Band 73, Heft 12

Bamboo is a natural bio-composite material that is both tough and possesses excellent mechanical properties. Its delicate structure and fiber arrangements play an important role in the mechanical properties and the fracture performance of bamboo. In this study, the fracture behaviors of bamboo due to tensile strength and bending were investigated using synchrotron radiation micro-computed tomography and environmental scanning electron microscopy (ESEM). Both tension and bending tests revealed that fibers are the key factors for bearing extra load and impeding crack propagation. Crack paths were found to be different in bamboo strips with different fiber percentages, and the interface between pulled-out fiber bundles and fiber break (in helix breaks and shearing break) was observed to be typical fracture characteristics in tension. Moreover, advanced micro-matrix cracking, interface debonding, fiber bundle bridging and the splitting of cell wall layers were found to be important fracture characteristics in bending. Both in tension and bending, a reasonably weak interface appeared to be beneficial for improving fracture toughness of fiber-reinforced bamboo. Thus, it is clear that “fiber bridging” mechanisms and “shear-lag theory” are well exhibited in bamboo.

The objective of the study was to investigate the water vapour sorption behaviour of thermally modified (TM) wood powder, e.g. ground wood prepared from waste streams of TM solid wood, and wood powder that was extracted in pressurised hot water. Solid spruce wood was TM in steam conditions (210°C for 3 h), milled and hot-water-extracted (HWE) at elevated pressure (140°C for 1 h). The results evidence that the hot-water extraction reduced the water sorption and the accessible hydroxyl group concentration by the removal of amorphous carbohydrates. In contrast, the enhanced cross-linking of the cell wall matrix and the annealing of amorphous matrix polymers during thermal modification reduced the sorption behaviour of wood additionally, without further reducing the hydroxyl accessibility. These additional effects of thermal modification were at least partially cancelled by hot-water extraction. The results bring novel insights into the mechanisms that reduce the water vapour sorption of wood by compositional and structural changes induced by heating.

Although weathering is usually carried out in outdoor conditions, even ageing in indoor conditions can cause significant changes to wood surfaces. We found these to notably impact wetting and coatability of surfaces of common beech ( Fagus sylvatica L.) and Norway spruce [ Picea abies (L.) Karst.] wood. These effects were well overcome and the surfaces reactivated using a novel kind of a plasma device. On both kinds of wood, ageing caused significant changes in their colour, opening of pits and cell wall destruction. Infrared spectra indicated a significant decrease of aromatic lignin and production of non-conjugated carbonyl groups. Surface free energies and spreading kinetics varied much upon this kind of ageing. In beech, pull-off strengths for a commercial waterborne wood coating slightly decreased for longer exposure times. For the reactivation of wood surfaces, we employed a novel approach using a floating electrode dielectric barrier discharge (FE-DBD) plasma, which had not been done on wood before. Similar to other plasma techniques, the surface free energy (SFE) increased after treatment. On beech wood, the plasma treatment (PT) led to higher pull-off strengths of the waterborne coating. On spruce wood, the coating mostly showed cohesive failures after ageing, and thus the PTs showed less improvements.

To discuss the macromolecular structure and properties of lignin, the complementary use of solid- and liquid-state nuclear magnetic resonance (NMR) combined with a specific 13 C-enrichment technique may provide useful information. The 13 C-enriched lignin was prepared by administering [guaiacyl ring-5(G5)- 13 C]-coniferin to a growing Ginkgo biloba L. shoot. The 13 C-enriched cellulolytic enzyme lignin (EL) and its acetate prepared from the ginkgo shoot were examined by solid- and liquid-state 13 C NMR spectroscopy. The 13 C NMR spectrum derived only from the G5 carbon was obtained as a difference spectrum based on the spectra of the G5- 13 C enriched and un-enriched (UE) samples. The condensed structures, including the enriched G5-carbon, were evaluated using difference spectra. The chemical shifts of the enriched G5 carbon in both the solid- and liquid-state 13 C NMR spectra agreed with each other in principle. The quantitative ratio of the condensed and uncondensed structures at G5 was found to be larger by solid-state cross polarization/magic angle spinning (CP/MAS) NMR than by liquid-state NMR.

The aim of this study was to have a better understanding of the phenomenon of resin exudation on pinewood boards due to the liquefaction of the resin caused by the heat of solar radiation. Resin exudation is a crucial problem for the maritime pine industry, as it may strongly decrease the wood’s aesthetics submitted to outdoor exposure. The softening temperature of dried pine resin was analyzed and the results were examined relative to its physico-chemical properties. To achieve this, resin samples were dried under different conditions (60°C, 120°C during 16 h and at 150°C during 24 h). After 3 months of post-drying at ambient temperature, their softening temperatures were measured by thermomechanical analysis. These same samples were subsequently studied to determine their chemical, structural and morphological properties. Samples with a low drying temperature (60°C) had high opacity attributed to crystallinity and porosity. These elements partially explained a higher softening temperature than for the other samples. Different chemical modifications of resin were also observed due to temperature, with a significant amount of oxidized derivatives for samples dried at low temperature and dehydrogenated compounds for samples dried at high temperature.

Dissolution of microcrystalline cellulose (MCC) in pure ionic liquids (ILs) and IL/dimethyl sulfoxide (DMSO) mixtures (mole fraction χ DMSO  = 0.2–0.9) was quantified using a specially constructed mechanical stirring system that allows reproducible agitation speed; temperature control, and minimum solution-air contact. The electrolytes employed were: 1-( n -butyl)-3-methylimidazolium acetate (C 4 MeIm AcO), 1-(methoxyethyl)-3-methylimidazolium acetate (C 3 OMeIm AcO), 1,8-diazabicyclo[5.4.0]undec-7-enium acetate (DBU AcO), tetramethylguanidinium acetate (TMG AcO), and tetra( n- butyl)ammonium fluoride hydrate (TBAF·xH 2 O). The effects on MCC dissolution of IL/DMSO composition, and temperature (50, 70°C) were studied. C 4 MeIm AcO and C 4 MeIm AcO/DMSO were more efficient solvents than their C 3 OMeIm AcO counterparts, due to “deactivation” of the ether oxygen of C 3 OMeIm AcO. MCC dissolution by C 4 MeIm AcO/DMSO was compared with DBU AcO/DMSO, TMG AcO/DMSO at χ DMSO  = 0.6, and TBAF·xH 2 O/DMSO at χ DMSO  = 0.95. The relative efficiency was (solutions in DMSO): C 4 MeIm AcO > C 3 OMeIm AcO > DBU AcO > TMG AcO > TBAF·xH 2 O. The efficiency of C 4 MeIm AcO relative to C 3 OMeIm AcO is due to higher solution basicity. Isothermal titration calorimetry was used to study cellobiose-solvent interactions. Except for TBAF·xH 2 O/DMSO, these interactions are exothermic; the relative solvent efficiency increases with increasing dissolution |enthalpy|. Using the mole fraction concentration scale to report cellulose dissolution avoids possible ambiguities.

To examine the methodology for determining the moisture diffusion behavior of lignocellulosic biomass in steady and unsteady states (two stages of a sorption isotherm), the diffusion coefficients in the steady and unsteady states ( D SS and D US ) were investigated over a range of relative humidity (RH) from 10 to 90% using a dynamic vapor sorption (DVS) apparatus and a specifically designed cell kit. Thin samples with a thickness of 50 μm were prepared from three lignocellulosic biomasses, i.e. poplar, Chinese fir and moso bamboo. Based on Fick’s first and second laws, D SS and D US were determined. An increase in D SS or D US was observed with increasing equilibrium moisture content (EMC) or transient status, regardless of the lignocellulosic biomass species. The moisture-dependent D SS of poplar, Chinese fir and moso bamboo was similar to values previously reported. Chinese fir and moso bamboo exhibited the highest and the lowest D SS values, respectively, when the same EMCs were achieved. The results of this study revealed that D SS and D US of lignocellulosic biomass (even with limited dimensions) could be determined during a sorption isotherm in a wide humidity range. Furthermore, the results are helpful for simulating moisture transport behaviors in the fields of drying, paper packaging and wooden building maintenance.

A study of tylosis in European Turkey oak ( Quercus cerris L.) shows correspondences in the formation of tyloses and of regular cell walls. The outer tylosis wall has a smooth, granular surface with simple perforations analogous to that of the primary wall of ordinary cells. The underlying wall stratum shows parallel oriented macro-fibrils, normally found in the secondary walls of regular cells. At the contact areas of tyloses, stabilizing seams can be observed. Various types of wall openings such as simple pits, blind pits and vestured pits were present. Also tylosis division was detected. The characteristics of parenchyma cell walls can be re-discovered in tyloses.