Tough, reusable adhesive can glue a variety of materials

Researchers at the Department of Energy's Oak Ridge National Laboratory have invented a reusable adhesive from waste polymers that is tougher than commercial glues, works underwater as well as in dry environments, and bonds a variety of materials, including wood, glass, metal, paper and polymers. Inspired by the way mussels stick stubbornly to surfaces, the innovative adhesive contains reversible chemical crosslinkers that allow the hardened glue to soften, detach and be reused, unlike current glues, which set permanently after one use.

Today's projects typically require different glues for different material surfaces - white glue for grade-school art projects, polyvinyl acetates for bookbinding, polyurethanes for shoemaking, silicones for sealing windows and affixing electronic parts, and industrial epoxies for joining aircraft and automobile components. A single adhesive that performs well across many applications could simplify manufacturing and repair while reducing waste. This advance may create big economic impact for the global adhesives and sealants market, valued at approximately $87 billion and projected to reach nearly $119 billion by 2032.

"Most adhesives are made for one specific application," said Anisur Rahman, a research and development staff member at ORNL who led a study published in Science Advances with former ORNL postdoctoral researcher Mary Danielson, now a research assistant professor with the University of Tennessee-Oak Ridge Innovation Institute. "Our adhesive can be used for diverse applications, including structural or pressure-sensitive uses, and it performs reliably in both wet and dry environments," he said. "None of the commercial adhesives can be used this way."

Beginning with common polymers from beverage bottles, fabric fibers and packaging films, the research team developed a process that saves materials, energy and money. "We took material destined for the landfill and turned it into something valuable," Danielson said.

The researchers have applied for a patent for their versatile glue.

How the reversible bonds work

Traditional structural adhesives rely on permanent crosslinks that make removal difficult. "You apply traditional adhesives once; you cannot reuse them," Rahman said.

"You basically have to rip an assembly apart to debond it," Danielson added. "You've damaged both the part you're glueing to and the part you're glueing from. If you make a mistake when you're gluing something and you allow it to cure, it's done."

In the ORNL adhesive, crosslinkers act like reversible attachments, akin to Velcro. Heating breaks dynamic chemical bonds in the polymer, allowing the adhesive to release without damaging surfaces. As the material cools, the bonds reform.

"If something is damaged or misapplied, you're able to completely remove it and put it back on with full integrity," Danielson said.

The team debonded and rebounded the adhesive more than 10 times with no loss in performance.

"Normally in the marketplace, structural adhesives typically have shear strength - a measure of adhesion - in the 7- to 10-megapascal range," Rahman said. "Our adhesives also stay well above that range but maintain reusability."

The researchers can also retrieve the glue chemically, using an excess of amine molecules to break the adhesive into its monomer subunits. "We can recover all chemicals used in this adhesive," Rahman said.

Mussel-inspired design

A polymer is a long chain or network made of monomers, or chemical subunits of one type. Using no solvents or catalysts, the scientists added amine, a nitrogen-containing chemical group, to the waste polymer and heated it to just below the polymer's melting temperature. Under these mild conditions, the amine broke the polymer down into monomers that each contained four amine groups.

Next, to design the adhesive, ORNL researchers mimicked mussel foot proteins, which contain both hydrophilic and hydrophobic components that enable strong adhesion even in wet environments.

"We used a crosslinker, or hardener, that has both water-loving (hydrophilic) and water-hating (hydrophobic) components together in the same molecule," Rahman said. "We mix the hardener and the monomer. It creates an adhesive resin that acts like a mussel foot protein."

"For any glue that is a cross-linked network of two components, it takes time to complete the reaction between the two components," Danielson said. "To repair boats, submarines and pipelines, our glue can be applied underwater using hand pressure until it sets."

Curing happens when a large four-armed monomer interacts with the crosslinking hardener. The monomer's amine group reacts with the hardener's acetoacetate group to produce a resin, or matrix with hydrophilic and hydrophobic characteristics. Whether the protein sticks or releases depends on the balance of those properties.

"Our glue maintained strong adhesion across different environmental conditions, including seawater, extremely low temperature (100 degrees Celsius below zero), and both acidic and basic conditions," Rahman said.

National lab capabilities enabled the achievement

Rahman conceived the concept of transforming deconstructed polymer waste into an adhesive. He and Danielson designed and led experiments and drafted the paper. Chuyi Pan, a summer intern from the University of Pennsylvania, assisted in synthesizing the adhesive. Tomonori Saito of ORNL and the University of Tennessee, Knoxville, reviewed and edited the manuscript drafts.

ORNL researchers performed vital characterizations. Bobby Sumpter simulated the energies with which the adhesive bound to different surface materials. Catalin Gainaru used rheology to characterize its stress and relaxation. Honghai Zhang and Vilmos Kertesz performed mass spectrometry to quantify different molecules. Zoriana Demchuk's lifecycle analysis of the ORNL glue showed it was more energy-efficient to make than commercial adhesives.

Toward strong and weak bonding applications

The team has also explored using this pioneering chemistry to advance vehicles. ORNL's glue maintained strong adhesion between dissimilar substrates - a crucial requirement in automotive and aerospace applications, where joining composites to aluminum or steel presents notable challenges.

Next, the scientists aim to tune crosslinking to enable weaker, temporary bonds for removable labels, adhesive bandages, drug-delivery patches and other applications.

ORNL's versatile, high-performance glue is poised to make an impact that sticks in situations from the mundane to the extraordinary. Potential uses range from household items that require gentle removal, like press-on nails and price tags, to repairs in remote or extreme environments, including underwater or outer space - settings where specialty glues may be unavailable.

The DOE Office of Science supported the research. The work used resources of the Center for Nanophase Materials Sciences, a DOE Office of Science user facility at ORNL.

UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Dawn Levy