POLYMER LATEX STABILITY MODIFICATION BY EXPOSURE TO HYDROPHOBIC SOLVENTS

The stability of latex particles toward coagulation in the presence of salt is modified by swelling the latex with toluene and chloroform va pors. Short-term stability was determined by turbidimetric titrations, and the long-term stability was evaluated by adding latex and salt so lutions, allowing the mixture to age for 24 or 48 h and determining th e characteristics of the supernatant and of the sediment. Nine differe nt latexes were examined, with variable results: in some cases, both a polar solvents stabilize the latex; in other cases, increased stabilit y is induced by only one of the solvents, either toluene or chloroform . There is also coherence, but not a strict correlation, between the s olvent effects on short- and long-term stability. For instance, in the case of a core-and-shell styrene-butyl methacrylate latex, chloroform has a small stabilizing effect in the titration experiment, but it pr events the formation of a coagulated latex sediment even 48 h after mi xing latex and salt. Two hypotheses are discussed to account for these observations: (i) swelling solvents decrease the particles ability to dissipate the collision kinetic energy, so that particles collide but without joining each other; (ii) the solvents induce the release of t rapped charged groups from the particle interior to the interface, enh ancing the usual (electrostatic, steric, hydration) stability factors. (C) 1998 Academic Press.