Glass Transition Of Lignin Research Articles

Dynamical mechanical behaviors of rubber-filled wood fiber composites with urea formaldehyde resin

Wood composites glued with thermosetting synthetic resins tend to show inadequate damping performance caused by the cured resinous matrix. Waste rubber maintains prominent elasticity and is feasible to be an optional modifier. To that end, composite panels of granulated tire rubber (GTR) powders and thermal-mechanically pulped wood fibers were fabricated in this study. Urea formaldehyde (UF) resin was applied as the bonding agent (10% based on wood/rubber total weight). Dynamical mechanical analysis (DMA) was conducted to disclose the thermo-mechanical behaviors of the rubber-filled wood fiber composites. Influence of two technical parameters, i.e., GTR powder size (0.55–1.09 mm) and addition content (10%, 20% and 30% based on wood/rubber total weight), was specifically discussed. The results showed that storage modulus (E') of the rubber-filled composite decreased while loss factor (tan δ) increased monotonously along with elevated temperature. A steady “plateau” region among 110–170 °C was found where both E' and tan δ keep constant. Accordingly, tan δ showed two peak values at 103–108 and 231–233 °C due to glass transition of lignin and thermal degradation of hemicellulose, respectively. Addition of rubber fillers resulted in lower bending and internal bonding strengths as well as storage modulus values. When the temperature was above 183 °C, all the rubber-filled composites showed higher tan δ values than the control. The findings above fully demonstrate the improved damping performance of the UF-bonded wood fiber composites on account of rubber component. Further work is still needed to optimize the rubber/fiber interfacial bonding strength.

Optimization of Component Yields and Thermal Properties by Organosolv Fractionation of Loblolly Pine (Pinus taeda) Using Response Surface Design

Lignin, a low value by-product of biomass fractionation, is of current particular interest for the production of value added materials such as carbon fibers within a biorefinery. Accordingly, we have isolated lignin, hemicellulose, and cellulose by organosolv fractionation of loblolly pine (Pinus taeda) under the influence different fractionation severities (2.50–3.31). A designed experimental matrix targeted maximum lignin and pulp yields and lignin glass transition temperature (Tg) as a function of several process parameters. Optimal fractionation conditions were estimated from an enhanced regression model of the statistically significant variables generated using response surface design (RSD). The most significant factor influencing lignin and pulp yields was solvent composition (p-value < 0.0001). A maximum lignin yield of 94.71 wt% and maximum cellulose yield of 74.96 wt% were determined at combined severities of 3.21 and 2.63, respectively. The lowest Tg of 132.83 °C was found at a combined severity of 2.82.

Rheology of transgenic switchgrass reveals practical aspects of biomass processing

BackgroundMechanical properties of transgenic switchgrass have practical implications for biorefinery technologies. Presented are fundamentals for simple (thermo)mechanical measurements of genetically transformed switchgrass. Experimental basics are provided for the novice, where the intention is to promote collaboration between plant biologists and materials scientists.ResultsStem sections were subjected to two stress modes: (1) torsional oscillation in the linear response region, and (2) unidirectional torsion to failure. Specimens were analyzed while submerged/saturated in ethylene glycol, simulating natural hydration and allowing experimental temperatures above 100 °C for an improved view of the lignin glass transition. Down-regulation of the 4-Coumarate:coenzyme A ligase gene (reduced lignin content and altered monomer composition) generally resulted in less stiff and weaker stems. These observations were associated with a reduction in the temperature and activation energy of the lignin glass transition, but surprisingly with no difference in the breadth and intensity of the tan δ signal. The results showed promise in further investigations of how rheological methods relate to stem lignin content, composition, and functional properties in the field and in bioprocessing.ConclusionsMeasurements such as these are complicated by small specimen size; however, torsional rheometers (relatively common in polymer laboratories) are well suited for this task. As opposed to the expense and complication of relative humidity control, solvent-submersion rheological methods effectively reveal fundamental structure/property relationships in plant tissues. Demonstrated are low-strain linear methods, and also nonlinear yield and failure analysis; the latter is very uncommon for typical rheological equipment.

Dynamics of the lignin glass transition.

The dynamics of lignin, a complex and heterogeneous major plant cell-wall macromolecule, is of both fundamental and practical importance. Lignin is typically heated to temperatures above its glass transition to facilitate its industrial processing. We performed molecular dynamics simulations to investigate the segmental (α) relaxation of lignin, the dynamical process that gives rise to the glass transition. It is found that lignin dynamics involves mainly internal motions below Tg, while segmental inter-molecular motions are activated above Tg. The segments whose mobility is enhanced above Tg consist of 3-5 lignin monomeric units. The temperature dependence of the lignin segmental relaxation time changes from Arrhenius below Tg to Vogel-Fulcher-Tamman above Tg. This change in temperature dependence is determined by the underlying energy landscape being restricted below Tg but exhibiting multiple minima above Tg. The Q-dependence of the relaxation time is found to obey a power-law up to Qmax, indicative of sub-diffusive motion of lignin above Tg. Temperature and hydration affect the segmental relaxation similarly. Increasing hydration or temperature leads to: (1) the α process starting earlier, i.e. the beta process becomes shortened, (2) Qmax decreasing, i.e. the lengthscale above which subdiffusion is observed increases, and (3) the number of monomers constituting a segment increasing, i.e. the motions that lead to the glass transition become more collective. The above findings provide molecular-level understanding of the technologically important segmental motions of lignin and demonstrate that, despite the heterogeneous and complex structure of lignin, its segmental dynamics can be described by concepts developed for chemically homogeneous polymers.

Effect of Process Parameters and Kraft Lignin Additive on The Mechanical Properties of Miscanthus Pellets

Miscanthus had a higher lignin content (19.5 wt%) and carbohydrate (67.6 wt%) than other herbaceous crops, resulting in higher pellet strength and positive effect on combustion. However, miscanthus also contains a high amount of hydrophobic waxes on its outer surface, cuticula, which limits the pellet quality. The glass transition of lignin and cuticula were related to forming inter-particle bonding, which determined mechanical properties of pellet. To determine the effects of surface waxes, both on the pelletizing process and the pellet strength were compared with raw and extracted samples through solvent extraction. In addition, to clarify the relationship between pellet process parameters and bonding mechanisms, the particle size and temperature are varied while maintaining the moisture content of the materials and the die pressure at constant values. Furthermore, kraft lignin was employed to determine the effect of kraft lignin as an additive in the pellets. As results, the removal of cuticula through ethanol extractions improved the mechanical properties of the pellet by the formation of strong inter-particle interactions. Interestingly, the presence of lignin in miscanthus improves its mechanical properties and decreases friction against the inner die at temperatures above the glass transition temperature (Tg) of lignin. Consequently, it could found that the use of kraft lignin as an additive in pellet reduced friction in the inner die upon reaching its glass transition temperature.

Effects of hot-washing process on structure and enzymatic hydrolysis of treated steam explosion corn stover

Cellulase adsorption of lignocellulosic materials is the key link during enzymatic hydrolysis. Hot-washing process (above lignin glass transition temperature) was used to change the physical structure of lignin, decrease covalent connection between cellulose and lignin, reduce the concentration of inhibitor, and explore the feasibility of enzymatic hydrolysis. The general objective of the paper was conducted to determine whether the hot-washing process has the potential to change the mechanism of lignin on enzyme hydrolysis. Hot-washing was carried out at 151 °C for 20 min. The ratio of acid insoluble lignin to acid soluble lignin was increased, while the formation of spherical lignin droplets on the cell wall surface was decreased. Enzymatic digestibility of hot-washed filter cakes showed enhanced digestibility over the control samples. The concentration of fermentation inhibitor (acetic acid, formic acid, furfural and 5-hydroxymethylfurfural) obviously decreased after hot-washing process. Hot-washing process significantly increased the adsorption ability of cellulase on the substrates and digestibility of biomass without removing much of the insoluble lignin content. Lignin distribution and/or physical property composition play a role.

The effects of sugar beet molasses on wheat straw pelleting and pellet quality. A comparative study of pelleting by using a single pellet press and a pilot-scale pellet press

The main aim of this paper is to investigate the effects of molasses on wheat straw pelleting and physical pellet quality. Molasses was added at weight fractions of 1.5% and 3%, while pure straw served as a control. The effects of molasses were examined by producing pellets in a single pellet press (SPP) and in a pilot-scale pellet press (PSPP). The second aim of this study was to compare the results obtained from the SPP and the PSPP, i.e., to understand how the information from the SPP can be used for the prediction of material behavior, process adjustments, and improvement of pellet quality in an upscale pelleting process. The production and pellet quality parameters were compared and information from the two pelleting methods was combined by response surface modeling. Pellet density was the response variable, while pelleting pressure and temperature were the independent variables. Large differences in pellet quality were observed between the two pelleting methods. These differences are discussed from the perspective of technical differences in the pelleting procedures and different fiber orientations in the pellets. The results indicate that pelleting temperature is a key factor for achieving good pellet quality of all the samples. Exceeding the glass transition temperature of lignin leads to significantly better pellet quality and facilitates pelleting. The results showed that molasses strengthens pellets produced at temperatures below the glass transition of lignin. Addition of molasses at higher pelleting temperature did not significantly affect the pellet quality and processability.

Effect of densification parameters on the properties of maize residue pellets

The effect of the conditioning process parameters on the properties of the pellets was investigated. Maize residues were collected locally from the field and crushed into small sizes, dried, and separated for compaction. The effects of the variation of the die temperature (30–80 °C), the compaction pressure (150–250 MPa), and the biomass type (cob, husk, and stalk) on compact density, relaxed density, and durability index were investigated. It was found that the compact density increased with pressure and temperature to around 950–1100% higher than the residue density. The relaxed density was stable at 60–80 °C, but at 30 °C, it was found to decrease from 800–1000 kg m−3 to 660–700 kg m−3. The durability index was observed to improve with increasing pressure and temperature by 30–60% and 70–90%, respectively. This corresponded well with the lignin glass transition temperatures being in the range of 60–80 °C at moderate pressure values between 150 MPa and 250 MPa. Pellet density was also found to increase with increasing compression pressure and temperature. Pellet density was three times higher than bulk density and similar to the particle density. Heating the feed materials during compression decreased the compaction pressure from 250 MPa to 150 MPa, resulting in the formation of pellets with a higher durability index and more stable relaxed density.

樹種による組織構造及び熱軟化特性の違いが木材の横引張変形特性へ及ぼす影響

To obtain the information about maximum possible tensile deformation of wood, deformation properties of various wood species in lateral tension were studied. In addition, we observed fracture sections of lateral tensile tests in water, in order to examine the effect of anatomy on the deformation properties. Tensile deformation properties were measured in water at 25℃ or 80℃. The test specimens have different angles of the tensile direction to the annual rings (0°, 45°, 90°). The results obtained were as follows. 1) Failure strain of the hardwood specimens were not larger than the specimens of Hinoki at 80℃ in water. 2) At the specimen of angle 45°, failure strain increased proportionally with the decrease in density. 3) In all of specimens, failure strain in water saturated condition were larger at 80℃ than 25℃. However, failure strain were not increased proportionally with the decrease in elastic modulus. This result suggests that softening of wood with moisture and temperature course not only capability for the deformation of cell shapes but also origin of the fracture. 4) Result of observation of the fracture sections of lateral tensile tests, specimen of Keyaki having large pores susceptible to fracture along the sequence of large pores. These results suggest that lateral tensile deformation properties were greatly affected by anatomy. Therefore, in order to increase the tensile deformation it is necessary to deform of cell shapes which affected by density, thermal-softening caused by glass transition of lignin and anatomy, i.e, ray tissue or large pores.

Experiences of Kraft Lignin Functionalization by Enzymatic and Chemical Oxidation

Linear hydrophilic derivatives are expected to soften lignin and improve its utilization in composite applications. Oxidation by means of laccase in the presence of oxygen was employed in an attempt to functionalize commercial kraft lignin by vanillic acid-PEG ester and ether derivatives. Thielavia arenaria and Melanocarpus albomyces laccases at pH 6 and 8 were used. According to O2 consumption and the increase in molar mass, the tested laccases were active toward the lignin and the vanillic acid derivatives and also formed corresponding phenoxyl radicals. However, homogenous polymerization instead of cross-coupling and functionalization took place. As an alternative, lignin functionalization by the ester derivative by chemical oxidation under alkali-O2 conditions was also tested. Efficient lignin polymerization was observed. However, functionalization was not detected. Interestingly, a clear decrease in lignin glass transition temperature was obtained by an isolation procedure that included freeze-drying. This suggests that functionalization may not be necessary to induce the desired softening effect.

A Study on the Morphology, Mechanical, and Electrical Performance of Polyaniline-modified Wood - A Semiconducting Composite Material

This study investigated the morphology, electrochemical modification with respect to the wood fiber direction, and mechanical properties of wood modified by in situ polymerization with polyaniline (PANI). This polymerization formed a composite material with applications as an anti-static, electromagnetic, anti-corrosion, and heavy metal purifying materials. The polymer was found throughout the entire structure of the wood and was quantified within the wood cell wall and middle lamella by SEM-EDX. The presence of PANI affected the conductivity of the composite specimens, which was found to be higher in the fiber direction, indicating a more intact percolation pathway of connected PANI particles in this direction. The PANI modification resulted in a small reduction of the storage modulus, the maximum strength, and the ductility of the wood, with decreases in the properties of specimens conditioned in an environment above 66% relative humidity. The in situ-polymerized PANI strongly interacted with the lignin component of the veneers, according to the decrease in the lignin glass transition temperature (Tg) noted in DMA studies.

DYNAMIC MECHANICAL THERMAL ANALYSIS OF MOSO BAMBOO (Phyllostachys heterocycla) AT DIFFERENT MOISTURE CONTENT

Bamboo is a type of biomass materials that has great potential as a bio-energy resource in China. The thermal-mechanical behavior of bamboo plays an important role in the formation process of pellets. To investigate the effect of moisture content (MC) on thermal-mechanical behavior of bamboo, the storage modulus and loss factor of moso bamboo was determined using dynamic mechanical thermal analysis (DMTA) from -50 to 150 oC. The experimental results showed that the general feature of bamboo thermal-mechanical properties with temperature is similar to other cellulosic materials, and they are affected by MC. A substantial decrease of storage modulus over the entire temperature range implies that bamboo underwent a glass to rubber transition. Bamboo, at lower MC, has a higher storage modulus, which decreases the mechanical strength of pellets. The loss factor exhibited two major transitions for all samples. There was an α-transition (α1), attributed to glass transition of lignin, peaking in a higher temperature range. The second major relaxation (α2), located in a lower temperature range, was attributed to glass transition of hemicelluloses. Activating lignin and hemicelluloses using moisture and temperature in the temperature range of glass transition can be very helpful to achieve durable particle-particle bonding.

Fuel Pellets from Wheat Straw: The Effect of Lignin Glass Transition and Surface Waxes on Pelletizing Properties

The utilization of wheat straw as a renewable energy resource is limited due to its low bulk density. Pelletizing wheat straw into fuel pellets of high density increases its handling properties but is more challenging compared to pelletizing woody biomass. Straw has a lower lignin content and a high concentration of hydrophobic waxes on its outer surface that may limit the pellet strength. The present work studies the impact of the lignin glass transition on the pelletizing properties of wheat straw. Furthermore, the effect of surface waxes on the pelletizing process and pellet strength are investigated by comparing wheat straw before and after organic solvent extraction. The lignin glass transition temperature for wheat straw and extracted wheat straw is determined by dynamic mechanical thermal analysis. At a moisture content of 8%, transitions are identified at 53°C and 63°C, respectively. Pellets are pressed from wheat straw and straw where the waxes have been extracted from. Two pelletizing temperatures were chosen—one below and one above the glass transition temperature of lignin. The pellets compression strength, density, and fracture surface were compared to each other. Pellets pressed at 30°C have a lower density and compression strength and a tendency to expand in length after the pelletizing process compared to pellets pressed at 100°C. At low temperatures, surface extractives have a lubricating effect and reduce the friction in the press channel of a pellet mill while no such effect is observed at elevated temperatures. Fuel pellets made from extracted wheat straw have a slightly higher compression strength which might be explained by a better interparticle adhesion in the absence of hydrophobic surface waxes.

The mechanisms of hydrothermal deconstruction of lignocellulose: New insights from thermal–analytical and complementary studies

Differential Scanning Calorimetry, Dynamic Mechanical Thermal Analysis, gravimetric and chemical techniques have been used to study hydrothermal reactions of straw biomass. Exothermic degradation initiates above 195°C, due to breakdown of the xylose ring from hemicellulose, which may be similar to reactions occurring during the early stage pyrolysis of dry biomass, though activated at lower temperature through water mediation. The temperature and magnitude of the exotherm reduce with increasing acid concentration, suggesting a reduction in activation energy and a change in the balance of reaction pathways. The presence of xylan oligomers in auto-catalytic hydrolysates is believed to be due to a low rate constant rather than a specific reaction mechanism. The loss of the lignin glass transition indicates that the lignin phase is reorganised under high temperature auto-catalytic conditions, but remains partially intact under lower temperature acid-catalytic conditions. This shows that lignin degradation reactions are activated thermally but are not effectively catalysed by aqueous acid.

Thermal transitions of the amorphous polymers in wheat straw

The thermal transitions of the amorphous polymers in wheat straw were investigated using dynamic mechanical thermal analysis (DMTA). The study included both natural and solvent extracted wheat straw, in moist (8–9% water content) and dry conditions, and was compared to spruce samples. Under these conditions two transitions arising from the glass transition of lignin and hemicelluloses have been identified. Key transitions attributed to softening of lignin were found at 53, 63 and 91 °C for moist samples of wheat straw, extracted straw and spruce, respectively. Transitions for hemicelluloses were determined at 2, −1 and 5 °C, respectively. Differences are likely due to different compositions of lignin and hemicelluloses from straw and spruce and structural differences between the raw materials. The high wax content in wheat straw resulted in a transition at about 40 °C which was absent in solvent extracted wheat straw samples and spruce. This specific transition was further investigated and confirmed by differential scanning calorimetry (DSC) of extracted wheat straw wax. Information about the thermal transitions is of great importance for the utilization of wheat straw in pelletizing, briquetting and fiber board manufacturing.

Rapid dissolution of lignocellulosic biomass in ionic liquids using temperatures above the glass transition of lignin

Rapid dissolution of bagasse and southern yellow pine has been achieved in the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([C2mim]OAc) by using a dissolution temperature above the glass transition of lignin (ca. 150 °C). When 0.5 g of bagasse or pine is added to 10 g of [C2mim]OAc, complete dissolution can be obtained in 5–15 min for bagasse at a temperature of 175–195 °C, compared to 15–16 h at 110 °C, and over 90% of added pine can be dissolved with heating at 175 °C for 30 min. Upon regeneration in acetone/water, lignin and carbohydrate can be partially separated as lignin and a cellulose-rich material (CRM, pulp). Compared to published methods with lower temperatures and longer times (e.g., 110 °C, 16 h), processing bagasse in [C2mim]OAc at 185 °C for 10 min results in higher yields of both recovered lignin (31% vs. 26% of the available lignin) and carbohydrate (carbohydrate yield = 66% vs. 63% of the available carbohydrate). In addition, the CRM pulp recovered using the higher temperature method has much lower residual lignin content (6% vs. 20%). Similar results were obtained for pine (lignin content in CRM with higher vs. lower temperature method = 16.1% vs. 23.5%). The IL was recycled and reused although the efficiency decreased and ca. 15% of the IL had degraded after the higher temperature process. These latter results suggest further optimization of the choice of IL and heating conditions might be needed to develop an energy and chemical efficient process.

Advancing the dynamic mechanical analysis of biomass: comparison of tensile-torsion and compressive-torsion wood DMA

Abstract In an effort to advance the dynamic mechanical analysis (DMA) of very small biomass specimens, and/or specimens having poor mechanical integrity, the functional equivalent of pendulum-torsion (tensile-torsion) DMA was compared to parallel-plate compressive-torsion DMA. The solvent-saturated lignin glass transition in yellow-poplar (Liriodendron tulipifera) was generally similar determined by both modes; however, direct data comparisons should be avoided or carefully considered. First-heat glass transition temperatures (T gs) were relatively similar; however, specimen densification elevated subsequent cooling-mode T gs by 5–8°C in compressive-torsion. Both modes revealed a first-heat tan δ shoulder; it was more prominent and had grain dependency in compressive-torsion. Below fiber saturation, subambient tensile-torsion DMA was superior; compressive-torsion resulted in an anomalous response, obscuring subambient secondary relaxations. With these differences and limitations in mind, compressive-torsion offers specific advantages. Solvent-submersion studies are simplified because solvent cups are easily devised for torsional rheometers. Specimens lacking mechanical integrity are more easily analyzed. Heavily biodegraded spruce (Picea sp.) was analyzed in the solvent-submersion mode as fibrous mats and the different actions of Gloeophyllum trabeum and Postia placenta were revealed. Very small specimens are easily analyzed in compressive-torsion; tissue maturity effects were revealed in minute sections of switchgrass (Panicum virgatum) stems. Applied appropriately, parallel-plate compressive-torsion DMA will provide new research opportunities.

Studies on bamboo for sustainable and advanced utilization

Relationship between thermal-softening properties and deformation appearing after heating and cooling processes with loading was investigated. In the heating process, an obvious decrease in relative relaxation modulus due to thermal-softening of lignin was found at around 60 °C. After the cooling process, around 70% of residual deformation was measured. Also, the residual set depended on the maximum temperature reached in the heating process and the unloaded temperature in the cooling process. It was thought that these results suggest that the glass transition of lignin from the rubbery to glassy state is important to fix the deformation.

A literature-based study on the loss tangent of wood in connection with mechanical pulping

In mechanical pulping, wood is dynamically loaded, which causes large heat losses due to wood viscoelasticity. The heat losses depend on the loss tangent (tan δ) of wood. The loss tangent has a temperature-dependent behaviour, especially in the lignin glass transition region. The glass transition softens wood, and is therefore necessary for gentle mechanical pulping, but at the same time, the loss tangent shows a maximum called the α-peak. The transient peak depends on temperature, loading frequency and moisture content. The temperature where the peak is found can be lowered with chemical treatments, but they also increase the magnitude of the peak. Thermal treatment in the presence of water also increases the magnitude. The loss tangent of wood depends, amongst other things, on the chemical structure of lignin, width of cellulose crystals, microfibril angle, and extractives in the cell wall.

Molecular mobility and relaxation process of isolated lignin studied by multifrequency calorimetric experiments

The glass transition of lignin has been studied by multifrequency calorimetric measurements in order to highlight the morphological changes and the dynamic aspects associated to this relaxation process. Influences of water sorption and thermal annealing on molecular mobility have been considered. Additional investigations by thermogravimetry, infra-red spectroscopy and rheometry have been performed to corroborate the claims. The relaxation process of annealed lignin shows a different behaviour as the consequence of micro-structural modifications of lignin. These are explained by redistribution of secondary bonds as well as formation of new interunit linkages. Concerning the dynamic aspects, apparent activation energy, E, and sizes of cooperatively rearranging region, V(crr), have been evaluated respectively from the frequency dependence and heat capacity measurements of the glass transition. Compared to dried lignin, both E and V(crr) significantly decrease in a water-sorbed matrix indicating that the three-dimensional structure presents a higher mobility and is less confined.