Bruno Fayolle

Oxidation induced embrittlement in polypropylene — a tensile testing study

Abstract The thermal oxidation of polypropylene films (100 μm) in air at 90°C was studied by IR spectroscopy, rheometry at 210°C and tensile testing. Tensile testing reveals a sudden embrittlement before the end of the induction period determined from carbonyl build-up curves. Embrittlement occurs at a very low conversion of the chain scission process (only 1 chain scission per 3 initial chains) and it can be demonstrated that it results from a decrease in polymer toughness rather than from the build-up of defects linked to a presumed heterogeneity of the oxidation process. Tensile testing is not necessarily a good tool to characterize embrittlement given that phenomena such as necking and crack initiation at geometric defects are not taken into account with the usual measuring methods. The sudden drop of ultimate elongation could be, at least partly, an artifact.

About a quasi-universal character of unstabilised polyethylene thermal oxidation kinetics

Abstract Literature values of the induction time ti and maximum rate rS of oxygen absorption for unstabilised polyethylene samples have been compiled. In Arrhenius plots all of their representative points are close to straight lines of parameters: Ln (t i0 )≈−24.4; E i ≈116 kJ mol −1 for t i and Ln (r S 0 )≈−34.2; E s ≈146 kJ mol −1 for r s It can be concluded that there is a quasi-universal behaviour, for the thermal oxidation of polyethylene, irrespective to branching and other structural irregularities or crystallinity. This result is consistent with the hypothesis that radical chain oxidation of PE is essentially initiated by the bimolecular decomposition of hydroperoxides provided that their initial concentration is not lower than 10−5 mol l−1 and not higher than 10−1 mol l−1.

Initial steps and embrittlement in the thermal oxidation of stabilised polypropylene films

The oxidation of 100 μm stabilised polypropylene films has been studied at 130 °C in air, using physico-chemical and mechanical characterisation methods. During the induction period, where no carbonyl build-up is observable by IR spectroscopy, both stabiliser consumption (UV spectrophotometry) and molecular weight decrease (steric exclusion chromatography) can be observed. These structural modifications induce a decrease of fracture properties as determined using the essential work of fracture (EWF) method whereas tensile ultimate elongation does not display significant variations. The results clearly indicate the interest of the EWF method to characterise degradation induced embrittlement of initially ductile polymers. The experimental data gathered in this work bring some new elements of discussion about the problem of spatial heterogeneity of the oxidation process.

Rate constants of oxidation of unsaturated fatty esters studied by chemiluminescence

This study deals with oxidation kinetics of three unsaturated fatty esters: methyl oleate, methyl linoleate and methyl linolenate at temperatures ranging from 90 to 150 °C. The reaction was monitored by chemiluminescence. A kinetic model was derived from a simple mechanistic scheme, in which initiation is due to hydroperoxides decomposition, whereas propagation results from the abstraction of the most labile hydrogen and termination results only from the bimolecular combination of peroxyl radicals. Analysis of induction period duration indicated that hydroperoxides mainly decompose by a bimolecular process. The model well predicts the main features of the experimental chemiluminescence curves. Kinetic parameters of the three unsaturated fatty esters were assessed from inverse method and discussed.

Oxidation induced changes in viscoelastic properties of a thermostable epoxy matrix

The thermal ageing of a neat epoxy matrix has been studied at 473 K in air by three complementary analytical techniques: optical microscopy, dynamic mechanical analysis and nano-indentation. Thermal oxidation is restricted in a superficial layer of about 195 μm of maximal thickness. It consists in a predominant chain scission process involving, in particular, chemical groups whose β motions have the highest degree of cooperativity and thus, are responsible for the high temperature side of β dissipation band. As a result, chain scissions decrease catastrophically the glass transition temperature, but also increase significantly the storage modulus at glassy plateau between T β and T α. This phenomenon is called “internal antiplasticization”. Starting from these observations, the Di Marzio and Gilbert’s theories have been used in order to establish relationships between the glass transition temperature and number of chain scissions, and between the storage modulus and β transition activity respectively. The challenge is now to establish a relationship between the β transition activity and the concentration of the corresponding chemical groups.

Study on the temperature dependence of the bulk modulus of polyisoprene by molecular dynamics simulations

The temperature dependence of the bulk modulus of polyisoprene has been studied using molecular dynamics simulations. Virtual polyisoprenes have been submitted to volume contractions above and below the glass transition. Bulk modulus has been observed to be linearly dependent on temperature both above and below the glass transition respectively, and it dropped by a factor of about 2 while temperature was raised above the glass transition. By monitoring the energy changes during volume contractions, it was observed that the bulk modulus arises mainly from the Van der Waals interactions. Nevertheless, the entropy contribution to the bulk modulus becomes significant above the glass transition. At a first order, the entropy part of the bulk modulus can be considered as independent of the temperature.

Methodology of Lifetime Prediction in Polymer Aging

Two approaches for lifetime prediction in polymer aging are compared: the “simulation approach” and the “ideal approach.” The widely used “simulation approach,” based on the hypotheses which were never clearly formulated, consists of finding a set of exposure conditions such that accelerated aging leads to the same structural states as natural aging. Using simple kinetic models it is demonstrated, that in the general case it is impossible to obtain a “good simulation.” Anyhow, in this approach, the problem of the relationship between accelerated aging and natural lifetimes remains unresolved. The “ideal approach” uses nonempirical kinetic models, taking into account structural changes at all the pertinent scales, and also uses polymer physics to establish the link between the polymer structure and the property under consideration. An important characteristic of this approach is that the accelerated aging serves only to determine the model parameters.

Investigation of Model Fuel Effects on Thermal Oxidation of Polyethylene

This work deals with a study of stabilizers extraction by ethanol‐cyclohexane mixtures simulating ethanol based biofuels. It appears that the extractive power of cyclohexane is considerably stronger then ethanol one. In other words, from this point of view, ethanol based biofuels are less aggressive than a pure hydrocarbon.

Influence of thermo-oxidation on dielectric and mechanical properties of epoxy/amine resins

This objective of this work is to compare the changes of electrical and mechanical properties during thermo-oxidation of epoxy, in order to improve the choice of end life criterion. The thermal oxidation of DGEBA totally cured with linear aliphatic hardener is investigated under several temperature and oxygen pressure. Chemical changes are followed thanks to infra-red spectroscopy, macromolecular changes thanks to differential scanning calorimetry (changes of Tg). During thermo-oxidation at 110°C in air, volume resistivity and tensile properties in terms of modulus and deformation at break changes are assessed. The results show a formation of polar groups corresponding to oxidation products and a decrease of molecular mobility associated to a crosslinking process. Furthermore, it appears that the oxidation process leads to an embrittlement process whereas electrical properties are not significantly modified.

New Developments in the Field of Radiochemical Ageing of Aromatic Polymers

Polymers having an aromatic backbone polymers have a high mechanical strength and a high modulus. Their aromaticity increases their resistance for use in relatively severe conditions especially in aerospace and nuclear industry for which lifetime prediction is a key issue. For example, a challenge for nuclear plants is to extend lifetime from the initially planned 40 years duration to 50 or 60 years, which makes necessary to determine lifetime by a non-empirical method. Since polymers mechanical failure originates from chain scission or crosslinking of the backbone, the ideal method of lifetime prediction would first involve the elaboration of a kinetic model for chain scission and crosslinking. Then, the changes of molecular mass would be related to the changes of mechanical properties using the available laws of polymers physics. Lifetime would be then determined using a pertinent lifetime criterion. A noticeable difficulty comes here from the fact that oxidation, which plays a key major role in chain scission, is diffusion controlled and thus heterogeneously distributed in sample thickness. It is crucial, indeed, to determine experimentally and to predict this depth distribution of chain scission and crosslinking because it will play a key role on fracture properties. This chapter will be henceforward devoted to the effect of aromaticity on radiostability, the effect on temperature on the chain scission/crosslinking competition, the diffusion limited oxidation (which will be illustrated by the effect of dose rate, atmosphere and sample thickness), then some concluding remarks on oxidative stability of aromatic polymers and the possible link with the absence of macromolecular mobility below Tg. We will start by some basics of radiochemistry which are necessary for the good understanding of this paper, and especially the quantitative treatment for crosslinking and chain scission.