Ageing and Degradation of Multiphase Polymer Systems

Abstract

Ageing can be defined as a slow and irreversible variation as a function of time (in use conditions) of a material structure, morphology or composition leading to a detrimental change in its use properties. The cause of this change can be the own material instability or its interaction with the environment of exposure. The definition so given is that viewed from an application point of view. There are issues associated with this definition that deserve to be mentioned. First, there are ageing mechanisms, essentially of a physical nature as detailed below, that are not irreversible in nature (e.g. crystallinity change, structural recovery, water uptake without loss of the integrity of the chemical structure, etc.), but may lead to a change in the use properties of these materials. The reversibility is in principle achievable by, e.g. thermal treatment or drying. However, this is not necessarily compatible with the use of materials as pieces, or the material will evolve again anyway when exposed to use environmental stresses. Second, irreversible material evolution in itself does not necessarily imply a detrimental change of use properties. It can even result in an improvement of properties. This represents indeed a marked difficulty when attempting to define so-called 'ageing markers' for materials, i.e. material properties to be monitored for health monitoring purpose: the marker must be 21.1.2 Classification of Ageing Types There is a wide variety of polymer ageing types. Each one is a complex phenomenon, but decomposable into a set of elementary mechanisms that can be interactive: chemical reactions, macromolecular motions, molecular species transport into the polymer, crystallinity change, etc. Unfortunately, there is no entirely satisfactory method for their classification, because the most rigorous criteria, in theory, are often open to criticism in practice. The simplest and most agreed method consists in distinguishing two main ageing families according to the nature of involved mechanisms: r chemical ageing is responsible for a variation of the macromolecule chemical structure r physical ageing affects the macromolecule conformation, the material morphology or composition, without altering the macromolecule chemical structure. Then, the different ageing types, defined in terms of the main ageing driving environmental stress, involve one of the two families of mechanisms defined above: r Thermal ageing is initiated by material's own instability. Its kinetics essentially depends on temperature and atmosphere composition. It can be a chemical or a physical ageing, or a combination of them. r Humid ageing is due to a chemical or a physical interaction of polymer with water, the latter being either in a liquid or a vapor state. r Photo-chemical ageing results from polymer interaction with light rays (in general, UV solar radiations). Atmospheric oxygen plays, in general, an important role (photo-oxidation). r Radio-chemical ageing is due to the polymer interaction with ionizing (γ, β, α, neutrons, etc.) rays. ...

Thermo‐oxidative degradation of additive free polyethylene. Part I. Analysis of chemical modifications at molecular and macromolecular scales

“…According to this scheme, aldehydes result exclusively from the β scission of PO° radicals (step VId). However, as the dissociation energy of their CH bond is significantly lower (≈368 kJ/mol) than in methylene groups (≈393 kJ/mol), aldehydes are highly sensitive to radical attacks. Therefore, their rapid oxidation leads to the formation of carboxylic acids, as proposed by many authors in the literature .…”

“…It must be recalled that thermal oxidation is a radical chain reaction initiated by the decomposition of its main propagation product: the hydroperoxide group (POOH), which is also the most instable product . Indeed, the dissociation energy of its OO bond is about 150 kJ/mol, against more than 325 kJ/mol for all other chemical bonds composing the polymer chain (including structural defects such as allylic CH bonds) . In its simplest form (i.e., in oxygen excess and in the absence of thermal and photochemical stabilizers), this reaction can be written such as:…”

QSPR studije karbonilnih, hidroksilnih, polienskih indeksa i prosječne molekulske težine polimera pod fotostabilizacijom pristupom ANN i MLR

“…The linear models obtained for the prediction of I CO , I OH , I PO , and v M of different polymers are represented by the following Eqs. (10)(11)(12)(13) with the reported statistical parameters: For each pair of descriptors, the value of the correlation coefficient (see Table S1 in the supplementary materials) of the four parameters studied was < 0.717, which leads to the independence of the selected descriptors. In addition, the multi-collinearity of the descriptors used in the MLR model can be evaluated by calculating their Inflation Variation Factors (VIF).…”

“…11 It should be noted that photostabilization and aging of polymers are a complex problem to study in practice, as they usually take place slowly, and their service life generally reaches several decades. [12][13][14][15][16] Several studies on photostabilization of polymers are reported in the literature. [17][18][19][20][21][22] The photostabilization activities of polymers compounds were determined by monitoring the carbonyl, polyene and hydroxyl indices, as well as the variations in the viscosity average molecular weight with the duration exposure.…”

Analyse de la dégradation thermique du Poly(éther éther cétone)

“…L'oxydation du PEEK se produit sur les seuls groupes hydrocarbonés de l'unité monomère, c'est-à-dire sur les cycles aromatiques, malgré la très faible labilité des atomes d'H. En effet, l'énergie de dissociation de la liaison C-H aromatique est de l'ordre de 465 kJ.mol -1 , contre 393 et 378 kJ.mol -1 pour les liaisons C-H méthylénique et méthynique respectivement [1]. Au-delà de 400°C, l'oxydation serait principalement amorcée par la rupture des liaisons éther et cétone de l'unité monomère [2][3][4].…”