Characterization of copper-plated conductive fibers after pretreatment with supercritical carbon dioxide and surface modification using Lyocell fiber

Abstract

Metal plating onto the fibers using supercritical fluid (SCF) is a manufacturing technology used to make highly conductive fiber; further research is being carried out actively on synthetic fibers such as aramid, polyester and nylon fibers. However, metal plating onto the fibers using SCF has some problems. The adhesion between the fiber and plate metal is not high, and this method requires heat stability because of the high temperature and pressure under supercritical state, so natural fibers are of limited use. This paper investigated Lyocell fiber, which is cellulose fiber and has good heat stability, to find the optimum condition of the supercritical pretreatment. After making conductive fibers, abrasion and washing tests were carried out to examine adhesion between the fiber and plated metal. Also, to improve the adhesion between the fiber and metal much more, an attempt has been made to change the surface modification of the fiber by oxygen plasma, acid or base. In particular, the copper-plated conductive fiber after plasma treatment had high conductive properties, even after abrasion and washing tests were conducted.

Keywords

supercritical fluid surface modification conductive fiber copper plate

Lyocell is a cellulose fiber that is recycled using non-toxic solvents. During this process, all of the spent solvent is collected, minimizing the risks to humans and the environment. The strength of Lyocell is about two times that of cotton fiber whether it is wet or dry, and there is limited contraction when it is wet. Therefore, Lyocell is a practical and environmentally friendly fiber,^1–3 which is one of the most attractive fibers for our future life. To fully exploit Lyocell fiber in our life, it is important to develop functional processing methods for the fiber. Recently, conductive processing has become important on processing of the fiber. In this study, we investigated how to give conductivity to Lyocell fiber.

Electroless metal plating, such as that using copper^4,5 or nickel,^6,7 has been studied in varying ways, and has been used as the most common processing method to give conductivity to the fiber. Palladium (Pd) is used as the catalyst widely for electroless metal plating.^8,9 A Pd catalyst is put on the polymer substrate by the redox reaction with metal ions through the pretreatment.^10–12

Pd is a powerful catalyst that is able to lower the activation energy of the initiation reaction and allow the deposition of any autocatalytic metal in the electroless metal plating process. An autocatalytic reaction starts on a polymer surface through putting the pretreated polymer substrate into the electroless metal plating solution. Once the plating reaction is started, it continues thanks to the autocatalytic property of the corresponding metal, which catalyses the further reduction of its own ions.¹³ Many processes have been developed to seed a polymer surface with Pd species as the catalyst for electroless metal plating. However, there were several problems associated with the conventional pretreatment process of electroless metal plating, such as the many complicated stages that include surface adjustment, neutralization, etching, catalysis and acceleration.¹⁴ To solve these problems, we have researched pretreatment with supercritical carbon dioxide (scCO₂) for giving Pd metal to a polymer substrate.

The use of supercritical fluids (SCFs) as solvents in the metal plating process^15–17 or dyeing process^18–20 has attracted considerable attention in recent years. A SCF is defined as “a substance existing above its critical temperature and pressure” and it has unique properties.^21,22 A solvent’s properties are determined from inter- and intra-molecular interactions because for incompressible liquid solvents, molecular distances are unchanged. It is therefore difficult to change the properties of a single solvent. When the materials are in the supercritical state, their properties, such as density, viscosity and diffusibility, are continually changed from near the gas state to near the liquid state according to changing pressure and temperature over the critical point.^23,24 The most frequently used SCF is carbon dioxide. Besides the advantage of having a low critical temperature, this particular SCF is also available at low cost and in high purity. It is also well known that scCO₂ is non-toxic, incombustible and can be easily recycled. So there are both economic and environmental advantages to be realized by using this fluid.^25–27 Recently, SCFs have been used in applications for which the end products have high economic value. Examples are classical extractions and sophisticated industrial processes. Many applications in the area of polymer processing involve impregnation with organic molecules using SCFs, such as impregnation with pharmaceutical products.^28–32 On account of its solvating ability towards non-polar or slightly polar organic molecules in the supercritical phase, CO₂ can be used to transport disperse precursor to a polymer substrate, without having to use the aqueous medium, thus avoiding pollution problems. The supercritical impregnation (or infusion) is that some ingredients are put in solid. Normal coating processes using liquid solvent cannot impregnate into narrow gaps, because of the high viscosity of liquid solvent. Also, gas solvent under critical temperature is condensed and becomes liquid when it is passed by narrow spaces. SCF has low viscosity and cannot be condensed. Further, a lot of precursors are able to impregnate effectively into a narrow space, because the solubility of the solute under SCF is able to be controlled by changing the temperature or pressure of fluid and adding entrainers.^33–35

In this article, we discuss the design of a pretreatment process of electroless metal plating, in which a Pd organometallic complex is impregnated into Lyocell fibers using scCO₂. A Pd metal as a catalyst for electroless metal plating is formed at the fiber surface through the removal of ligands, using appropriate reduction treatments that do not require hydrogen.³⁶ The electroless metal plating then conducts easily. In addition, no wastewater is produced by this pretreatment process. We also discuss the improvement in adhesion between the fiber and coating metal on the metal-plated Lyocell fiber. The adhesion between the fiber and coating material changes depending on the shape of the surface.^37,38 For normal fiber, the surface is very smooth, so the coating material easily falls from this surface. In order to improve the adhesion between the fiber and coating material, its surfaces are modified using acids, bases and plasma. In general, fibrillation is formed on the surface of Lyocell after treatment with bases. However, fibrillation is removed and the Lyocell surface becomes rough after treatment with acid.^39–41 The method used for plasma treatment results in improved moisture absorption because of changed chemical and physical surface properties. In other words, surface energy and adhesion are improved.^42–44

Experimental details

Materials

Carbon dioxide (purity: 99.99%) was purchased from the Uno Oxygen Co., and was used as received. Palladium (II) hexafluoro acetylacetonate (Pd(hfa)₂) was purchased from the Aldrich Chemical Co., and was used without further purification. Electroless copper-plating solutions containing ATS-ADDCOPPER IW-A, ATS- ADDCOPPER IW-M and ATS- ADDCOPPER C were bought from the Okuno Chemical Industry Co., Ltd. Untwisted Lyocell fiber (1500 denier/1000 filament, Kolon Co., Ltd) was used. HCl (080-01066) was purchased from Wako Pure Chemical Industries, Ltd, and NaOH (96.0%, 31511-05) was purchased from Nacalai Tesque, Inc.

Methods

The processes involved in the supercritical pretreatment, decompression and electroless deposition of copper on Lyocell fiber by magnet stirring are shown in Figure 1. The details for each process are described below.

Figure 1.

Schematic illustration of the processes involved in supercritical pretreatment, decompression, and electroless deposition of copper on fiber by magnet stirring.

Supercritical pretreatment

Lyocell fiber was cut into strips of 150 cm, and both ends of the fiber were tied to prevent the mono-filaments from becoming loose. All experiments were performed on a batch-type supercritical extractor (SFX™220, ISCO, USA), a diagram of which is shown in Figure 2. The impregnation was conducted in a 10 cm³ sample cartridge, which could be inserted or extracted from a high-pressure stainless steel vessel sealed with a plug at one end, and a high-pressure needle valve at the other end.

Figure 2.

Illustration of the reaction apparatus: (A) syringe pump; (B) cleaning pump; (C) heater and high-pressure stainless steel vessel; (D) organometallic complex; (E) sample; (F) sample cartridge; (G) washing solvent; (H) thermometer; (I) glass filter; (5, 8) pressure gauges; (1, 2, 3, 4, 6, 7, 9, 10) valves.

Before sealing the sample cartridge, a glass filter was placed both over and under the fiber. Pd(hfa)₂ powder, calculated as 5 wt.% of the sample fiber, was placed on the glass filter. After the desired temperature was reached, the sealed sample cartridge was placed in the stainless steel vessel, and carbon dioxide was then added via the high-pressure syringe pump until the desired pressure was achieved. The impregnation lasted for 20, 40, 60 and 100 min.

Electroless copper plating

First, 20 ml of ATS-ADDCOPPER IW-A were added to 344 ml of deionized water, followed by 32 ml of ATS-ADDCOPPER IW-M and 4 ml of ATS-ADDCOPPERC. The solution was stirred for 5 min using a magnet stirrer.

Lyocell fiber was fixed onto the preset holder and dipped into the electroless copper-plating solution at 42 ± 2℃ for 30 min under magnet stirring (Hot Stirrer HS-5BHSD/AS ONE Co., Ltd). The agitation rate of the mechanical stirring was set at about 200 rpm. After plating, the fibers were maintained at room temperature for 24 h in order to remove any remaining moisture. The main mechanism of copper plating on Lyocell fiber after scCO₂ pretreatment is as follows:

pd + {Cu}^{2} \to Cu + {pd}^{2}

(1)

The samples were coated with copper on the surface due to the redox reaction after copper ions reacted with activated Pd catalysts on the fiber surface in the electroless metal plating solution.

Surface modification

First, 1% and 2% HCl solutions were prepared after dilution, and Lyocell fiber was immersed in these solutions at room temperature for 30 and 60 min. Using the same method, 1% and 2% NaOH solutions were prepared after dilution, and the surface of the Lyocell fiber was treated at room temperature for 30 and 60 min. After surface treatment, these samples were dried for 24 h at room temperature. Then 100 and 200 W oxygen plasma was introduced to Lyocell fibers using a plasma reactor (PR300, Yamato, Inc.). The 100 W plasma samples reacted for 30 and 60 s, while the 200 W plasma samples reacted for 60 and 120 s. After each treatment, the samples were measured using a scanning electron microscope (SEM) and attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectroscopy.

Analysis details

Investigation of conductivity and structure

The structures of both neat and copper-plated Lyocell fibers were investigated using a SEM (S-3400 N, HITACHI Co., Ltd) and EMAX (250, HORIBA Co., Ltd). The EMAX in particular is capable of identifying dispersed copper on the fiber. In addition, the peaks of the polymer structure were compared using an ATR-FTIR spectrometer (NICOLET 380 FTIR, Smart Orbit, Diamond 30,000–200 cm⁻¹, Thermo SCIENTIFIC Co., Ltd) before and after surface treatment. Fibers were fixed for 24 h using epoxy resin, and cut from the ion beam using a cross-section polisher (SM-09010, JEOL DATUM. Ltd) for 24 h in order to measure cross-sectional images of Lyocell fiber.

The electrical conductivity of Lyocell fiber was measured at intervals of 10 cm using a micro ohmmeter (3440 A 7½ Digit Nano Volt/Micro Ohmmeter, Agilent Co., Ltd). When a multi-filament is coated, its cross-section may be irregular. In this article, however, the conductivity of Lyocell fiber was calculated with the following two assumptions:

the copper-plated Lyocell fiber has a regular cross-sectional area;

the densities of Cu and Lyocell (Cu: 8.94 g/cm³, Lyocell: 1.50 g/cm³) do not change.

The density of copper plated on the fibers or plastics using the electroless metal plating method is a little smaller than the normal metal copper. However, it is difficult to measure the density of the plated copper. To be able to compare conductivities among the fibers, the density of plated copper was assumed as the normal metal copper, and volume resistance was calculated for the each fiber.

Evaluation of abrasion and washing tests

For the measurement of the abrasion of copper-plated Lyocell fiber, a friction testing machine was designed and is shown in Figure 3. Copper-plated Lyocell fiber was arranged between the metal pins and supported a 20 g weight on one end.⁴⁵ Lyocell fiber was then stretched in both directions 50 and 100 times at regular speeds (6π cm/2 s) using a motor. After the abrasion test, the surface structure and conductivity of the fiber was analyzed.

Figure 3.

Schematic diagram of the abrasion test device.

Copper-plated Lyocell fiber was washed using a household washer (NA-F42S6, National. Co., Ltd). The samples were sewn on a fabric following the International Organization for Standardization (ISO; 105-C06) or Korean Industrial Standards (KS; K-0431). However, the copper-plated yarn could not be used for ISO methods, because ISO methods are too severe to evaluate the durability of copper-plated fiber. Therefore, a household washing machine was used for washing tests. After each washing test, samples were kept horizontal at room temperature for 12 h in order to remove any remaining moisture from the fiber surface. The conductivity of the fiber was then measured. The washing test was repeated 10 times.

Results and discussion

Surface properties of copper-plated Lyocell fiber

The SEM images are shown in Figure 4 and illustrate the surface structures of both neat Lyocell fiber and copper-plated Lyocell fiber. The surface of copper-plated Lyocell fiber is more irregular than that of neat Lyocell fiber. Figure 5 shows a cross-sectional diagram of Lyocell fiber after copper coating. This image shows that copper was uniformly dispersed on Lyocell fiber.

Figure 4.

Surface images of (a) neat Lyocell fiber and (b) copper-plated Lyocell fiber (magnification: 3000×).

Figure 5.

Cross-section images of copper-plated Lyocell fiber: (a) obtained using a scanning electron microscope; (b) Cu image by EMAX (magnification: 3000×).

The cross-sectional area was calculated from the weight increase after plating under the assumption that the specific gravity of the plated copper layer is the same as the metal copper (8.94 g/cm³) in order to measure the conductivity of copper-plated fibers. This was done using the formula in (2):

Cross - sectional area = \frac{Mass}{Density \times Length}

(2)

Table 1 shows the cross-sectional areas of copper and copper-plated Lyocell fiber that were manufactured under different temperature conditions and pretreated with Pd(hfa)₂– for different periods of time. According to the results, the weight of copper plating was 40–50 wt.% that of the entire plated fiber.

Table 1.

Supercritical pretreatment for preparing copper-plated Lyocell fibers and their characteristics

Supercritical pretreatment			Characteristics of copper-plated Lyocell fiber
No.	Temp. (℃)	Pres. (MPa)	Time (min)	Cross-sectional area (×10⁻³ cm²)	Conductivity 2(×10⁻⁴ Ω·cm)
1	120	15	20	1.551	34.628
2		15	40	1.575	20.409
3		15	60	1.573	8.183
4		15	100	1.583	3.455
5	130	15	20	1.564	25.701
6		15	40	1.581	9.382
7		15	60	1.585	3.527
8		15	100	1.584	3.580
9	140	15	20	1.585	7.032
10		15	40	1.593	5.528
11		15	60	1.583	3.037
12		15	100	1.597	2.852
13	150	15	20	1.583	4.918
14		15	40	1.597	3.152
15		15	60	1.583	2.458
16		15	100	1.583	1.986

Electric properties of copper-plated Lyocell fiber

The electrical conductivity (σ) of copper-plated Lyocell fiber was evaluated after measurements were taken at room temperature. Generally, the conductivity of the fiber is evaluated using volume resistance (Ω·cm) or sheet resistance (Ω/□, or Ω/square). In this article, volume resistance was used. Volume resistance can be compared with other materials, so it is selected. The conductivity was calculated using

σ = \frac{V}{I} \times \frac{A}{L}

(3)

where σ (Ω·cm) represents the volume resistance, and V, I, A, and L are the voltage, electric current, cross-sectional area and the distance between measuring probes, respectively. Electric resistance was measured using a micro ohmmeter. The volume resistance (σ) can be calculated after calculating the cross-sectional area and measuring the resistance (Ω) and distance of probes (10 cm).

All pretreated samples were pretreated at a pressure of 15 MPa, as shown in Table 1. The conductivity of the Lyocell fiber increased with time and temperature. In general, Pd(hfa)₂ begins to melt above 140℃, so it is easily absorbed by the fiber above 140℃, even though the reaction time is short. However, below 140℃, Pd(hfa)₂ took in excess of 40 min to completely permeate into Lyocell fiber. The best (lowest) conductivity of Lyocell fiber was 1.986 × 10⁻⁴ Ω·cm and was observed for sample no. 16 (temperature: 150℃; time: 100 min). In order to simulate the optimum conditions, the conductivity of the fiber was ignored, and the conductivity of copper-plated Lyocell fiber (cross-sectional area of copper: 1.24 × 10⁻⁴ cm²) was 1.41 × 10⁻⁵ Ω·cm. Assuming that the copper was evenly coated on the mono-filament, the thickness of the copper layer was calculated. For sample no. 16, the thickness was around 280 nm (about 1.5 denier), which is considered to be very thin. So the softness of the fibers would be little changed before and after the plating. Therefore, copper-plated fiber maintains the flexibility of the original fiber and also has high conductivity.

Surface treatment of Lyocell fiber

Lyocell fiber has thin and long-chain molecules that vary according to the direction of the fiber’s axis, resulting in a very high degree of orientation. Therefore, it swells after contact with water, since the amorphism of Lyocell fiber is also connected by thin and long-chain molecules. When it gets wet, it also becomes dense and stiff. For this reason, fibrillation is formed after rubbing the surface of the swollen fiber. Generally, fibrillation is formed after treatment with a base, while fibrillation is removed after acid treatment. The conductivity of copper-plated fiber and the improvement in the adhesion between the fiber and copper layer were investigated after surface treatment using an acid and a base. In addition, the effect of the surface treatment was investigated using oxygen plasma.

Treatment with base

For base treatment, NaOH was diluted in distilled water at concentrations of 1% and 2%. Lyocell fiber was placed in the diluted base solution for periods of 30 and 60 minutes. After base treatment, the samples were cleaned in distilled water and dried for 24 h at room temperature. The surface of the fiber was observed to be very slippery and sticky. Figure 6 shows the SEM images after base treatment. Some filaments had lost their original shape and had broadened. Lyocell fiber contracts when there is friction under base conditions. Meanwhile, Figure 7 shows the ATR-FTIR spectra for Lyocell fiber after base treatment. There are no remarkable changes of peaks, but meaningful differences were observed. At about 3400 cm⁻¹ (OH bond) the peak was decreased depending on time and concentration. OH groups of Lyocell fiber are easily attacked by other molecules, so some OH groups were broken from NaOH and the structures of fibrils were changed.

Figure 6.

Changes in the surface of Lyocell fiber after NaOH treatment: (a) 1%, 30 min; (b) 1%, 60 min (magnification: 3000×); (c) 2%, 30 min; (d) 2%, 60 min. (magnification: × 1000×).

Figure 7.

Attenuated total reflection–Fourier transform infrared spectra of Lyocell fiber after base treatment.

These samples after base treatment were pretreated for 60 min at 150℃ and 15 MPa by scCO₂. The color of the samples was lighter than that of untreated samples after supercritical pretreatment. Table 2 gives the conductivities after copper coating. The conductivities of base-treated samples were expected to be poor, based on the color of fiber after supercritical pretreatment. The sample is regarded as good when its color is dark after scCO₂ pretreatment. However, these conductivities were very high when compared with an untreated sample under the same conditions (no. 15 in Table 1, 2.458 × 10⁻⁴ Ω·cm). This is due to residual copper plating on broadened filaments and fibrils.

Table 2.

Conductivities of base-treated fibers

Conc.	Time of treatment (min)	Cross-section area (×10⁻³ cm²)	Conductivity (×10⁻⁴ Ω·cm)
1%	30	1.633	1.168
1%	60	1.605	1.345
2%	30	1.598	1.322
2%	60	1.614	1.932

Treatment with acid

For acid treatment, HCl was diluted in distilled water at concentrations of 1% and 2%. Lyocell fiber was placed in the diluted acid solution for 30 and 60 minutes. After acid treatment, the samples were cleaned in distilled water and dried for 24 h at room temperature. The surface of the fiber was soft and flexible. The SEM images in Figure 8 also show that the surface of the fiber was very smooth. Figure 9 shows the ATR-FTIR spectra of the fiber after acid treatment. The peaks of about 3400 cm⁻¹ (OH bond) and 1100 cm⁻¹ (CO single bond) were decreased as time and concentration increased. Some OH groups get out of the cellulose structure after the chemical reaction with HCl. Meanwhile, the ATR-FTIR spectra for the 1%, 30 min sample is similar to that of neat fiber. However, the ATR-FTIR spectra for the 2% sample had decreased significantly. This trend was also verified in Figure 8(c) and (d). In Figure 8(c) and (d), the surface had become very clean due to acid treatment.

Figure 8.

Different changes in the surface of Lyocell fiber after HCl treatment: (a) 1%, 30 min; (b) 1%, 60 min; (c) 2%, 30 min; (d) 2%, 60 min. (magnification: 3000×).

Figure 9.

Attenuated total reflection–Fourier transform infrared spectra of Lyocell fiber after acid treatment.

These samples after acid treatment were pretreated for 60 min at 150℃ and 15 MPa by scCO₂. The color of the samples was darker than that of untreated samples. After supercritical pretreatment, the color of the fiber is an indirect method for determining whether pd(hfa)₂ was properly injected into the fiber. Table 3 gives the conductivities after coating with copper. However, these values do not show any improvements over the values for untreated samples (2.458 × 10⁻⁴ Ω·cm). In addition, these fibers were very weak so they could be easily cut. This is because they have been weakened by acid treatment and subsequent high temperature and pressure conditions that exist during supercritical pretreatment. The smooth surface is due to the reduced adhesion between the fiber and coating material.

Table 3.

The conductivities of acid-treated fibers

Conc.	Time of treatment (min)	Cross-section area (×10⁻³ cm²)	Conductivity (×10⁻⁴ Ω·cm)
1%	30	1.598	9.675
1%	60	1.574	15.071
2%	30	1.565	9.435
2%	60	1.563	18.716

Treatment with plasma

The samples were treated with 100 W of power for 30 and 60 s, and with 200 W of power for 60 and 120 s using oxygen plasma for plasma treatment. Oxygen plasma is expected to have a chemical effect due to the presence of oxygen, and a physical effect due to the plasma. The surface of the fiber was rougher than that of untreated fiber. The SEM images in Figure 10 show damages to the fiber surface. Meanwhile, it was reported that the remarkable changed peak of ATR-FTIR spectra is a CO double bond after plasma treatment of cellulose fiber.⁴⁶ In Figure 11, ATR-FTIR spectra were largely not changed from plasma treatment, but at about 1720 cm⁻¹ (CO double bond) the peak was increase after 100 W plasma treatment. However, in the case of 200 W, the peak was rather decreased, because some chemical bonds were destroyed from the high temperature.

Figure 10.

Surface change of Lyocell fiber after oxygen plasma treatment: (a) 100 W, 30 s; (b) 100 W, 60 s; (c) 200 W, 60 s; (d) 200 W, 120 s (magnification: 3000×).

Figure 11.

Attenuated total reflection–Fourier transform infrared spectra of Lyocell fiber after plasma treatment.

After plasma treatment, these samples were pretreated for 60 min at 150℃ and 15 MPa by scCO₂. Table 4 gives the conductivities after coating with copper. The conductivities of plasma-treated samples were very high when compared with those of untreated samples (no. 15 in Table 1). This is due to the presence of copper plating on the embossed surface resulting from plasma damage.

Table 4.

The conductivities of plasma-treated fibers

Power	Time of treatment (s)	Cross-section area (×10⁻³ cm²)	Conductivity (×10⁻⁴ Ω·cm)
100 W	30	1.589	2.387
100 W	60	1.597	1.661
200 W	60	1.596	1.664
200 W	120	1.589	1.490

Abrasion and washing test

The normal abrasive test of the fiber involves measuring the number of stretching cycles between the fiber and friction device until the fiber is cut. However, this method reflects mainly the mechanical properties of the original fiber as opposed to the properties of the material used for coating. Therefore, in order to investigate the abrasion property of the material used for coating, the method used to measure the conductivity and surface conditions was used after the fiber was rubbed with pins at constant time intervals. This method can be used to evaluate the abrasion property, as well as the adhesive strength between the fiber and coating material. Figure 12 shows SEM-EMAX images of copper-plated Lyocell fibers after the abrasion test was carried out. Some of the copper layer was separated from Lyocell fiber or cracked during the abrasion test. Hence, carbon can be observed at the section that was separated from copper in Figure 12(c). This image could be decisive evidence copper had been removed by abrasion.

Figure 12.

Images of copper-plated Lyocell fiber after abrasion test using a scanning electron microscope (SEM-EMAX): (a) obtained using a SEM; (b) Cu image by SEM-EMAX; (c) image by EMAX (magnification: 1000×).

The washing fastness is a very important index for fiber, so the adhesion between the fiber and copper layer was investigated after performing the washing test. Prior to carrying out the washing test, Lyocell fiber was cut into strips of 10 cm and sewn onto fabric. The conductivity after the washing test was observed to have decreased, but this was due to damage to the mono-filament as opposed to the falling or breaking of the copper layer. Figure 13 shows SEM-EMAX images of copper-plated Lyocell fiber taken after the washing test. Some of the copper layer was separated from the Lyocell fiber during the washing test. Hence, carbon can be observed at the section that was separated from copper in Figure 13(c). After washing and abrasion tests, the conductivity was high and remained of the order of ×10⁻² Ω·cm. The conductivities are displayed in Table 5. After the abrasion and washing tests, conductivities became lower, but they were still in the range of the conductor. The base and plasma-treated fibers had higher conductivities than untreated fiber. In particular, the plasma-treated fiber had very good abrasion washing fastness. This is because the embossed surface increased the adhesion between the fiber and the copper layer.

Figure 13.

Images of copper-plated Lyocell fiber after washing test using a scanning electron microscope (SEM-EMAX): (a) obtained using a SEM; (b) Cu image by SEM-EMAX; (c) image by EMAX (magnification: 1000×).

Table 5.

The conductivities of copper-plated Lyocell fibers before and after abrasion and washing test

Conductivity(×10⁻⁴ Ω·cm)
Untreated	Base treated	Plasma treated
0 cycles	1.986	1.273	1.542
Abrasion test	50 cycles	4.363	3.242	2.823
100 cycles	6.252	4.782	3.672
0 times	2.452	1.205	1.543
Washing test	5 times	19.781	11.694	5.187
10 times	116.472	74.476	29.752

Conclusions

In this study, the conductive Lyocell fiber was produced by the electroless copper metal plating method. We investigated the scCO₂ pretreatment process to give the Pd catalyst for electroless metal plating to the fiber and the fiber surface treatment with acid, base and oxygen plasma to improve the adhesion between the fiber and the copper layer. During the scCO₂ pretreatment, the fiber was impregnated with Pd(hfa)₂ under scCO₂ atmosphere, and then Pd metal was deposited in and on the fiber from the complex by heat reduction treatment. When the fiber was immersed into the electroless copper-plating solution, the fiber was coated with a copper layer by Pd metal on the fiber activating the metal plating as a catalyst. Copper-plated Lyocell fiber had a much higher conductivity, of the order of 10⁻⁴ Ω·cm, compared with neat Lyocell fiber of ×10¹⁴ ∼ 10¹⁶ Ω·cm. The highest conductivity of 1.986 × 10⁻⁴ Ω·cm was observed for the fiber pretreated at 150℃ and 15 MPa for 60 min. For reference, the resistance of silver-plated cellulose fiber was 39.2 ± 1.5 Ω/cm.⁴⁷ However, the resistance of copper-plated Lyocell fiber using scCO₂ was very low (1.2 × 10⁻¹ Ω/cm).

Surface treatment of the fiber with base and oxygen plasma before scCO₂ pretreatment induced the embossed surface on the fiber, which lead to an increase in the adhesion between the fiber and copper layer after the metal plating. The conductivities of the surface treated fibers with base or oxygen plasma were maintained even after abrasion and the washing test. In particular, plasma-treated fiber at 200 W for 120 s kept a low resistibility of 3.672 × 10⁻⁴ Ω·cm after the abrasion test (100 cycles) and 29.752 × 10⁻⁴ Ω·cm after the washing test (10 times) compared with untreated fibers. Therefore, copper-plated Lyocell fiber using scCO₂ pretreatment can be used in the textile industry for applications such as smart textiles and E-textiles.

Footnotes

Acknowledgments

The authors would like to sincerely thank Dr Jeaho Kim from the Department of Material Science Engineering at Fukui University for the SEM-EMAX and ATR-FTIR analysis. The authors also thank Mr Nowoo Park from the Fiber Research Institute of KOLON Co., Ltd and Mr Kihyung Lee, the President of BOWOO Industry Co., for their kind assistance.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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