Polymer Melt Strength

Physical Mechanism

Extensional viscosity is the primary rheological driver of this resistance. High polymer melt strength is typically found in resins with high molecular weight and a high degree of long-chain branching. These branches act as temporary entanglements that resist being pulled apart when the melt is stretched.

This behavior is characterized by strain hardening, where the polymer becomes stiffer as it is drawn, preventing localized thinning and premature rupture of the strand.

Processing Impact

Performance in processes like blown film extrusion, blow molding, and thermoforming depends heavily on this property. In blown film extrusion, high polymer melt strength is required to maintain a stable bubble and prevent film tearing as the melt is drawn upward. In blow molding, it prevents the parison from sagging or thinning excessively before the mold closes.

Insufficient strength leads to wall thickness variations and part defects that compromise the structural integrity of the finished product.

Material Formulation

Formulators can adjust this property through resin blending or by incorporating specific additives. In polymer sourcing, high melt strength grades are selected for extrusion applications, while lower melt strength grades are chosen for injection molding where fast mold filling is required. Adding chain extenders or blending virgin resin with long-chain branched grades are common methods used to improve the melt behavior of recycled materials.

This compounding approach helps recover the processing performance of recycled polymers that have suffered degradation during previous heat cycles.