Rheological Determination of Ultra High Molecular Weight Tails in Bimodal Resins
Low-frequency storage modulus and creep recovery accurately quantify ultra-high molecular weight tails in bimodal resins, ensuring stress crack resistance.
Creep Recovery
Meaning
Time-dependent deformation of a material after the removal of a constant load defines the elastic return of a polymer. When a plastic part is released from stress, creep recovery measures how much of the initial strain is reversed by the internal energy stored in the molecular chains. This value distinguishes between purely viscous flow, which is permanent, and elastic deformation, which is reversible.
Deformation Response
Viscoelastic materials do not return to their original dimensions immediately upon the removal of force. Instead, the polymer chains slowly rearrange to their equilibrium state over a period that can last from seconds to hours.
Molecular Implication
Chain entanglements and cross-links act as the primary drivers for the restoring force observed during creep recovery. Long chains in high molecular weight resins provide more entanglements, which leads to a higher percentage of elastic return. In contrast, low molecular weight waxes or lubricants increase the permanent set of the material.
This measurement provides a sensitive probe into the long chain branching of the resin, which standard melt flow rate tests often miss.
Tooling Influence
Mould designers must account for the delayed dimensional changes that occur after a part is ejected. Because creep recovery is temperature dependent, the cooling rate in the mould determines the final precision of the part. Parts that are ejected too hot may undergo shape changes as the internal stresses relax outside the constraint of the tool.