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.
Elongational Viscosity
Meaning
Resistance to flow when a polymer melt is stretched rather than sheared determines the stability of many industrial processes. Elongational viscosity governs the behavior of the material in film blowing and fiber spinning where the melt is pulled into its final shape. Unlike shear viscosity, which typically decreases with rate, this property may increase rapidly in a phenomenon known as strain hardening.
Strain Hardening
Molecular chains with long branches resist being pulled apart because they form a more complex network of entanglements. This increase in elongational viscosity prevents the melt from thinning too much in a single spot, a process called localized necking.
Processing Effect
Bubble stability in blown film production depends on the melt being able to support the stresses of expansion. If the elongational viscosity is too low, the bubble will flicker or burst, leading to machine downtime and material waste. Conversely, if it is too high, the motor may struggle to pull the film at the required line speed.
Defect Prevention
Thermoformed parts often fail at the corners if the resin does not possess the correct extensional properties. By monitoring elongational viscosity, manufacturers ensure that the material distributes evenly across the mould surface. This control eliminates thin spots that compromise the structural integrity of the finished product.