thermal degradation of polymers mechanism of degradation effects of degradation remedies

2. INTRODUCTION
 In recent years polymeric materials has largely
replaced traditional materials like metals, ceramics
due to enhanced properties at low cost.
 One of the major disadvantage though is thermal
degradation of polymeric materials leading to its
limited applications.
 This makes polymeric materials less suitable then
others, specially so at elevated temperatures.
 A study has to be made having a in depth site on
mechanism of thermal degradation of polymeric
materials.

3. THERMAL DEGRADATION
 Degradation :
 Degradation of a material is extensive loss of useful
properties. This properties can be mechanical,
electrical, chemical or physical properties.
 Swelling and dissolution
 Bond rupture
 Weathering
 Thermal degradation :
 A process whereby the action of heat or elevated
temperature on a material, product or assembly
causes a loss of physical, mechanical or electrical
properties.

4. EFFECTS OF DEGRADATION
 Molecular weight change / distribution
 Reduced ductility
 Embrittlement
 Color changes
 Changes in desirable properties
 Changes in molecular structure
 Cracking

5. MECHANISM OF THERMAL DEGRADATION
 Initiation:
 The initiation of thermal degradation involves the loss of a hydrogen atom from
the polymer chain (shown below as R.H) as a result of energy input from heat or
light. This creates a highly reactive and unstable polymer ‘free radical’ (R*) and
a hydrogen atom with an unpaired electron (H*). The strength of the C-F bond
means that there it is much harder for thermal degradation to initiate.
 Propagation :
 The propagation of thermal degradation can involve a variety of reactions and
one of these is where the free radical (R*) reacts with an oxygen (O2) molecule
to form a peroxy radical (ROO*) which can then remove a hydrogen atom from
another polymer chain to form a hydroperoxide (ROOH) and so regenerate the
free radical (R*). The hydroperoxide can then split into two new free radicals,
(RO*) + (*OH), which will continue to propagate the reaction to other polymer
molecules. The process can therefore accelerate depending on how easy it is to
remove the hydrogen from the polymer chain.
 Termination :
 The termination of thermal degradation is achieved by ‘mopping up’ the free
radicals to create inert products. This can occur naturally by combining free
radicals or it can be assisted by using stabilizers in the plastic.

6. MECHANISM OF THERMAL DEGRADATION

7. WAYS OF POLYMER THERMAL DEGRADATION
 Depolymerisation : (poly methy methacrylate (perspex).)
 CH2C(CH3)COOCH3CH2C*(CH3)COOCH3 →
CH2C*(CH3)COOCH3+ CH2=C(CH3)COOCH3
 Random chain scission : (poly methylene)
 CH2CH2CH2CH2CH2CH2CH2’ →CH2CH2CH=CH2 + CH3CH2CH2’
or CH2’+CH2=CHCH2CH2CH2CH3
 Side group elimination : (PVC)
 CH2(Cl)CHCH2CH(Cl)→CH=CHCH=CH+2HCl
The research methods of thermal degradation of polymers
• TGA (THERMO GRAVIMETRI ANALYSIS )
• DTA ( DIFFERENTIAL THERMAL ANALYSIS )
• DSC ( DIFFERENTIAL SCANNING CALORIMETRY
)

8. REACTIONS

9. REMEDY
 Protecting Plastics with Stabilizers
 Protecting plastics with radical scavenger
stabilizers
 Testing Plastics for Thermal Stability and
Predicting Lifetimes
 Oxidation control by inert environment
 Use of stronger bonds like C – F in polymer

10. Thermal degradation of few known polymers

19. REFERENCES
• WWW . ZEUSINC.COM
• THERMAL DEGRADATION OF POLYMERS – WIKEPEDIA, FREE ENCYCLOPEDIA
• THERMAL AND OXIDATIVE DEGRADATION OF POLYMER – TAKASHI KASHIWAGI
• MATERIAL SCIENCE AND METTALLURGY – CALISTER
• THERMAL DEGRADATION OF POLYMERS AT HIGH TEMPERATURES – SAMUEL
L. MADORSKY AND SIDNEY STRAUS
• THERMAL DEGRADATION OF POLYMERS AS A FUNCTION OF MOLECULAR
STRUCTURE – S. L. MADORSKY AND S. STRAUS

20. Questions ?