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

Abstract

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. U nderstanding the mechanism and progress of UV degradation is one of the keys for predicting the service life of polymeric coatings. To achieve this objective, extensive research efforts 1-6 both in outdoorand indoor-accelerated weathering exposures have been carried out to investigate the influences of various climatic parameters on coating degradation, and to further establish the correlation between physical and chemical degradation. Typically, appearance-related measurements, such as gloss retention and color fading, are used to assess physical degradation and define the failure of the weathered coatings. Spectroscopic measurements, such as Fourier transform infrared (FTIR) spectroscopy, are often used for monitoring chemical degradation occurring in an exposed coating. Most studies 3-6 have focused on identifying primary chemical changes and assessing the rates of degradation under specific exposure conditions. The results have provided some understanding about the initial chemical degradation mechanism in the coatings.However, the link between specific chemical and physical changes (for example, gloss loss) in a coating has remained elusive, and inconsistent results as shown by investigators have been reported for various coatings and exposure conditions (both in outdoor-and indoor-accelerated weathering tests). 2,7 Consequently, the lack of correlation between gloss loss and chemical changes measured by spectroscopy has severely limited the ability to measure and predict service life accurately. Obviously, the difficulty in comparing FTIR results with gloss measurements is that no direct link has been established between the two measurements. The FTIR measures the concentration of chemical species in coa...

Multiscale physical characterization of an outdoor-exposed polymeric coating system

“…A detailed description of the LSCM measurements can be found elsewhere. 3,11 The laser wavelength was 543 nm. The LSCM images are 2D intensity projections with 512 pixels x 512 pixels.…”

“…1 With prolonged exposure, these photochemical processes lead to physical and mechanical changes in a coating. 2,3 In the physical properties, the change in surface topography and gloss are two highly interdependent parameters affecting the appearance of a coated system.…”

Scratch behavior of nano-alumina/polyurethane coatings

“…A detailed description of LSCM measurements can be found elsewhere. 11,12 The laser wavelength used in this study was 543 nm. LSCM images presented in this article are 2D intensity projections (an image formed by summing the stack of images over the z direction, 512 x 512 pixel) of the nanocomposite surface.…”

Adhesion and Durability of Coatings on Polypropylene Exterior Sidings

“…An increase in gloss is usually associated with a 'smoother' surface. Atomic force microscopy (AFM) could be used to determine the surface changes of the panels, as in the formation of pits and surface roughening or smoothing (Sung, 2004). Further XPS investigations could be completed on the weathered samples.…”

“…Further XPS investigations could be completed on the weathered samples. Laser scanning confocal microscopy (LSCM) can characterize ablation, formation of pits, and other surface defects, as well as measure film thickness over time (Sung, 2004). This technique would be helpful in determining the reason for the gloss increase as well.…”