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VARIABILITY
PVC is the most versatile plastic, with properties that can be tailored from rigid to flexible by mixing different liquid and solid additives with PVC resins prior processing.
It is well known that when a vinyl manufacturer replace a PVC resin in his/her process with another of the same grade / classification but from different supplier, differences are observed in processing speed, energy consumption (temperature, torque) and / or melting (degree and homogeneity of fusion) of PVC, which can range from aesthetic aspects to differences in mechanical properties of the product (Quality).
To eliminate negative effects it is necessary to make modifications in the processing conditions, which can vary from a minor adjustment to notable increases in the energy required for processing and / or the amount of additives necessary for correct processing.
The reason for these differences lies in the particular PVC resin Morphology since although all seems like powders at first glance, their complex nature can be clearly seen using Scanning Electron Microscopy (SEM).
It is important to mention that both internal and external morphologies are important for processing.
External morphology is directly related to Grain size distribution (or Sieve distribution) and Bulk Density, while internal morphology is related to Porosity, residual VCM level, liquid (plasticizer / thermal stabilizer) absorption, gels and easiness of processing.
Although mechanical properties of final products depend mainly on K-value (molecular weight) of PVC and used formulation (type and amount of additives), many processors have realized that PVC resins with same K-value from different suppliers perform slighthly (or very) differently during processing.
Some PVC resins absorb liquid additives very fast and homogeneously while absorption in other resins is slow and heterogeneous. Some resins melt easily and produce a homogeneous extrudate while others are difficult to melt and result in heterogeneities within extrudate.
The consequences of such fusion heterogeneity can range from aesthetic imperfections (such as fish eyes and gels in flexible applications) to lower mechanical properties in rigid applications.
When processors are faced in using such heterogeneous PVC resins in their processes, their only alternative is to increase processing energy and/or the amount of additives to avoid Quality issues.
The product from a batch produced by Suspension Polymerization is a PVC resin characterized by a population of grains with distributions of size (may contain from very small to very large grains), porosity (may contain from very porous to almost solid grains) and shape (may contain from very rounded to very irregular grains).

SEM images of PVC resin obtained with different polymerization process conditions
Extruders (either single or double screw) predominantly perform distributive mixing but much less dispersive mixing, so that grains with different levels of plasticizers would maintain that difference until they are melted, but this also depends on their plasticizer concentration.
Just as PVC grains with good plasticizer absorption have the targeted concentration of plasticizer and will melt homogeneously, PVC resins with high dispersion of grain types can cause problems in flexible applications such as:
- Grains with very little plasticizer: Fish eyes
- Grains with plasticizer deficit: Gels / imperfections visible in thin and transparent products, blockages in extruder meshes, partial segregation of mineral additives
- Very plasticized grains: Exudation of plasticizers in highly plasticized products.

Microscopy images of partially fused grains in plasticized film

Microscopy images of grains soaked with plasticizer

Some dispersity is also observed: resins with equal porosity show a range of densities. Although such dispersity could be attributed to variations in analysis, R & R study show that most of it is not methodological variability.
The reason is that original drops agglomerate to form grains in a distinctive way defined by charge and polymerization conditions, resulting in grains that pack differently depending on their shape. Rounded grains pack better (higher density) while more irregular grains pack poorly (lower density).
We can calculate the percentage of volume in a sample occupied by PVC grains by using this formula:

This percentage gives an indication of the shape of grains, since more rounded grains will pack better and occupy more volume of the sample. Also, heterogeneous grains will pack poorly compared with homogeneous grains.
By adding to the graph isolines corresponding to constant volumes occupied by grains, we can see the effect of grain shape on the density of PVC resins:

When comparing PVC resins with the same Porosity, Mercury Intrusion Porosimetry analysis show that resins with lower % volume of grains are more heterogeneous than resins with higher % volume of grains.
Then it is possible to compare the percentages of volume occupied by PVC grains of two samples of the same (or similar) Porosity and to be able to reach a conclusion of their heterogeneities.
The sample with higher % volume would have more rounded and homogeneous grains while the sample with lower % volume would possess more irregular grains with more heterogeneous internal structure.
You should not assume that your S-PVC resins are optimized even if you have no Quality complaints from your customers and there are no Sales issues.
It is only a matter of time before a new competitor arrives in your region and offers optimized S-PVC resins with competitive advantages to your Clients, and if you wait for them to realize such advantages then you will be putting the competitiveness of your products at risk.
Let me help you diagnose and optimize your S-PVC resins so that your Customers realize your throughput and ensure their loyalty.
Do you have more questions regarding PVC resins? I have many answers.
View my PROFILE and visit my CONSULTING SERVICES page.
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