Cyril Clerici

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

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.

Effect of Pigment Dispersion on Durability of a TiO 2 Pigmented Epoxy Coating During Outdoor Exposure

The effect of pigment dispersion on durability of a TiO 2 pigmented epoxy coating during outdoor exposure has been investigated. Well-dispersed and poorly dispersed coating samples were prepared through the addition or absence of a dispersant in the coating formulation. Ultra small angle neutron scattering (USANS) and scanning electron microscopy (SEM) showed that pigment aggregation occurs in the absence of dispersant. A thin, clear layer of epoxy was observed at the air/exposed surface interface in both the dispersed and non-dispersed samples. Chemical degradation and physical changes during UV exposure were measured by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), atomic force microscopy (AFM), and laser scanning confocal microscopy (LSCM). Results showed that the degree of pigment dispersion and the thickness of the clear layer contributed to weathering. Changes in surface topography and gloss loss during UV degradation were correlated with degree of pigment dispersion. Ripples and bumps on the top surface of the poorly dispersed coating greatly affected gloss. Bulk and surface mechanical properties were investigated using dynamic mechanical thermal analysis (DMTA) and instrumented indentation, respectively. Relative to the neat epoxy coatings, the addition of TiO 2 particles into the epoxy coatings increased elastic modulus but decreased the glass transition temperatures (T g ) of both of the pigmented coatings. Relationships between surface and bulk mechanical property changes and chemical degradation are discussed.

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D2O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO2 interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.

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

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.

Characterization of Ambient Temperature Cure Epoxies Used in Adhesive Anchor Applications

Thermo-viscoelastic properties of two commercial, ambient temperature-cure epoxy structural adhesives were analyzed and compared. The adhesives were formulated by the same manufacturer and appeared to have the same base chemistry; however, one system contained accelerators for shorter cure times. In the laboratory, dynamic mechanical temperature/frequency sweeps were performed on both systems to generate dynamic mechanical data and predict creep compliance master curves using frequency-temperature superposition principles. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and moisture sorption analysis were also used to assess the thermal and hygroscopic properties of the materials. Differences were observed in the thermal, hydrolytic, and dynamic mechanical properties of the two adhesive systems as well as in their estimated creep compliance behavior, which were attributed to differences in the curing agent(s) and accelerator(s) used in the adhesive systems. In most cases the differences in the properties of the epoxies were small, but a few properties, particularly the predicted creep behavior, exhibited very large differences. Results from laboratory creep testing confirmed the predicted difference in creep behavior. The data also suggest that dynamic mechanical testing combined with frequency-temperature superposition may be a useful metrology for predicting trends for in-service creep behavior from short term tests.

Metrologies for Characterizing Optical Properties of Clear and Pigmented Coatings

Coatings are designed to achieve a particular appearance effect and to protect consumer products and built structures. The ability to accurately characterize appearance attributes of a coating and to relate these attributes to material and performance properties continues to be of great academic and industrial research interest. Key material properties, such as surface topography, pigment dispersion, and heterogeneity in microstructure, affect the appearance and are believed to impact the service life and mechanical properties of a coated object. Currently, specular gloss measurements using commercially available glossmeters remain the primary measurement tool for assessing the appearance and durability of coated objects. However, it has not been possible to consistently correlate specular gloss measurements with the fundamental micro-physical surface properties, such as surface roughness, pigment dispersion, and subsurface microstructure of a material, that are felt to affect the appearance of a coating. These changes can only be monitored through changes in diffuse scattering intensity, which commercial glossmeters do not measure. Chapter 17