Capillary Rheometry Shear Rate Correction Protocols for Bimodal High Density Polyethylene
Correcting capillary rheometry data via Bagley and Rabinowitsch protocols is mandatory to prevent up to 50 percent viscosity errors in bimodal HDPE die design.
Processing Aid
Meaning
An additive incorporated into polymer formulations improves the processability of the melt during high-shear operations without altering the mechanical properties of the final product. By migrating to the metal surfaces of the processing equipment, the processing aid creates a low-friction boundary layer that reduces melt fracture and extrusion torque. This compound is typically used at low concentrations in resins like linear low-density polyethylene.
It is added to eliminate surface defects like sharkskin during high-speed extrusion.
Additive Action
Incompatible additives migrate to the interface between the polymer melt and the metal die wall. Once at the boundary, it coats the metal surface, allowing the polymer to slip more easily. This action lowers the shear stress at the die exit, preventing the melt from tearing as it emerges.
Extrusion Processing
High-speed production lines benefit from this additive to run at lower temperatures and pressures. Lowering these variables reduces energy consumption and minimizes the risk of thermal degradation of the resin. Additionally, it prevents die build-up, which is the accumulation of oxidized polymer at the die exit that can leave streak marks on the extruded product, thereby reducing the need for frequent shutdowns to clean the tooling.
Defect Elimination
Eliminating surface roughness is important for packaging films where clarity is required. With the help of the additive, operators can increase production throughput without risking melt fracture. This enables film manufacturers to achieve high clarity and uniform thickness.