Thermal Degradation of Polymers

1. Thermal Degradation of Polymers Michael Hess Department of Physical Chemistry University Duisburg-Essen Campus Duisburg 47048 Duisburg, Germany e-mail: hi259he@uni-duisburg.de

2. Principle scheme of a thermogravimetric system Balance Optional to analyzer: IR GC-MS etc. Zero control oven Conroller Analyzer Data output Mass compensation Carrier gas: N2,air, O2, … Thermo couple

3. 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  10-3 mg

4. 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

5. with n = 1 in this case thermal activated degradation (depolymerization) In polystyrene the depolymerization occurs randomly along the chain inert atmosphere, e. g. N2 e. g.: thermal depolymerization of poly(-methyl styrene): This reaction is (during a large part of the reaction) a simple “un-zipping” of the polymer chain from its end, monomer after monomer.

6. 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. thermo-oxidative degradation

7. 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

8. BUT: some important information can be obtained e. g.: different types of reactions or the influence of modifiers

9. AA + BB+… mM + LL +… reactants i  0 products i  0 • 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 Some basic reaction kinetics ni = ni0+ i r•= d/dt= - i-1dni/dt [mol s-1] (rX•= dX/dt= - i-1dci/dt [mol L-1 s-1])

10. isothermal experiments: w = f(t)T dynamic experiments: w = f (T)dT/dt = f (t) w = sample mass w0 = initial sample mass t = time T = temperature  = heating rate C = conversion In Thermogravimety Experiments isothermal experiments are straight forward but they are experimentally difficult 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)

11. r= kn cA(A) cB(B) … kn = f(T, p, catalyst, solvent,…) kn= rate constant (A), (B) … = partial formal order of component A, component B,… n = formal (total) order of reaction Formal kinetics rcA(A) rcB(B) . . .

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

13. 1-C lg  slope m = -0.457 Ea/R 1 1 2 3 2 3 T [K] 1T [K-1] Ozawa method

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

15. Residual material Process I Process IV Process II Process III

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

17. Further Application of Thermogravimetric Methods