INTRODUCTION

In recent years polymeric materials has largelyreplaced traditional materials like metals, ceramicsdue to enhanced properties at low cost.

One of the major disadvantage though is thermaldegradation of polymeric materials leading to itslimited applications.

This makes polymeric materials less suitable thenothers, specially so at elevated temperatures.

A study has to be made having a in depth site onmechanism of thermal degradation of polymericmaterials.

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 assemblycauses a loss of physical, mechanical or electricalproperties.

EFFECTS OF DEGRADATION

Molecular weight change / distribution

Reduced ductility

Embrittlement

Color changes

Changes in desirable properties

Changes in molecular structure

Cracking

MECHANISM OF THERMAL DEGRADATION

Initiation:

The initiation of thermal degradation involves the loss of a hydrogen atom fromthe polymer chain (shown below as R.H) as a result of energy input from heat orlight. This creates a highly reactive and unstable polymer ‘free radical’ (R*) anda hydrogen atom with an unpaired electron (H*). The strength of the C-F bondmeans that there it is much harder for thermal degradation to initiate.

Propagation :

The propagation of thermal degradation can involve a variety of reactions andone of these is where the free radical (R*) reacts with an oxygen (O2) moleculeto form a peroxy radical (ROO*) which can then remove a hydrogen atom fromanother polymer chain to form a hydroperoxide (ROOH) and so regenerate thefree radical (R*). The hydroperoxide can then split into two new free radicals,(RO*) + (*OH), which will continue to propagate the reaction to other polymermolecules. The process can therefore accelerate depending on how easy it is toremove the hydrogen from the polymer chain.

Termination :

The termination of thermal degradation is achieved by ‘mopping up’ the freeradicals to create inert products. This can occur naturally by combining freeradicals or it can be assisted by using stabilizers in the plastic.

MECHANISM OF THERMAL DEGRADATION

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)

REACTIONS

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

Thermal degradation of few known polymers

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

Questions ?