Volume 60, Issue 3

The stabilisation of a peroxide-bleached chemothermomechanical pulp against light-induced yellowing by novel sulfur-containing radical-scavenging compounds was investigated. Substances were identified that lack the unpleasant odour typical for compounds containing thiol groups. High inhibition effects were observed when the sulfur compounds were combined with small amounts of hindered amines and UV absorbers. A novel group of thiols isolated from biomatter was among the sulfur-containing inhibitors. Photoyellowing could be avoided for 3 years in real conditions when the inhibitors were applied as part of a coating. The ability of the thiol compounds to scavenge phenoxy radicals was demonstrated by ESR spectroscopy of irradiated chemithermomechanical pulp sheets. The results are encouraging for efficient brightness stabilisation of high-yield pulps.

The chemistry of thermomechanical pulp bleaching and brightness reversion was studied. First, UV-Vis reflectance spectroscopy was used to obtain information on the reactive structures in pulp. Based on these data, a Raman excitation wavelength was chosen close to the absorption bands of the chromophores formed to take advantage of the resonance enhancement (resonance Raman spectroscopy). Fluorescence was rejected with a picosecond Kerr gate. The results revealed that coniferyl aldehyde structures were partly removed by alkaline peroxide bleaching and these structures were further degraded during light exposure. However, this reaction was obviously not responsible for chromophore formation in the pulp. On the other hand, based on the resonance Raman spectra, formation of quinonoid structures, possibly para -quinones, was a more prominent explanation for the brightness reversion.

Characteristics of the NAEM [neutral alkali-earth metal; CaCl 2 and Mg(NO 3 ) 2 in this study] salt-catalyzed alcohol organosolv pulping of softwood were investigated and compared to AlCl 3 -catalyzed kraft pulping. The catalysts control the acidity of the cooking liquor, which is the key factor for pulping selectivity and fiber separation. The crucial role of catalysts in alcohol organosolv pulping is demonstrated. The pulp yield and quality were satisfactory. The dissolved wood components in the spent liquor have high potential to be converted into value-added co-products.

The effect of the molecular mass of poly(ethylene oxide) (PEO) on lignin-PEO blends was studied using thermal analysis and FT-IR. Differential scanning calorimetry (DSC) analysis revealed miscible blends over the entire blend ratio. A negative deviation in T g from a simple weighted average was observed, indicating the existence of relatively weak favorable interactions between blend components. Analysis of the data revealed no difference in the magnitude or propensity of intermolecular interactions with increasing PEO molecular mass in the kraft lignin-PEO blends. By contrast, the fitting parameters obtained for organosolv lignin were substantially different; the higher molecular mass PEO had a higher propensity to form slightly stronger intermolecular inter-actions than the lower molecular mass PEO. Low molecular mass poly(ethylene glycol) dimethylether (M-PEG)-lignin blends had a much higher degree of crystallinity than the PEO blends, resulting in an increase in T g at high PEG content. FT-IR analysis revealed the presence of strong intermolecular hydrogen bonding between lignin and PEO. However, the band shape of the ν OH region of the M-PEG blends was slightly different from the PEO blends; some of the original lignin inter- and intramolecular hydrogen bonding was still present in the M-PEG-lignin blends.

The fraction of the available fibre surface in a sheet that is bonded to other fibres is the relative bonded area (RBA), which is an important determinant of sheet mechanical properties. The main method for estimating RBA has been to extrapolate data for the light scattering coefficient as a function of tensile strength to zero tensile strength. This method can produce significant errors. In this paper, the light scattering coefficient, corrected for the total surface area of the fibres available for scattering, was plotted against sheet density, corrected for fibre shape. This was carried out for sheets made from several series of pulps, including single furnishes of long- and short-fibre bleached and unbleached pulps, as well as sheets made of blends of bleached long- and short-fibre pulps. The corrected scattering coefficient was inversely linearly correlated to the corrected density for each data set. A theory was developed to allow RBA calculation from the intercept and slope of the fitted line.

A method has been developed to analyse and quantify formic and acetic acids in archaeological and fresh wood. The method takes advantage of the fact that, in equilibrium, the gas-phase concentrations of formic and acetic acids over pulverised archaeological wood in a sulfuric acid solution are proportional to their absolute concentrations in wood. The method is based on automated solid-phase micro extraction (SPME) combined with gas chromatography-mass spectrometry (GC-MS). Good linearity of the calibration curves was observed. The content of formic and acetic acids in archaeological wood from ships conserved with polyethylene glycol (PEG) was determined. The content of formic acid is related to the intensity of conservation and, hence, the PEG content in wood. Accordingly, formic acid found in the hull of the ships may partly be a result of PEG degradation. The formic acid content in the Vasa warship is, on average, not higher than in the other ships analysed. In contrast, the acetic acid content in PEG-preserved archaeological wood is lower than in fresh wood. The acetic acid content is age-dependent and is lowest in 1000-year-old wood. The acetic acid probably originates from the wood.

The time domain of 1 H NMR spectroscopy allows straightforward editing of the T 2 relaxation profiles in maritime pine wood. A new method from the Carr-Purcell-Meiboom-Gill sequence is proposed to measure the amount and distribution of water in wood, as well as the location of major dissolved organic materials. A general calibration model giving reliable and precise identification of these parameters is described. The method presented for editing T 2 relaxation profiles (obtained by the Contin program) may be helpful in solving practical drying and gluing problems in the wood industry. It can be used for monitoring chemical modifications of wood fibers involved in the design of wood composite materials.

Chemical reaction between silanol-modified wood and methyltrimethoxysilane (MTMS) was carried out using two different approaches with dibutyltin dilaurate (DBTDL) or ethylamine (EtNH 2 ) as catalysts. Modifications were characterised by Fourier-transform infrared (FTIR) spectroscopy, as well as solid-state 13 C and 29 Si cross-polarisation with magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy and scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis. Silanol-modified wood was obtained by carbamoylation reaction with 3-isocyanatopropyltriethoxysilane (IPTES) and subsequent hydrolysis of the triethoxysilane end groups. 29 Si CP-MAS NMR spectroscopy indicated that SiOH groups existed mostly in the form of mono- and di-condensed structures in wood, whatever the level of modification (low or high). The attachment of methyltrimethoxysilane (MTMS) molecules to silanol sites was apparently achieved when ethylamine was used as a catalyst. The reacted MTMS remaining in wood was found to exist mostly in the form of polysiloxane polymers (or oligomers), as shown by 29 Si CP-MAS NMR results. Reactions performed with unmodified wood were much less effective, thereby demonstrating the importance of the IPTES pre-treatment.

The objectives of this study were to improve the dimensional stability of medium-density fibreboard (MDF) by heat treatment and to determine the effects of the treatment on the mechanical properties and vertical density profile of the panels. MDF panels were produced from untreated fibres and fibres treated at two different temperatures (150 and 180°C) for 15, 30 and 60 min. Panels produced from heat-treated fibres showed an important reduction in thickness swelling and water absorption after water soaking. Linear expansion and contraction were not improved by the treatment. Thickness swelling after repeated cycles of adsorption and desorption increased, while thickness shrinkage under the same conditions was not changed by the treatment. The springback of panels after repeated cycles of adsorption and desorption was not improved, either. Statistical analyses did not show significant differences in the modulus of rupture, modulus of elasticity and internal bond strength of panels following heat treatment. No significant effect was found for the vertical density profile of panels following heat treatment, although the profiles for panels produced from heat-treated fibres were flatter than those produced from untreated fibres.

Properties of medium density fiberboard (MDF) panels in relation to wood and fiber characteristics were investigated. Laboratory MDF panels were manufactured from raw fiber materials from black spruce [ Picea mariana (Mill.) BSP.], three hybrid poplar clones ( Populus spp.), two exotic larch ( Larix gmelinii and Larix sibirica ), and a mix of spruce, pine, and fir wood chips. The panels were evaluated for modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), linear expansion (LE), thickness swelling (TS), and water absorption (WA). These properties were analyzed as response variables. As predictor variables, various wood and fiber characteristics were measured, including wood density, pH, base buffering capacity and fiber coarseness. Multiple linear regression analysis was performed to develop functional relationships between panel properties (response variables) and wood fiber characteristics (predictor variables). Ten dummy variables were created and incorporated into the analysis to examine the effects of wood species or type on MDF panel properties. MOR was negatively related to arithmetic fine percentage. MOE was negatively affected by the percentage of small particles (>200 mesh) and wood pH. IB strength was negatively related to arithmetic fine percentage and fiber pH, but positively related to the percentage of small particles (>200 mesh). Wood density affected LE. TS was negatively affected by arithmetic mean fiber length. Arithmetic mean fiber width had a negative effect on panel WA. The presence of dummy variables in the models for MOE, IB and LE indicates that wood fiber characteristics other than those measured in this study significantly affected these panel properties. The study indicates that the refining process can play a significant role in manipulating MDF panel properties.

To understand the mechanism of bulk wood forming, compressive stress relaxation experiments were performed on radiata pine ( Pinus radiata D. Don) specimens with various moisture contents at a temperature of 200°C. The results aided in understanding the time-dependent viscoelastic behaviour with respect to springback and microstructural changes. This study clearly demonstrates the commercial feasibility of value-adding applications such as pattern forming on wood panels using a relatively simple, quick and inexpensive procedure. The experimental data were characterised by a three-component model with a spring and a Maxwell element in parallel, which showed very good agreement under different forming conditions.

The creep of wood increases remarkably during moisture changes, a phenomenon called mechano-sorptive creep. The microstructure in cell walls of wood is destabilized by changes in temperature and/or moisture content. The mechanical properties of destabilized wood change over long periods of time. In the present study, the influence of moisture conditioning history on bending creep was examined. During changes in moisture, greater creep occurred in wood subjected to the first moisture change after long-term moisture conditioning than immediately after desorption or adsorption. This result indicates a kind of memory effect immediately after moisture change. During desorption processes, greater creep occurred immediately after slow adsorption than immediately after rapid adsorption. In the course of adsorption, the reverse was observed: the creep was greater immediately after rapid desorption than immediately after slow desorption. Accordingly, greater instability immediately after a change in moisture does not always cause greater creep during the next moisture change. The size of the moisture change and the load level affect the recovery of creep during adsorption processes.

Extensive experiments were conducted to investigate the transverse (vertical) air permeability of trembling aspen ( Populus tremuloides ) veneer and phenol formaldehyde (PF) gluelines, as well as aspen plywood and strandboard. The theory for permeability through laminates was used to determine the relative contribution of the veneer and glueline to panel permeability. Based on the classic Carman-Kozeny theory for porous materials, a concept of effective porosity was proposed to explain the difference in panel permeability and the resulting hot-pressing behavior. The results show that, for veneer-based panels, the panel compression ratio (CR) was the most important factor affecting panel permeability, followed by the sapwood/heartwood composition and glueline. As a 3–7% CR was reached, the panel permeability substantially decreased by approximately 80%. On average, the permeability of sapwood veneer panels was approximately four-fold higher than that of heartwood veneer panels. The glueline permeability decreased during glue curing, but the reduction from the uncured to the cured state was only ∼15%. In addition, the glueline permeability also decreased with increasing glue spread. At the normal level of glue spread for plywood, the average permeability of cured gluelines was approximately 20% of that for non-compressed veneer, but was close to that for veneer with a 3–7% CR. Since the thickness of the glueline was only ∼5% of that for normally peeled veneer, the net contribution of the glueline to panel permeability was limited. During hot pressing, therefore, the small deformation of the veneer ply effectively acts as the main barrier to gas and moisture movement, rather than the curing glueline. The rate of convection is negligible. The effective porosity in veneer panels was only ∼0.05–0.5% compared to the total panel voids, which ranged from 50% to 70%. At the same panel density, veneer panels had much lower permeability than strandboards. Although these commercial panel types had almost the same average permeability, the veneer panels showed approximately two-fold less variation due to the formation of layered and uniform gas and moisture barriers. As a result, the temperature and gas pressure responses during hot pressing were essentially different between plywood and strandboard.

Mode I fracture cleavage testing was conducted on wood bonded with poly(vinyl acetate-co-NMA) latex adhesive containing two types/degrees of cross-linking: (1) cross-linking through AlCl 3 catalysis of N -methylolacrylamide (NMA) comonomer; and (2) additional cross-linking using a phenol-formaldehyde resol additive, in addition to AlCl 3 catalysis. The formulation lacking the phenolic additive performed well under dry conditions; but it completely failed during testing as a result of accelerated weathering. In contrast, fortification with the phenolic additive provided durability against accelerated weathering. In an effort to understand the effects of accelerated weathering, parallel-plate dynamic mechanical analysis was applied to freestanding neat adhesive films and to wood-bonded films (composites). Accelerated weathering dramatically altered the viscoelastic response of films and composites that lacked the phenolic additive; weathering caused a second, broad and reversible relaxation near 100°C, which might be because of softening of the poly(vinyl alcohol) interfacial agent used during emulsion polymerization. In contrast, samples formulated with the phenolic additive only displayed the typical base-polymer glass transition. Correlation of the fracture testing and the rheological analysis suggests that the phenolic additive promotes adhesive durability by cross-linking the interparticle boundaries, where poly(vinyl alcohol) is believed to reside.

This paper describes the influence of the addition of an organosilicon waterproof agent on the properties of gypsum particleboard (GPB). Test results showed that the additive at an appropriate concentration reduces the thickness swelling (TS) and water absorption (WA), and increases the mechanical properties. TS was lowered to ∼1.6% and internal bond strength (IB) was increased to ∼0.4 MPa when 3% organosilicon agent was added. Accordingly, the TS was reduced by 53% and the IB increased by 45% relative to control board without additives. The modulus of rupture (MOR) and modulus of elasticity (MOE) were also increased in the presence of the organosilicon waterproof agent. The effect was pronounced for MOR. On the other hand, too high a content of organosilicon additive had a negative effect and the corresponding property values were poorer. X-Ray diffraction revealed that the content of gypsum dihydrate increased in the presence of the additive, but other chemical changes were not observed. Scanning electron microscopy (SEM) confirmed that the crystal size in gypsum was reduced is the presence of the additive.

Images obtained by scanning electron microscopy (SEM) helped to clarify the question as to how anatomy influences the deformation on compression and the springback of densified wood on water soaking. Transverse sections of Norway spruce ( Picea abies ), Scots pine ( Pinus sylvestris ), black alder ( Alnus glutinosa ), Swedish aspen ( Populus tremula ), European birch ( Betula pubescens ), European beech ( Fagus sylvatica ) and pedunculate oak ( Quercus robur ) were studied. Wood is reinforced with rays in the radial direction and with dense latewood in the tangential direction. When strained radially, rays buckle or tilt tangentially. Softwoods were mainly compressed radially, owing to low number of rays and since latewood is much denser than earlywood. The diffuse-porous hardwoods with low density variation between latewood and earlywood were mainly deformed tangentially, except birch, which has high density at the annual ring border and is mainly compressed radially. The ring-porous hardwoods were relatively equally deformed in the radial and tangential directions because of the high number of rays and high latewood density. Moisture-induced springback (shape recovery) was proportional to the degree of compression. Rays remained deformed, which also influenced the surrounding wood. Longitudinal wood cells almost resumed their original shape. Wood with low density and a low degree of compression showed the highest structural recovery. Shearing deformation was particularly pronounced and permanent in woods with high strength anisotropy. Thin-walled and sheared cells, such as earlywood in softwood, tended to crack on compression. Cracks usually stopped at the middle lamella and had a lesser influence on strength properties than for lumen-to-lumen cracks.

Weight loss, specific gravity and strength are traditional measures of how wood changes after fungal exposure. This study investigated the effects of fungal decay on properties of oriented strand board (OSB) made of aspen including weight loss, specific gravity, dowel-bearing strength, shear strength, and alkali solubility. Shear strength and alkali solubility were strongly correlated with specific gravity. In addition, X-ray densitometry and near-infrared (NIR) spectroscopy were used to study the decay process. X-Ray densitometry was used to assess localized density around the dowel-bearing embedment zone of a nail. A statistical model using the specific gravity directly under the nail from dowel-bearing strength tests as the explanatory variable had a higher coefficient of determination than models using the gross specific gravity of the sample. Predictive models using NIR spectro-scopy, in combination with multivariate statistical methods, showed promise as predictors of weight loss, shear strength, dowel-bearing strength, and solubility.

The dry rot fungus, Serpula lacrymans , is one of the most destructive copper-tolerant fungi causing timber decay in buildings in temperate regions. Calcium and oxalic acid have been shown to play important roles in the mechanism of wood decay. The effect of calcium on growth and decay was evaluated for 12 strains of S. lacrymans and compared to five brown-rot fungi. This was done by treating copper citrate (CC)-treated Southern yellow pine (SYP) wood with a CaCl 2 solution and estimating the decay rate and amount of soluble oxalic acid in an ASTM soil block test. Decay by S. lacrymans was found to be significantly inhibited by treatment with CaCl 2 in the presence of copper. In addition, calcium showed no effect on two strains of S. lacrymans and one Serpula himantioides strain in non-copper-treated SYP wood blocks. The growth rate of S. lacrymans was not affected on malt extract agar containing CaCl 2 . In summary, a marked decrease was observed in the decay capacity of S. lacrymans in pine treated with CC+CaCl 2 . The amount of soluble oxalic acid was measured in CC-treated blocks and blocks also treated with CaCl 2 . Of the comparative brown-rot fungi, both Antrodia vaillantii (TFFH 294) and Postia placenta (Mad 698) displayed notable wood decay despite CaCl 2 treatment, while the remaining strains were inhibited.