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Abstract

In order to develop the functional fabric endowed with electromagnetic shielding and thermal protection ability, hollow glass microspheres were first deposited with silver by electroless plating using silver nitrate 0.03 mol/L, ammonium hydroxide 0.008 mol/L, sodium hydroxide 0.1 mol/L, glucose 0.005 mol/L, tartaric acid 0.0006 mol/L, and anhydrous alcohol 0.18 mol/L. The Ag-deposited hollow glass microspheres were subsequently coated on the surface of PET/cotton fabric by adhesive printing process. The particle integrity, morphology, composition, and crystal texture of Ag-deposited microspheres were investigated by means of size distribution, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The diffuse reflectance spectrum, electrical conduction, shielding effectiveness, and thermal radiant protection performances were also measured. The results showed that a specific performance of PET/cotton fabric coated with Ag-deposited glass microspheres was obtained.

Keywords

hollow glass microsphere electroless silver plating coating PET/cotton fabric

Introduction

The hollow glass microspheres are hollow, thin-walled microscopic spheres of glass having a dominant grain size of 1–1000 µm in diameter. Due to the unique characteristics of lightweight, high strength, low thermal conductivity, and inertness, hollow glass microspheres can not only be exploited as a lightweight filler in composite materials, but also be used in composites to fill polymer resins, such as syntactic foam, lightweight concrete, sealing surface, etc. [1,2]. Hollow glass microspheres with different particle sizes were incorporated with binder to develop the reflective and heat insulation coating, which could be used for roof and external walls [3]. A variety of metals including nickel, copper, silver, gold, palladium, and so on could be deposited on the surfaces of microspheres using different coating methods, which could be easily customized to the desirable properties [4 –8]. The resultant microspheres with a larger surface area are endowed with the expected properties of solid metals, but are lightweight, highly reflective, and inexpensive. After being added to paints, adhesives, plastic and resin materials at appropriate ratios, the coated microspheres could provide electrical conductivity and shielding of electronic devices against electromagnetic interference (EMI) [9,10]. The low particle density and large surface area facilitate slow phase separation in paints and adhesives compared with heavy metallic and inorganic fillers. Recently, many researchers mainly focused on the optimizing treatment conditions and mechanism studies of electroless plating on microspheres [11,12]. It was found that the pretreatment process, Ni film thickness, and postannealing had an effect on the microwave properties of Ni-plated microsphere [13,14]. The microwave absorption performances of a double-layer absorber made of NiCoZn ferrites and hollow glass microsphere electroless plated with FeCoNiB could be enhanced [15]. The hollow glass microsphere was first treated with the coupling agent (3-aminopropyltriethoxy silane), and then deposited with Ni-Fe-P by a modified electroless plating process [16]. However, the application of microsphere on textile industry is seldom taken into account.

In the present work, electroless plating of silver on hollow glass microsphere was first carried out. The Ag-deposited glass microspheres were then mixed with the printing paste, and subsequent coated on the surface of commercial PET/cotton fabric. It is expected that the Ag-deposited glass microspheres can reduce the weight of coated PET/cotton fabric. The particle integrity, morphology, composition, and structure were verified by size distribution, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analyses. The properties of diffuse reflectance, electrical conduction, electromagnetic interference shielding, and thermal protective performances were also investigated. The Ag-deposited glass microspheres may be utilized for producing conducting fabrics for EMI shielding and thermal radiant protection applications.

Experimental

Materials

The desized grey woven PET/cotton (65/35 w/w) fabric with a 3/1 weave construction was used as the substrate for coating, which was obtained from Shaanxi Tanghua No.4 Textile Co. Ltd. The linear densities of ends and picks are 24 tex and 28 tex, and the numbers of ends and picks are 532 and 280 per 10 cm, respectively. The fabric weight per unit area is 233.0 g/m² and the thickness is 0.53 mm. The chemicals used in this study are in analytical reagent grade, and include silver nitrate (AgNO₃), ammonium hydroxide (NH₃·H₂O), glucose (C₆H₁₂O₆), tartaric acid (H₂C₄H₄O₆), anhydrous alcohol (C₂H₅OH), hydrofluoric acid (HF), sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), sodium silicate (Na₂SiO₃), and distilled water. The printing paste UN-404 provided by Union Carbide Corporation was used for coating the PET/cotton fabric in this experiment. The white hollow glass microspheres having a diameter ranging from 15 to 90 µm were purchased from Shanghai FEP International Trade Co. Ltd, having a compressive strength of 5500 PSI (pounds per square inch), and an actual density of 0.38 g/cm³.

Electroless silver plating on hollow glass microsphere

First, the hollow glass microspheres were scoured with 70 g/L NaOH, 25 g/L Na₂CO₃, and 7.5 g/L Na₂SiO₃ of specified concentration under the mechanic stirring at 85°C for 20 minutes, and then rinsed with distilled water. Second, the degreased glass microspheres were etched in 4% HF solution for 15 minutes at room temperature, followed by washing with distilled water. Finally, the etched microspheres were deposited with silver coating at 20°C for 3 minutes based on the optimized electroless silver plating process without surface sensitization and activation. The loadage of glass microspheres was 5 g/L. The electroless silver plating solution was composed by 0.03 mol/L silver nitrate, 0.008 mol/L ammonium hydroxide, 0.1 mol/L sodium hydroxide, 0.005 mol/L glucose, 0.0006 mol/L tartaric acid, and 0.18 mol/L anhydrous alcohol. After the reaction was completed, the Ag-deposited microspheres were rinsed with distilled water for three times and were kept at 120°C for 2 hours.

Coating finish of PET/cotton fabric

PET/cotton fabric was first scoured and bleached, and then repeatedly washed with distilled water. The adhesive printing procedure was employed to coat fabric in the laboratory. The ivory white printing paste UN-404 was constituted with 60 g/L diffusing agent NNO, 40 g/L peregal O, 700 g/L kerosene, 20 g/L emulsifying thickener M, 10 g/L sodium hexametaphosphate, and 170 g/L water. The microspheres were added to the printing paste with stirring in different concentrations of 5%, 10%, 15%, and 20%. PET/cotton fabric was then coated with the above mixed printing paste. The coated fabric was immediately dried at 105°C for 2 minutes in an oven, followed by curing at 160°C for 1.5 minutes. The weight increasing percents relative to the base fabric were 80.3%, 111.2%, 116.3%, and 124.7%, respectively.

Characterization and measurement

Particle size distribution measurement

The size distributions of glass microspheres at different stages were determined by using a Mastersizer S laser particle size analyzer (Malvern Instruments Ltd.). The scanning range was from 0.05 µm to 3500 µm.

Scanning electron microscopy and energy dispersive spectroscopy analyses

The surface morphologies of glass microsphere and coated fabric were observed by field emission scanning electron microscope (FSEM, JEOL, JSM-6700F) equipped with an Oxford INCA Energy 400 energy-dispersive X-ray spectrometer. X-ray energy dispersive spectroscope (EDS) was used for analysis of the content of silver element in the coating.

X-ray diffraction analysis

The phase structure of microsphere before and after silver plating was characterized with X-ray diffraction technique (XRD-7000S, 40 kV, 40 mA, with a Cu Kα₁ X-ray source, λ = 0.154056 nm) in the 2θ ranging from 20° to 80° at a scan speed of 10°/min (sampling pitch 0.02^o). The apparent crystallite size of the sample was obtained by using the Scherrer formula D = Kλ/βcosθ (where D is the diameter of the particle, λ is the X-ray wavelength, β is the FWHM of the diffraction line, θ is the diffraction angle, and K is a constant 0.89).

Diffuse reflectance spectrum measurement

The diffuse reflectance curves of glass microspheres and coated fabric in the 240–2600 nm waveband were recorded by using a U-4100 UV-Visible-NIR spectrophotometer (Scan speed: 1200 nm/min at visible range, 1500 nm/min at NIR range. Detector: Photomultiplier (VIS), PbS (NIR). Inner face coated with BaSO₄. Incident angle on reflective sample: 10° on both standard and reference sides) with an integrating sphere (ø60 mm).

Specific resistance measurement

The Ag-deposited microspheres were filled into a plastic cylinder, and were then pressed with a pressure of 2 kg/cm². The positive and negative electrodes were embedded at the bottom of the cylinder, respectively. The volume resistance R₁ of the Ag-deposited microspheres was measured using a MASTECH MY-68 digital multimeter. The specific resistance ρ₁ (Ω cm) was calculated by Equation (1) [17].

ρ_{1} = (π R_{1} d^{2}) / 4 h

(1)

Where R₁ is the measured volume resistance (Ω), h and d are the length (10 cm) and inner diameter (0.5 cm) of the plastic cylinder, respectively.

The specific resistance ρ₂ (Ω) of the coated fabric with the size of 10 cm × 1 cm was also measured using the above mentioned resistance meter and was calculated by Equation (2) [18].

ρ_{2} = (R_{2} L) / D

(2)

Where R₂ is the measured surface resistance (Ω), D and L are the width (1 cm) and length (10 cm) of the coated fabric, respectively.

Shielding effectiveness measurement

The shielding effectiveness (SE) of the coated fabric to electromagnetic was performed using the dual chamber method for near-field cases on the electromagnetic radialization testing instrument, which is composed of a rectangular waveguide tube and a PAN3610 network analyzer, according to the method of ASTM ES 7-83. An electromagnetic shield is a conductive material which attenuates (through reflection and absorption) electromagnetic energy. The effective frequency for testing is in the range of 2250–2650 MHz, and the size of the sample is 11 cm × 6.5 cm. An SE is defined as the ratio of incident to transmitted electric intensity (or magnetic intensity, or power) and is usually expressed in decibels (dB) by Equation (3).

SE = 20 \times \log | \frac{E_{0}}{E_{1}} | = 20 \times \log | \frac{H_{0}}{H_{1}} | = 10 \times \log | \frac{W_{0}}{W_{1}} | (dB)

(3)

Where E₀ is the incident electric intensity (V/m), and E₁ is the transmitted electric intensity (V/m); H₀ is the incident magnetic intensity (A/m), and H₁ is the transmitted magnetic intensity (A/m); W₀ is the incident power (W), and W₁ is the transmitted power (W).

Thermal radiant protection measurement

The thermal radiant protection properties of the coated fabric were measured with reference to GB/T 18319-2001. An infrared ray lamp with main wavelength at 2.4 µm was employed as the infrared radiant source. The fabric sample, which was fixed on a specimen holder with an exposing area of 60 mm × 60 mm, was in the front of the lamp at a distance of 30 cm. The irradiance reached to the fabric surface was 1.88 kW/m². The transmitted radiant energy at regular interval time was recorded by a MR-4 digital infrared radiometer (the detection wavelength 0.8–10 µm), which was set behind the fabric sample. At the same time, the temperature of fabric itself was measured using a DM6801A digital temperature meter, of which its punctate sensor was tightly attached onto the fabric surface. The size of the tested fabric sample was 20 cm × 20 cm. The experiment was carried out in a laboratory at the temperature of 20°C and humidity of 65%. In order to ensure reproducibility, each experiment was repeated three times.

Results and discussion

Analysis of particle size distribution

Figure 1 shows the volume percentage of microsphere versus particle size at different stages. It is clear that when hollow glass microspheres were etched with the HF solution, the particle size distributions are shifted from 1–100 µm to 10–100 µm, indicating the removal of most crushed microspheres. The Debrouckere average diameters (volume moment average value) D[4,3] are increased from 65.56 µm (concentration 0.018 Vol%) to 69.03 µm (concentration 0.036 Vol%). The corresponding volume middle value diameters D(v, 0.5) are increased from 58.32 to 63.81 µm, and the sauter average values (surface moment average values) D[3,2] are increased from 11.49 to 43.03 µm. After being plated with silver deposition, the particle size distribution had almost no change. Most of microspheres still retain their original shape. The values of D[4, 3], D(v, 0.5), and D[3,2] (concentration 0.036 Vol%) are increased to 73.07, 67.40, and 41.19 µm, respectively.

Figure 1.

Particle size distributions of glass microspheres: (a) the original microsphere; (b) the etched microsphere; and (c) the Ag-deposited microsphere.

Scanning electron microscopy and energy dispersive spectroscopy analyses

Figure 2 shows the SEM pictures of glass microspheres at different stages and coated fabric. It can be seen that the surface of the original microsphere is very clean and smooth (Figure 2(a)). The rough grain scattered with small pits is attributed to the chemical etching in HF solution (Figure 2(b)). With respect to the Ag-deposited microsphere, it is covered with a layer of an inhomogeneous but compact deposition (Figure 2(c)). For the coated fabric (additive amount 20%), a number of Ag-deposited microspheres are adhered to each other. Some glass microspheres are broken due to the collision in the preparation process (Figure 2(d)).

Figure 2.

SEM pictures of glass microspheres and coated fabric: (a) the original microsphere; (b) the etched microsphere; (c) the Ag-deposited microsphere; and (d) PET/cotton fabric coated with Ag-deposited microsphere.

The chemical compositions for the original and Ag-deposited microspheres are also analyzed by using an EDS of an emissive-type electron microscope. Figure 3 shows the EDS survey spectra of microsphere before and after silver plating. The corresponding results are also given in Table 1. By comparison of Figure 3(a) and 3(b), the compositions by mass percent are O 51.51%, Na 0.83%, Al 21.72%, Si 25.32%, and K 0.62% for the original microsphere. The corresponding atomic percents are O 64.67%, Na 0.73%, Al 16.17%, Si 18.11%, and K 0.32%. As for the Ag-deposited microsphere, the compositions by mass percent are O 48.16%, Na 1.13%, Al 13.38%, K 1.13%, and Ag 19.65% and the corresponding atomic percents are O 69.12%, Na 1.13%, Al 11.38%, Si 13.53%, K 0.66%, and Ag 4.18%.

Figure 3.

EDS survey spectra of: (a) the original and (b) Ag-deposited microspheres.

Table 1.

The compositions of the original and Ag-deposited microspheres.

Element	Weight percent		Atom percent
Element	Original	Ag-deposited	Original	Ag-deposited
O K	51.51	48.16	64.67	69.12
Na K	0.83	1.13	0.73	1.13
Al K	21.72	13.38	16.17	11.38
Si K	25.32	16.55	18.11	13.53
K K	0.62	1.13	0.32	0.66
Ag L	–	19.65	–	4.18

X-ray diffraction analysis

Figure 4 shows the XRD patterns of the original and Ag-deposited microspheres. It is obvious that there are no typical diffraction peaks in the XRD pattern of the original microsphere. A series of characteristic peaks are observed at 2θ of 38.2°, 44.3°, 64.5°, and 77.4° after electroless silver plating. These are related to the {111}, {200}, {220}, and {311} planes of silver face-centered cubic structure. From the width of the peaks at 2θ = 38.2°, 44.3°, 64.5°, and 77.4°, the average crystallite size of the metal silver particle is calculated to be 22.1 nm by using Scherrer’s equation.

Figure 4.

X-ray patterns of: (a) the original and (b) Ag-deposited microspheres.

Diffuse reflectance spectrum analysis

Figure 5 shows the diffuse reflectance spectra for the original, etched and Ag-deposited microspheres and the coated fabric in the range of 240–2600 nm. It is found that the reflectance spectra for the original, etched, and Ag-deposited microspheres are very similar in shape. The reflectance is reduced about 20% after being etched by HF solution. After electroless silver plating, the reflectance is decreased further. Even though the reflectance is reduced to a great extent (about 35%), the electrical conductivity is gained. As for the coated fabric, the reflectance is decreased further. This is mainly due to the existence of the printing paste.

Figure 5.

Diffuse reflectance spectra of: (a) the original; (b) etched; and (c) Ag-deposited microspheres and the coated PET/cotton fabric.

Specific resistance analysis

The specific resistance ρ₁ for the Ag-deposited microsphere is 4.2 × 10⁻² Ω cm. For the coated fabric, the specific resistance ρ₂ decreases rapidly with increasing the additive amount of Ag-deposited microsphere, as depicted in Figure 6. The specific resistances are 18.23 Ω, 9.56 Ω, 6.41 Ω, and 0.78 Ω for the additive amounts of 5%, 10%, 15%, and 20%, respectively. This implies that a three-dimensional conductivity chain has been formed in the printing paste. The larger the additive amount of Ag-deposited microspheres, the nearer the distance between the microspheres.

Figure 6.

Effect of the amount of Ag-deposited microsphere on the specific resistance for the coated PET/cotton fabric.

Shielding effectiveness analysis

Figure 7 shows the shielding effectiveness of PET/cotton fabric coated with different additive amounts of Ag-deposited microspheres. It is evident that as the dosage of Ag-deposited microspheres increases, the shielding effectiveness of the coated fabric increases. The average shielding effectivenesses are 1.6 dB, 5.4 dB, 11.4 dB, and 32.8 dB for the additive amounts of 5%, 10%, 15%, and 20%, respectively. This is ascribed to the addition of Ag-deposited microspheres in the printing paste, which improves the ability of shielding the electromagnetic wave, and makes the coated fabric conductive.

Figure 7.

The shielding effectiveness of PET/cotton fabric coated with Ag-deposited microspheres.

Thermal radiant protection analysis

Figure 8(a) shows the changes of transmitted irradiance of PET/cotton fabric before and after treatment with the increase of irradiant time. It can be seen that the transmitted irradiances are constantly increased with the increase of the irradiant time for all samples. The thermal radiant protection ability of the coated fabric behavior is much better compared with the uncoated one. As the additive amount of Ag-deposited microspheres increases, the transmitted irradiance decreases gradually under the same irradiant time. For the same additive amount of microspheres (20%), the transmitted irradiance of coated fabric with the Ag-deposited microspheres is slightly higher than that of the original microsphere coated fabric. The result is consistent with the diffuse reflectance of microsphere before and after silver plating. Figure 8(b) shows the temperatures of the uncoated and coated fabrics at different irradiant times. The result does correlate with the irradiance. Compared with the coated fabric, the temperature of the uncoated fabric is the lowest for the same irradiant time as most of irradiant energy transmits through the fabric. The fabric’s temperature for the coated fabric decreases with increasing the additive amount of Ag-deposited microspheres. When the additive amount of glass microsphere is identical, the temperature of the Ag-deposited microsphere coated fabric is smaller than that of the original microsphere coated fabric, which is in accord with the irradiance.

Figure 8.

The relationships between irradiance and time (a), temperature and time (b) for PET/cotton fabrics.

Conclusions

In this article, electroless silver plating was employed to metallize the surface of hollow glass microsphere using silver nitrate 0.03 mol/L, ammonium hydroxide 0.008 mol/L, sodium hydroxide 0.1 mol/L, glucose 0.005 mol/L, tartaric acid 0.0006 mol/L, and anhydrous alcohol 0.18 mol/L. The particle integrity, surface morphology, and structure of the Ag-deposited microspheres were examined by the size distribution, SEM, EDS, and XRD techniques. The Ag-deposited microspheres were mixed with the printing paste, and then coated on the surface of PET/cotton fabric. The properties of diffuse reflectance, resistance, shielding effectiveness, and thermal radiant protection were also investigated. SEM observation showed that hollow glass microsphere was covered by the silver deposition. XRD result indicated that pure Ag phase was constituted of nanoparticle with a size in 22 nm. Diffuse reflectance spectrum result revealed that the light absorption change of microsphere in the range 240–2600 nm was due to the silver deposition. As the additive amount of Ag-deposited microspheres increased, the specific resistance of the coated fabric decreased, but the shielding effectiveness increased. Meanwhile the thermal radiant protection capability became better.

Footnotes

Funding

The work was supported by the Key Laboratory of Functional Fabric of Shaanxi Province Research Project (No.09JS007) from Shaanxi Education Department in China.

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Silver plating on hollow glass microsphere and coating finishing of PET/cotton fabric

Abstract

Keywords

Introduction

Experimental

Materials

Electroless silver plating on hollow glass microsphere

Coating finish of PET/cotton fabric

Characterization and measurement

Particle size distribution measurement

Scanning electron microscopy and energy dispersive spectroscopy analyses

X-ray diffraction analysis

Diffuse reflectance spectrum measurement

Specific resistance measurement

Shielding effectiveness measurement

Thermal radiant protection measurement

Results and discussion

Analysis of particle size distribution

Scanning electron microscopy and energy dispersive spectroscopy analyses

X-ray diffraction analysis

Diffuse reflectance spectrum analysis

Specific resistance analysis

Shielding effectiveness analysis

Thermal radiant protection analysis

Conclusions

Footnotes

Funding

References