## 3 Results
### Examination of HIPS and ABS Contact Angles by the Sitting Drop Method  
The averaged test results with the standard deviations are given in Table 9. The difference in the contact angles for HIPS and ABS using only tap water is 0.70\({}^{\circ}\). The largest differences in the contact angles occur at concentrations of 0.10 and 0.20 g/L SLES, being 10.76\({}^{\circ}\) and 10.10\({}^{\circ}\), respectively (Figure 14). The standard deviations for the contact angles range from 2.90 to 5.48. The results for the solution including only tap water (0.00 g/L SLES) confirm the information reported in the literature about the similar degrees of wettability of HIPS and ABS [15; 16; 17; 18; 19]. In the paper [15], the difference between the contact angles of HIPS and ABS using only distilled or deionized water was 3.10\({}^{\circ}\); in [16], 0.15\({}^{\circ}\); in [17], 1.16\({}^{\circ}\); and in [18],  
Figure 14: Plot of contact angle’s dependence on SLES concentration. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SLES** & **HIPS** & **Standard** & **ABS** & **Standard** \\
**Concentration** & **Contact Angle** & **Deviation** & **Contact Angle** & **Deviation** \\ \hline
0.00 & 72.30 & 4.36 & 73.00 & 4.36 \\
0.10 & 61.02 & 5.16 & 50.26 & 3.93 \\
0.15 & 54.71 & 2.90 & 61.57 & 4.52 \\
0.20 & 66.80 & 5.23 & 56.70 & 4.90 \\
0.25 & 52.64 & 3.38 & 55.67 & 3.50 \\
0.30 & 49.01 & 5.48 & 56.48 & 3.45 \\
0.40 & 53.38 & 4.42 & 53.16 & 4.16 \\
0.50 & 41.46 & 4.00 & 37.57 & 2.70 \\
0.60 & 41.45 & 3.64 & 38.32 & 3.11 \\
0.70 & 38.33 & 4.21 & 37.35 & 4.81 \\
0.80 & 52.37 & 3.19 & 56.88 & 5.58 \\
0.90 & 49.10 & 3.02 & 51.39 & 3.93 \\
1.00 & 57.42 & 3.33 & 54.07 & 4.23 \\
1.10 & 44.74 & 4.09 & 47.59 & 3.90 \\
1.20 & 36.20 & 4.31 & 39.19 & 3.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: The averaged test results of contact angles. 0.41\({}^{\circ}\), and that presented in the master's thesis [19] was 0.86\({}^{\circ}\). Comparing the results of the difference in the contact angles after the change in the nature of the HIPS and ABS surfaces presented in the papers [15; 16; 17; 18; 19], the result obtained in this paper is the smallest--amounting to 10.76\({}^{\circ}\). This result is similar to that obtained in the paper [18] (10.89\({}^{\circ}\)). It should be noted that the presented studies were carried out at room temperature, and in [15; 16; 17; 18], an elevated temperature (from 60 \({}^{\circ}\)C to 70 \({}^{\circ}\)C) was used. This reduces energy consumption in the process. The other advantages distinguishing the results obtained in these studies are the low concentration of NaCl (0.10 or 0.20 g/L NaCl), and the easy availability of salt and its favorable price. According to Table 9, the wettability for the other concentrations did not bring the expected results (the difference in the contact angles was too small). The high standard deviation ranges are due to unevenness and impurities on the surface of recycled plastics, which was one of the biggest challenges of this study (Figures 6-9). Such a surface has an impact on the obtained results of contact angle tests because a rough surface will mean that the contact angle will never be constant [42; 43]. The described surface topography (Figures 12 and 13) has a significant impact on the values of the measured contact angles, especially on the significant values of the standard deviations of the measurement (Figure 14). When proposing a model of surface wettability, the Wenzel model should be chosen [44]. This indicates that the droplets penetrate the rough surfaces, wetting them completely. This can be explained by the roughness scale, which is dominated by micro sizes and the low surface tension of the solutions used, which additionally promotes wetting. Another challenge is the material composition of HIPS and ABS, which is not constant in terms of composition. The reason for this is the processing of refrigerators in the Polish Recycling Corporation from different manufacturers and from different years of production.