Analytical Techniques for Non Intentionally Added Substances in Recycled Plastics
Non-target HRMS screening combined with toxicological threshold evaluation provides the empirical basis for verifying recycled plastic safety before market entry.
Thermal Degradation Pathways
Meaning
Chemical reactions involving the scission of molecular chains or the release of volatile compounds define these degradation routes during polymer processing. When exposure to excessive heat forces thermal degradation pathways, the resin loses molecular weight and physical strength. This phenomenon occurs when energy input exceeds the bond dissociation limits of a polymer backbone, leading to unintended side groups forming or chain fragments releasing into the environment.
Processing Limit
High melt temperatures during extrusion or injection moulding frequently accelerate these chemical breakdowns. Machine operators track residence time in the barrel to avoid excessive heating cycles that trigger such molecular shifts. A specification for a resin grade dictates the maximum safe processing window, but the actual thermal history depends on machine settings and the presence of residual moisture or shear stress.
Virgin resin generally tolerates higher temperatures than material containing regrind, as repeated heating cycles leave behind active free radicals that lower the temperature threshold for further degradation.
Product Defect
Visual evidence of material failure appears as yellowing, brown streaks or black specks on the surface of an injection moulded part. Mechanical properties often suffer before these aesthetic issues become visible to the eye. Reduced tensile strength and lower impact resistance result from the shortened molecular chains that populate the finished component.
A part specification defines the acceptable performance for the final item, but the molder manages the variables that prevent this structural loss.
Chemical Mechanism
Molecular chains break apart through random scission or de-propagation when kinetic energy vibrates bonds past their breaking point. Small volatile molecules evaporate from the melt, causing the formation of voids or bubbles that weaken the matrix of the plastic. Stabilizers like antioxidants and ultraviolet inhibitors scavenge the resulting free radicals to pause these reactions before they propagate through the entire melt volume.
The rate of this deterioration depends strictly on the chemical structure of the polymer chain and the concentration of available oxygen in the melt stream.