Outdoor weathering performance parameters of exterior wood coating systems on tropical hardwood substrates

Wood coating research almost exclusively focuses on softwood as substrate despite the fact that coatings applied on tropical hardwoods show a different weathering behaviour. The objective of this study was to quantify the impact of wood substrate and coating type on the weathering behaviour of finished wooden joinery in outdoor exposure using a set of statistical methods. Therefore, a broad range of white opaque paints and mid-oak semi-transparent stains were used. The coatings were industrially applied on window frames made of frequently used commercial tropical hardwood species. The general appearance of the wood/coating systems was assessed visually using a rating scale that takes into account the anatomical characteristics of hardwood. For the evaluation, a required minimum exposure time of 2.5 years outdoor weathering for joinery in vertical position and facing south-west was determined. At 30 months best performances were recorded for waterborne alkyd primers with an acrylic topcoat when dry coating film was sufficiently thick. The dry layer thickness also affected the impact of the hiding power on the performance of the wood/coating system. Within hardwood species the degradation rate of coatings on species with a fine texture was the lowest. The significant influence of the substrate’s texture on the weathering behaviour of coated hardwood stressed the need to evaluate the wood/coating system in total.     

Improvement of tensile shear strength and wood failure percentage of 1C PUR bonded wooden joints at wet stage by means of DMF priming

Tensile shear tests according to EN 302-1 for load-bearing timber structures were performed on European beech wood (Fagus sylvatica L.) and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] bonded by means of a one-component polyurethane adhesive (1C PUR). Results reveal a substantial loss of tensile shear strength (TSS) and wood failure percentage (WFP) at the wet stage compared to the dry stage. As can be seen from microscopic images, this is accompanied by a loss of adhesion at the boundary layer. Therefore, the aim of this work was to find a priming fluid that improves the load transmission between adhesive and adherend at the wet stage without introducing formaldehyde into the gluing process. A substantial improvement of TSS and WFP was achieved by means of the hygroscopic organic solvent N,N-dimethylformamide (DMF). In addition, contact angle measurements were carried out, revealing that DMF heavily enhances the wettability of the joining surface. Furthermore, it was attempted to integrate the outcomes into the swelling strain model stated by Frihart in 2009. By way of comparison a hydroxymethylated resorcinol coupling agent, a mixture of diphenylmethane-4,4′-diisocyanate isomers and water were also tested as priming fluids. The data confirm that TSS and WFP of 1C PUR bonded wooden joints do not correlate, whilst WFP is mostly not normally (at wet stage often bimodally) distributed.     

Relation of arabinogalactans to density, growth rate and shear strength in wood of cultivated Siberian larch

Present in high concentrations in the heartwood of Larix species, water-soluble arabinogalactans affect the density, moisture content and processing properties of larch wood. The aim of this study was to chart arabinogalactan concentration (AC) and to define the relations between arabinogalactans, and density and shear strength at different axial and radial locations in Larix sibirica Ledeb. trunks. In addition, growth rate (ring width, fibre dimensions) was compared with AC to determine the link between them. Although quite low AC levels were observed, values differed significantly between sapwood and heartwood, with a slight increasing trend from pith to mature heartwood. Variation in growth rate affected AC levels in juvenile wood only. In conclusion, AC levels were clearly lower in the studied cultivated, fast-grown Siberian larches than those measured earlier in trees of natural origin. AC had a weak correlation with growth rate and shear strength.     

Influence of steaming on selected wood properties of four hardwood species

Influence of steaming on various mechanical and physical properties of two European (Robinia pseudoacacia L., Quercus robur L.) and two tropical (Intsia bijuga, Hymenolobium petraeum) hardwood species were investigated. Each of these wood species requires adequate adhesive systems for the use as dimension stock because of their highly reactive surface chemistry. In order to optimize the gluing behaviour of the timbers involved, steaming processes with five different sets of parameters (steaming time and temperature) were carried out. In addition to the adhesive test, bending strength, hardness, and colour of the modified timbers were examined. The result of steaming highly depends on the wood species. For black locust, steaming is a suitable method of colour homogenization and colourization. Short term, low temperature treatment improves the adhesion performance also, whereas the colour change value reaches its maximum in the case of long term, high temperature steaming. Hue shift of oak and sapupira was inconsiderable for any applied set of parameters, only a small L* decrease was observed at higher temperatures. The colour of merbau samples shifted slightly during the treatment. Bending strength and hardness of wood samples in all of the four wood species decreased during the treatment. However, steaming time is more important than temperature while aspect of colour change both time and temperature has the same significance. Adhesive properties of sapupira can be greatly improved by hydrothermal treatment.     

提高漂白硫酸盐阔叶木浆强度之浅析

分别从原料蒸煮和漂白各个环节对阔叶木浆强度的影响及提高进行了阐述．     

An investigation of thermochemical changes in Canadian hardwood species during wood welding

Thermochemical changes during wood-dowel welding were investigated in two Canadian hardwood species commonly used for indoor appearance applications: sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis). The original reference wood sample and the welded interface between two bonded wood pieces, a dowel and a substrate, were compared to explain differences in mechanical properties between species. Pyrolysis gas chromatography–mass spectrometry (Py-GC/MS), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS) were used. The gases emitted during wood welding were determined by Py-GC/MS and gas chromatography coupled with a thermal conductivity detector and a flame ionization detector (GC-TCD/FID). Results showed that thermal welding of birch and maple woods degrades hemicelluloses and affects lignin polymer through depolymerisation. Welding effectiveness is therefore directly associated with the properties of the original wood constituents, primarily lignin and carbohydrates. The lignin-related changes at the welded interface were greater for sugar maple than for yellow birch, corroborating mechanical property differences observed between the two species. The gases proportions were similar for both species and no harmful gases were detected in the smoke released during welding process.     

Characterisation of hardwood fibres used for wood fibre insulation boards (WFIB)

European Journal of Wood and Wood Products - Wood fibre insulation boards (WFIB) are typically made from softwood fibres. However, due to the rapid decrease in softwood stands in Germany, the...     

竹木层积材层间剪切强度的研究

采用短梁法对竹木层积材料层间剪切强度进行测试,结果表明,层间剪切强度随着层积材密度的增加而增大,当密度为0.95g/cm3时,达到最大值22.86MPa;其破坏模式可归纳为三种:胶层开裂、胶层竹材混合破坏和竹材破坏     

木结构握钉力的研究与发展

阐述了国外木结构钉节点的握钉力研究现状.根据不同的钉杆形状,介绍了普通圆钢钉、螺钉、麻花钉三种常见钉形的握钉力特性,从握钉力计算公式、时间及含水率变化造成的松驰效应、表面涂层等强度影响因素以及结构用板材握钉力性能等五个方面分析了握钉力的研究发展,并对我国握钉力研究提出了建议.     

Effect of storage on extractives from particle surfaces of softwood and hardwood raw materials for wood pellets

When wood pellet raw material (dried sawdust and shavings) is stored for six months, wood pellet producers can observe an increased pellet mill capacity and pellet quality. To investigate the storage effect on the raw material and the possibilities for accelerating the effect, the effect of storage on surface extractives of the raw material was studied. Softwood and hardwood raw material was stored for six months, and the effect of the storage on the composition and concentration of the extractives on particle surface peelings was analyzed. Furthermore, the effect of storage at elevated temperatures was analyzed. The acetone extracts of peelings were analyzed by gas chromatography mass spectrometry (GC-MS), and multivariate regression modelling on the chemical composition was made to compare the effects of storage at elevated temperatures to the normal storage. The analysis showed that the extractive concentrations of the surface material were initially high and that these concentrations further increased during storage. Compound analyses showed that the concentrations of triterpenoid and steroidal structures increased during the storage period, whereas the concentrations of fatty acids and diterpenoid structures decreased during storage. It is suggested that this reflects oxidation and polymerization reactions of the fatty acids and diterpenoid structures. Modelling of the softwood storage proved good linear correlation of specific compound concentration changes to storage, whereas hardwood showed large uncertainties. It was shown that storage at higher temperatures accelerated the changes in extractive concentrations for softwood, whereas it was non-significant for hardwood. The study indicates that the improvement for the wood pellet production connected to raw material storage could be related to changes in the amount and composition of surface extractives for softwoods, whereas the effect for hardwoods is less certain.     

Moisture-dependent orthotropic elasticity and strength properties of Chinese fir wood

Elastic and strength properties are very important material characteristics in mechanical modelling. Due to the anisotropic and hygroscopic nature of wood, a characterization of wood mechanical behavior will require knowledge of its moisture-dependent properties in relation to the three principal axes of anisotropy. The influence of moisture content (MC) on the elastic and strength anisotropy of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook) wood was examined in the present study. Selected parameters, including the anisotropic Young’s moduli, shear moduli, Poisson’s ratios, yield and ultimate stress values in the longitudinal, radial and tangential directions, were determined in compression and tension tests at different moisture conditions. The results indicated that a distinct moisture dependency is exhibited for the elastic and strength behavior of Chinese fir wood. With the exception of some Poisson’s ratios, all investigated elastic and strength parameters were shown to decrease with increasing MC, whereby individual moduli and strength values were affected by the MC to different degrees. The two- and three-dimensional representation of the compliance matrix and the two-dimensional visualization of a yield surface give a valuable overview on the moisture-dependent elastic and strength anisotropy of Chinese fir wood.     

Assessment of phenomenological failure criteria for wood

Many different phenomenological failure criteria have been proposed. These type of criteria do not explain the mechanism of failure itself. They merely identify failure (yes or no) and are usually regarded in practice as a simple and reliable tool for design. Most of them were developed for composite materials, but are extensively applied for wood. In this study, existing phenomenological strength criteria for orthotropic materials were applied to clear wood data. Instead of fitting the criteria to available experimental data, the criteria were used to predict failure of biaxial tests based on uniaxial strength. This procedure is closer to practice, and hence an answer to the question “is there any reliable failure criterion for wood?” Predictability of the criteria was assessed using normalisation procedure, and statistical significance of the difference of the means analysed. Based on the results, it can be concluded that a general criterion cannot be applied to predict failure, but should be chosen according to the biaxial stress state.     

Derivation of the shear strength of continuous beams

The elastic??full plastic loading curve is for all materials sufficient to explain the strength of beams and beam columns loaded by bending and compression. This theory is extended for the influence of shear stress, and it is shown to be the only way to explain the combined bending-shear strength from test results. Also, the in the past derived bearing strength theory is extended here for bracing action. It will be shown for continuous beams as example, that besides moment redistribution by plastic flow in bending, a plastic shear flow mechanism exists that is also able to cause full moment redistribution. The derivations lead to requirements for the design rules and show how the shear stress may reduce the ultimate bending capacity.     

Thermal stability of glued wood joints measured by shear tests

The thermal stability of glued wood joints is an important criterion to determine the suitability of adhesives in the field of engineered wood. During their product life, glued wood joints can be exposed to high temperatures in various ways (direct exposure to the sun, fire, etc.). Thereby the cohesiveness of the adhesive must not degrade. This raises the question of how the strength of bonding changes under thermal load. The current investigation covers the influence of temperature (T=20 to 220°C) on the shear strength of glued wood joints. Different adhesive systems were investigated. With increasing temperature, the shear strength of solid wood and also of glued wood joints decreased. There were big differences in thermal stability and failure behaviour between the adhesive systems as well as within the polyurethane group. The thermal stability of one-component polyurethane systems can be greatly varied by modifying their chemical structure. Well adapted one-component polyurethane adhesives reach a strength similar to that of phenol resorcinol resin.