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PROCESSING &
RESTRUCTURING
It is expected that each S-PVC resin is designed and produced to meet the requirements for an specific application, be it for the manufacture of rigid or flexible products.
However, each producer uses different combinations of recipes, procedures and process conditions that result in particular Molecular characteristics as well as Internal and External Morphologies that may or may not be detectable by conventional characterization methods.
SEM images of PVC resin obtained with different polymerization process conditions
What is crucial is that vinyl processors "feel" these differences in their processing performance.
When a vinyl processor has only one supplier of S-PVC, their production and managerial responses "get used" to dealing with the characteristics of that resin.
However, those who normally change suppliers or are forced by market circumstances to try resins from other supplier realize that resins made for the same application but from different producer DO NOT perform in the same way, finding differences that can reach up to 30% in energy consumption and some additives necessary for correct processing without Quality issues.
To identify how S-PVC resins may perform differently in processing, first there is a brief review of the phenomena involved in Blending and Extrusion processes.
1. BLENDING PROCESS
PVC is an extremely versatile polymer with properties that can be adapted by mixing with additives, some of them are solid powders while others are liquid, some are highly compatible with PVC while others have limited compatibility, depending on their function to perform.
Blending or additive mixing process is sequentially designed to achieve a proper dispersion for each additive by mixing and heating with a high-shear blender to produce a free-flowing dry-blend powder suitable to be processed.
Liquid / molten additives with high compatibility with PVC such as plasticizers and heat stabilizers have key roles for PVC performance and must be homogeneously dispersed into polymer matrix.
As the absorption of large amounts of plasticizer for the production of flexible articles is extremely demanding, we will take it as a case to review the simultaneous processes that are carried out during Blending:
- Capillarity that fills pores (voids) and result from intermolecular forces between the liquid additive and the surrounding PVC internal surfaces.
- Diffusion that is the net migration of liquid additive from external PVC surface in contact with additive that fills pores towards the internal parts of Primary Particles and Agglomerates.
Capillarity is affected by plasticizer viscosity and pore geometry (Internal Morphology) while Diffusion is affected by PVC/plasticizer compatibility, Temperature, PVC Molecular Weight Distribution and pore geometry (Internal Morphology).
Only the effects associated with the properties of PVC grains will be reviewed and not other factors.
Effects of Internal Morphology
As commented, S-PVC Internal Morphology is determined by polymerization conditions and consists of a distribution of macromolecular microstructures known as Primary Particles and their agglomerates.
Some resins consist of Grains with high Porosity made of loose packing of Primary Particles, which rapidly absorb plasticizers first by Capillarity into their pores and then by Diffusion inside Primary Particles. These resins are optimal for flexible applications where large quantities of liquid additives have to be absorbed quickly and homogeneously to produce flexible articles.
Other resins consist of Grains with low Porosity, made with compact Primary Particles and Agglomerates, that would slowly absorb plasticizers by Capillarity and Diffusion. These resins are more suitable for the production of rigid articles.
SEM images of fractured PVC resins, one for rigid application and other for flexible application
Although it is assumed that plasticizer has been homogeneously dispersed once the mixture becomes dry (dry-blend time), this only means that Capillarity is finished but Diffusion may not have been completed.
Problems arise when not all Grains in a PVC batch are formed in the same way and any of the following types can be found:
- "Loose Grains"
Formed of individual Primary Particle and lightly fused Aggregates.
They have medium Porosity that absorb fairly good by Capillarity. and quickly by Diffusion.
- "Cave Grains"
They contain large voids inside that can quickly absorb by Capillarity high amounts of plasticizers, decreasing dry-blend time.
Stored plasticizer causes PVC within Grains to become over-plasticized after Diffusion which lead to processing issues.
- "Dense Grains"
Formed by packed and fused agglomerates with small internal voids that absorbs small quantities of plasticizer by Capillarity and then slowly by Diffusion.
As result this type of PVC Grain is underplasticized and would be difficult to melt, so they are known as "Gels" and "Fish-eyes".
- "Fused Grains"
Made from very fused Aglomerates with low Porosity and very small or occluded pores that prevent plasticizer absorption by Capillarity.
As the distance for plasticizer Diffusion is very large, PVC inside remains unplasticized which makes it essentially infusible under processing conditions.
Effects of Molecular Weight Distribution
Molecular Weight is assumed to be homogeneous and that a single value describes it, commonly expressed as "K-value", but in reality is a distribution of polymer chain lengths determined along vinyl chloride polymerization mainly by polymerization temperature and to a lesser extent by initiator loading and the conditions under which chain propagation takes place.
Molecular Weight is obtained by measuring the effects of hydrodynamic volume of polymer chains in dilute solutions, either by viscometry or chromatography. The limitation of these methods is that the resulting distribution "averages" all molecular weights present within all dissolved Grains.
Long polymer chains move with more difficulty than short polymer chains to expand and allow penetration of plasticizer molecules, so the distribution of polymer chain lengths is very important for Diffusion.
2. EXTRUSION PROCESS
Single or twin screw extruders used for PVC processing provide energy needed to transform dry-blends into a homogeneous molten material:
- Mechanical energy that generates pressure and mixing (both distributive and dispersive) on powder/melt.
- Thermal energy both from conductive transfer (heating elements in barrel) and from dissipation of viscoelastic stresses on powder/melt to increase chain mobility beyond glass transition and melt temperature.
Processing destroys PVC external and internal macromolecular structures (Grains, Aggregates and Primary Particles) while original nanocrystalline structure partially melts.
It is known that processing of PVC resins within twin screw extruders goes through some simplified stages known as the CDEF mechanism:
1) Compaction.- Mechanical forces compress gravity-fed dry-blend Grains and pack them more closely as temperature begins to increase. This lessens and eventually eliminates voids between dry-blend grains.
2) Densification.- Mechanical forces continue to press and temperature continues to raise until the interior of dry-blend grains is crushed and internal Porosity is lost.
3) Elongation.- Shear forces in melt front and barrel wall deform Grains while Primary Particles begin to lose identity as temperature is high enough to begin fusion of nano-crystalline 3D structure.
4) Fusion.- Shear forces and temperature equalization lead to complete obliteration of Grains and partial melting of original nano-crystallites resulting in a molten flow with both particle and viscoelastic behaviors whose balance depends on the "true" processing temperature.
Extrusion performance is affected by Dry-Blend Formulation, Temperature, Extruder design, screw speed, External and Internal Morphologies, as well as Molecular Weight Distribution and Chain Defects Distribution.
Effects of Molecular Weight Distribution
The length of polymeric chains influences both processing (melt viscosity, melt strength, die swell, etc.) and mechanical performance of product (tensile strength, impact, low temperature behavior, etc.) for an application, as shorter chains flow better but have lower opportunity to form crystallites that boost physical properties while longer chains flow with more difficulty due to entanglements and produce a stronger material due to its increased formation of crystallites.
Effects of External Morphology
External morphology clearly has an effect in extrusion as regular-shaped PVC grains flows quickly and packs well while irregular-shaped grains flows slowly and packs poorly in gravity-hoppers used to feed PVC extruders.
Highly regular grains then fills better screw flights, improving energy usage and increasing extrusion throughput while exhibiting more stable operation.
Dense packing of Grains is achieved with good polymerization process control and optimization, so that monomer droplets are homogeneous and form grains with homogeneous morphological properties.
On the other hand loose packing of grains is formed with unstable and not-optimized polymerization conditions, which result in heterogeneous monomer droplets and grains with inhomogeneous morphological structures.
Effects of Internal Morphology
As commented before, the way in which Primary Particles are packed has a direct effect on Internal structure. Loose packings contain larger "pores" with high Porosity while more packed/fused agglomerates contain smaller "pores" with low Porosity.
Internal morphology has a clear effect in Extrusion as loose packings of Primary Particles need less mechanical and thermal energy to crumble and melt than dense fused packings.
Beside, as loose packings absorb plasticizers more homogeneously, internal PVC would be easier to melt than less plasticized PVC from dense fused packings.
Optical microscopy images of partially fused and unfused Grains in flexible formulation
Any inhomogeneous dispersion of liquid/molten additive among PVC grains after dry-blending can only be equalized during Fusion (molten) stage but excess thermal and mechanical energy must be applied to avoid Quality issues such as aesthetic imperfections (gels) or regions prone to failure under stress.
After cooling, extruded PVC will form a homogeneous plastic reinforced with a new 3D nanocrystalline structure whose characteristics are defined by ...
- the nature of the PVC chains,
- the internal morphology of original resin,
- the amount of energy supplied in processing, and
- the cooling of the extrudate
and that determine the mechanical properties of final vinyl product.
Effects of Chain Defects Distribution
Thermal degradation occurs when PVC chains increase their temperature during processing until reaching a temperature higher than that necessary to initiate decomposition by dehydrochlorination from labile groups inside chains.
To limit the negative effects of Thermal degradation, recipes and polymerization conditions are chosen to minimize the formation of labile groups, mainly tertiary chlorines and internal allylic chlorines.
As PVC is a thermal insulator, processing temperature is not completely homogeneous while PVC heats up and melts, so Thermal Stability in final product results from the interaction between thermal susceptibility of PVC chains (concentration of labile sites ) and the amount of energy used to melt it (initiation of degradation).
Vinyl processors add chemical compounds called "Thermal Stabilizers" whose function is to penetrate PVC matrix and substitute labile functional groups to make them more stable and stop propagation of degradation reaction.
So Thermal Stabilizers must be added in sufficient amount and must be adequately dispersed to reach labile sites in a timely manner to achieve their required performance.
Thermal Stability during processing are inferred from methods that measure PVC resin degradation under different conditions such as conductive heat transmission alone (Static Thermal Stability) or combined with mechanical processing (Dynamic Thermal Stability) to melt PVC and expose it to degradation conditions.
Static Thermal Stability can be evaluated either by comparing an onset of degradation (change in color with forced convection heating) or measuring the kinetics of degradation (neutralization of released hydrochloric acid or with thermogravimetric analysis).
Dynamic Thermal Stability can be evaluated by measuring the strong change in rheological properties resulting from advanced degradation using a torque rheometer (ASTM D2538) or other processing devices at varying conditions of temperature and rotational speed.
All methods for measuring Thermal Stability have different contributions between heat transmitted by conduction and self-generated by mechanical stress. And they also offer a different vision of the degradation process, whether measuring its beginning, development or end point.
Do your Customers complain about the performance of your S-PVC resins
Do you want to match the extrusion performance of a competing resin?
Do you have problems of low Thermal Stability in your resins?
Do you have issues with low mechanical strength in your resins
(compared with resins of similar K-value?
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required to process your S-PVC resins?
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