Dynamic Covalent Polyurethane Networks with Excellent Property and Cross-Link Density Recovery after Recycling and Potential for Monomer Recovery

Abstract

Dynamic urethane chemistry, which involves both associative and dissociative mechanisms, can be used to achieve reprocessability in cross-linked polyurethane (PU) networks. We discovered that property recovery and thermal stability of dynamic covalent PU networks can be enhanced by incorporating excess free hydroxyl groups, thus performing polymerization reactions at slight stoichiometric imbalance of hydroxyl and isocyanate groups, and by increasing the cross-linker functionality from three to four. While free hydroxyl groups suppress the reversion of urethane links and minimize side reactions associated with liberated isocyanate groups under reprocessing conditions, tetrafunctional cross-linkers help to maintain network integrity in the presence of small levels of side reactions. Using these strategies, we developed a PU network that, within experimental error, exhibits 100% recovery of cross-link density and tensile properties after multiple reprocessing steps, which has not been reported before. We also demonstrated the potential of recovering monomer from PU networks by alcoholysis under relatively mild conditions. Our study not only offers a solution to the long-standing issue of PU network recycling but also provides simple strategies to improve property recovery of reprocessable networks.

Neighboring Group Participation and Internal Catalysis Effects on Exchangeable Covalent Bonds: Application to the Thriving Field of Vitrimer Chemistry

“…In contrast, only 15 mins at 120 °C were sufficient to reprocess a PU containing 20 % of free -OH. 258 In contrast, an increase of the relaxation time was observed in the case of thiourethane-based networks (Scheme 11) containing free thiols compared to the thiol-free analog. This counterintuitive result was explained by a change from a dissociative to a slower associative mechanism.…”

Recent advances to decrease shrinkage stress and enhance mechanical properties in free radical polymerization: a review

“…heat or light), and can reform. Using this dynamic process, covalent adaptable networks (CANs) have been developed, combining the desirable attributes of conventional thermosets with the dynamics of controllably bonded structures 68, 69 . In general, the CANs‐forming mechanisms, including reversible dissociative reactions and reversible associative reactions, are shown in Figure 10 70 .…”

“…12(A)). 69,[73][74][75] The networks containing these dynamic covalent bonds have network structures similar to those of traditional polyurethane materials but can be reprocessed at elevated temperatures. By introducing additional free functional groups such as thiols or hydroxyls, the reprocessed polymer networks show the same crosslinking density.…”

Crosslink density and mechanical property evolution during the curing of polyurethane-urea/sodium silicate hybrid composites

“…This can be explained by the fact that initiation of segmental relaxation of the soft segment due to the temperature increases. 27–29 Moreover, the presence of a plateau modulus after the storage modulus (E ′ ) sudden drop around 60°C suggests their crosslinked nature 1 and microphase separation, 30 and this is due to the physical crosslinking of the hard segment. At temperature higher than 160°C, the drop of E ′ is related to the hard segment disintegrate.…”

“…Polyurethane-ureas (PUs) and its composite materials are a family of polymeric materials widely used as elastomers, adhesives, and foams in construction including roadway, bridge, and underground engineering. 1 The wide application of PU-based materials comes from adjustable properties for their outstanding mechanical properties of the cured networks including toughness and strength. Among them, polyurethane-urea/sodium silicate (PU/SS) hybrid composites are currently one of the most commonly used grouting materials.…”