NATURAL-RUBBER PYROLYSIS - STUDY OF TEMPERATURE-DEPENDENCE AND THICKNESS-DEPENDENCE INDICATES DIMER FORMATION MECHANISM

The technique of pyrolysis-gas chromatography (Py-GC) has been used to study the thermal degradation of natural rubber. The monomer/dimer (M/D) ratio has been measured over the temperature range 300 to 500-degrees-C, using sample sizes of the order of 0.3 mg. The M/D ratio has also been measured as a function of sample thickness, using samples in the range 30 nm to 3-mu-m (0.1-mu-g to 10-mu-g). The dependence of M/D on temperature may be interpreted in terms of (a) different activation energies for depropagation versus intramolecular cyclisation, (b) dissociation of dimer to monomer at higher temperatures, or (c) reduced residence time of monomer in the melt at higher temperatures allowing less opportunity for its recombination. The results for the dependence of M/D on thickness (at constant temperature) indicate that interpretation (c) is the most probable. The present work therefore suggests that monomer recombination, possibly by a Diels-Alder mechanism is an important contributor to dimer formation in rubber pyrolysis.