Blending Polyurethane Thermosets Using Dynamic Urethane Exchange

Abstract

Recycling crosslinked polyurethanes (PUs) is accomplished through mechanical or chemical processes that are energy-intensive or produce plastics of lesser value. Polymer recycling processes are notably intolerant of polymer mixtures, yet the ability to reprocess and compatibilize two or more crosslinked PUs together will make this process more amenable to mixed waste streams while offering an opportunity to tune the properties of the recycled polymer products. Here, we blend a rigid polyester PU and a soft polyether PU using twin-screw extrusion to yield materials with tunable mechanical properties based on the feed composition. Their material properties were compared to those of compression-molded reprocessed blends and blends where the monomers were mixed prior to synthesis. The extruded materials showed similar mechanical and thermal properties to newly prepared blends and had higher-value mechanical properties compared to the samples reprocessed via compression molding. The morphologies of the blends were observed using phase imaging via atomic force microscopy to show that there is less phase separation in the extruded materials compared to compression-molded blends. The mechanical properties of these materials were tunable from soft to elastomeric to rigid based on the feed composition, and this tunability was demonstrated through four consecutive reprocessing cycles, through which the mechanical properties were steadily varied from rigid to soft by incorporating increasing amounts of soft polyether PU material. This blending method for reprocessing mixed waste compatibilizes different PUs and provides a means to tune the mechanical properties of a PU product, even if starting from waste streams of varying compositions. As such, this process represents an intriguing new approach for polymer reprocessing.

Divergent Aminolysis Approach for Constructing Recyclable Self-Blown Nonisocyanate Polyurethane Foams

“…28−30 Although now largely investigated for many thermosets, this concept was only recently applied for PU foams by promoting transurethanization reactions during extrusion when the foam was loaded by a tin-based catalyst. 31,32 Unfoamed PHU networks with recycling ability were also reported when transcarbamoylation reactions, 33 eventually combined to other reversible reactions (e.g., transesterifications), 34,35 were induced in the presence of an appropriate catalyst during thermal treatment. 33−35 In PHU thermosets, the pending hydroxyl groups were responsible for the transcarbamoylation reactions and, thus, for the thermoset reprocessability.…”

Transcarbamoylation in Polyurethanes: Underestimated Exchange Reactions?

“…In two recent publications, Dichtel and co-workers combined the use of tin-catalyzed TC and industrially relevant polymer processing techniques, twin-screw extrusion (Figure ). , They demonstrated that PU thermosets and foams could be reprocessed into new thermosets via blending networks containing DBTDL or by postintroduction of DBTDL in the foams (2.3–3.1 wt %, 1.5 mol % to NCO groups). They initially evidenced the potential of this recycling strategy by incorporating sequentially “soft” PU networks into “hard” PU networks using coextrusion.…”

Closing the Carbon Loop in the Circular Plastics Economy

“…[ 114 ] By dynamic urethane exchange reaction during twin‐screw extrusion, blends of rigid polyester PU and PU soft polyether PU networks yield materials with tunable mechanical properties that can vary from soft to elastomeric to rigid as a function of the feed composition. [ 115 ] Whether dynamic networks with the thermoresponsive bond exchange are, like static covalent networks, creep resistant and enable re‐processing of mixed waste streams, including formulated multicomponent thermosets, needs to be demonstrated.…”