Wood Moisture — Induced Swelling At The Cellular Scale

Wood continues to be widely used as a resource to meet humankind’s material needs. However, for wood to meet its full potential, researchers must overcome the challenge of understanding its fundamental moisture-related properties across its many levels of hierarchy spanning from the molecular scale up to the bulk wood level. By Xavier Arzola-Villegas and Roderic Lakes, University of Wisconsin–Madison, and Nayomi Z. Plaza and Joseph E. Jakes, USDA Forest Service


Wood remains a widely used material. Among its primary uses, wood is the principal resource used in the manufacture of paper and pulp. Wood possesses a strength performance index and structural stiffness per weight similar or superior to that for the best engineering composites when considered in bending, which makes it a favourable structural material. 

The interaction of wood with water is a major concern when it comes to wood and wood-based materials. The absorption of moisture causes dimensional instabilities and swelling forces in the wood that may cause splits. 

Additionally, moisture accumulation in wood is required for the growth and proliferation of fungi, which leads to wood degradation by wood-decay fungi. Excessive moisture accumulation can even lead to corrosion of fasteners in wood–metal connections used in constructions, especially in wood treated with copper-based preservatives.

Despite the well-established observations of swelling and shrinkage at the bulk level, many of the proposed explanations for the anisotropy in the transverse plane and density effects are not satisfactory.

Excessive moisture accumulation can even lead to corrosion of fasteners in wood–metal connections used in constructions, especially in wood treated with copper-based preservatives. 

Both wood decay and fastener corrosion can be prevented if wood is kept at moisture contents below about 15 percent. However, despite these negative effects, the interactions between wood and moisture can give inspiring ideas. For example, moisture changes in a bundle of wood cells cause a moisture-activated torsional behaviour. 

Wood cell bundles are capable of generating a specific torque higher than that of commercial motorsand carbon nanotubes yarn. Moreover, wood cell bundles possess moisture-activated shape memory twist capabilities. 

Thermally-induced shape memory effects are also observed in wood veneers.

A better understanding of wood moisture-induced swelling will provide insights into solving issues related to influences that cause susceptibility to degradation of wood products as well as inspiration for the development of advanced wood-based materials and bioinspired materials. 

Despite the well-established observations of swelling and shrinkage at the bulk level, many of the proposed explanations for the anisotropy in the transverse plane and density effects are not satisfactory. 

Although the swelling of bulk wood originates in the smaller scale levels of structure, the understanding of the complex swelling in these smaller structures is still incomplete. 

This review aims to advance the understanding of moisture-induced swelling and contraction in wood and provide ideas for future research.


Wood Structure

Wood is an anisotropic cellular material with a hierarchical structure spanning from molecular-scale up to tubular cells, which are cemented together to form the wood in a tree. To describe the wood structure is necessary to contemplate wood’s different components across different length scales.

Trees are divided into two main sub-categories, softwoods and hardwoods. Softwood is wood that comes from gymnosperm plants, which germinate from seeds that are not enclosed in an ovule, typically conifers with needle-type leaves. 

Hardwood is wood that comes from angiosperm plants, whose seed is surrounded by an ovule, mostly flowering and broadleaved trees. Besides the seeds from which they are derived, the most important distinction between softwoods and hardwoods is found in their cellular structure.


Cellular Structure Of Wood

The detailed cellular structure of wood has been shown on the following picture:

The softwood cellular structure is composed mainly of axial tracheids and ray cells. In a cross-sectional view, the tracheids appear as rectangular cells with 3–10 μm thick walls that are thicker and thinner in the latewood and earlywood, respectively.

The ray cells appear as rectangular prisms, typically 15 μm high by 10 μm wide and 120–250 μm long in the radial direction from the pith to the bark. The tracheids are elongated tubular cells (1 mm to 10 mm long on average depending on wood species) aligned along the longitudinal direction of the tree trunk. 

Their hollow interiors are called lumina. The tracheids, which compose up to 90 percent of the volume of the wood, are the most important cells in terms of the mechanical support and water transport in softwoods. 

The water-transport system between tracheids consists of cavities in the cell walls known as pits. The tracheids end overlap with each other and are interconnected by pairs of pits that allow water to flow from cell to cell. The ray cells are aligned along the radial direction, and their primary function is the synthesis, storage, and lateral conduction of biochemical products.

In hardwoods, three important types of cells are found: vessels, fibres, and ray cells. The vessels are long hollow tubes composed of stacks of specialised cells called vessel elements. 

Vessel’s primary function is the conduction of water, which flows through end to end vessel connections called perforations. Vessels have 100–1200 μm of length and appear as large openings in the transverse plane of wood with the diameter ranging from 50–200 μm. The fibres are 2–10 times longer than vessel elements. 

Hardwood fibres (composing around 24 percent of the volume of the wood) are similar to softwood tracheids, but because they only function as mechanical support, they have smaller lumina. The ray cells in hardwoods are much more diverse in terms of structure than in softwoods, but function in a similar manner.


Wood–Water Relations

Wood, like many other biological materials, is a hygroscopic material. Wood is constantly exchanging moisture with the ambient air based upon the temperature and relative humidity (RH) of the surrounding environment. 

Moisture in wood can be categorised as being either free or bound water. Free water is defined as water present in the macroscopic voids of wood, such as lumina, and can be in a vapor, liquid, or solid state. Bound water is water absorbed into the wood polymers. 

Below fibre saturation (which depending on the definition is typically 30 percent MC) moisture exists as water vapor in the macroscopic void spaces in the wood cellular structure and bound water. 

Changes in the amount of bound water cause wood dimensional instabilities (swelling and shrinking). The total amount of bound water depends on the ambient temperature and RH, and because of hysteresis whether the water is being absorbed or desorbed. 

It should also be noted that water sorption is a time-dependent process, and the amount of bound water will also depend on the conditioning time and size of the piece of wood being conditioned. At constant temperature and RH, wood will eventually reach a constant MC known as equilibrium moisture content (EMC). 

At a given temperature, the relationship between the EMC and RH is described by a moisture sorption isotherm. For each RH value, a sorption isotherm indicates the corresponding EMC. The EMC, as a function of RH, has a hysteretic behaviour. 

Measurements of MC in equilibrium with a given RH and temperature are typically higher when reached by desorption than by absorption. 

In practice, wood likely does not reach a completely steady-state EMC value, and when EMC values are reported, they would depend on the criteria of the experimenter to define EMC. 

Because swelling and shrinkage in wood are caused by changes in the amount of bound water, changes in the amount of free water do not cause dimensional instabilities.


Wood Moisture-Induced Swelling And Shrinking

Wood is a cellular structure with polymeric cell walls. Depending on conditions, free water can be present as ice, liquid water, or water vapor in the pores of the cellular structure. 

Water also exists as bound water, which is absorbed water held by inter-molecular attractions in the accessible wood polymers inside of the cell walls. All wood polymers are accessible except for the highly-ordered cellulose chains in the interior of the elementary fibrils.

In amorphous polymers, absorbed water is proposed to exist in different states classified by its local molecular environment. 

Water in the amorphous wood polymers is expected to exist in similar states as in other amorphous polymers. With the large number of hydroxyl groups on wood polymers, hydrogen-bonded water is expected to be especially prevalent. 

The amount of bound water in wood, and therefore the extent of swelling is controlled by thermodynamics. A steady-state amount of bound water is reached when the bound water chemical potential is in equilibrium with the free water chemical potential. Consequentially, changes in free water chemical potential result in changes in the amount of bound water. 

Much remains to be resolved about the molecular-scale sorption and swelling mechanism in wood. The molecular-scale processes have mostly been accessible by computation methods, such as molecular dynamics (MD) simulations, that typically investigate idealised wood polymers.

As mentioned before, crystalline cellulose does not absorb water molecules within its inner structure. However, synchrotron X-ray diffraction measurements have shown that the cellulose crystalline lattice is deformed with changes in wood’s MC. 

Thus, X-ray diffraction was used to measure the deformations in the cellulose crystals of the elementary fibrils and investigated the mechanical interactions between the cellulose elementary fibrils and the matrix substance in wood cell walls.

Given that the crystalline cellulose barely interacts with water molecules, they suggested that the matrix substance filling in the gaps between the cellulose elementary fibrils causes the cellulose elementary fibril to expand transversely during water desorption.

Besides, systematic studies are still need that can help elucidate how the swelling pressures are affected by the constraints present in the cell wall across length scales ranging from microfibrils in the nanostructure to the effect of neighbouring cells in the cellular structure. 

Systematic studies measuring moisture-induced elementary fibril lattice strains in different size specimens from the same piece of wood would aid in elucidating these constraint effects on elementary fibril deformations. 

Consequentially, if the elementary fibril deformations are directly related to swelling pressures, then the systematic studies would also provide information about the relationships between multi-scale constraint effects and swelling pressures. 

Furthermore, systematic studies are needed that probe the combined effects of temperature, moisture, and even type of wood (i.e., latewood vs. earlywood, tension vs. compression wood, softwood vs. hardwood) on the crystalline lattice structure. 

For example, variations in the source of wood will have an impact on the overall cellulose crystallinity of the samples, and thus the total swelling strains and pressures observed will likely be affected, yet experiments that address this issue are lacking. 

Last but not least, microscopy studies that can also measure the elementary fibril diameter might provide valuable insights in terms of the effects of sample preparation and variability in these measurements, particularly if they are combined with diffraction.

A majority of the studies at this length scale have relied on two-dimensional imaging capabilities to map the changes occurring in the cellular structure and lumen areas. 

Studies that can probe the unmodified earlywood and latewood inside of the three-dimensional bulk wood without changing the native cellular structure are needed to determine whether or not the lumen area changes during swelling. 

Such measurements could be made by analysing three-dimensional volumes of cellular structures obtained by XCT with in situ humidity chambers. 

It would also be valuable to compare results from the same wood using the X-ray tomography experiments and the more conventional two-dimensional studies on sections to better understand constraint effects. 

Another valuable piece of information that is often lacking in the literature is a more quantitative analysis of the structural features present in the samples studied. 

For example, the role of ray cells could be further elucidated if their spatial distribution was quantitatively correlated to the dimensional changes observed during the swelling of the cellular structure. 

Likewise, experiments that can allow determining the role of neighboring cells on the overall hygroscopic swelling coefficient would be valuable, particularly if it is also correlated to the samples’ MC and swelling pressure.


Summary And Future Research Directions

Wood is a complex hierarchical material, whose moisture-induced swelling must be understood and investigated across different length scales. 

Studies focusing on the swelling of the cellular structure will inevitably be affected by the smaller length scale features and vice-versa. 

Furthermore, the behaviour observed at larger length scales might not be representative of the smaller length scales. For instance, at the molecular level, 5 percent–20 percent MC swelling of the hydrophilic polymers is approximately linear with the increase in MC. 

However, crystalline cellulose in the elementary fibrils is mostly inaccessible to water, and as a result, it is transversely compressed upon moisture uptake. 

At the microfibril level, the spacing between the elementary fibrils increases with MC, leading to an overall increase in the microfibril cross-sectional area. 

The swelling and, consequently, the swelling of the cell wall can be constrained by the presence of the cell wall concentric layers and even neighbouring cells. 

Due to the complex structure of wood, swelling is anisotropic across all length scales. Longitudinal swelling is typically restrained by the microfibrils, and thus, deformations along this axis are minimal across all length scales. 

At the cell wall level, swelling in the transverse plane along the perpendicular direction to the cell wall is the largest. Whereas at the bulk scale, wood swells most in the tangential direction. 

Differences between the radial and tangential swelling have mostly been attributed to the arrangement of the cells, but it is likely that the cell wall ultrastructural features also play a role. 

Interestingly, the swelling of the helically embedded microfibrils leads to moisture-induced twisting in bundles of wood cells consisting of only a few wood cells. This behaviour has inspired the development of new materials and could be used in the future in accelerated moisture-durability tests.

Future research aiming to improve our understanding of swelling in wood across length scales and its implications would benefit from a more holistic approach.

Multi-scale studies and integration of the data across length scales will be necessary to develop predictive models that can conclusively explain the role of the cell wall ultrastructural features in the anisotropic swelling of the cellular structure. 

The use of XCT shows promise to further elucidate the cellular swelling, although future studies would benefit from improved models that can take into account the hygroscopic and complex polymer chemistry of the cell walls. Such models could help correlate changes in the dimensions of the lumina to the wood density and cell wall properties. 

The effects of sample preparation, as well as the role of mechanical constraints like the number of neighbouring cells on the measured swelling, should also be addressed. 

Not taking into account these factors could lead to overlooking artefacts that could compromise the accuracy of the swelling measurements, particularly at the cell wall level and below. 

Moreover, future research should also provide insights into the generation and dissipation of moisture-induced swelling pressures and their implications on the swelling across length scales. For example, we need experiments that can determine the role of these pressures on the molecular-scale wood–water interactions such as the deformation of the crystalline cellulose structure, the formation of water clusters, and overall swelling of the hydrophilic polymers. 

Likewise, swelling pressures might also affect the hydromechanical properties of the cell wall, yet experiments and computational studies that can provide insights on this topic are needed. 

Finally, integrating our knowledge of the cellular swelling ab intra will be valuable in establishing models that can predict the behaviour of the bulk wood as a function of environmental conditions, thus, unlocking the potential of wood beyond its role as a construction material.

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