1. Introduction
Maritime pine is one of the most common softwood found in South West of France, covering one million hectares of the country forestry area [
1]. It is mainly used for its wood, which possesses good mechanical properties; high elastic modulus (8800 MPa), low density (550 to 750 kg·m
−3), and long fibers (2.5 to 5 mm versus 0.8 to 2.5 mm for hardwood) [
2]. These characteristics make maritime pine suitable for packaging purposes (ex: wood pallets) or engineered wood products for indoor and decorative applications (wood flooring, wood paneling). Nevertheless, due to the consumers’ feedbacks and specifications, industries identified a major issue: the color difference between heartwood and sapwood. This issue is particularly present in the case of decorative wood where the visual aspect is an important parameter to control.
Wood color is mainly attributed to lignin [
3,
4] and extractives. Lignin is a polyphenolic macromolecule biosynthesized from coumaryl alcohol, coniferyl alcohol, and synapyl alcohol. Quinone and coniferaldehyde units are known to have the greatest impact on color [
5,
6,
7]. The latter, which are present only in a low amount in lignin (less than 4%), [
6] absorb light radiations between 300 and 450 nm (with a maximal absorption at 350 nm) [
8,
9]. Quinones are produced either during the biosynthesis of lignin or by oxidation of phenolic structures. The well-known structures are ortho-quinone (λ
max = 420 nm) and para-quinone (λ
max = 373 nm).
Extractives are secondary metabolites. They confer different properties to the wood such as protective effects against insects, mold, and also wood destructive fungi, wood fragrance (terpenes), and color. Phenolic extractives, which have several chromophoric moieties, play a major role on this parameter. According to heartwood type, the synthesis and evolution of those extractives might be different [
10]: for
Pinus-type heartwood, phenolic precursors are synthetized in sapwood and then modified in the transition zone.
It was suggested in several studies that those low molecular weight phenolic molecules present in the transition zone evolved into more complex structures within heartwood. This was observed for hardwood [
11,
12,
13,
14] as well as softwood [
15,
16,
17,
18,
19,
20]. Even if those complex materials are present in low amount, they can highly influence wood color as shown in the study of Johansson et al. on red cedar wood extractives. An insoluble fraction, with a molar mass between 1000 and 10,000 g·mol
−1, was recovered after extraction and fractionation in methyl t-butyl ether (MTBE), corresponding to only one third of dry matter but being responsible for 70% of the color of the sample.
Because of these chemical composition differences, maritime pine heartwood has a brown-reddish color while sapwood is yellow.
In addition to homogenization, lighter shades are now more and more appreciated by customers. Treatments based on paper processes chemistry make then suitable candidates for the modification of wood color. Oxidative compounds like oxygen, ozone, or hydrogen peroxide are preferred and commonly employed in Totally Chlorine Free (TCF) bleaching sequences [
21,
22,
23,
24,
25].
Hydrogen peroxide (H
2O
2) is usually found in aqueous solutions and, therefore, is more easily handled than the two other oxidants. Quite ineffective when used alone, an alkali as sodium hydroxide (NaOH) is added to produce perhydroxyl anions (HOO
-), which are better oxidizing agents than the native peroxide [
26,
27]. H
2O
2 stability in solution can be improved by adding metal chelatants or stabilizers like magnesium salts (magnesium sulfate, MgSO
4) or silicates (sodium metasilicate, Na
2SiO
3) [
28,
29]. The reaction between alkaline hydrogen peroxide and wood can be influenced by several parameters: H
2O
2 and alkali concentrations, pH, temperature, and treatment duration. Hydrogen peroxide treatment has been used since the 1930s to successfully modify wood color but very few studies [
30,
31] were made on the influence of the process on the color homogenization between heartwood and sapwood.
Herein, we report in the present study the results of wood color homogenization by alkaline hydrogen peroxide treatments. The experiments were done on wood powders to facilitate the chemical characterizations before and after the treatment. Soxhlet extractions were carried out to observe the effect of the bleaching on the wood extractives’ structure. Heartwood (Hw) and sapwood (Sw) were treated separately with a solution composed of hydrogen peroxide, sodium hydroxide, and sodium metasilicate. Color changes were studied with colorimetric analyses and chemical evolutions were observed with infrared analyses. The impacts on wood color of the variation of NaOH concentration or the substitution of NaOH by another alkali source were also studied. Finally, ageing experiments under UV irradiation were carried out to examine the behavior of the treated powders overtime.
2. Materials and Methods
2.1. Materials and Reagents
2.1.1. Wood Samples
A 60–70 years old maritime pine log with a high red heartwood content was selected in a local sawmill (Gascogne Bois sawmill).
After sawing, wooden boards (200 × 5 × 2 cm
3) were collected, cut into small pieces (10 × 5 × 2 cm
3), heartwood and sapwood part were visually determined, cut apart and then milled to reach a 40-mesh screen [
32]. Finally, wood powders were stored at −20 °C before further uses. Moisture contents for heartwood and sapwood were, respectively, 20 ± 2% and 42 ± 2%.
2.1.2. Chemicals
Hydrogen peroxide (H2O2, 35% in aqueous solution, ACROS Organics), sodium hydroxide (NaOH, 98% pellets, Alfa Aesar), sodium metasilicate (Na2SiO3, Sigma Aldrich), and magnesium hydroxide (Mg(OH)2, Honeywell Fluka) were used for powder color modification.
2, 10-dimethylphenanthroline (DMP, 98+%, Alfa Aesar) and pentahydrate copper(II) sulfate (CuSO4.5H2O, ≥98%) were used for spectroscopic monitoring of hydrogen peroxide concentration.
Acetone (99.5+%, ACROS Organics) was used as a solvent for Soxhlet extraction.
2.2. Hydrogen Peroxyde Treatment
For all the experiments, the wood/solvent mass ratio was 1/20. The bleaching solution was composed of H2O2 (4%v), alkali (NaOH or Mg(OH)2; 1%w), and Na2SiO3 (0.4%w). Alkali and Na2SiO3 were first solubilized in water in a round bottom flask. The latter was put in an oil bath and the temperature was set at 60 ± 1 °C. Then hydrogen peroxyde was added and finally heartwood or sapwood powders were put into the solution under vigorous stirring. After 1 h, the powder was collected, washed with distilled water, filtered, and finally dried in an oven at 50 °C under vacuum for 12 h. The powder was then stored at room temperature and relative air humidity and kept away from light to prevent any color change.
2.3. Hydrogen Peroxide Concentration Monitoring
A spectroscopic method based on Baga et al.’s work was developed [
33]. A calibration curve was established first to determine the concentration range for which Beer–Lambert’s law was respected (absorbance below 1). During the reaction, a 2.5 µL aliquot was collected from the batch and put into a 25 mL-volumetric flask and then filled up with water. Then, 10 µL of this solution were taken out, added to a 10 mL-volumetric flask as 1 mL of a copper (II) sulfate aqueous solution at 0.01 mol.L
−1 and 1 mL DMP alcoholic solution. A [2Cu(DMP)
2+] complex was formed with a maximal absorbance at 454 nm. The Equation (1), following, gives the correlation between hydrogen peroxide concentration and the absorbance:
2.4. Attenuated Total Reflectance Spectroscopy (ATR)
ATR analyses were carried out on Bruker Vertex 70 device fitted with a Pike GladiATR plate. Resolution was set at 4 cm−1 and 64 scans were accumulated between 400 and 4000 cm−1 for each experiment.
2.5. Soxhlet Extraction
Powders were extracted using a Soxhlet device and acetone as a solvent. Oil bath temperature was set at 75 ± 1 °C and the extraction was carried out for 8 h with a rate of 3–6 siphoning per hour. Extracts were then concentrated and let under dynamic vacuum during a night to remove the last solvent traces. Untreated and peroxide treated powders were extracted to compare the evolution of extractive composition.
2.6. Colorimetric Analyses
Wood powders color evolution was monitored by a Nix Pro Color 2 colorimeter. The device source was composed of three High-CRI LEDs and the illuminant D50 was chosen. Observation angle was set at 2° and optical aperture was fixed at 15 mm. Wood powders were pressed with a laboratory pellet press (Specac, 12.5 MPa applied for 5 min) to form 50 mm diameter discs. Five measurements were performed on the smoothest surface. CIEL*a*b* 1976 system was used for the study and the value of the three colorimetric parameters L* (lightness), a* (green-red coordinate) and b* (blue-yellow coordinate) could be determined for each sample.
The color difference between heartwood (Hw) and sapwood (Sw) is given by Equation (2):
It was aimed in this study to get ΔESw/Hw values as low as possible, so that the color differences are no longer discernible (ideally ΔESw/Hw ≤ 1 with the colorimetric system used).
2.7. UV Irradiation Ageing
Wood discs were irradiated with UV light produced by four black light tubes (TFWN18-Mazdafluor). Irradiance was determined with a CCD Thorlab spectrometer I
365nm = 0.45 mW.cm
−1. The temperature was kept under 40 °C by an air flux. Treated and untreated wood were irradiated for 69 days, and the color was monitored (colorimetric analyses as described in
Section 2.6) at regular intervals (measurements every seven days from day 0 to day 49, last measurement was done on day 69).
4. Conclusions
Maritime pine heartwood and sapwood colors were homogenized with paper bleaching processes based on alkaline hydrogen peroxide chemistry. First, H2O2/NaOH solution was used, and wood surface are lighter and brighter than the untreated ones. This behavior is attributed to the modification of chromophores (coniferaldehyde, quinones) and to the extraction of extractives by formation of anionic species due to the alkaline environment.
When the percentage of alkali is high, the peroxide may be totally consumed, which may promote secondary reactions between hydroxyl anions and wood and, thus, leads to alkali darkening. The best homogenization result is obtained for the 4% H2O2/1% NaOH system.
The substitution of NaOH by Mg(OH)2 leads to a decrease of b* parameter values and also to an increase of the L* values. Consequently, samples treated with this system appear brighter than the one bleached with a H2O2/NaOH solution.
Due to his poor solubility in water, the release of hydroxyl is progressive, which may prevent alkali darkening reactions. Best color homogenization was obtained for the 4% H2O2/0.5% NaOH/0.5% Mg(OH)2 system.
Finally, UV irradiation ageing experiments were carried out on both untreated and bleached samples. A color reversion was observed for all the samples irradiated and attributed to the formation of quinone derivatives. However, it was noted that the color between bleached heartwood and sapwood stayed homogeneous even after irradiation.
This preliminary study shows the effects of an alkaline hydrogen peroxide treatment on the color of maritime pine heartwood and sapwood powders. Facing these results, these experiments shall be applied on the treatment of veneer and solid wood considering the scale differences. Maritime pine wood is known to have a wide variability of extractives concentrations depending on the soil, the climate, or the position in the tree [
57,
58,
59,
60]. This parameter should be considered during the treatment and the composition of the hydrogen peroxide solution adapted to the material used.