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Annals of Warsaw University of Life Sciences - SGGW Forestry and Wood Technology № 88, 2014: 157-161 (Ann. WULS - SGGW, For. and Wood Technol. 88, 2014) The activation energy of swelling beech wood (Fagus sylvatica L.) in water WALDEMAR MOLIŃSKI, JAN KUCZYŃSKI, JAKUB PUSZYŃSKI, EDWARD ROSZYK Department of Wood Science, Poznań University of Life Sciences Abstract: The activation energy of swelling beech wood (Fagus sylvatica L.) in water. This paper shows the results of the activation energy of swelling beech wood in water. The results showed that activation energy depends on the density. Increasing that values increase activation energy of swelling. Keywords: activation energy, swelling, beech wood INTRODUCTION

Swelling of wood is considered to be its disadvantage. To minimize this effect, solid wood is processed into many kinds of wood based materials. Those types of materials characterized by a lower swelling in water values, witch increase activation energy. For OSB its 54.1, plywood 62.8 (Sinha et al. 2011) and for the wood-polymer composite (WPC) the activation energy is 590 kJ/mol (Ou et al. 2011). For wood, linear function of swelling in wetting time in initial period (after induction) of those process is used to mark activation energy. Carrying out this process at various temperatures allows to determine the initial swelling rate. Already in 1872, Svante Arrhenius, a Swedish scientist showed that the dependence of the reaction rate of the natural logarithm (ln k) as a function of the inverse absolute temperature (1/T) is a straight line. The directional factor of this compound is characteristic to a given reaction. According to West (1988) and Banks and West (1989) swelling of the wood is results of collisions of a particle solution with a wood and their binding to the active centres of wood substance. They have shown that kinetics of wood swelling strongly depends on temperature, and this relationship is subordinated classical Arrhenius equation. Recognizing wood swelling as a bimolecular chemical reaction that takes place only by colliding liquid molecules with the wood molecules, they proposed a model for illustrating the movement of the liquid into the wood of the zipper-like effect. React may, however, only molecules whose energy level exceeds the value of the activation energy (Bala 1997). The activation energy of the process of wood swelling in water test is relatively new, and this is due to the very high kinetics of the swelling even at room temperature. Only in 1994 Mantanis et al. (1994b) using a linear deflection sensor connected to the computer determined the activation energy of the four species of wood in the water. In this work, it was found that the value of activation energy of wood swelling in water depends, among others: from the species, content of the compounds extraction and wood density. Sahin (2010) studying the Castanea sativa Mill. wood also demonstrated that measured value of the wood swelling energy in water is partly depends from the direction of moisture deformation measurements. Resuming, the wood swelling is a complex process influenced by many factors (Mantanis et al. 1994a and b; Morisato et al. 2002; Ishimaru and Sakai 1998; Ishimaru and Iida 2001), the results may vary. No information about the activation energy of wood swelling beech (Fagus sylvatica L.) in water was the cause of these study.

MATERIALS

Determination of the swelling beech wood was carried out on samples with dimensions of 25 (R) x 25 (T) x 10 (L) mm. Samples were obtained from a single, central

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plank, with 60 mm thickness and 3 m length. Plank was obtained from close to stump zone of tree and was seasoned by two years under the roof. To prepare the samples only defect-free, straight grain wood was used. In next step, plank was sawed longitudinally through the pith. From both elements, two boards were handling with dimensions slightly larger than the size of the final samples. After conditioning in laboratory for two weeks, elements were cut using a planer. Rings were tangential to one of the edges. Subsequently, each planks were cut in half of its length, which gives a samples with 10 mm length. All samples like those were treated as a twin. A scheme of preparation of the samples is shown in Figure 1.

Figure 1. Scheme of preparation of the samples

Samples were properly described to permit their origin identification. Then, they were

laboratory kiln dried at 103±2°C to completely dry condition. After the seasoning to room temperature, their linear dimensions (±0.001 mm) and their mass (±0.001 g) were measured. Density of each sample were calculated. Its values were very similar to twin samples.

The method of determination kinetics of the wood swelling in water was similar to that used in the Mantanis′a et al. (1994a) work. This value was determine for the tangential direction. Samples were moistened in water with varying temperature: 20, 40, 60 and 80°C. KEST ELECTRONICS displacement meter with special software were used to measure kinetics of swelling. It allows for automatic registration of displacement inductive sensor in time with an accuracy of 0.001 mm. Moistening in water has been done on the triple water bath LWR RS-485. It allows maintaining the water temperature at the desired level during the experiment with accuracy of 0.5°C. The registration process of wood swelling continued out for 8 hours. The value of maximum swelling were taking after 24 hours of soaking in water. RESULTS

The results of the average wood swelling measurements in water with varying temperature are shown in Figure 2. Each average values were calculated from six reps for samples obtained from two planks located on opposite sides of the pith. Figure shows only the initial period of swelling process. It shows that increasing temperature of the water increase swelling process, and further, equilibrium of swelling establishes quicker. This fact should be associated with the kinetics of wetting samples (Figure 3), samples at higher temperatures

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moisturize faster. Increase maximum value of swelling with increasing temperature (Table 1) should be associated with leaching some of a extraction compounds of cell walls (Stamm and Loughborough 1942; Boiciuc and Petrician 1970; Mantanis et al. 1994a i b; Ishimaru and Iida 2001). An important aspect of the increase wood swelling as the temperature of immersion liquid increases is also softening the cellulose matrix, which facilitates swelling (e.g. Eriksson et al. 1991, Engelund et al. 2013). Furthermore, Mantanis et al. (1994a) found that a higher value for the maximum swelling of wood at increased temperatures may be the result of small changes in the structure of the cell wall.

Figure 2. Initial period of wood swelling in the tangential direction for the beech samples in varying

temperatures

Figure 3. Kinetics of changes in moisture content of wood

Swelling rate, calculated for the six samples (3 on each side, L and P), in the initial

linear range are summarized in Table 2. Table data shows a clear increase in the initial rate of wood swelling (in linear zone) with increased temperature of the process. The results of this study fully agree with Mantanisa et al. (1994a) results (on Sitka spruce, Douglas fir, sugar maple and aspen wood). They have shown a dramatic increase in the swelling kinetics of these species of wood in the water as the temperature increases. It should be noted that those study were modelled on the methodology developed by these authors and the obtained results confirmed earlier expectations in this regard. Activation energy of swelling in water (Table 2) ranged from 25.5 kJ/mol (L3/P4 samplers with the lowest average density) to 29.8 kJ/mol

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(L1/P2 with highest density). Those results confirm earlier observations (Mantanis et al. 1994a and b) that increasing wood density increase activation energy of swelling. Table 1. Average maximum values of swelling beech wood samples in a tangential direction, humidified in the water of different temperatures and different positions of the samples within the tree

Temperature, T [°C]

Samples mark L1/P2 L2/P3 L3/P4

Density, ρ0 [kg/m3] 730 695-720 645-660

Maximum values of swelling [%] 20 14.22 13.84 13.08 40 15.84 15.34 13.71 60 16.36 16.01 14.26 80 16.57 16.13 14.43

Table 2. Initial swelling kinetics of beech wood samples in water at different temperatures and position

Samples mark

Density, ρ0 [kg/m3]

Kinetics of swelling [%/min] 20°C 40°C 60°C 80°C

L1/P2 730 0.260 0.570 1.119 2.072 L2/P3 710 0.239 0.478 0.911 1.776 L3/P4 650 0.306 0.598 1.068 1.815

Correlation between activation energy of swelling wood in water with density is

shown at Figure 4. The data in this figure show that the correlation can be approximated by a straight line with a relatively high coefficient of determination (R2 > 0.85). In the range of wood density from 650 to 730 kg/m3, density increase of 10 kg/m3 gives activation energy increases about 0.5 kJ/mol.

Figure 4. Correlation between activation energy of swelling wood in water with density

CONCLUSIONS On the grounds of the results, it can be concluded that:

1. As temperatures increase, the swelling of beech wood in the water and its maximum values increases, equilibrium time of swelling establishes is dramatically shorten.

2. The activation energy of beech wood swelling in water range from 25.5 to 29.8 kJ/mol.

3. The value of activation energy of beech wood swelling is a linear function of its density.

Ea = 0.0514ρ0 - 7.8985 R² = 0.8544

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630 650 670 690 710 730 750

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2. BANKS W.B., WEST H., 1989: A chemical kinetics approach to the process of wood swelling, Proc. Tenth Cellulose Conf., C. Schuerich, ed., John Wiley & Sons, New York; 1215

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Streszczenie: Energia aktywacji pęcznienia drewna buka (Fagus sylvatica L.) w wodzie. W pracy przedstawiono wyniki oznaczeń energii aktywacji procesu pęcznienia drewna w wodzie. Uzyskane rezultaty wykazały, że wartość tej wielkości zależy od gęstości. Wraz ze wzrostem gęstości drewna buka wzrasta energia aktywacji jego pęcznienia. Corresponding author: Edward Roszyk Department of Wood Science Poznań University of Life Sciences Wojska Polskiego 38/42 60-627 Poznań, Poland e-mail: eroszyk@up.poznan.pl