In situ nylon salt formation enables the molecular recycling of thermoplastic polyurethane to nylon 6.6

Polyurethanes (PUs) are a class of polymers that currently constitute 5.3% of the total global plastic production. A variety of multifunctional isocyanates and polyols can be utilized in PU synthesis, resulting in materials with versatile properties and applications. This versatility, however, complicates end-of-life PU recycling. Mechanical recycling is convenient and inexpensive but results in downcycled products. Molecular recycling, however, can preserve the polymers’ value through depolymerization and subsequent repolymerization. Although recent studies have focused on the molecular recycling of polyurethanes, model PUs or polyether-based PU foams are often utilized in depolymerization studies, or diamines obtained from depolymerization are often repolymerized to non-commercial polymers. In contrast, this work reports the microwave-assisted chemical depolymerization of a commercial thermoplastic polyurethane (TPU) based on hexamethylene diisocyanate (HDI) and subsequent repolymerization of the recovered hexamethylene diamine (HDA) to nylon 6.6. The TPU was depolymerized via a multistage approach. First, TPU was depolymerized through glycolysis to obtain oligo-urethane, which was subsequently hydrolyzed to yield HDA. HDA was then converted to nylon salt via a two-step addition of adipic acid (AA) to the crude hydrolysis mixture. A two-step process was crucial for obtaining pure nylon salt by first removing sodium ions and then forming pure nylon salt upon the second addition of AA. The synthesized nylon salt was polymerized into nylon 6.6 with similar properties to nylon 6.6 obtained from commercial nylon salt polymerization. This work demonstrates a promising alternative technique for molecularly recycling HDI-containing PUs to polyamides, facilitating the transition toward a closed-loop economy of plastics.