**1. Introduction**

Regarding the antioxidant capacity of wood extracts, including phenolic compounds, numerous studies focus on the total phenolic content measurements in food research or medical biology [1]. Only a few researches deal with the investigation of extractive content of wood, while most researches report on the mechanical properties of wood. The main goal of this research was to detect the relation between changes of chemical components and the apparent contact angle of wood surface during long-term artificial radiation. To find these relations, total carbohydrate and total phenolic compound measurements were performed in parallel with contact angle measurements to identify decomposition products of the wood surface thin layer and to monitor the behavior of contact angle of the wood surface.

There are considerable di fferences between wood species regarding the ratio of lignin, as their structural material. While the lignin content of hardwoods is 18%–25%, for softwoods, it is between 25% and 35% generally [2]. The considerable diversity in quality and quantity of wood components is reflected in the amount of extractives. There can be significant di fferences in extractive content of samples coming from di fferent species [3]. Although extractives are present only in minor quantity relative to the quantity of main chemical wood components, their influence is intense on the chemical nature of di fferent wood species. Extractives a ffect the color and odor of wood, while even the e fficiency of such di fferent processing technologies as pulping and drying have and impact on the durability, adhesion, and hygroscopic behavior of wood, as noted by Umezawa [4]. Previous

researches described the relation between wood mechanical properties and phenolic compounds. Aloui et al. [5] demonstrated the e ffect of phenolic compounds on the durability of oak wood and defined that the higher the proportions of phenol, the higher the resistance of wood is. According to Trapp et al. [6], in terms of wood surface, the intensification of hydrophobic character can be primarily related to the increase of phenolic compounds quantity. The high degree of wood wettability can be attributed to the various hydrophilic components (e.g., hemicelluloses) of the wood surface [7]. Many studies sugges<sup>t</sup> that adhesion is highly a ffected by extractives of wood and the wood surface layer [7–11]. High concentration extract material on wood surfaces could become a physical obstacle, making di fficult the chemical bonding between, e.g., glue and wood. This type of extractive layer can decrease the surface energy and extent of wettability, and can degrade the penetration of glue into the wood material [12]. According to certain studies, there isn't a clear correlation between adhesion and the quantity of extractives [13], however Nguyen and Johns [14] stated that the percentage of wood extractive content is in inverse proportion to wettability. During contact angle measurements, wood species having the higher extractive and lignin content were found to have the higher contact angle values [15–17]. The di fferent parts of wood due to their chemical and structural complexity can be wetted in di fferent manner and with di fferent success, typically decreasing with the increase of the extract materials [18]. The quantitative di fference of extractives is able to cause even 40% variance with regard to wettability [19].

The wettability of wood depends on several factors due to its complexity, such as species, storage conditions (water, sunlight, biotic and abiotic factors), drying process, and cutting direction, as stated by Nguyen and Johns [20] and Kalnins et al. [21]. Wettability is also a ffected by the chemical composition of the surface, as stated by Kishino and Nakano [22], and the density of wood, as noticed by Amorim et al. [23].

One of the simplest methods to determine the surface energy of a solid surface is to measure the contact angle of di fferent liquid drops on the wood surface [24]. Theoretically, the contact angle instantaneously characterizes the feature of the solid-liquid system and is measurable in a given condition [25]. Assuming that instantaneous contact angle changes are consequence of hydrodynamic processes, it could be stated hypothetically that its most characteristic value can be measured in that moment, when the shape of the liquid droplet is formed on the surface (or at a very close time) [26]. Behavior of a liquid drop relaxing on a solid surface are influenced by the changes of the wood surface [27]. The reason for this is that the contact angle is a phenomenological parameter which is not influenced just by surface energy, but by surface roughness, surface heterogeneity, moisture content, and a lot of other factors of the wood material [28].

The photodegradation of wood makes it more complicated to find the influencing factors of wettability. Those research results are relevant for us which deal with the chemical changes of wood surface caused by artificial ageing, and with the influence of radiation-caused degradation and its effect on contact angle.

From the adhesion point of view, one of the most critical factors is the time elapsed since machining. In the case of wood, the most optimal surfaces for gluing and surface treatments are freshly prepared surfaces [9,29]. Accordingly, the wettability of di fferent wood species decreases in parallel with the age of machined surfaces because of the chemical transformation of extractives on wood surfaces [30]. Wood surfaces show significant changes in their surface free energy even after days of conditioning, as reported in di fferent studies [31,32]. Lignin is easily oxidisable by photodegradation and its structural changes can be detected right when it comes in contact with the air, or during long-term storage [33]. Surface changes of wood due to natural aging can be imitated using artificial ageing apparatus under laboratory conditions. Both processes can be blamed for decreasing the surface energy parameter, caused by the migration of hydrophobic extractives onto the surface of wood [34].

It is noted that lignin together with extractives is able to increase the hydrophobic character of wood surface. Unlike the cellulose and hemicellulose, lignin has relatively hydrophobic character [35]. Lignin is able to play a role in the increase of hydrophobicity, being active in moisture transport in wood [36,37]. Williams et al. [38] stated that light irradiation causes rapid colour changes followed by processes having a strong impact on surface wettability in the case of some wood species. Similarly, to other natural polymers lignin and polyphenols, as wood components, absorb UV radiation. Due to UV absorption, photolytic, photo-oxidative, and thermo-oxidative reactions start to develop in wood material and those reactions are responsible for the degradation caused by sunlight [39]. Radicals formed due to the photodegradation of lignin protect lignin and also the complex wood system from further photodegradation e ffects [40]. Besides lignin, the most intensive visible changes due to photodegradation are caused by extractives. The reason of this is that extractives have strong light absorption attribute [41,42] and in this manner protect the main wood components [43]. During artificial or natural ageing hydrophobic extractives migrate to the wood surface and cause decrease of the wettability [34]. The dissolution of phenolic compounds is result of surface structure also, which is typical when the wood surface comes into contact with liquids [44–47]. In case of some wood species due to artificial ageing wettability increases. The phenomenon of ablation caused by UV radiation is the reason for the mitigation of materials able to decrease wettability of wood surface [48]. Due to long-term UV radiation of the cell wall, researchers [49] detected significant quantity of water-soluble decomposition products, which can alter the quality of wettability.

Penetration depth of UV radiation and visible light are di fferent, and the photodegradation caused by them is of di fferent measure also. Based on Williams [50] and Pandey [41] the usual penetration depth of UV radiation is ~75 μm. These values depend on the density of wood and ratio of earlywood and latewood. Hon and Ifju [51] stated that the maximum UV penetration depth in wood is not more than 80 μm. According to the research of Németh and Faix [52], the upper 75-μm thin layer of wood should be monitored when examining photodegraded wood surfaces.

The present investigations are directed towards the development of a novel method for the evaluation of wood surface chemical compound changes under artificial ageing in the case of di fferent wood species.

#### **2. Materials and Methods**

During the investigation of artificially aged wood surfaces, contact angle measurements (using distilled water and diiodomethane), moisture content (MC) examinations, total phenolic and total soluble carbohydrate content measurements were performed on wood samples of beech (*Fagus sylvatica* L.), birch (*Betula pendula*), sessile oak (*Quercus petraea*), and spruce (*Picea abies*), as depicted in Figure 1.

**Figure 1.** Flow diagram of experimental procedures.

#### *2.1. Preparation of Wood Specimens*

Before ageing, all wood surfaces were sanded with sandpaper of grit size 150. From the di fferent wood species similar samples were prepared for di fferent measurements, including the specimens for contact angle and moisture content examinations from di fferent boards with dimensions

50 × 100 × 20 mm<sup>3</sup> and specimens for total phenol and total carbohydrate content measurements with dimensions 20 × 20 × 20 mm3.

#### *2.2. Technological Background of Artificial Ageing*

The artificial ageing of different wood species was performed with Original Hanau Suntest (HERAEUS, Hanau, Germany) equipped with xenon bulb and built-in UV mirror, started within 1 h after machining. The different measurements were performed in the following ageing periods: 0 h (control), 1 h, 3 h, 5 h, 8 h, 10 h, 15 h, 20 h, 30 h, 60 h, 96 h, 132 h, 174 h, and 240 h, respectively. During artificial ageing, the temperature of wood surfaces was monitored by using Maxwell MX 25-903 type (GLOBIZ, Ashford, Kent, UK) digital thermometer. Temperature monitoring was responsible for the chamber temperature do not exceed the 55 ± 5 ◦C value which was defined as maximum reachable ageing temperature. Based on practical experience of artificial ageing the temperature over 60 ◦C can induce major surface chemical changes.

For moisture content measurements HM8-WS5 Merlin Moisture Meter (MERLIN Technology GmbH, Tumeltsham, Austria) was used, with ~5 mm scanning depth, and ~20 cm<sup>2</sup> maximum measuring area to perform 5 moisture content examinations on each wood surface after ending different ageing periods.

#### *2.3. Contact Angle Measurements of Artificially Aged Wood Surfaces*

Dynamic contact angle measurements were performed on each wood surface by using PG-X goniometer (FIBRO SYSTEMS AG, Hagersten, Sweden): 20 by using distilled water and 20 by using diiodomethane (SIGMA-ALDRICH, St. Louis, MO, USA). Measurements were performed in case of all ageing periods, 1 sec after the drop release and formation of the liquid droplet on the surface, as previously agreed [26].

#### *2.4. Alcoholic Extraction and Determination of Total Phenolic and Total Soluble Carbohydrate Contents*

To measure the total phenolic and total soluble carbohydrate contents, a ~75-μm thick layer was collected by alcohol sterilized steel blade from sample surfaces, following previously agreed ageing periods.

For alcoholic extraction methanol: water (volume ratio 4:1) (with methanol from REANAL, Budapest, Hungary) mixture was used and BRANSON 3510 ultrasonic bath (EMERSON, St. Louis, MO, USA) was applied for 20 min, after 5 mL extracting agen<sup>t</sup> was mixed with the previously collected wood material (~0.05 g). The extract produced in this manner was centrifuged with a MiniSpin spinner (EPPENDORF, Hamburg, Germany) for 10 min (13,400 rpm).

The total phenolic content (TPC) was calculated based on the Folin–Ciocalteu method [53]. First, Folin–Ciocalteu reagen<sup>t</sup> (VWR International, Debrecen, Hungary) (2.5 mL, tenfold dilution) was mixed with the extract (0.5 mL), and after 1 min application time, 2 mL (with concentration 0.7 M) Na2CO3 (VWR International, Debrecen, Hungary) solution was blended with the mixture. The reaction mixture was warmed in Memmert WNB 200 water bath (MEMMERT GmbH, Buechenbach, Germany) at 50 ◦C for 5 min. After warming in water bath, the solutions were cooled in cold water bath, until the temperature of solutions reached ~25 ◦C. For determination of total phenol content Metertech SP 8001 spectrophotometer (METERTECH Inc., Taipei, China) was used at 760 nm wavelength and as standard, quercetin (SIGMA-ALDRICH, St. Louis, MO, USA) was chosen.

Total soluble carbohydrate content (TSCC) was calculated based on method of Dubois et al. [54]. Firstly, phenol solution (REANAL, Budapest, Hungary) (0.5 mL; dilution: 5%) was mixed to the extract (0.5 mL). After mixing to the solution 2.5 mL concentrated sulfuric acid (REANAL, Budapest, Hungary), sealed test tubes were hold for 10 min at room temperature, followed by a second cooling for 20 min, in a 25 ◦C temperature water bath. Total soluble carbohydrate content was measured by using Metertech SP 8001 spectrophotometer (METERTECH Inc., Taipei, China) at 490 nm. During the total soluble

carbohydrate content measurements, glucose was used as standard (SIGMA-ALDRICH, St. Louis, MO, USA). For the determination of TPC and TSCC, 3-3 replicates were analyzed.
