Jean-Louis Philippart

Influence of light intensity on the photooxidation of polypropylene

Abstract Polypropylene was photooxidized under polychromatic light (λ ≥ 300 nm) at different intensities of the incident light. The variations of intensity were obtained without changing the relative spectral distribution of the source or the temperature at the surface of the exposed samples. Oxidation curves were generated by FTIR analysis of the exposed polypropylene films. The light intensity dependence of both the induction period and the steady-state rate of oxidation was plotted. Simple kinetic behaviours were determined, showing a variation of the steady-state rate of oxidation with the square root of the light intensity and the variation of the induction period being directly proportional to the inverse of the light intensity.

Mass spectroscopy analysis of volatile photoproducts in photooxidation of polypropylene

Abstract Depending on their structure, photooxidising polymers can produce significant concentrations of low molecular weight products formed by multiple fragmentations of the main or branched chains. These photoproducts can migrate out of the polymer matrix and consequently are not taken into account by analysis only of the solid sample. In some cases this may complicate the elucidation of the photooxidation mechanisms of the corresponding polymers. This is why an analysis of the volatile products is sometimes required. We describe experiments based on the analysis by mass spectroscopy of the gas phase obtained by photooxidation of polypropylene. Because the photo-oxidation of this polymer is fairly well characterized, it was a good candidate to calibrate the apparatus and the analytical mode. One of the main advantages of using mass spectroscopy to analyse the gas phase is that this method requires virtually no separation of the different constituents before analysis. However, we show that by using the different volatilities of the products formed in the polymer and in the gas phase, separation of the constituents can be obtained, which facilitates further analysis by mass spectroscopy.

Influence of the exposure parameters on the mechanism of photooxidation of polypropylene

Abstract Samples of polypropylene were exposed to artificial accelerated photoageing in various conditions of light intensity. The aged samples were analyzed by FTIR techniques coupled with chemical derivatization to determine the degradation structures resulting from UV exposure. Chemical titrations of the hydroperoxides and determinations of the oxygen consumption were carried out to quantify the formation of the oxidation photoproducts. As expected, our results showed that modifying the exposure parameters induced notable changes of the kinetics of the reactions. These changes led also to some modifications of the relative concentrations of the various oxidation photoproducts, particularly those of the carbonylated products including esters and carboxylic acids. This light intensity dependent behaviour permitted the proposal of a mechanism to account for the formation of the final photoproducts resulting from the hydroperoxides. Our results indicated two possible mechanisms of decomposition of the hydroperoxides, whose relative importance is dependent on the light intensity.

Photo and thermal decomposition of iPP hydroperoxides

Products resulting from the photo and thermal decomposition under vacuum of a pre-oxidized isotactic polypropylene (iPP) containing a known content of hydroperoxide have been examined. In contrast to the case of polyethylene, few products were retained in the polymer matrix. Detailed analysis of the gas phase was performed by gas chromatography, Fourier transform infra-red spectroscopy and mass spectrometry. It was revealed that about 70% of the hydroperoxides were converted into gaseous products such as acetone, acetic acid and methanol. Mechanisms for their formation are suggested, and the consequences of such a phenomenon for the evaluation of ageing in PP are discussed.

Photooxidation of cured fluorinated polymers—I. Photooxidation of copolymers of fluorinated olefins and allyl or vinyl ethers

The photooxidation mechanisms of two copolymers of fluorinated olefins and allyl or vinyl ethers are studied. The ether groups that constitute the reactive part of both of the polymers are located on short pendant-chains. For this reason, the scissions due to the oxidation of the material lead to notable quantities of low molecular weight photoproducts that can be lost from the solid matrix by migration. The classical analysis of the evolution of the solid polymer matrix then has to be completed by an analysis of the gas phase formed during irradiation. Unanticipated results are obtained, since it is shown from the identification of the whole oxidation photoproducts that the presence of the fluorine strongly influences both the orientation of the reaction and the photooxidation kinetics. Due to the neighboring fluorine atoms, the methylene groups in α-position of the oxygen of the ether groups are not equivalent regarding oxidation and the secondary carbon becomes more oxidizable than the tertiary one. Moreover, these polymers show a very high resistance towards photooxidation, which is quite unusual with regard to polyethers in general.

Photo-oxidation of polydimethylsiloxane oils: Part I—Effect of silicon hydride groups

Abstract Photo-oxidations at wavelengths longer than 300 nm of polydimethylsiloxane (PDMS) oil with various contents of SiH and different configurations (H inside the chain (2·2 H equivalent kg−1) or at the end of the chain (1·2–4 and 15 H equivalent kg−1)) have been examined. Hydroperoxides groups are primarily formed on photo-oxidation and result from radical attack on the SiH bond. These photo-unstable and thermally unstable hydroperoxides are transformed essentially into silanols, absorbing at 3300 cm−1. Oxygen consumption measurements suggest a 1 1 stoichiometry between SiH and oxygen consumed, whereas the quantitative determination of SiOH reveals a deficit, mainly as a result of a condensation reaction which leads to a significant increase in viscosity.

Thermo-oxidation of isotactic polypropylene in 32O2-36O2: comparison of the mechanisms of thermo- and photo-oxidation

The formation of gaseous products resulting from thermo-oxidation of isotactic polypropylene (iPP) at 95 and 140°C in an atmosphere composed of a blend of 32O2 and 36O2 has been investigated using mass spectroscopy (MS). In contrast to the results obtained previously in the case of photochemical oxidation, MS analysis shows that thermo-oxidation does not generate 34O2. A possible interpretation is that, in thermo-oxidation, the mechanism involving the recombination of peroxy radicals POO• occurs only with a very low probability. As a consequence, it is suggested that, in thermo-oxidation conditions, the main pathway of oxidation involves the formation of hydroperoxides and not recombination of peroxy radicals as proposed in photo-oxidative conditions.

Perturbation of the photochemistry of poly(vinyl chloride) by a conventional aromatic plasticizer

Abstract Samples of poly(vinyl chloride) unplasticized or plasticized with di(octyl)phthalate were exposed to accelerated photo-aging. The presence of the plasticizer appeared to modify the behaviour of the polymer. Plasticized samples photocolourate more slowly than unplasticized ones. This effect is discussed in terms of a screening effect and an increased permeability to oxygen owing to the plasticizer, and of an increase in chain mobility owing to the reduction in the glass-transition temperature.

Photooxidation of cured fluorinated polymers—III. Quantitative study of the photooxidation by mass spectrometry

The photooxidation mechanisms of two copolymers of fluorinated olefins and allyl or vinyl ethers have been previously established. The different photoproducts have been identified, by completing analysis of the solid polymeric matrix by an analysis of the low molecular weight photoproducts that migrate towards the surrounding atmosphere. The present article describes an analytical method based on mass spectrometry that is developed to measure directly the concentrations of the volatile products. These analyses permit a verification of the mechanism and a quantification of the possible routes of oxidation.

Photochemical behavior of isotactic-polypropylene irradiated in 32O2-36O2

Abstract The photochemical oxidation of isotactic polypropylene (iPP) in an atmosphere composed of a blend of 32 O 2 and 36 O 2 has been investigated using mass spectroscopy (MS) and infra-red spectroscopy (IR). The samples were irradiated with an artificial accelerated photoageing unit at 60°C. MS analysis reveals the formation of several oxidation photoproducts previously identified, and unambiguously shows that photooxidation of iPP generates 34 O 2 . A possible interpretation is that, according to the results proposed in the literature, a pseudo-termination reaction by recombination of peroxy radicals POO occurs and undergoes a head-to-head reaction, which produces tetroxides. These intermediate species decompose to give molecular oxygen and two alkoxy macroradicals. On a kinetic point of view, two successive stages are observed. In the first, at low levels of oxygen incorporation, recombinations of POO are not favoured and peroxy radicals mostly generate hydroperoxides. In the second phase, the local concentrations of peroxy radicals are high enough to favour recombination.