Meaning
Chemical degradation processes alter the molecular weight distribution of thermoplastics during high-temperature processing. The term melt scission describes the cleavage of covalent bonds in the polymer backbone when the material is molten. This process leads to a reduction in average molecular weight and a change in physical properties.
Thermal Mechanism
Excessive heat and mechanical shear during extrusion break the long polymer chains into shorter fragments. This melt scission reaction is often accelerated by the presence of oxygen or trace moisture in the processing cylinder. Polypropylene is particularly susceptible to this degradation, resulting in a narrower molecular weight distribution and lower melt strength.
In contrast, some other polymers may undergo branching or cross-linking under similar conditions.
Moulding Impact
Molders observe a sharp increase in the melt flow rate when chain cleavage occurs. This high fluidity can lead to flash at the mold parting line or difficulty in controlling the melt during extrusion blow molding. The mechanical consequences of melt scission include reduced impact strength and lower elongation at break in the finished part.
Degradation Control
Processors mitigate this degradation by drying the resin thoroughly before processing and using heat stabilizers. Minimizing regrind consumption also prevents the cumulative effects of multiple thermal cycles. These precautions help maintain the molecular integrity of the polymer during production.
