A virtual design method for medium density fiberboards (MDF) is proposed with the aim to optimize the fiber orientation and lay-up of MDF. The new method estimates the stiffness and strength by using microstructure models of the MDF fiber network. The virtual design is used to improve the manufacturing technology of MDF plates with multilayer oriented fiber structure. Experimental investigations of the mechanical behavior of MDF microstructure for various fiber geometries, glue content and distribution are complicated, time consuming and expensive. On the other side, virtual microstructure design allows to develop a new wood fiber based material with less experimental work. Microstructure models help to better understand the non-linear damage mechanical behavior of a wood fiber network depending on fiber geometrical parameters. Such parameters as crack distribution and fiber deformation on micro-scale level are complicated to experimentally measure, but possible to model using computer simulations. The virtual design tool requires less empirical data. The model takes into account information on average wood fiber orientation, fiber diameter, fiber length and mechanical properties of wood fiber cell wall and glue. The numerical method for strength and stiffness analysis of MDF microstructure was calibrated using standard MDF with non-oriented fibers. It turned out that this method gives precise results for MDF with oriented fibers and even with multilayer structure. The proposed virtual microstructure design tool can significantly improve and speed-up the optimization manufacturing technology of MDF and other wood fiber based composites.

This paper presents numerical methods for the characterization of fiber orientation and fiber bundles of medium density wood fiberboards (MDF). The strength and stiffness of MDF is significantly affected by the fiber orientation and fiber bundles. Proposed methods and results are necessary to virtually generate realistic fiber networks and optimize MDF by using computer simulations. Based on 3D \(\mu\)CT images for laboratory manufactured MDF with oriented fibers, the fiber orientation is calculated in two ways. Firstly, we use an image processing method based on Hessian matrix directly on \(\mu\)CT image. Secondly, we computed the effective heat conductivity by solving PDEs on a segmentation of the \(\mu\)CT image to estimate the fiber orientation. A fiber bundle segmentation method based on local fiber orientations is introduced. Fiber bundles, which are segmented by this method show good agreement with manually segmented ones. It was observed that fiber bundles are oriented in MDF plane with log-normal distribution of bundle length. The proposed methods are general and can be used also to calculate fiber orientation and segment fiber bundles in fiber concrete, paper, glass and carbon fiber composites.

Study of the influence of treating pine wood strands with hydrogen peroxide as an oxidising agent and sodium sulphite as a reducing agent on the emission of volatile organic compounds revealed that hydrogen peroxide increases the emission of aliphatic aldehydes (e.g. hexanal) and reduces the emission of monoterpene compounds (like α-pinene). Sodium sulphite as a reducing agent decimated the emission of monoterpene compounds without noticeably affecting the emission of volatile aldehydes.

The main objective of this study was to evaluate the influence of moisture content on the formaldehyde release of particleboards and medium density fibreboards (MDF) bonded with tannin–formaldehyde (TF-) resins. The results revealed that increasing the moisture content in the TF-bonded boards elicited a decline in the content of extractable formaldehyde (perforator values according to EN 120). This is an opposing behaviour to the general state of knowledge related to the influence of moisture content on the formaldehyde content (EN 120) of particleboards and MDF bonded with urea–formaldehyde based resins. Moreover, the TF-bonded panels showed a decrease in the gas analysis values (EN 717-2), flask values (EN 717-3) as well as formaldehyde emission measured in the 1 m3-chamber (EN 717-1). EN 120 (now also pr EN ISO 12460–5, 2013) does not take into consideration that boards bonded with other binders rather than aminoplastic resins may behave differently at different moisture contents. Insofar, it leaves no leeway to consider different binders systems.

A numerical two scale method for the prediction of tensile and bending stiffness and strength of medium density fiberboards (MDF) is proposed with the aim to study the fiber orientation influence on mechanical properties of MDF. The method requires less experimental data to optimize MDF and to improve industrial manufacturing technology of MDF. A new method for computing orientation tensors of the compressed fiber network is proposed. First, the virtual microstructure is generated by simulations of a fiber laydown and a subsequent compression to obtain the necessary density. The density profile, fiber length, thickness, and orientation are used for the microstructure generation, which are obtained from mu CT images and image analysis tools. Then a new damage model for the wood fiber cell walls and joints is introduced. The microstructural problem is formulated as a Lippmann-Schwinger type equation in elasticity and solved by using Fast Fourier Transformation (FFT). The macroscopic three point bending test is simulated with hexahedral finite elements and analytical methods based on Euler Bernoulli theory. The difference between bending strength and stiffness numerically obtained and corresponding experimentally measured values is less than 10%. This study lays a foundation for the optimal design of MDF fiber structures and the optimization of industrial manufacturing processes. The first results show an increase of up to 60% for bending stiffness in the case of strongly oriented fibers. (C) 2014 Elsevier Ltd. All rights reserved.

The 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.

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.

Die Untersuchungen betreffen die Eignung von TMP (thermo-mechanisch hergestellte Fasern) aus gebrauchten Holzspan- und -faserplatten als Rohstoff für die Herstellung von mitteldichten Faserplatten. Die Untersuchungsergebnisse lassen folgende Schlüsse zu: TMP aus gebrauchten Holzspan- und Holzfaserplatten weisen charakteristische morphologische und chemische Unterschiede zu TMP aus Holz auf. TMP aus Gebrauchtfaser- und -spanplatten sind kürzer in der Faserlänge und verfügen über einen höheren Feinanteil. Die Kaltwasserextrakte der TMP aus Gebrauchtspanplatten und Gebrauchtfaserplatten weisen einen deutlich höheren pH-Wert und einen höheren Gehalt an Acetat- und Formiationen auf. Auch die Formaldehydabgabe der TMP aus gebrauchten Span- und Faserplatten ist höher als die des aus Holz hergestellten TMP. TMP aus Holz lässt sich bei der Herstellung von UF-Harz-gebundenen MDF zu 30% durch TMP aus Gebrauchtspan- und -faserplatten ersetzen, ohne dass die mechanisch-technologischen Eigenschaften der hergestellten Platten eine nennenswerte Beeinträchtigung erfahren. Auch die Formaldehydabgabe der hergestellten Platten wird hierdurch nicht erhöht. Es sind weiterhin keine signifikanten Unterschiede in der Abgabe an flüchtigen Säuren zwischen den direkt aus Holz hergestellten Platten und denen unter Zusatz von TMP aus Gebrauchtspan- und -faserplatten gefertigten MDF feststellbar. Dies kann als hinreichendes Kriterium dafür angesehen werden, dass zwischen dem eingesetzten Bindemittel und den chemischen Abbauprodukten der Gebrauchtholzspan- und -faserplatten chemische Wechselwirkungen bestehen.

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.

IR-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.

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.

Chips from pine wood were subjected to thermo-mechanical pulping (TMP) at 140 and 180 oC for 5 minutes, whereas the cooked chips were defibrated using a single disk pressurized refiner at the same temperatures (140 and 180 oC). The fibres were tested for some of their morphological properties including fibre length, fibre width, cellwall thickness. Moreover, the fine fibre fraction (zero fibres) and the content of splinters were also estimated. The results reveal, that increasing the temperature during thermo-mechanical pulping decreases the fibre length, the cell width and the fibre wall thickness. It also increases the amount of fine fibres and increases the curl factor.

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.

IR-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.