Abstract
Although nonionic surfactants show improved stabilization characteristics in emulsion polymerization as well as superior shear and freeze thaw stability of the final latex, their behaviour is generally not well understood. In the present paper a steric stabilization model is developed for describing particle stabilization in emulsion polymerization systems in the presence of nonionic oligomeric surfactants. The model takes into account the effect of unequal particle sizes on the steric interaction potential and the resulting enhanced heterocoagulation. An additional feature of the model is the incorporation of a possible surfactant lateral migration mechanism, which may be significant in the case of oligomeric nonionic surfactants. The proposed model can simulate reasonably well the experimentally observed conversion profiles and average particle sizes as a function of the surfactant molecular structure (i.e., lengths of stabilizing and adsorbing moieties) and surfactant concentration. As a result of the competing effects of the adsorbed volume fraction and surface coverage on particle stability, an optimum hydrophobicity/hydrophilicity ratio for the oligomeric nonionic surfactants can be derived.
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