Fluorination of Carbon Fiber Surface by Electrolysis in the Molten KF-HF System

Abstract

A basket type carbon electrode had the dimensions of 15 cm is diameter, a depth of 8 cm and a wall thickness of 0.5 cm with fifty holes (φ5cm) on the wall. A coil of iron net (5 mesh) was used as a cathode. An electrolytic cell made of maild iron had the dimensions of 30 cm in diameter, a depth of 17 cm and a wall thickness of 1 cm. The electrolyte used was 15 kg of KHF₂, and the bath temperature was kept at 300±10°C. A felt of carbon fiber (60g) was charged in the basket type electrode which was fluorinated in advance. Fluorination was conducted for about 4∼5 h with 10 A (about 11mA⋅cm^-2). The colors of fluorinated fibers varied from black through brown, blue, yellow, violet and gray to white with an increase in the degree of fluorination. The specific gravities of carbon fibers after the electrolysis were from 1.62 to 2.50g⋅cm-3. The heats for wetting for water at 25°C were from 2.06 to 0.59J⋅cm-2, and the value decreased with increasing the degree of fluorination. The contact angle against water was about 140°. The thickness of fluorinated fibers observed from 0.25 to 0.45μm by Scanning Electron Microscope. Identification of the products was not comfirmed yet, but formation of carbon fluoride was considered to be evident.