Properties and chemiluminescence of polypropylene stored for a long period

Regulating Degradation Pathways of Polymers by Radical-Triggered Luminescence

Understanding the radical behaviours during polymer degradation is beneficial to unveil and regulate the degradation pathways of polymers to achieve a sustainable polymer development. However, it is a long-standing challenge to study radical behaviours owing to the ultra-short lifetime of the transient radicals generated during the polymer degradation. In this contribution, we have proposed the radical-triggered luminescence to monitor the radical behaviours during polymer degradation without/with the addition of inorganic additives. It was disclosed that the pure polymers showed a single sigmoidal dynamic curve from peroxy radicals (ROO⋅) emissions, leading to the exponential proliferation for the degradation evolution. Alternatively, the degradation pathways with the addition of additives, layered double hydroxides (LDHs) with positively charged Al centers, could be modulated into a double sigmoidal dynamics, involving the main product of alkoxy radicals (RO⋅) with the activation energy of 40.2 kJ/mol and a small amount of ROO⋅ with 76.3 kJ/mol, respectively. Accordingly, the polymers with the additive-regulated pathways could exhibit prominently anti-degradation behaviours. This work is beneficial for the deep understanding of the radical mechanisms during polymer degradation, and for the rational design of anti-degradation polymers.

Spin Trapping Analysis of Radical Intermediates on the Thermo-Oxidative Degradation of Polypropylene

The purpose of this study is to investigate the thermo-oxidative degradation behavior of polypropylene (PP) by comparing three types of pristine PP granules (consisting of homopolymer, random copolymer, and block copolymer) with their corresponding oxidized analogues. These analogues were intensely oxidized under oxygen at 90 °C for 1000 h by using the electron spin resonance (ESR) spin trapping method that can detect short-lived radical intermediates during the degradation. The degrees of oxidation could be evaluated by chemiluminescence (CL) intensity, which was related to the concentration of hydroperoxide groups generated in the PP chain. In the pristine PP samples, a small amount of hydroperoxides were found to be formed unintentionally, and their homolysis produces alkoxy radicals, RO•, which then undergo β-scission to yield chain-end aldehydes or chain-end ketones. These oxidation products continue to take part in homolysis to produce their respective carbonyl and carbon radicals. On the other hand, in the oxidized PP granules, because of their much higher hydroperoxide concentration, the two-stage cage reaction and the bimolecular decomposition of hydroperoxides are energetically favorable. Carbonyl compounds are formed in both reactions, which are then homolyzed to form the carbonyl radical species, •C(O)-. PP homopolymer produced the largest amount of carbonyl radical spin adduct; thus, it was found that the homopolymer is most sensitive to oxygen attack, and the presence of ethylene units in copolymers enhances the oxidation resistance of PP copolymers.

Quantitative Analysis for Polymer Degradation in the Extrusion Process

Polymer degradation in the extrusion process decreases quality and productivity. For this reason, it is necessary to prevent polymer degradation. In the extrusion process, polymer degradation is caused by oxidation. It depends on the processing temperature and the amount of dissolved oxygen in the molten polymer. Therefore, a quantitative analysis of these factors is required.

As for the degradation characteristics of the material used in this study, temperature and oxygen concentration dependency of the oxidation rate could be quantitatively characterized with an apparatus to evaluate polymer degradation which utilized chemiluminescence generated by an oxidation reaction.

Moreover, an online measuring apparatus to analyze dissolved gas in the extruded molten polymer was developed. With this apparatus, the volume ratio of dissolved gases (N2, O2 etc.) to the extruded molten polymer could be quantitatively analyzed and the quality of the extruded molten polymer evaluated.

With this apparatus, dissolved nitrogen (an index of entrained air) was analyzed with a full-flight screw and a barrier screw. Furthermore, observation of cross sectional views in the screw channel obtained from the cooling experiment under the operating conditions was carried out for the full-flight screw and the barrier screw. With the full-flight screw, break up phenomenon (collapse of solid polymer) occurred in the screw channel and the amount of nitrogen increased. With the barrier screw, the amount of nitrogen decreased because of prevention of the break up phenomenon in the screw channel.

Consequently, it is shown that the use of the barrier screw is suitable for oxygen reduction in the molten polymer, which is a factor in causing polymer degradation.

Chemiluminescence as a Probe of Polymer Oxidation

Many oxidation reactions of organic materials, including polymers, are accompanied by the emission of weak chemiluminescence (CL). From a study of the mechanism of this weak CL, it is shown that the time development of the CL intensity may provide the kinetics of the oxidation reaction and is thus a sensitive probe of the degradation of the material. The intensity of emission reflects the concentration of peroxidic species in the material. Whereas the kinetics of the oxidation may be described by a series of elementary, homogeneous free radical reactions, the use of imaging techniques has shown that the oxidation of polymers such as polypropylene is highly heterogeneous. A model that describes the oxidation as spreading through the material as an infection from a number of initiating sites is able to rationalize these observations and provide a new approach to the prediction of the useful lifetime of a polymeric material.