Upcycling Polyolefin Blends into High-Performance Materials by Exploiting Azidotriazine Chemistry Using Reactive Extrusion

Abstract

The revalorization of incompatible polymer blends is a key obstacle in realizing a circular economy in the plastics industry. Polyolefin waste is particularly challenging because it is difficult to sort into its constituent components. Untreated blends of polyethylene and polypropylene typically exhibit poor mechanical properties that are suitable only for low-value applications. Herein, we disclose a simple azidotriazine-based grafting agent that enables polyolefin blends to be directly upcycled into high-performance materials by using reactive extrusion at industrially relevant processing temperatures. Based on a series of model experiments, the azidotriazine thermally decomposes to form a triplet nitrene species, which subsequently undergoes a complex mixture of grafting, oligomerization, and cross-linking reactions; strikingly, the oligomerization and cross-linking reactions proceed through the formation of nitrogen−nitrogen bonds. When applied to polyolefin blends during reactive extrusion, this combination of reactions leads to the generation of amorphous, phase-separated nanostructures that tend to exist at polymer−polymer interfaces. These nanostructures act as multivalent cross-linkers that reinforce the resulting material, leading to dramatically improved ductility compared with the untreated blends, along with high dimensional stability at high temperatures and excellent mechanical recyclability. We propose that this unique behavior is derived from the thermomechanically activated reversibility of the nitrogen− nitrogen bonds that make up the cross-linking structures. Finally, the scope of this chemistry is demonstrated by applying it to ternary polyolefin blends as well as postconsumer polyolefin feedstocks.

Advances in Nonreactive Polymer Compatibilizers for Commodity Polyolefin Blends

“…can be installed on postconsumer materials. − Although the resulting polymers exhibit decreased crystallinities due to the introduction of noncrystallizable moieties, they more importantly exhibit enhanced extensibilities, surface adhesion strength, and toughness. These reactive functionalization protocols are promising tools for tailored compatibility between polyolefin phases, and have been demonstrated in other polymer compositions using H-bonding, ionomers, and π–π stacking. − There have also been significant advances in applying the concepts of dynamic cross-links to reactive compatibilization, thereby preserving processability and improving compatibility. − Notable recent examples include the use of diazirines, azidotriazenes, and thiosulfonates to functionalize polyolefins with dynamic cross-linking moieties. The resulting blends exhibit improved compatibility, creep resistance, and in some cases enhanced mechanical toughness.…”