Polyurethane (PU) is a versatile and widely utilized material that offers essential benefits to society. However, it is also often perceived as non-sustainable due to challenges associated with recycling. The diversity in chemical compositions, along with the varying quantities and qualities of waste produced, complicates recycling processes. To address this issue, multiple recycling technologies are required. Here, we outline different technical pathways for recycling flexible and rigid PU foams. Achieving maximum circularity requires depolymerization into the respective isocyanate precursor and polyol, as demonstrated by a slab foam material. For rigid foams, such as vacuum insulation foam, moderate recycled content can be achieved by adjusting formulation parameters. In addition, a design-to-recycle approach for flexible foam, allowing for reprocessable materials, is delineated. Beyond recycling, our efforts in rendering PU more sustainable via benign additives, catalysts, etc. are presented.

Polyurethane (PU) flexible foams are reticulated materials extensively used in automotive and furniture and still poorly recycled to date. Current mechanical and chemical PU foam recycling processes allow limited incorporation into new PU foams of recycled material, whether it is grinded PU or recycled reactants.[1] Dichtel’s groups developed an alternative recycling process using dynamic covalent chemistry to convert PU foams into vitrimers: by introducing catalysts into PU foams, carbamate bond exchange was activated and PU foams were effectively processed through extrusion, and even foamed using chemical blowing agents.[2–4] Alternatively, Nettles et al. recently reported a recycling process using short carbamates to deconstruct the PU network allowing extrusion and obtaining a thermoplastic PU with tunable properties and extensive recycled content.[5]
The herein study demonstrated the extrusion reprocessing of PU foams with low amounts of bi-functional depolymerizing reagent, shifting the polycondensation equilibrium toward a thermoplastic PU. The effect of the amount of reactant on PU network density and viscosity was evaluated to limit reagent consumption. Then, a slow-kinetic PU curing chemistry and various loadings of blowing agent (5 to 20 wt%) were added during melt processing. The loaded thermoplastic PU was then concomitantly reticulated and foamed. The reticulation kinetics was evaluated rheologically to relate them to the obtained foam structures. PU foams with closed-cell structure and densities of 500 kg/m3 were obtained. The process allow obtaining either thermoplastic or thermoset PU depending on the chemistry used and mechanically resilient recycled PU foams with recycled content over 70 wt% were obtained.

Chemical recycling of polymers containing chemically labile bonds in the backbone, especially those that are difficult to reprocess, offers an alternative for the recovery of secondary raw materials suitable for the production of new polymer materials. Here, we present an ammonolysis degradation process for flexible polyurethane foams using a novel degradation agent that enables efficient degradation of the urethane groups in the foam structure without the use of a solvent or any/medium, while effectively preventing the formation of unwanted aromatic diamines during the degradation process. The process is energy efficient and allows for easy separation of the liquid polyol phase from the remaining PUF hard segments as well as easy purification of the recycled polyols. Finally, the results of a comprehensive characterization of the recycled polyols and the properties of the flexible foams synthesized from recycled polyols are presented.

N-Hydroxyphthalimide (NHPI) and its derivatives are versatile redox-active molecules that play different roles in electrochemically mediated hydrogen atom transfer (e-HAT) reactions and in the design of functional materials for polyolefin recycling. NHPI can be easily converted into a (meth)acrylic monomer, enabling its incorporation into copolymers by free radical polymerization, Atom Transfer Radical Polymerization and Reversible Addition Fragmentation chain transfer polymerization with tailored properties for reactive extrusion processes.1,2 These copolymers are thermally active, solid-state compatibilizers capable of generating Phthalimide N-oxyl (PINO●) radicals during reactive extrusion, facilitating selective hydrogen atom abstraction from polyethylene and polypropylene substrates, thus enhancing their compatibility when physically blended in the molten state.

NHPI and its derivative N-hydroxytetrachlorophthalimide are also efficient mediators of e-HAT reactions when reactive PINO● are generated upon anodic oxidation. PINO● selectively abstracts hydrogen atoms from relatively activated C-H bonds, enabling controlled oxidative transformations or oxidative depolymerization of weaker polymer substrates in the presence of inexpensive oxygen. Inspired by electrochemically mediated Atom Transfer Radical Polymerization driven by alternating current (AC),3 we used a similar AC electrolysis at a low frequency to enable the degradation of a series of homopolymers and copolymers into oxygenated products. Mechanistically, AC provides a tunable dynamic environment in which the reactive PINO● is better shielded from side reactions, especially dimerization and trimerization, unlike direct current electrolysis. This is an additional example of the untapped potential of AC electrolysis in polymer science. As expected, many more electrochemical processes using AC electrolysis will soon appear.

The mechanical recycling of the widely used thermoplastic, Poly Vinyl Chloride (PVC), is limited due to the use of hazardous additives in the past, i.e., phthalates. Solvent-based recycling (SBR) or dissolution precipitation is an innovative approach to remove unwanted substances from polymers which might represent a viable option for plasticized PVC recycling. According to this method, a suitable solvent is used to dissolve the polymer which is then precipitated with a non-solvent and easily recovered, obtaining the separation from the organic additives which remain in the liquid phase.[1] In this study SBR was applied to PVC samples containing Bis(2-ethylhexyl) phthalate (DEHP) as a plasticizer, to resemble common PVC waste from medical applications (DEHP ≥ 30 wt.%). To this end, Hansen Solubility Parameters were used for solvent and non-solvent selection.[2] Temperature, stirring speed, non-solvent flow rate and drying steps were optimized to obtain a PVC resin in close resemblance to virgin material on the market. GC-MS analysis was carried out to confirm DEHP extraction and a residual content below the legal limit of 0.1 wt.%[1] while the changes in molecular weight and morphology of recycled PVC were studied with GPC and SEM, respectively. To ensure circularity the completely dried material was reformulated with Bis(2-ethylhexyl) terephthalate (DEHP) and necessary additives. The properties of material recycled through SBR remained unexplored. The recycled plasticized PVC was comprehensively evaluated in comparison to virgin plasticized PVC, with similar formulation, in terms of chemical, morphological, thermal and mechanical properties.[3][4]