ROLE OF WATER IN PARTICLE DEFORMATION AND COMPACTION IN LATEX FILM FORMATION

The particle deformation and compaction stage of latex polymer film formation was investigated via minimum film formation temperature (MFT) measurements; variables included polymer composition, particle size, time, and, especially, the water content of the deposited film and in the drying environment. The water content of the system ranged from very low (latex film predried well below the MFT before imposition of the temperature gradient, with low relative humidity maintained throughout) to very high (wet latex cast on the gradient bar and high humidity maintained during drying). A film predried well below the MFT-a turbid deposit owing to interparticle voids-exhibits a ''dry MFT'' transition, from turbid to clear film, as it is heated. With hydrophobic polymer compositions, the dry MFT is virtually identical to that from a wet casting, indicating that so-called capillary forces associated with the presence of liquid water have little if any role. With hydrophilic compositions the ''wet MFT'' is lower than the dry MFT by as much as 10-degrees-C or more; this is ascribed simply to plasticization by water. Dry MFT values decrease with log time similar to the WLF glass temperature-time shift, consistent with viscoelastic relaxation driven by interparticle van der Waals attractive forces/polymer-air surface tension such as described by the Johnson-Kendall-Roberts model of particle adhesion and deformation. For a given polymer composition, the dry MFT data correlate with a simple model of surface tension driven collapse of the interstitial voids.