Formaldehydabgabe von mitteldichten Faserplatten (MDF) unter hydrolytischen Bedingungen

Subject Using a modified version of the WKI-bottle-method formaldehyde release from pine-MDF bonded with TF- and MUPF-resin was measured after 3, 24 and 48 hours at 40°C, 50°C and 60°C.The results reveal: With increasing time and increasing temperature the release of formaldehyde increased. Boards bonded with MUPF-resins emanated at 50°C and 60°C, in general, more formaldehyde than those bonded with TF-resins. From the preliminary results it can be assumed that MUPF-resins are less resistant to hydrolysis than TF-resins at high temperatures.

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Die Förderung des Forschungsprojektes erfolgte teilweise durch die AiF über die DGFH.     

Benetzung von TMP und CTMP aus Holz sowie gebrauchten mitteldichten Faserplatten (MDF)

TMP (thermo-mechanical pulps) and CTMP (chemo-thermo-mechanical pulps) were made from pine wood and recycling MDF, which has been prepared from pine wood too. The fibres were tested for their wettability using the method developed by Roffael et al (2002). The results reveal that TMP are much less wettable in comparison to CTMP from the same wood. Moreover, fibres made from UF-bonded MDF by the TMP- and CTMP-process are of much higher wettability compared to TMP and CTMP prepared directly from wood. No significant difference in the wettability between TMP and CTMP from UF-bonded MDF was detected. This behaviour has been attributed to the fact that the UF-resin in MDF degrades during thermohydrolytic pulping leading to the formation of ammonia and free urea. Therefore, pulping of UF-bonded boards takes place under chemo-thermo-mechanical conditions (CTMP).     

Einfluß von Holzaufschlußverfahren (TMP-, CTMP-Verfahren) und Aufschlußbedingungen auf die physikalisch-technologischen Eigenschaften von mitteldichten Faserplatten (MDF)

Ohne Zusammenfassung     

Einsatz von Faserstoffen aus chemo-thermo-mechanisch (CTMP) aufgeschlossenen, gebrauchten Span- und Faserplatten zur Herstellung von mitteldichten Faserplatten (MDF)

Ohne Zusammenfassung     

Einsatz von Faserstoffen aus chemisch-thermisch aufgeschlossenen, gebrauchten Faserplatten zur Herstellung von mitteldichten Faserplatten (MDF)

Ohne Zusammenfassung     

Chemische Eigenschaften von TMP und CTMP, hergestellt aus Kiefernholz und UF-Harz-gebundenen mitteldichten Faserplatten (MDF)

Thermo-mechanical (TMP) and chemo-thermo-mechanical pulps (CTMP) were prepared from pine wood and from UF-bonded MDF made from pine wood. For preparing TMP pine chips as well as UF-bonded MDF were digested under pressure at 170 °C. Thereafter, the digested chips and MDF were defibrated at 170 °C using a single disc refiner. CTMP was prepared from pine wood with 0.25% NaOH (based on dry wood) under the conditions set forth above. CTMP was also made from MDF under two conditions using 0.15% sodium hydroxide (% based on dry MDF) and a mixture of sodium sulphite (1%) and sodium hydroxide (0.25%). The pulps show distinct differences in their properties: TMP from UF-bonded MDF shows lower extractive content in cold and hot water, lower pH-value and higher buffering capacity towards alkali of the cold water extractives than the CTMP counterpart. Moreover, CTMP prepared by using sodium hydroxide alone as a pulping agent increases the content of formate and acetate ions in the cold water extractives. The use of a mixture of sodium sulfite and sodium hydroxide as a pulping agent decreases, however, significantly the content of formate and acetate ions in the cold water extractives. This may be due to the buffering action of sodium sulfite. In general, CTMP decreases the formaldehyde release of the fibres, as measured by the flask method. In presence of sodium sulfite as a pulping agent for recycled MDF, the formaldehyde release is slashed to almost 30% of its original value. The formaldehyde release of CTMP from UF-bonded boards seems to be in the same range as that of TMP from virgin wood.      

Zur Abgabe von flüchtigen Säuren aus mitteldichten Faserplatten (MDF)

Wood and wood-based panels emanate formic and acetic acid. The amount of acetic acid release is high in comparison to its concentration in cold water extracts of wood-based panels containing both formic and acetic acids. This is most probably due to the fact that formic acid is a stronger acid (pK_a 3.77) than acetic acid (pK_a 4.76). Insofar the dissociation of acetic acid is suppressed by the presence of formic acid and the undissociated acetic acid emanates in higher amounts than formic acid in the vapour phase.     

Paraffine unterschiedlicher Kettenlänge als Hydrophobierungsmittel in mitteldichten Faserplatten (MDF)

Ohne Zusammenfassung     

Über die Eignung von thermo-mechanischem und chemo-thermo-mechanischem Holzstoff (TMP und CTMP) aus Buchen- und Kiefernholz für die Herstellung von mitteldichten Faserplatten (MDF)

A condensed review on the state of the art of the producton of medium density fibreboards is given. Thereafter, results of investigations on pulping beech and pine wood under differnet conditions according to the TMP- and CTMP-process are presented. The pulps derived from both wood species were analysed for some of their physical and chemical properties. The results reveal: Within the TMP- and CTMP-process, both the chemical and biological oxygen demand increase with increasing pulping temperature. In general, however, waste water from the CTMP-process has higher chemical and biological oxygen demand. The CTMP-process seems to be milder than the TMP-process as it leads to pulps with higher fibre length, especially in case of beech. The content of acetyle groups is much lower in CTMP-pulps than in TMP-pulps, CTMP-pulps emit also more acetic acid than TMP-pulps. With increasing pulping temperature the pH-value decreases and the buffering capacity towards alkali increases. MDF were prepared from TMP- and CTMP-pulps and their properties were evaluated. The results are included in the next parts of the publication.     

Einfluß der Lagerung von Industrierestholz der Kiefer und Fichte auf seine Eignung für die Herstellung von Spanplatten und mitteldichten Faserplatten (MDF)

The residues of a saw mill with a profiling unit were stored in piles of about 40 m³ for different time periods. From time to time wood material was taken from the piles and disintegrated to chips using a ring flaker or pulped using the thermomechanical process and defibrated. Both chips and fibres were analysed chemically to detect any change in adhesion relevant properties, especially in pH-value, buffering capacity, amount of volatile acid content in the extractives. Moreover, the formaldehyde release and the emanation of volatile acids were assessed using the flask-method technique. The results led to the following conclusions: The pH-value of cold and hot water extractives of chips and fibres derived from saw mill residues increased slightly on seasoning of the residues. The hot extractives showed, however, lower pH-values and higher alkaline buffering capacities than the cold water extractives. The results of measuring the volatile acid content in extractives are in harmony with those of pH-value and buffering capacity. The content of volatile acids of fibres and flakes decreased on seasoning of saw mill residues as well as the release of volatile acids from flakes and fibres. Likewise, storage decreased the formaldehyde release from the chips on seasoning the saw mill residues. Pine flakes and fibres showed always lower pH-value, higher alkaline buffering capacity compared to Spruce chips and fibres. Moreover, the release of volatile acids from Pine flakes and fibres was much higher than from Spruce wood. The changes in adhesion related properties in saw mill residues of Pine wood were much pregnant than in Spruce.     

Einfluß der Lagerung von Industrierestholz der Klefer und Fichte auf seine Eignung für die Herstellung von Spanplatten und mitteldichten Faserplatten (MDF)

Particle- and medium density fibreboards (MDF) were prepared from chips of spruce and pine, a byproduct of sawing wood in a saw mill with a profiling unti. The wood residues were stored in piles of 40 m³ volume for different time periods. The physical-technological properties of the particle- and fibrebroads prepared therefrom were evalutated and the formaldehyde release of the boards was assessed. The results reveal: In general, there is a positive influence of storage on the thickness swelling and internal bond strength of UF-bonded particleboards; the bending strength of UF-bonded particleboards showed no pronounced response to storage. The prositive influence of storage onthe thickness swelling and internal bond strength was much higher in case of boards made from pine chips than in case of spruce boards. On using PMDI as a binder for particleboards no significant influence of storage on the thickness swelling, internal bond strength and bending strength was detected. The influence of storage on the thickness swelling of UF-bonded MDF from pine fibres was much less pregnant than on that of UF-bonded particleboards. On UF-bonded MDF from spruce the influence of storage, if any, was negligible. On using PMDI as a binder for MDF there was no detectable change in the thickness swelling due to storage. The internal bond strength of UF-bonded MDF experiences only subtle positive change due to storage of pine wood chips. However, no notable change was detected in case of MDF from spruce. Storage has almost no influence on the chemical properties of boards.     

Zur Hydrophobierung von mitteldichten Faserplatten (MDF) mit Paraffinen Teil 2: Einfluss des Beleimungsverfahrens (Blender-, Blowline-Verfahren) und der Art der Zugabe von Paraffindispersionen auf die Hydrophobierung von MDF

The main objective of this study was to evaluate the influence of resination process (blender and blowline technique) on paraffin sizing of MDF. Moreover, the method of adding paraffin dispersion within the blender- and the blowline-process on MDF properties was also investigated. For blender-resinated MDF it was found, that adding paraffin dispersion to the fibers before resination leads to lower thickness swelling and water absorption values as well as higher internal bond strength than adding the paraffin dispersion together with the resin to the fibers. Within the blowline-process paraffin dispersion was applied in two different ways. In one method the paraffin dispersion was added to the chips before thermo-mechanical pulping. In the other method the paraffin dispersion was added to the fibers together with the resin in the blowline. In both cases the chips were pulped at temperatures of 140°C, 160°C and 180°C. Irrespective of the method of adding the paraffin within the blowline-process it was found that an increase of pulping temperature from 140°C to 180°C leads to a reduction of thickness swelling and water absorption values of the MDF. Moreover, the results reveal that adding paraffin dispersion to the chips before pulping leads to more effective sizing than applying the paraffin dispersion to the fibers together with the resin in the blowline.     

Über die Eignung von thermo-mechanischem (TMP) und chemo-thermomechanischem (CTMP) Holzstoff aus Buchen- und Kiefernholz für die Herstellung von mitteldichten Faserplatten (MDF)

Medium density fibreboards (MDF) were made from thermomechanical (TMP) and chemo-thermo-mechanical pulps (CTMP) derived from pine wood. As a binder a melamine reinforced urea formaldehyde resin (UF-resin) and diisocyanate polymers (PMDI) were applied. The mechanical-technical properties of the boards as well as some of their chemical properties were evaluated. The results of the investigation led to the following conclusions: MDF from CTMP showed in general higher mechanical properties (bending strength) compared to MDF from TMP. Moreover, the formaldehyde release of the CTMP-boards was lower. This may be due to the formaldehyde scavenging properties of the used pulping chemicals during the process of CTMP. MDF from CTMP showed much higher release of acetic acid compared to MDF from TMP. This is likely to higher deacetylation of the wood substance during CTMP process compared to the TMP technique.     

Einfluß der Trocknung von thermo-mechanischem (TMP) und chemo-thermo-mechanischem Holzstoff (CTMP) der Fichte auf die physikalisch-technologischen Eigenschaften von daraus hergestellten mitteldichten Faserplatten (MDF)

Ohne Zusammenfassung     

Zur Hydrophobierung von mitteldichten Faserplatten (MDF) mit Paraffinen Teil 3: Methoden zur Erfassung der Hydrophobierung von Faserstoffen durch Paraffine

The first part of this publication deals with the influence of chain length and amount of n-paraffins as well as the chemical composition of technical paraffins (chain length and amount of n- and iso-alkanes) on the thickness swelling, water absorption and internal bond strength of MDF (Roffael et al. 2005a). The second part reports on the effect of resination process (blender- and blowline-technique) as well as different methods of adding paraffin dispersion in both resination processes on paraffin sizing of MDF (Roffael et al. 2005b). This paper describes two different methods for evaluation of paraffin sizing on thermo-mechanical pulps (TMP). In one method the sizing degree was measured by determining the wettability of handsheets made from paraffin sized TMP as previously described by Roffael et al. (2002). In the other method paraffin sizing of fibres was determined by using the well known Cobb test which quantifies the water absorption of handsheets. Industrial produced TMP of Scots pine was resinated in the blender by using an MUF-resin together with n-paraffin dispersions of different chain length (C₂₀, C₂₈, C₃₆). The MUF-resin was not cured. Handsheets were manufactured from the MUF-resinated TMP and kraft pulp (ratio 1:1). On using 0.25% n-paraffin (based on dried fibres) wettability of handsheets decreased with increasing chain length of n-paraffins (C₂₀ to C₃₆). Also the results of the Cobb test (Cobb₃₀) showed that water absorption of handsheets decreased by using n-paraffins with higher chain length. Further investigations were carried out on handsheets, manufactured from kraft pulp and TMP resinated in the bowline (ratio 1:1). Within the blowline-process three technical paraffins with different chemical composition were applied in two different ways. In one case the paraffin dispersion was added to the chips before thermo-mechanical pulping. In the other case the paraffin dispersion was added to the fibres together with the resin in the blowline. Results of wettability and of water absorption of handsheets (Cobb test) showed that adding paraffin dispersion to the chips prior to thermo-mechanical pulping leads to more effective paraffin sizing than applying the paraffin dispersion to the fibres together with the resin in the blowline. Moreover, the influence of type of softwood (Scots pine and Norway spruce) on the paraffin sizing of TMP was investigated. Irrespective of the chemical composition of the used technical paraffins it was found, that handsheets manufactured from TMP of Norway spruce showed better wettability and higher water absorption than handsheets manufactured from TMP of Scots pine. The lower water absorption of handsheets manufactured from TMP of Scots pine may be due to the higher content of hydrophobic extractives in pine.     

Vergleichende Untersuchungen an mittelharten Faserplatten (MDF) und Spanplatten

Mechanical and physical properties of commercial medium density fiberboard (MDF) and particleboard were determined and compared. The investigation showed that bending strength and face strength for MDF were considerably higher than corresponding particleboard properties. Concerning internal bond strength and screw holding ability no remarkable differences were found between the two board types. Medium density fiberboard showed lower hygroscopicity and thickness swelling but higher linear expansion than particleboard. Both board types were found to have higher bending strength and lower linear expansion parallel rather than perpendicular to feeding direction. Furthermore MDF showed lower surface roughness than particleboard.     

Zum thermohydrolytischen Abbau von UF-Harzen in Holzspanplatten

Zusammenfassung Über den thermohydrolytischen Abbau von UF-gebundenen Holzspanplatten ist umfangreiches Schrifttum bekannt geworden (vgl. z. B. Fleischer 2001, Kirchner und Kharazipour 2002). Einige der entwickelten Verfahren werden inzwischen in der einschlägigen Industrie praktiziert (Michanickl, Boehme1995). Durch den thermohydrolytischen Abbau der UF-Harze in Gebrauchtspanplatten entzieht sich ein Teil des UF-Harzes hartnäckig der Hydrolyse und verbleibt in den Hydrolyserückständen (vgl. Franke und Roffael 1998a, b).

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During thermal hydrolysis of UF-bonded boards more urea is dissolved than is equivalent to the molar ratio (F:U) in the used resin. Therefore, the molar ratio F:U of the resin in the hydrolysis residues increases.

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Teile dieser Arbeit wurden finanziell gefördert durch die Arbeitsgemeinschaft industrieller Forschungsvereinigung (AiF) mit Mitteln des Bundesministeriums für Wirtschaft und Arbeit (BMWA) über die Deutsche Gesellschaft für Holzforschung (DGfH). Die Autoren danken an dieser Stelle für die freundliche finanzielle Unterstützung.

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Mitteilung aus dem Lehrbereich Holzchemie und Holztechnologie des Instituts für Holzbiologie und Holztechnologie der Universität Göttingen