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.     

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)

Medium density fibreboards (MDF) were made from beech in laboratory and pilot plant scale from thermo-mechanical (TMP) and chemo-thermo-mechanical pulps (CTMP) using both melamine reinforced urea-formaldehyde resin (UF-resin) and diphenylmethane diisocyanate polymers (PMDI). The physicalmechanical and chemical properties of the boards were evaluated. From the results the following conclusions can be drawn:

1. Both TMP and CTMP led, on using UF-resin, to MDF with very high mechanical properties exceeding the required values in European standards.
2. Pulping temperature seems to have an influence on the mechanical properties of the boards, prepared from TMP and CTMP. Increasing the pulping temperature from 150°C to 170°C negatively affects the mechanical board properties, whereas the properties of MDF from CTMP (sulfonic group content between 0.2% and 0.3%) increases by elevating the maximum pulping temperature from 150°C to 170°C.
3. The pulping chemicals in the CTMP-process (Na₂SO₃ and/or NaOH) decrease the formaldehyde release from the boards, as they act as scavengers for formaldehyde.
4. Due to higher deacetylation degree during CTMP process, MDF made from CTMP release more than 4 times acetic acid than MDF from TMP. The release of formic acid is quite different, it is in MDF, made from TMP higher than in MDF from CTMP.
5. MDF with very high mechanical properties can also be made from beech pulps (TMP) using PMDI. PMDI in combination with a formaldehyde scavenger in the middle layer and UF-resin in the surface layer leads to boards with very low formaldehyde release.

     

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.      

On the release of volatile acids from wood-based panels – chemical aspects –

Wood-based panels release different amounts of volatile organic acids, which depend, among other factors, on the binder used. The volatile acids released from the boards can be measured using the flask method following the principle of measuring the formaldehyde release according to EN 717-3. Particleboards bonded with alkaline curing phenol-formaldehyde resins (PF-boards) release higher amounts of acetic and lower amounts of formic acid compared to those boards bonded by acid curing urea-formaldehyde reins (UF-resins) or binders based on polymeric diphenylemethandiisocyanates (PMDI). This has been explained by the equilibrium reaction between sodium hydroxide in PF-resins and volatile acids, which leaves less free formic acid than free acetic acid to emanate from the boards. Increasing the binder content in the boards also leads to an increase in the over all amount of sodium hydroxide in the boards. Accordingly, less free acids are left, which could be released from the boards. Fibres made by the chemo-thermo-mechanical process (CTMP) using sodium hydroxide and sodium sulphite as pulping agents release less formic acid than those made by the thermo-mechanical pulping (TMP). This behaviour was also attributed to the equilibrium reaction between the alkali in CTMP-boards and the volatile acids.     

Zum Verhalten des Splint- und Kernholzes der Lärche (Larix decidua) bei der Herstellung von feuchtebeständigen Spanplatten unter Einsatz verschiedener Bindemittel

Chips from sap- and heartwood of 20, 40 and 102-year-old larch (Larix decidua) were prepared and chemically characterized. From the chips one-layer boards of 20 mm thickness were prepared using melamine-urea-phenol-formaldehyde resin (MUPF-resin), phenolformaldehyde resin (PF-resin), polymeric diisocyanate (PMDI), and tannin-formaldehyde resin (TF-resin). The physical and chemical properties of the boards were determined. Moreover, the influence of hot water extractives on the pH-value, viscosity and gelation time of the resins was assessed. Sap- and heartwood chips are different in their chemical properties as well as in bonding behaviour. The age of the tree has also an influence on the bonding characteristics of the chips. Thickness swelling, water absorption and equilibrium moisture content of heartwood boards were always lower than those of sapwood boards. Moreover, bending and internal bond strength of heartwood boards made from 20- and 40-year-old trees are much higher than those produced from sapwood. The strength properties of the boards deteriorate, however, with increasing age of the tree. This applies for both heartwood and sapwood boards. The water extractives of heartwood boards are of lower pH-value and higher buffering capacity than the corresponding boards from sapwood. Moreover, the emanation of acetic acid from heartwood boards is much higher than that of formic acid. In addition, boards from heartwood are in general of lower formaldehyde release compared with boards from sapwood. The addition of heartwood extractives to MUPF-resin increases the viscosity and gelation time of the resin and decreases its pH-value, whereas in case of PF-resin no such increase in the gelation time was registered. The extracts of heartwood increase the viscosity and gelation time of TF-resin.     

Veränderungen der chemischen Eigenschaften von Fasern und MDF durch Waschen der aufgeschlossenen Fasern

Thermomechanical pulps from beech were in parts washed by water at room temperature to remove water soluble components. MDF were prepared from unwashed and washed fibres under exactly the same conditions. The chemical properties of the fibres as well as of the boards from unwashed and washed fibres were assessed using conventional methods. The results reveal that washing of the thermomechanical pulps (TMP) increased the pH-value and decreased the alkaline buffering capacity of their water extractives. Moreover, the emission of formaldehyde and furfural from the fibres and from the medium density fibreboard (MDF) prepared therefrom declined significantly. Also, the emission of volatile acids from fibres and boards dropped enormously. Moreover, the amount of extractable urea from the boards increased significantly.     

Use of UF-bonded recycling particle- and fibreboards in MDF-production

The work is related to the use of thermomechanical pulps (TMP) prepared from recycling UF-bonded particle- and fibreboards as a partial substitute for TMP made directly from wood in MDF. The results reveal that TMP from recycling boards show striking differences in their morphological characteristics and chemical properties compared to TMP prepared directly from wood. TMP from recycling boards are of shorter fibre length and higher content of fine fraction. Moreover, their cold water extractives are of higher pH-value and contain appreciably higher amounts of acetate and formate ions. TMP from recycling boards releases also more formaldehyde compared to TMP made directly from wood. Moreover, the results clearly show that TMP from recycling boards can be used up to 30% as a partial substitute for TMP from wood in making UF-bonded MDF without any noteworthy deterioration of the physical-mechanical board properties. Also, the formaldehyde release of the boards made using TMP from recycling boards experiences no negative change. Only small differences were found in the content of formate and acetate ions as well as in the release of volatile acids from MDF made from recycling fibre- and particleboards on the one side and boards directly made from wood on the other side. All the results indicate that chemical interaction seems to take place between the chemically degraded resin in UF-bonded recycling boards and the new binder used in making MDF.     

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).     

Über die Abgabe von Ameisen- und Essigsäure aus Deck- und Mittelschichten von aus CTMP hergestellten mitteldichten Faserplatten (MDF)

Volatile acids (formic and acetic acid) from the surface and core layers of medium density fibre-boards (MDF) made from chemo-thermo-mechanical pulps (CTMP) were measured following the flask method technique. The results indicate explicitly that the emission of acids from the core layer is much higher than from the surface layer. The results are indicative of the influx of the steam distillable volatile acids from the surface layers to the core layer during pressing operation.     

Abbau von UF-Harzen beim TMP- und CTMP-Aufschluss von mitteldichten Faserplatten (MDF)

Thermo-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.     

Charakterisierung einiger auswaschbarer Stoffe in thermomechanisch gewonnenen Holzstoffen der Buche

Chips of beech wood were digested to fibres by thermo mechanical pulping (TMP). Part of the obtained fibres was washed using cold water. The amount of water soluble material as well as its chemical oxygen demand (COD) were measured. Moreover, the formaldehyde release of washed and unwashed fibres was determined using the Flask-Method. The pH-value and the alkaline buffer capacity of the water extracts of the fibres were also measured. The results reveal: The formaldehyde release of the fibres decreased by washing the fibres. The pH-value of the washing medium in the Flask-test after a testing period of 72 hours was higher for washed fibres, correspondingly the alkaline buffering capacity of the water extracts of washed fibres was lower after the same period.     

Zum Einfluss des Holzes sowie des Holzaufschlussverfahrens auf das Benetzungsverhalten von Kiefern- und Fichten-Faserstoffen

The main objective of this research work was to study the wettability of thermomechanical pulps (TMP) prepared from heart- and sapwood of Scots pine (Pinus sylvestris [L.]). The TMP were prepared at different pulping temperatures (150 °C, 180 °C). Furthermore, thermomechanical (TMP) and chemo-thermomechanical pulps (CTMP) from Norway spruce (Picea abies [Karst.]) were also investigated. In this case both TMP and CTMP were prepared at 140 °C and 180 °C. Sheets of all pulps were prepared according to a method developed by Roffael et al. (2002). Sheets of TMP pulps from pine sapwood showed much better wettability compared with their counterparts from heartwoods. Pulps from heartwood experienced a dramatic decrease in their wettability when increasing the pulping temperature from 150 °C to 180 °C. In contrast increasing the temperature from 150° to 180 °C seems to have no deleterious effect on the wettability of pulps prepared from sapwood. This phenomenon has been explained by the higher extractive content in pine wood. Comparing TMP and CTMP pulps from Norway spruce leads to the result that in general CTMP pulps are of higher wettability. This can be attributed to the high hydrophobic extractive content in TMP pulps on the one hand and to the higher alkalinity of CTMP pulps on the other hand.     

Verringerung der Emission von Thermoholz durch nachtr?gliche Behandlung mit Ammoniak

Thermally modified timber (TMT) emits a high amount of aldehydes (furfural, formaldehyde) and volatile organic acids (formic and acetic acids). The release of aldehydes and acids, as assessed by the flask-method (EN 717-3), can be reduced significantly by ammonia fumigation. The emission of formaldehyde is attenuated by more than 90%. Emission of furfuraldehyde decreased in most cases by more than 70%. The pH-value of TMT increased due to ammonia fumigation above the neutral point indicating buffering of the volatile acids.     

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.     

Complex chemical interactions on thermo hydrolytic degradation of urea formaldehyde resins (UF-resins) in recycling UF-bonded boards

Thermohydrolytic treatment of chips from UF-bonded 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.     

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.     

Potential of pulp and paper secondary sludge as co-adhesive and formaldehyde scavenger for particleboard manufacturing

This study investigated the potential of secondary sludge (SS) as urea–formaldehyde (UF) co-adhesive for particleboard manufacturing. Three proportions of SS from three conventional pulping processes were added in the formulation of particleboard manufacturing. A 3³ factorial design was used. All panels were tested for thickness swell (TS), linear expansion (LE), internal bond strength (IB), flexural modulus of elasticity (MOE), flexural modulus of rupture (MOR) and formaldehyde emission. Results indicated that particleboards made with SS from thermomechanical pulp (TMP) and kraft pulp (Kraft) met the ANSI standards for LE, IB, MOE, and MOR (with 7 and 9 % UF). However, the TS of panels made with SS was higher than that of control panels and adding SS to the formulation affected negatively this property. Most of the properties studied in the particleboards made with SS from chemical–thermomechanical pulping (CTMP) process failed to meet the ANSI standards. The main advantage of using SS as co-adhesive is the reduction of formaldehyde emission, in the best case here, about 50 %, with CTMP sludge added, of the particleboards.     

Einfluß der Verkernung und des Baumalters auf die Eigenschaften von Spanplatten aus Kiefernholz (Pinus sylvestris)

Chips from sap- and heartwood of 2-- and 126-year-old pine of the same site were chemically characterized. One-layer boards of 20 mm thickness were prepared from the chips using melamine-urea-phenol-formaldehyde resin (MUPF-resin), phenolformaldehyde resin (PF-resin), tannin-formaldehyde resin (TF-resin) and adhesives based on polymeric diisocyanate (PMDI). The physical and chemical properties of the boards were determined. Moreover, the influence of hot water extractives on the pH-value, viscosity and gelation time of the resins was assessed. The results reveal: Sap- and heartwood chips differ in their chemical properties and bonding behaviour. The age of the tree has also a dominant influence on the bonding characteristics of the chips. The addition of heartwood water extractives to PF- and MUPF-resin decreases the pH value and increases the viscosity, the attained viscosity of the resins remained unchanged for 6 h. In addition, water extractives of heartwood increase significantly the gelation time of TF-resin. Thickness swelling, water absorption and equilibrium moisture content of heartwood boards were always lower irrespective of the binder used than those of the corresponding sapwood boards. Moreover, bending and internal bonding strength of heartwood boards from 40-year-old trees were much higher than those produced from corresponding sapwood. The strength properties of the heartwood boards deteriorate, however, with increasing age of the tree. In addition, MUPF- and TF-bonded boards from heartwood were in general of lower formaldehyde release compared with boards from sapwood. The chemical properties of heart- and sapwood boards depend on the binder and the age of tree.     

Influence of resin content and pulping temperature on the formaldehyde release from medium density fibreboards (MDF)

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.