Joseph W. Holubka

The application of x-ray photo-electron spectroscopy to a study of interfacial composition in corrosion-induced paint de-adhesion

Abstract Interfacial surfaces generated by corrosion-induced de-adhesion of organic coatings on bare steel have been analyzed by X-ray photo-electron spectroscopy (XPS). XPS elemental analyses have been used to provide an initial overview of changes in composition relative to untested surfaces; high resolution XPS spectra have been used to provide more detailed, sometimes semi-quantitative, analyses of molecular functional group composition. The high resolution spectra of specimen surfaces have been compared with each other and with reference compounds qualitatively and, in more detail, by calculating difference spectra and binding energy differences. Curve resolution methods have been employed to estimate functional group compositions. Corrosion-induced de-adhesion is associated principally with cathodic corrosion reactions and reaction products, including hydroxide. Epoxy ester-based coating interfacial surfaces have been demonstrated to bear (in addition to undegraded resin) carboxylate moieties indicative of ester saponification. For model epoxy-urethane and epoxy-amine coatings, a substantial carbonate residue is deduced. This residue is attributed to degradation of urethane and urea moieties. In addition, the presence of polymer residues on the interfacial substrate surfaces is demonstrated. It is concluded that de-adhesion involves substantial cohesive failure of the coating resin in the interfacial region.

Interfacial chemistry of corrosion-induced bond degradation for epoxy/dicyandiamide adhesive bonded to cold-rolled and galvanized steels

The interfacial chemistry of a conventional epoxy/dicyandiamide adhesive formulation applied to cold-rolled and galvanized steels has been studied by X-ray photoelectron spectroscopy (XPS). In both cases, the interfacial corrosion process appears to be dominated by attack on the metal substrate, and is reminiscent of crevice corrosion phenomena. On cold-rolled steel, analysis of the interfacial bond failure surfaces suggests that the contamination layer initially present on the substrate is not disturbed during bond formation. Bond failure appears to occur between the substrate and the contamination layer; the contamination layer remains largely intact on the interfacial surface of the adhesive after bond failure. On galvanized steel, the locus of failure appears to be at the adhesive-adherend interface, within a layer of zinc corrosion products. The contamination layer appears to have been partially displaced by the adhesive during bonding.

Interfacial chemistry of epoxy-modified poly(vinyl chloride) adhesive on cold-rolled and galvanized steels

—The interfacial reactions of galvanized steel with a poly(vinyl chloride) adhesive formulated with epoxy resin and dicyandiamide have been studied using X-ray photoelectron spectroscopy (XPS). The dicyandiamide was observed to segregate partially to the adhesive/substrate interface. A small amount of amine hydrochloride, evidently formed by reaction of dicyandiamide and HCI formed by dehydrochlorination of the poly(vinyl chloride), was observed near the adhesive/ galvanized steel interface. Dehydrochlorination of poly(vinyl chloride) appears to be enhanced in the adhesive/substrate interfacial region relative to the bulk, but less degradation was observed than for unmodified poly(vinyl chloride) formulations. In parallel studies on cold-rolled steel, neither thermal degradation nor amine hydrochloride formation was observed.

Interfacial chemistry of poly(vinyl chloride) adhesive on cold-rolled and galvanized steels

—The interfacial chemistry of a model poly(vinyl chloride) adhesive formulation applied to cold-rolled and galvanized steels has been studied by X-ray photoelectron spectroscopy (XPS). Chemical changes suggesting enhanced dehydrochlorination of poly(vinyl chloride) in the polymer/ metal interfacial region were observed on both substrates. An ionic chloride species (probably zinc chloride) and an increased level of hydrocarbon were observed at the adhesive/substrate interface on galvanized steel; the total amount of chlorine present near the interface was much less than in the bulk of the adhesive. On cold-rolled steel, the amount of chloride was only slightly lower near the metal surface, and no ionic chloride was observed. Polymer degradation is suggested by an increase in hydrocarbon observed at the interface. Changes in the oxidation state of the iron surface oxide were also observed.

Molecular modeling of reduction reactions of dicyandiamide on zinc: a theoretical study of epoxy adhesive/galvanized steel adhesion

Molecular orbital calculations have been used to model the interfacial reactions that occur during bond formation of epoxy adhesives to galvanized steel. The theoretical calculations were applied to interfacial reactions of generic dicyandiamide-crosslinked epoxy adhesives on zinc surfaces. Semi-empirical MNDO molecular orbital calculations were used to model the reduction reactions. The calculations were used to predict the most stable tautomer of dicyandiamide as well as to suggest the structure of the most likely zinc-dicyandiamide reduction products. The results of the calculations are consistent with infrared spectroscopic studies of dicyandiamide reduction on zinc surfaces that suggest the formation of carbon-nitrogen multiply bonded moieties. An MNDO geometry optimization of dicyandiamide suggests that one tautomeric form of dicyandiamide, the diamino form, is the most stable structure by about 9.3 kcal/mol. The optimized geometry of the diamino form of dicyandiamide is nearly planar, suggesting th...

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

Mechanism for Environmental Etch of Acrylic Melamine-Based Automotive Clearcoats: Identification of Degradation Products

This paper focuses on defining the reaction mechanism involved in the environmental etch of acrylic melamine-based automotive clearcoats via an identification of reaction products. This has been accomplished through a comparative study of products formed on acid treatment of neat crosslinkers, and those formed following laboratory acid treatment and field exposure of acrylic melamine clearcoat systems. Bulk elemental, X-ray photoelectron spectroscopy (XPS), and infrared (IR) analyses of sulfuric acid-treated melamine crosslinkers show that acid hydrolysis results primarily in the formation of melamine sulfate. Melamine sulfate formation was also observed following laboratory and field exposure of acrylic melamine clearcoat systems. These results confirm that the primary mode of melamine crosslink decomposition is through hydrolysis of acetal linkages and subsequent formation of melamine sulfates. However, data show that hydrolysis of pendent amino groups on the triazine ring also occurs.

A model for adhesion-producing interactions of zinc oxide surfaces with alcohols, amines, and alkenes

The interactions between paint/adhesive polymers and metal surfaces that are critical for adhesion have been studied theoretically. This study used zinc oxide as a model of a galvanized steel surface, and ammonia, water, and ethylene as models for amino, hydroxy, and unsaturated functionalities in paint/adhesive polymers. Ab initio molecular orbital calculations were carried out on zinc oxide and zinc oxide dimer. Geometries were optimized at the HF/3-21G level and relative energies were calculated by CASSCF/3-21G and by MP2 with the DZP basis set of Wachters and Hay. Ethylene forms a stable complex with zinc oxide dimer that has a stabilization energy of 24.9 kcal/mol. Insertion of ethylene into zinc oxide dimer to form a stable six-membered ring adduct occurs with a surprisingly low activation energy of 8.8 kcal/mol. The binding energy of ammonia with zinc oxide dimer is 38.5 kcal/mol and the activation energy for insertion of ammonia forming covalent Zn-NH2 and O-H bonds is calculated to be 9.6 kcal/mo...

Acid etch of automotive clearcoats II. Comparison of degradation chemistry in laboratory and field testing

A laboratory test procedure was developed to quantitatively evaluate the acid etch resistance of automotive clearcoats. Here we attempt to verify that the conditions used in the laboratory test reproduce the field degradation chemistry to assure that the test is evaluating realistic performance characteristics. In this study we applied X-ray photoelectron spectroscopy (XPS) as a tool to determine if degradation products are observable which can be used to verify the consistency of the field and laboratory degradation processes for acrylic melamine-based clearcoats.

Acid etch resistance of automotive clearcoats. I: Laboratory test method development

This paper reports on the development of a laboratory test procedure for the evaluation of the environmental etch resistance of clearcoats. The test evaluates the bulk acid hydrolysis resistance of clearcoats by gravimetrically following material weight loss as a function of exposure time to a sulfuric acid solution, under conditions that simulate outdoor exposure. The bulk hydrolysis resistance of five production clearcoat technologies including acrylic melamine, acrylic melamine-silane, carbamate, acrylic urethane, and epoxy acid were evaluated. Results from the weight loss measurements were consistent with those anticipated based on the coating systems bulk chemistry and inherent hydrolysis resistance, for clearcoat systems processed under nominal processing conditions. The relative rankings from the laboratory test were found to correlate with field etch ratings. The test method is inexpensive, quantitative, and generates repeatable results that are not subject to environmental variations associated with current field etch testing.