Hardwood from Softwood
The Inside Story
Cross-section (click to enlarge)
Resin duct (click to enlarge)
Cambium
Electron microscope images of wood showing its cellular structure
(click to enlarge)
Wood is a wondrous material – its stability, load-bearing capacity and convenience make it the most efficient structural material known. These properties do not derive from the constituent materials, but rather from their arrangement. Wood is composed of parallel columns of long hollow cells (fibres) joined end to end, their walls forming the load-bearing shell of the cells and of the whole plant.
The cell walls in a living tree consist of fibrous sheets laid across each other, like plywood. A helical criss-cross arrangement of fibres in adjacent layers gives the assembly toughness and strength. Each layer is a composite of cellulose crystalline fibres in a resinous matrix of organic polymers – lignin, hemicelluloses and non-crystalline cellulose. The fibres confer strength, while the matrix gives the sheet lateral stiffness and transfers stress from fibre to fibre, improving toughness.
The toughness of wood, as calculated from the theory of fibres in a resin matrix, is only one tenth of that actually observed. Toughness is largely about managing the fracture process, which wood does to perfection. Cracks are encouraged to wander around within the material, without being allowed to go right through it, more or less harmlessly using up the available strain energy.
A dry piece of wood consists of a rigid foam, an empty lattice of cell walls. The thickness of the walls determines the density of the wood and so its mechanical properties: balsa wood has thin cell walls, while the cells walls of lignum vitae, one of the densest woods, are so thick they almost occlude the cell cavity.
The solution
When FR scientists first thought about hardening pine, the available technique involved impregnating wood with chemicals using vinyl polymerisation chemistry. The result was a high-density product, so hard it was impossible to work using conventional techniques and plant. FR scientists reasoned that their product had to be workable with ordinary equipment. And if it was to succeed commercially, the process had to be simple and economical, making use of existing wood-treatment plant, such as pressure vessels and drying kilns.
Furthermore, they sought to replace the synthetic treatment with something more biologically-based and less toxic. Capitalising on their knowledge of plant cell-wall chemistry, they developed an elegant solution that essentially involves pouring wood into wood.
COP Outcome development and evaluation
By combining a hemicellulose analog from starch (maltodextrin) with a lignin analog, they produced an artificial ligno-hemicellulose polymer, which binds to the cell walls, thickening them. Precursors of the artificial polymer are impregnated into wood before kiln-drying, during which polymerisation of the impregnated material leaves the pine harder and denser, but still readily workable. The degree of hardness can be controlled by adjusting the strength of the formulation. This irreversible process was termed Indurite, after the Latin indurare, "to harden".
