Volume 64, Issue 3

Mixtures of stickies-containing and stickies-free pulps were investigated for their deposition potential in an attempt to better understand the interactions of dissolved and colloidal material (DCM) when mixing two different pulps. The filtered pulp mixtures were tested in a deposition apparatus which measures both the dynamic fluid as well as impact deposition by an established optical scanning method. A pulp filtrate of model stickies led to deposition according to the proportion of stickies-containing pulp in the mixture. Other mixtures of pulps, however, led to deposition that differed from the predicted value. It is theorised that pulp fibres present in the mixture could adsorb the polysaccharides that would otherwise keep the colloidal system electrostatically stable. This destabilisation was most prominent at lower proportions of the stickies-free pulp suspensions used for mixing. These results show that care must be taken when mixing different pulps for paper and board production, as even a small amount of replacement pulp can lead to destabilisation of DCM and formation of deposits. Furthermore, special care must be taken when mixtures of pulps are used to validate stickies measurements.

Western red cedar and Japanese beech are studied concerning their liquefaction behavior with an ionic liquid, 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]). Significant differences in liquefaction behavior were observed in terms of solubility and the lignin content of the insoluble residues. Thioacidolysis analysis of the residues confirmed that β-O-4 bonds in lignin were cleaved preferentially by the treatment with [C2mim][Cl]. The differences in liquefaction behavior can be explained by structural differences between softwood and hardwood lignins. Furthermore, gel permeation chromatography and GC-MS analyses revealed that cellulose and hemicelluloses were decomposed partly to monomeric sugars such as glucose, mannose, galactose, xylose, and arabinose.

The main objective of this work was to apply contact angle analysis to predict work of adhesion ( W a ) between some modified wood materials and certain thermoplastics and adhesives. Wetting properties, i.e., contact angles, were measured by the Wilhelmy method on both freshly prepared and aged veneer samples of unmodified and acetylated Scots pine, furfurylated radiata pine, and heat treated Norway spruce. The sessile drop method was used to measure contact angles on a phenol resorcinol formaldehyde, an emulsion polymer isocyanate, and a one-component polyurethane adhesive. Contact angle data were also collected from the literature on polyethylene, polyvinyl chloride, polymethyl methacrylate, polystyrene, and Nylon 6. Contact angle analysis based on the Chang-Qin-Chen model was then applied to determine so-called acid-base interaction parameters and W a between the wood samples and the selected thermoplastics and adhesives. Results show that the ageing process led to an increased hydrophobic character of unmodified, heat treated, and furfurylated wood samples. The freshly prepared acetylated wood samples had a pronounced hydrophobic character which remained approximately constant after ageing. The predicted W a between the wood and the adhesives was considerably higher than that between the wood and the thermoplastics. Furthermore, the predicted W a between the acetylated wood and both the thermoplastics and water was approximately unchanged when comparing the fresh and aged samples. In contrast, the ageing of all other wood samples resulted in a dramatic decrease of the wood-water W a and a moderate decrease of the wood-thermoplastics W a . The wood-adhesives W a , however, was unchanged for the unmodified and furfurylated wood when comparing the fresh and aged samples and even increased for heat treated and acetylated wood samples.

The main objective of this work was to study the chemical composition of surfaces and ageing effects on acetylated pine ( Pinus sylvestris ), heat treated spruce ( Picea abies ), and furfurylated radiata pine ( Pinus radiata ) in comparison to unmodified wood. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were the instruments of choice. Observation with a low-vacuum scanning electron microscope (LV-SEM) complemented the study. The spectroscopic information was also linked to a parallel wettability study on matched wood samples by the Wilhelmy method. The results show that XPS and ToF-SIMS are two powerful tools that in combination give complementary information, both quantitative and qualitative, and are well suited for observation of the ageing process of different wood surfaces. The hydrophobization process as a result of migration of extractives during ageing was well quantified by the XPS measurements and the results correlated well with wetting results. Several specific hydrophobic substances could be identified by ToF-SIMS measurements.

Desorption isotherms at 20°C for untreated, acetylated, and furfurylated Norway spruce [ Picea abies (L.) Karst.] sapwood were established in the 91.9–99.9% relative humidity (RH) range. Three methods were employed to secure various constant RH levels: saturated salt solutions, climate chambers, and the pressure plate technique. The curve form for the untreated samples did not show an upward bend, except perhaps above 99.5% RH, indicating that – contrary to what has hitherto been assumed – capillary condensation does not play a significant role for water sorption in wood below fiber saturation. Three additional results corroborate this conclusion: (1) calculation of the theoretical contribution of capillary condensation to the moisture content (MC) in wood based on idealized microstructural geometries by means of the Kelvin and Laplace equations resulted in very small contributions to the equilibrium moisture content (EMC), i.e., below 0.35% moisture at 99.9% RH. (2) The ratio between the EMC of acetylated and untreated samples did not show an increasing trend for increasing RH, as would have been the case if capillary condensation had taken place in both untreated and acetylated wood. (3) Low field time domain nuclear magnetic resonance results showed that only the relaxation curves from the furfurylated samples were affected systematically by freezing, indicating that neither untreated nor acetylated wood contained significant amounts of capillary condensed water.

A theoretical study of the amount of moisture held in wood as capillary condensed water in the relative humidity (RH) range of 90–99.9% is carried out. The study is based on idealized geometries of the softwood structure related to micrographs. It is confined to structural elements such as bordered pits and the pointed ends of tracheids. The theoretical amount of water in these elements is found by employing the Kelvin equation. An equal amount of earlywood and latewood cells with different geometries and with different amounts of pits is assumed. The effect of pit aspiration is considered, and different degrees of pit aspiration are assigned to earlywood and latewood. We suggest based on the results that capillary condensation makes only a very small contribution to the equilibrium moisture content. At 99.9% RH the contribution amounts to less than 0.0035 kg water per kg dry wood. This is in line with the experimental results presented in Part 1 of this study.

Cellobiose and lignin were plasma treated in synthetic air and argon using a dielectric barrier discharge at atmospheric pressure. Changes due to the plasma modification of the surfaces were studied by the techniques of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, and metastable impact electron spectroscopy. The combination of these techniques makes it possible to distinguish between hydroxyl and carbonyl groups at the surface, on the one hand, and conjugated and non-conjugated carbon bonds, on the other hand. This type of differentiation would not be easily possible based on XPS alone, even at very high resolution. The plasma treatment in oxygen containing atmospheres oxidizes the lignin surface by the generation of hydroxyl, carbonyl, and carboxyl groups and reduces cellulose surfaces by the degradation of hydroxyl groups and the formation of double bonds between carbon and oxygen. The plasma treatment in argon leads to the reduction of both lignin and cellulose by the formation of double bonds under degradation of hydroxyl groups.

The tensile shear strength of veneer lap joints was characterised. The joints were produced with an Automated Bonding Evaluation System (ABES) using urea-formaldehyde (UF) as well as melamine-urea-formaldehyde (MUF) adhesive formulated for particleboard production. At a fixed heating temperature of 110°C, a systematic increase in bond strength was observed for both adhesives with increasing cure time. The absolute bond strength was significantly higher for MUF compared to UF. Nanoindentation experiments with the same specimens used for ABES revealed a very hard, stiff and brittle character of the UF resin, whereas the MUF proved significantly less hard and stiff, and less brit-tle. Wood cell walls in contact with adhesive, i.e., where adhesive penetration into the cell wall was assumed, showed significantly altered mechanical properties. Such cell walls were harder, stiffer and more brittle than unaffected reference cell walls. These effects were slightly more pronounced for UF than for MUF. Comparing UF and MUF, the micro-mechanical properties of cured adhesive and interphase cell walls confirm earlier observations that tougher adhesives can lead to higher macroscopic bond strength. In strong contrast to that, no obvious correlation was found between micromechanical properties and the strong cure time dependence of macroscopic bond strength.

Macro-voids in the core of uncompressed particle mats and pressed particleboard manufactured from novel particleboard furnishes were characterized using a response surface method with mixture design and X-ray CT technology. Industrial particles were screened into core-fine, medium, and coarse size classes and their dimensions were measured. Wooden blocks measuring 10 times the mean dimensions of these particles were cut and used as surrogates for the industrial particles. Novel particle mixtures were prepared by mixing together various proportions from each particle size class. The mixtures were packed to simulate particleboard mat formation and a pre-pressed particle mat. Panels were then fabricated from the industrial furnish mixtures. The void fraction of the packed particles and the finished panels were measured and correlated with the IB strength and edge screw withdrawal resistance. Results indicated that densely packed hammer-milled industrial particles had a maximum void fraction of 63.2%. The void fraction of a randomly packed, dense particle mat was described using a full cubic model. In both particle mats without resin and the pressed panels, increasing the core-fine content decreased void volume, whereas increasing coarse particle fraction increased void volume in the mat only. The macro-void ratio in the pressed panels increased exponentially with void fraction for the randomly packed, loose particle mats. Particle mixtures that resulted in boards with the smallest void fraction were not necessarily the strongest boards; low density particleboard panels made from the novel 100% coarse mixture were found to have the highest mechanical properties.

An experimental evaluation of mixed mode fracture tests conducted on adhesively bonded wood specimens using a dual actuator load frame is presented. This unit allows the fracture mode mixity to be easily varied during testing of a given specimen, providing improved consistency, accuracy, and ease of testing over a range of loading modes. Double cantilever beam (DCB) type specimens made of southern yellow pine ( Pinus spp.) wood substrates bonded with a commercially available one part polyurethane adhesive were tested over a wide range of mode mixities from pure mode I to pure mode II. The critical strain energy release rate (SERR) values were calculated from the measured load, displacement, and crack length data, in combination with material properties and specimen geometric parameters, and compared on a versus fracture envelope plot. Mean quasi-static fracture energy values were calculated to be 390 J m -2 and 420 J m -2 for mode I and mode II fracture, respectively. For various mixed mode phase angles, the critical SERR values were partitioned into mode I and mode II components. In mixed mode loading conditions the cracks were typically driven along the interface, which resulted in lower total fracture energy values when compared with those measured under pure mode I loading conditions. A drop in measured fracture energy of approximately 45% was observed with mode mixity phase angles as small as 16°, implying that engineering designs based on results from the popular mode I DCB test could be nonconservative in some situations. Fracture surfaces obtained at different mode mixities are also discussed. An improved understanding of fracture behavior of adhesively bonded wood joints under mixed mode loading through generation of fracture envelopes could lead to improved designs of bonded wood structures.

Experiments concerning drying behaviour of ntholo ( Pseudolachnostylis maprounaefolia PAX) were conducted to find a suitable drying schedule. Two non-symmetrical drying (NSD) tests were carried out to determine the drying behaviour of ntholo in terms of drying rate and stress behaviour. A tentative drying schedule was selected for comparison of the test results with those of similar tests with other known species. The schedule was tested in a laboratory kiln on 28-mm thick boards. According to both NSD tests and laboratory tests, ntholo dries easily but slowly. The laboratory drying lasted 266 h and achieved standard drying quality characterised by 8.9% moisture content, a moisture gradient of 1.2% and a case-hardening (gap) of 1.2 mm. Twist was the largest deformation with 3.4 mm per 1000 mm on average. The assigned schedule provided standard drying quality and it could be tested further in industrial kilns.

A method is proposed for the automated quantitative analysis of vessel characteristics in birch and poplar species. The method combines image-processing techniques with robust statistical approaches for automatically identifying vessels from digital microscopy images obtained by transmitted red light. The proposed method has been tested over a wide range of birch and poplar samples from different growth environments. Performance of the automatic vessel identification routine was assessed using results obtained by manual counting. The automated method produced fast and reliable vessel measurements and was robust to variations within and between samples. The approach has been merged into the wood property measurement system SilviScan as a core component of the hardwood analysis set for research and commercial use.

Pine wood ( Pinus sylvestris ) veneer strips were incubated in acetate buffer containing hydrogen peroxide and Fe ions (Fenton's reagent) to mimic aspects of brown rot decay and to assess the degradation of cellulose in wood via measurement of tensile properties (measured in a zero-span mode). Varying the type of iron (ferrous or ferric sulfate) mixed with H 2 O 2 did not yield significant differences in the rates of H 2 O 2 concentration and tensile strength reduction. However, increasing the amount of wood material (the number of wood strips) in the reaction mixture elevated Fe(III) reduction in solution, indicating that wood constituents participated in this reaction. Increasing concentrations of Fe(III) in the reaction mixture resulted in a decrease in H 2 O 2 in solution. Despite an increase in iron concentration and H 2 O 2 decomposition under these conditions, a uniform and consistent strength loss of 30% was observed at all Fe(III) concentrations tested. At fixed Fe(III) concentrations, increasing the H 2 O 2 concentration linearly increased the strength loss of the veneers up to approximately 50% within 24 h. The addition of a low molecular weight, metal-binding, phenolic compound (2,3-dihydroxybenzoic acid) and of a non-chelating hydroquinone to the reaction mixtures entailed a more rapid consumption of H 2 O 2 ; however, the tensile strength loss of the veneers decreased with increasing concentration of the phenolics. Thus, in contrast to previous studies on cellulose degradation, phenolics reduced the degree of wood decay in a Fenton system.

A major drawback of wood in outdoor exposure is photodegradation of the surface. Nano zinc oxide (ZnO) has been reported to provide enhanced stability of wood against UV degradation. In this study, the formation of ZnO nanostructures on the surface of solid wood was achieved via a simple two-step process consisting of generation of ZnO seeds on the wood surface followed by a solution treatment to promote crystal growth. The morphology and chemical composition of the ZnO films was studied by field-emission scanning electron microscopy combined with energy dispersive X-ray and X-ray diffraction. Accelerated weathering was used to evaluate the photostability of treated wood. The morphology of ZnO nanofilm was dependent on the exposure time in the ZnO seeding solution. Greatly enhanced photostability was achieved with treated wood. UV stability was partially dependent on crystal morphology. This two-step method could probably also be employed to treat other biomaterials with rough surfaces such as bamboo.

Biocides currently employed to prevent termite damage face environmental and toxicity concerns. We recently proposed that heartwood which is naturally resistant to termites might be due to the extractives having both toxicity and antioxidant properties and we reported that the artificial and benign antioxidant butylated hydroxytoluene (BHT) had termite feeding deterrence and toxicity properties. Here, we investigate other antioxidants, both natural and synthetic, and analogs which lack antioxidant properties, on Reticulitermes flavipes Kollar. Laboratory no-choice tests according to AWPA Standard E1 were performed. Antioxidants that are benign to humans had feeding deterrence and mortality properties to R. flavipes . Conversely, the non-antioxidant analogs, with the exception of the heterocyclic flavanone which was previously shown to be active against decay fungi, had little effect on R. flavipes . We conclude that R. flavipes avoids wood which contain high levels of antioxidants, such as heartwood with phenolic extractives.

Vacuum-impregnation with copper azole was applied as a post-treatment to five commercially available wood-based composites with thicknesses of approximately 12 mm, such as softwood plywood (SWP), hardwood plywood (HWP), medium density fiberboard (MDF), oriented strand board (OSB), and particleboard (PB). Untreated and treated composites were tested for their resistance to decay fungi (brown rot fungus Fomitopsis palustris and white rot fungus Trametes versicolor ) and the subterranean termite Coptotermes formosanus by Japanese standardized laboratory test methods. Untreated MDF was highly resistant to both biological attacks and seemed to require no preservative treatment under less hazardous conditions, i.e., under protected and above-ground applications, with possible occasional wetting. PB was ranked second, and needed further protection only against C . formosanus . Copper azole did not adequately protect SWP from F . palustris and termite. OSB was not protected either against F . palustris and T . versicolor even at a concentration level of 1 kg copper azole per m 3 in contrary to blocks of Cryptomeria japonica sapwood, which was protected at these concentration levels. The biological resistance of HWP was reasonably improved by copper azole. The performance of wood-based composites treated with copper azole, which was inferior to the biological resistance of treated C . japonica sapwood blocks, might depend on the thickness or layer profile, density as a result of porosity, uneven distribution of the preservative in the composites, and the susceptibility of the component raw materials.

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