Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

TY - JOUR

T1 - Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

AU - Hoffmeyer, Preben

AU - Engelund, Emil Tang

AU - Thygesen, Lisbeth Garbrecht

PY - 2011/10/1

Y1 - 2011/10/1

N2 - It is a commonly accepted notion that the equilibrium moisture content (EMC) of wood at a given relative humidity (RH) is highest during initial desorption of green wood due to an irreversible loss of hygroscopicity during the 1 st desorption. The basis for this notion is investigated by assessing how drying and saturation procedures influence the differences between the 1 st and the 2 nd desorption curves for Norway spruce (Picea abies (L.) Karst.) sapwood. The study establishes 1 st and 2 nd desorption isotherms for a variety of initial conditions and it covers the RH range from 60.1% to 99.9%. The state of the water is not affected by oven-drying and rewetting as demonstrated by time domain low field NMR relaxometry. The results challenge the conclusions of earlier studies and indicate that in these studies the 2 nd desorption was initiated at much too low EMC and therefore fails to describe a boundary desorption isotherm. Instead, it becomes an intermediate desorption isotherm starting at the adsorption boundary curve and crossing over to eventually meet the desorption boundary curve. The results also show that vacuum drying at room temperature only gives a modest loss of hygroscopicity compared to the green state. Conversely, oven-drying at 103°C results in a more significant loss of hygroscopicity, except for RH above 96% where an increase in EMC surprisingly is seen.

AB - It is a commonly accepted notion that the equilibrium moisture content (EMC) of wood at a given relative humidity (RH) is highest during initial desorption of green wood due to an irreversible loss of hygroscopicity during the 1 st desorption. The basis for this notion is investigated by assessing how drying and saturation procedures influence the differences between the 1 st and the 2 nd desorption curves for Norway spruce (Picea abies (L.) Karst.) sapwood. The study establishes 1 st and 2 nd desorption isotherms for a variety of initial conditions and it covers the RH range from 60.1% to 99.9%. The state of the water is not affected by oven-drying and rewetting as demonstrated by time domain low field NMR relaxometry. The results challenge the conclusions of earlier studies and indicate that in these studies the 2 nd desorption was initiated at much too low EMC and therefore fails to describe a boundary desorption isotherm. Instead, it becomes an intermediate desorption isotherm starting at the adsorption boundary curve and crossing over to eventually meet the desorption boundary curve. The results also show that vacuum drying at room temperature only gives a modest loss of hygroscopicity compared to the green state. Conversely, oven-drying at 103°C results in a more significant loss of hygroscopicity, except for RH above 96% where an increase in EMC surprisingly is seen.

U2 - 10.1515/HF.2011.112

DO - 10.1515/HF.2011.112

M3 - Journal article

SN - 0018-3830

VL - 65

SP - 875

EP - 882

JO - Holzforschung

JF - Holzforschung

IS - 6

ER -