Elsevier

Progress in Polymer Science

Volume 27, Issue 7, September 2002, Pages 1283-1346
Progress in Polymer Science

Miniemulsion polymerization

https://doi.org/10.1016/S0079-6700(02)00010-2Get rights and content

Abstract

Miniemulsion polymerization has recently exploded in terms of publications and the development of a wide range of useful polymer materials only accessible through this polymerization technique. In the first part of this article, the fundamental aspects involved in the preparation and polymerization of monomer miniemulsions are reviewed. The second part deals with the application of miniemulsion polymerization for the production of high solids low viscosity latexes, dispersion of polymers of well defined microstructure through controlled radical polymerization, use of catalytic polymerization in aqueous media, encapsulation of inorganic solids, incorporation of hydrophobic monomers, preparation of hybrid polymer particles, implementation of anionic and step polymerization in aqueous dispersed media, and process intensification by using continuous reactors.

Introduction

Polymeric dispersions are used in a wide variety of applications such as synthetic rubber, paints, adhesives, binders for non-woven fabrics, additives in paper and textiles, leather treatment, impact modifiers for plastic matrices, additives for construction materials and flocculants [1], [2]. They are also used in biomedical and pharmaceutical applications such as diagnostic tests and drug delivery systems. The rapid increase of this industry is due to environmental concerns and governmental regulations to substitute solvent-based systems by water borne products, as well as to the fact that polymeric dispersions have unique properties that meet a wide range of market needs.

Commonly, these products are produced by means of conventional emulsion polymerization. In this process, monomer is dispersed in an aqueous solution of surfactant with a concentration exceeding the critical micelle concentration (CMC) and polymerization is started by means of an (most often water-soluble) initiator system. In principle, polymer particles can be formed by entry of radicals into the micelles (heterogeneous nucleation), precipitation of growing oligomers in the aqueous phase (homogeneous nucleation), and radical entry in monomer droplets. However, monomer droplets are relatively large (1–10 μm) compared to the size of monomer-swollen micelles (10–20 nm), and hence the surface area of the micelles is orders of magnitude greater than that of the monomer droplets. Consequently, the probability for a radical to enter into the monomer droplets is very low, and most particles are formed by either homogeneous or heterogeneous nucleation.

Once they are nucleated, the polymer particles undergo substantial growth by polymerization. The monomer required for the polymerization must be transported from the monomer droplets by diffusion through the aqueous phase. In some cases, this represents a severe limitation of the conventional emulsion polymerization. Thus, water resistance of coatings prepared from dispersed polymers is significantly improved if very hydrophobic monomers, e.g. lauryl and stearyl methacrylates are incorporated into the polymer backbone. However, mass transfer of these monomers from monomer droplets to polymer particles through the aqueous phase is diffusionally controlled, and hence they cannot be readily incorporated into the polymer in conventional emulsion polymerization.

The need of mass transport of monomer through the aqueous phase would be greatly diminished if all (or at least a large fraction) of the monomer droplets were nucleated. Prevalent droplet nucleation can only occur if the surface area of the monomer droplets is large compared with that of the micelles, and this requires submicron droplet size. The word miniemulsion was coined [3] to describe submicron oil-in-water dispersions that are stable for a period ranging from hours to months. Review articles by El-Aasser et al. [4], [5] summarized the work done up to 1995. The kinetics of miniemulsion polymerization has been recently reviewed by Capek and Chern [6]. This paper reviews the main aspects concerning the preparation and polymerization of monomer miniemulsions.

Section snippets

Early work

Ugelstad et al. [7] were the first to demonstrate that under conditions in which the droplet size is small enough, nucleation of monomer droplets could account for an important part of the particles formed. The method used to produce the small droplet size was inspired in previous reports showing that the presence of long chain fatty alcohols drastically increased the capacity of anionic surfactants to disperse and stabilize oil-in-water emulsions [8], [9], [10], [11]. In those works, it was

Preparation of monomer miniemulsions

Monomer miniemulsions suitable for miniemulsion polymerization are submicron monomer-in-water dispersions stabilized against both diffusional degradation and droplet coagulation by using a water-insoluble low-molecular weight (costabilizer) compound and an efficient surfactant. The key issues in the preparation of the monomer miniemulsions are the formulation and the method of preparation. These issues as well as the often-controversial methods for the characterization of monomer miniemulsions

Particle nucleation

Droplet nucleation is a unique feature of miniemulsion polymerization and, as shown below, the reason for the wide range of applications of miniemulsion polymerization. However, in spite of its importance, it is not well understood and conflicting results have been reported leading to different theories. Thus, while some authors claim that narrow PSDs are obtained in miniemulsion polymerization [22], others report very long nucleation periods [114], [115] and claim that miniemulsion

Applications

For quite a long time, miniemulsion polymerization remained for the most part as a scientific curiosity, kept alive by the determination of Professor El-Aasser, but in the last few years many of new applications have been discovered. These applications include:

  • (i)

    production of high solids low viscosity latexes,

  • (ii)

    continuous polymerization reactors,

  • (iii)

    controlled radical polymerization (CRP) in dispersed media,

  • (iv)

    catalytic polymerization,

  • (v)

    encapsulation of inorganic solids,

  • (vi)

    incorporation of hydrophobic monomers,

Summary

Miniemulsion polymerization combines some useful features of conventional emulsion polymerization (solvent-free, easy thermal control, radical compartmentalization that allows simultaneous high polymerization rate and high molecular weights) with the possibility of using water-insoluble reactants. Droplet nucleation is the distinctive feature of miniemulsion polymerization that allows dealing with hydrophobic reactants because in miniemulsion polymerization the need for mass transfer through

Acknowledgements

The financial support for this work by the University of the Basque Country (Programa de Grupos Consolidados) and the Ministerio de Ciencia y Tecnologı́a (PPQ2000-1185) is gratefully acknowledged.

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