Meaning
Polymer degradation mechanisms involve the breaking of covalent bonds along the backbone of a macromolecular chain. In chain scission degradation, thermal or mechanical stress splits the polymer chains into shorter segments. This reduction in molecular weight alters the physical and rheological characteristics of the material.
Reaction Pathway
Exposure to high melt temperatures, shear stress, or moisture during extrusion initiates the cleaving of the polymer backbone. Free radicals form at the broken chain ends, which propagates further breakdown of neighboring chains in the presence of oxygen. This self accelerating reaction continues until termination occurs through the coupling of two radicals.
The rate of this reaction increases with higher processing temperatures and longer residence times in the heated barrel of the machine.
Processing Consequence
Viscosity drops dramatically as molecular weight decreases, which changes the behavior of the melt during mold filling. Processors observe a sudden increase in melt flow rate, leading to flash at the parting lines and mold vents. In severe cases, the reduced melt strength causes the polymer to drool from the nozzle or sag during profile extrusion.
Mechanical Impact
Molded parts produced from degraded resin suffer from severe reductions in impact strength and elongation at break. This embrittlement occurs because the shorter chains cannot form the entanglements needed to distribute mechanical stress. Consequently, parts fail prematurely under impact loads that a non degraded material would easily survive.
