Band 54, Heft 2

During two years of natural weathering the course of the moisture content of test samples of pine sapwood ( Pinus sylvestris L.), Norway spruce ( Picea abies L.), oak heartwood ( Quercus robur L.) and Douglas fir heartwood ( Pseudotsuga menziesii Franco.) was recorded by means of 12,000 individual gravimetric readings. The samples were untreated, treated with a weather protection varnish, or impregnated with melamine resin. The data were evaluated with regard to the number of days per year on which the moisture content of wood exceeded the limit of 25%. From the results of subsequent laboratory investigations into the adsorption of liquid water and moisture vapour desorption using the same test material, it was possible to work out a mathematical relationship (MRI = m oisture induced r isk i ndex) which correlates closely with the number of days of more than 25% moisture content during the second year of natural weathering. The MRI is proposed as a parameter for the assessment of the protective effect of wood treatments which are intended to reduce moisture content and also for durability prediction of untreated wood in out of ground contact situations.

The interaction of copper ethanolamine (Cu-EA) preservative and wood components was studied by using Fourier transform infrared spectroscopy (FTIR). In Cu-EA treated wood, significant reduction was noticed on the band attributed to carbonyl vibration from carboxylic groups at 1735 ± 5 cm −1 and an increase in band intensity was obtained from carbonyl in carboxylate at 1595 ± 5 cm −1 . The same observation was made in Cu-EA treated holocellulose. Cu-EA treated lignin resulted in a reduction in the aromatic ester band at 1710 ± 5 cm −1 and an increase in carbonyl from carboxylate at 1595 ± 5 cm −1 . Bands at 1370 cm −1 and 1221 cm −1 , assigned to phenolic hydroxyl groups, exhibited a decrease in intensity after the treatment. From these data, it is concluded that Cu-EA interacts with carboxylic groups, phenolic hydroxyl groups and ester groups from lignin to form copper carboxylate and phenolate complexes.

The phenolic hydroxyl groups in the lignin model compounds, 2-methoxy-4-propylphenol and 4-hydroxy-3-methoxyacetophenone, were removed by first converting the hydroxyl groups to the trifluoromethanesulfonates (triflates) and then cleaving the triflate substituents via catalytic hydrogen transfer. The products, 1-methoxy-3-propylbenzene and 3-methoxyacetophenone, were characterized by 1 H and 13 C NMR, mass spectrometry and elemental analyses. The effect of the removal of the phenolic groups on the photostability of the model compounds was evaluated by impregnating the compounds into Whatman filter paper sheets, and subjecting them to an accelerated yellowing experiment in a UV chamber. The removal of the phenolic groups resulted in a significant yellowing inhibition, with a higher photostabilizing effect than methylation or acetylation of the hydroxyl, particularly for the model compound without an α-carbonyl group.

The stoichiometric validity of copper(II) salts in the assessing the cation exchange capacity of lignocellulosic materials with respect to sodium salts is determined on some materials like Populus euramericana , Pinus pinea , flax, hemp, cotton, and some of their derivatives. Copper exhibits the usual stoichiometry (bivalence) with saponified woods and Norman & Jenkins holocelluloses whereas it is present as bivalent and/or apparent monovalent ions in the other materials. The major groups responsible for cation exchange are the carboxyls of polyoses while hydroxyls seem inert at this reaction. Cation exchange of wood and raw fibers is, on the contrary, influenced by the structure of lignin. In fact, when it forms ester bonds with polyoses it causes the rising of copper apparently working as monovalent species. Lacking these bonds, lignin does not provoke any chemical interactions except for some isolated lignins. These data show that copper interacts mainly with polyoses, secondly with lignin if bonded to polyoses whereas cellulose seems to be weakly involved in these interactions. To improve the assumption of copper from preservative formulations mild saponification of woods is suggested since such treatment enhances the number of sites of exchange in polyoses.

In the present study, the biological, chemical and mechanical wood properties of “Wulstholz” as a response to mechanical stress of spruce trees ( Picea abies [L.] Karst.) were investigated. “Wulstholz” is formed locally on the compression side of overstressed tree trunks after slip planes were initiated. It is characterized by an increased xylem production leading to bulges. The S2-layers of “Wulstholz”-tracheids are thicker than the corresponding cell wall layers in normal wood, and the cell lumen diameter is significantly reduced. The tracheid length decreases gradually from an average of 4 mm to 2.5 mm compared to normal wood, and the tracheids conspicuously are shifted against each other longitudinally. “Wulstholz” contains a higher concentration of lignin than normal wood, whereas the concentration of glucose in the hydrolysates is reduced. The hemicelluloses-concentration, particularly the mannose-content, is significantly higher compared to normal wood. The compression strength and the modulus of elasticity, despite a higher density, are significantly reduced in “Wulstholz” in comparison to normal wood. But due to the specific anatomical and chemical properties, “Wulstholz”-tracheids can be deformed and compressed considerably without fracturing. The investigations demonstrate that “Wulstholz” can restabilize wind-exposed trees.

The reactions of chromophoric model compounds of the coniferaldehyde type with hydrogen peroxide and peracetic acid have been investigated in relation to lignin-retaining bleaching. Analysis of the main reaction products indicated that the side chain of coniferaldehyde could cleave either between the α, β double bond or between the β, γ bond. Comparison of possible reaction pathways to the formation of oxidation products from hydrogen peroxide and peracetic acid suggested that peracetic acid is more effective than hydrogen peroxide. Advantages of peracetic acid over hydrogen peroxide include a faster rate of reaction, more efficient utilization of the bleaching agent and less likelihood of producing new chromophores.

The reactions of three hydroxy- and/or methoxy-substituted stilbenes with hydrogen peroxide and peracetic acid have been investigated. Overall, the reactivity of stilbenes was greater towards peracetic acid than towards hydrogen peroxide. Among the stilbene model compounds studied, 4-hydroxystilbene ( I ) was virtually unreactive to hydrogen peroxide, but was oxidized by peracetic acid to a varying extent that was highly pH-dependent. 4,4′-Dihydroxy-3,3′-dimethoxystilbene ( II ) and 3,5-dimethoxy-4-hydroxystilbene ( III ) showed a greater reactivity to either hydrogen peroxide or peracetic acid than 4-hydroxystilbene ( I ).

Bench filtrates from three-stage conventional bleaching sequences (CEoD) with substitution degrees of chlorine by chlorine dioxide in the first stage equals to zero, 25%, 75% and 100% have been analysed for their content of chlorinated phenolic compounds (CPCs). Alternatively, CPCs filtrates content of an AZEoPD sequence were also determined. A cellulose pulp obtained from a mixture of 85% Eucalyptus saligna (young trees) and 15% Acacia mearnsii delignified with oxygen (kappa number 10.0) was employed in all bleaching tests. Levels of CPCs were determined by using an in situ acetylation method followed by GC/ECD technique. GC/MS of bleaching filtrates was employed in qualitative analyses. CPCs content in filtrates and the Kovacs' equivalent toxicity for the total filtrates of the studied bleaching sequences are presented. The lowest content of CPCs was found in AZEoPD filtrates, as expected due to the utilised bleaching agents. CPCs were not detected in the filtrates of stage D from the AZEoPD sequence probably due to the low lignin content of the pulp. Mixed filtrates of the sequence DEoD, showed the lowest CPCs content among conventional bleachings. Results are in accordance with those found in the literature for hardwood and softwood pulp. The equivalent toxicity in the sequences Dc(75/25), DEoD and AZEoPD was found to be lower than the limit given by Kovacs to characterise chronical toxicity.

Kinetics of the laccase-catalyzed oxidation of veratryl alcohol with dioxygen in the presence of 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamonium salt (ABTS), the mediator, were studied to elucidate the possible reaction mechanism and the role of the mediator in this reaction. The reaction follows a pseudo -first order reaction law. The first order rate constant ( κ ) is dependent on the Mediator/Substrate (M/S) ratio and has a maximum at M/S molar ratio of 0.15. The kinetic studies show that the mechanism of veratryl alcohol oxidation with dioxygen-laccase-ABTS is rather complex and includes different reaction pathways. The mediator is involved in competitive reactions. It has been suggested that at low mediator concentration, the veratryl alcohol is oxidized via the laccase redox cycle. The mediator acts mostly as a laccase activator at a M/S ratio lower than 0.15. With increasing ABTS concentration with respect to the substrate concentration, ABTS acts increasingly as a cosubstrate competing with the original substrate for active centers of the laccase. This results in inhibition of veratryl alcohol oxidation in the enzyme cycle and increases the role of substrate oxidation by an oxidized mediator.

The kinetics of dioxygen uptake in the laccase-catalyzed oxidation of veratryl alcohol with dioxygen in the presence of ABTS, the mediator, was studied. The kinetics of dioxygen uptake consists of two phases: (1) the initial phase up to a reaction time of one hour, and (2) the second phase, after a reaction time of one hour. In the initial phase, ABTS is mainly oxidized to the corresponding cation radical. The kinetics of dioxygen uptake follows a pseudo -zero order rate law. The dioxygen uptake under the reaction condition correlates with the initial ABTS concentration according to the stoichiometric relationship of 0.25 moles dioxygen per mole ABTS. In the second phase, veratryl alcohol is mainly oxidized to veratraldehyde. The kinetics of the dioxygen uptake follows a pseudo -first order rate law. The dioxygen uptake correlates linearly with the yield of veratraldehyde. The stoichiometric ratio between the formation of veratraldehyde and the consumption of dioxygen differs slightly at different M/S ratios. On average, however, it is 0.42 moles of dioxygen per one mole of veratraldehyde formed. The reaction mechanism is discussed on the basis of the kinetic data.

Due to the moisture and temperature gradients developed during hot pressing of medium density fibre-board (MDF), residual stresses occur within the board as it equilibrates to room conditions. It would be extremely useful to measure these residual stresses and to determine their effects on board properties such as moduli of elasticity and rupture in bending, internal bond strength and dimensional stability. In this article two methods, namely dissection and hole drilling, have been adapted to measure residual internal stress distributions in six different samples of industry produced MDF. The dissection method involves cutting several pieces of MDF perpendicular to the thickness direction at different depths. The residual stresses released by the dissection can be determined by measuring the curvatures of cut pieces and knowing their elastic moduli. The hole drilling method, on the other hand, involves mounting three strain gauges on the surface of a piece of MDF and drilling a hole to release residual stresses in close proximity. The released stresses are manifested as strains in the forms of which can be measured in three directions on the surface of the board. A theoretical model for predicting residual stresses involving various parameters has been developed and an excellent agreement with the experimental results from both the dissection and hole drilling methods has been achieved. Linear moisture expansion coefficient appears to have the greatest influence on residual stress. When compared against each other, the residual stresses measured by the hole drilling method show some shortcomings towards the centre of the board. While all six of the MDF boards exhibited similar trends in their residual stress distributions, significant differences were identified in the magnitudes of residual stress measured. Finally, some preliminary results linking the residual stress with the thickness swell of the samples and their surface densities have been presented.

The phenomenon of axial liquid penetration in hardwoods is modeled through two different mechanisms: in one of them the driving force is taken to be the empty space that remains unfilled inside the material; in the other one, it is the pressure differential between the interior of the material and its liquid environment. The equations obtained show an increase of liquid retention exponentially approaching an asymptote, which agrees with experimental results. By combining them with existing models for liquid permeability, an equation is obtained for predicting retention as a function of wood ultrastructure for a given liquid under fixed conditions.

This paper deals with the evaluation of wood—wetting process with coating materials on the basis of interactions of surface forces on phase boundary. The obtained results are compared with the actual state in the system wood—coating material. Primary ability of coating materials—spontaneous spreading over the wood surface has been proved. There is also the secondary influence of rheological properties of coating materials causing deformations of the phase boundary, non-perfect wetting of the wood surface and apparent lowering of adhesion work. The influence of rheological properties has been expressed by using the coefficient F the meaning of which follows from the comparison between results of adhesion work computed according to modified Young-Dupré equation and of adhesion work determined on the basis of the interactions of surface forces on the phase boundary between wood and liquid coating materials. A direct dependence between the values of the coefficient F and coating materials viscosity has been proved.

The effect of lignocellulosic materials on the board performance of thermoplastic polymer/wood composites was examined by using soft wood flours of 20 mesh- and 120 mesh-pass, steam-exploded beech flour, and two kinds of thermoplastic polymers, polyvinyl chloride and polystyrene. Steam-exploded wood flour was found to be one of the best lignocellulosic materials in terms of fracture strength and water resistance of the composite board. The properties of the composites are dependent not only on the lignocellulosic materials and polymers, but also on the average size of wood flour. Generally, a flour of 120 mesh pass gave composites of better performance than that of 20 mesh pass, but the tendency was reversed when steam-exploded beech flour was used.

In conventional wood dryers, air is dehumidified by condensation or by its partial remove. A new air dehydration process is proposed for low temperature convective wood drying. This process consists in using an absorber working with an organic absorbent, the triethylene glycol (TEG) that is pulverised in a venturi type of device. The adaptation of this process well known for hydrocarbon gas dehydration and its interest for wood drying operations are discussed. The prototype installation is presented and the impact of the absorption on the evolution on drying cycles are investigated. The performances of the absorber are evaluated quantitatively and qualitatively. The average drying kinetic is quantified with a global mass transfer coefficient and the drying quality is characterised through the measurements of defects generated during drying.

In this second part of the study, the ability of the dehydration unit to fix the organic compounds released by wood is evaluated in the particular case of green fir drying at 70° C. The organic compounds are condensed in a bubbling device and analysed with a flame ionisation detector. From these analysis are deduced the mass fraction of organic compounds in the gas phase, the mass flow rate of these compounds released by wood and the one absorbed by TEG. These last results permit to define an equivalent efficiency of the absorber to capture organic compounds present in the drying air.

Kenaf bast fiber bundles alternate radially in layers accompanied by sieve elements and phloem parenchyma cells. Fiber bundles are about 20 % in the area of a cross section of kenaf bast. Four sample preparation methods for kenaf bast fiber length analysis with an automated fiber length analyzer were compared and a new sample preparation method including three steps was proven to be accurate. Fiber bundles were obtained by acidified sodium chlorite treatment and an additional ammonium oxalate treatment. Following a 1% sodium hydroxide treatment, fiber bundles were completely separated into single fibers with no damage. Fiber length values of this method were more accurate and reliable than other three sample preparation methods and manual measurement with optical microscope.