Use of wood elemental composition to predict heat treatment intensity and decay resistance of different softwood and hardwood species

Heat treatment is an attractive alternative to improve decay resistance of low natural durability wood species. Decay resistance is strongly correlated to thermal degradations of wood cell wall components. Some recent studies proposed the use of wood elemental composition as a valuable marker to predict final properties of the material. These results, initially obtained with pine, have been extended to different softwood and hardwood species to check validity of the method using equipment specially designed to measure mass losses during thermal treatment. Heat treatment was performed on two softwood species (pine and silver fir) and three hardwood species (poplar, beech and ash) at 230 °C under nitrogen for different times to reach mass losses of 5, 10 and 15%. Heat-treated specimens were exposed to fungal decay using the brown rot fungus Poria placenta and the weight losses due to fungal degradation determined as well as initial wood elemental composition. Correlations between weight losses recorded after fungal exposure and elemental composition indicated that carbon content and O/C ratio can be used to predict wood durability conferred by heat treatment. Moreover, it was observed that for given curing conditions thermo-degradation patterns differed considerably according to the wood species. The sole analysis of wood physical properties like its density, thermal conductivity and diffusivity cannot allow explaining the observed differences, which should also depend on thermally activated chemical processes depending on wood chemical composition.     

Analytical pyrolysis study of biodelignification of cloned Eucalyptus globulus (EG) clone and Pinus pinaster Aiton kraft pulp and residual lignins

This study centred on the analysis of lignin in situ of cloned eucalypt and pine kraft pulps. Trametes versicolor laccase-violuric acid system (LMS) delignifications were performed on a softwood (Pinus pinaster) and a hardwood (Eucalyptus globulus) conventional kraft pulp with an initial kappa number of 34.5 and 15.5, respectively. The LMS treated pulps were then subjected to alkaline extraction stages (E). The kappa number data show that LMS is effective at biodelignifying both softwood and hardwood kraft pulps. However, under the conditions employed in this study, a greater level of biodelignification was obtained with LMS E. globulus (hardwood) than with LMS P. pinaster (softwood), but the amount of lignin removed was higher for the softwood pulp. The original milled wood samples, kraft pulps, biodelignified kraft pulps, and isolated residual lignin and milled wood lignins from the two wood samples have been characterized by pyrolysis-gas chromatography/mass spectrometry. The analysis of the pyrograms indicates that the lignin compositions of the two wood species and corresponding pulps are very different, as expected; however, the knowledge of the chemical mechanisms of delignification is very limited and requires additional work. Analytical pyrolysis is one the few degradative methods for the analysis of biopolymers that has shown a sufficient degree of success.     

Thermogravimetric analysis of wood, holocellulose, and lignin from five wood species

Thermogravimetry has been widely applied to the study of wood and cellulose materials. There is a general agreement that decomposition of hemicellulose, cellulose, and ligning take place in a relatively narrow range of temperature, partially overlapping. There is no a definitive demonstration of which thermal feature corresponds to each component. In this study, three hardwood and two softwood species were considered: Castannea sativa, Eucaliptus globulus, Quercus robur, Pinus pinaster, and Pinus sylvestris. Thermogravimetric analysis of wood powder, ethanol-extracted wood, holocellulose, and lignin, obtained from those species revealed some important differences between hardwood and softwood holocelluloses and an important role of the ethanol-extractives, which explain the different behavior observed in both kinds of wood. FTIR spectra obtained from the evolved gases helped to clarify some degradation steps.     

Enhanced Lignin Biodegradation by a Laccase-Overexpressed White-Rot Fungus Polyporus brumalis in the Pretreatment of Wood Chips

The laccase gene of Polyporus brumalis was genetically transformed to overexpress its laccase. The transformants exhibited increased laccase activity and effective decolorization of the dye Remazol Brilliant Blue R than the wild type. When the transformants were pretreated with wood chips from a red pine (softwood) and a tulip tree (hardwood) for 15 and 45 days, they showed higher lignin-degradation activity as well as higher wood-chip weight loss than the wild type. When the wood chips treated with the transformant were enzymatically saccharified, the highest sugar yields were found to be 32.5 % for the red pine wood and 29.5 % for the tulip tree wood, on the basis of the dried wood weights, which were 1.6-folds higher than those for the wild type. These results suggested that overexpression of the laccase gene from P. brumalis significantly contributed to the pretreatment of lignocellulose for increasing sugar yields.     

Adsorption of a Trichoderma reesei endoglucanase and cellobiohydrolase onto bleached Kraft fibres

Cellulases can be used to modify pulp fibres. For the development of biotechnical applications, a better understanding of the adsorption of cellulases onto commercial wood fibres is needed. In this work, the adsorption behaviour of purified CBH I and EG II on bleached Kraft fibres was investigated. Three variables were studied with respect to their effect on adsorption: fibre type (hardwood or softwood), fibre history (never-dried or once-dried), and ionic strength. The results showed that fibre history had the largest influence on the extent of adsorption of each enzyme. The effect of ionic strength was shown to be dependent on the enzyme and fibre type. At high ionic strength, CBH I exhibited a higher affinity for both once-dried and never-dried fibres at low enzyme concentrations; however, salt was shown to decrease the extent of adsorption at higher enzyme dosages. In contrast, salt increased the maximum adsorption of EG II, most notably on the once-dried hardwood fibres. Fibre type was also shown to affect adsorption behaviour. CBH I had a higher affinity for softwood fibres than for hardwood fibres at low enzyme concentrations. The maximum adsorption of EG II onto once-dried softwood fibres increased by 80% compared to the once-dried hardwood fibres. Interestingly, this did not correlate to in creased fibre hydrolysis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Microsphere valorization of forestry derived hydrolysates

A value-adding approach to the material utilization of non-cellulosic polysaccharides (NCPs) released from the lignocellulosic feedstock was realized via the formulation of renewable microspheres from wood hydrolysates using a purposely elaborated all edible water-in-oil emulsion technique. Four compositionally different hemicellulose rich wood hydrolysates were recovered from process waters in pulping and other hydrothermal treatments of hardwood and softwood. Multivariate screening designs were employed allowing for the identification and quantitation of significant process parameters and interaction effects governing the conversion of hydrolysates into small, smooth and well-defined microspheres with narrow size dispersity.     

Utilization of thermodesorption coupled to GC-MS to study stability of different wood species to thermodegradation

Thermodesorption coupled to gas chromatography coupled to mass spectroscopy (TD-GC-MS) has been investigated to identify volatile degradation products generated during wood heat treatment by mild pyrolysis. For this purpose, wood samples of different softwood and hardwood species have been heat treated under nitrogen for different temperatures comprised between 180 and 230 °C during 15 min in the glass thermal desorption tube of the thermodesorber and the volatile wood degradation products trapped. The trapped products were then thermodesorbed and analysed by GC-MS. Chromatograms of the different samples indicated the formation of different products resulting from degradation of lignin and hemicelluloses. Hardwoods were shown to be more sensitive to thermodegradation than softwoods, for which degradation products appear at slightly higher temperature. The important formation of acetic acid is concomitant with the formation of most of degradation products and at the origin of the difference of reactivity observed between softwoods and hardwoods.     

Thermal characterization of rubberwood-polymer composites

The thermal properties of five types of radiation-induced wood-polymer composites based on a tropical hardwood, rubberwood (Hevea braziliensis), was studied by oxygen index measurement, differential thermal analysis (DTA) and thermogravimetry (TG). The DTA and TG curves of composites were different from those of rubberwood, which can be attributed to the presence of the incorporated polymers. Of the five composites, the one impregnated with bis(2-chloroethyl)vinyl phosphonate reduced the initial temperature of decomposition, increased the peak temperatures of exothermic reactions, and increased the char yield. Comparison with physical blends of rubberwood and the corresponding polymer provided some evidence of chemical interaction of wood and polymer in some of the composites.     

Bound and free water distribution in wood during water uptake and drying as measured by 1D magnetic resonance imaging

Knowledge on moisture transport in wood is important for understanding its utilization, durability and product quality. Moisture transport processes in wood can be studied by Nuclear Magnetic Resonance (NMR) imaging. By combining NMR imaging with relaxometry, the state of water within wood can be identified, i.e. water bound to the cell wall, and free water in the cell lumen/vessel. This paper presents how the transport of water can be monitored and quantified in terms of bound and free water during water uptake and drying. Three types of wood from softwood to hardwood were selected covering a range of low to high density wood; pine sapwood and oak and teak. A calibration is performed to determine the different water states in each different wood type and to convert the NMR signal into moisture content. For all wood types, water transport appeared to be internally limited during both uptake and drying. In case of water uptake, free water was observed only after the cell walls were saturated with bound water. In case of drying, the loss of bound water starts only after vanishing of free water, irrespective of the position. Obviously, there is always a local thermodynamic equilibrium of bound and free water for both uptake and drying. Finally, we determined the effective diffusion coefficient (D _eff ). Experimentally determined diffusion constants were compared with those derived by the diffusion models for conceptual understanding of transport mechanism. We found that diffusion in the cell wall fibers plays a critical role in the transport process.     

Emission of gases and degradation volatiles from polymeric wood constituents in friction welding of wood dowels

Analysis of volatile compounds and gases emitted as smoke at the welding interface during rotational wood dowel welding of a hardwood (beech) and of a softwood (Norway spruce) has shown that the compounds in such a smoke are water vapour, CO₂, degradation compounds from wood polymeric carbohydrates and from amorphous lignin, as well as some volatile terpenes, these latter only for the softwood used, Norway spruce. The main carbohydrates contributing to the volatile compounds are xylans for beech and glucomannans for spruce. Numerous compounds, in very small proportions derived from the degradation and rearrangement reactions of lignin, have also been identified. The proportion of CO₂ emitted is very low, and neither CO nor methane is emitted due to the relatively low temperature of dowel welding. Experiments at temperature slightly higher than that of dowel welding but prolonged in time have shown that the main component of the smoke produced during welding appears to be water vapour.     

Comparative study of photodegradation of wood by a UV laser and a xenon light source

A detailed study of photodegradation of wood surfaces by xenon light source and a UV laser has been carried out. Silver birch, rubberwood, Scots pine and chir pine wood veneers were irradiated with a xenon light source or a 244 nm argon ion laser. The changes in chemical structure of wood surfaces were monitored by UV resonance Raman (UVRR), photoacoustic Fourier transform infrared (FTIR-PAS) and UV-vis reflectance spectroscopies. The depth profile of xenon lamp irradiated wood surfaces was carried out by measuring FTIR-PAS spectra at different moving mirror velocities. The UVRR and FTIR-PAS spectra of irradiated wood surfaces showed degradation of aromatic structure in lignin combined with strong formation of carbonyl structures. The FTIR-PAS spectra measured from xenon irradiated wood surfaces indicate that hardwood lignin degrades at a faster rate than softwood lignin. The UVRR spectra of xenon irradiated wood show a significant decrease in the intensities of aromatic structures at 1602 cm⁻¹. This is accompanied by a significant band broadening and notable shift towards longer wavenumbers, which has been attributed to the formation of o- and p-quinone structures as degradation products. The formation of quinone structures was also supported by the generation of a broad absorption band between 350 and 600 nm in UV-vis reflectance spectra of irradiated wood surfaces. There was a significant broadening in the region of 1500-1000 cm⁻¹ in UVRR spectra due to the formation of unsaturated structures as a result of lignin degradation. The UVRR spectra of laser irradiated wood showed similar behaviour i.e., overall broadening and a rapid reduction in the intensity of lignin aromatic structure. The rate of degradation by laser was very high. However, the extent of band broadening was higher in xenon irradiated wood indicating the generation of several different types of structures as compared to laser irradiation, which produces only particular type of structures. UVRR spectra of laser irradiated Whatman paper showed significant photodegradation of cellulose by UV laser. The UV degradation rate of lignin was much higher than cellulose.     

Morphological changes in never-dried kraft fibers under mechanical shearing

The modification of bleached never-dried cellulose fibers was studied under controlled compression and shearing conditions. Fibers were further treated in a high-intensity mixing device in low-consistency to determine if the fiber structure was weakened in the in-pad attrition. The difference between the development of the softwood and hardwood fibers was examined. The fiber properties were analyzed using a fiber morphology analyzer, fractional fiber analysis and an electron microscope. The results indicate that the shearing under the controlled compression at high consistency modified the softwood and hardwood fibers already at low-energy consumptions. The fiber length and width decreased, and the formation of curls and kinks was pronounced. However, the intensive mixing after in-pad attrition revealed that the fiber structure was not weakened under compression and shear forces; conversely, the fiber cell wall was more resistant for the intensive mixing. When comparing the results for hardwood and softwood fibers, the softwood fibers were more modified during in-pad attrition, whereas the fiber wall strengthening was more significant in the hardwood fibers.     

Fourier-transform Raman spectroscopic study of a Neolithic waterlogged wood assemblage

The use of Fourier-transform Raman spectroscopy for characterising lignocellulosics has increased significantly over the last twenty years. Here, an FT-Raman spectroscopic study of changes in the chemistry of waterlogged archaeological wood of Pinus sp. and Quercus sp. from a prehistoric assemblage recovered from northern Greece is presented. FT-Raman spectral features of biodeteriorated wood were associated with the depletion of lignin and/or carbohydrate polymers at various stages of deterioration. Spectra from the archaeological wood are presented alongside spectra of sound wood of the same taxa. A comparison of the relative changes in intensities of spectral bands associated with lignin and carbohydrates resulting from decay clearly indicated extensive deterioration of both the softwood and hardwood samples and the carbohydrates appear to be more deteriorated than the lignin. The biodeterioration of the archaeological timbers followed a pattern of initial preferential loss of carbohydrates causing significant loss of cellulose and hemicellulose, followed by the degradation of lignin.     

Physical and chemical characteristics of aging pyrolysis oils produced from hardwood and softwood feedstocks

Pyrolysis oils were produced from hardwood or softwood feedstocks in a vacuum batch reactor and trapped at 0 °C. The vacuum process was used to intentionally avoid the presence of entrained char particles. The hardwood feedstock was a pelletized mixture of various Eastern tree species. The softwood samples were de-barked Lodgepole pine (Pinus contorta) and Douglas Fir (Pseudotsuga menziesii) wood cut into the same dimensions as the pellets. The oils’ physical (viscosity) and chemical (speciation) properties were measured as-produced and after aging. The total liquid and char yields ranged from ∼50 to 55% and 25 to 27% respectively. Measured water contents were 30% or more, which are greater typically reported from fast pyrolysis oils produced in fluidized beds. Aging took place in covered glass containers at room temperature over a period of 5 months. Gas chromatography-mass spectrometry (GCMS) was used to characterize the oils’ volatile components. Since bio-oils are mixtures of hundreds of different compounds with wide-ranging molecular weights and polarities, the oils were extracted using benzene followed by methanol. Out of ca. 80 non-polar and 100+ polar compounds GCMS showed a few chemical species present in the freshly produced oils were absent in the aged oils. The oils’ viscosities at shear rates (measured between 1 and 1000 s⁻¹) increased by approximately a factor of 2.5 during aging. To determine if this was due to polymerization reactions during aging or simply water and other volatile material losses, freshly made oils were aged at an accelerated rate by using elevated temperatures (65 °C and 85 °C) in a water-saturated environment between 1 and 7 days. The oils are fairly stable with respect to aging both over long periods of time (months) at room temperature and at elevated temperatures, 65 °C and 85 °C for shorter time periods (days). It is concluded that high water content and char-free characteristics act to slow polymerization reactions.     

Evaluation of cellulase preparations for hydrolysis of hardwood substrates

Seven cellulase preparations from Penicillium and Trichoderma spp. were evaluated for their ability to hydrolyze the cellulose fraction of hardwoods (yellow poplar and red maple) pretreated by organosolv extraction, as well as model cellulosic substrates such as filter paper. There was no significant correlation among hydrolytic performance on pretreated hardwood, based on glucose release, and filter paper activity. However, performance on pretreated hardwood showed significant correlations to the levels of endogenous β-glucosidase and xylanase activities in the cellulase preparation. Accordingly, differences in performance were reduced or eliminated following supplementation with a crude β-glucosidase preparation containing both activities. These results complement a previous investigation using softwoods pretreated by either organosolv extraction or steam explosion. Cellulase preparations that performed best on hardwood also showed superior performance on the softwood substrates.     

Quantitative characterization of chemical degradation of heat‐treated wood surfaces during artificial weathering using XPS

The X‐ray photoelectron spectroscopy (XPS) study of three heat‐treated North American wood species (jack pine, birch and aspen) was carried out to evaluate chemical modifications occurring on the wood surface during artificial weathering for different times. The results suggest that the weathering reduces lignin content (aromatic rings) at the surface of heat‐treated wood, consequently, the carbohydrates content increases. This results in surfaces richer in cellulose and poorer in lignin. Heat‐treated wood surfaces become acidic due to weathering, and the acidity increases as the weathering time increases. Three possible reasons are given to account for the increase of acidity during weathering. The lignin content increases, whereas the hemicelluloses content decrease due to heat treatment. Heat‐treated woods have lower acidity to basicity ratios than the corresponding untreated woods for all three species because of the decrease in carboxylic acid functions mainly present in hemicelluloses. The wood composition changes induced by weathering are more significant compared to those induced by heat treatment at wood surface. Exposure to higher temperatures causes more degradation of hemicelluloses, and this characteristic is maintained during weathering. However, the wood direction has more effect on chemical composition modification during weathering compared to that of heat treatment temperature. The heat‐treated jack pine is affected most by weathering followed by heat‐treated aspen and birch. This is related to differences in content and structure of lignin of softwood and hardwood. The use of XPS technique has proved to be a reliable method for wood surface studies. Copyright © 2012 John Wiley & Sons, Ltd.     

How does soaking wood in various pH buffers impact ToF‐SIMS spectra?

During the production of biofuels and/or bioproducts, wood and other lignocellulosic materials are frequently exposed to buffers during enzyme treatments. Buffer pH varies according to the activity profiles of the enzyme(s) used. Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is an increasingly valuable analytical tool for the surface analysis of lignocellulosic solids, allowing for characterization of the lignin and polysaccharides at the surface, along with other components such as protein and inorganic salts. Despite the use of ToF‐SIMS to characterize dilute acid and alkali pretreatments of wood, the exposure of wood to buffers of intermediate pH range has not been studied as it relates to ToF‐SIMS analysis. This leads to the question: “How does soaking wood in various pH buffers impact ToF‐SIMS spectra?” Accordingly, a softwood (spruce) and hardwood (birch) were soaked in universal buffers ranging from pH 5 to 10, and then positive ion ToF‐SIMS spectra were acquired from the washed wood. Deacetylation was evident for both wood species above pH 8. Additionally, at higher pH, birch ToF‐SIMS spectra revealed a relative loss in polysaccharide peaks attributed to hemicellulose and an increase in lignin peaks. This study provides a basis for understanding the pH‐dependent alteration of wood solids in aqueous solution, which is important for understanding the controls in enzyme treatments.     

Structural analysis of photodegraded wood by means of FTIR spectroscopy

We report on the detailed analysis of chemical modifications and structural changes in the cellulose and lignin of Populus tremula (a hardwood) and Buxus sempervirens (a softwood), as a result of photodegradation in a Xenon test chamber. The results obtained by means of FTIR spectroscopy indicate that lignin is the most sensitive component to the degradation process for both woods examined. On a structural level, the virtual elimination of the amorphous cellulose was observed for both types of wood. The crystallised cellulose I component, which accounts for the whole crystalline phase, undergoes minor structural changes, this effect being more important in the case of Populus tremula that was less degraded than Buxus Sempervirens.     

Gas- and solid/liquid-phase reactions during pyrolysis of softwood and hardwood lignins

Pyrolytic reactions of Japanese cedar (Cryptomeria japonica, a softwood) and Japanese beech (Fagus crenata, a hardwood) milled wood lignins (MWLs) were studied with thermogravimetry (TG) and by pyrolysis in a closed ampoule reactor (N₂/600 °C). The data were compared with those of guaiacol/syringol as simple lignin model aromatic nuclei. Several DTG peaks were observed around 300-350, 450, 590 and 650 °C. The first DTG peak temperature (326 °C) of beech was lower than that (353 °C) of cedar. This indicates that the volatile formation from cedar MWL is slightly delayed in heating at 600 °C. The gas-phase reactions via GC/MS-detectable low MW products were explainable with the temperature-dependent reactions observed for guaiacol/syringol in our previous paper. The methoxyl groups became reactive at ∼450 °C, giving O-CH₃ homolysis products (catechols/pyrogallols) and OCH₃ rearrangement products (cresols/xylenols). The former homolysis products were effectively converted into gaseous products (mainly CO) at >550-600 °C. However, the GC/MS-detectable tar yields, especially syringyl unit-characteristic products, were much lower than those from guaiacol/syringol. Thus, contributions of higher MW intermediates and solid/liquid-phase reactions are more important in lignin pyrolysis. From the results of stepwise pyrolysis of char + coke fractions at 450 and 600 °C, the methoxyl group-related reactions (450 °C) and intermediates gasification (600 °C) were suggested to occur also in the solid/liquid phase. This was consistent with the DTG peaks observed around these temperatures. These solid/liquid-phase reactions reduced the tar formation, especially catechols/pyrogallols and PAHs. Different features observed between these two MWLs are also focused.