Correlating Zero-Shear Viscosity Drop with Antioxidant Depletion in Recycled Polypropylene
Zero-shear viscosity drops exponentially before MFR shifts, providing an early indicator of antioxidant depletion in recycled polypropylene.
Tertiary Carbon Cleavage
Meaning
Polymer degradation mechanisms describe the chemical breakdown of polymer chains at highly reactive molecular sites. In branched polyolefins, tertiary carbon cleavage represents the primary pathway for chain scission under thermal and shear stress. This chemical reaction leads to a reduction in molecular weight and a corresponding increase in the melt flow index of the polymer.
Radical Formation
Frictional heat and mechanical shear during extrusion can break the weak bonds at the tertiary carbon atoms, creating active polymer radicals. These radicals react rapidly with oxygen to form hydroperoxides, which propagate the degradation cycle. This radical formation is the first step in the thermal degradation of polymers like polypropylene.
Polymer Chain
Structural weakness is inherent in polymers that contain a high concentration of tertiary carbon bonds. The branch points in the polymer chain are more susceptible to radical attack than the linear segments. When these bonds are cleaved, the long chains are broken into shorter segments, which reduces the mechanical strength of the moulded parts.
This molecular breakdown can be monitored in the laboratory by measuring the change in viscosity over time at a constant processing temperature.
Material Degradation
Degraded resin exhibits a lower viscosity, which can lead to flashing and erratic cavity filling during the injection moulding run. In addition, the cleavage of the polymer chain causes a decline in impact resistance and elongation at break. This structural failure emphasizes the necessity of incorporating effective stabilizer packages during the compounding stage.