Švarcová Silvie, Kočí Eva, Bezdička Petr, Garrappa Silvia, Kobera Libor, Plocek Jiří, Brus Jiří, Šťastný Martin, Hradil David
Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-ŘeŽ 1001, 250 68 Husinec-ŘeŽ, Czech Republic.
Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Praha 6, Czech Republic.
Dalton Trans. 2020 Apr 28;49(16):5044-5054. doi: 10.1039/d0dt00327a.
Lead carboxylates are an extensive group of compounds studied for their promising industrial applications and for their risky behavior when they are formed in oil paintings as corrosion products of lead-based pigments, leading to serious deterioration of paintings. Although the processes leading to the formation of aggregates, protrusions or inclusions, affecting undesirably the appearance of paintings, are assumed to be long term, neo-formed lead carboxylates are detectable in the early stage of paint drying. To uncover the chemical changes in lead pigments during the drying of oil paint films, model systems consisting of minium (Pb3O4) and four common drying oils were studied by X-ray powder diffraction (XRPD), 13C and 207Pb solid state NMR (ssNMR) spectroscopy and Fourier-transformed infrared spectroscopy (FTIR). For the first time, a degradation mechanism of Pb3O4via the crystallization of lead formate (Pb(HCOO)2), at the end of oxidative polymerization of oil paint films, was uncovered. The formation of formic acid in oils was proved by gas chromatography-mass spectrometry (GC-MS). Vapor experiments evidenced the susceptibility of Pb3O4 to react with volatile formic acid released during the autoxidation of oils comparably to the direct pigment-binder interactions in paint films. The investigation of the local environment of lead atoms in the paint film by 207Pb WURST-CPMG NMR spectroscopy showed that Pb(ii) atoms reacted with linseed oil preferentially to form highly crystalline Pb(HCOO)2, while the local chemical environment of Pb(iv) atoms did not change. The results proved the co-existence of (i) highly crystalline Pb(HCOO)2, (ii) a highly mobile amorphous phase corresponding to free carboxylic acids or a nascent lead soap phase and (iii) the remaining Pb3O4 in the polymeric/ionomeric network. Pb(HCOO)2 is assumed to be an intermediate for the conversion of Pb3O4 to lead soaps and/or lead carbonates.
羧酸铅是一类广泛的化合物，因其具有潜在的工业应用价值而受到研究，但当它们在油画中作为铅基颜料的腐蚀产物形成时，却具有危险性，会导致画作严重受损。尽管导致聚集体、凸起或内含物形成从而对画作外观产生不良影响的过程被认为是长期的，但在油漆干燥的早期阶段就能检测到新形成的羧酸铅。为了揭示油画膜干燥过程中铅颜料的化学变化，通过X射线粉末衍射（XRPD）、13C和207Pb固态核磁共振（ssNMR）光谱以及傅里叶变换红外光谱（FTIR），对由铅丹（Pb3O4）和四种常见干性油组成的模型体系进行了研究。首次发现了在油画膜氧化聚合结束时，Pb3O4通过甲酸铅（Pb(HCOO)2）结晶而发生降解的机制。气相色谱 - 质谱联用（GC - MS）证明了油中甲酸的形成。蒸汽实验表明，与漆膜中直接的颜料 - 粘合剂相互作用类似，Pb3O4易于与油自氧化过程中释放的挥发性甲酸发生反应。通过207Pb WURST - CPMG NMR光谱对漆膜中铅原子的局部环境进行研究表明，Pb(ii)原子优先与亚麻籽油反应形成高度结晶的Pb(HCOO)2，而Pb(iv)原子的局部化学环境没有变化。结果证明了（i）高度结晶的Pb(HCOO)2、（ii）对应于游离羧酸或新生铅皂相的高度可移动无定形相以及（iii）聚合物/离聚物网络中剩余的Pb3O4共存。Pb(HCOO)2被认为是Pb3O4转化为铅皂和/或碳酸铅的中间体。
