Inhibition of the surface levelling of thermosetting polyester powder coatings caused by surface tension gradients

• Published in: Journal of materials science 2002-11, Vol.37 (22), p.4759-4768
• Main Authors: ZHAO, Y, CAREY, J. D, KNOOPS, N, MAETENS, D, HOPKINSON, I, HAY, J. N, KEDDIE, J. L
• Format: Article
• Language: English
• Subjects: Applied sciences; Atomic force microscopy; Coatings. Paints, varnishes and inks; Exact sciences and technology; Film formation and curing, properties, testing; Leveling; Materials science; Polyesters; Polymer industry, paints, wood; Polymer melts; Powder blending; Powder coating; Powder coatings; Surface defects; Surface roughness; Surface tension; Viscosity
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1023/A:1020866330857

Previous theoretical and experimental work has shown that surface tension gradients in liquid layers create surface defects and inhibit the levelling of an uneven surface. In coatings deposited from thermosetting polyester powders, which are studied here, small amounts of a low molecular-weight acrylate are incorporated to act as a “flow agent.” We find that this additive lowers the surface tension of the polymer melt and has a minor effect on the melt viscosity. A slower rate of levelling results from the decreased surface tension. We provide experimental evidence that lateral gradients in the surface tension of the polymer melt, resulting from the non-uniform distribution of the flow agent, inhibit the levelling of the surface. Specifically, the surface roughness of a powder coating is up to three times greater when a steep surface tension gradient is purposely created through powder blending. Surface tension gradients might also be responsible for the greater surface roughness (observed with atomic force microscopy on lateral length scales of 100 μm) that is found in coatings that contain flow agent.