Evaluation of Discoloration and Subterranean Termite Resistance of Four Furfurylated Tropical Wood Species after One-Year Outdoor Exposure

: The discoloration and resistance to subterranean termite attack of four furfurylated fast-growing tropical wood species were evaluated after outdoor exposure for 1 year in Bogor, Indonesia. For comparison purposes, imidacloprid-preserved and untreated wood samples were also prepared. Discoloration of all treated samples was measured before and after the furfurylation process. The wood specimens were then placed vertically to three-fourths of their length in the ground for 1 year, at which point they were evaluated for resistance to subterranean termite attack. After furfurylation, wood samples were darker in color than untreated wood, while imidacloprid-preserved wood was lighter. After 1-year exposure, furfurylated wood samples appeared to have the highest resistance to subterranean termite attack. These samples had minimal weight loss, indicating a substantial protection level. Imidacloprid-preserved wood had less resistance to termite attack, but was more resistant than untreated wood.


Introduction
Since 2000, log production in Indonesia has mostly been based on harvesting from plantation forests. In 2019, the wood industry was supplied with 57.9 million m 3 of logs, with more than 85% coming from plantation forests, including 32.6 million m 3 of mangium (Acacia mangium), 9.0 million m 3 of eucalypt (Eucalyptus spp.), and 5.4 million m 3 sengon (Falcataria moluccana) [1]. Mangium, sengon, jabon (Anthocephalus cadamba), and pine (Pinus merkusii) represent the most common fast-growing plantation species. The trees are cut from young-age stands, mostly less than 10 years old, and the timber has a high portion of juvenile wood. Consequently, it has inferior physical-mechanical properties and is susceptible to wood-destroying organisms, especially termites [2].
As a building material, wood needs to have a long service life, but the timber available on the market is mostly from fast-growing wood species, which are prone to termite attack. The economic losses due to termite damage of wooden buildings in Indonesia has been estimated to reach USD 1 billion per annum [3]. If wood currently on the market is to be used for buildings, it has to be preserved beforehand to obtain a longer service life. Wood preservatives are commonly used in the industry, but they pollute the environment and carry the risk of toxicity for living organisms. To reduce or eliminate these adverse effects, methods based on chemical modification of the wood, including furfurylation, have been developed.
Furfurylation involves the use of furfuryl alcohol (FA) and a catalyst that is used under specific conditions of temperature and curing time to accelerate the polymerization Forests 2021, 12, 900 3 of 16 that were 2 cm × 0.8 cm × 20 cm (width, thickness, and length, respectively) were kilndried to reach 12% moisture content (MC) [23], weighed, and then vacuumed at 0.80 bars for 30 min in a tank. For the impregnation process, tartaric acid as catalyst was added to FA (1:20; v/v), and the solution was inserted into the tank when the vacuum was released. Afterward, pressure at 9.81 bars was applied for 30 min.
Following the impregnation process, each wood specimen was wrapped with aluminum foil and then put in the oven at 100 • C for 24 h. The foil was then removed, and the specimen was weighed to calculate polymer loading. Conditioning of the specimens was conducted at room temperature for 3 weeks. For comparison purposes, untreated (control) and imidacloprid-preserved wood specimens were also prepared. For imidacloprid preservation, a 3% imidacloprid solution was used in the same vacuum-pressure treatment as described for wood furfurylation. Ten replications of test samples were done for each wood species and treatment combination.
To identify chemical molecules in the jabon wood before and after furfurylation, also after 1-year outdoor exposure, pyrolysis gas chromatography-mass spectrometry (Py-GCMS; Shimadzu ® GCMS-QP2010) was used. Energy dispersive X-ray detector (EDX) analyses (Zeiss evo 50 Bruker 133 eV) were also carried out to identify the different chemical components in the wood before and after the exposure. The other collected data were weight percent gain (WPG), retention, density, and MC, with the MC calculation following Thybring [24].

Discoloration Determination of the Samples
Color for all wood specimens was determined according to the CIELab method by measuring L* (lightness), a* (red to green), and b* (blue to yellow) values [25], and using a scanner machine (CanoScan 4400F) and the Adobe Photoshop CS5 application. The color change of wood specimens was also calculated by referring to CIELab, while classification was according to Hunter Lab [26] and Hrčková et al. [27].

Termite Resistance of the Samples
A 1-year field test was carried out in an arboretum of IPB University, Bogor, Indonesia. Bogor has a tropical climate with an average total rainfall of 4617 mm·year −1 and an average rainfall of 14.4 mm·day −1 (range, 0-140 mm·day −1 ). The average relative humidity (RH) is 84% (range, 65%-95%), average temperature is 26.4 • C (range, 24.2-28.6 • C), and average amount of sunlight is 5.4 h·day −1 (range, 0.0-10.6 h·day −1 ) [28]. An ombrothermic diagram during the test period is shown in Figure 1.   Each wood specimen was vertically inserted to three-fourths of its length into the ground in a 1-year graveyard test. The conditions of the test location are shown in Figure 2. After 1 year, the weight loss percentage [29] of each specimen and its protection level against subterranean termite attack [30] were determined. The protection level values varied from 0 to 10, where lower values meant less resistance to termite attack.

Analysis of the Data
The data were analyzed in a completely randomized block design using two factors, wood species and treatment. The wood species as a block factor consisted of sengon, jabon, mangium, and pine, while the factor of treatment consisted of untreated wood (control), imidacloprid-preserved wood, and furfurylated wood. Duncan's multiple range test was carried out for further analysis when the main factor was significantly different at p ≤ 0.05.

Furfurylation of the Samples
The results from Py-GCMS are presented in Tables 1 and 2 for untreated and furfurylated jabon woods, respectively. Based on the findings, the furfurylated wood contained new chemical compounds, such as methyl furfuryl compounds, in some peak numbers. The compounds were likely created through reactions between FA and wood components, especially lignin [30]. Furthermore, Gérardin [31] reported that FA molecules could polymerize with each other and generate poly-furfural or poly-furfurylated compounds that fill the void space and bind to the cell wall. These compounds could affect the weight gain and color change of wood.

Analysis of the Data
The data were analyzed in a completely randomized block design using two factors, wood species and treatment. The wood species as a block factor consisted of sengon, jabon, mangium, and pine, while the factor of treatment consisted of untreated wood (control), imidacloprid-preserved wood, and furfurylated wood. Duncan's multiple range test was carried out for further analysis when the main factor was significantly different at p ≤ 0.05.

Furfurylation of the Samples
The results from Py-GCMS are presented in Tables 1 and 2 for untreated and furfurylated jabon woods, respectively. Based on the findings, the furfurylated wood contained new chemical compounds, such as methyl furfuryl compounds, in some peak numbers. The compounds were likely created through reactions between FA and wood components, especially lignin [30]. Furthermore, Gérardin [31] reported that FA molecules could polymerize with each other and generate poly-furfural or poly-furfurylated compounds that fill the void space and bind to the cell wall. These compounds could affect the weight gain and color change of wood.
The density of each wood species according to treatment is shown in Figure 3. The variance analysis is summarized in Table 3, and the results of Duncan's multiple range test are presented in Table 4. Based on the variance analysis, wood density varied by wood species and treatment. Sengon and jabon wood were classified as low-density woods, while mangium and pine wood were medium-density woods. In terms of the treatment, furfurylation significantly increased the wood density, with the average weight percent gain of all wood species reaching 49.1%, and retention reaching 9.9 kg·m −3 (0.01 g·cm −3 ). The density of furfurylated wood was the highest, and it was significantly different from untreated and imidacloprid-preserved woods. However, the two latter types of wood were not different from each other based on the small density increment (0.01 g·cm −3 only) of imidacloprid-preserved wood compared with untreated wood.    furfurylation significantly increased the wood density, with the average weight percent gain of all wood species reaching 49.1%, and retention reaching 9.9 kg•m −3 (0.01 g•cm −3 ). The density of furfurylated wood was the highest, and it was significantly different from untreated and imidacloprid-preserved woods. However, the two latter types of wood were not different from each other based on the small density increment (0.01 g•cm −3 only) of imidacloprid-preserved wood compared with untreated wood.      Retention and the weight percent gain (WPG) values of each wood species are shown in Figure 4. According to the variance analysis, retention and WPG were affected by wood species. Mangium and pine wood had smaller retention and WPG values than the other two wood species. Untreated mangium and pine wood had higher densities, implying less void space and more cell wall components, consequently these types of wood had lower preservative retention and WPG than sengon and jabon. These results were in line with Hadi et al. [32]. Nevertheless, the level of preservative retention was satisfactory in all wood samples; that is, it was more than 8 kg·m −3 and fulfilled the Indonesian standard, which requires 8-14 kg·m −3 [33]. Sengon wood had lower retention and WPG than jabon wood, despite having lower density. This finding could be attributed to sengon wood having an interlocked grain [34]. Furthermore, sengon heartwood was previously found to contain 19.6% extractive substances soluble in NaOH, including carbohydrate derivatives (glucose, ribose, xylose, cellobiose, oligosaccharide, arabinose, and pentasaccharide), and long-chain fatty acids (myristic acid, capriate, pentadecylic acid, myristoleic acid, palmitic acid, stearic acid, margaric acid, linoleic acid, oleic acid, and arachidonic acid) [35].
wood, despite having lower density. This finding could be attributed to sengon wood having an interlocked grain [34]. Furthermore, sengon heartwood was previously found to contain 19.6% extractive substances soluble in NaOH, including carbohydrate derivatives (glucose, ribose, xylose, cellobiose, oligosaccharide, arabinose, and pentasaccharide), and long-chain fatty acids (myristic acid, capriate, pentadecylic acid, myristoleic acid, palmitic acid, stearic acid, margaric acid, linoleic acid, oleic acid, and arachidonic acid) [35]. The moisture content (MC) of each type of wood specimen according to treatment is shown in Figure 5. The variance analysis presented in Table 3 showed that the MC of the wood was affected by the wood species as well as the treatment. The average MC of the four wood species varied between 8.59% and 9.35%, and for untreated woods, the range in average MC was between 11.19% and 12.61%. These values were not significantly different and were still in ambient moisture content in the Bogor area.  The moisture content (MC) of each type of wood specimen according to treatment is shown in Figure 5. The variance analysis presented in Table 3 showed that the MC of the wood was affected by the wood species as well as the treatment. The average MC of the four wood species varied between 8.59% and 9.35%, and for untreated woods, the range in average MC was between 11.19% and 12.61%. These values were not significantly different and were still in ambient moisture content in the Bogor area.
having an interlocked grain [34]. Furthermore, sengon heartwood was previously found to contain 19.6% extractive substances soluble in NaOH, including carbohydrate derivatives (glucose, ribose, xylose, cellobiose, oligosaccharide, arabinose, and pentasaccharide), and long-chain fatty acids (myristic acid, capriate, pentadecylic acid, myristoleic acid, palmitic acid, stearic acid, margaric acid, linoleic acid, oleic acid, and arachidonic acid) [35]. The moisture content (MC) of each type of wood specimen according to treatment is shown in Figure 5. The variance analysis presented in Table 3 showed that the MC of the wood was affected by the wood species as well as the treatment. The average MC of the four wood species varied between 8.59% and 9.35%, and for untreated woods, the range in average MC was between 11.19% and 12.61%. These values were not significantly different and were still in ambient moisture content in the Bogor area.  The MC values of untreated and imidacloprid-preserved wood did not differ from each other, but they were different from furfurylated wood, which had very low MC. In addition to furfurylated wood having poly-FA in the void, FA could also react with wood components, especially lignin, leading to a reduction of hydroxyl compounds and creating methyl furfuryl. These changes could cause the wood to become more hydrophobic [31]. These outcomes were in line with other works reporting that furfurylated wood had a much lower MC and its equilibrium MC decreased by 40% compared with untreated wood [11,36].

Discoloration of Wood Specimens
Wood color was measured before and after the furfurylation process based on Col-orHexa [37]. Wood colors are indicated quantitatively with L*, a*, and b* values, while discoloration is indicated by ∆E [37]. The values of all wood species according to treatment before and after furfurylation are presented in Table 5. Remarks: Values in parentheses are standard deviations; L* is lightness, a* is red to green, and b* is blue to yellow.
Color characteristics of untreated wood in this study were similar to those in a previous study [21]. Table 5 shows that the furfurylated wood had lower values for L* and b* compared with the untreated wood. The L* value was decreased after the furfurylation process, but the inverse happened with the imidacloprid preservation process. The increased lightness after perseveration using imidacloprid was similar to Hadi et al. [22]. The color of wood specimens after 1-year test for each wood species and treatment is shown in Figure 6.
According to the variance analysis shown in Table 6, the wood species and treatment affected the values of L*, a*, and b*. Based on the Duncan's multiple range test presented in Table 7, mangium had the darkest color compared with the other three wood species, while sengon wood had more green and blue than the other woods. With regard to the treatment factor, furfurylated wood had the darkest color, as well as more red and blue, compared with untreated and imidacloprid-preserved woods, while imidacloprid-preserved wood was lighter and had more blue compared with untreated wood. Table 6. Variance analysis summary of wood discoloration after furfurylation.

Parameter Sengon Jabon Mangium Pine Untreated Imidacloprid FA
Remarks: Cells in a row with the same letter are not significantly different.
Remarks: Cells in a row with the same letter are not significantly different.
Furfurylated wood specimens had the lowest L* value, indicating that they had the darkest color. According to the results of py-GCMS in Table 8 and those of the EDX analysis in Table 9, the furfurylated samples contained the element bromine that could cause a darker color. The discoloration of furfurylated wood was in line with findings from previous work, which stated that furfurylated wood is characterized by a darker color than untreated wood [11].    Regarding the color change classification, the treated wood samples were different colors that the untreated wood, which were indicated by the ∆E value of imidaclopridpreserved wood being 17.1 and that of furfurylated wood being 27.7. According to Table 6, the color change was significantly affected by the treatment, but not by the wood species.

Weight Loss
Representative termites that attacked wood specimens during the field test are shown in Figure 7. The two species were identified as Microtermes inspiratus Kemner and Coptotermes curvignathus Holmgren.  Regarding the color change classification, the treated wood samples were different colors that the untreated wood, which were indicated by the ΔE value of imidaclopridpreserved wood being 17.1 and that of furfurylated wood being 27.7. According to Table  6, the color change was significantly affected by the treatment, but not by the wood species.

Weight Loss
Representative termites that attacked wood specimens during the field test are shown in Figure 7. The two species were identified as Microtermes inspiratus Kemner and Coptotermes curvignathus Holmgren. The weight loss percentages of the wood specimens after 1-year of exposure in the field for each wood species according to treatment are presented in Table 10. The variance analysis in Table 11 shows that wood species and treatment significantly affected the  The weight loss percentages of the wood specimens after 1-year of exposure in the field for each wood species according to treatment are presented in Table 10. The variance analysis in Table 11 shows that wood species and treatment significantly affected the weight loss of wood specimens. Jabon had the highest weight loss, indicating greater susceptibility to termite attack and poor resistance, which translates to the lowest class, V, with regard to the Indonesian standard [34]. According to the Duncan's multiple range test in Table 12, jabon differed from the other wood species, which did not differ from each other.  In terms of treatment, imidacloprid preservation and furfurylation appeared to enhance wood resistance to subterranean termite attack. This interpretation is supported by the weight loss (Table 12) of untreated wood being 67.3% compared with 38.2% for imidacloprid and 5.2% for furfurylated wood, or only 8% that of untreated wood. Both treatments effectively enhanced the wood resistance to subterranean termite after exposure for 1 year in the field.
In comparison with previous work [10], the furfurylated wood showed very consistent resistance to termite attack, as indicated by weight loss being 3.4% in the 3-month test and 5.2% in the 1-year test, an increase of about 1.5 times only. In comparison, imidaclopridpreserved wood had a weight loss of 6.6% at 3 months and 38.2% at 1 year (5.8 times), and untreated wood had a weight loss of 12.9% at 3 months and 65.4% at 1 year (5.1 times). Compared with previously reported findings, the furfurylated wood had similar resistance to subterranean termite attack as European beech wood that was impregnated with poly glycol-maleic anhydride and then heat treated [18].

Protection Level
Protection levels for each type of specimen are presented in Table 13 Analysis of  variance results are summarized in Table 14, and Duncan's multiple range test outcomes are shown in Table 12. Wood species and treatment significantly affected the protection level. Further analysis showed that jabon had the lowest protection level (value of 5.4, refers to Table 12), which was significantly different from the other three wood species. Sengon, mangium, and pine wood had protection levels of 6.0, 7.1, and 7.7, respectively. Table 13. Protection level of specimens.

Treatment
Wood Species Sengon Jabon Mangium Pine  In terms of treatment, after 1 year of exposure in the field, furfurylation had the highest protection level (value of 9.3, refers to Table 12), and the value was slightly smaller than that at 3 months (9.6). Imidacloprid-preserved wood had a value of 7.2 at 1 year and 8.7 at 3 months, while untreated wood had values of 3.1 and 7.4 at 1 year and 3 months, respectively. In other words, the furfurylated wood was the most resistant and had the smallest decrease in protection level over time with regard to subterranean termite attack.

Conclusions
Based on the findings in this work, it could be concluded that the density and moisture content of untreated and imidacloprid-preserved wood samples were found to be not significantly different from each other. However, they were significantly different from furfurylated wood, which had a much higher density and a much lower moisture content. In terms of wood color, mangium wood had a darker color compared with the other three wood species, which had lighter colors. After treatment, imidacloprid-preserved wood had a lighter color, but furfurylated wood had a darker color compared with untreated wood. Concerning termite attack, untreated jabon wood had the lowest resistance to subterranean termite attack, while untreated sengon, mangium, and pine had almost the same resistance. After 1-year exposure in the field in Bogor, Indonesia, furfurylated wood showed superior resistance to subterranean termite attack, followed by imidacloprid-preserved wood, and untreated wood which had the lowest resistance.