Properties Of Wood Cell Walls Research Articles

Quasistatic and Dynamic Nanoindentation Measurements of Pinus radiata D. Don S2 and CCML Cell Wall Layers

Quasistatic nanoindentation is a proven tool that provides information on the micromechanical behavior of wood cell walls. However, quasistatic tests cannot probe the time-dependent mechanical behavior shown by wood. Nanoindentation dynamic mechanical analysis (nanoDMA) can measure the viscoelastic properties of wood cell walls. This research aimed to study the quasistatic and viscoelastic properties of individual radiata pine wood (Pinus radiata D. Don) cell wall layers. To minimize variability and retrieve both properties at the same locations, a load function composed of a multiload-quasistatic function followed by dynamic reference frequency segments was developed. Nanoindentations were then performed on the S2 layer and compound corner middle lamella (CCML) of unembedded latewood cells. Because the S2 layer is anisotropic, both transverse and longitudinal–tangential wood planes were studied. In the transverse plane, the average results of the quasistatic elastic moduli Es for the S2 layer and CCML were 15.7 GPa and 4.6 GPa, respectively. In the longitudinal–tangential plane, the Es was 3.9 GPa. In the transverse section, the hardness H of the S2 layer and CCML were 331 MPa and 277 MPa, respectively, and in the longitudinal–tangential section H was 244 MPa. To acquire the viscoelastic properties, measurements were made over more than three decades of frequency. An increase of the storage modulus E′, and a reduction of the loss modulus E″ and loss factor tanδ as frequency increased were found in both wood orientations. The quasi-static and dynamic indentations equivalent at 0.1 Hz showed similar values for Es and E′. This study contributes to our knowledge of wood cell wall micromechanical properties.

Characterization of wood cell walls treated by high-intensity microwaves: Effects on physicochemical structures and micromechanical properties

High-intensity microwave treatment is an environmentally friendly and time-saving method with a great potential to generate a platform for wood product innovations by increasing the permeability and providing an excellent precondition for wood modification. This work aimed to evaluate the influence of high-intensity microwaves on the physicochemical structures and micromechanical properties of wood cell walls. At the same time, the different response behavior of earlywood and latewood cell walls to microwaves was determined separately. With the help of the optical microscopic, imaging FTIR microscopy, X-ray diffractometer and nanoindenter, the characteristic of the treated cell walls, such as morphology, chemical structures, crystalline structures, hardness and elastic modulus, was thoroughly investigated. The results showed that the physicochemical structures and properties of cell walls were significantly affected by the initial moisture content and microwave intensity with the earlywood being more sensitive to microwaves than latewood. The most damaged cell wall morphology occurred with the moisture content of 20% and microwave power density of 80 kWh/m3. Compared with cellulose and lignin, the hemicellulose showed much more considerable changes in the molecular structures. The crystallinity in microwave-treated samples fluctuated and increased by 15.5% under 60 kWh/m3 microwaves. The cell wall hardness and modulus could both be enhanced, the maximum increase for the former being 48.3% and 36.5% and for the latter being 50.5% and 33.3% in earlywood and latewood, respectively. The findings obtained from this study could help elucidate the mechanism and precisely manipulate the effect of high-intensity microwave treatment, and further extend the application of the wood microwave treatment.

Properties and durability of wood impregnated with high melting point polyethylene wax for outdoor use

In order to develop a new wood product suitable for outdoor applications, a high melting point polyethylene wax was used for high-pressure impregnation treatment of wood. Properties of impregnated wood were studied. Wax in molten state was stable with low viscosity, good fluidity, and easily penetrate into the wood under high pressure. Full-cell process included vacuum (-0.06 MPa for 30 min), high-pressure impregnation (0.75 MPa for 3 h) and pressure release (30 min). Wax contents of treated lodgepole pine (Pinus contorta Douglas ex Loudon.) and eucalyptus (Eucalyptus saligna) were 0.205 g/cm3 and 0.321 g/cm3, respectively. Cell gaps and cell cavities of wood were filled with polyethylene wax. Nanoindentation tests showed that longitudinal mechanical properties of wood cell walls were weakened after impregnation. After a 3 h high-pressure impregnation, hardness of lodgepole pine wood cell walls was reduced by 35.1%, and the modulus of elasticity was reduced by 4.9%. Treated wood had a much lower water absorption and swelling rate than untreated wood, its weathering resistance as well as insect resistance were better.

Comparison of the time-moisture and time-temperature equivalences in the creep properties of Chinese fir

ABSTRACT The present study investigated how the applications of the time-moisture superposition principle (TMSP) under a moisture content (MC) range of 0–14.2% and the time-temperature superposition principle (TTSP) under a temperature range of 30–150 °C would affect to Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) creep behaviors across grain orientations via a dynamic mechanical analysis instrument. The results showed that both TMSP and TTSP could describe the evolution of creep in the radial and tangential directions. Master curves constructed by TMSP and TTSP overlapped at a reference condition (30 °C, 0% MC), regardless of the grain orientation. In addition, an equivalent relation between the effects of moisture and temperature on the creep behavior was assumed based on TMSP and TTSP. Under the tested temperature, the effect of the per unit MC was equivalent to that at ca. 33 and 44 °C for the radial and tangential specimens, respectively. These findings elucidate the effects of moisture and temperature on the rheological properties of wood cell walls and are helpful for better utilizing wood in civil engineering applications.

Wood composites modified with waterborne hyperbranched polyacrylate dispersed organo‐montmorillonite emulsion and the permeability investigations by surface characterizations

AbstractA novel waterborne hyperbranched polyacrylate (HBPA) dispersed organo‐montmorillonite (OMMT) emulsion was synthesized in laboratory. The sapwood of Cathay poplar (Populus cathayana Rehd.) was modified with the emulsion via vacuum treatment for polymer impregnated wood composite with cross section sealing off. Some surface characterization was carried out to analyze the impregnation depth. The results showed that OMMT and HBPA successfully penetrated into the wood cell wall with completely exfoliated OMMT and formed a hydrophobic film covering on the inner surface of cell wall lumen, which improved the surface hydrophobicity and mechanical properties of wood cell wall. With increasing impregnation depth, the improvements decreased, which was associated with the limited impregnation ability of the modifiers only with the pathway of radial and tangential sections. Thus, for better modification, the cross section of the wood must be unobstructed.

Chemical Structure and Mechanical Properties of Wood Cell Walls Treated with Acid and Alkali Solution

The mechanical properties of individual fibers are related to the production and performance of papers and fiberboards. This paper examines the behavior of the microstructure constituents of wood subjected to acid and alkali treatments. The chemical structure and mechanical properties of wood cell walls with different acid or alkali treatments was analyzed. The results show that, compared with acid treatment, the crystal size and crystallinity index of cellulose increased after alkali treatment, resulting in an increase in the cell wall elastic modulus. The mechanical properties of the wood cell wall S2 region were higher than those of the compound middle lamella (CML) region. There was a topochemical effect between the CML and the S2 region in acid and alkali-treated samples, which provided a major threshold that facilitates the production/separation of wood fibers for better strength fiber properties.

Mechanical characterization of Pinus massoniana cell walls infected by blue-stain fungi using in situ nanoindentation

Characterizing the mechanical properties of wood cell walls will lead to better understanding and optimization of modifications made to wood infected by the blue-stain fungi. In this study, in situ nanoindentation was used to characterize the mechanical properties of the cell walls of Pinus massoniana infected by blue-stain fungi at the cellular level. The results show that in situ nanoindentation is an effective method for this purpose and that blue-stain fungi penetrate wood structures and degrade wood cell walls, significantly reducing the mechanical properties of the cell walls. The method can also be used to evaluate and improve the properties of other wood species infected by blue-stain fungi.

Creep and stress relaxation behavior for natural cellulose crystal of wood cell wall under uniaxial tensile stress in the fiber direction

We investigated the temporal changes in creep and stress relaxation behavior in both microscopic crystalline cellulose and macroscopic strain of wood specimen using Japanese cypress (Chamaecyparis obtusa Endl.) to understand the viscoelastic properties of wood cell walls. Specimens 600 µm in thickness were observed by the X-ray diffraction and submitted to tensile load. The crystal lattice strain of (004) plane and macroscopic strain of specimen were continuously detected during creep and stress relaxation tests. It was found that the creep compliance based on macroscopic strain showed a gradual increase after instantaneous deformation due to loading and then the parts of creep deformation remained as permanent strain after unloading. On the other hand, crystal lattice strain showed a different behavior for macroscopic strain; it kept a constant value after instantaneous deformation due to loading and then increased gradually after a certain period of time. These differences between macroscopic and microscopic levels were never found in the stress relaxation tests in this study. Relaxation modulus at the macroscopic level only showed a decreasing trend throughout the relaxation process. However crystal lattice strain kept a constant value during the macroscopic relaxation process. In addition, the microfibril angle (MFA) of wood cell wall has a role of mechanical behavior at microscopic level; crystal lattice strains were smaller with increasing MFA at both creep and relaxation processes. Creep compliance and stress relaxation modulus at the macroscopic level decreased and increased with increasing MFA, respectively. Our results on the viscoelastic behavior at microscopic level evidenced its dependency on MFA.

Effect of the penetration of isocyanates (pMDI) on the nanomechanics of wood cell wall evaluated by AFM-IR and nanoindentation (NI)

AbstractThe effects of the penetration of polymeric diphenyl methane diisocyanate (pMDI) on the chemical structure as well as the static and dynamic mechanical properties of wood cell walls (CWs) were investigated by atomic force microscopy with infrared radiation (AFM-IR) and nanoindentation (NI). Results indicated that the possible penetration of some pMDI molecules into the CW affected the mechanical properties of wood CW significantly. The physical and chemical interactions between pMDI and CW may strengthen the connections between the cell-wall materials and thus improved the static elastic modulus and short-term creep resistance of the CW. The elastic modulus (Er) of CWs was increased from 16.5 to 17.7 GPa; the creep ratio of the CWs decreased by 15% after the penetration of pMDI. Dynamic NI properties indicated that the effective penetration of pMDI had a positive effect on the reduced storage modulus (Er′), whereas it negatively affected the loss modulus (Er″) and the damping coefficient (tanδ) of wood CW in a large frequency scale.

Effects of accelerated aging treatment on the microstructure and mechanics of wood-resin interphase

AbstractPlywood panels prepared from loblolly pine with cured phenol resin (PF) and urea-formaldehyde resin (UF) were submitted to accelerated aging and the microstructures and mechanics of wood-resin interphase were studied by nanoindentation (NI) and nanoscale dynamic mechanical analysis (Nano-DMA). The mass loss (ML) of wood, PF and UF resins were 3.4, 5.0 and 4.6% after aging treatment, respectively, and a large amount of microcracks were observed on the surface of wood and resins after aging treatment, which also affected the static mechanics of the cell walls far from the interphase region and the resins in the interphase region. The elastic modulus (Er) and hardness (H) values of the cell wall decreased by 7.2 and 9.5%, respectively, against the untreated control. The storage and loss modulus of the resins decreased significantly after aging treatment. The significant inconsistency in the mechanics, shrinkage and swelling properties of wood cell wall and resin in the interphase region after aging treatment resulted in a decrease of about 47 and 51% on the average bonding strength of the plywood made of PF and UF resins, respectively.

Micromechanical properties of wood cell wall and interface compound middle lamella using quasi-static nanoindentation and dynamic modulus mapping

Micromechanical properties of individual phases within wood tissues are crucially important for the processing and utilization of this biomass material. The quasi-static nanoindentation and dynamic modulus mapping techniques were employed to study the micromechanical properties of wood cell walls and their interface regions. Nanoindentation results showed that the reduced modulus of secondary cell walls (18 GPa) was twice that of interface compound middle lamella (CML) (7 GPa). Modulus mapping, with advantages of high resolution and undamaged test over conventional nanoindentation, was able to analyse the subtle variations in micromechanical properties of individual phases within the whole wood tissues, especially interface layers. The variation tendency across adjacent cell walls was similar to that tested by nanoindentation, and the secondary cell walls exhibited maxima in the middle of the secondary wall and slightly reduction approaching to the S1 and S3 layers. Moreover, the storage modulus of interface region CML showed “W” distribution, which meant that the modulus first fell and then rose from the central area to edge regions.

Eucalyptus Cell Wall Architecture: Clues for Lignocellulosic Biomass Deconstruction

The architecture, composition, and chemical properties of wood cell walls have a direct influence on the process that occurs prior to fermentation in second-generation biofuel production. The understanding of the construction patterns of cell wall types is the key to the new era of second-generation biofuels. Eucalyptus species are great candidates for this purpose since these species are among the fastest growing hardwood trees in the world and they have been improved for biomass production. We applied the glycome profiling and other combined techniques to study xylem cell walls of three economically important species (Eucalyptus globulus, Eucalyptus grandis, and Eucalyptus urophylla). Glycome profiling analyses revealed that species differ in the same key aspects of cell wall polymer linkages, with E. globulus and E. urophylla presenting contrasting phenotypes, and E. grandis with intermediate characteristics. E. urophylla is known for high recalcitrance, that is probably determined by the strong associations between lignin and cell wall polymers, and also lignin content. On the other hand, E. globulus cell wall polymers are loosely linked, so its cell wall can be easily deconstructed. We have shown in this work that the composition of cell walls differs in quantity and quality among the Eucalyptus species and such variations in composition influence the process of lignocellulosic feedstock assessment. However, the greatest influence relies on the amount and type of associations between cell wall polymers. A high yield of cellulose, from any biomass source, directly depends on the cell wall architecture.

Contributions of Basic Chemical Components to the Mechanical Behavior of Wood Fiber Cell Walls as Evaluated by Nanoindentation

Selective chemical extraction was applied to gradually remove classes of chemical components from wood cell walls. Nanoindentation was performed on the control and treated wood cell walls to evaluate the contributions of the chemical components to the cell walls by measuring the elastic modulus, hardness, and creep compliance. Burger’s model was applied to simulate the process of nanoindentation and to gain insight into the response of visco-elastic properties to the chemical components. Wood extractives showed limited effects on the cell-wall mechanics; however, the removal of hemicelluloses and lignin resulted in reductions of 11.7% and 28.4%, respectively, in the elastic modulus and 14.8% and 30.4%, respectively, in the hardness. The extraction of hemicelluloses and lignin reduced the resistance of wood cell walls to creep. Furthermore, the extracted parameters from Burger’s modeling indicated that cellulose exhibited the greatest influence on the mechanical properties of wood cell wall, while hemicelluloses exhibited the greatest contribution to cell-wall viscosity, and lignin contributed extensively to cell-wall elasticity.

The Furfurylation of Wood: A Nanomechanical Study of Modified Wood Cells

Furfurylation of wood is of interest worldwide as an environmentally friendly modification process. It is widely assumed that low-molecular weight furfuryl alcohol (FA) can penetrate into wood cells and polymerize in-situ during the process, resulting in substantial improvement in the physical-mechanical properties and durability of wood. In this study, confocal laser scanning microscopy (CLSM) was used to visualize the microscopic distribution of polymerized FA resin in the Masson pine wood cavities, and a Nanoindenter was used to probe the mechanical properties of modified wood cells. The effects of catalysts (maleic anhydride and a mixed organic acid catalyst), FA concentration, curing time, and curing temperature on the nanomechanical properties of wood cell walls were investigated. An improvement in the indentation modulus and hardness of modified wood cells demonstrated indirectly but strongly that FA indeed penetrated wood cells during the modification process. Based on the results of the cell wall nanoindentation test, a combination of 50% furfuryl alcohol, 8 h curing time, and 95 °C curing temperature were proposed as the starting processing parameters for the development of a more practical and effective wood furfurylation process using a mixed organic acid catalyst.

Effect of Water on the Mechanical Properties of Wood Cell Walls – Results of a Nanoindentation Study

The paper presents a nanoindentation study on five different wood species in which the elastic and creep properties of the S2 cell wall layer and the middle lamella were determined. Measurements were carried out at relative humidities (RH) ranging from 10 to 80% as well as underwater. Indentation moduli were found to decrease by about a third in the S2 layer and by about half in the middle lamella between RH of 10 and 80%. Hardness dropped by 50 to 60% in this humidity range in both the S2 layer and the middle lamella. Creep parameters were almost constant up to a relative humidity of 40%, but they increased considerably at higher RH. The most pronounced change of reduced moduli and creep properties occurred between 60 and 80% RH, which is consistent with the expected softening of hemicellulose and amorphous parts of cellulose in this humidity region. Immersion into water resulted in a further decrease of the reduced moduli to about 20 to 30% of their values at 10% RH and to only about 10 to 20% for the hardness. This can be explained by additional softening of the less ordered regions of cellulose.

A combined view on composition, molecular structure, and micromechanics of fungal degraded softwood

Abstract Fungal decay alters the composition, microstructure, and mechanical properties of wood cell walls. To understand better the structure-function relationships during fungal decay, selected annual rings of fungal deteriorated Scots pine sapwood were analyzed in terms of their composition, microstructure, and micromechanical properties. The datasets were acquired separately for earlywood and latewood concerning the S2 cell wall layer and the cell corner middle lamella (CCML) and analyzed by means of principal component analysis and partial least squares regression analysis. Links between cell wall stiffness and hardness and the composition and microstructure could be established. Increased mechanical properties in the CCML, as obtained by nanoindentation, were correlated to the degradation of pectins. In the S2 layer, the altered data were related to the degradation of hemicelluloses and lignin modification during fungal decay.

Changes in the properties of wood cell walls during the transformation from sapwood to heartwood

Changes in the chemical, viscoelastic and hygroscopic properties of wood cell walls in Chinese fir (Cunninghamia lanceolata) during the transition from sapwood to heartwood were studied to provide insights into the formation of heartwood. In situ imaging FTIR measurements indicated that the composition of the main components of cell walls remained almost unaltered, but more extractives were deposited in the wood cell walls during the sapwood–heartwood transition. Compared to the sapwood and transition wood, the heartwood had a higher softening temperature and greater activation energy, suggesting that the mobility restrictions of cell wall biopolymers were due to extractives obstructing the accessing of the plasticizer (ethylene glycol). The moisture sorption was the same from the sapwood to heartwood at a low relative humidity (RH), while the heartwood adsorbed less water at a high RH, probably caused by the extractives deposited in the matrix and mesopores of heartwood cell walls.

Mechanical properties and failure mechanism of wood cell wall layers

In this paper, the deformation and failure behavior of a model nanocomposite that mimics the wood cell wall layer is studied using molecular dynamics simulation. The effect of microfibril angle (MFA) on the mechanical properties of the cellulose microfibril-reinforced nanocomposite is investigated. The measured elastic modulus in the simulations is found to be comparable with the estimation based on experimental measurements. Further more, the simulation results show that it is the local buckling of cellulose microfibril that causes the initiation of failure of the fiber-reinforced nanocomposites, i.e., the wood cell wall layers. Since kink band formation is the compressive failure mode of wood cell walls as well as the macroscopic wood, this suggests that the compressive failure of the micron-scale wood cell wall as well as the macroscopic wood may originate at the nanoscale as a consequence of the bulking of microfibrils. The effect of water on the mechanical properties of wood cell wall is also studied.

Effects of decreasing carbohydrate content on properties of wood strands

Hot-water pretreatment was performed on wood strands to investigate effects of the extraction of hemicellulose under different temperature (140, 155, and 170 °C) and time durations (30 and 60 min) conditions. Hydrolysate was analyzed by means of high-performance liquid chromatography. Chemical changes in the surface of wood strands were studied by infrared spectroscopy. The effects of hemicellulose extraction on the wettability of wood strands were studied by measuring the contact angle and surface free energy. The mechanical properties of wood cell walls before and after treatment were studied by nanoindentation. Among the extracted monosaccharides, mannose was found in the highest concentration. The mechanical properties of cell walls showed little decrease after extraction. The chemical changes in the surface of the wood strands reduced wettability of wood surface by water and produced hydrophobic characteristics after extraction.

The relation between indentation modulus, microfibril angle, and elastic properties of wood cell walls

Nanoindentation is a well known tool for identification of mechanical properties at the micrometer scale of materials. When applied to study wood cell walls the commonly used isotropic indentation theory is not applicable. In this study, anisotropic nanoindentation theory was employed for analyzing nanoindentation test results on wood cell walls. The influence of elastic stiffness components, microfibril angle, and cell wall composition on the indentation modulus was studied. The indentation modulus was found to depend on longitudinal, transverse, and shear modulus to a similar extent. A significant influence of the microfibril angle on the indentation modulus was observed and discussed with respect to experimental scatter and sample preparation. It is concluded, that application of anisotropic nanoindentation theory provides a tool for quantitative instead of qualitative investigation of wood cell walls, with the goal of identifying all elastic properties of the transversely isotropic cell wall from nanoindentation tests.