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

Chapter I : Bibliographic review
This may be achieved by using inhibitors of polymerization, chemical compounds such as phenols,
quinones, amines or nitro derivatives, which will rapidly react with the free radicals presents in the
system (Malmonge, 1996). The amount of inhibitor used must be stoichiometrically proportional to
the initiator amount. Nitro derivatives are preferentially used in the inhibition process, even though
they are toxic and slightly yellow, affecting resin quality after inhibition.
I.D.2.d
Agitation
As already mentioned early in this study, agitation also plays a key role in the S-PVC process. It
initially serves to break the vinyl chloride into droplets (control of the morphology), but also to
manage the heat transfer between the reaction medium and the cooled reactor walls.
Basically the higher the level of agitation, the smaller the VCM droplets and the greater the surface
area to be protected. At a certain agitation speed the droplets become too small and cannot be
stabilized well enough, resulting in the agglomeration and thus eventually the formation of larger
particles. The minimum agitation speed is defined as the minimum speed at which complete dispersion
of the organic phase in water can be achieved. Hence this agitation speed is sufficient to obtain a
complete dispersion of VCM in water at the beginning of the polymerization. However with
increasing conversion it may become insufficient, as the viscosity of the polymerization system
increases. In the worst case scenario, this could finally result in one big block of polymer. Therefore,
the minimum agitation speed is also defined by the speed at which the suspension is maintained during
the polymerization, particularly during the pressure drop stage.
I.D.2.e
Temperature
The polymerization temperature has an important impact on the porosity of PVC particles. A higher
polymerization temperature causes a lower porosity, as the internal particles tend to coalesce more,
which results in a more compact internal structure. With increasing temperature all reaction rate
constants increase, but to different extents.
However temperature variations may affect negatively the quality of the polymer produced, since
changes in the polymerization temperature may cause an increase in polydispersity (broadening of the
molecular weight distribution).
Commercial suspension-PVC is manufactured in the temperature range of 40-80 °C. As the resulting
polymers differ in molecular weight and morphology, they are suitable for different types of
28