Assessment of chemical degradation of epoxy resin binder used in the service of the Qin terracotta warriors | Scientific Reports

Scientific Reports volume 14, Article number: 17572 (2024) Cite this article

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Since the Qin terracotta warriors were unearthed, polyamide 650 cross-linked E-44 epoxy resin binder has been employed to bond and restore them. In this paper, the chemical aging of the binders service in indoor natural environment during the past 30 years in the terracotta warriors was studied by means of infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. The results indicated that the binders did not emerge the characteristic peak of carbonyl stretching vibration at 1700 cm−1 in the IR spectra of all determined binders, and their thermal decomposition curves did not emerge any abnormal changes, and the thermal decomposition mainly occurred above 300 °C. There are evident ceramic grains attached to the surface of the binders being peeled off for sampling. These results that the binders service in the Qin terracotta warriors did not exhibit an observable chemical aging and still has strong adhesion. Generally, discrepancies were observed between natural aging and accelerated artificial aging due to the ineffectiveness of the latter to reproduce the effects of complex weather conditions. Compared to artificially accelerated aging, the evaluation results in a long-term natural aging of the binder which is used for restoration of the life-size Qin terracotta warriors, providing in the present investigation, are more reliable in terms of predicting the safety of restored terracotta warriors.

The world-famous Emperor Qin Shihuang’s Terra-cotta Army is an array of life-sized, realistic ceramic figures representing warriors, stationed in three large pits within the mausoleum of Qin Shihuang (259–210 BCE), the first emperor of a unified China. Over two thousand ceramic warriors have been excavated so far, and it is estimated that several thousand more remain buried1. According to the historical records, Emperor Qin palace and his mausoleum were burned in 206 BC. So the construction of the Pits was damaged during the fire. The collapsed roof pressed the terracotta warriors and horses into fragments (as showed in Fig. 1a). None of them was completed when unearthed. The most severely damaged terracotta figures contain as many as 200 pieces. Mending broken figures becomes a painstaking work for archaeological workers. Today, in the Mausoleum Museum of Emperor Qin Shihuang, all terracotta figures in the military array we see have been restored (as showed in Fig. 1b). As same as large stone carvings, conservation and reassembly of such large breakage objects are particularly challenging because of the uncontrollable size and heavy weight2.

The typical state of the Qin terracotta warriors and horses when they were unearthed (a), and a situation of restoration by experts (b).

Since the excavation of the terracotta warriors in 1974, the main adhesive used for the restoration of the terracotta warriors and horses has been epoxy resin. Although the selection of binders was limited by the conditions at that time, from the bonding strength point of view, it is reasonable to use epoxy resin as the binder for the restoration of the life-size terracotta warriors and horses. Even now for the restoration of large-scale pottery or stone artifacts, epoxy resins are good adhesive candidates also. Generally, epoxy resins are materials with high adhesion strength, water and chemical resistance. Epoxy resins were discovered in 1909, and the development began in the late 1930s. By the 1950s, epoxy resins were widely used commercially3. Due to epoxy resins being excellent mechanical properties, high adhesion strength, and good heat and chemical resistances, currently they have been widely used in various fields for high-performance coatings or consolidation4,5. As early as the 1960s, epoxy resins were candidates of high interest as stone adhesive materials, consolidates, and gap-fillers4. However, it has been proposed that there were advantages as well as disadvantages to the use of epoxy resins in heritage relics conservation. The disadvantages include irreversibility, poor penetration, and a crust formed near the surface causing color problems6,7. Due to these shortcomings mentioned, so as a general rule, epoxy resins should be avoided in the heritage protection field8. Objectively, the refusal to use epoxy resin in the field of heritage protection is too absolute. In fact, many views emphasize the need to use epoxy, especially in cases of reassembly of massive broken stone-based or pottery-based cultural relics because nothing else has the necessary strength. Epoxy resins are excellent when a very strong, permanent bond is required9,10. Additionally, the epoxy resins are used as cross-section binders rather than coatings, and their color changes have almost no effect on the appearance of the cultural relics. Although there are some counterviews in epoxy resins used in conservation, conservation experts always confirm that they are usually the appropriate binders for the reassembly and reconstruction of massive stone or pottery artifacts11.

The choice of binder to reassemble and reconstruct massive stone and pottery cultural relics is not only related to suitability, including quick cure time, good mechanical strength, and easy processing but also related to the anti-aging performance12. The anti-aging property of epoxy resin has been extensively studied. Indeed, epoxies that are exposed to long-term sustained levels of elevated temperatures, moisture, electric fields, and other harsh environments, can lead to degradation of the overall thermos-mechanical properties13,14. Generally, the evaluation of the anti-aging performance of materials used for restoration mainly derives from the results of simulated samples treated under artificially intensive environment conditions.

The inability to generate precise service life for polymer systems exposed in the field has been a challenge for over a century. Prediction of the lifetime and performance of materials used requires accelerated testing under different conditions such as temperature, humidity, aqueous solutions, or UV exposure. Due to the difference between definiteness of artificial aging conditions and uncertainty of the natural environmental conditions, as well as some parameter values selected for accelerated aging being much higher than that of the natural environment, artificial accelerated aging is not truly accelerated natural aging. Generally, these tests have been developed based on empirical considerations and do not take into account the degradation mechanisms experienced in field conditions. Strategies to speculate about natural aging from artificial accelerated aging have been questioned15. For example, if the exposure temperature set in accelerated material aging is close to or exceeds the glass transition temperature of the polymer (Tg), the mechanism of degradation may be different from that under the field condition. Consequently, there is significant uncertainty in applying this mechanism model to predict the safety risks of materials. In any case, natural aging can provide valuable information on material behavior under the real environment.

Epoxy resin has been used as a binder for the restoration of Qin terracotta warriors for nearly 40 years, and the binder has experienced long-term effects in the indoor natural environment that over 120 million visitors intervened. In the present work, the chemical degradation of the binders served on the Qin terracotta warriors mentioned above was assessed based on analysis including infrared spectroscopy, XPS and thermal stability. This investigation particularly highlights the following two aspects. Firstly, these samples have experienced the long-term effects in the indoor natural environment that over 120 million visitors intervened, and it is rare to find samples that have experienced such a special environmental impact. Secondly, compared to the artificial accelerated aging of materials, these results originated from natural aging in long-term practical environments and have a more effective and reliable reference value in predicting the safety of restored cultural relics. We believe that the results and conclusions obtained from this investigation have important reference values for the screening and the chemical degradation evaluation of epoxy resin binders served in similar environments.

According to the reports of the Mausoleum Museum of Emperor Qin Shihuang, the adhesive and curing agents used for the restoration of the Qin terracotta warriors and horses are diglycidyl ether of bisphenol A type epoxy resin (E-44) and polyamide 650 (PA650) respectively16. They (Commercial brand named Phoenix) were produced by the Chinses Wuxi Resin Factory. According to the existing literature, the structures of E-4417,18 and PA65019,20 are showed in Fig. 2, respectively. E44 is used typically added with a lower molecular weight PA650, in a 3:2 ratio by weight16,21. This formulation has a working at room temperature.

The chemical structure of the resin E-44 (a) and hardener polyamide 650 (b) cited from the literature16. No data sheet specifies exactly the chemical structures of the resin and hardener as commercial product mentioned in the report.

All epoxy resin samples were taken from different parts of different Terracotta Warriors restored between 1989 and 2020 year in Mausoleum Museum of Emperor Qin Shihuang. These terracotta warriors were all in the museum's exhibition hall, and microscale samples were carefully stripped in minimal intervention way from these terracotta warriors. The sampling point and code as well as the year of binder used were showed in Table 1. To make the survey results more representative, the binder samples were taken from different parts of terracotta warriors located at different pits of the Emperor Qinshihuang's Mausoleum Site Museum.

Except for the ATR (Attenuated Total Reflectance) method to be used to measure IR spectra of the particularly small samples (IC-1996-3, MA-2018-2, ES-1989-–3) by using ATR accessory, for other samples, the common KBr disk technique was used. KBr plates were prepared by mixing 1 mg of adhesive powders with 200 mg of KBr. IR spectroscopy was recorded in the range from 500 to 4000 cm−1 using a Perkin Elmer FT-IR spectrophotometer.

XPS measurements were performed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA, Kratos Analytical Ltd., Manchester, UK) using Al Kα radiation (1486.6 eV) as the excitation source. The trace samples (all samples involved in these measurements) were detected directly without any treatment.

Thermogravimetric analysis data were obtained using a TA Instruments STD Q600. The temperature was increased from room temperature to 600 °C under nitrogen at a rate of 10 °C/min. The trace samples (all samples involved in these measurements) were detected directly without any treatment.

It has been more than 40 years since the exhibition of the Qin terracotta warriors and horses was opened in 1979. In addition to a short high temperature and low temperature (such as − 21.2 °C, on December 28, 1991; 41.8 °C, on June 2, 1998), the temperature and humidity in other periods are relatively regular. Here are the typical temperature and humidity of the pits of the terracotta warriors and horses in several years. In 1982, the temperature ranged from 30 to − 3 °C, the relative humidity is in the range of 24–88%22; In 2010, the temperature ranged from 41 to − 1 °C, the relative humidity is in the range of 14–86%23; In 2013, the temperature ranges from 38 to 2.8 °C, the relative humidity is in the range of 20–79%24.

The participants were informed about the study and signed informed consent forms. The participants and any identifiable individuals consented to publication of his/her image.

From the molecular structure of the epoxy resin adhesive shown in Fig. 2, the structure contains the amide bond, ether oxygen bond, benzene ring, hydroxyl group, imine bond, etc. The main characteristic peaks related to these bonds of all samples can be found in Fig. 3, indicating that the IR spectral characteristic peaks of the adhesive here measured match the bisphenol A epoxy resin cured with polyamide.

FTIR spectra of the epoxy resin binders served on the restored terracotta warriors. The spectra obtained using KBr plates (a, b, c and d) and ATR (e).

Although the infrared spectra of all samples shown in Fig. 3 are extremely similar, some differences still exist. For the spectra obtained by using the KBr pressed-plate method as shown in Fig. 3a,b,c,d, the following results were obtained. Firstly, the FTIR characteristic peaks from PA650 and E-44 are found in Fig. 3. The intensive and relative sharp absorption band centered at 3420 cm−1 ranging from 3685 to 3000 cm−1 corresponds to the overlapping stretching modes of the primary amine group form PA650 and the hydroxyl groups formed by the reaction of PA650 with epoxy groups of E-4425. Additionally, the stretching modes of the C–H bonds at near 2920 cm−1 and 2850 cm−1 bands, and the bending mode of the H–C–H angle at 1462 cm−1, originated from the CH2 groups in PA650 and E-44 also appeared. For PA650, the most prominent FTIR characteristic peak is the carbonyl of a primary amide vibration at 1650 cm−1 19,26. For E-44, the characteristic peaks of the bisphenol A epoxy resin, 1510 cm−1 elongation vibration associated with aromatic ring (–C=C), 1362 cm−1 (the CH3-bending vibration)27, the band at 1184 cm−1 to the stretching mode of Ar–C–Ar (Ar represents the aromatic ring), the band at 1038 cm−1 attributed to the stretching C–O–C of ether, the peak at 830 cm−1 of the benzene ring distribution, appear in the spectra of all samples28. Above results indicated that the adhesive used in the restoration of terracotta warriors belongs to a mixture of polyamide and bisphenol A epoxy resin.

Although the PA650 polyamide and E-44 epoxy resin respectively have the above typical characteristic peaks and the FTIR spectra of all samples are similar to some extent, there are overlap of some characteristic absorption peaks between PA650 and E-44, resulting in the difficulty to precisely assign these peaks. For example, the peaks at 1250 cm−1 (asymmetric C–C–H bending) and 1164 cm−1 (symmetric deformation C–C–H)29 from PA650 respectively overlap the peaks at 1250 cm−1 (the anti-symmetric stretching of aromatic ether –C–O–C) and 1164 cm−1 (symmetric stretching of aromatic ether) from E-4430,31,32. However, the relative content of E44 in the binders can be determined by using the relative intensity of E44 characteristic peaks. These peaks include the elongation vibration at peak 1510 cm−1 associated with the aromatic ring (–C=C) and the peak at 830 cm−1 of the benzene ring distribution28. In the infrared spectra of all samples, the samples (ES-2018-2, IC-1990-3, CT-1989-3, BF-1989-3) showed strong characteristic absorption at 1510 cm−1 and 830 cm−1, indicating that these samples contained higher E44, also reflecting that there was some degree of randomness in the relative amount of E44 and PA650 used in the restoration practice of the Qin terracotta warriors. In fact, the difference in the relative amount of PA650 and E-44 in the binders has some universality in practice33.

Additionally, there are some distinct differences in details of the FTIR spectra ranging from 1250 to 1000 cm−1 for the samples shown in Fig. 3. These differences mainly reflect changes of various vibration modes involving aromatic ether, fatty ether, and alcohol hydroxyl dominated by the reaction of PA650 with E-44 in a different relative amount34.

For MT-2015-3 (Fig. 3d), compared with all samples, the intensities of absorption mainly attributed to E-44 at 1510 cm−1, 1462 cm−1, 1184 cm−1 and 830 cm−1 are lower, indicating that the ratio of E-44 to PA650 used in MT-2015-3 is the lower among samples measured. Accordingly, the peaks related to the alkyl ether groups and the secondary alcohol groups at 1085 cm−1 (alkyl ether C–O symmetric stretching), 1038 cm−1 (C–O bending vibration), at 3420 cm−1 (O–H stretching vibration, N–H stretching vibration), formed by the reaction between E-44 and PA650 appear very obvious. For BF-1989-3 (Fig. 3d), compared with MT-2015-3, the absorption intensities at 1530 cm−1, 1462 cm−1, 1184 cm−1 and 830 cm−1 originated from E-44 are higher, indicating that the ratio of E-44 to PA650 used in MT-2015-3 is the higher among samples measured. Additionally, due to the presence of two epoxy groups in bisphenol A epoxy resin E-44, more ether and alcohol hydroxyl groups are present in the reaction products between E-44 and PA650. As a result, the peaks related to the alkyl ether groups and the secondary alcohol groups at 1085 cm−1 (alkyl ether C–O symmetric stretching), 1038 cm−1 (C–O bending vibration), the at 3420 cm−1 (O–H stretching vibration), exhibit more obvious.

Based on the infrared spectrum analysis mentioned above regarding the two typical ratios of E-44 and PA650 for preparation of binders, it can be inferred that samples ES-2018-2 and ET-1989-3 binders were prepared in a relatively high ratio of E-44 to PA650, while the binders (SF-2018-2, IC-1990-3, MT-2020-3, UC-1996-3, CF-1989-3) were prepared in a relatively lower ratio of E-44 to PA650. For the two-components epoxy resin binders that require on-site mixing of two components with strong viscosity, it is difficult to strictly control the amount of components in practical operation. In fact, the above deduction mentioned is quite similar to the relative change of aromatic ether and aliphatic ether in the bisphenol A epoxy resin system reported in the literature35. Therefore, it is not surprising that the binders used at different times have different ratios of E-44 and PA650. However, in view of practice requirements, it is necessary to develop a convenient method for on-site quantitative mixing of epoxy resins and cross-linking agents.

Because the amount of samples (MA-2018-2, ES-1989-3, IC-1996-3) obtained was not enough to be determined using the KBr method, these samples were measured by the ATR mode. As for the spectra obtained using ATR mode (Fig. 1S), due to the inherent feature of this method, the measured result obtained from this method is not exactly same as the KBr method. Firstly, a lower intensity at higher wave-numbers is one of the innate effects associated with ATR due to the decreased penetration depth of the evanescent wave36, this results in greater absorption on the longer wavelength side of an absorption band, contributing to band distortion and band broadening. However, the FTIR spectra obtained by using the KBr method are qualitatively similar to the ATR spectra in the fingerprinting region37. Secondly, ATR mode mainly responds to the surface information of a sample. For the surface composition of the sample may being different from that of the interior, the ATR mode has more advantages over the KBr method in detecting the FTIR spectrum. In any case, under the same detecting conditions, the differences in the spectra of monitored samples with different aging times can still provide some useful information. The ATR FTIR spectra of the samples (MA-2018-2, ES-1989-3, IC-1996-3) show in Fig. 4. For the samples of ES-1989-3 and MA-2018-2, their main characteristic peaks were basically similar to that found in Fig. 3. According to the similar analysis of the spectra shown in Fig. 3, there is no significant advantage in the relative amounts of E44 and PA650 used in samples (MA-2018-2, ES-1989-3, IC-1996-3) shown in Fig. 4. Unexpectedly, for ES-1989-3 and MA-2018-2, the strong peaks at 2920 cm−1 and 2850 cm−1 corresponding to the stretching modes of the C–H bonds of –CH2– groups appeared, unlike the spectrum determined by ATR mode to be commonly lower intensity at higher wave-numbers. This situation may be related to the relatively more abundant distribution of PA650 chain segments on the surface. PA650 has the hydrophobic chain segment38. As a result, such hydrophobic chains tend to move towards a surface, spontaneously, the groups of –CH2– groups as well as the corresponding functional groups in PA650 can be specially monitored by ATR mode (Fig. 4). It has been reported that the chemical composition of an epoxy resin is not uniform along the direction normal to the solid interface39. The primary amide vibration characteristic peak at 1650 cm−1 and N–H stretching vibration at 3300 cm−1 from PA650 also indicate the distribution advantage of PA650 on the surface of the binder. These results indicate that ATR FITR spectroscopy has a significant advantage in characterizing surface functional groups of materials. In general, the aging of materials first occurs on the surface. Therefore, ATR FITR has obvious advantages in characterizing material aging. By polishing the surface of the sample and examining the FTIR spectra of the bulk resin materials, Miller SG et al. confirmed that the chemical aging processes, including cure and oxidation, were limited to the surface of the resin40. Therefore, to some extent, the infrared spectroscopy detected by the ATR mode is more effective for evaluating the epoxy resin aging.

ATR FTIR spectra of the epoxy resin binders served on the restored terracotta warriors. The spectra obtained using ATR mode.

In particular, the intensity in IR spectrum of the sample IC-1996-3 is significantly weaker even under the same test conditions. Obviously, the result is related to the sample itself rather than others. It has been found that rigid materials can be problematic to measure with the ATR technique as it is difficult to create close optical contact with the diamond crystal. As contact is confined to small areas, weak spectra are produced, the effects of which are greatest at shorter wavelengths where the depth of penetration is lowest41. It is clear that aging causes an increase in relative rigidity, and it is explained by the reduction in free volume during the physical aging process42. Meanwhile, within the range of 1250–880 cm−1, the strong absorption peak indicates that the sample IC-1996-3 may have a high cross-linking density. This is also another reason why the sample has strong rigidity. Therefore, the sample IC-1996-3 may have too strong rigidity due to physical aging and high cross-link density so it is difficult to create close optical contact with the diamond crystal in ATR mode, resulting in decreased optical absorbance.

Although there are some differences in the specific details of the infrared spectrum caused by the difference in the relative content of E-44 and PA650 or the detection mode, all the samples have similar normal spectral features. Unfortunately, the lack of FTIR spectra of the corresponding original samples made it impossible to directly compare with present spectra. However, many investigated results regarding epoxy resin aging reported in the literature provided an important reference to estimate whether the measured samples have been aged. It has been reported that the carbonyl group was an important marker of epoxy resin aging25,43,44,45, and the formation of different carbonyl units might originate from the photo-oxidation of different alkyl and phenyl units. In terms of hygrothermal aging, most of the literature showed that hygrothermal aging can also cause the oxidation of α–CH2 or amines29,39,46,47,48 and isomerization of oxirane ring49 to form carbonyl groups. Fortunately, the characteristic peak at 1730 cm−1 assigned to the carbonyl functional groups was not present in the FTIR spectra of all samples shown in Fig. 3, indicating that all samples did not undergo visible chemical aging25,40,41,42,43,44. In fact, the results are not unexpected to some extent. All exhibited unearthed Qin terracotta warriors and horses are set in the exhibition hall of the Mausoleum Museum of Emperor Qin Shihuang, and this museum is located in the Guanzhong Plain, China, with a semi-arid and semi-humid climate. As described in the 2.3 section, the humidity and the temperature of the museum don’t exceed 90% and 43 °C, respectively. Generally, in a year, the humidity and the temperature in two extreme climatic conditions are respectively in summer 2.8–13.6 °C, 20.0–57.6%, in winter 23.2–37.5 °C, 4 3.0–78.5%24. It has been reported that epoxy resins were not prone to chemical aging in such environmental conditions. Many studies have shown that the UV photo radiation degraded materials much more compared to hygrothermal expose25. Additionally, the hygrothermal aging of epoxy resins at below 60 °C did not cause any chemical modifications, the chemical aging at this temperature was very slow even at 90% relative humidity36,47. For instance, the literature reported that the epoxy resins did not significantly change in colour upon in-room storage after 28 years50. Therefore, in a mild indoor temperature and humidity environment for 40 years, no significant chemical aging of the epoxy resins used for the restoration of the Qin terracotta warriors is not surprising.

X-ray photoelectron spectroscopy (XPS) is an indispensable technique in materials science for the determination of surface chemical bonding. To further investigate the possible chemical aging of the epoxy resin served in the Qin terracotta warriors, the XPS spectra of all samples involved in this investigation were detected. It can be found that there are three elements, C, N and O, in all samples, and N is related to PA650, and C and O are related to both E-44 and PA650. Taking into consideration of the molecular structure of the samples and the reports in reference51,52,53,54,55,56,57, here we tentatively performed curve-fitted high-resolution XPS spectra of C1s, O1s and N1s, and the components that correspond to the different functional groups. The corresponding results of the samples (SF-1989-3, IC-1990-3, MT-2020-2), (UC-1996–3, CT-1989-3), (ES-2018-2, ES-1989-3), (MT-2015-3, BF-1989-3), (MA-2018-2, ES-1989-3, IC-1996-3), were shown in Fig. 5a,b,c,d,e), respectively. For the C1s spectra, five different carbon environments were found, including 284.8 eV (C–H, C–C, C=C), 285.3 eV (C–N), 286.1 eV (C–OH, C–O–C), 288.2 eV (NC=O)58,59. For the O1s spectra, three different oxygen environments were found, including 533.0 eV (C–OH and C–O–C), 531.8 eV (NC=O, adsorbed water or oxygen). It can not be divided an unequivocal signal related to C=O groups at ~ 534 eV in the O1s spectra and to C=O groups at 289–290.1 eV in the C1s spectra60,61,62, further indicating no chemical aging occurred visibly in all samples. For the N1s signal in all samples, the peak centered at ~ 400 eV appeared, which can be assigned to C–N CONH species57. It is worth noting that the N1s spectra of the samples (IC-1990-3 Fig. 5a; CT-1989-3, Fig. 5b; T-1989-3, Fig. 5c; BF-1989-3, Fig. 5d; MA-2018-2, ES-1989-3, IC-1996–3, Fig. 5e) emerged a small shoulder peak at 401.8 eV attributed to the amine groups (N–H) species63, indicating that these samples have a certain number of uncross-linked amine groups.

(a) XPS of C1s, N1s and O1s spectra of the epoxy resin binders (SF-1989-3, IC-1990-3, MT-2020-2). (b) XPS of C1s, N1s and O1s spectra of the epoxy resin binders (UC-1996-3, CT-1989-3). (c) XPS of C1s, N1s and O1s spectra of the epoxy resin binders (ET-1989-3, ES-2018-2). (d) XPS of C1s, N1s and O1s spectra of the epoxy resin binders (MT-2015-3, BF-1989-3). (e) XPS of C1s, N1s and O1s spectra of the epoxy resin binders (MA-20187-3, ES-1989-3, IC-1996-3).

The XPS spectra shown in Fig. 5a,b,c,d,e effectively reflect the main chemical bonds of all samples detected in this study, which consistent with the conclusions obtained from the corresponding infrared spectra shown in Figs. 3 and 4. Figure 5a,b,c,d,e indicates that the peak attributed to C–O–H and C–O–C species in ES-2018-2, ET-1989-3, SF-1989-3, MT-2015-3 and BF-1989-3 is more obvious than that in IC-1990-3 and ES-1989-3, which is related to the fact that the ring-open reaction of epoxy group occurred in the former samples is more significant than in the latter ones. These results are consistent with the characteristic peaks ranging from 1200 to 1132 cm−1 and the centered 3400 cm−1 shown in Figs. 3 and 4. It is worth mentioning that there is an evident difference in the consistency of the relative intensity between the species CONH and the species (C–O–C, C–OH) in C1s and in O1s in Fig. 5 for all samples. For samples, the species CONH compared with the species (C–O–C, C–OH) in O1s are always dominant but not in C1s. The unexpectedly high portion of CONH in the deconvolution of O1s spectra may be associated with the oxygen species unconsidered. It has been found that the O2 binding energy (BEs) corresponds to various oxygenated species, as for example, C=O, having similar BEs (531.8 eV) and thus hardly discernable by curve-fitting64. This report implies that the variability mentioned above may be related to the presence of oxygen in the samples. In practice, air inclusion can occur when mixing two-part epoxy liquid systems by hand65. Only vigorous stirring can mix the sticky E-44 and PA650 due to both E44 and PA650 being highly viscous. These results imply that the binder served in Qin terracotta warriors has oxygen that had been randomly introduced during the restoration process.

Generally, a initial decomposition temperature is defined as the temperature at which the material loses 5% of its weight66. Therefore, thermogravimetric analysis (TG) is one of the important methods to assess material aging67,68.

Figure 6 showed the thermogravimetric curves (TG(A1, B1), DTG(A2, B2)) of the binders used restoration of Qin terracotta warriors and horses. As shown in Fig. 6, for all samples, their TG curves have the similar profile, and the initial temperatures at which the samples significantly lost weight were above 300 °C, being very similar to the results of unaged epoxy resins reported in the literature69,70,71. Generally, the loss weight ranging from 310 to 480 °C is mainly caused by the elimination reaction of the end groups (hydroxyl, amino) in the cured structure and the degradation of the epoxy resin backbone72. To make it easier to identify the thermal decomposition temperatures of all samples, the derivative results (DTG) of the TG curves are shown in Fig. 6A2,B2, evidently indicating that there are certain differences in the thermal decomposition temperature of the measured samples. The temperature difference of the sample during the significant weight loss stage may be related to the difference in the contamination of inorganic substances in the sample. Comparing the DTG curves (Fig. 6A2,B2) and the residual weight in the thermogravimetric curves (Fig. 6A1,B1), it can be observed that the turning point of the thermal decomposition curve at the high temperature end of the sample with higher residual weight shifts to the left, resulting in a decrease in the weight loss temperature. In any event, the thermal decomposition behaviors of all samples shown in Fig. 6 are quite consistent with the literature report, suggesting that there has been no noticeable change over time in the structural and compositional features of all detected samples. It can therefore be concluded that the binder served in Qin terracotta warriors and horses did not undergo obvious chemical aging.

TG/DTG curves of the epoxy resin binders.

It is worth noting here that there were still some subtle differences in the thermal decomposition behavior of these binders. Firstly, for the samples, including UC-1996-3, IC-1996-3, MA-2018-3, IC-1990-3, and MT-2020-3, exhibited the slight loss weight (approximately, 4%) ranging from 50 to 250 °C shown in Fig. 6A, and the corresponding curves were slightly different. These phenomena may be related to the volatilization of residual oligomers and residual moisture in epoxy resins72,73,74,75. In other words, it is normal that the slight loss weight at below 250 °C for the determined binder served in Qin terracotta warriors and horses. Secondly, a stabilization of mass percentage appeared after 500 °C for all samples, however, there were a significant difference in the final residue proportion between the determined samples. This difference is primarily attributed to the contamination of ceramics. Epoxy resin, as a masonry binder, has a so strong adhesion that the ceramic particles can be also peeled off in the sampling process of peeling epoxy resin16. Polarization micro-graphs and EDX spectrum of the corresponding samples respectively confirm this hypothesis. Using SF-1989-3A as a typical sample, the corresponding results are shown in Fig. 7. As expected, prominent characteristic peaks of the typical elements, such as Si, Al, K, Mg and Na related to ceramic materials, appeared in the EDX spectrum of the sample SF-1989-3. The bright regions originated from the bi-refraction of ceramic grains appeared in the polarization micrograph of the sample SF-1989-3 shown in Fig. 7, further demonstrating the presence of inorganic minerals in the sample SF-1989-3. The above results not only verify the rationality of the higher residual ratio observed in the thermogravimetric curves but also demonstrate that the binder still maintains a strong bonding ability. In other words, the performance degradation of the binder served in the Qin terracotta warriors is not significantly.

SEM image, EDX spectrum and polarization micro-graph of the sample SF-1989-3.

The binder being composted of bisphenol A epoxy resin E-44 cross-linked by polyamide (PA650) was used before more than 30 years in restoration of the Qin terracotta warriors. In this investigation, the chemical aging states of the binders served in the Qin terracotta warriors were investigated by FTIR, XPS and TG. Based on the results, the following conclusions are drawn.

The carbonyl formation is an efficient indicator for evaluating the aging extent of epoxy resin binders. For all binders used for restoration during the 30 year period, carbonyl functional group was not detected, indicating that the binders used for repairing the Qin terracotta warriors did not exhibit significant chemical aging.

The binder has so strong adhesion that the ceramic grains are also peeled off from the bonding surface of the terracotta warriors when the binder is peeled for sampling, sufficiently demonstrating that the binder served in the terracotta warriors still has strong adhesion.

The reason why the binders used in the restoration of the terracotta warriors did not undergo obvious chemical aging is related to environmental conditions its located at. The binder was served in Qin terracotta warriors locates at a mild indoor natural environment. This environment not only prevents the binder from being directly exposed to sunlight, but also kept the binder at a temperature and humidity much lower than that required for chemical aging of epoxy resin binders. In addition, the absence of transition metals that significantly catalyze the degradation of epoxy resin in the ceramic materials of the Qin terracotta warriors is also one of the important reasons why epoxy resin binders do not undergo chemical aging.

The performance degradation of epoxy resin binders strongly affects the reliability and safety of its served products, so it is important to predict its lifetime under the performance requirements. Artificially accelerated degradation is a common way to be employed. Generally, the accelerated degradation base on acquiesces the degradation mechanism in accelerated conditions to be consistent with that in field conditions. In fact, some parameters for accelerated degradation are commonly beyond that in the actual environment. As a result, the mechanism of degradation in accelerated testing conditions may be obviously different from that in field conditions, and the results obtained from the artificially accelerated degradation are likely to deviate significantly from reality. In this sense, the aging evaluation of real long-term epoxy resin binder served in the Qin terracotta warriors is more reliable and practical guidance. The results and conclusions obtained from this investigation have important reference value for the screening and the chemical aging evaluation of epoxy resin binders served on similar environment.

The datasets generated and/or analyzed during the current study are not publicly available due data use requires permission from experimental collaborators but are available from the corresponding author on reasonable request.

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The author thanks the National Natural Science Foundation (21773150) and Key R&D Program of Shaanxi Province of China (2021ZDLSF-06-01) for their project supports and the other members of the research team for their scientific research assistance.

Engineering Research Center of Historical and Cultural Heritage Protection, Ministry of Education, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710062, China

Siyu Tian, Wenhua Ma & Daodao Hu

Emperor Qin Shihuang’s Mausoleum Site Museum, Lintong, Xi’an, 710600, China

Hua Li, Chunyan Wang, Yin Xia, Desheng Lan & Ping Zhou

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S.T. conducted the experiments and analyzed the data. W.M. prepared the figures and drafted the manuscript. H.L., C.W., Y.X., D.L. and P.Z. contributed to the research plan determination and provided the research resources. D.H. designed the research scheme, conceived and supervised the project, analyzed the data and wrote the manuscript. All authors reviewed the manuscript.

Correspondence to Ping Zhou or Daodao Hu.

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Tian, S., Ma, W., Li, H. et al. Assessment of chemical degradation of epoxy resin binder used in the service of the Qin terracotta warriors. Sci Rep 14, 17572 (2024). https://doi.org/10.1038/s41598-024-68442-3

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DOI: https://doi.org/10.1038/s41598-024-68442-3

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