3.1. Morphology and Structural Analysis
The surface of Milife fabric with and without Cu/Ni-coating was observed from SEM images (
Figure 4 and
Figure 5). Control samples have plain surface without any metal addition, whereas the samples coated with Ni showed the deposition of metal particles on the surface. It was interesting to observe that the deposited Cu/Ni were on the surface of the fibers and did not fill in the interspace between the fibers significantly.
The structural change between the Milife fabric with and without Cu/Ni-coating is shown in
Figure 6. It was found that Cu/Ni coating layer was very thin. It was also found that the Cu/Ni coating amount could be assumed as almost same (
Table 1) and ranges from 0.47 to 0.60 g/m
2. Therefore, the Cu/Ni-coated Milife fabric had the GSM ranging from 10.47 to 10.60 g/m
2, which supported that the prepared samples were light.
The air permeability of the Milife fabrics with Cu/Ni deposition (N1–N8) was slightly smaller than the Milife fabric (N0). There was no obvious difference in the air permeability between N1, N2, N3, N4, N5, N6, and N7 by observing the value ranging from 700 to 1000 mm/s, while the air permeability of the sample (N8) was significantly decreased to 600 mm/s (
Figure 7). The difference may be caused by different Cu/Ni crystals on the surface of the fiber. Overall, the high air permeability (>600 mm/s) supported the breathability of the Cu/Ni-coated Milife fabric by comparing with other work [
24,
25].
3.2. Effect of Ni Content in the Cu/Ni-Coating Milife Fabric on EMI
EMI results including EM
SE and
SER with frequency ranging from 30 M–1.5 G are shown in
Figure 8, and the evaluation of the EMI is calculated in the
Table 2.
From the
Figure 8A, it was found that the EM
SE was obtained when there was Cu/Ni deposition on the Milife fabric since only the N0 (without Cu/Ni deposition) had the EM
SE around 0 dB. The sample N1 (only with Cu deposition) was assumed the highest EM
SE by observing the highest EM
SE curve and the other samples N2–N8 (Cu/Ni-coated Milife fabrics) had the lower EM
SE than the sample N1, which meant that the addition of Ni in the Cu coating on Milife fabric reduced the EM
SE. The EM
SE of the Cu/Ni-coated Milife fabric at 1.5 G decreased from 26.87 to 19.77 dB with the increasing w
Ni (
Table 2). The linear relationship between the
wNi and the EM
SE was modeled in the
Figure 8B, and
R2 = 0.97. In addition, it was found that the effect of
wNi on the
SER was not as significant as on the EM
SE, which was proved by the close
SER curves shown in
Figure 8C. The
SER at 1.5 G of the Cu/Ni-coated Milife fabric increased from 14.67 to 15.22 dB with the increasing
wNi (
Table 2).
The trend of the EM
SE and
SER of the Cu/Ni-coated Milife fabric over the
wNi was different, which suggested the EMI mechanism was affected by the
wNi. It was well-known that the EM wave passing through the Cu/Ni-coated Milife fabric (
Figure 8D) included the reflection
SER, adsorption
SEA, and transmittance EM
SE. To confirm the exact EMI mechanism in the Cu/Ni-coated Milife fabric, three parts including
SER,
SEA, and EM
SE at 1.5G were evaluated according to the Equation (2)–(4) and shown in
Table 2. It was found that
REM and
TEM increased while the
Aeff,EM and
Em,EM decreased. The linear relationship between
REM,
TEM,
Aeff,EM,
Em,EM, and
wNi was modeled separately in
Figure 8E,F. The details of the relationship are shown in
Table 3. The slope of the
Aeff,EM was much higher than
REM (0.0151 > 0.0002), which suggested that the
A eff,EM loss in the Cu/Ni-coated Milife fabric mainly accounted for the decrease of the EM
SE. Namely, addition of Ni in the Cu coating on the Milife fabric increased the EMI reflection while seriously reduced the EMI adsorption. Besides,
Em,EM of the Cu/Ni-coated Milife fabric linearly decreased from 0.034 to 0.030 with the increased
wNi.
Although the Ni had a negative effect on the EM
SE of the Cu/Ni-coated Milife fabrics, the EM
SE of the samples were above 20 dB except for the sample N8 whose EM
SE was around 19 dB. According to the classification, the prepared Cu/Ni-coated Milife fabrics were evaluated in the “very good” category for general use (
Table 4).
Furthermore, the EM
SE was in relation to the surface resistivity (
). As shown in
Table 5, the
of all the Cu/Ni-coated Milife fabrics (N1–N8) were smaller than 60
while the
of the pure Milife fabric was as large as 13.45 M
. The Cu/Ni deposition gave the Milife fabric the good conductivity. Besides, no linear relationship between
and
wNi was found. The
wNi did not change the surface electrical conductivity of the Cu/Ni coating on the Milife fabric.
3.3. UV Properties of Cu/Ni-Coated Fabric
The UV measurement of Cu/Ni-coated Milife fabric including both of ultraviolet radiation A (UVA) and ultraviolet radiation B (UVB) was shown in
Figure 9. It was obvious that UV transmittance percentage of Cu/Ni-coated Milife fabric decreased with higher
wNi over the UV electromagnetic spectrum. The UV transmittance percentage increased significantly in the fabrics N1, N2, N3, N4, and N5, and fabrics N6, N7, and N8 had less than 5% of transmittance all over the UV region (290–400 nm). The stable and good UV protective property was obtained when
wNi was higher than 15.2 wt %.
Figure 10 showed the UV protection capability of Cu/Ni-coated Milife fabric in terms of UPF values. As in sun protection factor (SPF) rating system used in case of sunscreens fabric, UPF rating is used to measure the UV protection [
27]. Usually if the UPF value of fabric has equal or more than 50, it can provide the better protection by blocking the 98% of UV radiations. The Cu-coated Milife fabric (N1) had the lowest UPF value around 9, which was assumed to have little UV protective ability as their UPF values are about 8–10 (<15). When the coated fabric contained 2 wt %
wNi (N2), the UPF value was 20.2. Similarly, the UPF was increased with the increased
wNi. On other hand, the fabric with higher
wNi (>7.1 wt %) showed 40+ UPF ratings. The UPF values increased till
wNi reached 15.2 wt % and thereafter it was saturated with no significant difference in the UPF values. It was found that the resistance against UV for the Cu/Ni-coated Milife fabric could be modified with increased
wNi while the resistance against EMI was reduced with more
wNi. The reason may be that the Ni was able to dissipate the electromagnetic energy ranging from 290–400 nm. With respect to the
wNi in the coated fabric, 16.6 wt % of Ni in the sample contributed to 46 UPF value and 19.2 wt % of Ni in the sample contributed to 48 UPF value. For statistical approach, we fitted the data between UPF value and
wNi by using exponential function (Equation (9)), where
Uf was set as 50 according to standard stating that excellent protection from UV when UPF was ≥50.
Ui was the initial UPF of the Cu-coated Milife fabric (N1).
kU was the estimated value giving the increasing rate of UPF with Ni deposition.
kU was estimated as 0.115 and the final fitting model had
R2 = 0.91, which confidently proved that there was an exponential relationship between
wNi and UPF value.
3.4. Infrared Analysis of Cu/Ni-Coated Milife Fabric
The infrared resistance evaluation on Cu/Ni-coated Milife fabric including reflectance (
Rinfrared), transmittance (
Tinfared) and adsorption (
Ainfrared) was schemed in
Figure 11 and the values at 1000 cm
−1 are shown in
Table 6.
Ainfrared and
Tinfared curves were used to characterize the components of the Cu/Ni-coated Milife fabrics. It was found that all the samples had the same peaks in the
Ainfrared except for the peak ranging from 2360 to 2340 cm
−1 (
Figure 11A). The ester groups of all the samples were proved by observing the strong peaks at 1712, 1090, and 1244 cm
−1 separately. The peaks in the range between 1712 to 1627 cm
−1 and 1555 to 1425 cm
−1 confirmed that there was a deposition of Cu and Ni particles on the surface of Milife fabric [
20]. The peak at 3430 cm
−1 represented the –OH group, which suggested that the hydrolysis and aminolysis happened during the electroless plating process and the strong interaction between metal particles and polyester was developed. However, the peaks of the samples ranging from 2360 to 2340 cm
−1 were different, which represented the stretching and vibration of –C=N=O– or –N=C=O–. It was found that the peaks from 2360 to 2340 cm
−1 became stronger with higher
wNi (>4.8 wt %). It may be caused by the unstable coating process when the higher Ni molar was introduced in the electroless plating process, and the balance between the chemical reaction was affected. For the
Tinfared curves (
Figure 11B), only the peaks ranging from 2360 to 2340 cm
−1 were changed until the
wNi was higher than 8.8 wt %.
Rinfrared values were used to evaluate the resistance of the Cu/Ni-coated Milife fabric against penetration of IR. As seen in
Figure 11C, all the samples had a similar
Rinfrared curve over the wave number from 516 to 5000 cm
−1 by observing the same peak position and the similar increasing trend that
Rinfrared values increased with decreasing wave number and tended to be stable after the 1000 cm
−1. The
Rinfrared at 1000 cm
−1 of different samples were different which proved that the
wNi affected the
Rinfrared. By observing the
Rinfrared at 1000 cm
−1 over the
wNi (
Figure 11C and
Table 6), it was found that no obvious decrease of
Rinfrared happened until the
wNi reached 5.5 wt %, and the
Rinfrared tended to be saturated when the
wNi reached 16.6 wt %. Therefore, the influence of the
wNi on the
Rinfrared was considered to be classified into two stages: the stable stage (I:
wNi < 5.5 wt % and II:
wNi > 16.6 wt %) and the negative stage (5.5 wt % <
wNi < 16.6 wt %) (
Figure 11C). On the one hand, the initial
Rinfrared was measured around 0.57 in the stable stage I and the lowest
Rinfrared was measured around 0.473 in the stable stage II. On the other hand, the negative linear relationship (
R2 = 0.96) between the
Rinfrared and
wNi was found in the negative stage. Similarly, the
Em,infrared values at 1000 cm
−1 was calculated according to the Equation (7) and schemed in
Figure 11D. The
Em,infrared values at 1000 cm
−1 tended to be saturated to be around 0.527 when
wNi reached 16.6 wt %. Although the addition of Ni had a negative effect on the resistance of the Cu/Ni-coated Milife fabric against penetration of IR, the highest
Em,infrared was around 0.527, which was still much smaller than the
Em,infrared of the normal fabric (
Em,infrared = 0.95 to 0.98). Therefore, the Cu/Ni-coated Milife fabrics had good resistance against penetration of IR.
Furthermore, the practical testing of the Cu/Ni-coated Milife fabrics for the thermal radiation resistance was carried out according to
Section 2.3.4. The results are shown in
Table 7. All the samples had the much lower
Ts (302–304 K) than the
Th (313.15 K). In addition, a small decrease was observed by comparing the samples with Ni (N2–N8) with Cu-coated Milife fabric (N1), while there was no similar linear relationship between the
Ts and
wNi, which was different from the aforementioned standard IR resistance analysis. The phenomena may be caused by the higher porosity of the samples, where the infrared transmittance and the unexpectable heat convection existed during the measurement. The change of the air components in the measurement also accounted for the difference as well.
Furthermore, by combining the results of the EMI and UV with the IR analysis, it was found that the addition of Ni only had a positive influence in the resistance against the penetration of electromagnetic waves when the wavelength only ranged from 290 to 400 nm in this case. Namely, the Ni had the selectivity on the resistance against the penetration of electromagnetic waves.
3.5. WCA on Ni-Coated Fabric
From the previous work, the WCA of the control sample (N0) was around 0°, while the Cu/Ni coating supported the WCA [
28].
Figure 12 described the outcomes of the
wNi on the wettability of the Cu/Ni-coated Milife fabrics. The Cu-coated Milife fabric (N2) had the WCA only of around 60.1 ± 1.0°. Meanwhile, the WCA of the Cu/Ni-coated Milife fabric increased from 77.7° to 114° with increasing
wNi. N7 and N8 had the WCA higher than 110° which confirmed the hydrophobicity of the surface. So, the erosion of the sample (N7 and N8) by the water could be relieved in reality [
29].
In addition, the relationship between the WCA and
wNi was roughly estimated by using exponential function (Equation (10)),
where
Cf was set roughly as 140° which was the highest value of the pure Ni surface coating among the various studies [
30],
Ci was the initial WCA of the Cu-coated Milife fabric without any Ni deposition (N1), and
kc [°/wt %] was the estimated increasing rate of WCA with Ni deposition. The
R2 was 0.80 and
kc was estimated as 0.032.