Volume 73, Issue 10

Wood ranks among the most valued resources in construction, for joinery and furniture. Rather than increasing the pressure on a limited number of species, we need to move towards a fit for purpose approach where the basis for selection of a material is a solid knowledge of its relevant properties. Therefore, knowledge about wood technological characteristics of a vast range of wood species is needed. Here, we exploit the potential of xylarium samples by mapping wood density and dimensional stability, using digital image correlation (DIC) on non-standardised samples, of 53 different tropical wood species from the Congo Basin. Furthermore, we attempt to link the results with quantitative anatomical features of the different species tested. DIC proved successful compared to standard calliper measurements on reference samples (R 2 -radial = 0.94, R 2 -tangential = 0.96). Vessel properties play a larger role towards explaining dimensional stability compared to wood density. Fibre wall thickness is positively related to wood density and volumetric swelling. We are able to differentiate between species and sample groups with similar behaviour, partially explained by their anatomical structure. Selecting species based on the required properties for the targeted end use as such can unlock the potential of currently unknown species.

Owing to technical difficulties, experimental assessment of wood cell wall pore size distribution (PSD) in the hygroscopic range still remains challenging. Here, a “trial-and-error” approach was proposed to calculate such distribution through bridging experimental and simulated sorption isotherms presented by the authors in the past. Two main assumptions were made in the calculations, namely, the generation of new and the swelling of existing cell wall pores during water sorption. The nanopore size distribution of dried cell wall derived from the experimental CO 2 gas sorption isotherms was used as the initial boundary condition. Predicted pore size distributions were assessed to be fairly reasonable by comparing them at 95% relative humidity with the PSD of fully saturated cell walls derived from the solute exclusion method. The predicted distribution was relatively wide with several major peaks evolving in the hygroscopic range. The present work also showed that confounded by a wide PSD that includes mostly micropores, the shape of the experimental sorption isotherms was not reliable in assessing the sorption mechanism. The simulations suggested an alternative water sorption mechanism for wood, i.e. micropore filling of cell wall nanopores.

Low-field nuclear magnetic resonance (LF-NMR) was used to clarify the difference between the fiber saturation point (FSP) of Southern pine ( Pinus spp.) determined at the hygroscopicity limit (HL) vs. cell wall saturation (CWS). The HL samples were conditioned at 100% relative humidity (RH) while the CWS samples were fully saturated with distilled water. Cell wall water was found to increase by around 10% in the CWS state. A possible explanation may be that the cell wall was further swollen under liquid saturation. The increase in water content located in the cell wall was determined using freezing-point depression. About 11% of the moisture content (MC) was found in cell wall nanopores with sizes ranging from 1.73 nm to 13.80 nm. The discrepancy between the HL and CWS values of the samples was attributed to freezable water in the nanopores of the matrix components and inter-microfibrils. In the drying process, wood shrinkage was further evaluated in the MC region corresponding to the discrepancy, where the tangential and radial dimensions changed slightly.

Reaction woods of three softwoods, Pinus merkusii , Cryptomeria japonica and Cedrus deodara , were investigated by alkaline nitrobenzene oxidation (NBO) to characterize the condensed-type structures in compression wood lignins. A novel biphenyl-type NBO product carrying guaiacyl (G)- and p -hydroxyphenyl (H)-units, dehydrovanillin- p -hydroxybenzaldehyde (HG-biphenyl product), was identified using the authentic standard compound. On the basis of the yield of this novel NBO product, as well as those of GG-biphenyl-, β-5-, and uncondensed-type products [e.g. dehydrodivanillin, 5-formylvanillin, vanillin and p -hydroxybenzaldehyde], the compression wood lignins contained more HG-type biphenyl and H-type β-5 structures than the opposite wood lignins. The increase in the condensed-type structure content was largely offset by the decreases in the content of GG-biphenyl and G-type β-5 structures. Consequently, the relative yields of biphenyl, β-5 and uncondensed-type NBO products were very similar between the compression wood and the opposite wood, even though the H-unit having no methoxy group on its aromatic ring can be assumed to have a greater probability to form condensed-type structures during lignin biosynthesis than the G-unit.

In this study, the formation of particles and evolution of the particle size distribution in the micron range were monitored in situ during acid precipitation of kraft lignin. The objective of this work was to study the influence of anionic specificity and the ion concentration level. The concentrations of ions in the solution were altered both in terms of the concentration of Na + and the type of anion in the salt added (SO42−${\rm{SO}}_4^{2 - }$ and Cl − ). The results indicate that a salting-out phenomenon occurred as NaCl was added (Na + ≥2 mol kg −1 water) to the kraft lignin solution at high pH, but not when Na 2 SO 4 was added. However, the onset pH of the formation of particles (≥1 μm), triggered by acidification, showed to be virtually non-specific to the anion but strongly dependent on the Na + concentration. As the pH decreased further to below the onset pH of the formation of particles ≥1 μm, the chord length distributions (particle-size related) indicated that most of the volume of the precipitated kraft lignin (and thus possibly also the mass) may be found among the micron-sized particles, despite the fact that a relatively large number of submicron particles may also be present. The volume-based distributions tended to be wide at relatively low pH and high Na + concentrations (e.g. pH 9.4 and 2.0 mol kg −1 water).

Xylem sap obtained from radiata pine sapwood using a supercritical CO 2 (scCO 2 ) dewatering process contained a complex mixture of solutes comprising carbohydrates, amino acids, alkanoic and diterpene acids, neutral diterpenoids, β-sitosterol and cyclitols. Sap also contained free phosphoric acid which is known to catalyse the condensation reactions between glutamic and aspartic amino acids and glucose, galactose and fructose to form Maillard products. When heated these Maillard products lead to the formation of melanoidins, which are partly responsible for the formation of kiln brown stain (KBS) when green radiata pine sapwood is kiln dried. The cyclitols, which are major components in sap, have been shown to undergo phosphoric acid-catalysed dehydration reactions under kiln drying conditions to give methoxyl derivatives of hydroquinone, catechol, resorcinol and O-methylphloroglucinol. These reactive phenols are known to exothermally condense with formaldehyde and could also react with other short carbon chain aldehydes generated during kiln drying. The products of these chemical reactions are extended π-orbital aromatic polymers which are darkly-coloured and would be expected to contribute to KBS. This paper describes the chemistry of sap from radiata pine sapwood using scCO 2 dewatering and proposes new theory on KBS formation invoking thermal dehydration and retro Diels-Alder chemical reactions of the sap cyclitols.

A high voltage electrostatic field (HVEF) was applied to enhance the bonding performance of wood composites prepared with phenol-formaldehyde (PF) adhesive and different wood species and radial cut combinations. Four wood species including Masson pine ( Pinus massoniana ), Chinese fir ( Cunninghamia lanceolata ), poplar ( Populus tomentosa ) and ayous ( Triplochiton scleroxylon ) were studied. The results of HVEF-treatment turn out to be species-dependent, and are related to the anatomical and chemical properties of wood. It was demonstrated by a statistical approach that the lignin content is the most significant parameter with a good correlation coefficient (R 2  > 0.8). High lignin content leads to high free radical concentration at the wood surface and the HVEF enhanced the adhesive penetration depth, the maximal density and the bonding strength (Bst) at the interphase. On the contrary, high extract contents and large lumina diameters negatively impacted the surface modification by HVEF. The magnitude of the effects was in the following order: ayous < poplar < Masson pine < Chinese fir.

Poplar wood ( Populus deltoides Bartr.) was modified by a combined two-step treatment with different chemicals to improve its properties. Maleic anhydride (MAN), 3-(trimethoxysilyl) propyl methacrylate (TMPS) and glycidyl methacrylate (GMA) were first employed to modify wood cell wall resulting in W MAN , W TMPS and W GMA . Then, in a second step, the vinyl acetate (VA) monomer was let to polymerize within the cell lumina resulting in WPC MAN/VA , WPC TMPS/VA and WPC GMA/VA (WPCs). Field emission scanning electron microscopy (FESEM) observations confirmed the bulking of modified cell walls. The thermal stability, mechanical properties and decay resistance of WPCs were remarkably improved compared to unmodified wood in the order WPC GMA/VA  >  WPC TMPS/VA  > WPC MAN/VA . WPC MAN/VA displayed a significant decay resistance increment, despite lower retention and reactivity than the WPC TMPS/VA , which is probably due to a better penetration into the cell wall and the higher degree of chemical modification of the wood components.