Michael Hess Department of Physical Chemistry University Duisburg-Essen Campus Duisburg Duisburg, Germany

Balance oven Thermo couple Conroller Analyzer Data output Zero control Mass compensation Optional to analyzer: IR GC-MS etc. Carrier gas: N 2, air, O 2, … Principle scheme of a thermogravimetric system

TGA-systems can be combined with: IR-spectrometry GC-MS gas phase absorption thinlayer chromatography DSC DTA Product identification Enthalpy, phase transitions Sample mass  1-20 mg Sensitivity  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, rate constant k apparent reaction energy E a apparent pre-exponential factor A (collision factor) formal (apparent) reaction order n

thermal activated degradation (depolymerization) inert atmosphere, e. g. N 2 e. g.: thermal depolymerization of poly(  -methyl styrene): with n = 1 in this case 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 O 2 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

n i = n i0 + i   A  A +  B  B+…   m  M +  L  L +… reactants i  0 products i  0 r  = d  /dt= - i -1 dn i /dt [mol s -1 ] (r  X = dX/dt= - i -1 dc i /dt [mol L -1 s -1 ]) i = stoichiometric coefficient n i = amount of substance n i0 = amount of substance at  =0 (initial amount of substance)  = extend of reaction c i =(molar) concentration X= conversion r=rate of reaction

isothermal experiments: w = f(t) T dynamic experiments: w = f (T) dT/dt = f (t)  w = sample mass w 0 = initial sample mass t = time T = temperature  = heating rate C = conversion The mass loss at any time is given by:  w = w 0 -w so that the conversion C is given by: C =  w/w 0 = (w 0 -w)/w 0 (1-C) = w/w 0 isothermal experiments are straight forward but they are experimentally difficult (mass-loss fraction)

r  c A   (A) r  c B  (B ). r= k n  c A   (A)  c B  (B)  … k n = rate constant  (A),  (B) … = partial formal order of component A, component B,… n = formal (total) order of reaction k n = f(T, p, catalyst, solvent,…)

In case of a pyrolytic reaction frequently the form: can be used:

1-C T [K] 1T [K-1] slope m = E a /R lg  11 22 33 22 11 33 Ozawa method

E a = (apparent) activation energy [kJ/mol] Arrhenius’ law: r  C = dC/dt= - dm/dt [mg s -1 ] C = conversion In thermogravimetric experiments:

Process I Process II Process III Process IV Residual material

(random) bond scission volatile products radical transfer (chain transfer) disproportionation Some examples of pyrolytic reactions