Introduction

Wood as a raw material is widely used for structural and nonstructural purposes both indoors and outdoors. In outdoor uses, exposure to sunlight radiation, moisture, and microorganisms can severely change wood characteristics, particularly the physical, mechanical, and color attributes. As a biodegradable material, wood can undergo oxidation and depolymerization of lignin and cellulose, which lead to material loss and discolored surfaces (Xie et al. 2005; Ghosh et al. 2009; Bhat et al. 2010).

Wood degradation due to outdoor exposure is also apparent in the characteristics of cellulose, particularly the crystalline fraction. The crystalline fraction determines numerous properties of wood-based materials, including their physical, mechanical, and chemical properties. These properties encompass Young’s modulus, tensile strength, hardness, density, dimensional stability, and acoustical properties, as well as moisture regain, dye sorption, and chemical reactivity (Lionetto et al. 2005; Bucur 2006; Lionetto and Maffezzoli 2008). Other factors under natural conditions, such as temperature, humidity, and precipitation, also affect wood performance through impacts on the hygroscopic properties of wood. These impacts induce changes in the physical and mechanical properties of wood when its moisture content is below the fiber saturation point (Kretschmann 2010).

Indonesia has thousands of tropical wood species, but only about 400 can be identified. In the tropical area of Southeast Asia, only two seasons are recognized, that is, a dry season and a rainy season. The environmental conditions have allowed sunlight radiation almost the same in a year with highly precipitation. Changes in wood properties are unavoidable since the climatic conditions favor the conditions that contribute to the deterioration process of wood products. For that reason, evaluation of wood properties as a function of outdoor exposure should be determined. The aim of this study was to evaluate wood density, moisture content, color change, ultrasonic wave velocity, and the dynamic modulus of elasticity of five commercially important wood species in Indonesia after 10 months (300 days) of outdoor exposure in field testing.

Materials and method

Wood material

Samples were prepared from sawn boards that were obtained from the market. The chosen boards were clear of defects and were already in an air-dried condition (moisture content about 13%). A total of 25 test samples of size 75 mm × 20 mm × 300 mm (width, thickness, length) were used from five wood species, namely sengon or albizia (Paraserianthes falcataria), meranti or shorea (Shorea spp.), mahoni or mahagony (Swietenia spp.), teak (Tectona grandis), and merbau (Intsia spp.). Five replications were done for each wood species.

Outdoor exposure

All samples were placed in an 80-cm-tall weathering rack facing south that was inclined at 45 °C. Since the sun rises directly in the east and sets directly in the west, positioning the weathering rack facing south ensured that the samples were equally exposed to sunlight radiation each day throughout the experimental period. The exposure lasted for 10 months, or 300 days, starting in February 2016 and ending in December 2016. Outdoor exposure occurred on a roof top, where there were no obstacles to sunlight radiation and other climatic factors (i.e., precipitation and wind; Fig. 1). The study was conducted in Darmaga districts, Bogor, West Java (06°33′23.3″S, 106°43′49.9″E). Data on temperature, relative humidity, and other climatic conditions (i.e., daily average precipitation, sunlight radiation, and wind velocity) were obtained from the Meteorological, Climatological, and Geophysical Agency.

Fig. 1
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Weathering rack for outdoor exposure

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Moisture content and density

The moisture content of wood samples was tested every 15 days in the morning (at 9 am) and afternoon (at 5 pm) using a portable moisture meter. In addition, the exposed samples were also weighed and measured for determining wood density. Wood density was calculated as the ratio of weight to volume.

Color change evaluation

Color change caused by natural weathering was evaluated through scanning the sample surface every month or 30 days. Images were acquired with a scanner (Hewlett-Packard Deskjet Ink Advant type k209a-z). Three scans were done for each conditioned sample without further preparation. The images were imported and analyzed using Adobe Photoshop® CS software (Adobe Systems Inc., USA) with 1330 × 2333 pixels at three consistent focusing points for every exposure period (Fig. 2). The software generated the L*, a*, and b* values. The color changes were determined by referring to the coordinates using CIE Lab system, in which L* denotes lightness or brightness (black to white), and a* and b* denote redness (green to red) and yellowness (blue to yellow), respectively. This system is the most common method for estimating the color of a material. The change in color (ΔE) of each sample was calculated with the following equation:

$$\Delta E = \sqrt {\left( {\Delta L^{*} } \right)^{2} + \left( {\Delta a^{*} } \right)^{2} + \left( {\Delta b^{*} } \right)^{2} }$$

where ΔE is the wood color change due to weathering, ΔL* is the difference between the values of initial L* and after exposure, Δa* is the difference between the values for the initial a* and after each exposure, and Δb* is the difference between the values for the initial b* and after each exposure.

Fig. 2
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Representative color change appearance on day 0 (a), day 30 (b), and day 300 (c) for five tropical species (A: merbau, B: mahagony, C: shorea, D: teak, E: sengon or albizia)

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Color change was determined by referring to the five color change levels defined by Cui et al. (2004): (1) the color change is not perceived (0 < ΔE* < 1.5); (2) the color change is barely perceived (1.5 < ΔE* < 3.0); (3) the change is perceived (3.0 < ΔE* < 6.0); (4) the color change is quite noticeable (6.0 < ΔE* < 12.0); and (5) the color is completely changed (ΔE* > 12.0).

Ultrasonic wave velocity evaluation

The ultrasonic transmitting pulse method was used to determine the ultrasonic wave velocity (Vus) based on time of flight. The longitudinal ultrasonic pulse was generated with SylvatestDuo® equipment, using a frequency of 22 kHz. The measurement was done by inserting one conical transducer at each end. One transducer acted as a transmitter, and the other as a receiver of ultrasonic waves. Every sample was measured at three points at its end, with ultrasonic velocity calculated three times. The samples were tested every month or 30 days. The ultrasonic wave velocity (Vus) was calculated by dividing the distance between the transducer (m) by the average of three repeated readings of time for ultrasonic wave transmittal (s).

Dynamic modulus of elasticity

Dynamic modulus of elasticity (MOEd) was determined based on the relationship between wood density and ultrasonic wave velocity:

$${\text{MOEd}}\, = \,\rho {\text{ Vus}}^{ 2}$$

where ρ is the wood density (g cm−3) and Vus is the ultrasonic wave velocity (m s−1).

Statistical analysis

Univariate variance analyses using a general linear model with the factors exposure time and species were conducted to evaluate moisture content, density, and color change (ΔE*), as well as the change in the ultrasonic velocity and MOEd associated with outdoor exposure for 10 months. The statistical package SPSS version 20.0 was employed in this study. All results were considered significant if the probability level was less than or equal to 5% (P ≤ 0.05).

Results and discussion

The Meteorological, Climatological, and Geophysical Agency reported the average environmental conditions at the study site. The daily temperature, humidity, precipitation, sunlight radiation, and wind velocity from February 2016 until December 2016 averaged 26.14 °C (standard deviation, SD = 0.82), 85.38% (SD = 3.88), 17.25 mm (SD = 20.27), 4.90 h (2.87), and 1.94 knots (SD = 0.83), respectively. All samples remained in an air-dried condition with a moisture content in the range of 7.65–14.76% during the experimental period (Table 1). Moisture content tended to decrease with longer exposure after the third month. Statistical analysis found that the moisture content was significantly different (α = 0.05) among species and duration of exposure and for the interaction of species and time exposure. Further analysis using Duncan’s test revealed that moisture content at the end of the exposure period was significantly different from other time points as a function of the length of exposure.

Table 1 Change in moisture content and wood density as a function of time exposure
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Outdoor exposure leads to the depolymerization of cellulose and the loss of hemicellulose and lignin content due to oxygen and humidity, as well as the effect of ultraviolet light, which induces photochemical reactions in wood (Mattos et al. 2014; de Cademartori et al. 2015; Oberhofnerová et al. 2017). Generally, weathering causes wood to become more hydrophilic because the extractives are leached from the surface, causing the wood to become less water-repellent (Willams 2005). In this study, the moisture content tended to decrease, but the values fluctuated up and down every month. The moisture content for all samples remained at an equilibrium, which may have been due to the ability of the wood to absorb and release water vapor from the environment and to the measurement time during the day. Rachmasari (2017) also found that moisture content decreased for jointed wood products made from two tropical species, meranti (Shorea spp.) and keruing (Dipterocarpus spp.), which had outdoor exposure for 210 days.

The five commercial wood species used in the current study were chosen because they are frequently used in outdoor applications such as garden furniture. The wood density ranged from low to high among the five species. Albizia or sengon wood possessed the lowest density; meranti or shorea, mahoni or mahagony, and teak were in the middle in terms of wood density; and merbau (Intsia spp.) had the highest wood density. The low-density wood was about 0.2 g cm−3, middle values were about 0.5–0.6 g cm−3, and high values were about 0.8 g cm−3 (Table 1). As a function of exposure time, the wood density decreased, especially after 300 days. Santos et al. (2015) reported decreasing wood density values of Eucalyptus hybrid, which were significant after 360 days of exposure.

The appearance and color changes were verified as outcomes of exposure time. Figure 3 shows the change in appearance, with all wood samples becoming darker by the end of the exposure period, with the exception of merbau wood. The lightness parameter (L*) showed a significant change in the first 90 days. In merbau wood, darkening occurred until the 90th day, after which the color become lighter. Regarding the redness parameter (a*) and yellowness parameter (b*), the color changed after 300 days or 10 months of exposure to natural weathering compared with the initial value. The redness parameter (a*) showed a lower value than the yellowness parameter (b*). Interestingly, the redness and yellowness values found in merbau wood were almost the same. Merbau is known as a wood with high physical and mechanical properties as well as durability and an attractive dark red-brown color. The wood contains extractive substances that act as water-soluble polymers (Hu et al. 2012). Exposure to natural weathering for 300 days was presumed to leach out these substances, causing the colors to become lighter as the duration of exposure increased.

Fig. 3
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Appearance and color change parameter of L*, a*, b*, and ΔE* for 10 months (300 days) of outdoor exposure (symbol: open circle = L*; open rectangle = a*; open triangle = b*), a albizia, b shorea, c mahagony, d teak

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The overall color change (ΔE*) in wood increased with longer exposure time and totally changed by 60 days, with a ΔE* value of > 2.0, with the exception of merbau wood (Fig. 3 and Table 2), which started changing after 120 days. Teak had the greatest overall color change since the start of outdoor exposure (ΔE* = 12.09), and it was noted to be sensitive to exposure with regard to natural weathering. Shorea wood had a relatively small change in color in terms of the value of ΔE* at 300 days. Meanwhile, merbau wood did not show much color change in the beginning, but start changing after exposure for 90 days. Statistical analysis showed that color change (ΔE*) was significantly different (α = 0.05) among species and exposure times and interactions of species and time exposure. According to the Duncan’s multiple range tests, teak wood was significantly different from other wood species based on grouping subsets, with 60 and 90 days being subsets that were separated from other exposure times.

Table 2 Wood color change (ΔE*) due to weathering for 10 months (300 days) of outdoor exposure
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Ultrasonic velocity (Vus), together with wood density, was measured to determine the MOEd as an estimation of wood stiffness, which is correlated with wood strength in terms of mechanical properties (Bucur 2006; Karlinasari et al. 2008; Baar et al. 2015; Güntekin and Aydin 2016). The average ultrasonic velocity of the five tropical wood species studied was 5700 m s−1 in initial condition (0 days). After outdoor exposure for 300 days, it was about 5800 m s−1 (Fig. 4). The decreasing ultrasonic velocity at 60 days for all wood species was presumed to be caused by environmental conditions. Based on data from the Meteorological, Climatological, and Geophysical Agency, at 60 days, or in April 2016, there was high daily precipitation compared with other months. In this case, the ultrasonic wave propagation was influenced by the moisture content of wood (Oliveira et al. 2005, Karlinasari et al. 2016). Bucur (2006) reported that the range of ultrasonic velocities in wood was 2500–5800 m s−1. Meanwhile, Karlinasari et al. (2005) found that ultrasonic velocity of four tropical hardwoods in an air-dried condition was in a range of 4000–6000 m s−1. The MOEd values of wood were varied (Fig. 5) because other wood characteristics were involved, especially wood density. As with the ultrasonic velocity, the MOEd decreased by 60 days of exposure. The fluctuating MOEd values were likely caused by sensitivity to the environmental conditions. Statistical analysis revealed that the factors species and time of exposure were significantly different as was the interaction between species and time of exposure (α = 0.05) for both ultrasonic velocity and MOEd value. The Duncan’s multiple range tests showed that there were four subsets of the time exposure function for which both Vus and MOEd were significantly different between the initial (0 days) and last (300 days) outdoor exposure.

Fig. 4
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Ultrasonic wave velocity of five tropical wood species as a function of exposure time in field test

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Fig. 5
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Dynamic modulus of elasticity (MOEd) of five tropical wood species as a function of exposure time in field test

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Conclusion

Outdoor exposure in terms of natural weathering for 10 months led to changes in all wood properties tested; that is, moisture content, wood density, color, ultrasonic wave velocity, and MOEd were altered over the course of 300 days. The color parameter of lightness (L*) changed by the 90th day, and the overall color change (ΔE* > 12.0) occurred by the 60th day, with the exception of merbau, which changed at the fourth month (120 days). All wood species darkened by the final exposure compared with initial condition (0 days), except for merbau, which became lighter. Shorea and teak were sensitive in terms of color change as a function of time exposure. The ultrasonic velocity and MOEd were sensitive to environmental conditions and changed as a function of species and time exposure, especially at the final time point of 10 months (300 days) of outdoor exposure.