RATES OF ACTIVATION AND SCANNING ELECTRON-MICROSCOPY OF POLYARYLAMIDE-DERIVED CHARS

Fibrous activated carbons were prepared from Kevlar(R) (poly (p-phenylene terephthalamide)) and Nomex(R) (poly (m-phenylene isophthalamide)) by carbonisation in nitrogen and subsequent activation in steam of carbon dioxide to varying burn-offs at 1123 K. The resultant chars were characterised by activation rate studies, scanning electron microscopy and, in the case of the Nomex chars, energy dispersive X-ray analysis. It was found for both the polyarylamide-derived chars that significant differences existed between the chars activated in steam and carbon dioxide. Steam reacts much faster than carbon dioxide with these chars at elevated temperatures. This may be due to its smaller molecular size, resulting in an increase in diffusion and accessibility in and out of the constricted char structure, its higher intrinsic reactivity, or to specific reactions with metallic residues. The catalytic activity of metallic residues, rather than the crystallinity differences between the two polymers, is the suggested reason why Kevlar chars are nearly three times more reactive than Nomex chars. The appearance of the external surfaces of all the chars can be related to the percentage burn-off and the activation gas used. The presence of metallic residues leads to pitting on activation, which increases with progressive burn-off. At high burn-offs, the action of metallic residues in steam and carbon dioxide results in homogeneous and localised surface pits, respectively.