Volume 62, Issue 2

Cellulosic aerogels are intriguing new materials produced by supercritical drying of regenerated cellulose obtained by solvent exchange of solid Lyocell moldings. From N -methylmorpholine- N -oxide (NMMO) solutions with cellulose contents between 1 and 12%, dimensionally stable cellulose bodies are produced, in which the solution structure of the cellulose is largely preserved and transferred into the solid state, the material having densities down to 0.05 g cm -3 and surface areas of up to 280 m 2  g -1 . In this study, several aspects of cellulosic aerogel production are communicated: the stabilization of the cellulose solutions against degradation reactions by agents suitable for later extraction and drying, a reliable extraction and drying procedure by supercritical carbon dioxide, the advantages of DMSO/NMMO in this procedure as a solvent/non-solvent pair, and some data on the physical properties of the materials.

In the present work, the formation of ester linkages in cellulosic materials during drying and heating was addressed. The results indicated that direct ester formation, without anhydride intermediate, occurs between carboxylic acids of suitable structure and cellulosic hydroxyls at 180°C, even after reaction times as short as 5 min. The 2(R)-hydroxybutyric acid is most effective in ester formation, followed by 2-ketobutyric acid and 3-hydroxybutyric acid. Hexanoic acid is less effective and veratric acid produces no esters at all. At lower temperatures, 2-ketobutyric acid is the most reactive compound. One conclusion is that a hydroxyl or keto group in α position of the carboxylic acids favours ester formation. Another finding is that three pyrrolidine structures are produced after esterification and aminolysis of 2-ketobutyric acid. One of the structures indicates that 2-ketobutyric acid is reactive in two positions and could therefore be a potential cross-linker. A critical parameter for ester formation is pH, and the results indicate that pH <2 strongly favours esterification, whereas almost no esters are produced at pH >5–6. All experiments were performed in a solid-state model system (on cellulosic filter paper) with low moisture content. Esterification of five carboxylic acids was studied. Samples were heated constantly at 180°C for 5 min or gradually from 22°C to 180°C over periods ranging from 5 to 45 min. Quantitative analysis of ester formation between the monocarboxylic acids and the hydroxyl groups of cellulose was performed by means of aminolysis and gas chromatography.

Alkaline sulfite/anthraquinone (AS/AQ) pulping and totally chlorine free (TCF) bleaching of bagasse was investigated. Depithed and wet cleaned bagasse was collected from the Pars Paper Company in southern Iran, dried and then pulped. The active alkali charge was chosen between 16 and 20% on oven-dry bagasse and the alkali ratio, Na 2 SO 3 /NaOH, was varied from 30/70 to 70/30. The AQ dose was kept constant. While keeping a constant cooking time at maximum temperature for 60 min, the maximum temperature was varied between 155 and 165°C. For bleaching, a pulp with a kappa number of 8.5 was produced under the following conditions: 18% active alkali, alkali ratio 30/70, 60 min cooking at 155°C. TCF bleaching was performed with oxygen and hydrogen peroxide. Oxygen delignification (O) with an alkali charge of 2% NaOH resulted in 50% degree of delignification. A brightness of 80% ISO was achieved with a chelating treatment (Q) and a subsequent oxygen reinforced peroxide bleaching stage (OP) under drastic conditions (4% H 2 O 2 , 2.75% NaOH, 90 min at 98°C). With an additional peroxide bleaching stage charging 2% H 2 O 2 , the brightness could be further improved to 83.5% ISO. Bleaching led to only small losses in tensile and burst strength at unchanged tear strength. Short beating in a Jokro mill was sufficient to attain good pulp strength. The results indicate that AS/AQ pulping and TCF bleaching can be successfully applied to bagasse to produce pulp for writing and printing paper.

The aim of the present study was to determine the effect of a variety of abnormal fibres on the mechanical properties of paper made from Norway spruce, Picea abies (L.) Karst. Fibres representing abnormality were obtained from trees treated by irrigation and fertilisation. Moreover, fibres from compression wood and its accompanying opposite wood were isolated. The effect of dislocations on paper quality was studied on four mixtures (20, 40, 60 and 80% fibres with induced dislocations) of untreated/compressed fibres. Two more groups consisting of control untreated samples and samples with 100%-induced dislocations were also included in the test. The mechanical properties of the paper were tested and the results were compared to those of control samples. Abnormal fibres reduced the desired mechanical properties of the final paper concerning tensile strength, modulus of elasticity and tear-tensile index. Irrespective of the type of treatment, all morphological changes introduced in fibre cell walls appear to directly affect changes in the mechanical properties of the paper. Control samples had a tear index of 25 compared to 10 mN m 2  g -1 of samples containing 100% dislocations. It is obvious that 20% of dislocations, an amount that is expected to be induced in pulp under mechanical processing and transport, will contribute to a decrease in tear index with an average of 3 mN m 2  g -1 , i.e., 10% of the total value.

Crystallized mesoporous carbon was produced from ligno- p -cresol, a lignin derivative obtained by phase separation with p -cresol and sulfuric acid, onto which 3% nickel and 2% calcium were co-loaded, by carbonization at 900°C. Subsequent pulverization followed by soaking in acid improved the crystallinity of carbon with an increased volume of mesopore, attaining approximately 90% and 99% removal of nickel and calcium, respectively. These aspects implied both enhancement of electroconductivity and liquid phase adsorption capacity of the carbon with a good outlook on the recovery and reuse of the two metals. In particular, combination of wet milling using small beads and mild soaking with diluted acid was appropriate in respect of relevant properties of the carbon, including the particle size. It can thus be concluded that incorporation of such post-treatments into the manufacturing process of carbon products is greatly advantageous.

The reaction route of a dimeric non-phenolic C 6 -C 2 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol ( VIII ), was kinetically examined under acidolysis conditions (0.2 mol l -1 HBr in 82% aqueous 1,4-dioxane at 85°C). The disappearance of ( VIII ) followed the pseudo-first-order rate law, and the rate constant k ( VIII ) was 0.00854. In the course of the reactions, the following compounds were produced quantitatively at any time: an enol ether, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethylene ( IX ), 2-methoxyphenol ( X ), and a Hibbert's ketone, 3,4-dimethoxyphenylacetaldehyde ( XI ). The substances ( X ) and ( XI ) are the result of the β-O-4 bond cleavage and their amounts were always equal during the whole reaction. When ( IX ) was subjected to the acidolysis under the identical conditions, its disappearance followed the pseudo-first-order rate law, and the rate constant k ( IX ) was 0.00825. Furthermore, ( VIII ) was not observed at all, and ( X ) was produced quantitatively at any time. Based on these results, the formation rate of ( IX ) during the acidolysis of ( VIII ) is expressed by the equation: d [( IX )]/ dt = A · k ( VIII ) [( VIII )]- k ( IX ) [( IX )], where A is the proportion of ( VIII ) that converted into ( IX ) when ( VIII ) degraded. It was confirmed by solving this differential equation that the formation and disappearance of ( IX ) is best simulated when A was assumed to be 1.00. Therefore, it was proven in this paper for the first time that ( VIII ) primarily converts into ( IX ), and subsequently the β-O-4 bond cleavage occurs and ( X ) and ( XI ) are yielded.

The etherification of phenolic groups has been found to inhibit photodegradation in wood and lignin rich pulps. The precise understanding of kinetics of chemical reaction between lignins or their model compounds and the etherifying agent is the first step for developing a viable modification procedure. In this study, we have investigated the reaction of lignin model compounds (namely, phenol and guaiacol) with propylene oxide in aqueous media. The kinetics of etherification reaction was studied under varying pH conditions in the temperature range 30–60°C. The etherified reaction products were characterized by gas chromatogram-mass spectrum (GC-MS). The extent of etherification of phenols and the rate of chemical reaction was followed by UV-Visible absorption spectroscopy. The reaction between lignin model compounds and propylene oxide was indicated by a rapid reduction in the absorbance accompanied by the development of a new band corresponding to etherified products. The reaction kinetics was investigated at pH ∼12 under the condition of excess concentration of propylene oxide. The reaction followed first order kinetics and rate constants increased linearly with an increase in the temperature and concentration of propylene oxide. The MS fragment data of reaction product support the proposed reaction scheme. The activation energy of the reaction of propylene oxide with phenol and guaiacol, calculated with the Arrhenius equation, was 56.2 kJ mol -1 and 45.4 kJ mol -1 , respectively.

In this work, near infrared (NIR) and pyrolysis-molecular beam mass spectra (MBMS) of loblolly pine ( Pinus taeda) were studied by means of generalized two-dimensional correlation spectroscopy in order to assign specific contributions of cellulose in the two spectral domains. Homo- and hetero-correlation techniques were employed to analyze the concentration-dependent spectral variations of cellulose. Specific bands of cellulose were assigned in the NIR and MBMS spectra, and moreover two masses m/z at 114 and 173, commonly assigned to cellulose fragments, were found to not originate from the pyrolysis of cellulose.

The effect of laser irradiation (Ar + ion laser, 244 nm) on photodegradation of lignin in silver birch and rubberwood as hardwoods and spruce and chir pine as softwoods has been investigated by UV resonance Raman spectroscopy (UVRRS). UVRR spectra showed degradation of aromatic structures accompanied by the formation of both ortho- and para -quinone structures as a result of photodegradation of wood surfaces. There was a rapid decrease in the intensities of all the lignin-associated bands accompanied by broadening of aromatic bands at 1602 cm -1 and in the region of 1500–1000 cm -1 due to formation of unsaturated structures arisen from lignin degradation. The rate of lignin degradation was observed by measuring the relative intensity of aromatic band at 1602 cm -1 during photodegradation. The rates of UV-degradation of hardwood and softwood surfaces were compared and it was found that hardwoods degrade at a faster rate than softwoods. The syringyl structures in hardwood lignin are more sensitive to photochemical degradation than guaiacyl structures. The rate of photodegradation increased with increasing intensity of the irradiated laser energy.

Yellow poplar ( Liriodendron tulipifera L.) wood flour was chemically modified with acetic anhydride at varying times and initial moisture contents at a constant temperature. All samples exhibited increasing weight gains in the range of 5–16% with increasing reaction or treatment time. The acetylated poplar wood flour was characterized by means of Fourier transform infrared spectroscopy and near infrared (NIR) spectroscopy. Principal component analysis performed on the spectral data revealed clusters according to the esterification level. Partial least squares regression models developed from NIR data were able to predict the weight gain and the percentage of reacted OH groups. The correlations show that there is a direct and linear relationship between the spectra and the weight percentage gain and, therefore, the degree of acetylation.

A generalized model for the prediction of mat pressure-density relationship of wood-based composites was developed. Based on the compression models for fiber assembly, this model treats the composite mat structure as a system of bending units, thus making element bending the dominant mechanism during early stage of mat consolidation. The consolidation behavior of fiber, strand, and particle mats were experimentally investigated. Satisfactory agreement was found between the model predictions and experimental results. Combined with the compression model, the entire strand mat consolidation can be predicted based on the properties of the wood constituents.

The main objective of this paper was to develop a mathematical model to predict the surface roughness and to determine the optimal cutting conditions during milling of medium density fiberboard. The milling experiments were carried out as per Taguchi orthogonal array with feed rate and cutting speed as the controlling parameters. A second-order surface roughness model has been developed using the methodology of response surface analysis. The paper presents the details of analysis of variance to validate the developed model. The Taguchi optimization results show that the surface roughness can be optimized with lower feed rate and higher cutting speed values.

In wood-based composite mats, mass and heat transfer are substantially influenced by their gas permeability. Thus, reliable permeability data are needed if one wants to model and simulate those processes taking place prior, during, and after hot-pressing operations of the mat. In this study, wood particles of surface and core layer type were obtained from two particleboard manufacturers, using impact mills and knife ring flakers, respectively. The permeability measurements were performed on the initial materials, as well as on subsets of it obtained by laboratory sieving. Additionally, medium density fibreboard (MDF) fibres from three species, namely pine ( Pinus sylvestris ), spruce ( Picea abies ) and rubberwood ( Hevea Brasiliensis ) were investigated. Cross-sectional and within-plane permeability was measured for densities from 250 to 1000 kg m -3 . The permeability of the core particle materials was determined on solidified samples. On the other hand, the surface particles and MDF fibres were investigated by means of a rapid testing method on loose furnish materials (without solidification). MDF mats exhibited lower permeability values than particle mats at low densities, but values were found in the same order of magnitude for high densities. Mainly, the small particles determine the pore structure and permeability of particle mats. Thickness and in-plane size of particles affect the mat permeability, with the in-plane size becoming primarily effective at elevated densities. The permeability of MDF fibre mats depends more on the anatomy of the raw material than on its size distribution. We suggest, based on computer simulations, that only the within-plane permeability has a strong effect on gas pressure and temperature conditions during hot pressing. The cross-sectional permeability does not play an essential role in this regard.

Enzymatic functionalization is an attractive tool to provide a reactive interface for further processing of lignocellulosic materials, such as wood particles and fibers. Here, spruce wood particles have been functionalized by fungal laccase combined with 4-hydroxy-3-methoxy-benzylamine (HMBA) or 4-hydroxy-3-methoxybenzylurea (HMBU). The expectation was crosslinking with resins in subsequent glueing processes, which should improve strength properties of particle boards. Essential process parameters, such as liquid to solid mass ratio and treatment time, were optimized on a laboratory scale resulting in HMBA and HMBU binding yields of 90% and above as determined by radiochemical mass balance analysis. We employed a multifactorial experimental design for board production from treated wood particles and urea/formaldehyde resin. Mechanical testing and multivariate data analysis revealed, for the first time, an increase of internal bond (IB) as a result of functionalization with HMBU. HMBA was not successful. Variance analysis of relevant parameters and their interactions demonstrated a highly significant difference (P>99.99%) between boards treated with laccase/HMBU versus untreated wood particles. Due to positive interactions, functionalization was most effective at high bulk density (750 kg m -3 ) and high resin content (10%) resulting in a calculated IB improvement of 0.12 N m -2 (21%).

Refined wood fibers of a 54-year-old loblolly pine ( Pinus taeda L.) mature wood were investigated by nanoindentation and atomic force microscopy (AFM). The effect of steam pressure, in the range of 2–18 bar, during thermo-mechanical refining was investigated and the nano-mechanical properties and nano- or micro-level damages of the cell wall were evaluated. The results indicate that refining pressure has important effects on the physical and mechanical properties of refined fibers. No obvious damage was observed in the cell walls at pressures between 2 and 4 bar. Nano-cracks (most less than 500 nm in width) were found in fibers at pressures in the range of 6–12 bar, and micro-cracks (more than 5 μm in width) were found in fibers subjected to pressures of 14 and 18 bar. The damages caused at higher pressures were more severe in layers close to the lumen than on the fiber surfaces. Under special circumstances, the S 3 layer was heavily damaged. The natural shape of the cross sectional dimensions of the cell walls was not changed at lower pressures (2 and 4 bar), but, as pressure was increased, the fibers tended to collapse. At pressures around 18 bar, the lumina were augmented again. The nano-mechanical properties in terms of elastic modulus and hardness were obviously decreased, while nanoindentation creep increased with refining pressure.

Norway spruce wood specimens covering a wide density range, including density samples between 300 kg m -3 (low density wood) and 750 kg m -3 (high-density compression wood), were joined at end grain surfaces and tested in tension perpendicular to the bonded surface to estimate the effect of mechanical interlocking on the adhesive bond strength. It was hypothesised that the higher the density of the specimens the lower will be the mechanical interlocking of the adhesive joints due to a reduced penetration of the adhesive into the cell cavities. An increase of tensile strength was observed with increasing density. These results demonstrate that in the specific case investigated here the cross-sectional cell wall area available for adhesive bonding, which correlates with density, is probably more important than mechanical interlocking.

This experimental study examines the influence of moisture variations on the load-bearing capacity of steel-timber dowel joints. The glulam specimens used to manufacture the joints were first exposed to controlled climate changes. After being stored in the climate chambers, holes were drilled and dowels were inserted. Then, the joints were loaded to failure. The main aim of this study was to investigate how and to what extent moisture variations and gradients in the joint area affects the load-bearing capacity in tension parallel to the grain. The load-bearing capacity was found to be reduced under these conditions when compared to reference joints. Moreover, the brittleness of the joints increased with the time the joints had been exposed to drying. Moisture-induced stresses and cracks in the joint area were found to be a possible explanation of the results. A similar interpretation was given in other studies where other structural timber elements have been studied. This raises an important question of how such moisture effects should be considered in design codes. One possibility is to include the influence of moisture-induced stresses in the k mod factor used in Eurocode 5. Another way could be to consider this type of moisture effect as an equivalent mechanical load case.