The colorimetric analysis performed in May 2019 (after twenty-two months from the loan) highlighted two different colors of the humanoid mask. As reported in Fig. 5, two clusters were recognized for the pale pink and yellow regions (for sampling spots and coordinate values, see Additional file 1: Fig. S2 and Table S1), which showed ΔE* values of 1.6 and 2.5 respectively, after 5 months of monitoring. These variations indicate an ongoing color changing progress meanwhile the mask was stored in laboratory conditions (ca. 22 °C, 40–60% RH, mostly kept in the dark and for short times exposed to 250 lx). PUR elastomer, especially polyether- and aromatic isocyanates-based, is known to be very sensitive to light [19, 37,38,39]. In the SAYA mask case, the points uncovered by the cloths and wig yellowed with time. A clear color change trend was not observed for the other spots, highlighting the inherent complexity of the PUR elastomer degradation and its related discoloration over time.
Microscopic and SEM–EDX investigations
The appearance of the pale pink core differed from the yellow surface of the mask. In Fig. 6a, the pinkish part appears smooth, homogeneous and translucent, with only a few fibers attached on it. Instead, the yellowish surface (Fig. 6b) appears dirty with several soiling particles. The stickiness, a degradation phenomenon also observed in plastics like PVC, is likely caused by the migration of plasticizers to the surface, mainly in the liquid form such as bis(2-ethylhexyl)phthalate [39, 40].
The observation of the sample under SEM emphasized the two different morphologies between the consistent pale pink core (P) and heterogeneous yellow surface (Y) (Fig. 6c). The EDX analysis identified mainly carbon and oxygen for the pale pink area. Whereas the yellowed surface is characterized by further elements as magnesium (Mg), aluminium (Al) and silicon (Si), indicating the likely presence of aluminosilicates adhering to the sticky surface.
Raman microscopy (µ-Raman)
Only whitish particles were observed within the PUR elastomer under the coupled microscope, identified as Pigment White (PW) 6 (titanium dioxide, TiO2, C.I. n. 77891) as rutile and anatase crystalline form by µ-Raman. As depicted in Fig. 7, overlapping signals from the two crystalline Raman signatures were detected. In the Raman spectrum, three bands of rutile (R) at 226 (multi-photon process), 441 (Eg) and 614 (A1g) cm−1 are observed, while only the most intense Raman-active optical phonon modes at 141 (Eg) cm−1 characterize the presence of anatase (A) [41,42,43,44]. Pigments based on TiO2 are the preferred white pigments used for polymers, including PUR . The detection of both TiO2 crystalline forms reveals the use of this polymorphic mixture for the mask’s pigmentation. The photochemistry of TiO2 has been subject of detailed research . Studies have demonstrated how its presence could influence the photo-stability of several polymers [47,48,49,50,51]. In coloring plastics, the different photo-active properties of the two crystalline forms are well known . Chen et al.  demonstrated the strong photo-sensitizing effect of anatase and, on the contrary, the photo-stabilizing properties of rutile against photo-oxidation of PUR under UV irradiation. The presence of the crystalline form anatase may impart a greater susceptibility of the elastomer to photo-oxidation.
No other pigments were identified in the PUR sample. A strong fluorescent background characterized the degraded polymeric matrix, limiting the µ-Raman investigation and probably detecting the red component of the pink color. However, a possible composition of the pink pigment could be a mixture of TiO2 with iron oxides, Pigment Red (PR) 101 and Pigment Yellow (PY) 42 (and traces of Pigment Black (PBk) 7, carbon black), as reported in the safety data sheet of the pink pigment produced by the Japanese manufacturer of the PUR elastomer . These pigments are known in the literature for the coloring of PUR . The inorganic pigments used for the pink coloring could have been dispersed in small quantities and are known for having a small size (10–30 µm) , which made their identification in polymeric matrix a challenge. No differences between the pale pink core and the yellow surface were observed.
Figure 8 shows the spectra of the pale pink core and the yellow surface of the mask. The band assignments are reported in Table 2. Similar absorption bands characterize the two samples, with the characteristic stretching vibration C–O–C at 1100 cm−1 of the polyether-based PUR [18, 39, 55]. Other typical features are: (1) the bands at 2966 cm−1, 2928 cm−1 and 2870 cm−1 of CH3 and CH2 stretching; (2) 1727 cm−1 band of the urethane C=O group; (3) 1530 cm−1 band of the N–H bend vibration with C–N stretch vibration in the –C–NH group (amide II band); 4) 1273 cm−1 band of the C–O stretch [19, 38, 52,53,54,55,56,57,58]. However, the typical band between 3340 and 3320 cm−1 of the stretching vibration of N–H group is weak. This could indicate the loss of the urethane structures due to UV exposure [19, 59] or the low amount of urethane bonds in the plastic formulation .
Besides the characteristic IR bands of the PUR elastomer, slight differences were recognized between the pale pink core and yellowed surface, as highlighted by the negative and positive intensities in the spectrum (Fig. 8). Especially for the yellowed surface, the band at 3674 cm−1 of hydroxyl stretching band and the peaks at 1039 cm−1 and 1015 cm−1 ascribed to in-plane Si–O vibrations, could indicate the presence of aluminum silicates (e.g. kaolinite) on the surface as attached dust particle , which confirms the EDX analysis (dust presence). Moreover, when compared with the bands of the pale pink core, the spectrum of yellowed sample denotes a slight decrease of intensity of the following bands: (1) C–H stretch vibrations in the 3000–2800 cm−1 region attributed to the vibrations of the methylene groups; (2) the urethane C=O stretch free at 1727 cm−1; (3) the band at 1100 cm−1 of the C–O–C stretching vibrations of the ether group (Additional file 1: Fig. S3) [19, 37]. These changes can be correlated with the beginning of the production of quinone structures, which led to yellowing . It is known that the aromatic structures of PUR are oxidized by UV radiation in the central methylene group, resulting in conjugated quinones and, therefore in the formation of colored products and loss of physical and mechanical properties [10, 19, 37].
Both spectra (pale pink core and yellowed surface) present characteristic absorption bands of phthalate. As shown for the reference spectrum of di(2-ethylhexyl)phthalate (DEHP) (Fig. 8, blue line), the main absorptions are: 1727 cm−1 (ν C=O of O=C–O), 1600 cm−1 and 1580 cm−1 (ν C=C of benzene ring), 1464 cm−1 (ν C–H), 1273 cm−1 (ν C–O of O=C–O), the shoulder at 1077 cm−1 (ν C–O of O–CH2) and 744 cm−1 (ν C–H) [15, 39, 60, 61]. The band at 744 cm−1, used to discriminate the presence of phthalate from the polymeric matrix, resulted slightly increased in the yellowed surface compared to the pale pink core, a signal of the plasticizer migration to the surface (Additional file 1: Fig. S4). No FTIR signals correlated with the presence of pigments were observed in PUR samples, probably because of the low pigment concentration, well below the detection limit of the technique (around < 3%) .
The spectrum of the glue found on the reverse side of the mask is also shown in Fig. 8. The absorbance bands are comparable to those reported for the mask samples, which determine its composition as a PUR ether-based adhesive. Further bands are at 3294 cm−1 (stretching vibration of N–H), 1534 cm−1 (amide II band) and at 1224 cm−1 (amide III) [19, 37, 63]. Moreover, the main absorption bands for phthalate were also identified at the glue’s surface. The Japanese team probably decided to use a PUR glue chemically compatible with the PUR elastomer. PUR adhesives are extensively used in several applications for their good adhesion on many substrates, good toughness, excellent flexibility, durability, resistance to water, and a broad range of chemicals [64,65,66]. PUR is known to form hydrogen bonds and covalent bonds when active hydrogen is present . As described in the previous section (see The SAYA mask), the areas with an internal layer of glue have a pale pink color similar to the original one. This fact could suggest the positive effect of the glue to act as a barrier against oxygen, slowing down the yellowing process that characterizes the other areas of the mask. Further research is needed to investigate this aspect.
The spectrum of the sample from the skull revealed to be made of glass fiber reinforced polyester, as presented and discussed in Additional file 1: Fig. S5 and Table S2.
Figure 9 (top) shows the EGA-MS thermograms of the pale pink core and the yellow surface. For both samples, two thermal decomposition zones are identified: zone 1 from 160 to 330 °C and zone 2 from 330 to 500 °C. The first band describes the evolved gas emitted by additives, with the most abundant ions in the mass spectrum of m/z 43, 57, 71, 85, 127, 149 167, 279 and 293 (Fig. 9a, c). These ions suggested the presence of phthalates used as plasticizers, in particular diisononyl phthalate (DINP, quantitative ion m/z 293, confirmation ions m/z 149, 267) and DEHP (quantitative ion m/z 279, confirmation ions m/z 149, 167) [67, 68].Footnote 2 The second band is related to the pyrolysis of the PUR, characterized by the ions m/z 43, 59, 73, 91, 104 and 117 (Fig. 9b, d). La Nasa et al.  and Neumann et al.  reported that these fragments are attributable to the soft segments of polyether polyols of PUR. The presence of the hard segment of PUR, whose volatilization presumably occurred between 250 and 300 °C , was not identified probably due to the too intense signal of the additives.
Focusing on the mass spectra, the thermal zones of the two samples are similar (Fig. 9a–d). However, their EGA profiles present differences. While the temperature of the second step of the polymer did not change with a peak centered at 404 °C, the temperature of first step related to the evolution of additives slightly shifted from 268 °C for the pale pink core to 273 °C for the yellowed surface. This shift could be due to the two main phthalates present in different ratios in the bulk and at the surface (i.e. their different speed of migration towards the surface or their different evaporation rates). Moreover, the ratio between the intensities of the bands changed: the pale pink inner core sample is characterized by a less intense band of additives than that of the polymer; the yellowed surface sample, instead, presents a more intense additive band and a reduction of the band related to the polymer. These variations could be due to the mask aging, especially regarding the migration of the plasticizer and, its consequent enrichment to the surface of the mask, causing the stickiness phenomenon . Plasticizers in the formulation were necessary to create soft and elastic PUR elastomer material to reproduce the human skin.
Figure 10 shows the volatile organic compounds detected with the first step of the analysis (TD-GC/MS) of the pale pink core and the yellow surface. The peak identification is reported in Table 3. The presence of the phthalates DEHP (n° 19) and DINP (n° 21) as plasticizers was confirmed and traces of diethyl phthalate (DEP, n° 12) and dibutyl phthalate (DBP, n° 16) were also identified. Moreover, the UV stabilizer 2-(2′-hydroxy,3′,5′di-tert-amylphenyl) benzotriazole (Tinuvin 328, n° 20) was detected, as well as the principal components of the polymer, e.g. propylenglycol (n° 3), diisopropyl ether (n° 4) and methylene diphenyl diisocyanate (MDI) (n° 17). The identification of MDI supports the finding of FT-IR analysis and corroborates the photo-oxidation of the PUR mask and, therefore, its visible yellowing . Besides PUR, specific fragments of another polymer were recognized: small peaks, which became more intense in the pyrograms, related to styrene-acrylonitrile (SAN, n° 6, 10, 11, 13–15, 18) were identified. The presence of SAN is discussed later in the text.
Considering the results of the pale pink core and the yellowed surface, a relative increase of the abundance of the main additives DEHP (n° 19), Tinuvin 328 (n° 20) and DINP (n° 21) was detected on the yellow surface (see Additional file 1: Table S3). These results confirm those obtained from the EGA-MS analysis regarding the additive migrations to the mask´s surface. This degradation process is already well known for plastics with a high amount of additives, such as PVC [39, 72, 73].
Figure 11 shows the pyrograms of the mask obtained in the second step (Py-GC/MS analysis) at 600 °C. The relative peaks are reported in the Table 3. Diisopropyl ether (n° 26) and 2-propanone, 1-(1-methylethoxy)-(n° 27) were the main compounds from the PUR polymer. All the other fragments, instead, belonged to the pyrolysis of the SAN amount, in particular: acrylonitrile (n° 23), styrene (n° 29), α-methylstyrene (n° 31), 4-phenylbutyronitrile (n° 33), SA: 4-phenylpent-4-enenitrile (n° 34), 2-methylene-4-phenethylpentadinitrile (n° 35), 2-methylene-4-phenylheptanedinitrile (n° 36), 2-(2-phenylallyl)pentanedinitrile (n° 37) and 2-phenethyl-4-phenylpent-4-enenitrile (n° 38) . The presence of SAN in PUR museum objects was already observed by Izzo et al. , as an additive capable of improving the mechanical properties of PUR polymers . Its presence, however, could have also influenced the photo-stability of the PUR elastomer negatively. SAN is known to turn yellow in color when exposed to light and oxygen due to photo-oxidative degradation of the polystyrene component [75, 76].
When comparing the two sample’s compositions, no significant differences were detected. This could mean that the degradation process of PUR mask is still at an initial stage and has not yet caused evident chemical alterations in the polymeric matrix composition.
Assessment of the adhesive treatment effectiveness
The results of the adhesion tests are summarized in Table 4. The cellulose-based adhesives gave good results on SAYA PUR elastomer, with the exception of Methocel A 15 VL, while not good adhesion was found for the PP mannequin. The best results were obtained with the wheat starch paste and PU 52 in both supports. The PUR dispersion was the unique adhesive with good flexibility. Because of its good adhesion properties and chemical compatibility with the mask’s composition, a complete treatment of the tears was suggested to the museum staff as adequate.