Assessment of Sorptive Properties of Selected Tropical Wood Species

This study is primarily focused on broadening the knowledge on sorptive properties of tropical wood species. The main objective of this research was to determine the equilibrium moisture content (EMC) of 17 selected tropical wood species vital to the fl ooring industry in Europe. For comparison, selected European wood species – European beech, European oak and Scots pine, were also tested. Differences in the EMC between wood from temperate and tropical zones were established, resulting in the necessity of revising the knowledge, especially in wooden fl oor production. The results show that the EMC of tropical wood species is generally lower than that of temperate region wood species. African padouk, Teak, Afzelia and Ipe showed the lowest values of moisture content. Furthermore these wood species showed the lowest values of fi bre saturation point (17.7 %, 22.5 %, 19.7 % and 18.7 %, respectively). Secondly, it was established that the basic density has signifi cant infl uence on sorption properties of tested wood species. Further to the above, it was concluded that the higher wood basic density, the larger are the changes of dimensions. Consequently, based on the results obtained, the possibility of supplementing the recommendations in industry standards regarding wooden fl oors should be considered.


INTRODUCTION 1. UVOD
The sorption isotherms are essential properties in the analysis and design of various biological material processes such as preservation, drying, storing, packaging, mixing and using in production fl oors, furniture and others (Salin, 2011).As a hygroscopic material, wood shows signifi cant changes in moisture content depending on climatic conditions.Due to relative humidity changes of the air, the phenomena of swelling and shrinkage of wood causes dimensional changes in fi nished products.In spite of a large number of wood species, data from the sorption isotherm of fi r wood is usually used for the evaluation of the sorption behavior of domestic wood species, (Popper et al., 2007).
The mechanism and rate of wood humidity change is directly dependent on the parameters characterizing the indoors climate, while climate interiors through all the year is molded by exterior conditions and heating system.The air temperature in heated rooms during winter time is much higher than outside temperature.Coming into buildings, the cold atmospheric air (with high relative humidity) causes gradual drying of the indoor air.Relative humidity of the indoor air in summer can be 70 % and decreases at the end of winter to 40 % or even lower.The lower the outside temperature, the greater is the drying effect of external air.The result of these changes is wood moisture content change.In buildings with central heating system, wood moisture content is at the level of approx.13 % at the end of summer and decreases to even 5 % at the end of winter (Krzysik and Sobczak, 1960).
Many studies were conducted on wood from moderate climate zones (Central Europe, North America).Due to a large number of wood species, the knowledge in this fi eld of expertise is still incomplete.Furthermore, the equilibrium moisture content (EMC) of many wood species varies within a relatively broad range (Popper et al. 2007(Popper et al. , 2009;;Adampoulos and Voulgarridis, 2012).Few studies on tropical woods indicate the occurrence of variations in the sorption properties between species, which was corelated to the presence of high extractives content (Wangaard and Granados, 1967;Hernandez, 2007;Popper et al., 2006Popper et al., , 2007;;Jankowska and Kozakiewicz, 2016;Jankowska et al., 2017).A comprehensive presentation of the sorption behavior of around 100 wood species was given by Keylwerth (1969).This list of wood species includes a lot of species from moderate climate and tropical zones.However, most of them are not present on European market of wood products, while wood species used nowadays were not included by this author.The studies conducted later give some information that should be included.Deliiski (2011) presents wood sorption isotherms, but no specifi c data is given on each species.Jannot et al. (2006) determined desorption isotherms and estimated specifi c areas of four selected tropical wood species from Africa (Doussié, Moabi, Ebony, Obeche and Iroko) at different temperatures.Sorption behavior of four tropical woods from Africa was investigated by Sigmo-Tagne et al. (2016).Popper et al. (2006Popper et al. ( , 2007) ) also carried out research about sorptive properties of tropical wood species.Most of the presented data does not fi nd use in practical application of tropical wood species in Europe, because wood species tested in the mentioned research are not currently present on European market.The knowledge in this area is lacking.This has also been confi rmed by a number of studies made for the Association of Engineers and Technicians of Forestry and Wood Industry (Poland) dealing with products made of tropical wood such as fl oor, elevation, furniture, etc.This problem was emphasized in trade press (eg.Wróżek and Romanovski, 2014).
Determining properties of commercial wood available on the European market is an important issue and results of research should be taken into account during the design stage.The knowledge in this area will help to avoid many problems in the use of wooden products such as fl oor.The main aim of this work was to determine and compare the EMC of wood species used in fl oors production in Europe as well as to improve the knowledge of the tropical wood sorptive properties.The sorption tests were combined with the measurements of physical properties providing the value of fi ber saturation point (FSP) of wood species selected for the tests.This group includes tropical woods from Africa, South American and tropical parts of Asia.

Materijali
The wood species used in this study are presented in Tab. 1.The selected species are widely used in fl ooring production in Europe.Selection was made in order to have a representative sample of hardwoods, with a wide range of density and different anatomical structures.According to Popper et al. (2006Popper et al. ( , 2007)), Jankowska et al. (2017), the selected species represent different types and amounts of extraneous substances.European wood species were used as a reference.All test materials were heartwood because it is commercially more important than sapwood.Wood from each species was acquired from DLH Poland, Warsaw, Poland.Material was identifi ed in the laboratory using both macroscopic and microscopic techniques.
The samples of each wood species were collected from one log.Thus, differences in tested properties caused by differences in wood anatomy were avoided.Each part was quarter-sawn to produce planks of 4 cm thickness.Prior to samples preparation, wood was conditioned to air-dry in a room with relative humidity of 40-60 % and temperature of approx.21 °C.The defectfree planks were sawn and sized to samples for moisture sorption and dimensional stability tests.For each wood species, 10 samples were used.Dimensions of the samples were 30 mm (tangential) × 30 mm (radial) × 5 mm (longitudinal).

Metode
The specimens were exposed to moisture sorption test in adsorption and desorption.Five different relative humidities ranging from 9 to 97 % were used followed by immersion treatment in water.As soon as each point of sorption was completed, the mass of specimens was measured to the nearest 0.001 g and their dimensions were taken to the nearest 0.01 mm.The conditioning of specimens to appropriate moisture content was performed with the use of sealed enclosures in which prescribed saturated salt solutions were placed at the temperature close to 20 °C.The relative humidity was monitored and recorded using a hygrom-eter.The salt solutions used to create various relative humidity of air at 20 ±2 °C are listed in Table 2.A criterion for equilibrium was established as three successive identical mass readings at 24-hour intervals.
The equilibrium moisture content (EMC) of samples was determined according to PN-D-04100:1977 and ISO 3130:1975.The wood basic density of samples was determined according to PN-D-04101:1977 and ISO 3131:1975.The volumetric shrinkage of wood was also determined according to PN-D-04111:1982 and ISO 4858:1982.Moreover, the fi ber saturation point (FPS) was estimated by the interpolation method between volumetric shrinkage and moisture content according to Jankowska and Kozakiewicz (2016).
Statistical analysis of the test results was carried out using Statistica v. 10 software (StatSoft, Inc.).Data were analyzed and provided as the mean ± standard deviation and minimum and maximum values.Regression analysis was used to evaluate relationships between the measured properties.The effects of wood basic density on equilibrium moisture content and dimensional stability were determined.Moreover, t-test was used to verify the signifi cance of differences among the average values.

REZULTATI I RASPRAVA
The average values of the measured EMC for adsorption and desorption at 20 °C are shown in Table 3.For each tested wood species, the EMC at different relative humidity of air is individual.The highest differences in moisture content between tested wood species can be seen at higher relative humidity (approx.97 %).The results show that the EMC of tropical wood   This study fi lls an important gap concerning moisture content of tropical wood species used in Europe.Due to a lack of information on sorptive properties of tropical species, according to the European standards on wooden fl oors -PN-EN 13227:2004, PN-EN 13629:2012, PN-EN 13226:2009, the proper value of wood moisture content should range from 7 to 11 % when air temperature is about 20 ±3 C and air humidity 50 ±5 %.Regarding the results obtained during this study, it can be said that such a wide range of recommended wood moisture content causes problems in choosing the proper value, especially due to the fact that tropical wood species are generally characterized by lower moisture content in comparison to European wood species.According to the results obtained, the EMC of wood in the air temperature close to 20 °C and relative humidity of 55 % can range from 7.50 to 12.96 % (Table 3).With so many differences in the sorption properties of wood, (especially in case of tropical wood species), the possibility of supplementing the recommendations in industry standards regarding wooden fl oors should be considered.
The results of density and shrinkage of the tested wood species are summarized in Table 4. High variation was observed in swelling properties among the tested  ysis indicated a statistically signifi cant relationship at the 95 % confi dence level (the signifi cance value of the linear regression analysis was less than 0.05).When Teak and Merbau were not included in this analysis, the relationship between the basic density and coeffi cient of volumetric total shrinkage of wood was higher (R 2 value of 0.70).This can be caused by the chemical composition of these wood species.According to former studies (Mantanis et al. 1994; Adamopoulos and Voulgaridis 2012; Jankowska et al. 2017), it could be predicted that Merbau wood will show relatively low moisture content, because of high content of water soluble extractives.This is due to the bulking effect of extractives, which are partly deposited on the cell walls.The low moisture content values observed in case of Teak wood is an effect of higher content of chloroform-ethanol extractives and relatively low contents of hot water extractives (Jankowska et al., 2017).Analysis of data given in Table 4 revealed that the values of the volumetric shrinkage of some wood species (African mahagony, Cumaru, Iroko, Massaranduba, Sucupira) are larger than the sum of radial and tangential shrinkage.This phenomenon can be explained by the fact that the above said wood species are characterized by irregular fi ber arrangement.Consequently, the main anatomical directions are distorted and as a result dimension changes can be higher than when wood exhibits a simple fi ber arrangement (as in case of European beech).Regarding the results presented in Table 4, it can be concluded that fi ber saturation point (FSP) of tested wood species varied within a broad range from 17.7 % (in case of African padouk) to 31.2 % (in case of European beech).FSP was nega-tively correlated with wood density, but the correlation (R 2 = 0.39) was rather low.According to previous studies (Jankowska et al., 2017), the relationship between wood density and its sorptive properties can be strongly affected by the high variability of extractives content.The observed volumetric shrinkage varied from 7.65 % to 21.0 %.The lowest value was observed in Teak wood, and the highest in case of Massaranduba wood.The relationship between volumetric shrinkage (sum of radial and tangential shrinkage) and specifi c density has been already confi rmed be several authors (Choong and Achmadi, 1991;Hernández, 2007).Moreover, wood density had an infl uence on the fi ber saturation point during the testing of nine tropical hardwoods from Peru and sugar maple wood from Quebec (Hernández, 2006).According to results of Jankowska et al. (2017), the relationship between wood basic density and its sorptive properties is strongly affected by high extractives content and it is clear only when material with high extractives content is analyzed.Despite the reported relationship between volumetric shrinkage and wood density, no signifi cant correlation was found in this study.It has been already confi rmed by Hernández (2007) that wood density has a positive effect on total volumetric shrinkage However, the analysis of total volumetric shrinkage and wood density did not give a clear confi rmation of signifi cant relationship.The relationship was not signifi cant (P < 0.1), and the correlation (R 2 = 0.24) was also rather low, indicating that only 24 % of the variation in volumetric shrinkage was accounted for by wood basic density.However, when European wood species were not included in this analysis, the relationship between the basic density and volumetric shrinkage of wood was statistically significant (R 2 = 0.44).According to Babiak and Kúdela (1995), the wood density and its structure play important roles.The species tested here revealed a similar structure of diffuse-porous wood.In some (Courbaril, Light red meranti) but not all cases, an expanded axial parenchyma was observed.

ZAKLJUČAK
In this study, sorptive properties of wood species used in fl oors production in Europe were determined and compared.In the course of research, 17 tropical wood species were tested and analyzed.For comparison, selected European wood species -European beech, European oak and Scots pine, were also tested.
Findings and results show that equilibrium moisture content of tropical wood species is lower than that of temperate zone wood species.African padouk, Teak, Afzelia and Ipe showed the lowest values of moisture content.Furthermore, these wood species showed the lowest values of fi ber saturation point (17.7 % for African padouk, 22.5 % for Teak, 19.7 % for Doussié and 18.7 % for Ipe).Among tropical wood species, Light red meranti and Massaranduba samples showed the highest values of moisture content at different air conditions.Furthermore, it was established that wood density affected positively volumetric swelling of tested tropical hardwoods.Regression analysis indicated a statistically signifi cant relationship when the coefficient of volumetric total shrinkage was considered.Thus, the higher wood density, the higher are the dimensions changes.
The results provide crucial knowledge applicable in practice in using many tropical wood species.Furthermore, this indicates that a common recommendation should be considered for all wood species.Based on the results obtained, the possibility of supplementing the recommendations in industry standards regarding wooden fl oors and other applications of wood should be considered, especially in case of recommended values of moisture content.It would help to avoid signifi cant problems during production, installation and exploitation of wooden products such as fl oors.

Fig. 1 -
3 show the adsorption and desorption isotherms for selected species used in this study.To avoid double presenting of the results, only isotherms for a few wood species are presented as examples.

Table 2
Relative

Table 3 )
(Choong and Achmadi, 1991;isture content were observed for European wood species, especially in case of European beech.The lowest values of moisture content were observed for African padouk, Teak, Doussié and Ipe.Among tropical wood species, Light red meranti, Merbau, Massaranduba, Sapele and Tauari showed the highest values of moisture content at different air conditions.This was probably caused by the relatively low content of nonpolar extractives of these species (Wanshura et al., 2014;Jankowska et al., 2017).According to t-test results, the difference between equilibrium moisture content of European oak, European beech and Scots pine are not signifi cant.The European wood samples displayed signifi cantly (the signifi cance value was less than 0.05) higher value of equilibrium moisture content than tropical wood samples when air humidity was 55 % and above.In case of low air humidity, the differences were not observed.As it is known from former studies(Choong and Achmadi, 1991; Popper et al. Jankowska et al., 2017)Voulgarridis, 2012;Jankowska et al., 2017), the difference between equilibrium moisture content of European wood and tropical wood is caused by the amount of extractives in wood structure, which can be hydrophobic.

Table 3
Change in equilibrium moisture content in adsorption and desorption of selected tropical and European wood species depending on relative humidity Tablica 3. Promjena ravnotežnog sadržaja vode u adsorpciji i desorpciji istraživanih tropskih i europskih vrsta drva ovisno o relativnoj vlazi zraka