Efficient aramid fiber monomerization using alkaline subcritical water

•Technora® was monomerized by subcritical water+NaOH at 250°C and 4 MPa for 3 h.•Filtration, extraction, and precipitation gave pure monomers with 88%–99% yield.•An adapted cylindrical-surface model describes Technora® hydrolysis kinetics. To establish the chemical recycling of copoly(p-phenylene-3,4′-oxydiphenylene terephthalamide) fibers (i.e., Technora®), their hydrolysis in pure supercritical, pure subcritical, and alkaline subcritical water was investigated, and an efficient purification method to recover the constituent monomers in high yields and > 99% purity was developed. Technora® hydrolysis in pure supercritical water required high temperatures, thus leading to monomer degradation and hence low monomer yields. Similarly, Technora® hydrolysis to monomers was incomplete in pure subcritical water, which also resulted in low monomer yields. In contrast, the addition of NaOH to subcritical water promoted fiber decomposition, rendered hydrolysis irreversible, and prevented degradation of the produced monomers by allowing the reaction to proceed at lower temperature. Thus, when subcritical water containing 10 equivalents of NaOH per Technora® unit was used at 250°C and 4 MPa for 3 h, the fibers decomposed with 100% efficiency, and the monomers were recovered in 88–99% yields. The rate of Technora® hydrolysis in a neutral medium of either pure supercritical or subcritical water was represented well using the cylindrical-surface reaction model. This model was adjusted to describe the reaction in an alkaline medium, i.e., subcritical water containing NaOH, by considering the decrease in both NaOH concentration and Technora® surface area.