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Article

Investigations of the Interface Design of Polyetheretherketone Filament Yarn Considering Plasma Torch Treatment

by
Toty Onggar
1,*,
Leopold Alexander Frankenbach
1 and
Chokri Cherif
1,2
1
Institute of Textile Machinery and High Performance Material Technology (ITM), Technical University Dresden, 01062 Dresden, Germany
2
Centre for Tactile Internet with Human-in-the-Loop (CeTI), TUD Dresden University of Technology, 01069 Dresden, Germany
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(11), 1424; https://doi.org/10.3390/coatings14111424
Submission received: 2 October 2024 / Revised: 26 October 2024 / Accepted: 6 November 2024 / Published: 8 November 2024
(This article belongs to the Special Issue Functional Polymer Coatings and Films: Synthesis, Properties and Applications)
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Abstract

:
Taking advantage of its high-temperature resistance and elongation properties, conductive-coated polyetheretherketone (PEEK) filament yarn can be used as a textile-based electroconductive functional element, in particular as a strain sensor. This study describes the development of electrical conductivity on an inert PEEK filament surface by the deposition of metallic nickel (Ni) layers via an electroless galvanic plating process. To enhance the adhesion properties of the nickel layer, both PEEK multifilament and monofilament yarn surfaces were metalized by plasma torch pretreatment, followed by nickel plating. Electrical characterizations indicate the potential of nickel-coated PEEK for structural monitoring in textile-reinforced composites. In addition, surface energy measurements before and after plasma torch pretreatment, surface morphology, nickel layer thickness, chemical structure changes, and mechanical properties were analyzed and compared with untreated PEEK. The thickness of the Ni layer was measured and showed an average thickness of 1.25 µm for the multifilament yarn and 3.36 µm for the monofilament yarn. FTIR analysis confirmed the presence of new functional groups on the PEEK surface after plasma torch pretreatment, indicating a successful modification of the surface chemistry. Mechanical testing showed an increase in tensile strength after plasma torch pretreatment but a decrease after nickel plating. In conclusion, this study successfully developed conductive PEEK yarns through plasma torch pretreatment and nickel plating.

1. Introduction

In today’s textile technology, electrically conductive filaments are becoming increasingly important in a wide range of applications. For example, textile sensors and sensor networks use electrically conductive filaments [1]. These yarns are also critical for information transmission. In the medical [2,3,4] and healthcare sectors, data gathered from mechanical structural monitoring of critical components or wound monitoring are transmitted using electrically conductive yarns [5,6]. In addition, the development of smart textiles and functional fiber composites relies heavily on electrically conductive yarns, as many of their functions depend on signal transmission and electrical energy storage [4,7]. For example, the development of smart clothing that can connect to smartphones to monitor vital signs, make entries, or display digital information highlights the need for high mechanical flexibility, as the garments need to adapt to the wearer’s movements [8,9,10,11]. Modern fiber-reinforced composites used in soft robotics also require electrical conductivity to sense and respond to environmental conditions. At the same time, it is critical that the electrically conductive textile components have a low Young’s modulus and yield strength to minimize movement and accommodate significant deformation [12,13].
Electrically conductive fibers, yarns, and textile structures are in high demand as industrial materials for various applications such as filters, electrostatic discharge prevention, electromagnetic interference shielding, sterile and dust-free garments, and data transfer in clothing. In addition to these emerging applications for functional textiles, there is a growing demand for improved functional integration in textile-reinforced composites. In glass fiber (GF) or aramid fiber reinforced composites, the integration of carbon fiber (CF), hybrid yarns containing CF [14], or carbon nanotube (CNT)-coated GF [15] facilitates real-time strain and damage monitoring by electrical measurements, thereby adding multifunctionality to the composites.
However, due to the brittleness and low elongation of CF and conductive-coated GF, their use is primarily effective in low elongation ranges. Reproducible sensor properties cannot be reliably achieved with these composites at elongations exceeding 1.5%. Although textile-reinforced composites are predominantly manufactured with a thermoset matrix, thermoplastic matrix composites have been developed due to their advantages over thermoset composites, including lower density, indefinite storage of preforms, recyclability, improved impact and shock resistance, and environmental benefits. However, the high temperature and pressure required during consolidation (e.g., producing a GF/Polypropylene (GF/PP) thermoplastic composite requires a temperature of 220 °C) make the integration of functional components into thermoplastic composites more challenging than thermoset composites. Consequently, there is an urgent need for a conductive fiber with high elongation and temperature resistance that can withstand the impregnation and consolidation processes used in the production of thermoplastic composites.
Polyetheretherketone (PEEK), known for its excellent thermal, chemical, and mechanical properties (e.g., tensile strength of 90–120 MPa and elongation of 16%–80% [16]), is of great interest to the aerospace and automotive industries. PEEK is an organic semi-crystalline (~35%) and robust high molecular weight thermoplastic polymer [17]. With a glass transition temperature (Tg) of approximately 143 °C, a continuous service temperature of 260 °C, a melt temperature (Tm) of around 343 °C, and degradation onset temperatures between 575 °C and 580 °C, PEEK is one of the most thermally stable thermoplastic polymers [3]. Its exceptional thermal properties are due to the stability of its aromatic backbone, which consists of ether and carbonyl groups in the monomer unit.
The incorporation of a conductive layer onto non-conductive fibers, filaments, or yarns can be achieved by coating with electrically conductive materials such as CNT, intrinsically conductive polymers, or metal particles and coating agents. Among the various coating techniques, metallization has gained considerable importance in achieving high conductivity. This process involves the absorption of metal salts (e.g., gold, silver, copper, nickel) by fiber and their reduction to metallic conductive forms using a reducing agent [18].
This paper discusses the conductive coating of PEEK achieved by electroless plating of plasma-torch-pretreated PEEK filament yarns [18,19,20]. The process uses sensitizing, activating, and nickel-plating solutions to deposit a nickel layer on the PEEK surface. Given the high-temperature resistance and elongation properties of PEEK, and considering that the melting temperatures of most engineering thermoplastics are in the range of 160 °C to 270 °C, nickel-coated PEEK filament yarns are suitable not only for thermoset composites but also as functional components in textile-reinforced thermoplastic composites. The intended application of such nickel-coated PEEK is as a strain sensor in textile-reinforced composites.
Nickel plating of PEEK filament yarns presents significant challenges, primarily due to the highly stable chemical structure, inertness, and hydrophobic surface of PEEK [21], which complicate the chemical bonding of the nickel layer. To achieve a uniform and well-adhering nickel layer, the stable chemical structure must be modified, and the low surface energy increased to improve wettability by the nickel solution and adhesion of the nickel layer. Traditional methods to increase surface roughness and energy include laser treatment [3,22], low-pressure plasma treatment [23,24,25,26], atmospheric pressure plasma [2,27,28,29], or chemical modifications, such as sulphuric acid treatment [30,31,32,33,34], air abrasion, and piranha etching treatments (sulphuric acid to hydrogen peroxide) [32,35,36,37,38]. Low-pressure plasma works under vacuum conditions (around 0.1 to 10 mbar), requires a vacuum chamber, and is suitable for temperature-sensitive materials, such as textiles [39,40]. Atmospheric pressure plasma, on the other hand, operates at normal atmospheric pressure and does not require a vacuum chamber. Compared with low-pressure plasma, the control of the plasma parameters is less precise. More suitable for large surfaces than for filament yarns [39,41]. Laser treatment is more suitable than plasma treatment for high-precision, selective surface modification or structuring. Chemical changes to the textile surface are limited with laser treatment; it is used more for finishing, cutting, marking, and structuring in the textile sector. Laser treatment, therefore, mainly changes the physical structure of the surface, e.g., through microroughness or ablation. While sulfuric acid treatment can effectively increase surface roughness and adhesion, it is hazardous, environmentally unfriendly, and potentially damaging to the material. Another approach is to use special adhesion promoters or primers to improve nickel adhesion by creating an interfacial layer that enhances the interaction between the materials. Adhesion promoters offer a more controlled method but require additional processing steps, increased cost, and demand compatibility and durability considerations. Long-term adhesion durability can be problematic under extreme conditions such as high temperature, humidity, or chemical exposure. Therefore, plasma torch pretreatment of the PEEK surface was used in this study. Plasma torch pretreatment offers advantages over sulfuric acid treatment and adhesion promoters by avoiding the use of solvents or toxic chemicals, thus providing a safer, more environmentally friendly, and more efficient method that can improve adhesion properties. Compared with low-pressure plasma technology, plasma torch pretreatment does not require a vacuum while providing a more intense plasma source compared with atmospheric pressure plasma. Plasma torch pretreatment with Relyon PG-31 is therefore a flexible, cost-effective, and efficient solution for the pretreatment of temperature-resistant textiles such as PEEK. In addition, plasma torch pretreatment of PEEK is a “dry” process that requires significantly less water and chemicals than conventional wet processes. This leads to a reduction in water consumption and minimization of waste, which in turn contributes to a reduction in environmental impact [40].
This paper utilizes the electroless plating method with nickel–phosphorus alloy for coating plasma-torch-pretreated PEEK filament yarns. Nickel–phosphorus alloy [42,43] and nickel–boron alloy [44,45,46,47] are commonly used alloys for electroless plating. Although electroless nickel–boron alloy plating is expensive, has a slow deposition rate, and offers poor corrosion resistance, electroless nickel–phosphorus plating offers excellent performance with lower test costs and higher efficiency [48,49].

2. Experimental Part

2.1. Materials

Two types of PEEK filament yarns were used in this study. The first type was a PEEK multifilament with a diameter of 15.01 μm and a tenacity of 22.40 tex, supplied by ZYEX Ltd., Stone House, UK (Figure 1a). The second type was a PEEK monofilament yarn spun by melt spinning at the Institute of Textile Machinery and High-Performance Material Technology (Figure 1b). Tin(II) chloride (MARAWE GmbH & Co. KG, Regensburg, Germany) was used for sensitization, palladium chloride (MARAWE GmbH & Co. KG) for activation, and a nickel star with reducing agent (MARAWE GmbH & Co. KG) for nickel plating.

2.2. Pretreatment of the PEEK Filament Yarn Surface with a Plasma Torch

The untreated PEEK multifilament and monofilament yarns were subjected to continuous pretreatment using a plasma torch (Relyon Plasma Generator PG-31) from Relyon Plasma GmbH, Regensburg, Germany. The principle of operation of the plasma torch is shown schematically in Figure 2. The untreated PEEK filament yarns, initially on a spool, were continuously pretreated with the plasma torch and then wound onto another spool.
Table 1 presents the set and optimized process parameters for the plasma torch pretreatment of PEEK multifilament yarns. The effectiveness of the plasma torch pretreatment depends on several factors, including power, frequency, air flow rate, the distance between the yarn surface and the plasma torch tip, and the yarn running speed. A frequency of 54 kHz was found to be optimal for the plasma torch system used and the surface pretreatment of the PEEK fibers. This frequency enabled homogeneous plasma generation without material damage, improved plasma stability, and efficient PEEK surface pretreatment. Optimum ion and electron densities were ensured in this frequency range. To ensure stable, homogeneous, and effective plasma generation and uniform yarn flow, the air flow rate for the PEEK filament yarns was also optimized. Too low an air flow rate (<35 L/min) resulted in an insufficient amount of air for ionization, leading to an unstable and insufficiently hot plasma. Too high air flow rate (>35 L/min), on the other hand, resulted in turbulence and insufficient plasma focusing of the PEEK filaments. An air flow rate of 35 L/min gave the best ionization rate, homogeneous plasma, and uniform yarn feed, which were the optimum process parameters for the PEEK pretreatment. At the same time, the necessary plasma torch pretreatment temperature was achieved to treat the PEEK filament yarns without damaging them and to keep the plasma process stable.
The air flow rate also affected the rate of the chemical reactions on the fiber surface of the PEEK material. Too low an air flow rate resulted in slow and incomplete reactions, while too high a flow rate diluted the plasma too much. In addition, for the pretreatment of PEEK multifilament and monofilament yarns, the yarn running speed and the distance between the yarn surface and the plasma torch tip were optimized in order to improve the desired adhesion and wetting properties of the PEEK filament yarns without damaging the material. The yarn running speed and the distance between the PEEK fiber surface and the plasma torch tip are critical parameters that must be carefully matched to achieve optimum pretreatment of the PEEK filament yarns. Low yarn running speeds and short distances can result in a very intensive pretreatment of the PEEK fiber surface, where the surface is highly activated or even damaged. Many functional groups can also be incorporated, but this can result in thermal damage to the PEEK filament yarns. High yarn speeds at short distances can lead to targeted and effective activation of the surface without overloading the material. At high yarn speeds and long distances, surface pretreatment may not be sufficient as the short exposure time combined with the low intensity of the plasma results in only slight modification. The aim of pretreating PEEK filament yarn with a plasma torch is to improve the desired adhesion, and wetting properties are improved without damaging the material through excessive thermal effects.
Detailed experimental parameters for the plasma torch pretreatment of PEEK monofilament yarns are provided in Table 2.

2.3. Nickel Plating of Plasma-Torch-Pretreated PEEK Filament Yarns

After pretreatment, the PEEK filament yarns were subjected to nickel plating. Prior to plating, the plasma-torch-pretreated PEEK filament yarns were sensitized by immersion in a 1% stannous chloride (SnCl2) solution at room temperature for 1 h, followed by rinsing with deionized water. Activation was then carried out in a 1% palladium chloride (PdCl2) solution at room temperature for 1 h. The sensitized and activated PEEK filament yarns were then metalized at 30 ± 2 °C in a nickel-plating solution consisting of 4 mL reduction solution and 50 mL nickel star solution (Figure 3) by immersion for 10 min. The bath temperature was maintained at 70 ± 2 °C while the nickel solution temperature was 30 ± 2 °C. The nickel-plated PEEK monofilament and multifilament yarns were then dried at room temperature.

2.4. Characterization

2.4.1. Surface Morphology by Light Microscope

The surface morphology of both untreated and nickel-plated PEEK filament yarns was examined using a Zeiss optical microscope (Axio Scope.A1, Jena, Germany).

2.4.2. Investigation of the Surface Morphology and Determination of the Nickel Layer Thickness Using Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) was used to assess changes in the surface topography of plasma-torch-pretreated and nickel-plated PEEK filament yarns. The thickness of the nickel layer was determined from the SEM images using Axiovision Rel. 4.8 software.

2.4.3. Fourier Transform Infrared Spectroscopy (FT-IR)

The chemical composition of the PEEK filament yarns was analyzed by Fourier Transform Infrared Spectroscopy (FT-IR) using a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with an attenuated total internal reflection (ATR) unit. The spectral range covered was from 4000 cm−1 to 650 cm−1, with a resolution of up to 0.25 cm−1. Infrared spectroscopic studies were carried out on the plasma-torch-pretreated and nickel-plated PEEK filament yarns to qualitatively elucidate the structure and quantify the chemical functions, and the results were compared with those of untreated PEEK filament yarns.

2.4.4. Surface Free Energy Measurement

The total surface free energy of untreated and plasma-torch-pretreated PEEK filament yarns was determined using the Krüss K100 tensiometer (Hamburg, Germany), which measures the contact angle of the yarn according to DIN 55660-2 [50]. The polar and dispersed components of the total surface energy were calculated using the Owens–Wendt–Rabel–Kaelble method. Water was used as the polar liquid and ethylene glycol as the non-polar liquid. The surface tensions of selected liquids, including water and diiodomethane, are given in Table 3.

2.4.5. Tensile Strength Measurement

The fineness of the PEEK filament yarns was measured prior to the tensile elongation tests. Tensile elongation tests were performed on untreated, plasma-torch-pretreated, and nickel-plated PEEK filament yarns according to DIN EN ISO 2062 using a Zwicki 2.5 Junior (Zwick GmbH & Co. KG, Ulm, Germany). The tests were performed with a force of 2.5 N, a clamping length of 250 mm, and a clamp type 8188/Vulkollan/4 bar.

2.4.6. Measurement of Electrical Resistance

The electrical resistance of the nickel-plated PEEK filament yarns was measured at various lengths using a Keithley DAQ 6510/7700 (Solon, OH, USA) four-wire ohmmeter, which provides direct resistance readings. The nickel-plated PEEK filament yarn was secured at both ends with a holder (Figure 4). Ten measurements were taken to determine the average resistance for each length.

3. Results and Discussion

3.1. Chemical Composition Analysis of PEEK Filament Yarn

PEEK is a semi-crystalline thermoplastic polymer characterized by repeating units of aromatic rings (benzene rings), ethers (-O-), and ketones (-CO-) (Figure 5). The chemical structure of PEEK can be represented by the repetition of a specific monomer unit consisting of two ether linkages and one ketone group. The C-O bond in the phenyl ether group is weaker than the C-C bond in the phenyl group due to the electronegativity of oxygen and a binding energy of approximately 84 kcal/mol. In contrast, the C-C bond in the phenyl groups has a binding energy of 112 kcal/mol, and the C-C bond in the benzophenone group (C=O) is about 102 kcal/mol, contributing to its strength [51]. In order to increase the polar fraction of the total surface energy and to improve the surface roughness for better adhesion of the nickel layer, the PEEK surface was pretreated with a plasma torch prior to nickel plating. The plasma torch pretreatment generates high-energy plasma particles (e.g., ions, radicals) that can activate the PEEK surface, potentially modifying or breaking existing chemical bonds, including C-O bonds.
FT-IR spectra of untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarns (Figure 6) were analyzed. The FT-IR spectra of untreated PEEK multifilament yarns (Figure 6) show characteristic absorption bands corresponding to the functional groups and structural elements of the PEEK molecules. PEEK, with its repeating structure of aromatic rings, ether, and ketone groups, shows a strong absorption band around 1647.41 cm−1, indicative of C=O stretching vibrations, characteristic of ketone functionality. Medium and strong bands at about 1592.39 cm−1 and 1487.47 cm−1, respectively, are typical of the aromatic ring structures (C=C) [52]. Bands at approximately 1276.81 cm−1 and 1304.48 cm−1 correspond to C-O-C stretching vibrations for ether linkages [52,53]. The bands at 1185.42 cm−1 and 1157.13 cm−1 are due to asymmetric stretching vibrations of C-O-C groups, while the band at 1096.62 cm−1 reflects symmetric stretching vibrations of C-O-C groups, possibly including aromatic C-H in-plane bending vibrations. Bands between 700–900 cm−1 are due to bending vibrations of aromatic C-H bonds. Bands in the range of 2849.21 cm−1 and 2916.83 cm−1 are characteristic of C-H bonds in aromatic rings and aliphatic parts of the polymer [52,53]. The C-H stretching vibration at 3041.55 cm−1 indicates aromatic structures in PEEK (Figure 6).
Untreated PEEK filament yarns lack O-H stretching vibrations (hydroxyl groups) that are typically absorbed in this region, which were observed in the 3670.92 cm−1 band after plasma torch pretreatment (Figure 6). Plasma torch pretreatment introduces new functional groups, such as hydroxyl (-OH) groups, on the PEEK surface through oxygenation (Figure 7). Oxygen enrichment in a plasma produces reactive oxygen species such as O2⁺, O3, and OH radicals. These radicals can break the C-O or C=O bonds in the PEEK surface and cause oxidation reactions on the PEEK surface. Plasma torch pretreatment cleaves the top layers of the PEEK filament yarn, breaking down ether groups into radicals [54,55]. These radicals then oxidize to form oxygen-containing functional groups (OH, COO, and C=O) on the PEEK surface (Figure 7) [56].
The presence of these functional groups enhances the adhesion of the nickel layer to the PEEK surface. After nickel plating, the FT-IR spectra show fewer absorption bands typical of PEEK (Figure 6 and Figure 8), indicating that the PEEK surface is completely covered by the nickel layer. The bands in the range of 2849.21 cm−1 to 2916.83 cm−1 are typical of C-H bonds in aromatic rings and the aliphatic segments of PEEK. The C-H stretching vibration at 3041.55 cm−1 indicates unchanged aromatic structures in PEEK after nickel plating (see Figure 6 and Figure 8). A peak at 2400 cm−1 in the infrared spectrum usually indicates the presence of CO2, which is often due to the absorption of carbon dioxide from the environment. However, carbon dioxide is not chemically bound to the nickel-plated PEEK multifilament yarn but is usually seen as an impurity or physical adsorption in the spectrum (Figure 6).

3.2. Surface Free Energy Analysis

The total surface energy of untreated and plasma-torch-pretreated PEEK filament yarns was determined by measuring the diameters of the yarns at multiple points on five samples using a light microscope (Figure 9). The average diameter value was calculated for each sample.
The average diameters for untreated and plasma-torch-pretreated PEEK multifilament yarns are shown in Table 4. A slight increase of 0.8 ± 0.2 µm in diameter was observed in 100% plasma-torch-pretreated PEEK multifilament yarns (samples V4–V10, V13) compared with 80% plasma-torch-pretreated samples (V1–V3 and V11–V12).
The total surface energy (SE) of the PEEK multifilament yarns, together with their polar and dispersed fractions, was measured using a contact angle meter. The total SE was calculated from these measurements. The results are summarized in Table 5. No data were available for samples V1 and V4 due to problems with the distance between the PEEK multifilament yarn and the plasma torch tip being too small (2 cm), resulting in the yarn burning. Similarly, sample V5 also suffered from excessive plasma torch conductivity (100%) and a short distance (2.5 cm), resulting in damage. The contact angles for untreated PEEK filament yarns were the highest with water (89.70°) and diiodomethane (61.17°), indicating a hydrophobic surface. The total surface energy for untreated PEEK multifilament yarns was 29.1 mN/m, with a polar fraction of 3.24 mN/m, confirming its inert hydrophobic nature. In contrast, samples V2, V6, and V9, which had a high polar fraction, exhibited lower contact angles with both water and diiodomethane (Table 5). In particular, sample V2 achieved the highest polar fraction of 12.18 ± 5.12 mN/m and a total SE of 36.36 ± 4.52 mN/m, showing a 27% increase in polar fraction and a 20% increase in total SE compared with untreated yarns. This indicates that a yarn running speed of 3 m/min and a plasma torch distance of 2.5 cm for 80% plasma torch pretreatment are optimal for achieving a high polar fraction. Theoretically, the dwell time of the PEEK filament yarn in the plasma zone is longer at a low yarn speed of 1.5 m/min (V10). This can lead to more intensive treatment and greater activation of the surface through the formation of more functional groups (e.g., hydroxyl, carboxyl, or carbonyl groups). The functional groups thus formed on the surface typically improve the wettability and adhesion of the PEEK surface. However, if the yarn speed of 1.5 m/min was too slow, the PEEK material was thermally damaged. A high total surface energy was observed at a higher yarn speed of 3 m/min for samples V2 and V6. Here, the exposure time of the plasma to the PEEK surface was shorter. This shorter exposure was sufficient to increase the total surface energy and achieve some activation of the PEEK surface without risking damage or excessive modification of the PEEK filament surface. A high polar fraction of the total surface energy was found for sample V2 at 80% power and 2.5 cm distance, resulting in a uniform, gentle, and intensive plasma torch pretreatment of the PEEK fiber surface. At high power (100%), a large distance should be selected, such as samples V6 and V9, to achieve a gentle treatment and achieve a high polar fraction of the total surface free energy.
For the PEEK monofilament yarns, the elliptical shape required the diameter to be calculated from the circumference. The average diameter for the PEEK monofilament yarns is shown in Table 6 and was used to determine the surface energy.
The contact angle values for untreated PEEK monofilament yarns were 80.78 ± 2.03° with water and 57.87 ± 1.82° with diiodomethane, similar to the untreated multifilament yarns (Table 7). For the monofilament yarn, increasing the yarn running speed (2 m/min < 2.5 m/min < 3 m/min) resulted in a higher polar fraction of the total surface energy, and reducing the distance between the yarn and the plasma torch tip (3 cm → 2.5 cm → 2 cm) also increased the polar fraction (7.45 mN/m < 7.70 mN/m < 11.78 mN/m). Sample V4 showed the lowest contact angles with water (42.40 ± 4.12°) and diiodomethane (32.10 ± 1.01°) and the highest polar fraction (18.01 ± 7.54 mN/m) with a total surface energy of 59.35 ± 9.44 mN/m.
In summary, the plasma torch pretreatment of PEEK monofilament yarns requires a 100% power setting, a treatment distance of 1.5 cm, and a yarn travel speed of 3 m/min to achieve a high polar fraction (18.01 ± 7.54 mN/m) of the total surface energy (59.35 ± 9.44 mN/m), compared with PEEK multifilament yarns. These optimized process parameters were selected for further plasma torch pretreatment of the PEEK monofilament yarns.

3.3. Surface Morphology

The surface topography of untreated and plasma-torch-pretreated PEEK multifilament yarns (Figure 10) and PEEK monofilament yarns (Figure 11) was examined by optical microscopy. No significant changes in the surface structure were observed between the untreated and plasma-torch-pretreated yarns, indicating that both types of PEEK filament yarns maintained a smooth surface. However, the untreated and plasma-torch-pretreated PEEK monofilament yarns showed some macroscopic roughness due to indentations from the filament production process (Figure 11).
For untreated PEEK multifilament yarns, the surface appeared smooth, but magnified images (Figure 12) revealed impurities, probably surface dirt or residues of sizing agents.
With a large distance of 3 cm between the yarn surface and the plasma torch tip, the plasma torch pretreatment was insufficient, and sizing agents and impurities remained on sample V3 (Figure 13c). In contrast, samples V1 (2 cm) and V2 (2.5 cm) exhibited a surface with grooves but no visible sizing agent or impurities (Figure 13a,b).
The SEM images of plasma-torch-pretreated PEEK multifilament yarns, influenced by different yarn running speeds (1.5 m/min (V10), 2 m/min (V9), and 2.5 m/min (V8)), showed that slower yarn running speeds resulted in better cleaning of the yarn surface (Figure 14).
The power of the plasma torch also affected the surface treatment. SEM images of PEEK multifilament yarns treated at 80% (Figure 15a) and 100% power showed no significant changes or damage to the PEEK surface (Figure 15b).
For untreated PEEK monofilament yarns, SEM confirmed a smooth surface with no sizing agents detected (Figure 16). The surface was smooth, with some underprinted areas visible by optical microscopy.
However, after the plasma torch pretreatment, no firmly underprinted areas were observed on the PEEK monofilament (Figure 17). Notably, the cross-sectional diameter of the PEEK monofilament yarns decreased compared with the untreated samples, probably due to residual stretching effects from the plasma torch heat (Table 6). Sample V6 had a diameter of 1391.78 µm after pretreatment, which was 1455.90 µm thinner than the untreated yarn.
After plasma torch pretreatment, the high polar fraction samples V2 (PEEK multifilament) and V4 (PEEK monofilament) were nickel-plated. SEM images of the nickel-plated PEEK multifilament yarn (sample V2) showed complete coverage of the nickel layer (Figure 18a). The nickel particles appeared round (Figure 18b), growing homogeneously and evenly distributed along the length of the filament yarn (Figure 18).
For the nickel-plated PEEK monofilament yarns (sample V4), SEM images at different magnifications (200× and 10,000×) showed that all yarns were coated with a nickel layer. However, the magnified images showed that the nickel layer was agglomerated and had partially burst (Figure 19). This indicates poor adhesion of the nickel layer to the PEEK monofilament yarn compared with the multifilament yarn. The low adhesion may be due to inadequate sensitization with SnCl2 and activation with PdCl2, resulting in unstable nickel nuclei and poor nickel particle growth. The high internal stress in the nickel layer resulted in visible cracks and dents (Figure 19).
The main aim of the plasma torch pretreatment was to remove organic impurities and change the chemical composition of the PEEK surface by introducing functional groups. This increased the polar component of the total surface energy of PEEK, which significantly improved the wettability and adhesion of the PEEK filament yarns. In conclusion, plasma torch pretreatment improved surface cleanliness and increased the polar fraction of surface energy for PEEK filament yarns. While the treatment effectively cleaned and activated the surface of PEEK multifilament yarns, the adhesion of nickel plating on PEEK monofilament yarns was less successful due to insufficient surface sensitization and activation. The challenge was the surface activation of the PEEK monofilament yarn compared with the PEEK multifilament yarn. The compactly bound polymer chains of PEEK monofilament yarns with a smooth and unstructured surface made it difficult to attach reactive groups and provided fewer anchoring points for the nickel coating, which also made the adhesion of the nickel layer more difficult. PEEK monofilament yarns have less contact area for the plasma, making surface activation less effective. The reduced introduction of functional groups to the surface reduces the chemical affinity for the nickel coating.

3.4. Nickel Layer Thickness

SEM images of the cross-section of nickel-plated PEEK filament yarns (Figure 20) show a complete nickel coating on both PEEK multifilament and monofilament yarns. The PEEK multifilament yarns have a round cross-section (Figure 20a), while the PEEK monofilament yarns have an oval cross-section (Figure 20b).
The average thickness of the nickel layer was found to be 1.25 µm for the multifilament yarn and 3.36 µm for the monofilament yarn (Table 8). This indicates that the monofilament yarns have a significantly thicker nickel coating compared with the multifilament yarns.

3.5. Tensile Strength

No major changes in elongation at break were observed for PEEK multifilament yarns after plasma torch pretreatment and nickel plating compared with untreated yarns (Table 9). However, there was a 14% increase in tensile strength after plasma torch pretreatment, whereas nickel plating resulted in a 35% decrease in tensile strength compared with untreated samples. The mechanical properties of PEEK monofilament yarns were not tested due to insufficient sample availability.
Figure 21 illustrates that PEEK multifilament yarns generally exhibit a non-linear elastic–plastic relationship. Initially, in the elastic phase, the stress–strain relationship is linear. After plasma torch pretreatment, the entry into the plastic phase occurs earlier, as indicated by flatter curves. The yield point shifts to the lower left, indicating a reduced elastic deformation range and an earlier transition to plastic deformation. The stepped breakage curves are due to the different properties of the individual yarns within the multifilament bundle, resulting in uneven breakage. The improvement in tensile strength could therefore be due both to the increased crystallinity caused by the heat and to chemical changes on the PEEK surface. PEEK is a high-temperature-resistant thermoplastic that can withstand temperatures up to about 340 °C without significant structural changes. At higher temperatures, particularly above 500 °C, the material can degrade or change its crystalline structure [40]. When the plasma torch reaches temperatures in this range, the crystallinity of PEEK can be increased by heat treatment alone, thereby improving tensile strength. Higher crystallinity generally means that the material becomes harder and more resistant to mechanical stress. In addition to the thermal effects, the plasma torch causes chemical changes on the surface of the PEEK due to the ionized gases. This can lead to an increase in the adhesive forces between the polymer chains or to a strengthening of the surface structure, which has a positive effect on the mechanical properties. The ionization effect can reorganize the molecular chains and harden the surface layers, resulting in improved tensile properties [40]. These chemical changes can be caused by the formation of C-H or C-O bonds, which increase the strength of the material. The temperature of the plasma torch can be very high near the material, but due to the short treatment time and thermal diffusion, the material itself cannot be fully heated to the same temperature. If the PEEK yarn was not heated much during the treatment, this would indicate that the improvements were due to the plasma chemistry rather than the heat treatment.

3.6. Electrical Resistivity

The electrical resistivity of nickel-plated PEEK filament yarns as a function of yarn length is shown in Figure 22. The electrical resistivity increases linearly with yarn length. The PEEK monofilament yarns exhibit better electrical resistivity due to their larger surface area compared with the multifilament yarns.

4. Conclusions

In this study, PEEK multifilament and monofilament yarns were pretreated with a plasma torch, which significantly improved the polar and dispersed fractions of their total surface energy, thereby improving wettability for nickel plating. Plasma torch pretreatment increased the polar fraction of the total surface energy by 27% for multifilament yarns and 21% for monofilament yarns. This improvement is attributed to the generation of free radicals or hydrophilic groups (e.g., OH, COOH) on the PEEK surface, as confirmed by FTIR spectra.
Electroless nickel plating was successfully applied to the plasma-torch-pretreated yarns. The PEEK multifilament yarns achieved a uniform nickel coating with round and evenly distributed nickel particles. The FTIR spectra confirmed complete coverage of the PEEK surface. The average nickel layer thickness was 1.25 µm for multifilament yarns and 3.36 µm for monofilament yarns.
Despite no significant change in elongation at break, the tensile strength of PEEK multifilament yarns increased by 14% after plasma torch pretreatment but decreased by 35% after nickel plating. The mechanical properties of PEEK monofilament yarns were not evaluated due to sample limitations. The electrical resistivity of the nickel-plated yarns increased linearly with length, with monofilament yarns showing higher resistivity than multifilament yarns.
Overall, the plasma torch pretreatment effectively improved the surface properties of the PEEK yarns, allowing for better adhesion of the nickel plating, although it affected the mechanical strength and electrical resistivity.

Author Contributions

Investigation, resources, and data curation, L.A.F.; formal analysis, data curation, and writing—original draft, T.O.; writing—review and editing, C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by German Research Foundation (DFG, Deutsche Forschungsgemeinschaft: Project ID CH174/40-249 3 and as part of Germany’s Excellence Strategy—EXC 2050/1—Project ID 390696704—Cluster of Excellence “Centre for Tactile Internet 250 with Human-in-the-Loop” (CeTI) of Technische Universität Dresden.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors thank the German Research Foundation (DFG) and the German Excellence Strategy—EXC 2050/1 for the financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Chen, G.; Li, Y.; Bick, M.; Chen, J. Smart Textiles for Electricity Generation. Chem. Rev. 2020, 120, 3668–3720. [Google Scholar] [CrossRef] [PubMed]
  2. Verma, S.; Sharma, N.; Kango, S.; Sharma, S. Developments of PEEK (Polyetheretherketone) as a biomedical material: A focused review. Eur. Polym. J. 2021, 147, 110295. [Google Scholar] [CrossRef]
  3. Sang, S.; Yuan, K.; Lv, S.; Wang, T.; Zhao, G.; Liu, P.; Gao, D.; Li, X.; Qiao, Y. Study on laser processing of medical polyether ether ketone surface texture and its improvement on service performance. AIP Adv. 2024, 14, 035349. [Google Scholar] [CrossRef]
  4. Soukup, R.; Hamacek, A.; Mracek, L.; Reboun, J. Textile based temperature and humidity sensor elements for healthcare applications. In Electronics Technology (ISSE), Proceedings of the 2014 37th International Spring Seminar in Electronics Technology (ISSE), Dresden, Germany, 7–11 May 2014; IEEE: Piscataway, NJ, USA, 2014; pp. 407–411. ISBN 978-1-4799-4455-2. [Google Scholar]
  5. Khan, S.; Ali, S.; Bermak, A. Recent Developments in Printing Flexible and Wearable Sensing Electronics for Healthcare Applications. Sensors 2019, 19, 1230. [Google Scholar] [CrossRef] [PubMed]
  6. Zhao, H.; Su, R.; Teng, L.; Tian, Q.; Han, F.; Li, H.; Cao, Z.; Xie, R.; Li, G.; Liu, X.; et al. Recent advances in flexible and wearable sensors for monitoring chemical molecules. Nanoscale 2022, 14, 1653–1669. [Google Scholar] [CrossRef]
  7. Hatamie, A.; Angizi, S.; Kumar, S.; Pandey, C.M.; Simchi, A.; Willander, M.; Malhotra, B.D. Review—Textile Based Chemical and Physical Sensors for Healthcare Monitoring. J. Electrochem. Soc. 2020, 167, 37546. [Google Scholar] [CrossRef]
  8. Nan, N.; He, J.; You, X.; Sun, X.; Zhou, Y.; Qi, K.; Shao, W.; Liu, F.; Chu, Y.; Ding, B. A Stretchable, Highly Sensitive, and Multimodal Mechanical Fabric Sensor Based on Electrospun Conductive Nanofiber Yarn for Wearable Electronics. Adv. Mater. Technol. 2019, 4, 1800338. [Google Scholar] [CrossRef]
  9. Shuvo, I.I.; Shah, A.; Dagdeviren, C. Electronic Textile Sensors for Decoding Vital Body Signals: State-of-the-Art Review on Characterizations and Recommendations. Adv. Intell. Syst. 2022, 4, 2100223. [Google Scholar] [CrossRef]
  10. Fan, W.; He, Q.; Meng, K.; Tan, X.; Zhou, Z.; Zhang, G.; Yang, J.; Wang, Z.L. Machine-knitted washable sensor array textile for precise epidermal physiological signal monitoring. Sci. Adv. 2020, 6, eaay2840. [Google Scholar] [CrossRef]
  11. Wu, R.; Ma, L.; Hou, C.; Meng, Z.; Guo, W.; Yu, W.; Yu, R.; Hu, F.; Liu, X.Y. Silk Composite Electronic Textile Sensor for High Space Precision 2D Combo Temperature-Pressure Sensing. Small 2019, 15, e1901558. [Google Scholar] [CrossRef]
  12. Hasan, M.M.B.; Diestel, O.; Cherif, C. Electro-mechanical properties of friction spun conductive hybrid yarns made of carbon filaments for composites. Text. Res. J. 2011, 81, 1603–1616. [Google Scholar] [CrossRef]
  13. Onggar, T.; Häntzsche, E.; Hund, R.-D.; Cherif, C. Multilayered Glass Filament Yarn Surfaces as Sensor Yarn for In-situ Monitoring of Textile-reinforced Thermoplastic Composites. Fibers Polym. 2019, 20, 1945–1957. [Google Scholar] [CrossRef]
  14. Hasan, M.M.B.; Matthes, A.; Schneider, P.; Cherif, C. Application of carbon filament (CF) for structural health monitoring of textile reinforced thermoplastic composites. Mater. Technol. 2011, 26, 128–134. [Google Scholar] [CrossRef]
  15. Rausch, J.; Mäder, E. Health monitoring in continuous glass fibre reinforced thermoplastics: Manufacturing and application of interphase sensors based on carbon nanotubes. Compos. Sci. Technol. 2010, 70, 1589–1596. [Google Scholar] [CrossRef]
  16. Cherif, C. (Ed.) Textile Werkstoffe für den Leichtbau: Techniken—Verfahren—Materialien—Eigenschaften; Springer: Berlin/Heidelberg, Germany, 2011; ISBN 9783642179921. [Google Scholar]
  17. Patel, P.; Hull, T.R.; Lyon, R.E.; Stoliarov, S.I.; Walters, R.N.; Crowley, S.; Safronava, N. Investigation of the thermal decomposition and flammability of PEEK and its carbon and glass-fibre composites. Polym. Degrad. Stab. 2011, 96, 12–22. [Google Scholar] [CrossRef]
  18. Akbarov, D.; Baymuratov, B.; Westbroek, P.; Akbarov, R.; de Clerck, K.; Kiekens, P. Development of electroconductive polyacrylonitrile fibers through chemical metallization and galvanisation. J. Appl. Electrochem. 2006, 36, 411–418. [Google Scholar] [CrossRef]
  19. Onggar, T.; Cheng, T.; Hund, H.; Hund, R.-D.; Cherif, C. Silvering of inert polyethylene terephthalate textile materials: The factors of adjustment on silver release, part II. Text. Res. J. 2011, 81, 1940–1948. [Google Scholar] [CrossRef]
  20. Onggar, T.; Cheng, T.; Hund, H.; Hund, R.-D.; Cherif, C. Metallization of inert polyethylene terephthalate textile materials: Wet-chemical silvering with natural and synthetic polyamine, part I. Text. Res. J. 2011, 81, 2017–2032. [Google Scholar] [CrossRef]
  21. Jha, S.; Bhowmik, S.; Bhatnagar, N.; Bhattacharya, N.K.; Deka, U.; Iqbal, H.M.S.; Benedictus, R. Experimental investigation into the effect of adhesion properties of PEEK modified by atmospheric pressure plasma and low pressure plasma. J. Appl. Polym. Sci. 2010, 118, 173–179. [Google Scholar] [CrossRef]
  22. Riveiro, A.; Soto, R.; Comesaña, R.; Boutinguiza, M.; Del Val, J.; Quintero, F.; Lusquiños, F.; Pou, J. Laser surface modification of PEEK. Appl. Surf. Sci. 2012, 258, 9437–9442. [Google Scholar] [CrossRef]
  23. Fu, Q.; Gabriel, M.; Schmidt, F.; Müller, W.-D.; Schwitalla, A.D. The impact of different low-pressure plasma types on the physical, chemical and biological surface properties of PEEK. Dent. Mater. 2021, 37, e15–e22. [Google Scholar] [CrossRef] [PubMed]
  24. Liston, E.M. Plasma Treatment for Improved Bonding: A Review. J. Adhes. 1989, 30, 199–218. [Google Scholar] [CrossRef]
  25. Piel, A. Plasma Physics: An Introduction to Laboratory, Space, and Fusion Plasmas, 2nd ed.; Springer International Publishing: Cham, Switzerland, 2017; ISBN 9783319634272. [Google Scholar]
  26. Yoshida, S.; Hagiwara, K.; Hasebe, T.; Hotta, A. Surface modification of polymers by plasma treatments for the enhancement of biocompatibility and controlled drug release. Surf. Coat. Technol. 2013, 233, 99–107. [Google Scholar] [CrossRef]
  27. Hasan, M.; Cherif, C.; Foisal, A.; Onggar, T.; Hund, R.D.; Nocke, A. Development of conductive coated Polyether ether ketone (PEEK) filament for structural health monitoring of composites. Compos. Sci. Technol. 2013, 88, 76–83. [Google Scholar] [CrossRef]
  28. Singh, S.; Prakash, C.; Wang, H.; Yu, X.; Ramakrishna, S. Plasma treatment of polyether-ether-ketone: A means of obtaining desirable biomedical characteristics. Eur. Polym. J. 2019, 118, 561–577. [Google Scholar] [CrossRef]
  29. Fricke, K.; Reuter, S.; Schroder, D.; Schulz-von der Gathen, V.; Weltmann, K.-D.; Woedtke, T. von. Investigation of Surface Etching of Poly(Ether Ether Ketone) by Atmospheric-Pressure Plasmas. IEEE Trans. Plasma Sci. 2012, 40, 2900–2911. [Google Scholar] [CrossRef]
  30. Choudhury, S.S.; Pandey, M.; Bhattacharya, S. Recent Developments in Surface Modification of PEEK Polymer for Industrial Applications: A Critical Review. Rev. Adhes. Adhes. 2022, 9, 401–433. [Google Scholar] [CrossRef]
  31. Binhasan, M.; Alhamdan, M.M.; Al-Aali, K.A.; Vohra, F.; Abduljabbar, T. Shear bond characteristics and surface roughness of poly-ether-ether-ketone treated with contemporary surface treatment regimes bonded to composite resin. Photodiagn. Photodyn. Ther. 2022, 38, 102765. [Google Scholar] [CrossRef]
  32. dos Santos, F.S.F.; Vieira, M.; da Silva, H.N.; Tomás, H.; Fook, M.V.L. Surface Bioactivation of Polyether Ether Ketone (PEEK) by Sulfuric Acid and Piranha Solution: Influence of the Modification Route in Capacity for Inducing Cell Growth. Biomolecules 2021, 11, 1260. [Google Scholar] [CrossRef]
  33. Dua, R.; Sharufa, O.; Terry, J.; Dunn, W.; Khurana, I.; Vadivel, J.; Zhang, Y.; Donahue, H.J. Surface modification of Polyether-ether-ketone for enhanced cell response: A chemical etching approach. Front. Bioeng. Biotechnol. 2023, 11, 1202499. [Google Scholar] [CrossRef]
  34. Jaafar, J.; Ismail, A.F.; Mustafa, A. Physicochemical study of poly(ether ether ketone) electrolyte membranes sulfonated with mixtures of fuming sulfuric acid and sulfuric acid for direct methanol fuel cell application. Mater. Sci. Eng. A 2007, 460–461, 475–484. [Google Scholar] [CrossRef]
  35. Alsmael, M.A.; Al-Khafaji, A.M. Evaluation of High-Performance Polyether Ether Ketone Polymer Treated with Piranha Solution and Epigallocatechin-3-Gallate Coating. BioMed Res. Int. 2024, 2024, 1741539. [Google Scholar] [CrossRef] [PubMed]
  36. Alsmael, M.A.; Mohammed Al-Khafaji, A. Improving Surface Properties of PEEK for Dental Applications by Using Piranha Solution. Int. J. Dent. 2023, 2023, 7840601. [Google Scholar] [CrossRef] [PubMed]
  37. Emara, M.H.; Wahsh, M.M.; Nour, M.M. Effect of manufacturing techniques and surface treatment of custom-made polyetheretherketone posts on the shear bond strength to resin cement versus customized fiber posts. Mansoura J. Dent. 2023, 10, 219–228. [Google Scholar] [CrossRef]
  38. Santos, F.S.F.d.; Rodrigues, J.F.B.; da Silva, M.C.; Barreto, M.E.V.; da Silva, H.N.; de Lima Silva, S.M.; Fook, M.V.L. Use of Piranha Solution as An Alternative Route to Promote Bioactivation of PEEK Surface with Low Functionalization Times. Molecules 2023, 28, 74. [Google Scholar] [CrossRef]
  39. Sarmadi, M. Advantages and disadvantages of plasma treatment of textile materials. In Proceedings of the 21st International Symposium on Plasma Chemistry (ISPC 21), Cairns, QLD, Australia, 4–9 August 2013. [Google Scholar]
  40. Malik, T.; Parmar, S. Use of Plasma Technology in Textiles Use of Plasma Technology in Textiles. Available online: https://www.researchgate.net/publication/326669117_Use_Of_Plasma_Technology_In_Textiles_Use_Of_Plasma_Technology_In_Textiles (accessed on 23 October 2024).
  41. Sparavigna, A. Plasma treatment advantages for textiles. arXiv 2008, arXiv:0801.3727. [Google Scholar]
  42. Duan, Q.; Sun, Q.; Lu, Y. Design of metalized polyether-ether-ether-ketone based on semi-additive manufacture for 5G applications: From bottom-up assembly to selective electroplating. J. Mater. Chem. C 2024, 12, 8873–8884. [Google Scholar] [CrossRef]
  43. Gerullis, S.; Gerschütz, A.; Beier, O.; Kretzschmar, B.S.M.; Pfuch, A.; Schmidt, J.; Grünler, B. Modification of powders by atmospheric pressure plasma and embedding into nickel coatings. Surf. Innov. 2020, 8, 304–314. [Google Scholar] [CrossRef]
  44. Delaunois, F.; Vitry, V.; Bonin, L. (Eds.) Electroless Nickel Plating: Fundamentals to Applications, 1st ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, USA, 2020; ISBN 9780429466274. [Google Scholar]
  45. Gao, S.; Wu, C.; Yang, X.; Cheng, J.; Kang, R. Study on Adhesion Properties and Process Parameters of Electroless Deposited Ni-P Alloy for PEEK and Its Modified Materials. Coatings 2023, 13, 388. [Google Scholar] [CrossRef]
  46. Sarkar, S.; Baranwal, R.K.; Biswas, C.; Majumdar, G.; Haider, J. Optimization of process parameters for electroless Ni–Co–P coating deposition to maximize micro-hardness. Mater. Res. Express 2019, 6, 46415. [Google Scholar] [CrossRef]
  47. Yazdani, S.; Vitry, V. RSM models approach for optimization of the mechanical properties of electroless Ni-B-nanodiamond coating: An experimental and molecular dynamic simulation study. Surf. Coat. Technol. 2023, 452, 129133. [Google Scholar] [CrossRef]
  48. Rajaguru, J.C.; Duke, M.; Au, C. Investigation of electroless nickel plating on rapid prototyping material of acrylic resin. Rapid Prototyp. J. 2016, 22, 162–169. [Google Scholar] [CrossRef]
  49. Reade, G.W.; Kerr, C.; Des Barker, B.; Walsh, F.C. The Importance of Substrate Surface Condition in Controlling the Porosity of Electroless Nickel Deposits. Trans. IMF 1998, 76, 149–155. [Google Scholar] [CrossRef]
  50. DIN Deutsches Institut für Normung e.V.; DIN German Institute for Standardization. DIN 55660-2; Beschichtungsstoffe-Benetzbarkeit-Teil 2: Bestimmung der Freien Oberflächenenergie Fester Oberflächen durch Messung des Kontaktwinkels; DIN Media GmbH: Berlin, Germany, 2024; Available online: https://nautos.de/O9G/search/item-detail/DE30035626 (accessed on 23 October 2024).
  51. Narushima, K.; Tasaka, S.; Inagaki, N. Surface Modification of Poly(aryl ether ether ketone) by Pulsed Oxygen Plasma. Jpn. J. Appl. Phys. 2002, 41, 6506–6516. [Google Scholar] [CrossRef]
  52. Lu, C.; Qiu, S.; Lu, X.; Wang, J.; Xiao, L.; Zheng, T.; Wang, X.; Zhang, D. Enhancing the Interfacial Strength of Carbon Fiber/Poly(ether ether ketone) Hybrid Composites by Plasma Treatments. Polymers 2019, 11, 753. [Google Scholar] [CrossRef] [PubMed]
  53. Al-Mufti, S.M.S.; Ali, M.M.; Rizvi, S.J.A. Synthesis and structural properties of sulfonated poly ether ether ketone (SPEEK) and Poly ether ether ketone (PEEK). In Proceedings of the 3rd International Conference on Condensed Matter and Applied Physics (ICC-2019), Bikaner, India, 14–15 October 2019; AIP Publishing: Melville, NY, USA, 2020; p. 20009. [Google Scholar]
  54. Flejszar, M.; Chmielarz, P. Surface Modifications of Poly(Ether Ether Ketone) via Polymerization Methods-Current Status and Future Prospects. Materials 2020, 13, 999. [Google Scholar] [CrossRef]
  55. Primc, G.; Mozetič, M. Surface Modification of Polymers by Plasma Treatment for Appropriate Adhesion of Coatings. Materials 2024, 17, 1494. [Google Scholar] [CrossRef]
  56. Sundriyal, P.; Pandey, M.; Bhattacharya, S. Plasma-assisted surface alteration of industrial polymers for improved adhesive bonding. Int. J. Adhes. Adhes. 2020, 101, 102626. [Google Scholar] [CrossRef]
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Figure 1. PEEK multifilament yarn (a) and PEEK monofilament yarn (b).
Figure 1. PEEK multifilament yarn (a) and PEEK monofilament yarn (b).
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Figure 2. Schematic representation of the simplified continuous plasma torch pretreatment system for PEEK filament yarns.
Figure 2. Schematic representation of the simplified continuous plasma torch pretreatment system for PEEK filament yarns.
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Figure 3. Electroless galvanic nickel plating of PEEK filament yarns.
Figure 3. Electroless galvanic nickel plating of PEEK filament yarns.
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Figure 4. Four-wire resistance measurement of nickel-plated PEEK filament yarns.
Figure 4. Four-wire resistance measurement of nickel-plated PEEK filament yarns.
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Figure 5. Chemical structure of PEEK.
Figure 5. Chemical structure of PEEK.
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Figure 6. FTIR spectrum of the untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarns.
Figure 6. FTIR spectrum of the untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarns.
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Figure 7. Possible degradation reactions on the surface of PEEK filaments during plasma torch pretreatment.
Figure 7. Possible degradation reactions on the surface of PEEK filaments during plasma torch pretreatment.
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Figure 8. FTIR spectrum of the untreated, plasma-torch-pretreated, and nickel-plated PEEK monofilament yarns.
Figure 8. FTIR spectrum of the untreated, plasma-torch-pretreated, and nickel-plated PEEK monofilament yarns.
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Figure 9. Determination of filament yarn diameter by light microscopy.
Figure 9. Determination of filament yarn diameter by light microscopy.
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Figure 10. Light microscopy image of untreated (a) and plasma-torch-pretreated (b) PEEK multifilament yarn (sample V2).
Figure 10. Light microscopy image of untreated (a) and plasma-torch-pretreated (b) PEEK multifilament yarn (sample V2).
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Figure 11. Light microscopy image of untreated (a) and plasma-torch-pretreated (b) PEEK monofilament yarn (sample V3).
Figure 11. Light microscopy image of untreated (a) and plasma-torch-pretreated (b) PEEK monofilament yarn (sample V3).
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Figure 12. Scanning electron microscopy (SEM) image (1000× (a) and 5000× (b)) of untreated PEEK multifilament yarn.
Figure 12. Scanning electron microscopy (SEM) image (1000× (a) and 5000× (b)) of untreated PEEK multifilament yarn.
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Figure 13. SEM image (5000×) of plasma-torch-pretreated PEEK multifilament yarn; influence of increasing treatment distance (sample V1: 2 cm (a); sample V2: 2.5 cm (b) and sample V3: 3 cm (c)) between the plasma torch tip and PEEK surface on the surface.
Figure 13. SEM image (5000×) of plasma-torch-pretreated PEEK multifilament yarn; influence of increasing treatment distance (sample V1: 2 cm (a); sample V2: 2.5 cm (b) and sample V3: 3 cm (c)) between the plasma torch tip and PEEK surface on the surface.
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Figure 14. SEM image (5000×) of plasma-torch-pretreated PEEK multifilament yarn; influence of increasing yarn speed (sample V10: 1.5 m/min (a), sample V9: 2 m/min (b) and sample V8: 2.5 m/min (c)) during plasma torch pretreatment on the PEEK surface.
Figure 14. SEM image (5000×) of plasma-torch-pretreated PEEK multifilament yarn; influence of increasing yarn speed (sample V10: 1.5 m/min (a), sample V9: 2 m/min (b) and sample V8: 2.5 m/min (c)) during plasma torch pretreatment on the PEEK surface.
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Figure 15. SEM image (5000×) of plasma-torch-pretreated PEEK multifilament yarn; influence of plasma torch power (sample V2 80% (a) and V12 100% (b)) during plasma torch pretreatment on the PEEK surface.
Figure 15. SEM image (5000×) of plasma-torch-pretreated PEEK multifilament yarn; influence of plasma torch power (sample V2 80% (a) and V12 100% (b)) during plasma torch pretreatment on the PEEK surface.
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Figure 16. SEM image (200× (a) and 5000× (b)) of untreated PEEK monofilament yarn.
Figure 16. SEM image (200× (a) and 5000× (b)) of untreated PEEK monofilament yarn.
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Figure 17. SEM image (200× (a) and 500× (b)) of plasma-torch-pretreated PEEK monofilament yarn (sample V4).
Figure 17. SEM image (200× (a) and 500× (b)) of plasma-torch-pretreated PEEK monofilament yarn (sample V4).
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Figure 18. SEM image (1000× (a) and 20,000× (b)) of nickel-plated PEEK multifilament yarn surface.
Figure 18. SEM image (1000× (a) and 20,000× (b)) of nickel-plated PEEK multifilament yarn surface.
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Figure 19. SEM image (200× (a) and 10,000× (b)) of nickel-plated PEEK monofilament yarn.
Figure 19. SEM image (200× (a) and 10,000× (b)) of nickel-plated PEEK monofilament yarn.
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Figure 20. SEM image of the cross-section of nickel-plated PEEK multifilament yarn (a) and PEEK monofilament yarn (b).
Figure 20. SEM image of the cross-section of nickel-plated PEEK multifilament yarn (a) and PEEK monofilament yarn (b).
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Figure 21. Tensile properties of untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarn. Lines of different colours mean that several measurements were carried out on one sample.
Figure 21. Tensile properties of untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarn. Lines of different colours mean that several measurements were carried out on one sample.
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Figure 22. Measured electrical resistivity of nickel-plated PEEK multifilament yarn and monofilament yarn as a function of yarn length.
Figure 22. Measured electrical resistivity of nickel-plated PEEK multifilament yarn and monofilament yarn as a function of yarn length.
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Table 1. Process parameters set for the plasma torch pretreatment of PEEK multifilament yarns.
Table 1. Process parameters set for the plasma torch pretreatment of PEEK multifilament yarns.
No.Power (%)Frequency (kHz)Actual Power (W)Air Permeability (L/min)Yarn Run Speed (m/min)Distance Between the Tip of the Plasma Torch and the Yarn Surface (cm)
V18054650 ± 103532
V28054650 ± 103532.5
V38054650 ± 103533
V410054750 ± 103532
V510054750 ± 103532.5
V610054750 ± 103533
V710054750 ± 103532.75
V810054750 ± 10352.52.75
V910054750 ± 103522.75
V1010054750 ± 10351.52.75
V118054650 ± 103532.75
V128054650 ± 103523
V1310054750 ± 103523
Table 2. Process parameters for plasma torch pretreatment of PEEK monofilament yarns.
Table 2. Process parameters for plasma torch pretreatment of PEEK monofilament yarns.
No.Power (%)Frequency (kHz)Actual Power (W)Air Permeability (L/min)Yarn Run Speed (m/min)Distance Between the Tip of the Plasma Torch and the Yarn Surface (cm)
V110054750 ± 103532
V210054750 ± 103532.5
V310054750 ± 103533
V410054750 ± 103531.5
V510054750 ± 10352.52
V610054750 ± 103522
Table 3. Surface tension of selected water and diiodomethane.
Table 3. Surface tension of selected water and diiodomethane.
LiquidSurface Tension (mN/m)Dispersive Part (mN/m)Polar Part (mN/m)
Water72.821.851.0
Diiodomethane50.850.80.0
Table 4. Determined average diameter of PEEK multifilament yarns.
Table 4. Determined average diameter of PEEK multifilament yarns.
UntreatedPlasma-Torch-Pretreated
SampleDiameter (μm)SampleDiameter (μm)
V038.51 ± 0.67V138.67 ± 0.87
V240.44 ± 0.59
V340.43 ± 0.76
V441.05 ± 0.72
V641.38 ± 0.88
V740.77 ± 0.97
V840.81 ± 0.83
V941.02 ± 0.79
V1040.53 ± 0.62
V1139.06 ± 0.75
V1240.12 ± 0.78
V1342.06 ± 0.92
Table 5. Contact angle and surface energy (SE) of untreated and plasma-torch-pretreated PEEK multifilament yarns.
Table 5. Contact angle and surface energy (SE) of untreated and plasma-torch-pretreated PEEK multifilament yarns.
VariantContact Angle θ°Surface Energy (mN/m)
θW°ΘD°Polar Part
γp		Dispersive Part
γd		Total SE,
γgesamt
Untreated89.7 ± 1.3361.17 ± 3.383.24 ± 1.4125.86 ± 5.1529.1 ± 6.56
V271.61 ± 8.8341.62 ± 3.0612.18 ± 5.1224.17 ± 6.1336.36 ± 4.52
V384.30 ± 2.6648.57 ± 3.812.98 ± 1.9635.17 ± 7.0238.15 ± 8.98
V672.38 ± 6.3431.5 ± 5.797.48 ± 6.8535.67 ± 14.943.15 ± 11.50
V782.19 ± 7.3447.29 ± 6.684.05 ± 5.7433.37 ± 15.0937.43 ± 12.83
V887.12 ± 1.8661.38 ± 8.575.25 ± 3.8921.94 ± 11.4227.19 ± 15.31
V980.19 ± 4.5954.99 ± 7.258.96 ± 6.1820.66 ± 10.3929.62 ± 16.56
V1088.51 ± 5.0952.00 ± 4.911.51 ± 2.4937.47 ± 11,7138.98 ± 14.20
V1174.11 ± 4.5525.66 ± 11.494.59 ± 4.0844.29 ± 13.4348.87 ± 13.51
V1280.26 ± 8,5645.26 ± 5.514.88 ± 7.0832.88 ± 15.6937.76 ± 9.77
V1383.73 ± 2.5344.42 ± 6.042.31 ± 1.9340.00 ± 9.6142.30 ± 11.54
Table 6. Monofilament yarn dimensions.
Table 6. Monofilament yarn dimensions.
No.Width (μm)High (μm)Extent (μm)Calculated Diameter (μm)
Untreated1755.36 ± 1.761121.41 ± 2.114573.84 ± 2.761455.90 ± 1.45
V11687.69 ± 1.451105.00 ± 1.984434.62 ± 2.441411.58 ± 1.53
V21714.67 ± 2.011100.26 ± 1.794474,51 ± 3.111424.28 ± 1.14
V31712.26 ± 1.821118.37 ± 1.814495.40 ± 2.561430.93 ± 1.52
V41674.81 ± 1.241106.21 ± 1.694414.19 ± 2.551405.08 ± 1.51
V51658.43 ± 1.691106.37 ± 1.824386.32 ± 2.481396.21 ± 1.23
V61670.25 ± 1.651081.76 ± 1.664372.40 ± 3.161391.78 ± 1.34
Table 7. Contact angle and surface energy (SE) of untreated and plasma-torch-pretreated PEEK monofilament yarns.
Table 7. Contact angle and surface energy (SE) of untreated and plasma-torch-pretreated PEEK monofilament yarns.
PEEK SampleContact Angle θ°Surface Energy (mN/m)
θW°θE°Polar Part
γp		Dispersive Part
γd		Total SE,
γgesamt
Untreated80.78 ± 2.0357.87 ± 1.823.83 ± 2.5118.07 ± 3.2127.89 ± 5.72
V155.35 ± 4.4238.52 ± 2.8611.78 ± 8.2233.39 ± 3.7445.17 ± 11.96
V277.23 ± 2.4668.25 ± 6.597.70 ± 3.4921.16 ± 1.9129.30 ± 5.39
V374.79 ± 1.7364.46 ± 2.087.45 ± 7.2920.14 ± 4.7027.59 ± 11.99
V442.40 ± 4.1232.10 ± 1.0118.01 ± 7.5441.34 ± 1.9059.35 ± 9.44
V558.17 ± 0.8530.73 ± 6.848.52 ± 3.8934.47 ± 4.6942.99 ± 8.58
V663.18 ± 5.1525.88 ± 8.285.38 ± 7.4137.48 ± 9.0642.86 ± 16.47
Table 8. Measured diameter of untreated, plasma-torch-pretreated, and nickel-plated PEEK monofilament and multifilament yarn.
Table 8. Measured diameter of untreated, plasma-torch-pretreated, and nickel-plated PEEK monofilament and multifilament yarn.
PEEKAverage Diameter (µm)Nickel Layer Thickness (µm)
UntreatedPlasma Torch TreatedNickeled
Multifilament yarn38.70 ± 0.3240.43 ± 0.2841.66 ± 0.161.25 ± 0.21
Monofilament yarn1455.21 ± 0.411404.28 ± 0.321407.64 ± 0.523.36 ± 0.52
Table 9. Tensile properties of untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarn.
Table 9. Tensile properties of untreated, plasma-torch-pretreated, and nickel-plated PEEK multifilament yarn.
PEEKBreaking Force (N)Tenacity (cN/tex)Elongation (%)
Untreated14.0 ± 0.6329.5 ± 1.3331.0 ± 1.93
Plasma-torch-pretreated16.3 ± 0.5334.7 ± 1.1232.8 ± 1.52
Nickel-plated16.0 ± 0.6722.6 ± 0.9433.3 ± 1.80
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Onggar, T.; Frankenbach, L.A.; Cherif, C. Investigations of the Interface Design of Polyetheretherketone Filament Yarn Considering Plasma Torch Treatment. Coatings 2024, 14, 1424. https://doi.org/10.3390/coatings14111424

AMA Style

Onggar T, Frankenbach LA, Cherif C. Investigations of the Interface Design of Polyetheretherketone Filament Yarn Considering Plasma Torch Treatment. Coatings. 2024; 14(11):1424. https://doi.org/10.3390/coatings14111424

Chicago/Turabian Style

Onggar, Toty, Leopold Alexander Frankenbach, and Chokri Cherif. 2024. "Investigations of the Interface Design of Polyetheretherketone Filament Yarn Considering Plasma Torch Treatment" Coatings 14, no. 11: 1424. https://doi.org/10.3390/coatings14111424

APA Style

Onggar, T., Frankenbach, L. A., & Cherif, C. (2024). Investigations of the Interface Design of Polyetheretherketone Filament Yarn Considering Plasma Torch Treatment. Coatings, 14(11), 1424. https://doi.org/10.3390/coatings14111424

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