L. Matisová-Rychlá

On a transition at 80 °C in polypropylene oxidation kinetics

Abstract The thermal oxidation of isotactic polypropylene homopolymer (PP) in air has been studied between 50 and 150 °C by means of infra-red spectrophotometry and gravimetry. The induction period, ti and oxidation rate in the steady state, rs, were determined from isothermal tests. The Arrhenius plot of rs is linear over the whole temperature range, whereas for ti it displays a slope change at about 80 °C. This transition has been discussed within the framework of ‘closed-loop’ kinetic schemes in which initiation is only due to POOH decomposition. The hypotheses involving a molecular mobility change that influences propagation and termination rate constants can be rejected. Only the initiation step is involved. The transition can be explained by the coexistence of many modes of POOH decomposition. Several hypotheses are compared.

Decomposition of peroxides of oxidised polypropylene studied by the chemiluminescence method

Abstract It was found that the initial increase in light emission from pre-oxidised polypropylene, measured in a nitrogen atmosphere, is due to decomposition of hydroperoxides in the system. Decomposition is a bimolecular reaction with the probable participation of chemisorbed oxygen. A new initiation reaction step is proposed which contributes to the deterioration of the polymer properties in the induction period of oxidation.

New approach to understanding chemiluminescence from the decomposition of peroxidic structures in polypropylene

It is suggested that the chemiluminescence from the thermal oxidation of polypropylene arises predominantly from the decomposition of associated hydroperoxides. Measurements of the chemiluminescence response to heating/cooling cycles applied to polypropylene oxidation in the induction period and in an advanced stage of the process is a novel approach to the extrapolation of the oxidation course to the lower temperature region.

Chemiluminescence in thermo-oxidation of polypropylene

Abstract Chemiluminescence in thermo-oxidation of polypropylene was interpreted, noting its course with time at different temperatures and the corresponding spectra. The light emission was attributed to decomposition of α-ketone-hydroperoxides and to chain oxidation of polymer proceeding via secondary peroxy radicals.

The effect of physical parameters of isotactic polypropylene on its oxidisability measured by chemiluminescence method. Contribution to the spreading phenomenon

Abstract The physical parameters of the polypropylene powdered samples like average molar mass, the degree of isotacticity and the resulting crystallinity and melting temperature have quite an important effect on the oxidisability of polypropylene expressed in induction time of oxidation or in maximum chemiluminescence intensity. It was shown that it is the increase of both the average molar mass and the degree of isotacticity which predominantly lead to the longer induction times of oxidation while the opposite is true for maximum chemiluminescence intensity for molar mass 40,000–180,000 and temperature interval 80–130°C. The combined model of the homogeneous increase of concentration of hydroperoxides in the oxidation micro-zones and the spreading of the oxidation from these micro-zones has been proposed which fits well the experimental runs of chemiluminescence intensity–time.

Effect of oxygen on the degradation of polypropylene initiated by ionizing irradiation

Abstract An attempt to differentiate the effect of direct chain scisson due to polypropylene irradiation by gamma rays and/or chain scission brought about by fragmenation of polymer peroxy radicals has been made. The results obtained quantify the dependence of the destruction rate of polypropylene on the intensity of irradiation taking into account specific features of cleavage of macromolecular chains in the presence of oxygen and with/without the inhibition of free radical reactions by ionol.

Chemiluminescence of polyamides: I. Luminescence accompanying autoxidation of lactams and thermolysis of lactam hydroperoxides

The intensity of luminescence accompanying autoxidation of lactams and the effect of lactam oxidation products on its time course were measured under conditions where the oxidation rate was independent of oxygen pressure. Luminescence during thermolysis of lactam hydroperoxides was measured. Products of the bimolecular reaction of lactam peroxy radicals appear to be a rather weak source of luminescence. A stronger light emission comes from the excited state of the cyclic hydrogen-bonded associate of lactam hydroperoxide with the ω-aldehyde of the amide of the respective acid. Thus a distinct luminescence appears at that oxidation stage which e.g. for Nylon 6 fibres, corresponds to the stage of significant reduction of mechanical properties. At the same time, luminescence accompanying thermolysis of oxidation products of an N-alkylamide does not correspond to the oxidation degree of amides.

Chemiluminescence of polyamides. III. Luminescence accompanying thermooxidation of lactam-based polyamides stabilized by antioxidants

Chemiluminescence accompanying oxidation of lactam-based polyamides stabilized with phenols and secondary aromatic amines as well as with a hindered nitroxy radical was measured. The yields of chemiluminescence per mol of oxygen absorbed were found to be higher for oxidation of polyamide in the presence of antioxidants than in their absence depending on the content of amino and carboxylic end-groups. Because antioxidants react with peroxy radicals by several consecutive inhibition reactions with different yields of chemiluminescence, the intensity of chemiluminiscence also depends on the rate of initiation of the oxidation. The relation between the intensity of chemiluminescence and the rate of oxygen consumption during oxidation of stabilized polyamides differed profoundly for various polyamides depending on their end-group content, on the concentration of antioxidant and also on the conditions of oxidation, especially on the rate of initiation determining the rate of antioxidant consumption. Redox inhibition reactions were found to be the predominant source of chemiluminescence accompanying the oxidation of N-alkylamides in the presence of antioxidants.

Chemiluminescence from oxidized polypropylene during temperature cycling

Abstract The chemiluminescence measured during temperature cycling over some temperature range reveals the complexity of the oxidation of polypropylene much better than isothermal or ramp experiments. It enables the characterization of the stages of oxidation within one temperature cycle by corresponding values of apparent activation energy and pre-exponential factor. The independence of the rate constant of oxidation on the extent of oxidation within certain limits is thus demonstrated. It is shown that the Arrhenius plot for the increase of temperature in the first cycle of decomposition of polypropylene hydroperoxides in nitrogen covers the rate constants of so-called faster decomposing hydroperoxides while that for subsequent cycles approaches the rate constants of so-called slower decomposing hydroperoxides. The bimolecular decomposition of hydroperoxides in initiation of oxidation provides a unified approach to the “heterogeneous’ and “homogeneous’ interpretation of polypropylene oxidation.

The effect of terminal groups on the kinetics of chemiluminescence in the oxidation of poly(ϵ-caprolactam)

Abstract The chemiluminescence (CL) technique was used to study the thermal oxidation of polyamide 6 samples with different ratios of terminal basic and acidic groups, which have a distinct effect on the kinetics of the isothermal CL-time relationships at 120–140°C. The isothermal experiments are complemented by nonisothermal ramp experiments where the CL intensity is recorded versus temperature at a programmed heating rate.