Unsaturated polyester microstructures: Morphology and relation to electrical properties by current sensing atomic force microscopy (CSAFM)

Abstract

Polymeric coatings offer a relatively simple and cost-effective method of protecting steel assets from corrosion. Without this protection, corrosion can weaken the structural integrity of the steel and jeopardise the lifespan of the asset. In some cases, corrosion can cause catastrophic failures that risk lives as well as economic and environmental damage. It has long been theorised that structural and crosslinking inhomogeneities in the cured polymer coating may represent micro- or nano-scale defects in the barrier properties of a coating, allowing the ingress of exogenous materials. This work confirms the feasibility of detecting these inhomogeneities with established analysis techniques, attempts to correlate the polymer structures found with electrical properties, understand the mechanisms that link polymer structure and electrical properties, and ultimately use this new understanding to enhance anti-corrosive coatings. Samples of cured isophthalic unsaturated polyester are exposed to salt fog before examination using an Alicona InfiniteFocus 3D optical microscope and MAC III Agilent 5500 Atomic Force Microscope in various modes. Other studies have reported microstructural features in unsaturated polyesters; the expected structure was a three dimensional cellular structure with microgels of unsaturated polyester connected by polystyrene branches. After only 15 minutes of salt fog exposure, the polymer began to show this structure. Current Sensing Atomic Force Microscope (CSAFM) scans attempt to correlate these structures with highly localised electrical properties but were unsuccessful. Theories of ion migration and conductivity in polymers are discussed with relation to CSAFM and a commercially available unsaturated polyester coating.

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ORCID for Julian Wharton: orcid.org/0000-0002-3439-017X

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