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Preparation and characterization of thin barrier layers
Blahová, Lucie ; Mráček, Aleš (referee) ; Buršíková, Vilma (referee) ; Krčma, František (advisor)
Combinations of different acrylic resins and microcrystalline waxes are most often used for the conservation and preservation of metallic archaeological artefacts these days. However, their properties are not sufficient and satisfactory in many ways. Therefore, the aim of this doctoral thesis is to develop a conservation system which will draw on the new knowledge gained in the field of advanced materials and technologies during last years. A conservation coating based on a thin barrier film appears the most promising. The conservation coating must fulfil particularly following rather contradictory requirements: good barrier function against oxygen, humidity and other corrosive agents; transparency because of colour appearance preservation; long term stability and easy removability; possibility to apply to more objects at the same time and reasonable financial requirements of a deposition process. Parylene C polymer was chosen as suitable material for this purpose. It was prepared via modified chemical vapour deposition. Parylene removability was ensured through the soluble interlayer made of Laksil silicone-acrylic lacquer which was applied between the protected metallic object and the parylene thin film. Initially, the deposition process of Laksil/parylene bilayer was optimized, then its physical and chemical characteristics were determined and eventually, they were compared with the conventional conservation coating composed of Paraloid B72 acrylic resin and Revax microcrystalline wax. Regarding to the demands of the conservation coating, we were interested especially in barrier properties, optical properties, surface morphology and removability of Laksil/parylene bilayer. The most useful method for description of coating barrier properties was corrosion testing in which coated metallic samples were exposed to highly corrosive environment of salt spray (made of 50 g•l–1 brine), 100% humidity and temperature of 35 °C (ISO 9227). The Laksil/parylene bilayer showed excellent barrier properties; samples treated this way sustained unchanged in the corrosion chamber for almost three months. The surface roughness measured by profilometry and surface morphology scanned by SEM illustrated the synergy between Laksil and parylene layer which leads to the exceptional barrier and anticorrosion function of bilayer. The Laksil layer is able to flatten out rough surface of a substrate. Furthermore, it can toughen the item surface which is, in case of a number of archaeological findings, created by corrosion product layers with variable mechanical strength. Thereon deposited parylene film becomes smooth and defect free and can act as a good diffusion barrier. The colouristic measurements confirmed that the Laksil/parylene bilayer does not almost change the appearance of protected item, in terms of colour. The caused colour difference (ISO 11664-4) ranged around 1 which value is defined as “perceivable only for experienced observers”. The Laksil/parylene bilayer is removable thanks to the Laksil solubility in xylene. It is necessary to carefully scratch the outer parylene film to enable access of xylene solvent to the lacquer. The successful removal of Laksil/parylene bilayer was first confirmed by the EDX on metallic iron samples. Afterwards, the possibility of bilayer complete removal even from a corroded surface of the original artefact was proved by TGA. Thus the conservation treatment can be considered fully reversible.
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Preparation and characterization of thin barrier layers
Blahová, Lucie ; Mráček, Aleš (referee) ; Buršíková, Vilma (referee) ; Krčma, František (advisor)
Combinations of different acrylic resins and microcrystalline waxes are most often used for the conservation and preservation of metallic archaeological artefacts these days. However, their properties are not sufficient and satisfactory in many ways. Therefore, the aim of this doctoral thesis is to develop a conservation system which will draw on the new knowledge gained in the field of advanced materials and technologies during last years. A conservation coating based on a thin barrier film appears the most promising. The conservation coating must fulfil particularly following rather contradictory requirements: good barrier function against oxygen, humidity and other corrosive agents; transparency because of colour appearance preservation; long term stability and easy removability; possibility to apply to more objects at the same time and reasonable financial requirements of a deposition process. Parylene C polymer was chosen as suitable material for this purpose. It was prepared via modified chemical vapour deposition. Parylene removability was ensured through the soluble interlayer made of Laksil silicone-acrylic lacquer which was applied between the protected metallic object and the parylene thin film. Initially, the deposition process of Laksil/parylene bilayer was optimized, then its physical and chemical characteristics were determined and eventually, they were compared with the conventional conservation coating composed of Paraloid B72 acrylic resin and Revax microcrystalline wax. Regarding to the demands of the conservation coating, we were interested especially in barrier properties, optical properties, surface morphology and removability of Laksil/parylene bilayer. The most useful method for description of coating barrier properties was corrosion testing in which coated metallic samples were exposed to highly corrosive environment of salt spray (made of 50 g•l–1 brine), 100% humidity and temperature of 35 °C (ISO 9227). The Laksil/parylene bilayer showed excellent barrier properties; samples treated this way sustained unchanged in the corrosion chamber for almost three months. The surface roughness measured by profilometry and surface morphology scanned by SEM illustrated the synergy between Laksil and parylene layer which leads to the exceptional barrier and anticorrosion function of bilayer. The Laksil layer is able to flatten out rough surface of a substrate. Furthermore, it can toughen the item surface which is, in case of a number of archaeological findings, created by corrosion product layers with variable mechanical strength. Thereon deposited parylene film becomes smooth and defect free and can act as a good diffusion barrier. The colouristic measurements confirmed that the Laksil/parylene bilayer does not almost change the appearance of protected item, in terms of colour. The caused colour difference (ISO 11664-4) ranged around 1 which value is defined as “perceivable only for experienced observers”. The Laksil/parylene bilayer is removable thanks to the Laksil solubility in xylene. It is necessary to carefully scratch the outer parylene film to enable access of xylene solvent to the lacquer. The successful removal of Laksil/parylene bilayer was first confirmed by the EDX on metallic iron samples. Afterwards, the possibility of bilayer complete removal even from a corroded surface of the original artefact was proved by TGA. Thus the conservation treatment can be considered fully reversible.
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