Prof. Petri P. Kärenlampi
Damage-Free Wood Drying
Timber drying in general induces moisture gradients within sawn goods, such
moisture gradients inducing gradients in shrinkage, resulting as internal
stresses. As a consequence of such internal stresses, or corresponding
mechanical strains, checks, cracks or shape distortions may appear.
It has recently been shown that in addition to visible checks and cracks, significant invisible damage happens in wood materials in the course of drying [1, 2]. The effect of drying damage on mechanical behavior of thin wood specimens is shown in Fig. 1 [2]. Moisture gradients are not necessary for the creation of such damage within the cell wall. Such damage is believed to be due to the anisotropic drying shrinkage of cell wall layers, inducing significant internal stresses within the cell walls [3, 4]. Such internal stresses obviously may well be large enough to damage the cell walls. In fact, it has been shown that impregnation with a chemical which reduces drying shrinkage may more than double the strength of dried wood [2, Fig. 1].
Figure 1. The effect of drying damage on mechanical behavior of thin wood
specimens. In this experiment, a reference specimen has been created using a
chemical impregnation which prevents drying damage [2]. Prevention of drying
damage has more than doubled the strength of the dried specimen.
It has been recently shown through electron microscopy that drying significantly changes the cell wall structure. Drying appears to close cavities of some nanometers of size, particularly in the S2-layer of the cell walls [5, 6]. On the other hand, new cavities appear to become created in the S3-layer of the cell walls, possibly also in the S1-layer [5, 6].
Changes in cell wall structure have been observed also using Differential Scanning Calorimetry. Pore size distribution within cell walls significantly changes in the course of drying and rewetting [7]. Closure of pores of diameter more than 50 nm in earlywood perhaps is not surprising; such a phenomenon obviously is related to cavities being related to the appearance of pits between fibers [8, 9]. The increment of non-freezing water content in earlywood may well be related to the new cavities becoming created in the S3-layer of the cell walls, possibly also in the S1-layer [5, 6].
It is widely believed that the only way of preventing the drying damage is to exchange water molecules to some non-swelling chemical like glycerol prior to drying [10, p. 16], this kind of a treatment being able to increase strength more than two-fold. Such impregnation however is troublesome in practice [11].
We believe drying damage in cell walls can be prevented without impregnation with chemicals. This obviously is possible if it is possible to avoid the fulfillment of some criteria governing the creation of mechanical damage in the wood cell walls. This kind of avoidance of damage obviously requires inconventional drying circumstances. Such inconventional circumstances may well be worth the effort in case the strength of the dried product is doubled.
Experimental facilities have been established as illustrated below. We will be happy to tell you more if you give us a lot of money.
1. Thuvander, F.
and Berglund, L. A., A multiple fracture test for strain to failure distribution
in wood. Wood Sci. Tech. 32(3):227-235 (1998).
2. Thuvander, F., Wallström, L., Berglund, L. A. and Lindberg, K. A. H., Effects
of an impregnation procedure for prevention of wood cell wall damage due to
drying. Wood Sci. Tech. 34(6):473-480 (2001).
3. Van den
Akker, J. A.: Some theoretical considerations on the mechanical properties of
fibrous structures. In: Bolam, F.(ed.), Formation and structure of paper Vol I.
Transactions of the symposium held at Oxford, September 1961. Technical Section
of the British Paper and Board Makers' Association, London 1962, pp. 205-241.
4. Thuvander, F., Kifetew, G., and Berglund, L. A., Modeling of cell wall drying
stresses in wood. Wood Sci. Tech. 36(3):241-254 (2002).
5.
Wallström, L., and Lindberg, K. A. H., Distribution of added chemicals in the
cell of high temperature dried and green wood of swedish pine, Pinus sylvestris.
Wood Sci. Tech. 34(4):327-336 (2000).
6. Wallström, L., and Lindberg, K. A. H., The diffusion, size and location of
added silver grains in the cell walls of Swedish pine, Pinus sylvestris. Wood
Sci. Tech. 34(5):403-415 (2000).
7. Tynjälä, P. and
Kärenlampi, P. P.: Cell wall porosity in Spruce wood - variation within annual
ring and drying response. Submitted for publication (10/2002).
8. Koran, Z., Intertracheid pitting in radial walls of black spruce tracheids.
Wood Sci. 7(2):111-115 (1974).
9. Sirviö, J. and Kärenlampi, P.: Pits as Natural Irregularities in Softwood
Fibers. Wood and Fiber Science 30(1):27-39 (1998).
10.
Thuvander, F., Fracture behaviour of green wood (Pinus Sylvestris L.). A
composite materials approach. Luleå University of Technology, Department of
Materials and Manufacturing Engineering, Doctoral Thesis 1998:34.
11.
Moriya, K., Method for drying wood and method for subjecting wood to
impregnative treatment. Patent US5970624, Oct. 26, 1999.