Conducting polymer coated fabrics for potential applications in microwave frequencies : a study of electromagnetic properties

The microwave reflection, transmission and complex permittivity of paratoluene-2-sulfonic acid doped conducting polypyrrole (PPy/pTSA) coated Nylon-Lycra textiles in the 1-18 GHz frequency were investigated. The real part of permittivity increased with polymerization time and dopant concentration, reaching a plateau at certain dopant concentration and polymerization time. The imaginary part of permittivity showed a frequency dependent change throughout the tested range. All the samples had higher values of absorption than reflection. The total electromagnetic shielding effectiveness exceeded 80% for the highly pTSA doped samples coated for 3 hours.

Characterization of conducting polymer coated fabrics at microwave frequencies

Purpose – The purpose of this paper is to investigate microwave reflection, transmission, and complex permittivity of p-toluene-2-sulfonic acid doped conducting polypyrrole coated nylon-lycra textiles in the 1-18?GHz frequency with a view to potential applications in the interaction of electromagnetic radiation with such coated fabrics.

Design/methodology/approach – The chemical polymerization of pyrrole is achieved by an oxidant, ferric chloride and doped with p-toluene sulfonic acid (pTSA) to enhance the conductivity and improve stability. Permittivity of the conducting textile substrates is performed using a free space transmission method accompanied by a mathematical diffraction reduction method.

Findings – The real part of permittivity increases with polymerization time and dopant concentration, reaching a plateau at certain dopant concentration and polymerization time. The imaginary part of permittivity shows a frequency dependent change throughout the test range. All the samples have higher values of absorption than reflection. The total electromagnetic shielding effectiveness exceeds 80 percent for the highly pTSA doped samples coated for 3?h.

Originality/value – A non-contact, non-destructive free space method thin flexible specimens to be tested with high accuracy across large frequency range. The non-destructive nature of the experiments enables investigation of the stability of the microwave transmission, reflection, absorption and complex permittivity values. Moreover, mathematical removal of the diffraction enables higher accuracy.

Generating heat from conducting polypyrrole-coated PET fabrics

Heating effects in polypyrrole-coated polyethyleneterephthalate (PET)-Lycra® fabrics were studied. Chemical synthesis was employed to coat the PET fabrics by polypyrrole using ferric chloride as oxidant and antraquinone- 2-sulfonic acid (AQSA) and naphthalene sulfonic acid (NSA) as dopants. The coated fabrics exhibited reasonable electrical stability, possessed high electrical conductivity, and were effective in heat generation. Surface resistance of polypyrrole-coated fabrics ranged from approximately 150 to 500 /square. Different connections between conductive fabrics and the power source were examined. When subjected to a constant voltage of 24 V, the current transmitted through the fabric decreased about 10% in 72 h. An increase in resistance of conductive fabrics subjected to constant voltage was observed

Development of a cooling fabric from conducting polymer coated fibres: proof of concept

It is supposed that there should be a thermal electric effect if a dc current is applied across two dissimilar conducting polymers, similar to so called “Peltier effect” in metals or semiconductors. However, this hypothesis has not been tested on conducting polymers and using these materials to make cooling fabrics has never been attempted before. Polypyrrole coated fabrics were used to test the hypothesis in this preliminary study. Seebeck and the Peltier effects were proven to exist. However, thermoelectricity effect between two conducting polymer coated fabric samples was only about 10 μV/°C. Cooling effect by conductive polymer powder was achieved but performance was unsteady due to electrical degradation of the conducting polymer. Nevertheless, the concept was demonstrated and the development of a cooling fabric is possible.

Conductive fabrics for heating applications

By coating textiles with electrically conductive organic polymers, we are able to produce functional, intelligent fabrics. These fabrics can be utilised in applications such as gas sensors, actuators, electromagnetic shielding, radar absorption, selected frequency filtering in indoor wireless applications, and heating applications where vital parts of the body can be heated without embedding any wiring through the fabric.

Heat generation in fabrics coated with the conductive polymer polypyrrole was investigated. The fabrics were coated by chemical synthesis methods by oxidizing the pyrrole monomer in the presence of the fabric substrate. Ferric chloride was selected as the oxidizing agent and anthraquinone-2-sulfonic acid (AQSA) sodium salt monohydrate as the dopant.

Conductive fabrics were characterized by resistivity measurements, scanning electron microscopy, thermal imaging, current transmission over a period of time and calculations of power density per unit area. Effects of reaction conditions on the electrical properties and heat generated are presented. Polypyrrole coated fabrics were stable and possessed high electrical conductivity. Resistivity values ranged from 100-500 ohms/square depending on the reaction parameters. When subjected to a constant voltage of 24V, the polypyrrole coated polyester-Lycra® fabric doped with AQSA reached a maximum temperature of 42°C and a power density per unit area of 430 W/m² was achieved.

Superhydrophobic fabrics prepared by silica coating

In this work, a silica sol prepared by co-hydrolysis and co-condensation of TEGS (Tetraethylrthosilicate) and alkyl silane under alkaline condition was applied to polyester, wool, and cotton fabrics. The water contact angle measurement indicated considerable increase in the surface hydrophobicity of the sol-treated fabrics. Five different alky silanes were used, namely methyltritthoxysilane (MTES), pheryl triethoxysilane (PTES), n-octyltricthoxysilane (OTES), hexadecyl trimethoxysilan (HDTMS), and tridecafluorooctyl triethoxysilane (FAS), and the water contact anglc (CA) for the coated fabrics ranged between 1300 and 174°. The alkyl silane used influenced the CA valuc, and the silica coating from FAS, HDTMS and PTES snowed CA value greater than ISO', indicating the formation of superhydrophobicity. The fabric coated by the fluorinated silica (TEOS/FAS) has a water contact angle as high as 174°. The treated polyester fabric showed a slightly higher CA value than the wool and cotton fabrics, under the same coating condition.
The coating surface was characterized by SEM, EDX, TEM, FTlR, XPS and AFM. The results showed that silica nanoparticles with thc sizc in the range of 50-ISOnm werc formed in the cohydrolyzed silica sol, and these particles had a core-shell structure with many alkyl groups gathering on the surface region. The formation of superhydrophobic surface was attributed to the nano-structured surface coating with a low surface energy.

Novel conducting polymer films and coated textiles

This research developed non-hazardous methods for coating wool with conductive polymers for thermal and anti-static clothing. Conductive polymers are black in colour, thus the synthesis of new conductive polymers was required to produce coloured or fluorescent conductive textile. Cross-linked conductive polymers were also synthesised to increase their durability.

Functional fabrics with controlled wettability

In this study, fabrics with novel super water-repellent and unidirectional water-transfer functions have been prepared using one-step wet-chemical coating processes. The mechanism of directional water transport across the fabrics having gradient superhydrophobicity to hydrophilicity has been elucidated.

Photochromic fabrics from hybrid silica surface coatings

In this study, hybrid silica prepared by a sol-gel technique and doped with a photochromic dye was used to produce photochromic coatings on fabric surfaces. The coated fabrics showed a strong photochromic effect with very fast optical response speed. Good coating adhesion was obtained on wool fabrics. The photostability of the photochromic fabrics was improved by three different processes: adding a photo stabilizer, adjusting the surface wettability and sealing off the dye-containing pores with additional silica coating. Four UV stabilizers were added separately to the photochromic silica coatings to investigate their influence on the photostability and photochromic behaviour. The addition of UV stabilizers retarded the photochromic response and reduced photochromic absorption, but increased photochromic lifetime. Among the four UV stabilizers studied, the quencher resulted in the best improvement to the photostability with minimal reduction in the photochromic absorption. Increasing the hydrophobicity of the coating, and sealing-off the dye-containing pores were also found to improve photostability.

Directional water-transport fabrics achieved by wettablity gradient from superhydrophobicity to hydrophilicity

In this study, we demonstrate that fabrics having a wettability gradient from superhydrophobic to hydrophilic through the thickness direction show a novel directional water transfer effect: water can transfer only from the superhydrophobic to the hydrophilic side, but not in the opposite direction unless an external force is applied. A sol-gel technology was used to prepare a superhydrophobic coating on fabrics, and the coated fabrics showed water contact-angle as high as 165°. When the coated fabric was subjected to a photochemistry treatment from one fabric side, the irradiated surface turned hydrophilic permanently, while the back side still maintained the superhydrophobicity. The treated fabric can transfer water droplet rapidly from hydrophobic to hydrophilic side, and the pressure allowing water breakthrough the fabric is different considerably between the two fabric sides. The directional water transfer effect is also affected by the wettability gradient. Such a directional water transfer coating may be useful to develop new functional fabrics for defence applications.

Unidirectional water transfer effect from fabrics having a superhydrophobic-to-hydrophillic gradient

In this study, we demonstrate that fabrics having a wettability gradient from superhydrophobic to hydrophilic through the thickness direction show a novel directional water transfer effect: water can transfer from the superhydrophobic to the hydrophilic side, but not in the opposite direction unless an external force is applied. A sol-gel technology was used to prepare a nano-structured superhydrophobic coating on fabrics, and the coated fabrics showed water contact-angle as high as 165 degrees. When the coated fabric was subjected to a photochemistry treatment from one fabric side, the irradiated surface turned hydrophilic permanently, while the back side still maintained the superhydrophobicity. The treated fabric can transfer water droplet rapidly from hydrophobic to hydrophilic side, and the pressure allowing water breakthrough the fabric are different considerably between the two fabric sides. The directional water transfer effect is affected by the wettability gradient. Such a directional water transfer coating may be useful to develop new functional fabrics for defence applications.

Antibacterial activity of capsaicin-coated wool fabric

Fabrics made from natural fibers, such as wool and cotton, are susceptible to attacks from micro-organisms, which may damage the fabrics and harm the human body. Antimicrobial finishing of natural textile products may involve harmful and non-environmentally friendly chemicals. In this study, a natural antibacterial agent, capsaicin, was coated on the surface of wool fabrics by a sol-gel process. The antibacterial properties of coated fabrics were evaluated against test bacteria Escherichia coli according to the American Association of Textile Chemists and Colorists (AATCC) method and standard American Society for Testing and Materials (ASTM) E2149-01. Compared with the control group (sol-gel coated fabric without capsaicin), the capsaicin-coated fabric inhibited bacterial growth markedly after 24 hours incubation at 37°C. The antibacterial efficiency after laundry washes was also investigated. Good durability to washing of capsaicin on fabric was achieved by the sol-gel coating technique.

Photochromic fabrics with improved optical performance

In this PhD project, the photostability and colour depth of photochromic fabrics prepared by a hybrid silica coating were improved and the relationship between the pore structure of the silica coatings and the photochromic properties of the coated fabrics was investigated.

Unidirectional water transfer effect from fabrics having a superhydrophobic-to-hydophilic gradient

In this study, we demonstrate that fabrics having a wettability gradient from superhydrophobic to hydrophilic through the thickness direction show a novel directional water transfer effect: water can transfer from the superhydrophobic to the hydrophilic side, but not in the opposite direction unless an external force is applied. A sol–gel technology was used to prepare a nano-structured superhydrophobic coating on fabrics, and the coated fabrics showed water contact-angle as high as 165 degrees. When the coated fabric was subjected to a photochemistry treatment from one fabric side, the irradiated surface turned hydrophilic permanently, while the back side still maintained the superhydrophobicity. The treated fabric can transfer water droplet rapidly from hydrophobic to hydrophilic side, and the pressure allowing water breakthrough the fabric are different considerably between the two fabric sides. The directional water transfer effect is affected by the wettability gradient. Such a directional water transfer coating may be useful to develop new functional fabrics for defence applications.

Robust, self-healing superamphiphobic fabrics prepared by two-step coating of fluoro-containing polymer, fluoroalkyl silane, and modified silica nanoparticles

A robust, superamphiphobic fabric with a novel self-healing ability to autorepair from chemical damage is prepared by a two-step wet-chemistry coating technique using an easily available material system consisting of poly(vinylidene fluoride-co-hexafluoropropylene), fluoroalkyl silane, and modified silica nanoparticles. The coated fabrics can withstand at least 600 cycles of standard laundry and 8000 cycles of abrasion without apparently changing the superamphiphobicity. The coating is also very stable to strong acid/base, ozone, and boiling treatments. After being damaged chemically, the coating can restore its super liquid-repellent properties by a short-time heating treatment or room temperature ageing. This simple but novel and effective coating system may be useful for the development of robust protective clothing for various applications.