*Article* **Effect of Sunlight on the Change in Color of Unsteamed and Steamed Beech Wood with Water Steam**

**Michal Dudiak \*, Ladislav Dzurenda and Viera Kuˇcerová**

Faculty of Wood Sciences and Technology, Technical University in Zvolen, T.G. Masaryka 24, 96001 Zvolen, Slovakia; dzurenda@tuzvo.sk (L.D.); viera.kucerova@tuzvo.sk (V.K.) **\*** Correspondence: xdudiak@tuzvo.sk; Tel.: +421-45-520-6367

**Abstract:** This paper presents the differences in the color changes of unsteamed and steamed beech wood (*Fagus sylvatica* L.) caused by long-term exposure to sunlight on the surface of wood in interiors for 36 months. The light white-gray color of the yellow tinge of native beech wood darkened under the influence of sunlight, and the wood took on a pale brown color of yellow tinge. The degree of darkening and browning is quantified by the value of the total color difference ∆*E\** = 13.0. The deep brown-red color of steamed beech under the influence of sunlight during the exposure brightened, and the surface of the wood took on a pale brown hue. The degree of lightening of the color of steamed beech wood in the color space CIE *L\*a\*b\** is quantified by the value of the total color difference ∆*E\** = 7.1. A comparison of the color changes of unsteamed and steamed beech wood through the total color difference ∆*E\** due to daylight shows that the surface of steamed beech wood shows 52.2% smaller changes than unsteamed beech wood. The lower value of the total color difference of steamed beech wood indicates the fact that steaming of beech wood with saturated water steam has a positive effect on the color stability and partial resistance of steamed beech wood to the initiation of photochemical reactions induced by UV–VIS wavelengths of solar radiation. Spectra ATR-FTIR analyses declare the influence of UV–VIS components of solar radiation on unsteamed and steamed beech wood and confirm the higher color stability of steamed beech wood.

**Keywords:** beech wood; thermal treatment; saturated water steam; natural aging; wood color; ATR-FTIR spectroscopy

## **1. Introduction**

The color of wood is a basic physical–optical property, which belongs to the group of macroscopic features on the basis of which the wood of individual woody plants differs visually. The color of the wood is formed by chromophores, i.e., functional groups of the type >C=O, -CH=CH-CH=CH-, -CH=CH-, aromatic nuclei found in the chemical components of wood (lignin and extractive substances such as dyes, tannins, resins and others), which absorb some components of the electromagnetic radiation of daylight and thus create the color of the wood surface perceived by human vision.

Using the coordinates of the color space CIE *L\*a\*b\** is one of the ways to quantify the given optical wood property objectively. Lab color space (according to CIE-Commission Internationale d'Eclairage) in accordance with ISO 11 664-4 is based on the measurement of three parameters: brightness *L\** represents the darkest black at *L\** = 0 and the brightest white at *L\** = 100. The value of *a\** is a measure of the red-green character of the color, with positive values for red shades (+*a\**), and negative values for green (−*a\**). The value of *b\** gives the yellow-blue character with positive values for yellow shades (+*b\**) and negative for blue (−*b\**).

Wood with long-term exposure to sunlight changes color on its surface. The surface of the wood darkens and mostly yellows and browns. This fact is also referred to in the professional literature as natural aging [1–3].

**Citation:** Dudiak, M.; Dzurenda, L.; Kuˇcerová, V. Effect of Sunlight on the Change in Color of Unsteamed and Steamed Beech Wood with Water Steam. *Polymers* **2022**, *14*, 1697. https://doi.org/10.3390/ polym14091697

Academic Editor: Jacques Lalevee

Received: 18 March 2022 Accepted: 18 April 2022 Published: 21 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Solar radiation is electromagnetic radiation with wavelengths in the range from 100 to 3000 nm [4], which consists of ultraviolet radiation, visible radiation (light) and infrared radiation. Ultraviolet radiation (UV) with wavelengths of 100–380 nm makes up about 2% of the daylight spectrum. According to the effect of UV radiation on biological materials and their effects on these materials, UV radiation is divided into: UV-A radiation, with a wavelength of 320–380 nm; UV-B radiation, with a wavelength of 280–320 nm; and UV-C radiation, with a wavelength below 280 nm. The spectrum of UV radiation falls on the Earth's surface from solar radiation, which is made up of 90–99% UV-A radiation and 1–10% UV-B radiation. The most dangerous UV-C radiation is completely absorbed by the atmosphere. The visible light spectrum, referred to as VIS, with wavelengths from 380 to 780 nm, represents approximately 49% of the daylight spectrum. The rest consists of infrared IR radiation with wavelengths of 780–3000 nm. The wavelengths of visible and infrared radiation are absorbed or reflected by the wood surface. The reflected wavelengths of the visible spectrum allow a person to perceive its color when looking at a given object. The absorbed wavelengths of infrared solar radiation change to heat on the surface.

UV–VIS components of solar radiation (daylight) initiate wood photodegradation processes when impacting on the wood surface (photolytic and photo-oxidation reactions with lignin, polysaccharides and wood accessory substances), and carbohydrates absorb 5–20% and 2% of the accessory substance [5]. These reactions cleave both lignin macromolecules with the simultaneous formation of phenolic hydroperoxides, free radicals, carbonyl and carboxyl groups, as well as polysaccharides into polysaccharides, with a lower degree of polymerization to form carbonyl, carboxyl groups and gaseous products (CO, CO2, H2) [1,3,6–8].

The aim of this work is to compare the effect of solar radiation on the surface of thermally treated beech wood with saturated steam (steaming) and unsteamed beech wood. Through changes in the coordinates *L\**, *a\**, *b\** in the color space CIE *L\*a\*b\** and the total color difference ∆*E\**, color changes of native and steamed beech wood caused by UV–VIS components of sunlight (daylight) are evaluated.

#### **2. Material and Methods**

**Mode** 

10%.

where ଵ

<sup>∗</sup> , ଵ <sup>∗</sup>, ଵ

*2.2. Conditions of Beech Wood Exposure* 

approximate value of 1.7 kWh/m2 per day.

diameter of the optical scanning aperture was 8 mm [10].

dried, milled beech wood before exposure, and ଶ

*2.3. Color Measurement of Beech Wood* 

#### *2.1. Material and Technology of Beech Wood Steaming*

Blanks with dimensions 32 × 60 × 600 mm made of beech wood had a moisture content *w<sup>p</sup>* = 56.4 ± 4.2% and were divided into 2 groups. The blanks of the first group were not thermally steamed prior to drying. The blanks of the second group were steamed to modify the color of the beech wood. Steaming was performed in an APDZ 240 pressure autoclave (Himmasch AD, Haskovo, Bulgaria) installed at Sundermann Ltd. (Banská Štiavnica, Slovakia). The steaming mode of beech wood with saturated water steam is shown in Figure 1, and the technological parameters of the steaming mode are given in Table 1. *Polymers* **2022**, *14*, x FOR PEER REVIEW 3 of 12

**Figure 1.** Mode of color modification of beech wood with saturated water steam. **Figure 1.** Mode of color modification of beech wood with saturated water steam.

**Table 1.** Mode of color modification of beech wood with saturated water steam.

**Temperature of Saturated** 

Mode 132.5 137.5 100 6.0 1.0 7.5

Unsteamed and steamed beech blanks were dried by a low-temperature drying mode preserving the original wood color to a moisture content *wk* = 12 ± 0.5% in a convection

Samples of the following dimensions were produced from native and steamed beech wood blanks: 20 × 50 × 400 mm. The planed surface of unsteamed and steamed beech wood samples was exposed to daylight for a long time at an angle of 45° in the northern temperate zone (Slovakia, Central Europe) locality for 36 months. The temperature and relative humidity of the indoor air during the exposure were *t* = 20 ± 2.5 °C and φ = 50 ±

The average density of incident solar radiation in Slovakia is 1100 kWh/m2 per year. The intensity of the sun's rays changes throughout the year. The highest intensity of solar radiation is in the summer months of June and July when it reaches a value of 5.9 to 6.0 kWh/m2 per day. During the autumn, the intensity of sunlight decreases and is lowest during the winter. In December, the intensity of solar radiation is the weakest, with an

The surface color of beech samples before and during the exposure was evaluated in the color space CIE *L\*a\*b\** at monthly intervals using the Color Reader CR-10 (Konica Minolta, Osaka, Japan) colorimeter was measured. A D65 light source was used, and the

The total color difference Δ*E\** of the beech wood surface change during the 36-month exposure to sunlight was determined according to the following ISO 11 664-4 equation:

<sup>∗</sup> − ଵ

<sup>∗</sup> ሻଶ + ሺଶ

<sup>∗</sup> are the coordinates of the color space CIE *L\*a\*b\** on the surface of the

<sup>∗</sup> , ଶ <sup>∗</sup>, ଶ

<sup>∗</sup> − ଵ

∗ሻଶ + ሺଶ

<sup>∗</sup> − ଵ

<sup>∗</sup> are the coordinates of the color

∗ሻଶ (1) (1)

<sup>∗</sup> = ඥሺଶ

space CIE *L\*a\*b\** on the surface of the dried, milled beech wood during exposure.

hot air dryer: KC 1/50 (SUZAR Ltd., Považany, Slovakia) [9].


**Table 1.** Mode of color modification of beech wood with saturated water steam.

#### *2.2. Conditions of Beech Wood Exposure*

Unsteamed and steamed beech blanks were dried by a low-temperature drying mode preserving the original wood color to a moisture content *w<sup>k</sup>* = 12 ± 0.5% in a convection hot air dryer: KC 1/50 (SUZAR Ltd., Považany, Slovakia) [9].

Samples of the following dimensions were produced from native and steamed beech wood blanks: 20 × 50 × 400 mm. The planed surface of unsteamed and steamed beech wood samples was exposed to daylight for a long time at an angle of 45◦ in the northern temperate zone (Slovakia, Central Europe) locality for 36 months. The temperature and relative humidity of the indoor air during the exposure were *t* = 20 ± 2.5 ◦C and ϕ = 50 ± 10%.

The average density of incident solar radiation in Slovakia is 1100 kWh/m<sup>2</sup> per year. The intensity of the sun's rays changes throughout the year. The highest intensity of solar radiation is in the summer months of June and July when it reaches a value of 5.9 to 6.0 kWh/m<sup>2</sup> per day. During the autumn, the intensity of sunlight decreases and is lowest during the winter. In December, the intensity of solar radiation is the weakest, with an approximate value of 1.7 kWh/m<sup>2</sup> per day.

#### *2.3. Color Measurement of Beech Wood*

The surface color of beech samples before and during the exposure was evaluated in the color space CIE *L\*a\*b\** at monthly intervals using the Color Reader CR-10 (Konica Minolta, Osaka, Japan) colorimeter was measured. A D65 light source was used, and the diameter of the optical scanning aperture was 8 mm [10].

The total color difference ∆*E\** of the beech wood surface change during the 36-month exposure to sunlight was determined according to the following ISO 11 664-4 equation:

$$
\Delta E^\* = \sqrt{\left(L\_2^\* - L\_1^\*\right)^2 + \left(a\_2^\* - a\_1^\*\right)^2 + \left(b\_2^\* - b\_1^\*\right)^2} \tag{1}
$$

where *L* ∗ 1 , *a* ∗ 1 , *b* ∗ 1 are the coordinates of the color space CIE *L\*a\*b\** on the surface of the dried, milled beech wood before exposure, and *L* ∗ 2 , *a* ∗ 2 , *b* ∗ 2 are the coordinates of the color space CIE *L\*a\*b\** on the surface of the dried, milled beech wood during exposure.

#### *2.4. Mathematical-Statistical Evaluation of Measured Data*

The measured values on the brightness coordinate *L\** and the chromaticity coordinates *a\*, b\*,* as well as the calculated values of the total color differences ∆*E\** during the observed exposure periods, were statistically and graphically evaluated using Excel and Statistica v.12 programs (V12.0 SP2, Palo Alto, CA, USA).

#### *2.5. Analysis of Changes in Lignin-Cellulose Matrix of Wood ATR-FTIR Spectroscopy*

Fourier-transform infrared spectroscopy (FTIR) was used to follow chemical changes in beech wood after radiation in unsteamed and steamed beech wood. The measurements were carried out using a Nicolet iS10 spectrometer (Thermo Fisher Scientific, Madison, WI, USA) equipped with the Smart iTR ATR accessory.

The spectra were collected in an absorbance mode between 4000 and 650 cm−<sup>1</sup> by accumulating 32 scans at a resolution of 4 cm−<sup>1</sup> using diamond crystal. All analyses were performed in four replicates. The spectra were evaluated using the OMNIC 8.0 software.

#### **3. Results and Discussion**

exposure

*Polymers* **2022**, *14*, x FOR PEER REVIEW 4 of 12

and Statistica v.12 programs (V12.0 SP2, Palo Alto, CA, USA).

*2.5. Analysis of Changes in Lignin-Cellulose Matrix of Wood ATR-FTIR Spectroscopy* 

*2.4. Mathematical-Statistical Evaluation of Measured Data* 

WI, USA) equipped with the Smart iTR ATR accessory.

Beech wood, according to [11–13], has a light white-brown-yellow color. In the steaming process with a steam–air mixture at atmospheric pressure, or thermal treatment of beech wood with saturated water steam, as reported by [13–16], depending on the temperature and length of the thermal treatment process, the wood darkens and acquires shades from a pale pink-yellow color shade to brown-red color. Table 2 shows the coordinates of the color space CIE *L\*a\*b\** of unsteamed and steamed beech wood at a moisture content of *w* = 12% on the planed surface before and after 36 months of dazzling. The values for the brightness coordinates *L\** and the chromatic coordinates *a\** and yellow *b\** of the color space CIE *L\*a\*b\** of unsteamed beech wood given in Table 1 are similar to those reported by [13,17,18]. **3. Results and Discussion**  Beech wood, according to [11–13], has a light white-brown-yellow color. In the steaming process with a steam–air mixture at atmospheric pressure, or thermal treatment of beech wood with saturated water steam, as reported by [13–16], depending on the temperature and length of the thermal treatment process, the wood darkens and acquires shades from a pale pink-yellow color shade to brown-red color. Table 2 shows the coordinates of the color space CIE *L\*a\*b\** of unsteamed and steamed beech wood at a moisture content of *w* = 12% on the planed surface before and after 36 months of dazzling. The values for the brightness coordinates *L\** and the chromatic coordinates *a\** and yellow

The measured values on the brightness coordinate *L\** and the chromaticity coordinates *a\*, b\*,* as well as the calculated values of the total color differences Δ*E\** during the observed exposure periods, were statistically and graphically evaluated using Excel

Fourier-transform infrared spectroscopy (FTIR) was used to follow chemical changes in beech wood after radiation in unsteamed and steamed beech wood. The measurements were carried out using a Nicolet iS10 spectrometer (Thermo Fisher Scientific, Madison,

The spectra were collected in an absorbance mode between 4000 and 650 cm−1 by accumulating 32 scans at a resolution of 4 cm−1 using diamond crystal. All analyses were performed in four replicates. The spectra were evaluated using the OMNIC 8.0 software.


**Table 2.** Coordinate values of color space CIE *L\*a\*b\** of unsteamed and steamed beech wood. *b\** of the color space CIE *L\*a\*b\** of unsteamed beech wood given in Table 1 are similar to those reported by [13,17,18].

The color of native and steamed beech wood before and after exposure to daylight glare is shown in Figure 2. The color of native and steamed beech wood before and after exposure to daylight glare is shown in Figure 2.

**Figure 2.** View of beech wood: native before and after exposure (**left**), steamed before and after exposure (**right**). **Figure 2.** View of beech wood: native before and after exposure (**left**), steamed before and after exposure (**right**).

The courses of the measured values of beech wood color on the coordinates: *L*\*, *a*\*, *b*\* of the color space CIE *L\*a\*b\** in individual months, during 36 months of dazzling by the sunlight of daylight are shown in Figures 3 and 4.

The course of the measured values on the light coordinate *L\**, the chromatic coordinates of color *a\**, and the yellow color *b\** in Figures 3 and 4 during 36 months of dazzling is not fluent. The fluctuations are attributed to the influence of the intensity of solar radiation during the individual seasons, causing photolytic and photo-oxidative reactions of daylight radiation with wood. Figures 5 and 6 show the magnitudes of changes in the average values of ∆*L\**, ∆*a\**, ∆*b\** in individual seasons during the exposure.

sunlight of daylight are shown in Figures 3 and 4.

sunlight of daylight are shown in Figures 3 and 4.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 5 of 12

The courses of the measured values of beech wood color on the coordinates: *L*\*, *a*\*, *b*\* of the color space CIE *L\*a\*b\** in individual months, during 36 months of dazzling by the

The courses of the measured values of beech wood color on the coordinates: *L*\*, *a*\*, *b*\* of the color space CIE *L\*a\*b\** in individual months, during 36 months of dazzling by the

**Figure 3.** Values on the *L\** coordinate of dazzled native and steamed beech wood over a period of 36 months (October 2019 to October 2021). **Figure 3.** Values on the *L\** coordinate of dazzled native and steamed beech wood over a period of 36 months (October 2019 to October 2021). **Figure 3.** Values on the *L\** coordinate of dazzled native and steamed beech wood over a period of 36 months (October 2019 to October 2021).

steamed beech wood over a period of 36 months (October 2019 to October 2021). The course of the measured values on the light coordinate *L\**, the chromatic **Figure 4.** Values on chromatic coordinates of red color *a\** and yellow color *b\** of dazzling native and steamed beech wood over a period of 36 months (October 2019 to October 2021). **Figure 4.** Values on chromatic coordinates of red color *a\** and yellow color *b\** of dazzling native and steamed beech wood over a period of 36 months (October 2019 to October 2021).

coordinates of color *a\**, and the yellow color *b\** in Figures 3 and 4 during 36 months of dazzling is not fluent. The fluctuations are attributed to the influence of the intensity of

The course of the measured values on the light coordinate *L\**, the chromatic coordinates of color *a\**, and the yellow color *b\** in Figures 3 and 4 during 36 months of dazzling is not fluent. The fluctuations are attributed to the influence of the intensity of

solar radiation during the individual seasons, causing photolytic and photo-oxidative reactions of daylight radiation with wood. Figures 5 and 6 show the magnitudes of changes in the average values of ∆*L\**, ∆*a\**, ∆*b\** in individual seasons during the exposure.

solar radiation during the individual seasons, causing photolytic and photo-oxidative reactions of daylight radiation with wood. Figures 5 and 6 show the magnitudes of

*Polymers* **2022**, *14*, x FOR PEER REVIEW 6 of 12

**Figure 5.** Magnitudes of changes in the ∆*L\**, ∆*a\**, ∆*b\** values in the color space CIE *L\*a\*b\** native beech wood during the 36-month exposure to sunlight, depending on the season. **Figure 5.** Magnitudes of changes in the ∆*L\**, ∆*a\**, ∆*b\** values in the color space CIE *L\*a\*b\** native beech wood during the 36-month exposure to sunlight, depending on the season. **Figure 5.** Magnitudes of changes in the ∆*L\**, ∆*a\**, ∆*b\** values in the color space CIE *L\*a\*b\** native beech wood during the 36-month exposure to sunlight, depending on the season.

steamed beech wood during the 36-month exposure to sunlight, depending on the season. In Figure 7, the degree of the color change of the surface of native and steamed beech **Figure 6.** The magnitudes of changes in the values of ∆*L\**, ∆*a\**, ∆*b\** in the color space CIE *L\*a\*b\** of steamed beech wood during the 36-month exposure to sunlight, depending on the season. **Figure 6.** The magnitudes of changes in the values of ∆*L\**, ∆*a\**, ∆*b\** in the color space CIE *L\*a\*b\** of steamed beech wood during the 36-month exposure to sunlight, depending on the season.

wood caused by solar radiation during 36 months is documented by the total color difference ∆*E\*.*  In Figure 7, the degree of the color change of the surface of native and steamed beech wood caused by solar radiation during 36 months is documented by the total color difference ∆*E\*.*  In Figure 7, the degree of the color change of the surface of native and steamed beech wood caused by solar radiation during 36 months is documented by the total color difference ∆*E\*.*

**Figure 7.** Values of the total color difference ∆*E\** of native and steamed beech wood during 36 months of dazzling (October 2019 to October 2021). **Figure 7.** Values of the total color difference ∆*E\** of native and steamed beech wood during 36 months of dazzling (October 2019 to October 2021).

From the comparison of wood colors in Figure 2 and the values presented at the coordinates *L\**, *a\**, *b\** of unsteamed and steamed beech wood during the exposure, Figures 3 and 4 show that while the surface of the unsteamed beech wood darkened and browned, the brown-red color on the steamed wood lightened. From the comparison of wood colors in Figure 2 and the values presented at the coordinates *L\**, *a\**, *b\** of unsteamed and steamed beech wood during the exposure, Figures 3 and 4 show that while the surface of the unsteamed beech wood darkened and browned, the brown-red color on the steamed wood lightened.

The darkening and browning of unsteamed beech wood numerically documents the shift of the brightness coordinate *L\** from the value *L0\** = 76.6 to *L*36*\** = 71.3, i.e., by the value ∆*L\** = −5.3, and changes in chromatic coordinates: red color a\* from *a*0*\** = 7.8 to *a*36*\** = 12.2, i.e., by the value ∆*a\** = +4.4, and the yellow color *b\** from the value *b*0*\** = 19.8 to *b*36*\** = 26.8, i.e., by the value ∆*b\** = +10.0. The largest darkening of unsteamed beech wood occurred during the first year of dazzling, when changes in the brightness coordinate Δ*L\** reached 76.9% of the total change in brightness of beech wood caused by daylight; in the second year, it reached 16.8 and in the third year 6.3%. The browning of unsteamed beech wood is described by changes in the chromatic coordinates: red *a\** and yellow *b\*.* The change in the red coordinate in the first year of exposure was 57.5% of the total change ∆*a\*,* in the second year of dazzling 42.5% and in the third year oscillated around *a\** = 12. In the yellow coordinate, the change ∆*b\** in the first year of dazzling was 57.7% of the total value of the change in ∆*b\** beech wood, 38.7% in the second year and 3.6% in the third year. The changes in the red *a\** and yellow *b\** coordinates of the color space CIE *L\*a\*b\** in the third year, as the measurements show, are small, and, in addition, the different seasons are contradictory, while in winter and spring, they show a decrease in values, so in summer, in times of more intense sunlight, they grow. The darkening of wood due to solar radiation is in line with the views of experts dealing with changes in the properties of wood due to long-term exposure to sunlight, who state that the wood surface darkens and mostly yellows and browns [2,3,8,19,20]. The darkening and browning of unsteamed beech wood numerically documents the shift of the brightness coordinate *L\** from the value *L*0*\** = 76.6 to *L*36*\** = 71.3, i.e., by the value ∆*L\** = −5.3, and changes in chromatic coordinates: red color a\* from *a*0*\** = 7.8 to *a*36*\** = 12.2, i.e., by the value ∆*a\** = +4.4, and the yellow color *b\** from the value *b*0*\** = 19.8 to *b*36*\** = 26.8, i.e., by the value ∆*b\** = +10.0. The largest darkening of unsteamed beech wood occurred during the first year of dazzling, when changes in the brightness coordinate ∆*L\** reached 76.9% of the total change in brightness of beech wood caused by daylight; in the second year, it reached 16.8 and in the third year 6.3%. The browning of unsteamed beech wood is described by changes in the chromatic coordinates: red *a\** and yellow *b\*.* The change in the red coordinate in the first year of exposure was 57.5% of the total change ∆*a\*,* in the second year of dazzling 42.5% and in the third year oscillated around *a\** = 12. In the yellow coordinate, the change ∆*b\** in the first year of dazzling was 57.7% of the total value of the change in ∆*b\** beech wood, 38.7% in the second year and 3.6% in the third year. The changes in the red *a\** and yellow *b\** coordinates of the color space CIE *L\*a\*b\** in the third year, as the measurements show, are small, and, in addition, the different seasons are contradictory, while in winter and spring, they show a decrease in values, so in summer, in times of more intense sunlight, they grow. The darkening of wood due to solar radiation is in line with the views of experts dealing with changes in the properties of wood due to long-term exposure to sunlight, who state that the wood surface darkens and mostly yellows and browns [2,3,8,19,20].

Steamed beech wood under the influence of sunlight for 36 months compared to unsteamed wood showed the opposite character of the color change, where the surface of the wood faded. Visually, this is documented in Figure 2, as well as the shift of the brightness coordinate *L\** from the value *L*0*\** = 55.3 to *L*36*\** = 57.2, i.e., by the value ∆*L\** = +1.9, on the red coordinate *a\** offset from *a*0*\** = 14.5 to *a*36*\** = 14.1, i.e., by the value ∆*a\** = −0.4, and on the chromatic coordinate of yellow color *b\** from the value *b*0*\** = 19.8 to *b*36*\** = 25.5, i.e., by the value ∆*b\** = +5.7. On the basis of comparison of individual changes ∆*L\*,* ∆*a\*,* ∆*b\** on the coordinates of the color space CIE *L\*a\*b\** of steamed beech wood caused by the action of sunlight with changes ∆*L\**, ∆*a\**, ∆*b\** on the coordinates of the unmatched beech wood caused by daylight, it can be stated that the values expressing the magnitude of changes Steamed beech wood under the influence of sunlight for 36 months compared to unsteamed wood showed the opposite character of the color change, where the surface of the wood faded. Visually, this is documented in Figure 2, as well as the shift of the brightness coordinate *L\** from the value *L*0*\** = 55.3 to *L*36*\** = 57.2, i.e., by the value ∆*L\** = +1.9, on the red coordinate *a\** offset from *a*0*\** = 14.5 to *a*36*\** = 14.1, i.e., by the value ∆*a\** = −0.4, and on the chromatic coordinate of yellow color *b\** from the value *b*0*\** = 19.8 to *b*36*\** = 25.5, i.e., by the value ∆*b\** = +5.7. On the basis of comparison of individual changes ∆*L\*,* ∆*a\*,* ∆*b\** on the coordinates of the color space CIE *L\*a\*b\** of steamed beech wood caused by the action of sunlight with changes ∆*L\**, ∆*a\**, ∆*b\** on the coordinates of the unmatched beech wood caused by daylight, it can be stated that the values expressing the magnitude of changes in

steamed beech wood are smaller. The magnitude of changes in the brightness coordinates *L\** and yellow *b\*,* similar to unsteamed beech wood, is largest in the first year of exposure. The red coordinate changes of *a\** oscillated around the value of *a\** = 14.0. In winter, at low sunlight intensity, the values on the red coordinate *a\** decreased, and from spring to autumn, they increased at higher sunlight intensity. The rate of decline or the increase in red coordinate values decreases over the years. Based on the above findings, it can be stated that the functional groups of chromophores in beech wood with absorption of the electromagnetic radiation spectrum with a 630–750 nm red wavelength causing reddening of steamed beech wood were steamed and strongly eliminated for photochemical reactions of wood caused by daylight.

The authors of [21,22] point out the effect of lightening the surface of steamed wood under the action of UV radiation. In the work, a team of authors [21] report the lightening of the surface color of steamed maple wood after its irradiation in Xenotest with a 450 xenon lamp emitting UV radiation with a wavelength of 340 nm, 42 <sup>±</sup> 2 W/m<sup>2</sup> intensity, for 7 days. The lightening of the red-brown color of steamed maple wood is declared by the increase in the values on the brightness coordinate from *L*1*\** = 65.3 to the value of *L*2*\** = 70.7, i.e., by the value ∆*L\** = +5.4, the increase in the value on the chromatic coordinate of the yellow color from *b*1*\** = 19.4 to the value *b*2*\** = 28.9, i.e., by the value ∆*b\** = +9.3, with a slight change in the red coordinate value from *a*1*\** = 10.8 to *a*2*\** = 10.3, i.e., by the value ∆*a\** = −0.5.

The influence of UV radiation on steamed acacia wood in [22] states that while the surface of steamed acacia wood darkened slightly at the steaming temperature *t* = 100 ◦C, the surface of acacia wood lightened at the steaming temperature *t* = 120 ◦C.

The authors [23] describe the positive effect of the steaming process on the decomposition of functional groups of maple wood chromophores manifested by the darkening and browning of maple wood and the elimination of photochemical reactions caused by daylight. They point to the fact that the greater the darkening of the maple wood in the steaming process, the smaller the color changes on the surface of the irradiated steamed maple wood by UV radiation. This is declared by the decrease in the total color difference from the value ∆*E\** = 18.5 for unsteamed maple wood to ∆*E\** = 7.2 and steamed maple wood with saturated water steam with temperature *t* = 135 ◦C, as well as the results of FTIR analyses.

The contribution of the influence of beech wood steaming on the color fastness and resistance to the effects of sunlight declares a decrease in the value of the total color difference ∆*E\** in Figure 7. While the change in the color of unsteamed beech wood caused by solar radiation expressed by the value of the total color difference over 3 years is ∆*E\** = 15.7, the change in the total color difference of steamed beech wood in the same period is ∆*E\** = 7.5, which is a decrease in color change by about 52.2%. This points to the fact that beech wood steaming has a positive effect on changes in the chromophore system of steamed beech wood and the partial resistance of steamed beech wood to the initiation of photochemical reactions induced by daylight wavelengths.

Chemical changes on the surface of unsteamed and steamed beech wood before and after solar irradiation were also monitored by ATR-FTIR spectroscopy. The FTIR spectra of the examined samples in Figure 8 show the whole range of wavenumbers 4000 to 650 cm−<sup>1</sup> . In Figure 9, only spectra in the range 1800 to 800 cm−<sup>1</sup> are presented, where most of the specific vibrations occurred.

During various thermal treatments of wood, the chemical composition changes in the wood, which depends on the experimental treatment conditions, such as the temperature, time and atmosphere used. However, many competitive reactions take place simultaneously, depending on experimental conditions. For these reasons, our results may differ from those of other authors.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 9 of 12

**Figure 8.** FTIR spectra of beech wood samples. **Figure 8.** FTIR spectra of beech wood samples. **Figure 8.** FTIR spectra of beech wood samples.

During various thermal treatments of wood, the chemical composition changes in the **Figure 9.** FTIR spectra of beech wood samples in the range 1800 to 800 cm<sup>−</sup>1. **Figure 9.** FTIR spectra of beech wood samples in the range 1800 to 800 cm−<sup>1</sup> .

wood, which depends on the experimental treatment conditions, such as the temperature, time and atmosphere used. However, many competitive reactions take place simultaneously, depending on experimental conditions. For these reasons, our results may differ from those of other authors. The bands in the range of 3800 to 2750 cm−1 are assigned to hydroxyl and methyl/methylene stretching vibrations [24]. In the FTIR spectra at 1730 cm−1 (assigned to unconjugated carbonyl groups), the increase in absorbance, between the original beech wood before and after irradiation and the steamed beech wood before and after irradiation, was due to an increase in carbonyl or carboxyl groups in lignin or carbohydrates (Figure 9). At this observed band, the highest increase in absorbance (by 34%) was recorded in the spectrum of the irradiated steamed beech wood sample. The results of the FTIR analysis also suggest that the lignin macromolecule changes after sun irradiation of the samples by reducing the absorption bands characteristic of lignin: 1594 cm−1 (belonging to aromatic skeletal vibration in lignin, and -C=O stretching [25]), 1506 During various thermal treatments of wood, the chemical composition changes in the wood, which depends on the experimental treatment conditions, such as the temperature, time and atmosphere used. However, many competitive reactions take place simultaneously, depending on experimental conditions. For these reasons, our results may differ from those of other authors. The bands in the range of 3800 to 2750 cm−1 are assigned to hydroxyl and methyl/methylene stretching vibrations [24]. In the FTIR spectra at 1730 cm−1 (assigned to unconjugated carbonyl groups), the increase in absorbance, between the original beech wood before and after irradiation and the steamed beech wood before and after irradiation, was due to an increase in carbonyl or carboxyl groups in lignin or carbohydrates (Figure 9). At this observed band, the highest increase in absorbance (by 34%) was recorded in the spectrum of the irradiated steamed beech wood sample. The results of the FTIR analysis also suggest that the lignin macromolecule changes after sun irradiation of the samples by reducing the absorption bands characteristic of lignin: 1594 The bands in the range of 3800 to 2750 cm−<sup>1</sup> are assigned to hydroxyl and methyl/methylene stretching vibrations [24]. In the FTIR spectra at 1730 cm−<sup>1</sup> (assigned to unconjugated carbonyl groups), the increase in absorbance, between the original beech wood before and after irradiation and the steamed beech wood before and after irradiation, was due to an increase in carbonyl or carboxyl groups in lignin or carbohydrates (Figure 9). At this observed band, the highest increase in absorbance (by 34%) was recorded in the spectrum of the irradiated steamed beech wood sample. The results of the FTIR analysis also suggest that the lignin macromolecule changes after sun irradiation of the samples by reducing the absorption bands characteristic of lignin: 1594 cm−<sup>1</sup> (belonging to aromatic skeletal vibration in lignin, and -C=O stretching [25]), 1506 cm−<sup>1</sup> (C=C stretching of the aromatic skeletal vibrations in lignin [26]), 1422 cm−<sup>1</sup> (assigned to aromatic skeletal vibrations combined with C-H deformation in carbohydrates [27]). The absorption band at 1506 cm−<sup>1</sup> almost disappeared with the irradiated steamed sample, and the reduction of theband intensity was by more than 93% compared to the steamed sample. The reduction of bandintensity for lignin in wood samples after UV irradiation was also observed by the authors [28].

cm−1 (C=C stretching of the aromatic skeletal vibrations in lignin [26]), 1422 cm−1 (assigned

cm−1 (belonging to aromatic skeletal vibration in lignin, and -C=O stretching [25]), 1506 cm−1 (C=C stretching of the aromatic skeletal vibrations in lignin [26]), 1422 cm−1 (assigned

In our research, we also recorded a decrease in absorbances at bands 1421 cm−<sup>1</sup> (aromatic ring vibration in lignin combined with C−H deformation in carbohydrates) and 1328 cm−<sup>1</sup> (C-O vibration in syringyl plus guaiacyl derivatives is characteristic for condensed structures in lignin) when comparing unsteamed beech wood samples before and after irradiation, as well as steamed wood and irradiated steamed beech wood. This decrease indicates cleavage of the methoxyl groups during sunlight, leading to gradual demethoxylation. The reduction in the intensity of the bands at 1124 cm−<sup>1</sup> (C-H vibration of syringyl units in lignin) irradiated wood samples indicates cleavage of the ether bond in the lignin structure.

The intensity of the band at 1370 cm−<sup>1</sup> (C-H deformations in polysaccharides) is not significantly affected by the treatment conditions of the wood samples, so this band was used as a reference to determine lignin degradation. To compare the changes in chemical composition on the surface of the wood samples used for the experiment, the ratios of intensities 1506/1370, 1730/1370 and 1730/1506 were calculated (Table 3).


**Table 3.** Ratios of absorption bands for the beech wood.

The decrease in the 1506/1370 ratio of intensities in the wood samples after solar irradiation demonstrates the decay of lignin, which occurs due to the interaction of radiation with wood. The increase in unconjugated carbonyl groups (carbonyl-containing chromophores) due to photo-oxidation is reflected in the increasing ratio of 1730/1370 (carbonyl/carbohydrates). These results are consistent with published research by other authors [29–31]. The relative increase in carbonyl groups in lignin or carbohydrates with the relative decrease in lignin in parallel is reflected in an increase in the 1730/1506 ratio. The authors of [32] explained the increase in this ratio by primarily lignin degradation and oxidation (photo-oxidation and thermo-oxidation resulting in an increase in carbonyl groups).

The authors of [33] used FTIR analysis of chemical changes in beech and oak wood after steaming and compression. Using FTIR, they observed changes in the hydroxyl groups, as well as in the C-O and C-H functional groups in the polysaccharides and in lignin on the surface of the samples. Other authors [34–38] state that the lightfastness of various woody plants to a large extent mainly affects lignin and extractives.

#### **4. Conclusions**

This paper presents the results of the color change of the surface of unsteamed and steamed beech wood with saturated water steam in a pressure autoclave, which was exposed to daylight interior for 36 months. The results of analyses of the effect of solar radiation on unsteamed and steamed beech wood showed:


**Author Contributions:** Conceptualization, M.D.; methodology, M.D., L.D. and V.K.; software, M.D. and V.K.; formal analysis, M.D., L.D. and V.K.; resources, M.D., L.D. and V.K.; data curation, M.D., L.D. and V.K.; writing—original draft preparation, M.D.; writing—review and editing, M.D., L.D. and V.K.; visualization, M.D.; project administration, L.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by project APVV-17-0456 "Thermal modification of wood with saturated water steam for purposeful and stable change of wood color".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** This experimental research was carried out under the grant project APVV-17- 0456 "Thermal modification of wood with saturated water steam for purposeful and stable change of wood color" as the result of the work of the author and the considerable assistance of the APVV agency. This publication is the result of the project implementation: Progressive research of performance properties of wood-based materials and products (LignoPro), ITMS: 313011T720 (25%) supported by the Operational Programme Integrated Infrastructure (OPII) funded by the ERDF.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Ladislav Dzurenda \*, Michal Dudiak and Eva Výbohová**

Faculty of Wood Sciences and Technology, Technical University in Zvolen, T.G. Masaryka 24, 96001 Zvolen, Slovakia; xdudiak@tuzvo.sk (M.D.); vybohova@tuzvo.sk (E.V.) **\*** Correspondence: dzurenda@tuzvo.sk; Tel.: +421-45-520-6365

**Abstract:** The wood of maple (*Acer Pseudopatanus* L.) was steamed with a saturated steam-air mixture at a temperature of *t* = 95 ◦C or saturated steam at *t* = 115 ◦C and *t* = 135 ◦C, in order to give a pale pink-brown, pale brown, and brown-red color. Subsequently, samples of unsteamed and steamed maple wood were irradiated with a UV lamp in a Xenotest Q-SUN Xe-3-H after drying, in order to test the color stability of steamed maple wood. The color change of the wood surface was evaluated by means of measured values on the coordinates of the color space CIE *L\* a\* b\**. The results show that the surface of unsteamed maple wood changes color markedly under the influence of UV radiation than the surface of steamed maple wood. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The positive effect of steaming on UV resistance is evidenced by the decrease in the overall color difference ∆*E\**. While the value of the total color diffusion of unsteamed maple wood induced by UV radiation is ∆*E\** = 18.5, for maple wood steamed with a saturated steam-air mixture at temperature *t* = 95 ◦C the ∆*E\** decreases to 12.6, for steamed maple wood with saturated water steam with temperature *t* = 115 ◦C the ∆*E\** decreases to 10.4, and for saturated water steam with temperature *t* = 135 ◦C the ∆*E\** decreases to 7.2. Differential ATR-FTIR spectra declare the effect of UV radiation on unsteamed and steamed maple wood and confirm the higher color stability of steamed maple wood.

**Keywords:** maple wood; color difference; ATR-FTIR spectroscopy; steaming; saturated water steam

## **1. Introduction**

The color of wood is a basic physical-optical property, which belongs to the group of macroscopic features on the basis of which the wood of individual woody plants differs visually. The color of the wood is formed by chromophores, i.e., functional groups of the type: >C=O, –CH=CH–CH=CH–, –CH=CH–, aromatic nuclei found in the chemical components of wood (lignin and extractive substances, such as dyes, tannins, resins, etc.), which absorb some components of the electromagnetic radiation of daylight and thus create the color of the wood surface perceived by human vision.

The color of wood changes in thermal processes, such as wood drying, wood steaming, and thermo-wood production technologies. The wood darkens more or less and, depending on the wood, acquires color shades of pink, red, and brown to dark brown-gray color [1–11].

Wood steaming is a physico-chemical process, in which wood placed in an environment of hot water, saturated water steam or saturated humid air is heated and changes its physical, mechanical, and chemical properties. The action of heat initiates the chemical reactions in wet wood, such as the extraction of water-soluble substances, degradation of polysaccharides, cleavage of free radicals, and phenolic hydroxyl groups in lignin, resulting in the formation of new chromophoric groups causing a change in the color of the wood. These facts are used for the full-volume modification of wood color into non-traditional color shades of wood of individual trees. Beech wood, depending on the length of the

**Citation:** Dzurenda, L.; Dudiak, M.; Výbohová, E. Influence of UV Radiation on the Color Change of the Surface of Steamed Maple Wood with Saturated Water Steam. *Polymers* **2022**, *14*, 217. https://doi.org/ 10.3390/polym14010217

Academic Editor: Antonios N. Papadopoulos

Received: 14 December 2021 Accepted: 31 December 2021 Published: 5 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

steaming time, acquires a pale pink to red-brown color shade [5,7,9,12–15]. Oak wood, as reported by the works [11,16,17] and depending on the steaming conditions, achieves color shades from a pale brown-yellow color to a dark dark-gray color. The light white-yellow color of maple wood in the process of steaming the wood with saturated water steam acquires shades of pale pink-brown to brown-red color [18,19].

The color of the wood also changes due to the long-term effects of sunlight on its surface. The surface of the wood darkens and is mostly yellow and brown. This fact is also referred to in the professional literature as natural aging [20–22].

Solar radiation falling on the wood surface is partly absorbed and partly reflected from the surface. The absorbed spectrum of infrared electromagnetic radiation is converted into heat. In addition, the photon flux of ultraviolet and part of the visible radiation of wavelengths *λ* = 200–400 nm are the source of initiation of photolytic and photooxidation reactions with lignin, polysaccharides, and accessory substances of wood. Of the chemical components of wood, lignin is the most subject to photodegradation, which captures 80–85% of UV radiation, while carbohydrates absorb 13–18% and 2% of accessory substances [23]. These reactions cleave the lignin macromolecule with the simultaneous formation of phenolic hydroperoxides, free radicals, carbonyl and carboxyl groups, and to a lesser extent depolymerize polysaccharides to polysaccharides with a lower degree of polymerization to form carbonyl, carboxyl groups, and gaseous products (CO, CO2, H2). Although the photodegradation of natural wood is a widely studied phenomenon [20,22,24–29], less attention has been paid to the issue of photodegradation and color stability of steamed wood.

The aim of the work is to investigate the color fastness of maple wood obtained by the process of steaming with a saturated steam-air mixture or with saturated water steam through a simulated aging process-UV radiation in Xenotest Q-SUN Xe-3-H. The color fastness of the wood is evaluated by changes in the coordinates *L\**, *a\**, *b\** of the color space CIE *L\* a\* b\*,* the total color difference ∆*E\**, and changes in the values of differential absorbance *A<sup>d</sup>* of selected bands in FTIR spectra.

#### **2. Material and Methods**

#### *2.1. Material*

The wet wood of maple blanks with the following dimensions: Thickness: *h* = 40 mm, width *w* = 100 mm, length *d* = 750 mm, and moisture content *w<sup>p</sup>* = 57.8 ± 4.8%, was steamed with a saturated steam-air mixture at a temperature of *t* = 95 ◦C or saturated steam at *t* = 115 ◦C and *t* = 135 ◦C for *τ* = 9 h, in order to obtain a pale pink-brown, pale brown, and brown-red color in a pressure autoclave: APDZ 240 in Sundermann s.r.o. (Banská Štiavnica, Slovakia). The steamed and unsteamed maple wood blanks were subsequently dried to a moisture content of *w* = 10 ± 0.5%. Samples measures with the following dimensions: Thickness: *h* = 15 mm, width *w* = 50 mm, and length *d* = 100 mm, were made to test the color fastness of the wood.

#### *2.2. Color Measurement of Maple Wood*

The color of steamed and unsteamed maple wood before and after irradiation was measured in the color space CIE *L\*a\*b\*.* To measure the color of maple wood, in the color space CIE *L\*a\*b\**, the color reader CR-10 (Konica Minolta, Osaka, Japan) was measured. A D65 light source was used and the diameter of the optical scanning aperture was 8 mm.

The color measurement was performed on a radial surface machined by planning. The color coordinates of maple wood samples in the color space CIE *L\*a\*b\** before irradiation are given in Table 1.


**Table 1.** Coordinate values of color space CIE *L\*a\*b\** of native and thermally treated maple wood.

The total color difference ∆*E*\* of the color change of the surface of the maple wood samples under the influence of UV radiation is determined according to the following equation ISO 11 664-4:

$$
\Delta E^\* = \sqrt{\left(L\_{298}^\* - L\_0^\*\right)^2 + \left(a\_{298}^\* - a\_0^\*\right)^2 + \left(b\_{298}^\* - b\_0^\*\right)^2} \tag{1}
$$

where *L* \* <sup>0</sup>, *a* \* <sup>0</sup>, *b* \* <sup>0</sup> are values on the coordinates of the color space of the surface of the dried milled native and thermally treated maple wood before exposure.

*L* \* <sup>298</sup>, *a* \* <sup>298</sup>, *b* \* <sup>298</sup> are values on the surface color coordinates of the dried milled native and thermal treated maple wood during UV exposure.

#### *2.3. Irradiation of Maple Wood in Xenon Test Chamber*

In the Q-SUN Xe-3-H Xenon test chamber, Q-Lab Corporation, Westlake, OH, USA (1800 W Xenon arc lamp-full spectrum, irradiation 0.35 W/m<sup>2</sup> -340 nm, black panel temperature 63 ◦C), the samples were irradiated for *τ* = 298 h. During the exposure, the color of the irradiated surface was measured regularly at *τ* = 24 h intervals.

#### *2.4. Analysis of Changes in Lignin-Cellulose Matrix of Wood ATR-FTIR Spectroscopy*

Infrared spectroscopy was used to monitor changes in maple wood components induced by UV radiation in unsteamed and steamed wood. The FTIR surface analysis of wood samples was performed on a Nicolet iS 10 FTIR spectrometer (Thermo Fisher Scientific, Madison, WI, USA) using the attenuated total reflectance (ATR-FTIR) technique. The measurements were performed on a diamond crystal in the range of 4000–650 cm−<sup>1</sup> . For each sample, 64 scans were performed at a resolution of 4 cm−<sup>1</sup> . The obtained spectral records were evaluated by the spectroscopic software OMNIC 8. The calculation of the values of the differential absorbance *A<sup>d</sup>* describes the relation:

$$A\_d = \frac{\left(A\_i - A\_{ref}\right)}{A\_{ref}} \cdot 100\tag{2}$$

where *A<sup>d</sup>* is the differential absorbance, *A<sup>i</sup>* is the absorbance at a given wavelength in the spectrum of the irradiated sample, and *Aref* is the absorbance at a given wavelength in the spectrum of the unirradiated sample.

#### **3. Results and Discussion**

#### *3.1. Color Analysis*

The color of unsteamed and steamed maple wood before and after UV irradiation in the Q-SUN Xe-3-H test chamber (Q-Lab Corporation, Westlake, OH, USA) is shown in Figure 1.

According to the visual evaluation of the color of maple wood before and after the UV radiation, it can be stated that while the light white-yellow color of untreated maple wood darkens and acquires a yellow-red-brown color shade due to photodegradation reactions induced by UV radiation, which is the pale pink-brown of court wood treated with a steam-air mixture with a temperature of *t* = 95 ◦C, it darkened slightly with UV radiation and took on a pale brown-yellow color shade. The brown-red color of steamed maple wood obtained by steaming with saturated steam with a temperature of *t* = 135 ◦C lightened.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 4 of 10

**Figure 1.** View of maple wood before and after UV irradiation: Native; steamed at *t* = 95 °C; steamed at *t* = 115 °C; steamed at *t* = 135 °C. **Figure 1.** View of maple wood before and after UV irradiation: Native; steamed at *t* = 95 ◦C; steamed at *t* = 115 ◦C; steamed at *t* = 135 ◦C. and took on a pale brown-yellow color shade. The brown-red color of steamed maple wood obtained by steaming with saturated steam with a temperature of *t* = 135 °C light-

According to the visual evaluation of the color of maple wood before and after the UV radiation, it can be stated that while the light white-yellow color of untreated maple wood darkens and acquires a yellow-red-brown color shade due to photodegradation re-The course of color changes of unsteamed and steamed maple wood in the color space CIE *L\*a\*b\** under the influence of UV radiation in Xenotest Q-SUN Xe-3-H for 298 h, are shown in Figures 2–4. ened. The course of color changes of unsteamed and steamed maple wood in the color space CIE *L\*a\*b\** under the influence of UV radiation in Xenotest Q-SUN Xe-3-H for 298 h, are shown in Figures 2–4.

actions induced by UV radiation, which is the pale pink-brown of court wood treated with

**Figure 2.** The course of changes of values on the light coordinate *L\** in the process of UV irradiation of samples of unsteamed and steamed maple wood. **Figure 2.** The course of changes of values on the light coordinate *L\** in the process of UV irradiation of samples of unsteamed and steamed maple wood.

**Figure 2.** The course of changes of values on the light coordinate *L\** in the process of UV irradiation

of samples of unsteamed and steamed maple wood.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 5 of 10

**Figure 3.** The course of changes of values on the coordinate of red color and *a\** in the process of UV irradiation of samples of unsteamed and steamed maple wood. **Figure 3.** The course of changes of values on the coordinate of red color and *a\** in the process of UV irradiation of samples of unsteamed and steamed maple wood. **Figure 3.** The course of changes of values on the coordinate of red color and *a\** in the process of UV irradiation of samples of unsteamed and steamed maple wood.

**Figure 4.** The course of changes of values on the coordinate of yellow color *b\** in the process of UV irradiation of samples. **Figure 4.** The course of changes of values on the coordinate of yellow color *b\** in the process of UV irradiation of samples. **Figure 4.** The course of changes of values on the coordinate of yellow color *b\** in the process of UV irradiation of samples.

Based on the experimentally determined values of color changes on the luminance coordinate *L\**, which are the chromaticity coordinates red *a\** and yellow color *b\** of maple wood samples induced by photodegradation reactions of individual maple wood components with UV radiation in Xenotest Q-SUN Xe-3-H, it can be stated that significant changes in the color of the wood occur in the first 72 h of UV radiation. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The change of color of unsteamed maple wood is greater than the steamed maple wood. Numerically, this is documented by the shifts on the individual coordinates of the color space CIE *L\*a\*b\** of the analyzed maple wood samples before and after UV irradiation, as shown in Table 2. Based on the experimentally determined values of color changes on the luminance coordinate *L\**, which are the chromaticity coordinates red *a\** and yellow color *b\** of maple wood samples induced by photodegradation reactions of individual maple wood components with UV radiation in Xenotest Q-SUN Xe-3-H, it can be stated that significant changes in the color of the wood occur in the first 72 h of UV radiation. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The change of color of unsteamed maple wood is greater than the steamed maple wood. Numerically, this is documented by the shifts on the individual coordinates of the color space CIE *L\*a\*b\** of the analyzed maple wood samples before and after UV irradiation, as shown in Table 2. Based on the experimentally determined values of color changes on the luminance coordinate *L\**, which are the chromaticity coordinates red *a\** and yellow color *b\** of maple wood samples induced by photodegradation reactions of individual maple wood components with UV radiation in Xenotest Q-SUN Xe-3-H, it can be stated that significant changes in the color of the wood occur in the first 72 h of UV radiation. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The greater the darkening and browning color of the maple wood by steaming, the smaller the changes in the values at the coordinates *L\**, *a\**, *b\** of the steamed maple wood caused by UV radiation. The change of color of unsteamed maple wood is greater than the steamed maple wood. Numerically, this is documented by the shifts on the individual coordinates of the color space CIE *L\*a\*b\** of the analyzed maple wood samples before and after UV irradiation, as shown in Table 2.


**Table 2.** Sizes of changes in ∆*L\**, ∆*a\**, ∆*b\** values in the CIE *L\*a\*b\** color space of unsteamed and steamed maple wood before and after UV irradiation in the Q-SUN Xe-3-H test chamber.

The degree of darkening and browning of the unmapped maple during 298 h of UV irradiation in the CIE *L\*a\*b\** color space is declared by a decrease in the luminance coordinate by ∆*L\** = −12.3 and an increase in points in the red chromatic coordinate by ∆*a\** = +5.8 a on the yellow coordinate b\* by the value ∆*b\** = +12.7. The above findings on darkening of wood due to UV radiation are in accordance with the opinions of experts dealing with changes in the properties of wood due to solar radiation, respectively UV radiation is shown in [20,22,24,25,29–31].

The differences in the color of steamed maple wood before and after UV irradiation on the light coordinate *L\** and the chromaticity coordinate of red *a\** and yellow *b\** are smaller compared to the changes of unsteamed maple wood.

While the darkness of the thermal treated maple wood with the steam-air mixture with the temperature *t* = 95 ◦C due to UV radiation increased, the darkness of the maple wood treated with the saturated steam increased by decreasing the values from *L*<sup>0</sup> \* = 75 to *L*<sup>298</sup> \* = 69.8, i.e., <sup>∆</sup>*L*\* = <sup>−</sup>5.2 temperature *<sup>t</sup>* = 115 ◦C did not change due to UV radiation and the brightness of steamed maple wood with saturated water steam with temperature *t* = 135 ◦C increased from *L*<sup>0</sup> \* = 61.1 to *L*<sup>298</sup> \* = 63.9, i.e., the value of ∆*L*\* = +2.8. The decrease in the darkening of maple wood steamed at higher steaming temperatures due to UV radiation or to achieving the opposite effect—lightening the surface of steamed maple wood with saturated steam with temperature *t* = 135 ◦C indicates changes in the chromatic system caused by steaming, which affects the photochemical reactions of UV radiation with functional groups of the chromophore system of steamed maple wood. The work of [32,33] also points that steamed wood, unlike unsteamed wood, is more or less resistant to UV radiation.

The effect of fading of the red-brown color of the beech wood surface is achieved by steaming with a saturated steam with temperature *t* = 120 ± 2 ◦C after UV irradiation in the Xenotest 450 Xenon lamp. This emits UV radiation with a wavelength of 340 nm and an intensity of 42 <sup>±</sup> 2 W/m<sup>2</sup> for 7 days, as stated in the work of [32].

In [33], the effect of UV radiation on steamed agate wood states that while the surface of steamed agate wood darkened slightly at a steaming temperature *t* = 100 ◦C, the surface of agate wood brightened at a steaming temperature *t* = 120 ◦C.

The positive effect of maple wood steaming on the resistance to the effects of UV radiation is declared by the decrease in values ∆*a\** and ∆*b\** on the chromatic coordinates given in Table 2. The pale pink-brown color obtained by steaming with a steam-air mixture with temperature *t* = 95 ◦C by absorbing UV radiation increased on the red coordinate by *a\** = +2.1. In addition, the pale-brown color of steamed maple wood formed by steaming with saturated steam with temperature *t* = 115 ◦C due to UV radiation increased by the value ∆*a\** = +0.6. Moreover, the effect of UV radiation on the surface of steamed maple wood steamed with saturated steam with a temperature of *t* = 135 ◦C was manifested by a decrease in the value on the red coordinate from *a*<sup>0</sup> \* = 12.6 to *a*<sup>298</sup> \* = 12.3, i.e., value ∆*a\** = −0.3. Similarly, at the yellow coordinate there are decreases in ∆*b\** values caused by UV radiation on steamed maple wood. The value of the change ∆*b\** on the yellow coordinate induced by UV radiation on the surface of maple wood steamed with a steam mixture with temperature *t* = 95 ◦C is ∆*b\** = +11.5. In addition, maple wood steamed with

saturated water steam with temperature *t* = 115 ◦C is ∆*b\** = +10.4 and saturated water steam with temperature *t* = 135 ◦C is ∆*b\** = +6.6. Based on the above findings, it can be stated that the functional groups of the maple wood chromophoric system absorbing electromagnetic radiation spectra with a wavelength of red 630–750 nm and a wavelength of 570–590 nm of yellow color were significantly eliminated for photochemical reactions of wood induced by UV radiation for the red color and to a lesser extent for the yellow color. water steam with temperature *t* = 115 °C is ∆*b\** = +10.4 and saturated water steam with temperature *t* = 135 °C is ∆*b\** = +6.6. Based on the above findings, it can be stated that the functional groups of the maple wood chromophoric system absorbing electromagnetic radiation spectra with a wavelength of red 630–750 nm and a wavelength of 570–590 nm of yellow color were significantly eliminated for photochemical reactions of wood induced by UV radiation for the red color and to a lesser extent for the yellow color.

The changes in the values at the individual coordinates of the color space CIE *L\*a\*b\** induced on the surface of unsteamed and steamed maple wood by UV radiation in Xenotest 450 are reflected in the quantification of the color change of the maple wood surface expressed by the total color difference ∆*E\*.* The influence of UV radiation on the magnitude of color changes of the analyzed maple wood samples in the form of the total color difference ∆*E\** is shown in Figure 5. The changes in the values at the individual coordinates of the color space CIE *L\*a\*b\** induced on the surface of unsteamed and steamed maple wood by UV radiation in Xenotest 450 are reflected in the quantification of the color change of the maple wood surface expressed by the total color difference ∆*E\*.* The influence of UV radiation on the magnitude of color changes of the analyzed maple wood samples in the form of the total color difference ∆*E\** is shown in Figure 5.

**Figure 5.** Influence of UV radiation on the size of the total color difference ∆*E\** of unsteamed and steamed maple wood. **Figure 5.** Influence of UV radiation on the size of the total color difference ∆*E\** of unsteamed and steamed maple wood.

The lower values of the total color difference ∆*E\** of steamed maple wood indicate the benefit of steaming on the resistance of steamed maple wood to UV radiation causing the color change in the process of natural aging. While the color change of unsteamed maple wood caused by UV radiation reaches the value ∆*E\** = 18.5, for steamed maple wood steamed with temperature *t* = 95 °C it is ∆*E\** = 12.6, which is a decrease of 31.8% compared to the total color difference of unsteamed maple wood, for steamed maple wood with saturated steam with temperature *t* = 115 °C is ∆*E\** = 10.4, which is a decrease of 43.8%, and for steamed maple wood steamed with saturated steam with temperature *t* = 135 °C is ∆*E\** = 7.2, which is a decrease of 61.1%. The lower values of the total color difference ∆*E\** of steamed maple wood indicate the benefit of steaming on the resistance of steamed maple wood to UV radiation causing the color change in the process of natural aging. While the color change of unsteamed maple wood caused by UV radiation reaches the value ∆*E\** = 18.5, for steamed maple wood steamed with temperature *t* = 95 ◦C it is ∆*E\** = 12.6, which is a decrease of 31.8% compared to the total color difference of unsteamed maple wood, for steamed maple wood with saturated steam with temperature *t* = 115 ◦C is ∆*E\** = 10.4, which is a decrease of 43.8%, and for steamed maple wood steamed with saturated steam with temperature *t* = 135 ◦C is ∆*E\** = 7.2, which is a decrease of 61.1%.

#### *3.2. ATR-FTIR Spectroscopy Analysis 3.2. ATR-FTIR Spectroscopy Analysis*

The color changes of maple wood samples caused by photodegradation reactions initiated by UV radiation are also documented by FTIR analyses of the surface of unsteamed and steamed maple wood after UV radiation in Figure 6 and Table 3. The color changes of maple wood samples caused by photodegradation reactions initiated by UV radiation are also documented by FTIR analyses of the surface of unsteamed and steamed maple wood after UV radiation in Figure 6 and Table 3.

The results of FTIR analysis indicate the formation of new carbonyl C=O groups in the spectra of the samples manifested by an increase in the intensity of the absorption band at a maximum of 1720 cm−<sup>1</sup> . In this section, an overlap of several absorption bands can be observed, which is a manifestation of vibrations of conjugated and unconjugated C=O bonds, as well as carboxyl groups. These can come not only from the main constituents of wood (lignin, cellulose, hemicelluloses), but also from extractives [33]. We recorded the most significant increase in the intensity of the C=O group bands in the spectrum of

the irradiated native sample, by more than 57%. In the case of irradiated steamed wood samples, this increase ranges from 16 to 21%. *Polymers* **2022**, *14*, x FOR PEER REVIEW 8 of 10

**Figure 6.** Differential ATR-FTIR spectra expressing the effect of UV radiation on unsteamed and steamed maple wood. **Figure 6.** Differential ATR-FTIR spectra expressing the effect of UV radiation on unsteamed and steamed maple wood.

**Table 3.** Values of differential absorbances of selected absorption bands expressing the influence of UV radiation on unsteamed and steamed maple wood. **Table 3.** Values of differential absorbances of selected absorption bands expressing the influence of UV radiation on unsteamed and steamed maple wood.


The results of FTIR analysis indicate the formation of new carbonyl C=O groups in the spectra of the samples manifested by an increase in the intensity of the absorption band at a maximum of 1720 cm−1. In this section, an overlap of several absorption bands can be observed, which is a manifestation of vibrations of conjugated and unconjugated C=O bonds, as well as carboxyl groups. These can come not only from the main constituents of wood (lignin, cellulose, hemicelluloses), but also from extractives [33]. We recorded the most significant increase in the intensity of the C=O group bands in the spectrum of the irradiated native sample, by more than 57%. In the case of irradiated steamed wood samples, this increase ranges from 16 to 21%. The performed FTIR analyses also indicate a significant degradation of the lignin macromolecule due to UV radiation. After irradiating the steamed wood sample at 135 ◦C, we recorded a complete loss of the absorption band at wavenumber 1504 cm−<sup>1</sup> , in order to be able to speak of complete degradation of lignin in the surface layer of the samples. After irradiation of the samples steamed at 95 and 115 ◦C, the intensity of the said characteristic lignin absorption band decreased by 96.24% or 97.37%. The reduction can also be observed by comparing the intensities of other absorption bands characteristic for lignin, at wavenumber 1593, 1462, 1422, 1225, and 830 cm−<sup>1</sup> . However, it should be noted that at 1462 and 1225 cm−<sup>1</sup> , not only vibrations of lignin, but also hemicelluloses, occur.

The performed FTIR analyses also indicate a significant degradation of the lignin macromolecule due to UV radiation. After irradiating the steamed wood sample at 135 °C, we recorded a complete loss of the absorption band at wavenumber 1504 cm−1, in order to be able to speak of complete degradation of lignin in the surface layer of the samples. After irradiation of the samples steamed at 95 and 115 °C, the intensity of the said characteristic lignin absorption band decreased by 96.24% or 97.37%. The reduction can also be Several studies have confirmed that lignin is the most sensitive to UV radiation among all of the wood components [24,29,34,35]. By absorbing energy, the bonds are cleaved and new functional groups (carbonyl and carboxyl) are formed, as well as radicals, which further induce lignin depolymerization and condensation reactions. Aromatic phenoxyl radicals react with oxygen to form unsaturated carbonyl compounds (quinones), which contribute to the color changes of wood [25,36].

observed by comparing the intensities of other absorption bands characteristic for lignin, at wavenumber 1593, 1462, 1422, 1225, and 830 cm−1. However, it should be noted that at 1462 and 1225 cm−1, not only vibrations of lignin, but also hemicelluloses, occur. Several studies have confirmed that lignin is the most sensitive to UV radiation among all of the wood components [24,29,34,35]. By absorbing energy, the bonds are Based on the decrease in the intensities of the absorption bands at the wavenumber 1328 and 1126 cm−<sup>1</sup> , we can state that in addition to lignin, polysaccharide degradation also occurs. While the first band corresponds to the vibrations of cellulose macromolecule, the second band belongs to the symmetric valence vibrations of the ether bond and the glucose ring [33,37].

cleaved and new functional groups (carbonyl and carboxyl) are formed, as well as radicals, which further induce lignin depolymerization and condensation reactions. Aromatic phenoxyl radicals react with oxygen to form unsaturated carbonyl compounds (qui-Since the formation of new C=O bonds is considered to be the main cause of wood color changes during its exposure to UV radiation, the results of FTIR analysis confirm the positive effect of thermal steaming of wood on its color stability.

nones), which contribute to the color changes of wood [25,36].

#### **4. Conclusions**

The surface color of unsteamed maple wood changes more markedly than the surface color of steamed maple wood due to UV radiation. The more pronounced the darkening and browning color of the steamed maple wood, the smaller the UV-induced changes in the color of the steamed maple wood. This is evidenced by the degree of darkening of the surface of unsteamed and steamed maple wood at *t* = 95 ◦C and *t* = 125 ◦C after UV irradiation by decreasing values on the luminance coordinate *L\**, as well as the rate of decrease of ∆*a\**, ∆*b\** values on chromatic coordinates. The decrease in ∆*a\** and ∆*b\** values on the chromatic coordinates indicates that the functional groups of the maple wood chromophore system absorbing electromagnetic radiation spectra with a wavelength of red of 630–750 nm and a wavelength of 570–590 nm were eliminated to a lesser extent by steaming for photochemical reactions of wood caused by UV radiation.

The positive effect of maple wood steaming on the limiting effect of initiating photo degradation reactions induced by UV radiation on the surface of steamed maple wood is evidenced by the decrease in the overall color difference ∆*E\*.* While the change in color of unsteamed maple wood caused by UV radiation expressed by the total color difference is ∆*E\** = 18.5, for steamed maple wood the stated changes in color difference values depending on the steaming temperature decrease from ∆*E\** = 12.6 to ∆*E\** = 7.2, which is a decrease from 31.8 to 61.1%.

This is confirmed by the results of FTIR analyses. While in the case of unsteamed wood we recorded an increase in the intensity of absorption bands of chromophoric C=O groups due to UV radiation by more than 57%, in the case of steamed wood samples this increase is lower and ranges from 16 to 21%.

**Author Contributions:** L.D. designed the whole study; M.D., L.D., and E.V. conducted data collection, modeling, and results analysis; L.D. wrote the original draft paper; M.D. and E.V. revised and edited the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by project APVV-17-0456 "Thermal modification of wood with saturated water steam for purposeful and stable change of wood color".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

