POLYMER CHEMISTRY

ASSIGNMENT – 1

TOPIC - EMULSION POLYMERISATION

SUBMITTED BY

NAME- ASHANK UPADHYAY

ROLL NO- 2K14/PS/011

EMULSION :- An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). Emulsions are part of a more

general class of two-phase systems of matter called colloids.

EMULSION POLYMERISATION

Emulsion polymerization is a complex process in which the radical addition polymerization proceeds in a heterogeneous system.

The polymerization of monomers in the form of emulsions (i.e., colloidal dispersions).

KEY POINTS

High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerisation, there is a tradeoff between molecular weight and polymerization rate.

The continuous water phase is an excellent conductor of heat, enabling fast polymerization rates without loss of temperature control.

Since polymer molecules are contained within the particles, the viscosity of the reaction medium remains close to that of water and is not dependent on molecular weight.

The final product can be used as is and does not generally need to be altered or processed.

Disadvantages of emulsion polymerization include:

Surfactants and other polymerization adjuvants remain in the polymer or are difficult to remove

For dry (isolated) polymers, water removal is an energy-intensive process

Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.

Can not be used for condensation, ionic or Ziegler-Natta polymerization, although some exceptions are known.

• Emulsion polymerization is a technique leading to colloidal polymer particles dispersed in a continuous medium, most often water.

• These polymeric dispersions are called latexes

• Polymer particles are mostly spherical

• Average diameter 80 to 300 nm (more than one order of magnitude smaller than suspension polymerization particles)

• The solids content spans from 40 to 65 wt%

• The dispersed system is thermodynamically unstable, and kinetic stability is provided by emulsifiers (ionic and non-ionic) and by incorporation of hydrophilic groups into the polymer.

Main products and markets of EMULSION POLYMERISATION

Main ingredients of emulsion polymerization

1) monomer

2) dispersion medium

3) emulsifier

4) initiator

Further auxiliaries, such as chain transfer agents, buffers, acids, bases, anti-aging agents, biocids, etc., can be used

Monomer

The major monomers used in emulsion polymerization include butadiene, styrene, acrylonitrile, acrylate ester and methacrylate ester monomers, vinyl acetate, acrylic acid and methacrylic acid, and vinyl chloride

Dispersion medium

The dispersion medium, for monomer droplets and polymer particles, is generally water as well as liquids other than water. Water is cheap, inert and environmentally friendly. It provides an excellent heat transfer and low viscosity. It also acts as the medium of transfer of monomer from droplets to particles

Emulsifier

These materials perform many important functions in emulsion polymerizations

(i) reducing the interfacial tension between the monomer phase and the water phase .

(ii)Micelle generating substances

(iii) Stabilizing the monomer droplets in an emulsion form

(iv) Serving to solubilize the monomer within emulsifier micelles

(v) Stabilizing the growing latex particles

(vi) stabilizing the particles of the final latex

(vii) Acting to solubilize the polymer.

The most used type of these emulsifiers is that with a head group of ethylene oxide (EO) units are Polyoxyethylenated alkylphenols, polyoxyethylenated straight-chain alcohols and polyoxyethylenated polyoxypropylene glycols

Initiator

The function of the initiator is to generate free radicals, which in turn lead to the propagation of the polymer molecules. The free radicals can be commonly produced by two main ways:

(i) thermal decomposition

(ii) redox reactions. In addition, the free-radical initiators can be either water or oil-soluble

The most commonly used water-soluble initiators are persulfates (peroxodisulfates). For example, potassium-, sodium-, and ammonium-persulfate

Kinetics and mechanism of emulsion polymerization

The three stages of the free-radical polymerization are shown in the following steps

Initiation: The reaction of initiation can be described as a two-stage process. In the first stage the initiator is decomposed to free-radicals, in the second stage the primary radicals react with the monomer, converting it to a growing radical

The most common method used in emulsion polymerizations is thermal initiation in which the initiator (I) dissociates homolytically to generate a pair of free-radicals

where kd is the rate constant for the initiator dissociation. The rate of this dissociation, Rd, is given by:-

where [I] is the concentration of the initiator and f is the initiator efficiency.

In the second stage, the free radicals generated from the initiator system attack the first monomer (M) molecule to initiate chain growth:

where ki is the rate constant for the initiation. The rate of initiation, Ri, is equal to the rate of dissociation of an initiator. Because the primary radical adds to monomer is much faster than the first stage, and so the dissociation of the initiator is the rate-determining step in the initiation sequence

Propagation: The propagation step is only one which produces polymer. This involves essentially the addition of a large number of monomer molecules (n) to the active centers (RM ) for the growth of polymer chain

where kp is the rate constant for propagation

The rate of polymerization, Rp, is known as the rate of monomer consumption. Monomer is consumed by the propagation reactions as well as by the initiation reaction. The corresponding rate of polymerization is then:

The amount of monomer consumed in the initiation step can be neglected due to the number of monomer molecules reacting in the initiation step is far less than

the number in the propagation step for a process producing high polymer, and a very close approximation of the polymerization rate can be given simply by the rate of propagation. Then, the polymerization rate can be written:

Termination: In last step of the polymerization, the growing polymer chain is terminated. There are two main mechanisms, recombination and disproportionation, for termination reactions. In these mechanisms, the growing polymer chain react with another growing chain or another free radical of some kind

Recombination;

in which two growing chains constitute the coupling with each other resulting in a single polymer molecule

Disproportionation;

in which one growing chain abstracts a hydrogen atom from another, leaving it with an unsaturated end group. This mechanism occurs more rarely than recombination. It results in the formation of two polymer molecules, one

saturated and one unsaturated.

The overall rate constant for termination reaction is given as kt=ktc+ktd

In the chain transfer reactions, some substances such as polymer, monomer, solvent, additives, impurities, or initiator can act as a chain transfer agent. An example of these reactions is given

The equation for the steady state conditions is:

rearranging

apply to propagation eq we get

Schematic representation for the mechanism of emulsion polymerization

OVERALL POLYMERISATION AND MONOMER CONVERSION

Surfactant-Free Emulsion Polymerization

The presence of surfactant is a disadvantage for certain applications of emulsion polymers such as

The presence of adsorbed surfactant gives rise to somewhat variable properties since the amount of adsorbed surfactant can vary with the polymerization and application conditions

Removal of the surfactant, either directly or by desorption, can lead to coagulation or flocculation of the destabilized latex

Surfactant-free emulsion polymerization, involving no added surfactant, is a useful approach to solving this problem

The process uses an initiator yielding initiator radicals that impart surface-active properties to the polymer particles. Persulfate is a useful initiator for this purpose

Applications

Polymers produced by emulsion polymerization can be divided into three rough categories

Synthetic rubber

Some grades of styrene butadiene (SBR) Some grades of polybutadiene polychloroprene Nitrile rubber Acrylic rubber Fluoroelastomer (FKM)

Plastics

Some grades of PVC Some grades of polystyrene Some grades of PMMA Acrylonitrile-butadiene-styrene terpolymer (ABS) Polyvinylidene fluoride Polyvinyl fluoride PTFE

Dispersions

polyvinyl acetate polyvinyl acetate copolymers latexacrylic paint Styrene-butadiene VAE (vinyl acetate - ethylene copolymers)