Relationship Between Interfacial Water Layer Adhesion Loss of Silicon/Glass Fiber–Epoxy Systems: A Quantitative Study

Abstract

Abstract Water at the polymer/substrate interface is often the major cause of adhesion loss in coatings, adhesives, and fiber-reinforced polymer composites. This study critically assesses the relationship between the interfacial water layer and the adhesion loss in epoxy/siliceous substrate systems. Both untreated and silane-treated Si substrates and untreated and silane-treated E-glass fibers were used. Thickness of the interfacial water layer was measured on epoxy/Si systems by Fourier transform infrared–multiple total internal reflection (FTIR-MTIR) spectroscopy. Adhesion loss of epoxy/Si systems and epoxy/E-glass fiber composites was measured by peel adhesion and short-beam shear tests, respectively. Little water accumulation at the epoxy/Si substrate interface was observed for silane-treated Si substrates, but about 10 monolayers of water accumulated at the interface between the epoxy and the untreated Si substrate following 100 h of exposure at 24 °C. More than 70% of the initial epoxy/untreated Si system peel strength was lost within 75 h of exposure, compared with 20% loss after 600 h for the silane-treated Si samples. Shear strength loss in composites made with untreated E-glass fiber was nearly twice that of composites fabricated with silane-treated fiber after 6 months of immersion in 60 °C water. Further, the silane-treated composites remained transparent, but the untreated fiber composites became opaque after water exposure. Evidence from FTIR-MTIR spectroscopy, adhesion loss, and visual observation strongly indicated that a water layer at the polymer/substrate interface is mostly responsible for the adhesion loss of epoxy/untreated siliceous substrate systems and epoxy/untreated glass fiber composites and that FTIR-MTIR is a viable technique to reliably and conveniently assess the adhesion loss attributable to water sorption at the interface. Keywords: AdhesionCompositeInterfaceQuantitativeSilane coupling agentSiliconWater Notes a Determined using epoxy density of 1.14 g/cm3, which was measured by the water displacement method. b Calculated using (γ s − γ p ) values given in this table for the epoxy, and γ p and (γ s − γ p ) values of 44.6 mJ/m2 and 29.4 mJ/m2, respectively, measured for solvent-cleaned Si wafers. All surface free-energy components were determined using the geometric mean method and contact angles of water and methylene iodide measured at ambient conditions (24°C and 40% RH). Coefficients of variation of all contact angles were < 2%. This article not subject to U.S. copyright law. Certain commercial products or equipment may be is described in this article to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that they are necessarily the best available for the purpose.