Moisture-dependent elastic and plastic properties of pine wood and bamboo cell wall layers measured by nanoindentation

1 - Wood microstructure – A cellular composite

For wood and forest products to reach their full potential as structural materials, experimental techniques are needed to measure mechanical properties across all length scales. Nanoindentation is uniquely suited to probe in situ mechanical properties of micrometer-scale features in forest products, such as individual wood cell wall layers and adhesive bondlines. However, wood science researchers most commonly employ traditional nanoindentation methods that were originally developed for testing hard, inorganic materials, such as metals and ceramics. These traditional methods assume that the tested specimen is rigidly supported, homogeneous, and semi-infinite. Large systematic errors may affect the results when these traditional methods are used to test complex polymeric materials, such as wood cell walls. Wood cell walls have a small, finite size, and nanoindentations can be affected by nearby edges. Wood cell walls are also not rigidly supported, and the cellular structure can flex under loading. Additionally, wood cell walls are softer and more prone to surface detection errors than harder inorganic materials. In this paper, nanoindentation methods for performing quasistatic Berkovich nanoindentations, the most commonly applied nanoindentation technique in forest products research, are presented specifically for making more accurate nanoindentation measurements in materials such as wood cell walls. The improved protocols employ multiload nanoindentations and an analysis algorithm to correct and detect errors associated with surface detection errors and structural compliances arising from edges and specimen-scale flexing. The algorithm also diagnoses other potential issues arising from dirty probes, nanoindenter performance or calibration issues, and displacement drift. The efficacy of the methods was demonstrated using nanoindentations in loblolly pine (Pinus taeda) S2 cell wall layers (S2) and compound corner middle lamellae (CCML). The nanoindentations spanned a large range of sizes. The results also provide new guidelines about the minimum size of nanoindentations needed to make reliable nanoindentation measurements in S2 and CCML.

To better understand how moisture content (MC) affects the longitudinal compressive mechanical properties of bamboo, mechanical tests on both the macroscopic and cellular levels were performed on Moso bamboo (Phyllostachys pubescens Mazei ex H. de Lebaie) at different MCs. At the macroscopic level, the compressive modulus of elasticity (CMOE) was determined using a common mechanical tester, while the indentation modulus of elasticity (EIT) and the hardness (HIT) of the bamboo fiber cell walls were obtained using nanoindentation. The results showed that CMOE, EIT, and HIT were all negatively correlated with the change in MC below the fiber saturation point (FSP) with strong linear relationships. However, the CMOE was found to be more sensitive to a change in MC than was EIT, which indicated that the bamboo was more sensitive to MC at the macro level than at the cellular level, at least in terms of longitudinal compression stiffness. Moreover, EIT was found to be much less sensitive to a change in MC than was HIT, which may explain why the longitudinal compression strength of bamboo was much more sensitive to changes in MC than was the compression modulus of elasticity on the macro scale.

Flax fibre composite is a bio-material that can meet the requirements in many engineering applications. It has been used especially in applications for the automotive industry due to the several excellent specific properties which are similar to composites manufactured with glass fibres. However, due to the hydrophilic nature of flax fibre, this material is prone to absorb water and moisture from the environment, and therefore, they are not reliable for outdoor applications. The growing demand for flax composites in the field of bio-material research enforces the researchers to have in-depth knowledge in their processing into a useful final product. However, the correlation between the environmental effects and the performance of the flax fibre bio-composites is not well established. The main focus of this study is to address the specific environmental issues and their effects on flax fibre composites manufactured by the vacuum infusion method. First of all, the effect of the moisture content in the fibres on the mechanical performance and overall quality of the manufactured composites is studied. Flax composites are manufactured with fibres having different moisture content and they are compared to composites manufactured with dry fibres. This work begins with measuring moisture absorption kinetics of the flax fibres. Flax fibres absorb moisture almost linearly from 0% to 70% RH (relative humidity) environments, however, this absorption almost double from 70% to 95% RH. The tensile strength of the flax fibre reinforced composites increases until a certain RH value (50% RH) and then it decreases as the RH of the environment is raised up to the maximum value (95% RH). However, the tensile modulus, flexural strength and modulus of the composite samples continuously decrease with the increasing RH values. The microstructural tests of the composites confirmed the presence of micro-cracks and pores and debonding in the fibre-matrix interface. In addition, a study about the dynamic response of the flax composites manufactured with fibres stored at different RH levels ensures that dry composites have better vibration and damping properties. The damping ratios of the flax composites manufactured with fibres with different RH values are slightly increased with the increasing rates of moisture absorption although this change is not notable. Moreover, the mechanical properties of the flax composites manufactured with fibres at different RH environments are predicted by analytical models. The geometric and displacement potential function approaches are proposed to estimate the tensile properties of the flax composites. The proposed models demonstrate greater accuracy when validated with experimental observations. Secondly, the consequence of different environmental conditions on the performance and durability of flax composites is analysed. Composites made by vacuum infusion using dry fibres are immersed in water, exposed to warm humid environments and subjected to freeze-thaw (F/T) cycles. During exposure to different environments, the mechanical performance (tensile and flexural properties), moisture content and physical changes (dimensional stability) of the composites are analysed. The water absorption trend is partially Fickian for the samples immersed in water at room temperature and these samples are greatly affected and degraded. Furthermore, the mechanical properties of the water saturated (WS) samples are clearly degraded in contrast to the as manufactured (AM) samples. Compared to AM samples, the tensile strength and modulus are decreased approximately by 9% and 57%, respectively for WS samples and their flexural strength and modulus is decreased by 64% and 70%, respectively. However, some of these properties could be recovered, to some extent, by drying the WS samples. On the other hand, the reduction of tensile strength and modulus for the humidity saturated (HS) samples is only 0.8% and 3%, respectively, when compared to the AM composite samples. It was found that freezing-thawing cycles have almost no effects on the flax bio-composites.

The moisture relations of spray dried skimmed milk

The feruloylarabinoxylan deposition was initiated at the formation of the secondary cell wall, especially S2 layer in moso bamboo, which may affect crosslinking between cell wall components and plant growth. Hemicelluloses, major components of plant cell walls that are hydrogen bonded to cellulose and covalently bound to lignin, are crucial determinants of cell wall properties. Especially in commelinid monocotyledons, arabinoxylan is often esterified with ferulic acid, which is essential to crosslinking with cell wall components. However, the deposition patterns and localization of ferulic acid during cell wall formation remain unclear. In this study, developing moso bamboo (Phyllostachys pubescens) culms were used to elucidate deposition patterns of hemicelluloses including feruloylarabinoxylan. Ferulic acid content peaked with cessation of elongation growth, and thereafter decreased and remained stable as culm development proceeded. During primary cell wall (PCW) formation, xyloglucan and (1,3;1,4)-β-glucan signals were detected in all tissues. Along with culm development, arabinoxylan and feruloylarabinoxylan signals were sequentially observed in the protoxylem, vascular fibers and metaxylem, and parenchyma. Feruloylarabinoxylan signals were observed slightly later than arabinoxylan signals. Arabinoxylan signals were observed throughout the compound middle lamella and secondary cell wall (SCW), whereas the feruloylarabinoxylan signal was localized to the S2 layer of the SCW. These results indicate that the biosynthesis of hemicelluloses is regulated in accordance with cell wall layers. Feruloylarabinoxylan deposition may be initiated at the formation of SCW, especially S2 layer formation. Ferulic acid-mediated linkages of arabinoxylan-arabinoxylan and arabinoxylan-lignin would arise during SCW formation with the cessation of elongation growth.

Willows (Salix spp.) are important as a potential feedstock for bioenergy and biofuels. Previous work suggested that reaction wood (RW) formation could be a desirable trait for biofuel production in willows as it is associated with increased glucose yields, but willow RW has not been characterized for cell wall components. Fasciclin-like arabinogalactan (FLA) proteins are highly up-regulated in RW of poplars and are considered to be involved in cell adhesion and cellulose biosynthesis. COBRA genes are involved in anisotropic cell expansion by modulating the orientation of cellulose microfibril deposition. This study determined the temporal and spatial deposition of non-cellulosic polysaccharides in cell walls of the tension wood (TW) component of willow RW and compared it with opposite wood (OW) and normal wood (NW) using specific antibodies and confocal laser scanning microscopy and transmission electron microscopy. In addition, the expression patterns of an FLA gene (SxFLA12) and a COBRA-like gene (SxCOBL4) were compared using RNA in situ hybridization. Deposition of the non-cellulosic polysaccharides (1-4)-β-D-galactan, mannan and de-esterified homogalacturonan was found to be highly associated with TW, often with the G-layer itself. Of particular interest was that the G-layer itself can be highly enriched in (1-4)-β-D-galactan, especially in G-fibres where the G-layer is still thickening, which contrasts with previous studies in poplar. Only xylan showed a similar distribution in TW, OW, and NW, being restricted to the secondary cell wall layers. SxFLA12 and SxCOBL4 transcripts were specifically expressed in developing TW, confirming their importance. A model of polysaccharides distribution in developing willow G-fibre cells is presented.

Pistachio nut (Pistacia vera L.) is one of the popular tree nuts in the world. Proper selection of packaging materials is necessary to prevent absorption of moisture and aflatoxin formation which will influence the overall product quality and safety. This research is undertaken to study the effect of different type of flexible packaging films on the moisture and aflatoxin contents of whole pistachio nuts during storage at ambient temperature (22-28°C) and relative humidity of 85-100%. Five types of plastic films tested were low density polyethylene (LDPE) which serves as the control, food-grade polyvinyl chloride (PVC), nylon (LDPE/PA), polyamide/polypropylene (PA/PP) and polyethylene terephthalate (PET). The moisture content and aflatoxin content of pistachio nuts were measured using oven drying method and HPLC, respectively. Sample were analysed at 0, 2, 4, 6, 8 and 10months during the storage period. Results showed that there was an increase in moisture content with the increase in storage time of pistachio nuts. The increase in moisture content was associated with the aflatoxin level of pistachio nuts during storage time. All the packaging materials except LDPE delayed the moisture absorption and aflatoxin formation of the product. The most suitable packaging materials for maintaining the quality and safety of pistachio nuts is PET films followed by nylon, PA/PP and PVC. The shelf-life of pistachio can be extended from 2months (Control) to 5months when PET is used as the packaging material.

Abstract. Using the right equilibrium moisture content (EMC) relationship is critical for implementing successful aeration strategies and for determining the safe storage moisture content of grains and oilseeds. The oil content of sunflower seeds substantially affects the moisture equilibrium relationship, implying that a specific set of model parameters for each oil content range should be obtained. To overcome this practical limitation, the Enhanced Halsey model was developed incorporating a new parameter (D) to characterize the effect of oil content on the original Modified Halsey model. The constants A, B, C, and D of the model were obtained for a wide range of temperatures, moisture and oil contents. The simplicity of the Enhanced Halsey model and the possibility of adapting EMC as a function of oil content make the Enhanced Halsey model valuable for engineering applications (e.g., aeration controllers) and for predicting the safe storage moisture content of seeds with different oil contents, such as sunflower. Keywords: Composition, Grain, Isotherms, Oilseed, Relative humidity, Sorption, Storage quality.

The moisture content of the residue obtained by evaporating sea water is shown here to be determined by the relative humidity of the air in which it is subsequently kept, provided that the storage temperature does not vary: samples of such sea salt corrode mild steel in proportion to their moisture contents. It follows that the extent of corrosion of mild steel by sea salt is governed by the relative humidity of the ambient air and that keeping the moisture content of the salt‐complex at 8% or less substantially prevents corrosion from this cause.

Determination of seed moisture content (MC) is a destructive method according to the Department of Standards Malaysia and the International Seed Testing Association (ISTA) where the seeds were dried at 103±2℃ for 16±1 hrs in a convection oven to know the amount of water within seeds. The present work analysed the relationship between the weight and MC of Coffea liberica seeds so that the seeds with MC estimates could subsequently be applied as planting materials or for germplasm storage for future varietal improvement for the beverage sector. From the fully ripe fruits, single seeds, two seed units, three seed units, and four seed units were studied for suitability in developing a predictive model for MC. In each context, seeds of varying MCs for model building were obtained with air drying for 0, 2, 4, 6 and 8 days at the ambient temperature of 28±2℃ and relative humidity (RH) of 55±5% in the laboratory. Prior to desiccation treatments, the seed units were tagged and noted for fresh weight (FW), length and breadth. After desiccation treatments, weight after desiccation (DesW) was recorded and the actual MC of the seeds was determined with the oven-drying method. Plots of MC - desiccation period showed that ambient air-drying reduced the MC of seeds progressively and consistently. With the initial MC of 0.4 g H2O g-1 Wt on a wet weight (Wt) basis, air drying of the seeds in the laboratory for up to 8 days reduced their MC to 0.15 g H2O g-1 Wt. Subsequent correlation analysis revealed that MC had a positive relationship with DesW, but was negatively associated with (Loss of water), which is the difference between FW and DesW. Stepwise linear regression analysis implied that the MC of C. liberica seeds could best be estimated as MC = 0.185 + 0.118 FW – 0.412 (Loss of water) using two seed units. The model was statistically significant (P < 0.001) with appropriate R 2 and SEE values of 0.95 and 0.0187 g, respectively.

Moisture gradients in wood subjected to relative humidity and temperatures simulating indoor climate variations as found in museums and historic buildings

The performance of a solar energy-assisted seed storage room was evaluated through an ordinary 22-m3 room that was retrofitted with a solar collector, inlets and chimney. The structure was made of a solar collector to heat the ambient air before entering the chimney. The chimney circulated the air inside the structure and inlets. To compare the performance of the modified storage room, a room with a similar capacity and without the retrofitted components (control storage room) was used. Twelve 8 kg bags of maize were stored in each storage room for a period of three months. Samples were taken every two weeks to determine germination rate, moisture content and seed vigour. The temperature and relative humidity (RH) was measured during storage. The RH in the control storage was significantly higher (P≤0.05) (60.6 ± 5.87%) than in the modified storage (40.1 ± 3.21%) during the day. However, at night, the RH in the control storage room was significantly lower (P≤0.05) (58.5 ± 7.32%) than in the modified storage (63.7 ± 6.28%). The RH in the modified storage room increased from 40.1% during the day to 63.7% at night. The RH in the control storage room decreased slightly from 60.6% to 58.5% during the day and night. The seed moisture content in the modified storage facility was significantly lower (P≤0.05) (12.6 ± 0.21%) than in the control storage room (13.3 ± 0.52%). The moisture content in the modified storage room decreased from 12.6% to 12.4%, whereas in the control room, moisture content increased from 12.6% to 13.8% in three months. The seed germination rate obtained after three months of storage in the modified storage room was significantly higher (P≤0.05) (98.5 ± 0.85%) than in the control storage room (96.8 ± 1.49%).The seed vigour obtained in the modified storage room was significantly higher (93.6 ± 0.35%) than in the control room (91.7 ± 2.08%) (P≤0.05). Seed stored in the control storage lost vigour at a faster rate, compared to the seeds stored in the modified storage room. Therefore, the modified naturally-ventilated seed storage room maintained seed quality better than the control storage room.

Stingless bee honey from Heterotrigona itama exhibits a high moisture content (>27.0%) and therefore, is highly susceptible to undesirable microbial fermentation unless it is reduced to less than 20%. This study aims to reduce the moisture content of honey using a novel method based on passive diffusion and monitor the subsequent changes in several properties of honey. To achieve this, approximately 50 mL of freshly harvested honey samples was placed in clay pots (4 cm diameter × 6 cm height), covered with lids, and stored at temperatures of either 25 ± 1°C, 60% relative humidity (RH), or 35 ± 1°C, 25% RH. The results showed that the moisture content and Aw of honey after storage were significantly reduced (p ≤ .05) from 25.8 to 19.5% and from 0.79 to 0.70, respectively. A similar level of reduction was achieved at 25°C, although this process took 21 days and resulted in some losses of honey solids due to cross‐wall diffusion. The reduction in moisture content also resulted in a decrease in pH and free acidity of honey and an increase in viscosity. The data obtained in this study indicate the feasibility of using clay pots as an alternative method to reduce the moisture content in honey and protect it against fermentative deterioration.Practical ApplicationsStingless bee honey is highly popular in Malaysia and many other countries as it is high in carbohydrates and possesses many health benefits such as antioxidants and antimicrobial properties. However, the honey is susceptible to fermentation as its moisture content often exceeds the level required for microbial stability. Traditionally, the moisture content of stingless bee honey is reduced by drying the honey in shallow trays at elevated temperatures. However, this leads to contamination if the drying is performed under unhygienic conditions. Therefore, this study proposed the application of a passive diffusion method that serves as an effective alternative for moisture reduction in honey. It is anticipated that this method will not only reduce the moisture content but also retain most of its properties when the filtration process is performed under appropriate conditions.

A study of fleece moisture content showed that it was not evenly distributed over the body of the sheep. The lower mid side of the fleece showed the highest average moisture content, while positions such as mid shoulder, lower shoulder, flank, and belly showed moisture contents above the mean value. The mid side and mid hindquarter positions showed means approximately equal to the average for the entire fleece. Positions more exposed to the air, such as upper shoulder, upper side, and upper hindquarter, showed lower amounts of moisture. Change in relative humidity during shearing affects slightly the initial moisture content of the wool. High-yielding wool shows a higher affinity for moisture. A strong wool strain of sheep had a significantly higher moisture content than medium and fine wool strains run together in the one environment. Wools of different origin in New South Wales on the showfloor of a wool store exhibited marked and statistically significant differences in moisture content both between and within districts. The variations within districts are greater than the variations between districts. Wool packed in bales, although stored in Sydney for a period of 6 or more weeks, had not reached equilibrium with the coastal atmosphere. Differences as large as 7 per cent. were found on the showfloor. Evidence collected in this study suggests that expert wool valuers and buyers cannot or do not take into consideration moisture content differences which are as large as 6 per cent. Although existing practice in commercial yield estimation apparently does not affect the wool merchants, individual growers selling wool grown, or shorn, or both grown and shorn in drier atmospheric conditions may be losing substantially.