Figure 2.2An illustration of the process of emulsion polymerisation

At the conclusion of the process, as well as polymer particles there are still some unreacted monomers, initiators and free radicals, which find it difficult to participate in reaction for reasons of steric hindrances. Surfactant and coagulant residues are hard to remove and also contribute to high impurity levels. Used mainly for the production of polyvinyl acetate, PMMA and PVC.

Introduction to Polymer Science and Technology

Polymerisation

2.2.6 Gas-phase polymerisation

The monomer is in the gaseous state and a heterogeneous coordination catalyst such as a Ziegler-Natta catalyst is used. In gas-phase and slurry processes, catalysts such as Ziegler-Natta and metallocenes need to be supported on a suitable substance, e.g., SiO₂, whereas they can be added directly in solution polymerisation. The polymer is formed on the active sites of the catalyst into a gradually expanding catalyst-polymer particle, and as in emulsion polymerisation there is one active centre in each particle. Fresh gaseous monomer diffuses through the polymer particle to reach the active site. In reactors, the catalyst is supported/uniformly dispersed by mechanical stirring or by fiuidization.

Gas-phase and slurry processing techniques are used mainly for the production of polyolefins such as HDPE. Table 2.1 shows some examples of polymers that are produced under various polymerisation mechanisms and processes covered in this section.

Table 2.1A comparison of different polymerisation methods (source: Asua 2007, p24)

2.3 Polymerisation reactors

Industrially employed reactors include horizontal/vertical stirred tanks, high-pressure tube, loop and fiuidised-bed reactors, as well as polymerisation in moulds, e.g., RIM (reaction injection moulding). The type of reactor used is dictated by the polymerisation process. For example stirred-tank reactors are suitable for suspension and emulsion polymerisations since agitation assists in controlling polymer particle size, e.g., in production of PVC.

Most of polyolefins are produced using a fluidised-bed reactor,illustrated in Figure 2.3.

Introduction to Polymer Science and Technology

Polymerisation

-Ci]—►bleed

compressor

heat exchanger (gas cooler)

ethylene, H₂, N₂

condensing agent

disengagement zone

reaction zone catalyst(s)

Figure 2.3Gas-phase fluidised-bed reactor (an animation of the UNIPOL process is presented at http://www.univation.com/unipol.animation.html)

The design of the fluidised distributer plate at the base of a reactor is important for the efficiency of the process: fluidisation should prevent the hot polymer particles from settling onto the plate, and cause agglomeration, by maintaining sufficient recycled gas and additional feed gas flow rate through the distributor.

Date: 2015-12-11; view: 887