Meaning
Organic chemical structures comprising long carbon chains display elevated melt viscosity and high thermal stability during thermoplastic processing. The presence of high molecular weight hydrocarbons in polyolefin resins determines structural integrity and environmental stress crack resistance during extrusion or blow moulding. This molecular fraction governs melt flow behavior and mechanical impact performance in finished plastic parts.
It stops applying when thermal degradation or mechanical shear breaks the carbon chains into lower molecular weight oligomers.
Viscosity Response
Extended chain entanglements restrict polymer chain mobility during melt flow, increasing shear viscosity under low shear rate conditions. Resins rich in high molecular weight hydrocarbons require higher processing temperatures and elevated extrusion pressure to achieve full cavity filling. Melt fracture occurs when extreme shear rates exceed chain entanglement relaxation speed.
Pressure drop along feed channels increases.
Structural Integrity
Long carbon backbones dissipate impact energy efficiently across the polymer matrix, preventing rapid crack propagation in structural mouldings. Incorporating high molecular weight hydrocarbons increases tensile strength and environmental stress cracking resistance in heavy-duty containers and pipe applications. Microscopic orientation during cooling locks the extended chains into crystalline domains, yielding enhanced toughness.
Short molecular chains cause premature brittle failure under environmental stress. Chemical resistance improves because dense molecular packing retards chemical migration.
Processing Behaviour
Narrow molecular weight distributions containing long chains create strong elastic melt behavior, enabling stable blow moulding parison inflation without sagging. Processing high molecular weight hydrocarbons demands robust drive motors and heavy-duty gearboxes to overcome high melt resistance within the extruder barrel. Temperature profiles must be controlled tightly to prevent thermal cleavage of long chains during extended residence times.
Polymer degradation reduces structural performance if localized melt temperatures spike during shear heating. Resin specifiers match molecular weight distribution to process equipment capability to prevent melt instability.
