Turning Waste Plastic into Structural Adhesive

Waste Plastic Converted to Structural Adhesive Researchers at Energy's Oak Ridge National Laboratory used polymer chemistry to convert a common household plastic into a reusable adhesive with a rare combination of strength and toughness, making it one of the strongest materials reported to date. A study published in Science Advances fundamentally advances approaches to designing a new class of durable adhesives with desired properties combined in a single material. The technology is suited to carrying heavy loads, tolerating extreme stress and heat, and reversibly bonding to various surfaces including glass, aluminum and steel. ORNL scientist and corresponding author Tomonori Saito said, "Designing strong, durable adhesives is difficult because they typically need to contain incompatible hard and soft properties." Structural adhesives like epoxy are largely designed for load-bearing strength but lack toughness—a property that helps materials distribute stress when pulled or stretched to prevent sudden failure. "The challenge was adding the toughness you get in flexible materials without compromising durability. Our approach uses dynamic chemical bonds to develop a new adhesive with extraordinary properties not seen in existing materials," he said. The researchers aimed to upcycle a meta thermoplastic, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene or SEBS, an easy-to-process but rubbery polymer material not designed for strong adhesion. The goal of upcycling is to add value to common plastics produced in high volume for general and often single-use applications such as food containers, toys and household items. By modifying SEBS's chemical structure through dynamic crosslinking, the team not only made it more robust but also created reuse pathways that enhance its performance for new and specialized applications beyond traditional plastic recycling. Crosslinking is a known strategy for designing materials with more stable properties. The approach can create a bridge between normally incompatible structures. In the study, boronic esters were used to combine SEBS with silica nanoparticles or SiNP, a filler material used to reinforce polymers. The combination yields a new crosslinked boronic ester-SiNP composite material. Conventional crosslinking typically results in permanent bonds that prevent removal or reprocessing of adhesives. The study found that boronic esters enable "dynamic" or reversible crosslinking, and that the new material's key to strong adhesion and recyclability lies in this property. These unique chemical compounds can create stable bonds that repeatedly form and break—an extraordinary property that makes them attractive for sustainable materials design. Md Anisur Rahman, lead author working with Saito at ORNL's Chemistry Sciences Division, said, "A fundamental discovery was that boronic esters on SEBS could rearrange bonds with hydroxyl groups (oxygen and hydrogen) on SiNP to adapt properties for demanding tasks. We also found the formation of similar reversible boronic ester bonds on various surfaces with hydroxyl groups." This two-fold finding was observed experimentally and computationally with density functional theory. The results show that the crosslinked bonds slide within the material to enable certain properties and adhered so strongly to surfaces that a thin square centimeter could hold approximately 300 pounds. Shear tests measuring toughness by attempting to forcibly separate materials were off the charts, greatly exceeding all commercial adhesives tested in the study. The material's adhesion to glass was so strong that the glass broke before the bond failed. The approach also raised thermal stability to 400 degrees Fahrenheit, making the adhesive attractive for ambient and high-temperature applications. Beyond extraordinary adhesion, a surprising property of the hard material is that it is recyclable. Rahman said, "It is rare for a high-performance adhesive to be removable, but ours is designed for reuse and recyclability." "It can be applied and separated with heat and pressure and reused several times." The development extends applications toward aerospace, automotive and construction adhesives. "There are benefits to industry and the environment in conserving resources and reducing waste. By design, this adhesive allows you to make repairs or correct costly mistakes, and can be reprocessed for new uses in very demanding applications," he said. The team plans to commercialize the technology and is investigating dynamic crosslinking with other fillers to develop robust adhesives optimized for specific bonding surfaces and functions. Source Md Anisur Rahman et al, Design of tough adhesive from commodity thermoplastics through dynamic crosslinking, Science Advances (2021). DOI: 10.1126/sciadv.abk2451 Journal information: Science Advances https://phys.org/news/2022-01-polymer-upcycling-common-plastic-toughness.html