Modelling of the behaviour of free water in sapwood during drying

Abstract

A stochastic three-dimensional model for the behaviour of free water in wood, which was described in Part I, is used for the simulation and analysis of some further features of wood drying. The kiln brown stain mechanism is studied and the connection to the condition of the wood surface is pointed out. Several drying rate questions and related aspects are analysed and compared with experimental results. Finally the risk of surface checking in an early stage of sapwood drying is shown theoretically.     

Comparison of moisture distribution along radial direction in a log cross section of heartwood and mixed sapwood and heartwood during radio-frequency/vacuum drying

The purpose of this study was to compare the distribution of moisture contents (MCs) along the radial direction during radio-frequency/vacuum (RF/V) drying of log cross sections of heartwood (HLC) where sapwood was removed from a green log cross section and log cross section of mixed sapwood and heartwood (MLC) prepared with debarked logs of Japanese larch (Larix leptolepis) and locust (Robinia pseudoacacia). For Japanese larch, an even distribution of MC was observed over the entire cross section in HLC not only at the initial stage of drying but also up to the completion of drying. Furthermore, the moisture gradient between the outermost slice and the adjacent inner slice was more gradual compared with that in MLC. For locust, the moisture gradient between the outermost slice and the adjacent inner slice became severe as drying progressed. It decreased after reaching the maximum during the middle stage of drying but continued until the late stage of drying. Furthermore, despite the fact that the average initial MC of mixed slice within MLC was higher compared with that in heartwood slice, this trend reversed immediately after drying started. It was suggested that the possibility of formation of border checking would be high during drying the MLC, since it would be so complicate that the sapwood and heartwood reach fiber saturation point together because of differences in their green MCs and permeability between them.     

Increase in the content of nitrogenous compounds at lumber surfaces during drying and possible biological effects

Summary Nitrogen contents have been determined at different depths from the surface of dried pine (Pinus sylvestris) and spruce (Picea abies) lumber. The effects of factors such as time of felling, storage of the timber, and drying process for the lumber, have been studied. Part of the selected lumber was characterized by surfaces which were yellowish after drying. At such surfaces, to a depth of about 2 mm, a high accumulation of nitrogen was always found. Yellowing is enhanced in lumber from wet-stored timber but also occurs in other lumber. Some possible contributive factors are suggested. More research in this field is proposed. The nitrogen gradients in outer sapwood without a yellow surface and in inner sapwood and in heartwood were much weaker. The effect which enrichment of nitrogenous compounds at surfaces may have on timber with regard to its disposition towards moulding is discussed. Although attention is drawn to the fact that strong nutrient gradients may occur, it must be emphasised that in most lumber nutrient gradients are weak and probably without practical consequence for its susceptibility towards fungal attack.     

Unexpected experimental results for capillary suction in wood: Analysis on the fibre level

Abstract

A series of simple experiments on capillary water uptake in rectangular sticks of Scots Pine (Pinus sylvestris L.) gave some unexpected results. Sticks of sapwood and heartwood with lengths of 100, 200 and 300 mm, with the four long sides sealed, were used in the experiments. The samples were allowed to suck water, one group for 1000 h and one group for 1900 h. After exposure, as anticipated, the heartwood samples had a moisture content (MC) distribution with high values at the bottom and steeply decreasing values towards the top. The sapwood samples, in contrast, had moisture distributions with high MC at the bottom and the top, and low MC in the middle. Additional experiments to study flow paths and timescales of the process were performed using dyed water. A Fickian approach is discussed in some detail. It is clear that the process represented by the experimental results cannot be explained within a Fickian framework. The analysis has to be done on the fibre level, where an explanation is proposed. The influence from bordered pits is of significant importance and their resistance dominates the timescales. A key factor in the proposed explanation is the high lateral capillary conductivity within the saw-damaged top surface. However, some questions remain unanswered.     

Proton magnetic resonance techniques for characterization of water in wood: application to white spruce

Summary Two new proton magnetic resonance techniques, relaxation spectra and relaxation selective imaging, have been used to investigate the distribution of water in samples of normal white spruce sapwood, heartwood, and juvenile wood as well as two rehydrated heartwood samples containing incipient decay and compression wood respectively. It is demonstrated that the spin-spin (T₂) relaxation behavior in wood is best presented as a continuous spectrum of relaxation times. Spectra of T₂ for white spruce show separate peaks corresponding to the different water environments. Bound water gives a peak with an T₂ time of about 1 ms and lumen water gives a distribution of T₂ times in the range of 10 to 100 ms. The lumen water T₂ time is a function of the wood cell radius. Consequently, the different cell lumen radii distributions for spruce sapwood, juvenile wood, and compression wood are readily distinguishable by the shape of their T₂ spectra. Water environments which are separable on a T₂ spectrum may be imaged separately. Imaging has been carried out in one dimension for bound water and lumen water of a spruce sapwood sample at four different moisture contents ranging from 100% to 17%. For the first time, we demonstrate that above the fibre saturation point the moisture density profile of the bound water is largely independent of moisture content. The feasibility and utility of using these techniques for internal scanning of logs and lumber is discussed. These techniques should provide new insights into the wood drying process.We would like to thank Michael Weiss of the Biological Science Electron Microscopy Facility at the University of British Columbia for his assistance with the microscopy and image analysis. This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the Canadian Forestry Service     

Characterisation of pore size distributions in variously dried Pinus radiata: analysis by thermoporosimetry

The pore size distribution and networks within the wood cell wall tend to decrease on drying wood from its green state having implications for water ingress and further processing such as in preservative treatment. Thermoporosity measurements using differential scanning calorimetry have been applied to determine the pore size distribution of green Pinus radiata wood and how well this pore network is retained on supercritical fluid (SCF) dewatering compared with kiln drying. Generally, in green sapwood, the majority of bound water was distributed in pore sizes less than 50 nm diameter with only a small proportion present in pore sizes between 50 and 200 nm. On SCF dewatering, most bound water was found to reside in pore sizes <20 nm, consistent with dewatered wood being at fibre saturation point (FSP). Generally, on rewetting dewatered, kiln- or oven-dried wood, proportionately less bound water was present compared with green wood, consistent with reduced pore sizes and network accessibility for water ingress. Furthermore, on rewetting, there were distinctions between earlywood and latewood (LW) sections with the LW sections having a statistically greater proportion of water occupying smaller-sized pores (<20 nm). There was also greater variability of pore size distributions on sample rewetting, inferring drying does not uniformly reduce pore size volumes. Analysis suggests the extent of variability was generally less in the rewetted dewatered wood sample compared to kiln- or oven-dried samples, which may be a consequence of SCF drying only to FSP.     

Wood drying process: impact on Scots pine lumber durability

There are indications that the drying process may have negative effects on the natural durability of wood. The impact of various drying processes on the durability of Scots pine lumber has been evaluated with mass loss in a decay test with brown rot fungus, Coniophora puteana, as measure of the decay resistance of sapwood and inner and outer heartwood. Drying with or without steam conditioning was performed in six different series: air drying, kiln drying at temperature ranges commonly used in Swedish sawmills at 70°C and 90°C with two different regulation principles, and one high-temperature drying at 110°C. Durability varied considerably both between and within boards. Sapwood showed considerable less durability than heartwood. No difference in durability was found between inner heartwood and outer heartwood. Air-dried heartwood showed the highest durability compared to other drying series. The lowest durability in sapwood and heartwood was found for series dried at the 90°C temperature level with high material temperature early in drying. The interpretation is that the duration of high material temperature at high moisture content (MC) is the critical combination for decay resistance in heartwood. Steam conditioning after drying decreased durability in sapwood.     

End grain water absorption and redistribution in slow-grown and fast-grown Norway spruce (Picea abies (L.) Karst.) heartwood and sapwood

Abstract

Wood is susceptible to decay by rot fungi if it is exposed to high-moisture contents during long periods of time and it is therefore important to limit the duration of such periods. Critical points in outdoor wood structures are, for example, end grain surfaces in joints where water can get trapped after a rain. It is therefore of interest to study both absorption and redistribution of moisture in wood. This paper presents moisture content profiles during end grain water absorption and redistribution in Norway spruce (Picea abies (L.) Karst.) measured by computed tomography with the specimens in individual climate boxes. Heartwood and sapwood of two provenances (slow-grown and fast-grown wood) were included. No major differences were seen between the water uptake of the slow-grown and the fast-grown wood since the densities were similar despite of the large difference in growth ring width. However, for the sapwood specimens, the moisture content was higher further into the specimens than for the heartwood specimens in agreement with previous studies. For the slow-grown wood, the redistribution was also generally more rapid for the sapwood specimens than for the heartwood specimens.     

Some factors influencing susceptibility to discoloring fungi and water uptake of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and Oriental spruce (Picea orientalis)

Abstract

The heartwood and sapwood from Scots pine (PS), Norway spruce (PA), and Oriental spruce (PO) were tested for susceptibility to discoloring fungi and water uptake. In addition, annual ring width and density were measured. The methods used were Mycologg for testing growth of fungi and a modified version of EN 927-5 to investigate water uptake. For pine, the heartwood showed a lower water uptake and no discoloring fungi growing in the tests. The heartwood had a significantly higher density and smaller annual ring width than the sapwood. In PA the heartwood had significantly lower discoloration than sapwood. The total water uptake in g/m² was significantly higher in sapwood, but not the calculated moisture content. As for wood properties, the density was significantly higher in sapwood compared to heartwood, although there were no differences in annual ring width. Regarding PO, differences in water uptake could be seen between sapwood and heartwood although the densities were similar. These results show that susceptibility to discoloring fungi and water uptake is hard to correlate to a single inherent property when looking at different wood species.     

Liquid water absorption in dried Norway spruce timber measured with CT scanning and viewed as a percolation process

Liquid flow in dried wood is complicated to study, since wood is a nonhomogeneous, hygroscopic-porous, anisotropic material. However, liquid flow is important to understand, since it has an influence on the durability of wood and on such processes like impregnation, drying, surface treatment, etc. In this study, simulations of liquid water absorption in wood as a fibre network, percolation, were compared with experimental water absorption in the longitudinal direction in spruce timber. With CT scanning, water distribution during liquid flow can be shown visually and measured by image processing. Liquid water absorption in end grain of spruce was measured with CT scanning after 1, 3, 7 and 14?days of liquid water absorption and shown as moisture content (MC) profiles in heartwood and sapwood. It was found that the amount of water absorbed could be expressed as a linear function of the square root of time. The slopes of the lines differed between sapwood and heartwood and also varied depending on the growth condition of the trees. The simulations according to the percolation method show generally good agreement with the measured results for sapwood.     

Experimental determination and modeling of equilibrium moisture content from the sapwood of Mexican pine (<Emphasis Type="Italic">Pinus pseudostrobus</Emphasis> Lindl.)

A desorption isotherm is a very important basis in the understanding of drying process. In this study, the desorption isotherms from the sapwood of Mexican pine (Pinus pseudostrobus Lindl.) were determined by the gravimetric method (discontinuous control of sample weight) at 30℃ and 50℃. The salt method was used in the range of water activities from 0.11 to 0.89. The results show that the desorption ability of Mexican pine sapwood increased with temperature at a given relative humidity. The experimental data was further simulated with the models, i.e., BET, GAB, Oswin and Henderson models. The GAB, Oswin and Henderson models allow the representation of the entire desorption isotherms. The BET model showed a better fit for water activity lower than 0.35, with a mean relative deviation of 0.0286 at 30℃ and 0.0167 at 50℃. Simultaneously, the BET model gave a better representation of moisture content in the monolayer saturation region. Overall, the GAB model ensured the best simulation of the entire isotherm, while the Henderson model displayed the worst simulation.     

Application of percolation modelling on end-grain water absorption in aspen (Populus tremula L.)

Abstract

The aim of this study was to improve our understanding of the capillary uptake of water in aspen sapwood and heartwood, which may help to explain the variable performance of aspen used outdoors. The study uses a percolation model developed for softwoods to examine capillary liquid absorption in aspen and compares predicted results with ones obtained experimentally using computed tomographic scanning. The study shows that there is an equilibrium saturation level where new flow paths are being found at the same rate as old ones are being blocked. This is seen as a plateau where the water content maintains a relatively constant height within the material. This makes it possible to model the capillary behaviour of aspen sapwood. In heartwood, however, the uptake of moisture seems to be mainly restricted to bound water, except for a short region in the sample wet end. This absorption is thus governed mainly by bound water diffusion. This improved understanding of the capillary behaviour of aspen may contribute to a future market expansion where aspen may be utilized to a greater extent outdoors and above ground.     

Degradation of Norway spruce (Picea abies) heartwood and sapwood during 5.5 years’ above-ground exposure

Abstract

Differences in durability between heartwood and sapwood of Norway spruce [Picea abies (L.) Karst.] were investigated to determine wood qualities most favourable for use in outdoor constructions above ground. Trees grown on sites with either good or poor access to water were used. Seventy-eight specimens measuring 20 × 50 × 300?mm³ separated into heartwood and sapwood, half untreated, half painted, were exposed horizontally outdoors above ground for 5.5?years with the pith side up and the bark side down. Crack length and crack number were measured. Fungus growth and surface changes were visually estimated. Fungus type was determined by microscopic analysis. The main finding was that spruce heartwood had fewer and shorter cracks and less surface-discolouring fungus growth than sapwood. This was valid for both painted and untreated wood. After 2?years’ exposure, the cracks in sapwood (upper surface) were more than three times longer and about five times more numerous than in heartwood for both painted and untreated boards. Microscopic study showed that surface discoloration was due mainly to Aureobasidium pullulans, together with a few other discolouring fungi. After 5.5?years, initial decay was established on the surface and in the end grain of four untreated test objects.     

Lumber Quality Model: The theory

Abstract

A new model for predicting moisture content, distortion and shrinkage distribution after lumber drying has been designed, implemented and tested. The model was implemented using Monte Carlo simulation, and it involves three empirical equations that were developed on the basis of experimental data. The model is referred as the Lumber Quality Model, and it is designed to be calibrated by knowing the initial and final moisture content, distortion and shrinkage distribution for a reference drying run. After calibration, the model can be used to predict the same information for other hypothetical drying scenarios. The present study explains the theoretical aspects of the model and the methodology for implementation. The model was validated with experimental data measured in a laboratory kiln. A full-scale industrial validation will be reported in a future paper.     

Resistance of Pinus leucodermis heartwood and sapwood against the brown-rot fungus Coniophora puteana

Abstract

This study assessed the decay resistance of Pinus leucodermis wood to the brown-rot fungus Coniophora puteana. Based upon the median weight losses of 30.65% for heartwood and of 34.68% for sapwood obtained in the biological tests, both the heartwood and sapwood material examined was classified as not durable (durability class 5) according to the CEN/TS 15083-1 classification. Total extractives were low, 3.93% in heartwood and 1.00% in sapwood, while lignin content was 22.60% and 25.41% in heartwood and sapwood, respectively. It is highly recommended to use protective treatments before using P. leucodermis wood in outdoor conditions.     

Combined application of computer tomography and light microscopy for analysis of conductive xylem area in coarse roots of European beech and Norway spruce

Axial water transport in trees is mainly determined by the gradient of negative water pressure and the structure of conductive xylem elements (i.e. conduits) connecting the fine roots with the foliage. There is still an essential lack of knowledge concerning the relationship between wood structure and hydraulic properties, especially of coarse roots. To this end, the study aimed (1) to work out a novel approach, based on the combination of computer tomography (CT) and light microscopy (LM), for determining the cumulative cross-sectional lumen area of conduits involved in the water transport of coarse roots in European beech (Fagus sylvatica) and Norway spruce (Picea abies) and (2) to demonstrate its adequacy in quantifying the functional relationship between sapwood anatomy and ascending water mass flow in the xylem. The cross-sectional sapwood area of coarse roots was assessed through CT. The cumulative cross-sectional lumen area of conduits in the sapwood (i.e. the lumen area of conductive conduits) was measured by LM in combination with interactive image analysis. The new approach was developed with coarse roots of both the tree species growing in a 60-year-old mixed forest in Bavaria, Germany. The combination of the two methods unveiled spruce to possess a distinct sapwood/heartwood boundary in small-diameter roots, whereas such roots of beech reflected a gradual transition zone; only large-diameter roots displayed a distinct boundary in beech. Additionally, the cumulative lumen area of conductive conduits was found to be approximately 12% of the total coarse root cross-sectional area in both the tree species. The new approach of measuring the conductive lumen area of coarse-root conduits yielded levels of specific sap flow (i.e. axial conductivity) that substantially differed from those derived from commonly applied methods, which were based on sap flow per unit of total cross-sectional root area or xylem cross-sectional area of individual roots. The combination of CT and LM will facilitate functional comparisons of woody roots differing in diameter and of tree species of different anatomical xylem structure.     

Impregnation of radiata pine wood by vacuum treatment II: effect of pre-steaming on wood structure and resin content

Radiata pine sapwood and heartwood were dried using high-temperature, conventional-temperature, and air drying schedules with and without pre-steaming. They were then impregnated by vacuum treatment with double-distilled water, toluidine blue, and fluorescein dye. For sapwood, there were only minor differences in uptake between drying methods and when pre-steaming was used. Using microscopy, the primary flow pathways in sapwood were found to be the resin canal network and ray parenchyma cells, which provided conduction without large resistance. In heartwood, uptake was strongly influenced by pre-steaming the green lumber. After pre-steaming heart-wood, there was an increase in uptake from all surfaces but especially from the radial surfaces. Lower extractive contents, disruption of epithelial and ray parenchyma cells, and alteration of the condition of bordered pits were also associated with pre-steaming. It was therefore possible to classify flow paths in radiata pine heartwood five ways, according to uptake values and wood anatomical features.This research was presented in part at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998.     

Local water vapor diffusion coefficient when drying Norway spruce sapwood

In this article, a one-dimensional and a two-dimensional approach to the evaluation of local diffusion coefficients for Norway spruce sapwood from measured moisture content (MC) values are presented. A studied wood sample was dried from the initial green condition to about 15% mean MC, but here only the diffusive part of the drying process between approximately 25% and 15% mean MC was treated. Measured local MC values were based on nondestructive X-ray computed tomography data. Finite element calculations were performed with two alternative diffusion coefficients to test the appropriateness of the diffusion coefficients that were evaluated from the measured MC values. The evaluated diffusion coefficients show interesting dependence on MC and distance from the evaporation surface. The advantage of using the methods presented is that the diffusion coefficient is calculated on a local level without having to define a function for the diffusion coefficient’s dependency on other parameters.