Electroless plating process for the manufacture of radiation protection suits for pregnant woman: Effect of bending on its electromagnetic shielding performance

Introduction

Scientific advances in various fields have increased the quality of human life. Besides making life easier and more prosperous, it exposes humans to various unexpected dangers.^1–8 The widespread popularity of mobile phones, computers, microwave ovens, and other household appliances has brought convenience to people, while the harm of electromagnetic radiation is also increasingly prominent, electromagnetic radiation protection for pregnant women has become a hot topic of public concern in recent years.^9–12

Silver-printed terylene is presented to fabricate anti-electromagnetic radiation Maternity Dress (AMD) with superior shielding effectiveness, preventing a high Electromagnetic interference (EMI) shielding effectiveness of 55–75 dB, which can meet the electromagnetic protection requirements of general equipment, such as home appliances, office automation and other equipment.^13,14 Chen et al. ¹⁵ reported silver-coated carbon fabric composites have been successfully fabricated by using electroless silver plating. Electroless plating (EP) is a free electrochemical deposition method. This technology is widely used in fabricating metal/polymer composite material due to its reliable capability for large-area deposition, less restriction in surface topography, and requirement for expensive facility.^16–20 Huang et al.^21,22 and Chang et al. ²³ found that the modified polyethylene terephthalate surface with activated groups (amino group (-NH ₂ ) and -SH) on its surface could adsorb silver (Ag) particles, acting as catalytic centers, so that the Cu coatings are deposited through oxidation–reduction reaction (REDOX) in the presence of catalytic centers. Generally, the catalyst particles, such as Au, Pd, Ag, and so on, adsorbed and then anchored on surface of modified substrate through the coordinate bond. Kwak et al. ¹⁹ designed and prepared a broadband microwave-absorbing honeycomb core structure using a EP-Nickel (Ni) on modified glass fabric, which can absorb the EM waves propagating along three orthogonal directions. In the case of the EP method, the metal coating on the fabric surface served to absorb EM wave. It is well known that an EMI shielding material with high shielding effectiveness (SE _t ) is a key indicator for AMD, it should also concurrently have the advantages of low density and exceptional flexibility as well.^6–9,19,23 The metal particles on the substrate surface without activated groups hold incompact structure and easy to break off in the plating solution.^23,24 It is noted that in most reports, the introduction of active group, the adsorption of catalytic centers, and electroless plating (three-step process) are the common technology.^5–10 In contrast, the cotton surface is a rich source of activated group, cotton has higher surface free energy because there are many -NH ₂ and hydroxyl groups in the cotton/polyester blend fiber surface. When cotton was sensitized by Sn ²⁺ , activated by Pd ²⁺ , and plated by Ni particle in sequence. The active group absorbed Sn ²⁺ and Pd ²⁺ , acting as catalytic centers, which could catalyze the reduction of Ni ²⁺ in the EP solution. As a result, the cotton could use as substrate to adsorb catalytic centers for electroless plating without modified part in order to simplify the conventional three-step process.

In this study, pure cotton with cotton/polyester blend fabric is used as substrate material. Ni-plated cotton was prepared by electroless plating process, and then a facile EP technology for fabricating radiation protection suits for pregnant woman was established. Flexibility of the Ni-plated cotton is important for AMD. The effects of bending times on its crystallinity, resistivity, and electromagnetic shielding performance were also studied. Ni-plated cotton was invented with a broad application prospect due to its advantageous properties of light weight, superior EMI shielding, and lower-cost performance.

Experimental details

Schematic illustration of the fabrication of Ni-plated cotton is displayed in Figure 1. At first, the cotton sheet was subjected to sensitized solution consisted of 8 g/L stannous chloride (SnCl ₂ ) (adjusted to pH 1 by hydrochloric acid (HCl)) at 60°C for 30 min (sensitized cotton), then immersed in an activated solution composed of palladium dichloride (PdCl ₂ ) 0.2 g/L and 20 g/L boric acid (adjusted to pH 2 by HCl) at 60°C for 30 min to achieve the absorption of Pd particle on fiber surface (activated cotton). After that, the sample was immersed into EP-Ni bath at 60°C for 15 min. The component of EP-Ni bath was reported in previous work. ²² After deposition, the sample was dried at 20°C for 120 min and then heated at 60°C for 60 min; Afterward, Ni-plated cotton was prepared. The EMI shielding effectiveness (EMI-SE _t ) of Ni-plated cotton was measured with a vector network analyzer (Agilent 8720ET) in a frequency range from 8.2 GHz to 12.4 GHz. The experiments' materials except cotton were purchased from Aladdin Chemical Reagent Company (China) and used without further purification. The cotton composed of 80% cotton and 20% polyester was bought from the Wanchang Weaving Co. Ltd (Jinzhou, China), the gram weight is 105 g/m².OPEN IN VIEWER

In order to evaluate the effect of bending times on its crystallinity, resistivity, and electromagnetic shielding performance, the Ni-plated cottons were bent 0 time, 300 times, 600 times, and 1000 times, respectively. The way to evaluate the bending test is depicted in Figure 5(a). R is sample volume resistivity before bending test, and R _B is its volume resistivity after bending test. The Ni-plated cotton bent to circular profile of radius (about 0.8 cm).

Surface morphology was observed by scanning electron microscopy (SEM, JEOL, JSM-5600LV). Chemical structure was measured by X-ray diffraction (XRD, Rigaku D/max-2550V) and X-ray photoelectron spectroscopy (XPS, Shimazu, AXIS ULTRADLD). XPS spectra were calibrated by using the C1s peak (284.5 eV).

Results

In order to investigate the surface characterization, we performed XPS analysis. As displayed in Figure 2(a), the O 1s peak at about 532.7 eV is assigned to -OH, the N 1s peak at 399.8 eV is assigned to -NH ₂ group in XPS spectrum of pristine cotton, which indicted active groups attached on cotton surface. As displayed in Figure 2(b), the characteristic peak of Sn 3d was detected (about 487.8 eV), which indicated that the pristine cotton chemisorp Sn particle by ion exchange and establish coordinate bond with surface active groups (Figure 2(e)).^20–25 Meanwhile, the Pd 3d signal was detected (about 338.4 eV), as shown in Figure 2(c). This result is because the Sn ²⁺ can reduce Pd ²⁺ (Figure 2(e), reaction equation is shown in equation (1)). Both of the two characteristic peaks disappear in XPS spectrum of Ni-plated cotton, and Ni characteristic peaks appeared simultaneously, as shown in Figure 2(d). In the electroless plating process, Pd particles acted as catalyst to promote the reduction of Ni ²⁺ (Figure 2(e)). It is indicated that nickel particle was deposited on the surface of cotton/polyester blend fabric.

P d 2 + + S n 2 + → S n 4 + + P d

(1)

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Figure 2.

XPS spectra of pristine (a), sensitized (b), activated (c) and plated (d) samples, structure diagrams of corresponding samples (e).

In order to study the crystal structure of Ni-plated cotton, we performed XRD analysis, as displayed in Figure 3 (XRD spectra). Compared with pristine cotton, Crystalline Bragg peaks are evident at ∼45^o, ∼52^o, and ∼76^o in XRD pattern of Ni-plated cotton. The three peaks are very close to the expected face-centered cubic (FCC) Ni (111), Ni (200), and Ni (220) diffraction peaks. It is indicated Ni particle deposited on the surface of cotton, which is consistent with the conclusion in Figure 2(d). Then, by observing the SEM images (Figure 3), it can be found that the pristine cotton has a smooth fiber structure (as displayed in Figures 2(e) and 3). After electroless plating, the surface structure becomes rough (as displayed in Figures 2(e) and 3). Reasonably, electroless plating could fabricate Ni coating on cotton surface in the form of “core-shell” structure (Ni-plated fiber), which has many advantages, such as light weight (0.021 g/cm²), electroconductivity (as displayed in Resistance test chart) and flexibility. It can attach to the clothe surface as electromagnetic shielding.OPEN IN VIEWER

The EMI shielding effectiveness (EMI-SE _t ) of the Ni-plated cotton is calculated using the following equation (2).

S E t ≈ S E r + S E a

(2)

where SE _r and SE _a are the reflection loss and absorption loss, respectively. The values of Ni-plated cotton are 1.12 dB and 28.39 dB, as displayed in Figure 4(a), which can meet the protection requirements of general equipment, such as home appliances, office automation, and other equipment. In order to intuitively explain the electromagnetic shielding mechanism of Ni-plated cotton, as shown in Figure 4(b), the reasons are analyzed as follows:OPEN IN VIEWER

On one hand, scattering point at the interface of Ni-plated fiber can prevent the electromagnetic wave from entering Ni-plated cotton by reflection. On the other hand, entered electromagnetic wave propagates along the conductive network which leads to energy degradation by means of ohmic loss. Lastly, influenced by the “core-shell” structure (Ni-plated fiber), the electromagnetic field would induce electrical currents in the “shell” surface due to interface polarization. Proper interface polarization of the material will enlarge the wave absorption frequency and improve the wave absorption performance. It is also indicated electroless plating process can be used to prepare the fabric with electromagnetic shielding properties.^1–3,5–8

In order to analyze the effect of bending on the electromagnetic shielding performance of cotton. The test diagram is shown in Figure 5(a).^21,22 Bending times of Ni-plated cotton were 0 times, 300 times, 600 times, and 1000 times, respectively. TG test was employed to evaluate Ni-deposited quantity on the plated cotton (M _{platedparticle} /M _{(PA12+platedparticle)} ), which are about 19%. TG curves showed there is no quality change of Ni particle after bending test, indicating bending test did not cause surface particles to fall off, as shown in Figure 5(b). We performed XRD spectrum of Ni-plated cotton after bending test, as shown in Figure 5(c). It is conducted that bending test did not affect crystalline form of Ni particle.OPEN IN VIEWER

In addition, as displayed in SEM image, the Ni-plated fiber arrays have two forms, one of which is parallel alignment, the other of which is bifurcation. More Ni-plated fibers are connected to the same pair of nodes, to be precise, the parallel connection model can be used in service analysis electrical resistivity. The diagram of effect of bending time on R _B /R is displayed in inset in Figure 5(d), where R and R _B represent the resistance before and after bending, n and m are the number of Ni-plated fiber and deteriorated Ni-plated fiber, respectively. But m much lower than n, as a result, we found that increasing bending time to 1000 would have no significant effect on R _B /R of sample. The SE _t of 0, 600, and 1000 times Ni-plated cotton are 29.5 dB, 30.4 dB, and 23.2 dB (Figure 5(e)), respectively. It is found that the shielding effectiveness does not change much when the bending times increase to 600 times, but decreases obviously when the bending times increase to 1000 times. Therefore, it can be concluded that bending operation has little effect on cotton surface crystallinity and resistivity, but it will decrease the electromagnetic shielding performance after 1000 times bending treatment.

In order to further analyze the reasons for the reduction of the electromagnetic shielding performance of cotton. We performed SEM images of Ni-plated cotton after bending 0 time, 300 times, 600 times, and 1000 times, as shown in Figure 6(a)–(d). As displayed in SEM images, the surface structure of Ni-plated coating also remains intact after 0, 300, and 600 times bending. When bending times increased to 1000 times, the surface of the Ni-plated cotton gradually began to crack after repeated buckling and destroy the core-shell (Ni-plated) structure, as shown in Figure 6(e). Fatigue cracking is one of main destruction forms of Ni-plated coating on fiber surface. Nevertheless, the crack has no effect on the crystallinity and electrical properties of Ni-plated cotton.OPEN IN VIEWER

The decline of the EMI-SE value could be attributed to three factors, as displayed in Figure 4(b). First, scattering point at the interface of Ni-plated fiber can prevent the electromagnetic wave from entering Ni-plated cotton by reflection. The presence of cracks in the Ni-plated cotton will partially weaken the surface reflection loss of the shielded material. Second, entered electromagnetic wave propagates along the conductive network, the presence of cracks in the Ni-plated cotton will reduce the relative conductivity of the plated fiber, which leads to less energy degradation by means of ohmic loss. Finally, the electromagnetic field will weaken interface polarization and reduce the adsorbing performance of the material, due to the formation of cracks in the core-shell structure (Ni-plated fiber), and then the transmission of electromagnetic wave through the Ni-plated cotton.^14,17,19,26

Conclusion

In this work, a facile EP-Ni process to fabricate radiation protection cotton was established. Sn ²⁺ was absorbed on surface of cotton/polyester blend fabric by active groups (with -NH ₂ and -OH). Pd ²⁺ was reduced to Pd particle with Sn ²⁺ desorption, the absored Pd particle acted as catalytic center for EP-Ni. Ni particle with (111) crystal plane preferential oriented crystal structure deposited on its surface with a coarse microstructure, the M _{plated particle} /M _{(PA12+platedparticle)} is about 19%. Bending operation has little effect on cotton crystallinity and resistivity, but it will decrease the electromagnetic shielding performance after 1000 times bending treatment with the reduction of the EMI-SE values by 24% due to the formation of crack in the EP-Ni coating.