Synthesis of high solid latices by thermally initiated emulsion polymerization

Abstract

Recent interest in synthetic emulsion products has shifted from mass-produced solid materials to specialty products such as paints, adhesives, and binders. High solid content latices are typically prepared in industrial practice, and used for coatings and adhesives. These latices offer definite advantages for most of the commercial applications, for example, low shipping costs and less energy to remove water. One of the major difficulties encountered in the preparation of these latices is the increase in viscosity once a certain value of the particle volume fraction is achieved. It is well known that this increase in the viscosity is due to the effect of particle–particle interactions and the formation of complex flow elements, which become crucially important when the volume fraction of the dispersed phase approaches the maximum packing density. The preparation of polymer emulsions with broad particle size distribution is an area of considerable industrial interest. High solid content, good filmformation characteristics, and improved mechanical and rheological properties can be obtained from polymer emulsions with broad particle size distribution. The polymer emulsions with broad size distribution have higher maximum packing concentration of polymer particles than that with monodisperse rigid spheres. Usually, semibatch and multiple-shot processes are the most common techniques for preparation of this type of latex to ease the extensive heat of polymerization by controlling the reaction rate. Inevitably, carefully controlled addition of monomers is required as well as use of sophisticated controlling devices. Molecular weight is lower than that of the batch process. High solid content emulsion copolymerization of styrene, butyl acrylate, and methacrylic acid has been carried out, and polymer latices with a multimodal particle size distribution (PSD) prepared by introducing additional surfactants during the semicontinuous emulsion polymerization. Bimodal and trimodal PSD polymer latices can be obtained by this method, and the secondary generation of particles is greatly affected by the nature and the amount of additional surfactants, as well as the moment at which they are introduced. High solid content ( 65%), low viscosity, and coagulum-free latices have been obtained through the secondary nucleation of polymer particles, and a minimum in viscosity was observed when the weight fraction of the large particles was around 80%. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2478– 2483, 2002Correspondence to: S. Omi (omi@cc.tuat.ac.jp).