Effect of PPTA pre-impregnation with phosphoric acid on the porous texture of carbons prepared by CO₂ activation of PPTA chars

Activated carbon fibers (ACFs) were prepared by CO₂ gasification to different burn-offs of chars from the polyaramid fiber, poly(p-phenylene terephthalamide) (PPTA) pre-impregnated with 5 wt.% H₃PO₄. Thermogravimetric analysis showed that the addition of phosphoric acid produces a significant increase in the yield of solid residue. While the char yield of the pure polymer at 1000 °C amounted to 39.5 wt.%, it rose to 52.8 wt.% in the case of 5 wt.% H₃PO₄-impregnated PPTA. The chars and derived ACFs were characterized by physical adsorption of N₂ (-196 °C) and CO₂ (0 °C). No significant differences were found as concerns chars porosity. At equivalent burn-offs, the surface area of the ACFs prepared by physical activation with CO₂ was up to 70% higher in the case of PPTA pre-impregnated with phosphoric acid, the pore size distribution being displaced towards somewhat wider pores within the 2-10 nm range. These results are in contrast with those obtained previously with the poly(m-phenylene isophthalamide) (PMIA) polymer, where H₃PO₄ pre-impregnation did not significantly affect the porosity of chars and ACFs.