Os aglutinantes da pintura contemporânea em Portugal e os seus problemas de conservação

Acabou de ser livremente disponibilizada a seguinte tese de doutoramento:

Joana Lia Ferreira, Liaisons dangereuses, conservation of modern and contemporary art: a study of the synthetic binding media in Portugal, Lisboa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2011.

Está aqui.

Resumo:

This project is focused on the study of synthetic materials used by Portuguese artists in the twentieth century. The aim of the study was the molecular characterization of vinyl and acrylic polymers used since the 1960s and the study of their photodegradation and evolution over time. In modern and contemporary art, water based synthetic emulsions have been widely used as painting binding media, especially acrylic and vinyl based emulsions. In some countries like Portugal where the first aqueous emulsions used as paint binders were the poly(vinyl acetate) – PVAc – ones, both as household and artist’s paints, the study of vinyl polymers is of particular importance. In Portugal these emulsions were used by outstanding artists like Joaquim Rodrigo and Ângelo de Sousa since the 1960s. In order to guarantee the preservation of the artworks for the future generations it is essential to understand how these paints will behave upon ageing. The molecular photodegradation of PVAc was studied and the presence of degradation products on reference and model samples, as well as on dated artworks was investigated. Paintings by Joaquim Rodrigo and Ângelo de Sousa, and also a hand painted catalogue for the Sabu vinyl artists’ paints, were selected as case studies. The characterization of micro-samples from the artworks provided useful information on the real evolution of the polymer binder over time. Accelerated photodegradtion studies were performed by the exposure of PVAc films on an ageing chamber equipped with a xenon-arc lamp. The behaviour of PVAc (homopolymer) was compared with a commercial emulsion (Vulcano V7) and paint reconstructions prepared in the laboratory. The results achieved by means of infrared microspectroscopy (FTIR) point out the considerably stable nature of these binding media as no molecular evidence was obtained concerning the formation of other carbonyl functions, the disappearance of the PVAc carbonyl or the formation of hydroperoxides. The polymer photodegradation process was further studied by means of size exclusion chromatography (SEC), following the molecular weight distribution and correspondent average molecular weights. It was concluded that no side-chain reactions are taking place and that main-chain scission is the foremost degradation mechanism, although not affecting the polymer performance significantly. The influence of pigments, such as titanium dioxide and iron oxide, on the polymer’s stability was also considered and it was concluded that they do not promote degradation, which might be explained due to pigment encapsulation in inert materials inhibiting its photocatalytic behaviour. The photodegradation quantum yield was determined as 7.4 x 10^-8 at 313 nm for PVAc homopolymer; therefore, degradation is quantified and comparisons may be performed for different polymers. A parallel study, according to a similar experimental approach, was undertaken on the stability of poly(methyl methacrylate) – PMMA – used as acrylic sheet by the Portuguese artist Lourdes Castro.