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Austin Maples - Solvent Type, Concentration and Molecular Weight Dependant Morphology and the Effect on Internal Stress and Corrosion Performance

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Austin Maples - Solvent Type, Concentration and Molecular Weight Dependant Morphology and the Effect on Internal Stress and Corrosion Performance

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Austin Maples

Ph.D. Candidate

The University of Southern Mississippi

Phone: 701-266-2511

Website: www.linkedin.com/in/austin-maples-b1410957

Co-Authors: Richard C. Ferguson, Eric B. Williams, James W. Rawlins

 

Abstract

The most ubiquitous corrosion protection of substrates is the use of organic coatings as barriers to environmental contaminants such as water, oxygen, and salt.  While literature has shown that water ingress can be correlated to the onset of corrosion of metallic substrates, most literature only focuses on the diffusion of water and electrolyte through the bulk coating, or lateral percolation along the coating-substrate interface.  What is unclear throughout most of the current literature, is the effect of coating morphology on water diffusion, and water-solvent interactions that drive polymer morphology with respect to corrosion performance.  Furthermore, internal stress measurements have been used to quantify the build-up and release of stresses within a film throughout the drying process, but lacks correlation to morphology, focusing only on solvent evaporation, viscosity profiles, and vitrification processes.  By understanding the role of solvent selection on induced morphology within coating design, formulators can adjust systems for targeted use environments.  Internal stress, if correlated with both diffusion of water and contaminants as well as the morphology, may allow for rapid screening of formulations, with an increase in understanding of the possible corrosion performance.
Herein we report on the characterization of low, medium, and high molecular weight thermoplastic Phenoxy® resins solubilized in a set of solvents, dried under four profiles to yield distinct residual solvent concentrations, followed by three exposure conditions in order understand the role that molecular weight, residual solvent, and environmental exposure has on the morphology, and subsequently, corrosion performance.  We found that molecular weight of the polymer did not impact the residual solvent concentration; however, it resulted in quantifiable differences in internal stress measurements.  Gas chromatography-mass spectroscopy was utilized to determine the solvent composition remaining in the film and the partition differences between molecular weight and solvent concentration.  FTIR-ATR was used to monitor the water diffusion rate, saturation limits, and the water type, e.g., the degree of hydrogen bonding of the water within the polymer network, as the challenge solution penetrated the sample from the top.  It was found that the salt concentration, residual solvent, and the molecular weight of the applied polymer all result in different transport rates and diffusion coefficients of liquid water and the contained electrolytes, and water hydrogen bonding characteristics.  This has important implications in determining choice of solvent and polymer types for various exposure or use environments when designing new coatings, or improving existing formulations.  Furthermore, internal stress correlated well with results obtained via FTIR-ATR, whereby it was consistently observed that molecular weight and residual solvent each altered the amount of internal stress and the degree of stress mitigation that can occur upon drying the film after exposure.  The internal stress measurements also distinguished important swelling characteristics and film breakdown during the wet exposure cycle.  SEM resulted in verifiable differences in the morphology of the films before and after exposure to various solutions that trended with molecular weight and solvent content.  While all of the films exhibited rapid corrosion during accelerated weathering, by correlating all of the measurements, distinct differences in failure mechanisms and rates were observed, allowing for the improved understanding of the complex interplay between all of the variables within a coating system.  All of these factors are directly applicable to the polymer chemical composition, the affinity or lack thereof for water, the polymer molecular architecture, the effect of water on the film morphology and the effect of film morphology on the water, molecular weight, the residual solvent composition, concentration, and mobility within model protective films.
 

 

Biography

Austin Maples is a Ph.D. candidate at the University of Southern Mississippi, in the School of Polymers and High Performance Materials.  His research focuses on 2 primary areas: 1)  the use of fluorescent molecules to track various corrosion processes, i.e. pH changes, metal dissolution, and water content; and 2) diffusion processes and morphology of polymeric thin films. He received his B.S. of Chemistry degree from North Dakota State University performing research in the area of electrochemical assessments of polymeric coatings on metallic substrates. in the area of electrochemical assessments of polymeric coatings on metallic substrates.

 

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