Altobelli, Annarita (2026) Hydrogel Green Gellan/γ-valero lactone for the Removal of Paraloid B72: Rheological Design and Applications in Conservation of Cultural Heritage. [Tesi di dottorato]

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Abstract

This doctoral thesis is situated within the broader framework of the green transition in cultural heritage conservation, which calls for a reduction of the environmental impact and health risks associated with conservation materials and processes, without compromising their technical effectiveness or compatibility with artworks. In this context, the controlled removal of aged Paraloid B72, an acrylic copolymer widely used as consolidant and protective varnish, represents a critical challenge, as long term ageing leads to reduced solubility, partial reversibility and the need for aggressive, difficult to control solvent mixtures. The research investigates the development of “green” hydrogels based on gellan gum, a natural origin biopolymer, loaded with the bio based solvent γ valerolactone (GVL) for the controlled removal of aged Paraloid B72 from substrates relevant to cultural heritage. The main objective is to quantitatively correlate the formulation and rheological/chemorheological properties of these hydrogels with their solvent release behaviour, cleaning efficacy and penetration into the substrate, in line with the principles of green chemistry and preventive conservation. After reviewing the literature on synthetic polymers used in conservation (including B72 ageing and removal methods as well as biopolymer hydrogels and green solvents) a series of gellan/GVL hydrogels were prepared by varying polymer concentration, solvent content and gelation conditions. The formulations were characterised using shear oscillatory and rotational rheology, chemorheology and rheology based 3D printing to map sol–gel transitions, structural stability and “easy peelability” as a function of composition. The cleaning performance was evaluated on laboratory specimens coated with aged Paraloid B72 through controlled protocols and a multi technique diagnostic approach, which include microscopy, spectroscopy, colour measurements and wettability tests. The solvent mass transfer was studied by diffusion cells, low field NMR and complementary methods to quantify GVL penetration and retention. The results show that suitably designed gellan/GVL hydrogels exhibit favourable viscoelastic properties for application and peelable removal and are capable of effectively softening and removing the superficial B72 layer. This can be performed with a reduced use of free solvents, limited colour changes and substrate alterations. Diffusion data indicate a significant reduction in solvent penetration depth compared to non gelled systems, highlighting an improved balance between acrylic film solubilisation and control of solvent action. Overall, the thesis demonstrates the feasibility of a rheology-driven and green chemistry-oriented design strategy for gellan based hydrogels aimed at the removal of aged acrylic coatings and provides conceptual and practical guidelines for the development of more controlled and sustainable cleaning systems in conservation.

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