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.     

X-ray photoelectron spectroscopy determination of urea formaldehyde resin coverage on MDF fibre

Quantification of fibre resin coverage is essential for the understanding of panel integrity in medium density fibreboard (MDF) and similar panel products. X-ray photo-electron spectroscopy (XPS) was used to determine the proportion of fibre surface covered by nitrogen containing UF resin in blowline resinated and dry-blended MDF. Coverage values were dependent on processing variables such as resin content, wax content, resin spray droplet size and for blowline resination, the position of the resin injection nozzle along the blowline. Comparison of XPS coverage values with those obtained using confocal laser scanning microscopy (CLSM) revealed substantially differing absolute resin coverage of fibre between the two techniques. For dry-blended panels there was a correlation between the two techniques. However, grouping resin coverage values having more than one processing variation did not establish any relationship between XPS and CLSM coverage values, whether for dry-blended or blowline resinated MDF. Wax movement as well as deposition of other materials on the fibre surface during hot pressing were observed to contribute to lower surface %N content, which may limit the usefulness of the XPS technique.     

Influence of hydrothermal modification of fibers on some physical and mechanical properties of medium density fiberboard (MDF)

Any treatment of fibers can influence properties of medium density fibreboard (MDF). In this research, the effects of hydrothermal treatment on physical and mechanical properties of MDF were studied. Industrial fibers were hydrothermally treated in a stainless steel reactor at 120, 150 and 180 °C for 0, 30 and 90 min as holding time. Test boards were made based on 0.7 g/cm³ target density, with 10 mm thickness under a pressure of 30 bar and at a temperature of 170 °C and a press time of 10 min. The boards were tested for internal bonding (IB), moduli of elasticity (MOE) and rupture (MOR), thickness swelling and water absorption. Results showed that the water absorption was not affected by the hydrothermal treatment; while the thickness swelling was improved and the boards became dimensionally stable. The MOE was slightly reduced due to the hydrothermal treatment. The MOR and the IB were significantly decreased by the hydrothermal treatment.     

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.     

Characterization of novolac type liquefied wood/phenol/formaldehyde (LWPF) resin

Novolac type liquefied wood/phenol/formaldehyde (LWPF) resins were synthesized from liquefied wood and formaldehyde. The average molecular weight of the LWPF resin made from the liquefied wood reacted in an atmospheric three neck flask increased with increasing P/W ratio. However, it decreased with increasing phenol/wood ratio when using a sealed Parr reactor. On average, the LWPF resin made from the liquefied wood reacted in the Parr reactor had lower molecular weight than those from the atmospheric three neck flask. The infrared spectra of the LWPF resins were similar to that of the conventional novolac resin but showed a major difference at the 1800–1600 cm^-1 region. These results indicate that liquefied wood could partially substitute phenol in the novolac resin synthesis. The composites with the liquefied wood resin from the sealed Parr reactor yielded higher thickness swelling than those with the liquefied wood resin from the three neck flask likely due to the hydrophilic wood components incorporated in it and the lower cross-link density than the liquefied wood resin from the three neck flask during the resin cure process.     

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.     

Effects of nanowollastonite on biological resistance of medium-density fiberboard against Antrodia vaillantii

The effect of wollastonite nanofibers (NW) on biological resistance of medium-density fiberboards (MDF) made from wood and chicken-feather fibers (CF) against Antrodia vaillantii was studied. NW content of 10 % and CF content of 5 and 10 %, based on the dry weight of wood fibers, were applied to the MDF matrix, giving a total of six different MDF mixing treatments. Mass loss values were measured roughly following the EN 113 specifications. The results showed that NW significantly decreased mass loss to a considerable extent in all mixing ratios, proving its potential to be used as a suitable preservative in wood-composite materials without environmental hazards. A CF content of 5 % showed improving effects on biological resistance, while CF of 10 % was too high and resulted in a weak MDF-matrix; eventually the biological resistance did not improve properly. NW ameliorated part of the undesirable effect of adding chicken feather fibers to the MDF-matrix. A significantly high correlation was found between mass loss versus water absorption (R ² of 81 %), implying that the penetration of water and fungal hyphae can have rather similar patterns. It can be concluded that NW not only can be used to improve the biological resistance in wood-composite materials against fungi attack, but it can also reduce some of the undesirable properties of chicken feathers, thus providing a reliable and renewable resource for natural fibers to be used in the MDF manufacturing industry.     

Mechanical properties of wood treated with PF-pyrolytic oil resins

Jack pine and sugar maple wood samples were treated in a two-step process that involved first a copper chloride or a copper chloride-sodium borate mixture and then a phenol-formaldehyde resin containing a certain percentage of softwood bark pyrolytic oil. Various controls were also prepared for comparison. The modulus of rupture (MOR) of jack pine samples was generally negatively affected by the treatment, the observed values being lower than those of the controls. These values were statistically lower or equal to those of untreated samples. On the other hand, the modulus of elasticity (MOE) of treated jack pine samples was not found to be statistically different from that of the untreated wood. Mechanical properties behaved in a similar way for sugar maple wood. Unlike the other treatments, copper chloride–sodium borate and the resin containing 85% of pyrolytic oil as phenol substitute appeared to slightly improve the mechanical properties of both wood species even if not significantly so. Treatment with PF-pyrolytic oil resin resulted in similar or slightly better mechanical properties when compared to CCA-treated wood. The amount of treating water soluble salts retained in samples after the first treatment had a significant impact on MOR of both wood species and on MOE of sugar maple.     

Influence of different climatic conditions on the roughness of uncoated medium density fiberboards (MDF)

The surface roughness is primarily a function of the raw material properties, but other factors like type and amount of resin, press cycle, sanding and moisture content of the boards may also affect the roughness and other surface properties. In this study the effect of the equilibrium moisture content on medium density fiberboards (MDF) surfaces was evaluated using different raw materials in the surface layers as well as different binders. The statistical analysis confirmed that the relationship between equilibrium moisture content and average roughness is a complex function of interactions between many variables.     

Gas pressure measurements during continuous hot pressing of particleboard

Knowledge about the development of the internal gas pressure during hot pressing of wood-based composites is important for the optimization of panel properties and production speed. The gas pressure heavily affects the thermodynamic conditions inside the wood furnish mat, and a too high maximum value at press opening might cause an impairment of the panel properties. In this paper, gas pressure and temperature measurements inside a particle mat while passing through a continuous hot press are presented for the first time. The measurements were performed with a transportable system, consisting of a steel tube attached to a miniature pressure transducer and a data logger. The particleboards had a target thickness of mainly 16 mm, but also of 28 mm and 38 mm, respectively. The measurements show a distinct horizontal gas pressure distribution in both directions, in production direction and across the mat’s width. In contrast, cross-sectional gas pressure gradients were only visible inside the panels after leaving the press. By comparing the gas pressure curves measured for particleboard with those for medium density fiberboard (MDF), characteristic differences became evident. Overall, the gas pressure is higher in MDF compared to particleboard. Finally, a comparison between the gas pressure levels measured for three different panel thicknesses showed a clear relation between panel thickness and gas pressure, with a decreasing panel thickness resulting in an increase in gas pressure. The results of this paper will contribute to our understanding about the events inside wood furnish mats during continuous hot pressing.     

Virtual characterization of MDF fiber network

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.     

Investigation of the interrelations between defibration conditions,fiber size and medium-density fiberboard (MDF) properties

Defibration conditions and raw material properties affect wood fiber characteristics, and thereby the properties of fiber-based panels such as high-density fiberboard (HDF), medium-density fiberboard (MDF) and wood fiber insulation board. This study investigates the influence of steaming conditions (time and temperature), grinding disc distance, wood species (pine, beech, birch and poplar), method of refiner discharging (radial and tangential stock outlet) and wood chip size on fiber length and fiber length distribution, and further the influence of fiber size on MDF properties. Fiber lengths were determined applying the recently developed image analysis-based fiber size measuring system FibreCube. This system enables an automated and nearly complete mechanical separation of woolly-felted fiber samples prior to image acquisition, software-supported post-separation of overlapped-lying fibers at the beginning of image analysis, and flow line tracing-based length measurement. It was found that grinding disc distance and wood species are the most influential parameters on fiber length characteristics. Especially the content of undefibrated fiber bundles (shives) was found to strongly correlate with the grinding disc distance. Wood anatomical differences between hardwood and softwood were reflected clearly by the fiber length characteristics. Fiber size was found to be one of the parameters influencing panel properties. However, other fiber characteristics—in particular the chemical nature of the fiber, which is responsible for its wettability with water (thickness swelling) and glue (mechanical properties)—have to be considered as important influencing parameters on panel properties.     

A semi-automatic method to determine the wood failure percentage on shear test specimens

An alternative method to quantitatively determine the wood failure percentage on shear test specimens was developed, experimentally tested and successfully applied to specimens with typical combinations of adhesive types and wood species used by the glulam industry in Switzerland. The method consists of a staining technique for a better differentiation of wood fibers and adhesive, and an image processing procedure for a standardized estimation of the fractured surface ratio covered by wood fibers and adhesive, respectively. The semi-automatic method allows for a more objective determination of wood failure percentage and is suitable for quality control in the glulam industry.     

On the performance of a melamine–urea–formaldehyde resin for decorative paper coatings

The decorative laminates industry is a highly competitive industrial sector. To be profitable, manufacturers of impregnated papers for surface laminated MDF and particleboards need to significantly reduce their production costs. Melamine formaldehyde resin (MF) formulations are commonly used for impregnation and coating of such papers, melamine being an important, but costly raw material used in high quantities. While MF is substituted by cheaper urea formaldehyde resins (UF) in the core impregnation, for paper surface films pure MF is used. Therefore, a further reduction in cost could be achieved if a portion of the melamine in the surface film was replaced by urea. In the present contribution, recent results of technological tests on paper laminates using a novel melamine–urea–formaldehyde resin (MUF) formulation are reported and their performance is compared to traditional surfaces made from MF.     

Use of aluminum oxide nanoparticles in wood composites to enhance the heat transfer during hot-pressing

In this study, the effect of Al₂O₃ nanoparticles on the heat-transfer properties of medium density fiberboard (MDF) was investigated. Al₂O₃ nanoparticles were added at two levels (0.5 and 1.0 %) by percentage weight fraction of dry wood fibers to urea formaldehyde (UF) resin. The core temperature profile and thermal conductivity tests showed higher rate of heat transfer after addition of nanoparticles, subsequently it has improved the bonding strength of MDF. Differential scanning calorimetry was used to estimate the heat evolved during the exothermic reaction of UF resin curing. Scanning electron microscopy shows good dispersion of nanoparticles. Thermal conductivity of UF resin improved after nanoparticles addition proved by KD2 pro results. Modulus of rupture, modulus of elasticity and thickness swelling also improved.     

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.     

Development of selectively densified surface laminated wood based composites

An attempt was made to develop selectively densified surface laminated wood based composites. Two types of wood composites were produced (veneer laminated Medium Density Fiberboard, MDF and wafer overlaid particleboard, PB) in this study. Veneer or wafer plasticized by NaClO₂ treatment and PF resin impregnated were laminated on both sides of the MDF or PB and compressed at ordinary pressure. The main observation was that overlaying plasticized veneer on MDF before hot pressing is a promising method for improving mechanical properties and dimensional stability of commercial MDF. The MOR and MOE of surface laminated MDF attained 177 MPa and 18 GPa, respectively. The water absorption percentage of laminated MDF decreased from 160.7 to 7.2%. Furthermore, it is interesting to note that NaClO₂ treated and resin impregnated wafer or powder laminated particleboards showed excellent performance especially in stiffness. The MOR and MOE reached 40 MPa and 7 GPa which is more than two times higher compared to untreated wafer laminated board and three times higher than particleboard. This value is comparable to commercial lauan plywood having a density of 0.78 g/cm³. The processing technique of the composite is simple and can be easily applied by the industry.     

Eigenschaften von extrudierten Holz-Kunststoff-Werkstoffen auf Basis von Refiner-(TMP-)Fasern und Hanffasern

At present, wood particles (wood flour) with a low aspect ratio are mostly used as fillers in wood-plastic composites (WPC). Reinforcement of WPC and improved strength properties may be achieved by using real wood fibres with a high aspect ratio. WPC based on 70% (wt.) refiner (TMP) wood fibres and mechanically processed hemp fibres were extruded in a two-step process. Eleven compounds based on the two natural fibre types were prepared using a thermokinetic mixer and extruded in a conical, counter-rotating twin-screw extruder. Additional formulation components were polypropylene fibres, maleic anhydride-modified polypropylene (MAPP) and lubricant. It was determined that compounding in a thermokinetic mixer is a useful step for processing of WPC with refiner and hemp fibres as little fibre damage occurred. However, during extrusion, both natural fibre types were severely shortened due to strong shear forces, and homogeneous dispersion of fibres in the matrix was not achieved. WPC based on hemp fibres displayed the best strength properties of the formulations tested. Current extruder screw and die configurations need to be modified to achieve improved fibre reinforcement and to create new, structurally demanding applications for WPC. Using dynamic mechanical analysis, fibre-matrix adhesion of WPC was investigated, and activation energies for glass transition of selected formulations were calculated. Activation energy for formulations containing MAPP was higher than for WPC without MAPP. This indicates that better fibre-matrix adhesion was achieved in formulations with MAPP.     

Effects of wood fiber content and coupling agent content on tensile properties of wood fiber polyethylene composites

This study investigated the effects of coupling agent content, wood fiber content and wood fiber type on the mechanical properties of wood fiber-reinforced polymer composites (WPC). This study adopted a response surface strategy of a 20 run optimal design for these three factors. The WPC modulus of elasticity was mainly influenced by wood fiber type and wood fiber content, and tensile stress at break depended on wood fiber type, wood fiber content and coupling agent content, whereas strain at break was significantly affected by wood fiber content and coupling agent content, but not significantly influenced by wood fiber type.     

Mechanical behavior of heat-treated spruce (Picea abies) wood at constant moisture content and ambient humidity

The mechanical behavior of heat-treated spruce wood was investigated in relation to the mass loss that occurs during thermal treatment. At constant wood moisture content, the strength, failure strain and toughness of wood were reduced by the heat-bath treatment, decreasing with increasing mass loss. The stiffness was unaffected up to a mass loss of about 3%, and thereafter it decreased. The mechanical properties, however, are not only dependent on the mass loss but also on the relative humidity in the heating atmosphere. As a function of mass loss, the inelastic ductility and the inelastic toughness were the lowest when wood was heated in a dry climate, as compared to a moist climate. On the other hand, the mechanical properties of heat-treated wood were tested at constant ambient humidity. In such circumstances, the failure strain and the toughness were still reduced, but the strength and the stiffness were actually improved up to a mass loss of about 2%–3%. The improvement is due to the lower equilibrium moisture content of heat-treated wood when placed in service conditions. As a function of mass loss, wood heated at intermediate relative humidity (in the vicinity of 50%) exhibited the best mechanical behavior, which surprisingly included inelastic ductility. This is believed to be due to some irreversible hydrogen bonding.