Water-based laquer on wood

Non-contact quality control (inline) of multi-layer lacquering using Chemical Sensing.

The properties of lacquers and coatings do not only depend on their compositions, but also on the homogeneity and thickness of the applied layers. It is important to inspect both parameters (homogeneity and thickness) of a coating layer. Ideally this can be done in one measurement, but currently, there is a lack of widespread availability of methods and systems for providing precise homogeneity information as well as thickness information for the coating of a surface.

In this example, Chemical Sensing is used to detect the thickness of a clear coating on spruce wood. The left image shows the pieces as they are. And the right image shows the Chemical Sensing result.

Chemical Sensing uses hyperspectral imaging technology and spectroscopic analysis and modelling algorithms to portrait chemical/molecular properties as colours in an image (see Wikipedia(1) for a brief introduction to hyperspectral imaging). Multiple lacquers and coatings show absorption bands in the spectral range between 1000 nm and 1700 nm, such as polyurethane lacquer coatings, who show a characteristic one from 1650 to 1720 nm (Riemeier(2)). Also inorganic (containing copper, chromium or arsenic) and organic (carbon based) preservatives show absorption bands in this spectral range (So(3)). The images were recorded using the measurement system Perception HEAD Model 2 and analysis and modelling were done using Perception STUDIO.

In the Chemical Sensing image the clear coat thickness is depicted in a heat map from blue (background) to red (3 layers of clear coat). This clearly shows the non-destructive and simultaneous measurement of thickness as well as homogeneity of a clear coat on spruce wood offered by Chemical Sensing.

(1) https://en.wikipedia.org/wiki/Hyperspectral_imaging

(2) Riemeier, Milan & Lienig, Jens & Reifegerste, Frank & Pfeil, Lutz & Zierke, Fritz. (2016). Two-Dimensional Thickness Distribution Measurement Using near Infrared Spectroscopy. Journal of Near Infrared Spectroscopy. 24. 467. 10.1255/jnirs.1249

(3) So, Chi-Leung; Lebow, Stan T.; Groom, Leslie H.; Rials, Timothy G. 2004. The application of near infrared (NIR) spectroscopy to inorganic preservative-treated wood. Wood and fiber science. Vol. 36, no. 3 (2004): Pages 329–336

Article written by: Christoph Miksits, Senior Application Engineer, April 2022