Evaluation of Perfluoropolyether Coatings for Environmental Protection of Stone
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Date
2002-04-24
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Abstract
The objective of this study was to develop and evaluate the rapid expansion of supercritical solutions (RESS) as an environmentally safe coating process. RESS allows formation of small particles or droplets of polymers dissolved in supercritical carbon dioxide (SC-CO2), by rapidly expanding the supercritical solution through an expansion nozzle. Due to the abrupt pressure decrease to atmospheric pressure, very high super-saturation values can be achieved. Furthermore, as the pressure change travels at the speed of sound, uniform operating conditions are maintained within the solution and a large number of small nuclei are formed. Previous RESS experimental results indicate it is an extremely attractive technology; small droplets and particles can be obtained with a narrow (mono-disperse) droplet size distribution. The present work is aimed at gaining an understanding of the relationship between droplet and spray characteristics and RESS process conditions. This was also extended to the evaluation of coating effectiveness at different RESS process conditions. Sandstone samples were coated with a perfluoropolymer (UNC diamide) by the RESS process for this study. The characteristics studied were transfer efficiency, liquid water contact angles, liquid water absorption and water vapor diffusivity.
The experimental results show that increases in process (i.e. supercritical solution) temperature reduce transfer efficiency. Increases in polymer coating thickness lead to higher contact angles, higher reduction of liquid water absorption and reduction in water vapor diffusivity.
The synthesis of new polymers to be used for stone coating applications was also considered. The polymers designed and synthesized for this investigation were EVE (15)-ba and EVE (15)-au. Both polymers are derivatives of the same base polymer with the only difference being the pendent functionality. Therefore, these two new polymers contain multiple pendent functionalities per chain so they are capable of forming associative networks via hydrogen-bonding interactions that can result as an important factor in the water absorption data. This work presents the performance of these compounds brush coated on sandstone. The evaluation was based on performance of these compounds exposed to de-ionized water. Contact angle, liquid water absorption and water transport rates were measured for varying amounts of polymer coating.
Since the RESS process requires a large amount of polymer to be synthesized, therefore small-scale brush coating evaluations were used technique to identify coating compounds which display good water protection characteristics. The experimental data shows that the coating compound EVE (15)-au exhibited lower water absorption and also lower water vapor transport rate than the EVE (15)-ba coating. Finally, it was observed that there was not a significant reduction in water vapor transport as a result of the brush coating process with either compound. Identifying how pendent functionality affects coating properties is an important factor in developing a compound we wish to study by RESS process coating.
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RESS, Rapid Expansion of Supercritical Solutions
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Degree
MS
Discipline
Mechanical Engineering
