Nomex polyaramid as a precursor for activated carbon fibres by phosphoric acid activation. Temperature and time effects

Abstract

This work concerns the preparation of activated carbon fibres (ACFs) by phosphoric acid activation of Nomex [poly( m-phenylene isophthalamide)] polyaramid fibres. The transformations involved in Nomex pyrolysis in the presence of phosphoric acid as a function of temperature were studied by infrared spectroscopy. Then, ACFs were prepared by impregnating Nomex with phosphoric acid, followed by pyrolysis and washing with water. The effects of activation temperature and activation time were addressed on the basis of the porous texture of the resulting ACFs, which was characterized by N 2 and CO 2 adsorption. Phosphoric acid has a strong effect on Nomex degradation, lowering the temperature range at which the decomposition takes place. Porosity development is at a maximum in the temperature interval corresponding to the degradation of the polymer (400–500 °C). Further trends in porosity at higher treatment temperatures have been justified on the basis of two concurrent phenomena with opposite effects: contraction of the porous network by rupture of phosphate and polyphosphate bonds, and release of P compounds by volatilisation. The effect of the soaking time was similar to that of increasing the activation temperature in the 700–800 °C range. Results suggest the practical feasibility of phosphoric acid activation of Nomex polyaramid fibres. ACFs are obtained with high yields and they exhibit a well-developed porosity restricted to the micropore range.