1Rapid Hydrolysis of Waste and Scrap PA6 Textiles to ε-Caprolactam

The great challenge for the chemical recycling of waste and scrap (Was) polyamide 6 (PA6) textiles is to achieve chemical recycling with short processes, low costs, and low energy consumption. An effective method was developed to convert waste PA6 textiles into high-purity ε-caprolactam (CPL) with a short process, low reaction temperature, and low water consumption. This work described in detail the feasibility of water as a catalyst for PA6 hydrolysis under subcritical water, enabling the degradation of roughly 100% of virgin PA6 (vPA6) and Was PA6 textiles into 89.9 and 91.6% of CPL under optimized conditions, i.e., 300 °C of temperature, a mass ratio (H2O/PA6) of 11:1, and 60 min of reaction time, respectively. Moreover, the purity of produced CPL was as high as 99.62%, similar to that of fresh CPL. Furthermore, the n-value of oligomers in liquid products ranged from 0 to 5. In contrast, the concentrations of cyclic oligomers (Cn) with n-value (n ≥ 1) and linear oligomers (Ln) with n-value (n ≥ 2) gradually decreased. In addition, the total contents of Cn were far less than those of Ln. In summary, Was PA6 textiles were easier to degrade than vPA6, and the concentration of the target monomer was higher. Overall, this study offers a green, elementary, and cost-effective technology to recycle and reuse CPL for hydrolysis of Was PA6 textiles.