Unexpected Toughening of Poly(lactic acid) by Microwave-Assisted Devulcanization of Waste Latex Rubber

Vulcanized, foamed latex rubber features high density of intercommunicated cells and controlled elastomeric properties, rendering wide applications from elastic sponges to rubbery coatings. This provides an emergent incentive for properly recycling, devulcanizing and reusing the waste latex rubber (WLR). Here, a microwave-assisted devulcanization (MAD) approach was disclosed to cleave the cross-links without severe rupture of backbone rubber chains, strategically involving the use of a microwave-assisted hydrothermal reaction at 180 °C for several minutes and incorporation of graphene nanosheets serving as the nanoabsorbers of microwave irradiation. The MAD processing duration was examined to be a vital parameter controlling the macromolecular characteristics: a high devulcanization degree of 83.6% was obtained with 8 min MAD, whereas the soluble content was undesirably increased to 76.2% after 16 min. The devulcanization efficacy was examined by melt compounding with poly(lactic acid) (PLA), displaying extensive interfacial interactions and thereby, creating strong and resilient interfacial adhesion within the PLA composites. With the addition of 10 wt % devulcanized WLR by 8 min MAD, the tensile strength and elongation at break of PLA composites were largely promoted to 58.2 MPa and 16.6%, increasing nearly 62% and 240% compared to those of pure PLA, respectively. The proposed MAD approach may open an environment-respecting and reliable pathway to sustainable recycling of waste rubber, yielding useful components to toughen PLA without sacrificing the strength.