23 research outputs found

    A versatile method to fingerprint and compare the oxidative behaviour of lipids beyond their oxidative stability

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    : In this work we propose the use of isothermal thermogravimetry to evaluate the oxidative stability of a lipid and to evaluate how the glyceride composition affects the entire oxidative process, to quantify the oxidation undertaken by the lipid, and numerically compare the oxidative behaviour of different lipids. The innovative aspect of the present method lies in the acquisition of a prolonged "oxygen uptake" curve (4000-10,000 min) of a lipid under oxygen and in the development of a semi-empirical fitting equation for the experimental data. This provides the induction period (oxidative stability), and allows to evaluate the rate of oxidation, the rate and the magnitude of oxidative degradation, the overall mass loss and the mass of oxygen taken by the lipid upon time. The proposed approach is used to characterize the oxidation of different edible oils with different degrees of unsaturation (linseed oil, sunflower oil, and olive oil) as well as chemically simpler compounds used in the literature to model the autoxidation of vegetable oils and lipids in general: triglycerides (glyceryl trilinolenate, glyceryl trilinoleate and glyceryl trioleate) and methyl esters (methyl linoleate and methyl linolenate). The approach proves very robust and very sensitive to changes in the sample composition

    A Thermal Analytical Study of LEGO® Bricks for Investigating Light-Stability of ABS

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    Acrylonitrile butadiene styrene (ABS) is a thermoplastic polymer widely used in several everyday life applications; moreover, it is also one of the most employed plastics in contemporary artworks and design objects. In this study, the chemical and thermal properties of an ABS-based polymer and its photo-degradation process were investigated through a multi-analytical approach based on thermal, mass spectrometric and spectroscopic techniques. LEGO(& REG;) building blocks were selected for studying the ABS properties. First, the composition of unaged LEGO(& REG;) bricks was determined in terms of polymer composition and thermal stability; then, the bricks were subjected to UV-Vis photo-oxidative-accelerated ageing for evaluation of possible degradation processes. The modifications of the chemical and thermal properties were monitored in time by a multi-technique approach aimed at improving the current knowledge of ABS photodegradation, employing pyrolysis online with gas chromatography and evolved gas analysis, coupled with mass spectrometric detection (Py-GC-MS and EGA-MS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and corroborated by external reflection FT-IR spectroscopy. The multimodal approach provided new evidence on the two-step degradation pathway proposed for ABS, defining molecular markers for polybutadiene oxidation and styrene-acrylonitrile depolymerization. Moreover, the results highlighted the feasibility of correlating accurate compositional and thermal data acquired by bulk techniques with external reflection FT-IR spectroscopy as a non-invasive portable tool to monitor the state of conservation of plastic museum objects in-situ

    The chemistry of artists’ oil paints

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    Oil paintings from the 20th and 21st centuries present a range of conservation issues different from those of paintings realised in the previous centuries. These issues are believed to be correlated with the use of manufactured artists’ oil paints, which contained driers, fillers and stabilisers, modified oils, semi-drying oils and a range of pigments. The chemical nature of the changes occurring in the film former as a consequence of degradation is, in most cases, not fully understood at the molecular level. It has been hypothesised that some degradation phenomena taking place in oil paintings - especially those produced with modern manufactured artists’ oil paints - are related to the chemical nature of the oil cross-linked network, and in particular, to a low degree of polymerization and a high degree of oxidation. The polymeric as well as the highly complex multi-component nature of paint films do not allow a straightforward characterization of its molecular features. However, a useful approach to improve our understanding of the chemical structure of the paint film is the characterisation of the process leading to its formation: the curing. In this context, the principal aim of my Ph.D. project was to implement a methodological approach to monitor physico-chemical changes of oil paints upon curing and ageing. In particular, great effort was made to design a methodological approach able to characterise the effect of different oils, pigments and driers on the autoxidation kinetics, to monitor the evolution of oxidation products, to evaluate the magnitude of oxidative degradation and cross-linking phenomena, and to determine the structure of the cross-linked fractions

    Unravelling the effect of carbon black in the autoxidation mechanism of polyunsaturated oils

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    Carbon black-based particles are widely employed as pigment and they are known to slow down the drying time of oil paints. In this work, the effect of amorphous carbon black on the mechanism and speed of autoxidation of a polyunsaturated oil was investigated. Moreover, the effect of the addition of aluminium stearate and zinc stearate was studied. These are two common additives used in the artists’ paint industry to facilitate pigment dispersion. The curing of the oil paints with and without the addition of the two stearates at 80 °C under airflow was followed by isothermal Thermogravimetry. The oxygen uptake profiles were fitted by a semi-empiric equation to comparatively study the kinetics of the oil oxidation and estimate oxidative degradation. Moreover, model paintings were left to cure at ambient conditions and Differential Scanning Calorimetry was then used to monitor their curing progress over time and to evaluate the stability of peroxides formed in the paint layers. Gas Chromatography–Mass Spectrometry was performed at 7 and 12 months of natural ageing of the model paintings, to investigate the non-covalently cross-linked fractions. Analytical Pyrolysis coupled with Gas Chromatography–Mass Spectrometry was used to characterise the whole organic fraction of the model paintings, including the cross-linked network. Amorphous carbon has an antioxidant effect and inhibits the radical chain propagation. The presence of aluminium and zinc stearates in the black paint affects the autoxidation process, by leading, in the first months, to a faster consumption of unsaturated moieties, and, accordingly, to accelerate and increase peroxides formation. After a few months though, the whole curing slows down, and active peroxides and radicals are still present even after 12 months

    Disclosing the chemistry of oil curing by mass spectrometry using methyl linoleate as a model binder  

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    The structure of the polymeric fraction in an oil painting is believed to be strongly connected to the stability of the paint layers over time, but its molecular the characterisation is extremely difficult given the complex composition of a vegetable oil-based polymer. In this study, we report the implementation of a methodological approach for the systematic mass spectrometric investigation of the molecular features of the products of oxidative degradation and cross-linking of oil paint layers upon curing. The approach is based on the use of methyl linoleate as a simplified model of an oil paint binder. Gas-chromatography coupled with mass spectrometry, solid phase microextraction gas chromatography-mass spectrometry, flow injection electrospray mass spectrometry and evolved gas analysis mass spectrometry, are used to analyse the evolution of compounds produced over seven months of natural ageing, from the volatile products to the macromolecular and cross-linked fractions. The aim is to improve our fundamental molecular understanding of the curing process of oil paints, and to investigate the balance between oxidative degradation and cross-linking when specific binder-pigment combinations are in place. Model paint layers were prepared using lead white and ultramarine blue as pigments. These two pigments are known to produce paint layers with different stability over time. The use of methyl linoleate as a model oil binder greatly simplifies the mass spectral features of the lipid paint fraction, enabling the detection of products of oxidation and cross-linking with a new high level of molecular detail. Data clearly show that that crucial differences between paints containing the two pigments establish with time, which are mostly related to the cross-linked fraction
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