Ove Söderström

Crack formation due to weathering of radial and tangential sections of pine and spruce

Abstract The development of cracks and changes in appearance have been investigated on radial and tangential sections of pine (Pinus sylvestris L.) and spruce (Picea abies Karst.) after exposure outdoors for 61 months. The degradation of the sections has also been studied at the micro-level. The annual ring orientation was the most important factor affecting crack development on weathering. After 61 months of outdoor exposure, the tangential sections of spruce had 1.7–2.2 times greater mean total crack length per area unit than the corresponding radial sections. In pine, the total crack length per area unit on the tangential sections was 2.2–2.6 times greater than that on the radial sections. Tangential and radial sections show the same colour change as a result of weathering. Tangential sections have more and deeper cracks than radial surfaces. The cracks on the tangential sections occur frequently in both earlywood and latewood. On radial sections, cracks occur primarily at the annual ring borders, but to a certain extent also in the earlywood. Decomposition of the cell wall takes place in both radial and tangential cell walls, and cracks tend to follow the fibril orientation in the S2-layer of the cell wall. The radial cell wall of the earlywood has a large number of pits which are degraded at an early stage.

Moisture content profiles and uptake kinetics in wood cladding materials evaluated by a portable nuclear magnetic resonance spectrometer

Abstract This study evaluated the capability of nuclear magnetic resonance (NMR) technology based on small portable magnets for in situ studies of the local moisture content in wood. Low-field and low-resolution [1H]NMR with a unilateral permanent magnet was used to monitor and map the moisture content of wood cladding materials of various types in a spatially resolved manner. The results show that portable NMR equipment based on small open-access permanent magnets can be successfully used for non-invasive monitoring of the moisture content in various extended wood specimens. The moisture content was measured with a depth resolution of 0.2 mm and a maximum penetration depth of 3 mm. This makes the technique suitable for in situ local moisture content measurements beneath a coating layer in the cladding, for example, and it is also possible to relate the moisture level to specific properties of the wood material.

Sorption Equilibrium in Relation to the Spatial Distribution of Molecules - Application to Sorption of Water by Wood

An application of the general equilibrium condition given in an earlier paper, to experimental values of the equilibrium moisture content for wood gives information on the probable physical nature of sorption of water by wood. Physical sorption, coupled to swelling and shrinkage of the sorbent, is coupled together with a chemical sorption of water to the free OH-group, which cannot be treated separately. A number of structural arrangements in the paracrystalline part of the cell walls, a steric hindrance for swelling in the high moisture content range and the nature of the chemical sorption determine the isotherms. There are two different types of hysteresis in the physical and in the chemical sorption. In addition there is a statistical range for an isotherm, probably due to the swelling mechanism. The chemical sorption is considerably changed, if fibres are cut to a high extent in test samples. The enthalpy of the sorbent itself is changed by the chemical sorption, which must be considered by use of the Clausius-Clapeyron relation for calculating the heat of sorption.

Surface phenomena during drying of MDF

Sorption experiments with wood based material were performed under conditions similar to those in industrial wood drying kilns. The main purpose of these experiments was to demonstrate the influence of the surface interaction with the ambient air stream on the moisture flow. Part of this work has been published earlier (Soderstrom and Salin 1993; Samuelsson and Soderstrom 1994). The analysis was done with a simple mathematical method for a directly measurable quantity and constant transport coefficients. For industrial applications the results showed good correspondence between measured and calculated values.

Surface conditions and Fick's law

The use of Ficks law for the description of the moisture content has been questioned for years by several scientists. A modern contribution was recently published with an interesting approach (Nakano 1994a, b). Unfortunately, these articles contained some mistakes that were corrected by another author (Babiak 1995). Parallel to that discussion, another discussion about the choice of a proper potential for the moisture transport in a hygroscopic material like wood has taken place (Bramhall 1995). In this latter reference the partial vapor pressure in the air that is in equilibrium with the material, was used as the potential for the moisture transport and it was shown that this potential is a function of the wood state. It has also been shown that this choice of potential is unique, because all other potential will contain gradients of the state variables in the description of the moisture transport (Hunter 1993). The problem with the partial pressure is that it is not directly measurable in an ordinary sorption experiment, where the weight of the material easily can be measured as a function of the time. The mean moisture content of the board is the most important quantity in drying of wood in practice and therefore, for applications the used potential must be transformed to the moisture content somewhere in the analysis in order to he useful. Ficks law is empirical but has been very successful in descriptions of transport processes. The validity of Ficks law can of course be questioned, but then the observations must clearly and unambiguously show that the real problem is the assumption of the Ficks law. This problem and similar ones have been discussed earlier, but no one seems to have been able to give a proper suggestion (Kayihan 1993). A new approach has recently been published and that is focused on the surface conditions appearing in the boundary conditions for the differential equation of Ficks law (Babiak 1995). The surface moisture content is assumed to approach equilibrium as an exponential. Another similar approach would be to introduce a surface emission factor in analogy with heat transfer in heat transport theory. This was mentioned in Babiaks letter to the editor, but it was not developed (Babiak 1995). A surface emission factor is introduced in the one dimensional case through:

Sorption Equilibrium in Relation to the Spatial Distribution of Molecules – Application to Desorption of Potato

ABSTRACT An analysis of desarption isotherms for potato shows the following probable qualities in the water-potato sysarn. in the natural staa the walcr is bound rather strongly due to loss of rotational degrees of freedom in the sorbed molecules, which is caused by small molecular voids in the cellulose structure. This strucmre therefore is formcd as a uniformly cross bound state of the OH-groups. This slate prevails up to a cenin temperature, above which a reaction sas in. whcre OH bonds are successively braken, giving passibility for a panial rotation of sorbed molecules. in the range of lower moisture conlcnt the cellulose strucmre is changed into n crystalline slate a in wood, although the dcgrcc of crystallinity is lower, which is thc