Abstract
This chapter describes some recent studies and applications of photochemistry in the physical–chemical characterization of two acrylic paint materials based on phthalocyanines and the study of the photodegradation (photobleaching) processes which could occur, caused by exposure to artificial irradiation, similar as in the museum. The studies in this paper has been conducted on phthalocyanines, these compounds being known as organic colorants in painting. Their color depends not only on the chemical nature of the colorant, which play an important role in the kinetics and degree of aging, but also on the compounds added to the paints (TiO2, micas, arylamide yellow). The techniques used in such studies involve UV–Vis spectroscopy, gloss, and colorimetric measurements, comparing our results with similar ones from the literature.
Highlights
Photochemistry is the science that deals with the study of physical and chemical processes that arise from the interaction between radiation used for irradiation and absorbing molecules [1]
The excited Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in the Phthalocyanine molecules (Pc), which increase the formation of free radicals
The main processes that occur during the light action on paints are identified in Jablonski diagram (Figure 1): (a) Non-radiative processes, where the excited-state species release the excess of energy as heat by three different processes: vibrational relaxation (VR), through which the excited molecule decreases its vibrational energy within a single electronic state, internal conversion (IC); meaning a transition between two electronic states with the same spin multiplicity, generally followed by vibrational relaxation; and intersystem crossing (ISC), which involves the transition between two electronic states with different spin multiplicity, generally followed by vibrational relaxation
Summary
Photochemistry is the science that deals with the study of physical and chemical processes that arise from the interaction between radiation used for irradiation and absorbing molecules [1]. The light radiation together in the presence of oxygen induces photooxidative reactions on synthetic organic materials, mostly based on chain scission and cross-linking reactions of polymers used as binders in artists’ paints, such as acrylics [2]. The additives present in the paint formulations might interfere with the photooxidative reactions and could act by catalyzing and promoting or preventing photodegradation reactions, depending on their chemical properties and color [6]. The influence of different types of phthalocyanines, such as Phthalo Blue and Phthalo Green (Table 1), on the photostability of acrylic paints [7] is studied. The green phthalocyanines are copper-based phthalocyanines which are completely chlorinated (Pigment Green 7) or chlorinated and brominated (Pigment Green 36) (Table 1)
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