Assessment of Water Penetration Resistance of Coatings on Wood by Droplet Dynamics Analysis.
- Baptista, Diogo
University of Maine Graduate School
Moisture uptake in wood or wood-based products reduces their performance and durability. Development of improved, more effective coating systems that provide durable protection and thus extend the service life of wood products depends largely on a good understanding and accurate methods to assess their effectiveness in resisting water penetration to wood.
The main objective of this thesis was to assess the suitability of modified droplet dynamics analysis to assess the water penetration resistance on coatings.
To achieve the main objective, the experimental method used for dynamic droplet analysis was modified so that water penetration and surface properties of coatings could be determined in the same test. In this method, a water droplet is deposited on a coated surface and changes in droplet volume due to evaporation and penetration into the coating are registered on a series of digital images.
The quantification of water droplet evaporation was done using an evaporation model based on experimental results measured on impermeable reference samples, covering a wide range of surface energies and thermal conductivities. The results allowed determining the decreasing droplet volume in time, with an accuracy of at least 90% and proved to be valid as long as either the base radius or the contact angle of the droplet remained constant. The findings were used for further studies to quantify residual droplet volumes in the same experimental conditions.
Wood is an anisotropic material and an accurate analysis of water droplets on wood and wood-based materials requires having a full view of the droplet that can assume oblong shapes. To address this problem, a three-dimensional methodology was developed to study the droplet dynamics on anisotropic surfaces. The method uses the images collected simultaneously from three cameras positioned orthogonally to each other, for reconstructing the droplet as an ellipsoid, allowing calculating contact angle, droplet volume, and base and cap area. A major advantage of the developed method is its applicability to anisotropic surfaces, even when the orientation of the droplet's axes changes throughout the droplet lifetime.
The effect of short term accelerated aging on the water penetration resistance of coatings was studied. However the accelerated weathering applied over limited time duration did not produce measurable effects on the water penetration resistance of coatings. In this way, it was not possible to determine the potential of using droplet dynamics analysis as a new method to assess water penetration resistance of coatings.
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