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Nd-YAG laser irradiation of pigments and binders in paint layers

von Dipl.-Ing. Alexander Schnell (Autor) Lothar Goretzki (Autor) Christian Kaps (Autor)

Wissenschaftlicher Aufsatz 2003 5 Seiten

Ingenieurwissenschaften - Bauingenieurwesen


Nd-YAG laser irradiation of pigments and binders in paint layers

Alexander Schnell, Lothar Goretzki, Christian Kaps

Bauhaus-University, Department of Building Chemistry, Coudraystr. 13c, 99423 Weimar, Germany

1. Abstract

Laser cleaning of polychrome surfaces is currently problematic due to the fact, that laser irradiation can cause damage of the paint layers. A test program was worked out to analyse the chemical and physical background of these typical “pigment blackening effects”. The analytical methods ESEM, XRD, DTA, FTIR and NMR were used to describe the reactions of inorganic pigments and organic binding media caused by laser irradiation. The used laser system is commercially available, is currently used for the cleaning of natural stone surfaces and works at a wavelength of 1064 nm. The discoloration of pigments and paint layers was documented by colour measurement. Further the measurement of discoloration thresholds of energy density (of pigments and paint layers) was a main part of the research.

2. Introduction

The use of laser technique for the cleaning of natural stone surfaces has been established for several years. Nd-YAG laser ablation is mainly based on thermal effects (infrared laser light). The cleaning of polychrome surfaces is currently not suitable without problems due to the damage and/or discoloration of many pigments in paint layers, that can be caused by laser irradiation. In most cases the colour of these pigments is changing to black or grey. Known reasons are phase changes and different decomposition reactions. At some pigments these effects can be observed already at low energy densities. The objective of the research project is the cleaning of polychrome natural stone surfaces by laser ablation without any damage of the contained pigments and binders.

3. Experimental

Based on past investigations a test program was worked out to analyse the chemical and physical background of typical reactions for different pigments. Primarily historically relevant pigments and binders were chosen for the tests. Most of the 45 pigments were of inorganic origin. The binding media used for the painting of sandstone samples were linseed oil, casein and gum arabic as organic binders and lime as the only inorganic binder.

The laser effects were tested on pure pigments (pellets of 10 and 20 mm diameter; pressed under a low pressure of 1.. 3 kN/cm²) and on paint layers on sandstone samples. Within this study a Q-switched Nd-YAG laser at different energy densities (laser fluence) was used for laser irradiation of the samples. The laser system “Palladio” (Quanta System) works at a wavelength of 1064 nm with a variable pulse repetition rate from 1 to 15 Hz. The beam delivery is realised by a 7 mirror system integrated in an articulated arm. The diameter of the elliptical laser spot is approximately 7 mm.

For each sample the typical discoloration threshold of laser fluence was defined. The colour measurement was done by a MINOLTA spectrophotometer (results: CIE-L*a*b* values and specular reflectance spectra). Various analysis techniques were further used to describe the reactions of pigments and organic binding media caused by laser irradiation. A general view of the applied techniques and instruments is given in table 1.

Tab. 1: Analysis techniques and instruments

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4. Results and discussion

4.1. Effects of laser irradiation on pigments

The analysis of laser treated inorganic pigments by Environmental Scanning Electron Microscope showed, that the small pigment particles are partially melted together. These melted surface layers are thin, usually 1 micron or less. The melting of pigment particles at the surface (micro molten areas) could be observed on almost all tested pigments including pigments with high melting points. Examples are given in Figure 1 and 2 with the pigments titanium white (TiO2; melting point at 1855 °C) and zinc white (ZnO; melting point at 1975 °C). The colour of zinc white was changing to brown / grey, titanium white turned to a blueish grey. The discoloration can be caused by the chemical decomposition of these metal oxides (example: 3 TiO2 (white) Æ Ti3O5 (blue, grey) + ½ O2) or by physical effects (increasing particle size caused by the melting). The precise analysis of the discoloured material is difficult due to the thin modified layers. EDX analysis of the melted surface of some metal oxide pigments is pointing at reduced oxygen contents. Only in case of the pigment cinnabar a phase change as reason for discoloration was detected by XRD (by grazing incidence diffraction GID).

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Fig. 1 and 2. Surface of pigment pellets after laser irradiation at 600 mJ/cm² (left: titanium white at 10000x; right: zinc white at 16000x)

4.2. Effects of laser irradiation on binding media in paint layers

The gas phase developed during laser irradiation of different paint layers was analysed by NMR spectroscopy. For this tests, linseed oil paint layers containing pigments with low discoloration thresholds have been laser irradiated at high energy densities (~ 600 mJ/cm²) in an enclosed system. The reaction products were collected in a cooling trap, which was mounted to the system. Deuterized chloroform was used as solvent for H-NMR sample preparation. The proton spectra in Figure 3 shows, that glycerin, alkenes, alkanes and some aldehydes were detected in the gas phase. The signal at 9,6 ppm is evidently originated from acrolein. This is confirmed by a crosspeak pattern in TOCSY spectra (total correlation spectroscopy). Further many low-molecular reaction products are emitted, but have not been clearly detected yet. The presence of acrolein and other aldehydes points on a incomplete “combustion” of the volatile compounds.

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Fig. 3. H-NMR proton spectra of solved gas phase emitted from paint layer containing linseed oil as binding media

4.2. Discoloration thresholds of laser fluence

A main part of the test program was the documentation of the discoloration by colour measurement and the determination of thresholds of laser fluence for the pure pigments and paint layers containing these pigments combined with different binders. A statistical evaluation of the thresholds of the tested pigments and paint layers is given in Figure 4, where the “sensitivity” of the pigment or paint layer towards laser irradiation is structured in three groups. A surface with a threshold of more then 200 mJ/cm² is usual “easy to clean”, what means that the most encrustations are removable by laser cleaning above this energy density. Thresholds below 100 mJ/cm² are critical due to the fact, that laser ablation of dark crusts is usually not possible at this low energy density. Laser ablation tests on samples with artificial black gypsum crusts on polychrome sandstone samples showed, that these crusts can be actually removed at energy densities above 200 mJ/cm². It was further possible to remove crusts at lower energy densities (~ 100 to 200 mJ/cm², depending on the encrustation) if water was sprayed on the crust right before laser cleaning.

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Fig. 4. Percentage distribution of discoloration thresholds of laser fluence (related to all tested pigments and paint layers)

4. Conclusions

Reasons for discoloration of pigments are chemical decomposition reactions, phase changes or physical effects (melting of pigment particles). The binders contained in paint layers are less sensitive against laser irradiation. However the partially decomposition of linseed oil binders under laser irradiation has been detected by NMR. The emitted gases are cooled down very fast due to adiabatic expansion caused by the short laser pulses. Oxidation is prevented by the reducing atmosphere.

Comprehensive tests showed, that laser cleaning using Q-switched Nd-YAG lasers at a wavelength of 1064 nm is suitable for the cleaning of polychrome surfaces. The main requirement for this is the limitation of energy density depending on the characteristics of the certain pigment. Therefore a technical improvement of the laser cleaning equipment is desirable for the future. The first step to get a better controllable laser cleaning system could be an online control of the average laser energy and maximum energy per pulse. In combination with a constant spot size and a homogenised laser beam (flat top beam profile) there could be a real online control of the energy density (laser fluence). However there are a few pigments with very low discoloration thresholds. Polychrome surfaces containing these pigments can not be cleaned by laser ablation.


The Thuringia Ministry for Science, Research and Art (TMWFK) in Erfurt / Germany is gratefully acknowledged for funding the research project B509-01013.


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Titel: Nd-YAG laser irradiation of pigments and binders in paint layers