Evaluation of Wood Resistance to Artificial Weathering Factors Using Compressive Properties

The purpose of the study was to determine the infl uence of artifi cial weathering on selected properties of wood. This paper presents the changes of incompressive strength of wood along fi bers. The study involved wood of different density and anatomy 17 species of tropical wood commercially available in Europe and Scots pine and European oak. The specimens were exposed to artifi cial weathering consisting of soaking wood in water, drying at 70 °C and exposure to UV radiation. Three-step aging cycle was repeated 140 times. In general, the tested wood species changed their compressive strength differently under the infl uence of artifi cial weathering. The process of artifi cial weathering caused a loss of strength of all tested wood species. The extent of changes depended on initial properties of wood (especially density) and anatomy. The changes were most pronounced at the beginning of the artifi cial weathering process.


INTRODUCTION 1. UVOD
The increasing demand for tropical wood products leads to expanded trade offers, and requires knowledge of the characteristics of exotic wood such as its physical, mechanical, technological properties and its resistance to external factors.Tropical wood species, having wide trunk diameters, attractive texture and high resistance to decay, are used in different industries e.g.furniture, timber, plywood, and outdoor applications such as garden furniture, fences, facades, terraces, etc. (Williams, 2005;Kilic and Niemz, 2012).The decisive element for the use of different wood species in the harsh outside environment is their durability.Natural durability of wood depends on work conditions (hazard classes and construction features), as well as on the type of wood.Variations of weather conditions and prolonged exposure to weathering elements cause the process called weathering.
Many researchers, involved in changes of wood properties caused by varying environmental conditions, describe the natural weathering of wood as a process of irreversible changes in the appearance and properties of a material as the effect of a long-term impact of weather: solar radiation, air and oxygen contained in it, changes in temperature and humidity, assuming no direct infl uence of biotic factors (Holz, 1981;Feist, 1990;Feistand Hon, 1984;Hon et al, 1986;Tolvaj and Faix, 1995;Colom et al, 2003;Williams, 1999Williams, , 2005)).Wood weathering is a complex phenomenon (multifactorial) caused by solar radiation and by hydrolysis and leaching of wood components.Due to cyclical changes in humidity, swelling and shrinkage have a signifi cant infl uence on changes in wood properties in the weathering process.Due to slowness and questionable repeatability of the process, it is diffi cult to examine wood weathering and its consequences.Substantial changes often appear in real terms after many years of using wood.For this reason, various methods of artifi cial weathering were developed in laboratories to simulate the natural effect of weather conditions and to determine changes occurring in wood in short time.These methods differ between themselves in order and intensity of effects of individual factors (eg.Temiz et al., 2007;Evans et al., 2008;Follrich et al., 2011;Miklečić and Jirouš-Rajković, 2011).In addition, the fact should be taken into account that the size of samples used to determine the mechanical properties of artifi cially weathered wood is unrestricted, one published study -right for a type of test samples and artifi cial weathering -cannot be directly related to the results obtained in other trials.
Degradation phenomena have been reported in literature, e.g.Evans et al. (1996)  In this paper, the effects of artifi cial weathering on compressive properties of several species of wood (assuming the absence of biotic interactions) were determined and compared to each other.The study includes seventeen species of wood from foreign forests (heartwood), commercially available in Europe.Parallel studies were performed on the control wood: pine Pinus sylvestris L. (individual sapwood and heartwood) and European oak Quercus sp.(heartwood).As the processs of weathering progressed, compression strength along fi bers was determined.

Preparation of test specimens 2.1. Priprema uzoraka
Wood species selected for the research (Table 1) are a group of materials used for production of elements used in external conditions (such as elevation, terrace boards, garden furniture, etc.).This wood group represents different types of structures (coniferous, deciduous ring-pours and diffuse-pours) and differs in some details related to anatomy and density (Figure 1).
Samples of each wood species were collected from one board to obtain "identical sample".Thanks to these samples, density was similar and the structure was kept in order to appearing changes in the weathering process, this being the main factor deciding on the examined properties.30 groups of 6 samples were prepared from each wood species.Dimensions of samples were 15.0 x 15.0 x 22.5 mm (the last dimension along fi bers).Each group was intended for the study of different stages of weathering.Before the determination of properties, each group was air conditioned at a temperature close to 20 °C (±2) and relative humidity (rH) around 60 % (±5).

Artifi cial weathering method 2.2. Izlaganje umjetnim atmosferskim uvjetima
The design of the artifi cial weathering cycle was based on literature (Matejak et al., 1983: Follrich, 2011).It took 30 hours to complete an artifi cial weathering cycle and it was divided into three steps (Figure 2).The fi rst step was soaking speciemens in water at 20 °C (16 h).The conditions of the second step (8 h) were 70 °C and 5-10 % rH and the third step was performed at 30 °C and 20-25 rH (6 h) with irradiation with UV rays.Four fl uorescent lamps100R's Lightech of 100 W each, and the spectrum 300 -400 nm (90 % of the radiation spectrum is a wavelength of 340 -360 nm) were used for irradiating.140 cycles of artifi cial weathering were conducted.

Mechanical testing 2.3. Određivanje tlačne čvrstoće
Examination of compressive strength of wood along fi bers was performed before weathering, and then The use of slightly smaller dimensions of samples was a deviation from the mentioned norm.The deviation concerning the sample size occurred in view of the fact that no relation exists between the compressive strength along fi bers and the size of samples when they are geometrically similar and when the section of the samples contained at least a couple of annual increments (Matejak et al., 1983).The aim of this treatment was to cause great changes in wood during the aging process.Examination of compressive strength of wood along fi bers (RC) was carried out on the 10-ton universal testing machine.Constant speed of loading samples of 2 mm/min was used during the compression tests.For individual groups of samples, average values as well as standard deviations were calculated.
In order to illustrate the effect of artifi cial weathering of wood, the approximate percentage of decrease of compressive strength along fi bers per one cycle of artifi cial weathering was determined by the formula: where: ΔRC -percent decrease in compressive strength along fi bers per one cycle of artifi cial weathering, RC 0 -compressive strength of wood along fi bers before artifi cial weathering, RC n -compressive strength of wood along fi bers after n number of cycles of artifi cial weathering, n -number of artifi cial weathering cycles.

REZULTATI I RASPRAVA
The study shows that the compressive strength of wood along fi bers was reduced due to the artifi cial weathering process.The same direction of changes was observed for every tested wood species.By analyzing the average values of compressive strength along fi bers, it has been observed that gaboon showed the greatest changes.140 cycles of artifi cial aging process caused the loss of strength c. 35 % (from an initial value of 43 MPa to the fi nal 28 MPa).Merbau also showed a large change -the change of c. 33 % occurred from 65 MPa to 43 MPa.Teak showed the greatest resistance to weathering stated by the smallest change in compressive strength along fi bers.Strength of wood was falling from the initial value of 58 MPa to 48 MPa (reduction c. 18 %).Massaranduba showed a similar range of changes of compressive strength due to aging factors.140 cycles of artifi cial weathering caused a loss of the compressive strength along fi bers from the initial value of 92 MPa to 74 MPa (reduction c. 20 %).European oak, Scots pine (sapwood and heartwood) showed the rate of change of compressive strength along fi bers similar to opepe and garapa (about 28 %).There was no difference in strength loss between Scots pine sapwood and heartwood.Both of them (heartwood and sapwood) were made from one board, which was the material of similar properties (similar density).It is important to emphasize European coniferous wood species, because sapwood is an important and often dominant part of trunk volume.
Based on the results, it can be assumed that, due to aging, the loss of compressive strength of wood along fi bers is mainly caused by changes in wood structure.Cyclical changes in humidity and temperature caused strong stress (sorption and thermal) exceeding internal cohesion of wood.It resulted in cracks and signifi cant loss of wood strength.There was also a mass loss due to leaching of a number of extractives from the cell walls and partial hydrolysis of hemicellulose and cellulose (relaxed frame of ligno-cellulosic, the disintegration of some long-chain tissue constituents of wood) in subsurface layers, which consequently led to the reduction of wood density.The main cause of deterioration of mechanical properties lies in cyclical changes of moisture conditions, whicih causes tissue destruction of wood (wood cracks).Wood cracks are the consequence of sorption stresses, which occur during rapid wetting and quick drying (Matejak et al., 1983;Feist, 1983Feist, , 1990;;Feist and Hon, 1984;Williams, 1999Williams, , 2005)).High frequency of changes in humidity causes cracking of wood -the larger amplitude and changes of frequency, the larger the damage.The cumulative impact of wood weathering factors leads to the damage of a wood structure, which is refl ected in changes of its initial mechanical properties.
It is hard to indicate one particular factor that affects the extent of loss of compressive strength along fi bers of tested wood species.This is probably the interaction of many factors with many interactions between them -this is a submicroscopic construction of wood (cell walls), microscopic (size, layout and contribution of individual structural elements -rays, parenchyma, fi bers, vessels), macroscopic (width and layout of annual growth, the share of earlywood and latewood) and its chemical composition (types of extractives -resins, tannins, oils, minerals and other).
Gaboon is a wood with tangled texture -wood shows variations of cross grain.Deviation of wood fibers from the direction parallel to the longitudinal axis of the trunk causes an additional loss of strength.The loss of strength rises with increasing slope and cracks.Similarly, merbau has an irregular arrangement of fibers.Any deviation from fi ber direction parallel to the direction of the destructive force causes the loss of strength (slope of 15° lowers the compressive strength of wood by 20 % -Kollmann and Côte, 1968).Additionally, in the case of merbau, a relatively large content of extractives negatively affects the change of strength (Grabner et al., 2005).
The change of the compressive strength along fibers of garapa and opepe, similar to European wood species of much lower density (Figure 1), can also be explained by a cross grain.Both opepe and garapa wood are characterized by variations of defl ected fibers.In the case of teak, the smallest change in strength can be explained by limited changes in wood moisture due to artifi cial weathering.Acting hydrophobically, oily substances limited alternating swelling and shrinking, after soaking in water and drying, which led to weakening of wood structure through desorption cracks.The presence of oils in cumaru had no effect on changes in strength.Cumaru, merbau, gaboon and opepe show a cross grain.The consequence of irregular structure is the reduction of the compression strength from 107 MPa to 78 MPa (approx.27 %).The loss of compressive strength of other wood species ranged between 22 and 32 %, and without taking into account the extreme case of teak wood, the volume change decreases with the increase of wood density.
Irrespective of wood species, the results of testing compressive strength along fi bers at different stages of artifi cial weathering process can be described by a straight line (a correlation coeffi cient above 0.91 in all cases) -Table 2. Regression analysis shows that the higher the density of wood is, the slope (gradient) value (a) decreases and the value of constatnt term (b) increases.
To illustrate the effect of artifi cial weathering of tested wood species, the approximate percent loss of the compressive strength along fi bers per one cycle of aging (ΔRC) was determined.Irrespective of wood species, the character of changes was similar.Because of this, a few examples of the calculation of results are presented in Figure 3.The fi rst cycles of artifi cial weathering process have the greatest infl uence on the loss of compressive strength.It could be due to the fact that the initial rapid changes in wood moisture caused the strongest stress sorption.As the number of conducted cycles of artifi cial aging rises, the loss of strength for one aging cycle is getting smaller and tends to a constant value (from about 45-50 cycles of the artifi cial aging process).Thus, the progressive aging changes in the strength of wood proceed more slowly.

ZAKLJUČAK
The presented results showed that the process of artifi cial weathering causes a reduction of the compressive strength along fi bers of all tested wood species.The Figure 3 The dependence of decrease of compressive strength along fi bers after one cycle of artifi cial weathering on the number cycles of artifi cial weathering for light red meranti, ipe, teak and European oak Slika 3. Ovisnost smanjenja tlačne čvrstoće uzduž vlakanaca nakon jednog ciklusa izlaganja umjetnim atmosferskim uvjetima o broju ciklusa izlaganja drva svjetlocrvenog merantija, ipe, tika i europskog hrasta umjetnim atmosferskim uvjetima weathering factors, expressed through the smallest decrease in the compressive strength along fi bers.It is followed by massaranduba (the heaviest wood in the group).while gaboon and merbau showed the lowest resistance (loss of strength was partly the result of irregular arrangement of fi bres).European wood species (oak and Scots pine) showed greater resistance to weathering factors as wood of similar density but of interlocked fi bres.At later stages of artifi cial weathering, the loss of compressive strength along fi bers, after one cycle of artifi cial weathering.gets smaller and tends to a constant value -the highest intensity of change takes place at the beginning of the artifi cial weathering process.The rate (intensity) of changes depends on the initial density of wood.

REFERENCE S 5. LITERATURA
studied the loss of mass and chemical changes occurring in wood Pinus radiata D. Don. during weathering.Evans et al. 2008, Bhat et al. (2010) tested wood and modifi ed wood materials including its mechanical properties during weathering.Changes of physical properties were mainly tested (Oltean et al., 2009; Schnabel et al., 2009).Researches of tropical wood in this fi eld are few.An example of the study of artifi cial weathering of tropical wood are tests made by Pastore et al.(2004) and Oltean et al. (2010) -change of appearance and color were tested.