SEM micrographs of PU1 (Figure 1) showed that the surface features were irregular, rugged and on the scale of several hundred micrometers. Many micrometer-sized wrinkles and, especially, cracks were also observed along with pigments and dust. It has been shown that whitened PVA becomes transparent when it is coated in water.6 This suggests that cracks in the PVA surface cause the whitening. Light scattering from the surface of a material is known to occur when the refractive indices are different for materials on either side of an interface. The light scattering intensity is also dependent on the magnitude of the refractive indices and size of the scattering material. The refractive index difference for PVA and air is large when the air is enclosed in a crack in PVA. This enhances the light scattering intensity. However, if the air is replaced with water, the scattering intensity becomes less intense because the refractive index difference for water and PVA is small. In this case, the difference between the refractive indices at the water–PVA interface decreases as the PVA swells because of the water. This means that less light will be scattered as time progresses. The micrometer-sized cracks in PU1 can efficiently scatter all visible light because their size is slightly larger than the wavelength of visible light.
Figure 1Scanning electron microscopy images of PU1: (a) magnified 80 times and (b) magnified 450 times.
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The FT-IR–ATR spectra of PU1 and PVA reference films are shown in Figure 2. Peak assignments are listed in Table 2.9, 10, 11, 12, 13, 14, 15, 17 PU1 was obtained from a painted wooden panel that had been restored in the 1950s, first with PVA to improve paint adhesion and then with acrylate resin (that is, PMMA) on top of the PVA layer to improve water resistance.2 The acrylate resin was removed using organic solvents when re-restoration was carried out at a later date.
Figure 2Fourier transform-infrared spectroscopy–attenuated total reflection spectra of PU1 and reference samples. PMMA, poly(methyl methacrylate); PVA, polyvinyl alcohol.
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Table 2 Assignment of the FT-IR–ATR characterization of the PVA samplesFull size table
The spectrum of PU1 (Figure 2) shows small peaks at 1245, 1265 and 1380 cm−1. These can be attributed to the acetyl groups in the unsaponificated PVA polymer chain or in acrylate resin (PMMA), which may have remained on the surface after the re-restoration.
The absorption at 1730 cm−1 was assigned to a C=O stretching vibration and could be attributed to the carbonyl group in acrylate resin, which is formed by the degradation of PVA. The absorption at 3000–3500 cm−1 corresponds to the O–H stretching vibration and hydrogen bonds, whereas that at 1320 cm−1 corresponds to C–H and O–H bending vibrations. Both of these peaks were smaller than those in the reference samples, which implies that interchain dehydration occurs in the PVA obtained from the painted wooden panel. The peaks at 1040 and 1170 cm−1 may be attributed to the C–O ether bond of the PVA polymer chain. The strong absorption at 1650 cm−1 could be assigned to the C=C stretching vibration. It is known that C=C bonds are formed by intrachain dehydration during thermal and photodegradation.10, 16
Akhter et al.13 reported that PVA degrades through the routes depicted in Figure 3, and dehydration and crosslinking may occur in PVA chains.13, 16 The aforementioned PU1 peaks are consistent with the result by Akhter et al.; thus, three-dimensional chemical networks associated with dehydration and crosslinking can be formed on the PU1 surface, and curing of the surface occurs simultaneously. It is known among conservators of Shohekiga that whitened PVA becomes insoluble in water; therefore, our guess coincides with this fact.
Figure 3Schematic diagram of polyvinyl alcohol degradation routes.
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On the other hand, animal glue has been used as a traditional binder of pigments in Japanese paintings. The elasticity of animal glue can vary significantly by changes in humidity.18 PVA was coated on a color layer that contains animal glue. Hence, we hypothesize that wrinkles and cracks can be formed on the PVA surface easily by the difference in elastic properties between cured PVA and animal glue on the color layer. As a result, cracks on the PVA surface can cause whitening as described above.
A wide scan spectrum was obtained of the surface of PU1 (Figure 4) and partially magnified (Figure 5). The peaks in Figures 4 and 5 could be assigned to calcium, sulfate and silicon electron orbitals. These elements may be contained in the pigments and dust observed on the surface of the PVA by SEM. The appearance of calcium indicates the existence of CaCO3, which is widely used as a white pigment in Japanese paintings.
Figure 4X-ray photoelectron spectroscopy wide scan spectrum of PU1 (0–1100 eV).
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Figure 5X-ray photoelectron spectroscopy wide scan spectra of PU1 at (a) 300–500 eV and at (b) 0–200 eV.
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Figure 6 shows the C1s spectra of PU1 and reference PVA films (RA1 and RA2). A small peak at 289.1 eV in the spectrum of RA2 could be ascribed to acetyl groups in the unsaponificated PVA chain.11, 12, 19 The lack of a clear peak at 289.1 eV in the spectrum of PU1 indicates that it has a higher degree of saponification than RA2. However, the C1s band of PU1 at around 287–288 eV was more intense than in the reference PVA samples. These peaks are assigned to carbonyl or formyl groups, and this suggests degradation of PU1. On the other hand, dehydration proceeds on PU1 because the C–OH peak at 286.5 eV in the spectrum is smaller than those of RA1 and RA2. Finally, the C–O–C peak at 285.8 eV appears in the spectrum of PU1, which indicates chemical crosslinking of PVA chains. These results are in good accord with FT-IR–ATR measurements.
Figure 6X-ray photoelectron spectroscopy C1s spectra of PU1 and reference samples.
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A piece of PVA that had been used in the restoration of a historic Japanese painted wooden panel was obtained for analysis. SEM observation and functional group analysis by FT-IR and XPS were used to study the mechanism that caused whitening of this PVA piece. SEM showed that the surface of the whitened PVA thin film was irregular and rugged on the scale of several hundred micrometers and covered with many micrometer-sized cracks, pigments and dust. The whitening seemed to be caused by the scattering of all visible light by the cracks in the PVA. FT-IR and XPS measurements revealed that inter- and intra-chain dehydration and chemical crosslinking (C–O ether linkage) occur on the surface of whitened PVA; thus, it seems that three-dimensional chemical networks associated with dehydration and crosslinking can be formed, and curing proceeds on the whitened PVA surface.
Moreover, the animal glue used as traditional binder of pigments in Japanese paintings may be the source of PVA cracking. The elasticity of animal glue can vary significantly with changing humidity. PVA was coated on a color layer that contains animal glue. Hence, we believe that wrinkles and cracks can be formed on the PVA surface easily by the difference in the elastic properties between cured PVA and animal glue on the color layer. As a result, cracks on the PVA surface cause refraction of light, which gives the appearance of whitening.
The information in this paper provides a basis on which to investigate how to remove whitened PVA from the surface of paintings without any damage.