emulsion polymerization of Styrene

Emulsion Polymerization Of Styrene

Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols , can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and comprise many individual polymer chains. The particles are stopped from coagulating with each other because each particle is surrounded by the surfactant ('soap'). When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

Polystyrene:-

Polystyrene is actually an aromatic polymer that is made from the monomer styrene. It is a long hydrocarbon chain that has a phenyl group attached to every carbon atom. Styrene is an aromatic monomer, commercially manufactured from petroleum. Polystyrene is a vinyl polymer, manufactured from the styrene monomer by free radical vinyl polymerization.

Polystyrene is a rigid, transparent thermoplastic, which is present in solid or glassy state at normal temperature. But, when heated above its glass transition temperature, it turns into a form that flows and can be easily used for molding and extrusion. It becomes solid again when it cools off. This property of polystyrene is used for casting it into molds with fine detail. Pure polystyrene polymer is colorless and hard with limited flexibility. 

Properties Of Polystyrene:-


The unique physical and chemical properties of polystyrene are responsible for its use in a wide range of applications. Let us have a look at some polystyrene properties.

Polystyrene is hard and brittle and has a density of 1.050 g/cm³. It is represented by the chemical formula, C₈H₈. It is made up of three chemical elements, carbon, hydrogen and oxygen. Most of the polystyrene properties are as a result of the unique properties of carbon. It is highly flammable and burns with an orange yellow flame, giving off soot, as a characteristic of all aromatic hydrocarbons. Polystyrene, on oxidation, produces only carbon dioxide and water vapor. Have a look at the physical properties of polystyrene given below:

·        Density - 1.05 g/cc

·        Dielectric constant - 2.4 to 2.7

·        Thermal conductivity - 0.08 W/(m.K)

·        Young's modulus - 3000 to 3600 Mpa

·        Tensile strength - 46 to 60 Mpa

·        Melting point - 240 ºC

·        Water absorption - 0.03 to 0.1

Polystyrene is chemically nonreactive and hence, used to make containers for other chemicals, solvents and even food items. The transformation of carbon-carbon double bonds into less reactive single bonds in polystyrene is the main reason for its chemical stability. Polystyrene is flexible and can be made into moldable solid or thick viscous solids. This is mainly because of the Van der Waal's forces of attraction that exist between the long hydrocarbon chains. However, when heat is applied, the chains can slide over each other. This property of intermolecular weakness along with the intermolecular strength, due to the strong hydrocarbon backbone, allows polystyrene to be flexible and stretchable. Polystyrene is soluble in solvents that contain acetone, such as most aerosol paint sprays and cyanoacrylate glues.

Functions Of The Reactants

·        Monomers:-

Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in water. Solid monomers are difficult to disperse in water. If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduces to that of solution polymerization.

·       Initiators:-

Both thermal and redox generation of free radicals have been used in emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50°C, providing a thermal source of initiation. Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50°C.

Although organic peroxides and hydroperoxides are used in emulsion polymerization, initiators are usually water soluble and partition into the water phase. This enables the particle generation behavior described in the theory section. In redox initiation, either the oxidant or the reducing agent (or both) must be water soluble, but one component can be water-insoluble.

·       Surfactants:-

Selection of the correct surfactant is critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as tensile strength, gloss, and water absorption.

Anionic, nonionic, and cationic surfactants have been used, although anionic surfactants are by far most prevalent. Surfactants with a low critical micelle concentration (CMC) are favored; the polymerization rate shows a dramatic increase when the surfactant level is above the CMC, and minimization of the surfactant is preferred for economic reasons and the (usually) adverse effect of surfactant on the physical properties of the resulting polymer. Mixtures of surfactants are often used, including mixtures of anionic with nonionic surfactants. Mixtures of cationic and anionic surfactants form insoluble salts and are not useful.Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha olefin sulfonate.

Initiation And Polymerization:-

Initiation takes place when an initiator fragment migrates into a micelle and reacts with a monomer molecule. Water soluble initiators, such as peroxides and persulfates, are commonly used (This also prevents polymerization in the big monomer droplets). Once polymerization starts, the micelle is referred to as a particle. Polymer particles can grow to extremely high molecular weights, especially if the initiator concentration is low. That makes the radical concentration and the rate of termination low as well. Sometimes a chain transfer agent is added to the mix to keep the molecular weight from getting too high.Decreasing the initiator concentration increases molecular weight and rate of polymerization.

Sodium Lauryl Sulphate:-

Soap a molecule in which one end is polar and water-soluble and the other end is non-polar and organic-soluble, such as sodium lauryl sulfate:

These form micelles in water, little balls in which the polar ends of the molecules point out into the water, and the non-polar ends point inward, away from the water. Water insoluble dirt can hide inside the micelle, so soapy water washes away dirt that plain water can't.