Designing functional wood materials for novel engineering applications

Wood has great potential to become a key material for future bio-economy, thanks especially to its intrinsic renewability and CO2-storing capacity. Improved functionalization treatments can make wood materials valid substitutes for less ecofriendly ones, expanding and widening their application range. However, further research is needed. This mini-review highlights some of the most recent developments in the design of functional wood materials, critically discussing their current limitations and obstacles to their implementation.

A brief overview on the development of wood research

Wood science covers in particular the areas of the formation and composition as well as the chemical, biological and physical-mechanical properties of wood. First comprehensive studies have already been published in the last century. Detailed knowledge of wood is required for the processing of wood, the production of wood-based materials, and the utilization of wood and wood-based materials as buildings and various other products such as furniture. This review gives a brief overview on the progress in wood chemistry, wood biology (including photosynthesis and biodeterioration), and physical-mechanical properties of wood and wood-based materials. These fundamentals are also essential for understanding technological processes and product development.

Improving the stability of beech wood with polyester treatment based on malic acid

The improvement of durability and dimensional stability of wood properties via modification of the microstructure and wood–water interaction has been widely utilised. This study investigated polyester treatments, a possible alternative, using environmentally friendly chemicals such as malic acid to improve the beech wood (Fagus sylvatica) properties. The modified properties have been studied with four treatments using malic acid, glycerol, butanediol and succinic anhydride, mixing polycarboxylic acids and polyols. Results showed that the anti-swelling-efficiency (ASE) improved up to 70%, and the bulking coefficient improved around 23%, exhibiting an efficient penetration within the cell walls. The leaching rates (LR) of treatments and the extractables remained low, between 0.05 and 2.4%. The equilibrium moisture content (EMC) decreased by 50% for the four treatments, compared to untreated beech wood.

Determination of chemical shifts in 6-condensed syringylic lignin model compounds

A small library of 6-substituted syringyl model compounds with aliphatic, carboxylic, phenylic, benzylic alcohols and brominated substituents were prepared. The influence of the substituents on the chemical shifts of the compounds was analyzed. All of model compounds showed a characteristic increase in the 13C NMR chemical shift of the methoxy group vicinal to the substitution. This 13C NMR peak and its corresponding correlation peak in HSQC could potentially be used to identify 6-condensation in syringylic lignin samples.

Valorization of waste bark for biorefineries: chemical characterization of Eucalyptus camaldulensis inner and outer barks

Bark wastes today are viewed as a high-value resource for biorefinery due to their chemical richness and diversity. This work presents a comprehensive chemical characterization of the inner bark and the outer bark of Eucalyptus camaldulensis cultivated in Algeria. The extractives were first isolated with an Accelerated Solvent Extractor (ASE) and then analyzed by Gas Chromatography-Mass Spectrometry (GC-MS). The content of pre-extracted bark in cellulosic polysaccharide and free sugar monomers was determined by Gas Chromatography (GC). The hemicellulose composition and amount was determined after the acid methanolysis and GC. The amount of lignin was determined gravimetrically by the Klason lignin method and the acid soluble lignin was determined by the UV method. Formic and acetic acids in the bark were determined by HPLC after alkaline hydrolysis. It was found that the extractives content were similar in the outer bark (0.85%) and the inner bark (0.88%). The cellulose content was higher in the outer bark (33.4%) than in the inner bark (28.7%). Lignin and the total hemicellulose contents were more abundant in the outer bark (31.7, 26.2%) than in the inner bark (28.6, 19.3%) whereas, sugar monomers were more abundant in the inner bark (4.4%) than in the outer bark (3.8%). The variation in acetic and formic acids and ash contents between the outer bark (1.5, 0.006 and 2.5%) and the inner bark (1.3, 0.005 and 2.4%) was small. The obtained results showed that the bark can be considered a suitable feedstock for lignocellulosic biorefinery and also for the extraction of bioactive compounds that can be used in different sectors.

Effect of solvent type on the formation rate of benzyl cation intermediate in acidolysis of lignin

The purpose of this study was to examine how the type of solvent among aqueous 1,4-dioxane, tetrahydrofuran, ethanol, iso-propyl alcohol, or ethylene glycol and its content (mol%) affect the formation rate of benzyl cation intermediate (BC) in the acidolysis of lignin, using a simple model compound, 1,2-dimethoxy-4-methoxymethylbenzene. Because the BC forms from the model compound via two steps, i.e., protonation of the benzyl methoxymethyl group as the pre-equilibrium step and liberation of the methanol as the rate-determining step, the observed variation of the formation rate with type of solvent and solvent content originates from the effects on both steps undistinguishably. The formation rate of BC decreased with increasing mol% of any of the organic solvents for a range of relatively low mol%, but increased with it for relatively high mol%. The formation rate varied more in the ether than in the alcohol systems. These results seem to be regulated by the effect of changing the mol% on the pre-equilibrium step, i.e., on the proton activity, rather than on the rate-determining step. Two reaction products, 4-alkoxymethyl-1,2-dimethoxybenzene and 4-hydroxymethyl-1,2-dimethoxybenzene, exclusively formed in the aqueous alcohol systems. The former compound was confirmed to be thermodynamically more stable and kinetically the more favorable product.

“Green technology” processing of pine (Pinus sylvestris L.) and larch (Larix sibirica Ledeb.) wood greenery to produce bioactive extracts

In order to explore the extractives of conifers an effective and environmentally friendly method of extraction with aqueous-alkaline solution allowing to isolate up to 10.4% of extractive substances (ES) from pine wood greenery and up to 6.9% from larch wood greenery was investigated. The component fractional composition of aqueous-alkaline extracts was studied. The antioxidant fraction activity of neutral and acidic components isolated from the produced extracts was evaluated. It was found that these fractions have a high antioxidant activity, where the activity of larch extract components was higher than that of the respective components of pine extract. Due to their rich chemical composition and high biological activity, extracts of pine and larch wood greenery produced by aqueous-alkaline extraction have good application prospects as biologically active preparations.

CNT@PDMS/NW composite materials with superior electromagnetic shielding

Lightweight materials with high electrical conductivity and hydrophobic mechanical properties are ideal materials for electromagnetic interference (EMI) shielding. Herein, the conductive composites with great EMI shielding effectiveness (SE) were successfully obtained by introducing multi-walled carbon nanotube (CNT) and polydimethylsiloxane (PDMS) based on the original structure of natural wood (NW). CNT@PDMS/NW composites were prepared via vacuum-pulse impregnation method and characterized by Fourier transform infrared (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) patterns, hydrophobicity analysis, and EMI shielding performance. As demonstrated, CNT nanosheets were successfully inserted into wood matrices, and hydrogen bonding between CNT nanosheets and cellulose nanofibers induced the fabrication of CNT@PDMS/NW composites. CNT@PDMS/NW composites exhibited excellent EMI SE values of 25.2 dB at the X-band frequency.

Evaluating efficacy of different UV-stabilizers/absorbers in reducing UV-degradation of lignin

Susceptibility of wood to UV degradation decreases the service life of wood products outdoors. Organic UV absorbers (UVAs) and hindered amine light stabilizers (HALSs), as well as inorganic UVAs, are added to coatings to improve the UV stability of coated-wood products. Although about 85% of UV radiation is absorbed by lignin in the wood, it is unclear which UV stabilizers can minimize lignin degradation. In this study, the photodegradation of softwood organosolv lignin was monitored over 35 days of UV exposure. Changes in lignin properties were assessed using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR). It was found that the aromatic rings of lignin underwent significant degradation, resulting in increased glass transition temperature and molecular weight of lignin. Subsequently, 18 different additives were mixed with lignin and exposed to UV irradiation. The analysis of samples before and after UV exposure with FTIR revealed that inorganic UVAs (cerium oxide and zinc oxide) and a mixture of organic UVAs and HALSs (T-479/T-292, T-5248, and T-5333) were the most effective additives in reducing lignin degradation. This study can help coating scientists to formulate more durable transparent exterior wood coatings.

Synthesis of hydroxycinnamoyl shikimates and their role in monolignol biosynthesis

Hydroxycinnamoyl shikimates were reported in 2005 to be intermediates in monolignol biosynthesis. 3-Hydroxylation of p-coumarate, originally thought to occur via coumarate 3-hydroxylase (C3H) from p-coumaric acid or its CoA thioester, was revealed to be via the action of coumaroyl shikimate 3′-hydroxylase (C3′H) utilizing p-coumaroyl shikimate as the substrate, itself derived from p-coumaroyl-CoA via hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyltransferase (HCT). The same HCT was conjectured to convert the product, caffeoyl shikimate, to caffeoyl-CoA to continue on the pathway starting with its 3-O-methylation. At least in some plants, however, a more recently discovered caffeoyl shikimate esterase (CSE) enzyme hydrolyzes caffeoyl shikimate to caffeic acid from which it must again produce its CoA thioester to continue on the monolignol biosynthetic pathway. HCT and CSE are therefore monolignol biosynthetic pathway enzymes that have provided new opportunities to misregulate lignification. To facilitate studies into the action and substrate specificity of C3H/C3′H, HCT, and CSE enzymes, as well as for metabolite authentication and for enzyme characterization, including kinetics, a source of authentic substrates and products was required. A synthetic scheme starting from commercially available shikimic acid and the four key hydroxycinnamic acids (p-coumaric, caffeic, ferulic, and sinapic acid) has been developed to provide this set of hydroxycinnamoyl shikimates for researchers.