Introduction to Polymer Science and Technology Polymerisation

2.2.2 Solution polymerisation

The monomer is added to an inert solvent with a boiling point that corresponds to the polymerisation temperature.

During the polymerisation process some solvent evaporates and thus helps to remove the heat of polymerisation. Since the boiling point of the solvent is constant, this ensures a constant polymerisation temperature. Some difficulty lies in the separation of the residual solvent from the polymer after completion of polymerisation.

In comparison with bulk polymerisation, solution polymerisation offers easier temperature control because of the added heat capacity of solvent and lower viscosity.

2.2.3 Suspension polymerisation

Suspension polymerisation is essentially a bulk polymerisation carried out in droplets in an aqueous solution in which the monomer is dispersed. The polymer precipitates as fine spherical particles with diameters of 0.01 to 1.0 mm.

The polymerisation begins by radical initiators in the monomer droplets.

Protective colloids are added in order to prevent coagulation of the particles and to produce uniform polymer

particles.

The water absorbs the heat of reaction.

It is used in the manufacture of PVC and PS.

Residual additives need to be removed.

Introduction to Polymer Science and Technology

Polymerisation

2.2.4 Slurry polymerisation

The process is mainly used in the production of polyolefins. The catalyst is dispersed/dissolved in a liquid diluent in which monomer is dissolved, or in the liquid monomer by itself. As in emulsion polymerisation, the polymer is not soluble in the reaction medium, and precipitates on the catalyst forming slurry.

2.2.5 Emulsion polymerisation

As in suspension polymerisation the monomer is also dispersed in water but in much smaller droplets created with the use of emulsifiers, e.g., soaps. In the presence of soapy water, the chemicals, such as the monomer droplets and subsequently formed polymer molecules are kept apart and dispersed rather than coalesce into a useless lump! The temperature control is easier, since the viscosity changes very little with conversion. Also, thermal conductivity and specific heat of water are higher than those of organic solvents.

Some of the surfactant (emulsifier) particles huddle together forming micelles, others surround monomer particles and isolate them as droplets. The micelles consist of 10 to 100 soap molecules with their hydrophilic groups on the outside in the water phase and their hydrophobic groups on the inside surrounding the hydrophobic monomers. Once the process begins, the temperature increases generating free radicals that monomers are attracted to and come out of the droplets and start adding on to that free radical site in a micelle and finally finish of producing polymer molecules that are cocooned in the micelle, see Figure 2.2 . Of course this process takes place simultaneously in many different micelles that result in many polymer particles.

Date: 2015-12-11; view: 829