A statistical theory of polymer network degradation

A statistical theory was proposed for the degradation (random scission of chains) of a network having f-functional nodes in the case where all chains contain equireactive groups and a chain scission event does not create new groups or suppress more than one group. Closedform relations were established between the conversion ratio of the degradation process and the crosslink density. Emphasis was put on the value of the conversion ratio for which the gel disappears. Some limited cases already considered in the literature were recovered, but a general solution was proposed for networks having any number of reactive groups per chain, be it uniform or not, and for conversion ratios up to the degelation point. The results were applied successfully to recent experiments regarding the hydrolysis of a polyester

Plastisol Gelation and Fusion Rheological Aspects

This study deals with the rheological aspects of poly-vinyl chloride (PVC) plastisol gelation and fusion processes in foamable formulations. Here, such processes are simulated by temperature-programmed experiment (5 K min−1) in which complex viscosity components are continuously recorded. Nineteen samples based on a PVC-VAC (vinyl acetate 95/5) copolymer with 100 phr plasticizer have been studied, differing only by the plasticizer structure. The sample shear modulus increases continuously with temperature until a maximum, long time after the end of the dissolution process as characterized by DSC. The temperature at the maximum varies between 345 and 428 K with a clear tendency to increase almost linearly with the plasticizer molar mass, and to vary with the flexibility and the degree of branching of the plasticizer molecule. The shear modulus increase is interpreted in terms of progressive “welding” of swelled particles by polymer chain reptation. The plasticizer nature would mainly affect the friction parameter of chain diffusio

Co-oxidation kinetic model for the thermal oxidation of polyethylene-unsaturated substrate systems

The thermal oxidation of polyethylene (PE) impregnated by the methyl esters of unsaturated fatty acids (UFEs) was studied using chemiluminescence, and infra-red spectrophotometry. It was shown that the presence of UFEs accelerates the PE aging process. This can be interpreted as a co-oxidation phenomenon. In this study, the previously established models for PE and UFEs self-oxidation have been coupled in order to develop a co-oxidation model. Using the existing rate constants for the PE and UFEs selfoxidations, this model can simulate the complex shape of the kinetic curves of PE-UFE co-oxidatio

Monitoring and simulations of hydrolysis in epoxy matrix composites during hygrothermal aging

In this paper, we studied the water transport in thermoset matrices. We used Fourier Transform Infrared analysis (FTIR) during sorption/desorption experiments to investigate the interaction between sorbed water and the epoxy network. Our results demonstrated that the polymer matrix undergoes hydrolysis. We found that the chemical species involved in the reaction process was the residual anhydride groups. These results support the physical basis of the three-dimensional (3D) diffusion/reaction model. We finally showed that this model is able to reproduce multi-cycle sorption/desorption experiment, as well as water uptake in hybrid metal/epoxy samples. We simulated the 3D distributions of the diffusing water and the reacted water

Aging of Organic Matrix Composite Materials

Among the wide variety of organic matrix composite (OMC) applications, very scarce are those for which dura-bility is not a key property. This is the reason why OMC aging raised a relatively abundant literature in the past half century. Aging can be defined as a slow (e.g., needing the recourse to accelerated aging tests) and irreversible (in use conditions) evolution of the chemical structure, mor-phology (in its widest sense), and/or composition of the material. It is a technically and economically important problem when evolution leads to a deleterious change of use properties. This problem can be crucial in highly demanding domains, for instance in aerospace applica-tions, which explains why a great part of the research on OMC aging was induced and supported by the aerospace industry

Organic polymers

Natural organic materials (wood, horn, skin, etc.) have been used since the dawn of humanity. Textile fabrication from vegetal fibers (linen, cotton, etc.) or from animals (silk, wool, etc.) reached an “industrial” stage from the Middle Ages onwards. The idea of changing raw matter by specific physical or chemical treatment (dyeing fibers, leather tanning, etc.) is practically as ancient as the applications themselves. In the 19th century, however, a new idea emerged: chemically modifying natural substances (fibers, skins, etc.) so as to make them into completely different from the natural forms

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

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

Ageing and Durability of Organic Polymers

“Ageing” is any slow and irreversible phenomenon (under usage conditions) of a material’s structure, morphology or composition, under the effects of its own instability and/or interaction with the environment. Any condition is considered to be slow when the kinetics are not apprehensible in a time scale which is compatible with economic restraints. The classic approach of predicting long-term behavior consists of carrying out accelerated tests simulating, as accurately as possible, natural ageing and to presume that the hierarchy of stability for several materials is the same in accelerated and natural ageing. The modern approach consists of developing a kinetic model derived from analyzing degradation materials. Accelerated ageing is, then, used to identify the model parameters

Thermooxidative and thermohydrolytic aging of composite organic matrices

This chapter deals with the main causes of chemical ageing in organic matrix composites: hydrolysis, essentially in polyesters and polyamides, and oxidation in all kinds of polymer matrices. The first section is devoted to common aspects of chemical degradation of organic matrices. It is shown that chain scission and, at a lesser extent, crosslinking are especially important because they induce embrittlement at low conversions. Quantitative relationships between structural parameters and mechanical properties are briefly examined. The second section deals with diffusion-reaction coupling. In the cases of hydrolysis and oxidation, kinetics can be limited by respectively water and oxygen diffusion. Then, degradation is confined in a superficial layer, that can carry important consequences on use properties. The third section is devoted to hydrolysis. The kinetic equations are presented in both cases of non equilibrated and equilibrated hydrolysis. Structure-stability relationships are briefly examined. Osmotic cracking process, very important in the case of glass fiber/unsaturated polyester composites, is described. The last section is devoted to thermal oxidation. The simplest kinetic models are presented. The main gravimetric behaviours are explained. A mechanism is proposed for the "spontaneous cracking" in the superficial layer of oxidized samples