High-Sulfur-Content Materials Derived from Postconsumer Polystyrene Wastes: Thermomechanical Properties, Environmental Impacts, and Microstructural Insights

Postconsumer polystyrene (PS) wastes are a major contributor to microplastic contamination of the biosphere, which could be reduced or eliminated by developing strategies to upcycle these wastes into useful materials. Four postconsumer PS waste streams from flatware (PSF), cups (PSC), lids (PSL), and packaging materials (PSP), as well as a mixture of all four streams (PSM), were reacted with elemental sulfur at 230 °C to afford the corresponding high-sulfur-content materials (HSMs) PSF₉₀, PSC₉₀, PSL₉₀, PSP₉₀, and PSM₉₀, respectively. Glass transitions in these HSMs were observed at temperatures ranging from −36 to −39 °C, with these values being characteristic of oligo/polysulfide chains. Compressional and flexural strength measurements revealed that these HSMs were competitive with ordinary Portland cement and C62 Brick. To gain insight into the microstructural features within these HSMs, cumene was reacted with sulfur at 230 °C and then depolymerized with LiAlH₄, yielding small-molecule products amenable to GC-MS analysis. These reactivity studies provided compelling evidence that PSF₉₀, PSC₉₀, PSL₉₀, PSP₉₀, and PSM₉₀ contain the expected oligo/polysulfide cross-links between PS chains at 3° benzylic and 2° aliphatic carbons in addition to the formation of benzothiophene moieties.