X. Colin

A new method for predicting the thermal oxidation of thermoset matrices: Application to an amine crosslinked epoxy

Abstract The thermal oxidation of an aromatic amine crosslinked epoxy network has been studied in air below its glass transition temperature by optical microscopy, IR spectroscopy and gravimetry. The superficial oxidised layer has a thickness of 147 μm at 150°C, 94 μm at 180°C and 62 μm at 200°C. A method is proposed to predict the oxidised thickness. It is based on a differential equation in which O 2 diffusion and its chemical consumption rate, r ( C ), are coupled, C being the O 2 concentration. The knowledge of equation parameters requires two sets of experiments: permeation tests to estimate both O 2 diffusivity and solubility into the polymer and also, gravimetric measurements on thin polymer films oxidised at various O 2 partial pressures to determine the kinetic parameters of r ( C ). Predictions are in good agreement with experimental results for all temperatures under consideration.

Interaction between Cracking and Oxidation in Organic Matrix Composites

The thermal oxidative degradation of carbon fiber/bismaleimide (Hexel T800H/F655-2) and carbon fiber/amine-cross-linked epoxy (Cytec Fiberite IM7/977-2) unidirectional composites and their corresponding neat polymers has been studied in air at temperatures ranging from 150 to 240 C by optical microscopy, using an interferential contrast, and IR microspectrophotometry. A coarse analysis of the damaged zone allowed the determination of main kinetic characteristics in the whole degradation process (oxidation, crack propagation, and their interaction) for each principal direction of the composite materials. It has been found that cracks initiate at the sample free surfaces when a critical value of embrittlement is reached in the oxidized layer. Then, cracks propagate differently toward the core according to both the fiber orientation and the nature of the composite system. The difference in behavior between T800H/F655-2 and IM7/977-2 systems can be explained by a difference in polymer toughness and oxygen permeability of the fiber-matrix interface.

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.

Kinetic modelling and simulation of gravimetric curves: application to the oxidation of bismaleimide and epoxy resins

Abstract A simplification of a previously presented “closed-loop” kinetic scheme of radical chain oxidation consisting in taking into account the substrate consumption is applied to the thermal oxidation (in oxygen excess) of a polybismaleimide (BMI) and an amine crosslinked epoxy (ACE). The model fits gravimetric curves in the 0–12% mass loss range well, for both polymers and constitutes a noticeable progress compared to the previously established “steady-state” models. The qualitative difference between BMI (presence of an initial maximum) and ACE (continuous mass decrease), is explained by the fact that ACE hydroperoxides are considerably less stable than BMI ones, which can be mechanistically justified. A generalisation of these results leading to a classification of gravimetric oxidation curves according to their shape, is tentatively proposed.

A theoretical model for the diffusion-limited thermal oxidation of elastomers at medium temperatures

Abstract A theoretical model is presented to describe the diffusion-limited thermal oxidation of stabilized elastomers, with no assumptions concerning the concentration of oxygen, the kinetic chain length, the ratio of termination rate constants, or the existence of a steady state for oxidation products. Model predictions are found to agree qualitatively with reported main features of elastomer thermal oxidation. The model is used to simulate the thermal ageing of thick natural rubber slabs under ambient air at 80–150 °C. Among others, the effect of stabilizer concentration and thickness of the aged parts on the evolution of the oxidation process is discussed. Conditions are also identified under which the oxidation process may be considered free of oxygen mass transfer limitations.

Humid Ageing of Organic Matrix Composites

In this chapter, several aspects of the ageing phenomena induced by water in organic matrix composites are examined, essentially from the physico-chemical point of view. It is first important to recognize that there are two main categories of humid ageing. First there are physical processes, mainly linked to the stress state induced by matrix swelling and sometimes matrix plasticization. This kind of ageing can occur in matrices of relatively high hydrophilicity (affinity with water). Highly crosslinked amine cured epoxies are typical examples of this behavior. The second category of humid ageing involves a chemical reaction (hydrolysis) between the material and water. Unsaturated polyesters are typical examples of this category. They display a low to moderate hydrophilicity, swelling and plasticization have minor effects, but hydrolysis induces a deep polymer embrittlement and, eventually, osmotic cracking. Whatever the ageing mechanism, it needs the water to penetrate into the material and depends on the water concentration and its distribution in the sample thickness. This is the reason why the first and second sections are respectively dedicated to water solubility and diffusivity in matrices, interphases and composites. In each case, the elementary processes are distinguished, to examine the effects of temperature and stress state and to establish structure–property relationships. It is shown that, in most of these aspects, research remains largely open. The last section is devoted to hydrolysis, its kinetic modeling, including the case of diffusion controlled hydrolysis, and its consequences on polymer properties. Structure reactivity relationships are briefly presented. The very important case of osmotic cracking, which can be considered as a consequence of hydrolysis, is also examined.

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.

Mechanisms and Kinetics of Organic Matrix Thermal Oxidation

It is now well recognized that during thermal aging at moderate temperatures, for example, typically below the glass transition temperature, organic matrix composites perish mainly by matrix embrittlement resulting from its thermo-oxidation. The present chapter aims to briefly introduce this domain. The chapter consists of a brief history of polymer oxidation and description of mechanisms and kinetics. The radical character of oxidation processes; the main elementary steps: propagation, termination, initiation processes, and initial steps; structure–property relationships; the nature of oxidation products; and experimental methods for the study of oxidation mechanisms are also discussed. The standard kinetic scheme, case of oxygen excess and general shape of oxidation kinetic curves, the induction period, departure from Arrhenius law, and case of oxygen lack are described. Consequences of oxidation on matrix thermomechanical properties including chain scission and cross-linking physical approaches are presented.

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.

Understanding the durability of advanced fibre-reinforced polymer (FRP) composites for structural applications

Abstract: Fibre-reinforced polymer (FRP) composites are increasingly being used in the field of civil engineering, either for the rehabilitation/retrofitting of existing infrastructures or for the construction of new structural elements. However, such applications are still recent and there are still unresolved questions regarding the long-term durability of FRP reinforcements or structural elements under service conditions, and their behaviour under accidental fire events as well. In this chapter, it is proposed to highlight the basic mechanisms involved in the environmental degradation of FRP composites, with a large emphasis on ageing mechanisms of the polymer matrix and their consequences on the mechanical properties. The last section is specifically devoted to the fire behaviour of polymer composites and also recalls existing fire-proofing solutions.