Download Thermal Degradation

Michael Hess
Department of Physical Chemistry
University Duisburg-Essen
Campus Duisburg
47048 Duisburg, Germany
e-mail: hi259he@uni-duisburg.de
Principle scheme of a thermogravimetric system
Optional
to
analyzer:
IR
GC-MS
etc.
Thermo
couple
Balance
Zero control
oven
Conroller
Analyzer
Data output
Mass
Carrier gas: N2, air, O2, … compensation
TGA-systems can be combined with:
IR-spectrometry
GC-MS
Product identification
gas phase absorption
thinlayer chromatography
DSC
Enthalpy, phase transitions
DTA
Sample mass  1-20 mg
Sensitivity  10-3 mg
Processes of interest in polymer science:
In general: m = f(T)dm/dt or m = f(t)T
thermal activated degradation (depolymerization)
thermo-oxidative degradation
Thermal stability i. e. upper limit of use under short-term heat-exposure
Determination of reaction-kinetical data such as:
reaction rate r,
apparent reaction energy Ea
apparent pre-exponential factor A (collision factor)
formal (apparent) reaction order n
rate constant k
thermal activated degradation (depolymerization)
inert atmosphere, e. g. N2
e. g.: thermal depolymerization of poly(-methyl styrene):
n
dm



 k   m(t ) dm 
dt

dT 


with n = 1 in this case
extend of reaction
This reaction is (during a large part of the reaction) a simple “un-zipping”
of the polymer chain from its end, monomer after monomer.
In polystyrene the depolymerization occurs randomly along the chain
thermo-oxidative degradation
More complex kinetics which is in particular influenced by
the diffusion process of O2 to the reaction site (char formation),
the activities of flame retardants and inhibitors etc.
In many cases
•there are complex kinetics
•there is influence of diffusion rates of reactants and products
•there are solid-state reactions
• there are incomplete polymerizations or crosslink reaktions (in thermosets)
•apparent reaction orders different from n = 1 can be observed
AA + BB+… mM + LL +…
reactants i  0
ni = ni0+ i
products i  0
r•= d/dt= - i-1dni/dt [mol s-1]
(rX•= dX/dt= - i-1dci/dt [mol L-1 s-1])
i= stoichiometric coefficient
ni = amount of substance
ni0 = amount of substance at =0 (initial amount of substance)
= extend of reaction
ci=(molar) concentration
X= conversion
r=rate of reaction
isothermal experiments: w = f(t)T
isothermal experiments are straight forward
but they are experimentally difficult
dynamic experiments: w = f (T)dT/dt = f (t)
dT

dt
The mass loss at any time is given by:
w = w0-w
so that the conversion C is given by:
C = w/w0 = (w0-w)/w0
(1-C) = w/w0 (mass-loss fraction)
w = sample mass
w0 = initial sample mass
t = time
T = temperature
 = heating rate
C = conversion
rcA (A)
rcB(B
)
.
.
.
r= kn cA (A)  cB(B)  …
kn = f(T, p, catalyst, solvent,…)
iz
n

i
ia
kn= rate constant
(A), (B) … = partial formal order of component A,
component B,…
n = formal (total) order of reaction
In case of a pyrolytic reaction frequently the form:


dw
n
 k n w0  w
dt
Ea 1
dw
 Aexp 
dt
RT
 
 w0  w 
n




Ea 1
 dT  dw  

log 
 n log w0  w
    log A 
2.302R T
dt
d
t



r 
 


can be used:
Ozawa method
1-C
1
2
3
lg 
slope m = -0.457 Ea/R
1
2
3
T [K]
1T [K-1]
Arrhenius’ law:
kn 

A

 
 Ea
RT
e
preexponential factor effectivit y factor
collision factor
Ea 1
 ln k n  ln A  
R T
Ea = (apparent) activation energy [kJ/mol]
In thermogravimetric experiments:
rC •= dC/dt= - dm/dt [mg s-1]
C = conversion
example of a complex depolymerization
8,000
7,000
sample mass [mg]
6,000
5,000
nitrogen
4,000
3,000
2,000
Residual material
1,000
0,000
0,0
100,0
200,0
300,0
400,0
500,0
600,0
700,0
800,0
temperature [°C]
Process I
Process IV
Process II
Process III
900,0
Some examples of pyrolytic reactions
(random) bond scission
disproportionation
volatile products
radical transfer (chain transfer)
volatile products