The effect of high-current pulsed electron beam modification on the surface wetting property of polyamide 6

3.2.1 Influence of surface elemental composition changes on surface wettability after HCPEB bombardment

Increases in non-polar functional groups like C-C and C-H can cause an increase in surface hydrophobicity; in contrast, increases in polar oxygen functional groups like C-O and C=O can cause an increase in surface hydrophilicity (12). This study applied XPS analysis to analyze the changes in surface elemental composition in PA6 before and after HCPEB modification (pulsed voltage, 34 kV). Figure 3 shows the results.

Figure 3:

PA6 surface XPS spectrogram before and after HCPEB modification [(A) Untreated, (B) C1s Spectra: 34 kV, (C) Untreated, and (D) O1s Spectra: 34 kV].

Figure 3 indicates that the C1s peak on the PA6 surface is mainly composed of three sub-peaks, namely C1 (284.6 eV), C2 (285.2 eV), and C3 (287.7 eV), which correspond to C-H and C-C, C-O, and C=O, respectively. On the other hand, the O1s peak is mainly composed of two sub-peaks, namely O1 (531. 2 eV) and O2 (532.6 eV), which correspond to C=O and C-O, respectively. The results demonstrate that the proportion of the three sub-peaks in the C1s peak before modification was 47.57% C1, 43.24% C2, and 9.18% C3. The proportion of the two sub-peaks in the O1s peak was 50.84% O1 and 49.16% O2. After modification, the proportion of the three sub-peaks in the C1s peak was 45.94% C1, 43.98% C2, and 10.08% C3, while the proportion of the two sub-peaks in the O1s peak was 52.26% O1 and 47.74% O2. After HCPEB bombardment, the relative amount of carbon on the PA6 surface decreased slightly, but the relative amount of oxygen increased; the O/C ratio increased from 25.1% before modification to 42.8% after modification, indicating that HCPEB modification can increase the content of oxygen-containing polar groups on the PA6 surface. Therefore, we can chart the proportion of carbon and oxygen elements with different valence states on the PA6 surface, as shown in Table 1.

Table 1 shows that the carbon element on the PA6 surface decreases slightly after HCPEB modification, because of the decrease in carbon in C-C and C-H valence bonds from the original 38.05%–32.15%. The corresponding increase in oxygen on the surface is caused by increased oxygen in O=C and O-C valence bonds. However, Table 1 also shows that the elements’ percentage changes on the surface are relatively small before and after HCPEB modification. This suggests that although using HCPEB to bombard PA6 can increase hydrophilicity, the change in PA6’s wetting property is not caused primarily by the changes in elemental composition on the surface.

3.2.2 The influence of microstructure changes on wettability after HCPEB modification

In addition to its association with the surface’s elemental composition, the material surface’s wettability is also closely tied to the surface microstructure and roughness, ambient temperature, and the properties of the liquid. This study maintained the ambient temperature at a constant level, and the liquid used was consistently water; therefore, it was possible to isolate the influence of the surface’s microstructure and roughness on PA6’s wettability. When a solid surface is hydrophobic, high roughness will make it more hydrophobic; similarly, when a solid surface is hydrophilic, high roughness will make it more hydrophilic (13).

Figure 4 shows the roughness change curve after etching modification with different pulsed voltages on the surface; as the graph indicates, the roughness increases with increasing pulse voltage before stabilizing in a certain range. A comparison with Figure 1 reveals that the increased roughness increases the hydrophilicity of the PA6’s surface. When the pulse voltage exceeds 28 kV, the roughness tends to remain at a constant value, while the water contact angle is further reduced.

Figure 4:

Roughness changes of PA6 surface.

Figure 5 displays the SEM photographs of the PA6 surface after etching modification with different pulsed voltages. When the pulsed voltage measures 26 kV, a small quantity of dimple-like structures appear on the surface, the glass fiber is interrupted, the roughness is enhanced, and the surface’s hydrophilic property improves. When the pulsed voltage exceeds 28 kV, the roughness on the PA6’s surface changes only slightly, but the hydrophilic property improves because of the increased amount of dimple-like structures. This result can be explained by the Wenzel theory.

Figure 5:

SEM photographs of PA6 surface after modification with different pulsed voltages.

(A) Untreated, (B) 26 kV, (C) 28 kV, (D) 30 kV, (E) 32 kV, and (F) 34 kV.

Wenzel (14) proposed that the presence of a rough surface makes the actual solid-liquid contact area greater than the apparent contact area, geometrically enhancing the hydrophobic or hydrophilic property of the surface. Assuming that liquid always fills groove structures on a surface, the apparent contact angle θw on a rough surface and the eigen contact angle θ on a smooth, flat surface have the following relationship:

where r is the ratio between the actual solid-liquid contact area S_a and the apparent contact area S_b, also known as roughness factor. This equation is called the Wenzel equation. As r is always >1, values of θ>90° indicate a hydrophobic surface; as θw increases along with increasing values of r, the surface becomes more hydrophobic. When θ<90, this indicates a hydrophilic surface; θw decreases with increasing values of r, and the surface becomes more hydrophilic.

Based on the dimple-like structures produced on PA6’s surface after etching modification with different pulsed voltages, as shown in Figure 5, Wenzel’s model was applied to this test in Figure 6. Figure 6 represents the rough microstructure surface of the cone, where the diameter of the retuse cone is indicated as R, the depth is h, and the retuse bodies pacing is a and b. These variables introduce the following surface characteristic values: α=a/R (transverse diameter ratio), β=b/R (longitudinal diameter ratio), γ=h/R (depth to diameter ratio), such that r can be expressed as follows:

Figure 6:

Wenzel model sketch with changes in pulsed voltage.

Substituting r into equation [1] Wenzel model yields the following:

When α and β are constant in equation [3], increasing values of γ make a hydrophobic material more hydrophobic, and a hydrophilic material more hydrophilic. When γ remains the same, α and β are reduced; in this situation too, a hydrophobic material becomes more hydrophobic, and a hydrophilic material becomes more hydrophilic.

The SEM photographs in Figure 5 show the number of dimple-like structures gradually increasing along with the increase in pulsed voltage; in other words, α and β in equation [3] are reduced, resulting in reductions in the water contact angle. Therefore, under certain conditions of pulsed voltage range modification, it is feasible to apply Wenzel’s theory to explain the water contact angle and wettability of PA6’s surface.