Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 192

Table 4.1 Kinetic parameters obtained [ 61 ] by fitting the E α versus T dependencies to Eq. 4.25
A 1 / A 2 E 1 /kJ mol − 1 E 2 /kJ mol − 1 m
DGEBA/PDA 41.5 69.8 45.2 1.1
DGEBA/PDA/MMT 88.8 75.9 52.9 1.1
DGEBA diglycidyl ether of bisphenol A, MMT montmorillonite, PDA 1,3-phenylene diamine

4.3 Thermal and Thermo-oxidative Degradation

of Polymers

It is a fire that burns to Destruction.

Job 31:12

4.3.1 Background

Polymers are the most versatile materials known to humankind. Because of this
versatility, the application area of polymers is enormous. Numerous applications
involve exposure of polymers to elevated temperatures. Elevated temperatures are
also used when polymers are processed by molding, extrusion, coating, etc. To make
a solid polymer pliable, one should turn it to the liquid state that is accomplished by
heating well above its temperature of the glass transition and/or melting. Exposure
to heat ultimately leads to breaking chemical bonds that hold together the atoms in
a polymer chain. Scission of the polymer chain leads to degradation of the polymer
material. If degradation is caused by heat alone without the presence of oxygen (or
other oxidant), it is referred to as thermal degradation or pyrolysis. The presence of
an oxidizing atmosphere usually accelerates thermal degradation and the respective
process is called thermo-oxidative degradation.
From a thermodynamic standpoint, the thermal stability of a polymer can be
characterized by the ceiling temperature, T c , which is the temperature above which
the polymer would degrade spontaneously to the monomer. In this case, degradation
is a process reverse to polymerization (see Eq. 4.2):

k

dp

M ⋅⇔ + M M. (4.40)

ﾷ

n + 1 n

k

p

The degradation mechanism represented by Eq. 4.40 is referred to as depolymeriza-
tion, or depropagation, or unzipping. Being opposite to polymerization, the process
is endothermic, which means that an increase in temperature favors the forward
process. At some temperature, the rate constants of depropagation and propagation
become equal so that degradation attains equilibrium. Then, a change in the Gibbs
free energy