To develop coatings which guarantee optimal protection against environmental influences (weathering, chemicals etc.), it is necessary to detect and to differentiate the related defects in coatings already during their development. Various accelerated weathering tests and other methods (e.g. salt spray test) have been developed to save costs and to speed up the material development. Despite of the progress in high throughput experimentation, visual inspection is still the dominant screening method for coatings. However, visual inspection has several disadvantages: (i) the defects must be visible by naked eyes, (ii) it is a highly subjective method, and (iii) it is limited to defects, which are visible at the coating surface. The latter is an important limit for early detection of defects. The detection of delaminations or air bubbles in pigmented coatings is only possible if defects are visible at the surface. Therefore, methods which allow automatic detection of defects (e.g. in early stages of weathering tests) are highly desired. Such a method should provide the potential to locate the defects also inside the coating and to differentiate between various types of defects.

 Scanning acoustic microscopy (SAM) is well known as a non-destructive imaging method for quality and process control and for research applications, especially in microelectronics. However, the applications to polymer materials are still rare. The principle of a SAM is similar to that of a scanning electron microscope. The individual points of the image are successively scanned and composed to the final picture. In SAM, high frequent acoustic pulses are excited and focused by an acoustic lens into the material. The area of interest is located in the focal plane. The reflected and/or transmitted acoustic signals are detected for each position and the images are processed from the digitized signals (selected amplitude and/or phase of the echo train). In this study SAM has been tested for detection of the early phase of delamination, air bubble distribution (>10 µm), agglomerates (e.g. pigments, fillers), speckles and surface defects (scratches, orange skin or pores). Fig. 1 shows delaminations spread out from a scratch after salt spray test. 

Figure1: SAM images of delaminations spread out from a scratch after salt spray test for a single (left) and double layer (right) polyurethane coating.

 SAM can be applied for clear coats as well as for pigmented single and multi layer coatings without specific sample preparation. The possibility of a quantitative analysis of weathering induced defects is one of the advantages of the method. The development and implementation of specific software for coating applications (e.g. “surface scan” for topography imaging) is in progress.

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