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SUSPENSION POLYMERIZATION &
EXTERNAL MORPHOLOGY

Suspension polymerization is carried out by dispersing a water-insoluble liquid monomer as small droplets in a continuous water phase using strong agitation and additives known as "Suspending Agents" which have partial affinity for both monomer and water to stabilize droplet size and prevent first their coalescence and later their agglomeration.
Each droplet then act as a small bulk polymerization reactor running almost independently from other droplets.

Droplet size is the result of a struggle between several acting forces, some of which change with time, conversion and / or position within reactor:
- Inertial forces that deform, break and make droplets collide such as shear, velocity and turbulence fields provided by stirrer (speed, number and geometry) and modified by baffles and reactor geometries.
- Viscoelastic forces that try to prevent droplet deformation such as bulk liquid properties, monomer/water interfacial tension and protective coverage with Suspending Agents.
The result of this dynamic and complex balance is a distribution of droplet sizes that changes through defined stages during polymerization until they result in a population of grains or pearls.
EVOLUTION OF VINYL CHLORIDE SUSPENSION POLYMERIZATION
At the beginning of reaction (0% conversion) mixing conditions quickly produces VCM droplets with dissolved initiator and Primary Suspending agent (PSA) dispersed in water.
Increase of temperature during reactor heat-up makes PSA molecules precipitate on droplet surfaces to form a loose protective layer that first reduce surface tension and help breakup.
VCM droplets are subject to a fast dynamic breakup vs. coalescence equilibrium, producing a droplet size distribution that change with conversion but also with other process conditions.

0% conversion, from Sanderson “Aspects of Vinyl Chloride Suspension Polymerisation”, 1980
As soon as reaction begins, copolymerization of VCM with PSA molecules increase protection of droplets eventually stopping droplet breakage and almost hindering coalescence at around 2% to 5% conversion depending on conditions.
Protective layer, called "Pericellular Membrane", continues to strengthen by accretion of Microdomains and copolymerization with VCM as conversion increases.

3% conversion, from Sanderson “Aspects of Vinyl Chloride Suspension Polymerisation”, 1980
Around 5% – 15% conversion begin a partial agglomeration of polymerizing droplets (known now as "Subgrains") caused by specific conditions that can be controlled.
At this time, both strength and nature of Pericellular Membrane play a role for controlling agglomeration of Subgrains.

6% conversion, from Sanderson “Aspects of Vinyl Chloride Suspension Polymerisation”, 1980
When Conversion reaches around 30% in commercial polymerizations, both internal structure of grains and Pericellular Membrane are rigid enough so Grains behave as elastic solids, essentially stopping agglomeration.
From this Identification Point onward, both Grain size and shape distributions are unchanged unless some specific (intended or unintended) changes are made.
Wrinkles and folds develop progressively in Grain surface due to reduction of liquid VCM and contraction over internal structure.
Final PVC resin resulting from the specific conditions of each polymerization batch is characterized by a population of Grains with defined distributions of sizes and shapes.

Different combinations of Primary Suspending Agents are used to control the external formation of Grains, each producer keeping their details confidential.
Each Primary Suspending Agent has specific effects on droplet protection that determine not only Grain size and shape (which affects Bulk Density) but also influences Internal Morphology.

Grain Size Distribution (Mesh) of 4 PVC resin samples made with the same polymerization conditions but different PSA

Effect of PSA type on Density and Porosity, all other polymerization conditions are the same
However, not only the type of Primary Suspending Agent (PSA) is important but also the way in which it is used. The following graphs show results of Grain Size Distribution (measured by sieving) as well as Density and Porosity of two PVC resins produced using the same conditions as well as the same type and quantity of PSA, but modifying the way of use.
The way of usage clearly delivers similar Grain Size distribution but different results in Bulk Density and Porosity.

Grain Size Distribution (Mesh) of 2 PVC resin samples made with the same polymerization conditions and PSA (but different way of use)

Effect of PSA way of use on Density and Porosity, all other polymerization conditions are the same
Do you have problems of poor control of Grain Size Distribution?
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External Morphology during your polymerization process to optimize
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