Degradation of Polymers: Oxidative

Course Module for ME 5XX - The Degradation of Polymers

Types of Degradation - Oxidative

There are two types of oxidation in regards to polymers; direct oxidation and auto-oxidation.

Direct oxidation refers to reactions that occur spontaneously under standard temperature and pressure, such as the oxidation of polymers with metal ions as functional groups. The exposure of these polymers to oxidizing agents such as potassium permanganate, sulfuric acid, or nitric acid lead directly to oxidation. The oxidation of these metal ions may deteriorate the mechanical properties of the polymers, since direct oxidation leads to preferential oxidation of crystallization sites in polymers which are only partially amorphous.

Auto-oxidation occurs with polymers with molecular oxygen in their structure. This leads to the presence of free radicals. In commerical plastics, the main concern is that of thermal degradation of hydroperoxides (R-OOH) that tend to be present in many polymers in small quantities. Since ROOH groups decompose at relatively low temperatures, the presence of extremely reactive hydroxide ions (OH-) is common. Hydroxide radicals are the initiation step for a multitude of of hydrolysis reactions or addition reactions, such as in aromatic compounds.

Trace quantities of metals in polymers, whether impurities or minor parts or the polymer chain, can also catalyze the oxidative process. Metals are often present in the polymeric matrix as ions, which can pull a hydrogen molecule off the matrix, leaving a free radical. Free radicals start the chain scission which leads to degredation.

This page prepared by Kimberly Farrell for Professor Sisson of WPI, for use in ME XXX, Corrosion and assorted fun times.