Meaning
A self-sustaining cycle of free radical reactions describes how polymers degrade when exposed to heat and oxygen. The auto oxidation mechanism begins with the formation of a carbon-centered radical that reacts with oxygen to produce a peroxy radical. This sequence continues in a loop, stripping hydrogen from the polymer chain and creating more radicals that further break down the material.
Initiation Phase
External energy from heat or ultraviolet light triggers the first break in the polymer bond. Once the auto oxidation mechanism starts, the polymer loses its structural integrity as the chains become shorter or more branched. This initial stage is often invisible to the eye but sets the stage for rapid loss of mechanical properties.
Degradation Cycle
Propagation occurs when the newly formed radicals attack neighboring polymer chains. This part of the auto oxidation mechanism generates hydroperoxides, which are unstable molecules that eventually split into even more radicals. Without chemical intervention, the process accelerates until the plastic becomes brittle, changes color, or develops a foul odor.
The transition from a flexible part to a fragile one happens quickly once the hydroperoxide concentration reaches a critical threshold, leading to catastrophic failure in the field.
Termination Event
Stability is only achieved when two radicals meet and form a stable bond or when an antioxidant neutralizes them. Inhibiting the auto oxidation mechanism requires the use of primary and secondary stabilizers that intercept radicals at different points in the cycle. Effective stabilization prevents the cracks and failures that would otherwise occur in parts used for outdoor or high temperature applications.
