In the SIRT project (Scottish Integrated Research on Timber) we are working with building engineers at the Centre for Timber Engineering, Napier University and foresters at the Northern Research Station of Forest Research, building up a detailed quantitative picture of how the engineering properties of Sitka spruce derive from its composition and structure.
Wood mechanical properties.
It is coming to light that biomaterials in general have mechanical properties different from anything that can be described within classical materials science. We have been working with the ‘Molecular Velcro’ model originating from the Fratzl group at Potsdam, which predicts the load-deformation curves of some kinds of wood rather well but is not consistent with what we know about wood structure at the nanometer scale. We have a modified version of this model based on hemicellulose bridging between cellulose microfibrils, which accords better with our NMR investigation of the structures involved.
Cellulose microfibril structure in wood.
This is part of our fundamental programme of cellulose research, and we are using Sitka spruce microfibrils as a model for wood microfibrils in general. We are narrowing down the best estimates for microfibril diameter, currently just under 3 nm, and exploring how the surface chains relate to the Ialpha or Ibeta crystalline lattice that forms the core of the microfibril. A new venture is to use FTIR bandshifts to determine how the internal structure of the micofibril deforms at the molecular level under load. Apart from its intrinsic interst this will have one very practical benefit – to model the mechanical performance of wood we need to know the tensile modulus of cellulose, and although this is known theoretically for highly crystalline celluloses we have very little idea what is the tensile modulus of ‘real’ celluloses such as that of wood.
Detection and function of compression wood.
Compression wood is formed by conifers on the lower side of the branches as they grow heavier, or on the lower side of the trunk when it needs to be returned to vertical after being tilted. Its formation then expands that part of the tree in length. This is a remarkable example of active morphogenesis by a secondary meristem: that is, parts of the tree that have ceased to grow in length by the conventional mechanisms used by plants seem to succeed in growing a little by a completely different mechanism. How it works is a mystery, but from the point of view of sawmillers and papermakers compression wood is a disaster. It disintegrates unpredictably under stress, and it makes paper lumpy. We are focusing on the presence of beta-(1,4)-D-galactans in compression wood. These polysaccharides are not normally detectable in wood at all – they are typical of potato and onion cell walls. But immunofluorescent labelling of the galactans identifies even tiny rings of compression wood just a few cells thick, representing a day or two of wood formation as might happen when the tree was bent by a prolonged gale. We are working on some hypotheses as to the role of the galactan in the function of compression wood.
Tree-ring analysis by X-ray densitometry
The annual rings of trees are formed through differences in the density of the wood formed at different times in the growing season. There is also a tendency for wood formed in older annual rings of a conifer tree to be more dense than the younger wood. This variation in density affects the mechanical performance of the timber that is cut from the tree. It is also a continuous record of the history of the tree and the weather to which it has been exposed at every point in its lifetime. We are using an X-ray densitometer system to look at genetic variation in the density of Sitka spruce timber and the resulting variation in its mechanical strength. In a separate project, we are using the same methods to identify influences of weather and other environmental factors on wood formation in Sitka spruce.
‘Extractives’ is the generic name for soluble components of wood. They include a great variety of chemical species, some of which cause problems during paper manufacture while others are of value in protecting timber against fungal attack. We are using FTIR and chromatographic methods to classify extractives and their variation in Scottish Sitka spruce, a timber much in demand for pulp manufacture due to its low extractives content and light colour.