Monitoring of paint drying process

 A quantitative evaluation of drying process for paints has been required in industry. A technique to study the drying of paints, based on phase-shifting digital holography, is presented.

The experimental setup is shown in Fig. 1. Water-based clear paint for a plastic model is applied to the copper (Cu) plate. A painted surface is illuminated with an expanded beam from a laser diode (LD). The recording of four phase-shifting holograms was repeated with the interval of 1 s.

Fig. 1 Experimental setup.

During a drying process, a painted surface changes due to its film formation. After the paint is completely dried, the surface becomes stable and shows no dynamic change. So, dynamic change can be seen in the reconstructed image of painted surface during the process. To investigate the change of image depending on the drying status, we took the difference of 1.9 s intervals and the results are shown in Fig. 2.


Fig. 2 Phase difference images for water-based clear paint.

For the detection of stationary state, we obtained the time history of a standard deviation σ of phase difference between the adjacent reconstructed complex amplitudes. The temporal variation of value of σ is shown in Fig. 3.

Fig. 3 Time dependence of σ for phase difference image.