From Treetops to Engineered Wood
Wood is a fascinating material with a complex hierarchical structure. New forms of wood such as densified wood and mass timber have been surfacing in recent years that bring this material to new heights.
Wood has been a valued material in construction for centuries. We build our homes and furniture using this naturally available resource, its presence often evoking feelings of nostalgia and comfort. As versatile and lightweight as it is, wood has never been seen as a high-performance material since it’s relatively soft compared to metals. Recently however, there has been a growing interest in new processing methods that may elevate the performance of wood even further.
One new form is densified or compressed wood, often referred to as super wood. Researchers at the University of Maryland have found that a simple process of chemically treating and compressing wood increases its properties by staggering proportions: an increase in strength and toughness by over ten-fold (Song, 2018). This exciting wood derivative has demonstrated a higher strength-to-weight ratio than many metals like steel (Song, 2018). But what exactly is densified wood and what about its microstructure causes the properties to differ so drastically from natural wood?
Let’s first take a closer look at wood.
Natural wood has a porous microstructure, with the majority of it composed of long tube-like cells. The cells themselves are composed mainly of three biopolymers: cellulose, hemicellulose, and lignin (Chen, 2020). Cellulose is a stiff polymer that provides mechanical strength by wrapping around each of the long wood cells to create a rigid cell-wall. Lignin and hemicellulose are softer polymers that act as fillers to provide flexibility and energy absorption to the system (Fratzle, 2007).
These three naturally-occurring polymers make up a hierarchical structure or a structure with components of various size scales. Think of a twisting rope of various strands rather than a solid tube of a single material. The interplay and bonding between different types of materials at the nanoscale creates an extremely strong bulk system that is less likely to fail from a flaw compared to a monolithic material. Scientists believe that this size scale hierarchy is an important explanation for the high strength of many natural materials. That’s why scientists have long studied different materials such as bones, shells and plants (Fratzle, 2007).
So, wood found in nature exhibits a complex structure and humans have mastered the engineering and art of building with this material. However, when it comes to high-performance applications such as high-rise buildings, natural wood does not compete with manufactured products like steel beams or concrete (Chen, 2020). Wood isn’t just comparatively weak, it can also catch fire or rot. In many cases, these dilemmas are caused by the porous nature of wood.
How can densification overcome the weaknesses of wood?
A processing method of modifying wood is through a simple densification process. First, the wood undergoes a chemical treatment in ammonia to partially remove lignin and hemicellulose, the compliant biopolymers mentioned previously. The chemical treatment is similar to some steps in turning wood pulp paper (Perkins, 2018). Next, the planks of majority cellulose are squeezed slowly under moderate heat to flatten the pores and thus hardening the wood.
The resulting product of flattened, nano-layered cellulose exhibits high strength without becoming too brittle (Song, 2018). This is a promising result because often there is a tradeoff: when a material becomes stronger it also loses flexibility and can fracture suddenly. Densified wood still shows over an eight-fold increase in impact resistance. This may be due to the remaining lignin that acts as a binder for the cellulose fibers (Chen, 2020). From other experiments, the densified wood exhibited high moisture and fire resistance. The lack of pores means there are no channels for water to wick through or to encapsulate oxygen that would help propagate a flame (Chen, 2020).
Although the densification process is still being developed, researchers believe this process could be scalable to larger, more practical applications. In the meantime, there is another wood product of rising interest that is actually being used in existing large-scale buildings.
Mass Timber: Engineered wood used in Architecture
From the perspective of an architect or designer, the opportunity to use more exposed wood in structures without sacrificing functionality is desirable. Over the past decade, there have been some architecture projects in the US that heavily involved certain engineered wood products, referred to as mass timber. Projects include Carbon 12, an 8-story mass timber building in Portland, Oregon and the 18 story Mjøstårnet in Norway (Roberts, 2020).
Mass timber can mean different types of engineered wood, but the most promising type has been cross-laminated timber or CLT (Ahmed, 2020). Flat planks of soft wood like pine or beech are glued together in stacks with each layer alternating in the grain direction of wood. CLT was invented by researcher Gerhard Schickhofer in Austria in 1994 where there is an abundance of soft wood species (Roberts, 2020). The sturdiness of mass timber allows wood to be the primary load-bearing element in large-scale buildings. For example, earthquake tests show that mass timber structures will perform well enough to meet construction standards (Roberts, 2020).
Of course, there are many issues to the adoption of mass timber and perhaps other engineered wood products like densified. Lack of regulation, uncertainty of mass timber sturdiness over long periods of time, and already-depleted forests, to name a few. However, the environmental benefits of replacing steel or concrete parts with wood have given people a significant motivation to overcome these concerns. Researchers and officials hope that with an increased incentive for using wood, there will be funds to support better forest management (Roberts, 2020).
Works Cited
Song, J., Chen, C., Zhu, S., Zhu, M., Dai, J., Ray, U., ... & Hu, L. (2018). Processing bulk natural wood into a high-performance structural material. Nature, 554(7691), 224-228. https://www.nature.com/articles/nature25476/f
Fratzl, P., & Weinkamer, R. (2007). Nature’s hierarchical materials. Progress in materials Science, 52(8), 1263-1334. https://www.sciencedirect.com/science/article/pii/S007964250700045X
Perkins, S. (2018). Stronger Than Steel, Able to Stop a Speeding Bullet - It’s Super Wood! https://www.scientificamerican.com/article/stronger-than-steel-able-to-stop-a-speeding-bullet-mdash-it-rsquo-s-super-wood/
Chen, C., Kuang, Y., Zhu, S., Burgert, I., Keplinger, T., Gong, A., ... & Hu, L. (2020). Structure–property–function relationships of natural and engineered wood. Nature Reviews Materials, 5(9), 642-666. https://doi.org/10.1038/s41578-020-0195-z
Roberts, D. (2020). The hottest new thing in sustainable buildings is, uh, wood. Vox. https://www.vox.com/energy-and-environment/2020/1/15/21058051/climate-change-building-materials-mass-timber-cross-laminated-clt
Ahmed, S., & Arocho, I. (2020). Mass timber building material in the US construction industry: Determining the existing awareness level, construction-related challenges, and recommendations to increase its current acceptance level. Cleaner Engineering and Technology, 1, 100007. https://www.sciencedirect.com/science/article/pii/S2666790820300070
Reference Images
This set of figures from Fratzle and Weinkamer [2] shows the wrapping of cellulose forming a single wood cell, as well as different views of the hollow cellular structure in real life.
This set of figures from Song et al. [1] compares the microstructure and specific strength of densified wood to natural wood.
