Photo-oxidation of Polymers - Photo-oxidation Protection - Chemical Mechanism

Chemical Mechanism

Aldehydes, ketones and carboxylic acids along or at the end of polymer chains are generated by oxygenated species in photolysis of photo-oxidation. The initiation of photo-oxidation reactions is due to the existence of chromophoric groups in the macromolecules. Photo-oxidation can occur simultaneously with thermal degradation and each of these effects can accelerate the other.

The photo-oxidation reactions include chain scission, cross linking and secondary oxidative reactions. The following process steps can be considered:

  1. Initial step: Free radicals are formed by photon absorption.
  2. Chain Propagation step: A free radical reacts with oxygen to produce a polymer peroxy radical (POO•). This reacts with a polymer molecule to generate polymer hydroperoxide (POOH) and a new polymer alkyl radical (P•).
  3. Chain Branching: Polymer oxy radicals (PO•) and hydroxy radicals (HO•) are formed by photolysis.
  4. Termination step: Cross linking is a result of the reaction of different free radicals with each other.

where PH = Polymer

P• = Polymer alkyl radical

PO• = Polymer oxy radical (Polymer alkoxy radical)

POO• = Polymer peroxy radical (Polymer alkylperoxy radical)

POOH = Polymer hydroperoxide

HO• = hydroxy radical

Read more about this topic:  Photo-oxidation Of Polymers, Photo-oxidation Protection

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