Vinyl chloride polymerization in microdroplet reactor - Les thèses en ...

Chapter I : Bibliographic review
2 hours at 80° C > t 1/2 > 2/3 hour at 40° C.
Organic peroxides are unstable chemical compounds, decomposing at relatively low temperatures.
They can, however, be handled and stored safely if proper precautions are followed. They can be solid
long-chain alkyl peroxydicarbonates, available in different formulations. Among the latter, the water
based peroxide suspensions and emulsions are much safer than the usual solid, liquid or solvent based
peroxides.
The concentration and type of initiator depends on the polymerization temperature and the required
polymerization rate. It is common to use different types and often combinations of two or more
initiators in order to optimize the reaction. For instance, at low temperatures, a fast initiator may be
employed in combination with a slower one. The fast initiator mainly steps in at the very beginning of
the reaction in order to boost the slow initial rate while the slow one enables a constant reaction rate
until the limiting conversion is reached. (Pinto et al., 2001).
During the first stages of the reaction, the small droplets of initiator rapidly diffuse into the VCM. The
break-up and coalescence processes of the VCM droplets at the beginning of the reaction positively
contribute to the initiator distribution. This distribution over the VCM droplets was found to influence
the morphology of the final PVC particles. If the initiator is not equally partitioned in each VCM
droplet, particles without any porosity can be formed, which are therefore called clear particles. These
non-porous particles have a glassy character, a high density and they are very difficult to process. The
presence of these particles causes the appearance of visible imperfections, the so-called fish-eyes, on
the surface of finished PVC products.
The distribution of initiator over VCM droplets can be homogenized in a very short period of time (5
minutes) by dosing the initiator in solution in an inert solvent or as a suspension instead of a dosage as
a solid material with non-uniform particle sizes (PVC Symposium 1990a, PVC Symposium 1990b).
The polymerization kinetics is influenced by the type of initiator, in particular by its decomposition
rate or by the rate of distribution between monomer and polymer phases as highlighted by Titova et al.
(1982).
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