of polymers is molecular deterioration as a result of overheating. At high temperatures the components of the long chain backbone of the polymer
can begin to separate (molecular scission) and react with one another to change the properties of the polymer. Thermal degradation
can present an upper limit to the service temperature of plastics as much as the possibility of mechanical property loss. Indeed unless correctly prevented, significant thermal degradation
can occur at temperatures much lower than those at which mechanical failure is likely to occur. The chemical reactions involved in thermal degradation lead to physical and optical property changes relative to the initially specified properties. Thermal degradation
generally involves changes to the molecular weight
(and molecular weight distribution
) of the polymer and typical property changes include reduced ductility and embrittlement, chalking, color changes, cracking, general reduction in most other desirable physical properties.
The Mechanism of Thermal Degradation
Most types of degradation follow a similar basic pattern. The conventional model for thermal degradation is that of an autoxidation process which involves the major steps of initiation, propagation, branching, and termination.
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.
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.
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.
The Research Methods of Thermal Degradation of Polymers
(Thermogravimetric Analysis) (TGA) refers to the techniques where a sample is heated in a controlled atmosphere at a defined heating rate whilst the samples mass is measured. When a polymer sample degrades, it mass decreases due to the production of gaseous products like carbon monoxide, water vapour and carbon dioxide.
(Differential thermal analysis) (DTA) and (differential scanning calorimetry) (DSC): Analyzing the heating effect of polymer during the physical changes in terms of glass transition, melting, and so on. These techniques measure the heat flow associated with oxidation.
Ways of Polymer Thermal Degradation
Under thermal effect, the end of polymer chain
departs, and forms low free radical which has low activity. Then according to the chain reaction mechanism, the polymer loses the monomer
one by one. However, the molecular chain doesn’t change a lot in a short time. The reaction is shown below. This process is common for polymethymethacrylate (perspex).
CH2-C(CH3)COOCH3-CH2-C*(CH3)COOCH3→CH2-C*(CH3)COOCH3 + CH2=C(CH3)COOCH3
Random Chain Scission
will be break down randomly, could be occurred at any position of the backbone
. The molecular weight
decreases rapidly, and cannot get monomer
in this reaction, this is because it forms new free radical
which has high activity can occurs intermolecular chain transfer and disproportion termination with the CH2’group.
CH2-CH2-CH=CH2 + CH3-CH2-CH2’ or
Groups that are attached to the side of the backbone
are held by bonds which are weaker than the bonds connecting the chain. When the polymer was being heated, the side groups are stripped off from the chain before it is broken into smaller pieces.
For example the PVC eliminates HCL, under 100-120o
Oxidation of the polymer
Polyphenylene oxide is well known for oxidation.