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

The particle deformation and compaction stage of latex polymer film fo rmation was investigated via minimum film formation temperature (MFT) measurements; variables included polymer composition, particle size, t ime, and, especially, the water content of the deposited film and in t he drying environment. The water content of the system ranged from ver y low (latex film predried well below the MFT before imposition of the temperature gradient, with low relative humidity maintained throughou t) 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'' transitio n, from turbid to clear film, as it is heated. With hydrophobic polyme r compositions, the dry MFT is virtually identical to that from a wet casting, indicating that so-called capillary forces associated with th e presence of liquid water have little if any role. With hydrophilic c ompositions 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 tempe rature-time shift, consistent with viscoelastic relaxation driven by i nterparticle van der Waals attractive forces/polymer-air surface tensi on 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.