Melt viscoelastic assessment of Poly(Lactic Acid) composting: Influence of UV Ageing

This study is devoted to the degradation pathway (bio, photo degradation and photo/bio) of Poly(Lactic acid) PLA polymers by means of melt viscoelasticity. A comparison was made between three PLA polymers with different microstructures (L, D stereoisomers). Biodegradability was determined during composting by burying the polymer films in compost at 58 _C. Melt viscoelasticity was used to assess the molecular evolution of the materials during the composting process. Viscoelastic data were plotted in the complex plane. We used this methodology to check the kinetics of the molecular weight decrease during the initial stages of the degradation, through the evolution of Newtonian viscosity. After a few days in compost, the Newtonian viscosity decreased sharply, meaning that macromolecular chain scissions began at the beginning of the experiments. However, a double molar mass distribution was also observed on Cole-Cole plots, indicating that there is also a chain recombination mechanism competing with the chain scission mechanism. PLA hydrolysis was observed by infra-red spectroscopy, where acid characteristic peaks appeared and became more intense during experiments, confirming hydrolytic activity during the first step of biodegradation. During UV ageing, polymer materials undergo a deep molecular evolution. After photo-degradation, lower viscosities were measured during biodegradation, but no significant differences in composting were found. © 2018 by the authors

Durability of crosslinked polydimethylsyloxanes: the case of composite insulators

Most applications of silicones are linked to their hydrophobic properties and (or) their high resistance to ageing (e.g. thermal ageing and photoageing). However, when placed in extreme environments, these materials can fail as in the case of epoxy/fiber glass composite powerlines insulators, where crosslinked polymethylsyloxanes (PDMSs) are used as the protective envelope (housing) of the insulator. We report on the behavior of both pure/noncrosslinked PDMSs and typical formulations used in industrial insulators, i.e. containing peroxide crosslinked PDMS, alumina trioxide hydrated (ATH) and silica. Special attention is paid on both (i) the sources of potential degradation and (ii) the best analytical methods that can be applied to the study of very complex formulations. (i) Aside from conventional types of ageing such as photo-ageing and thermal, hydrolytic, and service life ageings, treatments with acidic vapors, plasma and ozone possibly generating species from the reaction of a high electric field with air were also performed, which allowed to accelerate electrical and out-door ageings and to obtain differently aged materials. (ii) Aside from conventional analytical methods of polymer degradation such as FTIR/ATR spectroscopy and SEC, TG, hardness measurements, more specific methods like photo/DSC, TG/IR, thermoporosimetry, resistivity and density measurements were also performed to characterize the chemical and physical evolutions of polymer materials. In particular, it was found that treatment with nitric acid vapor has detrimental effects on the properties of both fire retardants (e.g. ATH) and PDMSs, affecting the hardness and resistivity of the formulated material

Approche expérimentale et théorique de la dégradation des polydiméthylsiloxanes

L objectif de cette thèse était d étudier suivant deux approches la dégradation des polydiméthylsiloxanes. L approche expérimentale a permis d étudier les mécanismes de rupture de chaîne et les phénomènes de réticulation du polymère lorsque ce dernier est soumis à des conditions de thermo-oxydation ou de photo-oxydation comparables à celles du vieillissement naturel. La seconde partie propose de modéliser les résultats expérimentaux par un modèle cinétique issu des calculs ab initio. L exploitation in vitro de la dégradation du polymère a permis de mettre en évidence la formation de monoxyde de carbone et de dioxyde de carbone, ceux-ci jouant le rôle de révélateurs de rupture de liaison Si-CH3. Toutefois le comportement des PDMS sur des temps de dégradation plus importants montre clairement la formation d oligomères cycliques par dépolymérisation de la chaîne macromoléculaire. Parallèlement la photo-oxydation de ces polymères entraîne la formation de réseaux multidimensionnels qui traduisent des phénomènes de réticulation avec formation d acide formique. Cette étude a permis de montrer que la dégradation des PDMS, dans des conditions de vieillissement naturel, procède de deux mécanismes antagonistes, l un favorisant la réticulation du polymère et le second sa dépolymérisation. La seconde partie de ce travail s intéresse aux mécanismes de formation des oligomères cycliques observés expérimentalement lors de la dégradation des PDMS. Des réponses précises ont pu être amenées grâce à une analyse théorique détaillée de la constante de cyclisation de ces polymères. Cette étude nous a permis de montrer le rôle essentiel du couplage des rotateurs internes dans le calcul de la constante de cyclisation en fonction de la taille des cycles obtenus. Nos résultats théoriques reproduisent avec exactitude l évolution de la courbe de la constante de cyclisation observée expérimentalement pour des cycles de taille inférieure à 20 motifs, à savoir un maximum global pour des cycles constitués de quatre motifs et un minimum local pour des cycles à dix motifs. Nous avons démontré que l origine de l évolution oscillatoire de la constante de cyclisation traduisait le changement de caractère de certains degrés de liberté entre des vibrations pures (petits cycles) et des rotations empêchées (cycles de plus grande taille).This work aimed at investigating the degradation pathways of polydimethylsiloxanes through two perspectives. The experimental approach studied bond scission and cross-linking degradation mechanisms when the polymer is exposed to thermo- or photo-oxidation conditions that are similar to that of the natural ageing. In the second part, the experimental results are modeled with a theoretical kinetic model, based on ab initio calculations.The in-vitro study of the polymer degradation showed that formation of carbon monoxide and carbon dioxide were by-products of Si-CH3 bond scissions. However, the degradation of PDMS for longer time periods showed clearly the formation of cyclic oligomers obtained through depolymerization of the macromolecule. In parallel, the photo-oxidation conditions yielded the formation of multidimensional cross-linked networks and formation of formic acid. This study showed that in the natural conditions, the degradation of PDMS proceeded through two opposite mechanisms, one that produced cross-linked networks while in the other, depolymerization was favored. In the second part, we investigated the mechanisms of the formation of cyclic oligomers that were observed experimentally during the degradation of the polymer. A detailed theoretical model was built in order to reproduce the dependence of the experimental cyclization constant with cycle size. This study showed the key role of the coupling in the treatment of hindered rotors. Our theoretical results reproduced accurately the oscillatory behavior of the cyclization constant for cycle sizes less than 20-mers, namely the global maximum for the 4-unit cyclic oligomers, and the local minimum for cycle sizes of 10 units. We have also shown that the origin of the oscillatory behavior of the cyclization constant revealed that some degrees of freedom underwent specific character transformation between a pure vibration for small cycles and hindered rotation for larger ring sizes.CLERMONT FD-Bib.électronique (631139902) / SudocSudocFranceF

Initial properties and ageing behavior of pineapple leaf fibre or palm fibre as reinforcement for polypropylene.

International audienceIn the furniture, automotive and contruction industries, there is increased demand for cost-effective and lightweight biocomposites. The objective of this work was to develop new natural fibre-based composites with specific properties. Palm and pineapple leaf fibres were chosen in association with polypropylene (PP). The first step was to investigate the effect of these natural fibres as reinforcement for PP. The evolution of chemical structure and crystallinity was proposed with infrared spectroscopy measurements and differential scanning calorimetry thermograms, respectively. The assessments of mechanical properties with tensile tests and melt viscoelastic behaviour were also investigated. The study enabled to distinguish the influence of fibre content. The second step in our work was to assess the composite durability after ultraviolet exposure or thermal ageing. The oxidation level was calculated. The long-term evolution of thermal and mechanical properties was also proposed. As a result, the PP/pineapple leaf composite revealed a promising biocomposite

Durability of crosslinked polydimethylsyloxanes: The case of composite insulators

International audienceMost applications of silicones are linked to their hydrophobic properties and (or) their high resistance to ageing (e.g. thermal ageing and photoageing). However, when placed in extreme environments, these materials can fail as in the case of epoxy/fiber glass composite powerlines insulators, where crosslinked polymethylsyloxanes (PDMSs) are used as the protective envelope (housing) of the insulator. We report on the behavior of both pure/noncrosslinked PDMSs and typical formulations used in industrial insulators, i.e. containing peroxide crosslinked PDMS, alumina trioxide hydrated (ATH) and silica. Special attention is paid on both (i) the sources of potential degradation and (ii) the best analytical methods that can be applied to the study of very complex formulations. (i) Aside from conventional types of ageing such as photo-ageing and thermal, hydrolytic, and service life ageings, treatments with acidic vapors, plasma and ozone possibly generating species from the reaction of a high electric field with air were also performed, which allowed to accelerate electrical and out-door ageings and to obtain differently aged materials. (ii) Aside from conventional analytical methods of polymer degradation such as FTIR/ATR spectroscopy and SEC, TG, hardness measurements, more specific methods like photo/DSC, TG/IR, thermoporosimetry, resistivity and density measurements were also performed to characterize the chemical and physical evolutions of polymer materials. In particular, it was found that treatment with nitric acid vapor has detrimental effects on the properties of both fire retardants (e.g. ATH) and PDMSs, affecting the hardness and resistivity of the formulated material

Thermal and photochemical ageing of eposy resin. Influence of curing agents

International audienc

Physico-chemical durability criteria of oils and linked bio-based polymers

The oxidative stability or durability is an important indicator of performance that depends on the composition of the sample. The fatty oil or polymer degradation processes have generally been established as being free radical mechanism yielding primary oxidation products. We propose to explain in detail all the analytical methods and tools used for the determination of the initial physico-chemical properties of oils and the properties in ageing conditions. Chemical titrations for acid or peroxide value, Rancimat method or thermogravimetric measurements are discussed. Accelerated ageing tools for thermal or photochemical exposures are also shown. After the assessment of oil durability, the development of new bio-based polymer with vegetable oil is tackled because of its industrial interest. It is essential to understand the long term behavior of oils and biopolymers and to assess exactly the durability which is useful to produce life cycle analysis of materials. At last we underline the advantages of a new Fourier transform infrared (FTIR) instrumentation with in-situ irradiation and gas cell to give a screening of the durability of various oils or polymers. Main durability criteria of oils and biopolymers are linked to the production of volatile organic compounds and the resistance to the oxidation process

Physico-chemical durability criteria of oils and linked bio-based polymers

The oxidative stability or durability is an important indicator of performance that depends on the composition of the sample. The fatty oil or polymer degradation processes have generally been established as being free radical mechanism yielding primary oxidation products. We propose to explain in detail all the analytical methods and tools used for the determination of the initial physico-chemical properties of oils and the properties in ageing conditions. Chemical titrations for acid or peroxide value, Rancimat method or thermogravimetric measurements are discussed. Accelerated ageing tools for thermal or photochemical exposures are also shown. After the assessment of oil durability, the development of new bio-based polymer with vegetable oil is tackled because of its industrial interest. It is essential to understand the long term behavior of oils and biopolymers and to assess exactly the durability which is useful to produce life cycle analysis of materials. At last we underline the advantages of a new Fourier transform infrared (FTIR) instrumentation with in-situ irradiation and gas cell to give a screening of the durability of various oils or polymers. Main durability criteria of oils and biopolymers are linked to the production of volatile organic compounds and the resistance to the oxidation process

Physico-chemical durability criteria of oils and linked bio-based polymers

The oxidative stability or durability is an important indicator of performance that depends on the composition of the sample. The fatty oil or polymer degradation processes have generally been established as being free radical mechanism yielding primary oxidation products. We propose to explain in detail all the analytical methods and tools used for the determination of the initial physico-chemical properties of oils and the properties in ageing conditions. Chemical titrations for acid or peroxide value, Rancimat method or thermogravimetric measurements are discussed. Accelerated ageing tools for thermal or photochemical exposures are also shown. After the assessment of oil durability, the development of new bio-based polymer with vegetable oil is tackled because of its industrial interest. It is essential to understand the long term behavior of oils and biopolymers and to assess exactly the durability which is useful to produce life cycle analysis of materials. At last we underline the advantages of a new Fourier transform infrared (FTIR) instrumentation with in-situ irradiation and gas cell to give a screening of the durability of various oils or polymers. Main durability criteria of oils and biopolymers are linked to the production of volatile organic compounds and the resistance to the oxidation process