Trees, and their derivative products, have been used by societies around the world for thousands of years. Contemporary construction of tall buildings from timber, in whole or in part, suggests a growing interest in the potential for building with wood at a scale not previously attainable. As wood is the only significant building material that is grown, we have a natural inclination that building in wood is good for the environment. But under what conditions is this really the case? The environmental benefits of using timber are not straightforward; although it is a natural product, a large amount of energy is used to dry and process it. Much of this can come from the biomass of the tree itself, but that requires investment in plant, which is not always possible in an industry that is widely distributed among many small producers. And what should we build with wood? Are skyscrapers in timber a good use of this natural resource, or are there other aspects of civil and structural engineering, or large-scale infrastructure, that would be a better use of wood? Here, we consider a holistic picture ranging in scale from the science of the cell wall to the engineering and global policies that could maximise forestry and timber construction as a boon to both people and the planet.

The wood from the trees: The use of timber in construction

Solar cell substrates require high optical transparency but also prefer high optical haze to increase the light scattering and consequently the absorption in the active materials. Unfortunately, there is a trade-off between these optical properties, which is exemplified by common trans- parent paper substrates exhibiting a transparency of about 90% yet a low optical haze (<20%). In this work, we introduce a novel transparent paper made of wood fibers that displays both ultrahigh optical transparency (∼96%) and ultrahigh haze (∼60%), thus delivering an optimal substrate design for solar cell devices. Compared to previously demonstrated nanopaper composed of wood-based cellulose nanofibers, our novel transparent paper has better dual performance in transmittance and haze but also is fabricated at a much lower cost. This high-performance, low-cost transparent paper is a potentially revolutionary material that may influence a new generation of environmentally friendly printed electronics.

https://terpconnect.umd.edu/~lit/publications/TengLi-Pub58-NL-2014.pdf

Novel Nanostructured Paper with Ultrahigh Transparency and Ultrahigh Haze for Solar Cells

Microfibril angle (MFA) is perhaps the easiest ultrastructural variable to measure for wood cell walls, and certainly the only such variable that has been measured on a large scale. Because cellulose is crystalline, the MFA of the S2 layer can be measured by X-ray diffraction. Automated X-ray scanning devices such as SilviScan have produced large datasets for a range of timber species using increment core samples. In conifers, microfibril angles are large in the juvenile wood and small in the mature wood. MFA is larger at the base of the tree for a given ring number from the pith, and decreases with height, increasing slightly at the top tree. In hardwoods, similar patterns occur, but with much less variation and much smaller microfibril angles in juvenile wood. MFA has significant heritability, but is also influenced by environmental factors as shown by its increased values in compression wood, decreased values in tension wood and, often, increased values following nutrient or water supplementation. Adjacent individual tracheids can show moderate differences in MFA that may be related to tracheid length, but not to lumen diameter or cell wall thickness. While there has been strong interest in the MFA of the S2 layer, which dominates the axial stiffness properties of tracheids and fibres, there has been little attention given to the microfibril angles of S1 and S3 layers, which may influence collapse resistance and other lateral properties. Such investigations have been limited by the much greater difficulty of measuring angles for these wall layers. MFA, in combination with basic density, shows a strong relationship to longitudinal modulus of elasticity, and to longitudinal shrinkage, which are the main reasons for interest in this cell wall property in conifers. In hardwoods, MFA is of more interest in relation to growth stress and shrinkage behaviour.

/pdf/microfibril-angle-measurement-variation-and-relationships-a-524gkdmy8w.pdf

Microfibril Angle: Measurement, Variation and Relationships – A Review

The cell wall of wood tracheids is made up of various layers, distinguished from one other by the alignment of the innumerable, fine crystalline cellulose microfibrils within each layer that helically wind about the cell lumen. Microfibrils themselves are embedded in a more compliant, water-reactive matrix of amorphous lignin and hemicelluloses. The average inclination of microfibrils relative to the axis of the cell affects axial rigidity and dimensional stability of wood which are the two most important properties of wood. High and variable microfibril angles can be found in juvenile and compression wood, thus resulting in variations in product performance of forest products. For instance, seemingly identical trees in a plantation can have moduli of elasticity that differ by a factor of two or more. This is why the future is often seen in engineered wood products, where wood may be chipped, fiberised and blended before being glued together again: the average property values are little changed, but the range—the variability—is greatly reduced. There is the opportunity for better wood allocation and processing of timber, if averaged values for individual log characteristics, such as average microfibril angle, can be identified before the processing. In parallel there is genetic potential to select trees with low average microfibril angles. Unfortunately, determination of the average microfibril angle is a time-consuming, laboratory-based task. Preferably, a non-destructive, simple, field-hardened method should be employed that reflects the average microfibril angle in a given piece of wood. For this reason, acoustic methods have been developed to measure the velocity of sound propagation directly related to the stiffness of wood and in turn is dependent on the ultrastructure of the tracheid cell wall. In the fundamental equation, Edynamic=ρV2, the acoustic modulus is derived from two components, density, ρ, and velocity of sound, V. The latter relates to the intrinsic wood quality and ultrastructure of the tracheid wall. It is shown that acoustic methods can sort and grade trees and logs according to their suitability for structural lumber and for a range of fiber properties of interest to papermakers. Thus, acoustic methods have applications in tree breeding, harvesting, and wood processing.

Cell wall structure and wood properties determined by acoustics—a selective review

Variations in acoustic velocity and density with age, and their interrelationships in radiata pine

clones with high modulus of elasticity (MOE). Various ways of measuring MOE are examined: traditional static bending, axial compression of bolts, existing sonic methods, as well as a tool specifically configured for resonance on short bolts. Wood characteristics and microfibril angle are measured on discs taken from each tree. Results show a good correlation between acoustic and static measurements of modulus of elasticity. Moreover, the selected seven radiata clones differ in stiffness by a factor of two, and much of this variation seems to relate to differences in microfibril angle. The results validate the assumption that there is potential to improve wood stiffness of radiata pine genetically. This work does not offer definitive solutions but explores a number of approaches that could be utilised as a selection tool in tree breeding for better product performance of radiata lumber. Here, development of methods based on acoustics is shown to assist in the necessary mass screening of clones for stiffness properties.

H. Lindström

Papers

Cell wall structure and wood properties determined by acoustics—a selective review

Methods for measuring stiffness of young trees