Highly Conductive Fiber with Waterproof and Self-Cleaning Properties for Textile Electronics

Background

Moisture and contamination can cause electrical breakdown of textile devices and their interconnections, and wet electronic textiles can also shock the user. The authors note that multilayer encapsulation adds thickness and mechanical mismatch, that water-repellent fluorinated fabric coatings were applied to nonconductive fabrics, and that a wash-durable conductive polymer complex had a resistance of 1.9 MΩ/cm, too high for advanced E-textile applications. Their earlier Ag-polymer nanocomposite fibers were conductive and durable but could be wetted and lose conductivity.

Design and approach

A commercial multifilament Kevlar fiber (Grade I K49, 100D), chosen for its high modulus, is coated with SBS and soaked in silver trifluoroacetate (CF3COOAg, 20 wt % in ethanol) for 20 min; the absorbed Ag+ ions are reduced with hydrazine hydrate, and this cycle is repeated eight times. The Ag NP clusters roughen the surface, and a SAM is then vapor-deposited at 50 °C for 2 h and annealed at 80 °C for 30 min. Four SAM reagents with different head groups and chain lengths were compared: 1-decanethiol (DT), 1H,1H,2H,2H-perfluorohexanethiol (PFHT), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and PFDT. The combination of roughness and low surface energy is intended to repel water and protect the Ag NPs from oxidation.

How it was evaluated

The fibers were characterized by electron microscopy with energy-dispersive spectroscopy, thermogravimetric analysis, atomic force microscopy, contact angle measurement, infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. Resistance was tracked during submersion in water and in diluted nitric acid at pH 3, during 150° folding cycles and during laundry cycles of fibers woven or sewn into fabric. Function was tested by lighting a light-emitting diode (LED) through fibers immersed in water, by rhodamine 6G staining, dust and ash removal, everyday liquids on coated canvas textile, and AATCC 118, 130 and 193 textile test methods.

Key results

After eight absorption and reduction cycles, the fiber resistance was 0.11 Ω/cm, and the Ag content measured by thermogravimetry rose from 61.4 to 83.9 wt % with increasing cycles. The coating changed the diameter from 0.2 to 0.3 mm and the weight from 0.15 to 0.3 g/m, and the Ag NPs raised the root-mean-square roughness from 0.675 to 1.171 μm.

All four SAMs made the coated Kevlar fabric hydrophobic (water contact angle over 90°), but only PFDT gave a contact angle over 150°. The thiol SAMs shifted the Ag binding energy more than the silane FDTS, which the authors relate to denser, more stable monolayers on Ag. In water and acid, the fluorine-terminated SAMs limited the resistance increase, with PFDT showing the least change. In pH 3 nitric acid, the resistance of the untreated fiber rose from 0.11 to 12.8 Ω/cm after 60 min, and after 1 h only the untreated fiber showed Ag oxide peaks at 32.1°, 57.4°, 67.4°, 74.4° and 76.6° in X-ray diffraction.

In the underwater LED test, the LED connected through the untreated fiber dimmed markedly after 4 h, whereas the PFDT-treated fiber kept the LED bright for longer. The PFDT-treated fiber showed no rhodamine 6G fluorescence after dipping, shed ash with a single water droplet without residue, and the coated textile beaded milk, coffee, orange juice and wine.

Resistance increased only from 0.11 to 0.15 Ω/cm after 10,000 folds at 150°. After 20 washing cycles (30 min each with a magnetic stirrer at 600 rpm), the untreated fiber rose from 0.11 to 11.44 Ω/cm, reported as a 10 400% increase, and was physically damaged by swelling, whereas the PFDT-treated fiber changed from 0.11 to 0.28 Ω/cm. In AATCC 118 the fabric repelled oil, milk, coffee, wine and orange juice with a contact angle of about 150°, AATCC 130 showed removal of oil stains by washing, and AATCC 193 showed repellency to water/isopropyl alcohol solutions.

How it connects to other research

Park and colleagues made highly stretchable circuits from a composite of silver nanoparticles and elastomeric fibers, and this paper cites that work for the ion-dipole interaction that drives absorption of the silver precursor; here the Ag NP-SBS composite is a coating on a high-modulus Kevlar fiber, and a fluorinated monolayer adds water repellency and protection against oxidation.