CHEMICAL DEGRADATION Whilst all polymers will be attacked by certain chemicals it is the reactive...
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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.
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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).
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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
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PROTECTIONPROTECTION
By preventing the production of free radicles.
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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.
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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
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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
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B) Multiple bond scission
CH3
----CH2 – C complex rearrangement.
CH3
hv
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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
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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
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----CH2 - CH
O
----CH2 – CH--
OH
+
Stable products
rearrangementChain
scission
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OXIDATIVE DEGRADATIONOXIDATIVE DEGRADATION
Unsaturated polymerEffect of oxygen attack far more serious with
unsaturated polymerse.g. natural rubber.
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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
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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
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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
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CH3
H - C – CH2 - CH2 – C
O O
CH = CH------+To stable
product
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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
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INHIBITORSINHIBITORS
React rapidly with radicals, therefore it stops the initiation stage.
ExampleBenzoquinone
ROhvR + OO
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LIGHT ABSORBERSLIGHT ABSORBERSAgain prevent initiation caused by radiation.Example 2 hydroxybenzophenones
hvC
O OH
C
O OH
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HYDROPEROXIDE DECOMPOSING HYDROPEROXIDE DECOMPOSING AGENTSAGENTS
ExampleSulphides.
R2S + ROOH R2SO + ROH
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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
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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+
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Thank YouThank You
See You Next Lecture