Microstructure and morphology of amine-cured epoxy coatings before and after outdoor exposures—An AFM study

Atomic force microscopy (AFM) has been used to study the morphology and microstructure of an amine-cured epoxy before and after outdoor exposure. Measurements were made from samples prepared in an essentially CO₂-free, H₂O-free glove box and from samples prepared in ambient conditions. For those prepared in a CO₂-free glove box, AFM imaging was conducted on (1) an unexposed air/coating surface, (2) an unexposed coating bulk, (3) an unexposed coating/substrate interface, and (4) a field exposed air/coating surface. For samples prepared in ambient conditions, only the unexposed air/coating surface was investigated. The same regions of the exposed samples were scanned periodically by the AFM to monitor changes in the surface morphology of the coating as UV exposure progressed. Small angle neutron scattering and Fourier transform infrared spectroscopy (FTIR) studies were performed to verify the microstructure and to follow chemical changes during outdoor exposure, respectively. The results have shown that amine blushing, which occurs only under ambient conditions, had a significant effect on the surface morphology and microstructure of the epoxy. The surface morphology of the samples prepared under CO₂-free, dry conditions was generally smooth and homogeneous. However, the interface and the bulk samples clearly revealed a two-phase structure consisting of bright nodular domains and dark interstitial regions, indicating an inhomogeneous microstructure. Such heterogeneous structure of the bulk was in good agreement with results obtained by small angle neutron scattering of unexposed samples and by AFM phase imaging of the degraded sample surface. The relationship between submicrometer physical changes and molecular chemical degradation is discussed. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL.     

Relating gloss loss to topographical features of a PVDF coating

Semi-gloss commerical poly(vinylidene fluoride) (PVDF) coatings typically have 60° gloss values between 20 and 50. Gloss is affected by PVDF crystallite structures and by the pigmentation. In this article, we have demonstrated that for some pigmented PVDF coatings, after 10 years of Florida exposure, the principal proximal cause of gloss changes is the formation of micron-scale pits, rather than the emergence of pigment particles at the coating surface. We have used laser scanning confocal microscopy (LSCM) and light scattering to characterize the surface topography and near-surface structure of weathered and unweathered PVDF coatings. Florida-weathered PVDF coatings show only a modest increase in the root mean square (RMS) roughness of the surface, even when oticeable gloss loss has occurred. Changes in gloss can be correlated with surface roughness and other topographical, features, including the formation of pits and the emergence of pigments. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 25–27, 2004, in Chicago, IL.     

Use of laser scanning confocal microscopy for characterizing changes in film thickness and local surface morphology of UV-exposed polymer coatings

Laser scanning confocal microscopy (LSCM) has been used to characterize the changes in film thickness and local surface morphology of polymer coatings during the UV degradation process. With the noninvasive feature of LSCM, one can obtain thickness information directly and nondestructively at various exposure times without destroying the specimens or deriving the thickness values from IR measurement by assuming uniform film ablation. Two acrylic polymer coatings were chosen for the study, and the physical and chemical changes of the two systems at various exposure times were measured and analyzed. Those measurable physical changes caused by UV exposure include film ablation, formation of pits and other surface defects, and increases in surface roughness. It was found in both coatings that changes in measured film thickness by LSCM were not correlated linearly to the predicted thickness loss using the changes in the CH band obtained by the Fourier Transform Infrared (FTIR) spectroscopy measurements in the later degradation stages. This result suggested it was not a uniform film ablation process during the UV degradation. At later stages, where surface deformation became severe, surface roughness and profile information using LSCM were also proven to be useful for analyzing the surface degradation process Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13–14, 2004 in Philadelphia, PA.     

Validation of the reciprocity law for coating photodegradation

Accelerating the photodegradation of commercial polymeric materials has great practical importance in the weathering community. However, questions exist as to whether high radiant flux exposure results can be extrapolated to in-service exposure levels. Based on the reciprocity law, the photoresponse of a material is dependent only on the total energy to which the specimen is exposed, and is independent of the exposure time and the intensity of the radiation taken separately. An experiment to validate the applicability of the reciprocity law for polymeric coatings has been carried out using the NIST integrating sphere-based ultraviolet (UV) weathering device. A nonpigmented, non-UV stabilized acrylicmelamine coating was exposed to six different UV radiation intensities ranging from 36 W/m² to 322 W/m², and in the spectral region between 290 nm and 400 nm. Chemical changes in the coating due to UV exposure were measured with transmission Fourier transform infrared (FTIR) spectroscopy and UV-visible spectroscopy. Using two dose-damage models, the reciprocity law photoresponse for this polymeric system was verified for different photodegradation mechanisms, including chain scission, oxidation, and mass loss. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL.     

Combination of atomic force microscopy and chemical hydrolysis to characterize degradable regions in polymer blends

In this study, we demonstrate the usefulness of chemical‐based method in combination with atomic force microscopy (AFM) to characterize the degradable regions in a wide range of polymer blends. This approach is based on selective hydrolysis of one of the components in a multiple‐phase system, and the ability of AFM to provide nanoscale lateral information about the different phases in the polymer system. Composite films containing different percentage of hydrolyzable polymer were either melt processed or solution casted and then exposed to a hydrolytic acidic environment. Tapping mode AFM was used to analyze the samples before and after hydrolysis. Dramatic topographic changes such as pits were observed on the acid exposed samples, indicating that the degradation was localized and the more susceptible component in the blend was hydrolyzed. Additionally, the progressive hydrolysis of the composites was studied by attenuated total reflection FTIR (ATR‐FTIR) analyses to confirm the AFM results. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 726–733, 2006