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 (H 2 O/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.

The hydrophilic copolyester polyethylene terephthalate (PET) (ENCDP-X) was successfully synthesized by chemical modification consisting of copolymerization and blending and the comonomers, including sodium isophthalate-5-sulfonate (SIPE), polyethylene glycol (PEG), 2,2-dimethyl-1,3-propanediol (NPG) and matting agent TiO2 with different content. Moreover, the structural characterization of sequential structure, crystallization and thermal properties were studied. The results showed that the comonomers were successfully embedded in the copolyester, the actual molar ratio in the copolyester was consistent with the relative feed ratio and the degree of randomness was calculated to be 0.99, showing that the random copolymers synthesized during the melt polycondensation process and the chemical structure was roughly consistent with the expected molecular chain sequence structure. The thermal parameters of the modified copolyester, containing the glass transition temperature (Tg), melting point (Tm), crystallinity (Xc) and thermal degradation temperature, were decreased, and the cold crystallization temperature (Tc) was increased. In addition, with the increasing of the TiO2 content, it improves the thermal performance of the copolyester and it is beneficial to processing and application. The above conclusion is further verified by non-isothermal kinetic analysis. In addition, the copolyester exhibited the better hydrophilicity than pure PET.

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Terephthalic acid and ethylene glycol were used to prepare poly(ethylene terephthalate)-co-amide salts with adipate pentamethylenediamine (AP salts) as the modified monomer, improving the hygroscopicity and softness of poly(ethylene terephthalate) fibers. One of the facets of concern is that the yellowing index of poly(ethylene terephthalate)-co-amide salts increases from 3.8 to 13.8 than poly(ethylene terephthalate) and increases from 2.9 to 11.7 for fibers, limiting its acceptance in textile applications. The convention underpinning the yellowing originated from thermal oxidation and Schiff base reaction in the blend of polyester/polyamide, generating yellowing substances with conjugated imine structure. However, in this paper, an unconventional yellowing mechanism was discovered, attributed to the reaction of AP salts and acetaldehyde (a by-product of poly(ethylene terephthalate)). The structure of yellowing substances was characterized and confirmed the role of conjugated diketoamine (O=C–CH2–CONH–) and conjugated enamines (O=C–C=C–NH–). It is difficult for fiber applications to overcome the yellowing issue. Nevertheless, an inhibiting method for the yellowing phenomenon was provided, end-capping technology, by a clearer understanding of the yellowing processes. Thus, poly(ethylene terephthalate)/APA was prepared with the end-capping product carboxyl-terminated diamide (APA) as the modified monomer, which was synthesized by the reaction of AP salts and adipic acid. The yellowing index decreased from 13.8 to 8.5 than poly(ethylene terephthalate)-co-amide salts and decreased from 11.7 to 6.9 for fibers. This significantly improves the yellowing problem and provides technical support for colorless copolymer fibers.

Recycling their trash has garnered significant scientific interest as the use of waste and scrap (Was) polyamide 6 (PA6) blended textiles rises. Based on the complexation and decomplexation of PA6 with Ca2+, an investigation was conducted into the possibility of selectively and nondestructively dissolving and recovering PA6 and separating another component from Was PA6 blended models using CaCl2/ethanol/water (CEW) solvents. It was found through experiments that the dissolution of PA6 in CEW was related to temperature and stirring speed; the optimum conditions were chosen to be 75°C, 2 h, and 300 r/min, under which 94.80 wt% of Was PA6 textiles could be dissolved. Results also revealed that PA6 could be reprecipitated by simply adding water, whereas Ca2+ could be significantly removed by washing. The PA6 may be separated without degradation by controlled complexation and decomplexation, and the performance of Recycled PA6 (RPA6) was nearly identical to Was PA6 textiles. In addition, the two-component blended models experience non-destructive separation and recovery, and CEW treatment has little impact on the other components (except PA6). In this study, it is demonstrated that different types of Was PA6 blended textiles could be successfully separated via CEW treatment. Furthermore, the solvent system comprises easily accessible, low-cost materials that are widely used in industrial-scale processes. Thus, the concept will make a significant contribution to a green textile recycling strategy.