Improving Sustainability through Covalent Adaptable Networks in the Recycling of Polyurethane Plastics

Abstract

The global plastic waste problem has created an urgent need for the development of more sustainable materials and recycling processes. Polyurethane (PU) plastics, which represent 5.5% of globally produced plastics, are particularly challenging to recycle owing to their crosslinked structure. Covalent adaptable networks (CANs) based on dynamic covalent bonds have emerged as a promising solution for recycling PU waste. CANs enable the production of thermoset polymers that can be recycled using methods that are traditionally reserved for thermoplastic polymers. Reprocessing using hot-pressing techniques, in particular, proved to be more suited for the class of polyurethanes, allowing for the efficient recycling of PU materials. This Review paper explores the potential of CANs for improving the sustainability of PU recycling processes by examining different types of PU-CANs, bond types, and fillers that can be used to optimise the recycling efficiency. The paper concludes that further research is needed to develop more cost-effective and industrial-friendly techniques for recycling PU-CANs, as they can significantly contribute to sustainable development by creating recyclable thermoset polymers.

Keywords: CAN; composites; mechanical recycling; poly(urethane-urea); polyhydroxyurethane; polythiourethanes; polyurethanes.

Figure 5

Schematic representation of the networks proposed by Dichtel et al. [41] (A,B) represent the network formed by only rigid and soft units; (C,D) underline the similarity of the combined networks obtained via the direct synthesis or reprocessing of the separated units. Reprinted with permission from [41], Copyright 2023 American Chemical Society.

Figure 8

(A) Synthesis route of a bio-based PU; (B) proposed topological rearrangement upon reprocessing [59]. Reprinted with permission from [59], Copyright 2023 American Chemical Society.

Figure 9

Synthesis route of a PHU and examples of exchange reactions involved in reprocessing [70]. Reprinted with permission from [70], Copyright 2023 American Chemical Society.

Figure 10

(a) Synthesis route of a PUU and distinction of the role of each component [80]; (b) main exchange reaction involved in PUU rearrangement [76]; (c) schematic representation of the distribution and roles of the dynamic components in network repairing [79]. Respectively reprinted with permission from [76,80]. Copyright 2023 American Chemical Society; and reprinted from [79], copyright 2023, with permission form Wiley and Sons.

Figure 10

(a) Synthesis route of a PUU and distinction of the role of each component [80]; (b) main exchange reaction involved in PUU rearrangement [76]; (c) schematic representation of the distribution and roles of the dynamic components in network repairing [79]. Respectively reprinted with permission from [76,80]. Copyright 2023 American Chemical Society; and reprinted from [79], copyright 2023, with permission form Wiley and Sons.

Figure 11

(a) Schematic of synthesis route for PTU networks [84]; (b) representation of spatial disposition of PTU components in the final network, underlining dynamic bonds and crosslinkers [89]; (c) exchange reactions proposed for PTU networks [84]; (d) proposed working paths for thiourethane bond-exchange linkers [89]. Respectively reprinted from [84,89], Copyright 2023 American Chemical Society.

Figure 1

Common route for polyurethane synthesis.

Figure 2

Example of covalent bond exchange process in epoxide network [30]. Reprinted from [30], and with permission from AAAS.

Figure 3

Reaction scheme (A) and graphical representation (B) of working principle of vitrimers and vitrimer-like materials; (C) proposed reaction for PU networks [31]. Reprinted from [31], copyright 2023, with permission form Elsevier.

Figure 3

Reaction scheme (A) and graphical representation (B) of working principle of vitrimers and vitrimer-like materials; (C) proposed reaction for PU networks [31]. Reprinted from [31], copyright 2023, with permission form Elsevier.

Figure 4

Ideal viscosity–temperature relationship of vitrimers with (i) T_g lower than T_v and (ii) Tv lower than T_g [31]. Reprinted from [31], copyright 2023, with permission form Elsevier.

Figure 6

(a) Example of a ground sample before and after the reprocessing process via hot-pressing; (b) comparison of stress–strain curves of pristine and reprocessed PUs [39]. Reprinted from [39], copyright 2023, with permission form Elsevier.

Figure 7

(a) Exchange mechanism of imine bond; (b) structural exchange mechanism proposed for a vanillin-bio-based PU; (c) example of pulverisation and reprocessing; (d) stress–strain curves of pristine and reprocessed vanillin-based PU [58]. Reprinted from [58], copyright 2023, with permission form Elsevier.

Figure 12

Synthesis route of a composite PU and example of using an external agent (organo-silicon) as a crosslinker [94]. Reprinted from [94], copyright 2023, with permission form Elsevier.

Figure 13

(a) Precursors used in synthesis and proposed reaction involved in the formation of the PU network; (b) representation of the reprocessing process and oxidation-strengthening process of the network [104]. Reprinted from [104], copyright 2023, with permission form Springer Nature.