Adsorption boundary curve influenced by step interval of relative humidity investigated by Dynamic Vapour Sorption equipment

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1 IRG/WP THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION Section 4 Processes and Properties Adsorption boundary curve influenced by step interval of relative humidity investigated by Dynamic Vapour Sorption equipment Emil Tang Engelund, Morten Klamer, Thomas Mark Venås Danish Technological Institute Wood Technology Gregersensvej 4 DK-2630, Taastrup, Denmark Paper prepared for the 42 nd Annual Meeting Queenstown, New Zealand 8-12 May 2011 Disclaimer The opinions expressed in this document are those of the author(s) and are not necessarily the opinions or policy of the IRG Organization. IRG SECRETARIAT Box 5609 SE Stockholm Sweden

2 Adsorption boundary curve influenced by step interval of relative humidity investigated by Dynamic Vapour Sorption equipment Emil Tang Engelund 1, Morten Klamer 2, Thomas Mark Venås 3 1 Danish Technological Institute, Gregersensvej 4, DK-2630 Taastrup, Denmark, ete@teknologisk.dk 2 Danish Technological Institute, mkl@teknologisk.dk 3 Danish Technological Institute, thv@teknologisk.dk ABSTRACT The adsorption of water vapour from dry conditions by Norway spruce sapwood has been investigated using Dynamic Vapour Sorption (DVS) instrumentation. This equipment allows a fast and easy data acquisition as well as enables detailed studies of sorption properties using very small sample masses. In this study, particular focus was paid to the effect of step size on the sorption isotherms. Furthermore, the influence of relaxation of swelling stresses was investigated. This was done by having relative humidity (RH) histories with different RH step sizes and by introducing prolonged periods of conditioning at constant climate. The adsorption isotherms constructed on the basis of acquired sorption data was not significantly influenced by the differences in relative humidity (RH) histories. Thus, for practical purposes a stability criterion of %/min was found to be adequate for acquiring wood adsorption isotherms using sample masses around mg. The different RH histories did, however, affect the sorption kinetics. The sorption proceeds slower if the sample was conditioned at constant climate for a prolonged period before being exposed to another RH level. This indicates that relaxation of swelling stresses affects the sorption kinetics. During the initial phase of adsorption after changing RH, the moisture uptake was found to be linear with the square-root of time. From sorption and swelling kinetic theory the diffusion coefficient of the wood cell wall could be estimated based on data from the initial phase of the adsorption processes. The diffusion coefficient was found to decrease with increasing RH and to be independent of step size, as expected. Keywords: dynamic vapour sorption, sorption kinetics, Norway spruce 1. INTRODUCTION The wood-water relationship has been the topic of numerous studies due to the influence of water on almost all engineering properties of wood. Many sorption studies have focused on deriving sorption isotherms for different wood species as well as for decayed and modified woods. Typically, these studies employ various salt solutions or chambers with controlled climate to generate desired ambient water vapour pressures. Such methods involve significant amounts of time and labour. Both of these can be reduced by the use of dynamic vapour sorption (DVS) instrumentation as described by Engelund et al. (2010). The faster data acquisition and precision of the instrument makes more detailed and larger studies possible within a reasonable 1

3 time frame. The use of DVS instrumentation also enables detailed studies of the sorption over time, i.e. the sorption kinetics. The sorption equilibrium at various climates is typically illustrated by two sorption isotherms; the adsorption and desorption boundary curves. These are derived based on sorption experiments with adsorption from dry condition and desorption from saturated condition, respectively. For a given ambient climate, the moisture content of the wood in question is thought to be within the limits given by the boundary curves. However, some studies have indicated that the method for obtaining the boundary curves influence these. This is apparent both for sorption by the wood cell wall constituents (Newns 1956; Sadoh and Christensen 1964) and for sorption by the composite wood cell wall (Christensen and Kelsey 1959). In this study, the effect of relative humidity (RH) step size in obtaining adsorption isotherms is investigated for Norway spruce sapwood. Furthermore, the sorption kinetics are evaluated based on concepts by Newns (1956). 2. EXPERIMENTAL METHODS The investigation of sorption properties reported in this study was all performed using dynamic vapour sorption (DVS) instrumentation of the type DVS Advantage 2 from Surface Measurement Systems, London, UK. The basic principle of the equipment is shown in Fig. 1. Figure 1: Schematic illustration of the basic principle behind the DVS Advantage. Reproduced with kind permission of Surface Measurement Systems, London, UK. The cantilever of the microbalance is continuously held in horizontal position by an applied moment. The electrical current used to uphold this moment is therefore an indirect measure of the mass loaded in the sample holder. The equipment is capable of keeping a stable temperature in the range 5-60 C. The RH of the air surrounding the loaded sample in the sample holder is controlled by mixing dry and water saturated air to a mixed air with a specified RH. Thus, in the range 0-96 % RH all levels of RH are possible. This is an advantage over the traditional method with saturated salt solutions, where the levels of RH were limited by the different available salt solutions. This type of DVS Advantage is capable of handling up to 5 g sample mass; however, for a highly hygroscopic material like wood, a sample mass of around mg is enough. The smaller the sample mass, the faster the data acquisition. However, in order to secure accuracy of the 2

4 obtained results, sample masses below 20 mg should be avoided. The accuracy of the balance of the DVS is given by the manufacturer to be mg. 2.1 Material and preparation The sample material used in this study covers mature Norway spruce sapwood originating from Jyderup Skov comp. 162, Denmark (planted 1938). The dry density of the material was found to be 460 kg/m 3. Since only a very small amount of material is necessary for the sorption experiments, measures must be taken in order to secure a representative sample. Therefore, the sample material was grinded down to a powder with a maximal particle size of 1 mm. This does not change the sorption properties of the wood (Seborg and Stamm 1931). Samples were taken from the powder, and the sample mass varied between mg (dry weight). 2.2 Experimental procedure All experiments in this study were performed at 25 C. Equilibrium between ambient climate and moisture content of the sample is obtained when the mass change of the sample is below a certain threshold per time unit (a dm/dt requirement). Thus, in order for the software to automatically change the surrounding RH when moisture equilibrium for the sample is obtained, a dm/dt stability criterion must be met. The dm/dt stability criterion selected for the experiments in the present study was %/min and were based on previous findings for a sample range of mg of wood. The RH histories for different samples of the same batch of material are shown in Table 1. The experimental runs A and B will be used to see the effect of step size when changing RH. The runs C, D and E will be used to evaluate the influence of time on the sorption properties. In them, a period of constant RH is inserted after the dm/dt criterion is met, i.e. when the DVS would otherwise continue to the next RH level. Table 1: Relative humidity (RH) histories for the different experimental runs. Run A Run B Run C Run D Run E 0 % 0 % 0 % 0 % 0 % 20 % 20 % 20 % 20 % 20 % 40 % 60 % 60 % 40 % 40 % 60 % 90 % 60 % (8 hours) 40 % (2 hours) 40 % (8 hours) 80 % 90 % 60 % 60 % 90 % 3. RESULTS AND DISCUSSION The basic results obtained from the DVS equipment are shown in Fig. 2. The mass as function of time is illustrated along with the RH surrounding the wood sample for experimental run A. Due to the small sample amount used, c. 42 mg dry weight, the entire experimental run is completed in just 20 hours. 3

5 Mass [g] Relative humidity Figure 2: Mass of spruce sapwood sample (solid line) and relative humidity (dotted line) in the DVS equipment as function of time for experimental run A. Mass Time [min] Equilibrium moisture content Based on the mass curves and the dry mass for the experimental runs, the average moisture content of each sample can be calculated. The moisture content when the mass is stable, i.e. when equilibrium is said to be attained is given in Table 2. It is clear that the results for 20 %RH are very similar, i.e. that the sorption behaviour are quite similar for all RH histories. Thus, the sorption isotherms which can be constructed for the different experimental runs are for practical purposes identical. Table 2: Moisture contents (MC) in equilibrium with different levels of relative humidity (RH) for all experimental runs. RH [%] Run A [% MC] Run B [% MC] Run C [% MC] Run D [% MC] Run E [% MC] Sorption kinetics The adsorption of moisture over time depends more on the previous RH history than the final EMC does. In Fig. 3 the fractional change in moisture content is shown for all experimental runs. In order for the results to be compared properly, the dm/dt stability criterion is used to evaluate when equilibrium is attained even though the samples in runs C and D continues to take up very small amounts of water. Comparison of the curves for runs A and B shows that the adsorption process %RH (run B) is completed in about the same time as the process %RH (run A). Thus, even though the process in run B involves the uptake of about 6 %MC and the process in run A involves about 3 %MC, the approach to equilibrium is almost similar. Furthermore, the process %RH (run B) seems to be even faster than the process %RH (run A) despite the wood in the previous has to accommodate about 10 %MC compared with only about 5 %MC in the latter. 4

6 Figure 3: Fractional change in moisture content upon changing the ambient relative humidity. This difference in sorption kinetics can also be illustrated by t ½MC ; the time for half of the moisture content change in each step. This parameter is illustrated in Fig. 4 as function of the final RH in each change in ambient climate. It should be noted that the continued slow adsorption of water in the experimental runs C, D and E does not change t ½MC significantly. 5

7 t ½MC [min] log(t ½MC ) log(t ½MC ) E D A C B Figure 4: Logarithm of time (minutes) for half moisture change to occur at each step in RH (left). Experimental results from (Christensen 1959) on klinkii pine (right). The experimental results of this study are also compared with those by Christensen (1959) on klinkii pine in Fig. 4. The same tendency can be seen with increasing t ½MC for decreasing step size and increasing t ½MC for higher RH. The values of t ½MC of this study and that of Christensen (1959) differ, however, which might be due to differences in the experimental setup and the sample material. The t ½MC parameter does also seem to be influenced by the time of conditioning at the initial RH of each step. This is clear from Fig. 5 that depicts t ½MC as function of the time of conditioning for the runs A, D and E at 40 %RH and for the runs B and C at 60 %RH %RH %RH Time of conditioning [min] Figure 5: Time (minutes) for half of moisture change when changing RH % and % as function of the time of conditioning at 40 %RH and 60 %RH, respectively. Christensen and Hergt (1969) found a positive linear relation between the logarithm to t ½MC and the logarithm to the time of conditioning at a given RH level. This cannot be confirmed by this study due to lack of experimental data. However, the present data does not contradict the findings of Christensen and Hergt (1969). According to Newns (1956) the adsorption process in thin films and tissues can be described as a two-phase process. In the initial phase, the sorption is controlled by diffusion into the film. Thereafter, relaxation of the swelling stresses induced during the first phase will control the rate of adsorption during the second phase. In the first phase, the fractional moisture change can be approached by a linear function of the square-root of time. As seen in Fig. 6 the adsorption is linear with the square-root of time in about the first 9 minutes (3 min ½ ) of the adsorption for all 6

8 Diffusion coefficient Fractional MC change Fractional MC change experimental runs. Thereafter, the process becomes more non-linear which can be related to the relaxation of induced swelling stresses. The relaxation is more pronounced at higher RH which is expected due to the increasing rate of the time-dependent mechanical processes with increasing moisture in the cell wall (Kojima and Yamamoto 2005) %RH %RH %RH %RH t ½ [min ½ ] Figure 6: Adsorption processes with a change in RH of 20 % for all runs as function of t ½. If a linear function is fitted to the first part of the curves in Fig. 6, the diffusion parameter D s /l 2, i.e. the ratio of the diffusion coefficient and the thickness of cell wall, can be found. The same can be done for the curves for changes in RH of 30 % (60-90 %RH) and 40 % (20-60 %RH). The diffusion parameter is illustrated in Fig. 7 (left). If an average cell wall thickness of 5 µm and an assumed swelling of 0.25 %/%MC (Keylwerth 1968), the diffusion coefficient D s of the cell wall can be calculated. This parameter is illustrated in Fig. 7 (right) where it is seen that the diffusion coefficient as expected is decreasing with increasing RH %RH %RH %RH %RH t ½ [min ½ ] D s / L RH change 20% 30% 40% RH change 20% 30% 40% Figure 7: Left: Diffusion parameter, D s /l 2 and as function of initial RH for all steps in RH in all experimental runs. Right: Diffusion coefficient with estimated swelling of the cell wall thickness. Fig. 7 shows that the predicted diffusion coefficient, as required from physical considerations, is independent of step size. Further data analysis is needed to evaluate the sorption kinetics in more detail

9 4. CONCLUSIONS Differences in relative humidity (RH) histories did not lead to significant differences in the acquired equilibrium moisture contents. Thus, a dm/dt stability criterion of %/min is adequate for acquiring wood adsorption isotherms using sample masses around mg. The different RH histories did, however, affect the sorption kinetics. Diffusion coefficients of the cell wall could be estimated on the basis of the first 9 minutes of the adsorption processes. The diffusion coefficient was shown to decrease with increasing RH, independent of the RH step size, as expected. 5. REFERENCES Christensen, G. N. (1959) The rate of sorption of water vapour by wood and pulp. Appita Journal. 13: Christensen, G. N., Hergt, H. F. A. (1969) Effect of Previous History on Kinetics of Sorption by Wood Cell Walls. Journal of Polymer Science Part A-1-Polymer Chemistry. 7: Christensen, G. N., Kelsey, K. E. (1959) The Rate of Sorption of Water Vapor by Wood. Holz als Roh- und Werkstoff. 17: Engelund, E. T., Klamer, M., Venås, T. M. (2010) Acquisition of sorption isotherms for modified woods by the use of dynamic vapour sorption instrumentation: Principles and practice. IRG/WP Keylwerth, R. (1968) Wood Species with Dimensional Stability. Holz Als Roh-und Werkstoff. 26: Kojima, Y., Yamamoto, H. (2005) Effect of moisture content on the longitudinal tensile creep behavior of wood. Journal of Wood Science. 51: Newns, A. C. (1956) The Sorption and Desorption Kinetics of Water in A Regenerated Cellulose. Transactions of the Faraday Society. 52: Sadoh, T., Christensen, G. N. (1964) Rate of Sorption of Water Vapour by Hemicellulose. Australian Journal of Applied Science. 15: Seborg, C. O., Stamm, A. J. (1931) Sorption of water vapor by paper-making materials I - Effect of beating. Industrial and Engineering Chemistry. 23:

Faculty of Mechanical Engineering Institute of Wood and Paper Technology

Faculty of Mechanical Engineering Institute of Wood and Paper Technology Investigations to the unsteady sorption behaviour of thermally modified wood and its connection to the alteration of pore structure