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Cell Walls in Wood Crack Like Concrete

Credit: PixieMe/Adobe

Credit: PixieMe/Adobe

By Adya Singh & Ramesh Chavan

The discovery that nanometre-scale cracks form in the cell walls of wood can be exploited to engineer high-performance floorboards from soft woods such as pine.

While taking a walk on a concreted footpath you may have noticed the presence of cracks of varying dimensions, ranging from those just visible to the unaided eye to some of much larger dimensions. If you take a closer look at the fine cracks in the footpath you will find that they follow a distinct, sinuous course along their length.

Our work on nanocracks in wood cell walls, visible only when viewed with a transmission electron microscope, suggests a similarity with cracks initiating in concrete. The cracks in both materials initiate and propagate along the interface between hard and soft components.

While much research on crack initiation and propagation has been carried out in concrete materials, little is understood about the nanometre-scale cracks in wood cell walls, which we think also develop at molecular interfaces.

Cracking of Concrete

Understanding crack propagation is an area of science in itself. Knowledge of the underlying principles of crack propagation can serve as a fundamental base for probing defects in construction materials such as concrete.

Concrete is a heterogenous material created from a mixture of gravel particles, cement and water. Thus, upon drying, concrete exhibits complex mechanical and physical behaviour.

Although hardened concrete possesses good compressive strength it is poor in tensile strength and thus susceptible to cracking, which can influence its mechanical properties and durability. It is therefore not surprising that much work has been carried out on the causes and mechanics of fracturing in concrete.

It has been suggested that cracks in concrete usually form at the interface between gravel (a hard material) and cement (a soft material). Our simple observation that the fine cracks follow a sinuous course (Fig. 1) supports the view that the properties at the gravel–cement interface have an impact on the initiation and course of cracking in concrete structures.

The sinuous nature of cracks in concrete intrigued us because a remarkable parallel exists with nanometre-scale cracks we have observed in the cell walls of radiata pine wood. We think these initiate and propagate along the interfaces of cell wall macromolecules with contrasting physical properties. Like the initiating cracks in concrete, we found that the initial cracks in radiata pine also follow a sinuous course.

Nanocracks in Cell Walls

Radiata pine consists of a complex three-dimensional arrangement of axial cells along the length of the tree and radial cells in a direction perpendicular to the length of the tree. These cells perform specific functions in the living tree.

Axial cells consist of an empty lumen enclosed by a cell wall, and perform water transport and support functions. We observed sinuous cracks about 10 nm wide in the walls of axial cells in dry radiata samples.

Cellulose, hemicellulose and lignin in the cell wall are organised in a complex three-dimensional fashion. Strong and stiff microfibrils of cellulose are embedded in a soft matrix of hemicellulose and lignin, and function as reinforcing elements – much like the steel rods used to reinforce cement. We propose that nanocracks represent the earliest stages of cell wall cracking, with cracks propagating along the interface between hard cellulose molecules and soft hemicellulose–lignin complexes.

The sinuous form of nanocracks in cell walls inform us about the macromolecular design of cell walls in wood, particularly the pattern of microfibril distribution in the radial direction (Fig. 2), which are arranged somewhat randomly rather than linearly. As a result, the initiating cracks follow a sinuous course, much like the fine cracks in the concrete.

Turning Wood into a High-Value Commodity

Environmental concerns will continue to drive greater use of wood, a natural renewable plant product of high strength, for building construction and other purposes. The discovery that nanometre-scale cracks can form at molecular interfaces in wood cell walls could find applications in high-performance wood- and fibre-based products with enhanced properties, such as stiffness, hardness, stability and durability.

An example would be to turn low- to moderate-performing radiata pine in New Zealand and Eucalyptus in Australia into a commodity that can effectively compete with tropical hardwood floors. Effective impregnation of these woods with monomeric and polymeric materials would be required for this purpose, but because intermolecular pores in commercially dried wood are extremely small (around 2 nm), the challenge has been to facilitate adequate and uniform impregnation of hardness-enhancing substances into cell walls without increasing the size of cell wall pores, which can compromise the strength of the wood.

Engineered nanocracks could serve as nanoscale channels that enable engineers to impregnate low-quality woods with substances that enhance desirable properties.

Adya Singh and Ramesh Chavan are Honorary Research Fellows at the University of Auckland.