Faculty of Mechanical Engineering Institute of Wood and Paper Technology

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1 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 due to thermal treatment Hamburg, Alexander Pfriem and Mario Zauer

2 INTRODUCTION Thermal modification of wood Chemical degradation and changes in wood leads to Improved decay resistance Dimensional stability Reduced sorption/desorption behaviour Reduced mechanical properties (MOR) Changed pore structure and cell wall density

3 MATERIALS AND METHODS Specimens Maple and spruce - untreated and thermally modified Thermal treatment at 180 C and 200 C for 4h Four pairs of twin samples from the same annual rings Tests Helium pycnometry for an exact cell wall density and porosity Mercury intrusion porosimetry to determine the pore size distribution and average pore diameter Unsteady sorption experiments

4 MATERIALS AND METHODS Investigation of pore size distribution Gas Pycnometry is an inert gas displacement method to measure the true volume of solid materials accurately. Pressure transducer (p A, p E ) Samples V F Gas in Valve1 Valve2 Valve3 Vent Sample chamber V K Additional chamber V Z

5 MATERIALS AND METHODS Investigation of pore size distribution Mercury Intrusion Porosimetry is a method based on the principle that mercury (non-wetting liquid) only intrudes into a porous system under pressure. Washburn equation: d p 4 cos p

different thicknesses of the specimens 2, 4, 6 mm in

6 MATERIALS AND METHODS Investigation of unsteady sorption behaviour Tangential and radial sorption direction 3 different thicknesses of the specimens 2, 4, 6 mm in tangential sorption direction 4, 6, 8 mm in longitudinal sorption direction

7 MATERIALS AND METHODS Investigation of unsteady sorption behaviour Sudden changes of always 10 % relative humidity Adsorption and desorption (range of rel. humidity: 33 %...93 %) Sorption process was discribed by 2. Fick s law A total of 144 samples were tested User change of rel. humidity (theo.) 1 0 climatic chamber sample change moisture c c Dc () 1 t x x

8 MATERIALS AND METHODS Exemplary position and time-dependent adjustment of the moisture content Calculated distribution of untreated and thermally modified spruce

9 RESULTS AND DISCUSSION Apparent (cell wall) density measured by pycnometry: of maple is higher than that of spruce lower at 180 C and higher at 200 C Specific density Lignin 1.30 g cm -3 Cellulose 1.50 g cm -3 Hemicelluloses 1.53 g cm -3

10 RESULTS AND DISCUSSION Pore size distribution measured by porosimetry With the intensity of thermal treatment a shift to larger pores can be observed

11 RESULTS AND DISCUSSION Average pore diameter measured by porosimetry: increase with higher degree of thermal treatment

12 CONCLUSIONS PORE STRUCTURE Changes in Properties Initiated alteration due thermal treatment Spruce Maple Cell wall density Porosity Average pore diameter 180 C degrease 200 C increase Tendency increase Increase 180 C degrease 200 C increase Increase for both treatments Increase

13 RESULTS AND DISCUSSION Dent-Sorption isotherms (steady state) Equilibrium moisture content of spruce > maple All curves clearly show the well-known hysteresis between adsorption and desorption

14 RESULTS AND DISCUSSION Dependence of the diffusions coefficients (tangential) on the wood moisture, and the sample thickness With higher thickness of the sample D increases Attention: thermal treatment influences the MC! Desorption in tangential sorption direction of unmodified and modified maple

15 RESULTS AND DISCUSSION Dependence of the diffusions coefficients (tangential) on the wood moisture at adsorption and desorption D adsorption < D desorption may be a hysteresis Sorption in tangential sorption direction of unmodified and modified maple

16 RESULTS AND DISCUSSION Dependence of the diffusions coefficients (long.) on the wood moisture, and the degree of thermal treatment Generally D long > D tang Clear dependence on the degree of thermal modification Desorption in longitudinal sorption direction of unmodified and modified maple and spruce

17 RESULTS AND DISCUSSION Dependence of the diffusions coefficients (long.) on the wood moisture, and the thickness of the sample With higher thickness of the sample D increases Desorption in longitudinal sorption direction of unmodified maple and spruce

18 CONCLUSIONS UNSTEADY SORPTION BEHAVIOUR Changes in Properties Initiated alteration of diffusion coefficients in Species of wood Increase of thickness Tangential sorption direction Maple higher than spruce Increase Longitudinal sorption direction Spruce higher than maple Increase Increase of wood moisture Adsorption / Desorption Increasing degree of modification Increase Desorption higher than adsorption No influence Decrease Desorption higher than adsorption Decrease

CONCLUSIONS Beside the alteration of pore structure other changes influences the sorption behaviour of thermally modified wood Transport of water

19 CONCLUSIONS Beside the alteration of pore structure other changes influences the sorption behaviour of thermally modified wood Transport of water in wood (according to Krabbenhoft and Damkilde, 2004) Bound water diffusion Sorption Vapour + air transfer in pores Krabbenhoft and Damkilde, 2004

20 CONCLUSIONS However the alteration of pore structure influences the diffusion behaviour in longitudinal sorption direction But with increasing wood moisture content the pore diameter decrease and bound water diffusion becomes dominant In tangential sorption direction the bound water diffusion is more important than the transport in the pore system chemical changes affect the diffusion behaviour in tangential sorption direction

21 Faculty of Mechanical Engineering Institute of Wood and Paper Technology Thank you for your attention! This publication is based on results and tests, which were financially supported by the German Research Foundation (DFG WA 1540/7-1 and WA 1540/7-2)

Analysis of external and internal mass transfer resistance at steady state diffusion experiments on small clear wood specimens

COST FP0802 Workshop Experimental and Computational Micro-Characterization Techniques in Wood Mechanics, Vila Real, 27 28 April 2011 Analysis of external and internal mass transfer resistance at steady