A facile alternative strategy of upcycling mixed plastic waste into vitrimers

Abstract

Chemical depolymerization has been identified as a promising approach towards recycling of plastic waste. However, complete depolymerization may be energy intensive with complications in purification. In this work, we have demonstrated upcycling of mixed plastic waste comprising a mixture of polyester, polyamide, and polyurethane through a reprocessable vitrimer of the depolymerized oligomers. Using poly(ethylene terephthalate) (PET) as a model polymer, we first demonstrated partial controlled depolymerization, using glycerol as a cleaving agent, to obtain branched PET oligomers. Recovered PET (RPET) oligomer was then used as a feedstock to produce a crosslinked yet reprocessable vitrimer (vRPET) despite having a wide molecular weight distribution using a solventless melt processing approach. Crosslinking and dynamic interactions were observed through rheology and dynamic mechanical analysis (DMA). Tensile mechanical studies showed no noticeable decrease in mechanical strength over multiple repeated melt processing cycles. Consequently, we have clearly demonstrated the applicability of the above method to upcycle mixed plastic wastes into vitrimers and reprocessable composites. This work also afforded insights into a potentially viable alternative route for utilization of depolymerized plastic/mixed plastic waste into crosslinked vitrimer resins manifesting excellent mechanical strength, while remaining reprocessable/ recyclable for cyclical lifetime use.

Recycling/Upcycling of Physically Aged Poly(butylene succinate) into PBS Vitrimers through Zn(II)-Catalyzed Melt Vitrimerization

“…It is clear that although PBS vitrimers are cross-linked, they are able to fully relax stress (G t /G 0 ≈ 0) at moderate to low time scales (depending on the temperature and cross-link density), proving that the network is fully dynamic and any residual stress is trivial. In preceding studies of this type, [30][31][32][33]36,37 the characteristic relaxation time, τ*, was approximately determined by the time scale required for relaxing down to G t /G 0 = 1/e (≈0.37) of the imposed stress, consistent with the hypothesis of a singleexponential decay response from the Maxwell's model. [27][28][29][30][31][32]60 However, the stress relaxation for both vitrimers and polymers should not always be described by a single characteristic relaxation time but with a spectrum of relaxation times.…”

“…It is clear that although PBS vitrimers are cross-linked, they are able to fully relax stress ( G t / G 0 ≈ 0) at moderate to low time scales (depending on the temperature and cross-link density), proving that the network is fully dynamic and any residual stress is trivial. In preceding studies of this type, − ,, the characteristic relaxation time, τ*, was approximately determined by the time scale required for relaxing down to G t / G 0 = 1/e (≈0.37) of the imposed stress, consistent with the hypothesis of a single-exponential decay response from the Maxwell’s model. − , However, the stress relaxation for both vitrimers and polymers should not always be described by a single characteristic relaxation time but with a spectrum of relaxation times. Consequently, it is more consistent to fit the stress relaxation data to a Kohlrausch–Williams–Watts (KWW) stretched exponential decay, − provided by eq : G t G 0 = G perm G 0 + ( 1 − G perm G 0 ) · exp [ prefix− true( t τ * true) β ] where G t / G 0 is the normalized relaxation modulus, G perm / G 0 is the residual stress fraction that remains at t → ∞ which is anticipated to reach close to zero values within experimental error for linear and branched polymers as well as vitrimers without permanent cross-links and high values (≤1) for classic thermosets, t (s) is the experimental time, τ* (s) the characteristic relaxation time and β (0 < β ≤ 1) is the exponent that determines the shape of the stretched exponential decay and reflects the broadness of the relaxation distribution, with β = 1 implying a single relaxation time, while a smaller β indicates an extremely broad distribution of relaxation times. − …”

“…Vitrimers�in contrast to permanently cross-linked networks that are extremely difficult to be rearranged due to restrictions in chain movement�can relax stress above the freezing transition temperature (T v ), 30,36,37 generating a transition from viscoelastic solid to liquid conventionally taken at viscosity values of 10 12 Pa s. These capabilities can be manifested by stress relaxation experiments at T > T m , in which a temperature-dependent drop-off in the relaxation modulus, G t , can be observed. Figures 12 and 13 contain the normalized relaxation modulus, G t /G 0 , decay as a function of time and temperature (120−160 °C) for both PDG 170 and PGL 150 materials, and the observed trends are standard of a viscoelastic liquid.…”

“…It involves the dynamic cross-linking of already formed polymers mainly via three types of associative exchange reactions: carboxylate transesterification, transamination of vinylogous urethanes, and transalkylation of triazolium salts. − Transesterification is largely studied due to its relatively low activation energy (≈70–150 kJ/mol), ,− easy functionalization, and abundance of ester bonds in well-established polymeric systems. Consequently, vitrimers from commercial thermoplastic polyesters like PBT, − PET, − PBAT, and PBS have been developed so far and an overview of their most important properties can be found in Table S1. In most of those cases, special chemical modification was needed to incorporate the necessary functionality by breaking the polymeric chain, such as alcoholysis by glycerol in the case of PBT − , or a polyol with tertiary amine structure (bis-tris methane) in the case of PET .…”

The thermodynamics and kinetics of depolymerization: what makes vinyl monomer regeneration feasible?

“…Lastly, a multitude of reports exist where different methods are used to generate small molecules with synthetic value from vinyl polymer waste. [165][166][167][168] As a specic example (Fig. 16), polyisobutylene was degraded at 0 °C with a strong acid by modifying monomeric products to form more stable bonds than in the polymer so the small molecule formation becomes exothermic, and monomer is removed from the equilibrium.…”