Removal of a synthetic soiling mixture on mastic, shellac & Laropal® K80 coatings using two hydrogels

This work investigates the effects of two hydrogels used for the selective removal of a synthetic soiling mixture on mastic, shellac and Laropal® K80 varnishes. The varnishes were spin-coated on aluminium tiles which were subjected to accelerated degradation in a xenon-arc fadeometer. A synthetic mixture, simulating urban pollution, was produced and applied on the degraded varnish films. The fabricated soil-resin film samples were further irradiated in the xenon-arc fadeometer. Subsequently, a CH3COONa hydrogel (pH=5.5) and a C23H39O2COONa hydrogel (pH=7.8) were produced using CH3COOH and C23H39O2COOH, NaOH, Triton X-100 and hydroxyl-propyl-methyl-cellulose. Both hydrogels were applied to the corresponding samples and the cleaning results were evaluated using UV-induced fluorescence imaging, FT-IR and Reflection Absorption Infrared Spectroscopy (RAIRS) and semi-quantitative data analysis. The experimental data indicated the removal of both the soiling layer as well as the partial removal of the uppermost layers of the degraded coatings. It is assumed that this phenomenon was caused by the penetration of the soiling layer into the substrates followed by partial ionisation and dissolution of the upper layers of the degraded films. A case study of surface cleaning of a 20th century panel painting from accumulated soils is also presented.