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Theses

Using neutrons to protect our works of art

Abstract : Artworks do not last forever. They are composed of evolving materials which get altered over time. Indeed, the varnishes of easel paintings lose their initial visual appearance by deposition of dust and soot suspended in the air, by oxidation and by structural modification, e.g. the effect of free radicals. In order to reverse the evolution of these processes, paints are periodically restored with replacement of old varnishes. It requires the swelling and dissolution of the layer of polymeric varnish by solvents that can be penetrating and polar. These solvents can significantly alter the composition and cohesion of the original materials of the painting located under the varnish layers. It is therefore urgent to better understand the processes used in restoration operations, in particular when replacing varnishes. Conscious of this urgency, conservators and scientists propose to use aqueous gels/solvents to only transfer the quantities of solvents that are necessary for the dissolution of the varnish. The macroscopic images obtained before and after application of these gels demonstrate the efficiency of this method. However, the physical and chemical processes involved are still poorly understood. The aim of this thesis is to better understand the physics of polymer films in contact with solvent/non-solvent and solvent/gel mixtures, following the same processes used in the restoration of works of art. To study the physical processes of this method, we have used neutron beam reflection, in our case neutron reflectivity NR and small angle neutron scattering (SANS) combined with local visualizations, in particular optical microscopy and Atomic Force Microscopy (AFM). The polymer resin chosen is Laropal A81 (LA81), the solvent is benzyl alcohol (BA) and the hydrogel is Pemulen TR2. The NR allows to observe in situ the physical behaviour of ultrathin polymer films deposited on silicon blocks. This behaviour is happening during the kinetics of swelling by the good solvent then by the bad solvent followed by the dissolution and disappearance of the varnish film. The water used is D2O (W), which has a very high diffusion length density (SLD) compared to those of the LA81 and the BA. It is therefore possible to differentiate by their contrasts the spatial distributions of the BA and the D2O. The SLD of the LA81 was first determined. Then the thickness of the LA81 was modelled, as well as the roughness of the various elements. By using these thickness and SLD values, the volume fractions of the various components in the varnish film were calculated. The variations of these fractions of BA ϕBA and D2O ϕW which have penetrated into the films have been discussed as a function of the solvent concentration and the temperature. In parallel, a ternary phase diagram using Flory-Huggins (F-H) interaction parameters was plotted for the solvent/non-solvent system using NR results, intrinsic viscosity measurements, and turbidity measurements. The interaction parameters, were obtained by calculating the phase diagram using a F-H model. The understanding of the phenomena was deduced from the analysis of the NR spectra, supplemented by the observations made in optical microscopy and AFM. The AFM allowed to obtain the depth profile of the objects appearing in the film in the presence of the BA/W mixture. A SANS experiment on thick films (0.5-1.5 µm) gave spectra of intensities as a function of evolved time. These spectra were modelled according to Porod’s law. The interface area was plotted according to the values of ΧLA/BA. These results were supplemented by QCM-D measurements showing the kinetics of swelling and dissolution over hours and days after contact with the solvent/no-filler system. In conclusion, our experimental studies associated to the F-H model allowed a better and detailed understanding of the swelling and dissolution of thick and thin LA81 films deposited on silicon blocks and exposed to a solvent/non-solvent binary mixture.
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Submitted on : Wednesday, October 14, 2020 - 3:22:08 PM
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Amélie Castel. Using neutrons to protect our works of art. Chemical and Process Engineering. Université Grenoble Alpes [2020-..], 2020. English. ⟨NNT : 2020GRALI032⟩. ⟨tel-02966909⟩

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