The Waterborne Symposium

Abstract
Understanding Polymer-based Coating Transport Property Differences between Hydrophilic Particles (hydrogels) and Semi-impermeable Particles (inorganic pigment)
Co-authors:  Eric B. Williams, Joseph Tyson, Gopinath Subramanian, and James W. Rawlins

Organic polymer coatings protect metal substrates through a variety of mechanisms depending on exposure time and environmental severity. For example, coatings behave as water, oxygen and electrolyte barriers (with varying degrees of success), and corrosion inhibiting pigments or electrically active additives can be incorporated. The aforementioned protection methods consistently support the need for limiting water, oxygen, and/or electrolyte content at the polymer-metal interface which is rarely achieved in practice.  Therefore, quantifying and understanding the path water takes and the momentary/equilibrium location of the sorbed water in relation to the coating-substrate interface has promise to afford mechanistic understandings of water management to facilitate increased coating performance longevity. Water interactions with water loving moieties, molecules, and various shaped particles offer a route to alter water management methods and mechanisms.  We utilized hydrogel particles to create distinct water-loving domains to allow for concentrated ionic content, defined particle diameter, and quantifiable swellability in localized regions to understand and intentionally alter water and electrolyte interactions with both the parent polymer and hydrogel particles.  
Previous literature highlighted the utility of hydrogel particles with defined size and hydrophilic character to limit the through-film water diffusion to the substrate.  By limiting water accumulation at the substrate, adhesion was maintained during wet-dry cycles when blended into dried and annealed thermoplastic polymer solutions.  Our data suggests that hydrophilic inclusions act as water scavengers/traps while applying increased hydraulic pressure on the parent polymer film through swelling, thereby reducing the equilibrium water content for the parent polymer.  The incorporation of hydrogel consistently resulted in limiting the propagation of corrosion at a defect to an overall 2% by area, suggesting an entrapment mechanism occurs when water diffuses into the polymer films likely electrolyte interactions.  The films that contain hydrogels possess a higher water concentration at equilibrium than control films (e.g., 10% more water held).  Comparisons with non-swellable pigment particles were performed to delineate between tortuosity and transport properties within the coating.  Specifically, reported herein are the characterization of low molecular weight thermoplastic Phenoxy® resin solubilized in a solvent blend with various pigment volume concentrations (PVC) of TiO2 and weight percent of hydrogel particles to understand the relationship between fillers and additives during environmental exposure. Computational and FTIR-ATR experimental results were utilized to compare non-pigmented to the pigmented counterparts to gain understanding in diffusivity differences from the various formulations.