JCJC SIMI 9 - JCJC - SIMI 9 - Sciences de l'ingéniérie, matériaux, procédés énergie

Polymeric COatings under COupled environmental STRESSes – COCOSTRESS

COCOSTRESS

Polymeric COatings under COupled environmental STRESSes

Influence of the structure of an epoxy network on the hygrothermal ageing and the coupling phenomena

Improved estimation of the anti corrosion polymer coating durability <br />The objectives are double: <br />- Revealing of the influence of the epoxy film structure about its physico-chemical and mechanical properties, and about the absorption parameters. <br />- Better understanding of the unfilled polymer coating degradation mechanisms subjected to coupling of various ageing factors (water, temperature, mechanical stress). <br />

- The preparation and the chemical, physico-chemical and mechanical characterization of film and coating in the initial state and in the aged condition.
- The determination of the relationship between the physico-chemical and mechanical parameters determined by various techniques.
- The establishment of original experimental protocols including in particular electrochemical measurements.
- The identification of the synergic effects between the various ageing factors
- The obtaining the laws of behavior of the model polymer coating during their degradation in monotonous or cyclic conditions.

The DGEBA / TETA and DGEBA / DAMP free films were realised after stoichiometric study and presents a maximum conversion rate. The chemical nature of the films is determined by FTIR. Then, the initial physico-chemical and mechanical properties are characterized (Tg, Young's modulus, free volume fraction, elastic, viscoelastic, viscoplastic domains, activation volume). Significant differences were observed between both systems. As for coatings, we control the sample elaboration, but no initial characterization was made.
DGEBA / TETA and DGEBA / DAMP free films were then immersed in ultrapure water and NaCl 3wt% solution at different temperatures (30, 40, 50, 60°C). In view of results, we decided to continue the study only with ultrapure water. The absorption and desorption curves obtained were modeled to extract the following parameters: diffusion coefficients and water mass content at equilibrium. The diffusion coefficients of DGEBA / DAMP films are higher than those of DGEBA / TETA films, reflecting a more rapid diffusion kinetic, although the water equilibrium content are lower. So we highlight the network structure effect on the diffusion properties during hygrothermal aging. In addition, it seemed appropriate to make two absorption / desorption cycles on the DGEBA / DAMP system. The first and second absorption exhibit different behavior, unlike the desorption. Furthermore, during the first cycle, the film water uptake does not follow a purely Fickian law contrary to what we expected. So we decided to realize sorption isotherms to help us in the sorption modeling. The Tg of DGEBA / TETA films was followed by DSC and the obtained values show a system plastification during immersion.

Sorption isotherms are in progress. This should help us in the modelling of the sorption phenomena.
Tg evolution of DGEBA / DAMP films is followed by DSC during sorption cycles. In addition, we have to see the evolution of the mechanical domains (EL / VE / VP) and wet activation volumes.
Finally, it plans to carry out permeation tests (4-electrode electrochemical assembly) on free films and aging tests on coating.

- “Analysis of the non-ideal capacitive behavior for high impedance organic coatings”, Geoffrey BOUVET, Dan NGUYEN DANG, Stéphanie MALLARINO, Sébastien TOUZAIN, Progress in Organic Coating, soumis.
- 2 international colloquia

The main environmental factors of organic coating degradation are well-known: water, temperature, UV, mechanical stress. Within the framework of sustainable development of metallic structures, formulators consequently use natural or accelerated ageing tests to estimate the life time of the coatings. These tests, either monotonous or, nowadays commonly cyclic, impose ageing conditions including generally two or three ageing factors but never simultaneously the four parameters cited above. Moreover, these approaches are mainly based onto ISO standards which are usually far from fundamental physic. The applied mechanical stress generally places the polymer in a plastic deformation state which is remote from the usual service conditions (viscoelastic domain).
In a recent study, we applied a visco-elastic stress to the coating in order to conserve its intrinsic properties. We showed that the simultaneous application of the four ageing factors modified in an important way the life time of the organic coatings. So, and for the first time in our knowledge, synergies of coupling are clearly identified. However, the complexity of the coatings formulations does not allow to correctly define the constitutive equations of the physico-chemical behaviour. Indeed, the paint includes charges and internal stresses may develop during the ageing. Furthermore, the polymer also contains various additives which probably interact with products and photo-products of degradation.
In order to pursue our research works, we wish to begin a more fundamental study by choosing a "simple", non-charged model polymer. The methodology previously used will be reproduced with the objective to reach the variations of the chemical, physico-chemical and mechanical parameters of the model polymer. First, we will characterize the model polymer, studied as a free film and as a coating applied onto a metallic substrate before ageing. Then, the influence of each ageing factor for the exposure will be clarified to determine the mechanisms of degradation. The various ageing factors will be first applied alone, then coupled between them gradually until the coupling of all of them. The various techniques of characterization (DSC, IRTF, DMA, SIE, mechanical tests) will then allow to reach the variations of the chemical, physico-chemical and mechanical parameters of the model material. So, constitutive equations of the physico-chemical behaviour will be described according to the environmental stresses applied alone or coupled. A complete mechanical study will be realized in order to determine the visco-elastic domain of the system before the ageing. For that purpose, we will perform simulations using finite elements analysis and the model will be adjusted with the constitutive equations of the physico-chemical behaviour.
Finally, the evidence of the synergies of coupling and the numerical modelling should allow us to refine the models of lifetime prediction of organic coatings. The expected results are situated upstream to a technological industrial research but the methodology and the conclusions of our study should be welcomed by the actors of the anticorrosion domain who are respectful of the new environmental recommendations.

Project coordinator

Madame Stéphanie MALLARINO (UNIVERSITE DE LA ROCHELLE) – stephanie.mallarino@univ-lr.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LEMMA UNIVERSITE DE LA ROCHELLE

Help of the ANR 183,040 euros
Beginning and duration of the scientific project: - 48 Months

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