A variety of industries rely on strong epoxy adhesive bonds, from dentistry to medicine and composites. Yet studying how these bonds can fracture and fail, and therefore how to make them more resistant to these failures, has proved difficult. Recently, researchers at MIT have found a way to study potential bonding failures directly, citing moisture as a catalyst for failure.

“The composite may be made of a strong and durable material bonded to another strong and durable material, but where you bond them doesn’t necessarily have to be strong and durable,” says MIT professor of civil and environmental engineering Oral Buyukozturk.

Standard methods exist for testing the strength of materials and how they may fail structurally, but bonded surfaces are more difficult to model. Buyukozturk says the best approach is to look at the bonded surfaces on the molecular level. When the surfaces are exposed to moisture, “it initiates new molecules at the interface,” Buyukozturk says, “and that interferes with the bonding mechanism.”

The MIT team used a combination of molecular simulations and laboratory tests in its assessment. The modeling was based on fundamental principles of molecular interactions, not on empirical data, Buyukozturk says. In the laboratory tests, the team controlled the residual stresses in a metal layer that was bonded and then forcibly removed, which revealed that moisture has a degrading effect.

The findings could lead to exploration of new ways to prevent moisture from reaching into the bonded layer, perhaps using better sealants. “I think this is going to be an important step toward assessment of the bonding, and enable us to design more durable composites,” Buyukozturk adds.