Miniemulsion polymerization
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.
References (251)
Self-bodying action of the mixed emulsifier sodium dodecyl sulfate/cetyl alcohol
J Colloid Interface Sci
(1968)- et al.
Viscosity of emulsifying agents at oil–water interfaces
J Colloid Interface Sci
(1968) - et al.
Physical degradation of emulsions via the molecular diffusion route and its possible prevention
J Pharm Sci
(1962) - et al.
Swelling of oligomer–polymer particles. New methods of preparation of emulsions and polymer dispersions
Adv Colloid Interface Sci
(1980) - et al.
Molecular-weight distributions in the miniemulsion polymerization of styrene initiated by oil-soluble initiators
Polymer
(1994) - et al.
Effect of Ostwald ripening on styrene miniemulsion stabilized by reactive cosurfactants
Colloid Surf A
(1998) - et al.
Miniemulsion polymerization of styrene in the presence of a water-insoluble blue dye
Polymer
(1998) - et al.
Kinetics of styrene miniemulsion polymerization stabilized by nonionic surfactant/alkyl methacrylate
Polymer
(1999) - et al.
Oil–acrylate hybrid emulsions, mini-emulsion polymerization and characterization
Colloids Surf A Physicochem Engng Aspects
(1999) - et al.
Determination of surface area and particle size of synthetic latex by adsorption. I. Latexes containing fat-acid soaps
J Colloids Sci
(1954)