Larry P. Haack

Identification of two types of oxidized palladium on γ-alumina by X-ray photoelectron spectroscopy

Abstract It was demonstrated by X-ray photoelectron spectroscopy (XPS) that oxidized palladium supported on γ-alumina can exist in two chemically different entities, distinguishable by their observed electron binding energies and by selective reduction. Larger oxidized particles are characterized by a binding energy attributed to PdO. This oxide is reduced to metallic palladium at room temperature in hydrogen, as would be expected for bulk PdO. On the other hand, at low palladium loadings of 0.5 wt.-% and less, the majority of the palladium is present at a binding energy shifted approximately 1.6 eV higher than that observed for PdO. The shift appears to be associated with a metal-support interaction at concentrations where the palladium is highly dispersed. Contrary to that observed at loadings greater than 0.5 wt.-%, this palladium remains mostly oxidized when exposed to hydrogen at room temperature. The results are compared with literature data reported for platinum/γ-alumina.

High Frequency Rheometry of Viscoelastic Coatings with the Quartz Crystal Microbalance

We describe a quantitative method for using the quartz crystal microbalance (QCM) to characterize the high frequency viscoelastic response of glassy polymer coatings with thicknesses in the 5-10 μm regime. By measuring the frequency and dissipation at the fundamental resonant frequency (5 MHz) and at the third harmonic (15 MHz), we obtain three independent quantities. For coatings with a predominantly elastic response, characterized by relatively low phase angles, these quantities are the mass per unit area of the coating, the density-shear modulus product, and the phase angle itself. The approach was demonstrated with a model polyurethane coating, where the evolution of these properties as a function of cure time was investigated. For fully cured films, data obtained from the QCM are in good agreement with results obtained from traditional dynamic mechanical analysis.

Interfacial chemistry of spontaneous disbonding in stress durability testing of adhesively-bonded galvanized steel

Epoxy adhesive/galvanized steel bonds subjected to corrosion testing show a gradual loss of strength. Bonds subjected simultaneously to a static mechanical load and corrosion testing rupture spontaneously at relatively short exposure times. The differences in interfacial chemistry that accompany these exposure conditions were studied using an XPS elemental mapping technique that allowed the interfacial composition to be resolved spatially over the entire bond failure surface. An interfacial anodic process reminiscent of crevice corrosion dominated the interfacial chemistry of specimens exposed to corrosion testing without application of a static load. Bonds exposed under high loads exhibited both anodic and cathodic corrosion sites within the bond failure area. The changes in interfacial chemistry and failure mode upon application of a load are attributed to the opening of an interfacial crack at the locus of the initial corrosive attack. The ingress of electrolyte and the formation of cathodic sites adja...

Chemistry of surface modification with UV/ozone for improved intercoat adhesion in multilayered coating systems

Surface modification using UV/ozone was explored as an approach towards ensuring robust intercoat adhesion in multilayered automotive coating systems. This study was directed toward reducing the variability in adhesion performance associated with changes in coating surface chemistry that can result from the surface migration of formulation additives. The evaluated coating system included a melamine-cross-linked polyester layer applied over a commercially available epoxy layer, which is known to become surface-enriched by a polyether-based crater-control additive (CCA) in its formulation. X-ray photoelectron spectroscopy established that UV/ozone treatment oxidizes the epoxy CCA overlayer and forms carboxyl species. Contact angle measurements identified a concomitant increase in surface wettability. Epoxy-to-polyester adhesion improved slightly after exposure to ozone alone and dramatically after exposure to UV/ozone. X-ray photoelectron spectroscopy of interfacial epoxy surfaces, exposed by delamination of the polyester coating, disclosed that bond-line fracture occurs within a CCA-enriched region, with a trend of decreasing CCA concentrations in proportion to the UV/ozone exposure time initially received by the epoxy. Copyright

Bake oven induced variation of surface chemistry on electrocoat paint: Effect on primer-electrocoat intercoat adhesion

The interplay of bake oven processing environment and surface/interfacial chemistry, and their effect on intercoat adhesion has been investigated for a model primer/electrocoat paint system. X-ray photoelectron spectroscopy (XPS) surface analysis established that a prominent ether component in the C 1s core level spectrum correlated with a polyether-based crater-control additive in the electrocoat that surface-segregates during cure. Laboratory-simulated bake oven experiments confirmed that better adhesion characteristics were realized by removal of this electrocoat overlayer through reaction with nitrogen oxides and high levels of moisture present in the cure environment of production direct-fired ovens. Details of the effect of bake oven atmosphere on cure chemistry for direct- versus indirect-fired ovens are presented.

Characterization of interfacial chemistries associated with polymer systems by spatially resolved surface analytical methodologies

Abstract Two examples of recent advances in spatially resolved surface analytical characterization of paint and adhesive chemistries are presented. The examples extend previous analytical capabilities by combining the molecular specificity and bonding information of static secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) with the ability to resolve features of interest spatially. In one example, a cross-section of two paint layers, only one of which contained a photostabilizer additive, was characterized by time-of-flight (TOF)-SIMS operated in a microscope mode. Results indicate possible migration of this additive into the bulk of the adjacent paint layer. The second example presents an XPS mapping technique which was used to study the interfacial corrosion chemistry of an epoxy adhesive applied to galvanized steel. The sample was subjected to a corrosive environment while under static load. This technique afforded spatially resolved chemical information over the entire interfacial failure surfaces and allowed localized regions of corrosion activity to be imaged. Comparisons of results and techniques are made, along with discussions of the limitations of each technique.

FAST-RESPONSE ZIRCONIA SENSOR-BASED INSTRUMENT FOR MEASUREMENT OF THE AIR/FUEL RATIO OF COMBUSTION EXHAUST

A fast-response zirconia sensor-based instrument has been developed to measure the air/fuel ratio (A/F) of combustion exhaust. This instrument uses a reduced-pressure sampling system which improves instrument response time (due to faster diffusion at lower pressures) and eliminates the need for a heated sample line. The measured response time of the described instrument is 170 ms (0-90%) for a step-change in oxygen concentration. The prototype instrument is easily calibrated in less than 2 min, requiring only nitrogen and room air for calibration. A complete description of the instrument is given. Results of tests comparing the instrument accuracy to a chemical balance technique are given. Also, a comparison study was conducted with the prototype instrument and a conventional zirconia sensor-based A/F monitor (Lamdascan, Sensors, Inc., Ann Arbor, Michigan) with respect to accuracy and response time.

Neat Zinc bis[O,O-bis(2-ethylhexyl)] dithiophosphate (ZDDP), by XPS

X-ray photoelectron spectroscopy was performed directly on neat zinc bis[O,O-bis(2-ethylhexyl)] dithiophosphate liquid. Data were collected on the liquid oil placed in a recessed gold-coated sample holder without any specific precautions relating to outgassing or charge compensation. The compound is of a class of zinc dialkyl dithiophosphates (ZDDP) employed as anti-wear additives in modern motor oils. Our lab is advancing the study of the interactions between ZDDP and other additives in fully formulated engine oils. ZDDP is manufactured by neutralization of dialkyldithiophosphates with zinc oxide. The O 1s core level was found to be composed of two peaks, a higher binding energy state attributed to the ZDDP complex, and a lower binding energy inorganic peak (about 10 percent contribution) attributed to the presence of unreacted zinc oxide. This lower-binding-energy zinc oxide peak did not increase after consecutive 10 min acquisitions. Thus, at the analysis parameters utilized in this work, XPS was achie...

Influence of coating formulation variables and processing on the adhesion of melamine-crosslinked polyester primer to urethane-crosslinked epoxy electrocoat

The role of spray-primer formulation chemistry towards robustness of adhesion to an epoxy-based electrocoat was examined for a range of coatings representative of those used in automotive OEM applications. For coatings exhibiting degraded adhesion performance, high-resolution X-ray photoelectron spectroscopy (XPS) identified the presence of a surface-active polyether-based anti-cratering agent (ACA) formulated into the epoxy-based electrocoat at the primer/electrocoat interface. The adhesion performance of the better-performing systems was found to be related to the spray-primer’s ability to accommodate the ACA by either absorbing or displacing it from the electrocoat surface, thus diluting it to concentrations no longer adversely affecting adhesion.