Evaluating Degradation Mechanisms in Post-Consumer Polyolefin Flake
Polyolefin degradation evaluation requires coupling multi-load melt flow testing and OIT analysis to quantify chain scission and restabilize flake lots.
Multi-Load Melt Flow Index
Meaning
A testing method measures the flow behavior of a polymer melt under multiple load weights to evaluate its shear-thinning characteristics and molecular weight distribution. This analytical technique, known as multi-load melt flow index, offers a rapid assessment of how a resin will behave under both low-shear and high-shear processing conditions. It provides a more complete rheological profile than a single-load measurement can offer.
Testing Procedure
The test applies different loads to a piston that forces molten polymer through a standardized capillary die at a constant temperature. By recording the flow rate under each weight, the instrument calculates the flow rate ratio. A high flow rate ratio indicates a wider molecular weight distribution, which generally improves the processability of the resin.
This testing procedure helps moulders identify batches with different molecular structures even when their single-point indexes are identical. The differences in molecular weight distribution affect the final physical properties of the part.
Process Characterization
Polymer melts are non-Newtonian fluids that exhibit shear-thinning behavior, meaning their viscosity decreases as shear rate increases. High-shear conditions occur during the injection step of moulding, while low-shear conditions occur during the subsequent cooling and holding stages. Using this testing method helps moulders select resins that fill thin-walled mould cavities easily while maintaining mechanical strength.
Resin Selection
Recycled polymers often contain blends of different grades, resulting in a broad and unpredictable molecular weight distribution. Virgin resins have highly controlled distributions that are easy to process across standard operating windows. Moulders utilize these multi-load measurements to adjust the injection speed and holding pressure when switching from virgin to recycled feedstocks.