Dihydroxy Polyethylene Additives for Compatibilization and Mechanical Recycling of Polyethylene Terephthalate/Polyethylene Mixed Plastic Waste

Abstract

Polymer blend compatibilization is an attractive solution for mechanical recycling of mixed plastic waste because it can result in tough blends. In this work, hydroxy-telechelic polyethylene (HOPEOH) reactive additives were used to compatibilize blends of polyethylene terephthalate (PET) and linear low-density polyethylene (LLDPE). HOPEOH additives were synthesized with molar masses of 1−20 kg/mol by ring-opening metathesis polymerization of cyclooctene followed by catalytic hydrogenation. Melt-compounded blends containing 0.5 wt % HOPEOH displayed reduced dispersed phase LLDPE particle sizes with ductilities comparable to virgin PET and almost seven times greater than neat blends, regardless of additive molar mass. In contrast, analogous blends containing monohydroxy PE additives of comparable molar masses did not result in compatibilization even at 2 wt % loading. The results strongly suggest that both hydroxy ends of HOPEOH undergo transesterification reactions during melt mixing with PET to form predominantly PET−PE−PET triblock copolymers at the interface of the dispersed and matrix phases. We hypothesize that the triblock copolymer compatibilizers localized at the interface form trapped entanglements of the PE midblocks with nearby LLDPE homopolymer chains by a hook-and-clasp mechanism. Finally, HOPEOH compounds were able to efficiently compatibilize blends derived solely from postconsumer PET and PE bottles and film, suggesting their industrial applicability.

Polymer Multi‐Block and Multi‐Block+ Strategies for the Upcycling of Mixed Polyolefins and Other Plastics

“…Assuming localization at the interface and appropriate segregation of the multi-block components to their respective homopolymer domains, the number of interfacial crossings should scale with the number of copolymer blocks. [34,[37][38][55][56][57] It is generally believed that more copolymer blocks will improve compatibilization, but it is challenging to synthesize well-defined multi-block copolymers. To generate more blocks in lMBCPs, we developed a multi-…”

“…[33] Compatibilizers are typically added to immiscible blends of polyolefins and other polymers to decrease the interfacial energy and enhance the interfacial adhesion. [34][35][36][37][38][39] The reactive compatibilization of polyethylene (PE) and polyethylene terephthalate (PET) blends involves the use of specially-designed PE additives containing functional groups such as maleic anhydride, [40,41] glycidyl [42,43] or hydroxyl groups. [40,44] Functional PE additives react with PET at the interface during melt-mixing to form a compatibilizer in situ.…”

“…[45] Alternatively, linear multiblock copolymers (lMBCPs) can also be used as compatibilizers by crossing the interface multiple times to "stitch" the domains together. [37] An example of this strategy was reported by the Ellison and Miller groups, in which novel PE-PET multiblock copolymers were synthesized and used as a compatibilizer and adhesive-promoting layer to recycle PE/PET mixed waste streams. [38] However, limited success has been achieved in this field due to the difficulty of synthesizing different types of welldefined lMBCPs.…”

“…The dynamic crosslinker reactivated mixed‐plastic chains of polyolefins and polar polyesters, that compatibilized them via the dynamic formation of graft multiblock copolymers ( g MBCPs) [45] . Alternatively, linear multiblock copolymers ( l MBCPs) can also be used as compatibilizers by crossing the interface multiple times to “stitch” the domains together [37] . An example of this strategy was reported by the Ellison and Miller groups, in which novel PE‐PET multiblock copolymers were synthesized and used as a compatibilizer and adhesive‐promoting layer to recycle PE/PET mixed waste streams [38] …”

Design of Copolymer‐Based Blend Compatibilizers for Mixed Plastic Recycling

“…Error bars represent the standard deviation of all samples tested for a given sample type. (Reproduced with permission [170]. Copyright 2022, American Chemical Society).…”