Meaning
Material degradation analysis techniques identify the breaking of covalent bonds along the backbone of a polymer molecule during processing or environmental exposure. Quality laboratories perform polymer chain scission detection to evaluate the extent of thermal or mechanical damage introduced during the compounding process. This analytical method is focused specifically on the identification of molecular weight reduction and cannot detect cross-linking or additive depletion.
Analytical Method
Gel permeation chromatography is utilized to measure the change in the molecular weight distribution of the polymer sample. In polymer chain scission detection, a shift toward lower molecular weights indicates that the long chain molecules have been broken into smaller fragments. This molecular change is accompanied by an increase in the melt flow index of the material, which can be measured on the factory floor.
Process Consequence
Extruding the material at excessive temperatures or shear rates accelerates the degradation of the polymer matrix. The polymer chain scission detection results allow engineers to adjust the screw speed and the temperature profile of the extruder to minimize degradation. This optimization helps to maintain the physical properties of the finished product, such as impact strength and long-term durability.
It is especially important when processing recycled polymers, which are more susceptible to thermal degradation than virgin resins due to prior heat histories, meaning that minor process adjustments can have a significant effect on the final product quality.
Quality Documentation
Certificates of analysis must include the molecular weight distribution parameters to prove that the compounding process did not compromise the material integrity. Polymer chain scission detection provides the primary evidence required to resolve disputes between the resin supplier and the compounder regarding material degradation. This objective data prevents the shipment of fragile plastic parts that would fail prematurely in the field.
