Volume 70, Issue 4

The structure of lignin and suberin, and ferulic acid (FA) content in cork from Quercus suber L. were studied. Extractive-free cork (Cork), suberin, desuberized cork (Cork sap ), and milled-cork lignins (MCL) from Cork and Cork sap were isolated. Suberin composition was determined by GC-MS/FID, whereas the polymers structure in Cork, Corksap, and MCL was studied by Py-TMAH and 2D-HSQC-NMR. Suberin contained 94.4% of aliphatics and 3.2% of phenolics, with 90% of ω-hydroxyacids and α,ω-diacids. FA represented 2.7% of the suberin monomers, overwhelmingly esterified to the cork matrix. Py-TMAH revealed significant FA amounts in all samples, with about 3% and 6% in cork and cork lignins, respectively. Py-TMAH and 2D-HSQC-NMR demonstrated that cork lignin is a G-lignin (>96% G units), with a structure dominated by β– O –4′ alkyl-aryl ether linkages (80% and 77% of all linkages in MCL and MCL sap , respectively), followed by phenylcoumarans (18% and 20% in MCL and MCL sap , respectively), and smaller amounts of resinols (ca. 2%) and dibenzodioxocins (1%). HSQC also revealed that cork lignin is heavily acylated (ca. 50%) exclusively at the side-chain γ-position. Ferulates possibly have an important function in the chemical assembly of cork cell walls with a cross-linking role between suberin, lignin and carbohydrates.

In our recent studies, it was demonstrated that the IONCELL-P process selectively dissolves hemicelluloses from bleached birch kraft pulp in a mixture of 1-ethyl-3-methylimidazolium acetate ([emim][OAc]) and water as a solvent system. The IONCELL-P method refines paper-grade pulp to dissolving pulp with <5% hemicelluloses and allows isolation of polymeric xylan without yield losses or polymer degradation. This paper is a comparative study where paper-grade pine, birch, and eucalyptus pulps are subjected to the IONCELL-P process with two [emim]-based ionic liquids (ILs), i.e. [emim]acetate and [emim]dimethylphosphate. Also, the effect of an endoglucanase pretreatment was investigated to check whether 1) the pulp viscosity could be adjusted for the following process steps before the hemicellulose extraction and 2) the decreasing pulp viscosity would open the fiber structure and thus enhance the extraction. Under optimum conditions, the birch xylan content could be reduced from 25.4% down to 1.3% and for eucalyptus from 16.6% to 2.4%. Pine pulp xylan and glucomannan were decreased from 8.1% and 7.1% to 0.9% and 2.2%, respectively. The residual hemicellulose contents of the pine pulp could be further decreased with a hemicellulase pretreatment. The selectivity of the dissolution towards hemicelluloses was better for hardwoods. Adjusting the pulp viscosity by endoglucanase prior to the IONCELL-P process reduced the selectivity as short-chain cellulose molecules were extracted along with the hemicelluloses.

A new family of water-soluble lignosulfonate polymers with ultrahigh molecular weight (Mw) was developed based on alkali lignin (AL) as starting material in a one-pot reaction in two steps: sulfomethylation of AL as raw material led to AL-S and this material was subsequently cross-linked via alkylation with 1,6-dibromohexane (alkAL-S). Gel permeation chromatography showed a significant increase of Mw from 5200 Da of AL-S to 201 000 Da of alkAL-S with high degree of alkylation. Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy and functional group analysis confirmed the efficient polymerization by a nucleophilic substitution reaction mechanism. Additionally, alkAL-S with high Mw showed unexpected viscosity-reducing effect, stability and good rheological properties on a low-rank coal-water slurry (CWS), which are even better than those obtained by naphthalene sulfonate formaldehyde (NSF) as additive. The adsorption properties of the new products were also characterized via a quartz crystal microbalance combined with dissipation monitoring (QCM-D method). Cross-linked structure, large steric hindrance from high Mw and suitable amphiphilic properties of alkAL-S polymers contribute together to the highly improved dispersion performances for CWS.

Scots pine sapwood was treated with a new formulation consisting of vinyl acetate (VAc) and epoxidized linseed oil (ELO) catalyzed by potassium persulfate to impart protection to wood. The effects of various curing temperatures, durations, and solution uptakes on dimensional stability (DS) and leachability were studied. The new formulation provided good anti-swelling efficiency (ASE) ranging from 35% to 47% with negligible leaching of the treating agent after four cycles of water soaking and oven drying (2%–2.5%). The extent of polymerization in wood was observed by FTIR-attenuated total reflectance (FTIR-ATR) by evaluation of the areas below typical IR bands as a function of curing temperature and time. Linear relationships were found with high R 2 values. The FTIR data of extracted samples were interpreted that chemical reactions took place between the resulting copolymer and wood components.

Analysis of thin slivers of juvenile (2-year-old) pine wood by pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) was explored as a fast and high-throughput method to assess compression wood (CW) via the ratio of the p -hydroxyphenyl to guaiacyl units (H/G ratio). Various chemical parameters such as contents of lignin, galactose, and p -hydroxyphenyl (H) units have been related to the CW in the tissue. The H/G ratio was calculated from all identified H and G lignin-derived pyrolysis products. The H/G ratio ranged between 0.019 and 0.052 for normal wood (NW) and between 0.133 and 0.227 for CW. There was a weak correlation between CW H/G ratio and Klason lignin content ( R 2 =0.4), but it was not correlated to the galactose content. Lignin- and carbohydrate-derived pyrolysis products were subjected to principal component (PC) analysis. The first two PCs discriminate the CW from the NW, accounting for about 41% of the total variance in the dataset. The model developed based on lignin-derived peaks explained 54% of the variance.

The penetration characteristics of phenol formaldehyde (PF) resin, modified by two different nanomaterials (PF mod ), has been studied by means of scanning thermal microscopy (SThM). The thermal conductivity (ThC) of the two PF mod was lower than that of the cell wall (CW), but the ThC of both PF resins was basically the same. SThM imaging revealed the penetration of parts of PF mod into the CW by a ThC transitional region, which exists between the CW and the resin. In the transitional zone, the ThC changed obviously in a region about 2 μm in width. This region includes two subregions, one about 0.7 μm and another 1.3 μm in width. The first one is the interface, where PF mod and the CW are in direct contact where the ThC changes rapidly. In the second subregion, the PF mod and CW are in interaction, and ThC changes slowly. Regarding the adhesives’ penetration into the cell lumen, the ThC of the penetrating adhesive was higher than that in the glue line, and this is an indication that SThM is a useful tool to detect the differences of adhesive penetration at the micro-scale level.

The aim of cell wall modification is to keep wood moisture content (MC) below favorable conditions for decay organisms. However, thermally modified, furfurylated, and acetylated woods partly show higher MCs than untreated wood in outdoor exposure. The open question is to which extent decay is influenced by the presence of liquid water in cell lumens. The present paper contributes to this topic and reports on physiological threshold values for wood decay fungi with respect to modified wood. In total, 4200 specimens made from acetylated, furfurylated, and thermally modified beech wood ( Fagus sylvatica L.) and Scots pine sapwood (sW) ( Pinus sylvestris L.) were exposed to Coniophora puteana and Trametes versicolor . Piles consisting of 50 small specimens were incubated above malt agar in Erlenmeyer flasks for 16 weeks. In general, pile upward mass loss (ML) and MC decreased. Threshold values for fungal growth and decay (ML≥2%) were determined. In summary, the minimum MC for fungal decay was slightly below fiber saturation point of the majority of the untreated and differently modified materials. Surprisingly, T. versicolor was able to degrade untreated beech wood at a minimum of 15% MC, and growth was possible at 13% MC. By contrast, untreated pine sW was not decayed by C. puteana at less than 29% MC.

The development of fire retardants (FRs) is an approximative process of optimization. In this context, a novel water-soluble formulation with poly(sodium silicate-aluminum dihydrogen phosphate) (PSADP) has been developed, aiming at reduced hygroscopicity and enhanced leaching resistance of poplar wood in combination with nitrogen-phosphorus (NP) FR (FR NP ). After treatment of wood with FR NP and PSADP in vacuum, the following data of the samples were determined: rate of hydroscopicity, leaching resistance, heat release rate (HRR), total HR (THR), effective combustion heat, mass loss, and concentration of flue gas. FR distribution in the wood’s inner surface was investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA). Results show that PSADP and FR NP have favorable synergistic effects on moisture resistance and flame retardance. Smoke density (SD) of NP and PSADP treated samples shows a significant reduction relative to that of NP. NP-PSADP treated samples form more char with carbon layers of higher density. At the same time, FR NP-PSADP is evenly distributed over the inner wood surfaces and penetrates the cell cavities of the poplar wood.

The electrical properties of thermally modified wood (TMW) of southern pine ( Pinus spp.) have been investigated, while the samples were modified at 150°C, 175°C, 200°C, and 225°C for 4 h. After conditioning the samples at five relative humidities, the constants for electrical resistance ( k R ) and dielectricity ( k P ) were measured and calculated according to the model “electrical properties vs. moisture content”. The correlations between k R and k P and mass loss (ML), CIEL * a * b * color and bending properties in terms of modulus of rupture and modulus of elasticity were established. The coefficients of determination ( R 2 ) between these parameters were found to be between 0.926 and 0.999. TM influenced both k R and k P via changing the concentration and mobility of ions inside the wood and decreasing amount of polar groups of wood polymers. The conclusion is that the severity of TM such as the mechanical properties of TMW is in a close relation to the electrical properties.

A transparent and protective multilayer coating composed of poly(allylamine hydrochloride) (PAH), poly(styrene sulfonic acid) sodium salt (PSS), and nano-TiO 2 films was fabricated on wood surfaces by layer-by-layer assembly method. The coating was formed through pre-immobilization of cationic PAH layer on wood substrate, followed by alternate soaking cycles in pH-controlled anionic PSS solution and TiO 2 colloidal solution. The structure and properties of the assembled coating were characterized through scanning electron microscopy (SEM), energy disperse X-ray analysis (EDXA), UV reflection and absorption, colorimetry, and contact angle (CA) measurement. Results revealed that the coverage uniformity and thickness were improved with increasing number of PSS/TiO 2 bilayers. The coating masked the cell wall ultrastructure while leaving the microscale features intact. The anatase TiO 2 in the assembled coating enhanced the UV stability of wood and resulted in a lowered photochromism. Furthermore, the photocatalytic capability of the nano-TiO 2 films in degrading dyes of rhodamine B and methylene blue was verified. The nano-TiO 2 film on the top surface of the coating embodied a superhydrophilicity, showing self-cleaning and anti-fog capabilities. Stearic acid modification altered the superhydrophilicity to hydrophobicity with CA of 140°.

The fracture behaviour of wood is affected by, amongst other factors, its microscopic structure, moisture content and temperature, as well as the direction of crack propagation. Spruce ( Picea abies [L.] Karst.) and birch ( Betula pendula Roth.) compact tension specimens were subjected to pure mode I loading in both the radial-tangential (RT) and tangential-radial (TR) orientations under four climatic conditions. The effect of microscopic structure on crack propagation was observed, and the “crack mouth opening displacement vs. load” histories were recorded, from which characteristic fracture parameters were calculated. The fracture surfaces were subsequently examined by optical microscopy. In both species, distinct changes in the fracture behaviour were observed as moisture content (MC) and temperature (T) were varied. The results were specific for the wood species, but MC and T influenced fracture behaviour in a similar way. The effects were more pronounced in birch than in spruce. Elevated T and MC did not affect the failure mode, except in the case of spruce in the RT orientation.

A three-dimensional reconstruction method has been developed for voids in oriented strandboards made of mixed hardwood flakes. The method relies on the combination of X-ray computer tomography, image processing, and computer-aided design. The fractal dimension of void volume (FDVV) in the manufactured samples was calculated based on Menger sponge fractal model. It was shown that the FDVV decrement is linear to the increased void volume ratio. At the same overall porosity level, FDVV varied as a function of internal void distribution. FDVV can be a useful parameter for characterizing the internal void structure of strand-based wood composites.