Abstract
The ability of Citrus limon (L.) Burm wood to be used for flooring applications was considered in this work. Selected wood properties were determined, and the response to weathering was tested. Janka hardness, density, screw withdrawal resistance, color, glossiness, and artificial weathering (144, 288, and 432 h) were determined. The wood from Citrus limon was shown to be a dense wood with air density around 830 kg/m3 and with a high Janka hardness of 138.0 N/mm2, 120.4 N/mm2, and 115.9 N/mm2 for cross, tangential, and radial sections, respectively. This opens good perspectives to this kind of wood being used for flooring. Statistical analysis showed that both Janka hardness and screw withdrawal resistance were significantly different between surfaces. Nevertheless, the homogeneity groups showed that the main differences were between the surfaces in the longitudinal direction when compared with the transverse direction. The wood is lighter than most common woods and it is mainly yellow. After weathering, the wood became darker, redder, and yellower. Glossiness decreased in the first hours of the weathering period.
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Determination of Some Wood Properties and Response to Weathering of Citrus limon (L.) Burm Wood
Sırrı Sahin,a Ümit Ayata,b,* Bekir Cihad Bal,c Bruno Esteves,d Ahmet Can,e and Hüseyin Sivrikaya e
The ability of Citrus limon (L.) Burm wood to be used for flooring applications was considered in this work. Selected wood properties were determined, and the response to weathering was tested. Janka hardness, density, screw withdrawal resistance, color, glossiness, and artificial weathering (144, 288, and 432 h) were determined. The wood from Citrus limon was shown to be a dense wood with air density around 830 kg/m3 and with a high Janka hardness of 138.0 N/mm2, 120.4 N/mm2, and 115.9 N/mm2 for cross, tangential, and radial sections, respectively. This opens good perspectives to this kind of wood being used for flooring. Statistical analysis showed that both Janka hardness and screw withdrawal resistance were significantly different between surfaces. Nevertheless, the homogeneity groups showed that the main differences were between the surfaces in the longitudinal direction when compared with the transverse direction. The wood is lighter than most common woods and it is mainly yellow. After weathering, the wood became darker, redder, and yellower. Glossiness decreased in the first hours of the weathering period.
Keywords: Lemonwood; Weathering; Color; Gloss; Janka hardness
Contact information: a: Department of Agricultural Buildings and Irrigation, Agriculture Faculty, Ataturk University, Erzurum, Turkey; b: Faculty of Arts and Design, Department of Interior Architecture and Environmental Design, Bayburt University, Bayburt, Turkey; c: Vocational School of Technical Sciences, Kahramanmaras Sutcu Imam University, Kahramanmaras, Turkey; d: Polytechnic Institute of Viseu and Research Center for Natural Resources, Environment and Society (CERNAS) Viseu, Portugal; e: Department of Forest Industrial Engineering, Faculty of Forestry, Bartin University, Bartin, Turkey;
* Corresponding author: umitayata@bayburt.edu.tr
INTRODUCTION
The lemon tree Citrus limon (L.) Burm is a member of the Rutaceae family that consists of approximately 140 genera and 1,300 species (Yilmaz and Güneser 2017). C. limon is planted for its fruit, which is one of the most known fruits all over the world. Turkey is a big producer of this fruit, amounting to a total of 710,400 tons/year of lemons. According to Yildirim et al. (2010), the major lemon producing areas are located in Turkey’s east Mediterranean plains and mainly coastal parts along the Cukurova region. In this region the main producers are located in Mersin and Adan, and the region accounts for almost 98% of the lemons produced in Turkey (Yildirim et al. 2010).
There is a lot of information about the fruit from the Citrus limon tree; however, there is little information about the wood. This might be one of the biggest problems in marketing this kind of wood, because its properties are not well known by consumers. The knowledge of some technological properties might be useful when exploring new uses for this kind of wood, such as flooring, as an example. Recently Berti et al. (2018) studied the possibility of using orange wood, which is also a member of the Rutaceae family, for flooring applications. These authors concluded that this kind of wood has excellent technical characteristics to be used as wood flooring and that the aesthetic characteristics were appreciated by professionals and end users.
According to Fang et al. (2012), the professional associations, such as the National Wood Flooring Association (NWFA) and flooring manufacturers, rank the wood species by their Janka hardness. They also report that architects usually prefer tropical hardwoods due to their high densities. Nevertheless, the decreased availability of these tropical woods leads wood engineers to look for alternative wood materials.
The most used wood for flooring worldwide is oak, probably due to its high density and hardness but also due to its aesthetic look. There are several oak species, but most of them have high hardness. For instance, Uzcategui et al. (2000) studied two different oaks, red oak and white oak and concluded that Janka hardness was 6300 N and 6600 N in tangential surface and 5800 N and 5900 N in radial surface, respectively. Meier (2015) presented the Janka hardness of 16 different species of oak and reported a Janka hardness ranging from 4720 N to 7310 N.
Another important property in wood is its color, which significantly affects its aesthetic look. Architects often choose wood species based on their color, and generally whiter colors are preferred. This has been reported in the work of Wahl (2000), where Japanese consumers stated that light color was preferred in wood floorings. Another advantage of light colored woods is that they can become darker by using a stain or varnish, while darker woods cannot become lighter.
Weathering is important when wood is exposed outside. The first changes to be noticeable are the changes in color. Initially darker woods become lighter, whereas lighter woods become darker and later on, both acquire a similar gray tone, which has been attributed to residual cellulose and fungal growth such as that of Aureobasidium pullulans (Kucuktuvek et al. 2017). Chemically, the wood polymer most sensible to weathering is lignin, which suffers photochemical damage by UV light, further exposing cellulose and hemicelluloses and leading to their degradation. The mechanical damage of the surfaces can be due to rain, hail, or wind but mostly is owed to shrinkage and swelling of wood.
The goal of this study was to determine some wood properties (screw withdrawal resistance, Janka hardness, glossiness, color properties, and physical properties) and examine the response to artificial weathering of Citrus limon wood grown in Mezitli-Mersin, Turkey.
EXPERIMENTAL
Materials
Wood boards of Citrus limon (L.) Burm were obtained from a timber company (Erdem Odun – Kömür) located in Mersin, Turkey.
Methods
Determination of physical properties
Equilibrium moisture content and air-dried density were determined following TS 2471 (1976) and TS 2472 (1976), respectively. Ten samples were tested.
Determination of Janka hardness
The samples were cut to 5 cm × 5 cm × 5 cm, totalizing 10 samples, for testing the Janka hardness. On these cut wood materials, the Janka hardness values of tangential, cross, and radial surface were made according to the TS 2479 (1976) standard (Fig. 1A).
Fig. 1. Instruments used for different property measurement: (A) Janka hardness, (B) screw withdrawal resistance, (C) QUV weathering tester, (D) gloss meter, and (E) spectrophotometer
To determine the measurements, 10 test samples were prepared. The Janka Hardness Hj (N) was determined using Eq. 1,
Hj = K × Pmax (N/mm2) (1)
where Pmax is the load (N) during the insertion of the loading tip into a certain depth within the test piece. The coefficient K is defined by whether the loading end reaches a depth of 5.64 mm, in which case it equals 4/3, and if it reaches 2.82 mm depth, then it is equal to 1. In this study, K values for each group were taken as 1 because there was no cracking in the test samples.
Determination of screw withdrawal resistance
Screw withdrawal resistance of the samples on the tangential, cross, and radial surface was performed following the TS EN 13446 (2005) standard. In this study, screws with dimensions of 4 mm x 50 mm were used. The screw was introduced on the wooden material 20 mm (Fig. 1B) and pulled back to determine the screw withdrawal resistance on wood.
The strength was calculated by the Eq. 2 given below,
f = Fmax / (d × lp) (2)
where f is the screw withdrawal resistance (N/mm2), lp is the entrance depth (mm), d is the diameter given by the manufacturer (mm), and Fmax is the maximum load (N). Ten replicates were used
Artificial weathering
Ten wood samples were exposed in a QUV weathering tester (Q-Lab, Corporation Westlake, USA) (shown in Fig. 1C) following the ASTM G154-06 (2006) standard that is comprised of 8 h ultraviolet exposure, 18 min water spray with 0.75 W/m2 light intensity, and 50 °C temperature using UV-A 340 nm. This cycle was repeated for a period of 144, 288, and 432 h.
{\displaystyle C^{\star }={\sqrt {a^{\star \,2}+b^{\star \,2}}},\qquad h^{\circ }=\operatorname {atan2} \left({b^{\star }},{a^{\star }}\right)}Determination of glossiness measurement
Glossiness parallel (//) and perpendicular (⊥) to grain at 20°, 60°, and 85° angles were recorded with a Gloss Meter Poly-gloss GL0030 TQC device (TQC BV, Neuss, Germany) (Fig. 1D) in 100 mm × 100 mm × 17 mm samples according to the ISO 2813 (1994) standard. Ten replicates were used for each test.
Determination of color measurement
Red color (a*) tone, yellow color (b*) tone, lightness (L*), hue (H°), chroma (C), and total color (ΔE*) of 10 wood samples were determined using an X Rite Ci62 series portable instrument (X-Rite Europe GmbH, Regensdorf, Switzerland) with a wavelength resolution of 10 nm and measurement geometry D/8°) (Fig. 1E) with a D65 standard illuminant as per ASTM D2244-3 (2007). The CIELAB system, characterized by the three axes L*, a*, and b*, was used. The L* axis represents lightness, varying from 100 (white) to zero (black); a* is the red (+) to green (–) tone; and b* is the yellow (+) to blue (–) tone (Esteves et al. 2019). The corresponding changes ΔL*, Δa*, and Δb* after weathering were determined using Eqs. 3 to 5:
ΔL* = L*weathered – L*un-weathered (3)
Δb* = b* weathered – b* un-weathered (4)
Δa* = a* weathered – a* un-weathered (5)
The total color difference (ΔE*) was calculated by Eq. 6:
ΔE* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2 (6)
Statistical analysis
In this study, minimum values, maximal values, standard deviations, maximum values, homogeneity groups, analysis of variance, and multiple comparisons using data obtained before and after weathering were all determined using SPSS 17 software (Sun Microsystems, Inc., Santa Clara, CA, USA). One-way ANOVA followed by Duncan’s multiple range test was used. The level of significance considered was 5%.
RESULTS AND DISCUSSIONS
The wood of Citrus limon has been shown to be a dense wood with an air density of approximately 834 kg/m3 which is similar to orange wood that also belongs to the Rutaceae family with 827 kg/m3 (Berti et al. 2018). The density of this kind of wood is comparable to that of Turkey oak (801 kg/m3) (Bajraktari et al. 2018) or Rowan wood (801 kg/m3) (Korkut et al. 2009), which are two dense woods from Turkey.
Janka hardness of Citrus limon wood was approximately 120.5 N/mm2 for tangential, 115.9 N/mm2 for radial, and 138.1 N/mm2 for cross surfaces (Table 1). This means that its hardness is higher on the cross surface, followed by tangential and radial surfaces. A similar result was reported by Peng et al. (2016) for six different species: Chinese fir, red pine, Mongolian Scotch pine, Manchurian walnut, Asian white birch, and Mongolian oak. Holmberg (2000) studied the influence of grain angle on Brinell hardness of Scots pine (Pinus sylvestris L.) and concluded that this higher hardness on the cross surface may be because, when the load was applied parallel to the fiber direction, it was spread over several fibers but when it was applied perpendicularly, the amount of fibers carrying the load was lower. Nevertheless, a more plausible explanation would be that cohesive strength inside a fiber is higher than adhesive strength between many fibers.
The hardness of Citrus limon wood was higher than all of the woods presented in the work of Helińska-Raczkowska and Molinski (2003) with 13 different woods where the hardest wood from that study was hornbeam with 116.7 N/mm2, 92.7 N/mm2, and 86.0 N/mm2 hardness in cross, tangential, and radial sections, respectively . Rowan wood is one of the hardest woods grown in Turkey with hardness of 259 N/mm2, 116 N/mm2, and 142 N/mm2 in cross, radial, and tangential surfaces, respectively (Korkut et al. 2009). Citrus limon wood hardness is smaller for the cross-section but not that much smaller for tangential and radial surfaces. High hardness has already been reported for orange wood, which is also a member of the Rutaceae family (Berti et al. 2018). This shows that Citrus limon wood has a good potential to be used for flooring regarding its hardness.
The screw withdrawal resistance was also determined for the cross, radial, and tangential sections, and the results are presented in Table 1. The screw withdrawal resistance was higher for the radial section (53.98 N/mm2) followed by the tangential (50.80 N/mm2) and cross-sections (43.62 N/mm2). Nail and screw withdrawal resistance is important when accessing the workability of wood. This is important for some kinds of flooring as for example stairs that usually require nails and screws.
Kiliç et al. (2007) studied the nail and screw withdrawal resistance of the most common woods used for flooring; the results were reported using a different unit (kgf) and using 5 mm x 50 mm screws with an entrance depth of 35 mm. Converting their results to N/mm2, the screw withdrawal resistance values are, for oak (36.6 N/mm2 (R), 35.4 N/mm2 (T)) and 27.3 N/mm2 (C)), walnut (32.9 N/mm2 (R), 32.8 N/mm2 (T) and 24.3 N/mm2 (C)) and for cherry (28.0 N/mm2 (R), 26.9 N/mm2 (T) and 17.6 N/mm2 (C)). Results show that the screw withdrawal resistance of Citrus limon is higher than all of these woods, showing that this wood has a good potential to be used for flooring that requires the using of screws.
The high screw withdrawal resistance of this wood was expected, since the parameter is dependent on density. It is known that there is a strong relationship between wood density and mechanical properties of wood. In previous studies, some researchers reported this relationship (Kollmann and Cote 1968; Malkoçoğlu 1994; Bektaş 1997; Bal and Bektaş 2018).
The color of Citrus limon wood was determined using the CIELAB system and presented in Table 1. According to the results, lightness was 83.4, showing that this wood is lighter than most commercial woods in Turkey. This wood has a lightness similar to poplar (83.5) or Uludag fir (84.3); however it is much lighter than chestnut (73.3), beech (69), black locust (61.3), red oak (60.5), Doussie (58.3), and Iroko (57.5) (Ayata et al. 2017a, 2018; Gurleyen et al. 2018). The a* and b* values show that this wood is mainly yellow.
The results of the variance analysis of the screw withdrawal resistance and Janka hardness strength in accordance to the surface are given in Table 2. According to the results, both Janka hardness and screw withdrawal resistance were considered to be significantly different between surfaces. Nevertheless the homogeneity groups presented in Table 3 show that the main differences were between the surfaces in the longitudinal direction (radial and tangential sections) compared to the transverse direction (cross-section). Similar results were obtained by Çavuş and Ayata (2018) related to screw withdrawal resistance of some wood species.
Table 1. Some of the Wood Properties of Citrus limon Wood
Table 2. Variance Analysis Results of Janka Hardness and Screw Withdrawal resistance Values for Citrus limon Wood Samples
Table 3. Statistical Data for Janka Hardness and Screw Withdrawal resistance of Citrus limon Wood
The results of the variance analysis of color and perpendicular glossiness at 60° of un-weathered and weathered lemon wood are shown in Table 4. The results of variance analysis on lightness (L*), red color tone (a*), yellow color tone (b*), and perpendicular glossiness at 20° and 60° angles were found significant according to α ≤ 0.05. That means the weathering period had a significant influence both on color and glossiness of the samples. The reason for the glossiness measured at 85° angle not being considered significant might have been due to more irregular results that generally occur at this angle, which may be due to surface roughness, as stated before (Ayata et al. 2017b).
Table 4. Results of the Variance Analysis of Color and Glossiness Before and After Weathering Applications
Table 5 shows the statistical data for perpendicular glossiness and color parameters before and after weathering. Lightness decreased with the increase of the weathering period, and exhibited a maximum 13.8% decrease, which means that the surface became darker. Similar results were presented before for artificial weathering of angelim pedra (Hymenolobium sp.), piquia (Caryocar sp.), Afzelia (Afzelia sp.), Iroko (Milicia excelsa), and Opepe (Nauclea diderrichii), although only for the first 30 to 50 cycles after which L* increased again (Jankowska 2013, 2015). Both a* and b* increased with weathering, which meant that wood surface became redder and more yellow. A similar increase in both a* and b* was presented before for heat treated Scots pine aged for 500 h by Kucuktuvek et al. (2017). Glossiness generally decreased with weathering even though the biggest changes happened in the first 144 h of weathering, after which glossiness stayed approximately the same.
The results for total color difference are given in Table 6. As expected, total color difference increased with weathering. The ΔE* reached its highest value in the first 144 h of weathering. Table 6 indicates that similar darkness was obtained at 144 and 288 h of accelerated weathering in lemon wood, then slightly increased after 288 h. In addition, total color change was remarkably increased at the final stage (432 h) of weathering.
Table 5. Statistical Data for Color Parameters and Perpendicular Glossiness Values Before and After Weathering Applications
Table 6. Changes in Color Parameters After Accelerated Weathering for Lemon Wood
CONCLUSIONS
- Citrus limon wood has been shown to be a dense wood with air density of 834 kg/m3. The Janka hardness for Citrus limon wood was determined as 138.0 N/mm2, 120.4 N/mm2, and 115.9 N/mm2 for cross, tangential, and radial sections, respectively, which along with density might allow this kind of wood to be used for flooring. Screw withdrawal resistance was good and higher in the radial surface (54.0 N/mm2), followed by tangential and cross-sections. Janka hardness and screw withdrawal resistance were significantly different between surfaces, although the main differences were between the surfaces in the longitudinal direction and transverse direction.
- Wood is lighter than most common woods with L* of 83.4 and it is mainly yellow. Weathering darkened the wood and increased both a* and b* values. Glossiness decreased in the first hours of the weathering period, and then stayed approximately the same afterwards. Total color change especially increased after 288 h of weathering.
- The wood properties determined show that Citrus limon wood as good characteristics to be used for flooring applications.
- It is thought that the knowledge of these properties might be useful when exploring new uses for this kind of wood. Nevertheless other properties have to be determined in order to access if this species can have other important applications.
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Article submitted: October 31, 2019; Peer review completed: February 13, 2020; Revised version received: June 5, 2020; Accepted: June 8, 2020; Published: July 20, 2020.
DOI: 10.15376/biores.15.3.6840-6850