Mild Acid-Assisted Separation and Closed-Loop Monomer-Level Regeneration of Polyamide 6 from Waste Wool/PA6 Carpet Blends

Efficient component separation is a prerequisite for high-value recycling of blended textile waste. Herein, a two-stage metal-free recycling process for wool/polyamide 6 (PA6) carpet waste is developed, combining mild acid-assisted selective separation with monomer-level PA6 regeneration. Acetic acid pretreatment at 80 °C removes surface dyes, followed by formic acid treatment for selective PA6 dissolution. Both acids are efficiently recyclable via low-temperature rotary evaporation, with an average PA6 recovery yield over 75% across five consecutive cycles. Recovered wool preserves its original scale structure and tensile strength comparable to virgin fibers, and reclaimed PA6 maintains its macromolecular and crystalline structure, verifying the mildness of the separation protocol. The recovered PA6 is further depolymerized via a microwave-assisted acetic anhydride/organobase system, where the phosphazene base tBuP4 achieves a maximum N-acetyl-ε-caprolactam yield of 74.6%. Depolymerization efficiency is governed by catalyst basicity, nucleophilicity, and acetic anhydride-mediated polyamide backbone activation. After deacetylation, the resulting ε-caprolactam is repolymerized with the same tBuP4 catalyst, producing regenerated PA6 with chemical structure and thermal properties nearly identical to virgin PA6.