Preparation of colloidal low-density polyethylene latexes by flow-induced phase inversion emulsification of polymer melt in water

The aim of this study is to prepare colloidal polymeric latexes by using the flow-induced phase inversion emulsification method given by G. Akay [Chem. Eng. Sci. 53, 203 (1998)] of polymer melts followed by the solidification of polymer melt droplets. We also investigate the mechanism of emulsification and stabilization in polymeric dispersions which undergo a phase change after emulsification. The history of the emulsification and emulsion structure are monitored by using a process rheometer and off-line scanning electron microscopy with energy-dispersive X-ray analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, and particle size measurements. It is shown that the molecular structure of the surface-active material is the most important parameter in achieving phase inversion emulsification in polymer melts. Molecular surfactants could not be used to provide surface activity in polymeric melts. Several experimental polymeric surfactants are used and their ability to form a [water-in-polymer melt] emulsion is tested. The successful polymeric surfactants are known as hydrophobically modified water-soluble polymers. It is postulated that the surface-active materials should conform at the water/polymer melt interface and not be removed from the interface by surface deformations. The ability of hydrophobically modified water-soluble polymers to remain at the interface is reduced if the hydrophobic moeties which anchor into the polymer melt have chain length approaching 18 carbons or more. After the first phase inversion and subsequent dilution of the [polymer melt-in-water], if mixing is carried out while cooling, a second phase inversion takes place from [polymer melt-in-water] to [water-in-solid polymer] despite high water content of the polymer/water system. If the water content is high (25-40% investigated) the second phase inversion yields a powdered material with encapsulated water. A third phase inversion occurs if the powdered microcapsules from the second phase inversion is heated while mixing to yield a [(water-in-polymer)-in-water] multiple emulsion which can be inverted back to [polymer melt-in-water] emulsion by increasing the temperature and subjecting the emulsion to high deformation rate flows. However, if this last phase inversion is not allowed to proceed to completion, and the [(water-in-polymer)-in-water] multiple emulsion is cooled, microporous polymeric particles are obtained. (C) 2001 Academic Press.