Unlike thermoplastics, conventional thermosets and thermoset composites cannot be effectively recycled for high-value applications because the permanent nature of the covalent crosslinks prevents melt-state reprocessing. As a result, there are major sustainability and economic losses associated with conventional thermosets reaching their end of life. We have developed and/or applied three dynamic chemistries as replacements for permanent crosslinks. These dynamic chemistries include alkoxyamine chemistry (associated with nitroxide mediated polymerization) in which dynamic bonds are reversible, with an -N-O-C- bond, which is robust at low temperature, dissocating at high temperature to a nitroxide radical and a carbon-based radical. Using the polymers and fillers used in the tire industry and employing TEMPO methacrylate and a multifunctional initiatior, we have molded/melt-processed polymers and composites at temperatures near those use in tire-molding operations with crosslinks resulting on cooling. These network (composites) can be reprocessed multiple times with full crosslink density recovery. In accord with the robust nature of the crosslinks, these networks exhibit no creep at low temperature. We have also developed dynamic chemistries to provide for reprocessable polyurethane-like networks. Urethane crosslinks have insufficient dynamic characteri to allow for reprocessing with full crosslink density recovery. We have shown that hydroxyurethane and thiourethane crossslinks are effective for reprocessing with full recovery of crosslinks. These chemistries exhibit a dual nature of reversible reactions and exchange reactions. With these dynamic chemistries involving functional groups, we have also determined that incorporation of nanofiller in the network can lead to deleterious effects on reprocessability if the nanofiller surfaces contain functional groups that can participate in the dynamic reactions associated with the crosslinks.

Reprocessable polymer networks exhibiting full crosslink density recovery after recycling: Three eco-friendly alternatives to conventional thermosets based on dynamic covalent bonds

2019 Fall National Meeting

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