Chapter Two

11 июля, 2015

Malyar

Resin-water emulsions can be produced by the post-reaction emulsification of a condensation or addition polymer, by the use of surfactants or emulsifiers to form an “oil-in-water” emulsion of the polymer in the aqueous phase. This is simply a method of obtaining a polymer in an aqueous medium. This type of emulsion is different from a true emulsion polymer made by the process of emulsion polymerisation.

The process of emulsion polymerisation consists of dispersing a water insoluble acrylic monomer phase in an aqueous phase with the aid of surfactants.

Polymerisation is then brought about by free radical initiation using the water phase as a heat sink to help dissipate the heat of reaction. The polymer formed is stabilised within the emulsion by absorption onto surfactants and protective colloids as the polymerisation proceeds. The resultant product, unlike that obtained with suspension polymerisation is an homogenous, relatively stable emulsion of high molecular weight polymer in water.

An emulsion latex is a two phase system composed of mutually insoluble or sparingly soluble entities to which an emulsifier is added. The emulsifier bridges the solubility gap between the components and allows a dispersion to be produced.

The external or aqueous phase consists of water, salts, buffers, initiators and emulsifiers. The internal phase initially contains micelles containing monomer and monomer droplets.

After polymerisation the internal phase consists of polymer particles coated with adsorbed emulsifier.

The emulsion can be destroyed by the addition of electrolyte or water miscible solvents which bring about flocculation of the polymer particles by changing the interfacial tension of the system.

Any alteration in the internal energy of the system (such as evaporation, freezing, mechanical stress etc.) can lead to flocculation of the polymer particles.

Acrylic polymers manufactured by this technique are widely used in surface coatings applications. Emulsion polymerisation gives polymers with high molecular weight and allows greater flexibility in the preparation of copolymers. It also allows good reproducibility, rapid reactions, high monomer conversion and low cost relative to other polymerisation techniques. However, the presence of surfactants and colloids detracts from optimum film performance and film integrity.

Advantages of the emulsion polymerisation technique are:

a) high molecular weight polymers with a narrow molecular weight distribution can be produced in a controlled environment

b) the emulsion polymer may be used in latex form (latex paints etc.) or may be precipitated from the emulsion and used in solutions with polar solvents or neutralised to form a water dispersible resin

c) the latex viscosity is relatively low and is independent of the polymer molecular weight

d) water may be used as the sole solvent but water soluble coalescing solvents are often also present to assist in the film forming process.

Disadvantages of this approach are:

a) where the polymer is not required in latex form, it has to be precipitated from the emulsion and washed free of salts, surfactants and colloids

b) films formed from the latex have a tendency to be sensitive towards strong electrolytes

c) emulsion latices have poor mechanical stability. Special methods are required for transferring, filtering and handling the products in bulk (e. g. low shear pumps, vibratory sieves etc.)

The classical definition of an emulsion is ‘the colloidal dispersion of one liquid in

During the polymerisation of liquid monomer droplets, a physical change occurs whereby the very low viscosity monomer(s) is converted to either a high viscosity liquid, a soft and rubbery gel or a hard, tough or brittle solid.

These variants, due to the Tg of the polymer which is determined by the monomer composition, has given rise to terms which are often used synonymously, yet by definition are incorrect. For this purpose of this chapter, the term ‘emulsions’ is used in its widest sense, but:

Подпись: latices Подпись: dispersions contain viscous liquid droplets