CHEMICAL DEGRADATION Whilst all polymers will be attacked by certain chemicals it is the reactive chemicals in the atmosphere which must be considered. Of these probably the most important are: 1)Oxygen which leads to oxidative degradation. 2)Ozone - which leads to ozonolysis 3)Water - which leads to hydrolytic degradation. Protection A range of ANTIOXIDANTS etc. Have been developed. Combinations of antioxidants often used to obtain a synergistic effect.

OXIDATIVE DEGRADATION Oxidative degradation normally initiation by 1)radiation e.g. UV 2)heat 3)direct O 2 attack (not too important with saturated polymer) 4)initiator residues (proxides).

UNSATURATED POLYMERS 1)Can be easily initiated 2)Can be sujected to repeated O 2 attack 3)Must always be protected - CH 2 - C = CH - CH 2 CH 3 H 2 allylic hydrogen’s – easily replaced - CH 2 - C = CH - CH - CH 2 - C - CH = CH- CH 3 CH 3

PROTECTION By preventing the production of free radicles.

MECHANISMS 1- system which cross-link. Example Polyethylene. Changes observed on exposure to radiation are: a) H 2 and low molecular mass hydrocarbons evolved. b) cross – linking occurs. c) unsaturation Increases. d) crystallinity decreases. e) polymer yellows. f) in air surface oxidises.

hv -----CH CH H H CH CH H 2 H CH CH CH 4 CH 3 branch 2----CH CH cross – link forms ----CH by radical combination H H hv unsaturation increases ----CH--CH CH = CH H CH O CH ocidative ----O – O breakdown

2- System which show reduction in molecular mass. Mechanism still obscure, two main theories a) fracture of main chain followed by disproportionation of radicles. CH 3 CH 3 hv CH 3 CH CH 2 – C - CH 2 – C CH 2 – C + CH 2 – C CH 3 CH 3 CH 3 CH 3 CH 2 CH CH 2 – C + CH 3 – C ---- CH 3 CH 3 disproportionation

B) Multiple bond scission CH CH 2 – C complex rearrangement. CH 3 hv

OXIDATIVE DEGRADATION MECHANISM Initiative:- Formation of free radicales. e.g. from catalyst residue, effect of radiation and heat. Propagation: radical formed on polymer chain as consequence of initiation step (R o ) is attacked by oxygen. R + O 2 ROO ROO + RH ROOH + R Hydroperoxide decomposition RO + OH Rearrangement to stable product i.e. termination

example Saturated polymer (polystyrene) ----CH 2 - CH--CH 2 – CH-hv----CH 2 - C--CH 2 – CH--- rearrangement ----CH 2 – CH CH 2 - CH O - O O2O2 ----CH 2 – C O OH OH +O Farther chain activated ROOH hydroperoxide

----CH 2 - CH O ----CH 2 – CH-- OH + Stable products rearrangement Chain scission

OXIDATIVE DEGRADATION Unsaturated polymer Effect of oxygen attack far more serious with unsaturated polymers e.g. natural rubber.

MECHANISM CH 3 CH CH 2 – C = CH – CH 2 - CH 2 – C = CH – CH H CH 3 CH CH 2 – C = CH – CH 2 - CH 2 – C = CH – CH Rearrangement to stablilise radical CH 3 CH CH 2 – C = CH – CH 2 - CH 2 – C - CH = CH O 2 attack

Hydroperoxide formation cyclization CH 3 CH CH 2 – C - CH C - CH = CH O - O CH 2 – CH 2 further O 2 attack CH 3 CH CH 2 – C - CH C - CH = CH O - O CH 2 – CH 2 O - O Further cyclization Hydroperoxide formation CH 3 CH CH 2 – C = CH – CH 2 - CH 2 – C - CH = CH O - O

CH 3 CH CH 2 – C - CH C - CH = CH O - O CH 2 – CH 2 HO - O Hydroperoxide decomposition CH 3 CH CH 2 – C - CH C - CH = CH O - O CH 2 – CH 2 HO + O Chain scission CH 3 H - C – CH 2 - CH 2 – C - CH = CH O O CH 3 CH 2 - C – O + Further chain scission

CH 3 H - C – CH 2 - CH 2 – C O O CH = CH To stable product

ANTIOXIDANTS It is a must in some way to stop the breakdown process. 1- inhibitors 2- light absorbers 3- hydroperoxide decomposing agents 4- propagation interrupturs. 5- metal deactivators

INHIBITORS React rapidly with radicals, therefore it stops the initiation stage. Example Benzoquinone RO hv R + OO

LIGHT ABSORBERS Again prevent initiation caused by radiation. Example2 hydroxybenzophenones hv C O O H C O O H

HYDROPEROXIDE DECOMPOSING AGENTS Example Sulphides. R 2 S + ROOH R 2 SO + ROH

PROPAGATION INTERRUPTURS Example Phenols and amines + ROO CH 3 tBu O CH 3 tBu O OOR tBu CH 3 tBu OH ROO + CH 3 tBu O + ROOH 2, 6 – ditertiary butyl 4 methyl phenol

METAL DEACTIVATORS Multivalent metal ions accelerate oxidation and generate the unwanted peroxy radical. This must be prevented by complexing out the metal ions using chelates. If this is not done the hydroperoxide breakdown is accelerated and occurs as shown below. M 2+ + ROOH M 3+ + RO + OH - M 3+ + ROOH M 2+ + RO O + H + Overall 2 ROOH RO + RO O + H +

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