Meaning
Thermal decomposition thresholds define the temperature where a solid polymer experiences irreversible chemical breakdown and mass loss under controlled thermal exposure. Thermogravimetric analysis records degradation onset temperature as the point where the baseline mass begins its downward drop, typically calculated via the intersection of baseline and degradation tangent lines. Within resin purchasing and processing, this parameter sets the absolute thermal boundary for melt stability.
Exceeding this boundary cleaves the backbone chains of virgin or recycled pellet stocks. The measurement ceases to apply once volatilization is complete or when testing polymers that undergo crosslinking without immediate volatile release.
Processing Window
Safe melt processing requires maintaining cylinder zones, manifold channels, and hot runner tips well below the critical cleavage point. Operating within fifty degrees of degradation onset temperature introduces severe molecular weight loss, manifesting as sudden viscosity drops and severe flash at tool parting lines. Sourcing contracts specify this value alongside melt flow rate to guarantee thermal stability during unexpected machine stoppages.
Regrind Penalty
Repeated heat cycles lower the threshold through shear-induced chain scission and accumulated radical formation. Post-industrial regrind exhibits a downward shift in degradation onset temperature relative to virgin material, narrowing the permissible barrel residence time. Moulding facilities blending regrind at high ratios encounter sudden outgassing, splay defects, and charred black specks when residence times extend during cycle interruptions.
Testing Protocol
Standardized testing under nitrogen atmospheres isolates pure thermal rupture, whereas atmospheric testing with oxygen introduces thermo-oxidative degradation at substantially lower temperatures. Dynamic thermogravimetry running at ten degrees Celsius per minute yields a higher apparent onset than isothermal runs held at steady processing temperatures for an hour. Quality agreements must specify both atmosphere and heating rates to ensure repeatable incoming lot inspection.
