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Hydroplasticization of Styrene-Acrylic Resins by DSC
Co-Author: Evan P. Kangas
This work describes glass transition temperature (Tg) measurements made on two styrene-acrylic resins and their respective sodium salts under various conditions of water infusion. The work enables quantification of the hydroplasticization behavior for the acid-functional resins, RESIN #1 of acid value 55, and RESIN #2 of acid value 140. For instance, we find that the Tg for RESIN #1 drops from 64 ºC to about 47 ºC when fully hydrated, while the higher acid-containing RESIN #2 undergoes a similar drop from 65 to 50 ºC. We interpret this in terms of the co-monomers used, which are on balance considerably more hydrophilic for RESIN #1. We also find that the amount of water actually contributing free volume, i.e. hydroplasticization, is less than the amount estimated by Barrie’s group contribution method, which has been utilized by other researchers. Our analysis leads to a substantially lower effective Tg for water of about -190 ºC. A large increase in hydrophilicity is seen upon conversion to the sodium salt, with several noteworthy conclusions. Once reacted with NaOH, RESIN #2 becomes more hydrophilic than RESIN #1, presumably because of its higher acid number. Also, we find that full removal of water requires higher temperatures, especially for the RESIN #2 salt. However, once done, a much higher Tg is recorded. In other words, these ionized resins at roomtemperature pick up enough measureable water at typical ambient humidity to soften by 30 to 60 ºC or more. Careful measurement of the whole Tg vs weight percent water curve has again enabled usto estimate the Tg of water, this time in the -170 ºC range, assuming Fox-Flory type behavior. It also appears that the neutralized resins pick up ~ 2 strongly bound water molecules per ion pair, which can be released as the temperature is increased under a low humiditycondition. Additional water molecules can absorb and lead to further softening, albeit at a lower rate.