Abstract
The concept of color is not always suitable for some trees according to customers’ preferences. The color of the wooden material changes with processes such as heat treatment, impregnation, bleaching, etc. When the color of the wooden material changes, the surface also undergoes changes in its inherent properties. African ebony (Diospyros crassiflora Hiern.), black ebony (Diospyros ebenum), and Macassar ebony (Diospyros celebica Bakh.) wood species were treated using a single-component bleaching chemical [oxalic acid (C2H2O4)] and double component bleaching chemical [hydrogen peroxide (H2O2) + sodium hydroxide (NaOH)] using a brushing technique. Subsequently, the characteristics of the changes that occurred were determined, including color parameters and whiteness index (WI*) values. The wood species exhibited different behaviors (increases and decreases specific to each test) in response to both bleaching agents. There was an observed increase in the a*, b* and C* values, while the ho values decreased for all wood species. Furthermore, WI* values in directions perpendicular and parallel to the fibers were reduced in all wood species by oxalic acid treatment.
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Effects of Various Bleaching Chemicals on Some Surface Properties of Ebony Species (African, Black, and Macassar)
Ümit Ayata 
The concept of color is not always suitable for some trees according to customers’ preferences. The color of the wooden material changes with processes such as heat treatment, impregnation, bleaching, etc. When the color of the wooden material changes, the surface also undergoes changes in its inherent properties. African ebony (Diospyros crassiflora Hiern.), black ebony (Diospyros ebenum), and Macassar ebony (Diospyros celebica Bakh.) wood species were treated using a single-component bleaching chemical [oxalic acid (C2H2O4)] and double component bleaching chemical [hydrogen peroxide (H2O2) + sodium hydroxide (NaOH)] using a brushing technique. Subsequently, the characteristics of the changes that occurred were determined, including color parameters and whiteness index (WI*) values. The wood species exhibited different behaviors (increases and decreases specific to each test) in response to both bleaching agents. There was an observed increase in the a*, b* and C* values, while the ho values decreased for all wood species. Furthermore, WI* values in directions perpendicular and parallel to the fibers were reduced in all wood species by oxalic acid treatment.
DOI: 10.15376/biores.21.3.7304-7316
Keywords: Whiteness index; Color; Bleaching; Black ebony; African ebony; Macassar ebony
Contact information: Bayburt University, Faculty of Arts and Design, Department of Interior Architecture and Environmental Design, Bayburt, Turkey; *Corresponding author: umitayata@bayburt.edu.tr
Graphical Abstract
INTRODUCTION
No two tree species are alike, and even within the same tree species, the characteristics vary. In fact, when wood samples taken from different parts of the same tree are examined, different properties are observed. These distinct characteristics are apparent not only in their outward appearance but also in aspects such as density, pattern, color, and odor when the material is examined. These differences in trees are said to stem from factors such as the region where they grow, the environmental conditions they are exposed to, and their genetic structure. The properties of wood materials obtained from the same tree differ depending on their source (Gürtekin and Oğuz 2006).
One useful characteristic for determining the genus of trees is their color. The color of some trees may fade due to the effect of sunlight, while others may darken to a certain extent over time, but each tree still possesses its unique color. The darkening of color over time occurs in trees with a high tannin content in their structure (Şanıvar and Zorlu 1980).
Bleaching involves the application of chemicals to cellulosic fibers to enhance their brightness. This brightness enhancement can be achieved through either the removal of lignin or the reduction of lignin coloration. Despite undergoing chemical pulping digestion, lignin continues to be a significant component of the wood pulp used to manufacture different grades of paper (Anonymous 1989). Various chemicals are used for bleaching in different industries. Some examples of these chemicals include; acetic acid (CH3COOH), sodium hypochlorite (NaClO), carbamide peroxide (CH6N2O3), sodium dithionite (Na2S2O4), hydrogen peroxide (H2O2), sodium hydrosulfite (NaSH), sodium percarbonate (Na2CO3·1.5H2O2), chlorine dioxide (ClO2), chlorine (Cl), sodium hydroxide (NaOH), peracetic acid (CH3COOOH), oxalic acid (H2C2O4), and calcium hydroxide (Ca(OH)2). Hypochlorite is also utilized in certain recycling facilities to bleach the pulp and remove its color (Suess 2010). The objective of chemical wood bleaching is to disrupt the conjugated double bonds within the color-causing compounds. This entails halting the sharing of electrons between atoms, rather than breaking all double bonds. By doing so, the absorption of visible light is interrupted, leading to the elimination of color. Chemical wood bleaching can be accomplished through either oxidation or reduction (Kristiansson 2012).
The term “color” refers to the sensory impression transmitted by the eye, which is an optical phenomenon (Loos 1989). In the literature, there seems to be insufficient research on the bleaching of wood material surfaces. These studies have focused on investigating changes in color parameters, where both increases and decreases in all color parameters have been reported. For instance, it has been reported that sipo (Ulay and Ayata 2023a), sapele (Ulay and Ayata 2023b), satinwood ceylon (Ayata and Çamlıbel 2023), doussié, hornbeam, merbau, iatandza, mahogany (Çakıcıer and Ulay 2023), olon (Peker and Ayata 2023), Scots pine (Ulay and Ayata 2023c), and Anatolian chestnut (Ulay and Ayata 2023d) woods, when treated with NaClO, showed changes in color parameters. For woods treated with NaOH, such as Scots pine and oriental beech (Özçifçi and Özbay 2010), ayous, lime, and poplar (Lu et al. 2023) changes in color parameters were observed.
These studies indicate that the application of different chemical bleaching agents [oxalic acid (C2H2O4) and the combination of hydrogen peroxide (H2O2) + sodium hydroxide (NaOH)] lead to alterations in color parameters for various wood species.
In this study, three types of wood, namely African ebony (Diospyros crassiflora Hiern.), black ebony (Diospyros ebenum), and Macassar ebony (Diospyros celebica Bakh.) were subjected to the application of two different bleaching chemicals on their surfaces, and the resulting color and whiteness index values were investigated. Although wood bleaching processes have been extensively studied, comparative information on the bleaching response of different ebony species under identical chemical and experimental conditions is limited. In particular, systematic evaluation of African ebony, black ebony, and Macassar ebony in terms of color change on the CIELAB scale is underreported. Therefore, this study contributes to a better understanding of species-specific bleaching behavior by providing a direct comparison of these three commercially important ebony species under identical bleaching processes.
EXPERIMENTAL
The wood species African ebony (Diospyros crassiflora Hiern.), black ebony (Diospyros ebenum), and Macassar ebony (Diospyros celebica Bakh.) were selected for this study. The experimental material was obtained from a commercial timber supplier in Türkiye in first-class quality boards with dimensions of 100 mm × 100 mm × 10 mm.
Before bleaching, the wood surfaces were sanded with 80, 100, 120, 150, 180, and 220 grit sandpaper respectively using a vibrating sander to obtain a smooth and even surface.
For each wood species, a total of 30 specimens were prepared and divided into three groups: control group (n = 10), single-component bleach (single-com-bleach) treatment group (n = 10), and double-com-bleach treatment group (n = 10). The specimens were randomly selected, with straight and uniform fiber orientation, and were free from visible defects such as knots, cracks, and discoloration.
The specimens were supplied in a pre-conditioned state under standard laboratory conditions; therefore, no direct moisture content measurement was performed. All samples were kept under standard laboratory conditions prior to testing in accordance with TS ISO 642 554 (1997).
Two different bleaching agents were used in this study: a single-component aqueous solution of oxalic acid (C2H2O4) (odorless, liquid, and colorless, with a pH value of 2.0±0.5), and a dual-component system consisting of Part A: hydrogen peroxide (H2O2) and Part B: sodium hydroxide (NaOH), mixed in a ratio of 100 mL to 50 mL (2:1). The reagents were purchased from a private trading company. The bleaching solutions were applied as a single layer to the wood surfaces using a sponge brush until full surface coverage was achieved. After application, the treated specimens were dried under standard laboratory conditions for 2 weeks prior to further testing.
The color changes of the samples were measured according to the ASTM D 2244-3 (2007) standard using the CIELAB color system with a CS-10 (CHN Spec, China) device [CIE 10° standard observer; CIE D65 light source, illuminating system: 8/d (8°/diffuse illumination)] (ho: hue, L*: lightness, a*: red color tone, b*: yellow color tone, C*: chroma). In this study, the determination of whiteness index (WI*) values [perpendicular (⊥) and parallel (║) to the fibers] was carried out using the Whiteness Meter BDY-1 device according to the ASTM E313-15e1 (2015) standard.
The results for the total color differences were determined using the following formulas.
The definitions of ∆H*, ∆C*, ∆a*, ∆L*, and ∆b* are as follows (Lange 1999): ∆b*: A positive value of ∆b* indicates that the sample is more yellow than the reference, while a negative value indicates that the sample is more blue than the reference. ∆L*: A positive value indicates that the sample is lighter than the reference, while a negative value indicates that the sample is darker than the reference. The parameter ∆H* represents the hue difference or shade variation. ∆a*: A positive value indicates that the sample is redder than the reference, while a negative value indicates that the sample is greener than the reference. ΔC* represents the difference in chroma (color saturation), with positive values indicating increased vividness and brightness, and negative values indicating a duller and less clear appearance compared to the reference.
The comparison of ΔE* color difference through visual evaluation is provided in Table 1 (Cividini et al. 2007).
Table 1. Comparison Criteria Related to ΔE* Evaluation (Cividini et al. 2007)
A statistical program (IBM SPSS Statistics 27) was used to calculate variance analyses, minimum and maximum values, means, percentage change rates, standard deviations, and homogeneity groups.
RESULTS AND DISCUSSION
The color parameters of all the specified woods before and after the bleaching process are presented in Table 2. The L* values exhibited decreases in African ebony and black ebony wood species with both chemicals, while an increase was observed in Macassar ebony wood. The extent of the reduction in black ebony wood was found to be nearly double with the H2O2/NaOH bleaching system application compared to the oxalic acid chemical application. The highest L* value was obtained from the specimens treated with H2O2/NaOH bleaching system in this wood species, while the lowest value was found in the control (unbleached) samples (Table 2).
The a* values showed an increase in all wood types with both chemicals. The lowest a* values for all wood species were obtained from the control samples. Whereas a* values were negative only in African ebony under the control condition, bleaching treatments increased a* values toward zero in African ebony and increased positive a* values in black ebony and Macassar ebony compared to the control, indicating a shift toward redness in all wood species. For the control groups, highest a* values were recorded as follows: -2.33, 0.57, and 0.66 for African ebony, Macassar ebony, and black ebony wood, respectively (Table 2).
The b* values increased in all wood species with the application of both chemicals. The lowest b* values for all wood species were observed in the control group. For the control groups, the highest b* values were found to be 6.12, 5.72, and 4.26 for black ebony, Macassar ebony, and African ebony wood, respectively. Positive results were obtained for all wood species. In Macassar ebony wood, the b* values showed a moderate increase with oxalic acid chemical and a larger increase with hydrogen peroxide/sodium hydroxide mixture. The chemical treatment with oxalic acid showed a higher b* value for black ebony wood compared to the H2O2/NaOH bleaching system treatment (Table 2).
Table 2. Color Parameters Results for Wooden Materials Before and After Bleaching
The C* values increased in all wood species with the application of both chemicals. The lowest C* values for all wood species were obtained from the control group. For the control groups, the highest C* values were determined to be 4.87, 5.75, and 6.16 for African ebony, Macassar ebony, and black ebony wood, respectively. The H2O2/NaOH bleaching system chemical resulted in approximately 10 times more increase in C* parameter for African ebony wood compared to the oxalic acid chemical (Table 2).
The ho values decreased in all wood species with the application of both chemicals. The highest ho values for all wood species were obtained from the control group samples. The oxalic acid chemical application resulted in a higher reduction in ho values for black ebony wood compared to the H2O2/NaOH bleaching system application (Table 2).
Total color differences are presented in Table 3. The ∆L* values were negative (darker than the reference) for African ebony and black ebony wood species, while they were positive (lighter than the reference) for Macassar ebony wood (Table 3).
The bleaching process showed its strongest impact on Macassar ebony when using the H2O2/NaOH system which produced the most substantial color variation (ΔE* = 7.50). The value indicates that the color difference between the two samples reached a level that most people can see. The increases in Δa* (more red), Δb* (more yellow), and ΔC* (clearer and more vivid) values indicate notable changes in chromatic coordinates, reflecting alterations in surface color properties. The oxalic acid treatment produced a smaller change in color (ΔE* = 2.12), which indicates that it reacts less forcefully with the components of wood. It is proposed that the three species show different bleaching behaviors because their chemical makeup contains different amounts of extractives and phenolic compounds and tannins. The extractive content of African ebony and black ebony reaches higher levels, which creates problems for bleaching operations and results in uneven color changes. The chemical structure of Macassar ebony differs from others which enables better oxidation of its chromophoric groups when treated with alkaline hydrogen peroxide. The research findings show that bleaching treatment success depends on species type with H2O2/NaOH system achieving best color change for Macassar ebony (Table 3).
Table 3. Total Color Difference Results Determined in Different Wood Species after Bleaching Applications
When comparing the results obtained in this study with the color criteria, it was found that both bleaching chemicals from African ebony wood met the “Color difference visible with high-quality screen (3>ΔE*>2)” criterion, while both bleaching chemicals from black ebony wood met the “color difference visible with medium-quality screen (6>ΔE*>3)” criterion. In the case of Macassar ebony wood, oxalic acid chemical application resulted in meeting the “color difference visible with high-quality screen (3>ΔE*>2)” criterion, while the H2O2/NaOH bleaching system application met the “high color difference (12>ΔE*>6)” criterion (Table 3). The ∆E* values obtained from oxalic acid and H2O2/NaOH bleaching on olon wood were found to be 2.07 for the single-component treatment and 10.30 for the two-component treatment (Peker and Ayata, 2023).
The results of the variance analysis calculated for different wood species regarding color parameters are given in Table 4. According to these results, all parameters except the C* parameter in Africa ebony wood, and all color parameters in the measurements taken for the other two wood types, were determined to be statistically significant (Table 4).
Table 4. Analysis of Variance Results for Color Parameters
Table 5. Whiteness Index (WI*) for Wooden Materials Before and After Bleaching
All the whiteness index values for the identified woods before and after the bleaching process are presented in Table 5. The two different wood bleaching chemicals used in the study significantly altered the whiteness index values of different wood species due to their distinct chemical properties. These results are supported by variance analysis (ANOVA). In all wood species, both wood bleaching chemicals exhibited a decrease in WI* values in the perpendicular (⊥) direction to the fibers. The WI* ⊥ values for the African ebony, black ebony, and Macassar ebony wood species were obtained from the highest control group samples and determined as 4.79, 5.21, and 6.62, respectively. With both bleaching chemicals, reductions were observed in all wood species in directions perpendicular to the fibers (Table 5).
The results of the variance analysis calculated for whiteness index (WI* ⊥ and ║) values in different wood species are given in Table 6. According to these results, it is seen that significant data were obtained in both directions (⊥ and ║) in WI* measurements for all wood types (Table 6).