The lotus leaf, celebrated across cultures as a symbol of purity, owes its self-cleaning abilities to a phenomenon known as superhydrophobicity. Lotus leaves repel water so effectively that droplets skitter off, taking dirt and impurities with them. Inspired by this, materials researchers have turned to a similar strategy to address a pressing environmental issue: polypropylene (PP) recycling.
PP is ubiquitous in everyday items like plastic bags, yet despite its prevalence, barely one percent of it is recycled each year. The remainder accumulates in landfills, where it slowly degrades, releasing hazardous gases and contributing to pollution. Traditional PP recycling methods are resource-intensive and often fall short on a large scale.
Xiukai Li and Yugen Zhang from A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2) have pioneered a more sustainable approach to upcycling PP, enhancing its properties to add value. “PP is difficult to dissolve in most organic solvents, making traditional recycling methods like dissolution less effective,” the researchers explained.
Their study aimed to transform PP into microspheres that emulate the hierarchical nanostructures of lotus leaves, creating a superhydrophobic material. “Adapting the nanostructures inspired by lotus leaves posed challenges in achieving uniformity and stability in the synthesised microspheres,” said Li and Zhang.
Rising to the challenge, the team subjected waste PP to a catalytic oxidation process, then dissolved it in various solvents to create microspheres. Their method not only enhances PP's hydrophobicity, but also does so in a way that is both cost-effective and scalable.
“Theoretically, our oxidised PP should be more hydrophilic,” the researchers remarked, yet their results showed that the microspheres were exceptionally water-repellent: with a contact angle reaching up to 164 degrees, they mirrored the superhydrophobic properties of natural lotus leaves. Their performance versus similar materials exceeded the researchers’ expectations.
Moreover, this process proved effective with PP waste from a diverse array of sources, including medical waste and food packaging. “The synthesis process is simple and can be applied to waste PP from diverse sources, offering a versatile solution for a wide range of applications,” the researchers added, highlighting its potential in environmental remediation for oil spill clean-ups, the development of water-repellent textiles and self-cleaning surfaces.
Encouraged by these promising results, the team has filed a patent application for their microspheres and is actively seeking industry partnerships to further explore the technology's potential.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).