CHEMICAL DEGRADATION 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 ozonolysis3) 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 Oxidative degradation normally initiation by1) radiation e.g. UV2) heat 3) direct O2 attack (not too important with

saturated polymer)4) initiator residues (proxides).

UNSATURATED POLYMERSUNSATURATED POLYMERS

1) Can be easily initiated 2) Can be sujected to repeated O2

attack3) Must always be protected

- CH2 - C = CH - CH2

CH3

H2 allylic hydrogen’s – easily replaced

- CH2 - C = CH - CH - CH2 - C - CH = CH-

CH3 CH3

PROTECTIONPROTECTION

By preventing the production of free radicles.

MECHANISMSMECHANISMS1- system which cross-link.ExamplePolyethylene.Changes observed on exposure to radiation are:a) H2 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-----CH2-------- ----CH------ + H

H + ----CH2---- ----CH------ + H2

H + ----CH2---- ----CH------ + CH4

CH3 branch

2 ----CH---- ----CH------ cross – link forms

----CH------ by radical combination

H H hv unsaturation increases----CH--CH--- ----CH = CH---- + H2

----CH---- + O2 ----CH------ ocidative

----O – O ----- breakdown

2- System which show reduction in molecular mass.Mechanism still obscure, two main theoriesa) fracture of main chain followed by disproportionation of

radicles.

CH3 CH3 hv CH3 CH3

----CH2 – C - CH2 – C - ----CH2 – C + CH2 – C -----

CH3 CH3 CH3 CH3

CH2 CH3

----CH2 – C + CH3 – C ----

CH3 CH3

disproportionation

B) Multiple bond scission

CH3

----CH2 – C complex rearrangement.

CH3

hv

OXIDATIVE DEGRADATIONOXIDATIVE DEGRADATIONMECHANISMMECHANISM

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 (Ro) is attacked by oxygen.

R + O2 ROO

ROO + RH ROOH + R

Hydroperoxide decomposition

RO + OH

Rearrangement to stable product

i.e. termination

exampleexampleSaturated polymer (polystyrene)

----CH2 - CH --CH2 – CH- hv ----CH2 - C --CH2 – CH---

rearrangement

----CH2 – CH--------CH2 - CH

O - O

O2

----CH2 – C-- -------CH2 – C-- -------CH2 – C-- -------CH2 – C-- --- +

O OHOH +O

Farther chainactivated

ROOHhydroperoxide

----CH2 - CH

O

----CH2 – CH--

OH

+

Stable products

rearrangementChain

scission

OXIDATIVE DEGRADATIONOXIDATIVE DEGRADATION

Unsaturated polymerEffect of oxygen attack far more serious with

unsaturated polymerse.g. natural rubber.

MECHANISMMECHANISM CH3 CH3

--------CH2 – C = CH – CH2 - CH2 – C = CH – CH2---------

- H

CH3 CH3

--------CH2 – C = CH – CH2 - CH2 – C = CH – CH---------

Rearrangement to stablilise radical

CH3 CH3

--------CH2 – C = CH – CH2 - CH2 – C - CH = CH---------

O2attack

Hydroperoxide

formationcyclization

CH3 CH3

--------CH2 – C - CH C - CH = CH---------

O - O

CH2 – CH2

further O2attack

CH3 CH3

--------CH2 – C - CH C - CH = CH---------

O - O

CH2 – CH2

O - OFurther

cyclization Hydroperoxide formation

CH3 CH3

--------CH2 – C = CH – CH2 - CH2 – C - CH = CH---------

O - O

CH3 CH3

--------CH2 – C - CH C - CH = CH---------

O - O

CH2 – CH2

HO - O

Hydroperoxide decomposition

CH3 CH3

--------CH2 – C - CH C - CH = CH---------

O - O

CH2 – CH2

HO + O

Chain scission

CH3

H - C – CH2 - CH2 – C - CH = CH------

O O

CH3

CH2 - C –

O

+

Further chain scission

CH3

H - C – CH2 - CH2 – C

O O

CH = CH------+To stable

product

ANTIOXIDANTSANTIOXIDANTS

It is a must in some way to stop the breakdown process.

1- inhibitors2- light absorbers3- hydroperoxide decomposing agents4- propagation interrupturs.5- metal deactivators

INHIBITORSINHIBITORS

React rapidly with radicals, therefore it stops the initiation stage.

ExampleBenzoquinone

ROhvR + OO

LIGHT ABSORBERSLIGHT ABSORBERSAgain prevent initiation caused by radiation.Example 2 hydroxybenzophenones

hvC

O OH

C

O OH

HYDROPEROXIDE DECOMPOSING HYDROPEROXIDE DECOMPOSING AGENTSAGENTS

ExampleSulphides.

R2S + ROOH R2SO + ROH

PROPAGATION INTERRUPTURSPROPAGATION INTERRUPTURS Example Phenols and amines

+ ROO

CH3

tButBu

O

CH3

tButBu

O

OOR

tBu

CH3

tBu

OH

ROO+

CH3

tButBu

O

+ ROOH2, 6 – ditertiary butyl 4 methyl phenol

METAL DEACTIVATORSMETAL DEACTIVATORSMultivalent 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.

M2+ + ROOH M3+ + RO + OH-

M3+ + ROOH M2+ + RO O + H+

Overall 2 ROOH RO + RO O + H+

Thank YouThank You

See You Next Lecture