Process aids two mechanisms
2022-08-20
1. By improving adhesion in the first step of fusion
S-PVC grain (secondary particle) of size 90-250 micron has a hard skin of 0.2 – 1.0 micron. This hard skin makes fusion very difficult. Unless this skin breaks, dispersion of secondary particle to primary particle and in turn fusion cannot take place effectively.
During extrusion, some degree of adhesion is necessary to facilitate shearing and convey the material along the flights of the screw.
In the initial stages of fusion, PVC tends to exhibit reduced adhesion to it and to metal surfaces.
This delays heating and breakdown of the grains.
Process aids do not have hard skin like PVC. Due to this reason, when heated under shear and stress, process aids soften earlier than PVC and then adhere to the surrounding PVC particles. Owing to good compatibility with PVC, they transmit elongational and shearing forces to PVC grains.
In this way process aids work as shear transfer agents, resulting in faster and uniform breakdown of PVC grains to primary particles (dispersion), leading to faster and uniform fusion.
Generally, lower the molecular weight of process aid, lower is the temperature of onset of fusion.
2. By providing longer chains to entangle with PVC chains in the second stage of fusion:
Acrylic processing aids have very good compatibility with PVC because of their main component PMMA.
They also have higher molecular weight and longer chains than PVC.
Thus, when heated under shear stress during processing, entanglement appear between PVC chains and process aid chains resulting in viscoelastic network.
These entanglements work as cross links during processing and add rubbery characteristics to PVC compounds enabling increase in drawdown for extrudate or blow up ratio for blown films.
Process aid chains are elastic at processing temperature. Their action depends upon molecular weight or chain length and evaluated through die swell. Higher molecular weight process aids give higher die swell.
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