Volume 57, Issue 5

In order to define the role of lignin for stress transfer in the fiber wall, its orientation and mechanical interaction with other cell-wall components were investigated. Polarized infrared spectra of oriented thermomechanical pulp fiber sheets were used to determine the molecular orientation, and dynamic infrared spectroscopy was used to study the rheology of lignin in the cell wall. The results indicate that there is a preferred orientation of the phenyl-propane units of lignin along the fiber axis in spruce tracheids. This implies there is an ordered structure of lignin in the secondary wall of tracheids analogous to that of the cell-wall polysaccharides. The dynamic IR-spectra also indicated that lignin exhibits a much more viscoelastic behavior than do the carbohydrates, a knowledge of importance in the interpretation of transverse fiber properties.

The formation of two different β-O-4 lignin models is investigated by means of density functional calculations. It is found that the coupling of two coniferyl alcohol radicals forming a quinone methide proceeds by an energy barrier of ~2–5 kcal/mol, and that the associated reaction energy is negative by more than 20 kcal/mol. On the basis of the corresponding results obtained for the coupling of a coniferyl alcohol radical to a coniferyl alcohol, it is argued that the resulting radical, albeit being formed in an energetically less favourable process, might play an important role in lignin polymerisation. Finally, two different reaction mechanisms for the conversion of a quinone methide into a guaiacylglycerol-β-coniferyl ether dilignol through the addition of water are explored.

In order to analyse the fundamentals of alkaline chemi-mechanical pulping of hardwoods, the chemical state of the wood was related to both the swelling level of fibres and the papermaking properties of pulp. Wafers of poplar wood were alkali treated following a factorial experimental design for two variables: temperature and alkali concentration. Treated wafers were hot defibrated in a 300-mm disk mill at 15% consistency, and then refined in PFI mill at 20% consistency. Results show how fibre swelling gradually increases as alkaline action is increased. The significant improvement in tensile and tear strength of the pulp can, in great part, be ascribed to the development of fibre bonding capacity. A limited effect of ion content on cell wall swelling was found. Swelling correlates well with deacetylation level, and is a major factor in determining the tensile strength and scattering ability of the pulp.

In this study the molecular structure of birch xylan was altered by ozonation. Longer ozonation times increased the cleavage of aromatic rings in the lignin residues attached to the xylan molecules, resulting in the formation of muconic acid derivatives and other carboxylic acid structures. Size exclusion chromatography showed a decreasing molar mass of xylan after ozonation. The ozonation improved xylan solubility as evidenced by the reduced amounts of aggregates in aqueous solution. Atomic force microscopy showed increases in the sizes of the aggregates. Furthermore, the ozonation resulted in a decreased ability of xylan to assemble into particle-like structures on cellulose surfaces. We therefore believe that there is a relationship between xylan aggregation in aqueous solution and the formation of xylan surface structures on cellulose.

The structural changes of E. globulus wood extractives during bleaching with chlorine dioxide (D), oxygen (O), ozone (Z) and hydrogen peroxide (P) were studied. The detailed characterisation of the extractive derivatives detected in the partially bleached D, O, P and Z pulps was achieved by performing reactions of pure reference compounds with the different bleaching agents. The results show that the unsaturated sterols and fatty acids are extensively degraded during chlorine dioxide and ozone bleaching and only partially degraded during oxygen and hydrogen peroxide bleaching. The corresponding saturated extractives as well as the long chain aliphatic alcohols and ω-hydroxyfatty acids were stable during bleaching. The main oxidation products of β-sitosterol and oleic and linoleic acids, including one chlorinated derivative of linoleic acid, were identified here for the first time in E. globulus bleached pulps and bleaching filtrates.

Flax pulp obtained by anthraquinone-soda pulping, resulting in a kappa number of 11.1, a viscosity of 950 ml/g and 36.7% ISO brightness, was bleached in a totally chlorine-free sequence using the enzyme laccase from the fungus Pycnoporus cinnabarinus and 1-hydroxybenzotriazole (HBT) as redox mediator (stage L), followed by a hydrogen peroxide stage (P). The laccase treatment was optimized using a three-variable sequential statistical plan over the following ranges: 1–20 U/g o.d.p. (oven-dried pulp) laccase dose, 0.5–7.5% o.d.p. HBT dose and 1–24 h reaction time. The influence of these variables on several pulp properties after the P stage of the LP sequence was examined. The models defined from the results obtained predicted variations in ISO brightness, viscosity and kappa number of 57.6–74.8%,590–955 ml/g and 0–6.2, respectively. The variables most strongly influencing these pulp properties were found to be the reaction time and the enzyme dose. A compromise was adopted as regards the operating conditions in order to ensure optimum results. The study was completed by conducting a biobleaching assay in a pressurized reactor (590 kPa) to assess the effect of oxygen pressure. The high pressure level resulted in improved pulp properties by the laccase-mediator system.

The estimation of a range of Pinus radiata D. Don bark properties by calibrated near infrared (NIR) spectroscopy is reported. A series of P. radiata samples were characterised in terms of hot water extractives, NaOH extractives and Stiasny value. NIR spectra were obtained from the milled bark of each sample and used to develop calibrations for each parameter. Coefficients of determination (R 2 ) ranged from 0.84 (NaOH extractives) to 0.94 (Stiasny value). Standard errors of calibration ranged from 0.96 (NaOH extractives) to 2.47 (Stiasny value). When applied to a separate test set, the hot water extractives and Stiasny value calibrations performed well, while the NaOH calibration was disappointing. The calibration developed for Stiasny value could be of considerable practical importance as the method used to determine Stiasny value is particularly time consuming.

The analysis of two sets of Acacia mearnsii De Wild (Black Wattle) samples by near infrared (NIR) spectroscopy is reported. Set 1 samples were characterised in terms of hot water extractives, Stiasny value and polyflavanoid content. Set 2 samples were characterised by nine different parameters, including tannin content. NIR spectra were obtained from the milled bark of all samples and calibrations developed for each parameter. Calibrations developed for hot water extractives and polyflavanoid content (set 1) gave very good coefficients of determination (R 2 ) and performed well in prediction. Set 2 calibrations were generally good with total and soluble solids, tannin content, Stiasny value-2 and UV-2, all having R2 greater than 0.8. Owing to the small number of set 2 samples, no predictions were made using the calibrations. The strong relationships obtained for many parameters in this study indicates that NIR spectroscopy has considerable potential for the rapid assessment of the quality of extractives in A. mearnsii bark.

An inverse identification method for characterization of wood sorptive properties is presented. The method relies on a computer simulation of a real experiment, in our case a desorption experiment, where spruce heartwood samples were dried from 27% to 8% moisture content. Three samples, distinguished by the respective moisture flow pattern through the specimen, were investigated. A computer aided material characterization using the so-called inverse problem identification method was performed on the measurements. The solution of the specified inverse problem enabled us to estimate the moisture diffusion coefficients of wood and to determine the moisture content field in the sample simultaneously. The method is first verified on two simple cases of uniaxial moisture flow, and then is used to characterize the diffusion coefficients on a biaxial moisture flow sample. In the latter case some salient features of the proposed method are exhibited.

In this study the effect of high temperature on mechanical properties, dimensional stability and color of spruce was investigated. Wood specimens conditioned at different relative humidities (50, 65, 80 and 95%) were subjected to heat treatment at 200°C for 2, 4, 8, 10 and 24 h and at 100, 150 and 200°C for 24 h. Color changes were measured in the Minolta Croma-Meter CR-300 color system. Bending strength and modulus of elasticity were determined according to DIN 52186. The results show that heat treatment mainly resulted in a darkening of wood tissues, improvement of the dimensional stability of wood and reduction of its mechanical properties. The darkening accelerated generally when treatment temperature exceeded approximately 200°C. Most of the darkening occurred within the first 4 h of exposure. For the specimens heated to high temperatures, the average decrease in bending strength was about 44–50%, while modulus of elasticity was reduced by only 4–9%. We found that treatment time and temperature were more important than relative humidity regarding the color responses. Strong correlations between total color difference and both modulus of elasticity and bending strength were found. Thus, the color parameters can be estimated quantitatively and used as a prediction of wood strength.

In this study, the effects of moisture content and bulk density on the ultrasonic velocity of Taiwania ( Taiwania cryptomerioides Hay.) plantation wood in the longitudinal and radial directions are investigated. The dynamic modulus of elasticity is adjusted above the fiber saturation point by a simple method. It was observed that ultrasonic velocity tended to increase with a decrease in moisture content, and the effect of moisture content on the ultrasonic speed below the fiber saturation point was stronger than that above. Moreover, for this species ultrasonic speed in the longitudinal direction decreased with increasing bulk density, while that in the radial direction increased. The correlations between ultrasonic velocity, moisture and bulk density could be represented by a polynomial regression model. The K value for ultrasonic wave was affected by density. The adjusted dynamic modulus of elasticity remained fairly constant above the fiber saturation point using this simple method. Therefore, the results obtained by the ultrasonic-wave technique should be adjusted before estimating some physical properties of standing trees.

The microfibril angle (MFA) of latewood tracheids and its radial variation at breast height in Cryptomeria japonica D. Don (sugi) were investigated with twelve clones collected from three sites in the Kyushu region, Japan. Large variations both between clones and between sites were observed. The MFAs were well correlated to the stiffness of the logs collected from the sample trees. A simulation of clonal selection according to the ranking of the clones in MFA demonstrated that the log stiffness of the selected population was much improved even when the selection relied on MFAs in the second ring from the pith. The improvement in log stiffness by MFA selection was not different from the selection by log stiffness itself. The results indicate that early selection by MFA is very effective in improving log stiffness in this species.