Post on 26-Jan-2017
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 ?