Abstract Naturally occurring diterpenoid resins were extensively applied, mainly as oily mixtures, through the ages as protective and decorative coatings on paintings, metals and wood artifacts. When these coatings age, tend to generate tougher films than triterpenoid resins, completely insoluble, increasingly subject to darkening, and then very hard to remove using conventional methods. In this regards, laser sub-micrometric ablative techniques are being increasingly used in the cultural heritage field also for the treatment of synthetic and natural polymer coatings. Here, in the wake of the positive outcomes achieved on triterpenoid resin films, the present approach has been applied and extended to diterpenoid resin coatings, which have never been thoroughly studied yet. In detail, colophony, sandarac and Manila copal resin films, prepared as solvent and linseed oil formulations, were subject to light-ageing and then systematically irradiated at various exposure conditions using the 4th (266 nm) and 5th (213 nm) harmonics of a Q-Switched Nd:YAG laser. UV–Vis absorption spectroscopy was used as preliminary characterization of the films optical properties. The assessment of physico-chemical modifications induced by artificial light-ageing and ns UV laser irradiation were assessed non-invasively by μ-Raman spectroscopy, Laser Induced Fluorescence (LIF) and microscopic examination. The results underlined that, due to the presence of a polymer network, sandarac and copal resin coatings showed higher Fth than those composed of colophony. Morphologically, all the coatings were subject to bubbling upon 266 nm irradiation, while damage-free at 213 nm. At molecular level, irradiation at 266 nm produced minor changes to ν(CH3)/ν(CH2), ν(C O) and ν(C C) modes, thus confirming an ablation mechanism mainly driven by photo-thermal bond-breaking through the ejection of gaseous by-products. Raman background fluctuations along with shifting and broadening of LIF maxima were supportive in the assessment of laser-induced surface modifications. Finally, the work performed indicates that the 213 nm wavelength is the most indicated for the treatment of aged solvent and oil diterpenoid-based coatings, as no side effects occurred. This outcome, corroborated by the previous results obtained in triterpenoid resin films, may have important implications in the field of cultural heritage conservation, as it extends the application range of solid-state Nd:YAG lasers to all types of protective and decorative terpenoid coatings.

中文翻译：

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使用微拉曼光谱和激光诱导荧光检测紫外激光烧蚀油/二萜树脂涂层中的分子变化

摘要 天然存在的二萜树脂被广泛应用，主要作为油性混合物，历来作为绘画、金属和木制品的保护和装饰涂层。当这些涂层老化时，往往会产生比三萜树脂更坚韧的薄膜，完全不溶，越来越容易变黑，然后使用传统方法很难去除。在这方面，激光亚微米烧蚀技术越来越多地用于文化遗产领域，也用于处理合成和天然聚合物涂层。在这里，随着在三萜树脂薄膜上取得的积极成果，本方法已被应用并扩展到二萜树脂涂料，但尚未对其进行彻底研究。详细地，松香、sandarac 和马尼拉柯巴树脂薄膜，制备为溶剂和亚麻籽油制剂，经过光老化，然后使用 Q 开关 Nd:YAG 激光器的第 4 次 (266 nm) 和第 5 次 (213 nm) 谐波在各种曝光条件下进行系统照射。紫外-可见吸收光谱用作薄膜光学性能的初步表征。通过μ-拉曼光谱、激光诱导荧光 (LIF) 和显微镜检查非侵入性地评估人工光老化和 ns 紫外激光照射引起的物理化学修饰。结果强调，由于存在聚合物网络，sandarac 和 Copal 树脂涂层的 Fth 高于由松香组成的涂层。从形态上讲，所有涂层在 266 nm 照射下都会发生鼓泡，而在 213 nm 处无损伤。在分子水平上，266 nm 的辐照对 ν(CH3)/ν(CH2)、ν(CO) 和 ν(CC) 模式产生微小变化，从而证实了消融机制主要由光热键断裂通过气体喷射产品。拉曼背景波动以及 LIF 最大值的移动和扩大有助于评估激光诱导的表面改性。最后，所做的工作表明，213 nm 波长最适合用于处理老化的溶剂和油类二萜类涂层，因为没有发生副作用。这一结果得到了之前在三萜树脂薄膜中获得的结果的证实，可能对文化遗产保护领域具有重要意义，因为它将固态 Nd:YAG 激光器的应用范围扩展到所有类型的保护性和装饰性萜类涂层。