HOW PVC RESINS CAN BE DIFFERENT: PROCESSING

As expressed in previous articles, both internal and external Morphology of grains affect the performance of PVC resins in processing for the manufacture of vinyl products.

PVC is a very versatile polymer with properties that are improved and tailored by combining with additives. Some of them are solid powders while others are liquid, some are very compatible with PVC while others have limited compatibility, depending on their function to perform.

Blending or “additive mixing” process is sequentially designed to achieve the proper dispersion of each additive by heating with high-shear mixing to produce a free-flowing dry-blend powder to be extruded.

Liquid/molten additives with high compatibility with PVC such as plasticizers and heat stabilizers have key roles in the performance of PVC and must be rapidly and homogeneously absorbed into polymer matrix as temperature increases, first by filling in pores and then by diffusion inside primary particles.

More porous grains would absorb greater amounts of liquid/molten additives than less porous grains, while less dense agglomerations of primary particles will absorb liquid/molten additives more homogeneously than dense agglomerations. Absorption rate then depend on both processes.

So it is easy to realize that when using a PVC resin consisting of grains with different porosities and agglomerations of primary particles with different packing densities, the result will be an inhomogeneous distribution of liquid/molten additive among PVC grains that could result in observable differences in additive’s performance that would affect final-product Quality such as aesthetic imperfections, thermal degradation or regions prone to failure under stress.

The way in which these differences are commonly "fixed" is by adding more additive to compensate for its deficiency in hard-to-reach zones, which represents an additional cost for the processor.

After Blending process follows Extrusion process that transforms dry-blend into a homogeneous melt that will be shaped into the finished product either immediately or in a subsequent process.

It is well known that during their processing PVC resins go through distinctive stages within the extruder:

1) Compaction.- Loss of external voids among grains

2) Densification.- Loss of internal pores inside grains

3) Elongation.- Loss of identity of grains by initial melting of primary particles

4) Fusion.- Homogenization of plastic melt by partial to total melting of primary particles

On top of this article you can see SEM images from PVC processed in a torque rheometer showing transformations from dry-blend to homogeneous melt.

As any differential in liquid/molten additive concentration among PVC grains can only be equalized during Fusion stage, excess energy (temperature and stress) must be applied to viscous molten PVC to compensate for any poor initial dispersion of additives and avoid Quality issues such as aesthetic imperfections (gels) or regions prone to failure under stress.

 External morphology has also an effect in extrusion as regular-shaped PVC grains flow quickly and pack well while irregular-shaped grains flow slowly and pack poorly when gravity-hoopers are used to feed PVC extruders. Highly packed grains then fill better screw flights, improving energy usage and increasing extrusion throughput while exhibiting stable operation.

Use of inconsistent PVC resins with inhomogeneous grains would then cause any of the following:

a) Variations of dispersion for liquid/molten additives

b) Variations of feeding rate

c) Variations of PVC fusion

... resulting in Quality issues and cost overruns in energy / additives when trying to fix them.

On the other hand, using consistent PVC resins with homogeneous grains allows stable processing conditions without using excess additives and consuming the energy necessary to melt primary particles enough that optimizes mechanical properties without damaging unnecessarily its thermal stability.

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