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Wednesday, February 19 • 3:30pm - 4:00pm
Solvent Trapping Effect on Pull-off Adhesion of Model Epoxy-amine Coatings to Steel - USM

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Solvents continue to be a large part of the surface coating industry, making it important to know how the chosen solvents and their quantities affect the final product and performance.  One of the defects seen in solventbased systems is solvent trapping.  Rapid solvent loss from a freshly applied coating could result in the surface layer forming a skin, which significantly decreases the solvent diffusion coefficient at the air interface and traps the solvent remaining in the film.  The residual solvent has significant influence on the film's properties. Primarily, residual solvent induces film plasticization, which is manifest as reduction in modulus and glass transition temperature.  Due to skin formation, these changes are noted more towards the substrate interface.  
The effects of residual solvent on interfacial properties such as adhesion are virtually unexplored.  In the absence of residual solvent, increasing film thickness leads to increased internal stress and predictably lower adhesion.  As a consequence, there is a critical thickness for any formulation above which the coating is unable to be applied because it will spontaneously delaminate.
This solvent trapping process can be modeled to predict the amount of residual solvent over a range of time and temperatures.  The present work demonstrates a reversal of the relationship in the presence of residual solvent leading to better adhesion with increased thickness and elimination of the spontaneous delamination thickness.  Numerous factors contribute to this outcome and it is the goal of this work to investigate internal stress, modulus and interfacial interactions to explain what factors contribute in what way to the solvent trapping effect on adhesion.
The coatings in this study are model epoxy-amine networks formulated to cover a range of crosslink densities and glass transition temperatures.  Material properties, cure temperature, solvent boiling point and solvent-resin affinity encompass the processing knobs available to control the residual solvent content and its distribution.  Solvents that were used include ethanol, isopropanol, butanol, di(propylene glycol) propyl ether, xylene, tetrahydrofuran and p-chlorobenzotrifluoride. At each film thickness, the films were evaluated for residual solvent content, adhesion, modulus, internal stress and glass transition temperature. Residual solvent was determined by thermal gravimetric analysis. Adhesion was quantified using the pull-off method on a MTS load frame, while tensile testing was performed on the same load frame to collect modulus data. Internal stress of formation was calculated based on substrate deflection. The glass transition temperature was quantified from the tan d peak measured via dynamic mechanical analysis. These studies will help with the prediction and planning of interfacial adhesion properties of coatings and better formulation skills.

Speakers
DG

Diana Gottschalk

Graduate Student, University of Southern Mississippi



Wednesday February 19, 2020 3:30pm - 4:00pm
Rhythms II-III