Studying the Sorption Properties of Wood using the IGAsorp

Wood is a key natural resource and a versatile material obtained from renewable sources, holding promise for continued use in building and construction applications. The structural properties of wood may vary abruptly or gradually with respect to the environmental changes. Wood can also be used as a renewable fuel, with energy being extracted. The combustion process used for releasing this energy, however, needs to be optimized for the moisture content of the wood. Hence, it is necessary to understand the water sorption mechanism in wood fuels.

As a biological material, wood is subject to natural decay processes, mould and fungus growth and parasite infection if not preserved properly. The preservation processes are carried out by preventing water transport across the material, by sealing the surfaces and treating the wood so that it does not absorb moisture. The analysis of the susceptibility of wood to natural decay, and the suitability of wood for construction or fuels, can be performed through Dynamic Vapour Sorption (DVS) measurements.

The following article demonstrates an experiment to determine the specific water sorption properties and other mechanisms of Scotch pine wood using the Hiden Isochema IGAsorp.

Experimental Procedure

Wood is a hygroscopic material with the ability to absorb and desorb water with respect to the humidity level in the environment. This property is thought to be due to the polar hydroxyl- chemical groups in the water and cell walls. However, the extent of its response to the sorption kinetics and humidity changes is based on the type of wood and its surface characteristics.

This study investigated both bulk and splintered forms of Scotch pine (Pinus Sylvestris) wood using a standard gravimetric IGAsorp instrument that facilitated isothermal vapor sorption measurements. A refrigerated water bath was used to thermostat a sample chamber. The measurement was performed by adjusting the relative humidity (% RH) of the chamber atmosphere using combinations of dry (0% RH) and vapor-saturated (100% RH) nitrogen streams, controlled by mass-flow controllers and RH sensor feedback. The experimental parameters, including balance stability, pressure, total gas flow rate and temperature, were precisely maintained during the experiment.

37.143 mg of sample was dried in-situ for 24 hours at 25°C in the presence of a dry nitrogen stream, until its mass levelled at 34.690 mg. Then, a sorption and desorption isotherm was determined in increments of 5% RH, up to to 95% RH. Using the intelligent real-time analysis software of IGAsorp, the kinetic mass response of the sample was monitored until 99% of the predicted asymptotic uptake was achieved. After this, the next isotherm step was automatically activated, and the measurement was repeated. The temperature was set at 25°C ± 0.1°C throughout the measurement.

Results and Discussion

Figure 1 shows the complete kinetic profile of the sorption and desorption isotherms. The time it took for the wood to reach 99% of its predicted moisture sorption equilibrium at each isothermal point was fairly consistent at around 400 minutes. Figure 2 shows the graph of weight % uptake plotted against % RH. The predicted asymptotic uptake worked out from each fitted kinetic mass response gives the equilibrium weight % uptake.

The Hailwood-Horrobin and Dent models have both been used to fit moisture sorption data in wood and other porous materials. The Dent model was obtained from the Brunauer, Emmett and Teller (BET) method of surface area determination, while the Hailwood-Horrobin model is based on a first principles approach. Both the models assume two separate mechanisms of water sorption, primary and secondary types. The primary sorption is due to the interaction between the hydroxyl cellular structures within the wood and water, while the secondary sorption is a result of interaction with the already sorbed moisture. Thus, the secondary sorption interaction is much weaker than the primary sorption.

According to BET theory, the secondary sorption energetics are similar to the non-sorbed molecule interactions. The HH and Dent models deviate from this assumption during water sorption. Figure 3 shows the plot of the ratio of the relative humidity to the uptake, (RH/m), against % RH, which can be modeled using the following equation:

(1)

where m is the mass uptake (wt.%) and A, B and C are material dependent constants.

Figure 1. Kinetic Profile for DVS Isotherm

Figure 2. Sorption and desorption isotherms showing experimental data and theoretical fits

Figure 3. %RH/m vs %RH plot from sorption data, used for HH theory fitting

A number of physical properties can be determined using the constants A, B and C that fit the HH theory to the experimental data. The property of molecular mass of dry wood per sorption site is most relevant to various fields, including building and constructions applications. This indicates the susceptibility of wood to physical changes due to moisture uptake. This measure, W, can then be calculated from equation 1 using the constants A, B and C as follows:

(2)

Further information can be determined using this measure by deconvoluting the HH model into primarily and secondarily sorbed layer contributions, m1 and m2 respectively, for each humidity step as follows:

(3)

where K₁ and K₂ are calculated using the constants A, B and C as follows:

(4)

(5)

Figure 4 shows the determination of the sorption contributions for primary and secondary sorption sites with respect to %RH. This clearly shows that moisture is initially sorbed onto primary sites that are limited by the number of hydroxyl- sites for bonding. The subsequent sorption is a result of the interaction between the sorbate and water vapor.

Figure 4. Primary and secondary sorption contributions to total moisture uptake

Conclusion

This experiment involved the determination of moisture sorption isotherms for Scotch pine using the Hiden Isochema IGAsorp. The IGAsorp is an automated instrument that provides efficient and accurate results on equilibrium kinetics and dynamic vapor sorption isotherms of materials. The obtained data closely fits that predicted by Hailwood-Horrobin theory. The properties determined from the theoretical data produced and the corresponding material constants used to model the extent of primary and secondary sorption effects were evaluated. These properties helped to determine the susceptibility of wood to natural decay processes, and suitability of the wood as building and construction material.

This information has been sourced, reviewed and adapted from materials provided by Hiden Isochema.

For more information on this source, please visit Hiden Isochema.