**3. Results**

#### *3.1. Contact Angle Changes due to Artificial Ageing*

The changes occurring in the development of the contact angles measured with different test liquids are different (bi-distilled water and diiodomethane) during ageing. The character of the development of contact angles is similar on the different wood species (Figure 2).

**Figure 2. Figure 2**. Contact angle values of distilled water and diiodomethane during artificial ageing, L—left y-axis, R—right y-axis.

There is some difference within the control values (0h) of distilled water contact angles measured on different wood species (Figure 2 and Table 2). Higher contact angle values were detected on sessile oak material, associated with higher extractive content, according to previous studies [15–17]. Since it has significant influence on liquid contact angle results, moisture content determinations were performed in order to monitor wood surface layer MC changes due to artificial ageing. MC results clearly show that, after the first three hours of artificial radiation, there was no difference between two consecutive measurements, in the case of all wood species. The results clearly show the process of changes during the total duration of artificial ageing, which takes place similarly in the case of all

wood species: contact angle of distilled water increases in the first 8 h. Consequently, until the end of ageing (240 h), it has a slightly decreasing tendency.


**Table 2.** The average values of contact angle and its spread.

According to control values (0 h) and finally measured (after 240 h artificial radiation) distilled water contact angle values of di fferent wood species can be concluded that the control values (0 h) are higher in case of all four wood species. Those results lead to the conclusion that higher wettability is characteristic to the wood surfaces at the end of artificial ageing (Figure 2). In addition, evaluating the total duration of artificial ageing, contact angle of both polar and disperse distilled water changes to a greater extent than the contact angle of the solely disperse diiodomethane.

 1.72

 1.54  11.33

> 9.04

 1.86

 1.58  11.63

 11.22  0.97

 1.62

#### *3.2. Total Phenolic and Total Soluble Carbohydrate Content of Artificially Aged Wood Surfaces*

 11.51

 10.33

 10.52

 12.60

174

240

 1.99

 1.54

Total phenolic content examinations were performed to detect quantity changes of phenolic extractive compounds of di fferent wood materials (Figure 3). Phenolic compounds significantly influence the wetting of polar and disperse liquid drops, together with the wettability of wood surfaces.

**Figure 3.** Total phenolic and total soluble carbohydrate contents as function of artificial ageing.

The results of total phenolic and total soluble carbohydrate content examinations show major difference in their measure. Total phenolic content of different wood species, except sessile oak, increases from a value about 1 mg/g to about 15 mg/g, regarding the total 240 h of artificial radiation. Parallel with the changes of total phenolic content, the total soluble carbohydrate content (for all four wood species) increases from a value of 7 mg/g to about 45–60 mg/g. The results clearly show that the carbohydrate content changes in 3–4 times higher measure than the phenolic compounds (Table 3).


**Table 3.** The average values of TPC and TSCC and its spread.
