Wolfgang Gindl

A comparison of different methods to calculate the surface free energy of wood using contact angle measurements

Abstract A number of controversies concerning the correctness of the different approaches to determine the wood surface free energy and its components from contact angle measurement arose in the past, and it is not clear which approach one should follow. In this work the advancing contact angles of pure liquids on microtomed spruce surface were used to determine the surface free energy of wood. The most widely applied approaches for the surface energy of wood: the Zisman approach, the equation of state, the harmonic mean equation, the geometric mean equation and the acid–base approach were compared and the usefulness of this approaches referring to wood as material were discussed. It was found that the acid–base approach delivers a maximum of information about chemical composition of the natural polymer wood, which consists mainly of cellulose, lignin and a variety of hemicelluloses. Therefore, the acid–base approach is most suitable to explain the coating properties (adhesion) of wood surfaces. Furthermore, it is possible to improve the accuracy of the determination of the surface free energy of wood by increasing the number of different liquids applied. A computerized method for the calculation of surface energy parameters from all five liquids used to obtain an approximate solution for disperse, acid and base components of surface free energy greatly facilitated this task.

The influence of temperature on latewood lignin content in treeline Norway spruce compared with maximum density and ring width

Abstract. The latewood lignin content, maximum density and total ring width of ten consecutive annual increments were determined in treeline Norway spruce (Picea abies [L.] Karst.) using ultraviolet (UV) microscopy and radiodensitometry, respectively. A positive correlation between the total ring width and the mean temperature of mid-July to August was identified, as was one between the maximum density and the temperature of August–September. Lignin content in the secondary cell wall layer of the terminal latewood tracheids was positively correlated with the temperature for the period running from the beginning of September until the third week of October. It can, therefore, be concluded that lignification of the cell wall is susceptible to the influence of climatic variability, as is the case with ring width and maximum density.

The relationship between near infrared spectra of radial wood surfaces and wood mechanical properties

Near infrared (NIR) spectra taken from solid European larch wood samples subjected to axial bending and compression tests revealed an excellent ability to model the variability of mechanical properties using NIR spectroscopy. By including compression wood specimens, whose strength and elasticity is overestimated when modelled by density, in the investigated sample it could be demonstrated that the model is not just based on the measurement of density, but on density, surface geometry and possibly lignin content and composition. It is concluded that NIR spectroscopy shows considerable potential to become a tool for the non-destructive evaluation of small clear wood specimens, e.g. increment cores.

Impregnation of softwood cell walls with melamine-formaldehyde resin

Melamine-formaldehyde (MF) resin impregnation has shown considerable potential to improve a number of wood properties, such as surface hardness and weathering resistance. In this study, selected factors influencing the uptake of MF resin into the cell wall of softwood were studied. Using UV-microspectroscopy, it could be shown that water soluble MF diffused well into the secondary cell wall and the middle lamella. Concentrations as high as 24% (v/v) were achieved after an impregnation of 20 h. High cell wall moisture content, high water content of the resin used for impregnation, and low extractive content are factors which are favourable for MF resin uptake into the cell wall. For dry cell walls, solvent exchange drying improved resin uptake to a similar extent, as was the case when cell walls were soaked in water.

Adhesive penetration of wood cell walls investigated by scanning thermal microscopy (SThM)

Abstract Cross sections of wood adhesive bonds were studied by scanning thermal microscopy (SThM) with the aim of scrutinizing the distribution of adhesive in the bond line region. The distribution of thermal conductivity, as well as temperature in the bond line area, was measured on the surface by means of a nanofabricated thermal probe offering high spatial and thermal resolution. Both the thermal conductivity and the surface temperature measurements were found suitable to differentiate between materials in the bond region, i.e., adhesive, cell walls and embedding epoxy. Of the two SThM modes available, the surface temperature mode provided images with superior optical contrast. The results clearly demonstrate that the polyurethane adhesive did not cause changes of thermal properties in wood cell walls with adhesive contact. By contrast, cell walls adjacent to a phenol-resorcinol-formaldehyde adhesive showed distinctly changed thermal properties, which is attributed to the presence of adhesive in the wood cell wall.

Effects of thermal modification on the adhesion between spruce wood (Picea abies Karst.) and a thermoplastic polymer

In order to study the effect of thermal treatment on the adhesion between wood and a nonpolar thermoplastic polymer, spruce wood boards (Picea abies Karst.) were heat-treated and bonded with polyethylene. As a result the contact angle of an applied water droplet increased significantly. A strong improvement of the adhesion between the modified wood surface and polyethylene was found by mechanical tests on unmodified and thermally modified samples. ZusammenfassungUm die Auswirkungen einer thermischen Behandlung auf die Adhäsion zwischen Holz und einem Thermoplasten zu untersuchen, wurden Fichtenproben (Picea abies Karst.) thermisch modifiziert und mit einer Polyethylenfolie verklebt. Als Folge der Wärmebehandlung wurde ein Anstieg des Kontaktwinkels eines aufgebrachten Wassertropfens beobachtet. In mechanischen Tests vor und nach der thermischen Behandlung wurde eine stark verbesserte Adhäsion zwischen der modifizierten Holzoberfläche und Polyethylen festgestellt.

Actual versus apparent within cell wall variability of nanoindentation results from wood cell walls related to cellulose microfibril angle

Hardness and elastic modulus of spruce wood cell walls parallel to their axial direction were investigated by means of nanoindentation. In the secondary cell wall layer S2 of individual earlywood and compression wood tracheids, a systematic pattern variability was found. Several factors potentially affecting nanoindentation results were investigated, i.e. specimen orientation related to the indenter tip, cutting direction during specimen preparation, tip geometry, specimen and fibre inclination, respectively, and finally micro fibril orientation. Mechanical property measurements were correlated with structural features measured by confocal Raman spectroscopy. It was demonstrated that very high variability in the measurement of micromechanical cell wall properties can be caused by unintentional small fibre misalignment by few degrees with regard to the indentation direction caused by sub-optimal specimen preparation.

Mechanical characterisation of wood-adhesive interphase cell walls by nanoindentation

Abstract The elastic modulus, hardness, and creep factor of wood cell walls in the interphase region of four different adhesive bonds were determined by nanoindentation. In comparison with reference cell walls unaffected by adhesive, interphase cell walls from melamine-urea-formaldehyde (MUF) and phenol-resorcinol-formaldehyde (PRF) adhesive bonds showed improved hardness and reduced creep, as well as improved elastic modulus in the case of MUF. In contrast, cell walls from the interphase region in polyvinylacetate (PVAc) and one-component polyurethane (PUR) bonds showed more creep, but lower elastic modulus and hardness than the reference. Considering the different cell-wall penetration behaviour of the adhesive polymers studied here, it is concluded that damage and loss of elastic modulus to surface cells occurring during the machining of wood is recovered in MUF and PRF bond lines, whereas damage of cell walls persists in PVAc and PUR bond lines.

Comparing Mechanical Properties of Normal and Compression Wood in Norway Spruce:The Role of Lignin in Compression Parallel to the Grain

Summary Cell-wall lignin content and composition, as well as microfibril angle of normal and compression wood samples were determined prior to mechanical testing in compression parallel to the grain. No effect of increased lignin content on the Youngs modulus in compression wood was discernible because of the dominating influence of microfibril angle. In contrast, compressive strength of compression wood was not negatively affected by the high microfibril angle. It is proposed that the observed high lignification in compression wood increases the resistance of the cell walls to compression failure. An increased percentage of p-hydroxyphenylpropane units observed in compression wood lignin may also contribute to the comparably high compressive strength of compression wood.

Axial compression strength of Norway spruce related to structural variability and lignin content

Variability in the axial tensile strength and Youngs modulus of wood is mostly due to changes in the main orientation of the cellulose microfibrils with respect to the cell axis. By contrast, the causes of variability in the axial compression strength of wood are less well understood. Therefore, the axial compression strength and density as well as microfibril angle and lignin content of Norway spruce specimens were examined. 84% of the variability of compression strength could be explained by density. After normalisation of compression strength for density, the experimental results showed that variability in the microfibril angle in the secondary cell wall is not responsible for variability in the axial compression strength of the cell wall. This finding is supported by theoretical considerations using a composite failure criterion. Deviations of the microfibrils from a strictly axial alignment in the vicinity of rays are most probably the cause for the initiation of compression failure in Norway spruce. The lignin content of the secondary cell wall showed a positive relationship at low statistical significance with the compression strength of the cell wall. A positive effect of increasing lignin content on compression strength seems therefore possible, but very weak.