823 research outputs found
On the influence of binder content in particleboards bonded with resins of high and low molar ratio on the formaldehyde release measured by the perforator method
Full text linkA comprehensive literature survey and the results of investigations clearly show that the influence of resin level in particleboards on the perforator value (EN 120) depends on the molar ratio of the resin used as a binder. In case of high molar ratio resins (F:U 1.6:1) increasing the binder content in the boards leads to an increase in the perforator value of the boards. In contrast, boards bonded with very low molar ratio aminoplastic resins with molar ratio F:U of 1:1 (F:NH2 0.5:1) or even lower do not show any noticeable increase in the perforator value on raising the binder level. On the contrary, the measured perforator values may sometimes decline due to increase in the resin content in the boards. The perforator test seems to include two main processes taking place alongside each other: the physical extraction of free formaldehyde and the chemical hydrolysis process of the resin itself, induced by the moisture content of the boards. In case of low molar ratio resins, particularly at very low molar ratios (F:U 1:1 or even lower), free urea may also be present in the resin or added to the resin as a scavenger. Urea seems to react with extracted formaldehyde at high temperature during the extraction process. This lowers the perforator value without necessarily decreasing the emission at ambient conditions. Insofar, inextricably intertwined chemical processes seem to unfold during the extraction process. Additionally, the results show that subtle differences in the perforator value in the very low formaldehyde emission region may remain without noticeable impact on the emission measured by the chamber method (EN 717-1)
Einfluss der Alterung UF-Harz gebundener Holzspanplatten auf die Abgabe flüchtiger Säuren
No full textThe release of volatile organic acids from UF-bonded particleboards made from Scots pine was assessed after production as well as after aging for 13 years. The results reveal that aging significantly decreased the release of formic acid and substantially increased the emanation of acetic acid. The change in the release of volatile acids by aging seems to be highly independent of the hardener type used (ammonium chloride, ammonium nitrate) as well as of the amount of added hardener
Influence of binders on the water retention value (WRV) of glued recycling fibers and chips
No full textThe hydrophobing influence of binders on the equilibrium moisture content (EMC) of wood and fibers is well known in the literature (Roffael und Schneider 1978, 1979). Preliminary experiments reveal, that this effect can be assessed by measuring the water retention value (WRV) of glued recycling fibers and chips
Water support power of recycling shavings and of shavings extracted directly from wood
No full textDas Wasserrückhaltevermögen nimmt mit Verlängerung der Lagerungsdauer im Wasser zu, wobei die Zunahme im Falle der mechanisch gewonnenen Recyclingspäne (M) besonders ausge- prägt und im Falle der thermohydrolytisch gewonnenen Späne (T) auffallend gering war. Gegenüber den direkt aus Holz gewonnenen Spänen weisen die mechanisch gewonnenen (M) besonders niedrige WRV-Werte auf. Durch die thermohydrolytische Behandlung der Späne, bei der erhebliche Teile des Bindemittels abgebaut werden, nimmtdas Wasserrückhaltevermögen deutlich zu und erreicht sogar Werte, die höher liegen als die von „frischen“ Spänen. Für den Abbau des UF-Harzes infolge der thermischen Behandlung spricht auch der gegenüber den mechanisch gewonnenen Spänen (M) niedrigere Stickstoffgehalt der thermohydrolytisch erzeug- ten Späne (T). Es ist nicht auszuschließen, dass während der ther- mohydrolytischen Behandlung ebenfalls Acetylgruppen im Holz abgespalten werden, die zu einer zunehmenden Hydrophilie der Späne führen. Auf einen Abbau von Acetylgruppen deutet der vergleichsweise hohe Gehalt an Essigsäure in den Kaltwasserex- trakten der Späne hin, der nach der Thermohydrolyse festgestellt wurde. </p
Wettability of recycling shavings and "fresh" wood shavings with various adhesive agents
No full textNach allen Messzeiten zeigten die „frischen“ Holzspäne eine bessere Benetzung mit UF-Harz, PF-Harz und PMDI-Klebstoff als die Recyclingspäne. Besonders durch mechanische Zerklei- nerung gewonnene Recyclingspäne weisen eine geringere Benet- zung auf als die „frischen“ Holzspäne. Dies ist womöglich auf das den Recyclingspänen anhaftende Binde- und Hydrophopie- rungsmittel zurückzuführen. Die bessere Benetzung der durch thermohydrolytischen Aufschluss gewonnenen Späne ist möglicherweise darauf zurückzuführen, dass während des Aufschlus- ses Teile des Bindemittels chemisch abgebaut und wasserlöslich werden. Die Benetzung der Recyclingspäne mit PMDI ist auffallend besser als mit UF- und PF-Harz. Dies ist unabhängig davon, ob die zu benetzenden Späne durch mechanische Zerkleinerung oder durch thermohydrolytischen Aufschluss gewonnen wurden.</p
Abbau von UF-Harzen beim TMP- und CTMP-Aufschluss von mitteldichten Faserplatten (MDF)
No full textThermo-mechanical (TMP) and chemo-thermo-mechanical pulping (CTMP) of UF-bonded MDF induce considerable degradation of the UF-resin in the board and lead to a conspicuous increase in the content of water extractives of the fibres. Moreover, the water extractives of the fibres have a higher pH-value and a lower buffering capacity as well as higher acetate and formate ions content than the extractives of the original board. However, the lignin content seems rather to increase than to decrease. This is possibly due to reaction between lignin and formaldehyde from the resin
On the formaldehyde release of wood particles
Full text linkThe emission of formaldehyde from softwood particles, as measured by the flask method (EN 717-3), depends highly on the particle size. Therefore, no definite value for the formaldehyde release from wood can be given
IR-Spektren von TMP und CTMP, hergestellt aus Kiefernholz und mitteldichten Faserplatten (MDF)
No full textIR-spectra of TMP and CTMP prepared from pine wood and UF-bonded MDF made from pine wood show characteristic differences. The typical band for carbonyl groups (C=O-groups) at wave number about 1740 cm(-1) is strong in pine wood, but appears relatively weak in TMP and especially in CTMP made thereof. In UF-bonded MDF made from pine wood the band at about 1740 cm-1 is very weak compared to that in pine wood. Moreover, no significant difference in the IR-spectrum between TMP and CTMP made from UF-bonded MDF seems to exist. This is attributed to the fact that during thermo-mechanical pulping of UF-bonded boards hydrolytic degradation of the resin takes place enhancing the formation of ammonia which increases the pH-value of the fibres and leads therefore to chemo-thermo-mechanical pulping
Benetzung von TMP und CTMP aus Holz sowie gebrauchten mitteldichten Faserplatten (MDF)
No full textSubject 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)
Chemische Eigenschaften von TMP und CTMP, hergestellt aus Kiefernholz und UF-Harz-gebundenen mitteldichten Faserplatten (MDF)
No full textSubject 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 degrees C. Thereafter, the digested chips and MDF were defibrated at 170 degrees 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
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