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Article

Artificial Weathering Effects on the Physical and Chemical Properties of Paulownia tomentosa and Pinus koraiensis Woods Heat-Treated in Oil and Air

by
Intan Fajar Suri
1,2,†,
Byantara Darsan Purusatama
3,†,
Jong-Ho Kim
1,
Wahyu Hidayat
2,
Apri Heri Iswanto
4,
Se-Yeong Park
1,
Seung-Hwan Lee
1 and
Nam-Hun Kim
1,*
1
Department of Forest Biomaterials Engineering, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon 24341, Republic of Korea
2
Department of Forestry, Faculty of Agriculture, University of Lampung, Bandar Lampung 35145, Indonesia
3
Institute of Forest Science, Kangwon National University, Chuncheon 24341, Republic of Korea
4
Department of Forest Products Technology, Faculty of Forestry, Universitas Sumatera Utara, Medan 20155, Indonesia
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2023, 14(8), 1546; https://doi.org/10.3390/f14081546
Submission received: 10 July 2023 / Revised: 25 July 2023 / Accepted: 27 July 2023 / Published: 28 July 2023
(This article belongs to the Section Wood Science and Forest Products)
Browse Figures
Versions Notes

Abstract

:
This study evaluated the effect of artificial weathering on the surface characteristics, physical properties, and chemical compositions of oil heat-treated (OHT) and air heat-treated (AHT) Paulownia tomentosa and Pinus koraiensis woods. The untreated and heat-treated samples at 180, 200, and 220 °C for 2 h were exposed to ultraviolet (UV) and water for artificial weathering for 168 and 336 h, respectively, according to ASTM G53-96. The weathering experiment comprised 2 h of UV-light irradiation followed by 2 h of condensation cycles. Color change, volumetric swelling, and chemical components of weathered samples were determined. In both species, heat-treated woods showed a significantly lower total color change and volumetric swelling than the untreated woods. The total color change and VS of OHT woods were significantly lower than those of AHT. Fourier transform infrared spectra indicated that the lignin peaks in untreated and AHT woods decreased after weathering exposure, whereas the peaks were stable in OHT woods. Oil heat treatment is an effective method to stabilize dimensional change and color quality during weathering.

1. Introduction

Paulownia tomentosa is a fast-growing species that is widely distributed worldwide. The wood has low density and shrinkage and high dimensional stability [1]. Therefore, Paulownia wood is widely used as a material for furniture, construction, pulp and paper, particleboard, handicrafts production, bioenergy applications, and in making musical instruments [1,2,3,4,5].
Pinus koraiensis is a native and major plantation tree species used commercially in Korea, with a high growth rate, low density, and shrinkage. These characteristics make the wood suitable for furniture, particleboard, fiberboard, and wooden boats [1,6]. However, both wood species have low aesthetic value due to their light creamy or pale brown color [1].
Some modification methods, such as chemical, thermal, and impregnation, can improve wood properties. Thermal modification represents a popular method and has been generally used for wood modification [7,8]. Air, nitrogen, steam, oil, and vacuum are used as heating media for heat treatment [9,10,11,12,13,14,15,16].
Air heat treatment is a slow pyrolysis process between 180 °C and 260 °C under an oxidative atmosphere, resulting in higher hemicellulose degradation and a shorter time to alter the physical and chemical properties of wood. In addition, heat treatment in hot air is a low-cost method to improve the dimensional stability, hydrophobicity, durability, and color properties of wood [7,14,17,18,19,20,21,22,23,24]. Esteves et al. [23] reported that AHT Pinus pinaster wood at 170–200 °C had lower equilibrium moisture content and higher dimensional stability than steam heat-treated wood. In contrast, the mechanical properties of AHT wood were slightly lower than the steam heat-treated wood. Quirino et al. [24] reported that Populus nigra wood air heat-treated at 235 °C showed darker color than the wood heat-treated with water vapor and nitrogen.
Heat treatment with hot oil also has been commonly used for improving wood properties through the synergetic effects of oil and heat. During oil heat treatment, oil separates oxygen from wood and minimizes oxidative processes, which can slow down the weakening in the strength of the treated wood. In addition, oil heat treatment generates a fast and uniform heat transfer rate to wood and surface protection by the absorbed oil. Oil heat treatment can improve wood properties for outdoor use, such as uniformly darkening wood surfaces, improving fungal resistance, increasing density and compression strength, and reducing shrinkage [12,25,26,27,28]. Dubey et al. [29] explained that the weathering properties of oil-heat-treated (OHT) wood, such as color and dimensional stability, improved because the oil protected the wood surfaces from ultraviolet (UV) light and water absorption.
So far, the physical and mechanical properties of air- and oil-heat-treated P. tomentosa and P. koraiensis woods have been studied. Hidayat et al. [17,18] reported that the equilibrium moisture content and wettability of AHT P. tomentosa and P. koraiensis woods at 160, 180, 200, and 220 °C decreased, and the color of the wood surface became darker as the temperature increased, which was more preferred by consumers than the lighter colors of untreated wood. Kim et al. [19] explained that the lightness (L*) of AHT P. tomentosa wood at 160, 180, 200, and 220 °C decreased with increasing temperature. The density and relative crystallinity of heat-treated P. tomentosa wood remained constant and increased slightly. Suri et al. [20,21,22] reported that OHT P. tomentosa and P. koraiensis woods showed a darker color, lower weight loss in abrasion and volume shrinkage, and higher density, compressive strength, and hardness than AHT woods.
Conclusively, compared to air heat treatment, oil heat treatment is a more effective method for darkening wood color and improving the dimensional stability, compression strength, and hardness of P. tomentosa and P. koraiensis woods [20,21,22]. To date, no information on the weathering properties of OHT and AHT P. tomentosa and P. koraiensis woods is available to fully understand the properties of both for exterior use. Therefore, we investigated the weathering properties of OHT and AHT P. tomentosa and P. koraiensis woods to provide valuable information for effectively using both wood species.

2. Materials and Methods

2.1. Materials

Table 1 lists the basic information about the sample trees. In the present study, three trees of each P. tomentosa (15–20 years old) and P. koraiensis (30–35 years old) were harvested from Kangwon National University (KNU) research forest, Chuncheon, Korea (N 37°77′, E 127°81′). Seven defect-free quarter-sawn boards, each from near the bark of both species, with dimensions of 160 mm (longitudinal) × 50 mm (radial) × 20 mm (tangential) were prepared for heat treatment. The samples were oven-dried and put in a conditioning room at 25 ± 5 °C under 60 ± 5% RH until equilibrium moisture content was established.

2.2. Heat Treatment

A commercial palm oil (Lotte Foods, Seoul, Korea) was used as heating media for oil heat treatment. The wood samples were fully soaked in a lab-scale oil bath with 5 L volume (C-WHT-S2; ChangShin Science, Seoul, Korea), and then the temperature was raised from 25 ± 5 °C to the target temperatures of 180, 200, and 220 °C using 2 °C/min heating rate. The target temperature was maintained for 2 h. The samples were removed from the oil bath after the bath naturally reached 25 ± 5 °C, and then the samples were drained in a room for 24 h.
For air heat treatment, the samples were treated in an electric furnace (Supertherm HT16/16; Nabertherm GmbH, Lilienthal, Germany). The temperature was raised from 25 ± 5 °C to 180, 200, and 220 °C at a heating rate of 2 °C/min and maintained at the target temperature for 2 h. The samples were removed from the furnace after the furnace chamber naturally reached room temperature (25 ± 5 °C). All heat-treated samples were oven-dried at 105 ± 3 °C for 24 h and then kept in a desiccator with silica gel for a week.

2.3. Artificial Weathering Test

The artificial weathering test was conducted according to the ASTM G53-96. Three wood samples 50 (L) mm × 50 (R) mm × 20 (T) mm dimensions for each treatment were prepared for the weathering test, as shown in Table 2. The AHT, OHT, and untreated wood were exposed to UV lamps in a QUV accelerated weathering tester (QUV/se Accelerated Weathering Tester, Q-LAB, Westlake, OH, USA) for 168 h and 336 h. The weathering cycle involved continuous light irradiation with UV exposure for 2 h and condensation for 2 h. The average irradiance for UV exposure was 0.83 W/m2 at 340 nm wavelength at a constant chamber temperature of 60 °C. The condensation process was performed at 50 °C. After the weathering test, the exposed samples were stored in a room at 25 ± 5 °C until further evaluation.

2.4. Color Change

The color before and after the weathering was evaluated with the CIE L*a*b* system using a chroma meter (CR-10 Plus, Konica Minolta Inc., Tokyo, Japan). In addition, the color of each sample was measured at three points on the radial surface, as shown in Figure 1. The color change before and after the weathering test was measured with three replications and determined according to the following equation:
ΔE* = (ΔL*2+ Δa*2+ Δb*2)½
where ΔE*, ΔL*, Δa*, and Δb* represent the changes in overall color, lightness, red/green chromaticity, and yellow/blue chromaticity, respectively.

2.5. Volumetric Swelling (VS) Measurement

To measure VS, UV-exposed and oven-dried samples were immersed in water and weighed every 24 h until the weight was constant. VS was determined using the following formula:
VS = (V1 − V0)/V0 × 100 (%)
where V0 and V1 are the volume of the oven-dried specimens before water immersion (mm3) and the volume of the specimens after water immersion (mm3), respectively.

2.6. Fourier Transform Infrared Spectroscopy (FTIR)

The wood powder was prepared from the surface of the wood using 800 grit sandpaper to investigate the chemical composition of each untreated and heat-treated sample before and after the artificial weathering tests. The spectra were measured with the attenuated total reflectance (ATR) method in the wavenumber of 400–4000 cm−1 using an FTIR spectrometer (FT-IR, Nicolet Summit, Thermo Fisher Scientific, Waltham, MA, USA) installed at the Department of Forest Biomaterials Engineering at KNU.

2.7. Statistical Analyses

In the present study, the color change and volumetric swelling experiment were performed with three replications. Differences in color change and volumetric swelling after the weathering test among heat treatment temperatures and between heat treatment methods were statistically analyzed at a 5% significance level using univariate analysis of variance, followed by post hoc Duncan’s multiple range tests using SPSS (ver. 26, IBM Corp., New York, NY, USA).

3. Results and Discussion

3.1. Color Change after Artificial Weathering

Figure 2 and Figure 3 show the appearance of the heat-treated and untreated P. tomentosa and P. koraiensis wood, respectively, after the artificial weathering test. Macroscopically, both control and AHT woods at 180 °C showed a golden-yellow color after weathering. AHT P. tomentosa woods at 200 °C and 220 °C showed a grayish color, whereas AHT P. koraiensis woods at 200 °C and 220 °C showed dark-yellow color. In addition, the AHT woods at 200 °C and 220 °C showed an uneven color distribution after weathering exposure. In contrast, the color difference before and after the weathering of the OHT woods at all temperatures was negligible. As reported by Yildiz et al. [30], the weathered control woods (ash, Scots pine, and spruce) turned gray and dark, whereas the steam heat-treated woods turned silver-gray due to weathering. Furthermore, they mentioned that the light-colored woods generally tended to turn yellow or brown, and the dark-colored woods tended to bleach due to weathering. In the present study, OHT woods, after weathering exposure, showed a more uniform color than AHT woods, which could be due to heat treatment with oil providing even surface protection. As mentioned in previous studies, oil as heating media in heat treatment generated a fast and uniform heating transfer and surface protection. In addition, linolenic acid in could protect the OHT wood surface from UV ray and water penetration during weathering [20,26,29].
Figure 4 and Figure 5 show the lightness (L*), red/green chromaticity (a*), and yellow/blue chromaticity (b*) of the OHT and AHT P. tomentosa and P. koraiensis woods after the artificial weathering tests. The L* values of untreated and heat-treated P. tomentosa woods at 180 °C decreased with increasing weathering duration (Figure 4). There was no significant difference in L* values of AHT and OHT P. tomentosa woods at 200 and 220 °C between before and after weathering exposure.
In P. koraiensis, the L* values of untreated and AHT woods at 180 and 200 °C significantly decreased as the duration of weathering increased, whereas AHT woods at 220 °C showed no significant change in L* values after weathering exposure. The L* values of OHT woods at 180 °C significantly decreased with increasing weathering duration, whereas those of OHT woods at 200 and 220 °C hardly changed (Figure 5).
In both species, the a* values of untreated and heat-treated wood at 180 °C significantly increased after weathering exposure. The a* values of heat-treated woods at 200 and 220 °C hardly changed after weathering exposure.
The b* value of untreated and heat-treated P. tomentosa wood at 180 °C significantly increased after weathering for one week. Furthermore, the b* values of untreated and AHT woods at 180 °C were comparable between one and two weeks of weathering. There was no significant difference in the b* values before and after weathering exposure of AHT woods at 200 and 220 °C. The b* value of OHT P. tomentosa wood at 180 and 200 °C significantly increased with increasing exposure duration, whereas that of OHT P. tomentosa wood at 220 °C tended to remain constant with increasing weathering exposure. In P. koraiensis, the b* value of untreated and heat-treated woods greatly increased after one week of exposure and became constant after two weeks.
Several studies support our results regarding the color change of heat-treated wood after artificial weathering. Nemeth et al. [31] reported that the dry heat-treated Pannonia poplar wood at showed a higher decrease in L* value than the OHT wood after weathering for 40 h. Huang et al. [32] reported that the L*, a*, and b* values of steam heat-treated Pinus banksiana wood slightly increased after weathering for 168 h and rapidly decreased after 336 to 1500 h of weathering.
The overall color changes (ΔE*) after the artificial weathering test for one and two weeks of P. tomentosa and P. koraiensis are shown in Figure 6 and Figure 7, respectively. The ΔE* values of the heat-treated woods after the weathering test were significantly smaller than those of the untreated woods. The ΔE* value significantly decreased with increasing treatment temperature. The OHT woods had significantly lower color change than the AHT woods after the weathering test, indicating higher color stability in OHT woods. In addition, heat-treated P. tomentosa wood showed a smaller total color change than heat-treated P. koraiensis wood. Deka et al. [16] reported that the total color change of thermally modified Picea abies wood after UV light irradiation for 400 h was noticeably lower than that of untreated wood. Ahajji et al. [33] also mentioned that the color of heat-treated P. abies wood in nitrogen circumstance at 210 °C to 250 °C for 1 h showed improved stability to UV radiation than that of untreated samples. Additionally, Dubey et al. [29] reported that the color of OHT Pinus radiata wood after the weathering test was more stable than that of the untreated sample. They explained that after oil heat treatment, the oil forms a protective layer on the wood surfaces and acts as a barrier for UV and water absorption during artificial weathering.

3.2. VS in the Weathered Woods

Figure 8 shows the VS in the heat-treated and untreated P. tomentosa and P. koraiensis wood exposed to artificial weathering.
In untreated woods of both species, VS tended to increase after weathering exposure, but the difference was not significant. The VS in AHT P. tomentosa wood at 180 and 200 °C increased after weathering exposure, whereas there was no significant difference before and after weathering. The VS in AHT P. tomentosa wood at 220 °C significantly increased after weathering exposure. In addition, there was no significant difference in VS of AHT P. tomentosa between one and two week exposure. The VS in AHT P. koraiensis wood at 180 and 200 °C significantly increased after weathering exposure, whereas there was no significant difference at 220 °C before and after weathering.
The VS in OHT P. tomentosa wood at 180 °C significantly increased after weathering exposure. The VS in OHT P. tomentosa wood at 200 °C and 220 °C increased after weathering, but the difference was not significant. The VS in OHT P. koraensis wood at all temperatures significantly increased after weathering exposure.
The VS in AHT, the untreated and AHT P. tomentosa woods after one week of weathering at all temperatures was comparable. After two weeks of weathering of AHT wood, the AHT P. tomentosa wood showed a significantly lower VS than the untreated woods, and the VS significantly decreased with increasing treatment temperature. After one and two weeks of weathering, the untreated and AHT P. koraensis wood at 180 °C showed a comparable VS, and the VS significantly decreased at 200 °C.
The OHT woods exposed to artificial weathering for one and two weeks showed significantly lower VS than the AHT woods, revealing higher dimensional stability in OHT wood. Among the OHT woods, the OHT wood at 220 °C showed the lowest VS, whereas there was no significant difference between the OHT woods at 180 and 200 °C.
In the present study, the OHT woods demonstrated better dimensional stability than the untreated and AHT woods, which is supported by several previous studies. Dubey et al. [29] reported that after accelerated UV weathering, the VS in OHT Pinus radiata wood increased; however, the increase was smaller than that in the untreated wood, indicating improved dimensional stability in the heat-treated wood samples. Bal et al. [34] reported that Fagus orientalis wood treated in hot sunflower oil showed significantly better dimensional stability than that treated in hot air. Yang et al. [35] observed the properties of the Phyllostachys edulis bamboo heat-treated in three different media, including air, nitrogen, and oil, at 150, 170, 190, and 210 °C for 1, 2, and 3 h, and revealed that heat treatment in oil was the most effective method to improve the dimensional stability of the bamboo.
There are some plausible explanations for the dimensional stability of OHT wood. Oil usually contains high levels of linolenic acid, which tends to dry quickly and can harden on the wood surface after heat-treatment with oil [36]. Linolenic acid preserves the OHT wood surface from UV ray and water penetration during weathering [20]. The higher dimensional stability of OHT wood after the artificial weathering test may be due to the increase in lignin stability due to condensation during the thermal modification process [16,29,36].

3.3. Chemical Composition

The FTIR spectra of heat-treated P. tomentosa and P. koraiensis woods before and after weathering are presented in Figure 9 and Figure 10, respectively.

3.3.1. Chemical Composition of AHT Woods before and after Weathering

AHT woods at 180 °C for both species showed comparable FTIR spectra to the untreated woods. The peaks at 1650 cm−1 and 809 cm−1 disappeared in AHT P. tomentosa wood at 200 and 220 °C, whereas the peaks were not detected in AHT P. koraiensis wood at 220 °C. The peak at 1157 cm−1 of both AHT woods became smaller at 220 °C, whereas the peaks at 1105 cm−1 and 1050 cm−1 became greater at 200 and 220 °C. The lignin peak intensity at 1502 cm−1 of both AHT woods, 1230 cm−1 of AHT P. tomentosa wood, and 1260 cm−1 of AHT P. koraiensis wood decreased at 220 °C.
In the present study, AHT woods showed a noticeable change in the carbohydrate peaks with increasing temperature, which is consistent with previous studies. Lin et al. [37] reported that the peak intensity at 1730−1740 cm−1 representing C=O stretching vibrations of acetyl, carbonyl, and carboxyl groups of heat-treated poplar and fir wood decreased with increasing temperature. Yildiz et al. [30] reported that the decrease in the water peak at 1650 cm−1 for heat-treated pine and ash woods may be related to the acid-catalytic hydrolysis of wood polyose esters at elevated temperatures. Huang et al. [38] reported an increase in the C–O peak at 1103 cm−1 after heat treatment due to the formation of new alcohols and esters, which could be related to the improvement of the hydrophobicity of wood.
After weathering exposure for 2 weeks, the lignin peak intensity at 1590 cm−1, 1502 cm−1, 1460 cm−1, 1260 cm−1, and 1230 cm−1 of untreated and AHT woods at 180 and 200 °C decreased, whereas the lignin peaks of the AHT woods at 220 °C showed no noticeable change after weathering exposure. In this study, untreated and AHT woods showed a decrease in the peak of aromatic-skeletal vibration in lignin, which is consistent with a few earlier studies. Yildiz et al. [30] reported that lignin peaks at 1600 cm−1 and 1510 cm−1 decreased significantly due to UV irradiation during weathering in both untreated and heat-treated ash, iroko, Scots pine, and spruce woods. Huang et al. [38] reported that all lignin peaks at 1600 cm−1, 1510 cm−1, 1465 cm−1, 1263 cm−1, and 1103 cm−1 in untreated and heat-treated Pinus banksiana, Populus tremuloides, and Betula papyrifera woods decreased significantly due to weathering.

3.3.2. Chemical Composition of OHT Woods before and after Weathering

The carbohydrate peaks at 1720 cm−1, 1365 cm−1, 1157 cm−1, and 1105 cm−1 in OHT woods of both species were noticeably higher than those in the untreated and AHT woods of both species. Additionally, the peaks of aromatic-skeletal vibration in lignin at 1460 cm−1 in the OHT woods of both species were noticeably greater than those in the untreated and AHT woods of both species. Carbohydrate peaks at 1183 and 750 cm−1 were detected in OHT woods, whereas they were absent in untreated and AHT woods. Some of the sharp peaks in the OHT woods could be related to the functional groups of edible palm oil. Some previous studies [39,40,41,42] have explained the contribution of functional groups of edible oil to FTIR peaks, such as carbonyl esters of the triglycerides at 1742 cm−1, bending vibrations of the CH2 and CH3 aliphatic groups at 1465 cm−1, symmetrical bending of CH3 at 1377 cm−1, stretching vibration of the C–O ester groups at 1238 and 1163 cm−1,–C–O and CO stretching, CH2 bending vibration at 1155 cm−1, and methylene rocking vibration at 720 cm−1.
In OHT P. tomentosa woods, the peak at 1650 cm−1 disappeared at 200 °C and 220 °C, whereas the peak remained unchanged in OHT P. koraiensis wood. The carbohydrate peaks at 1157 cm−1 and 890 cm−1 and lignin peaks at 1502 cm−1, 1230 cm−1, and 1260 cm−1 of OHT woods remained constant with increasing temperature. It seems that there was no significant change in FTIR spectra of OHT woods with increasing temperature due to the contribution of functional groups of edible oil to FTIR peaks, which confirmed the contribution of palm oil to the improvement of color and dimensional stability of OHT woods.
After weathering exposure, the peak intensity of aromatic-skeletal vibration in lignin in the OHT woods of both species did not exhibit a noticeable change, indicating an improvement in color and dimensional stability. Dubey et al. [21] reported that linolenic acid in oil protects the surface of OHT wood from UV penetration and water absorption during weathering. The higher color and dimensional stability of OHT wood after the artificial weathering test may be due to the increase in lignin stability due to condensation during the thermal modification process [16,29,36]. The carbohydrate peaks at 1183 and 1157 cm−1 in the OHT woods of both species decreased after weathering exposure, which could be related to oil degradation due to weathering exposure. As previously reported [39,40,41,42], these peaks may be due to the functional groups in edible palm oil. In OHT P. koraiensis wood, the peak at 1650 cm−1 decreased after two weeks of weathering exposure, whereas the peak remained constant in OHT P. tomentosa wood. This decrease may be due to the leaching or water removal of previously formed oxidized compounds, such as quinones [42].

4. Conclusions

After weathering exposure, the color of untreated and AHT woods at 180 °C of both species changed from whitish to golden-yellow. AHT P. tomentosa woods showed a grayish color at 200 °C and 220 °C, whereas AHT P. koraiensis woods showed a dark-yellow color. There was no noticeable color difference in the OHT woods of both species after weathering. The AHT and OHT woods of both species had significantly lower total color change and VS than the untreated woods. However, OHT woods showed significantly higher color and dimensional stability than AHT.
In the FTIR spectra of AHT woods, the intensity of carbohydrate peaks noticeably changed with increasing temperature, whereas those peaks of OHT woods remained constant with increasing temperature. The peaks at 1720 cm−1, 1365 cm−1, 1157 cm−1, and 1105 cm−1 1460, 1183 cm−1, and 750 cm−1 in OHT woods were sharper than those in AHT, which related to palm oil substances. The FTIR spectra of untreated and AHT woods of both species showed a noticeable decrease in lignin peak intensity after weathering exposure, whereas in OHT woods, the peaks were unchanged, exhibiting color and dimensional change stability. The intensity of some carbohydrate peaks in untreated and AHT woods increased after weathering exposure, but the peaks decreased in OHT P. koraiensis wood and remained constant in OHT P. tomentosa wood.
In conclusion, oil heat treatment at 200 °C and 220 °C effectively improved the color and dimensional stability of both P. tomentosa and P. koraiensis woods by weathering. These results will assist in determining the outdoor application and suitable heat treatment for woods of both species. In addition, to overcome the cost of palm oil, further study on the reusability of palm oil used for heat treatment should be considered for future research.

Author Contributions

Conceptualization, I.F.S. and N.-H.K.; methodology, I.F.S., S.-Y.P. and N.-H.K.; validation, N.-H.K.; formal analysis, I.F.S. and B.D.P.; investigation, I.F.S. and B.D.P.; resources, S.-Y.P. and N.-H.K.; data curation, I.F.S. and B.D.P.; writing—original draft preparation, I.F.S. and B.D.P.; writing—review and editing, B.D.P. and N.-H.K.; visualization, I.F.S. and B.D.P.; supervision, S.-Y.P., W.H., A.H.I. and N.-H.K.; project administration, J.-H.K.; and funding acquisition, S.-H.L. and N.-H.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF2016R1D1A1B01008339; NRF-2018R1A6A1A03025582), Science and Technology Support Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Science and ICT (MSIT) (NRF-2022R1A2C1006470), and the R&D Program for Forest Science Technology (Project Nos. 2021350C10-2323-AC03) by the Korea Forest Service (Korea Forestry Promotion Institute).

Data Availability Statement

The datasets generated and analyzed during the current study are not publicly available but are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 14 01546 g001 550
Figure 1. Color change measurements of samples were taken at three points (red circles).
Figure 1. Color change measurements of samples were taken at three points (red circles).
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Forests 14 01546 g002 550
Figure 2. Appearances of air- and oil-heat-treated P. tomentosa wood after the weathering. Unexposed (A) and exposed to weathering for one (B) and two (C) weeks.
Figure 2. Appearances of air- and oil-heat-treated P. tomentosa wood after the weathering. Unexposed (A) and exposed to weathering for one (B) and two (C) weeks.
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Forests 14 01546 g003 550
Figure 3. Appearances of air- and oil-heat-treated P. koraiensis wood after weathering. Unexposed (A) and exposed to weathering for one (B) and two (C) weeks.
Figure 3. Appearances of air- and oil-heat-treated P. koraiensis wood after weathering. Unexposed (A) and exposed to weathering for one (B) and two (C) weeks.
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Forests 14 01546 g004 550
Figure 4. Color changes in oil heat-treated (OHT) and air heat-treated (AHT) P. tomentosa woods during the artificial weathering. (L*), Lightness; (a*), red/green chromaticity; and (b*), yellow/blue chromaticity. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
Figure 4. Color changes in oil heat-treated (OHT) and air heat-treated (AHT) P. tomentosa woods during the artificial weathering. (L*), Lightness; (a*), red/green chromaticity; and (b*), yellow/blue chromaticity. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
Forests 14 01546 g004
Forests 14 01546 g005 550
Figure 5. Color changes in oil-heat-treated (OHT) and air-heat-treated (AHT) P. koraiensis woods during the artificial weathering. (L*), lightness; (a*), red/green chromaticity; and (b*), yellow/blue chromaticity. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
Figure 5. Color changes in oil-heat-treated (OHT) and air-heat-treated (AHT) P. koraiensis woods during the artificial weathering. (L*), lightness; (a*), red/green chromaticity; and (b*), yellow/blue chromaticity. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
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Forests 14 01546 g006 550
Figure 6. Overall color changes (ΔE*) in P. tomentosa wood after weathering for one (A) and two (B) weeks. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
Figure 6. Overall color changes (ΔE*) in P. tomentosa wood after weathering for one (A) and two (B) weeks. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
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Forests 14 01546 g007 550
Figure 7. Overall color changes (ΔE*) in P. koraiensis wood after weathering for one (A) and two (B) weeks. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
Figure 7. Overall color changes (ΔE*) in P. koraiensis wood after weathering for one (A) and two (B) weeks. The different letters on the histogram is a significant outcomes at a 5% significance level for comparisons between the samples using the Duncan’s multiple range tests.
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Forests 14 01546 g008a 550Forests 14 01546 g008b 550
Figure 8. Volumetric swelling in water-immersed P. tomentosa (A) and P. koraiensis (B) woods exposed to artificial weathering. The same capital and lowercase letters on the histogram represent the insignificant outcomes at the 5% significance level between weathering duration and between treatment conditions (AHT, OHT, and control), respectively.
Figure 8. Volumetric swelling in water-immersed P. tomentosa (A) and P. koraiensis (B) woods exposed to artificial weathering. The same capital and lowercase letters on the histogram represent the insignificant outcomes at the 5% significance level between weathering duration and between treatment conditions (AHT, OHT, and control), respectively.
Forests 14 01546 g008aForests 14 01546 g008b
Forests 14 01546 g009 550
Figure 9. FT-IR spectra of air-heat-treated (AHT) and oil-heat-treated (OHT) P. tomentosa woods before and after weathering. Before weathering (A), one-week weathering (B), and two-week weathering (C).
Figure 9. FT-IR spectra of air-heat-treated (AHT) and oil-heat-treated (OHT) P. tomentosa woods before and after weathering. Before weathering (A), one-week weathering (B), and two-week weathering (C).
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Figure 10. FT-IR spectra of air-heat-treated (AHT) and oil-heat-treated (OHT) P. koraiensis woods before and after weathering. Before weathering (A), one-week weathering (B), and two-week weathering (C).
Figure 10. FT-IR spectra of air-heat-treated (AHT) and oil-heat-treated (OHT) P. koraiensis woods before and after weathering. Before weathering (A), one-week weathering (B), and two-week weathering (C).
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Table
Table 1. Basic sample tree information.
Table 1. Basic sample tree information.
Common NameScientific NameAge
(Years)		D.B.H.
(mm)		Air-Dried Density (g/cm3)Location
Royal paulowniaPaulownia tomentosa (Thunb.) Steud.15–20280–3300.30 (0.05) *Chuncheon, Korea
(37°77′ N, 127°81′ E)
Korean white pinePinus koraiensis Siebold & Zucc.30–35280–3200.42 (0.05) *
* Numbers in parentheses are standard deviations.
Table
Table 2. Heat-treated wood sample information for artificial weathering tests.
Table 2. Heat-treated wood sample information for artificial weathering tests.
Sample DimensionSpeciesHeating
Media		Temperature and DurationSample Number 1Total
50 (L) × 50 (R) × 20 (T) mm3Paulownia tomentosa (Thunb.) Steud.Oil and airControl
180 °C for 2 h
200 °C for 2 h
220 °C for 2 h 		3 × 2
3 × 2 × 2
3 × 2 × 2
3 × 2 × 2		42
Pinus koraiensis Siebold & Zucc.
1 Sample number = replication × species × heating media.
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MDPI and ACS Style

Suri, I.F.; Purusatama, B.D.; Kim, J.-H.; Hidayat, W.; Iswanto, A.H.; Park, S.-Y.; Lee, S.-H.; Kim, N.-H. Artificial Weathering Effects on the Physical and Chemical Properties of Paulownia tomentosa and Pinus koraiensis Woods Heat-Treated in Oil and Air. Forests 2023, 14, 1546. https://doi.org/10.3390/f14081546

AMA Style

Suri IF, Purusatama BD, Kim J-H, Hidayat W, Iswanto AH, Park S-Y, Lee S-H, Kim N-H. Artificial Weathering Effects on the Physical and Chemical Properties of Paulownia tomentosa and Pinus koraiensis Woods Heat-Treated in Oil and Air. Forests. 2023; 14(8):1546. https://doi.org/10.3390/f14081546

Chicago/Turabian Style

Suri, Intan Fajar, Byantara Darsan Purusatama, Jong-Ho Kim, Wahyu Hidayat, Apri Heri Iswanto, Se-Yeong Park, Seung-Hwan Lee, and Nam-Hun Kim. 2023. "Artificial Weathering Effects on the Physical and Chemical Properties of Paulownia tomentosa and Pinus koraiensis Woods Heat-Treated in Oil and Air" Forests 14, no. 8: 1546. https://doi.org/10.3390/f14081546

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