact on the internal bond strength. UF-bonded MDF with higher amounts of recycled fibres (67 and 100%) could not be produced without applying the hybrid bonding technology, i.e. adding small quantities of polymeric diphenylmethane diisocyanate to the UF resin. MDF made from 100% recycled fibres showed significantly lower thickness swelling values as well as a drastic drop in the formaldehyde release compared with boards made from virgin wood fibres.

Thermohydrolytic treatment of chips from UFbonded particleboards using the flask method technique at temperatures of about 103°C resulted in a notable increase in the pH-value of aqueous absorption medium after a reaction period of 24 h, most likely due to degradation of the UF-resin. This leads to neutralisation of the stronger formic acid to a much higher extent than acetic acid. Accordingly, the release of formic acid using the flask technique decreases to a much higher extent compared to that of acetic acid under the same conditions. The release of acetic acid could also be enhanced by possible deacetylation of the wood material induced by the hardener usually present in recovered UF-boards. This indicates that inextricably intertwined reactions unfold on hydrothermic degradation of UF-bonded particleboards. Chemische Wechselwirkungen bei dem thermohydrolytischen Abbau von UF-Harzen in RecyclingplattenZusammenfassung Die thermohydrolytische Behandlung von UF-Holzspanplatten bei 103°C unter Verwendung des Flaschenverfahrens führt nach einer Reaktionsdauer von 24 h zu einer deutlichen Erhöhung des pH-Wertes. Dies äu-ßert sich in einer stärkeren Neutralisation von Ameisensäure im Vergleich zu Essigsäure. Entsprechend wird die Abgabemenge von freier Ameisensäure aus den Recyclingplatten E. Roffael ( ) Georg-August