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SELF-STRUCTURING &
INTERNAL MORPHOLOGY

Unlike other polymer-monomer pairs, PVC is almost insoluble in VCM which gives rise to the formation of a complex Internal Morphology that is a very important for PVC performance.
For example, in the production of flexible articles it is necessary that PVC absorb rapidly and homogeneously large amounts of liquid plasticizers to decrease Glass Transition Temperature (Tg) so it can function as flexible thermoplastic.
As mentioned Suspension Polymerization consist of the formation and stabilization of VCM droplets, each one performing as mostly independent bulk polymerization reactors that collide continuously.
Droplets have dissolved initiator that thermally decompose according to their nature, to form two radicals per initiator molecule.
Very fast heat-to-tail propagations steps followed by head-to-head addition will produce a PVC chain and a new growing radical which goes into a new very fast head-to-tail propagation and a new chain transfer by head-to-head addition and so on until a chain termination reaction takes place.

Each decomposed initiator molecule will produce around 50 PVC chains in close proximity, so they will agglomerate as a very small 10-20 nm diameter colloidal particle called "Microdomain".
Microdomains appear at < 0.01% conversion and are very unstable, so they quickly flocculate (around 1,000 microdomains each) at < 2% conversion to form particles known as "Domains", which are the nucleus of Primary Particles and have enough surface charges to stabilize their size (0.1 – 0.2 μm) and maintain their number.
Domains thereafter grow into Primary Particles by
:a. PVC/VCM gel polymerization
b. Accretion of microdomains and polymer chains from liquid VCM polymerization.

3% conversion showing turbidity associated with formation of Domains, from Sanderson “Aspects of Vinyl Chloride Suspension Polymerisation”, 1980

Growth of Primary Particles eventually leads to their destabilization (5% – 15% conversion) depending on specific process conditions that can be adjusted in some degree.
6% conversion showing phase separation associated with aggregation of Primary Particles, from Sanderson “Aspects of Vinyl Chloride Suspension Polymerisation”, 1980
Upon reaching a conversion of 15% to 30%, a 3D agglomerate skeleton is formed within each drop (now called "Subgrain" depending on recipe and polymerization conditions. Since PVC is denser than its monomer, voids will form among Primary Particles.
As conversion increases, Primary Particles and Agglomerates continue growing until reaching their final size (around 1 micron for Primary Particles) at full conversion, when PVC "dries" as VCM is consumed in gel phase and voids partially fill.

The final product is a PVC grain with defined internal structure that can be characterized in a simple way by the total volume of voids among Primary Particles and Agglomerates, known as "Porosity".
Internal pore structure can be observed by Scanning Electron Microscopy (SEM) but interpretation is complex and quantification most times impractical as gives information of just a few Grains inside a sample.

SEM images of two PVC resins, one with processing problems and the other with good processing performance.
Mercury Intrusion Porosimetry (MIP) allows quantification of some aspects of internal structure which are known to affect S-PVC processability.
a. Large cavities.
b. Packing of Primary Particles.
c. Voids among packed Primary Particles.

Secondary Suspension Agents (SSA) are commonly used to modify Internal Morphology by increasing Porosity for the production of PVC resins suitable for flexible applications. However, not all SSAs perform in the same way and might require specific optimization to generate the best results.
The following figure shows MIP intrusion curves for PVC resins produced with 4 different types of SSAs (total Porosity is shown in parentheses). As all 4 Secondary agents have similar active chemical species according their Technical datasheets, the differences in PVC internal structure are related to the way are applied in polymerization.

Do you have problems with the residual VCM in your resins?
Do you have low Porosity in your resins?
Do you have issues with Gels (partially fused grains) during processing?
Do you want to have a faster and homogeneous absorption of plasticizers?
Do you want to improve chlorine diffusion during CPVC production?
Do you want to select a more cost-efficient Secondary Suspending Agent?
Do you want to replace your Secondary Suspending Agent without causing Quality issues?
Do you want to change polymerization conditions without having Gel problems?
Do you want to increase your polymerization rate keeping or improving PVC's Quality?
Let me help you understand and safely control the formation of
Internal Morphology during your polymerization process to optimize
the performance of your S-PVC resins and Production Plant.
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