Volume 73, Issue 8

The influence of tree height on the composition of bark extractives from Eucalyptus globulus and Eucalyptus nitens was investigated for its potential in the sourcing of valuable compounds. The bark of these trees was extracted at different heights to obtain lipophilic and hydrophilic compounds, which were chemically characterized. The extraction yield of lipophilic compounds was higher for E. globulus than for E. nitens . The type and proportion of the compounds changed with the tree variety and height. The main compounds detected were triterpenic acid derivatives, such as lupane, oleanane and ursane types, followed by fatty acids. The yield extraction of the hydrophilic compounds was higher for E. nitens than E. globulus . The total phenolic content, proanthocyanidins and antioxidant activity of the extracts increased with the tree height in both cases. However, the E. globulus extracts [half maximal inhibitory concentration (IC 50 ) = 13.8 ± 0.2 μg ml −1 ] showed higher antioxidant activity than did the E. nitens extracts (IC 50  = 22.8 ± 0.2 μg ml −1 ). Differences in the extract composition and the spatial conformation of the molecules (steric effect) could also help to obtain a higher antioxidant capacity from E. globulus extracts. These results show that differentiation by tree height can contribute to the selective enrichment of these valuable compounds.

The effects of variation in humidity coupled with long-term loading give rise to dimensional changes and creep effects in wooden elements. Many wooden products such as cross-laminated timber (CLT) plates as well as many common structural details used in timber engineering are vulnerable to variations in moisture content (MC) as well as to creep effects. This paper addresses the long-term effects in the material modelling of timber by the finite element method (FEM), also considering the viscoelastic and mechanosorptive effects in wood. The model was calibrated using both relaxation tests and creep tests. The results from both long-term compression perpendicular- to-grain tests (relaxation and creep) performed on glulam (GL30c) from Norway spruce ( Picea abies ) with moisture control are presented in this paper. The material model considers the effect of loading and moisture changes. For realistic comparison, the pith location of each lamella was specified in the numerical analyses. Ultimately, a comparison between the numerical results and the experimental results has been provided, exhibiting an overall good estimation of timber response.

To elucidate the frequency-dependent viscoelasticity of wood under a moisture non-equilibrium state, changes in stiffness and damping as a function of frequency were investigated during the moisture adsorption process. The moisture adsorption processes were carried out at six temperatures (30–80°C) and three relative humidity levels (30, 60 and 90% RH). During the moisture adsorption process, the wood stiffness decreased, and damping increased with the increment of moisture content (MC). Regardless of the moisture adsorption time, the wood stiffness increased, and damping decreased with the increasing testing frequency. Based on the re-organized Williams-Landel-Ferry (WLF) model, the time-moisture superposition (TMS) relation was assumed to be applicable for developing a master curve of wood stiffness during the moisture adsorption process. The frequency ranges of the stiffness master curves spanned from 16 to 23 orders of magnitude at temperatures ranging from 30 to 80°C. However, the TMS relation was not able to predict the wood damping properties during the moisture adsorption process due to the multi-relaxation system of the wood and the non-proportional relationship between free volume and MC at transient moisture conditions.

The frequency-dependent viscoelasticity of Chinese fir ( Cunninghamia lanceolata ) during moisture desorption was investigated and the applicability of the time-moisture superposition (TMS) relation on wood stiffness and damping during the moisture desorption was verified. The hygrothermal conditions for the moisture desorption were set up as six constant temperatures ranging from 30 to 80°C and three relative humidity (RH) levels at 0, 30 and 60%. Due to the elimination of water during the moisture desorption, the stiffness of the Chinese fir increased, whereas the damping decreased. With the increase in frequency, increased stiffness and decreased damping were observed. Utilizing the TMS relation, it was possible to construct master curves of wood stiffness at temperatures ranging from 30 to 80°C. The linear relationship between the shift factor and the moisture content (MC) manifested a low intermolecular cooperativity between the polymers and a narrow relaxation window. However, the TMS relation was not able to predict the wood damping properties during the moisture desorption, because wood is a multi-relaxation system. The non-proportional relationship between the free volume and MC during the moisture desorption may also explain why the TMS relation failed to construct master curves of the wood damping properties.

In order to clarify the evolution and role of extractives in thermally modified wood during the process of weathering, the compositions of acetone extractives from thermally modified Scots pine after exposure in an accelerated weathering tester for different durations were determined using gas chromatography-mass spectrometry (GC-MS). Fatty acids and phenolics were proved to be the main types of extractives in weathered thermally modified Scots pine, and some sugars, terpenes and alcohols were also collected. With the progress of weathering, the content of fatty acids decreases significantly while that of phenolics increases. The reduction or even elimination of the fatty acids is presumed to be a physical process, namely, a discharge from wood during exposure to radiation and elevated temperature. The increase of phenolic extractives is mainly due to the photodegradation of other wood components. Phenolic degradation products play the role as a “barrier” against further photodegradation of thermally modified wood, among which vanillin or its derivatives appeared to be the most predominant and important ones.

The viscoelastic properties of wood samples treated with various consolidants [polyethylene glycols (PEGs), lactitol and trehalose, Kauramin, polydimethylsiloxane (PDMS) and trimethoxymethylsilane (MTMS), rosin and polyethylene-L-tartaramide] were evaluated by means of dynamic mechanical analysis (DMA) in order to clarify their effects with regard to the conservation of wood objects. Two softwoods, medium-decayed silver fir ( Abies alba ) and highly decayed maritime pine ( Pinus pinaster ), and two differently decayed oak hardwoods ( Quercus spp.) were used. In addition, the selected parameters [retention, volume variations and equilibrium moisture content (EMC)] were evaluated to assess the consolidation efficacy. The results showed that in treated wood, both the storage modulus and tanδ depend upon the level of degradation. The impact of the consolidating product was greater in more degraded wood (a higher product amount inside the artefact) compared to well-preserved material, where also wood played a role (also when the products were applied at high concentrations). The values of the different viscoelastic parameters also depend upon the ability of the products to interact with the wood matrix; this characteristic is fundamental to the nature of each consolidant. Lactitol-treated samples showed the highest stiffness increases, whereas those treated with selected PEG combinations and rosin displayed noticeable tanδ increases (high dissipative behaviour).

Medium dense hardwoods (HWs) show higher tensile strength (TS) values than softwoods (SWs). These advantages cannot be utilised effectively because HW grading is not well developed. The aim of the present paper was to analyse the utilisation potential of European ash ( Fraxinus spp.) and maple ( Acer spp.) grown in Central Europe, which were graded by different methods. The visual grading characteristics of 869 HW boards were determined and the dynamic modulus of elasticity (MOE dyn ) and X-ray attenuation (XRA) were measured by an industrial scanner. The specimens were subsequently tested in tension according to EN 408:2010 and according to German visual grading rules show strength values of 28 MPa and 30 MPa, respectively. Machine strength grading and for a combination of manually assessed boards and MOE dyn give rise to higher strength data. MOE dyn , in particular, results in lamella data with 62 MPa for ash and 42 MPa for maple. There is good agreement with recently presented HW tensile profiles. Machine grading with a multisensor system allows better strength prediction compared to the MOE dyn or visual strength grading. Best performance is achieved by a combined grading approach.

Mass timber panels (MTPs) have a great potential in the construction of mid- and high-rise buildings. Evaluation of the face-bond strength of MTPs is of great practical interest for this kind of products. This study aimed at developing an appropriate test procedure for evaluating the adhesive bond strength of cross-laminated laminated strand lumber (CL-LSL). Three-point short-span bending tests were conducted on two-layer asymmetric CL-LSL specimens (2L asym CL-LSL), which were adhesively bonded by two-component resins of the type polyurethane (PUR) or polyvinyl acetate (PVAc). For comparison, block shear specimens were tested as well. It was found that the 2L asym CL-LSL assembly was better suitable under the short-span bending for differentiating between good and poor bond quality of MTPs.