Introduction

Wood as a porous system adsorbs water from the air by sorption and liquid water or other liquids such as varnishes or adhesives by capillary forces. Free water is stored in the voids or cavities, and bound water is found in the cell walls. If the cell wall system is fully filled with water but there is no water in lumens or macro voids, this is called fibre saturation, whereas, if the cell wall and macrosystem are saturated to the maximum with water, this is called water saturation (^{Hernandez 2007}, ^{Niemz 1993}, ^{Niemz and Sonderegger 2021}, ^{Keylwerth 1969}, ^{Popper et al 2006}, ^{Almeida and Hernandez 2007}). The fibre saturation range lies between 22% and 35% and is strongly dependent on the wood species. Many tropical woods have a very low equilibrium moisture content under normal climatic conditions (20^oC/65%RH), which is often due to their high extractives content (^{Willeitner and Schwab 1981}, Popper et al. 2006).

The sorption varies greatly between individual wood species. Tropical wood species and also selected wood species occurring in Europe with a high extractives content such as yew (Taxus baccata) or Black locust (Robinia pseudoacacia), have a significantly lower equilibrium moisture content at the same RH and temperature in comparison to most European wood species such as spruce or copper beech (^{Willeitner and Schwab 1981}, ^{Keunecke 2008}, ^{Keunecke et al. 2007}). ^{Keylwerth (1969}) investigated the sorption behaviour of 124 different wood species as well as the dimensional change with moisture changes in the hygroscopic range. This finding is probably one of the most complete overviews of the sorption behaviour of different wood species. Several percent of difference were observed between the adsorption and desorption equilibrium moisture content, where the desorption equilibrium moisture content was higher than the adsorption one. ^{Popper et al. (2009}) determined the sorption behaviour of selected European and overseas wood species in adsorption and desorption. The clear differences in the sorption behavior were found among the investigated wood species. A clear influence of the extractives content on the equilibrium moisture content was also found in Popper et al. (2006). The equilibrium moisture content tends to decrease with increasing extractives content.

^{Simo-Tagne et al. (2016}) investigated the sorption behavior of 4 tropical wood species by means of DVS at adsorption and desorption at 20ºC and 40^oC. The results were analyzed using the HH model. ^{Almeida and Hernandez (2007}) investigated the influence of the pore structure on the sorption behavior. Hernandez (2007) discusses the influence of the extract substances on the sorption behavior of wood from Peru. Both factors influence the sorption behavior.

Within the scope of the present study, the sorption behaviour during adsorption and desorption as well as the chemical composition were determined for 25 different wood species from Cameroon, and the influence of the extractives on the sorption behaviour was determined. For this purpose, a complex analysis of the chemical composition was carried out. Only a few studies have been conducted on the wood species investigated in this study, and the aim is to close this gap.

Materials and methods

Moisture content

From green condition, the specimens were cut and air dried in a climate chamber (circulating air mode) at a temperature of 20 ^oC. Prior to the test, the specimens were dried in a box containing silica gel (SiO₂), which may reach to 5% RH ± 5%. After one month, the specimens were moved to a chamber which accurately controlled the relative humidity and temperature. While the temperature was maintained at 20 °C, the relative humidity was changed stepwise from 20 % to 95 % with a step of 15 % for the adsorption test and then directly followed by a desorption test by changing the RH from 95 % to 20 % with the same RH step of 15 %. At each humidity level, the specimens were stored for at least 30 days until the equilibrium moisture contents of the specimens were reached. At the end of the experiment, the oven-dry mass of the specimens was determined, and the wood moisture can be calculated accordingly (Equation 1).

(1)

= moisture content in %

= mass wet in g

= mass oven dry in g

Raw density and oven-dry density

The density at a certain in level of moisture content (ꙍ) and the oven-dry density were determined according to ^{DIN 52182 (1976)}. The following applies (Equation 2):

(2)

Depending on the reference quantity of mass and volume, a distinction is made between the following types of density:

- the at defined wood moisture content (often at 20 ºC and 65% relative humidity, referred to as normal raw density).

- the oven-dry density (in the absolutely dry state)

Results and discussion

Table 1 shows all the tested wood species in this study, their density at normal climatic condition (20^oC/65% RH) and oven-dry density. The dry density varies between 309 kg/m³ (cedrela wood) and 1113 kg/m³ (ebene wood). The equilibrium moisture contentin the normal climate varies between 10,7 % (doussie wood) and 15,6 % (kosipo wood). Figure 1a and Figure 1b shows an overview of the sorption isotherms during adsorption and desorption of the 25 wood species. Table 3 shows the chemical composition of the wood species, which also varies greatly, especially the proportion of extractives. The total proportion of extractives varies from 3,3 % for ebiara to 20,3% for doussie, the proportion of hemicelluloses and lignin also varies greatly.

Figure 1a: Sorption isotherm with adsorption and desorption of the tested wood species. 

Figure 1b: Sorption isotherm with adsorption and desorption of the tested wood species. 

Figure 2: Relation total extractive content and moisture content for 20^oC/65%RH. 

The equilibrium moisture content in the normal climate decreases with increasing extractives content (total extractives content) (Figure 2). However, a strong variability of the moisture content was observed, e.g. at high extractives contents. Here, further detailed chemical analyses including the pore structure are necessary. An influence of the density is neither recognisable on the equilibrium moisture in the normal climate nor on the maximum differences in the sorption isotherm between adsorption and desorption. Furthermore, it can be also observed that the maximum difference between adsorption and desorption varies between 2,3 % and 3,6 % (Table 2). The influence of density on the equilibrium moisture content and the maximum difference between adsorption and desorption was small (Figure 3 and Figure 4).

Figure 3: Relation between density and moisture content (20^oC/65%RH) for the 25 species. 

Figure 4: Relation between max delta for ad- and desorption for the 25 species. 

Table 1: Overview about the tested species 

ρ_ꙍ is the density at moisture content ω; ρ₀ is the oven dry density

Table 2: Average moisture content in normal climatic conditions (20^oC/65%RH) and maximal delta EMC between and-and desorption. 

Table 3: Chemical composition. 

Conclusions

The equilibrium moisture content was determined for 25 wood species from Cameroon (including 3 species from plantations in Togo) at 20 ^oC and varying relative humidity. The proportion of extractives was also determined. The extractives content varied between 3,3 % and 20,3 %, the density in a normal climate between 309 kg/m³ and 1113 kg/m³. A very strong variability of the equilibrium humidity (e.g. in the normal climate at 20^oC/65% relative humidity) but also in the maximum of the hysteresis between adsorption and desorption is shown. A gap existing in this field for these wood species has been closed. The equilibrium moisture content and the extractives content are important for the processing of the wood, such as bonding, surface coating but also fungal resistance.