IR-laser effects on pigments and paint layers

used for laser treatment of the samples. The beam delivery of the laser system 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 the energy density was defined. The colour measurement was done by a spectrophotometer (results: CIE- L*a*b* and specular reflectance). 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.

Table 1. Analysis techniques and instruments

3. Results and discussion

The analysis of some laser treated inorganic pigments by Environmental Scanning Electron Microscope showed, that the small pigment particles are partially molten together at the surface of the pigment. These molten layers are thin, approximately 1 micron or less. The melting of pigment particles at the surface 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 (TiO 2 ; 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 TiO 2 (white) → Ti 3 O 5 (blue, grey) + ½ O 2 ) 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 on the surface of the pigment. EDX analysis of the melted surface of some metal oxide pigments is pointing at reduced oxygen contents. A phase change as reason for discoloration was detected by XRD (by grazing incidence diffraction GID) for the pigment cinnabar only.

Fig. 1. Surface of titanium white pellet after laser irradiation at 600 mJ/cm² (magnification 10000x)

Fig. 2. Surface of zinc white pellet after laser irradiation at 600 mJ/cm² (magnification 16000x)

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 the crosspeak pattern in TOCSY spectra (total correlation spectroscopy, not shown here). Further many low-molecular reaction products are emitted, but have not been clearly detected yet. The presence of