On the Origins of Sudden Adhesion Loss at a Critical Relative Humidity: Examination of Bulk and Interfacial Contributions

Abstract

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.

Role of salt on adhesion of an epoxy/aluminum (oxide) interface in aqueous environments

“…These measurements of the interfacial structure provide a mechanism to better understand the underlying mechanisms by which interfaces are attacked by environmental factors. We have previously used the combination of NR and fracture mechanics to elucidate a probable mechanism for humidity‐induced failure: this is swelling stress driven that is induced by reorganization of adsorbed water at the polymer adhesive‐inorganic interface . This explanation, aided by NR measurements to elucidate the distribution of water within the adhesives, is consistent with many of the prior explanations that independently may appear to be contradictory as some have proposed the bulk swelling to be important for humidity failure , while others have solely focused on the interface .…”

“…The width of the epoxy‐aluminum oxide interface is very similar for the dry and salt cases, but immersion in D 2 O leads to a significant broadening of this interface, which has been attributed to water accumulation . Additionally prior studies on the role of humidity on adhesion illustrated a decrease in adhesion as the width of this interface increases . However, the ambiguity associated with two possible absorbing species (D 2 O and NaCl) does not allow for the concentration profiles to be elucidated from these experiments without additional information.…”

“…Water (moisture) is a commonly observed agent for environmental attack to promote failure in adhesive joints , but its effect depends on the humidity , surface treatments prior to bonding , physical properties of the adhesive , and salts present during direct liquid exposure. On exposure to moisture, the failure mechanism for the joint can switch from cohesive to adhesive , which demonstrates the changing role of the interface in determining the durability of the joint. This interfacial sensitivity presents a major challenge in fundamental understanding of environmental impact on adhesive joints.…”

Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: State of the art

“…On the other hand, Lefebvre et al [136] found that a completely different mechanism, which only involves the bulk adhesive properties, was responsible for the sudden loss of adhesion in an epoxy/glass system. Inspired by the importance of critical moisture content and contradictory literature regarding its underlying mechanisms, Tan et al [137][138][139] used detailed characterisation methods and proposed a combined bulk-interfacial mechanism for the loss of adhesion at or above the critical moisture content. As illustrated in Figure 8, the interface moisture content increases as the surrounding humidity level increases and this in turn reduces the contact area between adhesive and substrate.…”

Ubiquitous distribution of salts and proteins in spider glue enhances spider silk adhesion

“…While most synthetic adhesive systems are negatively affected by increased humidity10, viscid-silk glue generates higher adhesion forces as humidity increases, and in some species adhesion continues to improve at conditions close to 100% relative humidity (R.H.)8. Therefore, understanding the glue composition and its adhesion mechanism can provide a framework to design synthetic adhesives that function in humid environments.…”