Pieter Gijsman

Comparison of the UV-degradation chemistry of polypropylene, polyethylene, polyamide 6 and polybutylene terephthalate

The photodegradation mechanism of polymers highly depends on the type and concentration of chromophores present. This influence is studied by making a comparison between the UV-degradation of PE, PP, PA6 and PBT and the thermooxidative degradation of PP at a comparable temperature. The degradation processess are followed by determining the oxygen uptake, CO, CO2 and peroxide formation. During UV-degradation of PP, PE, PA6 and PBT the oxygen uptake is linear in time without an induction period. The oxidation rate of PP is higher than that of PBT, which is slightly higher than the oxidation rate of PA6, while that of PE is the lowest. The determined amount of peroxides depends on the ageing time. For the UV-degradation this does not lead to the expected increase of the oxidation rate, as is the case with thermooxidation of PP. Only a limited proportion of the peroxides formed during the UV-degradation decomposes into radicals. The oxidation rate is mainly determined by other radical forming reactions. In the case of PE and PP this is probably initiation by polymer-oxygen Charge Transfer Complexes (CTCs) and for PA6 and PBT direct photolysis of the amide or the ester linkage, respectively. Thickness degradation profiles show that the UV-degradation processes of PP and PBT are heterogeneous. For PP this is due to oxygen diffusion limitation, while in the case of PBT it is the result of absorption of the UV-light.

Studies on thermal and thermo-oxidative degradation of poly(ethylene terephthalate) and poly(butylene terephthalate)

Abstract A comparative study was conducted into the thermo-oxidative degradation of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). Degradation of the polymer films and model compounds, ethylene dibenzoate (EDB) and butylene dibenzoate (BDB), was carried out in an oxygen atmosphere at 160°C. On the basis of the compounds identified by GC–MS a mechanism is proposed for the degradation of the model compounds that involves the oxidation at the α-methylene carbon with formation of unstable peroxides and carboxylic acids. From the studies performed under N 2 at 160 °C it could be concluded that benzoic acid and esters are products of the thermal degradation, while benzoic and aliphatic acids, anhydride and alcohols are due to thermo-oxidative degradation. In contrast to the thermo-oxidative degradation of other polymers, for PET and PBT, especially at the beginning, thermal degradation plays an important role. The results clearly showed that PET is more stable towards degradation than PBT.

The mechanism of the low-temperature oxidation of polypropylene

Abstract The peroxides (as detected by an iodometric titration) formed by the oxidation of polypropylene (PP) between 50 and 90°C consist of two types: fast-decomposing and slow-decomposing. It is shown that when both of these peroxides are present the oxidation rate is controlled by the fast-decomposing peroxides. In the induction period the slow-decomposing peroxides are formed initially; the decomposition of these peroxides leads to primary oxidation products. Due to the restricted mobility, these oxidation products will preferentially be oxidized, which leads to fast-decomposing peroxides. Chemical analyses show that these peroxides are peracids. In the latter stage of the oxidation the peracids determine the oxidation rate. This causes a tremendous increase in the oxidation rate. Thus, the increase in the oxidation rate of PP after the induction period between 50 and 90°C is not, as generally accepted, due to the accumulation of hydroperoxides, but can be ascribed to the faster decomposition of peracids which result from the oxidation of primary oxidation products.

The mechanism of action of hindered amine light stabilizers

Abstract Since the discovery of hindered amines as effective stabilizers against the photodegradation of polymers, the stabilization mechanism of Hindered Amine Light Stabilizers (HALS) has been a subject of discussion. Nevertheless, a complete mechanism has not yet been formulated. In this paper, the degradation of an unstabilized and two HALS-stabilized polyethylene (PE) films is described. The degradations are characterized by measuring: the oxygen uptake, the formation of CO and CO2, the FT-IR spectra, the stabilizer concentration and the oxygen content. The oxygen uptake of the unstabilized PE film led to the expected changes in the IR spectra and to the embrittlement of the film. The oxygen uptake by the HALS-stabilized films caused only minor changes. At an oxygen uptake of 900 mmol per kg of polymer, the HALS-stabilized PE films showed only minor differences in their IR spectra and in their mechanical properties, while the unstabilized material showed an enormous change in its IR spectrum and became totally brittle. Thus, for the HALS-stabilized materials and for the unstabilized materials, the mechanism of oxygen uptake must be different. The difference between the results for the unstabilized and the HALS-stabilized polymers are explained assuming that the initiation of the photodegradation of PE is due to charge transfer complexes.

The influence of temperature and catalyst residues on the degradation of unstabilized polypropylene

Abstract The influence of temperature and of residues of a TiCl 3 -based polymerization catalyst on the oxidation rate of polypropylene was determined. The plots of the logarithm of the induction period versus the reciprocal temperature (Arrhenius plot) are curved. The influence of the concentration of the polymerization catalyst on the stability depends on the degradation temperature. At 50°C polymers containing less than 2 or 8 ppm of a killed Ti polymerization catalyst show a longer induction period than polymers containing 64 and 180 ppm Ti. At 130°C there is almost no difference in the stability of these four different polymers. The different influences for the different polymers at high and low temperatures as well as the curvature of the Arrhenius plots can be explained assuming a change in the hydroperoxide decomposition mechanism. The hydroperoxide decomposition is probably thermal at high temperature and Ti-catalysed at low temperature.

The role of peroxides in the thermooxidative degradation of polypropylene

Abstract The nature of the peroxides formed during thermooxidative degradation of polypropylene is still a subject of discussion. It is clear that these peroxides consist of a fast and a slowly decomposing fraction. This paper describes the results of two different iodometric determination methods, as well as results of dimethylsulfide, acid and nitroxide treatments to determine the difference between the fast and the slowly decomposing fractions. It is concluded that the fast-decomposing peroxides are peracids and that the slowly decomposing peroxides are probably tertiary hydroperoxides.

New synergists for hindered amine light stabilizers

Abstract The development of hindered amine light stabilizers (HALS) in the seventies led to a tremendous increase in the outdoor use of polyolefins. Although since that time a great deal of insight has been gained into the UV-degradation mechanism of polyolefins and the mechanism of action of HALS, no new UV-stabilization chemistry has been discovered. In this paper, the most recent thoughts about the UV-degradation mechanism of polyolefins (initiation of photo-oxidation by polymer oxygen charge transfer complexes (CTCs)) and new insights into the mechanism of action of HALS stabilizers (quenching of these CTCs) are explored as a basis for designing new UV-stabilizers. A mechanism is proposed that explains the action of HALS as a quencher. Based on this mechanism several other possible quenchers (bridged amines) have been suggested. It is shown that these bridged amines are active as UV-stabilizers in films and plaques and that they show a synergism with HALS stabilizers. These new stabilizers are the most effective when they are present at concentrations higher than 0.1% and at higher concentrations of HALS. In such cases the addition of these synergists to a HALS stabilized system can lead to an improvement of a factor of 2–3.

Differences and similarities in the thermooxidative degradation of polyamide 46 and 66

Abstract A comparison is made between the thermooxidative degradation of polyamide 46 (PA46) and polyamide 66 (PA66), using oven ageing, oxygen uptake and viscosity measurements on the two polymers. Model experiments were used to determine the degradation mechanism of PA46. Although on a chemical basis it is expected that PA46 would be less stable than PA66, the opposite is found. The time until the tensile strength becomes 50% of its initial value is twice as long for unstabilized PA46 compared with PA66 at 145 °C. It is shown that the degradation of PA46 is a surface phenomenon and thus oxygen diffusion limited. PA46 is more stable than PA66 due to its lower oxygen permeability, probably caused by its higher crystallinity and density of the amorphous phase.

Comparison of the UV-degradation chemistry of unstabilized and HALS-stabilized polyethylene and polypropylene

The UV-degradation of unstabilized and HALS-stabilized polyethylene (PE) and polypropylene (PP) is described. The degradations are characterized by measuring the oxygen uptake, the formation of CO and CO2, the FT-IR spectra, the mechanical properties and the oxygen content of the films. The oxygen uptake of the unstabilized polymers led to the expected changes in the IR-spectra and embrittlement of the films, while oxygen uptake by the HALS-stabilized PE and PP caused only minor changes. The results are explained assuming that the initiation of the photodegradation of PP and PE is due to charge transfer complexes.

The mechanism of action of hindered amine stabilizers (HAS) as long-term heat stabilizers

Abstract Hindered amine stabilizers (HAS) are promoted as long-term heat stabilizers. A difficulty for this application is that at low temperatures (below 100°C) the effectiveness of HAS is more pronounced than at high temperatures (150°C). In order to measure the effectiveness of HAS, prolonged experiments are necessary. Another difficulty is that for HAS-containing polypropylene (PP) the mechanical properties drop gradually, while for phenolic antioxidants a fast decline of the mechanical properties is observed after a long period of no change. These two difficulties are due to different mechanisms of action for phenolic antioxidants and HAS and to a change of the degradation mechanism of PP with temperature. At high temperatures the degradation of PP is mainly due to the oxidation of the polymer itself, while at low temperatures the oxidation of oxidation products is more important. This secondary oxidation results in peracids, via aldehydes, which determines the oxidation rate at low temperatures. Model experiments show that HAS are capable of preventing this aldehyde oxidation, thereby giving good performance at low temperatures. These experiments also show that HAS are not able to prevent the oxidation of hydrocarbons, which takes place at high temperatures. The slow decline of the mechanical properties of HAS-containing PP is due to the oxidation of the polymer itself, which is not stopped by the HAS.